Bimolecular Charge Carrier Recombination Research Articles

Peculiarities of the effect of different types of SOR nanoimpurities on the value of ionic component of the electrical conductivity of the homeotropically aligned nematic liquid crystal 6 СВ

In this paper, the effects of SOR5, SOR10, and SOR15 nanoimpurities on the value of the ionic conductivity of the homeotropically aligned nematic liquid crystal 6CB are reported. Electrical measurements are carried out within a wide frequency range (from 6 Hz to 106 Hz) at room temperature (293 K). The largest changes in the electrical conductivity occur in the low-frequency range (less than 103 Hz) and depend on the type of nanoimpurity and on its concentration. Despite the similarity in the chemical composition of SOR5, SOR10, and SOR15, the measured dependences of the electrical conductivity of the studied samples on the concentration of nanoimpurities are substantially different. In the case of the SOR10 impurity, the ionic component of the electrical conductivity depends on the concentration of SOR10 according to a power law with an exponent approximately equal to 0.5, which is typical for the bimolecular recombination of charge carriers (i.e., the behavior of a weak electrolyte). Liquid crystal samples containing SOR5 and SOR15 nanoimpurities behave in a similar way in a low concentration region (between 0.01 and 0.05 mass %). Interestingly, further increase in the concentration of nanodopants (>0.05 mass %) results in a dramatically different behavior. The electrical conductivity of liquid crystals doped with SOR5 undergoes a sharp increase whereas the electrical conductivity of samples containing SOR15 decreases.

Optimizing the Photovoltaic Performance of Organic Solar Cells for Indoor Light Harvesting

Organic solar cells (OSCs) harvesting indoor light are highly promising for emerging technologies, such as internet of things. Herein, the photovoltaic performance of PTB7-Th:PC71 BM solar cells constructed using "optimized (with 1,8-diiodooctane (DIO))" and "non-optimized (without DIO)" processing conditions are compared for indoor and outdoor applications. We find that in comparison to the "optimized" solar cell, the "non-optimized" solar cell is less efficient under simulated solar light illumination (100 mW cm-2 , spectral range 350-1100 nm), owing to significant bimolecular charge carrier recombination losses. However, under simulated indoor illumination (3.28 mW cm-2 , spectral range 400-700 nm), bimolecular recombination losses are effective suppressed, thus the power conversion efficiency of the solar cell without DIO was increased to 14.7 %, higher than that of the solar cell with DIO (14.2 %). These results suggest that the common strategy used to optimize the OSCs could be undesired for indoor OSCs. We demonstrate that the efforts for realizing the desired "morphology" of the active layer for the outdoor OSCs may be unnecessary for indoor OSCs, allowing us to realize high-efficiency indoor OSCs using a non-halogenated solvent.

Enhanced Bimolecular Recombination of Charge Carriers in Amorphous Organic Semiconductors: Overcoming the Langevin Limit

We consider the bimolecular charge carrier recombination in amorphous organic semiconductors having a special kind of energetic disorder where energy levels for electrons and holes at a given transport site move in the same direction with the variation of some disorder governing parameter (the parallel disorder). This particular kind of disorder could be found in materials where the dominant part of the energetic disorder is provided by the conformational disorder. Contrary to the recently studied case of electrostatic disorder, the conformational disorder, if spatially correlated, leads to the increase of the recombination rate constant which becomes greater than the corresponding Langevin rate constant. Probably, organic semiconductors with the dominating conformational disorder represent the first class of amorphous organic semiconductors where the recombination rate constant could overcome the Langevin limit.

Bimolecular Recombination of Charge Carriers in Polar Amorphous Organic Semiconductors: Effect of Spatial Correlation of the Random Energy Landscape

We present a simple model of the bimolecular charge carrier recombination in polar amorphous organic semiconductors where the dominant part of the energetic disorder is provided by permanent dipoles and show that the recombination rate constant could be much smaller than the corresponding Langevin rate constant. The reason for the strong decrease of the rate constant is the long range spatial correlation of the random energy landscape in amorphous dipolar materials, without spatial correlation even strong disorder does not modify the Langevin rate constant. Our study shows that the significant suppression of the bimolecular recombination could take place in homogeneous amorphous organic semiconductors and does not need large scale inhomogeneity of the material.

Bimolecular recombination in methylammonium lead triiodide perovskite is an inverse absorption process

Photovoltaic devices based on metal halide perovskites are rapidly improving in efficiency. Once the Shockley–Queisser limit is reached, charge-carrier extraction will be limited only by radiative bimolecular recombination of electrons with holes. Yet, this fundamental process, and its link with material stoichiometry, is still poorly understood. Here we show that bimolecular charge-carrier recombination in methylammonium lead triiodide perovskite can be fully explained as the inverse process of absorption. By correctly accounting for contributions to the absorption from excitons and electron-hole continuum states, we are able to utilise the van Roosbroeck–Shockley relation to determine bimolecular recombination rate constants from absorption spectra. We show that the sharpening of photon, electron and hole distribution functions significantly enhances bimolecular charge recombination as the temperature is lowered, mirroring trends in transient spectroscopy. Our findings provide vital understanding of band-to-band recombination processes in this hybrid perovskite, which comprise direct, fully radiative transitions between thermalized electrons and holes.

Charge carrier concentration dependence of encounter-limited bimolecular recombination in phase-separated organic semiconductor blends

Understanding how the complex intermolecular- and nano-structure present in organic semiconductor donor-acceptor blends impacts charge carrier motion, interactions, and recombination behavior is a critical fundamental issue with a particularly major impact on organic photovoltaic applications. In this study, kinetic Monte Carlo (KMC) simulations are used to numerically quantify the complex bimolecular charge carrier recombination behavior in idealized phase-separated blends. Recent KMC simulations have identified how the encounter-limited bimolecular recombination rate in these blends deviates from the often used Langevin model and have been used to construct the new power mean mobility model. Here, we make a challenging but crucial expansion to this work by determining the charge carrier concentration dependence of the encounter-limited bimolecular recombination coefficient. In doing so, we find that an accurate treatment of the long-range electrostatic interactions between charge carriers is critical, and we further argue that many previous KMC simulation studies have used a Coulomb cutoff radius that is too small, which causes a significant overestimation of the recombination rate. To shed more light on this issue, we determine the minimum cutoff radius required to reach an accuracy of less than $\pm10\%$ as a function of the domain size and the charge carrier concentration and then use this knowledge to accurately quantify the charge carrier concentration dependence of the recombination rate. Using these rigorous methods, we finally show that the parameters of the power mean mobility model are determined by a newly identified dimensionless ratio of the domain size to the average charge carrier separation distance.

Hole transport and bimolecular recombination of charge carriers in polystyrene molecularly doped with 6 wt % tritolylamine

The general problems of hole transport and bimolecular recombination of charge carriers in molecularly doped polystyrene with an ultimately low dopant concentration (6 wt %) were considered. The experimental studies were performed by the radiation-induced time-of-flight method with bulk generation of charge carriers. The transient current curves were calculated using the multiple trapping model. Good agreement between the calculated and experimental curves of the transient current was achieved. The hole transport in the molecularly doped polymer was shown to be nonequilibrium. The bimolecular recombination was close to Langevin recombination when interpreted using the multiple trapping model.

Hole transport and bimolecular recombination of charge carriers in polycarbonate molecularly doped with aromatic hydrazone

General questions about hole transport and bimolecular recombination of charge carriers in molecularly doped polycarbonate with a low dopant concentration (10 wt %) are considered. The experiment is performed via a radiation-induced time-of-flight technique with bulk generation of charge carriers. Transient-current curves are calculated numerically via a multiple-trapping model. There is good agreement between the calculated and experimental results on the transient-current waveform. Nonequilibrium hole transport is observed in the studied molecularly doped polymer, and the bimolecular recombination is close to the Langevin recombination as described by the multiple-trapping model.

Bimolecular recombination of charge carriers in pure and molecularly doped branched polyphenylenevinylenes

General problems of bimolecular recombination of charge carriers in both pure and molecularly doped branched polyphenylenevinylenes are addressed. Experiments are performed via the nonstationary radiation-induced conductivity method. Transient-current curves are numerically calculated in terms of the multiple-trapping model. Good agreement between the calculated and experimental curves is attained. In the investigated polymers, the Langevin mechanism of bimolecular recombination is active.

Bimolecular recombination of charge carriers in molecularly doped polycarbonate

The general problems of the bimolecular recombination of charge carriers in molecularly doped polycarbonate are discussed. Experimental studies are performed via the transient radiation-induced conductivity method. Transient-current curves are numerically calculated via the multiple trapping model. The calculated and experimental curves are in good agreement. It is shown that the studied molecularly doped polymer undergoes bimolecular recombination via the Langevin mechanism.

Double injection in organic bulk-heterojunction

We have used double injection current (DoI) transient technique for investigation of mobility, recombination and deep trapping of charge carriers in bulk-heterojunction organic solar cells, where bimolecular charge carrier recombination is reduced compared to Langevin recombination. Experimental DoI transient’s investigation results of RRP3HT/PCBM blends were compared with ones of a-Si:H solar cells, and with results obtained by both charge carrier extraction (CELIV) and time-of-flight (TOF) techniques. Numerical modeling of DoI transient, taking into account charge carrier trapping, demonstrated that in bulk-heterojunction organic material blends the influence of charge carrier deep trapping is insignificant. The bimolecular recombination coefficient is strongly reduced compared to Langevin recombination one and depends on the density of charge carriers.

Time-dependent Langevin-type bimolecular charge carrier recombination in regiorandom poly(3-hexylthiophene)

We have studied transport and recombination of photogenerated charge carriers in thin films of regiorandom poly(3-hexylthiophene) (RRaPHT). Time-of-Flight studies show space charge perturbed current transients with a saturation of current level at high light intensities as a result of diffusion controlled Langevin-type bimolecular charge carrier recombination. We have been able to measure the time-dependent charge carrier mobility by using the carrier extraction by linearly increasing voltage technique. We found that the hole mobility follows a power law dependence over a few orders of magnitude in time as μ = μ 0 t −0.42. By using the carrier extraction by pulsed voltage technique we measured the photogenerated charge carrier concentration decay as a function of delay time and found that the decay can be fitted using a dispersive time-dependent bimolecular recombination coefficient governed by the time-dependent hole mobility.

Bimolecular Recombination Coefficient as a Sensitive Testing Parameter for Low-Mobility Solar-Cell Materials

Bimolecular charge carrier recombination has been clarified in bulk-heterojunction solar cells based on a blend of regioregular poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene using the time-of-flight method. We show how bimolecular recombination influences the charge carrier transport, how it limits the efficiency of low-mobility solar cells, and how to estimate the bimolecular recombination coefficient. We found that bimolecular recombination in these efficient photovoltaic materials is orders of magnitude slower as compared to Langevin recombination expected for low-mobility materials. This effect is inherent to the nanomorphology of the bicontinuous interpenetrating network creating separate pathways for electrons and holes, and paves the way for the fabrication of bulk-heterojunction solar cells where bimolecular recombination is not the limiting factor.

Observation of Bimolecular Carrier Recombination Dynamics in Close-Packed Films of Colloidal CdSe Nanocrystals

We study the intensity dependence of the photoconductivity in close-packed films of CdSe quantum dots in which the photoconductivity is characterized by a low density of trapped charges. We find that the intensity dependence of the photocurrent is nonlinear, which is indicative of bimolecular charge carrier recombination dynamics. We present a physical model for the intensity dependence of the photocurrent that is used to fit the data and extract a ratio of the density of carriers to the density of trapped charges. As our model predicts, we show that the current vs intensity plot becomes more linear when the carrier density is decreased.

Ultrafast Charge Carrier Recombination in a-Si:H and μc-Si:H

The ultrafast nonradiative bimolecular recombination of charge carriers was investigated in amorphous and microcrystalline silicon. The photoelectric method based on measurement of extraction time of the charge carrier reservoir in the space-charge-limited photocurrent mode was used. A series of models (Auger, diffusion-controlled, charge-carrier-fluctuation dominated) were experimentally tested but were not able to explain all features of bimolecular recombination in a-Si:H. The model of diffusion limited recombination taking into account the intrinsic random potential seems most suitable for a description of the observed suppressed bimolecular recombination.

Photoemission from Silver into AgCl, KBr, NaCl, and New Bands of Photosensitivity in AgCl

The work required to move an electron from a metal into an insulator has been investigated photoelectrically. The work function thus found was about 4.3 ev for Ag evaporated in vacuum on KBr (Fowler plot) as well as on NaCl (threshold wavelength). The photoelectrons become trapped in these crystals at vacant anion sites and may be released by $F$-band illumination. For Ag on AgCl a work function of 1.1 ev is measured at the low temperatures used to freeze out ionic conductivity.In AgCl seven new bands of photosensitivity were found; the corresponding optical absorption was too small to be measured directly. These bands have a half-width of $\mathrm{ca}$ 0.2 ev at 90\ifmmode^\circ\else\textdegree\fi{}K and are bleached by irradiation. The 4400, 5000, 5700, 6600, and 7500A bands increase in intensity with an increasing supply of electrons provided by photoelectric emission. These bands are identified by their location and behavior, with the $F$, $E$, $D$, $C$, and $G$ bands (Petroff's notation) in the alkali halides. The 4800A ($A$ band) and 9200A bands are favored by a scarcity of electrons and may correspond to trapped holes. Photosensitivity in all these bands can be created by irradiation in the intrinsic optical-absorption edge of AgCl at low temperature. They may not be characteristic of a perfect crystal.The intrinsic band-like photoconductive response of AgCl and the absorption edge on which it occurs shift toward shorter wavelengths with decreasing temperature. The maximum of response which appears with decreasing wavelength may arise from a bimolecular recombination of charge carriers released by light. The absorption edge steepens as it shifts toward shorter wavelength with decreasing temperature.