Novel process and materials for high efficiency solar cells v2

Novel Processes & Materials towards High Efficiency Inorganic Solar Cells (>20%) Dr. K. Balachander PSGIAS

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Solar Cell Development…

Three Generations of solar cell technology:

1. Single-crystal silicon based photovoltaic devices • Good efficiency • High Cost  Higher than traditionally-produced electricity 2. CuInGaSe2 (CIGS) polycrystalline semiconductor thin films • Low Cost • Less Efficiency 3. Nanotechnology-enhanced solar cells • Low Cost • Medium Efficiency • Emerging – Industrially not proven 5

Outline…

• PV Trend • Nanoimprint Lithography – Front Side Metallization – Texturization

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Black Silicon Solar Cells Plasmonic Solar Cells AgNW + ALD AZO HIT/HHJ Solar Cells GaAs Epitaxial Lift off (ELO) 6

Top 10 countries in 2014

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Price Trends‌

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Efficiency Comparison

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PV Deployment by Technology

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C-Si Silicon Solar Cell Structure

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Process Technology – Emitter & Metallization

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Industrial Solar Cell Process

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PV Road Map

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Outline

• PV Trend • Nanoimprint Lithography – Front Side Metallization – Texturization

• • • • •

Black Silicon Solar Cells Plasmonic Solar Cells AgNW + ALD AZO HIT/HHJ Solar Cells GaAs Epitaxial Lift off (ELO) 15

Economic Strategy for Solar Cells

Proportion of silver per cell (156 X 156 mm2) and the predicted share of electroplating based metallization technologies in the cell process. (Color coding refers to silver reduction) Alternate Low-Cost Metallization Strategy 1) Ni/Cu/Sn Electroplating 2) Ag Electroplating

Parameters

Ag

Cu

Ni

Conductivity (106 S/m) 61.39 59.1

13.9

Density (gm/cm3)

10.49 8.92

8.90

Cost ($/troy oz)

27.26 0.23

0.50

in April 2014 16

State of the Art (Motivation) Taguchi et al., IEEE JOURNAL OF PHOTOVOL., VOL. 4, NO. 1, 2014

HIT Cell : 23.7% (2011)24.7% (2014) (AIST, JAPAN) Continuously developing different printing technology (finer contact width) to minimize the resistive loss and the shadow loss at the grid electrode.

*P. Cousins et al., Generation 3: Improved performance at lower cost” Photovoltaic Specialists Conference (PVSC), 35th IEEE, Honolulu, HI, USA 2010; pp275 -278

2) World Highest efficiencies of 25.2% - expensive photolithography multi-metal evaporated stacks (Ti,Pd,Ag) - very low contact resistance with typical line width of 10-30 μm*.

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Optimized Shading & Ohmic losses Power Loss : Standard SP Solar Cell Resistive loss Front Metallization

Shading/Shadowing loss •Prevents light from entering the solar cell •Determined by width of the metal lines & spacing of the metal lines •Narrow line-width technology closer finger spacing, reducing the shadow loss & emitter resistance losses

Min Loss=

0.06417

Preliminary Simulated Results

Min Loss=

0.03129

•50% reduction in power loss •Reduced finger width  more fingers  Rise in FF  Rise in efficiency Expected   1%

Fractional power loss versus number of fingers for 120µm finger width

Fractional power loss versus number 18 of fingers for 20µm finger width

Front Side Metallization Success of the different front side metallization techniques boils down to five main factors :

NIL scores good figure of merit & can be used for patterning Can be combined with Ni/Cu electroplating Silver less (Ni/Cu EP) or Less Silver ( Ag EP)

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Nanoimprint lithography - Basics

Spin coat polymer (PMMA) Create Imprint with Pre-fabricated Template Etch the Residual layer (O2 plasma) Pattern Transfer : Further deep etch or metal deposition20

Nanoimprint Lithography - Comparison

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NIL Front Side Metallization Fabrication of in-house low cost Nanoimprint Lithography tool Feasibility analysis & process development of the advanced metallization (less silver or silver-less process) scheme for high efficiency crystalline silicon solar cells and to develop industry ready process using Nanoimprint Lithography tool To use this project as an enabler to promote Nanoimprint Lithography as a low cost patterning tool for the fabrication of other electronic devices PSGIAS Thrust Areas : Sensors, Solar cells, Health care devices & Smart Textiles 22

Process Flow Chart SP Solar Cell Wafering Saw damage removal Texturization & Cleaning Phosphorous Diffusion PSG removal & edge isolation PECVD SiN (ARC ) Deposition Silver Screen Printing Front & Drying

NIL Solar Cell

Comparison

Wafering Saw damage removal

Baseline - wafers Screen Printed Front Side Metallization Will be compared With Nanoimprinted Front side Metallization

Texturization & Cleaning Phosphorous Diffusion PSG removal & edge isolation PECVD SiN (ARC ) Deposition Al /Ag Screen Printing of rear bus bars and drying

Al /Ag Screen Printing of rear bus bars and drying

Al Screen Printing of rear and drying

Al Screen Printing of rear and drying

Firing rear contacts

Cofiring front & rear contacts

NIL Imprinting Front & Electroplating

I-V Measurements & Sorting

I-V Measurements & Sorting

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Combo Process Scheme : NIL (Narrow line width) – Electroplating – Industry ready : less silver or silver-less process

Ni or Ag Electroless Plating

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Self-Aligned vs NIL contact NIL Patterned Ni/Cu/Sn

•Self Alignment  Non-controlled •NIL  Controlled Contact width of Contact width of Cu (Ni Wc < Cu Wc ) Cu (Ni Wc  Cu Wc )  decreased Shading Loss Increased Shading Loss •Chances of Copper Migration into •Chances of Copper Migration into Active layer is not possible 25 Active layer is possible

NIL Texturization of C-Si Solar Cells Nanoimprint lithography offers a large variety of achievable structure types and profile shapes with dimensions ranging from 10nm to mm, and seamless structured areas up to 1.2 1.2 m2 in a single exposure/imprint step*.

NIL Texturization â&#x20AC;&#x201C; Nano scale *Benedikt et al, SPIE Newsroom. DOI: 10.1117/2.1201205.004241

Conventional Acid Texturization â&#x20AC;&#x201C; Micron Scale 26

NIL Texturization

Left: Honeycomb textured multi-crystalline silicon wafer Right: Crossed rear side grating (period 1μm, depth 270nm).

•NIL helps in Nano-texturization in c-Si fabrication - reducing the total thickness of wafer •Typically uses only 0.5µm of Silicon instead of 5-10µm for traditional texturization of silicon for effective light management in solar cells.

27 •Reduced usage material, leads to cost saving aspect.

Outline • PV Trend • Nanoimprint Lithography – Front Side Metallization – Texturization

• • • • •

Black Silicon Solar Cells Plasmonic Solar Cells AgNW + ALD AZO HIT/HHJ Solar Cells GaAs Epitaxial Lift off (ELO) 28

Black Silicon Solar Cells The nanostructuring of silicon surfaces—known as black silicon—is a promising approach to eliminate frontsurface reflection in photovoltaic devices without the need for a conventional antireflection coating Black silicon (b-Si)—absorbs light very efficiently for a wide range of wavelengths and, as a result, appears black to the naked eye.

In addition to boosting efficiency, b-Si can provide significant savings in manufacturing costs as there is no need to deposit a separate antireflection coating.

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Black Silicon Solar Cells Challenges - Increased surface recombination due to the larger surface area of the nanostructures, and the situation is even more challenging in a conventional front-contacted solar cell structure due to Auger recombination at these highly doped nanostructures. Auger recombination can be avoided by using an interdigitated back contact (IBC) solar cell design where the junction and the contacts are placed at the back of the cell, but the recombination problem due to the larger surface area remains unsolved. Surface passivation – PECVD SiN or SiO2, resulting in tradeoffs between reflectance and recombination. The final efficiency has therefore been limited by recombination at the increased b-Si surface, and the reported. Reported efficiencies have remained well below 20% (18.2% with a surface area of 0.8 cm2 ).

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Black Silicon Solar Cells

How to Solve â&#x20AC;&#x201C; pin-hole-free and highly conformal atomic layer deposited (ALD) thin films, combined with the chemical and field-effect passivation ability of Al2O3 - Overcoming the problematic surface recombination issue in b-Si. Different methods b-Si : laser texturization, plasma immersion ion implantation, metal-assisted wet etching & Reactive Ion Etching (RIE) Multiple Advantages of DRIE: Fast, reproducible, inexpensive, no dependence on crystalline orientation, and no requirement for 31 lithography mask for random texturization.

Black Silicon Solar Cells

Typical height of a silicon pillar, ∼800 nm; diameter at the bottom of the pillar, ∼200 nm. The 20 nm Al2O3 layer can be seen as a brighter layer on top of the pillars Measured reflectance spectra in the 300–1,000 nm wavelength range. The dashed line - reflectance of a bare b-Si sample and the black solid line shows the reflectance of b-Si with 20 nm of Al2O3 (inset: zoomed view).

The reflectance of random pyramids coated with 90-nm-thick Al2O3 film (dotted line) is shown as a reference.

Hele et al, Nature Nanotechnology (2015), DOI : 10.1038/NNANO.2015.89

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Black Silicon Solar Cells Key Findings

Successfully developed a high-efficiency, b-Si IBC solar cell, with greater than 22% efficiency with a surface area of 0.9 cm2.

The use of a surface sensitive 280-Îźm-thick IBC structure proves that surface recombination, which has been hindering the use of b-Si in photovoltaics, is no longer a limiting factor.

This should pave the way for even higher efficiencies in b-Si cells, not only in IBC structures but also in existing and new solar cell concepts.

Hele et al, Nature Nanotechnology (2015), DOI : 10.1038/NNANO.2015.89

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Outline

• PV Trend • Nanoimprint Lithography – Front Side Metallization – Texturization

• • • • •

Black Silicon Solar Cells Plasmonic Solar Cells AgNW + ALD AZO HIT/HHJ Solar Cells GaAs Epitaxial Lift off (ELO) 34

Plasmonics  Improved Photovoltaic Devices High energy conversion efficiency & Cost Reduction – DRIVING FORCE to make solar electricity more competitive with fossil fuels.

Thinner wafer (wafer cost accounts for approximate 50% of the solar cell cost) Wafer thickness 400µm to current 180µm  Trend of reduction is continuing Thinner wafer - leads to a higher Voc  lower bulk recombination & more efficient 35 electron-hole pair extraction

Plasmonics  Improved Photovoltaic Devices

• Thickness reduction  Challenge for efficiency maintenance  Significant light absorption decline at the longer wavelengths, especially when the wafer thickness is reduced below 50 µm • The longer wavelength light  lower absorption coefficient, requiring a larger travelling distance in Si to be completely absorbed.

• For wafer-thickness-reduction-induced cost savings and higher electrical performance  Advanced light trapping technology targeting the longer wavelength region of the solar spectrum is pressingly required to be developed 36

Plasmonics ď&#x192; Improved Photovoltaic Devices

(a) Standard 180Âľm solar cell with SiNx antireflection coating (ARC) layer, the Si layer and the Al back reflector. (b) Ultra-thin solar cell with spherical NPs located on the front surface of the SiNx ARC (c) The hemispherical NPs embedded in a SiO2 layer between the Si layer and the Al back reflector.

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Y. Zhang et al., Nature (2015) DOI: 10.1038/srep04939

Ultra-thin Plasmonic Silicon Wafer Solar Cells Front nanoparticles Reduction in light reflection  front surface due to the optical impedance matching & light path length increase, particularly, for the longer wavelengths Rear nanoparticles Enhances the light path lengths by scattering.

Scattered light with angles larger than the critical angle of the front interfaces can be trapped inside the Si due to total internal reflection.

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Ultra-thin Plasmonic Silicon Wafer Solar Cells Advanced light trapping strategy with a properly designed nanoparticle architecture ď&#x192; Wafer thickness can be dramatically reduced to around 1/10 (18 Âľm) of the current thickness (180 Âľm) without any solar cell efficiency loss at 18.2%

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Y. Zhang et al., Nature (2015) DOI: 10.1038/srep04939

Ultra-thin Plasmonic Silicon Wafer Solar Cells â&#x20AC;˘ Nanoparticle integrated ultrathin solar cells with only 3% of the current wafer thickness (180Âľm) can potentially achieve 15.3% efficiency combining the absorption enhancement with the benefit of thinner wafer induced open circuit voltage increase. â&#x20AC;˘ This represents a 97% material saving with only 15% relative efficiency loss in solar cells with Plasmonic light trapping.

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Y. Zhang et al., Nature (2015) DOI: 10.1038/srep04939

Outline • PV Trend • Nanoimprint Lithography – Front Side Metallization – Texturization

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Black Silicon Solar Cells Plasmonic Solar Cells AgNW + ALD AZO – Transparent Contacts HIT/HHJ Solar Cells GaAs Epitaxial Lift off (ELO) 41

Silver nanowire networks by atomic layer deposition for indium-free transparent electrodes  Highly transparent electrodes with low electrical resistivity are an essential part of optoelectronic devices. In solar cell industry 80% rely on silicon solar cell with silver screen printing Bottlenecks  Silver grid electrodes [reflect parts of incident light leading to efficiency deterioration]

Silver Nanowire Network The Silver nanowire (AgNW) is a promising candidate as a transparent conducting material. This method consume less material -20gm of Ag42/ m2

Silver nanowire networks by atomic layer deposition for indium-free transparent electrodes Low cost solution AgNW network processing: exhibits 1) Spin coating •Low sheet Advantage of AgNW 2) Drop casting or spray resistance coating •High transmittance 3) Roll to roll •Low silver consumption Chemically and thermally stable to guarantee long term stability Deposition  Al doped ZnO + (PLD, Sputtering, sol-gel) AgNW •Wide band gap •Electron donor (releases additional electron) •Non toxicity •Low cost

(but) ALD  high quality AZO layers with good conformity

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SEM images of annealed AgNWs encapsulated by a 100 nm thick layer of AZO: (a) overview of the AgNW network, (b) close-up of a FIB cut cross section of the AgNWs and (c) close-up of an encapsulated AgNW junction.

Electrode type

Voc (mV)

ISC (mA)

JSC (mA/cm2)

FF (%)

Ag-grid

553

7.0

16.3

68.4

AgNWs+ AZO+ Ag-grid

556

8.2

18.2

73.6

AZO+ Ag-grid

559

8.7

19.8

72.1

AZO

547

10.6

25.1

30.1

AgNWs(16.36%)+AZO

559

13.8

27.0

31.3

AgNWs(34.04%)+AZO

559

14.0

28.0

60.1 44

M. Gobelt et al, Nanoenergy (2015). DOI: 10.1016/j.nanoen.2015.06.027

Outline • PV Trend • Nanoimprint Lithography – Front Side Metallization – Texturization

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Black Silicon Solar Cells Plasmonic Solar Cells AgNW + ALD AZO HIT/HHJ Solar Cells GaAs Epitaxial Lift off (ELO) 45

HJT/HIT Solar Cells Silicon wafer KOH surface preparation KOH surface preparation i/p a-Si PECVD i/p a-Si PECVD i/n a-Si PECVD i/n a-Si PECVD Resist apply by inkjet Front TCO PVD (wafer edge masked)

Front TCO PVD

Back contact PVD Back contact PVD Front grid screen printing ITO Lift off in Water

Front grid screen printing

HJT solar cell

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Hetrojunction solar cells Amorphous/ crystalline silicon hetrojunctionon (a-Si:H/C-Si)

Achieve high conversion efficiency with low temperature process

Voc = above 720 mV, Efficiency 24.7%

(SANYO)

Rear Contact HET Solar cells : Efficiency: 25.6% 47

Hetrojunction solar cells Emitter Passivation

Improve the quality of interface [ (p) a-Si : H/ (i) a-Si : H/ (n) C-Si ]

Interface Density of State(Dit ) reduced by (i)a-Si : H

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Homo-Heterojunction (HHJ) Solar cells* Presence of one type of charge carrier layer

Decreases interface recombination (Leading to more efficiency ) One type of charge carrier accumulate on the top of the layer. It repels the minority charge carriers & acts as a barrier layer.

Reduces Electron Hole recombination ď&#x192;&#x2DC; The Simulation results from AFORS-HET yields an efficiency increase by 0.7% absolute (abs) compared to the HET cell. *T. Carrere et al., JRSE (2015) doi: 10.1063/1.4908189

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Outline • PV Trend • Nanoimprint Lithography – Front Side Metallization – Texturization

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Black Silicon Solar Cells Plasmonic Solar Cells AgNW + ALD AZO HIT/HHJ Solar Cells GaAs Epitaxial Lift off (ELO) 50

GaAs Epitaxial Lift off (ELO) for flexible Solar Cells

GaAs Solar Cells are extremely expensive and currently used only for space quality solar cells!!

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TRADITIONAL PROCESS

DEPOSITION (EPITAXY) CELL STRUCTURE (4 µm)

DEPOSIT CONTACTS & ARC

CRYSTAL WAFER (GaAs)

CRYSTAL WAFER (GaAs)

CRYSTAL WAFER (GaAs)

(150-350μm)

(150-350μm)

(150-350μm)

Modified PROCESS*

CRYSTAL WAFER (GaAs)

(150-350)

REUSABLE

DEPOSITION (EPITAXY) AlAs SEPARATION LAYER (10nm) & CELL STRUCTURE (2 µm)

Epitaxial Lift off

CRYSTAL WAFER (GaAs)

CRYSTAL WAFER (GaAs)

(150-350μm)

(150-350μm)

TRANSFER TO CARRIER DEPOSIT CONTACTS & ARC

EPITAXIAL LIFT-OFF (ELO)

CRYSTAL WAFER (GaAs) (150-350μm)

HIGH EFFICIENCY lll-V CELL ON CHEAP CARRIER 52 * Proprietary of Alta devices, Microlink devices & Radboud Univ

Epitaxial Lift off (ELO) Advantages Weight reduction – Critical for Space quality solar cells  2) Cheaper option - Substrate reuse  3) Possibility to Compete with Si, CIGS for use in terrestrial applications

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Final Summary Black Silicon Solar Cells oUse of surface sensitive 280μm thick IBC structure proves that surface recombination, which has been hindering the use of b-Si in Photovoltaics, is no longer a limiting factor. oThis should pave the way for even higher efficiencies in b-Si cells, not only in IBC structures but also in existing and new solar cell concepts

Plasmonic Solar Cells Nanoparticle with ultra-thin solar cells with 5.4µm can achieve 15.3% efficiency This represents a 97% material saving with only 15% relative efficiency loss in solar cells with Plasmonic light trapping.

AgNW with ALD AZO  AgNW is a promising candidate as a transparent conducting material. This method consume less material -20gm of Ag / m2  AgNW + ALD AZO achieves higher efficiency

Homo-Heterojunction Solar Cells  Low interface state density & interface combination + 1 type of charge carrier  AFORS-HET Simulation Yields  0.7% increase in absolute efficiency compared to HIT solar cells >25%

PSGIAS Contributions  Race towards High Efficiency:-

 Nanoimprint Lithography for Fine Line Width Front Side Metallization  Nanoimprint Lithography for Nano-Texturization (ICP-RIE)  Equipment (Ultrasonic Spray Pyrolysis) & Process Development for Large Scale  

Antireflection Coating on Glass Black Silicon & HHJ Solar Cells with ICP-RIE/ICP-CVD Nanostructured Thin Film Solar Cells (Hybrid PVD)

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Collaborators Competent Partner in Process Development & Equipment Building We invite potential partners to collaborate with us to set-up their business in Nanotech Research Innovation & Incubation Centre @ PSGIAS Potential areas : Crystalline Silicon, HIT Solar Cells, Thin film (CIGS, CZTS), Organic Solar Cells, & Antireflection coating on glass(Compensating C2M loss) Existing Collaborators : Udhaya Semiconductors, Moserbaer Solar, Azure power, IIT-Mumbai, IISc Bangalore & Anna University

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