Abstract
We investigate the charge-trapping dynamics in hybrid nanocrystal-polymer systems and their effect on performance in photovoltaic devices. Employing various steady-state spectroscopy techniques and ultrafast, three-pulse transient absorption methods, we identify the depth of electron trap states in the nanocrystal band gap and measure their population dynamics. Our findings show that photogenerated electrons are trapped at midgap states on the nanocrystal within hundreds of picoseconds. The trapping of the majority of charge carriers before charge extraction results in a lowering of the quasi-Fermi level of the electrons which limits the device open-circuit voltage, thereby underlining the significance of these processes in conjugated polymer/nanocrystal hybrid photovoltaics.
Highlights
Because of the prospect of cheap large-area production, organic photovoltaics have attracted great interest in the past decade
The efficiency of hybrid bulk heterojunction photovoltaics still lags that of polymer:fullerene devices, largely because of operating voltages that are lower than expected from the intrinsic electronic energy levels of the materials involved.[8,9]
This is largely due to the complexity of processes occurring at the nanoparticle surfaces and their interfaces with the organic component.[16−18] Dynamical studies of electron trapping in various nanoparticle systems have been reported using transient absorption and time-resolved photoluminescence measurements, with trapping times ranging from picoseconds to nanoseconds.[11,12,19,20]
Summary
Because of the prospect of cheap large-area production, organic photovoltaics have attracted great interest in the past decade. We study the energetics and dynamics of electron trapping in hybrid nanocrystal-polymer films and solar cells using a combination of steady-state and transient spectroscopic techniques including three-pulse pump−push−probe measurements.
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