Why Inverted Small Molecule Solar Cells Outperform Their Noninverted Counterparts

It is shown that the effect of substrate heating on the photo conversion efficiency in vacuum‐deposited small molecule organic solar cells is closely related to the improved free charge generation in ordered C₆₀ regions. The formation of these ordered regions strongly depends on the deposition sequence in the device and differs therefore between inverted and noninverted cells. Substrate‐induced local fullerene ordering is found in small molecule:C₆₀ bulk heterojunctions (BHJs) deposited on pristine C₆₀ at elevated temperatures. This does not occur for BHJs deposited under identical conditions on pristine donor molecule layers, despite similar degrees of phase separation in both cases. These findings point to a hitherto unidentified advantage of inverted over noninverted solar cells that manifests itself in a higher charge separation efficiency.