Transition metal oxide absorbers for photovoltaics

Abstract

Several oxides are non-toxic, stable, and can be deposited using inexpensive techniques on a variety of substrates. There is a compelling case for use of oxide in photovoltaics, referred to as all-oxide photovoltaics . Traditional photovoltaics technologies often use oxides, e.g. as transparent electrodes (SnO2: F. In2O3: SnO2, etc.) or carrier-selective contacts (TiO2, MoOx, etc.). However, oxides are rarely used as light-absorbers because oxides tend to have large bandgap (gt 2 eV), low hole mobility (lt 10-4 cm2/Vs), and low carrier diffusion lengths (lt100 nm). Despite considerable effort, few oxides have been demonstrated as efficient solar absorbers. One class of oxides still poorly investigated, are the multi-cation transition oxides. This is a very large library, so it is conceivable that good solar absorbers are waiting to be discovered. In this work, we present a study of three multi-cation transition metal oxides for their application in solar cells: Zn2Mo3O8, Mn2V2O7, and Ag2CrO4. Zn2Mo3O8 is a non-centrosymmetric oxide with a low-bandgap of 2.1 eV. Polycrystalline films deposited at room temperature are n-type with Hall mobility of 0.6 - 0.7 cm2V-1s-1. DFT calculations suggest that the valence band is composed of both O 2p and Mo 3d orbitals which could lead to higher hole mobility than that typical oxides. The valence band composition was experimentally determined using resonant photoelectron spectroscopy, which confirms this assertion. Unfortunately, DFT calculations also show that ZMO has high energy Frenkel excitons (0.78 eV), which will not dissociate at room-temperature, leading to reduced voltage in solar cells. Au/ZMO/TiO2 Schottky diodes with ZMO films deposited at room temperature show photoresponse but no photovoltage. This shows that ZMO is a carrier conducting semiconductor that can be used in a photodetector but not as a solar absorber. Mn2V2O7(MVO) was previously demonstrated as a photocatalyst with a low bandgap of 1.6 eV...