Negative capacitance in organic semiconductor devices: bipolar injection and charge recombination mechanism

We report negative capacitance at low frequencies in organic semiconductor based diodes and show that it appears only under bipolar injection conditions. We account quantitatively for this phenomenon by the recombination current due to electron-hole annihilation. Simple addition of the recombination current to the well established model of space charge limited current in the presence of traps, yields excellent fits to the experimentally measured admittance data. The dependence of the extracted characteristic recombination time on the bias voltage is indicative of a recombination process which is mediated by localized traps.Comment: 3 pages, 3 figures, accepted for publication in Applied Physics Letter

Direct observation of the ultrafast electron transfer process in a polymer/fullerene blend

Photoinduced electron transfer in organic molecules is an extensively investigated topic both because of fundamental interest in the photophysics and for applications to artificial photosynthesis. Highly efficient ultrafast electron transfer from photoexcited conjugated polymers to C60 has been reported, the back transfer is heavily hindered, thus providing an intrinsic stabilization mechanism of the photogenerated charges. Although an upper limit for the forward electron transfer time of 1 ps has been reported, its detailed time resolution is still missing and is highly needed to shed light on the photophysics of the charge transfer mechanism. We perform ultrafast experiments on conjugated polymer/C60 blends with sub-10-fs time resolution. We are able to time resolve for the first time the charge transfer process, obtaining a forward electron transfer time constant τct≈45 fs

Direct observation of the ultrafast electron transfer process in a polymer/fullerene blend

Photoinduced electron transfer in organic molecules is a topic of fundamental interest in photophysics and for applications to artificial photosynthesis. The detailed time resolution of this process is needed to shed light on the charge transfer mechanism. We perform ultrafast experiments on conjugated polymer/C60 blends with sub-10-fs time resolution. We are able to time resolve for the first time the charge transfer process, obtaining a time constant ≈45 fs