Improved materials and device architectures for polymer solar cells
Prof. René Janssen, University of Technology Eindhoven
The recent significant increase in power conversion efficiency (PCE) of polymer-fullerene solar cells largely originates from the successful development of new electron donor polymers. The donor-acceptor (D-A) or push-pull design, where electron rich and electron deficient units alternate along the copolymer chain is commonly used to tune the HOMO and LUMO energy levels and the optical band gap of these polymers. While structure-property relations for energy levels are well established, these are less clear for the actual photovoltaic performance. Creating morphologies in which nanometer-sized, interconnected, semi-crystalline domains of both polymer and fullerene exist seems crucial for high photovoltaic performance. These semi-crystalline domains optimize the conjugation along the polymer backbone and allow delocalizing the carrier wave functions to assist efficient charge separation. High molecular weight and a tendency to crystallize are important in achieving such morphologies.
For a range of diketopyrrolopyrrole-based small band gap polymers it will shown how the molecular weight of semiconducting polymers and the nanomorphology are crucial parameters in obtaining high power conversion efficiencies in the range of 6-8% for single junctions, with optical band gaps down to 1.3 eV. When the new semiconductor materials are combined with a wide band gap material it is possible to make create tandem and multi-junction devices in tandem or triple layer configurations with efficiencies close to 9%. The favorable efficiency of the tandem cell is achieved by an almost perfect complementarity of the absorption spectra of the different absorber layers that reduce thermalization losses. Because of their high voltages, triple junction solar cells can be used for photo-electrochemical water splitting.
The last part of the lecture will address our first attempts to predict bulk heterojunction morphologies that are obtained when casting polymer/fullerene blends from solution. The crucial role of the solvent will be highlighted.