Silicon solar cells are approaching their theoretical efficiency limit of 29%. This limitation can be exceeded with advanced device architectures, where two or more solar cells are stacked to improve the harvesting of solar energy. In this work, we devise a tandem device with a perovskite layer conformally coated on a silicon bottom cell featuring micrometric pyramids—the industry standard—to improve its photocurrent. Using an additive in the processing sequence, we regulate the perovskite crystallization process and alleviate recombination losses occurring at the perovskite top surface interfacing the electron-selective contact [buckminsterfullerene (C 60 )]. We demonstrate a device with an active area of 1.17 square centimeters, reaching a certified power conversion efficiency of 31.25%.

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Perovskite-Si tandem solar cells are the most prominent contenders to succeed commercial single-junction Si cells that dominate the market today. Yet, to justify the added cost of inserting a perovskite cell on top of Si, the tandem devices should first exhibit sufficiently high power conversion efficiencies (PCEs). Here, we present two key developments with a synergetic effect that boost the PCEs of our tandem devices with front-side flat Si wafers - the use of 2,3,4,5,6-pentafluorobenzylphosphonic acid (pFBPA) in the perovskite precursor ink that suppresses non-radiative recombination near the perovskite/C60 interface, and the use of SiO2 nanoparticles under the perovskite film that suppresses the enhanced number of pinholes and shunts introduced by pFBPA, while also allowing the use of [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid as a hole transport layer. Integrating a perovskite cell featuring these developments with a Si cell, reproducible PCEs of 30±1% and a certified maximum of 30.9% are achieved for an active area of 1cm2.

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Germanium (Ge) is a promising material for the development of a light source compatible with the silicon microfabrication technology, even though it is an indirect-bandgap material in its bulk form. Among various techniques suggested to boost the light emission efficiency of Ge, the strain induction is capable of providing the wavelength tunability if the strain is applied via an external force. Here, we introduce a method to control the amount of the axial strain, and therefore the emission wavelength, on a suspended Ge nanobeam by an applied voltage. We demonstrate, based on mechanical and electrical simulations, that axial strains over 4% can be achieved without experiencing any mechanical and/or electrical failure. We also show that the non-uniform strain distribution on the Ge nanobeam as a result of the applied voltage enhances light emission over 6 folds as compared to a Ge nanobeam with a uniform strain distribution. We anticipate that electrostatic actuation of Ge nanobeams provides a suitable platform for the realization of the on-chip tunable-wavelength infrared light sources that can be monolithically integrated on Si chips.

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