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We present a first-principles-based multiscale simulation framework for quantitative predictions of the high-energy part of the ultraviolet photoelectron spectroscopy (UPS) spectra of amorphous molecular solids. The approach combines a deposition simulation, many-body Green's function theory, polarizable film embedding, and multimode electron-vibrational coupling and provides a molecular-level view on the interactions and processes giving rise to spectral features. This insight helps bridging the current gap between experimental UPS and theoretical models as accurate analyses are hampered by the energetic disorder, surface sensitivity of the measurement, and the complexity of excitation processes. In particular, this is relevant for the unambiguous determination the highest occupied molecular orbital energy (HOMO) of organic semiconductors, a key quantity for tailoring and engineering new optoelectronic devices. We demonstrate the capabilities of the simulation approach studying the spectrum of two isomers of 2-methyl-9,10-bis(naphthalen-2-yl)anthracene as archetypical materials showing a clearly separated HOMO peak in experiment. The agreement with experiment is excellent, suggesting that our approach provides a route for determining the HOMO energy with an accuracy better than 0.1 eV.

For 19 diketopyrrolopyrrole polymers, the highest occupied molecular orbital (HOMO) energies are determined from i) the oxidation potential with square-wave voltammetry (SWV), ii) the ionization potential using ultraviolet photoelectron spectroscopy (UPS), and iii) density functional theory (DFT) calculations. The SWV HOMO energies show an excellent linear correlation with the open-circuit voltage (Voc) of optimized solar cells in which the polymers form blends with a fullerene acceptor ([6,6]-phenyl-C61-butyl acid methyl ester or [6,6]-phenyl-C71-butyl acid methyl ester). Remarkably, the slope of the best linear fit is 0.75 ± 0.04, i.e., significantly less than unity. A weaker correlation with Voc is found for the HOMO energies obtained from UPS and DFT. Within the experimental error, the SWV and UPS data are correlated with a slope close to unity. The results show that electrochemically determined oxidation potentials provide an excellent method for predicting the Voc of bulk heterojunction solar cells, with absolute deviations less than 0.1 V.

The efficiency of phosphorescent organic light emitting diodes (OLEDs) shows a decrease with increasing luminance (“roll-off”). One of the contributions to the roll-off is triplet–triplet annihilation (TTA). TTA is the process of energy transfer from one triplet exciton to another, after which the excited exciton decays nonradiatively to the lowest triplet state. In this study, the TTA-rate is measured for a large number of emissive materials consisting of a small concentration of phosphorescent “guest” molecules, with emission colors across the entire visible range, embedded in various host materials. It is found that the TTA-rate does not only depend on the direct interaction rate between the excitons on the guest molecules, but also on the difference in triplet energy ΔE T of the host and guest molecules: when ΔE T is smaller than about 0.20 eV, diffusion of excitons via the host molecules leads to a significant enhancement of the TTA-rate. By varying the guest concentration and using kinetic Monte Carlo simulations, the roles of the direct interaction, guest-mediated diffusion, and host-mediated diffusion are disentangled.

Photovoltaic retinal prostheses show great potential to restore sight in patients suffering from degenerative eye diseases by electrical stimulation of the surviving neurons in the retinal network. Herein, organic photodiodes (OPDs) sensitive to near‐infrared (NIR) light are evaluated as photovoltaic pixels for future application in retinal prostheses. Single‐junction and tandem OPDs are compared. In the latter, two nominally identical single‐junction cells are processed on top of each other, effectively doubling the open‐circuit voltage (V OC). Both single‐junction and tandem OPD micropixels can deliver the required charge to stimulate neurons under pulsed NIR light at physiologically safe intensities when connected to stimulating microelectrodes in a physiological saline solution. However, only tandem OPD pixels can cover the entire charge per pulse neural stimulation window due to their higher V OC (≈1.4 V). This demonstrates the viability of high‐resolution retinal prostheses based on flexible OPD arrays.