Effect of thermal annealing in vacuum on the photovoltaic properties of electrodeposited Cu₂O-absorber solar cell

¹ AIT-Austrian Institute of Technology, Energy Department, Giefinggasse 2, 1221 Vienna, Austria
² NanoTecCenter Weiz Forschungsgesellschaft mbH, Franz-Pichler-Str. 32, 8160 Weiz, Austria
³ Graz University of Technology, Institute of Solid State Physics, Petergasse 16, 8010 Graz, Austria

^a e-mail: theodoros.dimopoulos@ait.ac.at
^b Present address: EV Group E. Thallner GmbH, DI Erich Thallner Str. 1, 4782 St. Florian am Inn, Austria.

Received: 24 October 2013
Received in final form: 15 April 2014
Accepted: 15 April 2014
Published online: 7 July 2014

Abstract

Heterojunction solar cells were fabricated by electrochemical deposition of p-type, cuprous oxide (Cu₂O) absorber on sputtered, n-type ZnO layer. X-ray diffraction measurements revealed that the as-deposited absorber consists mainly of Cu₂O, but appreciable amounts of metallic Cu and cupric oxide (CuO) are also present. These undesired oxidation states are incorporated during the deposition process and have a detrimental effect on the photovoltaic properties of the cells. The open circuit voltage (V_OC), short circuit current density (j_SC), fill factor (FF) and power conversion efficiency (η) of the as-deposited cells are 0.37 V, 3.71 mA/cm², 35.7% and 0.49%, respectively, under AM1.5G illumination. We show that by thermal annealing in vacuum, at temperatures up to 300 °C, compositional purity of the Cu₂O absorber could be obtained. A general improvement of the heterojunction and bulk materials quality is observed, reflected upon the smallest influence of the shunt and series resistance on the transport properties of the cells in dark and under illumination. Independent of the annealing temperature, transport is dominated by the space-charge layer generation-recombination current. After annealing at 300 °C the solar cell parameters could be significantly improved to the values of: V_OC = 0.505 V, j_SC = 4.67 mA/cm², FF = 47.1% and η = 1.12%.