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KAMLESH PATEL

Amorphous and Micro-Crystalline Silicon Solar Cells

Physics of Energy Harvesting

NATIONAL PHYSICAL LABORATORY

Development and performance statusDevelopment and performance status

of HIT Solar cellsof HIT Solar cells

Advances and Progress of HIT Solar cells- Efficiency 18.1% to 23% a journey by AERC at SANYO Electric Co. Ltd., Japan

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Basics of Solar cell

3http://zone.ni.com/devzone/cda/tut/p/id/7229-30

Simplified Equivalent Circuit Model for a Solar Cell

Band diagram of a silicon solar cell

I-V Curve of a Solar Cell

a p-n junction Solar Cell

Photon excitation Electron-hole pair creation Current generation

Photon energy Ep > Eg Bandgap energy

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Efficiency Losses in Solar cellEfficiency Losses in Solar cell

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(1) Photons with Energy less than Eg are

transmitted, not absorbed.(2) Relaxation, heat lost to phonons and

environment

(1 and 2) - due to mismatch of Eg and

photon energy

(3) Junction voltage loss

(4) contact voltage loss(5) recombination recombination

Focus of R & D in Solar cells:

new type of solar cells with potentially lower production costs

reduction of Silicon material and cell processing costs

solar cell withhigher efficiencies

Material selection for Solar cell optimization :

a. Bandgap selection to cover appropriate spectrum

b. Lattice matching to produce optical transparency and maximum current

c. Thickness of semiconductor material as per their absorptivity

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Material selection for Solar cell optimization :

a. Bandgap selection to cover appropriate spectrum

b. Lattice matching to produce optical transparency and maximum current

c. Thickness of semiconductor material as per their absorptivity

losses not taken into account in this paper surface reflection contact shadowing series resistance incomplete collection of photogenerated carriers absorption in the inactive window layers

nonradiative recombination above ambient cell temperatures

Practical limitationsGeneral limitations in solar cells

Surface reflection Series resistance at contacts

Recombination losses voltage lossesLimitations specific to tandem solar cells Series-connection Current matching constrains! Variation of solar spectrum throughout the day Resistance in intermediate recombination layer Transmittance of top cells A challenge: Light management with textured substrates

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Silicon Solar cellsSilicon Solar cells

Earths solar spectra

a-Si:H solar cell

Bandgaps of some common semiconductors and photon e

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Multi-Junction Thin film Solar cells

Single junction

a-Si:H solar cell

GaInP/GaAs/Ge

triple-junction solar cell

Solar spectrum

splitting per junction

Quintuple-junction thin film structure with theoreticalconversion efficiency of 35% or higher.First cell, a-Si1xOx, a-Si1xCx, a-Si1xNx, Ag(InGa)Se2, etc.;second cell, a-Si, SiGe clathrate, etc.;third cell, a-SiGe, Si quantum dot, CdTe, Cu(InGa)Se2, etc.;fourth cell, c-Si, Cu(InGa)Se2, etc.; fifth cell, c-SiGe, Ge, CuInTe2,

Makoto Konagai,

Jpn J. of Appl Physics50 (2011) 030001-1-12

F. Dimroth, IEEE (2005). S. P. Bremneret. al., Prog. Photovolt.16, 225 (2008)

R. King, Appl. Phys. Lett. 90, 183516 (2007)

A double junctionthin film solar cell

A. Brown, Physica E14, 96 (2002)

S. Bremner, Prog.Photovol. 16, 225(2008)

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Conversion efficiency, annual production volume,Conversion efficiency, annual production volume,and future prospects for application to powerand future prospects for application to power

generation of various solar cellsgeneration of various solar cells

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Makoto Konagai, Jpn J. of Appl Physics 50 (2011) 030001-1-12

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Crystalline (Wafer-Based) and Thin-Film Photovoltaic Cells

NPL, INDIA 10

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Heterojunction Solar cells

Band offset Eg2 > Eg1

a-Si:H passivates the surface very well with surface recombination

velocities as low as 3 cm/s

Surface passivation by reducing surface dangling bond density and by

field effect passivation of doped a-Si:H

Low-temperature approach (< 230 C)

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Key features of silicon-heterojunction technology:

very simple fabrication process

important cost-reduction capability

relatively high efficiencies, with a high potential for significant improvements

Limitations specific to Heterojunction solar cells:

Series-connection Current matching constrains

Variation of solar spectrum throughout the day

Resistance in intermediate recombination layer

Transmittance of top cells

A challenge: Light management with textured substrates

n-on-p silicon heterojunction solar cellY. Hamakawa, Appl. Surf. Science 142, 215226 (1999).

Highest deposition temperature 210 rC

Heterojunction Solar cells

L. Korte et al. / Solar Energy Materials & Solar Cells 93 (2009) 905910

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HIT = Heterojunction with Intrinsic Thin-layer

HIT solar cells: first used by Sanyo in 1992 and now used forhigh-efficiency

solar cells (250 MWp in 2007)

Heterojunction emitter: two different semiconductor materials (a-Si:H and c-Si)

create the pn-junction

diffused homojunction emitter

Intrinsic layer : between the p and n type material there is an undoped

(intrinsic) amorphous Si layers

Thin layer: total a-Si:H layer is typically less than 20 nm

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HeterojunctionHeterojunction to HITto HIT

14M. Tanaka, et al, Development of New a-Si/c-Si Heterojunction Solar Cells: ACJ-HIT (Artificially ConstructedJunction-Heterojunction with Intrinsic Thin-Layer), Jpn. J. Appl. Phys., 31 (1992) 3518-3522

Thin intrinsic a-Si layer introduced, better

passivation of silicon wafers

Maximum L: 14.8%

Voc increased by 30 mV and FF > 0.8

Isc decreases with i-layer thickness

With thickness, Isc decreasesOptical absorption increases in p-a-

Si:H - film should be thin (

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Effect ofEffect of PassivationPassivationReduction of dangling bondsReduction of dangling bonds

Reduction of surface recombination (velocities)Reduction of surface recombination (velocities)Increase of minority carriers lifetimeIncrease of minority carriers lifetime

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Passivation by doped a-Si:H layer

Wider bandgap material stops carriersto move and reducing recombination

a-Si:H has direct bandgaphigh absorption coefficient

Minimise thickness to reduce theabsorption in the a-Si:H(lost for collection)

Passivation by i-a-Si:H layer

Provides extremely good surfacepassivation (3 orders ofmagnitude less defects than doped a-Si:H)

Reducing the recombination of carriersnear the interface

Reducing surface defects

Allows carriers to pass through passivatinglayer without any significant loss

M. Tanaka, et al, Development of New a-Si/c-Si Heterojunction SolarCells: ACJ-HIT (Artificially Constructed Junction-Heterojunction withIntrinsic Thin-Layer), Jpn. J. Appl. Phys., 31 (1992) 3518-3522

Dark I-V characteristics of HIT structure and p-nheterojunction

This suppression of backward current densitysuggests the better surface passivation at the a-Si/c-Siheterointerface with the HIT structure.

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Higher efficiency in HIT solar cellHigher efficiency in HIT solar cell

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a. Textured substrate- optical confinement effects

Two technologies with optimization in a-Si:H deposition

Structured surfaces to effect light trapping in silicon solar cells

Campbell, P. and Keevers, M., 2000. Light Trapping and Reflection Control for Silicon Thin Films Deposited onGlass Substrates Textured by Embossing. Proc. 28th IEEE Photovoltaic Specialists Conf., Anchorage,pp. 355-358.

Tom Markvart and Luis Castaner , Solar cells: Materials, Manufacture and Operation, Elsevier

Light trapping effects, which can offset the relatively weakabsorption near-bandgap energy photons by increasingthe optical path length of light within the solar cell structure

the generation of minority carriers would be relatively closeto thep-n junction formed near the top surface of the solar cell,thus providing a high collection efficiency.

Texturing one or both surfaces, and maximising thereflection at the back surface obtains optical path lengthsgreater than the thickness of the device.

Texturing results in oblique paths for internally confinedlight andmaximises total internal reflection at the illuminated devicesurface.

Isc Increases

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b. Back surface field: depositing n type a-Si:H on the rear surfaceof c-Si substrate.

The sharing of the applied voltage among the two junctions (the p-i-nand the n-n junction) decreases the dark currentThe reflection of minority carriers by the built-in electron field of the n-c-Si/n-a-Si junction increases Isc

Holes are repelled by c-Si (n)/a-Si:H(n)junction field

Voc increases

Texturing of c-Si needs to be smooth

The texturing of the c-Si needsto be rounded so thatcontinuous a-Si film isdeposited

N. Takuo, et al.

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14.8% to 18.1% HIT Solar cell14.8% to 18.1% HIT Solar cell

18M. Tanaka, et al, Development of New a-Si/c-Si Heterojunction SolarCells: ACJ-HIT (Artificially Constructed Junction-Heterojunction withIntrinsic Thin-Layer), Appl. Phys., 31 (1992) 3518-3522

Effect oftextured substrate andback surface field (BSF)

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20.7% HIT CELL in 100.5 cm20.7% HIT CELL in 100.5 cm22 areaarea

HitoshiSakata,TakuoNakai,ToshiakiBaba,MikioTaguchi,SadajiTsuge, Kenji Uchihashi, SeiichiKiyama,20.7%HIGHESTEFFICIENCYLARGEAREA (100.5cm2) HITTM CELL, in:Proceedingsofthe28thIEEEPhotovoltaic SpecialistsConference,2000,pp.712.

the measurement results according to JQA(Japan Quality Assurance organization)

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The better surface passivation , High Voc and thus highL of solar cell

Study of Passivation effect with the carrier lifetime of c-Si wafers

Carrier lifetime is measured by the -PCD Method.

Increasing the carrier lifetimesof the HIT structure

Higher the VOC of the HIT cell

Optimizations of deposition processes to get thehigher lifetime carriers

HitoshiSakata,TakuoNakai,ToshiakiBaba,MikioTaguchi,SadajiTsuge, Kenji Uchihashi, SeiichiKiyama,20.7%HIGHESTEFFICIENCYLARGEAREA (100.5cm2) HITTM CELL, in:Proceedingsofthe28thIEEEPhotovoltaic SpecialistsConference,2000,pp.712.

the standard PCD theory for ahomogeneously doped, defect-free wafer with aspatiallyuniform bulk carrier lifetime, the relationshipbetween T~~and 2, is given by:

where 2, is the effective lifetime(the measured lifetime),2, is the bulk lifetime and 2 , is thesurface recombinationlifetime component

Future study of surface passivation exists at interface, as where bothhydrogen passivationand carrier separation caused by the strong electrical field can exist,

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M. Tanaka, et al, Development of New a-Si/c-Si Heterojunction Solar Cells: ACJ-HIT (Artificially

Constructed Junction-Heterojunction with Intrinsic Thin-Layer), Appl. Phys., 31 (1992) 3518-3522

In HIT cell,

Voc and FF both increases significantly, representing improvements ininterface properties

Isc increases nominally due to very less improvement in optical absorption

S. Taira, Y. Yoshimine, T. Baba, M. Taguchi, H. Kanno, T.Kinoshita, H. Sakata, E. Maruyama, and M.Tanaka, "Our approaches for achieving hit solar cells withmore than 23% efficiency," presented at Proceedings ofthe 22nd European Photovoltaic Solar Energy ConferenceMilan, Italy, 2007

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22Takahiro Mishima n, MikioTaguchi,HitoshiSakata,EijiMaruyama, Solar Energy Materials & Solar Cells 95 (2011) 1821

Structure of a HIT solar cell.

History of the HIT cells conversion efficiency (R&D).

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Inside HIT structureInside HIT structure

Energy band gap

Electron and holes

Spectral response Effect of i-layer, H2 layer

Effect on Voc, Isc, effeciency

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THANKSTHANKS

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KP's 11-04-11