Microcavity‐Enhanced Semitransparent Electrodes for Oligothiophene Small‐Molecule Organic Solar Cells

Transparent electrodes (TEs) with high optical transparency (>85%) are usually considered to be prerequisite for realizing high‐efficiency solar cells. On the contrary, here an indium‐free TE is reported which is semitransparent with an average visible transparency of 66.7% only. The electrode consists of Ag 9 nm with TiO2 (30 nm) undercoat layer designed specifically for small molecule organic solar cell (SM‐OSC) based on methylated oligothiophene derivatives. The electrode also contains a polyethyleneimine (PEI) wetting layer which promotes the growth of continuous, ultrasmooth, and highly conductive Ag films on the TiO2 layer even at very low thicknesses. The developed TiO2/PEI/Ag (TPA) electrode exhibits much better mechanical properties with only <8% increase in sheet resistance after 200 bending cycles when a tensile strain of 2.1% is applied. The role of TiO2 undercoat layer has been investigated in detail and is shown to enhance the microcavity effect, leading to increased light coupling especially in the lower wavelength regime. With comparable photocurrent, and high fill‐factor values owing to much better electrical properties, the semitransparent TPA‐TE based SM‐OSC outperforms the state‐of‐art indium tin oxide (ITO) based reference device with photon conversion efficiency of 8.1% compared to 7.7% despite having lower transmittance (≈21%) relative to ITO.