Temperature-Dependent Aggregation Donor Polymers Enable Highly Efficient Sequentially Processed Organic Photovoltaics Without the Need of Orthogonal Solvents

The conventional method to prepare bulk-heterojunction organic photovoltaics (OPVs) is a one-step method from the blend solution of donor and acceptor materials, known as blend-casting (BC). Recently, an alternative method was demonstrated to achieve high efficiencies (13%) comparable to state-of-the-art BC devices. This two-step-coating method, known as sequential processing, (SqP) involves sequential deposition of the donor and then the acceptor from two orthogonal solvents. However, the requirement of orthogonal solvents to process the donor and acceptor constrains the choice of materials and processing solvents. In this paper, an improved version of SqP method without the need for using orthogonal solvents is reported. The success is based on donor polymers with strong temperature-dependent aggregation properties whose solution can be processed at a high temperature, but the resulting film becomes completely insoluble at room temperature, which allows for the processing of overlying acceptors from a wide range of nonorthogonal solvents. With this approach, efficient SqP OPVs is demonstrated based on a range of donor/acceptor materials and processing solvents, and, in every single case, SqP OPVs can outperform their BC counterparts. The results broaden the solvent choices and open a much larger window to optimize the processing parameters of SqP method.