Nongeminate Recombination Research Articles

Understanding the High Performance of over 15% Efficiency in Single-Junction Bulk Heterojunction Organic Solar Cells.

The highly efficient single-junction bulk-heterojunction (BHJ) PM6:Y6 system can achieve high open-circuit voltages (VOC ) while maintaining exceptional fill-factor (FF) and short-circuit current (JSC ) values. With a low energetic offset, the blend system is found to exhibit radiative and non-radiative recombination losses that are among the lower reported values in the literature. Recombination and extraction dynamic studies reveal that the device shows moderate non-geminate recombination coupled with exceptional extraction throughout the relevant operating conditions. Several surface and bulk characterization techniques are employed to understand the phase separation, long-range ordering, as well as donor:acceptor (D:A) inter- and intramolecular interactions at an atomic-level resolution. This is achieved using photo-conductive atomic force microscopy, grazing-incidence wide-angle X-ray scattering, and solid-state 19 F magic-angle-spinning NMR spectroscopy. The synergy of multifaceted characterization and device physics is used to uncover key insights, for the first time, on the structure-property relationships of this high-performing BHJ blend. Detailed information about atomically resolved D:A interactions and packing reveals that the high performance of over 15% efficiency in this blend can be correlated to a beneficial morphology that allows high JSC and FF to be retained despite the low energetic offset.

Kinetic Monte Carlo Study of the Role of the Energetic Disorder on the Open-Circuit Voltage in Polymer/Fullerene Solar Cells.

One major factor limiting the efficiency in organic solar cells (OSCs) is the low open-circuit voltage (Voc). Existing theoretical studies link the Voc with the charge transfer (CT) state and nonradiative recombination. However, also morphology and energetic disorder can have a strong impact on the Voc within realistic bulk-heterojunction OSCs. In this work, we present a kinetic Monte Carlo study on the role of the energetic disorder on the maximum Voc. We compute the quasi-Fermi level splitting for different energetic disorder and analyze the impact of the energetic disorder at the donor-acceptor interface as well as correlations in the site energies on the Voc. Our results show that the interface strongly controls the maximum Voc. For a higher interface disorder, charge densities and nongeminate recombination increase and the Voc is reduced. Furthermore, the correlated morphologies show an increase in the maximum Voc and a reduced impact of the energetic disorder.

An impact of higher fullerene on charge recombination in organic solar cell studied by magnetoconductance

Abstract In order to clarify roles of higher fullerenes on the charge recombination in organic solar cells based on the bulk heterojunction (BJ) of polymers as electron donors (Ds) and fullerenes as electron acceptors (As), we measured magnetoconductance (MC) effects for the BJ-cells consisted of a blend film of regioregular poly[3-hexylthiophene] (P3HT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), hereafter referred to as C71-cell, and compared them with the results for P3HT:PC61BM-cell (C61-cell) reported in our previous paper. As with C61-cell, three components (MCS,M,B) with half-field-at-half-maximums of 4 ± 1, 28 ± 12 and >400 mT were detected in the magnetic field dependence of the MC effects for the C71-cell under dark and light conditions. From the incident light power and time dependences of the positive MCS and MCM components in light, it was assinged that the light MC effects observed for the C71-cell is attributed to geminate recombination of the electron-hole (e-h) pair, whereas the C61-cell shows positive light MC effects due to nongeminate recombination. This is explained by decrease of the possiblity to form nongeminate e-h pair because C71-cells have wide D/A junction caused by uniform mixing of P3HT and PC71BM more than C61-cells. The positive low-field MC effects due to the geminate e-h pair in the C71-cell indicates that the recombination rate constant for the 3e-h pair (kT) to the triplet exciton (3ex) is greater than that for the 1e-h pair (kS) to the singlet ground state (1gr). The magnitude relationship of kS

Quantifying the Nongeminate Recombination Dynamics in Nonfullerene Bulk Heterojunction Organic Solar Cells

AbstractIn this study, a comprehensive analytical model to quantify the total nongeminate recombination losses, originating from bimolecular as well as bulk and surface trap‐assisted recombination mechanisms in nonfullerene‐based bulk heterojunction organic solar cells is developed. This proposed model is successfully employed to obtain the different contributions to the recombination current of the investigated solar cells under different illumination intensities. Additionally, the model quantitatively describes the experimentally measured open‐circuit voltage versus light intensity dependence. Most importantly, it is possible to calculate the experimental results with the same fitting parameter values from the presented model. The validity of this model is also proven by a combination of other independent, steady‐state, and transient experimental techniques. This new powerful analytical tool will enable researchers in the photovoltaic community to take into account the synergetic contribution from all relevant types of nongeminate recombination losses in different optoelectronic systems and target their analysis of recombination dynamics at any operating voltage.

Improved Charge Transport and Reduced Non-Geminate Recombination in Organic Solar Cells by Adding Size-Selected Graphene Oxide Nanosheets.

Size-selected graphene oxide (GO) nanosheets were used to modify the bulk heterojunction (BHJ) morphology and electrical properties of organic photovoltaic (OPV) devices. The GO nanosheets were prepared with sizes ranging from several hundreds of nanometers to micrometers by using a physical sonication process and were then incorporated into PTB7:PC71BM photoactive layers. Different GO sizes provide varied portions of the basal plane where aromatic sp2-hybridized regions are dominant and edges where oxygenated functional groups are located; thus, GO size distributions affect the GO dispersion stability and morphological aggregation of the BHJ layer. Electron delocalization by sp2-hybridization and the electron-withdrawing characteristics of functional groups p-dope the photoactive layer, giving rise to increasing carrier mobilities. Hole and electron mobilities are maximized at GO sizes of several hundreds of nanometers. Consequently, non-geminate recombination is significantly reduced by these facilitated hole and electron transports. The addition of GO nanosheets decreases the recombination order of non-geminate recombination and increases the generated carrier density. This reduction in the non-geminate recombination contributes to an increased power conversion efficiency of PTB7:PC71BM OPV devices as high as 9.21%, particularly, by increasing the fill factor to 70.5% in normal devices and 69.4% in inverted devices.

Realizing 20% External Quantum Efficiency in Electroluminescence with Efficient Thermally Activated Delayed Fluorescence from an Exciplex.

The investigation of nondoped exciplex blends of 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T), working as the one-electron acceptor molecule, with different electron donors is reported. The emissions of these exciplexes span from the blue to orange-red regions, showing clear contribution from thermally activated delayed fluorescence (TADF) and delayed fluorescence originated from nongeminate recombination of charge carriers created by the dissociation of optically generated exciplexes. We focus our studies on the properties of TADF in these systems, covering in particular the physical meaning of the different transient components observed in their luminescence decays. Our results unravel the intricate role of reverse intersystem crossing due to spin-orbit coupling and possibly also due to hyperfine interactions and internal conversion, which affect the efficiency of the TADF mechanism. Remarkable performances are obtained in prototype organic light-emitting diodes fabricated with some of these blends. Green exciplex blends, in particular, exhibited the current efficiency of 60 cd A-1, power efficiency of 71 lm W-1, and external quantum efficiency of 20%. We believe that our results will contribute significantly to highlight the potential advantages of intermolecular exciplexes in the area of organic light-emitting diodes.

Key Parameters Requirements for Non-Fullerene-Based Organic Solar Cells with Power Conversion Efficiency >20.

The reported power conversion efficiencies (PCEs) of nonfullerene acceptor (NFA) based organic photovoltaics (OPVs) now exceed 14% and 17% for single‐junction and two‐terminal tandem cells, respectively. However, increasing the PCE further requires an improved understanding of the factors limiting the device efficiency. Here, the efficiency limits of single‐junction and two‐terminal tandem NFA‐based OPV cells are examined with the aid of a numerical device simulator that takes into account the optical properties of the active material(s), charge recombination effects, and the hole and electron mobilities in the active layer of the device. The simulations reveal that single‐junction NFA OPVs can potentially reach PCE values in excess of 18% with mobility values readily achievable in existing material systems. Furthermore, it is found that balanced electron and hole mobilities of >10−3 cm2 V−1 s−1 in combination with low nongeminate recombination rate constants of 10−12 cm3 s−1 could lead to PCE values in excess of 20% and 25% for single‐junction and two‐terminal tandem OPV cells, respectively. This analysis provides the first tangible description of the practical performance targets and useful design rules for single‐junction and tandem OPVs based on NFA materials, emphasizing the need for developing new material systems that combine these desired characteristics.

Equilibrated Charge Carrier Populations Govern Steady-State Nongeminate Recombination in Disordered Organic Solar Cells.

We employed bias-assisted charge extraction techniques to investigate the transient and steady-state recombination of photogenerated charge carriers in complete devices of a disordered polymer-fullerene blend. Charge recombination is shown to be dispersive, with a significant slowdown of the recombination rate over time, consistent with the results from kinetic Monte Carlo simulations. Surprisingly, our experiments reveal little to no contributions from early time recombination of nonequilibrated charge carriers to the steady-state recombination properties. We conclude that energetic relaxation of photogenerated carriers outpaces any significant nongeminate recombination under application-relevant illumination conditions. With equilibrated charges dominating the steady-state recombination, quasi-equilibrium conceptsappear suited for describing the open-circuit voltage of organic solar cells despite pronounced energetic disorder.

Photovoltaics literature survey (no. 149)

Photovoltaics literature survey (no. 149)

Over 12% Efficiency Nonfullerene All-Small-Molecule Organic Solar Cells with Sequentially Evolved Multilength Scale Morphologies.

In this paper, two near-infrared absorbing molecules are successfully incorporated into nonfullerene-based small-molecule organic solar cells (NFSM-OSCs) to achieve a very high power conversion efficiency (PCE) of 12.08%. This is achieved by tuning the sequentially evolved crystalline morphology through combined solvent additive and solvent vapor annealing, which mainly work on ZnP-TBO and 6TIC, respectively. It not only helps improve the crystallinity of the ZnP-TBO and 6TIC blend, but also forms multilength scale morphology to enhance charge mobility and charge extraction. Moreover, it simultaneously reduces the nongeminate recombination by effective charge delocalization. The resultant device performance shows remarkably enhanced fill factor and Jsc . These result in a very respectable PCE, which is the highest among all NFSM-OSCs and all small-molecule binary solar cells reported so far.

In-depth investigation of the charge extraction efficiency for thermally annealed inverted bulk-heterojunction solar cells

The performance of bulk-heterojunction solar cells is significantly affected by several factors among which are the nano-morphology of the photoactive layer and the properties of interfacial layers promoting charge extraction and collection at the electrodes. In this work, we investigate the correlation between the MoOx layer thickness and the thermal annealing procedure on the device performance and on the charge extraction efficiency of inverted ITO/PEI/P3HT:PCBM/MoOx/Ag bulk heterojunction solar cells. The surface morphology of pristine and annealed P3HT:PCBM photoactive layers is examined by atomic force microscopy before and after the exposure to dichloromethane revealing that the distribution of the amorphous regions within the P3HT:PCBM layers can be related to the applied thermal annealing procedure. The chemical and molecular composition profiles in the photoactive layer and at the interfaces are investigated through depth profile analyses combining X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry showing a decreasing oxidation gradient of the MoOx and low diffusion of the MoOx species in the P3HT:PCBM layer. Additionally, we show that the voltage dependent photocurrent is significantly affected by non-geminate recombination for devices with a too thin MoOx layer thickness, while applying a non-optimal thermal annealing procedure leads to increased geminate recombination of charge carriers. The highest charge extraction efficiency and device performance is reached for devices with a MoOx layer above 5 nm while thermal annealing procedure has to be applied before the deposition of the MoOx/Ag layers.

Effects of DIO on the charge recombination behaviors of PTB7:PC71BM photovoltaics

Abstract The influence of the solvent additive 1, 8-diiodooctane (DIO) on the charge carrier recombination behaviours of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo [1,2-b:4,5-b0] dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b] thiophenediy1]] (PTB7): phenyl-[6, 6]-C71-butyric acid methyl ester(PC71BM) organic photovoltaics (OPVs) was investigated. The measurement of transient photocurrent (TPC) implies that the devices processed with 3% DIO have less charge carrier recombination loss than the devices without DIO. The photo-induced charge extraction by linearly increasing voltage (photo-CELIV) technique shows that the device with DIO has less charge carrier trap population and thus the bimolecular recombination can be reduced. The modified Onsager–Braun model with bimolecular recombination rate γ b i , calculated from the photo-CELIV, indicated that by adding DIO, the binding energy of the charge-transfer exciton at the interface of PTB7 and PC71BM reduced, thus the geminate recombination loss decreased. Atomic Force Microscope (AFM) images show that DIO can optimize the morphology of the active layer, which is beneficial to the exciton dissociation and the reduction of the charge carriers trapping. The reduction of the charge carrier recombination including geminate recombination and non-geminate recombination lead to the large increase of current density (JSC), fill factor (FF), and power conversion efficiency (PCE) of PTB7:PC71BM OPVs.

Impact of Structural Polymorphs on Charge Collection and Nongeminate Recombination in Organic Photovoltaic Devices

The formation of different types of structural polymorphs of poly(3-hexyl-thiophene) (P3HT) affects the performance of organic photovoltaic (OPV) devices that use thermally annealed P3HT:PCBM[60] blend films as a photoactive layer. Here it is demonstrated that when densely packed and nondensely packed P3HT polymorphs coexist in the P3HT:PCBM[60] layer, nongeminate charge recombination is fast; however, in a device nongeminate recombination is effectively overruled by efficient and fast charge carrier extraction. In stark contrast, when only a less densely packed P3HT polymorph is present in the blend, nongeminate charge recombination losses are less pronounced, and the charge carrier extraction efficiency is lower. The antagonistic nongeminate charge recombination and charge carrier extraction processes in these systems are monitored by time-delayed collection field (TDCF) and ultrafast transient absorption (TA) experiments. Furthermore, resonance Raman spectroscopy reveals that in the absence of the densely packed P3HT polymorph the energetic disorder present in the P3HT:PCBM[60] blend is higher. High-resolution atomic force microscopy imaging further identifies pronounced differences in the layer morphology when the polymorph distribution varies between unimodal and bimodal. These results indicate that less densely packed P3HT polymorphs increase disorder and impede charge collection, leading to a reduction of the device fill factor.

Preparation and Two-Photon Photoluminescence Properties of Organic Inorganic Hybrid Perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4

Organic inorganic hybrid perovskites have potential applications in solar cells, electroluminescent devices and radiation detection because of their unique optoelectronic properties. In this paper, the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4 were synthesized by solvent evaporation. The crystal structure, morphology, absorption spectrum, laser power dependence of the photoluminescence (PL) intensity and lifetime were studied. The results showed that the perovskites (C6H5CH2NH3)2PbBr4 and (C6H5CH2NH3)2PbI4 display a layered stacking structure of organic and inorganic components. The absorption peaks are located at 392 nm (3.16 eV) and 516 nm (2.40 eV), respectively. It was observed that the PL intensity and photoluminescence quantum yield (PLQY) increases with increasing laser power, and that the PL lifetime decreases with increasing laser power, which is mainly due to the non-geminate recombination.

An analytical model for organic solar cells incorporating non-geminate monomolecular and bimolecular recombinations

There is still a debate concerning the non-geminate recombination processes in organic bulk heterojunction solar cells. There are reports that suggest either only the non-geminate bimolecular recombination or the non-geminate monomolecular recombination is important, and also reports that suggest both non-geminate processes are important. In this paper, a physics-based analytical model for describing the current–voltage characteristics of organic bulk heterojunction solar cells is presented. The model incorporates both non-geminate recombination processes, which is an improvement compared with previous analytical models. This makes the model versatile by allowing us to choose whether to include both recombination processes or exclude one of the processes, depending on the studied device. In developing the model, a new method for calculating the non-geminate recombination and the current density in organic bulk heterojunction solar cells is proposed and employed. The model is validated with experimental current–voltage characteristics taken from the literature. Using the model, we show that considering the non-geminate recombination is important in order to properly predict the current–voltage characteristics of organic bulk heterojunction solar cells. The model can be useful for understanding the working operation of organic bulk heterojunction solar cells, and thus for predicting and improving the device performance. Finally, the method employed to develop the presented model can also be extended and employed to develop models for other devices.

Overcoming the trade-off between exciton dissociation and charge recombination in organic photovoltaic cells

The electron donor-acceptor (D-A) interface is an essential component for realizing efficient exciton dissociation and charge generation in organic photovoltaic cells (OPVs). It can also however enable rapid charge recombination due to the close spatial proximity of electrons and holes. To frustrate recombination losses, attempts have been made to separate charge carriers by introducing an insulating blocking interlayer at the D-A interface. It is challenging to realize increased efficiency using this approach as the relative similarity of interlayer optical and transport energy gaps may also frustrate exciton harvesting and charge generation. To overcome this trade-off, the interlayer must block charge carriers while continuing to permit exciton migration to the dissociating interface. In this work, we demonstrate this configuration in archetypical copper phthalocyanine (CuPc)-C60 planar OPVs containing a rubrene interlayer to frustrate charge recombination. Critically, the similarity in triplet exciton energy levels between rubrene and CuPc allows the interlayer to be permeable to excitons. Devices containing an interlayer show a reduction in the charge transfer state binding energy and non-geminate recombination rate with increasing interlayer thickness. For thin interlayers, geminate recombination is also suppressed. Thus, devices containing an exciton permeable interlayer show a simultaneous increase in open-circuit voltage, short-circuit current, and power efficiency.

Ultrafast Charge Dynamics in Mixed Cation - Mixed Halide Perovskite Thin Films.

Perovskite based photovoltaic devices are popularised by the rapid increase in their efficiencies. Understanding the fundamental physics and chemistry processes occurring upon excitation is key. We monitored the temporal evolution of the population and depopulation dynamics of various electronic states in FA0.85 MA0.15 PbI2.55 Br0.45 by means of ultrafast transient absorption spectroscopy in the visible and near infrared spectral regions in order to build a fully consistent charge dynamics model of the initial photoprocesses. Upon photoexcitation with 3.2 eV photon energy, hot electrons and holes are generated in the lowest conduction and highest valence bands, away from the bandgap, and cool to the band edges with a time constant of 500 fs. Geminate recombination of excitons occurs with a time constant of 66 ps, which increases to approximately 130 ps at the optical bandgap. From a systematic study of the excited state population dynamics and its dependence on charge carrier density, we determined the nonlinear recombination rate constants characteristic to FA0.85 MA0.15 PbI2.55 Br0.45 . The coefficient describing the non-geminate recombination of free electrons and holes is independent of the k vector as well as the charge carrier density and equal to 1×10-10 s-1 cm3 , while the Auger recombination coefficient decreases with increasing charge carrier density in the range of (2-50)×10-32 s-1 cm6 .

Balance Between Light Absorption and Recombination Losses in Solution‐Processed Small Molecule Solar Cells with Normal or Inverted Structures

AbstractComparison of the photovoltaic properties of F3:fullerene bulk heterojunction solar cells (where F3 is a donor molecule of intermediate dimensions) with normal and inverted structures as a function of photoactive layer thickness is carried out by using optical modeling and light‐dependent photoelectrical characterization. The larger short circuit current of inverted devices can be explained by higher light absorption in the active layer, relative to normal devices. However, worse charge extraction conditions and trap‐assisted recombination, in the bulk heterojunction blend in inverted devices, give rise to larger nongeminate recombination losses and an opposite thickness dependence of the short circuit current and fill factor than what is observed in counterparts with the conventional device architecture.

Suppression of trap assisted non-geminate recombination by incorporation of multilayer graphene in P3HT:PCBM for stable and efficient photovoltaic device

In the present work, multilayer graphene (MLG) has been demonstrated to be a promising material for improvement in power conversion efficiency (PCE) as well as stability of the polymer based photovoltaic (PV) devices. MLG, when incorporated into the active layer of the P3HT:PCBM based PV device provides additional 2D pathways for long distance electron transport, avoiding interaction with holes and hence leading to suppression of non-geminate recombination in the device. PCE of the device is shown to improve by ~ 54% due to improvement in absorbance as well as by reduction in non-geminate recombination leading to ~ 8.9% increase in short circuit current density. Also, open circuit voltage improves by ~ 13.75% owing to increase in the fermi-level splitting due to large number of charge accumulated at the electrodes. MLG also reduces the rate of degradation of the device by the factor of ~ 1.5 by suppressing the effect of generation of deep trap levels in the active layer which in turn results in improving the stability of the device.

Magnetoconductance Study on Nongeminate Recombination in Solar Cell Using Poly(3-hexylthiophene) and [6,6]-Phenyl-C61-butyric Acid Methyl Ester.

The magnetoconductance (MC) effect was investigated for two types of organic solar cells with single junction (SJ) and bulk junction (BJ) of poly(3-hexylthiophene) (P3HT) as donor (D) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as acceptor (A). Three components with different half-field-at-half-maximums (B1/2) of 4 ± 1, 20 ± 15 and >400 mT, hereafter referred to MCS,M,B in a sequence, were observed in the magnetic field dependence of the MC effects measured under dark and light conditions. The magnitude of the MCS,M,B components is sensitive to not only the junction structure of the cell but also the presence or absence of incident light. The bias voltage (V) dependence of the MC effect in the dark for the SJ-cell is maximized around the turn-on voltage (VON) of the dark current, where a flat band condition of the active layer is achieved. The B1/2 for the MCM component of the SJ-cell increases with V beyond VON. In light, the BJ-cell exhibits the MC effect, whereas no effect is detected for the SJ-cell. The MCS,M components for the BJ-cell in light increase with the incident light power. The transient MCS,M components for the BJ-cell measured using a nanosecond pulse laser increases with the delay time after the flash. By integrating these phenomena and the phase of the MC effect, it is concluded that all of the MC components arise from the magnetic field effect on the spin conversion of nongeminate electron (e)–hole (h) pairs with spin-dependent charge recombinations at the D/A-interface. The B1/2 values for MCS,M,B are, respectively, understood by the spin conversion due to the hyperfine interaction, the spin relaxation, and the g-factor difference for e (PCBM–) and h (P3HT+). Kinetic simulations of the MCS,M components for the BJ-cell observed at the short-circuit condition in light yield an efficiency of ca. 40% for the nongeminate recombination, which is accompanied by the generation of triplet excitons as well as relaxation to a ground singlet state. The loss mechanism of moderate triplet recombination suggests an important possibility to improve the power conversion efficiency by harvesting of the triplet excitons.