Solution processed single-phase Cu2SnS3 films: structure and photovoltaic performance
High quality microcrystalline tetragonal p-Cu2SnS3 (CTS) films are prepared by spin-coating a single precursor of metal salts and thiourea on to 5 × 5 cm2 Mo substrates. The best of the solar cells completed on these films with a substrate configuration: glass/Mo/CTS/CdS/i-ZnO/Al:ZnO/Ni–Al and a total area of 0.5 cm2 displays an open-circuit voltage of 206 mV, a fill factor of 34.5%, a short circuit current density of 27.6 mA cm−2 and a power conversion efficiency of 1.9% under simulated AM1.5 illumination. This the best performance reported for such solar architectures obtained by solution processing, with dispersion below 20% for 24 devices. For the first time, the key factors limiting CTS device efficiency are quantitatively established based on temperature dependent current–voltage curves and impedance spectroscopy, namely: (i) carrier recombination at the CTS/CdS interface, (ii) MoS2 non-ohmic back contact, formed due to sulfurization of the top Mo layer, with a barrier height of up to 216 ± 14 meV and (iii) the presence of two trap levels with activation energies 41 ± 0.4 meV and 206 ± 7 meV. The shallower trap is linked Cu vacancies, while the deeper trap is associated with Sn in Cu antisite defects based on DFT supercell calculations.