Highly Efficient AgBiS₂ Nanocrystal Solar Cells Enabled by Cation Disorder Engineering

Silver bismuth sulfide nanocrystals (AgBiS₂ NCs) are extremely promising absorber materials for low-cost solar cells, owing to their solution-processability, environmentally friendliness, earth abundance as well as high absorption coefficients.<br/>Previously, over 6% power conversion efficiency has been demonstrated in ultrathin (35 nm) AgBiS₂ NCs solar cells. However, even in such thin devices, incomplete charge extraction was observed, due to short diffusion length. Nevertheless, calculating from the absorption coefficient, high current density (&gt; 25 mA/cm²) can only be achieved with absorber thickness over 200 nm. The gap between short diffusion length and the large thickness of absorbing layer severely limits the device performance.<br/>Instead of directly increasing the diffusion length in AgBiS₂ NC films, we propose that by enhancing the film absorption coefficient, &gt; 25 mA/cm² can also be obtained with a thickness of only ~30nm. In order to do so, simple annealing process, suggested by theoretical simulations, was introduced to facilitate cation disorder homogenization in AgBiS₂ NCs and thus, enhanced optical transition moment matrix and absorption coefficient. An absorption coefficient that is higher than any other commonly used photovoltaic materials was obtained.<br/>Furthermore, the conventionally used hole transporting layer PTB7 was found possessing an unsatisfying morphology, leading to high leakage current, low shunt resistance and limited device performance. We further find replacing PTB7 with PTAA yields improved uniformity, better charge extraction and reduced interface recombination. Together with the ultrahigh absorption coefficients, a high <i>J</i>_sc of 27 mA/cm² and a record efficiency up to 9.17% (8.85% certified) were obtained with 30 nm AgBiS₂ NC film. The devices also showed superior shelf and photo-stability under ambient conditions. Our work not only establishes the extraordinary potential of solution-processable, RoHS-compliant, ultrathin AgBiS₂ NC solar cells, but also demonstrates the importance and power of cation disorder engineering in multinary systems.