OPV is a rapidly emerging PV technology with improving cell efficiency (currently ~11.5% and greater in tandem cells), encouraging initial lifetime (>5,000 hours un-encapsulated), and potential for roll-to-roll manufacturing processes. The building-integrated PV market may find OPV especially attractive because of the availability of absorbers in several different colors and the ability to make efficient transparent devices.

OPV's great strength lies in the diversity of organic materials that can be designed and synthesized for the absorber, acceptor, and interfaces. As part of NREL’s OPV research, we develop and apply new high-performance absorber materials for improved performance and lifetime, focusing on improving absorption of the solar spectrum, photo-voltage and stability to photo-oxidation.

Combinatorial generation of candidate materials and electronic structure prediction

Candidate materials are generated by combining elements from a library of organic multi-ring molecular units (building blocks) to produce complex molecules and polymers. The building blocks may be functionalized in different ways to tune the electronic properties of the resulting molecules. Code that can automatically generate molecular structures given appropriately annotated files describing building blocks is included in the STREAMM toolkit, available at http://streamm.nrel.gov.

Once a candidate molecule, or oligomer, has been generated, the geometry is locally optimized using density functional theory (DFT) and the excited states are computed using time-dependent DFT (TDDFT). All calculations were performed with the Gaussian09 electronic structure program, revisions B.01, C.01 and D.01.¹ A variety of exchange-correlation functionals and basis sets have been used but the most common combination is the B3LYP hybrid functional with a 6-31g(d) basis set.

To predict the properties of polymers, we have computed the electronic structure for monomers, dimers and (optionally) tetramers (n=1, 2, 4) and applied the extrapolation procedure described by Larsen.²

Functionalized building blocks are combined into materials according to different backbone motifs. Following convention, different building blocks are denoted as donors (D) or acceptors (A) according to whether they are electron rich or electron deficient, respectively. Backbone repeat motifs included here are the alternating structures shown at right: (D-A)n, (D-A-D)n, (D-D’)n, (A-A’)n. In all cases additional spacer (sp) molecules can be included, leading to repeat units such as: (D-sp-A-sp)n. We also have computed small molecule structures capped by terminal (T) building blocks based on a T-core-T architecture, where the core can have a variety of alternating donor/acceptor patterns, as shown schematically at right. The bottom-most figure at right shows examples of polymer structures in the database extrapolated from calculations on oligomers.