Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
DOI: https://doi.org/10.29363/nanoge.hopv.2020.128
Publication date: 6th February 2020
The fast rise of organic-inorganic hybrid perovskites is based on their excellent optoelectronic material properties, combining long charge carrier lifetimes, very low non-radiative recombination rates and a high absorption coefficient. In addition, the material class of mixed-halide organic-inorganic hybrid perovskites exhibits a tunable bandgap from 1.2 - 3.1 eV, simply by adjusting the ratio of the halide precursors. This property makes these materials excellent candidates for low-cost multi-junction photovoltaics (PV). In particular, wide-bandgap perovskites (WBP) with a bandgap ranging between EG ~ 1.7 - 1.8 eV are attractive top-cell materials to improve the power conversion efficiency (PCE) of single-junction silicon or thin-film solar cells in multi-junction PV. However, obtaining high open-circuit voltage (VOC), which is a mandatory requirement to achieve sufficient PCE, is still a key challenge for WBP solar cells.
In this contribution, we report on wide-bandgap perovskite solar cells with a stable power output efficiency of up to 19.4% and a remarkable VOC of up to 1.31 V. The WBP solar cells in focus of this study employ a double-cation perovskite absorber layer based on FA and Cs in the composition FA0.83Cs0.17Pb(I0.6Br0.4)3 with a bandgap of 1.72 eV. By solution processing ammonium derivatives on top of the perovskite absorber layer, an interlayer is introduced between the bulk 3D perovskite absorber layer and the hole transport layer (spiro-OMeTAD). As we will show by means of XRD studies, this interlayer is composed of 2D Ruddlesden-Popper perovskites in intermediate phases of n = 2, resulting in a thin 2D/3D perovskite heterostructure at the hole extracting side of the solar cell. The devices with 2D/3D heterostructure achieve an enhancement in VOC of up to 80 mV, leading to a stable record VOC for WBPs (EG ~ 1.72 eV) of up to 1.31 eV. This very remarkable VOC reaches > 90% of the Shockley Queisser (SQ) limit and corresponds to one of the highest ratios of VOC-to-EG (0.76) reported for any perovskite solar cell with decent PCE. Since the relation of VOC to the SQ limit as well as the ratio VOC-to-EG serve as key figure of merits for the quality of PV absorber materials, our results highlight the very high quality of the presented 2D/3D perovskite heterostructure. Along with an improvement of the VOC, also the fill factor (FF) increased up to 78% without losing in short-circuit current density (JSC). The devices with 2D/3D perovskite heterostructure show negligible hysteresis and demonstrate very high PCE of 19.8% with corresponding stable power output efficiency of 19.4% under continuous illumination of one sun irradiation intensity and maximum power point tracking. The stable performance and high reproducibility of the perovskite solar cells employing the 2D/3D perovskite heterostructure was proven further by providing data on the statistics of > 50 devices.
Next to the material characteristics also the photophysics of the devices and 2D/3D heterostructure are studied, in order to explain the causes for the performance increase and strong enhancements in VOC. They show that the 2D/3D perovskite heterostructure at the hole extracting side of the solar cell reduces non-radiative recombination which results in a high VOC. Given the very limited conductivity of disordered 2D perovskite layers, only for an optimized 2D Ruddlesden-Popper perovskite interlayer thickness the passivation of the 2D/3D perovskite heterostructure appears in combination with fast hole extraction.
The authors are grateful to the great spirit of the taskforce perovskite photovoltaics at KIT and the scientific discussions with Ihteaz M. Hossain, Amjad Farooq, Fabian Schackmar, Dirk Hauschild, and Helge Eggers. The discussion on the interpretation of the XRD data of 2D Ruddlesden-Popper perovskites with Amir-Abbas Haghighirad, Marc C. Weidman, and Christoph Sürgers were very enlightening. The authors thank Emilio Gutiérrez-Partida for EQE measurements and Simon Geisert for AFM measurements. The financial support by the Federal Ministry for Research and Education through the projects PRINTPERO and PeroSol, the Initiating and Networking funding of the Helmholtz Association (HYIG of Dr. U.W. Paetzold; Recruitment Initiative of Prof. B.S. Richards; the Helmholtz Energy Materials Foundry (HEMF); PEROSEED; the European Union’s Horizon2020 Program (ACTPHAST); and the Science and Technology of Nanostructures research program) as well as the Karlsruhe School of Optics & Photonics (KSOP) is gratefully acknowledged.
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