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Encapsulated organic solar cells often show a burn-in behaviour under illumination. This burn-in manifests itself as a rapid performance loss followed by a much slower progression of the degradation. Here we investigate the burn-in for PCDTBT:PC70BM solar cells under a wide range of illumination intensities. We find that increasing the sunlight concentration from 1 Sun to up to 100 Suns does not change the degradation behaviour, i.e. the dependence of all principal photovoltaic parameters on the dose of solar exposure (in Sun hours). This suggests that the degradation mechanisms under solar concentration (≤100 Suns) are the same as those observed under 1 Sun. This result makes it possible to use concentrated sunlight for accelerated stability assessment of these devices. We also find that devices with PEDOT:PSS as hole transport material show a rapid drop in open-circuit voltage of around 100 mV during the first Sun hour of light exposure. By replacing PEDOT:PSS with MoO3 this initial process can be prevented and only the much slower part of the photo-degradation takes place.

We present an overview of opto-electronic characterization techniques for solar cells including light-induced charge extraction by linearly increasing voltage, impedance spectroscopy, transient photovoltage, charge extraction and more. Guidelines for the interpretation of experimental results are derived based on charge drift-diffusion simulations of solar cells with common performance limitations. It is investigated how nonidealities like charge injection barriers, traps and low mobilities among others manifest themselves in each of the studied cell characterization techniques. Moreover, comprehensive parameter extraction for an organic bulk-heterojunction solar cell comprising PCDTBT:PC70BM is demonstrated. The simulations reproduce measured results of 9 different experimental techniques. Parameter correlation is minimized due to the combination of various techniques. Thereby a route to comprehensive and accurate parameter extraction is identified.

There is increasing evidence that the presence of mobile ions in perovskite solar cells can cause a current-voltage curve hysteresis. However, it is still subject of ongoing debates how exactly mobile ions influence the device operation. We use drift-diffusion simulations incorporating mobile ions to describe IV-curves of preconditioned methylammonium lead iodide perovskite solar cells and compare them with experimental results. Our simulation results show that the hysteresis depends on the extent of surface recombination and on the diffusion length of charge carriers. We provide a detailed explanation for the reduced hysteresis of perovskite solar cells with high power conversion efficiencies. We find that in high-efficiency solar cells ion migration is still present, but does not cause a hysteresis effect. In these devices charge extraction is mainly driven by diffusion of free electrons and holes.

If not oriented perfectly isotropically, the strong dipole moment of polar organic semiconductor materials such as tris-(8-hydroxyquinolate)aluminum (Alq3) will lead to the buildup of a giant surface potential (GSP) and thus to a macroscopic dielectric polarization of the organic film. Despite this having been a known fact for years, the implications of such high potentials within an organic layer stack have only been studied recently. In this work, the influence of the GSP on hole injection into organic layers is investigated. Therefore, we apply a concept called dipolar doping to devices consisting of the prototypical organic materials N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) as nonpolar host and Alq3 as dipolar dopant with different mixing ratios to tune the GSP. The mixtures are investigated in single-layer monopolar devices as well as bilayer metal/insulator/semiconductor structures. Characterization is done electrically using current-voltage (I-V) characteristics, impedance spectroscopy, and charge extraction by linearly increasing voltage and time of flight, as well as with ultraviolet photoelectron spectroscopy. We find a maximum in device performance for moderate to low doping concentrations of the polar species in the host. The observed behavior can be described on the basis of the Schottky effect for image-force barrier lowering, if the changes in the interface dipole, the carrier mobility, and the GSP induced by dipolar doping are taken into account.

We report a cooperative study conducted between different laboratories to investigate organic solar cell degradation with respect to P3HT material properties and different solar cell architectures. Various batches of P3HT were collected from different suppliers reflecting commercial availability as well as properties variability. Among the materials properties explicitly considered were the molar mass, dispersity, regio-regularity, impurities by trace metals and intrinsic doping evaluated from radical concentrations. Each of the participating laboratories contributing test devices applied their own layer stack, i.e. their own device architecture and layout. This variation was appreciated as another parameter for evaluation. Even though a large amount of devices failed due to extrinsic degradation effects, indeed, some materials properties were found to be more important than others for obtaining long lifetimes and high stability of P3HT-based polymer solar cells., All authors are grateful for support from COST Action MP 1307, “StableNextSol”. This article is based upon work from COST Action StableNextSol project MP1307, supported by COST (European Cooperation in Science and Technology). RM, TF, DF, RR and HH are grateful for financial support within the frame of “AIMS in OPV” junior research group by the BMBF, Germany. L.C. is thankful to ENEA (Ente Nazionale Energia e Ambiente) and the Italian Ministry of Foreign Affairs for a visitor post-doc fellowship to BGU. FG and MD are grateful for financial support by the National Center for Research and Development within the Project POSCIS under Grant No. LIDER/09/129/L-3/11/NCBR/2012). IVF and EAK acknowlegde financial support from Adelis Foudation. N. Blaubach is acknowledged for ICP measurements. MP thanks the Bundesministerium für Bildung und Forschung, Germany (BMBF FKZ 03EK3507).

​This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Applied Materials & Interfaces, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://pubs.acs.org/doi/10.1021/acsami.9b04991, A variety of experiments on vacuum-deposited methylammonium lead iodide perovskite solar cells are presented, including JV curves with different scan rates, light intensity-dependent open-circuit voltage, impedance spectra, intensity-modulated photocurrent spectra, transient photocurrents, and transient voltage step responses. All these experimental data sets are successfully reproduced by a charge drift-diffusion simulation model incorporating mobile ions and charge traps using a single set of parameters. While previous modeling studies focused on a single experimental technique, we combine steady-state, transient, and frequency-domain simulations and measurements. Our study is an important step toward quantitative simulation of perovskite solar cells, leading to a deeper understanding of the physical effects in these materials. The analysis of the transient current upon voltage turn-on in the dark reveals that the charge injection properties of the interfaces are triggered by the accumulation of mobile ionic defects. We show that the current rise of voltage step experiments allow for conclusions about the recombination at the interface. Whether one or two mobile ionic species are used in the model has only a minor influence on the observed effects. A delayed current rise observed upon reversing the bias from +3 to -3 V in the dark cannot be reproduced yet by our drift-diffusion model. We speculate that a reversible chemical reaction of mobile ions with the contact material may be the cause of this effect, thus requiring a future model extension. A parameter variation is performed in order to understand the performance-limiting factors of the device under investigation.

Charge carrier transport in organic semiconductor devices is thermally activated with characteristic activation energies in the range of 0.2–0.6 eV, leading to strongly temperature-dependent behaviour. For designing efficient organic semiconductor materials and devices, it is therefore indispensable to understand the origin of these activation energies. We propose that in bilayer organic light-emitting diodes (OLEDs) employing a polar electron transport layer, as well as in metal-insulator-semiconductor (MIS) devices, the hole injection barrier Einj and the hole mobility activation energy Eμ can be decoupled from each other if temperature-dependent capacitance-frequency (C-f-T) and MIS-CELIV (charge extraction by linearly increasing voltage) experiments are combined. While the C-f-T signal contains information of both injection and transport, the CELIV current is expected to be insensitive to the electrode injection properties. We employ numerical drift-diffusion simulations to investigate the accuracy of...

Article 1800474 Appreciable progress has been achieved in the development of organic photovoltaics (OPV) over the last decade. However, further improvement of operational stability remains a challenge. In this contribution, focus is placed on corrosion and delamination of the metal contact, which are mainly caused by oxygen or water vapor ingress but in other cases also via mechanical wear and different thermal expansion coefficients. So‐called pinholes and electrode edges provide pathways for ingress of water vapor and oxygen, which may attack the metal–organic interface. Thus, electrical insolation via formation of insulating metal oxide and concomitant mechanical delamination occurs. As charge injection and extraction is suppressed at insulated and delaminated areas, the active area contributing to power conversion gets reduced. This work links analytical and numerical predictions about the active area in contact with the electrode to experimentally observe dependencies. Spatially and time‐resolved electroluminescence measurements provide information on location, size, and growth‐rate of insulated areas. Area loss rates for dark spots depend either sub‐linear (for early stages and edge‐ingress) or linear (later stages) on time. The initial defect size has a clear impact on growth rates. Furthermore, it has possible to demonstrate titanium oxide interlayers to slow down this type of extrinsic degradation.

Single-carrier devices are an excellent model system to study charge injection and charge transport properties of (doped) transport layers and to draw conclusions about organic electronics devices in which they are used. By combining steady-state and impedance measurements at varying temperatures of hole-only devices with different intrinsic layer thicknesses, we are able to determine all relevant material parameters, such as the charge mobility and the injection barrier. Furthermore, the correlation and sensitivity analyses reveal that the proposed approach to study these devices is especially well suited to extract the effective doping density, a parameter which cannot be easily determined otherwise. The effective doping density is crucial in organic light-emitting diodes (OLEDs) for realizing efficient injection, charge balance, and lateral conductivity in display or lighting applications. With the fitted drift-diffusion device model, we are further able to explain the extraordinary two-plateau capacitance–frequency curve of these hole-only devices, which originates from charges that flow into the intrinsic layer at zero applied offset voltage. We demonstrate that the observation of this behaviour is a direct indication for ideal charge injection properties and the observed capacitance–frequency feature is linked to the charge carrier mobility in the intrinsic layer. The extracted material parameters may directly be used to simulate and optimize full OLED devices employing the investigated hole-injection and -transport materials.

We demonstrate the application of the CELIV (charge carrier extraction by linearly increasing voltage) technique to bilayer organic light-emitting devices (OLEDs) in order to selectively determine the hole mobility in N,N0-bis(1-naphthyl)-N,N0-diphenyl-1,10-biphenyl-4,40-diamine (α-NPD). In the CELIV technique, mobile charges in the active layer are extracted by applying a negative voltage ramp, leading to a peak superimposed to the measured displacement current whose temporal position is related to the charge carrier mobility. In fully operating devices, however, bipolar carrier transport and recombination complicate the analysis of CELIV transients as well as the assignment of the extracted mobility value to one charge carrier species. This has motivated a new approach of fabricating dedicated metal-insulator-semiconductor (MIS) devices, where the extraction current contains signatures of only one charge carrier type. In this work, we show that the MIS-CELIV concept can be employed in bilayer polar OLEDs as well, which are easy to fabricate using most common electron transport layers (ETLs), like Tris-(8-hydroxyquinoline)aluminum (Alq3). Due to the macroscopic polarization of the ETL, holes are already injected into the hole transport layer below the built-in voltage and accumulate at the internal interface with the ETL. This way, by a standard CELIV experiment only holes will be extracted, allowing us to determine their mobility. The approach can be established as a powerful way of selectively measuring charge mobilities in new materials in a standard device configuration.

Organic light-emitting diodes (OLEDs) rely on the use of functional materials with suitable energy levels and mobilities for selective charge carrier injection and transport of one species only at the respective electrode. Until recently, however, the dipolar nature of many organic semiconductors has been largely ignored in this context. In particular, electron transports layers (ETLs) often exhibit spontaneous orientation polarization leading to interfacial charges that modify the electrical potential landscape inside a hetero-layer device. Here we demonstrate that the effect of polar ETLs can be simulated using the well-established Poisson and drift-diffusion formalism, if these interfacial charges are taken into account. Impedance spectroscopy is used in order to validate our approach and to characterize the polarity of the material. Finally, simulations allow to quantify the impact of polar ETLs on device performance.

et al., The process of accurately gauging lifetime improvements in organic photovoltaics (OPVs) or other similar emerging technologies, such as perovskites solar cells is still a major challenge. The presented work is part of a larger effort of developing a worldwide database of lifetimes that can help establishing reference baselines of stability performance for OPVs and other emerging PV technologies, which can then be utilized for pass-fail testing standards and predicting tools. The study constitutes scanning of literature articles related to stability data of OPVs, reported until mid-2015 and collecting the reported data into a database. A generic lifetime marker is utilized for rating the stability of various reported devices. The collected data is combined with an earlier developed and reported database, which was based on articles reported until mid-2013. The extended database is utilized for establishing the baselines of lifetime for OPVs tested under different conditions. The work also provides the recent progress in stability of unencapsulated OPVs with different architectures, as well as presents the updated diagram of the reported record lifetimes of OPVs. The presented work is another step forward towards the development of pass-fail testing standards and lifetime prediction tools for emerging PV technologies., This work has been supported by the European Commission's StableNextSol COST Action MP1307. The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 609788 (CHEETAH) and n° 290022 (ESTABLIS). ICN2 acknowledges the support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant Nos. SEV-2013-0295 and ENE2013-48816-C5-4-R. Thanks goes to Agencia de Gestio d'Ajuts Universitaris i de Recerca for the support through the Xarxa de Referencia en Materials Avançats per a l'Energia (XaRMAE) and consolidated research group No. 2014SGR-1212. This work has been carried out under the Materials Science Ph.D. Degree for A.M. of the Universitat Autònoma de Barcelona. R.R. and H.H. are grateful for financial support from BMBF (grant number 03EK3502) and ESF. F.P. received support from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 659747. L.S. and M.R. received supported from the Portuguese Foundation for Science and Technology in the framework of the Strategic Program UID/FIS/04650/2013.

From s-polarized, angle-dependent measurements of the electroluminescence spectra in a three-layer phosphorescent organic light-emitting diode, we calculate the exciton distribution inside the 35 nm thick emission layer. The shape of the exciton profile changes with the applied bias due to differing field dependencies of the electron and hole mobilities. A split emission zone with high exciton densities at both sides of the emission layer is obtained, which is explained by the presence of energy barriers and similar electron and hole mobilities. A peak in the transient electroluminescence signal after turn-off and the application of a reverse bias is identified as a signature of a split emission zone.

Typically, organic light-emitting diodes (OLEDs) are characterized only in steady-state to determine and optimize their efficiency. Adding further electro-optical measurement techniques in frequency and time domain helps to analyze charge carrier and exciton dynamics and provides deeper insights into the device physics. We, therefore, first present an overview of frequently used OLED measurement techniques and analytical models. A multilayer OLED with a sky-blue thermally activated delayed fluorescent dopant material is employed in this study without loss of generality. Combining the measurements with a full device simulation allows one to determine specific material parameters such as the charge carrier mobilities of all the layers. The main part of this tutorial focuses on how to systematically fit the measured OLED characteristics with microscopic device simulations based on a charge drift-diffusion and exciton migration model in 1D. Finally, we analyze the correlation and sensitivity of the determined material parameters and use the obtained device model to understand limitations of the specific OLED device.Typically, organic light-emitting diodes (OLEDs) are characterized only in steady-state to determine and optimize their efficiency. Adding further electro-optical measurement techniques in frequency and time domain helps to analyze charge carrier and exciton dynamics and provides deeper insights into the device physics. We, therefore, first present an overview of frequently used OLED measurement techniques and analytical models. A multilayer OLED with a sky-blue thermally activated delayed fluorescent dopant material is employed in this study without loss of generality. Combining the measurements with a full device simulation allows one to determine specific material parameters such as the charge carrier mobilities of all the layers. The main part of this tutorial focuses on how to systematically fit the measured OLED characteristics with microscopic device simulations based on a charge drift-diffusion and exciton migration model in 1D. Finally, we analyze the correlation and sensitivity of the determined...