Design of Transparent Anodes for Resonant Cavity Enhanced Light Harvesting in Organic Solar Cells

Organic solar cells offer an attractive approach to low-cost solar energy conversion, due to a combination of abundant materials and high throughput fabrication processes.[1] However, organic semiconductors suffer from short exciton diffusion lengths and low charge carrier mobility, which necessitates the use of thin photoactive films and intercalated networks of donor and acceptor molecules in a so-called bulk heterojunction.[2,3] On the other hand, thinner photoactive regions in organic solar cells cause a reduction in optical absorption. This leads to the well known tradeoff in organic solar cells between internal quantum efficiency (IQE) and absorption efficiency. Several approaches have been previously used to enhance absorption in organic solar cells,[4] including plasmonics,[5] photonic crystal approaches[6] and external coatings.[7] Moreover, in recent years significant research efforts have been directed towards replacing the tin-doped indium oxide (ITO) transparent electrode in optoelectronic devices, owing to its poor mechanical flexibility, the necessary performance enhancing thermal treatment unsuitable for low temperature substrates, as well as the increasing cost of indium.[8] Potential alternative transparent conductors include high conductivity polymers,[9] unpatterned metal films,[10] patterned metal grids,[11] random metal nanowire meshes,[12] graphene,[13] and random carbon nanotube meshes.[14] In addition, several groups have shown promising results for tri-layer dielectricmetal-dielectric electrodes.[15–18] Here, building on the latter approach, a tri-layer electrode is proposed composed of a thin film of silver (Ag) sandwiched between two layers of molybdenum trioxide (MoO3). The MoO3/Ag/MoO3 transparent electrode is ITO-free, compatible with low-temperature substrates, and capable of alleviating the absorption-IQE trade-off by creating a resonant optical cavity to coherently trap light in the photoactive absorber. The thin Ag film dominates the lateral conductive properties of the electrode and therefore provides a means to obtain a sheet resistance below 10 Ω per square. Silver is known to prefer 3D island growth and therefore the percolation threshold of Ag layers