Revealing the Dynamics of Charge Carriers in Polymer:Fullerene Blends Using Photoinduced Time-Resolved Microwave Conductivity

Abstract

During the past decade, time-resolved microwave conductivity (TRMC) has evolved to an established, powerful technique to study photoactive layers. With this feature paper, we aim to fulfill two goals: (1) give a full description of the photoinduced TRMC technique, including experimental details and data analysis, and discuss to what extent the TRMC technique differs from more conventional DC techniques and (2) illustrate the potential of this technique for probing charge carrier dynamics in photoactive materials. For these reasons recent studies on conjugated polymer:fullerene blends will be presented and discussed. The findings from these studies have advanced the insight into the mechanism of charge carrier generation and decay in polymer:fullerene blends, which allows us to improve the efficiency of organic photovoltaic cells based on this active layer architecture. In short, it is shown how the TRMC technique can be used as a versatile method to screen the potential of new photovoltaic materials.

Enhancing Charge Carrier Lifetime in Metal Oxide Photoelectrodes through Mild Hydrogen Treatment

“…[17,[39][40][41][42] In short, the BiVO4 films are excited with a nanosecond laser pulse and the light-induced change in the reflected microwave power, which is sensitive to the presence of mobile carriers, is monitored (see Experimental Methods for more details). The TRMC signal can then be expressed as the product of the absorbance-normalized quantum yield () and the combined mobility of the charge carriers (∑μ, sum of the electron and hole mobilities).…”

“…Assuming that the internal quantum yield of electron-hole generation approaches 100% and no charge carrier recombination has taken place within the response time of the setup, the carrier mobility () and the carrier lifetime () can be obtained from the peak and the decay of the TRMC signal, respectively. [17,40] [43] , the individual electron (e) and hole mobility (h) of each films can be calculated ( Table 1). The carrier lifetime, however, shows a much larger change and increases from 43 ns to 109 ns.…”

“…[39,40] During the measurements, a change in the microwave power reflected by the cavity upon sample excitation by 6 ns (full-width at halfmaximum) pulses of a frequency-tripled Q-switched Nd:YAG laser at a wavelength of 355 nm (10 Hz repetition rate), ΔP/P, was monitored and correlated to the photoinduced change in the conductance of the sample, ΔG, by…”

Charge Generation and Transport Phenomena in Disordered Organic Semiconductors and Photovoltaic Diodes

“…The recombination from the chargeseparated (CS) state back to the CT-state with rate constant r is assumed to be described by [68]. Similarly, the mobilities on a nm length scale obtained from Transient Microwave Conductivity measurements is typically 1, 2 orders of magnitude higher than the bulk mobility [69]. Possible reasons are that charges have to cross less or no domain boundaries in the nm regime, or charge delocalization over the polymer chain or several molecules.…”

“…Charge generation is often studied using transient absorption spectroscopy (TAS) [174], transient microwave conductivity (TRMC) [69] or with the relatively new Time-Delayed Collection Field (TDCF) technique [90,91]. TAS allows the assessment and quantification of the initial population of exciton and charge transfer states prior to recombination of free carriers and TRMC measures the product of the generation yield and the sum of the "local" charge carrier mobilities.…”