Towards a photoelectrochemical tool for comprehensive quality assessment of solar cell absorber layers

Being able to predict the optoelectronic properties of thin film solar cell by analysis of the absorber layers – i.e. before a number of deposition steps are carried out – would be a clear advantage both at academic research level and for the implementation of procedures for process monitoring in industry. Recently we have demonstrated that the short circuit current density of CIGSe devices can be reasonably predicted by assessing photoelectrochemically the photocurrent density of the respective absorber layers on conductive substrates through a Eu3+ electrolyte junction. In such a junction the Eu3+ acts as a scavenger for the electrons generated on the p type semiconductor upon irradiation with photon whose energy is greater than the band-gap of the semiconductor. In presence of a redox couple with suitable standard potential with respect to the semiconductor Fermi level, the junction can also be assessed in forward bias and important information can be extracted. In this work we demonstrate that the reverse saturation current of cise solaf cell devices can be peedicteb by meeasuring ghe forward bias characteristics of the cise eu2+/3+ juncfion The aim of this work is to identify photoelectrochemical parameters for the reliable assessment of the optoelectronic properties of thin absorber films for photovoltaic applications. The ultimate goal is to develop an on-line testing tool capable of evaluating the suitability of solar cell absorbers such as Cu(In,Ga)(S,Se)2 or Cu2ZnSn(S,Se)4 before these are processed further into complete devices by addition of n-type, window and front contact layers. This would allow monitoring the stability of part of the production plant process with inherent advantages in terms of material usage, energy and time. By formation of virtually reversible electrolyte (Schottky) junctions [1] it is possible to interrogate semiconductor thin films and extract information about properties such as majority carrier type [2, 3], band-gap and flat-band potential [4], doping density [5], as well as insights on the presence of optically absorbing phases on the film surface [6]. In solid state devices the short circuit current density is related to the generation of carriers, their transport and their subsequent collection at the interface. This is also true for a Schottky junction. The parallel resistance of solar cells is associated to the presence of conducting (shorting) paths; the dark current measured in solution under reversed bias should give an estimate of such paths. The voltage of a device measured under open circuit conditions is proportional to the quasi-Fermi level splitting within the absorber. The consequence of this statement applies also to the semiconductor/electrolyte case [7]. In this work we investigate if a sound correlation between the solid state device properties (short circuit current, parallel resistance and open circuit voltage) and the corresponding parameters accessible through a transparent electrolyte junction can be established. To this end three Cu(In,Ga)Se2 absorber layers obtained by physical vapour deposition were split into two. Half were completed into solar cell devices and the other half were tested photoelectrochemically. The chosen absorber layers gave solid state device power conversion efficiencies of 6, 9 and 12.5% [8]. The photoelectrochemical experiments were performed with a three electrode setup in an equimolar solution of Eu3+/2+ and consisted of chronoamperometric and voltammetric analyses under pulsed illumination, as well as photocurrent spectroscopy. The theoretical correlations are complicated by experimental issues including the non-ideality of surface structures and of the current collection [9]. In fact, in agreement with the literature [5, 10, 11], our work shows that photoelectrochemical assessments can also be performed in the absence of active electrolyte redox species. Nevertheless, we highlight their importance if reversibility and longtime reproducibility of the measurements are a strict requirement. References [1] L.M. Peter, Semiconductor Electrochemistry, in: J.M. Feliu-Martinez, V.C. Paya (Eds.) Electrochemistry, Encyclopedia of Life Support Systems, Oxford, 2010. [2] P. Dale, A. Samantilleke, G. Zoppi, I. Forbes, S. Roncallo, L. Peter, Deposition and Characterization of Copper Chalcopyrite Based Solar Cells using Electrochemical Techniques, Electrochemical Society Transactions, 6 (2007) 535-546. [3] J. Kessler, D. Schmid, R. Schaffler, H.W. Schock, S. Menezes, Electro-optical and photoelectrochemical studies of CuIn3Se5 chalcopyrite films, in: Photovoltaic Specialists Conference (PVSC), 1993 23rd IEEE, 1993, pp. 549 - 554. [4] J.J. Scragg, P.J. Dale, L.M. Peter, Towards sustainable materials for solar energy conversion: Preparation and photoelectrochemical characterization of Cu2ZnSnS4, Electrochemistry Communications, 10 (2008) 639-642. [5] D. Lincot, H. Gomez Meier, J. Kessler, J. Vedel, B. Dimmler, H.W. Schock, Photoelectrochemical study of p-type copper indium diselenide thin films for photovoltaic applications, Solar Energy Materials, 20 (1990) 67-79. [6] D. Colombara, L.M. Peter, K. Hutchings, K.D. Rogers, S. Schäfer, J.T.R. Dufton, M.S. Islam, Formation of Cu3BiS3 thin films via sulfurization of Bi–Cu metal precursors, Thin Solid Films, 520 (2012) 5165-5171. [7] R. Memming, Semiconductor Electrochemistry, Wiley, 2001. [8] V. Depredurand, Y. Aida, J. Larsen, T. Eisenbarth, A. Majerus, S. Siebentritt, Surface treatment of CIS solar cells grown under Cu-excess, in: Photovoltaic Specialists Conference (PVSC), 2011 37th IEEE, 2011, pp. 000337-000342. [9] H. Gerischer, The role of semiconductor structure and surface properties in photoelectrochemical processes, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 150 (1983) 553-569. [10] C. Guillen, J. Herrero, D. Lincot, Photovoltaic activity of electrodeposited p-CuInSe2/electrolyte junction, Journal of Applied Physics, 76 (1994) 359-362. [11] O. Solorza-Feria, R. Rivera-Noriega, Photoelectrochemical response and characterization of p-CuInSe2 electrodeposited with different citrate ion concentrations, Journal of Materials Science, 30 (1995) 2616-2619. Acknowledgements LPV, LEM and Nexcis team members are gratefully acknowledged for help and discussion. Prof. Laurence M. Peter is thanked for initiating the authors’ interest in Photoelectrochemistry. The research leading to these results has received funding from the European Union’s Seventh Framework Programme FP7/2007-2013 under grant agreement nº 284486