Low-temperature growth of single-crystal Cu(In,Ga)Se 2 films by pulsed electron deposition technique S. Rampino a,n , M. Bronzoni a , L. Colace b , P. Frigeri a , E. Gombia a , C. Maragliano c , F. Mezzadri a , L. Nasi a , L. Seravalli a , F. Pattini a , G. Trevisi a , M. Motapothula d , T. Venkatesan d , E. Gilioli a a IMEM-CNR Institute, Parco Area Delle Scienze 37/A, 43124 Parma, Italy b Department of Engineering, University “Roma Tre”, Via Vito Volterra, 62, 00146 Rome, Italy c LENS Laboratory, Masdar Institute of Science and Technology, Masdar City, PO Box 54224, Abu Dhabi, United Arab Emirates d NUSNNI-NanoCore, National University of Singapore, Singapore 117576, Singapore article info Article history: Received 16 July 2014 Received in revised form 16 October 2014 Accepted 27 October 2014 Keywords: CIGS Thin film solar cells Epitaxial thin films Pulsed Electron Deposition PED abstract High quality epitaxial crystalline Cu(In,Ga)Se 2 (CIGS) films were grown on n-type (1 0 0)—Germanium (Ge) substrates using pulsed electron deposition (PED) technique at a remarkably low substrate temperature of 300 1C, thanks to the high-energy of adatoms arriving to the substrate. The crystalline quality was confirmed by X-ray diffraction techniques and from Transmission Electron Microscopy and the only defects found were twin boundaries along the (1 1 2) direction in these CIGS films; surprisingly neither misfit dislocations nor Kinkerdall voids were observed. A 100 meV optical band located below the band edge was observed by Photoluminescence technique. Current–voltage and capacitance–voltage measurements confirm an intrinsic p-type conductivity of CIGS films, with a free carrier concentration of E3.5  10 16 cm 3 . These characteristics of crystalline CIGS films are crucial for a variety of potential applications, such as more efficient absorber layers in single-junction and as an integral component of multi-junction thin-film solar cells. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Thin film solar cells based on polycrystalline Cu(In,Ga)Se 2 (CIGS) are considered to be among the most promising photovoltaic devices due to their high energy harvesting efficiency, long term stability and industrial scalability. Recent developments include record lab-scale cells with conversion efficiency above 20% on both rigid [1] and flexible substrates [2]. Even though the performance of CIGS devices is approaching the crystalline Silicon solar cell efficiency, the best lab results are still quite far from the Shockley–Queisser theoretical limit of 33% [3]. Despite extensive work carried out on the loss mechanisms in CIGS solar cells to achieve the theoretically predicted limit, this topic is still under intense debate. The most relevant losses are related to the following effects [4,5]: (i) Shockley–Read–Hall recombination in the space charge region and at the grain boundaries, (ii) bandgap fluctuations induced by lateral stoichiometry non-uniformity, (iii) low electron mobility due to impurity scattering and band bending at charged grain boundaries and (iv) fluctuations of the electrostatic potential caused by extended defects such as disloca- tions and grain boundaries. Although the beneficial role of grain boundaries as hole barriers in polycrystalline films is widely accepted, their presence induces strong fluctuations on the electrostatic potential, leading to an increase of the diode saturation current and to the consequent net reduction of the open circuit voltage [4,5]. From these considera- tions, the enhancement of the crystal quality is a crucial require- ment for reducing efficiency losses in CIGS solar cells. Aiming to investigate the role of the CIGS crystal structure and grain size on the solar cell performance, in previous studies we have developed a three-dimensional model for ZnO/CdS/CIGS/Mo solar cells to be included in Sentaurus Technology Computer Aided Design (TCAD) semiconductor device simulation tool [6]; the simulations strongly indicated that the cell efficiency would benefit from larger grain size over a wide range of values of the photogenerated carrier’s surface recombination rate, both at the CdS/CIGS heterojunction (S n ), and at the grain boundaries (V r ), as summarized in Fig. 1. The results suggest that significantly improved solar cell absorbers can be expected from single crystal CIGS films, obtainable only by epitaxial growth on substrates with reduced lattice mismatch. GaAs has been exploited as epitaxial substrate in previous works [7–9]: from these studies, it turned out that substrate temperatures above Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells http://dx.doi.org/10.1016/j.solmat.2014.10.048 0927-0248/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: rampino@imem.cnr.it (S. Rampino). Solar Energy Materials & Solar Cells 133 (2015) 82–86