Photovoltaic conversion is regarded as the ultimate solution to the mankind's ever growing demand for energy, yet the traditional silicon-based solar cells are expensive to produce, and the production itself involves intensive energy consumption. The emerging hybrid organic-inorganic solar cells based on perovskite CH3NH3PbI3, on the other hand, are not only inexpensive to process but also flexible, and thus are widely pursued as one of the most promising next generation photovoltaic conversion technologies.

Since its first report in 2009, the photovoltaic conversion efficiency of perovskite solar cells has increased spectacularly from 3.81% to 22.1% in just 7 years, and such unprecedented rise has fueled worldwide pursuit for its efficiency record.

Nevertheless, in the last two years, the pace in perovskite solar cell efficiency increase has slowed down considerably despite that it is still far away from the projected theoretical limit of 31%. Therefore, researchers are exploring new strategies to further enhance the perovskite solar cell performance.

The current perovskite solar cells are based on polycrystalline CH3NH3PbI3 films, and thus inevitably have many defects in grains and grain boundaries that affect the device performance. Efforts have been made to produce bulk CH3NH3PbI3 crystals that exhibit exceptional photovoltaic properties such as long diffusion length and lifetime of photo-generated charge carriers, though the integration of bulk crystal into perovskite solar cell device architecture proves rather challenging.

Now a team of Chinese and US scientists from Shenzhen Institute of Technology, Shijiazhuang Tiedao University, Peking University, Argonne National Laboratory, Institute of Metal Research, and University of Washington, led by Profs. Jiangyu Li and Jinjin Zhao, has successfully grown single crystalline film of CH3NH3PbI3 directly on electron-collecting FTO/TiO2 substrate, as shown in Fig. 1.

They took advantage of temperature gradient and capillary effect during the growth process, enabling them to produce high quality single crystalline film tightly integrated on FTO/TiO2. This proves critical, as FTO/TiO2 is the most widely used electron-collecting substrate for perovskite solar cells, making the subsequent device fabrication straightforward.

Indeed, the single crystalline CH3NH3PbI3 film shows excellent photovoltaic properties. Measured directly on FTO glass substrate with poor electron extraction, the time-resolved photoluminescence shows much longer carrier lifetime in single crystalline CH3NH3PbI3 film compared to polycrystalline one, as seen in Fig. 2(a).

When a TiO2 electron collecting layer is added on top of FTO glass, then the charge carrier lifetime drops substantially, thanks to the efficient electron extraction at the TiO2/perovskite interface. As a result, the device exhibits photovoltaic conversion efficiency of 8.78%, the highest reported to date for a single crystalline perovskite solar cells.

The team pointed out that the system has much room for further improvement, and with continuous optimization of materials and devices, they believe that the single crystalline perovskite solar cells will rival their polycrystalline counterparts in the foreseeable future.

Jinjin Zhao, Guoli Kong, Shulin Chen, Qian Li Boyuan Huang, Zhenghao Liu, Xingyuan San, Yujia Wang, Chen Wang, Yunce Zhen, Haidan Wen, Peng Gao, Jiangyu Li. Single crystalline CH3NH3PbI3 self-grown on FTO/TiO2 substrate for high efficiency perovskite solar cells. Science Bulletin, 2017.

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