Synthetic strategies for modifying dielectric properties and the electron mobility of fullerene derivatives

The goal of this PhD research project was to develop fullerene derivatives with enhanced dielectric properties for photovoltaic applications. Organic solar cells suffer from relatively low power conversion efficiency mainly due to charge recombination, which stems from the low dielectric constant of these materials. Donor and acceptor combinations are necessary to avoid this, but that approach leads to other types of losses. Increasing the dielectric constant would be the fundamental way to cure the problem. In this thesis the effects of several synthetic strategies on the dielectric constant of fullerene derivatives are described. These strategies includes installing strong permanent dipole groups, incorporating flexible ethyleneoxy-type side chains with small dipoles, installing side chains with highly polarizable heavy atoms such as bromine and iodine and, finally, installing side chains which inherently have a high dielectric constant, such as cyclic carbonates. Theoretical calculations predicted enhanced charge separation upon replacing PCBM by fullerenes with side groups containing strong permanent dipoles, but interestingly their experimentally determined dielectric constant remained similar. Among the proposed strategies, installing ethyleneoxy-type side chains is shown to be a promising way to increase the dielectric constant by ~46 percent without devaluation of optical properties, electron mobility, and orbital energy levels of the compound. However, the length of these chains did not show a considerable effect on dielectric constant

Elimination of the light soaking effect and performance enhancement in perovskite solar cells using a fullerene derivative

In this work, we investigate how electron extraction layers (EELs) with different dielectric constants affect the device performance and the light-soaking phenomenon in hybrid perovskite solar cells (HPSCs). Fulleropyrrolidine with a triethylene glycol monoethyl ether side chain (PTEG-1) having a dielectric constant of 5.9 is employed as an EEL in HPSCs. The commonly used fullerene derivative [60] PCBM, which has identical energy levels but a lower dielectric constant of 3.9, is used as a reference. The device using PTEG-1 as the EEL shows a negligible light soaking effect, with a power conversion efficiency (PCE) of 15.2% before light soaking and a minor increase to 15.7% after light soaking. In contrast, the device using [60] PCBM as the EEL shows severe light soaking, with the PCE improving from 3.8% to 11.7%. Photoluminescence spectroscopy and impedance spectroscopy measurements indicate that trap-assisted recombination at the interface between the hybrid perovskite and the EEL is the cause of the light soaking effect in HPSCs. The trap-assisted recombination is effectively suppressed at the perovskite/PTEG-1 interface, while severe trap assisted recombination takes place at the perovskite/[60] PCBM interface. We attributed these experimental findings to the fact that the higher dielectric constant of PTEG-1 helps to screen the recombination between the traps and free electrons. In addition, the electron donating side chains of PTEG-1 may also contribute to the passivation of the electron traps. As a consequence, the devices using PTEG-1 as the EEL display a considerable increase in the efficiency and a negligible light soaking effect