Recombination Of Free Charges Research Articles

A New Framework for Understanding Recombination-Limited Charge Extraction in Disordered Semiconductors.

Recombination of free charges is a key loss mechanism limiting the performance of organic semiconductor-based photovoltaics such as solar cells and photodetectors. The carrier density-dependence of the rate of recombination and the associated rate coefficients are often estimated using transient charge extraction (CE) experiments. These experiments, however, often neglect the effect of recombination during the transient extraction process. In this work, the validity of the CE experiment for low-mobility devices, such as organic semiconductor-based photovoltaics, is investigated using transient drift-diffusion simulations. We find that recombination leads to incomplete CE, resulting in carrier density-dependent recombination rate constants and overestimated recombination orders; an effect that depends on both the charge carrier mobilities and the resistance-capacitance time constant. To overcome this intrinsic limitation of the CE experiment, we present an analytical model that accounts for charge carrier recombination, validate it using numerical simulations, and employ it to correct the carrier density-dependence observed in experimentally determined bimolecular recombination rate constants.

Efficiency Boost in All‐Small‐Molecule Organic Solar Cells: Insights from the Re‐Ordering Kinetics

AbstractAchieving high‐performance in all‐small‐molecule organic solar cells (ASM‐OSCs) significantly relies on precise nanoscale phase separation through domain size manipulation in the active layer. Nonetheless, for ASM‐OSC systems, forging a clear connection between the tuning of domain size and the intricacies of phase separation proves to be a formidable challenge. This study investigates the intricate interplay between domain size adjustment and the creation of optimal phase separation morphology, crucial for ASM‐OSCs’ performance. It is demonstrated that exceptional phase separation in ASM‐OSCs’ active layer is achieved by meticulously controlling the continuity and uniformity of domains via re‐packing process. A series of halogen‐substituted solvents (Fluorobenzene, Chlorobenzene, Bromobenzene, and Iodobenzene) is adopted to tune the re‐packing kinetics, the ASM‐OSCs treated with CB exhibited an impressive 16.2% power conversion efficiency (PCE). The PCE enhancement can be attributed to the gradual crystallization process, promoting a smoothly interconnected and uniformly distributed domain size. This, in turn, leads to a favorable phase separation morphology, enhanced charge transfer, extended carrier lifetime, and consequently, reduced recombination of free charges. The findings emphasize the pivotal role of re‐packing kinetics in achieving optimal phase separation in ASM‐OSCs, offering valuable insights for designing high‐performance ASM‐OSCs fabrication strategies.

Highly-Active Chiral Organic Photovoltaic Catalysts with Suppressed Charge Recombination.

Recombination of free charges in organic semiconductors reduces the available photo-induced charge-carriers and restricts photovoltaic efficiency. In this work, the chiral organic semiconductors (Y6-R and Y6-S with enantiopure R- and S- chiral alkyl sidechains) are designed and synthesized, which show effective aggregation-induced chirality through mainchain packing with chiral conformations in non-centrosymmetric space groups with tilt chirality. Based on the analysis of spin-injection, magnetic-hysteresis loop, and thermodynamics and dynamics of the excited state, we suggest that the aggregation-induced chirality can generate spin-polarization, which suppresses charge recombination and offers more available charge-carriers within Y6-R and Y6-S relative to the achiral counterpart (Y6). Then the chiral Y6-R and Y6-S show enhanced catalytic activity with optimal average hydrogen evolution rates of 205 and 217 mmol h-1 g-1 , respectively, 60-70 % higher than Y6, when they are employed as nanoparticle photocatalysts in photocatalytic hydrogen evolution under simulated solar light, AM1.5G, 100 mW cm-2 .

Efficient Nanoscale Exciton Transport in Non-Fullerene Organic Solar Cells Enables Reduced Bimolecular Recombination of Free Charges.

The highest-efficiency organic photovoltaic (OPV)-based solar cells, made from blends of electron-donating and electron-accepting organic semiconductors, are often characterized by strongly reduced (non-Langevin) bimolecular recombination. Although the origins of the reduced recombination are debated, mechanisms related to the charge-transfer (CT) state and free-carrier encounter dynamics controlled by the size of donor and acceptor domains are proposed as underlying factors. Here,a novel photoluminescence-based probe is reported to accurately quantify the donor-acceptor domain size in OPV blends. Specifically, the domain size is measured in high-efficiency non-fullerene acceptor (NFA) systems and a comparative conventional fullerene system. It is found that the NFA-based blends form larger domains but that the expected reductions in bimolecular recombination attributed to the enhanced domain sizes are too small to account for the observed reduction factors. Further, it is shown that the reduction of bimolecular recombination is correlated to enhanced exciton dynamics within the NFA domains. This indicates that the processes responsible for efficient exciton transport also enable strongly non-Langevin recombination in high-efficiency NFA-based solar cells with low-energy offsets.

Isomeric acceptors incorporation enables 18.1% efficiency ternary organic solar cells with reduced trap-assisted charge recombination

The donor–acceptor miscibility is one of the most critical factors in controlling the active layer morphologies with multiple components. However, how to practically select the third component with the desired miscibility for efficient ternary organic solar cells remains unresolved. Herein, we demonstrate that the surface energy of thin films could be manipulated by altering the weight ratios of the isomeric acceptors in the blend films and, therefore, the regulation of donor–acceptor miscibility. When the acceptor weight ratio (M−Cl:O-Cl) is 1:0.3, low miscibility (χ = 5.43 × 10-2K) with PM6 is achieved, and the power conversion efficiency of organic solar cells composed of PM6:M−Cl (17.1%) is enhanced to 18.1% (PM6:M−Cl:O-Cl). The increased power conversion efficiency resulted from the surface energy-driven active layer morphology and donor–acceptor phase separations, associated with the significantly lower energetic disorder, trap density, and thus reduced free charge recombination. Our findings underline the significance of the isomeric approach to managing the blend film surface energy and, therefore, the donor–acceptor miscibility for improving the device performance, providing guidelines for the fabrication of high-performed ternary OSCs and material designs.

Interface property–functionality interplay suppresses bimolecular recombination facilitating above 18% efficiency organic solar cells embracing simplistic fabrication

The donor/acceptor interface properties play vital roles not only for singlet exciton dissociation but also to suppress the free charge recombination enabling state-of-the-art device fill factors (FFs).

Unveiling the Valence State of Interstitial Bromine on Charge Carrier Lifetime in CH3NH3PbBr3 by Quantum Dynamics Simulation.

Interstitial halogens are detrimental to the optoelectronic properties of metal halide perovskites. Using nonadiabatic (NA) molecular dynamics, we demonstrate that the valence state of interstitial bromine strongly changes the carrier lifetimes of MAPbBr3 (MA = CH3NH3+). Both neutral and negatively charged interstitial bromine create no midgap states, and they decrease the bandgap, weaken the NA coupling, and accelerate decoherence in a different extent with respect to pristine MAPbBr3, making free charge recombination either slow down about a 3-fold or remain largely unchanged. In contrast, a positively charged interstitial bromine forms a Br trimer and introduces a deep electron trap state, causing a 1.4-fold increase of charge recombination followed by a rapid electron trapping or across the bandgap because of an enhanced NA coupling. The simulations uncover the influence of different charged interstitial bromine defects on MAPbBr3 carrier lifetimes and provide rational guidelines for optimizing perovskite solar cells.

Orders of Recombination in Complete Perovskite Solar Cells – Linking Time‐Resolved and Steady‐State Measurements

AbstractIdeally, the charge carrier lifetime in a solar cell is limited by the radiative free carrier recombination in the absorber which is a second‐order process. Yet, real‐life cells suffer from severe nonradiative recombination in the bulk of the absorber, at interfaces, or within other functional layers. Here, the dynamics of photogenerated charge carriers are probed directly in pin‐type mixed halide perovskite solar cells with an efficiency >20%, using time‐resolved optical absorption spectroscopy and optoelectronic techniques. The charge carrier dynamics in complete devices is fully consistent with a superposition of first‐, second‐, and third‐order recombination processes, with no admixture of recombination pathways with non‐integer order. Under solar illumination, recombination in the studied solar cells proceeds predominantly through nonradiative first‐order recombination with a lifetime of 250 ns, which competes with second‐order free charge recombination which is mostly if not entirely radiative. Results from the transient experiments are further employed to successfully explain the steady‐state solar cell properties over a wide range of illumination intensities. It is concluded that improving carrier lifetimes to >3 µs will take perovskite devices into the radiative regime, where their performance will benefit from photon‐recycling.

General Rules for the Impact of Energetic Disorder and Mobility on Nongeminate Recombination in Phase-Separated Organic Solar Cells

State-of-the-art organic solar cells exhibit power conversion efficiencies of 18% and above. These devices benefit from the suppression of free charge recombination with regard to the Langevin limit of charge encounter in a homogeneous medium. It is recognized that the main cause of suppressed free charge recombination is the reformation and resplitting of charge-transfer (CT) states at the interface between donor and acceptor domains. Here, we use kinetic Monte Carlo simulations to understand the interplay between free charge motion and recombination in an energetically disordered phase-separated donor-acceptor blend. We identify conditions for encounter-dominated and resplitting-dominated recombination. In the former regime, recombination is proportional to mobility for all parameters tested and only slightly reduced with respect to the Langevin limit. In contrast, mobility is not the decisive parameter that determines the nongeminate recombination coefficient, ${k}_{2}$, in the latter case, where ${k}_{2}$ is a sole function of the morphology, CT and charge-separated (CS) energetics, and CT-state decay properties. Our simulations also show that free charge encounter in the phase-separated disordered blend is determined by the average mobility of all carriers, while CT reformation and resplitting involves mostly states near the transport energy. Therefore, charge encounter is more affected by increased disorder than the resplitting of the CT state. As a consequence, for a given mobility, larger energetic disorder, in combination with a higher hopping rate, is preferred. These findings have implications for the understanding of suppressed recombination in solar cells with nonfullerene acceptors, which are known to exhibit lower energetic disorder than that of fullerenes.

Numerical Analysis of Selective ITO/a-Si:H Contacts in Heterojunction Silicon Solar Cells: Effect of Defect States in Doped a-Si:H Layers on Performance Parameters

The effects of defect states in doped thin-film amorphous-silicon layers (p-a-Si:H and n-a-Si:H) and at heterointerfaces of the passivated crystalline-silicon (c-Si) wafer on the performance of heterojunction silicon solar cells are investigated using optoelectrical simulations. We used a state-of-the-art numerical model of a silicon heterojunction solar cell, including indium-tin-oxide (ITO) contacts as semiconductor layers, applying the accompanying tunneling mechanisms. We show that increasing the dangling-bond density in the p-a-Si:H reduces the conversion efficiency of the device more than in the n-a-Si:H, independent of the doping type of the c-Si wafer and illumination side of the device. Simulations revealed that this is due to: a larger ITO work-function mismatch with the p-a-Si:H than the n-a-Si:H; a larger valence-band offset at i-a-Si:H/n-c-Si interface at the p-side compared to the conduction-band offset at the n-side i-a-Si:H/n-c-Si; and asymmetric distribution of defects in the doped a-Si:H layers. The effects of individual defect-state parameters in doped a-Si:H layers and c-Si heterointerfaces are shown. We demonstrate that the decrease in conversion efficiency due to increased dangling-bond states in doped layers is primarily a consequence of a reduced fill-factor and open-circuit voltage. We explain the phenomenon by charge-redistribution linked to increased dangling-bond density in p-a-Si:H, whereas the increased free-charge recombination has only little effect on the short-circuit-current density over a wide range of defect-state variation. Besides quantitative results of defect-state variation on solar cell performance, this article aims to offer a thorough understanding of the physical processes in the device, governing these effects.

Deducing transport properties of mobile vacancies from perovskite solar cell characteristics

The absorber layers in perovskite solar cells possess a high concentration of mobile ion vacancies. These vacancies undertake thermally activated hops between neighboring lattice sites. The mobile vacancy concentration N0 is much higher and the activation energy EA for ion hops is much lower than is seen in most other semiconductors due to the inherent softness of perovskite materials. The timescale at which the internal electric field changes due to ion motion is determined by the vacancy diffusion coefficient Dv and is similar to the timescale on which the external bias changes by a significant fraction of the open-circuit voltage at typical scan rates. Therefore, hysteresis is often observed in which the shape of the current–voltage, J–V, characteristic depends on the direction of the voltage sweep. There is also evidence that this defect migration plays a role in degradation. By employing a charge transport model of coupled ion-electron conduction in a perovskite solar cell, we show that EA for the ion species responsible for hysteresis can be obtained directly from measurements of the temperature variation of the scan-rate dependence of the short-circuit current and of the hysteresis factor H. This argument is validated by comparing EA deduced from measured J–V curves for four solar cell structures with density functional theory calculations. In two of these structures, the perovskite is MAPbI3, where MA is methylammonium, CH3NH3; the hole transport layer (HTL) is spiro (spiro-OMeTAD, 2,2′,7,7′- tetrakis[N,N-di(4-methoxyphenyl) amino]-9,9′-spirobifluorene) and the electron transport layer (ETL) is TiO2 or SnO2. For the third and fourth structures, the perovskite layer is FAPbI3, where FA is formamidinium, HC(NH2)2, or MAPbBr3, and in both cases, the HTL is spiro and the ETL is SnO2. For all four structures, the hole and electron extracting electrodes are Au and fluorine doped tin oxide, respectively. We also use our model to predict how the scan rate dependence of the power conversion efficiency varies with EA, N0, and parameters determining free charge recombination.

Studies of MnO2/g-C3N4 hetrostructure efficient of visible light photocatalyst for pollutants degradation by sol-gel technique

MnO2 and g-C3N4 heterostructure were constructed by simple sol-gel technique combined with annealing process were characterized different variety of techniques to understand the structural, morphological, optical and elemental composition were using XRD, TEM, Raman, UV, PL, BET and EDAX analysis. In fact, XRD and FESEM results shows that α-phase MnO2 structure were individually spherical shaped - like morphology nanostructures the average diameter 25 nm to 40 nm was observed apparently. In order to understand, the optical bandgap properties and specific surface area of MnO2/g-C3N4 nanocomposites were ranging from 2.55 eV - 2.78 eV & 67.23 - 105.21 m2/g, which is confirmed through UV–vis DRS and N2 nitrogen absorption desorption study. Therefore, the photocatalytic activity of the photocatalysts were estimated by degradation effect of methyl orange (MO) and toxic phenol pollutants. Among the results reveal that the MnO2/g-C3N4 hybrid catalyst have showed to owing the degradation effect 98% and high stability (only loss 3.5%) towards phenol dye concentration. This can be attributed to the separation of photogenerated electron-hole (e−/h+) pairs, overwhelms the recombination of free charges were reported in detail.

A novel synthesis method for Ag/g-C3N4 nanocomposite and mechanism of enhanced visible-light photocatalytic activity

A novel synthesis approach toward Ag/g-C3N4 composite photocatalyst is reported. First, g-C3N4 nanoflakes (CNNs) were fabricated by thermal polymerization of melamine precursor combined with gas bubble exfoliation method. Second, Ag nanoparticles were homogeneously dispersed on CNNs by photo-reduction method to assemble Ag/CNNs nanocomposites. The structure of Ag/CNNs nanocomposites was verified by SEM, EDS, TEM, FTIR and XRD. The photocatalytic properties of Ag/CNNs nanocomposites were verified by degradation of Rhodamine B (RhB) under visible light irradiation. The increase of photocatalytic activity of Ag/CNNs nanocomposites was mainly due to the Schottky barrier between Ag and CNNs and the local surface plasmonic resonance (LSPR) effect of Ag NPs, which was conducive to improving the separation and transportation efficiency of photogenerated electrons-holes as well as the absorption of visible light, reducing recombination of free charges. Thus, the Ag/CNNs nanocomposites would show excellent photocatalytic activity in organic waste water treatment.

Enhanced Radiative Recombination of Excitons and Free Charges Due to Local Deformations in the Band Structure of MAPbBr3 Perovskite Crystals

We have imaged the emissions from excitons and free charges in a methylammonium lead bromide perovskite (MAPbBr3) crystal. In a direct band gap semiconductor, dynamics of excited electrons and holes in hybrid lead-halide perovskites is rather complex because of the formation of excitons and the presence of traps and structural inhomogeneities. A recent report by Nah et al. (Nah, S.; Spokoyny, B.; Stoumpos, C.; Soe, C. M. M.; Kanatzidis, M.; Harel, E. Nat. Photonics 2017, 11, 285-288) has identified spatially segregated populations of excitons and free charges in perovskites. Understanding the cause of segregation and how such distributions contribute to the photoresponse has remained a challenge. Here, we have used phase-modulated two-photon photoluminescence microspectroscopy to separately quantify the emissions from excitons and free charges in a MAPbBr3 crystal. Our results show that while most of the emission at room temperature is from the recombination of free charges, there are also localized spots where the emissions from excitons dominate. By analyzing the enhancement of the emissions at low temperatures, we show that the trap-mediated nonradiative recombinations are suppressed by local deformations in the band structure. The deformations that are induced by local strains in the crystal prevent excitons and free charges from reaching the traps. Our results indicate that strain engineering through structural designs can improve the performance of perovskite light-emitting diodes. (Less)

Investigating Free Charge‐Carrier Recombination in Organic LEDs Using Open‐Circuit Conditions

AbstractAlthough organic light‐emitting diodes (LEDs) are widely used in commercial applications, a comprehensive knowledge about the occurring recombination pathways is still absent. Especially access to the bandgap is needed to figure out which path the charge carriers take, key to further understand and optimize the latest generations of thin film LED technology. Here, state‐of‐the‐art organic LEDs are treated like solar cells by measuring the open‐circuit voltage under UV illumination. A variation of the illumination intensity and temperature gives access to the effective bandgap, the ideality factor and the recombination strength. A differential evaluation approach is implemented, yielding the full temperature and intensity dependence of these parameters. A radiative and a non‐radiative recombination process are identified and used to reconstruct the current density dependence of the external quantum efficiency. All recombination events are related to an effective bandgap that suggests free charge recombination between the guest and the host material of the emission layer. This approach will thus help to identify and eliminate non‐radiative recombination losses in LEDs based on other type of molecules emitters and materials, such as perovskites or quantum dots, as well. Knowing the effective bandgap will also be essential to realize LEDs with the lowest possible driving voltages.

Enhanced photocatalytic bactericidal performance and mechanism with novel Ag/ZnO/g-C3N4 composite under visible light

The Ag/ZnO/g-C3N4 composite photocatalyst, which was synthesized by thermal polymerization of a melamine precursor combined with solvothermal reaction, was used as a visible-light-driven biocide toward Escherichia coli (E. coli). The Ag/ZnO/g-C3N4 composite displayed considerably increased visible-light-driven bactericidal activity compared to g-C3N4, Ag/g-C3N4, and ZnO/g-C3N4 powders. The photocatalytic destruction of E. coli cells was verified by fluorescence-based cell live/dead analysis and SEM. The mechanism of increased bactericidal effect was studied by UV–vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, photocurrent response, and electrochemical impedance spectroscopy (EIS). The enhanced photocatalytic bactericidal effect was attributed to the matching conduction band levels of g-C3N4 and ZnO and the surface plasmon resonance (SPR) effect of Ag nanoparticles, which resulted in enhanced adsorption of visible light, reduced recombination of free charges, and rapid separation and transportation of photogenerated electrons-holes. The disinfection mechanism was investigated using different chemical scavengers and electron spin-resonance spectroscopy (ESR), and indicating the important role of h+ and H2O2. Furthermore, taking into account the bulk availability and better bactericidal effect of Ag/ZnO/g-C3N4, this work provides a new pathway to develop advanced materials for water disinfection.

Superior photocatalytic disinfection effect of Ag-3D ordered mesoporous CeO2 under visible light

Ordered mesoporous Ag/CeO2 nanocomposites were prepared by combining nanocasting with photo-assisted reduction. The heterogeneous photoreactivity of the prepared samples was tested for inactivating Escherichia coli (E. coli). These mesoporous Ag/CeO2 nanocomposites exhibited superior photocatalytic disinfection efficiency than mesoporous CeO2 and bulk CeO2, respectively. Doping Ag species could extend the absorption range to visible light area and enhance the concentration of Ce3+ ions and oxygen vacancies, which had significant influence on the surface properties of CeO2. Moreover, it is the first time to verify that created midgap states through doping Ag can significantly lower the threshold of incident photon energy and broaden the adsorption spectrum demonstrated by experimental studies and density functional theory (DFT) calculations. The enhanced disinfection mechanism was attributed to the fact that the novel mesoporous structure can promote the mass transfer process and the interfacial interaction between Ag species and CeO2 and lead to reduced recombination of free charges, rapid separation and transportation of photogenerated electrons-holes. This work provides a new method to develop novel mesoporous plasmonic photocatlysts for disinfection using solar radiation.

Intercalated vs Nonintercalated Morphologies in Donor-Acceptor Bulk Heterojunction Solar Cells: PBTTT:Fullerene Charge Generation and Recombination Revisited.

In this Letter, we study the role of the donor:acceptor interface nanostructure upon charge separation and recombination in organic photovoltaic devices and blend films, using mixtures of PBTTT and two different fullerene derivatives (PC70BM and ICTA) as models for intercalated and nonintercalated morphologies, respectively. Thermodynamic simulations show that while the completely intercalated system exhibits a large free-energy barrier for charge separation, this barrier is significantly lower in the nonintercalated system and almost vanishes when energetic disorder is included in the model. Despite these differences, both femtosecond-resolved transient absorption spectroscopy (TAS) and time-delayed collection field (TDCF) exhibit extensive first-order losses in both systems, suggesting that geminate pairs are the primary product of photoexcitation. In contrast, the system that comprises a combination of fully intercalated polymer:fullerene areas and fullerene-aggregated domains (1:4 PBTTT:PC70BM) is the only one that shows slow, second-order recombination of free charges, resulting in devices with an overall higher short-circuit current and fill factor. This study therefore provides a novel consideration of the role of the interfacial nanostructure and the nature of bound charges and their impact upon charge generation and recombination.

The Effects of Electronic Impurities and Electron–Hole Recombination Dynamics on Large‐Grain Organic–Inorganic Perovskite Photovoltaic Efficiencies

Hybrid organic‐inorganic perovskites have attracted considerable attention after promising developments in energy harvesting and other optoelectronic applications. However, further optimization will require a deeper understanding of the intrinsic photophysics of materials with relevant structural characteristics. Here, the dynamics of photoexcited charge carriers in large‐area grain organic‐inorganic perovskite thin films is investigated via confocal time‐resolved photoluminescence spectroscopy. It is found that the bimolecular recombination of free charges is the dominant decay mechanism at excitation densities relevant for photovoltaic applications. Bimolecular coefficients are found to be on the order of 10−9 cm3 s−1, comparable to typical direct‐gap semiconductors, yet significantly smaller than theoretically expected. It is also demonstrated that there is no degradation in carrier transport in these thin films due to electronic impurities. Suppressed electron–hole recombination and transport that is not limited by deep level defects provide a microscopic model for the superior performance of large‐area grain hybrid perovskites for photovoltaic applications.

A New Figure of Merit for Organic Solar Cells with Transport-limited Photocurrents.

Compared to their inorganic counterparts, organic semiconductors suffer from relatively low charge carrier mobilities. Therefore, expressions derived for inorganic solar cells to correlate characteristic performance parameters to material properties are prone to fail when applied to organic devices. This is especially true for the classical Shockley-equation commonly used to describe current-voltage (JV)-curves, as it assumes a high electrical conductivity of the charge transporting material. Here, an analytical expression for the JV-curves of organic solar cells is derived based on a previously published analytical model. This expression, bearing a similar functional dependence as the Shockley-equation, delivers a new figure of merit α to express the balance between free charge recombination and extraction in low mobility photoactive materials. This figure of merit is shown to determine critical device parameters such as the apparent series resistance and the fill factor.