Perovskite solar cells have been intensively examined with a rapid increase in power conversion efficiency (PCE) up to 26.7% [1]. To overcome Shockley-Queisser limit of a single-junction solar cell, tandem-type solar cells are developed to utilize a wide range of the solar spectrum. One of the most interesting tandem solar cells is the perovskite/silicon (Si) tandem solar cell.  Presently, there are two principal intriguing tandem structures: two-terminal (2T) and four-terminal (4T) configurations. The 2T connection requires current matching, whereas the 4T configuration does not. The top cell and bottom cell in a 4T tandem can be developed separately. The 4T tandem is thus interesting. Currently, the record PCE of rigid 4T perovskite/Si tandem solar cell is 30.9% under 1 SUN [2]. It is additionally known that lightweight flexible photovoltaic (PV) modules are intriguing since they are readily applied on rooftops and also important for car automotive applications. In this work, flexible and semi-transparent perovskite solar cells are developed as the top cell of 4T perovskite/Si tandem solar cell, which should have not only high PCE but also high transmittance in the long wavelength region (infrared) from 800 nm to 1100 nm to increase J_SC of the Si bottom cell, resulting in a high PCE (>30%) of 4T tandem solar cell.

              The flexible, semi-transparent, and inverted perovskite solar cells with an area of 0.1 cm² is fabricated, where its structure is composed of flexible substrate/ITO/thin transparent conductive oxide (TCO)/hole transport layer (HTL)/passivation layer/perovskite absorber/electron transport layer/buffer layer/TCO. Here, hole-collecting monolayer material was used as HTL. In this work, flexible substrate was used for a flexible and semi-transparent perovskite solar cell. However, the sheet resistance of flexible substrate is too high to obtain high fill factor (FF). Consequently, thin TCO was proposed and added on flexible substrate to act as the layer adjusting the sheet resistance of the substrate to increase FF.

              In the result, the sheet resistance was decreased by the insertion of a thin TCO on the flexible substrate, thus increasing FF of the device. High transmittance of the flexible and semi-transparent perovskite solar cells with a thin TCO is still realized with average transmittance in a wavelength region from 800 nm to 1100 nm (Averaged T_{800-1100 nm}) of 80.9% in Figure (a). The high PCE of 22.4% (V_OC of 1.18 V, J_SC of 23.1 mA/cm², and FF of 0.82) for the flexible and semi-transparent perovskite solar cell is demonstrated with high Averaged T_{800-1100 nm} of 80.9% in the infrared, which is suitable as top cell of 4T tandem. Under a semi-transparent perovskite cell filter, which has all the layers similar to the active semi-transparent perovskite cell, the PCE of the Si HIT bottom cell under the perovskite filter is 8.0%. Ultimately, a PCE of 30.4% for the 4T perovskite/Si tandem solar cell is achieved, as shown in Figure (b) and Table. This is the first report to use the flexible and semi-transparent perovskite top cell and realize the high PCE of over 30% for 4T perovskite/Si tandem solar cell, promising for lightweight flexible PV modules.