Thin film solar cells on semitransparent substrates are attracting much attention due to new application scenarios including building-integrated photovoltaics (BIPV). Environmentally-benign element constituted and highly stable kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) thin film solar cells are ideal candidates for such applications. However, the efficiency of kesterite solar cells on semitransparent substrates is far behind that on opaque Mo-based substrates. Here, fabrication of CZTSSe solar cells on fluorine-doped tin oxide (FTO) substrates from molecular solution and how step-by-step absorber engineering improves device performance is reported. A power conversion efficiency of 7.02% is obtained when the absorber is fabricated on bare FTO, which is improved to 9.56% after adding a MoO 3 interfacial layer. Investigations show the enhancement originates from the transformation of MoO 3 to MoSe 2 during film selenization which initiates crystallization at the back contact and at the same time prevents oversize grains at the absorber surface. Na-doping and Ag alloying further facilitate grain growth and mitigate band tailing, resulting in a certified effective area efficiency of 11.43% with all device parameters comparable to that on an Mo-substrate. This is the first time highly efficient kesterite solar cells are demonstrated on transparent electrodes, which opens up new opportunities for these earth-abundant elements composed of thin film photovoltaics.
The vapor transport deposition of quasi-one-dimensional antimony selenosulfide (Sb 2 (S,Se) 3 ) has recently attracted increasing research interest for the inexpensive, high-throughput production of thin film photovoltaic devices. Further improvements in Sb 2 (S,Se) 3 solar cell performance urgently require the identification of processing strategies to control the orientation, however the growth mechanism of high quality absorbers is still not completely clear. Herein, a facile and general vapor transport deposition approach to precisely control the growth of large-grained dense Sb 2 (S,Se) 3 films with good crystallization and preferred orientation via the source vapor speed is utilized. It is found that defect activation energy rather than the defect concentration plays a decisive role in the Sb 2 (S,Se) 3 photovoltaic performance. Admittance spectroscopy analysis is used to obtain efficient Sb 2 (S,Se) 3 solar cells. By employing dual-source coordinations to optimize the absorber layer a power conversion efficiency of 8.17% is obtained which is the highest efficiency for Sb 2 (S,Se) 3 solar cells fabricated by vapor transport technology. This study suggests that there are other opportunities for gaining deeper a understanding of the defect physics and carrier recombination mechanisms in other highly oriented low-dimensional materials to achieve improved device performance.
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