Proceedings of Perovskite Thin Film Photovoltaics (ABXPV16)
Publication date: 14th December 2015
In 2015, perovskite based solar cells reached a conversion efficiency of more than 20 %[1]. However, the low chemical stability of lead-based perovskite and the toxicity of lead could prevent this type of solar cells from commercialization. Lead-free devices such as the related tin-based compound (CH3NH3SnI3) exhibit moderate power conversion efficiencies of up to 6% but with an even more pronounced instability under ambient atmosphere due to the rapid oxidation of Sn2+ to Sn4+ [2],[3].
In an attempt to replace lead by a non-toxic element, solution processed bismuth-based zero-dimensional perovskite compounds A3Bi2X9 (A = CH3NH3+, CH(NH2)2+, Cs+; X = I-, Br-) were fabricated and their opto-electronic properties were investigated. The Bi perovskite thin films were prepared similar to the Pb perovskite absorbers by spin-coating a precursor solution of MAI and BiI3 (or related compounds) followed by a low temperature thermal treatment.
The hexagonal crystal structure of the (CH3NH3)3Bi2I9 compound was determined using x-ray diffraction measurements (XRD). Scanning electron microscopy (SEM) images showed hexagonal shaped micro-crystals but an incomplete coverage of the substrate. In order to get a closed and homogenous Bi perovskite layer solvent engineering techniques were used.The absorption spectra of the Bi perovskites were determined by photo thermal deflection spectroscopy (PDS) and UV-VIS measurements. All of the compounds showed a relatively high band gap >2.0 eV and strong exciton peaks where as weak photoluminescence (PL) emission is observed.
In addition we present as proof of concept planar hetero junction devices with Bi perovskite absorber layers. Up to now the best device provided a Voc > 0.6 V but suffered from low short circuit current density Jsc < 1 mA/cm² due to the high absorption onset, the strong exciton binding energy and the large energy offset between the Bi perovskite and the HTM (PEDOT:PSS) in the planar device.
[1]Yang, Woon Seok, et al., Science 2015, 348,1234 – 1237
[2]N.K. Noel et al., Energy & Environmental Science 2015, 7 3061-3068
[3]F. Hao et al., Nature Photonics, 2015 8 489-494
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