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CdTe Solar Cells
- 1. Jon Major Stephenson Institute for Renewable Energy University of Liverpool
- 2. Outline • Why CdTe? • CdTe cell structure • Deposition • Current research issues • Industrial manufacture
- 3. Efficiency vs Eg for single band gap solar cell AM 1.5 300KSi InP GaAs CdTe Ge CdS • Max efficiency vs Eg is trade-off between current and voltage. • Lower bandgap higher current – use more of the spectrum • Higher bandgap higher voltage – Voc linked to Eg • CdTe at ~1.5eV get best combined power
- 4. CdTe vs Si • Si is intrinsically expensive due to purification costs - High purity – controlled doping - need thick wafers due to indirect gap of Si • CdTe is a direct gap thin film compound semiconductor - 100 times more absorbing than Silicon - Can be used as a polycrystalline thin film • Typically need >100µm of high purity crystalline Si but 1µm of CdTe will absorb >90% of above bandgap light. • Cost of CdTe absorber layer is much lower cost than Si
- 6. Explodes in water! Used in WWI!
- 7. Safety Cd environmental risk? CdTe cell production contributes far less Cd to the atmosphere than fossil fuels or Si
- 8. Safety V. Fthenakis et al, Progress in PV, 2004 • Tests up to 1100⁰C of small scale glass encapsulated modules • ~99.5% Cd remained in module • Loss was through ends of exposed ends of module • In real world modules loss expected to be <0.04% • Risk is very low
- 9. Current output Glass superstrate Transparent conducting oxide (~ 200nm) CdS layer (~ 80nm - 400nm) CdTe layer (~ 2 - 10µm) Incident light Back contact Superstrate cell structure
- 10. TCOs Requirements • Transparency > 85% over the visible range • Sheet resistance <10 Ω/□ • Stable In2O3:SnO2 (ITO) • Widely used in range of applications – PV to flexible electronics. • Easy to achieve good electrical and optical properties. • Considered expensive due to reliance on In.
- 11. TCOs ITO can be unsuitable if using high temperature CdTe deposition techniques. In-diffusion into CdTe causes n-type doping! • Cd2SnO4 (CTO) - gives best performance, <3 Ω/□ and transparency >90% - requires high temperatures >600⁰C. • ZnO:A (ZAO) – Low cost, good performance – breaks down at temperatures >400⁰C.
- 12. TCOs SnO2:F (FTO) • FTO is the best compromise. • Deposited as an “on-line” coating during float glass manufacture. • Industrial choice • NSG ltd (Pilkington) “TEC” glass is commonly used.
- 13. CdS deposition routes Chemical bath deposition (CBD) -Low temperature, oxygen rich small grained films. -Uses aqueous cadmium. RF sputtering -Wide range of deposition control - Industrially scalable -Slow deposition rate Close space sublimation (CSS) - High deposition rates - Requires higher temperature deposition
- 14. CdS • n-type “window” layer – 2.4eV bandgap • Typically 60-300nm thick • Depletion region resides wholly within the CdTe – carriers generated in CdS recombine. • Desirable to minimise CdS thickness to maximise Jsc - without compromising FF and Voc Wavelength (nm) 400 500 600 700 800 900 NormalisedEQE 0.0 0.2 0.4 0.6 0.8 1.0 1.2 300nm CdS 250nm CdS 200nm CdS
- 15. J V Increasing Rs Slope = 1/Rs J V Increasing Rsh Slope = 1/Rsh a) b) Too thin CdS will lead to voids and shunt related losses - Jsc may increase but FF will decrease. CdS CdTe In severe cases where voids lead to TCO/CdTe interface regions VOC is also decreased
- 16. CdTe deposition routes Close space sublimation (CSS) - High deposition rates - Large grain size - Highest efficiency devices RF sputtering - Low temperature deposition - Ultrathin CdTe - Low deposition rates MOCVD - Allows elemental control and introduction of dopants i.e. As. - Low deposition rates
- 17. CdTe • CdTe thickness typically in 1-8μm range • <1μm is desirable but difficult to achieve without performance loss Paudel et al – Solar energy materials (2012) • >8μm see losses due to Rs increase
- 18. CdCl2 “activation” step • CdCl2 activation is vital to high efficiency CdTe solar cells • Typically converts a cell of <1% efficiency to >10% • A thin layer (20-200nm) of CdCl2 is deposited on the CdTe free back surface • Sample then annealed in a tube furnace under an air ambient • Can also be done with MgCl2 as an alternative Glass superstrate Transparent conducting oxide (~ 200nm) CdS layer (~ 80nm - 400nm) CdTe layer (~ 2 - 10µm)
- 19. • It isn’t a single process and not fully understood - Grain boundary passivation - Grain growth and recrystallization - Intermixing of CdS and CdTe layers - p-type doping of CdTe layer: presumed to be via [VCd - ClTe] (A-centre) What does the CdCl2 treatment do?
- 20. As-deposited CdCl2 treated Low temp deposition High temp deposition Grain growth
- 21. CdTe a c b Spectrum image Where does the chlorine go? See little evidence of Cl at the interface with CdS Chlorine instead segregates at the grain boundaries changing their electrical behaviour
- 22. Normalised Wd 0.0 0.2 0.4 0.6 0.8 1.0 NA(cm -3 ) 1e+13 1e+14 1e+15 1e+16 CdCl2 MgCl2 As-grown Doping density As-deposited ~1013cm-3 Chloride treated ~1015cm-3
- 23. As grown Air anneal CdCl2 treatment Device performance
- 24. Front contact Back contact Glass superstrate TCO/buffer layer CdS window layer CdTe layer e- e- e- Back wall EBIC Front wall EBIC Cross section EBIC Gold back contact Electron beam induced current (EBIC)
- 25. CdCl2 treatment forms the CdS/CdTe hetero-junction
- 26. Back contact • Forming back contact to CdTe is problematic owing to the high electron affinity χS = 4.5eV • For Ohmic contact require a work function of > 6eV – No such metal exists! • May contact with high work function metals (e.g. Au ~5.1eV) but a barrier still exists.
- 27. Cu addition to the back contact • Solution to the contacting problem is the use of an interface layer between CdTe and metal. • Typically done via formation of CuxTe layer at the back surface – decreases barrier width to allow tunnelling. • Number of routes but typically - Etching CdTe in nitric/phosphoric (NP) acid etch to create Te-rich layer - Deposition of 1-10nm of Cu via thermal evaporation - Annealing at 100-250⁰C to diffuse in Cu • Optimisation of Cu thickness and annealing is key • See improvement in FF and Voc (if ɸb >0.5eV)
- 28. Stability of the back contact • Alternative contacts are still being investigated. • Sb2Te3, CdTe:As, ZnTe, MoOx, NiTe have all been demonstrated. • Industrially Cu is still used but <2nm • Cu is a fast diffuser in CdTe • If it reaches the CdS layer - leads to photoconductivity and performance loss • Leads to long term instability of performance
- 29. • High Voc and FF are more believable • Jsc values are very sensitive to calibration or contact size errors. • Contacts should be minimum of 0.25cm2 • If it looks to good to be true it usually is! 00 Etch time (s) 0 100 200 300 400 500 600 Fillfactor(%) 10 20 30 40 50 60 00 Etch time (s) 0 100 200 300 400 500 600 Jsc(mA/cm 2 ) 0 10 20 30 40 50 60 Contact errors
- 30. The CdTe VOC problem CdTe single crystal cell Voc 1007mV Theoretical limit ~1400mV. CdTe polycrystalline cell today Voc - 876mV 1992 - Max 855mV A decade worth of cells from NREL (around 2500) Why the VOC shortfall?
- 31. Carrier density in CdTe • Doping density of CdTe films limited to ~ 1015cm-3 • Due to strong self compensation of CdTe • Limits Voc Single crystal work with >1V
- 32. Carrier lifetime • Voc linked to carrier lifetime in CdTe • By improving lifetime Voc has surpassed previous assumed limit of ~855mV • Achieved via formation of high quality MBE junctions • Still someway short of 1.4V theoretical limit
- 33. CdTe grain boundaries CdTe grain boundaries are highly complex as chemical composition will depend on deposition method, deposition conditions, post-growth treatment and contacting. Boundaries may be Te-rich, Cd-rich or have Cl, O, Cu, and S segregated there. All of these are likely to change their behaviour.
- 34. Negative effects of grain boundaries Grain boundary diffusion Current transport limitation Carrier recombination Some cell work shows link between grain size and performance.
- 35. CdCl2 treatment changes the behaviour of grain boundaries Positive effects of grain boundaries
- 36. Substrate solar cells • Up to 13.6% substrate cells produced on glass achieved • Up to 11.5% on flexible Mo foil • Allows lower cost and flexible substrates to be used. • Considerable challenges to be overcome – modification of basic process and back contacting
- 38. Year 1990 1995 2000 2005 2010 2015 2020 Efficiency(%) 15 16 17 18 19 20 21 22 23 0.9% increase in 18 years 5.4% increase in 5 years Past few years have seen a dramatic increase in CdTe performance after years of stagnation How has this been achieved?
- 39. CdTe PV industry
- 40. CdTe PV industry • CdTe industry dominated by first solar • More than 8GW of installed PV worldwide • More than 100M modules manufactured • Worlds single largest PV manufacturer
- 41. Record efficiency Date Produce by 15.8% May 1993 Univ. Florida 16.7% Oct 2001 NREL 17.3% Aug 2012 First Solar 18.3% Jan 2013 G.E global research 19.6% Aug 2013 G.E global research 20.4% Feb 2014 First Solar 21.0% Jan 2015 First Solar 21.5% Jun 2015 First Solar 22.1% Feb 2016 First Solar • If cost ~1$/Wp at 16.7% • Becomes ~0.75$Wp at 22.1% • First Solar predict ~ 25% efficiency achievable with current cell structure ~0.67$/Wp
- 42. 0% 1% 2% 3% 4% 5% 10% • Increased Eg of window layer – not typical CdS. • Potentially CdS:O –nanostructured CdS where Eg shifts via quantum confinement.
- 43. Bandgap grading
- 44. In the next 3 years CdTe expected to match Si photovoltaics in efficiency but at much lower cost!
- 45. Summary • CdTe solar cells are poised to become lowest cost mass production PV technology with efficiency surpassing that of multicrystalline Si • Number of questions still remain – Doping? Bandgap grading? Grain boundaries? • Cost of power to be further reduced through performance increases and improvements in cell structure or processing.