Diffusion-enhanced exciton dissociation in single-material organic solar cells.

Single-material organic solar cells have recently attracted research attention due to their simplicity, morphological robustness and high yield of exciton dissociation. Using α-sexithiophene as a model system, we show that the single-event probability of the exciton dissociation at the boundaries of polycrystalline domains with different molecular orientation is extremely low (∼0.5%), while a high efficiency of charge generation is gained via hundred-fold crossings of the domain boundaries due to the long exciton diffusion length (∼45 nm).

Optical Distance Measurement Based on Induced Nonlinear Photoresponse of High-Performance Organic Near-Infrared Photodetectors.

Extraction barriers are usually undesired in organic semiconductor devices since they lead to reduced device performance. In this work, we intentionally introduce an extraction barrier for holes, leading to nonlinear photoresponse. The effect is utilized in near-infrared (NIR) organic photodetectors (OPDs) to perform distance measurements, as delineated in the focus-induced photoresponse technique (FIP). The extraction barrier is introduced by inserting an anodic interlayer with deeper highest occupied molecular orbital (HOMO), compared to the donor material, into a well-performing OPD. With increasing irradiance, achieved by decreasing the illumination spot area on the OPD, a higher number of holes pile up at the anode, counteracting the built-in field and increasing charge-carrier recombination in the bulk. This intended nonlinear response of the photocurrent to the irradiance allows determining the distance between the OPD and the light source. We demonstrate fully vacuum-deposited organic NIR optical distance photodetectors with a detection area up to 256 mm2 and detection wavelengths at 850 and 1060 nm. Such NIR OPDs have a high potential for precise, robust, low-cost, and simple optical distance measurement setups.

Enhancing sub-bandgap external quantum efficiency by photomultiplication for narrowband organic near-infrared photodetectors.

Detection of electromagnetic signals for applications such as health, product quality monitoring or astronomy requires highly responsive and wavelength selective devices. Photomultiplication-type organic photodetectors have been shown to achieve high quantum efficiencies mainly in the visible range. Much less research has been focused on realizing near-infrared narrowband devices. Here, we demonstrate fully vacuum-processed narrow- and broadband photomultiplication-type organic photodetectors. Devices are based on enhanced hole injection leading to a maximum external quantum efficiency of almost 2000% at -10 V for the broadband device. The photomultiplicative effect is also observed in the charge-transfer state absorption region. By making use of an optical cavity device architecture, we enhance the charge-transfer response and demonstrate a wavelength tunable narrowband photomultiplication-type organic photodetector with external quantum efficiencies superior to those of pin-devices. The presented concept can further improve the performance of photodetectors based on the absorption of charge-transfer states, which were so far limited by the low external quantum efficiency provided by these devices.

Reducing Non-Radiative Voltage Losses by Methylation of Push-Pull Molecular Donors in Organic Solar Cells.

Organic solar cells are approaching power conversion efficiencies of other thin-film technologies. However, in order to become truly market competitive, the still substantial voltage losses need to be reduced. Here, the synthesis and characterization of four novel arylamine-based push-pull molecular donors was described, two of them exhibiting a methyl group at the para-position of the external phenyl ring of the arylamine block. Assessing the charge-transfer state properties and the effects of methylation on the open-circuit voltage of the device showed that devices based on methylated versions of the molecular donors exhibited reduced voltage losses due to decreased non-radiative recombination. Modelling suggested that methylation resulted in a tighter interaction between donor and acceptor molecules, turning into a larger oscillator strength to the charge-transfer states, thereby ensuing reduced non-radiative decay rates.