1 CdTe solar cells Master in Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali Francesco Biccari
2 Cadmium telluride (CdTe) Chalcogenide semiconductor Zincblend structure Direct energy gap 1.44 ev Can be growth both p-type (V Cd acceptors) or n-type (Cd i donors) m e = 0.1 m 0 µ e = 1100 cm 2 /Vs in single crystals Difficult extrinsic doping η th = 31% Source: Wikipedia Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 2/39
3 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 3/39 CdTe solar cells. Brief history CdTe based solar cells are studied since 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 of CdS 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!
4 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 4/39 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!
5 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 or CISe. 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/cd 2 SnO 4 /Zn 2 SnO 4 /CdS/CdTe/metal Typical cell: glass/ito/sno 2 /CdS/CdTe/metal or FTO instead of ITO In principle AZO is cheaper than ITO. But AZO degrades during the other steps (especially CdCl 2 ) giving a high series resistance
6 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 6/39 CdS CdS is a semiconductor with E g = 2.42 ev. It is yellow! It should be a window layer but it should be 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 Glass TCO CdTe Mo
7 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
8 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 8/39 CdS deposited by CBD or VTD Chemical Bath deposition (CBD) is not used for CdTe Cd 2+ source (CdSO 4, CdI 2 ) + NH 3 + S 2- source (thiourea) + H 2 O T = 70 C, reaction of Cd 2+ with S 2- 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)
9 CdS effects. CdS/CdTe interdiffusion 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, CdTe 1 x S x, in the CdTe absorber, and a tellurium rich CdS phase, CdS 1 y Te y, in the CdS layer. The bandgap E g (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 CdS 1 y Te y in CdS enhances the absorption (this is a loss) Around λ = 860 nm, the CdTe 1 x S x 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 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 9/39
10 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 O 2 and Cl.
11 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 11/39 CdTe: Phase diagram CdTe(s)+Cd(s) CdTe(s)+Te(s) CdTe is the only stable compound in the phase diagram!
12 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 12/39 CdTe. Self stabilization Equilibrium vapor pressure of elemental Cd and Te is much higher than that of CdTe: therefore the pure phases tend to re-evaporate 2 CdTe (s) 2 Cd (g) + Te 2 (g) Log(p Te2 (atm))= /T Low temperature congruent sublimation. The composition is self stabilizing.
13 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 13/39 CdTe. Close Spaced Sublimation 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 (600 C) (700 C) Deposition rate: 1 µm/min!
14 CdTe. Vapor Transport Deposition Similar to CSS but the source and the substrate environments are decoupled: the temperature difference can be larger! Industrially simpler Deposition rate: Up to 1 µm/s! P 1 = P 2 + cost Φ He Limited by surface kinetics (sticking coefficient) Limited by dilution of the source Kestner (2004) Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 14/39
15 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 15/39 VTD. First Solar and NREL The key enabler for low-cost, high-throughput manufacture is rapid deposition of high-quality semiconductor films
16 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 16/39 CdTe. Intrinsic defects CdTe is a ionic material with a large interatomic distance and low cohesive strength. The vacancy formation energy is therefore low. Te = Te = Te = Te = Te = Te = Cd ++ Cd ++ Cd ++ Te = Te = Te = Te = Te = Cd ++ Te = Cd ++ Cd ++ Cd ++ Cd ++ Te = Te = Te = Te = Te = Te = Cd Vacancy (V Cd, acceptor) Cd Interstitial (Cd i, donor) For PV application p-type CdTe is preferred. The density of cadmium vacancies is in the range of to cm -3 for a typical PV material.
17 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 17/39 CdTe: intrinsic defects and dopants Copper in CdTe is an acceptor (it sits on a cadmium site: Cu Cd ) Poortmans (2006) Chlorine resides on a tellurium site (Cl Te ), 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.
18 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 18/39 CdTe. The magic CdCl 2 treatment CdCl 2 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 CdCl 2 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 TeCl 2 ) Subgrains disappear, grain-boundary passivation p-type doping Passivation of recombination defects: longer minority carriers lifetimes. Interaction with Cl, O and V Cd can however generate other deep defects Increased CdS/CdTe interface alloying: reduced lattice mismatch between the CdS and CdTe layers CdCl 2 overtreatment can result in adhesion loss problems, deep defects formation
19 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 19/39 CdTe. Post deposition treatment Solution of methanol and CdCl 2 sprayed on CdTe and subsequentely heated at 450 C for few minutes CdCl 2 thin film over CdTe applied by evaporation, CSS or VTD Other methods (gaseous CdCl 2 ) or other compound containing Cl (HCl, NaCl, ) are under study CdCl 2 is highly toxic and soluble in water and alcohol!
20 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! Φ Au = 5.4 ev and Φ Pt = 5.7 ev). Ideal theory! Fermi level pinning! Poortmans (2006)
21 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 21/39 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 CdTe extrinsic doping is not simple (autocompensation). The p + region is obtained by etching (C 2 H 5 BrO, HNO 3 :H 3 PO 4, 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, Cu x Te: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 Cu i acts as a donor!) Moreover Cu decreases lifetime of minority carries Cu free: Ni-P, Sb 2 Te 3 /Mo, HgTe/graphite, Ni/Al, Sb/Mo
22 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 22/39 Effects of Cu from the back contact Unfortunately copper can diffuse D Cu 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 by temperature and illumination Most contact processes used for CdS/CdTe devices are optimized (often unknowingly) to result in an optimal depletion width
23 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 23/39 CdTe modules 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 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. 12% efficiency
24 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 24/39 First Solar 40 MW solar field installed in Germany (First Solar). Completed in December 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)
25 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 25/39 First Solar process (in 2000) Substrate cleaning APCVD SnO 2 undoped VTD CdS VTD CdTe (3 µm) CdCl 2 treatment Cu x Te formation Sputtering Back contact Laser scribing Laser scribing Laser scribing High throughput: 1 module 120 cm x 60 cm in 15 s, 17 kwp/h, 100 MWp/year for 3 shifts Doped SnO 2 (ITO) coated soda lime glass substrate Undoped SnO 2 deposition by APCVD and buffer layer (CdS) by VTD CdCl 2 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 Cu x Te (a good p-type semiconductor) Sputtering of metal for the back contact
26 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 26/39 First Solar CdTe module stability Tucson Electric/First Solar 480 kw thin film CdTe solar field installed in 2003
27 First Solar. Module cost per Wp 2009 december. Manufacturing module cost 0.84 $/Wp 2012 december: target of 0.7 $/Wp! targer of BOS, 1 $/Wp! $/W Last update: 2011 Module cost (2002 $/Wp) 10 1 a-si modules CdTe modules FOSSIL FUEL COMPETITIVE LEVEL $/W 2005 c-si shortage c-si modules 81% learning curve Grid parity at our latitudes is near! 0.1 1E Cumulative production (GWp) Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 27/39
28 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 28/39 CdTe solar cells. Matsushita process Borosilicate glass is used as a substrate. Spray pyrolysis for SnO 2 :F film (500 nm, ρ sheet <10Ωsq). Spray pyrolysis for CdS (100 nm). Close Spaced sublimation for CdTe film (3 7 µm) 0.3M CdCl 2 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, and dried in an N 2 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.
29 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 29/39 Arendi. Italian CdTe solar modules Glass cleaning Front TCO Sputtering CdS sputtering With CHF 3 CSS CdTe (Ar+O 2 ) CHF 3 Cl treatment Sputtering As 2 Te 3 /Cu/Mo Laser scribing Laser scribing Laser scribing 400 ºC 250 ºC 500 ºC 400 ºC 300 ºC RT RT RT RT Improvements: 1.New deposition process for the CdS: sputtering in Ar + CHF 3 (better reproducibility) 2.Substitution of the CdCl 2 step by treating CdTe films at 400 C, for a few minutes in an atmosphere containing HCFCl 2, (a Freon which is non toxic and inert at room temperature): no risk of stocking CdCl 2, faster process 3.Elimination of the acid etch of the CdTe surface. 4. Back contact: deposition on top of a not etched surface of nm of As 2 Te 3 followed by the deposition of 10-20nm of Cu at C. A reaction between Cu and As 2 Te 3 happens forming a Cu x Te layer by a substitution reaction. This type of contact resulted to be stable and non rectifying.
30 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
31 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 31/39 CdTe Roth & Rau turnkey line Production steps for CdTe modules 1. CdTe deposition 2. Activation 3. Back contact sputtering 4. Encapsulation
32 CdTe. Environmental issues Elemental cadmium is highly toxic. Detrimental effects on kidney and bone. Carcinogen for lungs. High energies of the CdTe and the CdS bonds, extremely low water solubility and the low vapor pressure of CdTe and CdS. CdTe and CdS are not so toxic! A 1 m 2 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 32/39
33 CdTe. Environmental issues Mining and production of CdTe and CdS Safe Production of solar modules Safe, deep studies from NREL, First Solar and Antec Active life of solar modules CdTe melts at 1041 C, CdS melts at 1750 C The 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 modules Even 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 33/39
34 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 34/39 Temperature coefficient With the increasing temperature, j sc sligthly increases while V oc decreases V V oc oc ( T 0 + dt ) ( T0 + dt ) V ( T ) oc 0 V oc 1+ ( T 0 dt dvoc ( T ) ) + dt dt V oc 1 ( T 0 ) T 0 dvoc ( T ) dt T 0 1+ β ( E g ) dt V oc temperature coefficient β 1 T 0 E g C C with E g > C Try to demonstrate this expression valid for an ideal solar cell. Approx. E g >> kt 0, j sc >> j S The higher the band gap the lower the temperature coefficient First solar module: β = %/ C. Same percentage for efficiency. For c-si modules: β = - 0.5%/ C
35 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 35/39 Materials availability: a future problem? Indium requirement: 0.03 gr/wp: the price is still acceptable The 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)
36 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 36/39 Materials availability: a future problem? Modules (Eff 10%) Metal Required (g/m 2 ) Reserves 1998 (Gg) Production 1999 (Gg/yr) Limit power (TWp) 1999 limit annual prod (GWp/yr) 2020 limit annual prod (GWp/yr) CdTe (2 µm) Cd Te CIGS (2 µm) Se Ga In asige (0.2 µm) Ge Dye Ru Source: B. A. Andersson, Prog. Photovolt. Res. Appl. 8, 61 (2000) 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. (10 5 TWh 2006 world consumption: 10 TWp of PV) 11 20
37 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
38 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.
39 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
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