We consider the two-dimensional bimolecular recombination of charge carriers in amorphous organic semiconductors having the lamellar structure. We calculate the dependence of the effective recombination rate constant on the carrier density taking into account the correlated nature of the energetic disorder typical for organic semiconductors. Resulting recombination kinetics demonstrates a very rich variety of behaviors depending on the correlation properties of the particular semiconductor and relevant charge density range.
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…
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 th…
The original Shockley-Read-Hall recombination statistics is extended to include recombination of localized excitations. The recombination is treated as a bimolecular process rather than a monomolecular recombination of excitons. The emphasis is placed on an interplay between two distinct channels of radiative recombination (shallow localized states vs extended states) mediated by trapping of photogenerated charge carriers by non-radiative centers. Results of a numerical solution for a given set of parameters are complemented by an approximate analytical expression for the thermal quenching of …
Charge carrier recombination is studied in operational organic solar cells made from the polymer:fullerene system PCDTBT:PC71BM (poly[N-9"-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] : [6,6]-phenyl-C$_{70}$-butyric acid methyl ester). A newly developed technique High Intensity Resistance dependent PhotoVoltage (HI-RPV) is presented for reliably quantifying the bimolecular recombination coefficient independently of variations in experimental conditions, thereby resolving key limitations of previous experimental approaches. Experiments are performed on sola…
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…
We propose a model to explain the reduced bimolecular recombination rate found in state-of-the-art bulk heterojunction solar cells. When compared to the Langevin recombination, the experimentally observed rate is one to four orders of magnitude lower, but gets closer to the Langevin case for low temperatures. Our model considers the organic solar cell as device with carrier concentration gradients, which form due to the electrode/blend/electrode device configuration. The resulting electron concentration under working conditions of a solar cell is higher at the cathode than at the anode, and vi…
The photoluminescence in amorphous semiconductors decays according to power law $t^{-delta}$ at long times. The photoluminescence is controlled by dispersive transport of electrons. The latter is usually characterized by the power $alpha$ of the transient current observed in the time-of-flight experiments. Geminate recombination occurs by radiative tunneling which has a distance dependence. In this paper, we formulate ways to calculate reaction rates and survival probabilities in the case carriers execute dispersive diffusion with long-range reactivity. The method is applied to obtain tunnelin…
The photoluminescence decay in amorphous semiconductors is described by power law $t^{-delta}$ at long times. The power-law decay of photoluminescence at long times is commonly observed but recent experiments have revealed that the exponent, $delta sim 1.2-1.3$, is smaller than the value 1.5 predicted from a geminate recombination model assuming normal diffusion. Transient currents observed in the time-of-flight experiments are highly dispersive characterized by the disorder parameter $alpha$ smaller than 1. Geminate recombination rate should be influenced by the dispersive transport of charge…
Apparent recombination orders exceeding the value of two expected for bimolecular recombination have been reported for organic solar cells in various publications. Two prominent explanations are bimolecular losses with a carrier concentration dependent prefactor due to a trapping limited mobility, and protection of trapped charge carriers from recombination by a donor--acceptor phase separation until reemission from these deep states. In order to clarify which mechanism is dominant we performed temperature and illumination dependent charge extraction measurements under open circuit as well as …
We studied the recombination dynamics of charge carriers in organic bulk heterojunction solar cells made of the blend system poly(2,5-bis(3-dodecyl thiophen-2-yl) thieno[2,3-b]thiophene) (pBTCT-C12):[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) with a donor--acceptor ratio of 1:1 and 1:4. The techniques of charge carrier extraction by linearly increasing voltage (photo-CELIV) and, as local probe, time-resolved microwave conductivity (TRMC) were used. We observed a difference in the initially extracted charge carrier concentration in the photo-CELIV experiment by one order of magnitude, w…
We performed temperature dependent transient photovoltage and photocurrent measurements on poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methylester bulk heterojuction solar cells. We found a strongly charge carrier concentration and temperature dependent Langevin recombination prefactor. The observed recombination mechanism is discussed in terms of bimolecular recombination. The experimental results were compared with charge carrier extraction by linearly increasing voltage (photo-CELIV) measurements done on the same blend system. We explain the charge carrier dynamics, following an ap…
In a recent paper, Street et al. [Phys. Rev. B 81, 205307 (2010)] propose first order recombination due to interface states to be the dominant loss mechanism in organic bulk heterojunction solar cells, based on steady-state current--voltage characteristics. By applying macroscopic simulations, we found that under typical solar cell conditions, monomolecular or bimolecular recombination cannot be inferred from the slope of the light intensity dependent photocurrent. In addition, we discuss the validity of calculating a mobility--lifetime product from steady-state measurements. We conclude that …
We present experimental evidence showing that the effective carrier diffusion length ($L_d$) and lifetime ($τ$) depend on the carrier density in MAPbBr$_3$ single crystals. Independent measurements reveal that both $L_d$ and $τ$ 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 $μ$m and ~ 13.8 $μ$m respectively. A marginal increase in uniform ligh…
Recombination of photogenerated charge carriers in polymer bulk heterojunction (BHJ) solar cells reduces the short circuit current (Jsc) and the fill factor (FF). Identifying the mechanism of recombination is, therefore, fundamentally important for increasing the power conversion efficiency. Light intensity and temperature dependent current-voltage measurements on polymer BHJ cells made from a variety of different semiconducting polymers and fullerenes show that the recombination kinetics are voltage dependent and evolve from first order recombination at short circuit to bimolecular recombinat…
THz Time-Resolved Photoconductivity is used to probe carrier dynamics in the dilute III-V nitride GaP0.49As0.47N0.036. In these measurements a femtosecond optical pump-pulse excites electron-hole pairs, and a delayed THz pulse measures the change in conductivity. We find the photoconductivity is dominated by localized carriers. The decay of photoconductivity after excitation is consistent with bimolecular electron-hole recombination with recombination constant r = 3.2E-8 +/-0.8E-8 cm3/s. We discuss the implications for applications in solar energy.
The power conversion efficiency of organic solar cells based on donor--acceptor blends is governed by an interplay of polaron pair dissociation and bimolecular polaron recombination. Both processes are strongly dependent on the charge carrier mobility, the dissociation increasing with faster charge transport, with raised recombination losses at the same time. Using a macroscopic effective medium simulation, we calculate the optimum charge carrier mobility for the highest power conversion efficiency, for the first time accounting for injection barriers and a reduced Langevin-type recombination.…
Suppressing charge recombination is key for organic solar cells to become commercial reality. However, there is still no conclusive picture of how recombination losses are influenced by the complex nanoscale morphology. Here, new insight is provided by revisiting the P3HT:PCBM blend, which is still one of the best performers regarding reduced recombination. By changing small details in the annealing procedure, two model morphologies were prepared that vary in phase separation, molecular order and phase purity, as revealed by electron tomography and optical spectroscopy. Both systems behave ver…
Carrier and spin recombination are investigated in p-type GaAs of acceptor concentration NA = 1.5 x 10^(17) cm^(-3) using time-resolved photoluminescence spectroscopy at 15 K. At low pho- tocarrier concentration, acceptors are mostly neutral and photoelectrons can either recombine with holes bound to acceptors (e-A0 line) or form excitons which are mostly trapped on neutral acceptors forming the (A0X) complex. It is found that the spin lifetime is shorter for electrons that recombine through the e-A0 transition due to spin relaxation generated by the exchange scattering of free electrons with …
In photovoltaic diodes recombination of photogenerated electrons and holes is a major loss process. Biological light harvesting complexes (LHCs) prevent recombination via the use of cascade structures, which lead to spatial separation of charge-carriers1. In contrast, the nanoscale morphology and high charge densities in organic photovoltaic cells (OPVs) give a high rate of electron-hole encounters, which should result in the formation of spin triplet excitons, as in organic light emitting diodes (OLEDs)2. OPVs would have poor quantum efficiencies if every encounter lead to recombination, but …
Using cross-correlation current noise spectroscopy, we have investigated carrier dynamics in methylammonium lead triiodide solar cells. This method provides a space selectivity for devices with planar multi-layered structure, effectively amplifying current noise contributions coming from the most resistive element of the stack. In the studied solar cells, we observe near full-scale shot noise, indicating the dominance of noise generation by a single source, likely the interface between the perovskite and the spiro-OMeTAD hole-transport layer. We argue that the strong 1/f noise term has contrib…
We have used transient absorption spectroscopy to measure carrier lifetimes in the multiband band semiconductor GaPAsN. These measurements probe the electron populations in the conduction band, intermediate band and valance band as a function of time after an excitation pulse. Following photoexcitation of GaP0.32As0.67N0.01 we find that the electron population in the conduction band decays exponentially with a time constant 23ps. The electron population in the intermediate band exhibits bimolecular recombination with recombination constant r = 2 10^-8 cm-3/s. In our experiment an optical pump …
Nanowires have unique optical properties [1-4] and are considered as important building blocks for energy harvesting applications such as solar cells. [2, 5-8] However, due to their large surface-to-volume ratios, the recombination of charge carriers through surface states reduces the carrier diffusion lengths in nanowires a few orders of magnitude,[9] often resulting in the low efficiency (a few percent or less) of nanowire-based solar cells. [7, 8, 10, 11] Reducing the recombination by surface passivation is crucial for the realization of high performance nanosized optoelectronic devices, bu…
The sub-gap density of states of amorphous indium gallium zinc oxide ($a$-IGZO) is obtained using the ultrabroadband photoconduction (UBPC) response of thin-film transistors (TFTs). Density functional theory simulations classify the origin of the measured sub-gap density of states peaks as a series of donor-like oxygen vacancy states and acceptor-like Zn vacancy states. Donor peaks are found both near the conduction band and deep in the sub-gap, with peak densities of $10^{17}-10^{18}$ cm$^{-3}$eV$^{-1}$. Two deep acceptor-like metal vacancy peaks with peak densities in the range of $10^{18}$ …
Energy carrier transport and recombination in emerging semiconductors can be directly monitored with optical microscopy, leading to the measurement of the diffusion coefficient (D), a critical property for design of efficient optoelectronic devices. D is often determined by fitting a time-resolved expanding carrier profile after optical excitation using a Mean Squared Displacement (MSD) Model. Although this approach has gained widespread adoption, its utilization can significantly overestimate D due to the non-linear recombination processes that artificially broaden the carrier distribution pr…
Cross-sectional profile of photocarrier mobility in thin film solar cell via nongeminate recombination and charge extraction by linearly increasing voltage (cs-p-CELIV)
Although it has been recently demonstrated that the laser-assisted (LA) crystallization process leads to the formation of perovskite absorber films of superior photovoltaic performance compared to conventional thermal annealing (TA), the physical origin behind this important discovery is missing. In this study, CH$_3$NH$_3$PbI$_3$ perovskite thin films have been synthesized by means of LA and TA crystallization on the surface of two hole transport layers (HTL) namely poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) and poly(triarylamine) (PTAA). A systematic study of the eff…
In organic bulk heterojunction solar cells, the open circuit voltage ($V_\mathrm{oc}$) suffers from an ultra-high loss at low temperatures. In this work we investigate the origin of the loss through calculating the $V_\mathrm{oc}-T$ plots with the device model method systematically and comparing it with experimentally observed ones. When the energetic disorder is incorporated into the model by considering the disorder-suppressed and temperature-dependent charge carrier mobilities, it is found that for nonselective contacts the $V_\mathrm{oc}$ reduces drastically under the low temperature regim…
The energetic landscape at the interface between electron donating and accepting molecular materials favors efficient conversion of intermolecular charge-transfer states (CTS) into free charge carriers in high-performance organic solar cells. Here, we elucidate how interfacial energetics, charge generation and radiative recombination are affected by structural ordering. We experimentally determine the CTS binding energy of a series of model, small molecule donor-acceptor blends, where the used acceptors (B2PYMPM, B3PYMPM and B4PYMPM) differ only in the nitrogen position of their lateral pyridi…
A promising route for the development of opto-elelctronic technology is to use terahertz radiation to modulate the optical properties of semiconductors. Here we demonstrate the dynamical control of photoluminescence (PL) emission in few-layer InSe using picosecond terahertz pulses. We observe a strong PL quenching (up to 50%) after the arrival of the terahertz pulse followed by a reversible recovery of the emission on the time scale of 50ps at T =10K. Microscopic calculations reveal that the origin of the photoluminescence quenching is the terahertz absorption by photo-excited carriers: this l…
The active layer crystallinity has long been associated with favourable organic solar cells (OSCs) properties such as high mobility and Fill Factor. In particular, this applies to acceptor materials such as fullerene-derivatives and the most recent Non-Fullerene Acceptors (NFAs), which are now surpassing 19% of Power Conversion Efficiency. Despite these advantages are being commonly attributed to their 3-dimensional crystal packing motif in the single crystal, the bridge that links the acceptor crystal packing from single crystals to solar cells has not clearly been shown yet. In this work, we…
