Bimolecular Recombination Research Articles

Optical Gain of Lead Halide Perovskites Measured via the Variable Stripe Length Method: What We Can Learn and How to Avoid Pitfalls

AbstractIn the search for novel photonic materials, the recent focus on metal halide perovskites (MHPs) has revealed their promise to become groundbreaking low‐threshold, tunable coherent light sources. An accurate determination of the optical gain coefficient (g) would help to screen for materials and design highly efficient perovskite lasers. Nevertheless, contradictory numbers are continuously reported, making this figure of merit unreliable. To address this issue, the present work outlines a meticulous analysis to retrieve g of MAPbI3, based on the variable stripe‐length (VSL) method. This method is often preferred due to its apparent simplicity; however, one can arrive at incorrect conclusions without the adequate considerations. Therefore, here the experimental implementation and numerical treatment of the data are thoroughly discussed to establish a robust VSL methodology. The obtained power dependence and spectral gain evolution point to the role of electron–hole bimolecular recombination dictating the stimulated emission properties of MAPbI3, with a behavior resembling that of bulk GaAs. Beyond providing further knowledge on the procedure to carry out pertinent VSL measurements, this work also outlines a meticulous methodology to study the underlying photophysical gain properties of MHPs and consequently, to obtain a deeper understanding of the lasing properties of these complex materials.

Nonhalogenated Solvent‐Processed Thick‐Film Ternary Nonfullerene Organic Solar Cells with Power Conversion Efficiency >13% Enabled by a New Wide‐Bandgap Polymer

Although several donor polymers have been synthesized for use in nonfullerene organic solar cells (NFOSCs), the number of efficient π‐conjugated donor polymers compatible with nonhalogenated solvent‐processed thick active layer NFOSCs is limited. Two wide‐bandgap π‐conjugated donor polymers functionalized with a siloxane side chain, P1 (chlorine‐free) and P2 (chlorinated), are designed and synthesized. The siloxane‐functionalized side chains and/or Cl π‐conjugated donor polymers increase the absorption coefficients, reduce the energy losses, increase the charge‐carrier mobility, and suppress the bimolecular recombination, which are beneficial to achieve high‐performance thick‐film ternary NFOSCs. Toluene‐processed devices based on P2:IT‐4F:BTP‐4Cl, and P2:IT‐4F:BTP‐4F exhibit high power conversion efficiencies (PCEs) of 13.25% and 11.02% with fill factors (FFs) of 70.03% and 71.60%, respectively. A P2:IT‐4F binary NFOSC exhibits a PCE of 10.38% with an FF of 69.78%, lower than that of the ternary NFOSC. The ternary device PCE of 13.25% is achieved using a 300 nm‐thick active layer, indicating that the siloxane‐functionalized side‐chain π‐conjugated polymer easily controls the bulk heterojunction blend film thickness of the NFOSC. The findings may potentially aid the development of nonhalogenated solvent‐processed thick‐film ternary NFOSCs that can satisfy future production requirements.

Nanosecond dynamics in intrinsic topological insulator Bi2−xSbxSe3 revealed by time-resolved optical reflectivity

$\text{Bi}_2\text{Se}_3$ is an ideal three-dimensional topological insulator in which the chemical potential can be brought into the bulk band gap with antimony doping. Here, we utilize ultrafast time-resolved transient reflectivity to characterize the photoexcited carrier decay in $\text{Bi}_{2-x}\text{Sb}_x\text{Se}_3$ nanoplatelets. We report a substantial slowing of the bulk carrier relaxation time in bulk-insulating $\text{Bi}_{2-x}\text{Sb}_x\text{Se}_3$ nanoplatelets as compared to $n$-type bulk-metallic $\text{Bi}_2\text{Se}_3$ at low temperatures, which approaches $3.3 \text{ ns}$ in the zero pump fluence limit. This long-lived decay is correlated across different fluences and antimony concentrations, revealing unique decay dynamics not present in $n$-type $\text{Bi}_2\text{Se}_3$, namely the slow bimolecular recombination of bulk carriers.

Investigation of charge carrier mobility and recombination in PBDTTPD thin layer structures

Abstract In this paper we present investigation of hole transport properties in sandwich and OFET structures with single active layer of PBDTTPD (Poly[(5,6-dihydro-5-octyl-4,6-dioxo-4H-thieno[3,4-c]pyrrole-1,3-diyl)[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]]). Sandwich structures were investigated by photo-CELIV and TOF techniques, obtained results showed strongly time dependent hole mobility and non-Gaussian DOS tail. Photogenerated charge carrier decay experiment demonstrated that bimolecular recombination coefficient is smaller than calculated Langevin recombination coefficient and this was explained by faster holes escaping recombination area and not participating in Langevin recombination process. Organic field-effect transistor structure was investigated by current transients technique to find hole mobility near the dielectric layer and to study OTS treatment influence on hole transport. The study of hole mobility dependence on temperature was performed in order to evaluate energetic disorder of interface DOS in the channel of OFET structures.

Advances on tin-based perovskite solar cells

<p indent=0mm>Organic–inorganic hybrid perovskites have been a focus in the photovoltaic research field due to their remarkable advantages such as tunable direct bandgap, low exciton binding energy, high light harvesting, and so on. The power conversion efficiency (PCE) of lead (Pb)-based perovskite solar cells (PSCs) has been rapidly raised from 3.8% to 25.5% over the decades. With the advantages of a simple preparation process and low production cost, PSCs have great potential in commercialization. However, the toxicity of lead in traditional PSCs limits their large-scale production. Among all lead-free perovskites, the tin-based one inherits the majority of the excellent photoelectric properties of lead-based perovskites. However, the maximum PCE of tin-based PSCs has only exceeded 13%. Although the electronic structure of Sn²⁺ is similar to that of Pb²⁺, it is easy to be oxidized to Sn⁴⁺, and the formation energy of Sn vacancy defects is low, resulting in heavy p-doping in tin-based perovskite films. The background carrier concentrations of tin-based perovskites are as high as <sc>10¹⁸–10²⁰ cm^–3,</sc> which makes photo-generated carriers easy to be recombined. Tin-based perovskites usually experience single-molecule recombination, accompanied by a small amount of bi-molecular recombination, and Auger recombination is negligible. Therefore, reducing recombination in tin-based perovskites is to suppress the single-molecule recombination. At present, additive engineering and component modification strategies are mainly adopted to inhibit carrier recombination in tin-based PSCs. Additive engineering includes tin compensating additives, reducing additives and some other additives which interact with perovskites. Theories and experiments show that adding tin compensating additives in precursor solution can effectively improve the chemical potential of Sn and increase the formation energy of tin vacancies. Although the work mechanism of stannous halide as an additive is not fully understood, it has become an indispensable part to prepare high performance tin-based PSCs. The introduction of reducing additives in the precursor can effectively reduce Sn⁴⁺ and inhibit the oxidation of Sn²⁺, thereby inhibiting the generation of tin vacancies, optimizing the morphology of the film, and improving the efficiency and stability of the device. In addition, some additives which interact with perovskite through coordination bonds, ionic bonds and hydrogen bonds can optimize the nucleation rate of tin-based perovskites and passivate the unsaturated coordination of Sn²⁺ at the grain boundaries. The composition modification of tin-based perovskites mainly includes A-site and X-site substitutions. Since the band structure of tin-based perovskites is mainly determined by Sn and X ions, A-site cation substitution only fine-tunes the bandgap of the perovskite. However, A-site cation substitution can tune the tolerance factor of perovskite, thereby improving the efficiency and stability of perovskite. As the alkyl chain of the A-site cation gradually increases, low-dimensional perovskites will be formed. The low-dimensional perovskite can inhibit the self-doping and ion migration, improve the efficiency and stability of the PSCs. It is worth noting that the current efficient tin-based PSCs are mainly focused on low-dimensional perovskites. In addition, since the np² electrons of halogens directly participate in the construction of the electronic structure, the bandgap of tin-based perovskites can be directly changed by adjusting the X-site anions. Compared with lead-based perovskites, tin-based ones have more suitable bandgap and higher theoretical efficiency. At the same time, the low toxicity of tin-based perovskites is also conducive to their large-scale industrial production. It is believed that with the in-depth research, tin-based perovskite will surely exert its full potential in the photovoltaic field.

Precise fluorination of polymeric donors towards efficient non-fullerene organic solar cells with balanced open circuit voltage, short circuit current and fill factor

By increasing the fluorination sites of polymer donors, the JSC, VOC and FF are enhanced for solar cells, because of fine-tuned energy levels, reduced trap-assisted or bimolecular recombination, and improved charge carrier mobility.

Cyclometalated Ir(III) complexes as potential electron acceptors for organic solar cells.

Cyclometalated iridium(iii) complexes have been investigated as promising electron donor (D) materials in organic solar cells (OSCs) due to their unique octahedral configuration for optimized morphology and their significantly long lifetimes potentially for enhanced exciton dissociation. However, the application as electron acceptor (A) materials has never been reported. In order to fill this blank, herein, two cyclometalated heteroleptic Ir complexes, TRIr and 2TRIr, based on electron donating-accepting type organic ligands with different π-conjugation lengths are reported as electron acceptor materials in comparison with their corresponding main organic ligands. The two Ir complexes exhibit suitable HOMO/LUMO energy levels of -5.55/-3.47 eV and -5.44/-3.48 eV, which are ∼0.1 eV higher in the HOMO and ∼0.15 eV deeper in the LUMO than the TR and 2TR ligands, respectively. 2TRIr with extended ligand π-conjugation displays a poor triplet feature, while TRIr demonstrates obvious metal-to-ligand charge transfer (MLCT) transition absorption, with a triplet component photoluminescence (PL) lifetime of 85 ns in neat films. When blended with PBDB-T in bulk heterojunction (BHJ) OSCs, the power conversion efficiencies (PCEs) are 2-3 times higher than their relevant ligands, with values of 1.20% and 1.62% for TRIr and 2TRIr, and 0.58% and 0.47% for the TR and 2TR ligand-based devices, respectively. TRIr and 2TRIr based active layer blends exhibit poorer hole and electron mobilities, whereas compared with their relatively linear planar ligands, both of the two octahedral Ir complexes exhibit an optimized surface morphology for less bimolecular recombination and more efficient exciton dissociation, thus contributing to improved photovoltaic performance.

Исследование кинетики гибели неравновесных носителей заряда в четверных соединениях меди Cu-=SUB=-2-delta-=/SUB=-NiSnS-=SUB=-4-=/SUB=- (0&amp;le;delta&amp;le;0.2)

By solid-phase synthesis from elemental Cu, Ni, Sn, and S samples Cu2 - δNiSnS4 was prepared . The parameters of their crystal lattice have been refined. The lifetimes of photogenerated current carriers in Cu2 - δNiSnS4 were estimated for the first time by the contactless time-resolved microwave photoconductivity method. This times turned out to be τ ≈ 7 ns, which is comparable with the literature data for the times in kesterites CZTS. Herewith, in kinetics of loses of photogenerated cureent carriers is observed predominance of bimolecular recombination processes over capture processes.

Increasing Molecular Packing Density and Dielectric Constant in Non-Fullerene Acceptors Enables to Boost the Fill Factor Approaching 0.82 in Organic Solar Cells

SUMMARYThe fill factor (FF) of organic solar cells (OSC), a critically important photovoltaic parameter, is yet sub-optimal, often less than 0.8. To further reduce the FF gaps with regard to the Shockley-Queisser upper limit, we present a study unveiling the impacts of dielectric properties on obtaining high FFs and photovoltaic efficiencies in OSCs. With increasing the dielectric constant (e) of non-fullerene acceptors (NFAs) and bulk heterojunction (BHJ) films, afforded by the increase of molecular packing density (MPD) of NFAs in solid states, we show that the FF penalties related to non-geminate charge recombination in OSCs can be appreciably mitigated. This conceptual modulation on FFs is embodied by a series of NFA-based BHJs where the differentiated MPDs and resultant changes of e in BHJs are reflected by the difference in FFs. In particular, as exemplified by an NFA model system L8-BO tailored with branched side chains, the enlargements of MPD and e allow to achieve an impressive FF of 0.815 with efficiency of 18.23% in OSCs featuring an impressively low bimolecular recombination and reduced sensitivity of FF on the BHJ film thickness.

On the Understandings of Dielectric Constant and Its Impacts on the Photovoltaic Efficiency in Organic Solar Cells

AbstractDielectric constant (ε) is an important parameter affecting the power conversion efficiency of organic solar cells (OSC). Increasing ε of bulk heterojunctions in general can benefit the performance of OSCs, as an increased ε will reduce the influence of Coulomb interaction between weakly bound electron‐hole pairs on charge‐transfer states or bimolecular recombination involving mobile carriers to reduce geminate and nongeminate losses. In this review, we overview the current understandings on dielectric constant and its impacts on exciton dissociation and voltage losses in OSCs and summarize recent efforts attempting to modify the dielectric properties of OSC materials through synthetic approaches. We further discuss the commonly adopted techniques for determining the parameter of ε with stressing the testing conditions that may affect the accuracy of results. At last, we suggest that novel experimental methods to improve the dielectric constant and resultant physical processes in OSCs will be appreciated, which helps enrich the existing strategy reservoir toward enhancement of photovoltaic efficiencies.

Exciton Transport in Molecular Semiconductor Crystals for Spin-Optoelectronics Paradigm.

Organic semiconductors with long-range exciton diffusion length are highly desirable for optoelectronics but currently remain rare. Here, the estimated diffusion length of singlet excitons (LD ) in 2,6-diphenyl anthracene (DPA) crystals grown by solvent evaporation was shown to be up to approximately 124 nm. These crystals showed a previously unseen parallelogram morphology with layer-by-layer edge-on molecular stacking, isotropic optical waveguiding, radiation rate and non-radiation rate constants of 0.15 and 0.26 ns-1 respectively, as well as good field-effect transistor hole mobility and theoretically computed strong electronic couplings as high as 109 meV. Photoresponse experiments revealed that the photoconductivity of DPA crystals is surprisingly not related to the radiative pathway but associated with rapid exciton diffusion to the crystal surface for charge separation and carrier bimolecular recombination. Taken together, DPA was shown to be a promising semiconducting material for a new organic optoelectronics paradigm.

Experimental Evidence Relating Charge-Transfer-State Kinetics and Strongly Reduced Bimolecular Recombination in Organic Solar Cells

Significantly reduced bimolecular recombination relative to the Langevin recombination rate has been observed in a limited number of donor-acceptor organic semiconductor blends. The strongly reduced recombination has been previously attributed to a high probability for the interfacial charge-transfer (CT) states (formed upon charge encounter) to dissociate back to free charges. However, whether the reduced recombination is due to a suppressed CT-state decay rate or an improved dissociation rate has remained a matter of conjecture. Here we investigate a donor-acceptor material system that exhibits significantly reduced recombination upon solvent annealing. On the basis of detailed balance analysis and the accurate characterization of CT-state parameters, we provide experimental evidence that an increase in the dissociation rate of CT states upon solvent annealing is responsible for the reduced recombination. We attribute this to the presence of purer and more percolated domains in the solvent-annealed system, which may, therefore, have a stronger entropic driving force for CT dissociation.

Effect of aromatic π-bridges on molecular structures and optoelectronic properties of A-π-D-π-A small molecular acceptors based on indacenodithiophene

Investigation on the relationship between molecular structure and device performance is of great important to develop highly efficient A-π-D-π-A small molecular acceptors (SMAs). However, there is still lack of a complete and in-depth study on effects of π-bridge on molecular structure, optoelectronic properties and photovoltaic performances. Herein, we reported the design, synthesis and photovoltaic application of four A-π-D-π-A type SMAs, denoted as IDT-Py-IC , IDT-Fu-IC , IDT-Th-IC , and IDT-Ph-IC , which possess an identical central D unit of indacenodithiophene and the terminal A group of 3-(dicyanomethylidene)indol-1-one, linked by various aromatic π-bridges of pyrrole, furan, thiophene, and benzene, respectively. The impact of the different aromatic π-bridge on molecular structures, optoelectronic and photovoltaic properties as well as active layer morphologies was comprehensively explored. Results show that both molecular co-planarity and electron-donating ability of aromatic π-bridges distinctly affect optical bandgaps ( E g opt ) and HOMO/LUMO levels of these SMAs. The poor backbone planarity of pyrrole-bridged IDT-Py-IC observed by theory calculation leads to a blue-shifted absorption and up-shifted HOMO/LUMO levels. The E g opt of these SMAs is gradually increased and HOMO levels are gradually down-shifted with the decrease of the electron-donating ability of aromatic π-bridges. Polymer solar cells (PSCs) based on these SMAs exhibit a high V oc over 0.93 V, especially for PBDB-T: IDT-Py-IC -based PSCs, producing a rather high V oc up to 1.06 V due to the high-lying LUMO level. After optimizations, the PBDB-T: IDT-Th-IC -based PSC outperforms the other three SMAs with a high PCE up to 8.72% mainly due to the large J sc and FF, which could be ascribed to better absorption characteristics, higher and more proportional carrier mobility, efficient exciton dissociation and charge collection, reduced bimolecular recombination and superior active layer morphology. This finding demonstrates that the π-bridge plays a crucial role in tailoring molecular structures, optoelectronic properties and device performance of A-π-D-π-A type SMAs. • Four A-π-D-π-A SMAs with the same D and A groups, but various π-bridge were developed. • Effect of aromatic π-bridges on structures and optoelectronic properties of SMAs was studied. • Pyrrole-bridged IDT-Py-IC showed a blue-shifted absorption and a high V oc over 1.0 V. • IDT-Th-IC -based PSCs outperformed other three SMAs, along with a high PCE up to 8.72%.

Characterization of nano-bio silicon carbide

Plasma-enhanced chemical vapor deposition, reactive magnetron sputtering, hot-wire chemical vapor deposition and radio frequency plasma-enhanced chemical vapor deposition were used to develop technology for preparation of nano-bio silicon carbide coating of ceramic materials for dental applications. The effect of the bias voltage applied to the ceramic prostheses and dental crowns on the crystallization processes have been recognized. The optimal bias voltage applied to conductive substrate was –200 V, whereas for dielectric substrate the bias voltage Vbias did not affect the properties of SiC coating. The analysis of CVCs and spectroscopic diagnostics as the methods for studying the mechanism of interfacial rearrangements to investigate SiC phase transition in nano silicon carbide coatings were used. The conductivity of the SiC coating coincided with the conductivity on the dielectric (µn0 = 1012…1013 сm–1·s–1·V–1). The conductive substrate had a significant effect on the properties of the coating and thus depended on the bias voltage Vbias. The conductivity increased by three-four orders of magnitude (µn0 = 3·1017 сm–1·s–1·V–1), if the bias voltage Vbias = –200 V. The increase of the bias voltage (Vbias = –600 V) led to a decrease in the conductivity (µn0 = 1011…1012 сm–1·s–1·V–1). It was found that there was the double injection regime with bimolecular recombination in this structure with the dependence I = V3/2 for CVCs of SiC. The luminescence spectrum of SiC coating on non-dielectric ceramics (if Vbias = – 200 V during deposition) was significantly different from the luminescence spectrum of SiC coating on dielectric ceramics. Increasing the applied voltage to the substrate Vbias during deposition led to increasing the fraction of hexagonal polytypes. Directions in the crystal lattice according to the photoluminescence spectra were identified from the comparing the values of the width of the non-phonon parts of stacking faults and deep level spectra in the low-temperature photoluminescence with arrangements of atoms in the SiC lattice structure. The displacement of each atom participating in photoluminescence allowed to find the correlation with technology of SiC deposition and to develop technology of SiC coating on the dental materials.

Performance of asymmetric non-fullerene acceptors containing the 4,4,9,9-tetramethyl-4,9-dihydroselenopheno[2',3':5,6]-s-indaceno[1,2-b]thiophene core

Through changing the terminated groups, three asymmetric non-fullerene acceptors containing a 4,4,9,9-tetramethyl-4,9-dihydroselenopheno[2',3':5,6]-s-indaceno[1,2-b]thiophene fused-ring core were designed and synthesized. The resultant acceptor-donor-acceptor (A-D-A) type of small molecules all show broad absorption spectra with tunable molecular energy levels and distinct molar extinction coefficients in solutions. The 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile terminated molecule (T-Se), has the smallest molar extinction coefficient of 1.83 × 105 M−1cm−1 with a bandgap of 1.62 eV, and a high-lying lowest unoccupied molecular orbital (LUMO) energy level (−3.83 eV). For the fluorinated molecule T-Se-4F, its molar absorption coefficient is enhanced to 2.05 × 105 M−1 cm−1, and its bandgap is narrowed to 1.58 eV, while its LUMO energy level is down-shifted to −3.93 eV. As for the molecule T-Se-Th having thiophene functionalized end-group, although the acquired LUMO energy level (−3.88 eV) is in between that of T-Se and T-Se-4F, it possesses the largest molar extinction coefficient (2.40 × 105 M−1cm−1), and a moderate bandgap of 1.60 eV. In solar cells with PM6 as electron-donor, the T-Se-4F- and T-Se-Th-based devices have superior exciton dissociation and charge collection properties, more weakly bimolecular recombination probabilities, and better electron transportability. As the results of the preceding characteristics, the T-Se-4F- and T-Se-Th-based devices achieve a higher current of 18.09 and 16.90 mA cm−2, better fill factors of 66.6 and 66.8%, relative to the 12.80 mA cm−2 and 62.1% of the T-Se-based device. Benefitting from the medium open-circuit voltage (VOC, 0.912 V), the T-Se-Th-based device successfully achieved the best power conversion efficiency of 10.29% among the three molecules. In contrast, the T-Se- and T-Se-4F-based devices owned the highest VOC of 0.935 V and the lowest VOC of 0.781 V, leading to the low efficiency of 7.44% and moderate efficiency of 9.41%, respectively, revealing the significant impact of the terminal groups on the properties of 4,4,9,9-tetramethyl-4,9-dihydroselenopheno[2',3':5,6]-s-indaceno[1,2-b]thiophene based asymmetric non-fullerene acceptors.

Nonexponential Photoluminescence Dynamics in an Inhomogeneous Ensemble of Excitons in WSe2 Monolayers

The spectral and spatiotemporal dynamics of photoluminescence in monolayers of transition metal dichalcogenide WSe2 obtained by mechanical exfoliation on a Si/SiO2 substrate is studied over a wide range of temperatures and excitation powers. It is shown that the dynamics is nonexponential and, for times t exceeding ∼50 ps after the excitation pulse, is described by a dependence of the form 1/(t + t0). Photoluminescence decay is accelerated with a decrease in the temperature and in the energy of emitting states. It is shown that the observed dynamics cannot be described by a bimolecular recombination process, such as exciton—exciton annihilation. A model that describes the nonexponential photoluminescence dynamics by taking into account the spread of radiative recombination times of localized exciton states in a random potential gives good agreement with experimental data.

Understanding the Role of Ion Migration in the Operation of Perovskite Light-Emitting Diodes by Transient Measurements.

Perovskite light-emitting diodes have been gaining attention in recent years due to their high efficiencies. Despite of the recent progress made in device efficiency, the operation mechanisms of these devices are still not well understood, especially the effects of ion migration. In this work, the role of ion migration is investigated by measuring the transient electroluminescence and current responses, with both the current and efficiency showing a slow response in a time scale of tens of milliseconds. The results of the charge injection dynamics show that the slow response of the current is attributed to the migration and accumulation of halide ions at the anode interface, facilitating hole injection and leading to a strong charge imbalance. Further, the results of the charge recombination dynamics show that the slow response of the efficiency is attributed to enhanced charge injection facilitated by ion migration, which leads to an increased carrier density favoring bimolecular radiative recombination. Through a combined analysis of both charge injection and recombination dynamics, we finally present a comprehensive picture of the role of ion migration in device operation.

A Pyrrole‐Fused Asymmetrical Electron Acceptor for Polymer Solar Cells with Approaching 16% Efficiency

Selecting suitable aromatic rings to develop asymmetric small‐molecule acceptors (SMAs) is of great significance. Herein, two asymmetric SMAs, namely, BTN‐4F and BTSe‐4F, are successfully synthesized by incorporating pyrrole and selenophene unit into the cores, respectively. Compared with the selenophene‐fused counterpart BTSe‐4F, BTN‐4F displays slightly upshifted energy levels and red‐shifted absorption spectrum (about 40 nm in film) due to the strong electron‐donating ability of N atoms in pyrrole. When matching with PM6, the BTN‐4F‐based device gives a higher power conversion efficiency (PCE) (15.82%) due to the simultaneously enhanced open circuit voltage (VOC), short‐circuit current density (JSC), and fill factor (FF), compared with those of the BTSe‐4F‐based device. The higher VOC coincides with the shallower lowest unoccupied molecular orbital (LUMO) energy level of BTN‐4F; the enhanced JSC mainly results from the broadened and enhanced absorption; the increased FF can be ascribed to the weaker bimolecular recombination, more balance charge transport, and more favorable morphology. It should be mentioned that the BTN‐4F‐based device yields a lower energy loss than that of the BTSe‐4F‐based device. Overall, the work demonstrates that pyrrole‐fused SMAs shows great potential in realizing low Eloss while yielding high JSC and PCE, which paves a new way to develop high‐performance SMAs.

Putting Order into PM6:Y6 Solar Cells to Reduce the Langevin Recombination in 400 nm Thick Junction

Increasing the active layer thickness without sacrificing the power conversion efficiency (PCE) is one of the great challenges faced by organic solar cells (OSCs) for commercialization. Recently, PM6:Y6 as an OSC based on a non‐fullerene acceptor (NFA) has excited the community because of its PCE reaching as high as 15.9%; however, by increasing the thickness, the PCE drops due to the reduction of the fill factor (FF). This drop is attributed to change in mobility ratio with increasing thickness. Furthermore, this work demonstrates that by regulating the packing and the crystallinity of the donor and the acceptor, through volumetric content of chloronaphthalene (CN) as a solvent additive, one can improve the FF of a thick PM6:Y6 device (≈400 nm) from 58% to 68% (PCE enhances from 12.2% to 14.4%). The data indicate that the origin of this enhancement is the reduction of the structural and energetic disorders in the thick device with 1.5% CN compared with 0.5% CN. This correlates with improved electron and hole mobilities and a 50% suppressed bimolecular recombination, such that the non‐Langevin reduction factor is 180 times. This work reveals the role of disorder on the charge extraction and bimolecular recombination of NFA‐based OSCs.

Impact of trap filling on carrier diffusion in MAPbBr3 single crystals

We present experimental evidence showing that the effective carrier diffusion length ($L_d$) and lifetime ($\tau$) depend on the carrier density in MAPbBr$_3$ single crystals. Independent measurements reveal that both $L_d$ and $\tau$ decrease with an increase in photo-carrier density. Scanning photocurrent microscopy is used to extract the characteristic photocurrent $I_{ph}$ decay-length parameter, $L_d$, which is a measure of effective carrier diffusion. The $L_d$ magnitudes for electrons and holes were determined to be ~ 13.3 $\mu$m and ~ 13.8 $\mu$m respectively. A marginal increase in uniform light bias ($\leq 5 \times 10^{15}$ photons/cm$^2$) increases the modulated photocurrent magnitude and reduces the $L_d$ parameter by a factor of two and three for electrons and holes respectively, indicating that the recombination is not monomolecular. The $L_d$ variations were correlated to the features in photoluminescence lifetime studies. Analysis of lifetime variation shows intensity-dependent monomolecular and bimolecular recombination trends with recombination constants determined to be ~ 9.3 $\times 10^6$ s$^{-1}$ and ~ 1.4 $\times 10^{-9}$ cm$^{3}$s$^{-1}$ respectively. Based on the trends of $L_d$ and lifetime, it is inferred that the sub-band-gap trap recombination influences carrier transport in the low-intensity excitation regime, while bimolecular recombination and transport dominate at high intensity.