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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

n–i–p Type perovskite solar cells generally require air oxidation of the Spiro-OMeTAD layer to achieve high power conversion efficiency (PCE). However, the detailed oxidation mechanism is still not fully understood. In this paper, oxidation of Spiro-OMeTAD was demonstrated via a non-contact electrochemical route using UV-Vis absorption, laser beam induced current (LBIC) imaging and secondary ion mass spectrometry (SIMS) profiling of the Spiro-OMeTAD films. At the cathode, oxygen is reduced to form OH− with the help of H2O, while the anodic reaction is the oxidation of Spiro-OMeTAD to form Spiro-OMeTAD+. Diffusion of Li+ towards the surface of the Ag electrode completes the electrochemical cycle and increases the conductivity of the hole-transporting layer. SIMS analyses of the completed devices demonstrate that the oxidation of Spiro-OMeTAD also leads to migration of Li+ through the perovskite layer into SnO2, which supposedly leads to an increase of the built-in voltage. We verify these results by incorporation of the experimentally measured Li+ concentration into a numerical drift-diffusion simulation, to replicate solar cell J–V-curves. This work provides a new insight into the oxidation of Spiro-OMeTAD in perovskite solar cells, and demonstrates that Li+ migration is involved in the oxidation of Spiro-OMeTAD.

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.

Bioelectronic transducing surfaces that are nanometric in size have been the main route to detect single molecules. Though enabling the study of rarer events, such methodologies are not suited to assay at concentrations below the nanomolar level. Bioelectronic field-effect-transistors with a wide (μm

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.

Early diagnosis in a premalignant (or pre-invasive) state represents the only chance for cure in neoplastic diseases such as pancreatic-biliary cancer, which are otherwise detected at later stages and can only be treated using palliative approaches, with no hope for a cure. Screening methods for the purpose of secondary prevention are not yet available for these cancers. Current diagnostic methods mostly rely on imaging techniques and conventional cytopathology, but they do not display adequate sensitivity to allow valid early diagnosis. Next-generation sequencing can be used to detect DNA markers down to the physical limit; however, this assay requires labeling and is time-consuming. The additional determination of a protein marker that is a predictor of aggressive behavior is a promising innovative approach, which holds the potential to improve diagnostic accuracy. Moreover, the possibility to detect biomarkers in blood serum offers the advantage of a noninvasive diagnosis. In this study, both the DNA and protein markers of pancreatic mucinous cysts were analyzed in human blood serum down to the single-molecule limit using the SiMoT (single-molecule assay with a large transistor) platform. The SiMoT device proposed herein, which exploits an inkjet-printed organic semiconductor on plastic foil, comprises an innovative 3D-printed sensing gate module, consisting of a truncated cone that protrudes from a plastic substrate and is compatible with standard ELISA wells. This 3D gate concept adds tremendous control over the biosensing system stability, along with minimal consumption of the capturing molecules and body fluid samples. The 3D sensing gate modules were extensively characterized from both a material and electrical perspective, successfully proving their suitability as detection interfaces for biosensing applications. KRAS and MUC1 target molecules were successfully analyzed in diluted human blood serum with the 3D sensing gate functionalized with b-KRAS and anti-MUC1, achieving a limit of detection of 10 zM and 40 zM, respectively. These limits of detection correspond to (1 ± 1) KRAS and (2 ± 1) MUC1 molecules in the 100 μL serum sample volume. This study provides a promising application of the 3D SiMoT platform, potentially facilitating the timely, noninvasive, and reliable identification of pancreatic cancer precursor cysts. Graphical abstract

Organic water-gated thin-film transistors (WG-TFTs) are of great interest in developing low-cost and high-performance biosensors. The device's sensitivity to changes in measurement conditions can impair long-term operation, and care must be taken to ensure that the WG-TFT sensor response is due to an actual biorecognition event occurring on the sensing electrode. This work aims to clarify the long-term stability of a poly-3-hexylthiophene (P3HT) WG-TFT operated intermittently over two months during 5750 measurement cycles. We have evaluated the device figures of merit (FOM), such as threshold voltage, mobility, and trap density, during the whole measurement period. Short-term changes in the FOM are mainly attributed to work function changes on the gate electrode, whereas long-term changes are consistent with an increase in the semiconductor trap density. The shift in threshold voltage and decrease in mobility are found to be linear as a function of measurement cycles and caused by electrical stress, with time immersed in water having a negligible effect on the device. The trap density-of-states estimated using the subthreshold slope is similar to earlier reported values for P3HT OFETs and exhibits a gradual increase during device use and a partial recovery after rest, indicating the formation of shorter- and longer-lived traps., Comment: Eight pages and five figures

Titanium dioxide (TiO

Sensitivity response is a critical parameter that decides the domain of dielectric materials to be implemented as piezocapacitive sensors for low- or high-pressure sensing applications. Here, we have clarified the sensitivity response behavior of three low-cost dielectric materials, two biodegradable paperboards, and one acoustic polymeric foam. The devices are fabricated in the form of a metal–insulator–metal structure, and the capacitive response of the devices is measured using the charge extraction by linearly increasing voltage technique. The sensitivity response curve (ΔC/C o vs. pressure) reveals that the paperboard materials are sensitive enough to detect low-pressure regimes (45 kPa), whereas the acoustic foam is quite promising for high-pressure monitoring (above 150 kPa). Using a multiplexer circuit, we demonstrated the sensitivity response via 2 by 2 matrix structure both as a steady-state and transient response. Our results show that the passive matrix structure interference between different pixels can be minimized after increasing the spacing between electrodes strip. Finally, a full-scale demonstrator (dimension 120 cm × 400 cm) with a 2 × 8 matrix structure laminated under floor tiling has been demonstrated. We show how such a floor sensor utilizing the low-cost substrates can be used to recognize single-stepping, walking, and falling.

Non-radiative losses to the open-circuit voltage are a primary factor in limiting the power conversion efficiency of organic photovoltaic solar cells. The dominate non-radiative loss is intrinsic to the active layer which, along with the thermodynamic limit to the open-circuit voltage, define the quasi-Fermi level splitting (QFLS). Quantification of the QFLS in organic photovoltaic devices is challenging due to the excitonic nature of photoexcitation and device-related non-radiative losses. In this presentation I will outline an experimental approach based on electro-modulated photoluminescence to quantify the QFLS in organic solar cells. Drift-diffusion simulations are used to verify the accuracy of the method, while state-of-the art PM6:Y6 solar cells are created with varying non-radiative losses. This method quantifies the QFLS in organic photovoltaics, fully characterizing the magnitude of different contributions to the non-radiative losses of the open-circuit voltage.

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.

Nonradiative losses to the open-circuit voltage are a primary factor in limiting the power-conversion efficiency of organic photovoltaic devices. The dominate nonradiative loss in the bulk is intrinsic to the active layer and can be determined from the quasi-Fermi-level splitting (QFLS) and the radiative thermodynamic limit of the photovoltage. Quantification of the QFLS in thin-film devices with low mobility is challenging due to the excitonic nature of photoexcitation and additional sources of nonradiative loss associated with the device structure. This work outlines an experimental approach based on electromodulated photoluminescence, which can be used to directly measure the intrinsic nonradiative loss to the open-circuit voltage, thereby quantifying the QFLS. Drift-diffusion simulations are carried out to show that this method accurately predicts the QFLS in the bulk of the device regardless of device-related nonradiative losses. State-of-the-art PM6:Y6-based organic solar cells are used as a model to test the experimental approach and the QFLS is quantified and shown to be independent of device architecture. This work provides a method to quantify the QFLS of organic solar cells under operational conditions, fully characterizing the different contributions to the nonradiative losses of the open-circuit voltage. The reported method will be useful not only in characterizing and understanding losses in organic solar cells but also in other device platforms such as light-emitting diodes and photodetectors.

Approaches to increase the efficiency in developing drugs and diagnostics tools, including new drug delivery and diagnostic technologies, are needed for improved diagnosis and treatment of major diseases and health problems such as cancer, inflammatory diseases, chronic wounds, and antibiotic resistance. Development within several areas of research ranging from computational sciences, material sciences, bioengineering to biomedical sciences and bioimaging is needed to realize innovative drug development and diagnostic (DDD) approaches. Here, an overview of recent progresses within key areas that can provide customizable solutions to improve processes and the approaches taken within DDD is provided. Due to the broadness of the area, unfortunately all relevant aspects such as pharmacokinetics of bioactive molecules and delivery systems cannot be covered. Tailored approaches within (i) bioinformatics and computer-aided drug design, (ii) nanotechnology, (iii) novel materials and technologies for drug delivery and diagnostic systems, and (iv) disease models to predict safety and efficacy of medicines under development are focused on. Current developments and challenges ahead are discussed. The broad scope reflects the multidisciplinary nature of the field of DDD and aims to highlight the convergence of biological, pharmaceutical, and medical disciplines needed to meet the societal challenges of the 21st century.

Uniform and pinhole-free electron-selective TiO2 layers are of utmost importance for efficient perovskite solar cells. Here we used a scalable and low-cost dip-coating method to prepare uniform and ultrathin (5–50 nm) compact TiO2 films on fluorine-doped tin oxide (FTO) glass substrates. The thickness of the film was tuned by changing the TiCl4 precursor concentration. The formed TiO2 follows the texture of the underlying FTO substrates, but at higher TiCl4 concentrations, the surface roughness is substantially decreased. This change occurs at a film thickness close to 20–30 nm. A similar TiCl4 concentration is needed to produce crystalline TiO2 films. Furthermore, below this film thickness, the underlying FTO might be exposed resulting in pinholes in the compact TiO2 layer. When integrated into mesoscopic perovskite solar cells there appears to be a similar critical compact TiO2 layer thickness above which the devices perform more optimally. The power conversion efficiency was improved by more than 50% (...

The generation and recombination of long-lived photoexcitations is clarified in TQ1 films and TQ1:PC71BM 1:1 and 1:3, by weight, blends using photoinduced absorption measurements. At 80 K triplets are formed in TQ1 films, while both triplets and polarons are formed in the 1:1 and 1:3 blends. We suggest that the triplet state acts as a loss mechanism for generation of free charges in these blends and suggest an energy diagram for the photoexcitations in the blends. We estimate the triplet polaron annihilation (γTPA) constant to be 1.12 × 10–14 and 3.10 × 10–13 cm3 s–1 for the TQ1:PC71BM 1:1 and 1:3 blends, respectively. At 300 K triplets are mainly formed in the TQ1 films, while only polarons are present in the TQ1:PC71BM blends. Using frequency measurements we show that the TQ1:PC71BM 1:1 blend shows nondispersive kinetics (ζ = 1), while the 1:3 blend exhibits dispersive kinetics (ζ = 0.87). Using intensity-dependent measurements, we show that trap-assisted recombination is the dominating recombination me...

We demonstrate roll-to-roll fabrication of ultraviolet A light-activated colorimetric oxygen indicators on paper and plastic substrates. Such large-scale, cost-effective and non-toxic oxygen indicator and production method can find applications as a very low-cost leakage indicator for modified atmosphere packages by printing the material directly or gluing the pre-printed indicator label inside the lid. The introduction of leakage indicators onto all modified atmosphere packages could result in improved food safety and reduced food spoilage. Copyright © 2017 John Wiley & Sons, Ltd.

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.

A paper-based platform was developed and tested for studies on basic cell culture, material biocompatibility, and activity of pharmaceuticals in order to provide a reliable, robust and low-cost cell study platform. It is based upon a paper or paperboard support, with a nanostructured latex coating to provide an enhanced cell growth and sufficient barrier properties. Wetting is limited to regions of interest using a flexographically printed hydrophobic polydimethylsiloxane layer with circular non-print areas. The nanostructured coating can be substituted for another coating of interest, or the regions of interest functionalized with a material to be studied. The platform is fully up-scalable, being produced with roll-to-roll rod coating, flexographic and inkjet printing methods. Results show that the platform efficiency is comparable to multi-well plates in colorimetric assays in three separate studies: a cell culture study, a biocompatibility study, and a drug screening study. The color intensity is quantified by using a common office scanner or an imaging device and the data is analyzed by a custom computer software without the need for expensive screening or analysis equipment.

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...

Several three-terminal organic bioelectronic structures have been proposed so far to address the needs for a variety of biosensing applications. The most popular ones utilized organic field-effect transistors operated in an electrolyte, to detect both proteins and genomic analytes. They are endowed with selectivity by immobilizing a layer of bio-recognition elements. These features along with the foreseen low-cost for their production, make them very appealing for point-of-care biomedical applications. However, organic bioelectronic transistors do not always exhibit a performance level beyond state-of-the-art electrochemical sensors, which have been dominating the field for decades. This review offers a perspective view based on a systematic comparison between the potentiometric and amperometric electrochemical sensors and their organic bioelectronic transistor counterparts. The key-relevant aspects of the sensing mechanisms are reviewed for both, and when the mathematical analytical expression is actually available, the amplification factors are reported as the ratio between the response of a rationally designed transistor (or amplifying circuit) and that of a homologous electrochemical sensor. The functional dependence of the bioelectronic sensor responses on the concentration of the species to be detected enabling their correct analytical quantification, is also addressed.

Organic bioelectronic sensors based on an electrolyte gated field-effect transistor are gaining momentum due to their extraordinary high-performance level that enables label-free selective single-molecule detection of both genomic and protein biomarkers with a millimeter-wide electrolyte-gated field-effect transistor (EGOFET) device. The organic semiconductor channel material used so far is a spin-coated regio-regular poly(3-hexylthiophene) (P3HT). Of paramount importance is to design an EGOFET sensor that is stable and cost-effective. To address the latter feature, an ink-jet printed regio-regular P3HT film is here investigated as the channel material. Moreover, the EGOFET device structure is intended to comprise a coplanar lateral gate electrode that enables mechanical and electrical stability. Overall, the structure is compatible with large area processing so it can be fabricated at low-costs and can be operated continuously for eight days. Additionally, systematic optical and Raman characterization of the P3HT film proves that the printing process results in a film with a low energetic disorder (better π–π stacking in the crystalline regions) that likely enables stable operation. However, the higher quality crystalline regions (as compared to a spin-coated film) are dispersed in a more significant fraction of the amorphous disordered material with a larger amount of trap states. The higher crystalline order is ascribed to the higher boiling point and slower evaporation of the ortho-dichlorobenzene solvent used in the printing process. Overall, the present study provides a systematic insight into the structure–property correlations, essential to design a well-functioning and cost-effective EGOFET for high-performance electronic biosensing. It also provides one of the few investigations comparing the features characterizing a spin-coated and an ink-jet printed P3HT film.

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)

Space charge effects can significantly degrade charge collection in organic photovoltaics (OPVs), especially in thick-film devices. The two main causes of space charge are doping and imbalanced transport. Although these are completely different phenomena, they lead to the same voltage dependence of the photocurrent, making them difficult to distinguish. In this work, a method is introduced how the build-up of space charge due to imbalanced transport can be monitored in a real operating organic solar cell. The method is based on the reconstruction of quantum efficiency spectra and requires only optical input parameters that are straightforward to measure. This makes it suitable for the screening of new OPV materials. Furthermore, numerical and analytical means are derived to predict the impact of imbalanced transport on the charge collection. It is shown that when charge recombination is sufficiently reduced, balanced transport is not a necessary condition for efficient thick-film OPVs.

Surface recombination has a major impact on the open-circuit voltage ($V_\text{oc}$) of organic photovoltaics. Here, we study how this loss mechanism is influenced by imbalanced charge transport in the photoactive layer. As a model system, we use organic solar cells with a two orders of magnitude higher electron than hole mobility. We find that small variations in the work function of the anode have a strong effect on the light intensity dependence of $V_\text{oc}$. Transient measurements and drift-diffusion simulations reveal that this is due to a change in the surface recombination rather than the bulk recombination. We use our numerical model to generalize these findings and determine under which circumstances the effect of contacts is stronger or weaker compared to the idealized case of balanced charge transport. Finally, we derive analytical expressions for $V_\text{oc}$ in the case that a pile-up of space charge is present due to highly imbalanced mobilities., 9 pages, 7 figures; Supplemental Material: 13 pages, 7 figures

Author(s): Bonnassieux, Y; Brabec, CJ; Cao, Y; Carmichael, TB; Chabinyc, ML; Cheng, KT; Cho, G; Chung, A; Cobb, CL; Distler, A; Egelhaaf, HJ; Grau, G; Guo, X; Haghiashtiani, G; Huang, TC; Hussain, MM; Iniguez, B; Lee, TM; Li, L; Ma, Y; Ma, D; McAlpine, MC; Ng, TN; Osterbacka, R; Patel, SN; Peng, J; Peng, H; Rivnay, J; Shao, L; Steingart, D; Street, RA; Subramanian, V; Torsi, L; Wu, Y | Abstract: This roadmap includes the perspectives and visions of leading researchers in the key areas of flexible and printable electronics. The covered topics are broadly organized by the device technologies (sections 1–9), fabrication techniques (sections 10–12), and design and modeling approaches (sections 13 and 14) essential to the future development of new applications leveraging flexible electronics (FE). The interdisciplinary nature of this field involves everything from fundamental scientific discoveries to engineering challenges; from design and synthesis of new materials via novel device design to modelling and digital manufacturing of integrated systems. As such, this roadmap aims to serve as a resource on the current status and future challenges in the areas covered by the roadmap and to highlight the breadth and wide-ranging opportunities made available by FE technologies.

The optimization of the hole-blocking layer in perovskite solar cells (PSC), typically based on TiO2, is crucial, as it strongly affects the device performance. In this work, we thoroughly characterize the thickness, roughness, and crystal structure of a set of TiO2 compact layers produced by spin coating of different precursor sols and correlate the choice of the TiO2 precursor to the photovoltaic performance of the PSC. By replacing the commonly used titanium isopropoxide (TTIP) blocking layer precursor with titanium tetrachloride (TiCl4), a clear enhancement in the PSC performance was observed, particularly in the hysteresis behavior and stability. The results from the morphological/structural analysis and transient photoluminescence studies clarify the different behavior of the compact layers in PSCs.

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

Space charge effects can significantly degrade charge collection in organic photovoltaics (OPVs), especially in thick-film devices. The two main causes of space charge are doping and imbalanced transport. Although these are completely different phenomena, they lead to the same voltage dependence of the photocurrent, making them difficult to distinguish. In this work, a method is introduced how the build-up of space charge due to imbalanced transport can be monitored in a real operating organic solar cell. The method is based on the reconstruction of quantum efficiency spectra and requires only optical input parameters that are straightforward to measure. This makes it suitable for the screening of new OPV materials. Furthermore, numerical and analytical means are derived to predict the impact of imbalanced transport on the charge collection. It is shown that when charge recombination is sufficiently reduced, balanced transport is not a necessary condition for efficient thick-film OPVs., Comment: replaced by final version; license changed to CC BY 4.0