The impact of driving force on electron transfer rates in photovoltaic donor-acceptor blends

Understanding and Suppressing Non-Radiative Recombination Losses in Non-Fullerene Organic Solar Cells

Organic solar cells benefit from non-fullerene acceptors (NFA) due to their high absorption coefficients, tunable frontier energy levels, and optical gaps, as well as their relatively high luminescence quantum efficiencies as compared to fullerenes. Those merits result in high yields of charge generation at a low or negligible energetic offset at the donor/NFA heterojunction, with efficiencies over 19% achieved for single-junction devices. Pushing this value significantly over 20% requires an increase in open-circuit voltage, which is currently still well below the thermodynamic limit. This can only be achieved by reducing non-radiative recombination, and hereby increasing the electroluminescence quantum efficiency of the photo-active layer. Here, current understanding of the origin of non-radiative decay, as well as an accurate quantification of the associated voltage losses are summarized. Promising strategies for suppressing these losses are highlighted, with focus on new material design, optimization of donor-acceptor combination, and blend morphology. This review aims at guiding researchers in their quest to find future solar harvesting donor-acceptor blends, which combine a high yield of exciton dissociation with a high yield of radiative free carrier recombination and low voltage losses, hereby closing the efficiency gap with inorganic and perovskite photovoltaics.

Correlating Charge Transfer Dynamics with Interfacial Trap States in High-Efficiency Organic Solar Cells

The charge transfer between the donor and acceptor determines the photogenerated carrier density in organic solar cells. However, a fundamental understanding regarding the charge transfer at donor/acceptor interfaces with high-density traps has not been fully addressed. Herein, a general correlation between trap densities and charge transfer dynamics is established by adopting a series of high-efficiency organic photovoltaic blends. It is found that the electron transfer rates are reduced with increased trap densities, while the hole transfer rates are independent of trap states. The local charges captured by traps can induce potential barrier formation around recombination centers, leading to the suppression of electron transfer. For the hole transfer process, the thermal energy provides a sufficient driving force, which ensures an efficient transfer rate. As a result, a 17.18% efficiency is obtained for PM6:BTP-eC9-based devices with the lowest interfacial trap densities. This work highlights the importance of interfacial traps in charge transfer processes and proposes an underlying insight into the charge transfer mechanism at nonideal interfaces in organic heterostructures.

Quantum Mechanical Assessment of Optimal Photovoltaic Conditions in Organic Solar Cells

Recombination losses contribute to reducing J_SC, V_OC, and the fill factor of organic solar cells. Recent advances in non-fullerene organic photovoltaics have shown, nonetheless, that efficient charge generation can occur under small energetic driving forces (ΔE_DA) and low recombination losses. To shed light on this issue, we set up a coarse-grained open quantum mechanical model for investigating the charge generation dynamics subject to various energy loss mechanisms. The influences of energetic driving force, Coulomb interaction, vibrational disorder, geminate recombination, temperature, and external bias are included in the analysis of the optimal photovoltaic conditions for charge carrier generation. The assessment reveals that the overall energy losses are not only minimized when ΔE_DA approaches the effective reorganization energy at the interface but also become insensitive to temperature and electric field variations. It is also observed that a moderate reverse bias reduces geminate recombination losses significantly at vanishing driving forces, where the charge generation is strongly affected by temperature.

Efficient Hot Electron Capture in CuPc/MoSe2 Heterostructure Assisted by Intersystem Crossing

Efficient hot electron extraction is a promising approach to develop photovoltaic devices that exceed the Shockley-Queisser limit. However, experimental evidence of hot electron harvesting employing an organic-inorganic interface is still elusive. Here, we reveal the hot electron dynamics at a CuPc/MoSe₂ interface using steady-state spectroscopy and transient absorption spectroscopy. A hot electron transfer efficiency of greater than 78% from MoSe₂ to CuPc is observed, comparable to that achieved in quantum dot hybrid systems. The mechanism is proposed as follows: the photogenerated hot electrons in MoSe₂ transfer to CuPc and form singlet charge transfer states, which subsequently transform into triplet charge transfer states assisted by the rapid intersystem crossing, inhibiting back-donation of electrons and facilitating exciton dissociation into CuPc polarons with a nanosecond lifetime. Our results demonstrate that the intersystem crossing of the hybrid electronic state at organic-inorganic interfaces may serve as a scheme to enable efficient hot electron extraction in photovoltaic devices.

Direct Observation of Increased Free Carrier Generation Owing to Reduced Exciton Binding Energies in Polymerized Small-Molecule Acceptors

Energy loss caused by exciton binding energy (E_b) has become a key factor that restricts further advancement of organic solar cells (OSCs). Herein, we used transient mid-IR spectroscopy to study direct photogeneration of free charge carriers in small-molecule acceptors (SMAs) Y6 and IDIC as well as polymerized SMAs (PSMAs) PYFT and PZ1. We found that free carrier concentration is higher in PSMAs than in their corresponding SMAs, indicating reduced exciton E_b, which is then confirmed by ultraviolet photoelectron spectroscopy, low-energy inverse photoemission spectroscopy, and film absorption spectra measurements. The measured E_b values of PYFT and PZ1 are 0.24 and 0.37 eV, respectively, smaller than those of Y6 (0.32 eV) and IDIC (0.47 eV). This work not only provides a method to directly monitor the photogenerated free carriers in OSC materials but also demonstrates that polymerization is an effective strategy to reduce the E_b, which is crucial to decrease the energy losses in high-performance OSCs.

Molecular Insight into Efficient Charge Generation in Low-Driving-Force Nonfullerene Organic Solar Cells

ConspectusFor organic solar cells (OSCs), charge generation at the donor/acceptor interfaces is regarded as a two-step process: driven by the interfacial energy offsets, the excitons produced by light absorption are first dissociated into the charge-transfer (CT) states, and then the CT states are further separated into free charge carriers of holes and electrons by overcoming their Coulomb attraction. Meanwhile, the CT states can recombine through radiative and nonradiative decay. Owing to the emergence of narrow-band-gap A-D-A small-molecule acceptors, nonfullerene (NF) OSCs have developed rapidly in recent years and the power conversion efficiencies (PCEs) surpass 18% now. The great achievement can be attributed to the high-yield charge generation under low exciton dissociation (ED) driving forces, which ensures both high photocurrent and small voltage loss. However, it is traditionally believed that a considerable driving force (e.g., at least 0.3 eV in fullerene-based OSCs) is essential to provide excess energy for the CT states to achieve efficient charge separation (CS). Therefore, a fundamental question open to the community is how the excitons split into free charge carriers so efficiently under low driving forces in the state-of-the-art NF OSCs.In this Account, we summarize our recent theoretical advances on the charge generation mechanisms in the low-driving-force NF OSCs. First, the A-D-A acceptors are found to dock with the D-A copolymer or A-D-A small-molecule donors mainly via local π-π interaction between their electron-withdrawing units, and such interfacial geometries can provide sufficient electronic couplings, thus ensuring fast ED. Second, the polarization energies of holes and electrons are enhanced during CS, which is beneficial to reduce the CS energy barrier and even leads to barrierless CS in the OSCs based on fluorinated A-D-A acceptors. Moreover, the exciton binding energies (E_b) are substantially decreased by the strong polarization of charge carriers for the A-D-A acceptors; especially for the Y6 system with three-dimensional molecular packing structures, the remarkable small E_b can enable direct photogeneration of free charge carriers. Accordingly, the excess energy becomes unnecessary for CS in the state-of-the-art NF OSCs. Third, to simultaneously decrease the driving force and suppress charge recombination via the triplet channel, it is imperative to reduce the singlet-triplet energy difference (ΔE_ST) of the narrow-band-gap A-D-A acceptors. Importantly, the intermolecular end-group π-π stacking is demonstrated to effectively decrease the ΔE_ST while keeping strong light absorption. Finally, hybridization of the CT states with local excitation can be induced by small interfacial energy offset. Such hybridization will result in direct population of thermalized CT states upon light absorption and a significant increase of luminescence quantum efficiency, which is beneficial to concurrently promote CS and reduce nonradiative voltage loss. We hope this Account contributes to the molecular understanding of the mechanisms of efficient charge generation with low driving forces and would be helpful for further improving the performance of organic photovoltaics in the future.

Photoinduced Charge Transfer and Recombination Dynamics in Star Nonfullerene Organic Solar Cells

Nonfullerene acceptors (NFAs) are regarded as star candidates for efficient organic solar cells with power conversion efficiency (PCE) over 18%. In contrast to the rapid development of NFA materials, however, the underlying excited-state dynamics which fundamentally govern the device performance remains unclear. In this Perspective, we discuss recent advances and provide our insights on photoinduced charge transfer and combination dynamics in NFA-based organic solar cells (OSCs), including the biphasic hole-transfer process and its correlation with morphology, the role of driving force and Marcus normal region behavior on interfacial hole-transfer properties, and charge recombination energy loss by NFA triplet formation. We also discuss our understanding of how to control the charge-transfer and recombination processes by phase morphology and molecular design to improve OSC performance. Finally, we suggest a few research directions, including the interfacial charge transfer and separation mechanism, the origin of low fill factor, and complex excited-state dynamics in multicomponent OSCs.

Narrowband organic photodetectors - towards miniaturized, spectroscopic sensing

Omnipresent quality monitoring in food products, blood-oxygen measurement in lightweight conformal wrist bands, or data-driven automated industrial production: Innovation in many fields is being empowered by sensor technology. Specifically, organic photodetectors (OPDs) promise great advances due to their beneficial properties and low-cost production. Recent research has led to rapid improvement in all performance parameters of OPDs, which are now on-par or better than their inorganic counterparts, such as silicon or indium gallium arsenide photodetectors, in several aspects. In particular, it is possible to directly design OPDs for specific wavelengths. This makes expensive and bulky optical filters obsolete and allows for miniature detector devices. In this review, recent progress of such narrowband OPDs is systematically summarized covering all aspects from narrow-photo-absorbing materials to device architecture engineering. The recent challenges for narrowband OPDs, like achieving high responsivity, low dark current, high response speed, and good dynamic range are carefully addressed. Finally, application demonstrations covering broadband and narrowband OPDs are discussed. Importantly, several exciting research perspectives, which will stimulate further research on organic-semiconductor-based photodetectors, are pointed out at the very end of this review.

Short and long-range electron transfer compete to determine free-charge yield in organic semiconductors

Understanding how Frenkel excitons efficiently split to form free-charges in low-dielectric constant organic semiconductors has proven challenging, with many different models proposed in recent years to explain this phenomenon. Here, we present evidence that a simple model invoking a modest amount of charge delocalization, a sum over the available microstates, and the Marcus rate constant for electron transfer can explain many seemingly contradictory phenomena reported in the literature. We use an electron-accepting fullerene host matrix dilutely sensitized with a series of electron donor molecules to test this hypothesis. The donor series enables us to tune the driving force for photoinduced electron transfer over a range of 0.7 eV, mapping out normal, optimal, and inverted regimes for free-charge generation efficiency, as measured by time-resolved microwave conductivity. However, the photoluminescence of the donor is rapidly quenched as the driving force increases, with no evidence for inverted behavior, nor the linear relationship between photoluminescence quenching and charge-generation efficiency one would expect in the absence of additional competing loss pathways. This behavior is self-consistently explained by competitive formation of bound charge-transfer states and long-range or delocalized free-charge states, where both rate constants are described by the Marcus rate equation. Moreover, the model predicts a suppression of the inverted regime for high-concentration blends and efficient ultrafast free-charge generation, providing a mechanistic explanation for why Marcus-inverted-behavior is rarely observed in device studies.

Intrinsic efficiency limits in low-bandgap non-fullerene acceptor organic solar cells

In bulk heterojunction (BHJ) organic solar cells (OSCs) both the electron affinity (EA) and ionization energy (IE) offsets at the donor-acceptor interface should equally control exciton dissociation. Here, we demonstrate that in low-bandgap non-fullerene acceptor (NFA) BHJs ultrafast donor-to-acceptor energy transfer precedes hole transfer from the acceptor to the donor and thus renders the EA offset virtually unimportant. Moreover, sizeable bulk IE offsets of about 0.5 eV are needed for efficient charge transfer and high internal quantum efficiencies, since energy level bending at the donor-NFA interface caused by the acceptors' quadrupole moments prevents efficient exciton-to-charge-transfer state conversion at low IE offsets. The same bending, however, is the origin of the barrier-less charge transfer state to free charge conversion. Our results provide a comprehensive picture of the photophysics of NFA-based blends, and show that sizeable bulk IE offsets are essential to design efficient BHJ OSCs based on low-bandgap NFAs.

Ultrafast Hole Transfer and Carrier Transport Controlled by Nanoscale-Phase Morphology in Nonfullerene Organic Solar Cells

Nonfullerene acceptors (NFAs) have attracted great attention in high-efficiency organic solar cells (OSCs). While the effect of molecular properties including structures and energetics on charge transfer has been extensively investigated, the effect of macroscopic-phase properties is yet to be revealed. Here we have performed a correlation study of the nanoscale-phase morphology on the photoexcited hole transfer (HT) process and photovoltaic performance by combining ultrafast spectroscopy with high temporal resolution and photo-induced force microscopy (PiFM) with high spatial and chemical resolution. In PM6/IT-4F, we observe biphasic HT behavior with a minor ultrafast (<100 fs) interfacial process and a major diffusion-mediated HT process until ∼100 ps, which depends strongly on phase segregation. Because of the interplay between charge transfer and transport, a compromised domain size of 20-30 nm for NFAs shows the best performance. This study highlights the critical role of phase morphology in high-efficiency OSCs.

Sub-picosecond charge-transfer at near-zero driving force in polymer:non-fullerene acceptor blends and bilayers

Organic photovoltaics based on non-fullerene acceptors (NFAs) show record efficiency of 16 to 17% and increased photovoltage owing to the low driving force for interfacial charge-transfer. However, the low driving force potentially slows down charge generation, leading to a tradeoff between voltage and current. Here, we disentangle the intrinsic charge-transfer rates from morphology-dependent exciton diffusion for a series of polymer:NFA systems. Moreover, we establish the influence of the interfacial energetics on the electron and hole transfer rates separately. We demonstrate that charge-transfer timescales remain at a few hundred femtoseconds even at near-zero driving force, which is consistent with the rates predicted by Marcus theory in the normal region, at moderate electronic coupling and at low re-organization energy. Thus, in the design of highly efficient devices, the energy offset at the donor:acceptor interface can be minimized without jeopardizing the charge-transfer rate and without concerns about a current-voltage tradeoff.

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%.

Impact of the donor polymer on recombination via triplet excitons in a fullerene-free organic solar cell

The greater chemical tunability of non-fullerene acceptors enables fine-tuning of the donor-acceptor energy level offsets, a promising strategy towards increasing the open-circuit voltage in organic solar cells. Unfortunately, this approach could open an additional recombination channel for the charge-transfer (CT) state via a lower-lying donor or acceptor triplet level. In this work we investigate such electron and hole back-transfer mechanisms in fullerene-free solar cells incorporating the novel molecular acceptor 2,4-diCN-Ph-DTTzTz. The transition to the low-driving force regime is studied by comparing blends with well-established donor polymers P3HT and MDMO-PPV, which allows for variation of the energetic offsets at the donor-acceptor interface. Combining various optical spectroscopic techniques, the CT process and subsequent triplet formation are systematically investigated. Although both back-transfer mechanisms are found to be energetically feasible in both blends, markedly different triplet-mediated recombination processes are observed for the two systems. The kinetic suppression of electron back-transfer in the blend with P3HT suggests that energy losses due to triplet formation on the polymer can be avoided, regardless of favorable energetic alignment.

Rational Tuning of Molecular Interaction and Energy Level Alignment Enables High-Performance Organic Photovoltaics

The performance of organic photovoltaics (OPVs) has rapidly improved over the past years. Recent work in material design has primarily focused on developing near-infrared nonfullerene acceptors with broadening absorption that pair with commercialized donor polymers; in the meanwhile, the influence of the morphology of the blend film and the energy level alignment on the efficiency of charge separation needs to be synthetically considered. Herein, the selection rule of the donor/acceptor blend is demonstrated by rationally considering the molecular interaction and energy level alignment, and highly efficient OPV devices using both-fluorinated or both-nonfluorinated donor/acceptor blends are realized. With the enlarged absorption, ideal morphology, and efficient charge transfer, the devices based on the PBDB-T-F/Y1-4F blend and PBDB-T-F/Y6 exhibit champion power conversion efficiencies as high as 14.8% and 15.9%, respectively.

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.

Local Excitation/Charge-Transfer Hybridization Simultaneously Promotes Charge Generation and Reduces Nonradiative Voltage Loss in Nonfullerene Organic Solar Cells

High power conversion efficiencies in state-of-the-art nonfullerene organic solar cells (NF OSCs) call for elucidation of the underlying working mechanisms of both high photocurrent densities and low nonradiative voltage losses under small energy offsets. Here, to address this fundamental issue, we have assessed the nature of interfacial charge-transfer (CT) states in a representative small-molecule NF OSC (DRTB-T:IT-4F) by time-dependent density functional theory calculations. The calculated results point to the fact that the CT states can borrow considerable oscillator strengths from the energy-close local excitation (LE) states or be fully hybridized with these LE states by molecular aggregation at the donor-acceptor interfaces. The LE/CT hybridization can promote charge generation by direct population of thermalized CT or LE/CT states under illumination. At the same time, the increased oscillator strengths of the lowest CT state will improve the luminescence quantum efficiencies and thus reduce nonradiative voltage losses. Our work suggests that it is crucial to tune the LE/CT hybridization by optimization of the donor and acceptor molecular and interfacial structures to further improve the NF OSC performance.

Unveiling Excitonic Dynamics in High-Efficiency Nonfullerene Organic Solar Cells to Direct Morphological Optimization for Suppressing Charge Recombination

Nonfullerene acceptors (NFAs)-based organic solar cells (OSCs) have recently drawn considerable research interests; however, their excitonic dynamics seems quite different than that of fullerene acceptors-based devices and remains to be largely explored. A random terpolymer of PBBF11 to pair with a paradigm NFA of 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC) such that both complementary optical absorption and very small offsets of both highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels are acquired is designed and synthesized. Despite the small energy offsets, efficient electron/hole transfer between PBBF11 and ITIC is both clearly observed from steady-state photoluminescence and transient absorption spectra and also supported by the measured low exciton binding energy in ITIC. Consequently, the PBBF11:ITIC-based OSCs afford an encouraging power conversion efficiency (PCE) of 10.02%. Although the good miscibility of PBBF11 and ITIC induces a homogenous blend film morphology, it causes severe charge recombination. The fullerene acceptor of PC71BM with varying loading ratios is therefore added to modulate film morphology to effectively reduce the charge recombination. As a result, the optimal OSCs based on PBBF11:ITIC:PC71BM yield a better PCE of 11.4% without any additive or annealing treatment.

Electronic couplings and rates of excited state charge transfer processes at poly(thiophene-co-quinoxaline)–PC71BM interfaces: two- versus multi-state treatments

Multi-state effects should be considered when calculating electronic couplings at local polymer–fullerene interfaces with the non-tuned and optimally tuned long-range corrected functionals.

Optimum driving energy for achieving balanced open-circuit voltage and short-circuit current density in organic bulk heterojunction solar cells

Organic bulk heterojunction solar cells generally suffer from a trade-off between the open circuit voltage (Voc) and the short circuit current density (Jsc) under a given donor/acceptor (D/A) interfacial energetic offset (or the so-called driving force). Here we theoretically investigate the optimum driving energy required for achieving the balanced Jsc and Voc simultaneously. To this end, the Jscversus the driving force ΔE curves are calculated under two different charge separation mechanisms by employing the drift-diffusion method. For the Marcus incoherent mechanism, the curve features a high plateau in a broad range of ΔE starting from 0.2 eV, which is due to the accumulation of undissociated excitons within their lifetime and signifies the possibility of obtaining a sizable Jsc under a ΔE value much smaller than the reorganization energy. After incorporating both the electron and hole transfer pathways into the device model, the calculated J-V curves are comparable to experimentally measured ones foractual blended systems of different driving forces. For the coherent mechanism, it is demonstrated that the maximum Jsc can also be achieved under the ΔE of 0.2 eV if a large proportion of the high-lying delocalized states are harvested through tuning the density of states for the charge transfer excitons to reduce the sub-gap states. This theoretical work revealed quantitatively the relationship between the interfacial energy offsets and device performance, and also provides some guidelines for identifying the macroscopic features of the actual charge separation mechanisms in bulk heterojunction solar cells.

Revealing the Importance of Energetic and Entropic Contributions to the Driving Force for Charge Photogeneration

Despite significant recent progress, much about the mechanism for charge photogeneration in organic photovoltaics remains unknown. Here, we use conjugated block copolymers as model systems to examine the effects of energetic and entropic driving forces in organic donor-acceptor materials. The block copolymers are designed such that an electron donor block and an electron acceptor block are covalently linked, embedding a donor-acceptor interface within the molecular structure. This enables model studies in solution where processes occurring between one donor and one acceptor are examined. First, energy levels and dielectric constants that govern the driving force for charge transfer are systematically tuned and charge transfer within individual block copolymer chains is quantified. Results indicate that in isolated chains, a significant driving force of ∼0.3 eV is necessary to facilitate significant exciton dissociation to charge-transfer states. Next, block copolymers are cast into films, allowing for intermolecular interactions and charge delocalization over multiple chains. In the solid state, charge transfer is significantly enhanced relative to isolated block copolymer chains. Using Marcus Theory, we conclude that changes in the energetic driving force alone cannot explain the increased efficiency of exciton dissociation to charge-transfer states in the solid state. This implies that increasing the number of accessible states for charge transfer introduces an entropic driving force that can play an important role in the charge-generation mechanism of organic materials, particularly in systems where the excited state energy level is close to that of the charge-transfer state.

Disentangling overlapping high-field EPR spectra of organic radicals: Identification of light-induced polarons in the record fullerene-free solar cell blend PBDB-T:ITIC

We present a combined high-field EPR and DFT study of light-induced radicals in the bulk heterojunction blend of PBDB-T:ITIC, currently one of the highest efficiency non-fullerene donor:acceptor combinations in organic photovoltaics. We demonstrate two different approaches for disentangling the strongly overlapping high-field EPR spectra of the positive and negative polarons after charge separation: (1) relaxation-filtered field-swept EPR based on the difference in T₁ spin-relaxation times and (2) field-swept EDNMR-induced EPR by exploiting the presence of ¹⁴N hyperfine couplings in only one of the radical species, the small molecule acceptor radical. The approach is validated by light-induced EPR spectra on related blends and the spectral assignment is underpinned by DFT computations. The broader applicability of the spectral disentangling methods is discussed.

Hot kinetic model as a guide to improve organic photovoltaic materials

The model yields that the most promising ways to increase the OSC performance are decreasing the reorganization energy, increasing the dielectric permittivity and enhancing the charge delocalization.

Impact of interfacial molecular orientation on radiative recombination and charge generation efficiency

A long standing question in organic electronics concerns the effects of molecular orientation at donor/acceptor heterojunctions. Given a well-controlled donor/acceptor bilayer system, we uncover the genuine effects of molecular orientation on charge generation and recombination. These effects are studied through the point of view of photovoltaics—however, the results have important implications on the operation of all optoelectronic devices with donor/acceptor interfaces, such as light emitting diodes and photodetectors. Our findings can be summarized by two points. First, devices with donor molecules face-on to the acceptor interface have a higher charge transfer state energy and less non-radiative recombination, resulting in larger open-circuit voltages and higher radiative efficiencies. Second, devices with donor molecules edge-on to the acceptor interface are more efficient at charge generation, attributed to smaller electronic coupling between the charge transfer states and the ground state, and lower activation energy for charge generation.

Molecular orientation profoundly affects the performance of donor-acceptor heterojunctions, whilst it has remained challenging to investigate the detail. Using a controllable interface, Ran et al. show that the edge-on geometries improve charge generation at the cost of non-radiative recombination loss.

Absorption Tails of Donor:C60 Blends Provide Insight into Thermally Activated Charge-Transfer Processes and Polaron Relaxation

In disordered organic semiconductors, the transfer of a rather localized charge carrier from one site to another triggers a deformation of the molecular structure quantified by the intramolecular relaxation energy. A similar structural relaxation occurs upon population of intermolecular charge-transfer (CT) states formed at organic electron donor (D)-acceptor (A) interfaces. Weak CT absorption bands for D-A complexes occur at photon energies below the optical gaps of both the donors and the C₆₀ acceptor as a result of optical transitions from the neutral ground state to the ionic CT state. In this work, we show that temperature-activated intramolecular vibrations of the ground state play a major role in determining the line shape of such CT absorption bands. This allows us to extract values for the relaxation energy related to the geometry change from neutral to ionic CT complexes. Experimental values for the relaxation energies of 20 D:C₆₀ CT complexes correlate with values calculated within density functional theory. These results provide an experimental method for determining the polaron relaxation energy in solid-state organic D-A blends and show the importance of a reduced relaxation energy, which we introduce to characterize thermally activated CT processes.

Light Harvesting for Organic Photovoltaics

The field of organic photovoltaics has developed rapidly over the last 2 decades, and small solar cells with power conversion efficiencies of 13% have been demonstrated. Light absorbed in the organic layers forms tightly bound excitons that are split into free electrons and holes using heterojunctions of electron donor and acceptor materials, which are then extracted at electrodes to give useful electrical power. This review gives a concise description of the fundamental processes in photovoltaic devices, with the main emphasis on the characterization of energy transfer and its role in dictating device architecture, including multilayer planar heterojunctions, and on the factors that impact free carrier generation from dissociated excitons. We briefly discuss harvesting of triplet excitons, which now attracts substantial interest when used in conjunction with singlet fission. Finally, we introduce the techniques used by researchers for characterization and engineering of bulk heterojunctions to realize large photocurrents, and examine the formed morphology in three prototypical blends.

Organic chemistry meets polymers, nanoscience, therapeutics and diagnostics

The atom-by-atom control provided by synthetic organic chemistry presents a means of generating new functional nanomaterials with great precision. Bringing together these two very disparate skill sets is, however, quite uncommon. This autobiographical review provides some insight into how my program evolved, as well as giving some idea of where we are going.