Available online at www.sciencedirect.com Electrochimica Acta 53 (2008) 2226–2231 Electrodeposited ZnO/Cu 2 O heterojunction solar cells S.S. Jeong, A. Mittiga ∗ , E. Salza, A. Masci, S. Passerini ∗ Agency for the New Technologies, Energy and the Environment (ENEA), Casaccia Research Center, Via Anguillarese 301, 00123 Rome, Italy Received 19 June 2007; received in revised form 12 September 2007; accepted 18 September 2007 Available online 22 September 2007 Abstract In this paper the fabrication and the characterization of heterojunction solar cells based on electrodeposited ZnO and Cu 2 O is described. The effect of the electrodeposition conditions (pH and temperature) on the cell performance has been investigated. The cells made with a Cu 2 O layer deposited at high pH (12) and moderate temperature (50 ◦ C) have shown conversion efficiency as high as 0.41%. © 2007 Elsevier Ltd. All rights reserved. Keywords: Zinc oxide; Copper oxide; ZnO/Cu 2 O; Electrodeposition; Photovoltaic cells 1. Introduction In view of the severe future ecological impacts of energy pro- duction by combustion of fossil fuels, solar energy is seriously considered as an alternative. However, the development of new solar energy converters with improved performance and lower cost requires new approaches focused on the use of cheap and non-toxic materials prepared via low energy intensity processes such as electrodeposition. The possibility of using electrodeposition to realize solar cells with energy conversion efficiencies larger that 10% is witnessed by the work regarding CdS/CdTe heterojunctions [1]. However, the well-known toxicity of cadmium represents a serious obsta- cle to the development of industrial processes for the production of electrodeposited solar cells. On the other hand, cuprous oxide (Cu 2 O) is a non-toxic direct energy gap semiconductor that can be easily produced with a minority carrier diffusion length suited for the use as solar cell absorber layer. Despite that, however, the highest energy conversion efficiency of a Cu 2 O solar cell obtained up to now is much lower than the Shockley–Queisser theoretical limit which is about 20%. The main hindrance to the optimization of Cu 2 O solar cells is the difficulty in the doping process. Cu 2 O is spontaneously a p-type semiconductor and all ∗ Corresponding author. E-mail addresses: Alberto.mittiga@casaccia.enea.it (A. Mittiga), passerini@casaccia.enea.it (S. Passerini). the efforts to form homojunctions by n-doping of Cu 2 O have, so far, failed. An exception is a very recent report [2] in which, however, no photovoltaic action is claimed. The best approach is therefore to use a heterojunction between Cu 2 O and a n-type oxide. Among all possible combinations, ZnO/Cu 2 O hetero- junctions have recently attracted a renewed interest of several researchers [3–7] because of the favorable alignment of the con- duction band edges. As a result of the new research efforts, cells with photovoltaic conversion efficiencies as high as 2% [3] have been recently made in our laboratories. The best performing cells [3,4] were fabricated on Cu 2 O substrates obtained by oxidation of copper sheets at high temperature. Their fill factor, however, was still limited by the low electric conductivity of the thick Cu 2 O substrates. The use of a thin film structure could solve this problem and, also, avoid the use of large amounts of high purity copper. Thin film solar cells based on Cu 2 O were obtained by depositing cuprous oxide and zinc oxide by reactive rf mag- netron sputtering [5]. In that work it was demonstrated that the two deposition sequences, ZnO deposited on Cu 2 O and Cu 2 O deposited on ZnO, were not equivalent with the latter sequence giving significantly better current–voltage characteristics. The difference was ascribed to a better crystallographic matching due to the spontaneous high orientation of the ZnO crystals in the (0 0 0 1) orientation that induced the growth of the Cu 2 O crystals with the (1 1 1) preferential orientation. Since ZnO is hexago- nal (wurtzite) while Cu 2 O is cubic (cuprite) this matching gives a similar hexagonal atomic arrangement at the interface with 0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.09.030