Perovskite solar cells (PSCs) are promising low-cost photovoltaic technologies with high solar-to-electric power conversion efficiency (PCE). Inverted-structured devices typically employing a perovskite–fullerene (such as, [6,6]–phenyl–C61–butyric acid methyl ester (PCBM)) planar heterojunction structure owe many advantages like simple device fabrication, high stability and small hysteresis.[1] However, in spite of numerous studies, the PCEs of inverted-PSCs are still lower than those of normal-structured PSCs, especially on the reporting of independently certified results.[2] Further device structure designs and materials development are essential for improving the performance of inverted-PSCs. The inverted-PSCs are featured with the photoelectron collection layer behind the light absorbing material of perovskite. As the lead halide hybrid perovskites have very high light absorption coefficients, a large proportion of photoelectrons are generated near the front electrode from which the light is incident. These photoelectrons have to travel a relatively long distance through the perovskite layer to be collected in the inverted planar heterojunction PSCs (PHJ). Consequently, the electron transporting characters of the semiconducting perovskite layer as well as the structure of electron extraction heterojunction play important roles on affecting the photovoltaic performance of inverted-PSCs.

  Here, we report efficient inverted-structured PSCs with a perovskite-fullerene graded heterojunction (GHJ), which is featured with a gradient distribution of electron accepting material in the light absorption perovskite layer.[3] This structure is found capable of enhancing the PCE of inverted structured PSCs as it improves the photoelectron collection and reduces recombination loss, especially for the formamidinium (FA) cation based perovskites that have superior spectra response and thermal stability. The conformal fullerene coating on perovskite during the GHJ deposition facilitates a full coverage of fullerene with reduced layer thickness, thus minimize the resistive loss in larger size devices. This GHJ structure enable the fabrication of centimeter-scale PSCs showing high efficiency with small hysteresis and good stability. A remarkable PCE of 18.21% with FF as high as 0.784 was certified by a public test center (Calibration, Standards and Measurement Team at the Research Center for Photovoltaics, AIST, Japan) based on a cell with an aperture area of 1.022 cm². To our best knowledge, this is the best results of inverted-PSCs to date.

Reference:

[1] Chen, Wei, et al., Science 350.6263 (2015): 944-948.

[2] Bi, Dongqin, et al., Science advances 2.1 (2016): e1501170.

[3] Wu, Yongzhen, et al., Nature Energy 1 (2016): 16148.