Derek R. Kozub

Direct measurements of exciton diffusion length limitations on organic solar cell performance

Through a combination of X-ray scattering and energy-filtered electron microscopy, we have quantitatively examined the relationship between the mesostructure of the photoactive layer and device performance in PBTTT/PC(71)BM solar cells. We can predict device performance from X-ray structural data through a simple morphological model which includes the exciton diffusion length.

Elemental mapping of interfacial layers at the cathode of organic solar cells.

One of the limitations in understanding the performance of organic solar cells has been the unclear picture of morphology and interfacial layers developed at the active layer/cathode interface. Here, by utilizing the shadow-Focused Ion Beam technique to enable energy-filtered transmission electron microscopy imaging in conjunction with X-ray photoelectron spectroscopy (XPS) experiments, we examine the cross-section of polythiophene/fullerene solar cells to characterize interfacial layers near the semiconductor-cathode interface. Elemental mapping reveals that localization of fullerene to the anode interface leads to low fill factors and S-shaped current-voltage characteristics. Furthermore, the combination of elemental mapping and XPS depth profiles of devices demonstrate oxidation of the aluminum cathode at the active layer interface for devices without S-shaped characteristics and fill factors of 0.6. The presence of a thin dielectric at the semiconductor-cathode interface could minimize electronic barriers for charge extraction by preventing interfacial charge reorganization and band-bending.

Organic solar cells: to mix or not to mix

Polymer-based organic photovoltaics (OPV) are a promising option for light-weight, cost-effective solar cells, especially if they can be processed in solution.1 In photovoltaic devices, electron transfer occurs predominantly at the interface between two materials that differ in their electron affinities. However, exciton diffusion lengths (the distance that an excited state can travel before decaying back to the ground state) of the materials used in the photoactive layer of these devices is limited to approximately 5-10nm.2, 3 Electron donor and acceptor molecules must be in close proximity to ensure excitation results in a photocurrent before recombination of the electron-hole pairs, and so this imposes considerable restrictions on device morphology. Research has focused on two broad architectures for creating efficient devices: a donor-acceptor bilayer, typically built through vacuum deposition of the components, and a structure in which the two materials are highly intercalated, referred to as a bulk heterojunction (BHJ). The mixed nature of the photoactive layer in BHJs greatly increases the interface between the high-affinity and low-affinity regions solving the problem of short diffusion lengths in these materials. They also offer the advantage of being able to be processed in solution in a single step. In fact, the key enabling characteristic of organic semiconductor mixtures targeted for use in solar cells is this ability to self-assemble into nanostructured morphologies. In this manner, photogenerated excitons can find donor-acceptor interfaces that promote dissociation prior to exciton decay. The morphology of the BHJ active layer is critical for device performance. A large interface between the two components must exist and the domains of donor and acceptor regions must be approximately 10nm. In addition, each domain must remain a continuous structure since electrons travel within the acceptor phase while holes travel through the donor phase. Currently, Figure 1. Bright field (BF), sulfur elemental (S), and carbon elemental (C) maps obtained from energy-filtered electron microscopy of poly(3hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM) mixtures. The light regions in the elemental maps correspond to the presence of the element of interest. The cloudy fibrous image under BF is revealed to be a matrix of crystalline P3HT (which contains sulfur) running through an amorphous layer of mixed P3HT and PCBM. The scale bar is 200nm.