CdTe solar cells
Master in Ingegneria del Fotovoltaico
Corso di Tecnologie Fotovoltaiche Convenzionali
Francesco Biccari
2012-04-25
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 2/39
Cadmium telluride (CdTe)
• Chalcogenide semiconductor
• Zincblend structure
• Direct energy gap 1.44 eV
• Can be growth both p-type (VCd acceptors) or n-type (Cdi
donors)
• me = 0.1 m0
• µe = 1100 cm2/Vs in single crystals
• Difficult extrinsic doping
• ηth = 31%
Source: Wikipedia
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 3/39
CdTe based solar cells are studied since 1950. pn-homounction, both poly- and single-crystal give poor efficiency (3%)
1960: n-CdS/p-CdTe, 1972: Bonnet and Rabenhorst obtain 6% efficiency
1981: Kodak introduces Close Spaced Sublimation method
1991: Ting L. Chu introduces a front window layer reducing the thickness ofCdS 15% efficiency! Born of Solar Cell Incorporated (now First Solar)
2002: NREL obtains 16.5% efficiency (current world record)
2005: First Solar reaches 25 MWp/y of production
2009: EMPA Labs show 13.5% efficiency on flexible polyimide substrates
2010: First Solar production cost: 0.75 €/Wp! Capacity 1.5 GWp/y!
CdTe solar cells. Brief history
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CdTe solar cells. Superstrate
• The superstrate configuration is used in most CdTe solar cells
• This is due to the particular difficulty in making the rear contact (we will see why)
• The back contact is usually deposited at the end of the cell to have a better control
Superstrate configuration!
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 5/39
TCO
• The free carrier absorption in the infrared is less important for CdTe because of its higher gap with respect to, for example, CIGSe orCISe.
• The window layer is usually divided in two layers: a highly conductive and thick TCO and a diffusion barrier between the first TCO and CdS.
• Record NREL cell: borosilicate glass/Cd2SnO4/Zn2SnO4/CdS/CdTe/metal
• Typical cell: glass/ITO/SnO2/CdS/CdTe/metal or FTO instead of ITO
• In principle AZO is cheaper than ITO. But AZO degrades during the other steps (especially CdCl2) giving a high series resistance
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 6/39
CdS is a semiconductor with Eg = 2.42 eV. It is yellow!
It should be a window layer but it shouldbe as thin as possible (we will see why)and it is called buffer layer.
Deposition methods:
Evaporation
Sputtering
Close Spaced Sublimation (CSS)
Vapour Transient Deposition (VTD)
Chemical Vapour Deposition (CVD)
Chemical Bath Deposition (CBD)
CdS
TCO
CdTe
Glass
CdS
Mo
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 7/39
CdS
• Compact to reduce shunts
• It can suffer from the subsequent processes (in superstrate configuration)
• Lattice mismatch with the absorber: defects
• Partecipation to carrier collection? Probably no
• High absorption in blue: usage of a TCO as window layer. CdS very thin!
• High resistance: usage of a TCO as window layer. CdS very thin!
Poortmans
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CdS deposited by CBD or VTD
• Chemical Bath deposition (CBD) is not used for CdTe
Cd2+ source (CdSO4, CdI2) + NH3 + S2- source (thiourea) + H2O
T = 70°C, reaction of Cd2+ with S2- to form CdS
• First Solar uses Vapor Transport Deposition (VTD)where the CdS is “evaporated” in an inert atmosphere and carried toward the glass with an inert gas flux(The same technique is used for CdTe, see below)
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 9/39
• CdS/CdTe: ~10% lattice mismatch and different crystal structures (wurtzite CdS vs zincblend CdTe)
• However the junction shows good electronic properties!
• The explanation is the possibility of CdTe and CdS to mix.A sulfur rich CdTe phase, CdTe1−xSx, in the CdTe absorber,and a tellurium rich CdS phase, CdS1−yTey, in the CdS layer.
• The bandgap Eg(x) of the mixed phases has a minimum value 1.40 eV at a composition of around 25% (atomic) CdS in CdTe. This effect shifts the QE of CdTe/CdS cells to longer wavelengths with a few tens of nm.
– Around λ = 520 nm, the CdS1−yTey in CdS enhances the absorption(this is a loss)
– Around λ = 860 nm, the CdTe1−xSx in CdTe enhances the absorption(this is a gain)
– Normally, the gain in the infrared does not compensate for the loss in the green region
CdS effects. CdS/CdTe interdiffusion
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 10/39
CdTe deposition techniques
• Close Spaced Sublimation (CSS). High temperatures, up to 650°C, give the best cells but borosilicate glass is needed. High cost.Commercial scale systems use soda lime glass (550°C).(Antec Solar, Mitsubishi).
• Vapor Transport Deposition (VTD). Low temperatures. Very fast.(First Solar)
• PVD, MOCVD, sputtering. Intermediate temperatures: 250 °C to 350°C.
• Electrodeposition. At about 90°C.
• All of these methods have yielded cells with performance well above 10%. Why CdTe has this unparalleled flexibility ?– Great stability of the binary compound with a tendency to self
compensate with intrinsic defects to form quite stable p-type CdTe. – Use of post-deposition activation treatments which involves an anneal
step at 400°C in the presence of some O2 and Cl.
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 11/39
CdTe is the only stable compound in the phase diagram!
CdTe(s)+Te(s)CdTe(s)+Cd(s)
CdTe: Phase diagram
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2 CdTe (s) ↔ 2 Cd (g) + Te2 (g)Log(pTe2(atm))=6.346 -104/T
Equilibrium vapor pressureof elemental Cd and Te is much higher than that of CdTe: therefore the pure phases tend to re-evaporate
CdTe. Self stabilization
Low temperature congruent sublimation. The composition is self stabilizing.
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 13/39
• The driving force for the deposition is the temperature difference
• Substrates and sources are very close together. The film growth occurs close to equilibrium condition. This small difference in temperature limits the deposition rate
• In rough vacuum or in inert gas
• For CdTe and CdS
(700 C)
(600 C)
Deposition rate:1 µm/min !
CdTe. Close Spaced Sublimation
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Limited by surface kinetics(sticking coefficient)
Limited by dilution of the source
Deposition rate:Up to 1 µm/s!
P1 = P2 + cost ΦHe
Similar to CSS but the source and the substrate environmentsare decoupled: the temperature difference can be larger!Industrially simpler
Kestner (2004)
CdTe. Vapor Transport Deposition
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 15/39
The key enabler for low-cost, high-throughput manufacture is rapid deposition of high-quality semiconductor films
VTD. First Solar and NREL
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 16/39
Te= Te=
Te= Te=
Te=
Te=
Te=
Te= Te=
Cd++
Cd++ Cd++
Cd Vacancy (VCd, acceptor)
Cd++
Te= Te=
Te= Te=
Te=
Te=
Te=
Te= Te=
Cd++
Cd++
Cd++
Cd++
Cd Interstitial (Cdi, donor)
CdTe is a ionic material with a large interatomic distance and low cohesive strength. The vacancy formation energy is therefore low.
CdTe. Intrinsic defects
For PV application p-type CdTe is preferred. The density of cadmium vacancies is in the range of 1017 to 1018 cm-3 for a typical PV material.
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 17/39
Copper in CdTe is an acceptor (it sits on a cadmium site: CuCd)
Chlorine resides on a tellurium site (ClTe), acts as a shallow donor. It forms, however, a complex with a doubly negatively charged cadmium vacancy, and this negatively charged (Cl+Te − V 2−
Cd) complex acts as a
single acceptor.
CdTe: intrinsic defects and dopants
Poortmans (2006)
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 18/39
CdTe. The “magic” CdCl2 treatment
CdCl2 treatment of CdTe (called post-deposition treatment) is fundamental to obtain good solar cells. Presence of oxygen is beneficial.
– Increased grain size in CdTe and in CdS when the initial grains are small (not with CSS and VTD). Grain growth, which can occur during CdCl2 treatment, introduces stress at the interface between the CdS and TCO layer, resulting in film blistering or peeling. (Cl solubility in CdTe is low: diffusion along the grain boundaries with the formation of CdO and TeCl2)
– Subgrains disappear, grain-boundary passivation
– p-type doping
– Passivation of recombination defects: longer minority carriers lifetimes. Interaction with Cl, O and VCd can however generate other deep defects
– Increased CdS/CdTe interface alloying: reduced lattice mismatch between the CdS and CdTe layers
– CdCl2 overtreatment can result in adhesion loss problems, deep defects formation
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 19/39
CdTe. Post deposition treatment
• Solution of methanol and CdCl2 sprayed on CdTe and subsequentely heated at 450°C for few minutes
• CdCl2 thin film over CdTe applied by evaporation, CSS or VTD
• Other methods (gaseous CdCl2) or other compound containing Cl (HCl, NaCl, …) are under study
• CdCl2 is highly toxic and solublein water and alcohol!
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 20/39
Back contact problems
• CdTe is a material with high electron affinity (χ = 4.28 eV)A metal with a high work function is needed (only noble metals! ΦAu= 5.4 eV and ΦPt = 5.7 eV).
Fermi level pinning!
Poortmans (2006)
Ideal theory!
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Back contact problems
• The strategy is to form a highly doped p+ region at the CdTe back contact in order to permit the tunnelling of the holes. But CdTeextrinsic doping is not simple (autocompensation). The p+ region is obtained by etching (C2H5BrO, HNO3:H3PO4, etc…) the back surface of CdTe leaving a Te-rich layer. Etching can introduce shunt paths due to preferential etching at grain boundaries
• Most commonly used back-contact materials are:
– Cu based: Cu:Au, ZnTe:Cu, CuxTe:HgTe/graphite, Cu/graphite, HgTe:Cu/graphite paste/Ag paste (record cell)Copper is used because of its acceptor character (when introduced in larger quantities, however, part of the copper will occupy an interstitial place, and this Cui acts as a donor!) Moreover Cu decreases lifetime of minority carries
– Cu free: Ni-P, Sb2Te3/Mo, HgTe/graphite, Ni/Al, Sb/Mo
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 22/39
• Unfortunately copper can diffuseDCu in CdTe = 3.7 x 10-4 exp (-0.67 eV/kT)
• If Cu diffusion is insufficient, the entire CdTe layer is depleted
• if Cu diffusion is excessive:– the depletion width can become
too narrow– Cu may segregate into the grain
boundaries forming shunting paths – Cu can arrive to CdS increasing its resistivity
• The Cu diffusion and therefore the degradation is accelerated bytemperature and illumination
• Most contact processes used for CdS/CdTe devices are optimized (often unknowingly) to result in an optimal depletion width
Effects of Cu from the back contact
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 23/39
Manufacturing Capacity in 2009:
First Solar (USA): 1100 MW/yr
Calyxo (Q-cells DEU): 25 MW/yr
Antec Solar (DE): 10 MW/yr
PrimeStar Solar
Arendi (Italia)
First Solar is the first company for production capacity in 2009.
Roth& Rau has announced in February 2009 that it will be able, by the end of the year, to sell complete production lines for CdTe Modules.
CdTe modules
12% efficiency
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 24/39
First Solar40 MW solar field installed in Germany (First Solar). Completed in December 2008. Estimated total price 130 M€ (3.25 €/W). It is one of the largest solar fields in the world and also one with a low price.
First solar production lines:
Malaysia (1.5 GWp)
Germany (0.5 GWp)
USA (0.25 GWp)
France (0.1 GWp)
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 25/39
Substratecleaning
VTD CdS
APCVD SnO2
undopedVTD CdTe
(3 µm)CdCl2
treatmentSputtering
Back contact
Laser scribing
Laser scribing
Laser scribing
CuxTeformation
First Solar process (in 2000)
• High throughput: 1 module 120 cm x 60 cm in 15 s, 17 kWp/h, 100 MWp/year for 3 shifts
• Doped SnO2 (ITO) coated soda lime glass substrate
• Undoped SnO2 deposition by APCVD and buffer layer (CdS) by VTD
• CdCl2 aqueous solution is sprayed on the CdTe formed on the glass substrate, and subsequently treated in a belt furnace
• First Solar process to make the back ohmic contact while reducing the Cu available: – A chemical etch of CdTe to create a Te-rich surface– Deposition of only 2 nm of Cu – Annealing to form the compound CuxTe (a good p-type semiconductor)
• Sputtering of metal for the back contact
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 26/39
Tucson Electric/First Solar
480 kW thin film CdTe solar field installed in 2003
First Solar CdTe module stability
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 27/39
• 2009 december. Manufacturing module cost 0.84 $/Wp
• 2012 december: target of 0.7 $/Wp!targer of BOS, 1 $/Wp!
First Solar. Module cost per Wp
1E-3 0.01 0.1 1 10 100 1000 100000.1
1
10
FOSSIL FUEL COMPETITIVE LEVEL
Last update: 2011
CdTe
modules
a-Si
modules2005
c-Si shortage
c-Si modules
81% learning curve
1979
30 $/W
2003
3.03 $/W
Module
cost
(2002 $
/Wp)
Cumulative production (GWp)
Grid parity at our latitudesis near!
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 28/39
Borosilicate glass is used as a substrate.
Spray pyrolysis for SnO2:F film (500 nm, ρsheet<10Ωsq).
Spray pyrolysis for CdS (100 nm).
Close Spaced sublimation for CdTe film (3–7 µm)
0.3M CdCl2 aqueous solution is sprayed on the CdTe formed on the glass substrate, and subsequently treated in a belt furnace at 420 C for 30 min in air. After the heat treatment, the substrate is rinsed in de-ionized water, anddried in an N2 atmosphere.
Sandblast technique to pattern the CdTe film.
Screen printing of Carbon paste (containing Cu and/or Pb) for the ohmic contact.
Screen printing of Ag paint as a metal electrode.
CdTe solar cells. Matsushita process
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 29/39
Improvements:1.New deposition process for the CdS: sputtering in Ar + CHF3 (better reproducibility)
2.Substitution of the CdCl2 step by treating CdTe films at 400°C, for a few minutes in an atmosphere containing HCFCl2, (a Freon© which is non toxic and inert at room temperature): no risk of stocking CdCl2, faster process
3.Elimination of the acid etch of the CdTe surface.
4. Back contact: deposition on top of a not etched surface of 100-200nm of As2Te3 followed by thedeposition of 10-20nm of Cu at 150-200°C. A reaction between Cu and As2Te3 happens forming a CuxTe layer by a substitution reaction. This type of contact resulted to be stable and non rectifying.
Glasscleaning
Front TCOSputtering
CdS sputteringWith CHF3
CSS CdTe(Ar+O2)
CHF3Cltreatment
Sputtering As2Te3 /Cu/Mo
Laser scribing
Laser scribing
Laser scribing
RT RT RT RT
400 ºC
250 ºC
500 ºC
400 ºC300 ºC
Arendi. Italian CdTe solar modules
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 30/39
CdTe modules. Roth&Rau turnkey line
• Roth & Rau has recently completed the development of the first CdTe turnkey production line.
• Nominal capacity: 80 MWp (glass-glass modules 1.2 m x 1.6 m)
• Targets: Conversion efficiency at 10%, production yield of 95%, production cost less than 1 €/Wp
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 31/39
Production steps for CdTe modules
1. CdTe deposition2. Activation3. Back contact sputtering4. Encapsulation
1
23
4
CdTe Roth & Rau turnkey line
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 32/39
CdTe. Environmental issues
• Elemental cadmium is highly toxic. Detrimental effects on kidneyand bone. Carcinogen for lungs.
• High energies of the CdTe and the CdS bonds, extremely low watersolubility and the low vapor pressure of CdTe and CdS.CdTe and CdS are not so toxic!
• A 1 m2 solar module contains about 6 g of Cd in CdTe and CdS.A typical AA Ni-Cd battery contains 4 g of metal Cd!
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 33/39
CdTe. Environmental issues
• Mining and production of CdTe and CdSSafe
• Production of solar modules Safe, deep studies from NREL, First Solar and Antec
• Active life of solar modulesCdTe melts at 1041°C, CdS melts at 1750°CThe modules are completely safe during normal operation and even during a fire the thin layers of CdTe and CdS would be encapsulated inside the molten glass, so any Cd vapor emissions are unlikely
• Dismantling, disposal and recycling of modulesEven cracking a module does not produce any relevant Cd contamination. Specifically recycling programs from all companies
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 34/39
• The higher the band gap the lower the temperature coefficient
• First solar module: β = - 0.25%/°C. Same percentage for efficiency.For c-Si modules: β = - 0.5%/°C
Temperature coefficient
• With the increasing temperature, jsc sligthly increases while Voc
decreases
dTEdT
TdV
TVdT
TV
dTTV
dT
TdVdTTVdTTV
T
T
)(1)(
)(
11
)(
)(
)()()(
goc
0oc0oc
0oc
oc0oc0oc
0
0
β+≈+≈+
+≈+Voc temperature
coefficient
CECE
C
T>
−−≈ g
g0
with 1
βTry to demonstrate this expressionvalid for an ideal solar cell.Approx. Eg >> kT0, jsc >> jS
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 35/39
Indium requirement: 0.03 gr/Wp: theprice is still acceptableThe entire In production would give a maximum of 10 GWp/yr PV production.
J.J. Scragg et al, Phys. stat. sol. (b) 245, 1772 (2008)
Materials availability: a future problem?
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 36/39
201160.01160.1RuDye
200140.50.06320.44GeaSiGe (0.2 µm)
7070.090.292.62.9In
0.0541100.53Ga
2.2704.8SeCIGS (2 µm)
2050.30.29206.5Te
206006.3CdCdTe (2 µm)
2020 limitannual prod
(GWp/yr)
1999 limitannual prod
(GWp/yr)
Limitpower(TWp)
Production1999
(Gg/yr)
Reserves1998(Gg)
MetalRequired
(g/m2)
Modules(Eff≈10%)
Total PV 2010 production ≈ 27 GWp (2 GWp due to CdTe and CIGS)No availability problems in the next 5-10 years
On the long term availability problems for In and Te could arise.(105 TWh 2006 world consumption: 10 TWp of PV)
Materials availability: a future problem?
Source: B. A. Andersson , Prog. Photovolt. Res. Appl. 8, 61 (2000)
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 37/39
CdTe solar cells. Conclusions (1)
• High efficiency
– The polycrystalline nature of the thin film is not detrimental and poly thin film solar cells give higher efficiency compared to their single crystal counterparts
• Stability
– The polycrystalline nature tolerate quite high concentration of impurities
– Even if some problems could exist with the diffusion of Cu, First Solar modules show a very good stability
• Low cost
– Effective use of raw materials
– Small energy pay-back time (less than two years)
– No doping: the p-type conductivity due to intrinsic defects is used
• Adaptable to various applications
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 38/39
CdTe solar cells. Conclusions (2)
• CIGSe modules have reached 1.44 GWp of production in 2010!
• CdTe cost per Wp is similar or lower than a-Si but CdTe efficiency is higher!
• At the moment the main product type is a glass monolithic module but probably a large production increase will derive from the introduction of flexible modules.
• Materials availability is not going to be a big problem in the next years and it will improve in response to demand and price increase.
• Environmental and safety problems are manageable in the production phase and almost irrelevant for the user.
Francesco Biccari – Master Ingegneria del Fotovoltaico – Corso di Tecnologie Fotovoltaiche Convenzionali 39/39
Acknowledgments
• Thanks to Dr. Alberto Mittiga for providing several figures, numbers and slides of this presentation
• Thanks to Dr. Rosa Chierchia for useful discussions
• Thanks to Dr. Shenjiang Xia for pointing me out some mistakes