Effects of CdCl 2 treatment on ultra-thin MOCVD-CdTe solar cells

Transcription

1 Effects of CdCl 2 treatment on ultra-thin MOCVD-CdTe solar cells A.J. Clayton, S. Babar, M.A. Baker, G. Kartopu, D.A. Lamb, V. Barrioz, S.J.C. Irvine Functional Thin Films, Thursday 17 th October 2013

2 Centre for Solar Energy Research (CSER)

3 Part I Thin film CdTe solar cell devices at CSER Metal Organic Chemical Vapour Deposition Purpose of CdCl 2 activation treatment Part II Ultra-thin CdTe absorber Wide band gap window layer Variable CdCl 2 activation treatment

4 Part I MOCVD-CdTe solar cell device Complete cell structure is produced in a single growth chamber using metal organic chemical vapour deposition (MOCVD) from transparent conducting oxide (TCO) to CdTe absorber. Atmospheric pressure (AP) process Glass substrate In situ CdCl 2 activation treatment No chemical etching required Baseline CdTe absorber = 2.25 m TCO n- Cd 1-x Zn x S p- CdTe:As p + - CdTe:As Front contact Back contact Superstrate configuration

5 Device evolution at CSER Early devices were ~1.5 2 cm 2 with 0.04 cm 2 Au back contacts (or cells). 2. Au back contacts were increased to 0.25 cm 2 with improvements to solar cell performances. 3. CdTe solar cell devices are currently 5 5 cm 2 and produced on 0.7 mm thick aluminosilicate/ito or 3 mm thick NSG TEC (FTO-coated) glass. Devices with cells 1 cm 2 in area have also been produced. Cell performances more susceptible to inhomogenities across the device.

6 Cerium-doped micro-sheet glass (CMG) EPSRC funded collaboration between CSER the University of Surrey and industrial partners Qioptiq Space Technology and Surrey Satellite Technology Ltd. Lightweight, flexible and low-cost energy supply for space-based power: Platform CdTe solar cell device deposited onto ZnO:Al/CMG ZnO:Al 700nm, 85% ave T, 6-9 Ω/ Chemically toughened CMG 100 µm thick CdTe solar cell device on CMG deposited by MOCVD

7 Metal Organic Chemical Vapour Deposition (MOCVD) - Batch Non-return valve Carrier gas Valve Substrate Extracted vent Precursor bubbler Deposition chamber Metal-organic chemical RF heater Graphite susceptor Activated carbon scrubber Transport of reactants (in the vapour phase) to deposition chamber by carrier gas. Gas phase & surface reactions with nucleation, island growth & coalescence resulting in polycrystalline film growth on heated substrate. MOCVD offers tight control of alloy/dopant parameters for all layers in the PV device structure.

8 Metal Organic Chemical Vapour Deposition (MOCVD) - Inline Exhaust TCO Buffer CdZnS CdTe CdTe p+ CdCl 2 Loading & pre-heating Annealing & cooling Separate deposition zones using cm 2 (max cm 2 ) substrates. Faster growth rates & better materials utilisation relative to batch process.

9 CdCl 2 activation treatment 5 µm 5 µm 5 µm CdTe after CdCl 2 deposition & anneal CdCl 2 deposition onto the CdTe followed by high temperature anneal. Fast diffusion of Cl along grain boundaries Recrystallisation & grain growth reducing density of grain boundaries. Interdiffusion between the CdZnS/CdTe interface reducing defects related to strain from lattice mismatch. Defect passivation by complex formation with Cl donor species.

10 Spectral response after CdCl 2 activation treatment IPCE 0.3 IPCE Wavelength (nm) Wavelength (nm) EQE analysis done by Dr Anura Samantilleke (Bath University) Non-treated CdTe solar cells have large carrier recombination at longer photon wavelengths (deeper into the absorber). CdCl 2 activation treatment increases carrier lifetime improving photocurrent generation at longer wavelengths.

11 Part II - Ultra-thin CdTe photovoltaics Continued large scale production of thin film CdTe PV modules may face challenges of materials supply due to limited global Tellurium stock. Consideration towards improved sustainability is necessary:- Improved utilisation of materials; Reduced consumption of materials; Recycling of waste material and/or end-of-life modules. Ultra-thin (< 1 µm) CdTe PV cells Majority of carriers generated close to the junction; Photo-absorption becomes optically limited for CdTe thicknesses below 1 µm. Marwede & Reller, Res. Conserv. Recycl. 69 (2012) Amin et al., SOLMAT 91 (2007) Compaan et al., SOLMAT 90 (2006)

12 Cd 1-x Zn x S window layer Wider band gap window layer improves short wavelength photocurrent generation. Thick window layer (0.24 µm) necessary due to absence of HRT layer. Kartopu et al., Prog. in PV (2012) DOI: /pip.2272

13 Spectral response vs. CdTe thickness For CdTe absorber thicknesses 1µm CdCl 2 thickness was halved.

14 Spectral response vs. CdTe thickness For CdTe absorber thicknesses 1µm CdCl 2 thickness was halved. Cd 1-x Zn x S window layer absorption edge red-shifted for all ultra-thin CdTe solar cells. The degree to which the Cd 1-x Zn x S window layer absorption edge red-shifted increased as the CdTe absorber thickness reduced. Clayton et al., SOLMAT 101 (2012) 68-72

15 Variable CdCl 2 activation treatment Ultra-thin CdTe thickness of 0.5 µm selected for CdCl 2 treatment investigation:- Sensitive to the spectral response red-shift after CdCl 2 treatment; Devices not dominated by shunting. Baseline CdTe thickness = 2.25 µm Annealing time kept at 600 seconds for majority of device sets. Control device w/o CdCl 2 activation treatment produced for comparison.

16 Spectral response (CdTe absorber = 0.5 µm) Reduction of CdCl 2 thickness prior to annealing resulted in recovery of solar cell blue response. Anneal time had less influence on Cd 1-x Zn x S window layer absorption edge.

17 XPS profile measurements General trend Zn content in the window layer reduced with greater CdCl 2 thickness. S content in the absorber layer increased with CdCl 2 thickness.

18 XPS profile measurements Cd low throughout device structure for Set 3. A low Cd precursor partial pressure = greater Zn incorporation in Cd 1-x Zn x S window layer.

19 XPS profile measurements Cd low throughout device structure for Set 3. A low Cd precursor partial pressure = greater Zn incorporation in Cd 1-x Zn x S window layer. Difference in anneal time between device Sets 3 & 4 was small and did not have significant effect on S content in CdTe. S interdiffusion should increase with longer anneal times (e.g. 30 minutes). Gibson et al., Surf. Interface Anal. 33 (2002)

20 Mean J-V parameters ( cm 2 cells) Variable CdCl 2 activation treatment did not significantly affect overall device efficiency. V oc improvement related to S interdiffusion & strain relaxation. No benefit from improved device blue response. Defects related to the lattice mismatch between Cd 1-x Zn x S & CdTe layers are reduced. Device blue response needs to be preserved w/o sacrificing CdCl 2 thickness.

21 Part II - Summary Zn is leached out of the Cd 1-x Zn x S window layer during activation treatment of ultra-thin CdTe solar cells when thick CdCl 2 layers are used. Intermixing across the Cd 1-x Zn x S/CdTe interface after CdCl 2 activation treatment leads to strain relaxation & reduces related defects improving V oc. Optimised level of Zn required in the Cd 1-x Zn x S window layer to compensate for the Cl leaching effect w/o sacrificing the degree of CdCl 2 activation treatment.

22 Acknowledgements Funding / support XPS Dr. Rossana Grilli Team effort Thank you for listening