Rubidium (Rb) is explored as an alternative cation to use in a novel multication method with the formamidinium/methylammonium/cesium (Cs) system to obtain 1.73 eV bangap perovskite cells with negligible hysteresis and steady state efficiency as high as 17.4%. The study shows the beneficial effect of Rb in improving the crystallinity and suppressing defect migration in the perovskite material. The light stability of the cells examined under continuous illumination of 12 h is improved upon the addition of Cs and Rb. After several cycles of 12 h light–dark, the cell retains 90% of its initial efficiency. In parallel, sputtered transparent conducting oxide thin films are developed to be used as both rear and front transparent contacts on quartz substrate with less than 5% parasitic absorption of near infrared wavelengths. Using these developments, semi‐transparent perovskite cells are fabricated with steady state efficiency of up to 16.0% and excellent average transparency of ≈84% between 720 and 1100 nm. In a tandem configuration using a 23.9% silicon cell, 26.4% efficiency (10.4% from the silicon cell) in a mechanically stacked tandem configuration is demonstrated which is very close to the current record for a single junction silicon cell of 26.6%.

Thin TiO2 films are demonstrated to be an excellent electron-selective contact for crystalline silicon solar cells. An efficiency of 21.6% is achieved for crystalline silicon solar cells featuring a full-area TiO2 -based electron-selective contact.

The performance of state-of-the-art perovskite solar cells is currently limited by defectinduced recombination at interfaces between the perovskite and the electron and hole transport layers. These defects, most likely under-coordinated Pb and halide ions, must either be removed or passivated if cell efficiencies are to approach their theoretical limit. In this work, we introduce a universal double-side polymer passivation approach using ultrathin poly(methyl methacrylate) (PMMA) films. We demonstrate very high-efficiency (~20.8%) perovskite cells with some of the highest open circuit voltages (1.22 V) reported for the same 1.6 eV bandgap. Photoluminescence imaging and transient spectroscopic measurements confirm a significant reduction in non-radiative recombination in the passivated cells, consistent with the voltage increase. Analysis of the molecular interactions between Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff))

Reducing interface recombination boosts the Voc for perovskite solar cells.