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Trap states in organic semiconductors are notoriously detrimental to the performance of organic electronics. However, the origin and energetics of trap states remain largely elusive and under debate, especially for bulk‐heterojunction (BHJ) photodiodes consisting of electron donor and acceptor materials. Combining three sensitive techniques now enables locating the origin and energy of trap states in six state‐of‐the‐art polymer – non‐fullerene acceptor organic photodiodes (OPDs) with noise‐based specific detectivities exceeding 1013 Jones. Analyzing the temperature dependence of the reverse‐bias dark current density (Jd) identifies intra‐gap states in the polymers, lying 0.3−0.4 eV above the energy of the highest occupied molecular orbital, as being responsible for Jd. Sub‐bandgap external quantum efficiency spectra of donor‐only and acceptor‐only diodes confirm that intra‐gap states are much more abundant in the polymers. Likewise, responsivity measurements at ultra‐low light intensities (10−7 mW cm−2) show trap‐mediated charge recombination in BHJ and polymer‐only diodes, but not in acceptor‐only devices. The results imply that to further improve the specific detectivity of near‐infrared OPDs, the intra‐gap state energy, and density need to be reduced. [ABSTRACT FROM AUTHOR]

Lead–tin (Pb–Sn) hybrid perovskite materials possess ideal narrow bandgaps (1.2–1.4 eV) for efficient single‐junction and tandem solar cells. Poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is commonly used as hole transport layer (HTL) for Pb–Sn perovskite solar cells (PSCs), despite its poor stability with these perovskites. Here, two new octacyclic heteroaromatic molecules, pyrenodiindole (PDI) and pyrenodi‐(7‐azaindole) (PDAI), are presented as the HTL for narrow‐bandgap (1.23 eV) p–i–n Pb‐Sn PSCs. The self‐assembled reciprocal hydrogen‐bonded solid‐state structure of PDAI bestows robustness compared to PDI, making it less vulnerable in processing the perovskite film on top, and improves the reproducibility of device fabrication. Transient photocurrent measurements and light‐intensity‐dependent device characteristics indicate that PDI and PDAI possess similar hole extraction properties to PEDOT:PSS. As a result, similar open‐circuit voltages and fill factors are obtained in the PSCs. Interestingly, the use of thin PDI and PDAI as HTL in PSCs changes the optical interference and reduces parasitic absorption in the near‐infrared region, resulting in an improved short‐circuit current density. Consequently, a higher power conversion efficiency of 16.1% is obtained for PDI and PDAI, compared to 15.1% for PEDOT:PSS. In addition, the self‐assembled structure of PDAI led to a notable enhancement of device stability. [ABSTRACT FROM AUTHOR]

The chemistry of the interface between the metal halide perovskite absorber and the charge transport layer affects the performance and stability of metal halide perovskite solar cells (PSCs). The literature provides several examples of poor PSC conversion efficiency values, when electron transport layers (ETLs), such as SnO2 and TiO2, are processed by atomic layer deposition (ALD) directly on the perovskite absorber. In the present work, we shed light on the chemical modifications occurring at the perovskite surface, during ALD processing of SnO2 and TiO2, in parallel with the evaluation of the PSC cell performance. The ALD processes are carried out on a (Cs,FA)Pb(I,Br)3 perovskite by adopting tetrakis(dimethylamino)tin(IV) and tetrakis(dimethylamino)titanium(IV) as metal precursors and H2O as the coreactant for SnO2 and TiO2, respectively. Perovskite surface modification occurs in the form of an ultrathin PbBr2 layer. Furthermore, in the case of SnO2, halogen molecules are detected at the interface, in parallel with the initial growth of an oxygen-deficient SnO2. Subgap defect states just above the valence band maximum of SnO2 are also detected. These states act as hole traps at the perovskite/SnO2 interface, subsequently promoting charge recombination and deteriorating the performance of the cell. We hypothesize that a redox reaction between the perovskite, or its decomposition products, and the Sn metal center of the ALD precursor takes place: I− and Br− are oxidized to I2 and Br2, respectively, and Sn(IV) is reduced to Sn(II). In contrast, the Ti(IV) metal center does not undergo any redox process, and, as a result, a promising 11% power conversion efficiency is measured with TiO2 as the ETL. This result strongly suggests that TiO2 may be a more suitable ETL, when processed directly on the perovskite absorber. [ABSTRACT FROM AUTHOR]

Abstract: Perovskite solar cells (PSCs) are emerging among the photovoltaic (PV) technologies due to their high power conversion efficiency (PCE) in combination with potentially low cost manufacturing processing. In this contribution, the fabrication of efficient planar n‐i‐p PSCs by the modification of the electron transport layer (ETL) adopted as n‐type contact is demonstrated. Specifically, a fluorine‐based plasma treatment prior to perovskite deposition leads to surface fluorination of the TiO2 ETL. The presence of fluorine on the TiO2 surface drastically improves the adhesion between the ALD layer and the methylammonium lead iodide perovskite film, and leads to a more favourable energy band alignment, accompanied by a faster electron carrier extraction at the interface. As consequence of surface fluorination, we observe a significant reduction in the current density‐voltage curve hysteresis with respect to the ALD based reference sample, as well as a remarkable improvement in power conversion efficiency from 4% up to a stable 14.8%. [ABSTRACT FROM AUTHOR]

Minimizing the reverse bias dark current while retaining external quantum efficiency is crucial if the light detection sensitivity of organic photodiodes (OPDs) is to compete with inorganic photodetectors. However, a quantitative relationship between the magnitude of the dark current density under reverse bias (Jd) and the properties of the bulk heterojunction (BHJ) active layer has so far not been established. Here, a systematic analysis of Jd in state‐of‐the‐art BHJ OPDs using five polymers with a range of energy levels and charge transport characteristics is presented. The magnitude and activation energy of Jd are explained using a model that assumes charge injection from the metal contacts into an energetically disordered semiconductor. By relating Jd to material parameters, insights into the origin of Jd are obtained that enable the future selection of successful OPD materials. [ABSTRACT FROM AUTHOR]

Interfaces between the photoactive and charge transport layers are crucial for the performance of perovskite solar cells. Surface passivation of SnO2 as electron transport layer (ETL) by fullerene derivatives is known to improve the performance of n–i–p devices, yet organic passivation layers are susceptible to removal during perovskite deposition. Understanding the nature of the passivation is important for further optimization of SnO2 ETLs. X‐ray photoelectron spectroscopy depth profiling is a convenient tool to monitor the fullerene concentration in passivation layers at a SnO2 interface. Through a comparative study using [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) and [6,6]‐phenyl‐C61‐butyric acid (PCBA) passivation layers, a direct correlation is established between the formation of interfacial chemical bonds and the retention of passivating fullerene molecules at the SnO2 interface that effectively reduces the number of defects and enhances electron mobility. Devices with only a PCBA‐monolayer‐passivated SnO2 ETL exhibit significantly improved performance and reproducibility, achieving an efficiency of 18.8%. Investigating thick and solvent‐resistant C60 and PCBM‐dimer layers demonstrates that the charge transport in the ETL is only improved by chemisorption of the fullerene at the SnO2 surface. [ABSTRACT FROM AUTHOR]

For 19 diketopyrrolopyrrole polymers, the highest occupied molecular orbital (HOMO) energies are determined from i) the oxidation potential with square‐wave voltammetry (SWV), ii) the ionization potential using ultraviolet photoelectron spectroscopy (UPS), and iii) density functional theory (DFT) calculations. The SWV HOMO energies show an excellent linear correlation with the open‐circuit voltage (Voc) of optimized solar cells in which the polymers form blends with a fullerene acceptor ([6,6]‐phenyl‐C61‐butyl acid methyl ester or [6,6]‐phenyl‐C71‐butyl acid methyl ester). Remarkably, the slope of the best linear fit is 0.75 ± 0.04, i.e., significantly less than unity. A weaker correlation with Voc is found for the HOMO energies obtained from UPS and DFT. Within the experimental error, the SWV and UPS data are correlated with a slope close to unity. The results show that electrochemically determined oxidation potentials provide an excellent method for predicting the Voc of bulk heterojunction solar cells, with absolute deviations less than 0.1 V. To predict the open‐circuit voltage (Voc) of polymer–fullerene solar cells, three independent methods, square‐wave voltammetry (SWV), ultraviolet photoelectron spectroscopy, and density functional theory, are compared. For 19 diketopyrrolopyrrole polymers, SWV gives the best correlation. Remarkably, the slope of Voc with the blend's electrochemical gap is less than unity and possible reasons for this result are discussed. [ABSTRACT FROM AUTHOR]