Small band-gap conjugated polymers based on monofluoro- and difluoro-substituted benzothiadiazole were developed. Highly efficient polymer solar cells (PCE as high as 5.40%) could be achieved for devices made from these polymers.

Organic photovoltaics (OPVs) have recently attracted extensive interest due to their potential for low cost, high throughput manufacturing to solve the scalability problem in solar energy. A typical OPV is based on the bulk-heterojunction (BHJ) device confi guration, which sandwiches a layer of polymer donor and fullerene acceptor blend between a transparent electrode (such as indium tin oxide (ITO)) and an opaque, refl ective metal electrode. These devices have shown high power conversion efficiencies (PCE) greater than 8%. [ 1 ] Semi-transparent organic photovoltaic cells (STOPVs), an extension of OPVs which utilize transparent conductive materials as both electodes, offer an extensive spectra of applications such as power-generating windows for buildings and automobiles, foldable solar curtains, and other aesthetic architectural uses. Furthermore, the capability of converting incident light to electric power shows the potential of STOPVs in the fi eld of effi cient energy conservation.When compared to vapor deposited small molecule STOPVs [ 2a ] or silicon-based STPVs, [ 2b ] the development of solution processed polymer-based STOPVs is lagging due to the absence of effi cient donor polymers and electrodes with proper transparency. STOPVs require more sophisticated materials and refi ned device engineering to simultaneously optimize both PCE and device transmittance, which are often contradictory to each other. In addition, for STOPVs to have practical solar window applications, good transparency perception (at least ≥ 20%) and color rendering properties are required under regular scene illumination. [ 2c ] To date, the state-of-the-art polymer-based STOPVs have either relatively low performance ( ≤ 3%) or unsatisfactory average visible transmittance (AVT) and color purity. [ 3 − 6 ] Therefore, there is a strong need to develop suitable polymer materials and novel device confi gurations to achieve improved PCE and transmittance for practical applications.In order for a device to have high PCE and transmittance in addition to proper color rendering index, it is critical to utilize appropriate thin BHJ layers that can effi ciently harvest the proper spectrum of light and electrodes with high transmittance and electrical conductivity. Until now, the frequently reported STOPVs still use a rather thick (at least 100 nm) poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC 61 BM) based BHJ layer. The dominant absorption of P3HT is located in the yellow-green wavelength region (500 ∼ 600 nm) where human eyes have the highest sensitivity, therefore leading to poor color perception and rendering properties. [ 3,4,7,8 ] The optimal materials for solar window applications should have ample light absorption outside of regions where human eyes are the most sensitive; at the same time allowing transmittance of visible light extensively. As a result, polymers with a band-gap smaller than P3HT have become a viable option for STOPVs. [ 4 ] Although these STOPVs showed improved transparency perce...

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