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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

Interfacial layers in conjunction with suitable charge-transport layers can significantly improve the performance of optoelectronic devices by facilitating efficient charge carrier injection and extraction. This work uses a neat C60 interlayer on the anode to experimentally reveal that surface recombination is a significant contributor to nonradiative recombination losses in organic solar cells. These losses are shown to proportionally increase with the extent of contact between donor molecules in the photoactive layer and a molybdenum oxide (MoO3) hole extraction layer, proven by calculating voltage losses in low- and high-donor-content bulk heterojunction device architectures. Using a novel in-device determination of the built-in voltage, the suppression of surface recombination, due to the insertion of a thin anodic-C60 interlayer on MoO3, is attributed to an enhanced built-in potential. The increased built-in voltage reduces the presence of minority charge carriers at the electrodes-a new perspective on the principle of selective charge extraction layers. The benefit to device efficiency is limited by a critical interlayer thickness, which depends on the donor material in bilayer devices. Given the high popularity of MoO3 as an efficient hole extraction and injection layer and the increasingly popular discussion on interfacial phenomena in organic optoelectronic devices, these findings are relevant to and address different branches of organic electronics, providing insights for future device design.

Efficient exciton dissociation and subsequent generation of free charge carriers at the organic donor-acceptor interface requires a number of electron-transfer processes. It is a common view that these steps result in an unavoidable energy loss in organic photovoltaic devices that is not present in other types of solar cells. The currently best performing organic solar cells with power conversion efficiencies over 16% challenge this view, and no interfacial charge-transfer states with energy significantly lower than the strongly absorbing singlet states are detected within the gap of the used donor and acceptor materials. This Perspective will discuss implications, the remaining sources of energy loss, and open questions to be solved to achieve power conversion efficiencies over 20%.

Charge generation and recombination processes at interfaces between electron donating (donor, D) and accepting molecules (acceptor, A) are mediated by intermolecular charge-transfer (CT) states. Si...

An understanding of the factors limiting the open-circuit voltage (V-oc) and related photon energy loss mechanisms is critical to increase the power conversion efficiency (PCE) of small-molecule organic solar cells (OSCs), especially those with near-infrared (NIR) absorbers. In this work, two NIR boron dipyrromethene (BODIPY) molecules are characterized for application in planar (PHJ) and bulk (BHJ) heterojunction OSCs. When two H atoms are substituted by F atoms on the peripheral phenyl rings of the molecules, the molecular aggregation type in the thin film changes from the H-type to J-type. For PHJ devices, the nonradiative voltage loss of 0.35 V in the J-aggregated BODIPY is lower than that of 0.49 V in the H-aggregated device. In BHJ devices with a nonradiative voltage loss of 0.35 V, a PCE of 5.5% is achieved with an external quantum efficiency (EQE) maximum of 68% at 700 nm.

The involvement of charge-transfer (CT) states in the photogeneration and recombination of charge carriers has been an important focus of study within the organic photovoltaic community. In this work, we investigate the molecular factors determining the mechanism of photocurrent generation in low-donor-content organic solar cells, where the active layer is composed of vacuum-deposited C

Diketopyrrolopyrrole (DPP)-based donor acceptor copolymers have gained a significant amount of research interest in the organic electronics community because of their high charge carrier mobilities in organic field-effect transistors (OFETs) and their ability to harvest near-infrared (NIR) photons in solar cells. In this study, we have synthesized four DPP based donor-acceptor copolymers with variations in the donor unit and the branching point of the solubilizing alkyl chains (at the second or sixth carbon position). Grazing incidence wide-angle X-ray scattering (GIWAXS) results suggest that moving the branching point further away from the polymer backbone increases the tendency for aggregation and yields polymer phases with a higher degree of crystallinity (DoC). The polymers were blended with PC70BM and used as active layers in solar cells. A careful analysis of the energetics of the neat polymer and blend films reveals that the charge-transfer state energy (E-CT) of the blend films lies exceptionally close to the singlet energy of the donor (E-D*), indicating near zero electron transfer losses. The difference between the optical gap and open-circuit voltage (V-OC) is therefore determined to be due to rather high nonradiative 418 +/- 13 mV) and unavoidable radiative voltage losses (approximate to 255 +/- 8 mV). Even though the four materials have similar optical gaps, the short-circuit current density (J(SC)) covers a vast span from 7 to 18 mA cm(-2) for the best performing system. Using photoluminescence (PL) quenching and transient charge extraction techniques, we quantify geminate and nongeminate losses and find that fewer excitons reach the donor-acceptor interface in polymers with further away branching points due to larger aggregate sizes. In these material systems, the photogeneration is therefore mainly limited by exciton harvesting efficiency. The authors acknowledge support by the German Excellence Initiative via the Cluster of Excellence EXC 1056 "Center for Advancing Electronics Dresden" (cfaed). For the GIWAXS measurements, the authors acknowledge KMC-2 diffraction beamline of the Photon source BESSY-II, Helmholtz Zentrum Berlin. Additionally, for TEM microscopy, the authors acknowledge Dr. Petr Formanek from the Leibniz-Institut fur Polymerforschung Dresden. T.E. acknowledges support by the German Alexander von Humboldt foundation. J.B., S.U., and K.V. acknowledge support from the German Federal Ministry for Education and Research (BMBF) through the InnoProfile Projekt "Organische p-i-n Bauelemente 2.2" (03IPT602X). Furthermore, S.U. acknowledges support by the graduate academy of the TU Dresden, financed by the excellence initiative of the German federal and state governments. E.C.-F. and U.H. and D.N. acknowledge funding by the BMBF (UNVEIL, FKZ 13N13719) and the DFG (SFB 951 "HIOS").

We present organic near-infrared photodetectors based on the absorption of charge-transfer (CT) states at the zinc-phthalocyanine–C60 interface. By using a resonant optical cavity device architecture, we achieve a narrowband detection, centered around 1060 nm and well below (>200 nm) the optical gap of the neat materials. We measure transient photocurrent responses at wavelengths of 532 and 1064 nm, exciting dominantly the neat materials or the CT state, respectively, and obtain rise and fall times of a few nanoseconds at short circuit, independent of the excitation wavelength. The current transients are modeled with time-dependent drift-diffusion simulations of electrons and holes which reconstruct the photocurrent signal, including capacitance and series resistance effects. The hole mobility of the donor material is identified as the limiting factor for the high-frequency response. With this knowledge, we demonstrate a new device concept, which balances hole and electron extraction times and achieves a ...

Organic spectrometers are attractive for biomedicine and industrial process monitoring but are currently limited in terms of spectral selectivity and the accessible wavelength range. Here, we achieve narrowband enhancement of the below-gap near-infrared response of charge-transfer (CT) excitations in organic photodiodes by introducing them into a high-quality microcavity. The device architecture includes a nonconductive distributed Bragg reflector and thin metal electrodes, leading to the formation of sharp Tamm plasmon-polariton resonances. We demonstrate how to tailor the arising multimode spectra for spectroscopic photodetectors and present efficient single-resonance devices with remarkable line widths below 22 nm, which are partially transparent for visible wavelengths. Taking advantage of the spectrally broad CT band, we vary the resonator thickness to provide a proof of concept that benefits from the spectral selectivity of our high-quality microcavities. Finally, utilizing transfer-matrix calculati...

In disordered organic semiconductors, the transfer of a rather localized charge carrier from one site to another triggers a deformation of the molecular structure quantified by the intramolecular relaxation energy. A similar structural relaxation occurs upon population of intermolecular charge-transfer (CT) states formed at organic electron donor (D)-acceptor (A) interfaces. Weak CT absorption bands for D A complexes occur at photon energies below the optical gaps of both the donors and the C-60 acceptor as a result of optical transitions from the neutral ground state to the ionic CT state. In this work, we show that temperature-activated intramolecular vibrations of the ground state play a major role in determining the line shape of such CT absorption bands. This allows us to extract values for the relaxation energy related to the geometry change from neutral to ionic CT complexes. Experimental values for the relaxation energies of 20 D:C-60 CT complexes correlate with values calculated within density functional theory. These results provide an experimental method for determining the polaron relaxation energy in solid-state organic D-A blends and show the importance of a reduced relaxation energy, which we introduce to characterize thermally activated CT processes. This work was supported by the German Federal Ministry for Education and Research (BMBF) through the InnoProfille project "Organische p-i-n Bauelemente 2.2". F.O. would like to thank the German Research Foundation (DFG) for financial support (Grant OR 349/1). This work was partly supported by the DFG within the Cluster of Excellence "Center for Advancing Electronics Dresden." F.P. and D.N. acknowledge funding by the DFG via the SFB 951 "HIOS". T.V. acknowledges the Agency for Innovation by Science and Technology in Flanders (IWT) for funding his Ph.D. The work at Georgia Tech was supported by the Department of the Navy, Office of Naval Research Award No. N00014-14-1-0580 (CAOP MURI) and through a state-sponsored scholarship for graduate students to Y.F. from the China Scholarship Council. We acknowledge the Center for Information Services and High Performance Computing (ZIH) at TU Dresden for computational resources. We thank Prof. Bauerle from the University of Ulm for the supply of DH4T and DH6T and Markus Hummert for P4-Ph4-DIP and BP-Bodipy.