The FF is defined as the ratio of the maximum power (P max ) divided by the product of V OC and J SCThe second important parameter of ST-OSCs is transparency. The transparency or transmittance of ST-OSCs is usually characterized by measuring the average visible transmittance (AVT) of the device in the visible region (370-740 nm) with UV-vis spectrophotometer. The requirement of AVT is up to real applications, for example, an AVT of 25% is a basic requirement for the window application. [47] Color is the third parameter of ST-OSCs, which is determined by the photoactive layers and electrodes. CIE 1931 xy chromaticity diagram [53] can be used to characterize the color properties of ST-OSCs. The color coordinate (x,y) of ST-OSCs is calculated from the corresponding transmitted light. The color rendering property is the fourth parameter of ST-OSCs. Generally, the color rendering property is determined by color rendering index (CRI). CRI is an indicator of neutral color degree, which can be calculated from the transmitted light. [54] To realize semitransparency of OSCs, both electrodes must be transparent, allowing not only the light to get in, but also the visible light to partially pass through. A number of transparent electrodes were reported, such as indium tin oxide (ITO), thin metals (Ag or Au), [55][56][57][58][59] poly(ethylenedioxythiophene):poly(st yrene-sulfonate) (PEDOT:PSS), [60][61][62] Ag nanowires, [63][64][65][66][67] and graphene. [68,69] Because there are a couple of reviews focused on the transparent electrodes, [47,52] we will not discuss transparent electrodes in this Research News.Since there is a compromise between the PCE and the AVT of ST-OSCs, in order to achieve high PCE and high AVT simultaneously, an ideal active layer should have strong nearinfrared (NIR) absorption but weak visible absorption. This kind of active layer can sufficiently utilize NIR solar irradiation to achieve high PCE, while maintain high transparency of visible light. Very recently, ST-OSCs based on narrow-band-gap polymer donors and narrow-band-gap nonfullerene acceptors exhibited PCEs up to >10%. [70] However, ST-OSCs were mainly based on blends of fullerene acceptors and poly(3-hexylthiophene) (P3HT) and exhibited low PCEs (<3%) with lower J SC at the early stage. [71,72] In order to extend absorption of the active layer and enhance the J SC of OSCs, a number of donors with medium and low band gaps have been synthesized (Figure 1), and their absorption edges extend from 600 nm (P3HT) to NIR region, [73][74][75][76][77][78][79][80][81][82][83] for instance, PBDTTT-C-T, [84] PTB7, [85][86][87] Semitransparent organic solar cells (ST-OSCs) have appealing features, such as flexibility, transparency, and color in addition to generating clean energy, and therefore show potential applications in building integrated photovoltaics and photovoltaic vehicles. Concerted efforts in materials synthesis (particularly low-band-gap polymer donors and nonfullerene acceptors) and device optimization (particularly incorporating transpa...