Dr. Sushil Kumar

Principal Scientist CSIR-National Physical Laboratory

New Delhi Email: skumar@nplindia.org

2nd Annual Conference-Expo-Awards“Roadmap for Innovations in Solar Energy” - RISE 2017, 20-21 July 2017, Hotel Taj, New Delhi

Silicon based Solar Cell Technologies: Trends & Challenges

Outline

Various solar cell materials &

technologies

c-Si solar cell technology, its advantage &

limitations

Improved silicon cell device structures

Amorphous silicon solar cell: limitations &

away to HIT cell

Apex level solar cell testing & validation at

CSIR-NPL

Conclusions

Choice of Solar Cell Materials

Three Generations of Solar Cells

• 1st: Wafer based:

– Monocrystalline silicon 25%

– Polycrystalline silicon 20%

– Multi-junction cell (different band-gap materials) 40%

• 2nd : Thin Films:

– Amorphous thin film silicon 13%

– CdTe (Cadmium Telluride) 17%

– CIGS (Copper Indium Gallium Selenium) 20%

• 3rd : Low Cost and high Efficient:

– DSSC (Dye-sensitized solar cells)

– QDSSC (Quantum Dot-sensitized solar cells)

– OPV (Organic photovoltics)

– QDs-Polymer Hybrid solar cells

– Perovskite Solar Cells

Efficiency

Various Solar Cells Efficiency

Prog. Photovolt Res. & Appl, 25 (2017)

Types of cell junction

N-type

P-type

Eg: c-Si cell

N-type

P-type

Eg: CdTe, CIGS cell

Cell 1, Eg1

Cell 2, Eg2

Cell 3, Eg3

Eg1 > Eg2 > Eg3

N-type

P-type

Intrinsic, i, layer

Eg: a-Si:H cells

Homo-junction

Hetro-junction

P-i-N junction

Multi-junction

Junction is required to facilitate charge separation for PV operation

Eg: GaAs, a-Si cells

Band gap Efficiency of multi-junction cell

Appropedia, 2012

Triple-junction cells

Appl. Phys. Lett. 90, 183516 (2007) Emcore Corporation, May (2006)

1.7-1.9 eV

1.3-1.4 eV

0.67 eV

The cells are in series;

current is passed through

device

The current is limited by

the layers that produces

the least current.

The voltages of the cells

add

The higher band gap

must see the light first.

Multi-junction solar cells

Low-end tandems

Turning poor solar cells into average ones !

Multilayer amorphous silicon

-Different bandgaps via process control or a-SiGe alloy

• The semiconductor junctions can be:

– Single Junction p-i-n

• a-Si

• µc-Si

– Tandem Junction p-i-n/p-i-n

• a-Si/a-Si, a-Si/a-SiGe, a-Si/ µc-Si

– Triple Junction p-i-n/p-i-n/p-in

• a-Si/a-SiGe/a-SiGe

• a-Si/ a-SiGe/ µc-Si

• nc-Si/a-Si:H/ µc-Si

Efficiency: 10-11%

Efficiency: 12-14%

Limitation: a-Si:H Light-induced degradation & Low efficiency

Roll-to-Roll Process for Thin Film Silicon Solar Cell

Roll-to-Roll a-Si deposition machine (UNI-SOLAR)

Production-sized cells

Cross-section of the triple-junction a-Si alloy solar cell

Efficiency Comparison of Technologies: Best Lab Cells vs. Best Lab Modules

9.13 %

9.08 %

8.84 %

8.85 %

7.78 %

Source: PV Report, Fraunhofer ISE, 12 July 2017

Will the 1st-generation c-Si

PV technology be replaced by

2nd- and 3rd-generation PV?

Annual PV Production by Technology

(Worldwide in GWp)

Source: PV Report, Fraunhofer ISE, 12 July 2017

Fact: c-Silicon Share of Production 93% in 2016

PV Production by Technology

Source: PV Report, Fraunhofer ISE, 12 July 2017

Current Efficiencies of Selected Commercial PV

Modules

Source: PV Report, Fraunhofer ISE, 12 July 2017

Crystalline silicon solar cells: Better than ever

Silicon-based photovoltaics dominate the market. A study now sets a new record efficiency for large-area crystalline silicon solar cells, placing the theoretical efficiency limits within reach

What is best efficiency possible in silicon solar cells

Shockley, Queisser (JAP 1961): Efficiency limit of solar cell made of one material = 33% (AM1.5)

Theoretical efficiency for

silicon = 29.4%

Best Silicon cell = 26%

88 % of theoretical efficiency

Different solar cell technologies strives to

maximize the efficiency

It requires

- Absorption of photon

- Separation of electron-

hole pair

- Collection of the charges

at electrodes

Three operations in different way

P-N Jn –separation force

Metal contact

Metal contact

The theoretical limit for a crystalline silicon solar cell is ~ 29%.

C. Battaglia, Energy Environ. Sci., 2016, 9, 1552--1576

Analysis of recombination losses of the exemplary

n+pp+ silicon solar cell

P-type -c-si wafer

Phosphorus diffusion

a-SiN:H

Al-doped

BSF (back surface filed)

Aluminum doping from the back contact into the silicon wafer is confined to the small openings in the rear passivation layer, minimizing recombination.

The dielectric passivation layer simultaneously improves the reflectivity of the aluminum back reflector.

Localized rear contact solar cell with dielectric rear surface passivation

Significant electronic and optical benefits

C. Battaglia, Energy Environ. Sci., 2016, 9, 1552--1576

Ways:

Restricting the aluminum alloyed p+ region to a small

fraction (about 1–5%) of the rear surface while

passivating the rest with an insulating material

High Efficiency Concept of c-Si cell: Ways & Approach

Three Approaches (also referred to with the acronyms):

PERC (Passivated Emitter and Rear Cell)

PERL (Passivated Emitter, Rear Locally-doped)

PERT (Passivated Emitter, Rear Totally diffused)

Method: Laser ablation. 1130 local contact openings (in

industry typically as lines) are made in the insulator

PERL

PERC PERT

HIT Solar cell

Wafer based Crystalline

Silicon Silicon solar cell

Amorphous thin film

silicon Solar cell

Highest Conversion

Efficiency

Module Efficiency: 18-20%

Cost reduction is main

overall challenge

Low cost Potential

Module Efficiency: 6-9%

Efficiency enhancement is

major challenge

Hybrid Technology HIT Solar cell

HIT: Hetrojunction with intrinsic thin layer

Present status of efficiency of HIT solar cell

(INTERNATIONAL STATUS)

Institute/ Company Year Efficiency (%) Area (cm2)

Kaneka Corp., Japan 2016 26.3 180.0

Panasonic, Japan 2014 25.6 143.8

Sanyo, Japan 2014 24.7 124.2

AIST, Japan 2013 18.2 0.2

RRS, Switzerland 2011 21.9 4

EPFL, Switzerland 2011 21.8 4.5

HZB, Germany 2008 19.8 1

NREL, USA 2009 18.2 0.9

LG, Korea 2010 19.1 1

Delft, Netherlands 2011 15.8 2

CEA, INES, France 2010 15.7 25

BHEL, India (at R&D level on industrial wafer): 18%

HIT – “Hetrojunction with intrinsic thin layer”

Structure of HIT solar cell

Intrinsic a-Si:H used for c-Si surface passivation: dangling bond passivation (surface recombination becomes crucial when device thickness decreases)

Doped a-Si:H for diode

creation (front end) and BSF (back contact)

TCO: Optimized optical

performances

Advantages of HIT solar cell over C-Si solar cell

Non regular Grid area = Defective area = relatively high surface recombination

Clean surface area = Suppressed surface recombination

http://www.posharp.com/hit-240hde4-solar-panel-from-sanyoelectric_p2075938320d.aspx

Processing Temperature ~900 0C Processing Temperature < 200 0C

Temperature coefficient of HIT solar module

Higher than standard

conditions temperatures

can actually mean

losses in maximum

output power

c-Si solar module:

-0.5% / degree Celsius

Power loss: 1% for

every 2°C increase

Simulation & Fabrication of HIT Solar Cells

Float zone c-Si (1-2 Ω cm, > 2 µsec, 200 µ thick, <100> orientation) wafers.

Standard RCA cleaning. Deposition of pure & doped a-

Si:H layers – PECVD (For n - PH3 and for p - B2H6).

TCO (AZO) deposition – Sputtering.

Contact formation – E-beam evaporation of Ag/Al.

Front grid –Screen printing/Inkjet printing/Photographic lift-off process

Market Share for different cell technologies

ITRPV 2017

Perovskite/silicon-heterojunction tandem solar cells

Journal of Optics 18 (2016)

Projected Efficiency 30% , however, stability of

Perovskite remain a challenge

Why validation needed for solar cell efficiency ?

“Most of the measurement

errors a researcher would

make would tend to make

the efficiency higher, not

lower”- Keith Emery,

NREL

An error in solar cell/module

efficiency leads to error in the

over all assessment of related

product & system.

The largest source of error in estimating the efficiency of a solar cell is arguably the correct

identification of its active area.

Effect of active area on efficiency of organic solar cells

(Measurement performed at CSIR-NPL)

Cell Area: 2 mm x 3 mm = 6.0 mm2

(Actual Area: 1.970 mm x 2.968 mm = 5.84696 mm2

Dimension accuracy in NPL @0.16 µm (in 1mm)

Pin : 1000 W/m2

(< 25% spectral mismatch)

Eff. measured at temp. 25 °C

NPL - NMI

(Apex Calibration)

NPL has expertise in calibration of the parameters

needed for validation of the solar cell efficiency

Optical Irradiance

Current

Voltage

Temperature

Dimension (Active Area)

Parameters Capabilities in Measurement Uncertainty

Spectral

irradiance

Wavelength: 350 - 2500 nm

(0.06 – 0.026 nm)

DC Voltage 1V (± 1µV), 1 mV (± 1µV), 1µV (± 0.1 µV)

DC Current 1mA (± 5nA ), 1µA (± 7pA),

1nA (± 0.1pA),1 pA (±0.01pA)

Temperature ±0.17 x10-3 °C at TPW (0.01 °C)

Dimension 1.0 mm (± 0.16 µm)

Validation of solar cell efficiency at CSIR-NPL

• Primary Standard Facility for Solar Cell

Calibration is under development at NPL

…….Recently MNRE has sponsored a Project

Measurement Traceability Pyramid

Efficiency Validation for organic solar cells

Secondary Standard Facility

Conclusions

c-Si solar cell is dominant technology, will remain for long….

Close Competition between PERC & HIT solar cells technology, perhaps HIT may make it winner in long run…..

Emerging & newer Thin-film solar cell materials for solar cells (OPV, DSSC, Perovskite, tandem…) present interesting advantages & showing high potential, however, its promising future for commercialization depends on its arrest of environmental instability