Interfacial modification for perovskite solar cells using a novel fused-ring electron acceptor

Abstract

Perovskite solar cells have recently attracted significant attention as the promising candidate of commercial photovoltaic devices with high power conversion efficiency, low cost and large-scale processing. Anatase-type TiO2 is often used as an electronic transport layer in a state-of-the-art perovskite solar cells due to its excellent optical transmittance, semiconductor characteristics, and chemical stability. However, its rough surface with a large number of surface defects leads to poor crystallization of the perovskite film and serious hysteresis effect. Recent studies have shown that suitable interfacial modification plays an important role in enhancing power conversion efficiency and long-term stability of the perovskite solar cells. Several fused-ring electron acceptors have been introduced into n-i-p perovskite solar cells as the interfacial modifications for the electronic transport layer, which dramatically improve the performance and the long-term stability of the devices. The fused-ring electron acceptors are widely used as the electron acceptors in organic solar cells, and their structures are based on an electron donating fused ring with strong electron withdrawing groups, which contributes to adjustable optical band gap and energy level structure, good electronic transfer capability, excellent thermal and photochemical stability. Therefore, a facile interface engineering method for all-solution-processed perovskite solar cells is reported in this paper. The fused-ring electron acceptor ITIC-Th containing alkylthiophene side chains is applied to modify TiO2 electronic transport layer to prepare the planar perovskite solar cells with excellent performance and stability. The main results and discussions are summarized as follows: (1) The ITIC-Th modified TiO2 film is much denser and smoother than the original TiO2 film. The images of scanning electron microscope show that the interfacial modification using ITIC-Th improves the morphology and increases the water contact angle of the electronic transport layer, which effectively enhances the quality and grain size of the perovskite crystals. (2) The characterizations of the electronic transport layer by UV-Vis transmission and photoluminescence spectroscopy indicate that ITIC-Th produces additional light absorption in the wavelength interval from 550 to 750 nm, so the transmittance of the modified electronic transport layer is slightly lower than that of ITO/TiO2 substrate; however, ITIC-Th has a strong fluorescence quenching effect, resulting in beneficial properties on the extraction, transportation and collection of photogenerated carriers for the modified electronic transport layer, which greatly reduces the surface recombination of charge carriers. (3) Planar solar cells with the structure of ITO/electronic transport layer/(FAPbI3)x(MAPbCl3)1-x/spiro-OMeTAD/Ag are successfully prepared. Consequently, the champion power conversion efficiency of the perovskite solar cells increases from 15.43% to 18.91% after the introduction of ITIC-Th. Moreover, the stability of the perovskite solar cells without encapsulation is investigated. As a result, the TiO2/ITIC-Th based device exhibits excellent stability with only 10% degradation after 1000 h in ambient humidity of 30% at room temperature, which is superior to that of the TiO2 based device. This work provides an important candidate in practical application for improving the performance of perovskite solar cells.