Interfacial Charge Transfer States Research Articles

Long-lived and disorder-free charge transfer states enable endothermic charge separation in efficient non-fullerene organic solar cells

Organic solar cells based on non-fullerene acceptors can show high charge generation yields despite near-zero donor–acceptor energy offsets to drive charge separation and overcome the mutual Coulomb attraction between electron and hole. Here, we use time-resolved optical spectroscopy to show that free charges in these systems are generated by thermally activated dissociation of interfacial charge-transfer states that occurs over hundreds of picoseconds at room temperature, three orders of magnitude slower than comparable fullerene-based systems. Upon free electron–hole encounters at later times, both charge-transfer states and emissive excitons are regenerated, thus setting up an equilibrium between excitons, charge-transfer states and free charges. Our results suggest that the formation of long-lived and disorder-free charge-transfer states in these systems enables them to operate closely to quasi-thermodynamic conditions with no requirement for energy offsets to drive interfacial charge separation and achieve suppressed non-radiative recombination.

Exciton and Charge Carrier Dynamics in Highly Crystalline PTQ10:IDIC Organic Solar Cells

AbstractHerein the morphology and exciton/charge carrier dynamics in bulk heterojunctions (BHJs) of the donor polymer PTQ10 and molecular acceptor IDIC are investigated. PTQ10:IDIC BHJs are shown to be particularly promising for low cost organic solar cells (OSCs). It is found that both PTQ10 and IDIC show remarkably high crystallinity in optimized BHJs, with GIWAXS data indicating pi‐pi stacking coherence lengths of up to 8 nm. Exciton‐exciton annihilation studies indicate long exciton diffusion lengths for both neat materials (19 nm for PTQ10 and 9.5 nm for IDIC), enabling efficient exciton separation with half lives of 1 and 3 ps, despite the high degree of phase segregation in this blend. Transient absorption data indicate exciton separation leads to the formation of two spectrally distinct species, assigned to interfacial charge transfer (CT) states and separated charges. CT state decay is correlated with the appearance of additional separate charges, indicating relatively efficient CT state dissociation, attributed to the high crystallinity of this blend. The results emphasize the potential for high material crystallinity to enhance charge separation and collection in OSCs, but also that long exciton diffusion lengths are likely to be essential for efficient exciton separation in such high crystallinity devices.

Charge Separation from an Intra-Moiety Intermediate State in the High-Performance PM6:Y6 Organic Photovoltaic Blend.

Bulk-heterojunction organic photovoltaic devices with nonfullerene acceptors (NFAs) exhibit efficient hole transfer with small interfacial energy offset, which results in power conversion efficiencies above 17% in single junction devices using the high-performance NFA of Y6. However, the underlying mechanism responsible for the hole transfer channel in the polymer/Y6 blends remains poorly understood. Herein, we report that the hole transfer channel of photocharge generation is mediated by an intra-moiety excited state in a blend of donor polymer PM6 and NFA Y6 using broadband transient absorption (TA) spectroscopy. By comparing the TA data recorded from the solution and film Y6 samples, we identify the ultrafast formation of an intra-moiety excimer state together with the conversion from the primary local excitation on a time scale of ∼0.2 ps in the Y6 film. The intra-moiety excimer state acts as the intermediate for the hole transfer channel, which dissociates into free polarons on a time scale of ∼15 ps in the PM6/Y6 blend at room temperature. The intra-moiety intermediate state, arising from the intermolecular coupling in Y6 domains, is markedly different from the interfacial charge transfer state, which is commonly accepted as the intermediate state for the electron transfer channel. These findings suggest that manipulating the interplay between intra-moiety and interfacial excited species can provide a promising route for further improving device performance.

Geminate recombination in organic photovoltaic blend PCDTBT/PC71BM studied by out-of-phase electron spin echo spectroscopy.

The key process in organic solar cell operation is charge separation under light illumination. Due to the low dielectric constant of organic materials, the Coulomb attraction energy within the interfacial charge-transfer state (CTS) is larger than the thermal energy. Understanding the mechanism of charge separation at the organic donor/acceptor interface still remains a challenge and requires knowledge of the CTS temporal evolution. To address this problem, the CTS in the benchmark photovoltaic blend PCDTBT/PC71BM was studied by the out-of-phase Electron Spin Echo (ESE). The protocol for determining the CTS geminate recombination rate for certain electron-hole distances was developed. Simulating the out-of-phase ESE trace for the CTS in the PCDTBT/PC71BM blend allows precise determination of the electron-hole distance distribution function and its evolution with the increase in the delay after the laser flash. Distances of charge separation up to 6 nm were detected upon thermalization at a temperature of 20 K. Assuming the exponential decay of the recombination rate, the attenuation factor β = 0.08 Å-1 is estimated for the PCDTBT/PC71BM blend. Such a low attenuation factor is probably caused by a high degree of hole delocalization along the PCDTBT chain.

The Cost of Converting Excitons into Free Charge Carriers in Organic Solar Cells.

Efficient exciton dissociation and subsequent generation of free charge carriers at the organic donor-acceptor interface requires a number of electron-transfer processes. It is a common view that these steps result in an unavoidable energy loss in organic photovoltaic devices that is not present in other types of solar cells. The currently best performing organic solar cells with power conversion efficiencies over 16% challenge this view, and no interfacial charge-transfer states with energy significantly lower than the strongly absorbing singlet states are detected within the gap of the used donor and acceptor materials. This Perspective will discuss implications, the remaining sources of energy loss, and open questions to be solved to achieve power conversion efficiencies over 20%.

How Does Polymorphism Affect the Interfacial Charge-Transfer States in Organic Photovoltaics?

The bulk heterojunction in organic photovoltaic (OPV) devices is a mixture of polymer (electron donor) and an electron acceptor material (typically functionalized fullerenes), and it is crucial for the device operation, as this is where excitons are split into electrons and holes to produce current. Nonfullerene acceptors (NFAs) are promising new materials for improving the device efficiency, and their solid-state arrangement with respect to the electron donor polymer is critical for the charge mobility and the performance of OPV devices. Although there have been numerous studies on NFAs, most of the current understanding comes from empirical considerations, with little atomistic-level interpretation of why and how the packing influences the charge-transport properties of these materials. In this work, we describe large-scale (with up to 3462 atoms) density functional theory simulations for ground and excited states on a number of polymer–NFA interfaces of realistic size, whose NFA domains consist of poly...

Charge Transfer State in the Composite DTS(FBTTh2)2:PC71BM: Dynamics of Electron–Hole Distance Distribution After Light Absorption

Light-induced charge separation in an organic photovoltaic (OPV) composite DTS(FBTTh2)2:PC71BM was studied. DTS(FBTTh2)2 or DTS is a non-polymer electron donor and PC71BM is a fullerene-based electron acceptor. Electron spin echo (ESE) technique has been developed to separate the signal of interfacial charge transfer state (CTS) from that of trapped charges. Pronounced out-of-phase ESE signal was observed within first few microseconds after a laser flash exciting the composite at cryogenic temperatures. This implies correlation of unpaired electron spins of DTS+ and PC71BM– species constituting CTS. The distribution of distances between these species is derived from out-of-phase ESE envelope modulation (ESEEM). Out-of-phase ESEEM traces were numerically simulated by the model assuming both magnetic dipolar and electron–hole exchange interactions within CTS. The most probable distance between DTS+ and PC71BM– within CTS increases from 4.9 nm and 5.7 nm with delays after the laser flash increase from 200 ns to 30 µs at the lowest temperature studied 20 K. This is caused by faster recombination of CTS with shorter electron–hole distance. The electron–hole exchange interaction is about J/h = 1.15 MHz for the smallest interspin distance obtained r0 = 2.5 nm. The overall similarity of the initial electron–hole distance and CTS recombination rate for DTS:PC71BM and polymer/fullerene OPV composites studied previously points to similar photoinduced charge separation mechanisms for these systems.

All-Polymer Solar Cells: Impact of the Length of the Branched Alkyl Side Chains on the Polymer Acceptors on the Interchain Packing and Electronic Properties in Amorphous Blends

All-polymer solar cells are attracting increasing attention because polymer acceptors present specific advantages, especially over fullerene derivatives. The length of the branched alkyl side chains on the polymer acceptors has been reported to have different impacts in the crystalline versus amorphous domains of donor/acceptor blends. Given that amorphous domains are difficult to characterize experimentally, here, molecular dynamics simulations are combined with density functional theory calculations to examine at the molecular scale the role that the side-chain length plays on the interchain packing and electronic properties. As representative examples, blends of the poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-carboxylate-2-6-diyl)] (PCE10) donor with the poly(thieno[3,4-c]1-dione-alt-3,4-difluorothiophene) (PTPD[2F]T) acceptor are discussed; two different branched side chains on the thieno[3,4-c]pyrrole-4,6-dione (TPD) moieties of the acceptor polymer are considered, that is, 2-hexyldecyl and 2-decyltetradecyl. Increasing the side-chain length is found to decrease the PTPD[2F]T backbone planarity and importantly to bring a higher extent of interactions between the side chains and their own TPD moieties, which plays a critical role in determining the interchain packings. The nature of these packings are then correlated with the following: (i) the electron-transfer rates between neighboring PTPD[2F]T chains; (ii) the energetic distribution of the interfacial charge-transfer states; and (iii) the nonradiative recombination processes from the charge-transfer states to the ground state and the associated voltage losses. Overall, our findings point to a higher electron mobility and a lower nonradiative voltage loss in the PCE10/PTPD[2F]T blends where the polymer acceptor has shorter side chains.

Anatomy of the energetic driving force for charge generation in organic solar cells

Eliminating the excess energetic driving force in organic solar cells leads to a smaller energy loss and higher device performance; hence, it is vital to understand the relation between the interfacial energetics and the photoelectric conversion efficiency. In this study, we systematically investigate 16 combinations of four donor polymers and four acceptors in planar heterojunction. The charge generation efficiency and its electric field dependence correlate with the energy difference between the singlet excited state and the interfacial charge transfer state. The threshold energy difference is 0.2 to 0.3 eV, below which the efficiency starts dropping and the charge generation becomes electric field-dependent. In contrast, the charge generation efficiency does not correlate with the energy difference between the charge transfer and the charge-separated states, indicating that the binding of the charge pairs in the charge transfer state is not the determining factor for the charge generation.

What is the Binding Energy of a Charge Transfer State in an Organic Solar Cell?

AbstractThe high efficiencies reported for organic solar cells and an almost negligible thermal activation measured for the photogeneration of charge carriers have called into question whether photoinduced interfacial charge transfer states are bound by a significant coulomb attraction, and how this can be reconciled with very low activation energies. Here, this question is addressed in a combined experimental and theoretical approach. The interfacial binding energy of a charge‐transfer state in a blend of MeLPPP:PCBM is determined by using energy resolved electrochemical impedance spectroscopy and is found to be about 0.5 eV. Temperature‐dependent photocurrent measurements on the same films, however, give an activation energy that is about one order of magnitude lower. Using analytical calculations and Monte Carlo simulation the authors illustrate how i) interfacial energetics and ii) transport topology reduce the activation energy required to separate the interfacial electron–hole pair, with about equal contributions from both effects. The activation energy, however, is not reduced by entropy, although entropy increases the overall photodissociation yield.

Simulations on photovoltaic conversion in perovskite solar cells by solving hierarchical equations of motion

Understanding the mechanism of photovoltaic conversion in perovskite solar cells (PSCs) remains a great challenge. In this work, we have simulated the charge generation, transport and recombination in PSCs under standard AM 1.5G sunlight on the basis of solving hierarchical equations of motion with model Hamiltonian. Reasonable current/voltage characteristics, power conversion efficiency (PCE), and incident photon to current conversion efficiency (IPCE) can be obtained by investigating different interaction with respect to the interfacial recombination. Moreover, it is found that the interfacial charge transfer (CT) states can greatly affect the current/voltage characteristics, which is elucidated by analyzing the density of states. Dynamics response of current to sunlight or bias is demonstrated and the intrinsic temperature effect on PCE is also discussed.

Magnetic Field Modulation of Recombination Processes in Organic Photovoltaics

Polymer–Fullerene photovoltaics have potential in small-scale power production, but low open-circuit voltages limit their efficiency. Understanding the processes affecting the charge recombination rate is key to increasing device efficiency through optimizing open-circuit voltage. Most of the polymer–fullerene systems have an intramolecular triplet exciton state lower in energy than the interfacial charge-transfer state, and its formation can provide a terminal recombination pathway that may limit device performance. We used magnetic fields to modulate intersystem crossing in a prototypical system and monitored the effect on the open-circuit voltage to infer changes in the steady-state carrier density and hence in the net recombination rate constant. We analyzed these effects using density matrix modeling and quantified the various recombination rate constants for a working device.

Spectroscopic Investigation of the Effect of Microstructure and Energetic Offset on the Nature of Interfacial Charge Transfer States in Polymer: Fullerene Blends.

Despite performance improvements of organic photovoltaics, the mechanism of photoinduced electron–hole separation at organic donor–acceptor interfaces remains poorly understood. Inconclusive experimental and theoretical results have produced contradictory models for electron–hole separation in which the role of interfacial charge-transfer (CT) states is unclear, with one model identifying them as limiting separation and another as readily dissociating. Here, polymer–fullerene blends with contrasting photocurrent properties and enthalpic offsets driving separation were studied. By modifying composition, film structures were varied from consisting of molecularly mixed polymer–fullerene domains to consisting of both molecularly mixed and fullerene domains. Transient absorption spectroscopy revealed that CT state dissociation generating separated electron–hole pairs is only efficient in the high energy offset blend with fullerene domains. In all other blends (with low offset or predominantly molecularly mixed domains), nanosecond geminate electron–hole recombination is observed revealing the importance of spatially localized electron–hole pairs (bound CT states) in the electron–hole dynamics. A two-dimensional lattice exciton model was used to simulate the excited state spectrum of a model system as a function of microstructure and energy offset. The results could reproduce the main features of experimental electroluminescence spectra indicating that electron–hole pairs become less bound and more spatially separated upon increasing energy offset and fullerene domain density. Differences between electroluminescence and photoluminescence spectra could be explained by CT photoluminescence being dominated by more-bound states, reflecting geminate recombination processes, while CT electroluminescence preferentially probes less-bound CT states that escape geminate recombination. These results suggest that apparently contradictory studies on electron–hole separation can be explained by the presence of both bound and unbound CT states in the same film, as a result of a range of interface structures.

Charge-transfer processes and carrier dynamics at the pentacene—C60 interface

Heterostructures of pentacene (PEN) and buckminsterfullerene (C60) are frequently attracting scientific interest as a well-defined small-molecule model system for the study of internal interfaces between two organic semiconductors. They are prototypical representatives forming a donor–acceptor combination for studies of fundamental optoelectronic processes in organic photovoltaics. Despite their importance in exciton dissociation, the energetics of their interfacial charge-transfer (CT) states and their microscopic excitation dynamics are not yet clarified and still being discussed. Here, we present steady-state and time-resolved photoluminescence measurements on stacked heterostructures composed of these two materials. All experiments are performed in the visible and near-infrared spectral regions as CT states are expected at energies below the fundamental electronic transitions of the respective bulk materials. A characteristic, interface-specific emission at around 1.13–1.17 eV is found, which we attribute to an interfacial CT state. Its excitation energy dependence reveals the intricate relaxation dynamics of excitons formed in both constituent materials. Moreover, the analysis of the dynamics of the C60 excitons shows that the lifetime of this state is reduced in the presence of an interface with PEN. This quenching is attributed to a long-range interaction, i.e. the relaxation of excitations into the interfacial CT state.

Non-Markovian Quantum-Classical Ratchet for Ultrafast Long-Range Electron-Hole Separation in Condensed Phases

In organic photovoltaic systems, a photogenerated molecular exciton in the donor domain dissociates into a hole and an electron at the donor-acceptor heterojunction, and subsequently separates into free charge carriers that can be extracted as photocurrents. The recombination of the once-separated electron and hole is a major loss mechanism in photovoltaic systems, which controls their performance. Hence, efficient photovoltaic systems need built-in ratchet mechanisms, namely, ultrafast charge separation and retarded charge recombination. In order to obtain insight into the internal working of the experimentally observed ultrafast long-range charge separation and protection against charge recombination, we theoretically investigate a potential ratchet mechanism arising from the combination of quantum delocalization and its destruction by performing numerically accurate quantum-dynamics calculations on a model system. We demonstrate that the non-Markovian effect originating from the slow polaron formation strongly suppresses the electron-transfer reaction back to the interfacial charge-transfer state stabilized at the donor-accepter interface and that it plays a critical role in maintaining the long-range electron-hole separation.

Gold Ultrathin Nanorods with Controlled Aspect Ratios and Surface Modifications: Formation Mechanism and Localized Surface Plasmon Resonance.

We synthesized gold ultrathin nanorods (AuUNRs) by slow reductions of gold(I) in the presence of oleylamine (OA) as a surfactant. Transmission electron microscopy revealed that the lengths of AuUNRs were tuned in the range of 5-20 nm while keeping the diameter constant (∼2 nm) by changing the relative concentration of OA and Au(I). It is proposed on the basis of time-resolved optical spectroscopy that AuUNRs are formed via the formation of small (<2 nm) Au spherical clusters followed by their one-dimensional attachment in OA micelles. The surfactant OA on AuUNRs was successfully replaced with glutathionate or dodecanethiolate by the ligand exchange approach. Optical extinction spectroscopy on a series of AuUNRs with different aspect ratios (ARs) revealed a single intense extinction band in the near-IR (NIR) region due to the longitudinal localized surface plasmon resonance (LSPR), the peak position of which is red-shifted with the AR. The NIR bands of AuUNRs with AR < 5 were blue-shifted upon the ligand exchange from OA to thiolates, in sharp contrast to the red shift observed in the conventional Au nanorods and nanospheres (diameter >10 nm). This behavior suggests that the NIR bands of thiolate-protected AuUNRs with AR < 5 are not plasmonic in nature, but are associated with a single-electron excitation between quantized states. The LSPR band was attenuated by thiolate passivation that can be explained by the direct decay of plasmons into an interfacial charge transfer state (chemical interface damping). The LSPR wavelengths of AuUNRs are remarkably longer than those of the conventional AuNRs with the same AR, demonstrating that the miniaturization of the diameter to below ∼2 nm significantly affects the optical response. The red shift of the LSPR band can be ascribed to the increase in the effective mass of electrons in AuUNRs.

The effect of PbS nanocrystal additives on the charge transfer state recombination in a bulk heterojunction blend

Abstract A persistent limitation of organic semiconductors is their low dielectric constant єr, which limits the performance of bulk heterojunction (BHJ) solar cells. One way to increase єr is to employ high-єr additives, such as PbS nanocrystals (QDs) to BHJ blends. In this work, we use the recombination of the interfacial charge transfer (CT) state as a means to study the effects of PbS nanocrystals on blends of a narrow bandgap copolymer: poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1- b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), and phenyl-C61-butyric acid methyl ester (PCBM). We show that at low dilution levels (0.25% - 0.75% by weight), there is a decrease in the relative weight of the CT recombination lifetime (longer decay component); suggesting that there is an increase in the local єr of the ternary blend.

Control of Geminate Recombination by the Material Composition and Processing Conditions in Novel Polymer: Nonfullerene Acceptor Photovoltaic Devices.

Herein, we report on the charge dynamics of photovoltaic devices based on two novel small-molecule nonfullerene acceptors featuring a central ketone unit. Using ultrafast near-infrared spectroscopy with optical and photocurrent detection methods, we identify one of the key loss channels in the devices as geminate recombination (GR) of interfacial charge transfer states (CTSs). We find that the magnitude of GR is highly sensitive to the choice of solvent and annealing conditions. Interestingly, regardless of these processing conditions, the same lifetime for GR (∼130 ps) is obtained by both detection methods upon decomposing the complex broadband transient optical spectra, suggesting this time scale is inherent and independent of morphology. These observations suggest that the CTSs in the studied material blends are mostly strongly bound, and that charge generation from these states is highly inefficient. We further rationalize our results by considering the impact of the processing on the morphology of the mixed donor and acceptor domains and discuss the potential consequences of the early charge dynamics on the performance of emerging nonfullerene photovoltaic devices. Our results demonstrate that careful choice of processing conditions enables enhanced exciton harvesting and suppression of GR by more than 3 orders of magnitude.

Thousand-atom ab initio calculations of excited states at organic/organic interfaces: toward first-principles investigations of charge photogeneration.

Predicting electronically excited states across electron-donor/electron acceptor interfaces is essential for understanding the charge photogeneration process in organic solar cells. However, organic solar cells are large and disordered systems, and their excited states cannot be easily accessed by conventional quantum chemistry approaches. Moreover, a large number of excited states must be obtained to fully understand the charge separation mechanism. Recently, we have developed a novel fragment-based excited state method which can efficiently calculate a large number of states in molecular aggregates. In this article, we demonstrate the large-scale excited-state calculations by investigating interfacial charge transfer (ICT) states across the electron-donor/electron acceptor interfaces. As the model systems, we considered the face-on and edge-on configurations of pentacene/C60 bilayer heterojunction structures. These model structures contain approximately 1.8 × 105 atoms, and their local interface regions containing 2000 atoms were treated quantum mechanically, embedded in the electrostatic potentials from the remaining parts. Therefore, the charge delocalization effect, structural disorder, and the resulting heterogeneous electrostatic and polarizable environments were taken into account in the excited-state calculations. The computed energies of the low-lying ICT states are in reasonable agreement with experimental estimates. By comparing the edge-on and face-on configurations of the pentacene/C60 interfaces, we discuss the influence of interfacial morphologies on the energetics and charge delocalization of ICT states. In addition, we present the detailed characterization of excited states and highlight the importance of hybridization effects between pentacene excited states and ICT states. The large-scale ab initio calculations for the interface systems enabled the exploration of the ICT states, leading to first-principles investigation of the charge separation mechanism in organic solar cells.

Origin of Charge Separation at Organic Photovoltaic Heterojunctions: A Mesoscale Quantum Mechanical View

The high efficiency of charge generation within organic photovoltaic blends apparently contrasts with the strong "classical" attraction between newly formed electron-hole pairs. Several factors have been identified as possible facilitators of charge dissociation, such as quantum mechanical coherence and delocalization, structural and energetic disorder, built-in electric fields, nanoscale intermixing of the donor and acceptor components of the blends. Our mesoscale quantum-chemical model allows an unbiased assessment of their relative importance, through excited-state calculations on systems containing thousands of donor and acceptor sites. The results on several model heterojunctions confirm that the classical model severely overestimates the binding energy of the electron-hole pairs, produced by vertical excitation from the electronic ground state. Using physically sensible parameters for the individual materials, we find that the quantum mechanical energy difference between the lowest interfacial charge transfer states and the fully separated electron and hole is of the order of the thermal energy.