| Organic photovoltaic devices (OPVs) have become a promising research field. OPVs have intrinsic advantages over conventional inorganic technologies: they can be produced from inexpensive source materials using high-throughput techniques on a variety of substrates, including glass and flexible plastics. However, organic semiconductors have radically different operation characteristics which present challenges to achieving high performance OPVs.;To increase the efficiency of OPVs, knowledge of fundamental operation principles is crucial. Here, the photocurrent behavior of OPVs with different heterojunction architectures was studied using synchronous photocurrent detection. It was revealed that photocurrent is always negative in planar and planar-mixed heterojunction devices as it is dominated by photocarrier diffusion. In mixed layer devices, however, the drift current dominates except at biases where the internal electric field is negligible. At these biases, the diffusion current dominates, exhibiting behavior that is correlated to the optical interference patterns within the device active layer.;Further, in an effort to increase OPV performance without redesigning the active layer, soft-lithographically stamped microlens arrays (MLAs) were developed and applied to a variety of devices. MLAs refract and reflect incident light, giving light a longer path length through the active layer compared to a device without a MLA; this increases absorption and photocurrent. The experimentally measured efficiency enhancements range from 10 to 60%, with the bulk of this value coming from increased photocurrent. Additionally, because the enhancement is dependent on the substrate/air interface and not the active layer, MLAs are applicable to all organic material systems.;Finally, novel architectures for bifunctional organic optoelectronic devices (BFDs), which can function as either an OPV or an organic light emitting device (OLED), were investigated. Because OPVs and OLEDs have inherently opposing operation principles, BFDs suffer from poor performance. A new architecture was developed to incorporate the phosphorescent emitter platinum octaethylporphine (PtOEP) into a rubrene/C60 bilayer BFD to make more efficient use of injected carriers. While the emission was localized to a PtOEP emitter layer by an electron permeable exciton blocking layer of N, N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPB), total performance was not improved. From these experiments, a new understanding of the material requirements for BFDs was obtained. |
