Date2020-02-17/ Research Highlight
Keywords: Small, hole transport material, dopant-free, perovskite solar cells, electrical, electronic engineering
Recently, a visiting student Lai Xue form the Department of Electrical and Electronic Engineering published a paper with the title of ‘High-Performance Inverted Planar Perovskite Solar Cells Enhanced by Thickness Tuning of New Dopant-Free Hole Transporting Layer’ as the first author on Small(impact factor 10.856). This paper was also highlighted by Materials Views China.
Under the guidance of Prof Aung Ko Ko Kyaw from the department of Electrical and Electronic Engineering of SUSTech and Prof Li Gongqiang from Nanjing Tech University, Lai Xue developed a dopant-free hole transporting material (HTM) using a strategy of flexible core with tunable conformation (FCTC), based on the concept that the rotation of molecular structural units can affect the spatial distribution of molecular conformation and hence charge carrier recombination and mobility. The research team applied this strategyto design a new type of HTM (called DMZ) based on bifluorenylidene core unit and triphenylamine side armand employed in inverted planar perovskite solar cells (PSCs).The highest power conversion efficiency (PCE) of 18.61% (JSC = 22.62 mA/cm2, VOC = 1.02 V and FF = 81.05%) can be obtained by using the undoped DMZ ashole transporting layer (HTL). Compared with the corresponding device based on commercial PEDOT:PSS as HTL, the PCE of the DMZ-based device is increased by about 50% and there is almost no hysteresis in J-V curve. In addition, the PCE of DMZ-based device remains 90% after 556 hours in air with 50% humidity, showing a good stability.
Moreover, systematic studies demonstrated that the modulating the thickness of HTL can effectively enhance the morphology and crystallinity of the perovskite layer, leading to low series resistance and less defects in the crystal. With increasing thickness of the HTL from 3.5 nm to 30 nm (the corresponding DMZ solution concentration is 0.5m/ml to 6.0mg/ml), the surface contact angle of HTL increases from 21.1° to 84.0°, which directly affects the grain size and crystallinity of the perovskite layer. The defect density in the perovskite layer also changes greatly, which affected the VOC and FF of the device. The best performance of device with PCE as high as 18.61% was obtained when the thickness of the HLT was tuned to ~ 13 nm (2 mg / ml).
Figure 1. (a) the chemical structure of DMZ, (b) device structure, (c) J-V curves (forward and reverse scans) of the best PSCs prepared by PEDOT:PSS and DMZ HTL under AM1.5G illumination, showing negligible hysteresis. The trend in PCE (d) and Voc & FF (e) of the devices upon reducing the concentration (thickness) of DMZ HTL from 6 mg/mL to 0.5 mg/mL. The reduction in VOC at DMZ concentrations below 2 mg/mL counteract the improvements in the fill factor. The optimum performance is reached at a DMZ concentration of 2 mg/mL. (f) the PCE for unsealed devices based on PEDOT: PSS and DMZ HTL, which were kept in dark under average relative humidity of 48% for 556 h.
Therefore, the cost effective and facile material named DMZ offers an appealing alternative to PEDOT:PSS or PTAA for highly efficient and stable inverted planar PSCs, and the tuning the thickness of HTL proposes an effective way to maximize the performance of the corresponding devices.The research outcomes are significant for future development of new hole transport materials and optimization of perovskite solar cells.
Lai Xue joined in Professor Aung Ko Ko Kyaw’s group in 2017, the Department of Electrical and Electronic Engineering. Her research interests are focused on highly efficient and long-term stable perovskite solar cell by using new small organic materials.