Abstract:

Tin oxide (SnO₂) thin films are widely used as electron transport layers in planar perovskite solar cells (PSCs) and commonly prepared using solution-processed spin-coating. However, obtaining full coverage and pinhole-free surfaces for the spin-coated (SC) SnO₂ is challenging because the nanocrystals in the precursor solution can undergo aggregation, wherein the precursor solution may contain dust particles, and the desired film thickness is rater small. Since dense electron transport layer films without pinholes are crucial in suppressing the non-radiative recombination of charge carriers in PSCs, developing deposition methods to prepare high-quality SnO₂ films is important to improve the performance of planar PSCs. In this study, we investigated the application of electrophoresis (EP) in depositing compact and pinhole-free SnO₂ thin films. We conducted electrophoresis to deposit a dense nanocrystalline film on the surface of Indium tin oxide (ITO) and employed a spin-coating step to adjust the thickness of the film and remove the residual SnO₂ nanocrystalline precursor solution. This method was denoted as EP-SC. In the electrophoresis, the negatively charged SnO₂ nanocrystals, caused by a strong electric field, migrated towards the surface of the ITO anode and formed more compactly packed thin films than that of the spin-coated SnO₂. The atomic force microscopy (AFM) measurements demonstrated that the surface of EP-SC SnO₂ was more uniform than that of SC SnO₂ and there were no streaks and aggregated particles on the surface. This may have been due to the fact that the surface charge properties of the aggregated and dust particles in the precursor solution was different from that of the desired SnO₂ nanocrystals. Hence, electrophoresis can selectively deposit SnO₂ nanocrystals. Specifically, high-quality perovskites and electron transport layer interfaces can be achieved using this method. Both the electrochemical impedance spectroscopy (EIS) and dark J–V measurements showed that the PSCs using SnO₂ prepared by electrophoresis followed by spin-coating demonstrated remarkably suppressed non-radiative recombination. As a result, the photoelectric power conversion efficiency increased from 18.17% (based on SC) to 19.52% (based on EP-SC) due to the enhanced short-circuit current and open-circuit voltage. The hysteresis of the device was eliminated. More importantly, the long-term stability measurements demonstrated that our device can maintain up to 71% of the initial efficiency after 960 h of continuous operation at the maximum power point (MPP) under one sun illumination. Whereas the device based on spin-coated SnO₂ can maintain only up to 70% of the initial efficiency after working for 100 h. The results of this study can help in preparing electron transport layers to construct long-term stable planar PSCs, which are favorable for fabricating large-area PSCs and modules in future researches.

Key words: Electrophoresis, Tin oxide, Electron transport layer, Perovskite solar cell