Review of Epitaxial Quantum Dots Solar cells By

Eddie Benitez-Jones Mani Hossien-Zadeh

ECE 475 Introduction to Optoelectronics and Photonics 4/18/2016

2. Abstract

Quantum dot intermediate band solar cells (QD-IBSC) have been an attractive technology for the

enhancement of GaAs solar cell efficiencies. The intermediate energy levels created by the quantum dots

allows for extra absorption in the infrared regime increasing the short circuit current (Jsc). However, any

gain in the photocurrent is counterbalanced with the decrease in the open circuit voltage (Voc). Despite all

the work to improve the open circuit voltage, the performance of quantum dot solar cells still hindered.

The reasons to pursue QD-IBSC and the challenges to the technology are discussed in this paper.

3. Introduction

In 2010, the U.S. Department of Energy put into effect a SunShot Initiative with the purpose to solar

energy cost-competitive with traditional forms of electricity [1]. Lowering the cost of solar electricity,

allows for an increase of clean energy usage reducing the emissions of greenhouse gases and other

pollutants that are threatening to our planet [1]. The SunShot Initiative has been funding private

companies, universities, and national laboratories to do research in all solar technologies to drive their

cost down [1]. The quantum dot solar cells presented here fall in the third generation solar cell

technology. Third generation solar cells are usually made of high price III-V materials. Even though

they are expensive to fabricate, the cost can be alleviated by the enhancement in efficiency and the small

area needed to achieve such efficiencies in comparison to cheaper solar technologies.

The two main loss mechanisms that limit Schockley-Queisser efficiency to 31% are thermalization losses

from photons with energies greater than the semiconductor bandgap and transmission losses from photons

with energies lower than the semiconductor bandgap [2]. Several approaches to overcome Schockley-

Queisser limit had been proposed and are an active area of research. The most successful approach uses a

vertical stack of solar cells with different bandgaps that can absorb different photon energies reducing

transmission losses. The current state of the art multi-junction solar cell is a 46% efficient 4 junction

solar cell under concentration [3]. The main drawbacks of this cells is their requirement for lattice

matching and current matching [2].

Another approach to overcome Schockley-Queisser limit is by introducing low dimensional

nanostructures in the intrinsic region of a p-i-n single junction solar cell [4]. The electron confinement of

such low dimensional structures leads to the formation of an intermediate band (IB) within the forbidden

bandgap of the host semiconductor. In these case, transmission losses are also expected to be reduced as

the sub-bandgaps formed are expected to absorb the photons with energies lower than the energy of the

host bandgap [4]. Theoretical calculations for this approach predicts an 63.2% efficiency under maximum

concentrated sunlight [4]. This theoretical efficiency is higher than the current state of the art multi-

junction solar cell efficiency. Therefore, there is a chance for this kind of single junction solar cells to

compete with multi-junction solar cells.

In this paper, I have chosen to review the effect of 0-D quantum dot nanostructures in single junction

solar cells. Although, quantum dots (QDs) can be used to enhance the efficiency of solar cells by

collection of hot carriers and by creating multiple exciton generation [5], I am going to concentrate on the

use QDs in intermediate band solar cells. In section 4 & 6, I explain what epitaxial QDs are and how they

are fabricated. Then I go on into how QDs are incorporated in intermediate band solar cells and what is

the working principle of such cells. In section 5 & 8, I give an overview of the techniques used to

improve the QD-IBSC design to increase its efficiency. In section 7, I go into detail of the differences

between state of the art GaAs solar cells and state of the art QD-IBSC.

4 & 6. Background: Epitaxial Quantum Dots and Applications in Intermediate Band Solar Cells

4.1 Fabrication of Epitaxial Quantum Dots

An epitaxial QD is a three-dimensional nanoscale island formed by self-organized epitaxy on the surface

of a III-V semiconductor [6]. They are fabricated by a bottom-up self-organized Stranski-Krastanov

growth method either by Molecular Beam Epitaxy (MBE) or by Metal-Organic Chemical Vapor

Deposition (MOCVD). This method relies in a strain-relaxation mechanism [6]. Hence, the QD material

must have a different lattice constant than the host or substrate material. The growth starts with a

deposition of thin film of the QD material on a lattice-mismatched substrate. The layer-by-layer growth

continues until the film reaches a critical thickness. Up to the critical thickness, the thin film, called

wetting layer, is strain free. Once the critical thickness is reached, strain energy starts to build up and so

3-Dimensional islands form to elastically relax the film/substrate misfit strain [2, 6]. If the growth

continues, the islands coalesce and strain is relieved by misfit dislocations. Ivan Stranski and Lyubomir

Krastanov proposed this method in 1938. However, it was not until 1985 that the formation of self-

assembled semiconductor QDs based on strained hetero-structures was experimentally observed in the

InAs/GaAs system [7].

6.1 QDs Intermediate Band Solar Cells

An intermediate band solar cell (IBSC) is a solar cell with an intermediate energy level within the

semiconductor band gap [4]. The main idea is that intermediate band will allow the absorption of photons

with energies lower than the semiconductor band gap energy enhancing the photocurrent of the cell

increasing the efficiency [4]. Theoretical calculations by Luque and Marti showed that a solar cell of this

kind can achieve a maximum efficiency of 63.2% under maximum concentrated sunlight [4]. Several

assumptions which were made to achieve such efficiency include carrier mobilities to be infinite, only

radiative transitions are allowed, no carriers can be extracted from the intermediate band, 100 %

absorption of photons with required energy to make transitions between the bands, and radiation produced

by the cell can only escape through the front of the cell [4].

In 2000, Marti introduced the idea of using QDs as the impurity to form the intermediate band energy

level [8]. QDs is a good option because its properties avoid non-radiative recombination between bands

and because the formation of quasi-Fermi levels is possible. The QD-IBSC is a p-i-n structure with the

quantum dots placed in the intrinsic region. The QDs form an intermediate band (IB) that splits the

semiconductor bandgap, EG, into two sub-bandgaps, EL and EH. In addition to the absorption of phonons

with energy greater than EG, two photons, hv1 and hv2, with lower energy than EG can be absorbed [8]. To

create an electron-hole pair from hv1, an electron from the valance band (VB) must jump to the IB and to

create an electron-hole pair from hv2, an electron from the IB must jump to the conduction band (CB).

Hence, to be able to create electron-hole pairs, the IB must be half-full with electrons so that it can

receive electrons from the VB and supply electrons to the CB [8, 9]. To half-fill the IB with electrons, the

QDs can be doped with n-type dopants [8, 9].

Figure 1: Sketch of the QD IBSC [8].

Figure 2: Sketch of the gaps and photon absorption processes involved in the operation of the IBSC [8].

In 2006, Martin published the results of QD-IBSC with InAs QDs and GaAs barrier layers [9]. In

comparison to a reference GaAs solar cell, the InAs/GaAs QD-IBSC showed a decrease in open circuit

voltage, Voc, and no photocurrent enhancement. The QD-IBSC structure had 10 layers of delta doped

InAs QDs. Structures with un-doped QDs showed further degradation of Voc. In order to improve the

photocurrent, it is believed that the number of QD layers should increase. However, there is a tradeoff

with the addition of QD layers, since each layer results in an additional compressive strain that is relieved

by inducing misfit dislocations that reduces the performance of the cell [9].

5 & 8. Technology Overview & Future

5.1 Improvement Techniques to Boost QD-IBSC efficiency

In order to increase absorption, Hubbard and et al studied a strain compensation technique to be able to

increase the number of QD layers without degradation of the material [10-12]. After the growth of the

QD layer and the capping GaAs layer, a tensile strain compensation layer of GaP is grown. The tensile

strain offsets the compressive strain leaving a strain neutral QD stack. The results show an enhancement

of photocurrent as the number of QD layers can be increased. The Voc is also benefited from the

introduction of GaP strain-compensation layers as shown in figure 3 [12]. It is noticeable that GaP strain-

compensation layer has an optimal thickness before the short circuit current, Jsc, is degraded.

Figure 3: Current Voltage characteristics of QD devices with different strain compensation GaP layer thickness [12]

In their next study, Hubbard and et al optimized the number of QD layers that the intrinsic region should

have [13]. For this, QD-IBSC with 10, 20, 40, 60 and 100 layers of QDs in the GaAs I-region were

tested. Figure 4 shows the results the I-V characteristics of the solar cells under one sun AMO

illumination. The QD cell with 40 layers shows the highest photocurrent enhancement with a 12.3 %

efficiency. Nonetheless, this efficiency is below the 14.1% efficiency of the baseline GaAs solar cell.

Figure 4: One sun AMO light J-V curve for the baseline GaAs p-i-n cell and 5X-100X QD cells [13]

Another parameter that requires optimization is the InAs coverage. That is the thickness of the of the

InAs layer while the InAs islands are forming. Hubbard and his group found that by reducing the InAs

coverage, the Voc of a QD solar cell is improved [14]. By reducing the InAs coverage to 1.8 monolayers

(ML), the efficiency of the solar cell with 40 layers of QDs increased from 12.3 % to 14.3 %.

Another technique which has been used to improve efficiency of QD solar cells involves positioning and

doping of the InAs QDs. The position of QDs within the intrinsic region of the pin GaAs solar cells is

expected to be most effective in the regions where SRH recombination rates are lower [15]. The table

below gives measured and simulated values for the devices I-V characteristics for different placement of

QDs within the GaAs solar cell. As shown in Table 1, even though simulations predicted that the

placement of the QDs was better near the emitter or near the base was better, actual measurements

showed that the placement of the QD layers in the center is a better option. This is due to the fact that

SRH rate of recombination is strongest when then electron and hole densities are most similar (at the

center region).

Table 1: Simulated & measured device I-V characteristic values for baseline & 5x QD p-i-n GaAs solar cells[15]

Up to this point, many techniques in attempt to improve the efficiency of QD-IBSC have been covered.

Photocurrent enhancement and improvement of Voc degradation was observed throughout the different

techniques. However, none of the techniques show a better efficiency than the GaAs baseline solar cell.

7. Comparison of State-of-the-art InAs/GaAs QDs solar cells to plain GaAs solar cells

GaAs is a direct bandgap III-V semiconductor often used as a substrate for epitaxial growth with other

III-V semiconductors. It has a great temperature coefficient and radiation resistance, making it perform

well under situations where the cell needs to operate at high temperature such as space applications. GaAs

can be prepared as either p-n or n-p designs, where the emitter needs to be as thin as possible. Some of the

major problems with GaAs solar cells include high front surface recombination, high series resistance,

and high substrate cost [16]. The record for the single-junction solar cell with highest efficiency is held by

GaAs solar cell with 28.8% efficiency [17]. This high efficiency was reached by an extremely high

quality GaAs epitaxial growth, with surface passivation by AlGaAs window layers and the promotion of

photon recycling [16].

An InAs/GaAs QD solar cell has a p-i-n structure instead of the typical p-n structure of a GaAs solar cell

as the one mentioned above. The introduction of the intrinsic region between the base and emitter is done

with the purpose to fill it with QDs layers. Figure 5 illustrates the structure of a p-i-n GaAs solar cell with

InAs dots grown in organometallic vapor-phase epitaxy [18]. Figure (b) below illustrates a comparison

between a control p-i-n GaAs solar cell without QD, and a p-i-n InAs/GaAs QD solar cell with different

number of InAs QD layers. As you can see from Figure (b), the current density increases with more layers

of QD incorporated within the intrinsic region of the cell, while the open circuit voltage decreases with

increasing QD layers.

Figure 5:(a) QD enhanced solar cell design;(b) J-V curves for a control, 5x, 10x, & 20x layer QD solar cell at AM1.5G [18]

As you can see in Figure 5, the External Quantum Efficiency is increased over a longer span of

wavelengths for the solar cell with QDs. This addition in wavelength absorption is due to transitions

between confined states of the wetting layer and the ground state of the QD which clearly illustrates the

short circuit current density will be bigger for a larger amount of QD layers compared to the baseline

[18]. The increase of EQE for QD is an improvement over GaAs solar cells, since they do not absorb as

much energy above 880nm as a cell with QDs [18].

Figure 6: Comparison of baseline GaAs to QD solar cell EQE graph [18]

9. Conclusion

I have reviewed all the attempts to make QD-IBSC an efficient solar cell. Although transmission loses

are reduced by the absorption of photon in the infrared region, the photocurrent enhancement by this

absorption do not seem to be sufficient to compensate for the reduction in open circuit voltage. Even if

the Voc is maintained or closely maintained to baseline GaAs Voc, the efficiency achieved of 14% is still

way below the 28.8% maximum efficiency of the free-QD complication p-n GaAs single junction solar

cell. The only way for QD-IBSC to be feasible is if they can bypass the 28.8% efficiency of their free-QD

GaAs solar cells counterpart and get closer to their theoretical 63% efficiency.

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