Intrinsic Diffusion Length Research Articles

Investigation of bulk carrier diffusion dynamics using β-Mn2V2−xMoxO7 photoanodes in solar water splitting

The realized efficiencies of hydrogen generation through photoelectrochemical water splitting are substantially low for commercialization. Efforts to define the loss mechanisms prevalent therein are centered on the carriers generated within the space charge region. However, the research attention directed at carriers generated beyond the space charge layer has been insufficient. In this study, n-type β-Mn2V2O7 was applied as a model photoanode to investigate the influence of the diffusion current generated by bulk carriers on the total device performance. A combination of electrochemical approaches and time-resolved photoluminescence with carrier quenching layers were implemented to determine the dynamics of bulk carriers and elucidate their major loss mechanisms. The intrinsic diffusion length of carriers yields a considerable number of bulk carriers at the electrode/electrolyte interface, even in the absence of a surface catalyst that results in the generation of diffusion photocurrent. These findings highlight the importance of bulk carriers for achieving the theoretical maximum efficiency in the design of unassisted or low-bias overall water splitting devices.

Intrinsic measurements of exciton transport in photovoltaic cells

Organic photovoltaic cells are partiuclarly sensitive to exciton harvesting and are thus, a useful platform for the characterization of exciton diffusion. While device photocurrent spectroscopy can be used to extract the exciton diffusion length, this method is frequently limited by unknown interfacial recombination losses. We resolve this limitation and demonstrate a general, device-based photocurrent-ratio measurement to extract the intrinsic diffusion length. Since interfacial losses are not active layer specific, a ratio of the donor- and acceptor-material internal quantum efficiencies cancels this quantity. We further show that this measurement permits extraction of additional device-relevant information regarding exciton relaxation and charge separation processes. The generality of this method is demonstrated by measuring exciton transport for both luminescent and dark materials, as well as for small molecule and polymer active materials and semiconductor quantum dots. Thus, we demonstrate a broadly applicable device-based methodology to probe the intrinsic active material exciton diffusion length.

Intrinsic diffusion length of excitons in long single-walled carbon nanotubes from photoluminescence spectra

Evaluation of intrinsic diffusion length of excitons in a long single-walled carbon nanotube (SWNT) is reported by analyzing the tube-length dependence of photoluminescence (PL) emission intensity. The PL emission intensities depending on the tube length are all obtained from one individual long SWNT suspended on a groove-patterned substrate. The exciton dynamics is described by a one-dimensional diffusion equation. The diffusion length in the clean and pristine long SWNTs is found to be constant, about 200 nm at a lower laser power regime, indicating an intrinsic diffusion behavior. The threshold laser power that is consistent with the onset of exciton-exciton annihilation is about 10${}^{21}$ photons/(cm${}^{2}$ s) for air-suspended SWNTs.