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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

Solution-processable interlayers are an important building block for the commercialization of organic electronic devices such as organic solar cells. Here, the potential of cross-linking to provide an insoluble, stable and versatile charge transport layer based on soluble organic semiconductors is studied. For this purpose, a photo-reactive tris-azide cross-linker is synthesized. The capability of the small molecular cross-linker is illustrated by applying it to a p-doped polymer used as a hole transport layer in organic solar cells. High cross-linking efficiency and excellent charge extraction properties of the cross-linked doped hole transport layer are demonstrated. However, at high doping levels in the interlayer, the solar cell efficiency is found to deteriorate. Based on charge extraction measurements and numerical device simulations, it is shown that this is due to diffusion of dopants into the active layer of the solar cell. Thus, in the development of future cross-linker materials, care must be taken to ensure that they immobilize not only the host, but also the dopants., Comment: 24 pages, 5 figures, supplemental material

Perovskite solar cells (PSCs) longevity is nowadays the bottleneck for their full commercial exploitation. Although lot of research is ongoing, the initial decay of the output power - an effect known as "burn-in" degradation happening in the first 100 h - is still unavoidable, significantly reducing the overall performance (typically of20%). In this paper, the origin of the "burn-in" degradation in n-i-p type PSCs is demonstrated that is directly related to Li

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.

We consider dynamic route planning for a fleet of Autonomous Mobile Robots (AMRs) doing fetch and carry tasks on a shared factory floor. In this paper, we propose Stochastic Work Graphs (SWG) as a formalism for capturing the semantics of such distributed and uncertain planning problems. We encode SWGs in the form of a Euclidean Markov Decision Process (EMDP) in the tool Uppaal Stratego, which employs Q-Learning to synthesize near-optimal plans. Furthermore, we deploy the tool in an online and distributed fashion to facilitate scalable, rapid replanning. While executing their current plan, each AMR generates a new plan incorporating updated information about the other AMRs positions and plans. We propose a two-layer Model Predictive Controller-structure (waypoint and station planning), each individually solved by the Q-learning-based solver. We demonstrate our approach using ARGoS3 large-scale robot simulation, where we simulate the AMR movement and observe an up to 27.5% improvement in makespan over a greedy approach to planning. To do so, we have implemented the full software stack, translating observations into SWGs and solving those with our proposed method. In addition, we construct a benchmark platform for comparing planning techniques on a reasonably realistic physical simulation and provide this under the MIT open-source license. We consider dynamic route planning for a fleet of Au-tonomous Mobile Robots (AMRs) doing fetch and carry taskson a shared factory floor. In this paper, we propose StochasticWork Graphs (SWG) as a formalism for capturing the seman-tics of such distributed and uncertain planning problems. Weencode SWGs in the form of a Euclidean Markov DecisionProcess (EMDP) in the tool UPPAALSTRATEGO, which em-ploys Q-Learning to synthesize near-optimal plans. Further-more, we deploy the tool in an online and distributed fashionto facilitate scalable, rapid replanning. While executing theircurrent plan, each AMR generates a new plan incorporat-ing updated information about the other AMRs positions andplans. We propose a two-layer Model Predictive Controller-structure (waypoint and station planning), each individuallysolved by the Q-learning-based solver. We demonstrate ourapproach using ARGoS3 large-scale robot simulation, wherewe simulate the AMR movement and observe an up to 27.5%improvement in makespan over a greedy approach to plan-ning. To do so, we have implemented the full software stack,translating observations into SWGs and solving those withour proposed method. In addition, we construct a benchmarkplatform for comparing planning techniques on a reasonablyrealistic physical simulation and provide this under the MITopen-source license.

Publisher Copyright: © 2022 The Royal Society of Chemistry Despite the outstanding power conversion efficiency of triple-cation perovskite solar cells (PSCs), their low long-term stability in the air is still a major bottleneck for practical applications. The hygroscopic dopants traditionally used in hole transport materials (HTMs) severely degrade the perovskite film. The p-type F4-TCNQ doping of the well-known spiro-OMeTAD HTM enables hydrophobicity-induced protection of the perovskite layer underneath. Nevertheless, the mechanism of F4-TCNQ doping in stabilizing PSCs is still rather unclear. Herein, when F4-TCNQ was adopted as the sole dopant of spiro-OMeTAD, highly stable mesoporous triple-cation PSCs were developed, with a very long T80 lifetime of more than 1 year (∼380 days) for devices stored in air (RH ∼ 40%). The present comprehensive experimental and theoretical studies on F4-TCNQ-doped spiro-OMeTAD reveal that the hydrophobic protection of the perovskite layer underneath is not the only reason for the increased long-term stability of the devices. The highuniformity of F4-TCNQ doping in the spiro-OMeTAD layer and less dopant aggregation and dopant migration towards the anode are key factors responsible for the increased stability of the perovskite solar cells when compared to conventional hygroscopic dopants. This work paves the way for future doping engineering of HTMs for PSCs with competitive stability.

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.

Titanium dioxide (TiO

Vacuum-thermal evaporation (VTE) is a highly relevant fabrication route for organic solar cells (OSCs), especially on an industrial scale as proven by the commercialisation of organic light emitting diode (OLED) based displays. While OSC performance is reported for a range of VTE-deposited molecules, a comprehensive assessment of donor:acceptor blend properties with respect to their photovoltaic performance is scarce. Here, we fabricate organic thin films and solar cells of three select systems and measure ellipsometry, external quantum efficiency with high dynamic range, as well as OTRACE to quantify absorption, voltage losses, and charge carrier mobility. These parameters are key to explain OSC performance and will help to rationalize the performance of other material systems reported in literature as our methodology is applicable beyond VTE systems. Furthermore, it could help to judge the prospects of new molecules in general. We find large differences in the measured values and find that today's VTE OSCs can reach high extinction coefficients, but only moderate mobility and voltage loss compared to their solution-processed counterparts. We outline what needs to improve for VTE organic solar cells to again catch up with their solution-processed counterparts in terms of power conversion efficiency. This article is protected by copyright. All rights reserved.

The main losses in perovskite solar cells (PSCs) are often related to their charge-transport layers (CTLs). While it has been shown how losses related to charge extraction and recombination at the CTL interfaces can be minimized individually, a comprehensive picture is still lacking. In this work, we describe how recombination at interfaces is related to the distribution of electric potential within the device based on drift-diffusion simulations. Our results show that interface recombination in PSCs can only be fully understood when the whole device is considered. We find that devices with poor conductivity in the CTLs or small built-in potential will suffer from fast recombination at interfaces and that the interface recombination can be avoided by improving these. To illustrate our results, we present a framework that provides an intuitive way to understand electrical potential and compare losses in PSCs. We show that the simulation data can be well understood within the framework for the whole range of parameters studied.

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 C

Implantasjon av hjertestarter (implantable cardioverter defibrillator, ICD) er etablert behandling hos pasienter med høy risiko for plutselig hjertedød. Studiens formål var å kartlegge pasientkarakteristika, indikasjoner, hyppigheten av ICD-støt, komplikasjoner, reoperasjoner samt endringer over tid i ICD-behandlingen ved St. Olavs hospital.Alle pasienter som fikk implantert hjertestarter ved St. Olavs hospital i perioden 2006-15 ble inkludert. Pasientene ble identifisert i pacemakerregisteret. Data ble hentet fra pacemakerregisteret og elektronisk pasientjournal.Studien inkluderte 598 pasienter (82 % menn, medianalder 65 år). Tidligere hjertestans eller alvorlig arytmi forelå hos 401 (67 %) av dem som fikk implantert hjertestarter. Koronarsykdom (n = 383) var vanligste underliggende årsak. I oppfølgingstiden (median 3,6 år) fikk 203 (34 %) av pasientene ICD-støt, 154 (26 %) fikk berettigede og 65 (11 %) fikk uberettigede støt. Hos 139 (23 %) pasienter oppstod komplikasjoner. 101 (17 %) pasienter døde i oppfølgingsperioden.Studien gir et godt grunnlag for kvalitetssikring av implantasjonsvirksomheten ved St. Olavs hospital. Kjønns- og aldersfordeling, indikasjon og underliggende årsaker for implantasjon samt hyppighet av støt og komplikasjoner samsvarer godt med tidligere publiserte data.

The influence of an interfacial dipole at the donor/acceptor interface on the generation and bulk recombination in P3HT:fullerene blends is clarified. We use P3HT:PC 60 BM and P3HT:ICBA blends which have a large and small interfacial dipole, respectively. We show that polarons are generated using above and below gap excitations in both blends. The polaron density generated using below gap excitation is lower in P3HT:ICBA blends which is suggested to be due to a higher lying CT state compared to P3HT:PC 60 BM blends. Using intensity and frequency dependent measurements, we show that the bulk recombination is surprisingly similar in both blends. The bulk recombination is dominated by bimolecular recombination at all light-intensities for the delocalized polarons, while for the localized polaron trap-assisted recombination dominates at low intensities which becomes bimolecular at high intensities. For the localized polaron we find that the characteristic energy for the trap depth to be E c h = 24 . 4 ± 0 . 7 meV and E c h = 21 . 3 ± 0 . 3 for the P3HT:ICBA and P3HT:PC 60 BM blends, respectively. Comparing the ratio between the bimolecular recombination constant and the mobility, we find the recombination to be practically the same in the two blends. However, while the localized polaron does show a difference of a factor of two, suggesting faster bimolecular recombination and stronger trap-assisted recombination in the P3HT:ICBA blend having a small interfacial dipole, we conclude that interfacial dipoles have little or no effect on the bulk recombination of long-lived photoexcitations in these blends.

The recently introduced perovskite solar cell (PSC) technology is a promising candidate for providing low-cost energy for future demands. However, one major concern with the technology can be traced back to morphological defects in the electron selective layer (ESL), which deteriorates the solar cell performance. Pinholes in the ESL may lead to an increased surface recombination rate for holes, if the perovskite absorber layer is in contact with the fluorine-doped tin oxide (FTO) substrate via the pinholes. In this work, we used sol-gel-derived mesoporous TiO2 thin films prepared by block co-polymer templating in combination with dip coating as a model system for investigating the effect of ESL pinholes on the photovoltaic performance of planar heterojunction PSCs. We studied TiO2 films with different porosities and film thicknesses, and observed that the induced pinholes only had a minor impact on the device performance. This suggests that having narrow pinholes with a diameter of about 10 nm in the ESL is in fact not detrimental for the device performance and can even, to some extent improve their performance. A probable reason for this is that the narrow pores in the ordered structure do not allow the perovskite crystals to form interconnected pathways to the underlying FTO substrate. However, for ultrathin (~20 nm) porous layers, an incomplete ESL surface coverage of the FTO layer will further deteriorate the device performance. publishedVersion

The field of organic solar cells has recently gained broad research interest due to the introduction of non-fullerene small-molecule acceptors. The rapid improvement in solar cell efficiency put in...

BAKGRUNN Implantasjon av hjertestarter (implantable cardioverter defibrillator, ICD) er etablert behandling hos pasienter med hoy risiko for plutselig hjertedod. Studiens formal var a kartlegge pasientkarakteristika, indikasjoner, hyppigheten av ICD-stot, komplikasjoner, reoperasjoner samt endringer over tid i ICD-behandlingen ved St. Olavs hospital. MATERIALE OG METODE Alle pasienter som fikk implantert hjertestarter ved St. Olavs hospital i perioden 2006-15 ble inkludert. Pasientene ble identifisert i pacemakerregisteret. Data ble hentet fra pacemakerregisteret og elektronisk pasientjournal. RESULTATER Studien inkluderte 598 pasienter (82 % menn, medianalder 65 ar). Tidligere hjertestans eller alvorlig arytmi forela hos 401 (67 %) av dem som fikk implantert hjertestarter. Koronarsykdom (n = 383) var vanligste underliggende arsak. I oppfolgingstiden (median 3,6 ar) fikk 203 (34 %) av pasientene ICD-stot, 154 (26 %) fikk berettigede og 65 (11 %) fikk uberettigede stot. Hos 139 (23 %) pasienter oppstod komplikasjoner. 101 (17 %) pasienter dode i oppfolgingsperioden. FORTOLKNING Studien gir et godt grunnlag for kvalitetssikring av implantasjonsvirksomheten ved St. Olavs hospital. Kjonns- og aldersfordeling, indikasjon og underliggende arsaker for implantasjon samt hyppighet av stot og komplikasjoner samsvarer godt med tidligere publiserte data.

The recently introduced perovskite solar cell (PSC) technology is a promising candidate for providing low-cost energy for future demands. However, one major concern with the technology can be traced back to morphological defects in the electron selective layer (ESL), which deteriorates the solar cell performance. Pinholes in the ESL may lead to an increased surface recombination rate for holes, if the perovskite absorber layer is in contact with the fluorine-doped tin oxide (FTO) substrate via the pinholes. In this work, we used sol-gel-derived mesoporous TiO2 thin films prepared by block co-polymer templating in combination with dip coating as a model system for investigating the effect of ESL pinholes on the photovoltaic performance of planar heterojunction PSCs. We studied TiO2 films with different porosities and film thicknesses, and observed that the induced pinholes only had a minor impact on the device performance. This suggests that having narrow pinholes with a diameter of about 10 nm in the ESL is in fact not detrimental for the device performance and can even, to some extent improve their performance. A probable reason for this is that the narrow pores in the ordered structure do not allow the perovskite crystals to form interconnected pathways to the underlying FTO substrate. However, for ultrathin (~20 nm) porous layers, an incomplete ESL surface coverage of the FTO layer will further deteriorate the device performance.

Perovskite solar cells with record efficiencies already above 24% are a highly promising clean energy technology. However, the reproducibility in their fabrication has proven to be challenging and needs more attention. Here we demonstrate that surface activation of the mesoscopic titanium dioxide (TiO2) scaffold, utilized in the two-step perovskite synthesis process, significantly affects the final device performance. Irradiating the mesostructured substrate with ultraviolet (UV) light prior to lead iodide (PbI2) deposition has a positive effect on the short-circuit current density and on the overall device performance (leading to a >20% increase in efficiency in our devices). As most of the UV light is absorbed in the topmost TiO2 layer, the interior of the scaffold remains less activated. This results in a sparsely packed PbI2 structure that facilitates an efficient conversion to the perovskite, while the activated topmost surface improves the perovskite capping layer. On the contrary, plasma treatment of the scaffold also activates the interior parts of the scaffold, which leads to a dense PbI2 structure that hampers the conversion and causing a >25% efficiency drop. We show that also minor changes in the surface properties of the mesoporous TiO2 scaffold can affect the device performance, which could explain some of the large efficiency variations observed between laboratories. acceptedVersion

Unintentional doping of the active layer is a source for lowered device performance in organic solar cells. The effect of doping is to induce a space-charge region within the active layer, generally resulting in increased recombination losses. In this work, the impact of a doping-induced space-charge region on the current-voltage characteristics of low-mobility solar cell devices has been clarified by means of analytical derivations and numerical device simulations. It is found that, in case of a doped active layer, the collection efficiency of photo-generated charge carriers is independent of the light intensity and exhibits a distinct voltage dependence, resulting in an apparent electric-field dependence of the photocurrent. Furthermore, an analytical expression describing the behavior of the photocurrent is derived. The validity of the analytical model is verified by numerical drift-diffusion simulations and demonstrated experimentally on solution-processed organic solar cells. Based on the theoretical results, conditions of how to overcome charge collection losses caused by doping are discussed. Furthermore, the presented analytical framework provides tools to distinguish between different mechanisms leading to voltage dependent photocurrents., Main text (29 pages, 7 figures) and supplemental material (7 pages, 4 figures)

Organic hole-transporting materials (HTMs), AZO-I and AZO-II, were synthesized via Schiff base chemistry by functionalizing a phenothiazine core with triarylamine(s) through azomethine bridges. Substantial enhancements in the power conversion efficiency (PCE = 12.6% and 14% for AZO-I and AZO-II, respectively) and stability (68% or 91% of PCE retained after 60 days for AZO-I or AZO-II, respectively) of perovskite solar cells (PSCs) were achieved when switching from mono- (AZO-I) to disubstituted (AZO-II) HTMs. The extremely low production costs (9 and 12 $/g for AZO-I and AZO-II, respectively), together with the Pd-catalyst-free synthesis, make these materials excellent candidates for low-cost and eco-friendly PSCs. publishedVersion

The charge extraction by a linearly increasing voltage (CELIV) technique is a well-known and commonly used method to characterize charge transport in low-mobility materials. In the original CELIV theory it is assumed that one type of charge carrier is mobile and the other fixed and that recombination during the extraction pulse is negligible. However, this is in general not the case, especially in photo-CELIV where both electrons and holes are generated by light excitation. Moreover, RC effects induced by the series or load resistance of the external circuit are typically assumed to be negligible. In this work, we use drift-diffusion modelling and analytical derivations to show that the standard equations used for calculating the mobility in the moderate conductivity regimes generally leads to errors in the mobility determination in the case when i) two carrier types of similar mobility, ii) recombination, iii) an electric-field-dependent mobility, and iv) RC effects are present in the device. The effect of the external series resistance on the mobility determination becomes of particular importance in devices with relatively large mobilities and/or high carrier concentrations, where the original CELIV theory might give rise to an underestimation of the mobility by several orders of magnitude.

The charge-carrier mobility and the built-in potential are important device parameters for optimizing the performance of thin-film solar cells. Charge extraction by a linearly increasing voltage (CELIV) is a commonly used technique for mobility measurements, but the original CELIV method applies only to devices with blocking contact---a severe limitation, since most operating devices have Ohmic contacts. Here the CELIV method is extended to determine these parameters in thin-film metal-insulator-metal ($m\ensuremath{-}i\ensuremath{-}m$) or $p\ensuremath{-}i\ensuremath{-}n$ diodes, rendering it a powerful tool for such devices.

Organic solar cells with an electron donor diluted in a fullerene matrix have a reduced density of donor-fullerene contacts, resulting in decreased free-carrier recombination and increased open-circuit voltages. However, the low donor concentration prevents the formation of percolation pathways for holes. Notwithstanding, high (>75%) external quantum efficiencies can be reached, suggesting an effective hole-transport mechanism. Here, we perform a systematic study of the hole mobilities of 18 donors, diluted at similar to 6 mol % in C-60, with varying frontier energy level offsets and relaxation energies. We find that hole transport between isolated donor molecules occurs by long-range tunneling through several fullerene molecules, with the hole mobilities being correlated to the relaxation energy of the donor. The transport mechanism presented in this study is of general relevance to bulk heterojunction organic solar cells where mixed phases of fullerene containing a small fraction of a donor material or vice versa are present as well. The work was funded by the German Federal Ministry for Education and Research (BMBF) through the InnoProfile Projekt "Organische p-i-n Bauelemente 2.2" (03IPT602X). This work was supported by the Deutsche Forschungsgemeinschaft (project OR 349/1-1), by the Department of the Navy, Office of Naval Research Award No. N00014-14-1-0580 (CAOP MUM), by the German Ministry of Science and Education (BMBF), project UNVEiL, and through a state sponsored scholarship for graduate students to Y.F. from the China Scholarship Council. M.N. acknowledges funding from "Svenska Tekniska Vetenskapsakademien i Finland". Grants for computing time from the Center for Information Services and High Performance Computing (ZIH) are gratefully acknowledged.

Processes taking place at contacts are of particular importance in organic and perovskite solar cells where selective contacts that are able to efficiently collect majority carriers, simultaneously blocking minority carriers are desired. The surface recombination velocity S_R, describing the quality of the contact interface, is a key parameter in obtaining an increased understanding of the kinetics taking place at contacts in thin-film devices [1]. We have extended the analytical framework of the charge extraction by linearly increasing voltage (CELIV) theory taking the effect of built-in voltage, diffusion and band-bending into account [2] and show how we can experimentally quantify loss mechanisms in charge collection [3-4]. We have derived analytical expressions describing the effective reduction of the built-in voltage and the (effective) open-circuit voltage providing means to quantify and distinguish various (loss) mechanisms for contact related effects in thin film solar cells [2-4]. References [1] O. Sandberg, M. Nyman, R. Osterbacka, Physical Review Applied 1, 024003 (2014) [2] O. Sandberg, M. Nyman, R. Osterbacka, Organic Electronics 15, 3413-3420 (2015) [3] A. Sundqvist, M. Nyman, O. Sandberg, S. Sanden, J.-H. Smatt, and R. Osterbacka, Advanced Energy Materials, 1502265 (2016) [4] O.J. Sandberg, et. al, Physical Review Letters, 118, 076601 (2017).

We present charge transport properties of intercalated and non-intercalated PBTTT:fullerene based bulk heterojunctions measured with the photo-CELIV technique. The mobilities are seen to vary greatly with fullerene loading in intercalated systems whereas the mobility in non-intercalated blends is fairly constant. The transport activation energies and widths of the Gaussian density of states (DOS) indicates lower energetic disorder in intercalated blends. The highest room temperature mobility of 2 × 10−3 cm2/Vs was obtained for a blend of 1:4 weight ratio PBTTT:PCBM, which is an order of magnitude higher than the other blends studied. We attribute the high mobility to lower energetic disorder and stronger electronic coupling between the transport sites due to shorter inter-site distances in the intercalated blend.

The kinetics at contacts plays a crucial role in sandwich-type thin-film devices based on organic semiconductors. This is of particular importance in organic and perovskite solar cells where selective contacts that are able to efficiently collect majority carriers, simultaneously blocking minority carriers, are desired. Despite the vast progress made, a comprehensive understanding, needed for developing new electrode materials to improve and optimize device performance is still lacking. A key parameter for obtaining information about processes taking place at the contacts is the effective surface recombination velocity.[1] However, means to quantitatively measure surface recombination velocities at contact interfaces in sandwich-type thin-film devices based on organic semiconductors are lacking. The Charge Extraction by a Linearly Increasing Voltage (CELIV) technique is one of the most common methods to measure the charge transport properties in organic semiconductor devices. In this work, we show how CELIV can be used to determine surface recombination velocities at selective and/or blocking contacts in thin-film devices. The analytical framework behind the method is presented, and confirmed by numerical drift-diffusion simulations. We furthermore demonstrate the method on organic semiconductor devices, employing TiO2 and SiO2 as cathode buffer layers. The method allows for an increased understanding of contact properties in sandwich-type thin-film devices based on organic semiconductors. [1] O. J. Sandberg, A. Sundqvist, M. Nyman, and R. Osterbacka, Phys. Rev. Appl. 5, 044005 (2016).

We have extended the analytical framework of the charge extraction by linearly increasing voltage (CELIV) theory to the case with doping, taking the effect of built-in voltage, diffusion and band-bending into account. Not taking the built-in voltage into account in dark CELIV will lead to an underestimation of the charge carrier density, while the mobility is overestimated. Furthermore, based on the analysis we propose to use CELIV in the doping-induced capacitive regime for direct determination of doping concentration and built-in voltage from extraction current transients in the time-domain of doped thin-film semiconductor devices. The analytical framework is confirmed numerically with a one-dimensional drift–diffusion model and experimentally on aged P3HT:PCBM bulk heterojunction solar cell devices. An excellent agreement between the experimental extraction current transients and the analytical prediction is found. The presented analytical treatment is not limited to sandwich-type thin-film devices, but is more general and the technique can also be extended to pn-junctions.

Leakage currents through the gate dielectric of thin film transistors remain a roadblock to the fabrication of organic field-effect transistors (OFETs) on ultrathin dielectrics. We report the first investigation of a self-assembled monolayer (SAM) dipole as an electrostatic barrier to reduce leakage currents in n-channel OFETs fabricated on a minimal, leaky ∼10 nm SiO2 dielectric on highly doped Si. The electric field associated with 1H,1H,2H,2H-perfluoro-octyltriethoxysilane (FOTS) and octyltriethoxysilane (OTS) dipolar chains affixed to the oxide surface of n-Si gave an order of magnitude decrease in gate leakage current and subthreshold leakage and a two order-of-magnitude increase in ON/OFF ratio for a naphthalenetetracarboxylic diimide (NTCDI) transistor. Identically fabricated devices on p-Si showed similarly reduced leakage and improved performance for oxides treated with the larger dipole FOTS monolayer, while OTS devices showed poorer transfer characteristics than those on bare oxide. Comparison of OFETs on both substrates revealed that relative device performance from OTS and FOTS treatments was dictated primarily by the organosilane chain and not the underlying siloxane-substrate bond. This conclusion is supported by the similar threshold voltages (VT) extrapolated for SAM-treated devices, which display positive relative VT shifts for FOTS on either substrate but opposite VT shifts for OTS treatment on n-Si and p-Si. Our results highlight the potential of dipolar SAMs as performance-enhancing layers for marginal quality dielectrics, broadening the material spectrum for low power, ultrathin organic electronics.

Thin-film solar cells are at the forefront of current energy research. A detailed theory to define how charges recombine---and so how performance is reduced---at electrical contacts to these films is lacking. This study derives analytic expressions that directly relate the open-circuit voltage to contact, material, and device properties. Drift-diffusion simulations verify their analytical expressions, and agree well with experiments.

We have clarified the origin of equilibrium charges in poly(3-hexylthiophene):[6,6]-phenyl-C 61 -butyric acid methyl ester devices by using charge extraction by a linearly increasing voltage. By varying the work function of the contacts we find that the equilibrium charges are injected from the contacts due to energy level alignment caused by Fermi-level pinning.

In the pursuit of developing new materials for more effi cient bulk-heterojunction solar cells, the blend poly (3-hexylthiophene):[6,6]-phenyl-C 61 -butyric acid methyl ester (P3HT:PCBM) serves as an important model system. The success of the P3HT:PCBM blend comes from effi cient charge generation and transport with low recombination. There is not, however, a good microscopic picture of what causes these, hindering the development of new material systems. In this report UV photoelectron spectroscopy measurements on both regiorandom- (rra) and regioregular- (rr) P3HT are presented, and the results are interpreted using the Integer Charge Transfer model. The results suggest that spontaneous charge transfer from P3HT to PCBM occurs after heat treatment of P3HT:PCBM blends. The resulting formation of an interfacial dipole creates an extra barrier at the interface explaining the reduced (non-)geminate recombination with increased charge generation in heat treated rr-P3HT:PCBM blends. Extensive photoinduced absorption measurements using both above- and below-bandgap excitation light are presented, in good agreement with the suggested dipole formation.

Using drift-diffusion simulations we have clarified the effect of 2-dimensional (2D) lamellar ordering on the device performance and, in particular, the open circuit voltage in donor-acceptor type organic solar cells. The simulations are performed both in systems where direct (band-to-band) recombination dominates and in systems where trap-assisted recombination dominates. Results show that lamellar ordering reduces both the amount of direct and trap-assisted recombination, which is beneficial for device performance. The effect is particularly prominent for small lamellar thicknesses (~ 1 nm). It is furthermore shown that in the case of s-shaped current-voltage characteristics due to electrostatic injection barriers the s-shape becomes less prominent for thinner lamellar thicknesses.

The charge extraction (of injected carriers) by linearly increasing voltage in metal-insulator-semiconductor structures, or MIS-CELIV, is based on the theory of space-charge-limited currents. In this work, the validity of MIS-CELIV for mobility determination in organic thin-film devices has been critically examined and clarified by means of drift-diffusion simulations. It is found that depending on the applied transient voltage, the mobility might be overestimated by several orders of magnitude in the case of an ohmic injecting contact. The shortcomings of the MIS-CELIV theory can be traced back to the underlying assumption of a drift-dominated transport. However, the effect of diffusion can be taken into account by introducing a correction factor. In the case of non-ohmic injecting contacts, the extracted mobility becomes strongly dependent on device parameters, possibly leading to large deviations from the actual mobility.

Charge selective interlayers are of critical importance in order for solar cells based on low mobility materials, such as polymer-fullerene blends, to perform well. Commonly used anode interlayers consist of high work function transition metal oxides, with molybdenum trioxide (MoO3) being arguably the most used. Here, it is shown that a thin interlayer of MoO3 causes unintentional bulk doping in solar cells based on polymers and polymer-fullerene blends. The doping concentrations determined from capacitance–voltage measurements are larger than 1016 cm−3 and are seen to increase closer to the anode, reference devices without MoO3 are undoped. Using time of flight secondary ion mass spectroscopy it is furthermore shown that molybdenum is present on the surface of all films with an interfacial layer of MoO3 beneath the active layer. Doping concentrations of this magnitude are detrimental for device performance, especially for active layers >100 nm.

We have measured the effect of degradation on the I-V characteristics in APFO3:PCBM solar cells. The solar cell devices were subject to ambient air under simulated solar illumination. We found that the degradation resulted in a lowering of the fill factor and short circuit current while the open circuit voltage remained unchanged. In order to gain insight into what has caused the degraded I-V characteristics we have studied the active layer film using various techniques. We found clear spectral changes both in absorption and in photoinduced absorption spectroscopy correlated with increased carrier lifetimes and lowered mobility when comparing the degraded film with a pristine one. The results show a significant degradation of the active layer causing a lower fill-factor and short circuit current.

Inverted organic solar cells generally exhibit a strong s-shaped kink in the current–voltage characteristics (JV curve) that may be removed by exposure to UV light (light-soaking) leading to a drastically improved performance. Using in-device characterization methods the origin of the light-soaking issue in inverted solar cells employing titanium dioxide (TiO2) as an electron selective layer is clarified. An injected hole reservoir accumulated at the TiO2/organic interface of the pristine device is observed from extraction current transients; the hole reservoir increases the recombination and results in an s-shape in the JV curve of pristine devices. The hole reservoir and the s-shape is a result of the energetics at the selective contact in the pristine device; the effect of UV exposure is to decrease the work function of the indium tin oxide/TiO2-contact, increasing the built-in potential. This hinders the build-up of the hole reservoir and the s-shape is removed. The proposed model is in excellent agreement with drift-diffusion simulations.

The impact of trapping on the recombination dynamics in polymer:fullerene blends is clarified using the highly ordered bulk heterojunction (BHJ) blend poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) at different weight ratios as a model system. The recombination dynamics are determined using both transient charge extraction and steady-state techniques. The results show that both the decay of photogenerated charge and the light ideality factor at a polymer:fullerene weight ratio of 1:4 are fully consistent with 2D Langevin recombination; in the 1:1 case the recombination is seen to be affected by electron trapping. The theory of 2D Langevin recombination is extended to the case with high trap density in agreement with the observations in the 1:1 case. The recombination capture coefficients are derived both for trap-assisted and band-to-band recombination and it can be seen that anisotropic charge transport reduces the capture coefficients in both cases resulting in a reduced overall recombination.

We present charge extraction by a linearly increasing voltage measurements on diodes based on an n-channel naphthalenetetracarboxylic acid diimide semiconductor and an aluminum oxide blocking layer. Results show a large displacement current (roughly 15 times that expected from the geometrical capacitance), which we associate with trap filling in the oxide. The trap density is calculated to be on the order of 1019 cm−3, in agreement with preceding work. We present a way of using the displacement current as a tool for probing the vacuum level shift caused by modifying the oxide surface with self-assembled monolayers in operating devices.

Using drift-diffusion simulations, we have clarified the effect of two-dimensional lamellar ordering on the device performance and, in particular, the open circuit voltage in donor–acceptor type organic solar cells. The simulations are performed both in systems where direct (band-to-band) recombination dominates and in systems where trap-assisted recombination dominates. Results show that lamellar ordering reduces both the amount of direct and trap-assisted recombination, which is beneficial for device performance. The effect is particularly prominent for small lamellar thicknesses (∼ 1 nm ). It is furthermore shown that in the case of s-shaped current–voltage characteristics due to electrostatic injection barriers the s-shape becomes less prominent for thinner lamellar thicknesses.