Bimolecular Charge Research Articles

Correlating the Photovoltaic Performance and Stability of the All-Small-Molecule Organic Solar Cells to Their Intermixed Phases Determined by Concentration-Dependent Ultraviolet-Visible Absorption Spectroscopy.

In addition to the donor-acceptor nano phases, the intermixed phase within the organic blends is crucial for the photovoltaic performance and stability of the bulk-heterojunction organic solar cells (OSCs). Here, the intermixed phase of a representative M-PhS:BTP-eC9 all-small-molecule organic solar cell was investigated by a concentration-dependent ultraviolet-visible (UV-vis) absorption spectroscopy method, where a shift of the absorption maximum wavelength was measured for the acceptor component with the increase of the acceptor concentration. The blend ratios of the acceptor to the donor in the intermixed phase, corresponding to the critical concentration for the formation of the acceptor nanophase (CAP), were determined to be 0.35, 0.20, and 0.15 for the as-cast, thermal annealing (TA), and the combined TA and solvent vapor annealing films. These results indicated that M-PhS and BTP-eC9 are kinetically well intermixed during spin coating, whereas TA and the following solvent annealing promote the crystallization of BTP-eC9 molecules out of the intermixed phase. The photovoltaic performance of the M-PhS:BTP-eC9 cells with different blend ratios was investigated. The formation of the BTP-eC9 nano phase in the blend film leads to stable VOC and fast increased JSC, which can be understood by the reduction of bimolecular charge recombination and the formation of electron transporting pathways within the photoactive layer. Similarly, the critical concentration for the formation of the donor phase was estimated to be 0.15 by measuring the stabilized VOC and increased JSC values of the cells with different donor blending ratios. More importantly, after a fast "burn-in" thermal degradation, the M-PhS:BTP-eC9 cell showed excellent thermal stability aging at 85 °C for over 1128 h, which is in good accordance with the unchanged intermixed phases measured by the UV-vis spectra of the annealed films. The current work demonstrates the feasibility of the spectroscopy method to investigate the intermixed phases for organic bulk-heterojunction solar cells and proves that all-small-molecule solar cells can be intrinsically very stable.

Bimolecular photoinduced symmetry-breaking charge separation of perylene in solution

Photoinduced symmetry-breaking charge separation (SB-CS) results in the generation of charge carriers through electron transfer between two identical molecules, after photoexcitation of one of them. It is usually studied in systems where the two reacting moieties are covalently linked. Examples of photoinduced bimolecular SB-CS with organic molecules yielding free ions remain scarce due to solubility or aggregation issues at the high concentrations needed to study this diffusion-assisted process. Here we investigate the excited-state dynamics of perylene (Pe) at high concentrations in solvents of varying polarity. Transient absorption spectroscopy on the subnanosecond to microsecond timescales reveal that self-quenching of Pe in the lowest singlet excited state leads to excimer formation in all solvents used. Additionally, bimolecular SB-CS, resulting in the generation of free ions, occurs concurrently to excimer formation in polar media, with a relative efficiency that increases with the polarity of the solvent. Moreover, we show that SB-CS is most efficient in room-temperature ionic liquids due to a charge-shielding effect leading to a larger escape of ions and due to the high viscosity that disfavours excimer formation.Graphical abstract

Impact of Intermolecular Interactions between Halogenated Volatile Solid Additives and the Nonfullerene Acceptor in Organic Solar Cells

AbstractThe halogenated volatile solid additives can delicately optimize the active layer morphology of organic solar cells, improving the devices' performance, stability, and reproducibility. However, what type of intermolecular interaction occurs between the solid additives and the active layer and whether the interaction truly impacts the donor or acceptor remains debatable. Herein, the focus is on halogenated volatile solid additives with conjugated benzene rings and their influence on the morphology of the active layer composed of PM6:Y6 as they evaporated. The absorbance spectra exhibit apparent red‐shift features in Y6 absorption regions, while the donor part is unaffected. The theoretical calculation results reveal that the additives can stay between two Y6 molecules and form halogen bonds, affecting the π–π aggregation properties of Y6. As a result, the crystalline features of the active layer are altered, leading to increased charge carrier mobilities, extended charge carrier diffusion lengths, reduced bimolecular charge recombination, and thus the device performance. Especially when 1,3,5‐tri bromobenzene is used, a champion power conversion efficiency of 17.9% is attained, among the best‐performed organic solar cells comprising PM6:Y6. The findings shed light on theoretical and experimental guidelines for designing and developing volatile solid additives for highly efficient nonfullerene organic solar cells.

Vertical Phase Regulation with 1,3,5-Tribromobenzene Leads to 18.5% Efficiency Binary Organic Solar Cells.

The sequential deposition method assists the vertical phase distribution in the photoactive layer of organic solar cells, enhancing power conversion efficiencies. With this film coating approach, the morphology of both layers can be fine-tuned with high boiling solvent additives, as frequently applied in one-step casting films. However, introducing liquid additives can compromise the morphological stability of the devices due to the solvent residuals. Herein, 1,3,5-tribromobenzene (TBB) with high volatility and low cost, is used as a solid additive in the acceptor solution and combined thermal annealing to regulate the vertical phase in organic solar cells composed of D18-Cl/L8-BO. Compared to the control cells, the devices treated with TBB and those that underwent additional thermal processing exhibit increased exciton generation rate, charge carrier mobility, charge carrier lifetime, and reduced bimolecular charge recombination. As a result, the TBB-treated organic solar cells achieve a champion power conversion efficiency of 18.5% (18.1% averaged), one of the highest efficiencies in binary organic solar cells with open circuit voltage exceeding 900mV. This study ascribes the advanced device performance to the gradient-distributed donor-acceptor concentrations in the vertical direction. The findings provide guidelines for optimizing the morphology of the sequentially deposited top layer to achieve high-performance organic solar cells.

Excited-State Dynamics in All-Polymer Blends with Polymerized Small-Molecule Acceptors.

Polymerizing small-molecular acceptors (SMAs) is a promising route to construct high performance polymer acceptors of all-polymer solar cells (all-PSCs). After SMA polymerization, the microstructure of molecular packing is largely modified, which is essential in regulating the excited-state dynamics during the photon-to-current conversion. Nevertheless, the relationship between the molecular packing and excited-state dynamics in polymerized SMAs (PSMAs) remains poorly understood. Herein, the excited-statedynamics and molecular packing are investigated in the corresponding PSMA and SMA utilizing a combination of experimental and theoretical methods. This study finds that the charge separation from intra-moiety delocalized states (i-DEs) is much faster in blends with PSMAs, but the loosed π-π molecular packing suppresses the excitation conversion from the local excitation (LE) to the i-DE, leading to additional radiative losses from LEs. Moreover, the increased aggregations of PSMA in the blends decrease donor: acceptor interfaces, which reduces triplet losses from the bimolecular charge recombination. These findings suggest that excited-state dynamics may be manipulated by the molecular packing in blends with PSMAs to further optimize the performance of all-PSCs.

Difluoro and dicyano substituted sexithiophene − isoindigo terpolymers enabling ternary polymer solar cells with a bulk heterojunction thickness 100 nm → 300 nm and a minor decline of efficiency 14.5 % → 14.0 %

In this work, difluoro and dicyano substituted terpolymers, P6TI-3N1F, P6TI-1N1F, and P6TI-1N3F, based on isoindigo − sexithiophene π-conjugated polymers have been designed and characterized. Together with two only cyano and only fluoro substituted copolymers, P6TIN and P6TIF, we have demonstrated that P6TI-1N1F is the champion terpolymer in terms of the ratio of electron/hole mobility, nanofibril and its agglomeration, monomolecular and bimolecular charge recombination, polymer chain face-on alignment, and photoluminescence quenching in a binary thin film of polymer:PC71BM or polymer:IT4F. In the binary polymer solar cells (PSCs) based on a bullk heterojunction (BHJ) of either polymer:PC71BM or polymer:IT4F, P6TI-1N1F exhibits the highest fill factor of 67.62 % or 76.86 %, and hence the highest power conversion efficiency (PCE) of 10.4 % or 11.9 %. Corresponding ternary PSCs with a 100 nm thick BHJ of P6TI-1N1F:IT4F:PC71BM or P6TI-1N1F:Y6:PC71BM exhibit PCE as high as 12.8 % or 14.5 %. It is rarely seen in literature that the PCE of the ternary PSCs shows only minor decline to 12.4 % or 14.0 % when the thickness of BHJ increases from 100 nm to 300 nm.

Suppressing Bimolecular Charge Recombination and Energetic Disorder with Planar Heterojunction Active Layer Enables 18.1% Efficiency Binary Organic Solar Cells

The performance of organic solar cells has been dramatically enhanced due to the use of bulk heterojunction active layer structure. However, blending donor and acceptor in the bulk volume results in a large extent of bimolecular charge recombination and increasing energetic disorder. Herein, we fabricated organic solar cells with polymer donor PM6 and nonfullerene acceptor N3 in bilayer structure to investigate the impact of planar heterojunction configurations on charge recombination and energetic landscapes. We find superior crystallinity features in the bilayer composed of pure donor and acceptor layers. The bimolecular charge recombination and energetic disorder are effectively suppressed compared with the bulk heterojunction counterparts. Thus, a power conversion efficiency as high as 18.1% (17.8% averaged) is achieved, which stands among the top values of planar heterojunction organic solar cells. In addition, improved device shelf stability is observed because of the fewer donor–acceptor interfaces in the active layer. Our results suggest that a planar heterojunction structure could efficiently suppress the bimolecular charge recombination and energetic disorder, providing an alternative pathway for developing solution-processed organic solar cells.

Charge Carrier Dynamics in Planar Heterojunction Organic Solar Cells

The ionization energy (IE) offset of a donor–acceptor pair provides the driving force for hole transfer and subsequent free charge carrier generation in low‐bandgap nonfullerene organic solar cells (OSCs). However, the interfacial energetic landscape in bulk heterojunction OSCs is determined by the materials’ electronic structure and intermolecular interactions at the donor/acceptor interface, causing local energy‐level shifts and disorder. Herein, the impact of the IE offset on the charge transfer efficiency and charge carrier dynamics is systematically evaluated by characterizing PM6/ITIC, PM6/IT‐2Cl, and PM6/IT‐4Cl planar heterojunction (PHJ) solar cells. Ultrafast spectroscopy and time‐resolved charge carrier density measurements reveal that an IE offset of about ≈0.5 eV leads to efficient hole transfer and subsequent free charge generation. Furthermore, bimolecular charge recombination and consequently triplet generation are significantly reduced in systems with high IE offset. This work underlines the importance of sizeable donor–acceptor IE offsets in PHJ nonfullerene OSCs as critical for high‐efficiency donor/acceptor material and device design.

Reduced bimolecular charge recombination in efficient organic solar cells comprising non-fullerene acceptors

Bimolecular charge recombination is one of the most important loss processes in organic solar cells. However, the bimolecular recombination rate in solar cells based on novel non-fullerene acceptors is mostly unclear. Moreover, the origin of the reduced-Langevin recombination rate in bulk heterojunction solar cells in general is still poorly understood. Here, we investigate the bimolecular recombination rate and charge transport in a series of high-performance organic solar cells based on non-fullerene acceptors. From steady-state dark injection measurements and drift–diffusion simulations of the current–voltage characteristics under illumination, Langevin reduction factors of up to over two orders of magnitude are observed. The reduced recombination is essential for the high fill factors of these solar cells. The Langevin reduction factors are observed to correlate with the quadrupole moment of the acceptors, which is responsible for band bending at the donor–acceptor interface, forming a barrier for charge recombination. Overall these results therefore show that suppressed bimolecular recombination is essential for the performance of organic solar cells and provide design rules for novel materials.

Competing dynamics of intramolecular deactivation and bimolecular charge transfer processes in luminescent Fe(iii) N-heterocyclic carbene complexes.

Steady state and ultrafast spectroscopy on [FeIII(phtmeimb)2]PF6 (phtmeimb = phenyl(tris(3-methylimidazol-2-ylidene))borate) was performed over a broad range of temperatures. The intramolecular deactivation dynamics of the luminescent doublet ligand-to-metal charge-transfer (2LMCT) state was established based on Arrhenius analysis, indicating the direct deactivation of the 2LMCT state to the doublet ground state as a key limitation to the lifetime. In selected solvent environments photoinduced disproportionation generating short-lived Fe(iv) and Fe(ii) complex pairs that subsequently undergo bimolecular recombination was observed. The forward charge separation process is found to be temperature-independent with a rate of ∼1 ps-1. Subsequent charge recombination takes place in the inverted Marcus region with an effective barrier of 60 meV (483 cm-1). Overall, the photoinduced intermolecular charge separation efficiently outcompetes the intramolecular deactivation over a broad range of temperatures, highlighting the potential of [FeIII(phtmeimb)2]PF6 to perform photocatalytic bimolecular reactions.

What's Next for Organic Solar Cells? The Frontiers and Challenges

The power conversion efficiency (PCE) of organic solar cells (OSCs) is improved dramatically in recent years and now approaches >19% for single‐junction cells and >20% for tandem cells. Therefore, the practical use of OSCs is becoming a reality. This perspective summarizes the state of the art of OSC characteristics and discusses the challenges that remain in further improving PCE. The short‐circuit current density (J SC) of the state‐of‐the‐art OSCs almost approaches 30 mA cm−2. As the internal quantum efficiencies of these devices exceed 90%, for further improvement in J SC, it is necessary to suppress reflection at interfaces and increase the active layer thickness to absorb as many photons as possible. Further suppression of the nonradiative voltage loss is imperative, as there is still large room for improvement compared to inorganic and perovskite counterparts. Hence, increasing the exciton lifetime and photoluminescence quantum yield of nonfullerene acceptors are pivotal to improving the open‐circuit voltage of OSCs. The fill factors of the latest OSCs approach 80%; however, because the optimized active layer thickness remains ≈100 nm, further suppression of bimolecular charge recombination is needed. Finally, this perspective discusses to what extent PCE can be improved and what can be done to achieve this.

Charge Carrier and Exciton Dynamics in Perovskites Revealed by Time‐Integrated Photoluminescence after Double‐Pulse Excitation

AbstractThe rate constants that describe the decays of excited states (free charge carriers, excitons) are important parameters for perovskite materials. Typically, these rates are determined using detectors with high time‐resolution to record the time‐dependent photoluminescence on the nanosecond scale. Herein, a method is applied that uses two excitation pulses with a variable delay and an inexpensive detector capable of measuring only the quasisteady state photoluminescence. Based on how the time‐integrated photoluminescence varies as a function of the delay time between the two excitation pulses, the rate constants for exciton–exciton annihilation, monomolecular exciton decay, bimolecular free charge carrier recombination, and monomolecular trapping of free charge carriers can be extracted. To demonstrate the method quasi‐2D perovskites are investigated as they are known to exhibit excited‐state populations that can be exciton dominated, free charge carrier dominated, or a mixture of the two. The method leads to unique curves and accurate extracted parameters in all cases. The introduced method of determining excited‐state lifetimes can be used with detectors that only have a low temporal resolution. This opens the path to the spatial mapping of excited‐state dynamics using standard cameras which would be attractive for quality control of photovoltaic layers.

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.

Chain aggregation dictates bimolecular charge recombination and fill factor of all-polymer blend solar cells

Small bimolecular charge recombination coefficients resulted in all-polymer blend solar cells with high fill factors. The preferred blend morphology was characterized by a well-ordered local structure, exhibiting chain aggregation by both the polymer donor and acceptor.

Effect of Palladium‐Tetrakis(Triphenylphosphine) Catalyst Traces on Charge Recombination and Extraction in Non‐Fullerene‐based Organic Solar Cells

AbstractThe effect of the cross‐coupling catalyst tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) on the performance of a model organic bulk‐heterojunction solar cell composed of a blend of poly([2,6′‐4,8‐di(5‐ethylhexylthienyl)benzo[1,2‐b;3,3‐b]dithiophene]{3‐fluoro‐2[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl}) (PTB7‐Th) donor and 3,9‐bis(2‐methylene‐((3‐(1,1‐dicyanomethylene)‐6,7‐difluoro)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (IOTIC‐4F) non‐fullerene acceptor is investigated. The effect of intentional addition of different amounts of Pd(PPh3)4 on morphology, free charge carrier generation, non‐geminate bulk trap‐ and surface trap‐assisted recombination as well as bimolecular recombination and charge extraction is quantified. This work shows that free charge carrier generation is affected significantly, while the impact of Pd(PPh3)4 on non‐geminate recombination processes is limited because the catalyst does not facilitate efficient trap‐assisted recombination. The studied system shows substantial robustness towards the addition of Pd(PPh3)4 in small amounts.

Molecular dispersion enhances photovoltaic efficiency and thermal stability in quasi-bilayer organic solar cells

In comparison to widely adopted bulk heterojunction (BHJ) structures for organic solar cells (OSC), exploiting the sequential deposition to form planar heterojunction (PHJ) structures enables to realize the favorable vertical phase separation to facilitate charge extraction and reduce charge recombination in OSCs. However, effective tunings on the power conversion efficiency (PCE) in PHJ-OSCs are still restrained by the currently available methods. Based on a polymeric donor PBDBT-2F (PBDBT= Poly [[4,8-bis [5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo [1,2- b :4,5- b ′]dithiophene-2,6-diyl]-2,5-thiophenediyl [5,7-bis (2-ethylhexyl)-4,8-dioxo-4 H ,8 H -benzo [1,2- c :4,5- c ′]dithiophene-1,3-diyl]-2,5-thiophenediyl]) and a non-fullerene (NF) acceptor Y6, we proposed a strategy to improve the properties of photovoltaic performances in PHJ-based OSCs through dilute dispersions of the PBDBT-2F donor into the acceptor-dominant phase with the sequential film deposition. With the control of donor dispersions, the charge transport balance in the PHJ-OSCs is improved, leading to the expedited photocarrier sweep-out with reduced bimolecular charge recombination. As a result, a PCE of 15.4% is achieved in the PHJ-OSCs. Importantly, the PHJ solar cells with donor dispersions exhibit better thermal stability than corresponding BHJ devices, which is related to the better film morphology robustness and less affected charge sweep-out during the thermal aging.

Contrasting Electron and Hole Transfer Dynamics from CH(NH2)2PbI3 Perovskite Quantum Dots to Charge Transport Layers

In this work, the ultrafast transient absorption spectroscopy (TAs) was utilized to first investigate the charge transfer from the emerging FAPbI3 (FA = CH(NH2)2) perovskite quantum dots (PQDs) to charge transport layers. Specifically, we compared the TAs in pure FAPbI3 PQDs, PQDs grown with both electron and hole transfer layers (ETL and HTL), and PQDs with only ETL or HTL. The TA signals induced by photoexcited electrons decay much faster in PQDs samples with the ETL (~20 ps) compared to the pure FAPbI3 PQDs (>1 ns). These results reveal that electrons can effectively transport between coupled PQDs and transfer to the ETL (TiO2) at a time scale of 20 ps, much faster than the bimolecular charge recombination inside the PQDs (>1 ns), and the electron transfer efficiency is estimated to be close to 100%. In contrast, the temporal evolution of the TA signals in the PQDs with and without HTL exhibit negligible change, and no substantive hole transfer to the HTL (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine], PTAA) occurs within 1 ns. The much slower hole transfer implies the further potential of increasing the overall photo-carrier conversion efficiency through enhancing the hole diffusion length and fine-tuning the coupling between the HTL and PQDs.

Fast Field-Insensitive Charge Extraction Enables High Fill Factors in Polymer Solar Cells.

Fill factor (FF) is a determining parameter for the power conversion efficiency (PCE) of organic solar cells (OSC). So far, nonfullerene (NF) OSCs with state-of-the-art PCEs exhibit FFs <0.8, lower than the values of Si or perovskite solar cells. The FFs directly display the dependence of photocurrent on bias, meaning that the competition between charge extraction and recombination is modulated by internal electric potential (Vin). Here, we report a study to understand key parameters/properties affecting the device FF based on seven groups of NF-OSCs consisting of widely used PBDBT-2F or PTB7-Th donors and representative NF-acceptors with FFs ranging from 0.60 to 0.78 and PCEs from 10.27 to 16.09%. We used field-dependent transient photocurrent measurements to reveal that fast and field-insensitive charge extraction at low Vin is an essential prerequisite for obtaining high FFs (0.75-0.8), which is enabled by balanced charge transport in steady and reduced bimolecular charge recombination in high purity phases. With bias-dependent quantum efficiency analysis, we further show that the recombination loss at low Vin in the devices with low FFs tends to be more significant involving excitons generated in the donor phase of blends. Our results provide relevance for how to improve the FF toward the boost of photovoltaic performance in NF-OSCs.

Tracing the Full Bimolecular Photocycle of Iron(III)-Carbene Light Harvesters in Electron-Donating Solvents.

Photoinduced bimolecular charge transfer processes involving the iron(III) N-heterocyclic carbene (FeNHC) photosensitizer [Fe(phtmeimb)2]+ (phtmeimb = phenyltris(3-methyl-imidazolin-2-ylidene)borate) and triethylamine as well as N,N-dimethylaniline donors have been studied using optical spectroscopy. The full photocycle of charge separation and recombination down to ultrashort time scales was studied by investigating the excited-state dynamics up to high quencher concentrations. The unconventional doublet ligand-to-metal charge transfer (2LMCT) photoactive excited state exhibits donor-dependent charge separation rates of up to 1.25 ps–1 that exceed the rates found for typical ruthenium-based systems and are instead more similar to results reported for organic sensitizers. The ultrafast charge transfer probed at high electron donor concentrations outpaces the solvent dynamics and goes beyond the classical Marcus electron transfer regime. Poor photoproduct yields are explained by donor-independent, fast charge recombination with rates of ∼0.2 ps–1, thus inhibiting cage escape and photoproduct formation. This study thus shows that the ultimate bottlenecks for bimolecular photoredox processes involving these FeNHC photosensitizers can only be determined from the ultrafast dynamics of the full photocycle, which is of particular importance when the bimolecular charge transfer processes are not limited by the intrinsic excited-state lifetime of the photosensitizer.

Role of Polar Protic Solvents in the Dissociation and Reactivity of Photogenerated Radical Ion Pairs.

The UV photolysis of bimolecular charge transfer complexes is employed to yield reactive radical ions in their solvent-equilibrated electronic ground state. In polar protic media, noncovalent complexes of 1,2,4,5-tetracyanobenzene and toluene undergo efficient, ultrafast dissociation to ion pairs and equilibrate with their solvent environment before the resulting radical ions engage in electron transfer and proton abstraction on subnanosecond time scales. Solvent molecules play a critical role in these reactive pathways and in the dissociation and relaxation processes that precede them. We report a clear separation of time scales for these relaxation and reactive processes, which implies that solvent-solute interactions can be used as a tool for tuning the reaction pathways of equilibrated radical ions in solution.