No abstract

Chemical composition and film quality are two key figures of merit for large-area high-efficiency perovskite solar cells. To date, all studies on mixed perovskites have used solution-processing, which results in imperfect surface coverage and pin-holes generated during solvent evaporation, execrably influencing the stability and efficiency of perovskite solar cells. Herein, we report our development using a vacuum co-evaporation deposition method to fabricate pin-hole-free cesium (Cs)-substituted perovskite films with complete surface coverage. Apart from the simplified procedure, the present method also promises tunable band gap, reduced trap-state density and longer carrier lifetime, leading to solar cell efficiency as high as 20.13%, which is among the highest reported for planar perovskite solar cells. The splendid performance is attributed to superior merits of the Cs-substituted perovskite film including tunable band gap, reduced trap-state density and longer carrier lifetime. Moreover, the Cs-substituted perovskite device without encapsulation exhibits significantly higher stability in ambient air compared with the single-component counterpart. When the Cs-substituted perovskite solar cells are stored in dark for one year, the PCE remains at 19.25%, degrading only 4.37% of the initial efficiency. The excellent stability originates from reduced lattice constant and relaxed strain in perovskite lattice by incorporating Cs cations into the crystal lattice, as demonstrated by the positive peak shifts and reduced peak width in X-ray diffraction analysis.

Two novel two-dimensional quinoxaline derivatives, DTPQx and DTDBPz, based copolymers, P2 and P3, were synthesized through Suzuki polycondensation reaction. Meanwhile, DPQx based polymer P1 was also designed for comparison. The UV-vis absorptions, thermal stability, energy levels, field-effect carrier mobilities and photovoltaic characteristics of the se three copolymers were systematically evaluated to understand the relationship between the polymer structure at the molecular level and the photovoltaic performances. Photovoltaic cells based on the two-dimensional quinoxaline-based polymers, P2 and P3, with a structure of ITO/PEDOT:PSS /Polymer:PC71BM /Ca/Al exhibited PCEs of over 3% compared to the PCE of less than 2% in the P1-based device.