Bimolecular Recombination Constant Research Articles

Revisiting open-circuit photovoltage decay in organic solar cells for the determination of bimolecular recombination constants

Open-circuit photovoltage decay from the steady state for the determination of bimolecular recombination constants has been studied in organic solar cells (OSCs) with three prototypical bulk heterojunctions including fullerene and non-fullerene acceptors. A simple theory for the determination of recombination constants from the initial decay rate of the open-circuit photovoltage was shown. The effective lifetimes were experimentally determined from the initial decay rates and were proportional to the −0.5 power of the excitation light intensity, indicating that the recombination process under the open-circuit condition in the OSCs is bimolecular. The experimental setup was also simple, and hence the experiment and the analysis are applicable to a variety of OSCs under operation. Information on the transport properties including bimolecular recombination constants is useful for the design and the optimization of the device architecture of OSCs.

19‐5: Late‐News Paper: Characterization of carrier transport properties in working polymer light‐emitting diodes

Charge carrier transport properties of polymer semiconductors (electron and hole drift mobilities, localized‐tail‐state distributions, and bimolecular recombination constants) in working polymer light‐emitting diodes (PLEDs) are determined simultaneously using impedance spectroscopy (IS). The IS technique is also applicable to degradation analysis of the PLED.

Determination of bimolecular recombination constants in organic double-injection devices using impedance spectroscopy

A method for determination of the bimolecular recombination constant in working double-injection diodes such as organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs) using impedance spectroscopy is proposed. The proposed method is based on the theory that was developed to interpret the negative capacitance that has previously been observed in both OLEDs and OPVs. The determination of the bimolecular recombination constants is demonstrated using working polymer light-emitting diodes based on poly(9,9-dioctylfluorene-alt-benzothiadiazole). These impedance spectroscopy measurements thus allow us to determine the required bimolecular recombination coefficients along with the electron and hole drift mobilities in working organic double-injection diodes.

Full characterization of electronic transport properties in working polymer light-emitting diodes via impedance spectroscopy

The electron and hole drift mobilities of organic semiconductor layers, localized tail state distributions, and bimolecular recombination constants in working polymer light-emitting diodes (PLEDs) are determined simultaneously using impedance spectroscopy (IS). The organic light-emitting layers of these PLEDs are composed of poly(9,9-dioctylfluorene-alt-benzothiadiazole). Electron and hole transit time effects are observed in the capacitance-frequency characteristics of the PLEDs, and their drift mobilities are determined over wide temperature and electric field ranges. The drift mobilities exhibit thermally activated behavior, and the localized tail state distributions from the conduction band and valence band mobility edges are then determined from analysis of the electric field dependences of the activation energies. The bimolecular recombination constants are determined from the inductive response of the impedance-frequency characteristics. The IS technique is also applicable to degradation analysis of the PLEDs; changes in the mobility balance, the localized tail state distributions, and the bimolecular recombination constant caused by aging are all shown.

Observation of bimolecular recombination in high mobility semiconductor Bi2O2Se using ultrafast spectroscopy

Bi2O2Se is emerging as a high mobility functional material for optoelectronics, but its fundamental optical properties remain less well studied. Here, ultrafast photocarrier dynamics in single crystal Bi2O2Se is investigated by pump fluence-dependent, broadband ultrafast spectroscopy. Our results reveal that bimolecular recombination plays an important role in the photocarrier relaxation process, and a room-temperature bimolecular recombination constant of (1.29 ± 0.42) × 10−9 cm−3 s−1 is obtained for Bi2O2Se. Such a level of the recombination constant combined with a high mobility (∼1006 cm2 V−1 s−1 at 200 K for Bi2O2Se) suggests that Bi2O2Se can be a promising material for photovoltaic applications.

Large-Area, Highly Uniform Evaporated Formamidinium Lead Triiodide Thin Films for Solar Cells

Perovskite thin-film solar cells are one of the most promising emerging renewable energy technologies because of their potential for low-cost, large-area fabrication combined with high energy conversion efficiencies. Recently, formamidinium lead triiodide (FAPbI3) and other formamidinium (CH(NH2)2) based perovskites have been explored as interesting alternatives to methylammonium lead triiodide (MAPbI3) because they exhibit better thermal stability. However, at present a major challenge is the scale-up of perovskite solar cells from small test-cells to full solar modules. We show that coevaporation is a scalable method for the deposition of homogeneous FAPbI3 thin films over large areas. The method allows precise control over film thickness and results in highly uniform, pinhole-free layers. Our films exhibited a high charge-carrier mobility of 26 cm2 V–1s–1, excellent optical properties, and a bimolecular recombination constant of 7 × 10–11 cm3 s–1. Solar cells fabricated using these vapor-deposited laye...

A Shockley‐Type Polymer: Fullerene Solar Cell

AbstractCharge extraction rate in solar cells made of blends of electron donating/accepting organic semiconductors is typically slow due to their low charge carrier mobility. This sets a limit on the active layer thickness and has hindered the industrialization of organic solar cells (OSCs). Herein, charge transport and recombination properties of an efficient polymer (NT812):fullerene blend are investigated. This system delivers power conversion efficiency of >9% even when the junction thickness is as large as 800 nm. Experimental results indicate that this material system exhibits exceptionally low bimolecular recombination constant, 800 times smaller than the diffusion‐controlled electron and hole encounter rate. Comparing theoretical results based on a recently introduced modified Shockley model for fill factor, and experiments, clarifies that charge collection is nearly ideal in these solar cells even when the thickness is several hundreds of nanometer. This is the first realization of high‐efficiency Shockley‐type organic solar cells with junction thicknesses suitable for scaling up.

Photon Reabsorption Masks Intrinsic Bimolecular Charge-Carrier Recombination in CH3NH3PbI3 Perovskite.

An understanding of charge-carrier recombination processes is essential for the development of hybrid metal halide perovskites for photovoltaic applications. We show that typical measurements of the radiative bimolecular recombination constant in CH3NH3PbI3 are strongly affected by photon reabsorption that masks a much larger intrinsic bimolecular recombination rate constant. By investigating a set of films whose thickness varies between 50 and 533 nm, we find that the bimolecular charge recombination rate appears to slow by an order of magnitude as the film thickness increases. However, by using a dynamical model that accounts for photon reabsorption and charge-carrier diffusion we determine that a single intrinsic bimolecular recombination coefficient of value 6.8 × 10-10 cm3s-1 is common to all samples irrespective of film thickness. Hence, we postulate that the wide range of literature values reported for such coefficients is partly to blame on differences in photon out-coupling between samples with crystal grains or mesoporous scaffolds of different sizes influencing light scattering, whereas thinner films or index-matched surrounding layers can reduce the possibility for photon reabsorption. We discuss the critical role of photon confinement on free charge-carrier retention in thin photovoltaic layers and highlight an approach to assess the success of such schemes from transient spectroscopic measurement.

Characterization of recombination in P3HT:fullerene blends: Clarifying the influence of interfacial states

The influence of an interfacial dipole at the donor/acceptor interface on the generation and bulk recombination in P3HT:fullerene blends is clarified. We use P3HT:PC60BM and P3HT:ICBA blends which have a large and small interfacial dipole, respectively. We show that polarons are generated using above and below gap excitations in both blends. The polaron density generated using below gap excitation is lower in P3HT:ICBA blends which is suggested to be due to a higher lying CT state compared to P3HT:PC60BM blends. Using intensity and frequency dependent measurements, we show that the bulk recombination is surprisingly similar in both blends. The bulk recombination is dominated by bimolecular recombination at all light-intensities for the delocalized polarons, while for the localized polaron trap-assisted recombination dominates at low intensities which becomes bimolecular at high intensities. For the localized polaron we find that the characteristic energy for the trap depth to be Ech=24.4±0.7meV and Ech=21.3±0.3 for the P3HT:ICBA and P3HT:PC60BM blends, respectively. Comparing the ratio between the bimolecular recombination constant and the mobility, we find the recombination to be practically the same in the two blends. However, while the localized polaron does show a difference of a factor of two, suggesting faster bimolecular recombination and stronger trap-assisted recombination in the P3HT:ICBA blend having a small interfacial dipole, we conclude that interfacial dipoles have little or no effect on the bulk recombination of long-lived photoexcitations in these blends.

Effects of Intramolecular Donor–Acceptor Interactions on Bimolecular Recombination in Small-Molecule Organic Photovoltaic Cells

To enhance the device performance of bulk heterojunction organic photovoltaic (OPV) cells, recombination processes must be suppressed. Here, we prepared OPV cells using donor materials composed of different electron-rich donor and electron-deficient acceptor units and the acceptor material 6,6-phenyl-C61-butyric acid methyl ester. The effects of the polarization of the donor materials which was induced by the intramolecular donor–acceptor (D–A) interactions on the bimolecular recombination constant of each device were studied using a time-of-flight method. A large D–A interaction decreased the bimolecular recombination constant, increasing the fill factor of the device. An active layer with a large dielectric constant led to a small Coulomb capture radius between the radical cations and anions. Overall, a larger D–A interaction enhanced the device performance of small-molecule OPV cells.

Interplay between stimulated emission and singlet-singlet annihilation in oligothiophene dioxide thin films

We have studied the optical properties of different thienyl-S,S-dioxide oligothiophenes under strong excitation. No stimulated emission is observed in neat films due to singlet-singlet bimolecular annihilation. The bimolecular recombination constant is quantified from a rate equation model in the limit of constant annihilation rate. We demonstrate that tunable stimulated emission in the range 490–660nm, due to optical gain with an estimated cross section of the order of σg≈10−17cm2, can be obtained by blending the active molecule with small quantities of inert polycarbonate poly(bisphenol-A-carbonate) (PC). The presence of amplified spontaneous emission (ASE) for a PC:active molecule relative concentration as small as 1:500 suggests that the polycarbonate role is to reduce the intermolecular diffusion rate rather than to isolate the active molecules. Moreover, for higher PC content, a continuous decrease of the bimolecular quenching role is observed. These results demonstrate that the absence of stimulated emission in neat films is not necessarily due to intrinsic molecular properties, as strong ASE can be obtained by slightly modifying the molecule interaction during the deposition process. This approach could allow a considerable extension of the number of molecules showing stimulated emission for organic laser applications.