Highly doped fullerene C60 thin films as transparent stand alone top electrode for organic solar cells

•New concept for transparent top electrodes for OPV: highly n-doped fullerene C60.•C60 electrodes show high conductivity (5 S/cm) and high transmittance over 70% in red.•Potentially metal-free, low cost, fast, high-purity roll-to-roll production of OPV.•OPV employing C60 stand alone top electrodes achieve 1.5% efficiency.•Electrical simulations reveal efficient charge transport in C60 over a few 100 µm.

We report on transparent stand alone C60 top electrodes for small molecule based organic photovoltaic cells. All device layers (including bottom and top electrode) are thermally evaporated in vacuum, guaranteeing fast, reliable, high-purity and low-cost processing. Employing an organic compound as electrode avoids refractive index mismatch between contact and photo-active layer which results in an increased number of photons reaching the absorber layers. The conductivity of C60 is improved from 10−8 S/cm to remarkable 4.97 S/cm by optimized molecular n-type doping. Furthermore, it is shown that high doping concentrations change the absorption behavior of C60, leading to transmittance values over 70% for wavelengths larger than 600 nm with a weak dependence on layer thickness. Organic solar cells employing such an optimized n-doped C60 top electrode (thickness 200 nm, doping concentration 16 wt%) exhibit power conversion efficiencies of 1.53%. Electrical device simulations based on a discretized equivalent circuit model reveal an efficient lateral charge transport in n-doped C60 electrodes over a few 100 µm. In combination with a supporting grid electrode (usually employed for large area photovoltaic applications), n-doped C60 might replace ITO and fill the lack of transparent conductive top electrodes required for roll-to-roll processed semi-transparent organic devices or devices on opaque substrates.