Improved Reproducibility of PbS Colloidal Quantum Dots Solar Cells Using Atomic Layer–Deposited TiO2

Thanks to their broadly tunable bandgap and strong absorption, colloidal lead chalcogenide quantum dots (QDs) are highly appealing as solution-processable active layers for third-generation solar cells. However, the modest reproducibility of this kind of solar cell is a pertinent issue, which inhibits the exploitation of this material class in optoelectronics. This issue is not necessarily imputable to the active layer but may originate from different constituents of the device structure. Herein, the deposition of TiO2 electron transport layer is focused on. Atomic layer deposition (ALD) greatly improves the reproducibility of PbS QD solar cells compared with the previously optimized sol–gel (SG) approach. Power conversion efficiency (PCE) of the solar cells using atomic layer–deposited TiO2 lies in the range between 5.5% and 7.2%, whereas solar cells with SG TiO2 have PCE ranging from 0.5% to 6.9% with a large portion of short-circuited devices. Investigations of TiO2 layers by atomic force microscopy and scanning electron microscopy reveal that these films have very different surface morphologies. Whereas the TiO2 films prepared by SG synthesis and deposited by spin coating are very smooth, TiO2 films made by ALD repeat the surface texture of the fluorine-doped tin oxide (FTO) substrate underneath.