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W e would like to start off by thanking the authors of the comment, Wetzelaer and Blom, for their very helpful and constructive analysis. They present an interesting alternative view on an important and timely research topic. Before discussing the mechanism suggested in their comment, from the viewpoint of our experimental results, we would like to summarize our findings. In our work, 1 we experimentally showed that • The contact between donor molecules in the active layer of organic solar cells (OSCs) and a molybdenum oxide (MoO 3) hole extraction layer (HEL) causes an increase in nonradiative recombination losses, proportional to the extent of contacts. • We attributed these losses to surface recombination, and we proved that the losses can be suppressed by inserting a thin interfacial fullerene layer at the anode side. • Analyzing various donor−acceptor mixing ratios, with and without a fullerene-modified HEL, we decoupled and quantified the contribution from surface recombi-nation on the total nonradiative losses occurring in these devices. In the best case, we showed an improvement of 150 meV in V OC , as compared to the reference device. This demonstrates that surface recombination is a considerable contributor to nonradiative voltage losses in these solar cells, which are otherwise commonly occurring through charge-transfer states or energetic trap states because of defects in the bulk. We consider this the main result of our work. • Measurements by a modified charge extraction by linearly increasing voltage (CELIV) technique provided evidence that the improvement in V OC could be attributed to an enhanced built-in potential (V bi), reducing the presence of minority charge carriers at the respective electrodes. Although the authors of the comment in general agree with our experimental findings, they argue that the V bi does not play a direct role in suppressing the surface recombination of minority carriers. The introduction of a C 60 interlayer, they argue, renders the MoO 3 contact ohmic. 2 The reduced anodic injection barrier simultaneously increases the V bi , minimizes nonradiative voltage losses upon the extraction of majority carriers (holes), and suppresses minority-carrier (electron) surface recombination, the latter being the result of hole accumulation and associated band bending near the ohmic hole contact. Therefore, the ohmic contact formation suppresses both majority-and minority-carrier surface recombination losses, whereas the built-in voltage per se, they reason, does not play a major role. It is our opinion that the authors of the comment provide a very reasonable alternative explanation for the reduced surface recombination. Injection barriers at the contacts are well-known to be detrimental for the performance of OSC as being a major cause of a reduced V bi and increased surface recombination. Injection barriers have been shown to reduce not only the V OC but also the fill factor (FF), sometimes even leading to s-shaped JV curves. 3−5 It has been suggested that, in the case of very large injection barriers, the V OC is given by V bi. 6,7 Although V bi is determined by the difference in the work functions of the contacts, because of Fermi level pinning and the associated band bending, the built-in potential across the active layer, the effective V bi , typically cannot exceed the effective gap of the bulk-heterojunction blend. In other words, the anode Fermi level pins to the highest occupied molecular orbital (HOMO) of the donor, whereas the cathode pins to the lowest unoccupied molecular orbital (LUMO) of the acceptor. In addition, because of disorder, Fermi level pinning typically occurs to discrete gap or tail states, causing additional band bending, which further limits the effective V bi. 8,9 If there is an injection barrier at one contact, an increase or decrease in this barrier is directly reflected in the V bi. 5 Sachsische Aufbaubank [100325708]; Academy of FinlandAcademy of Finland

Omnipresent quality monitoring in food products, blood-oxygen measurement in lightweight conformal wrist bands, or data-driven automated industrial production: Innovation in many fields is being empowered by sensor technology. Specifically, organic photodetectors (OPDs) promise great advances due to their beneficial properties and low-cost production. Recent research has led to rapid improvement in all performance parameters of OPDs, which are now on-par or better than their inorganic counterparts, such as silicon or indium gallium arsenide photodetectors, in several aspects. In particular, it is possible to directly design OPDs for specific wavelengths. This makes expensive and bulky optical filters obsolete and allows for miniature detector devices. In this review, recent progress of such narrowband OPDs is systematically summarized covering all aspects from narrow-photo-absorbing materials to device architecture engineering. The recent challenges for narrowband OPDs, like achieving high responsivity, low dark current, high response speed, and good dynamic range are carefully addressed. Finally, application demonstrations covering broadband and narrowband OPDs are discussed. Importantly, several exciting research perspectives, which will stimulate further research on organic-semiconductor-based photodetectors, are pointed out at the very end of this review.

Wavelength-discriminating systems typically consist of heavy benchtop-based instruments, comprising diffractive optics, moving parts, and adjacent detectors. For simple wavelength measurements, such as lab-on-chip light source calibration or laser wavelength tracking, which do not require polychromatic analysis and cannot handle bulky spectroscopy instruments, lightweight, easy-to-process, and flexible single-pixel devices are attracting increasing attention. Here, a device is proposed for monotonously transforming wavelength information into the time domain with room-temperature phosphorescence at the heart of its functionality, which demonstrates a resolution down to 1 nm and below. It is solution-processed from a single host-guest system comprising organic room-temperature phosphors and colloidal quantum dots. The share of excited triplet states within the photoluminescent layer is dependent on the excitation wavelength and determines the afterglow intensity of the film, which is tracked by a simple photodetector. Finally, an all-organic thin-film wavelength sensor and two applications are demonstrated where this novel measurement concept successfully replaces a full spectrometer.

Charge selective interlayers are crucial in thin-film photovoltaics, such as organic and Perovskite solar cells. Charge transporting layers (doped and undoped) constitute perhaps the most important class of charge selective interlayers; however, it is not well understood how a charge transporting layer should be designed in order to ensure efficient extraction of majority carriers while blocking minority carriers. This work clarifies how well charge-transporting layers with varying majority carrier conductivities block minority carriers. We use the Charge Extraction by a Linearly Increasing Voltage technique to determine the surface recombination velocity of minority carriers in model system devices with varying majority carrier conductivity in the transporting layer. Our results show that transporting layers with high conductivity for majority carriers do not block minority carriers - at least not at operating voltages close to or above the built-in voltage, due to direct bi-molecular recombination across the transporting layer-absorber layer interface. We furthermore discuss and propose design principles to achieve selective charge extraction in thin film solar cells using charge transporting layers.

Charge injection from electrodes into doped organic films is a widespread technology used in the majority of state-of-the-art organic semiconductor devices. Although such interfaces are commonly considered to form Ohmic contacts via strong band bending, an experiment that directly measures the contact resistance has not yet been demonstrated. In this study, we use a simple metal/doped organic semiconductor/metal stack and study its voltage-dependent resistance. A transport layer thickness variation proves that the presented experiment gains direct access to the contact resistance of the device. We can quantify that for an operating current density of 10mA/cm2 the investigated material system exhibits a voltage drop over the metal/organic interface of about 200mV, which can be reduced by more than one order of magnitude when employing an additional injection layer. The presented experiment proposes a simple strategy to measure the contact resistance between any metal and doped organic film without applying numerical tools or elaborate techniques. Furthermore, the simplistic device architecture allows for very high, homogeneous, and tunable electric fields within the organic layer, which enables a clear investigation of the Poole-Frenkel effect.