Sustainable PV technologies for future mass deployment: Kesterites Dr. Edgardo Saucedo

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1 Sustainable PV technologies for future mass deployment: Kesterites Dr. Edgardo Saucedo Catalonia Institute for Energy Research (IREC) Sant Adrìà de Besòs (Barcelona), Spain IX Barcelona Global Energy Challenges June 19 th 2014

2 Catalonia Institute of Energy Research Founded in 2008, and located in Barcelona, Spain: [objective of] creating a more sustainable future for energy usage and consumption Divided into six main areas: Advanced materials for energy Offshore wind energy Electrical engineering Lighting Bioenergy and biofuels Thermal energy and building systems - Solar energy materials and systems - Functional nanomaterials - Materials and catalysts - Nanoionics and fuel cells - Energy storage and harvesting 2

3 Solar Energy Materials and Systems Lab Group leader: Prof. Alejandro Pérez-Rodríguez Head of processes lab: Dr. Edgardo Saucedo Head of characterization lab: Dr. Victor Izquierdo-Roca Coordination of three European FP-7 projects KESTCELLS, SCALENANO, INDUCIS and involvement in numerous other national and international projects through public and private ventures 3

4 Solar Energy Materials and Systems Lab Preparation of entire solar cell structure, from the back contact to the completed device Main research lines include: New materials and concepts for high efficiency PV devices, focus on chalcopyrite and kesterite absorbers Advanced characterization processes, specifically with Raman scattering spectroscopy Low cost processes for industrially viable chalcogenide based technologies, such as spray pyrolysis and electrodeposition 4

5 Outline Introduction: First, second, and third generation PV Thin film device structure Cu 2 ZnSn(S,Se) 4 review and properties Secondary phase formation and identification and main Kesterite Raman modes Front interface Back interface Conclusions 5

6 Three generations of PV First generation PV: mono- and polycrystalline silicon (c-si) - Highest cell (25%) and module efficiencies (15%) - Expensive materials (monocrystalline silicon in particular) - Indirect bandgap of 1.1 ev >150 μm active layer Second generation PV: thin film (Cu(In,Ga)Se 2, CdTe, a-si:h) - Relatively high cell and module efficiencies, lower than first gen: CIGS (20.9%), CdTe (20.6%) and a-si-h (12%) - Direct bandgap materials <5 μm absorber layer - Reduced material cost and weight, useable with flexible substrates, facilitates building integration Third generation PV: organic, tandem, intraband, photon up/down converter, quantum dots, etc. - Still under research, some ideas not practically proven - Expensive 6

7 Thin film PV device structure Three main thin film absorbers: Cu(In,Ga)Se 2 CdTe a-si:h In-Ga scarce elements, usage of H 2 Se Te scarce element, Cd toxicity Long-term stability, conversion efficiency Sustainable materials based on earth abundant elements: KESTERITES Cu 2 ZnSn(S,Se) 4 (CZTS) are formed by earth abundant and low toxic elements Use similar technologies than those developed for Cu(In,Ga)Se 2 : Back contact layer Mo Buffer layer CdS Front contact layer In 2 O 3 :Sn, ZnO:Al, SnO 2 :F ZnO-i and ZnO:Al front contact CdS buffer CZTS absorber Mo back contact 7 Substrate

8 Cu 2 ZnSn(S,Se) 4 Noted as a PV active material in 1988, first device in 1996 (0.7%). Very limited research till In 2010 breakthrough result published by IBM, a 9.6% device Gaining interest as a mid- to long-term alternative to CIGSe because it is composed of earth abundant elements Current record efficiency of 12.6% (2013) Some of the main challenges include secondary phase formation and identification, elemental loss during annealing (Zn, Sn-S/Se), front and back interface optimization 8

9 Properties structure I-III-(II-IV)-VI compounds based on ZnS structure ZnS sphalerite Zn(II) CuInSe 2 chalcopyrite Cu(I) In(III) Cu 2 ZnSnSe 4 kesterite Cu(I) Zn(II) Sn(IV) Non-equilibrium and defect structures: CIGSe OVC, Cu-Au CZTSSe disordered kesterite, stannite, wurtzite Blue Cu Orange Zn Red Sn Yellow S/Se [1] S Schorr, Sol Eng Mat Sol Cells 95, 2011, pp

10 Properties optical Absorption coefficient ~ above bandgap CZTSe 1.0 ev Band-gap CZTS 1.5 ev Absorbing layers of <2 μm Ideal range for sun light absorption [1] J He, et al. J Alloy and Comp 511, 2012, pp

11 But we have a problem: to much elements... Band structure and point defects Secondary phases Morphology Crystal structure Contact Materials SOLAR CELLS PROPERTIES 11

12 Outline Introduction: First, second, and third generation PV Thin film device structure Cu 2 ZnSn(S,Se) 4 review and properties Secondary phase formation and identification and main Kesterite Raman modes Front interface Back interface Conclusions 12

13 Secondary phase formation Absorber Eff. (%) Cu/(Zn+Sn) Zn/Sn Method Reference Cu 2 ZnSn(S,Se) Hybrid sol-part. M Winkler, et al, Energ & Envir Sci 2013 Cu 2 ZnSn(S,Se) Hybrid sol-part. T Todorov, et al, Adv Energ Mat 2010 Cu 2 ZnSnS Co-sputter H Katagiri, et al, Appl Phys Exp 2008 Cu 2 ZnSnSe Sputtering I Repins, et al, Sol Energ Mat Sol Cells 2012 Non-stoichiometric compositions used for both CIGSe and CZTSSe: CZTSSe: Cu-poor and Zn-rich (high efficiency devices) Formation of binary sulfides/selenides Cu-S/Se can form locally, even in Cu-poor conditions [1] C Platzer-Björkman C, et al. Sol Eng Mat Sol Cells 98, 2012, pp

14 Secondary phase identification Sqrt. Int. (arb. units) Processing under Zn-excess conditions leads to the formation of ZnS/Se in addition to CZTS/Se, though other phases can form depending on the processing conditions ZnS is a high bandgap (3.7 ev) and resistive phase, potentially deteriorating cell performance KCN-based etches are often used with CZTS, but they are ineffective at removing ZnS/Se Cu 2 ZnSnSe 4 ZnSe Distinction between some phases in XRD difficult, namely for CZTS/Se, ZnS/Se, and CTS/Se Cu 2 SnSe

15 Sqrt. Int. (arb. units) Intensity (arb.units) Intensity (arb.units) Secondary phase identification: resonant Raman scatering X-ray diffraction Cu 2 ZnSnSe 4 Raman with λ exc = 514 nm ZnSe 2nd and 3rd order ZnSe peaks λ exc = 458 nm ZnSe 173 CZTSe CZTSe ZnSe 747 ZnSe ZnSe Cu 2 SnSe CTSe ZnSe Raman shift (cm -1 ) Raman shift (cm -1 ) Common laser wavelengths (nm) Bandgaps of Cu-Zn-Sn-S-Se phases (nm) 15

16 Main Kesterites Raman modes Simultaneous fitting of spectra allowed identification of 18 peaks, that are attributed to the 27 optical modes theoretically expected for this crystalline structure [1]. Detection of 5 peaks not observed previously, but theoretically predicted. Well resolved peak at cm -1, that according to simulation data, has been identified as the third A symmetry mode from the CZTS kesterite phase The peak at cm -1 usually attributed as ZnS peak, in this case is identified as CZTS mode due to the absence of second order peak of ZnS. Polarization measurements enabled determination of the symmetry of the modes. [1] M. Dimitrievska, Appl. Phys. Lett., vol. 104, no. 2, p , Jan

17 Outline Introduction: First, second, and third generation PV Thin film device structure Cu 2 ZnSn(S,Se) 4 review and properties Secondary phase formation and identification and main Kesterite Raman modes Front interface Back interface Conclusions 17

18 Front interface: CdS CdS most common buffer layer, alternatives: ZnS, In 2 S 3 Cliff alignment Band alignment at junction can be a cliff or spike: Spike increases barrier to charge carriers Cliff increases interface recombination, and reduces V OC CIGSe has ev offset (spike) with CdS Conflicting theoretical and experimental results on the type of alignment, which may vary between CZTS and CZTSe Secondary phases? [1] M Bär, et al. J Elec Spec Relat Phen, in press 2012 [2] M Bär, et al. Appl Phys Lett 99, 2011, pp

19 Front interface: CdS current density [ma/cm 2 ] current density [ma/cm 2 ] current density [ma/cm 2 ] (a) (d) before white light illumination dark curve before longpass filter 550 nm 1 sun illumination dark curve after CZTSe/CdS1/i-ZnO+ITO J sc = 31.6 ma/cm 2 V oc = 387 mv FF = 52.3% eff. = 6.4% R shunt = 154 cm 2 R s = 0.89 cm 2 crossover red kink voltage [V] after light soaking dark & illuminated curve longpass filter 550 nm CZTSe/CdS1/i-ZnO+ITO J sc = 31.9 ma/cm 2 V oc = 377 mv FF = 57.9% eff. = 7.0% R shunt = 225 cm 2 R s = 0.40 cm 2 crossover voltage [V] (b) (e) current density [ma/cm 2 ] before white light illumination dark curve before CZTSe/CdS2/ longpass filter 550 nm i-zno+ito 1 sun illumination dark curve after 1 sun J sc = 31.9 ma/cm 2 V oc = 386 mv FF = 61.8% eff. = 7.6% R shunt = 313 cm 2 R s = 0.46 cm 2 crossover voltage [V] after light soaking dark & illuminated curve longpass filter 550 nm CZTSe/CdS2/i-ZnO+ITO J sc = 32.4 ma/cm 2 V oc = 392 mv FF = 64.4% eff. = 8.2% R shunt = 302 cm 2 R s = 0.32 cm voltage [V] Elimination of JV distortions: red kink and cross over (soft thermal annealing and light soaking) Using different precursors for the CdS allows obtaining improved JV curves, increasing devices efficiency. M. Neuschitzer, Y. Sánchez et al., Optimization of CdS Buffer Layer for High Performance CZTSe Solar Cells and the Effects of Light Soaking: Elimination of Cross Over and Red Kink, 2014, submitted. 19

20 Front interface: ZnS etching [1] A Fairbrother, et al. J Amer Chem Soc 134, 2012, pp HCl etching to remove ZnS from CZTS films λ exc = 325 nm

21 Front interface: ZnSe etching Oxidizing based agent etching to remove ZnSe from CZTSe films KMnO 4 /H 2 SO 4 Na 2 S Se 0 CZTSe CZTSe ZnSe SEM confirms small aggregates (ZnSe) dissolve with KMnO 4 /H 2 SO 4 leaving porous aggregates( Se 0 ) to further be removed with Na 2 S. J sc increases due to ZnSe removal, higher EQE for etched samples in the ZnSe absorption region V oc drastically increased, up to 100 mv, EQE higher in the p-n junction region ( nm) η increased due to improvement of the p-n junction. Chemical passivation!! [1] M. López-Marino et al., Chemistry A European Journal, 2013, 19,

22 Front interface: Sn(S,Se) etching Simple chemical etching based in (NH 4 ) 2 S to remove Sn(S,Se) from CZTSSe films Two types of Sn(S,Se) overgrowths: those related with the processes parameters (type 1) and those condensed from the annealing atmosphere (type 2) Nevertheless the etching is effective in removing both of them SnSe 0.9 S 0.1 [1] H. Xie et al., ACS Materials and Interfaces, 2014, submitted. 22

23 Front interface: Sn(S,Se) etching Simple chemical etching based in (NH 4 ) 2 S to remove Sn(S,Se) from CZTSSe films J-V: large improvement of the J-V curves after etching (J sc, V oc, FF) EQE: nm there are significant increases in external quantum efficiency EQE(-1V)/EQE(0V): better carrier collection efficiency after etching. Increase in the short wavelength range for both samples (low electron lifetime, or low field strength at the p-n interface), relatively well solved in the case of the best cell. [1] H. Xie et al., ACS Materials and Interfaces, 2014, submitted. 23

24 Outline Introduction: First, second, and third generation PV Thin film device structure Cu 2 ZnSn(S,Se) 4 review and properties Secondary phase formation and identification and main Kesterite Raman modes Front interface Back interface Conclusions 24

25 Back interface Intensity (arb. units) Mo is the conventional back contact material for both CIGSe and CZTSSe For CIGSe it is stable and forms ohmic contact (with MoS/Se 2 ) Evidence of CZTSe decomposition, which has been previously shown for CZTS, and predicted for CZTSe More stable materials for CZTSSe? Substrate λ exc = 532 nm ZnSe Barriers or interfacial layers? MoSe 2 Cu x Se MoSe2 MoSe Raman shift (cm -1 ) [1] S Lopez-Marino, et al. J Mat Chem A 1, 2013, pp

26 Back interface ZnO barrier Intensity (arb. units) Zinc oxide layer can present decomposition by formation of a thin ZnSe layer during annealing, and also significantly improve the interface quality No ZnO 10 nm ZnO Substrate λ exc = 532 nm ZnSe MoSe 2 Cu x Se w/o ZnO MoSe 2 MoSe 2 10 nm ZnO layer Raman shift (cm -1 ) [1] S Lopez-Marino, et al. J Mat Chem A 1, 2013, pp

27 Back interface TiN barrier Titanium nitride acts as a diffusion barrier to reduce formation of MoSe 2 layer at high annealing temperatures and pressures 20 nm TiN T = 570 ºC p Se = 0.2 bar η = 3.0% η = 8.9% [1] B Shin, et al. Appl Phys Lett 101, 2012, pp

28 Summary To date CZTSSe-based device structure and processes have been mostly based on CIGSe After a rapid increase in efficiency, unique problems are being identified, indicating that the optimized conditions for CIGSe are not ideal for CZTSSe In addition to an incomplete knowledge of the basic material properties, other aspects of CZTSSe related to processing need to be understood, including the formation of secondary phases and their influence on device properties, and instabilities at both the front and back contact regions The major challenge of Kesterites is their advantage: earth abundant but to much elements 28

29 Acknowledgements This work has been carried out with the collaboration of my colleagues in the Solar Energy Materials and Systems Group at IREC: Prof. A. Pérez- Rodríguez, Dr. V. Izquierdo-Roca, Dr. Marcel Placidi, Dr. Diouldé Sylla, Y. Sánchez, Dr. A. Fairbrother, Dr. X. Fontané, M. Espíndola-Rodríguez, S. López-Marino, C. Insignares, M. Dimitrievska, M. Neuschitzer, H. Xie This research was supported by the Framework 7 program under the project KESTCELLS (FP7-PEOPLE-2012-ITN ) I would like to thanks the MINECO for the Ramón y Cajal fellow (RYC ) 29

30 Patrons: With financial support from: