Solar RRL: Efficient Perovskite Solar Cells Based on Green Solvent-Processed Undoped Spiro-OMeTAD

• In recent years, perovskite solar cells have received widespread attention due to their low cost and high efficiency, and have aroused great research interest. In this device structure, Spiro-OMeTAD is the most classic and commonly used organic small molecule hole transport material. According to reports, the highest efficiency of devices based on Spiro-OMeTAD is 19-22%. The upright device structure, or n-i-p structure, is currently the most studied type of perovskite cell, where the perovskite is located between the upper hole transport layer (HTL) and the lower electron transport layer (ETL). However, Spiro-OMeTAD can only have excellent hole transport performance under the premise of adding additives, and common additives such as Li-TFSI, tBP, FK209 are very easy to absorb moisture, which will cause the decomposition of perovskite materials and reduce the performance of devices. stability. In addition, it has also been reported that these additives may form recombination centers and reduce the open circuit voltage of the device. When non-doped Spiro-OMeTAD is used as a hole transport material, the device efficiency is generally not high, and the literature reports are generally around 10% (up to 13.2%).
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• Zhu Linna and Wu Fei, associate professors of the School of Materials and Energy of Southwestern University, and Yan He, professor of the Hong Kong University of Science and Technology, and Dr. Jiang Kui, tried to prepare perovskite batteries using undoped Spiro-OMeTAD as a hole transport material. The device structure is ITO/C60/perovskite (MAPbI3-xClx)/dopant-free Spiro/MoO3/Ag. C60 is used as the electron transport material, and non-doped Spiro-OMeTAD is used as the hole transport material. Spiro-OMeTAD film (Spiro-CB) was prepared with conventional chlorobenzene solvent, and Spiro-OMeTAD film (Spiro-THF) was prepared with halogen-free low boiling point tetrahydrofuran (THF) as solvent by dynamic spin coating. Through grazing incidence wide-angle X-ray diffraction (GIWAX) test, the crystallinity of Spiro-THF film prepared by dynamic spin-coating with THF as solvent is improved compared with that of Spiro-CB film prepared by conventional chlorobenzene solvent. The surface roughness of the films prepared under different conditions was characterized by atomic force microscopy. Spiro-THF and Spiro-CB films have lower roughness, which is conducive to charge transfer at the interface. The hole mobility of Spiro thin films prepared under different conditions was measured by SCLC method. It was found that the hole mobility of Spiro-THF thin films was significantly higher than that of Spiro-CB thin films. The steady-state and transient fluorescence spectra show that Spiro-THF film has better hole extraction ability. The perovskite cell device made of Spiro-THF as the undoped hole transmission material has a photoelectric conversion efficiency of up to 17%, an open-circuit voltage of 1.023V, a short-circuit current density of 21.29 mA cm-2, and a filling factor of 77.78%. This is also the highest efficiency achieved by non-doped Spiro-OMeTAD so far, and Spiro is treated with halogen-free green solvent. The device efficiency with Spiro-CB as HTL is up to 15.27%. In addition, in the forward device structure with TiO2 as the electron transport layer, the hysteresis is often obvious. In this work, the hysteresis of the forward device is very small. Since the hole transport layer does not need to add additives that are easy to absorb moisture, the device stability is also improved. Therefore, this study provides a simple, efficient, cheap and environmentally friendly preparation method for orthorhombic perovskite batteries.

  
  

  Relevant results were published on Solar RRL (DOI: 10.1002/solr. 201900061).