Kiarash Vakhshouri

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.

Effect of Crystallization Kinetics on Microstructure and Charge Transport of Polythiophenes

We have examined the effects of crystallization kinetics of poly(3-hexylthiophene) and poly[2,5-bis(3-hexadecylthiophen-2-yl)thieno(3,2-b)thiophene] on microstructure and charge transport. Rapid crystallization increases the density of tie molecules in polythiophenes. As a consequence, ordered regions are better connected resulting in higher charge carrier mobilities. Our results suggest that controlling the crystallization kinetics might be an important factor for maximizing the charge mobility in semicrystalline polythiophene thin films.

Tuning the Dielectric Properties of Organic Semiconductors via Salt Doping

Enhancing the dielectric permittivity of organic semiconductors may open new opportunities to control charge generation and recombination dynamics in organic solar cells. The potential to tune the dielectric permittivity of organic semiconductors by doping them with redox inactive salts was explored using a combination of organic synthesis, electrical characterization, and time-resolved infrared spectroscopy. The addition of the salt, LiTFSI (lithium bis(trifluoro-methyl-sulfonyl)imide), to a conjugated polymer specifically designed to incorporate ions into its bulk phase increased the density of holes and enhanced the static dielectric permittivity of the polymer blend by more than an order of magnitude. The frequency and phase dependence of the real dielectric function demonstrates that the increase in dielectric permittivity resulted from dipolar motion of bound ion pairs or clusters of ions. Interestingly, the increases in the hole density and dielectric permittivity were associated with enhancement of the hole mobility by 2 orders of magnitude relative to the undoped polymer. The charge recombination lifetime also increased by an order of magnitude in the blend with a fullerene electron acceptor when ions were added to the polymer. The findings indicate that ion doping enables organic semiconductors with large increases in low frequency dielectric permittivity and that these changes result in improved charge transport and suppressed charge recombination on the microsecond time scale.

Ultrathin body poly(3-hexylthiophene) transistors with improved short-channel performance.

The microstructure and charge transport properties of binary blends of regioregular (rr) and regiorandom (RRa) poly(3-hexylthiophene) (P3HT) are investigated. X-ray diffraction of the blended films is consistent with a vertically separated structure, with rr-P3HT preferentially crystallizing at the semiconductor/dielectric interface. Thin film transistors made with these blended films preserve high field effect mobility with rr-P3HTcontent as low as 5.6%. In these dilute blends, we estimate that the thickness of rr-P3HT in the channel is only a few nanometers. Significantly, as a result of such an ultrathin active layer at the interface, short channel effects due to bulk currents are eliminated, suggesting a new route to fabricate high-performance, short-channel, and reliable organic electronic devices.

Controlling crystallization to improve charge mobilities in transistors based on 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene

Long-range order at multiple length scales in small molecule semiconductors is critical to achieve effective electrical charge transport. As a consequence, processing strategies are often important for the fabrication of high-performance devices, such as thin-film transistors. We demonstrate that melting followed by quenching at a fixed temperature can obviate prior processing, control the crystallization process, and lead to enhanced charge mobilities in thin-film transistors based on 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene active layers. Melting followed by quenching to 80 °C yields films with higher degrees of orientational order, and therefore charge mobilities in devices that are higher by a factor of five over films annealed at the same temperature directly after film casting.

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.