Charge-carrier transport and recombination in thin insulating films studied via extraction of injected plasma

Abstract

We show how charge-carrier transport and recombination in thin insulator films are directly measured using the technique of extraction of injected plasma. This technically simple technique is complementary to the well-known time-of-flight technique. We use this technique on bulk-heterojunction solar cells, where the double-injection current into an insulator is found, and we show how to use the extraction of the injected plasma to independently and simultaneously measure the charge-carrier mobility and bimolecular recombination coefficient in these films. A simple analytical solution to calculate the bimolecular recombination coefficient from an injected charge is derived. We found that the extracted charge follows a linear dependence as a function of applied voltage and saturates as a function of offset voltage, leading to the conclusion that almost all of the injected charge is extracted at high offset voltages. Therefore, we can directly measure the charge-carrier mobility and bimolecular recombination coefficient from the extracted charge as a function of voltage pulse duration. Moreover, the charge-carrier bimolecular recombination coefficient $(\ensuremath{\beta}=2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}∕\mathrm{s})$ is found to be strongly reduced compared to Langevin-type coefficient, as previously shown.