Nanoscale Structure of Solar Cells Based on Pure Conjugated Polymer Blends

This paper gives an overview of the status of photovoltaic devices based on blends of semiconducting polymers. The polymer blends form the bulk heterojunction in these photovoltaic devices. The fundamental mechanisms governing the performance of these devices are discussed as well as the specificities of these all-polymer solar cells. The morphology of the polymer blend layer is expected to influence the device performance of these bulk heterojunctions. An overview is presented of factors that influence the morphology of the active layer of polymer blend photovoltaic devices together with a summary of tools available to study the structure of this layer. An advanced electron microscopy technique is applied to study the morphology of polymer blends of MDMO-PPV and PCNEPV with differing molecular weights. It is shown that the molecular weight of the polymer influences the typical domain size from 200 nm down to less than 5 nm in these bulk heterojunction PV devices. No strong relation is observed between the typical length scale of the phase separated domains and the measured external quantum efficiency, indicating that the phases are intermixed.