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(1)

Amorphous silicon thin film solar cells

a-SI c-SI

1

large concentration of intrinsic defects

N

T

>10

16

cm

-3

(„dangling bonds” D

+

, D

-

, D

o

)

doping more difficult, e.g. if we increase a number of free electrons by adding P the concentration of D

-

defects increases also

passivation of defects by hydrogen – a-Si:H - decreases NT to ~1015 cm-3 doping and creation of pn junction becomes possible!

(2)

Absorption in a-Si

λ(µm)

direct bandgap 1.7 eV, E

g

>E

g

(c-Si)

no well-defined E(k) dependence

no conservation of momentum k

absorption coefficient ~ 10-100 times higher thin film solar cell cell (~5 µm) possible

λ(µm)

(3)

D+ D-

Gap states in a-Si

3

passivation of defects by hydrogen

doping possible in Si:H!

very high density of defect levels in the gap

doping not effective

(4)

device degradation: efficiency loss due to photo-generation of defects

Steabler-Wronski effect

Best modules:

η=10.5 % (stabilized)

power

(5)

Deposition process

•large area deposition (more than 1 m2)

• low deposition temperature (100°C < Ts

< 400°C)

• use of any cheap and arbitrarily shaped substrates

• effective p- and n-type doping and alloying

• deposition of composition-graded layers

• deposition of multi-layer structures by

5

rf PECVD deposition system

control of gas mixtures in a continuous process

• easy patterning and integration technology

• low cost

• good mass-producibility

(6)

a-Si cell parameters

short diffusion length ~100 nm and decreases more with doping minority carrier lifetime 10 ns solution: p-i-n cell

extended electric field region

Jsc

[mA/cm2

Voc [V]

Fill factor Efficiency [%]

UNSW

mono c-Si PERL structure

42.2 0.706 0.828 24.7

USSC

a-Si:H p-i-n structure

14.3 0.965 0.672 9.3

(7)

a-Si cell – basic design

p i n p-i-n cell

7

band diagram

TCO - transparent conducting oxide (ZnO, SnO2) Thin doped regions – reduced degradation

(8)

a-Si solar cell

Fabrication steps:

TCO (spray deposition) laser scribing

p-aSi (10 nm) i-a-Si (500 nm) n-aSi (10 nm) laser scribing

metalization, laser scribing, polymer coating sputtering SiH4, glow

discharge process

metalization, laser scribing, polymer coating

(9)

HIT solar cells

(Heterojunction with intrinsic thin layer) Sanyo

good surface passivation

9

η= 25,6%

modules up to 19%

good surface passivation low temperature processing (<200 C)

(10)

Multijunction devices

better stability double junction

(11)

Thin film heterojunction solar cells

window(Eg1)

absorber (Eg2)

11

 polycrystalline materials

 good absorption properties – 2-5 µm absorbers

 p-n heterojunctions

 low costs, low energy- and material -consuming technologies 100 x less material than for c

100 x less material than for c 100 x less material than for c 100 x less material than for c----SiSiSiSi

(12)

absorber okno

CdTe, Cu(In,Ga)Se CdTe, Cu(In,Ga)SeCdTe, Cu(In,Ga)Se CdTe, Cu(In,Ga)Se2222

CdS, Zn(O,S), In CdS, Zn(O,S), In CdS, Zn(O,S), In

CdS, Zn(O,S), In2222SSSS3...3...3...3...,.. ,.. ,.. ,..

Heterojunction photovoltaic structure

absorber:

bandgap close to 1.5 eV window

advantages:

maximum generation in the electric field region of absorber:

no problem with front surface recombination disadvantages:

interface recombination, band edge discontunuities at interface

Eg2

Eg1 p

n

bandgap close to 1.5 eV p-type doping ~1016 cm-3 high absorption coeffient window:

large bandgap (transparent)

good conduction band alignement with absorber

n-type doping 1017-1018 cm-3

(13)

Heterojunction – advantages of window-absorber concept

pn junction heterojunction

13 Max. generation density of e-h pairs

at at at

at thethethe front thefront front surfacefront surfacesurfacesurface

surface recombination losses

•necessity of surface passivation

•Generation at maximum outside of the juntion field

Max. generation density in the absorber

• far from the front surface

• in the region of maximum electric field

E E

(14)

Current transport in the heterojunction

o recombination in the bulk of absorber o recombination via interface states o tunnelling to interface states

Conduction band alignement Conduction band alignement

cliff spike

bad: bad:

(15)

Material requirements

absorber

direct bandgap Eg≈ 1.4 eV

• p-type electrical conductivity

• high diffusion barrier

• long diffusion length

window

high energy gap

• high conductivity

• good ohmic electrical contacts

15

• good ohmic electrical contacts

• similar lattice constants

• good band alignement

• stable junction

• availability and low cost of materials and technology

• low toxity of materials

There is not so many materials meeting these requirements!

Highest efficiencies achieved with CIGS and CdTe-based solar cells

(16)

direct bandgap, very high absorption coefficient CuInSe2 Eg = 1.04 eV

CuGaSe2 Eg = 1.68 eV CuInS2 Eg = 1.55 eV Solid solutions Cu(In,Ga)Se2 i CuIn(Se,S)2

Semiconducting chalcopyrites from CIGS family

Highest efficiency (record 22.3%)

achieved with CuIn

1-x

Ga

x

Se

2

x = 0.2 - 0.3 E

g

= 1,12 - 1,20 eV

(17)

CuGa

x

In

1-x

Se

2

solid solutions as absorbers

17

Lower efficiency for Ga/(In+Ga)>0.3 Bad conduction band alignement?

(18)

Mo

p-CuIn

1-x

Ga

x

Se

2

(2 µm) n-CdS (50 nm)

n

+

- ZnO:Al (0.3 µm) i - ZnO (0.1 µm)

Baseline CIGS device

window

buffer

Mo

nCdS – buffer

•good alignement of conduction bands

•lattice constant matches that of CIGS

•electrochemical treatment of the absorber surface

(19)

absorber

CIGS cell band diagram

3.4 eV

19

„okno”

buffor

(20)

ZnO - sputtering ZnO - sputtering

CdS - CBD (chemical bath deposition) Cu(In,Ga)Se

2

- co-evaporation

- selenization of metal layers in Se lub H

2

Se vapour - bilayer, 3 stage process,

Mo - RF sputtering

(21)

Preparation of CIGS absorber layer

21

(22)

Absorber preparation- „3 stage process”

(23)

GB GB

c

v

c

Grain boundaries in CuInSe

2

Si i GaAs

Grain boundaries in CIGS

23

GB

in CIGS neutral grain boundaries, lower Ev?

policrystalline material makes better cells than single crystal segregation of impurities at GB?

h+ v

(24)

Specific problems

Native defects and doping

p-doping by Cu vacancies: Cu-poor composition Cu/(In+Ga)<1

large concentration of native defects: Se vacancies, InCu antisites, etc large deviation from stechiometry (Cu/(In+Ga) down to 0.8) tolerated!

metastable changes of photovotaic parameters – light soaking improves the efficiency!

Influence of sodium diffusing from soda-lime glass

Na increases net doping and improves morhology of the layer:

grain boundaries passivated? compensating donors passivated? growth improved by better incorporation of Se (Na2Se topotaxy)?

NaF – source of sodium when deposition on sodium-free substrate Cd-free buffers:

worse performance (???)

(25)

Metastabilities of junction parameters

„morning sickness”

-15 0 15 30 45 60 75

300 K

current [mA]

I-V after a long time in darkness - I-V after white light soaking:

25 0,2 0,4 0,6 0,8

-30

-15 300 K

voltage [V]

I-V after white light soaking:

Voc and FF improves!

Recent theoretical and experimental findings:

specific properties of VSe and InCu-related defects

(26)

Flexible substrates

20.4% on polyimide foil (2013) EMPA (Switzerland)

NaF precursor provides neccesary sodium

(27)

New record CIGS cell – 22.6% ZSW Germany

•new alkali (Na,K) post-deposition treatment

•very thin CdS

•steep Ga gradient towards back electrode

27

Modules with K-treatment – 17.5% (Solibro, Sweden)

(28)

Stability

s

irradiation by proton flux

exceptionally good if protected against humidity

radiation hardeness more than 1 order of magnitude better than crystalline absorbers good for space application!

(29)

Kesterites – new photovoltaic materials

Cu Cu

Cu CuIn In InSe In Se Se Se

2222

Cu Cu Cu Cu

2222

ZnSn( ZnSn(S,Se) ZnSn( ZnSn( S,Se) S,Se) S,Se)

4444

ηη

ηη=12.6% (=12.6% (=12.6% (=12.6% (IBM 2014) IBM 2014) IBM 2014) IBM 2014)

Eg=1.5 eV

remedy for shortage of indium, earth-abundant matrials

29 (from hydrazine solution)

Problems:

• many secondary phases

• very defected material

(30)

CdS/CdTe heterojunction solar cell

superstrate configuration

efficiency 22.6 (First Solar 2016) (modules up to 18.6%)

absorber:

CdTe 3-5 µm

intrinsic p-type doping ~1015 cm-3 direct bandgap Eg =1.5 eV

(31)

s Absorber preparation:

CdTe: PVD, CSS, electrodeposition, spray pyrolysis

CdCl2 treatment at T>400 C:

recrystallisation of CdTe grains passivation

interface improvement

ohmic back contact: difficult Cu-Au alloy or ZnTe:Cu, Sb2Te3,

Close space sublimation process

31

s

process company efficiency(%)

CSS Solar Cell Inc.

(USA)

15.8 / 1.05 cm2 8.4 / 7200 cm2 electrodeposition BP Solar 14.2 / 0.02 cm2 10.1 / 706 cm2 Screen printing Matsushita 12.8 / 0.78 cm2

8.1 / 1200 cm2 Spray pyrolysis Golden Photon 12.7 / 0.3 cm2

8.1 / 832 cm2

VTD First Solar 18,6 / 1200 cm2

sputtering 10.4 / 0.1 cm2

(32)

Life cycle assesment of Cd

Cd is a by product of production of metals from zink ores (~80%) and lead ores (~20%) (from wastes)

V.M. Fthenakis 2004

0.02 g of Cd per GWh during life time of CdTe module

power plants based on coal emit minimum of 2 g of Cd/GWh!

Obraz

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