Publication date: 1st July 2014
Nanostructures and interfaces are keys to the new generation photovoltaics, such as perovskite solar cell. One example is the interface between perovskite and semiconductor oxide (eg. TiO2) in perovskite solar cells. By inserting an ultrathin graphene quantum dots (GQDs) layer in between, we have recently succeeded in improving the perovskite solar cell efficiency from 8.81% to 10.15%. By combining with transient absorption measurements, we have found that the improved cell efficiency can be attributed to the much faster electron extraction time with the GQDs (～100 ps) than without the GQDs (~300 ps). This highlights the GQDs as a super fast electron tunnel, which may also find use in other optoelectronic devices.
On the other hand, these perovskite solar cells have so far largely relied on small-molecule hole transport materials (HTMs) such as spiro-OMeTAD, which commonly suffer from high cost and low mobility. Herein, we demonstrate polyfluorene derived polymers, which contains fluorine and arylamine groups, can indeed outperform spiro-OMeTAD as efficient hole-conducting materials for perovskite solar cells, which achieved a 10.92 % power conversion efficiency. By Photoluminescence (PL) and impedance spectroscopy technique, we have uncovered the higher electrical conductivity and lower series resistance than spiro-OMeTAD, which naturally explain the significantly higher fill factor, photocurrent and open-circuit voltage of the TFB-derived cells than with spiro-MeOTAD. Our systematic results will expedite the design of new HTMs, especially the design of high efficiency and low cost HTMs, for hybrid solar cells.
ACKNOWLEDGMENTSThis work was supported by the UGC Research Travel Grant (RTG) and HK-RGC General Research Funds (GRF No. HKUST 606511 and 605710)
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