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Semi-transparent (ST) tandem organic solar cells (OSCs) are typically exhibiting a trade-off between efficiency and average visible transmittance (AVT), which can be minimized by the appropriate selection of transparent contacts and absorbing materials in the sub-cells. Herein, we demonstrate efficient and highly transparent hetero-tandem OSCs constructed by stacking PBDT[2F]T:PC 71 BM and PCE10:PC 61 BM active layers as front-cell and back-cell, respectively. Optical and electrical simulations show that the PBDT[2F]T and PCE10-based sub-cells possess potential as ST and reflective tandem OSCs. With the aid of the simulations, we could fabricate reflective tandem OSCs with an efficiency of up to 8.3%. Furthermore, a ST tandem OSC, employing conductive silver nanowire top contact, achieved an AVT of 49.2%, efficiency of 5.3%, and high light utilization efficiency (LUE) of 2.61% under 1 sun, AM1.5G spectral illumination. This study demonstrates the importance of the sub-cells choices for developing ST tandem solar cells with a good balance of AVT and PCE, paving the way for solar windows of high performance. [Display omitted] • Optical and electrical simulations performed to aid reflective and semi-transparent tandem OSCs fabrication. • Fabrication of tandem based on PBDT[2F]T:PC 71 BM and PCE10:PC 61 BM blends with PCE higher than their single-cells (8.2%). • Semi-transparent tandem device with a very high AVT of nearly 50%, in agreement with optical simulation. • The semi-transparent tandem based on PBDT[2F]T:PC 71 BM and PCE10:PC 61 BM blends achieved high PCE of 5.3% and LUE of 2.61%. [ABSTRACT FROM AUTHOR]