A new technique has produced the highest performing inverted perovskite solar cell ever recorded. A team of researchers from Peking University and the Universities of Surrey, Oxford and Cambridge detail a new way to reduce an unwanted process called non-radiative recombination, where energy and efficiency is lost in perovskite solar cells.

The team created a technique called Solution-Process Secondary growth (SSG) which increased the voltage of inverted perovskite solar cells by 100 millivolts, reaching a high of 1.21 volts without compromising the quality of the solar cell or the electrical current flowing through a device. They tested the technique on a device which recorded a PCE of 20.9 percent, the highest certified PCE for inverted perovskite solar cells ever recorded.

Researchers are still working towards increasing efficiency and stability, prolonging lifetime and replacing toxic materials with safer ones. Researchers are also looking at the benefits of combining perovskites with other technologies, like silicon for tandem cells.

The hope is to bring perovskite solar power down to pennies per watt. The average module sold to a utility scale project in 2017 was about 35 cents, it will fall to 32 cents in 2018 and 31 in 2019.

Science – Enhanced photovoltage for inverted planar heterojunction perovskite solar cells by Deying Luo, Wenqiang Yang, Zhiping Wang, Aditya Sadhanala, Qin Hu1, Rui Su, Ravichandran Shivanna, Gustavo F. Trindade, John F. Watts, Zhaojian Xu, Tanghao Liu, Ke Chen1, Fengjun Ye, Pan Wu1, Lichen Zhao, Jiang Wu, Yongguang Tu, Yifei Zhang, Xiaoyu Yang, Wei Zhang, Richard H. Friend, Qihuang Gong, Henry J. Snaith, Rui Zhu
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Science 29 Jun 2018:
Vol. 360, Issue 6396, pp. 1442-1446
DOI: 10.1126/science.aap9282

Perovskite layers make the grade
Inverted planar perovskite solar cells offer opportunities for a simplified device structure compared with conventional mesoporous titanium oxide interlayers. However, their lower open-circuit voltages result in lower power conversion efficiencies. Using mixed-cation lead mixed-halide perovskite and a solution-processed secondary growth method, Luo et al. created a surface region in the perovskite film that inhibited nonradiative charge-carrier recombination. This kind of solar cell had comparable performance to that of conventional cells.

Abstract
The highest power conversion efficiencies (PCEs) reported for perovskite solar cells (PSCs) with inverted planar structures are still inferior to those of PSCs with regular structures, mainly because of lower open-circuit voltages (Voc). Here we report a strategy to reduce nonradiative recombination for the inverted devices, based on a simple solution-processed secondary growth technique. This approach produces a wider bandgap top layer and a more n-type perovskite film, which mitigates nonradiative recombination, leading to an increase in Voc by up to 100 millivolts. We achieved a high Voc of 1.21 volts without sacrificing photocurrent, corresponding to a voltage deficit of 0.41 volts at a bandgap of 1.62 electron volts. This improvement led to a stabilized power output approaching 21% at the maximum power point.