Nongeminate Recombination Research Articles

Molecular Control of the Donor/Acceptor Interface Suppresses Charge Recombination Enabling High-Efficiency Single-Component Organic Solar Cells.

Single-component organic solar cells based on double cable polymers have achieved remarkable performance, with DCPY2 reaching a high efficiency of over 13%. In this study, DCPY2 is further optimized with an efficiency of 13.85%, maintaining a high fill factor (FF) without compromising the short circuit current. Despite its intermixed morphology, DCPY2 shows a reduced recombination rate compared to their binary counterpart (PBDB-T:Y-O6). This slower recombination in DCPY2 is attributed to the reduced wavefunction overlap of delocalized charges, achieved by spatially separating the donor and acceptor units with an alkyl linker, thereby restricting the recombination pathways. Adding 1,8-diiodooctane (DIO) into DCPY2 further reduced the recombination rate by facilitating acceptor aggregation, allowing free charges to become more delocalized. The DIO-assisted aggregation in DCPY2 (5% DIO) is evidenced by an increased pseudo-pure domain size of Y-O6. Fine molecular control at the donor/acceptor interface in the double-cable polymer achieves reduced non-geminate recombination under efficient charge generation, increased mobility, and charge carrier lifetime, thereby achieving superior performance. Nevertheless, the FF is still limited by relatively low mobility compared to the blend, suggesting the potential for further mobility improvement through enhanced higher-dimensional packing of the double-cable material.

An Analytical Model for Describing Transient Photocurrents in Bias‐Assisted Charge Extraction for Low‐Mobility Organic Solar Cells

Bias‐assisted charge extraction (BACE) is a powerful technique for measuring the carrier density in organic solar cells under operational conditions and to deduce information on the charge recombination properties. Hereby, the carrier density in the active layer device is determined by integrating the transient extraction current, while nongeminate recombination during the charge extraction processes is generally neglected. This assumption becomes questionable for low mobilities, for example, at low temperatures and in case of high energetic disorder. Herein, the extraction process in BACE measurement is investigated by incorporating both drift and bimolecular recombination of charges during charge extraction into a unified framework. An explicit analytical model is proposed to describe the time dependence of the transient photocurrent in BACE measurement and excellent agreement is found with drift‐diffusion simulations. It is shown that global fitting of transient photocurrents measured at various collection biases with this analytical model allows to determine the carrier density in the device with high accuracy. Furthermore, the analytical model is demonstrated to provide accurate mobilities for both the fast and slow carriers within the device, rendering it a valuable supplement to other mobility measurement techniques such as resistance‐dependent photovoltage and space–charge‐limited current.

Theoretical exploration of the molecular stacking and charge transfer mechanism of PBQx:Y6 OSCs

The stacking morphology of the active layer molecules of organic solar cells is closely related to the PCE performance. However, experimental techniques are difficult to observe the molecular conformation and dynamic evolution at the atomic scale, which limits the understanding of the active layer charge transfer mechanism and the design of high-performance material molecules. DFT and AIMD methods can simulate the dynamic evolution of molecules at the atomic scale, and we obtained the local molecular stacking morphology of the active layers of PBQx:Y6 (x = 5,6,7) systems by AIMD simulations. The results reveal that PBQ6:Y6 exhibits the highest number of continuous π-π stacking molecules and the best ordering. The relatively high ΔES1CTx (> 0.1 eV) of PBQ7:Y6 results in increased open-circuit voltage loss. Interestingly, we found that the near-overlapping stacking of the D/A interface not only enables S1/CTX hybridization to reduce ΔES1CTx, but also enables energy level inversion of T1 and 3CT1 state to inhibit the non-geminate exciton recombination pathway. This work provides a systematic theoretical exploration of the molecular stacking and charge transfer mechanisms in PBQx:Y6 OSCs, which offer valuable theoretical insights for the development of high-performance photovoltaic devices.

Suppressing nongeminate recombination with two well-compatible polymer donors enables 16.6% efficiency all-polymer solar cells

The power conversion efficiency of all-polymer solar cells is rising quickly due to the development of polymerized small molecule acceptors. However, optimizing the active layer morphology remains challenging owing to the long and entangled polymer chains; therefore, the power conversion efficiency still lags behind that of organic solar cells composed of small molecule acceptors. Herein, we adopted a ternary strategy to improve the performance of all-polymer solar cells consisting of PM6:PY-IT by introducing D18-Cl as the third component to fine-tune the nanoscale morphology of the active layer. Because D18-Cl has good compatibility with PM6 and more robust crystalline features, adding D18-Cl regulates the donor and acceptor aggregations in the ternary blend films, leading to stronger crystallinity than the PM6:PY-IT films. Consequently, the ternary devices attained a champion power conversion efficiency of 16.6% (16.5% averaged), outperforming their binary counterparts. The improved performance was associated with enhanced fill factor and short circuit current density, resulting from significantly suppressed nongeminate charge recombination. Our results provide an in-depth understanding of the relationship between the active layer morphology and the nongeminate recombination in the ternary system composed of two well-compatible donors.

Anticorrelated photoluminescence and free charge generation proves field-assisted exciton dissociation in low-offset PM6:Y5 organic solar cells

Understanding the origin of inefficient photocurrent generation in organic solar cells with low energy offset remains key to realizing high-performance donor-acceptor systems. Here, we probe the origin of field-dependent free-charge generation and photoluminescence in non-fullereneacceptor (NFA)-based organic solar cells using the polymer PM6 and the NFA Y5—a non-halogenated sibling to Y6, with a smaller energetic offset to PM6. By performing time-delayed collection field (TDCF) measurements on a variety of samples with different electron transport layers and active layer thickness, we show that the fill factor and photocurrent are limited by field-dependent free charge generation in the bulk of the blend. We also introduce a new method of TDCF called m-TDCF to prove the absence of artifacts from non-geminate recombination of photogenerated and dark charge carriers near the electrodes. We then correlate free charge generation with steady-state photoluminescence intensity and find perfect anticorrelation between these two properties. Through this, we conclude that photocurrent generation in this low-offset system is entirely controlled by the field-dependent dissociation of local excitons into charge-transfer states.

High-Performance Small Molecule Organic Solar Cells Enabled by a Symmetric-Asymmetric Alloy Acceptor with a Broad Composition Tolerance.

Using a combinatory blending strategy is demonstrated as a promising path for designing efficient organic solar cells (OSCs) by boosting the short-circuit current density and fill factor. Herein, a high-performance ternary all-small molecule OSC (all-SMOSCs) using a narrow-bandgap alloy acceptor containing symmetric and asymmetric molecules (BTP-eC9 and SSe-NIC) and a wide-bandgap small molecule donor MPhS-C2 is reported. Introducing the synthesized SSe-NIC into the MPhS-C2:BTP-eC9 host system can broaden the absorption spectrum, modulate energy offsets, and optimize the molecular packing of the host materials. After systematically optimizing the weight ratio of MPhS-C2:BTP-eC9:SSe-NIC, a champion efficiency of 18.02% is achieved. Impressively, the ternary system not only delivered a broad composition tolerance with device efficiencies over 17% throughout the whole blend ratios, but also exhibited less non-geminate recombination and energy loss, and better-light-soaking stability than the corresponding binary systems. This work promotes the development of high-performance ternary all-SMOSCs and heralds their brighter application prospects.

Molecular orientation-dependent energetic shifts in solution-processed non-fullerene acceptors and their impact on organic photovoltaic performance

The non-fullerene acceptors (NFAs) employed in state-of-art organic photovoltaics (OPVs) often exhibit strong quadrupole moments which can strongly impact on material energetics. Herein, we show that changing the orientation of Y6, a prototypical NFA, from face-on to more edge-on by using different processing solvents causes a significant energetic shift of up to 210 meV. The impact of this energetic shift on OPV performance is investigated in both bilayer and bulk-heterojunction (BHJ) devices with PM6 polymer donor. The device electronic bandgap and the rate of non-geminate recombination are found to depend on the Y6 orientation in both bilayer and BHJ devices, attributed to the quadrupole moment-induced band bending. Analogous energetic shifts are also observed in other common polymer/NFA blends, which correlates well with NFA quadrupole moments. This work demonstrates the key impact of NFA quadruple moments and molecular orientation on material energetics and thereby on the efficiency of high-performance OPVs.

Enhanced charge separation by interchain hole delocalization in nonfullerene acceptor‐based bulk heterojunction materials

AbstractBulk heterojunction (BHJ) composites show improved power conversion efficiencies when optimized in terms of morphology using various film processing methods. A reduced carrier recombination loss in an optimized BHJ was characterized previously. However, the driving force that leads to this reduction was not clearly understood. In this study, we focus on the decreased carrier recombination loss and its driving force in optimized nonfullerene acceptor‐based PTB7‐Th:IEICO‐4F BHJ composites. We demonstrate that the optimized BHJ shows deactivation in the sub‐nanosecond nongeminate carrier recombination process. The driving force for this deactivation was determined to be the improved interchain hole delocalization between the polymers. An enhanced interchain hole delocalization was observed using steady‐state photoinduced absorption (PIA) spectroscopy. In particular, increased splitting between the polaron PIA bands was noted. Moreover, improved interchain hole delocalization was observed for other state‐of‐the‐art BHJ materials, including D18:Y6 with optimized morphologies.

Self‐Organized Anode Interlayer Enables PEDOT:PSS‐Free Structure for Thick‐Film Organic Solar Cells over 16% Efficiency

Thickness‐insensitivity of organic solar cells (OSCs) is a crucial issue for transformation from the laboratory‐scale devices to large‐area solar panels. Generally, the low carrier mobility of organic semiconductors hindering charge transport process leads to severe recombination losses and further poisons the fill factor (FF) in thick‐film OSCs. Herein, a PEDOT:PSS‐free pseudoplanar heterojunction OSC from sequential deposition method with self‐organized [2‐(9H‐carbazol‐9‐yl)ethyl]phosphonic acid interlayer is fabricated. The notable power conversion efficiency (PCE) enhancement from PEDOT:PSS‐based device of 17.0% to PEDOT:PSS‐free device of 17.8% is obtained in thin‐film devices (≈100 nm), which is correlated to the improved incident photons absorption by the absence of PEDOT:PSS. Besides, the PEDOT:PSS‐free device exhibits excellent thickness tolerance, with FF only degrades from 78.4% to 72.9% when the active layer thickness increases from ≈100 to ≈300 nm. The insights of remained FF are further traced to notably mitigated nongeminate recombination losses of PEDOT:PSS‐free structure. The strengthened charge extraction is also verified by reinforced built‐in potential and desirable and balanced mobilities. Consequently, the PEDOT:PSS‐free device with ≈300 nm active layer yields an inspiring PCE of 16.4%, much higher than that of PEDOT:PSS‐based device of 14.5%.

Unraveling Device Physics of Dilute-Donor Narrow-Bandgap Organic Solar Cells with Highly Transparent Active Layers.

The charge generation-recombination dynamics in three narrow-bandgap near-IR absorbing nonfullerene (NFA) based organic photovoltaic (OPV) systems with varied donor concentrations of 40%, 30%, and 20% are investigated. The dilution of the polymer donor with visible-range absorption leads to highly transparent active layers with blend average visible transmittance (AVT) values of 64%, 70%, and 77%, respectively. Opaque devices in the optimized highly reproducible device configuration comprising these transparent active layers lead to photoconversion efficiencies (PCEs) of 7.0%, 6.5%, and 4.1%. The investigation of these structures yields quantitative insights into changes in the charge generation, non-geminate charge recombination, and extraction dynamics upon dilution of the donor. Lastly, this study gives an outlook for employing the highly transparent active layers in semitransparent organic photovoltaics (ST-OPVs).

Temperature-Dependent Recombination of Triplet Biexcitons in Singlet Fission of Hexacene

Singlet fission is a spin-conserving process for the multiplication conversion of one singlet exciton into two individual triplet excitons by absorbing one photon. Such a multiplication is believed to circumvent the Shockley–Queisser thermodynamic limit for improving efficiency of solar energy conversion. A mechanistic understanding of generation and yields of triplet excitons from singlet fission materials is essential for efficient exploitation of solar energy. Here we employ temperature-dependent transient absorption spectroscopy to examine the dynamical nature of singlet fission and triplet excitons in hexacene. The generation and dissociation rates of the intermediate correlated biexciton, 1(TT), are independent of temperature from 77 K to the room temperature. On the other hand, the triplet excitons in spatially separated biexcitons, 1(T···T), relax via geminate and nongeminate recombination. The former was found to be temperature-dependent, whereas the latter is temperature-independent. Quantitative analyses of the temperate-dependent rates for the two recombination processes yield the energy difference between the 1(T···T) and 1(TT), which were further confirmed by our density functional theory (DFT) calculations.

Geminate and Nongeminate Pathways for Triplet Exciton Formation in Organic Solar Cells

AbstractOrganic solar cells (OSCs) have recently shown a rapid improvement in their performance, bringing power conversion efficiencies to above 18%. However, the open‐circuit voltage of OSCs remains low relative to their optical gap and this currently limits efficiency. Recombination to spin‐triplet excitons is a key contributing factor, and is widely, but not universally, observed in donor–acceptor blends using both fullerene and nonfullerenes as electron acceptors. Here, an experimental framework that combines time‐resolved optical and magnetic resonance spectroscopies to detect triplet excitons and identify their formation mechanisms, is reported. The methodology is applied to two well‐studied polymer:fullerene systems, PM6:PC60BM and PTB7‐Th:PC60BM. In contrast to the more efficient nonfullerene acceptor systems that show only triplet states formed via nongeminate recombination, the fullerene systems also show significant triplet formation via geminate processes. This requires that geminate electron–hole pairs be trapped long enough to allow intersystem crossing. It is proposed that this is a general feature of fullerene acceptor systems, where isolated fullerenes are known to intercalate within the alkyl sidechains of the donor polymers. Thus, the study demonstrates that engineering good donor and acceptor domain purity is key for suppressing losses via triplet excitons in OSCs.

High fill factor organic solar cells with increased dielectric constant and molecular packing density

<h2>Summary</h2> The fill factor (FF) of organic solar cells (OSCs), a critically important photovoltaic parameter, is still sub-optimal, often less than 0.8. To further reduce the FF gaps with regard to the Shockley-Queisser upper limit, we present a study unveiling the impacts of dielectric properties on obtaining high FFs and photovoltaic efficiencies in OSCs. By increasing the dielectric constant (ε) of non-fullerene acceptors (NFAs) and bulk heterojunction (BHJ) films, afforded by the increase in molecular packing density (MPD) of NFAs in solid states, we show that the FF penalties related to non-geminate charge recombination in OSCs can be appreciably mitigated. In particular, as exemplified by an NFA model system L8-BO tailored with branched side chains, the enlargements of MPD and ε allow achieving an impressive FF of 0.815 with an efficiency of 18.23% in OSCs, featuring an impressively low bimolecular recombination and reduced sensitivity of FF on the BHJ film thickness.

Mechano-optical Modulation of Excitons and Carrier Recombination in Self-Assembled Halide Perovskite Quantum Dots.

Mechanically modulating optical properties of semiconductor nanocrystals and organic molecules are valuable for mechano-optical and optomechanical devices. Halide perovskites with excellent optical and electronic properties are promising for such applications. We report the mechanically changing excitons and photoluminescence of self-assembled formamidinium lead bromide (FAPbBr3) quantum dots. The as-synthesized quantum dots (3.6 nm diameter), showing blue emission and a short photoluminescence lifetime (2.6 ns), form 20-300 nm 2D and 3D self-assemblies with intense green emission in a solution or a film. The blue emission and short photoluminescence lifetime of the quantum dots are different from the delayed (ca. 550 ns) green emission from the assemblies. Thus, we consider the structure and excitonic properties of individual quantum dots differently from the self-assemblies. The blue emission and short lifetime of individual quantum dots are consistent with a weak dielectric screening of excitons or strong quantum confinement. The red-shifted emission and a long photoluminescence lifetime of the assemblies suggest a strong dielectric screening that weakens the quantum confinement, allowing excitons to split into free carriers, diffuse, and trap. The delayed emission suggests nongeminate recombination of diffusing and detrapped carriers. Interestingly, the green emission of the self-assembly blueshifts by applying a lateral mechanical force (ca. 4.65 N). Correspondingly, the photoluminescence lifetime decreases by 1 order of magnitude. These photoluminescence changes suggest the mechanical dissociation of the quantum dot self-assemblies and mechanically controlled exciton splitting and recombination. The mechanically changing emission color and lifetime of halide perovskite are promising for mechano-optical and optomechanical switches and sensors.

Reduction of Electric Current Loss by Aggregation-Induced Molecular Alignment of a Non-Fullerene Acceptor in Organic Photovoltaics.

Y6 is a recently developed non-fullerene electron acceptor (NFA) with dithienothiophen[3.2-b]-pyrrolobenzothiadiazole as the central unit and improves the performance of organic photovoltaics (OPVs) in combination with many electron-donor polymers. Although Y6 has desirable electronic properties for OPVs, the origin of its superiority as an acceptor is unclear. This study empirically investigates why Y6 is an excellent acceptor by comparing Y6 with F8IC, an analogue with a similar electronic structure, in bulk heterojunction (BHJ) OPVs with various electron-acceptor concentrations. The difference in the performance between Y6 and F8IC appears only at high concentrations, suggesting that it originates from the aggregation structures of the NFAs in the BHJs. Electric current loss analysis reveals that the exciton loss and non-geminate recombination are suppressed more strongly in Y6 OPVs than in F8IC OPVs. Variable angle spectroscopic ellipsometry shows that Y6 molecules align in the in-plane direction at high concentrations, contributing to the high charge generation rate via efficient exciton collection.

Oligoethylene Glycol Side Chains Increase Charge Generation in Organic Semiconductor Nanoparticles for Enhanced Photocatalytic Hydrogen Evolution.

Organic semiconductor nanoparticles (NPs) composed of an electron donor/acceptor (D/A) semiconductor blend have recently emerged as an efficient class of hydrogen-evolution photocatalysts. It is demonstrated that using conjugated polymers functionalized with (oligo)ethylene glycol side chains in NP photocatalysts can greatly enhance their H2 -evolution efficiency compared to their nonglycolated analogues. The strategy is broadly applicable to a range of structurally diverse conjugated polymers. Transient spectroscopic studies show that glycolation facilitates charge generation even in the absence of a D/A heterojunction, and further suppresses both geminate and nongeminate charge recombination in D/A NPs. This results in a high yield of photogenerated charges with lifetimes long enough to efficiently drive ascorbic acid oxidation, which is correlated with greatly enhanced H2 -evolution rates in the glycolated NPs. Glycolation increases the relative permittivity of the semiconductors and facilitates water uptake. Together, these effects may increase the high-frequency relative permittivity inside the NPs sufficiently, to cause the observed suppression of exciton and charge recombination responsible for the high photocatalytic activities of the glycolated NPs.

Investigation of tunable halogen-free solvent engineering on aggregation and miscibility towards high-performance organic solar cells

Eco-friendly and sustainable organic solar cells (OSCs) have become the inevitable choice for the commercialization, which specifically dictates the use of non-halogenated solvents. Although a series of non-halogenated solvents have been introduced to meet such need, non-halogenated OSCs with high-performance are still rare due to the uncontrollable aggregation and penetration behaviors. To mitigate these drawbacks, tunable non-halogenated solvent engineering (TSE) strategy is proposed via mixing carbon disulfide (CS2) and o-xylene (o-XY) solvents with 0.3:0.7 vol ratio. By the merits of solubility and volatilization gradient of TSE strategy, the aggregation and interdiffusion dynamics of donor (D) and acceptor (A) are finely tailored. More crucially, by applying this TSE approach, the non-geminate recombination loss is drastically eliminated along with charge generation/transport/extraction processes are considerably improved by a series of photoelectric measurements. Consequently, the PM6-BTP-eC9 blend based OSCs yield a maximum performance of 17.6% without any additive and/or post treatments, with an improvement of 23.1% compared to that of neat o-XY based counterparts (14.3%), which is among the highest values in the field of halogen-free-solvent-processed OSCs to our best knowledge. All these pronounced that TSE is a feasible and promising strategy to construct the efficient and eco-friendly OSCs for industrial commercialization.

Analysis of electrical transients in the fourth quadrant of thin films photovoltaics: The case of organic bulk heterojunction solar cell

Current-voltage (J-V) measurements under illumination are essential for studying solar cells. They directly provide fundamental parameters of the solar cell, as the short-circuit current (JSC), the open-circuit voltage (VOC), and the fill-factor (FF); but a deeper investigation of transport phenomena along the fourth quadrant is hampered by the tight linear scale, limited between zero to ~1 V. In this work, we present transient measurements carried out in an organic solar cell (ITO/PEDOT:PSS/PTB7-Th:PC71BM/Ca/Al), which allows to analyze J-V curves, by extending the scale from a linear to a logarithmic scale. For this, we used a circuit similar to that used in transient measurements, by replacing the voltage source by a variable load resistance RL (from 50 Ω to 1 MΩ). A voltage transient ΔV(t) and its time decay τ are measured over each RL, and τ-RL curves were obtained, keeping the device under different intensities of sunlight. Each τ-RL curve is divided in three distinct zones: one in which the extraction dominates the photocurrent; the second where a competition between extraction and non-geminate recombination is established; and the third, dominated by recombination and diffusion. An equivalent circuit is used to analyze the τ-RL curves, in which the diode resistance plays a relevant role, especially in the region close to VOC. Not only recombination mechanisms can be better analyzed through this new approach, but also the distinction between drift and diffusion transports.

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.

Structural Dynamics of C2F4I2 in Cyclohexane Studied via Time-Resolved X-ray Liquidography.

The halogen elimination of 1,2-diiodoethane (C2H4I2) and 1,2-diiodotetrafluoroethane (C2F4I2) serves as a model reaction for investigating the influence of fluorination on reaction dynamics and solute–solvent interactions in solution-phase reactions. While the kinetics and reaction pathways of the halogen elimination reaction of C2H4I2 were reported to vary substantially depending on the solvent, the solvent effects on the photodissociation of C2F4I2 remain to be explored, as its reaction dynamics have only been studied in methanol. Here, to investigate the solvent dependence, we conducted a time-resolved X-ray liquidography (TRXL) experiment on C2F4I2 in cyclohexane. The data revealed that (ⅰ) the solvent dependence of the photoreaction of C2F4I2 is not as strong as that observed for C2H4I2, and (ⅱ) the nongeminate recombination leading to the formation of I2 is slower in cyclohexane than in methanol. We also show that the molecular structures of the relevant species determined from the structural analysis of TRXL data provide an excellent benchmark for DFT calculations, especially for investigating the relevance of exchange-correlation functionals used for the structural optimization of haloalkanes. This study demonstrates that TRXL is a powerful technique to study solvent dependence in the solution phase.