CuIn 1-x Ga x Se 2 solar cells

Transcription

1 CuIn 1-x Ga x Se 2 solar cells Master in Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali Francesco Biccari [email protected]

2 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 2/47 Chalcopyrite semiconductors The term chalcopyrite refers to the crystal structure of several chalcogenides (chalcogen (element of group 16) + electropositive elements). In common language chalcopyrite is identified with CuFeS 2, the first discovered compound with this structure. In photovoltaic community chalcopyrite indicates in particular 4 compounds: Chalcopyrite a (Å) c (Å) E g (ev) η th (%) η exp (%) CuInSe 2 (CISe) CuInS 2 (CIS) CuGaSe 2 (CGSe) CuGaS 2 (CGS) d(se-cu) d(se-in) Source: Wikipedia

3 Electronic and optical properties All these compounds are naturally p type because of the presence of copper vacancies V Cu which behave as acceptors CISe: Χ = 4.6 ev, m e = 0.09 m 0, ε r = 13.6 CGSe: Χ = 4.0 ev, m e = 0.14 m 0, ε r = 11.0 CIS:? CGS:? Good electron effective mass! Direct gap and one of the highest absorbtion coefficient! Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 3/47

4 The n-type doping problem in CGSe CISe can be grown both n and p type! CGSe can be obtained p type only. Explanation (ab-initio calculations): Critical E F value at which the V Cu formation energy becomes zero. V Cu are acceptors! Zhao Y., Persson C., Lany S., Zunger A. Appl. Phys. Lett., 85, 5860, (2004) Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 4/47

5 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 5/47 CISe: stoichiometry variations Ternary Cu-In-S phase diagram at 500 C. The most relevant phases lie on the quasi-binary intersection: Cu 2 Se-In 2 Se 3. In 2 Se 3 Cu 2 Se

6 CISe: stoichiometry variations [Cu]/[In]=0.66 CIGS has a good tolerance to stoichiometry variations: Cu/(In+Ga) from 0.95 to 0.82 The cell efficiency change only slightly if: 0.8< [Cu]/([In]+[Ga])<0.95 while they do not work if >0.95 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 6/47

7 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 7/47 Chalcopyrites solar cells. Brief history Hahn. Synthesis and characterization of CISe First single crystal CISe/CdS solar cell at Bell Labs CISe/CdS 11% at Bell Labs Univ. of Maine. First polycrystal CISe/CdS and CIS/CdS solar cells Univ. of Maine. First p-n CISe homojunction. 3% Boeing company obtains polycrystalline CISe/CdS 11.4% (coev.) 1980 s. Single crystals are abandoned in favor of thin films. Boeing (coevaporation) vs ARCO (selenization) Thick CdS abandoned. Thin CdS + thick doped ZnO Gallium alloying. Born of CIGSe The positive effect of sodium is discovered (soda lime glass) and the Ga gradient is introduced NREL CIGSe/CdS 19% on glass EMPA 17.4% on polyimide

8 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 8/47 CIGSe and CIGS alloys CISe pn homojunction have poor efficiency! (Why?) The lattice parameters of chalcopyrites are similar. I can make alloys in order to increase V oc. Why the Voc increases with the band gap? Using alloys CuIn 1-x Ga x Se 2 (CIGSe) with Ga content between 25% and 30% they reached 20.3% efficiency Using alloys CuIn 1-x Ga x S 2 (CIGS) with Ga content of 10% they reached good efficiency For larger Ga fractions the photovoltage saturates leading to a decrease of conversion efficiencies.

9 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 9/47 CIGSe alloy Ga/(Ga+In) Efficiency (%) The V oc does not increase linearly with E g! Open Circuit voltage (V) Usually CIGSe refers to the composition CuIn 1-x Ga x Se 2 with x = 0.3 CuIn 0.7 Ga 0.3 Se 2 E g = 1.15 ev Absorber band gap (ev)

10 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 10/47 Explanation of the V oc saturation If the CIGSe, like pure CGSe, is able to equilibrate at the temperatures used during the junction formation it will react to the E F upward shift creating a large amount of V Cu (acceptor defects). This effect will reduce the band bending in the absorber, it will introduce a large interface recombination and therefore it will give a lower V oc.

11 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 11/47 CIGSe solar cell structure Substrate configuration Need an encapsulating glass Max eff 20%. Currently used Superstrate configuration No transparent encapsulation on the back! Max eff 12.8% (Cd diffusion). Abandoned Source: Poortmans (2006)

12 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 12/47 CIGSe solar cell structure Substrate configuration

13 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 13/47 Back contact The work function of CIGSe is 4.6 ev. A metal with an high work function is needed. Mo has a work function of 4.6 ev, insufficient, in principle.? However Mo back contact seems to be ohmic (with strange I-V)! Explanation: MoSe 2? This layer is formed during CIGSe deposition. Tunnelling? Variable Mo work function? Back contact is usually made of molybdenum and deposited by DC sputtering Important role of MoSe 2 (adhesion)

14 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 14/47 Features of CIGSe growth Copper. We need a deficiency of copper (p type). But Cu excess: grain growth, less defect concentration, Cu x Se segregation. In Cu deficiency the best cell has 14% of efficiency. Several process were developed to use the good impact of Cu excess. Sodium. Diffuse from (soda lime) glass trough the Mo grain boundaries. Better morphology, p-type doping (V oc, FF). It can be put in definite quantities with a blocking layer and NaF deposition. Or by a soda lime thin film on flexible substrates SIMS Profile for Na

15 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 15/47 Features of CIGSe growth Gallium. In the highest-efficiency cells, the gallium content increases from 5 8% near the front junction to about 10 15% at the back contact, while the indium content correspondingly decreases toward the back. This composition gradient yields an increasing conduction band energy that produces a driving force to aid collection of electrons (minority carriers in the p-cigs) by the n-cds. These composition gradients act similarly to the back surface field in the silicon structure. Gallium enhances the MoSe 2 formation on the back contact

16 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 16/47 Deposition of CIGSe Co-evaporation or co-sputtering of elements on heated substrate. Se in excess. (best composition control, graded composition, Cu x Se aid) Bilayer process (Boeing) Inverted bilayer process Three stage process (the best process! Eff. record) Evaporation or sputtering of Cu, Ga, In and subsequent reaction in H 2 Se atmosphere or Se atmosphere (selenization) in tube furnace or RTP. Nanoparticle inks (or paints ) that are applied using non-vacuum coating techniques and then reacted into semiconductor films Electrodeposition and subsequent selenization/sulfurization From liquid phase, η = 10.3% (D. B. Mitzi. Solution processing of inorganic materials. Wiley-Interscience (2009)) CVD

17 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 17/47 Deposition of CIGSe. Co-evaporation Bi-layer process (Boeing) It starts with the deposition of Cu-rich Cu(In,Ga)Se 2 (grain growth) and ends with an excess In rate (no extra phases). Inverted process Deposition without copper of (In,Ga) 2 Se 3 (likewise In,Ga, and Se from elemental sources to form that compound) at low temperatures (typically around 300 C). Then, Cu and Se are evaporated at an elevated temperature (550 C) until an overall composition close to stoichiometry is reached. This process leads to a smoother film morphology when compared to the bi-layer process. Some Cu x Se remains. Three-stage process Deposition of In, Ga, and Se at the end of an inverted process to ensure the overall Inrich composition of the film even if the material had been Cu-rich during the second stage (grain growth, no extra phases). This process allows also to introduce Ga gradients into the absorber and thus enables us to design a graded band-gap structure (Ga gradient). During the third stage it is possible to detect Cu-rich and Cu-poor condition from a change in the thermal properties or optical properties

18 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 18/47 Deposition of CIGSe. Co-evaporation A. Romeo et al. Progress in photovoltaics, 12, 93 (2004)

19 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 19/47 Deposition of CIGSe. Co-evaporation Example of bilayer industrial process (Boeing) Source: Poortmans (2006)

20 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 20/47 Deposition of CIGSe. Co-evaporation Example of three-stage industrial process: Würth Solar. Other companies: Global Solar, Ascent Solar, Solibro.

21 Deposition of CIGSe. Selenization More industrial troughput. Difficult optimization due to the importance of kinetics of several reactions Solar Frontier (ex Showa Shell) does not use CdS! Annealing in H 2 S. η = 11% Shell Solar (Avancis) use CdS by CVD. η = 9% Miasolé. Unknown process. η = 9% Energy Photovoltaics (EPV). Hybrid process (coevaporation for In and Ga in Se atmopshere, sputtering of Cu, selenization). Low defect concentration! Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 21/47

22 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 22/47 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 ZnO CIGSe Mo Glass, Metal foil, plastics

23 CdS Compact to reduce shunts It can suffer from the subsequent processes (usually in superstrate configuration) Lattice mismatch with the absorber: defects Ordered Vacancy Compound (OVC) near the interface! CuIn 3x Ga 3(1-x) Se 5 Cd donor dopant? Partecipation to carrier collection? High absorption in blue: usage of ZnO as window layer. CdS very thin! High resistance: usage of AZO as window layer. CdS very thin! Protection of the absorber against sputter damage during AZO window deposition Poortmans Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 23/47

24 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 24/47 CdS deposited by CBD Best results are obtained by Chemical Bath deposition (CBD) Solution: 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

25 R&D automated setup for CBD Side channel blower Pre Rinse Intermediary transport unit Final Rinse Dosage Unit Temp. Control Stand alone modules Robot Handling Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 25/47

26 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 26/47 CdS alternatives CdS is usually thicker than necessary in industrial process: high absorption in blue region. The buffer could be changed from CdS (E g = 2.42 ev) to ZnS (E g = 3.7 ev) or In 2 S 3 :Na (E g = ev) to improve carrier collection in the blue.

27 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 27/47 CIGSe solar cells. TCO The cell is completed with two layers of ZnO: A first thin layer 100 nm of intrinsic ZnO useful for reducing pin holes A thick 400 nm layer of AZO (aluminium doped ZnO) ZnO and AZO are deposited by RF sputtering at relatively low temperature (150 C or less) to avoid Cd diffusion in CIGSe The best resistivity of AZO is 5 x 10-4 Ω cm (higher doping reduces the mobility and increases the free carrier absorption). Poortmans Alternatives: BZO (high mobility)

28 CIGSe modules Laser or mechanical scribing to create stripes connected in series Encapsulation with EVA Poortmans Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 28/47

29 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 29/47 CIGSe commercial modules Manufacturing capacity: Solar Frontier (Showa Shell company) (Japan): 900 MW in 2012! Global Solar (USA) 140 MW (roll to roll) 10% efficiency Avancis (Germany) (15.5% on 30 cm x 30 cm module!) Würth Solar (Germany) 30 MW Ascent Solar (USA): 1.5 MW (8% on plastic) 12% efficiency

30 CIGSe commercial modules Manufacturing capacity: Solibro (Q-Cells company) (Germany): 13% efficiency! Solyndra (USA) Heliovolt (USA) 12 % efficiency Solarion (Germany) 13% (?) roll to roll on plastic and rigid modules Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 30/47

31 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 31/47 Avancis process SiNx barrier before Mo layer Sodium dopant by DC sputtering Alternate layers Cu (85%) + Ga (15%) and In Thermally evaporated Se layer Selenization in RTP (IR lamps) 10 C/s! The chamber has a minimized volume to ensure high pressure of Se and S. S is introduced by an additional gas feed. The process has probably a duration of about 10 minutes. Time resolved photoluminesce and other advanced techniques for quality control!

32 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 32/47 Centrotherm turnkey line Process: precursor/selenization Cost: 60 M for a capacity of 50 MW/y First module produced in January 2010 in a line installed in Taiwan Module eff. 12%, 170 Wp

33 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 33/47 Flexible CIGS Modules: Nanosolar Founded in 2002 Production capacity = 430 MW Production 2009 = 4 MW Aluminium foil + Mo CIGSe printed with ink containing nanoparticles + RTP Eff.: 11.6% module, 15.3% cell Wrap-Through back contact on a second aluminium foil

34 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 34/47 CIGSe solar cells stability Adverse effect of humidity on unprotected ZnO: degradation was shown to come from a resistivity increase but this drawback was overcome with proper encapsulation. No formation of Cu 2 S at the interface between CdS and Inrich CIS. Cd in-diffusion is kinetically limited and poses no threat to the interface stability (D(Cd in CIS) = cm 2 /s at room temperature). ZnO CIGSe Mo/CIGS interface: very stable with no evidence of Mo diffusion into the CIGS film. An interfacial MoSe 2 layer of a 10 nm thickness can be formed. This layer is possibly beneficial in producing an ohmic back contact at room temperature. Mo Glass, Metal foil, plastics

35 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 35/47 CIGSe solar cells stability Why Cu outdiffusion is very likely in Cu 2 S/CdS cells but highly unlikely in CIGSe/CdS cells? 1. µ Cu (Cu 2 S) > µ Cu (CdS) 2. The electric potential drop across the Cu 2 S/CdS interface is small, so that under forward bias an overpotential results in a driving force for Cu outdiffusion 1. µ Cu (CIS) < µ Cu (CdS) 2. The electric potential drop across the CIS/CdS interface is high, so that even under forward bias Cu outdiffusion is very unlikely

36 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 36/47 CIGSe solar cells stability Test solar frontier Test Global Solar S. Wiedeman, TFPP 2006

37 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 37/47 CIS solar cells Sulfurcell in Germany has developed a manufacturing process for this cell technology Using Cu-rich CIS thin films efficiencies of over 12% have been achieved. No sodium needed! However the Cu x S phase has to be removed in a further step which can easily be done by KCN treatment. A solution is the usage of Cu-poor film with Na doping. E g greater than in CIGSe: smaller temperature coefficient

38 CIS solar cells. Development V oc improvement Ga alloying of CIS (larger concentration in the back of the cell) Lower electron affinity buffer layer. n-cds is considerably less appropriate for CIS because the electron affinities between the materials differ by about 0.4 ev, thus limiting the contact potential difference and thus the achievable V oc jsc improvement Larger E g window layer (i.e. Zn(S,O)) Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 38/47

39 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 39/47 CIS solar cells. Sulfurcell process Sulfurization of precursors in RTP furnace. Only 5 minutes in RTP!!! Max eff. 8.6%

40 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 40/47 CIGSe and others. Production CIGS 2009 MW/yr 2010 MW/yr Capacity Production Capacity Production Solar Frontier (Showa Shell) JPN Solibro (Q-cells) DEU Solindra USA Wurth Solar DEU Global Solar USA+DEU Avancis (Shell+Saint Gobain) DEU Sulfurcell DEU Nanosolar USA total production: 210 MWp 2010 total production: 435 MWp

41 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 41/47 Materials availability: a future problem? Indium requirement: 0.03 g/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)

42 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 42/47 Materials availability: a future problem? Source: B. A. Andersson, Prog. Photovolt. Res. Appl. 8, 61 (2000) 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 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)

43 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 43/47 Grain boundaries role in CIGSe Why are polycrystalline thin-film PV devices more efficient than their single crystal counterparts? Maybe the grain boundaries in the columnar grains act as preferential minority carrier paths (barrier for holes and well for electrons)

44 Minority carrier lifetimes in CIGSe Time resolved photoluminescence 2 ns < τ n < 200 ns. If µ n = 100 cm 2 /(Vs) and τ n = 10 ns then L n =1.6 µm L n = Dτ = n kt µ n τ n q This is partly confirmed by direct EBIC measurements Fast τ n decrease in air! B. Ohnesorge et al. Appl. Phys. Lett., 73, 1224 (1998) Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 44/47

45 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 45/47 CIGS 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 large stoichiometry variations (a few percents) and defect density are not detrimental Impurities in large concentrations (i.e. Fe at cm -3 ) do not cause any performance degradation or stability problem and in the case of Na (at cm -3 ) a performance improvement is observed. 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 Large supporting research and development community

46 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 46/47 CIGS solar cells. Conclusions (2) CIGSe modules have reached 431 MWp of production in 2010 CIGSe has the highest efficiencies and potentialities, but also the highest cost with respect to CdTe and a-si. 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. A few companies are starting to deliver turnkey production lines. 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.

47 Francesco Biccari Master Ingegneria del Fotovoltaico Corso di Tecnologie Fotovoltaiche Convenzionali 47/47 Acknowledgments Thanks to Dr. Alberto Mittiga for providing several figures, numbers and slides of this presentation Thanks to Dr. Rosa Chierchia for useful discussions