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Radiation hardness of thin film solar cells

Thesis report

Paraskevi Danaki

Supervisor: Charlotte Platzer-Bjorkman

Department of Physics and AstronomyUppsala University

SwedenJune 18, 2019

Abstract

In this master project, the proton radiation effects in thin film solar cells were studied.Electrical characterization was performed before and after radiation to determine theeffects on Cu2ZnSnS4 (CZTS) and CuInxGa(1−x)Se2 (CIGS) cells. Proton radiationcaused degradation on both types of cells. Room temperature annealing gave improvedperformance, but the recovery was slow. Thermal annealing at 100 C had notable im-pact on the basic current-voltage parameters. In CZTS cells, after 17 hours annealing,the open circuit voltage recovered completely, short-circuit current recovered almost com-pletely and the fill factor had minor recovery. Quantum efficiency measurements showeddegradation for both CIGS and CZTS devices in all wavelengths after radiation, andpartial recovery after annealing. Doping density measurements were also implementedwith two different methods, capacitance-voltage and drive level capacitance profiling. Theradiation did not cause any significant change in the doping density. Additionally, admit-tance measurements that were performed in both types of cells, could not give conclusiveresults. Lastly, a model was also created before and after the radiation for a CZTS cell,in order to quantify the caused defects. To fit the experimental data after the radiation,an increase of defect density in the CZTS layer of 87% was needed.

Acknowledgments

First of all, I would like to thank my supervisor, Charlotte Platzer-Bjorkman. She trustedme from the very beginning and supported me throughout the whole project. She gaveme guidance and valuable feedback whenever I needed, while allowing this project to bemy own work. Thank you Lotten.

Secondly, I would like to offer my special thanks to my mentor on my master studies,Matthias Weiszflog. His valuable suggestions and discussions during my studies helpedme choose the path I wanted to take.

I am also particularly grateful for the assistance given by the solar cell group at Up-psala University. In particular Volodymyr Kosyak shared with me his expertise on theexperimental devises and Sethu Saveda Suvanam on radiation effects and the space en-vironment.

My special thanks are extended to my parents for supporting me throughout my life.Finally, my master thesis would have been impossible without the aid and enthusiasticencouragement of Anastasios Gorantis. He helped me stay focused, organized and evolveas a physicist.

Contents

1. Sammanfattning pa svenska 5

2. Introduction 6

3. Theory 113.1. Solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.1.1. Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.1.2. pn junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.1.3. One diode model . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.1.4. Quantum efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . 163.1.5. Capacitance measurements . . . . . . . . . . . . . . . . . . . . . . 173.1.6. Main losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.2. Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.2.1. Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.2.2. Effects of radiation on solar cells . . . . . . . . . . . . . . . . . . 20

4. Experimental Method 254.1. Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 254.2. Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.2.1. IV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.2.2. QE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.2.3. CV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284.2.4. DLCP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284.2.5. TAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.3. Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5. Results 345.1. Electrical Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . 34

5.1.1. CZTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345.1.2. CIGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

5.2. Device Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455.2.1. Before radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455.2.2. After radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

6. Discussion and Conclusions 536.1. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 536.2. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 546.3. Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Appendices 59

A. Admittance 59

4

1. Sammanfattning pa svenska

Solceller ar en vaxande del av energitillforseln inte bara i Sverige utan i hela varlden.En av fordelarna med solceller ar att de kan anvandas i fristaende system. En typ avfristaende system ar rymdtillampningar. Rymduppdrag som utfors narmare an planetenMars, anvander solceller for energitillforseln. Ett problem ar dock att i rymden finns detgammastralning och andra partiklar som kan skada solceller och for att skydda solcelleroch forlanga deras livstid, maste man kanna till stralningseffekter.

Pa Jorden anvands kisel solceller, men de ar tunga och degraderas av stralning. Darforanvands andra material i rymden som har hog effektivitet och ar tunnare och lattare an ki-sel. I det har examensarbetet, undersoktes om tva material kan anvandas, CuInxGa(1−x)Se2(CIGS) och Cu2ZnSnS4 (CZTS). Solcellerna utsattes for stralning med protoner och de-ras elektriska egenskaper mattes fore och efter stralningen. Dessutom, utvecklades en mo-dell for CZTS beteende fore och efter stralningen for att forsta och kvantifiera stralningenseffekter pa solcellerna.

Bada materialen degraderades efter stralningen eftersom CIGS effektivitet sjonk med58% och CZTS med 40%. Proven varmebehandlades i 100 C och genom detta okadederas effektivitet delvis. Den utveklade modellen visar att defektdensiteten i CZTS okademed 87% efter stralningen.

Sammanfattningvis har CZTS och CIGS solceller hog potential att anvandas i rymd-tillampningar eftersom de visar en hog tolerans for stralning, om deras effektivitet skulleokas.

5

2. Introduction

In the last 60 years the world has changed significantly, with electrical devices being anintegral part of our daily life. Electrical energy is required for all of them to work, so mostof us cannot imagine living without a reliable energy supply. The increasing demandsrequest increased supply, but one source is not enough as nothing comes without anydisadvantages. The sun is the main source of energy on planet Earth. It heats air massescreating wind, which powers wind turbines. It sets the water cycle in motion, poweringhydroelectric power plants. It plays a crucial role in photosynthesis feeding organismsand vegetation that are now used as fossil fuel, and of course, it provides energy directlywith the help of solar cells.

The history of solar cells started in 1839 when Alexandre Edmond Becquerel discoveredthe photovoltaic effect, the main principle behind solar cells. The first solar cell, with anefficiency of around 1%, was produced in 1885 by C. Fritts [1]. In 1905, Albert Einsteinexplained the photoelectric effect, an effect similar to the photovoltaic, which laid thetheoretical foundation for the study and development of solar cells. Some years later,in 1954, a working solar cell with 6% efficiency was developed at Bell laboratories [2].The next revolution came in 1976, when Carlson and Wronski fabricated the first thinfilm solar cell from amorphous silicon [3]. In the next decade, the two sibling materialsthat will be our focus, namely CuInxGa(1−x)Se2 (CIGS) and Cu2ZnSnS4 (CZTS), werefabricated for thin film solar cell uses.

The number of applications is increasing dramatically as the costs drop. From calcu-lators to solar parks, photovoltaics are an essential and growing part of energy supply.The ability to build off-grid systems is one of the reasons the field of solar cells is grow-ing. Space applications are a type of off-grid systems, although probably not the mostcommon ones. Most missions that are closer than the orbit of Mars, use solar cells aselectricity supply. Further than Mars, sunlight is not enough to power missions, so othermethods are used.

The first satellite with solar cells was launched in 1958 named Vanguard 1 [4]. Inthe beginning, crystalline silicon solar cells were used for space missions, since Si was theonly available material. The disadvantages however were many. Silicon cells are heavy, donot have high efficiency compared to their mass, and are affected by solar flare particles.When solar cells based on other materials with better properties were manufactured, theydominated. Now, triple junction solar cell are mainly used, although they also have bothadvantages and disadvantages. The research for radiation hard cells with high efficiencyis still ongoing.

The aim of this project was to investigate the effects of proton radiation on thin filmsolar cells. Understanding the effects of radiation is essential in order to prevent damagesand improve efficiency and lifetime in space applications. To that end, one needs to mea-sure the cells’ electrical characteristics before and after the radiation. There are manycharacterization methods that can be used, such as electroluminescence, capacitance mea-surements, optical techniques etc. In this work we focused on electrical characterizationand capacitance measurements. The effects of radiation on capacitance based measure-

6

CHAPTER 2. INTRODUCTION

ments have not been included in radiation hardness research of CZTS cells yet, so partof this project was to attempt to determine them. Modeling the device before and afterthe radiation was also performed, since it can offer insights in the damages that radiationcauses.

The rest of this thesis is organized as follows. First, in chapter 3, we review the workingprinciples of solar cells and the characterization methods we used. Radiation and why itaffects solar cells will also be revised. In chapter 4 we explain the experimental procedurewe followed, and review the equipment that we used. Then, in chapter 5 we present theexperimental results. Finally, in chapter 6 we discuss these results, draw some conclusionsand provide an outlook.

7

3. Theory

3.1. Solar cells

The main principle behind solar cells is the photovoltaic effect, which was first demon-strated by Becquerel in 1839. When materials are exposed to light, electric current andvoltage are created. This happens because light transfers its energy to the electrons ofthe material, which get excited. The photovoltaic effect is similar to the photoelectriceffect, but in the photoelectric phenomenon the emitted electrons escape the materialwhile in the photovoltaic, they stay inside it.

3.1.1. Semiconductors

The energy of electrons bound to an atom is quantized, so it can only take certain valuescalled energy levels, which are given by the Schrodinger equation. The lowest energylevel that an electron can occupy is the ground state, and if the electron absorbs enoughenergy, it can go to a higher, excited state.

In crystalline materials, the energy levels are so close to each other that they createcontinua called bands. The band that contains the ground states with higher energy iscalled the valence band and the band that contains the excited states with the lowestenergies is the conduction band. If a range of energies has no allowed states, it is calleda band gap. Depending on the size of the band gap, materials are divided into thecategories metal, insulator and semiconductor. In metals, the conduction and valencebands are very close or overlap, and there is a small or no band gap. In insulatorsthere is a large band gap, which prevents electrons from going from the valence to theconduction band. Semiconductors are in the middle, with small band gaps that, in certaincases, allow the excitation of electrons from valence to conduction band.

There are two types of band gaps: direct and indirect (Figure 3.1). The type of bandgap is an intrinsic property of the material. An example of a direct band gap materialis GaAs and an example of an indirect one is Si. In a direct band gap material, themaximum energy of the valence band and the minimum energy of the conduction bandhave the same crystal momentum. Therefore, only a change in energy is required for anelectron to go from the valence to the conduction band. In an indirect band gap material,on the other hand, in addition to a photon, an electron needs to interact with a phonon,making the excitation to the conduction band less likely. However, the requirement ofinteraction among more particles, also makes it more difficult for an electron and hole torecombine, as the presence of a phonon with the appropriate momentum is required.

8

CHAPTER 3. THEORY

Momentum

Energ

y

Eg

Momentum

Energ

y

Eg

Direct band gapIndirect band gap

Photon absorption

Phonon absorption

Valence band

Conduction band

Valence band

Conduction band

Photon absorption

Figure 3.1.: Direct and indirect band gap

The occupation probability of the energy states follows the Fermi-Dirac distributiondescribed with:

F (E) =

[exp

(E − EF

kT

)+ 1

]−1

(3.1)

where F is the occupation probability, E is the energy of the state, EF is the energy atwhich the occupation probability is 0.5 (Fermi level), k is the Boltzmann constant, andT is the temperature.

When the material is doped with impurities, the Fermi level moves towards the valenceor the conduction band, depending on the type of the doping, and the electrical propertiesof the semiconductor are modified. Pure silicon has four valence electrons (electrons inthe outer shell of the atom), which bind to the neighboring atoms creating the lattice.If silicon is doped with a group III element, such as boron (B), which has three valenceelectrons, one electron of the neighboring silicon atoms is unpaired creating a vacantstate of electron hole. If, on the other hand, silicon is doped with a group V element,like phosphorus (P), which has five available electrons, there is one extra free electronin the lattice. When the material contains more holes it is called “p-type” and when ithas more free electrons it is called “n-type”. The type of semiconductor indicates whichis the material’s majority carrier. It should be clarified here that there is no additionalcharge, so the crystal is still neutral.

The addition of impurities can help to increase the number of free carriers. When thematerial is at temperature 0K, carriers do not have enough energy to move in the lattice.With increasing temperature, some electrons become thermally excited and are free tomove in the lattice, leaving behind empty holes, which can also move. Doping atomsadd energy levels inside the band gap, giving the opportunity to electrons to move to orfrom these states. At room temperature, there are enough electrons and holes availableto move freely, so the material conducts current.

3.1.2. pn junction

Things become even more interesting when a p-type material comes into contact with ann-type material, creating a pn junction. When this happens, the excess electrons fromthe n-type diffuse into the p-type and the holes from the p-type diffuse into the n-type.

9

CHAPTER 3. THEORY

When they meet, they recombine, leaving ions of opposite charge in each side. The ionscreate an electric field and the area in which they are located, is called the depletionregion or space charge region (Figure 3.2). The electric field created by the depletionregion drives a current named drift current, that counteracts the diffusion of electrons tothe p-type and holes to the n-type. The process continues until an equilibrium betweenthe two forces is achieved.

++ + + + + + + + + + + + + + +

---

--

--

--

--

--

--

-

n-type

p-type

Space

charge

region

Diffusion

length

Figure 3.2.: The pn junction

There are two options to connect a pn-junction to a circuit. Either the positive terminalcan go to the p-type and the negative terminal to the n-type or the opposite. The firstis called forward bias and the last reverse bias.

When the junction is connected with a forward bias, the voltage drives the holes inthe p-type and the electrons in the n-type to the depletion region. The charge carriersrecombine with each other reducing the width of the space charge region and the resistivityof the junction. With the shortened depletion region, it is easier for majority carriers tocross the junction and recombine with the minority carriers in the other side. The resultis a constant carrier flow, i.e. a current. Electrons and holes are oppositely charged, butthey also flow in opposite directions so their current is added and the junction behavesas a conductor.

In a reverse bias connection of a pn junction, the majority carriers are pulled awayfrom the depletion region increasing its width. The majority carriers cannot cross thejunction, the resistivity increases and the junction behaves as an insulator.

The current of an ideal pn junction is described with:

J(V ) = J0

[exp

(qV

kT

)− 1

](3.2)

where J is the current density, J0 is the saturation current density, q is the electroncharge and V is the applied voltage. A solar cell in the dark is actually a pn junction,which means a diode, and it follows this equation. The ideal IV curve is shown in

10

CHAPTER 3. THEORY

Figure 3.3. The saturation current density is given by:

J0 = A

(qDen

2i

LeNA

+qDhn

2i

LhND

)(3.3)

where A is the cross-sectional area, De and Dh are the diffusion coefficients for electronsand holes, ni is the intrinsic carrier concentration, Le and Lh are the diffusion lengths forelectrons and holes, and NA and ND are the acceptor and donor densities in the n and ptype respectively.

I

V

Forward

BiasReverse

Bias

Break-down

voltage

Knee

voltage

Figure 3.3.: Ideal IV curve

When a solar cell is illuminated, there are three possibilities. Firstly, the photons canbe reflected in the surface and not interact with the cell at all. Secondly, the photonscan pass through the solar cell, also without interacting. In the last case, energy fromthe light can be deposited into the electrons in the valence band, exciting them to theconduction band. The photon should have energy greater than the band gap to excite anelectron. An electron-hole pair is generated and the carriers are added to the majority andminority carriers. As the concentration of minority carriers increases, the drift currentincreases. The minority carriers cross the junction, become majority carriers and exit thecell recombining in the external circuit and depositing their energy.

The generation of electron-hole pairs and their flow to the external circuit, creates anextra current named light current. The new equation that describes the diode underillumination is:

J(V ) = J0

[exp

(qV

kT

)− 1

]− JL (3.4)

where JL is the light current density. The light IV curve is shown in Figure 3.4.

11

CHAPTER 3. THEORY

I

V

Voc

Jsc

MPP

Vm

Jm

FF = VmJm

JscVoc

Dark

Light

Figure 3.4.: Ideal IV curve

3.1.3. One diode model

The most fundamental characterization method is to measure the current of the cell whilealtering the applied voltage. The IV curve under illumination can give very useful infor-mation about the solar cell, since important parameters such as Jsc (short circuit currentdensity), Voc (open circuit voltage) and Pmp (maximum power point) are determined fromit. The open circuit voltage is given by:

Voc =kT

qln

(JLJ0

+ 1

)(3.5)

Another helpful parameter extracted by the IV curve is FF (fill factor, Equation 3.6)which has no physical meaning but describes the “squareness” of the IV curve, i.e. howclose to the maximum power point the cell works.

FF = FF0(1− rs) (3.6)

where

rs =Rs

Rch

, with Rch =VocJsc

(3.7)

where Rs is the series resistance, and

FF0 =voc − ln(voc + 0.72)

voc + 1(3.8)

where

voc =Voc

nkT/q(3.9)

12

CHAPTER 3. THEORY

If one has to choose only one parameter that will judge the performance of a solar cell,then this parameter must be the efficiency (η). It describes how well the cell converts theenergy of the incoming light into electric energy. All the parameters mentioned previouslyare included in η which is described with:

η =Pout

Pin

=Pmp

Pin

=JscVocFF

Pin

(3.10)

However, when a deeper analysis is needed, η is not enough because it does not giveinformation about losses.

Another parameter that characterizes solar cells is the ideality factor (n). It describeshow close is the solar cell to an ideal one. For an ideal solar cell, where there is norecombination in the space charge region, n = 1, but for experimental devices, n can takeother values.

The equivalent circuit of a solar cell according to the one diode model is shown inFigure 3.5. The diode is the pn junction in the dark, the current source is the lightgenerated current (JL) and there are two resistances that represent losses. The shuntresistance (Rsh) gives JL an alternative path so it needs to be as high as possible, becausewe want the current to go out of the circuit, not return back. The series resistancesymbolizes the resistances in the materials and the surrounding circuit and needs to beminimized because there is voltage drop in it. Both resistances affect the fill factor andthus the efficiency of the solar cell. The current in this case is given by:

J(V ) = J0 exp[ q

nkT(V −RsJ)

]+GshV − JL (3.11)

where Gsh is the shunt conductance.

IL

ID

Rsh

Rs

Ish

I

V

+

-

Ideal cell

Figure 3.5.: Solar cell equivalent circuit

3.1.4. Quantum efficiency

Quantum efficiency is another characterization method, used among other things to mea-sure the band gap of the material and locate losses. The tested solar cell is exposedto photons of different energies, and the number of carriers collected by the cell aremeasured. Photons with energy below the band gap cannot create electron-hole pairstherefore quantum efficiency (QE) is zero in those wavelengths.

13

CHAPTER 3. THEORY

There are two types of quantum efficiency, external (EQE) and internal (IQE). ExternalQE takes into account all the incident photons. Internal QE on the other hand, takesinto account only the photons that are absorbed by the solar cell, so it only depends onphotogeneration and collection. If the reflection and transmission profile of the cell isavailable, IQE can be obtained from EQE.

The QE curve of an ideal solar cell has a square shape. However, real cells have manylosses as we will discuss later. If losses happen in the front surface of the cell, QE dropsin high energy photons (blue light) because they are absorbed only in the front surface(Figure 3.2). The green portion of the QE curve is affected by losses in the bulk, andthe red part, which can penetrate more, can be absorbed in the rear surface. Otherfactors that reduce QE are shading losses, which are independent of the wavelength, andreflection.

3.1.5. Capacitance measurements

When we discussed previously about the formation of the pn junction, we mentioned thatin the depletion region, there is an electric field created by ions. The concentration ofnegative ions on the one side and of positive ions in the other side, can also be seen as acapacitor. Measuring the conductance and capacitance of the junction can give valuableinformation about doping density, free carriers, deep states and other.

In a standard capacitor C = Q/V , where C is the capacitance, Q is the charge andV is the voltage. But in a solar cell, Q does not vary linearly with voltage so a moregeneral equation in needed, and this is C = δQ/δV . A small ac voltage of the formV = Vac[cosωt+ j sinωt] is applied to the cell, and the capacitance is measured. We willdiscuss the information that can be obtained from this data in chapter 4.

3.1.6. Main losses

The world record efficiency of a single junction solar cell is 29.1% [5] which means that70.9% of the incident energy is lost. There are many mechanisms through which theenergy is lost during the process and we will discuss some of them.

• Optical losses When we talk about optical losses we mainly talk about reflectionand transmission. Reflection can occur at the front contact, on the front surfaceof the cell or on the rear surface. In the first two cases photons are not absorbed,and therefore get lost. Shading by the grid, dead areas and parasitic absorption arealso important causes of optical losses. They mostly effect the current generatedby the solar cell. There are a number of ways to reduce these kind of losses. Anti-reflection coating, surface texturing and appropriate top contact design are the mostimportant ones.

• Generation losses Generation losses are the ones that are caused by materialproperties. When incoming photons have less energy than the band gap of thematerial, they cannot excite electrons and create electron-hole pairs. They mightbe absorbed as heat or simply not interact.

• Thermalization lossesWhen an electron absorbs energy from an incoming photon and goes from the va-lence to the conduction band, it initially goes to a higher state inside the conduction

14

CHAPTER 3. THEORY

band. However, it rapidly relaxes to an energy equal to the band gap. The rest ofthe energy of the photon is lost in the material through thermalisation.

• Recombination losses When an electron in the conduction band loses its energyand goes to the valence band, it can meet a hole. In this case, they recombine andtheir energy is lost. Recombination losses are of the more complex and difficult toprevent and they affect both the current and the voltage. There are three types ofrecombination: radiative, Shockley-Read-Hall (SRH) and Auger recombination.

Radiative In this type of recombination, the energy of the electron hole pair is emittedas a photon with energy equal to the band gap. The photon is not likely to beabsorbed because its energy is exactly in the threshold, so the energy is mostprobably lost. Radiative recombination is more probable in direct band gapmaterials, because in indirect ones there must be a phonon interaction as well.

SRH When defects with energy states in the forbidden region are present in thecrystal, it is possible for two carriers to meet there and recombine. The defectscould be added on purpose (dopants) or through contamination during themanufacturing process. Radiation affects solar cells because it increases thedefect density. Grain boundaries and interfaces can also work as recombinationcenters. The released energy is in the form of a photon or heat. It is moreprobable than radiative recombination because the energy difference is smaller.

Auger Auger is the recombination that happens when an electron and a hole recom-bine and transfer their energy to another electron in the conduction band or ahole in the valence band, which gets excited to a higher energy state. It loosesthe extra energy by collision with other atoms. This type of recombination ismore probable when the material has higher minority carrier concentration,such as materials with higher doping.

• Resistive lossesThe existence of parasitic resistances (series and shunt), results resistive losses dueto voltage drop in the series resistance and current loss in the shunt resistance.The series resistance has its root in bulk resistances of the semiconductor, metalliccontacts and interconnections while shunt resistance is mainly due to leakage atthe edges of the cell across the pn junction. Crystal defects and impurities in thejunction can also cause decrease of shunt resistance. The parameter that is mostlyeffected by resistances is the fill factor.

3.2. Radiation

3.2.1. Theory

Radiation in space can be divided into electromagnetic and particle radiation. The first iswave radiation and will not be discussed in detail. The other type, particle radiation, willbe the focus of this project. The effects of each type depends on properties of the incomingparticles such as energy, mass, and charge and on properties of the target material suchas atomic mass and density. However, the effects are difficult to separate, because manyof the incoming particles induce secondary effects.

15

CHAPTER 3. THEORY

Radiation

Electromagnetic Particles

Ionizing Non Ionizing Charged Uncharged

Protons

...Electrons

...Neutrons

...

Figure 3.6.: Types of radiation in space

Electromagnetic radiation can be divided into ionizing and non-ionizing particles ofzero rest mass, with the border between the two types roughly defined. Non-ionizingparticles do not have sufficient energy to excite atoms in the target material, but inducephotochemical reactions and have thermal effects. Ionizing radiation has more energyand interacts with matter through the photoelectric effect, Compton effect, and pairproduction depending on the energy.

Particles with non-zero rest mass can either be charged or uncharged. The interactionof uncharged particles, such as neutrons, with matter is similar to the electromagnetic one.Charged particles on the other hand, transfer their energy through Coulomb interaction,therefore behave differently when interacting with matter. When they enter a material,they gradually lose their energy by ionizing material atoms. How deep into the materialthey will penetrate, depends on the initial energy, the type of particle and the material itinteracts with. The energy loss per unit path length is called stopping power and usuallyshows a peak before the particle completely stops (Bragg peak, Figure 3.7). The existenceof this peak means that a large part of the energy is deposited at a specific depth in thematerial. Taking into account the material properties, the energy of the radiation can beadjusted so that most of it is deposited to the desired depth.

Path Length

Sto

pin

g P

ow

er

Figure 3.7.: Bragg peak

16

CHAPTER 3. THEORY

3.2.2. Effects of radiation on solar cells

Charged particles can affect solar cells in two ways. Depending on their energy they canionize lattice atoms or they can cause displacement damages. The incoming particlescan remove lattice atoms out of their positions creating point defects (Figure 3.8). Theatom might leave its position (vacancy) or it can exchange positions with another latticeatom (antisite). Other types of point defects are interstitials, where atoms take positionsthat are not normally occupied, substitutions, where lattice atoms are substituted withother elements, and Frenkel pairs, which are vacancy-interstitial pairs. Radiation canproduce Frenkel pairs or move already existing defects such as substitutes, interstitials, orantisites. The creation of Frenkel pairs is more important because in the other case thereis no creation of new defects but rather conversion of one type of defect into another [6].As an example, if an already existing vacancy or substitute moves to a different positiondue to radiation, there are no new defects created, so only minor changes are expected.

Vacancy

Antisite

Substitute

Frenkel pair

Interstitial

Figure 3.8.: Defect types

Of the two materials that we investigated, namely CZTS and CIGS, the first one has notbeen thoroughly studied yet regarding radiation effects. The other one is more mature,therefore defects and stability issues are better known. The three possible defects thatradiation could cause are Cui − VCu, Ini − VIn and Sei − VSe [6]. Cu is highly mobile atroom temperature so Cu defects can easily be self-annealed. Although In is less mobile,the In vacancy can also possibly be annealed, with the help of Cu-exchanging reactions.Lastly, Se defects are much less likely because they have high formation energy, and theycan possibly also be annealed with the help of Cu-exchanging reactions.

Regarding CZTS, the probable defects and their effects have been studied, but notradiation caused defects. However, even these studies can give us important information.According to Baranowski et al [7], the CuZn defect is the most dominant one, and itsenergy level is 0.12eV above the valence band maximum, so it is an acceptor level. TheVCu, VZn, ZnSn and CuSn defects have also relatively low formation energies so they arealso probable. The VCu, VZn and ZnSn have shallow acceptor levels, but experimentalresults have found deep acceptor defects that were attributed to CuSn [8]. This meansthat CuSn could function as a recombination center.

An overview of radiation studies of thin film solar cells in literature is given in Table 3.1.In most cases, cells show degradation under radiation. However, depending on the energy

17

CHAPTER 3. THEORY

and flux of the radiation, even some performance improvement is possible. This is thecase for lower fluxes. The effect is considered to be equivalent to light-soaking [9], [10],[11] and to hydrogen-passivation of recombination centers [12].

Degradation starts when flux increases and the higher the flux, the more the cellsdegrade. This can be attributed to many causes, and since it is a complex matter, itis difficult to be certain which of them is the real cause of the degradation. Ionizationeffects [9], [13], recombination centers generation [13], [12], [14] and optical transmittancelosses [12] are some of the proposed reasons.

Annealing was also used in some of the experiments in order to accelerate any cellrecovery. Lamb et al [15] used 100C annealing and a dramatic change was observed inthe performance. Jasenek et al [13] used room temperature annealing for six months, andthe cells recovered partially.

Table 3.1.: List of studied papers. “e” represents electron radiation and “p” protonradiation.

Energy Dose (cm−2) Cell Characterization method Reference

e

60-250 keV 1014 − 1017 CIGS IV [9]100 keV 1015 CIGS EL [10]

60-600 keV 1014 − 1017 CIGS IV, EL, CV [11]0.5-3 MeV 1011 − 5 · 1018 CIGS IV, CV, CF [13]

2 MeV 1014 − 2 · 1017 CZTS IV, PL [16]

p

380 keV 1012 − 3 · 1016 CZTS IV, PL [16]0.5 MeV 1012 − 1014 CdTe IV, CV, QE, SCAPS [15]3 MeV 1010 − 1013 CZTS IV, QE [14]4 MeV 1011 − 1014 CIGS IV, CV, CF [13]15 MeV 1012 − 1015 CdTe IV, Optical Transmittance [12]

18

4. Experimental Method

4.1. Experimental procedure

The radiation was performed at The Tandem Laboratory in Angstrom. The solar cellswere irradiated with proton radiation of 250 keV and a dose of 1012 protons/cm2. Thereason we chose this energy is because the Bragg peak (Figure 3.7) in this case is withinthe device, therefore most of the energy would be deposited there. The stopping rangecan be calculated with SRIM simulation [17], but it was considered beyond the scopeof this project, so the energy was chosen based on previous research [14]. As discussedin section 3.2, protons induce both ionization and displacement damages, but at lowenergies the displacement damages dominate. The type of particle was chosen basedon what causes problems in space applications and what is feasible as an experimentalprocedure in Angstrom. Most satellites orbit Earth in a low orbit (∼ 2.000 km aboveEarth), where there is strong influence by the inner Van Allen radiation belt. This areacontains large numbers of protons that are trapped by Earth’s magnetic field. The dosewas chosen to be 1012 protons/cm2, which corresponds to ∼ 10 years in low Earth orbit.

We attempted to investigate two different thin film materials: CZTS and CIGS, withthe second considered more as reference. An overview of the samples and the measure-ments we performed in each of them is shown in Figure 4.1. For each material, we hadtwo samples in which we performed various experiments. The reason we chose to havetwo samples of each material, is because we wanted to perform frequent measurements inat least one of them. However, in order to measure, one needs to connect the sample tothe measurements devices and this is done with small metal probes. Multiple connectionscan damage the cell’s surface, therefore, we decided to take the risk for only one, andhave a second as reference sample. Furthermore, it was safer to have two, in case one ofthem got damaged at any point during the process.

CZTS1 CZTS2CZTS

CIGS

IV, QE IV, TAS,

CV, DLCP

CIGS1 CIGS2

Figure 4.1.: The samples and the measurements we performed on them. The fabricationnames of the samples are CZTS1: 23SSe8, CZTS2: 23SSe7, CIGS1: T62a2and CIGS2: T62a12

19

CHAPTER 4. EXPERIMENTAL METHOD

The CZTS samples were manufactured in Angstrom by Ross et al [18]. The two sampleshad slightly different properties before getting exposed to radiation, because they haddifferent S/(S + Se) ratio. The CIGS samples were also manufactured in Angstrom byFrisk et al [19]. It was originally one sample that was cut in two pieces for the reasonswe mentioned before.

For CZTS1 we performed IV before the radiation, and after the radiation we measuredIV every day for 2 weeks. After that, thermal annealing was performed at 100C andan IV measurement followed each annealing step. 100C was chosen based on previousresearch [15] and because this temperature can be reached in space. The annealing stepswere: 10 minutes, 30 minutes, 1 hour, 2 hours, 5 hours and 8 hours. The annealingprocess lasted about 2 weeks and, when it was completed, we decreased the frequency ofthe IV measurements finishing the measurements after 2 weeks. QE was measured beforethe radiation, after it, and after 2 hours annealing.

For CZTS2 we had a different approach. The measurements (IV, TAS, CV, DLCP)were not frequent but after certain points. The points we chose were: before radiation,after radiation, 2 weeks after radiation, after a 10-minute thermal annealing, after a 1.5-hour annealing and after a 17-hour total annealing. We continued measuring IV twomore times but with the Newport IV setup, which was easier to use and had a lampthat simulates better the sun spectrum. A timeline of the electrical characterization weperformed to the CZTS samples, is shown in Figure 4.2.

CZTS1

Before radiation Week 1 Week 2 Week 3

IV, QE IV every day IV every day

Radiation

QE QE

IV after each

annealing step

IV after each

annealing step

Week 4 Week 5 Week 6

IV x2 IV

CZTS2


IV, TAS,

CV, DLCP

Radiation

anneal

10 min

Week 4 Week 5 Week 6

Newport IV

* * * * **: IV, TAS

CV, DLCP

anneal

17 h

anneal

1.5 hNewport IV

Figure 4.2.: Timeline for CZTS samples.

For CIGS1 we performed IV before the radiation, and after the radiation we measuredIV every day for 2 weeks. QE was measured before the radiation, after it, and 2 weeksafter it. The annealing process started one month after that, and was done in the sameway as in CZTS1.

For CIGS2 we started by measuring IV, TAS, CV and DLCP before radiation, afterradiation, and 2 weeks after radiation. The investigation was stopped after this pointbecause we didn’t see any improvement. Furthermore, the admittance measurements didnot give clear results even before the radiation. Besides that, during the measurementafter the radiation, we faced some problem with the LCR meter and our data were notreliable. Due to the problem with the LCR meter we could not trust either of the methodsto get the doping after the radiation. Therefore, our analysis for CIGS was based onlyon the IV curves of CIGS2 and the rest of the measurements will not be discussed. Atimeline of the electrical characterization we performed on them, is shown in Figure 4.3.

20


CIGS1


IV, QE IV every day IV every day

Radiation

QE QE

IV after each

annealing step

IV after each

annealing step

Week 15 Week 16

IV x2

CIGS2

Before radiation Week 1 Week 2

IV, TAS,

CV, DLCP

Radiation

* *

*: IV, TAS,

CV, DLCP

Figure 4.3.: Timeline for CIGS samples.

4.2. Equipment

In order to electrically characterize our solar cells as thoroughly as possible, we useddifferent characterization methods described below.

4.2.1. IV

In order to have a fair comparison of cells, the cell parameters should be measured in thesame conditions. The standard test conditions (STC) is a standard according to whichall solar cells and panels should be measured at temperature T = 25C, solar irradiancePin = 1000W/m2 and an air mass 1.5 (AM1.5) spectrum. The last condition is thespectrum that corresponds to the spectrum of the sun, when light has passed through1.5 atmospheres thickness.

The basic IV parameters (Jsc, Voc, FF and η) are easy to extract based on Figure 3.4.The rest of the parameters, however, need more effort and are extracted gradually by theone diode equation (Equation 3.11) [20]. First, a plot of the derivative g(V ) = dJ/dVagainst V in the neighborhood of Jsc, gives the shunt conductance and therefore theshunt resistance. Secondly, the intercept of the plot of r(J) = dV/dJ against (J +Jsc)

−1 gives the series resistance and the slope gives the ideality factor. The last stepis a semilogarithmic plot of (J + Jsc) against (V − RJ), the intercept of which givesthe saturation current density. The slope of the same plot is the ideality factor, soa comparison of the two values is possible. Writing the codes needed to extract theparameters, was also part of the project.

Two different setups were used to measure IV. The first will be denoted as Newport IVand the equipment it included were: a Keithley 2401 multimeter and a Newport Sol2AClass ABA Solar Simulator. The second will be denoted as ICVT and the equipment itincluded were: an Agilent 4284A LCR meter and a Keithley 2401 multimeter.

4.2.2. QE

Quantum efficiency measurements are used to extract an estimation of the band gap ofthe material [21]. Quantum efficiency is given by:

QE = 1− exp (αW )

αL+ 1(4.1)

21


where α is the absorption coefficient, L is the diffusion length and W is the depletionregion width. Close to band gap, where the absorption is weak, the denominator canbe neglected. By Taylor expansion of the equation QE = 1 − exp (αW ), we get thatQE(λ) ∝ α(λ), where λ is the wavelength. The absorption coefficient for direct bandgap materials is α(λ) ∝

√E − Eg therefore QE(λ) ∝

√E − Eg. Plotting QE2 against

energy around the band gap and extrapolating to the x-axis, yields an estimation of theband gap.

The equipment we used to measure QE were: an Oriel Apex Monochromator Illumi-nator, a CornerstoneTM 130 1/8m Holographic Grating Monochromator, a Stanford Re-search system SR570 low-noise current preamplifier, a Stanford Research system SR810Lock-in amplifier and a Stanford Research system SR830 Lock-in amplifier.

4.2.3. CV

The Capacitance-Voltage measurements were used to determine the doping of our sam-ples [22], [23]. In the interest of analyzing our data, we had to accept the depletionapproximation, which is not accurate for thin film cells. According to this approxima-tion, the depletion region is defined precisely, ends abruptly, and is fully depleted of freecarriers. In this case, the capacitance that comes from the depletion edges is given by:

C =εε0A

W(4.2)

where ε is the semiconductor relative permittivity, ε0 is the vacuum permittivity, A isthe cell’s area, and W is the width of the depletion region. For an one-sided (Schottky)diode the width is:

W =

√2εε0(Vbi − Vdc)

qNB

(4.3)

where Vbi is the built-in voltage, Vdc is the applied voltage, q is the electron charge, andNB is the doping concentration. Combining these equations, we get that the doping is:

NCV = − 2

qεε0A2

(dC−2

dV

)−1

(4.4)

In the depletion approximation, the capacitance only comes from the depletion region,so the previous equation is correct even when the doping is not constant across thesemiconductor. The position in this case is:

xCV =εε0A

C(4.5)

The equipment we used to measure CV were: an Agilent 4284A LCR meter and aKeithley 2401 multimeter.

4.2.4. DLCP

Drive level capacitance profiling is another method to determine the doping [24]. Theapplied ac voltage has varied amplitude and the capacitance is:

C = C0 + C1dV + C2dV2 + . . . (4.6)

22


The doping density is given by:

NDLCP = − C30

2qεε0A2C1

(4.7)

and the position is x =εε0A

C0

.

If the sample has deep traps, DLCP gives more accurate results at high frequencies,since the depletion region approximation assumed for CV is no longer valid. At lowfrequencies, where the traps have time to respond to the applied ac voltage, the twomeasurements should give similar results.

The equipment we used to measure DLCP were: an Agilent 4284A LCR meter and aKeithley 2401 multimeter.

4.2.5. TAS

The temperature dependent admittance spectroscopy measurement as a function of fre-quency is another helpful measurement, since it can yield the thickness of the film, theposition of the Fermi level and the density of defect states. However, it can only detectdefects between the edge and mid-gap.

The frequency range of the applied voltage is chosen so that it crosses the transitionfrequency of trapped states. When the temperature is low or the frequency high, carriersdo not have time to respond to the frequent voltage change, so they do not shift in and outof the depletion edge. As the temperature increases or frequency decreases, trap statesrespond to the applied voltage, creating a cut-off step, characteristic for every certain(T, f) point. The derivative of capacitance will have a peak in the capacitance step.Plotting the frequency of the peak and the temperature in an Arrhenius plot returns astraight line, the slope of which is the activation energy and the intercept is the attempt-to-escape frequency. The code required to extract these information was written as partof the project. The temperature range was from 85K to 305K, and the frequency rangewas from 102 to 106 Hz.

The equipment we used to measure TAS were: an Agilent 4284A LCR meter, a Keithley2401 multimeter and a LakeShore 325 Temperature controller. Liquid nitrogen was usedto cool the sample.

4.3. Modeling

In order to understand the changes that radiation caused to our solar cells, we createda model of our devices before radiation and compared it with our experimental resultsfor CZTS1. After that, we tried to modify our model to fit the experimental data thatwe obtained after radiation. In this way we could get an indication of which parameterswere affected by the radiation and resulted in the degradation of the cells. The modelwas built with the Solar Cell Capacitance Simulator (SCAPS-1D) [25] version 3.2.01 andwas based on the model developed by Frisk et al [26].

SCAPS 1D is a free numerical tool that can simulate thin film solar cells with up toseven layers plus their interfaces and contacts. For the derivation of the model, it uses thePoisson equation (Equation 4.8a, relationship between charge and electric field) and con-tinuity equations (Equation 4.8b and Equation 4.8c, tracking of carriers in terms of move-ment, generation, and recombination), taking into account the current density equations

23


(Equation 4.8d and Equation 4.8e). The equations are solved in their one-dimensionalform. It can simulate current-voltage, capacitance-voltage, capacitance-frequency andquantum efficiency.

The input parameters required for each layer are the ones in Equation 4.8. Furthermore,the user can define absorption, reflection and recombination. Optical filters at the frontand back contact can also be added. Lastly, there is the possibility to specify illumination,temperature, resistances, voltage working point and many other parameters.

dξ

dx=q

ε(p− n+ND −NA)

1

q

dJedx

= U −G

1

q

dJhdx

= −(U −G)

Je = qµenξ + qDedn

dx

Jh = qµhpξ + qDhdp

dx

(4.8a)

(4.8b)

(4.8c)

(4.8d)

(4.8e)

The equations are combined with boundary conditions at the interfaces between thematerials and at the contacts, producing a system of coupled differential equations. Thestructure is discretized creating a mesh with more points close to interfaces and contacts,and fewer points in areas where properties are fairly constant. For the given structure, thebasic equations are solved by a combination of a Gummel iteration scheme and Newton-Raphson, and steady states are calculated. Small signal analysis is used for capacitancemeasurements.

24

5. Results

5.1. Electrical Characterization

5.1.1. CZTS

IV

The two different CZTS samples (CZTS1 and CZTS2) had similar IV parameters beforethe radiation. For CZTS1 we used only the Newport IV setup and the curves are shown inFigure 5.1. We can see the IV curves of CZTS2 with ICVT in Figure 5.2 and with NewportIV in Figure 5.3. Their IV parameters before the radiation are shown in Table 5.1.

−0.4 −0.2 0.0 0.2 0.4 0.6 0.8Voltage (V)

(40

(20

0

20

40

60

Curre

nt (m

Acm

2)

IV curve with Newp rt IV in sample CZTS1After radiati nAfter 10 minutes annealAfter 17 h urs anneal2 weeks after radiati nBef re radiati n

Figure 5.1.: IV curve

25

CHAPTER 5. RESULTS

−0.4 −0.2 0.0 0.2 0.4 0.6 0.8Voltage (V)

−20

0

20

40

60Cu

rrent (m

Acm

2)

IV curve with ICVT in sam le CZTS2After radiationAfter 10 minutes annealAfter 17 hours anneal2 weeks after radiationBefore radiation

Figure 5.2.: IV curve with ICVT

−0.4 −0.2 0.0 0.2 0.4 0.6 0.8Voltage (V)

(20

0

20

40

60

Curre

nt (m

Acm

2)

IV curve with Newp rt IV in sample CZTS2After 10 minutes annealAfter 17 h urs anneal2 weeks after radiati nBef re radiati n

Figure 5.3.: IV curve with Newport IV

A world record CZTS cell with 12.6% efficiency was reported in 2014 [27], but oursamples were far from that. Before the radiation, we had a sample with 7.55% efficiency(CZTS1) and one with 7.28% (CZTS2). We need to notice at this point the differencebetween the measurements in the two devices (ICVT and Newport IV) for CZTS2. ICVTgives less reliable results than Newport IV, since the position of the sample relative to

26

CHAPTER 5. RESULTS

Table 5.1.: IV parameters of CZTS samples before radiation

Sample Device Voc(mV) Jsc(mAcm2

)FF(%) η(%) Rsh(Ω) Rs(Ω) n J0

(10−3 mA

cm2

)CZTS1 Newport IV 430 29.9 58.8 7.55 216 2.15 1.87 0.99CZTS2 Newport IV 440 28.5 58.1 7.28 340 2.46 2.07 1.58CZTS2 ICVT 440 29.5 60.3 7.83 314 1.42 1.72 0.39

Table 5.2.: IV parameters of CZTS samples after radiation



(10−3 mA

cm2

)CZTS1 Newport IV 380 26.0 50.7 5.02 212 2.45 2.48 20CZTS2 ICVT 400 28.7 49.4 5.67 292 6.22 2.16 1.1

the lamp is not as good as the one in Newport IV. Furthermore, the spectrum of thelamp in Newport IV is closer to the spectrum of the sun. Therefore, we will rely moreon that and use ICVT only in the cases that we didn’t measure with Newport IV for thereasons described in chapter 4.

The radiation caused a clear degradation on both our samples. As we can see inTable 5.2, the efficiency of CZTS1 went from 7.55% to 5.02%, and for CZTS2 it droppedfrom 7.28% to 5.67%. The effect of the radiation can also be seen in Table 5.3. We canalso see that the parameter that suffered most was FF.

The results 2 weeks after radiation can be seen in Table 5.4. Comparing to the previouspoint, both devices seem to get closer to their initial state as there is an improvement inmost of the parameters.

Then, we continued with the annealing process. The IV parameters after 10 minutesare shown in Table 5.5. We can see a small improvement, similar to the one achievedafter 2 weeks in room temperature.

The next annealing step was 1.5 hours. Some improvement of the parameters wasachieved as we can see in Table 5.6. However, the effects of radiation were still obvious.

As a last step, we annealed for 17h. As we can see in Table 5.7, we got close to ourinitial state and this is one of the reasons we stopped annealing. The other reason wasthat Jsc and FF started degrading again.

The effect of annealing in the basic IV parameters can be seen in Table 5.8. We usedthe values given by Newport IV and compared each result to what we had 2 weeks afterthe radiation.

For CZTS1 we had a more frequent IV analysis as explained in chapter 4. Voc recoveredfaster than all the other parameters. It responded to annealing directly and the improve-

Table 5.3.: Effect of radiation on the basic IV paremeters of CZTS samples

Sample ∆Voc(mV) ∆Jsc(mAcm2

)∆ FF(%)

CZTS1 -50 (12%) -3.9 (13%) -8.04 (14%)CZTS2 -40 (9%) -0.8 (3%) -10.86 (18%)

27

CHAPTER 5. RESULTS

Table 5.4.: IV parameters of CZTS samples 2 weeks after radiation



(10−3 mA

cm2


Table 5.5.: IV parameters of CZTS samples after 10 minutes anneal



(10−3 mA

cm2


Table 5.6.: IV parameters of CZTS samples after 1.5 hours anneal



(10−3 mA

cm2


Table 5.7.: IV parameters of CZTS samples after 17 hours anneal



(10−3 mA

cm2


Table 5.8.: Effect of annealing on basic IV parameters of CZTS samples

Anneal time Sample ∆Voc(mV) ∆Jsc(mAcm2

)FF(%)

10 min CZTS1 20 0.8 2.010 min CZTS2 10 0.7 0.21.5 h CZTS1 30 0.8 2.71.5 h CZTS2 20 1.3 5.27 h CZTS1 40 0.5 -0.67 h CZTS2 40 0.9 2.5

28

CHAPTER 5. RESULTS

ment was constant. Voc after annealing was almost the same as before radiation. Jsc alsoshowed very good improvement. Although it did not completely recover, the annealinghelped a lot. FF on the other hand did not improve. Although at the first steps of theannealing we saw a clear improvement, later its value declined again to the state beforethe annealing process. η is a combination of these three parameters so because of theimprovement in Voc and Jsc, η shows also recovery.

0 10 20 30 40 50Day

0.37

0.38

0.39

0.40

0.41

0.42

0.43

0.44

V oc (V)

Voc in sample CZTS1

(a) Development of Voc

0 10 20 30 40 50Day

26.0

26.5

27.0

27.5

28.0

28.5

29.0

29.5

J sc(m

A/cm

2 )

Jsc in sample CZTS1

(b) Development of Jsc

0 10 20 30 40 50Day

50

52

54

56

58

FF (%

)

FF in sample CZTS1

(c) Development of FF

0 10 20 30 40 50Day

5.0

5.5

6.0

6.5

7.0

7.5

η (%

)

η in sample CZTS1

(d) Development of efficiency (η)

Figure 5.4.: Development of IV parameters of CZTS1. Annealing started on day 22 andthe state before radiation is day -1.

QE

The QE curves are shown in Figure 5.5. We can see that the radiation caused a decreaseof QE in all wavelengths and that 2 hours annealing was enough to get almost back tothe initial point. In the middle and long wavelengths the cell did not recover completely.This indicates defects in the bulk of the material. We also used the QE data to extractan estimation of the band gap energy for our sample, and we found Eg = 1.1 eV.

29

CHAPTER 5. RESULTS

400 600 800 1000 1200Wavelength (nm)

0.0

0.2

0.4

0.6

0.8

1.0

Quan

tum

Effi

cienc

y

QE in sample CZTS1After radiationAfter 2 hours annealBefore radiation

Figure 5.5.: QE curve

CV and DLCP

The last parameter we estimated was the doping density. The values extracted with CVare shown in Figure 5.6 and with DLCP in Figure 5.7. The difference between the twomeasurements is substantial regarding the shape. According to Heath et al [24], DLCPprovides a more accurate and correct picture of the doping density.

30

CHAPTER 5. RESULTS

0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32Position (μm)

1016

9×1015

Ne( a

ccep

(or c

once

n(ra(io

n (cm

−3)

Doping concen(ra(ion in sample CZTS2 wi(h CV

Af(er radia(ionAf(er 10 min)(es annealAf(er 17 ho)rs anneal2 wee s af(er radia(ionBefore radia(ion

Figure 5.6.: Doping concentration with CV

0.18 0.20 0.22 0.24 0.26 0.28 0.30Position (μm)

1016

1017

Net acceptor concentration (cm)3)

Doping concentration in sa ple CZTS2 (ith DLCPAfter radiationAfter 10 inutes annealAfter 17 hours anneal2 (eeks after radiationBefore radiation

Figure 5.7.: Doping concentration with DLCP

31

CHAPTER 5. RESULTS

5.1.2. CIGS

IV

Since the two CIGS samples were originally one, we expect the same behavior. The IVcurves of CIGS1 and CIGS2 are shown in Figure 5.8 and Figure 5.9 respectively. The IVparameters before radiation for both of them are shown in Table 5.9.

−0.4 −0.2 0.0 0.2 0.4 0.6 0.8Voltage (V)

−40

−20

0

20

40

60

Curre

nt (m

Acm

2)

IV curve with New ort IV in sam le CIGS1After radiationAfter 10 minutes annealAfter 17 hours anneal1 week after radiationBefore radiation

Figure 5.8.: IV curve for CIGS1

−0.4 −0.2 0.0 0.2 0.4 0.6 0.8Voltage (V)

−20

0

20

40

60

Curre

n (m

Acm

2)

IV curve wi h ICVT in sample CIGS2Af er radia ionBefore radia ion

Figure 5.9.: IV curve with ICVT for CIGS2

32

CHAPTER 5. RESULTS

Table 5.9.: IV parameters of CIGS samples before radiation



(10−3 mA

cm2

)CIGS1 Newport IV 670 33.5 74.5 16.7 2106 104 1.84 0.01CIGS2 Newport IV 670 33.5 74.7 16.75 4303 77.1 1.72 0.004CIGS2 ICVT 670 31.0 73.1 15.2 3766 23.9 2.23 0.41

Table 5.10.: IV parameters of CIGS samples after radiation



(10−3 mA

cm2

)CIGS1 Newport IV 470 30.2 64.4 9.15 2388 1.94 1.78 0.63CIGS2 ICVT 490 29.7 66.7 9.70 3547 1.98 1.92 0.43

CIGS had originally higher efficiency than CZTS. The effect of radiation on it thoughwas in general larger than in CZTS as we can see in Table 5.10 and Table 5.11.

The results of the IV measurement one week after the radiation are shown in Table 5.12.The annealing process started approximately one month after the previous measure-

ments. The IV parameters for CIGS1 before the annealing and after a 10-minute annealare shown in Table 5.13 and Table 5.14 respectively. We can see that this annealing stepmade almost no improvement.

The next annealing step was 1.5 hours. The results are shown in Table 5.15 and wecan see some improvement.

The last step was annealing for 17h (Table 5.16). The cells showed improvement butnot as much as in CZTS.

The effect of annealing in the basic IV parameters of CIGS1 can be seen in Table 5.17.We compared each result to the one we had before the annealing process (Table 5.13).

The evolution of the basic IV parameters for CIGS1 are shown in Figure 5.10. Voc againrecovered faster than all the other parameters as it responded to the annealing directly.However, it did not reach the values it had before radiation. In this case, Jsc showedvery good improvement and recovered almost completely. The annealing process did nothelp FF as it increased a bit at first, but dropped after the process was completed. η isa combination af these three parameters so because of the improvement in Voc and Jsc, ηalso showed recovery.

Table 5.11.: Effect of radiation on the basic IV parameters of CIGS samples

Sample ∆Voc(mV) ∆Jsc(mAcm2

)∆ FF(%)

CIGS1 -200 (30%) -3.3 (10%) -10.1 (14%)CIGS2 -180 (27%) -1.3 (4%) -6.4 (9%)

33

CHAPTER 5. RESULTS

Table 5.12.: IV parameters of CIGS samples 1 week after radiation



(10−3 mA

cm2

)CIGS1 Newport IV 510 31.8 66.5 10.77 2574 1.94 1.50 0.04

Table 5.13.: IV parameters of CIGS1 before the annealing process



(10−3 mA

cm2

)CIGS1 Newport IV 530 32.5 66.9 11.5 220 2.43 1.41 0.009

Table 5.14.: IV parameters of CIGS1 sample after 10 minutes anneal



(10−3 mA

cm2

)CIGS1 Newport IV 530 32.7 66.9 11.6 241 2.95 1.35 0.005

Table 5.15.: IV parameters of CIGS1 sample after 1.5 hours anneal



(10−3 mA

cm2

)CIGS1 Newport IV 580 33.1 70.4 13.5 302 7.89 1.69 0.019

Table 5.16.: IV parameters of CIGS1 sample after 17 hours anneal



(10−3 mA

cm2

)CIGS1 Newport IV 590 33.1 68.6 13.4 217 25.1 2.44 1.34

Table 5.17.: Effect of annealing on basic IV parameters of CZTS samples

Anneal time Sample ∆Voc(mV) ∆Jsc(mAcm2

)FF(%)

10 min CZTS1 0 0.2 01.5 h CZTS1 50 0.6 3.57 h CZTS1 60 0.6 1.7

34

CHAPTER 5. RESULTS

0 5 10 15 20

0.475

0.500

0.525

0.550

0.575

0.600

0.625

0.650

0.675

85 90 95 100

Day

V oc (V)

Voc in sample CIGS1

(a) Development of Voc

0 5 10 15 20

30.5

31.0

31.5

32.0

32.5

33.0

33.5

85 90 95 100

Day

J sc(m

A/cm

2 )

Jsc in sample CIGS1

(b) Development of Jsc

0 5 10 15 20

57.5

60.0

62.5

65.0

67.5

70.0

72.5

75.0

85 90 95 100

Day

FF (%

)

FF in sample CIGS1

(c) Development of FF

0 5 10 15 209

10

11

12

13

14

15

16

17

85 90 95 100

Day

η (%

)

η in sample CIGS1

(d) Development of efficiency (η)

Figure 5.10.: Development of IV parameters for CIGS1. Annealing started on day 86and the state before radiation is day -1.

QE

We also performed quantum efficiency measurements before and after the radiation andthe plots are shown in Figure 5.11. As we can see, after the radiation we noticed apeculiar behavior. This could be an effect of the radiation exposure, so the cell didn’twork properly in that area of the spectrum.

35

CHAPTER 5. RESULTS

400 600 800 1000 1200Wavelength (nm)

0.0

0.2

0.4

0.6

0.8

1.0

Quantum Efficie

ncy

QE in sample CIGS1

After radiation1 week after radiationBefore radiation

Figure 5.11.: QE curve of CIGS1

5.2. Device Modeling

5.2.1. Before radiation

The model developed by Frisk et al was based on a different CZTS sample so we needed tomodify it to fit the sample investigated here. First, the model was adjusted by changingthe thickness of the CZTS, the CdS, and the Al:ZnO layers based on [18]. Then, basedon our experimental data as described in section 5.1, the series and shunt resistances, andthe doping density were changed. For the doping we used the minimum value of the CVprofile [22]. The model after all these modifications was much closer to our experimentaldata, but we did some additional changes to get even closer. We changed the S/(S+Se)ratio in order to have the correct band gap, and the defect density in the CZTS layerto fit the rest of the QE curve and the IV curve. We also tried to modify the defectdensity at the interface between CZTS and CdS, but the result was not better so we keptthe initial value. After that, we tried to adjust more than one parameter at a time andthe results were even better. The parameters that were readjusted were the S/(S+Se)ratio and the defect density in the CZTS layer. The results were the IV and QE curvesshown in Figure 5.12 and Figure 5.13 respectively. As we can see, the model fits theexperimental data even if it is not perfect.

Besides the parameters that were set based on the experimental data, the parameterthat mostly affects the IV parameters is the defect density of the CZTS layer. Its effecton the parameters can be seen in Figure 5.14.

36

CHAPTER 5. RESULTS

−0.4 −0.2 0.0 0.2 0.4 0.6 0.8Voltage (V)

−40

−30

−20

−10

0

10

20

Cu e

nt (m

Acm

2)

IV cu ves in sample CZTS1 befo e adiationLight IV f om dataLight IV f om model


400 600 800 1000 1200Wavelength (nm)

0

20

40

60

80

Quan

tum

Effi

cienc

y

QE in sample CZTS1 before radiationQE from dataQE from model


37

CHAPTER 5. RESULTS

1014 1015 1016Defect density (1/cm3)

0.38

0.40

0.42

0.44

0.46

0.48

0.50

0.52

0.54

V oc (V)

Voc vs defect density in CZTS1modeldata

(a) Effect of defect density on Voc


26

27

28

29

30

31

32

33

J sc(m

A/cm

2 )

Jsc vs defect density in CZTS1modeldata

(b) Effect of defect density on Jsc


48

50

52

54

56

58

60

FF (%

)

FF vs defect density in CZTS1modeldata

(c) Effect of defect density on FF


5

6

7

8

9

10

η (%

)

η vs defect density in CZTS1modeldata

(d) Effect of defect density on (η)

Figure 5.14.: Effect of defect density on IV parameters for CIGS1.

5.2.2. After radiation

As a next step, we tried to modify our model to fit the experimental data after theradiation. Firstly, we changed the series and shunt resistances based on Table 5.2. Sincewe expected more defects than before, we increased the defect density of the CZTS layerand the results are shown in Figure 5.15 and Figure 5.16. The defect density was increasedfrom 8 · 1014 to 6.3 · 1015 1/cm3 so 87%. The increase of the interface defect density didnot change the model. The changes that we made are shown in Table 5.18. In Table 5.19we can see the basic IV parameters of the experimental and simulated data before andafter the radiation.

38

CHAPTER 5. RESULTS

−0.4 −0.2 0.0 0.2 0.4 0.6 0.8Voltage (V)

−40

−30

−20

−10

0

10

20

Curre

nt (m

Acm

2)

IV curve in ample CZTS1 after radiationLight IV from dataLight IV from model


400 600 800 1000 1200Wavelength (nm)

0

20

40

60

80

Quantum Efficie

ncy

QE in sample CZTS1 after radiationQE from dataQE from model


39

CHAPTER 5. RESULTS

Table 5.18.: Changes in the model parameters before and after the radiation

CZTS defect density (1/cm3) Rs (Ω) Rsh (Ω)

Before radiation 8 · 1014 2.15 215After radiation 6.3 · 1015 2.45 212

Difference +87% +12% -1%

Table 5.19.: Experimental and simulated basic IV parameters

Data Voc(mV) Jsc(mAcm2

)FF(%)

Before radiationexperimental 430 29.9 58.8

simulated 482 29.1 54.9

After radiationexperimental 380 26.0 50.7

simulated 401 25.9 48.2

40

6. Discussion and Conclusions

6.1. Discussion

Radiation hardness is a growing part of solar cell research. This project attempted tocontribute to this research by showing and discussing the effects of proton radiation inCZTS and CIGS cells.

Radiation caused a decrease in the open circuit voltage by ∼10%, which was recoveredwith 17 hours thermal annealing. Voc mainly depends on the light-generated current andthe diode saturation current as indicated by Equation 3.5. The fact that Voc decreased,indicates that recombination increased. This could be caused by vacancies and other trapsin the depletion region or in the quasi neutral region that cause SRH recombination, andare generated by radiation. However, annealing raised Voc almost to the initial value, sowe can assume that some of the radiation generated defects were repaired. This resultagrees with previous research ([15], [13]) according to which, thermal annealing recoversthe cell efficiency. The results for the two different materials (CIGS and CZTS) weresimilar qualitatively, but not quantitatively, since the CIGS efficiency degraded by 58%while CZTS by 40%, and CIGS did not recover as much.

The short circuit current, Jsc, also suffered degradation due to radiation, but partiallyrecovered with annealing. We can therefore deduce that both the collection probabilityand the recombination rate were affected by radiation. Our results coincide with previousresearch as discussed in subsection 3.2.2. The difference between the two materials wasnot as substantial as with Voc. Jsc had similar degradation in the two materials andshowed partial recovery.

The last basic IV parameter, the fill factor, underwent severe degradation and onlypartially recovered with annealing. FF is affected by recombination in the depletionregion, as it depends on the ideality factor (Equation 3.6), which in CZTS1 increasedby 24% after the radiation. Another reason for the degradation could be the decreaseof open circuit voltage. Parasitic resistances can also influence FF and this could be athird cause for the degradation of FF, since series resistance in CZTS1 increased by 12%,while shunt resistance decreased by only 1%. The result was anticipated, since previousresearch had similar findings. Regarding the difference between CIGS and CZTS, FFdegraded more in CIGS, but the difference was not as serious as in Voc.

The fact that quantum efficiency dropped in all wavelengths after radiation, impliesthat collection in all areas of the solar cell was decreased. It improved with the annealingprocess, but towards the back of the cell it did not recover completely. This indicatedthat annealing did not recover the defects in the bulk of the material. Quantum efficiencymeasurements were part of only two of the papers that we studied ([15], [14]) and ourresults are consistent with them. QE in CIGS and CZTS had similar behavior with lessdegradation in CZTS.

Doping density calculations in previous research suggests a drop of doping density, butin our investigation there was no significant change. Lattice imperfections with shallowstates can increase the doping density, because they make thermal excitation easier. In

41

CHAPTER 6. DISCUSSION AND CONCLUSIONS

contrast, lattice imperfections with deep states assist recombination processes. Radiationmay have caused both, so this counteraction could be the reason we did not see importantdifferences. The semiconductor materials studied in previous research (CIGS, CdTe) arenot the same as in this investigation (CZTS), so different results are to be expected. Sincethe doping density measurements in CIGS were not reliable, we cannot have a comparisonbetween the two material.

In the model that was developed to explain the effects of radiation, we had to modifythe resistances based on the experimental results. Furthermore, the defect density wasincreased by 87% in the scope of fitting the experimental data after the radiation. Themodel was developed using SCAPS, which is an 1D modeling tool, therefore the resultsit gives are only approximations of real devices. Moreover, we used an optical filter thatwas measured for another CZTS cell, not the one that we investigated. Another possibleproblem in our model was the difference between the measured and modeled S/(S+Se)ratio. We had to adjust the value to fit the data, which was different from the oneindicated in the manufacturing paper ([18]).

The IV curves for CZTS2 and CIGS2 were measured with two different setups (ICVTand Newport IV) and slightly different results were obtained (∼ 5% for the basic param-eters and ∼ 30% for the rest). The main reason that could lead to this problem is thedifferent lamp. The lamp used in Newport IV has a spectrum that simulates sunlightbetter than the one in ICVT. In addition, small variations in the relative position of thesample towards the lamp, can cause changes in the current which are then transferred tothe rest of the parameters.

6.2. Conclusions

Two different semiconductor materials (CIGS and CZTS) were irradiated with 250 keVprotons with dose of 1012 protons/cm2. The cells were characterized electrically, byquantum efficiency, and doping density. Both types of solar cells were affected by radiationand all basic IV parameters degraded. Quantum efficiency was also influenced acrossall wavelengths. Doping density of CZTS did not show severe change after radiation.Room temperature annealing helped towards recovery and thermal annealing of 100Chad notable results on recovery. Comparing the efficiency before radiation and afterthe annealing process, the efficiency of CZTS decreased by 18% and of CIGS by 22%.Furthermore, the quantum efficiency recovered completely in the short wavelengths afterthe annealing. The model that was implemented suggested that the cause of degradationwas the increase of defect density. The series and shunt resistances were also modifiedaccording to the experimental data. Although CZTS cells do not have high efficiency, theirradiation hardness is remarkable, so if the efficiency is improved, they can be preferred toother absorbing material. CIGS cells are less radiation hard than CZTS, but have higherefficiency.

6.3. Future work

Radiation hardness is a field that has still a lot to offer. CZTS is a material that has showngood qualities and potential for space applications but has not been fully studied yet.There is an increasing interest in low cost solar cells for space, and part of the work fromthis thesis will be presented together with other studies performed in the research group

42

CHAPTER 6. DISCUSSION AND CONCLUSIONS

in an oral presentation at the EUROMAT 2019 conference [28]. To continue this work,different energy and dose ranges could be tested to investigate the effect of radiation.Furthermore, the identification of exact defects would give important insights on how tomake the cells radiation harder and increase their efficiency.

43

A. Admittance

One of the characterization methods we used was admittance measurements. It was usedto extract the attemp-to-escape frequency and the energetic distance. The temperaturerange was from 85K to 305K, but we only used a part of that. The admittance, itsderivative and the Arrhenius plot before radiation are shown in Figure A.1, Figure A.2and Figure A.3 respectively. The energetic distance and attempt-to-escape frequencyare shown in Table A.1. These variables are not easily interpreted and could not giveconclusive results, so they were not investigated further.

102 103 104 105Frequency (H()

1.0

1.5

2.0

2.5

3.0

3.5

Capa

citan

ce (F

)

1e)8Admittance bef re radiati n in sample CZTS2

T = 163.00T = 168.00T = 173.00T = 177.00T = 182.00T = 187.00T = 192.00T = 196.00T = 201.00T = 206.00T = 211.00T = 216.00T = 220.00T = 225.00T = 230.00T = 235.00T = 240.00

Figure A.1.: Capacitance-Frequency

44

APPENDIX A. ADMITTANCE

103 104 105 106ω(1/s)

0.0

0.2

0.4

0.6

0.8

1.0−ω

dC dω(F/eV)

1e−8Derivative of Ca acitance before radiation in sam le CZTS2

T = 163.00T = 168.00T = 173.00T = 177.00T = 182.00T = 187.00T = 192.00T = 196.00T = 201.00T = 206.00T = 211.00T = 216.00T = 220.00T = 225.00T = 230.00T = 235.00T = 240.00

Figure A.2.: Derivative of capacitance

4.25 4.50 4.75 5.00 5.25 5.50 5.75 6.00 6.251000/T

−3

−2

−1

0

1

2

ln(ω

0T2

)

energetic distance = 229 meVattempt-to-escape frequency = 3.33e+05 1/sK2

















Arrhenius plot before radiation in sample CZTS2

Figure A.3.: Arrhenius plot before radiation

45

APPENDIX A. ADMITTANCE

Table A.1.: Effect of radiation in energetic distance and attempt-to-escape frequency

Energetic distance (meV) Attempt-to-escape frequency (105/sK2)

Before radiation 229 3.33After radiation 247 2.85After 10 min anneal 210 2.97After 1.5 h anneal 249 17.4

46

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