Limits for Recombination in a Low Energy Loss Organic Heterojunction

Exploring the photovoltaic properties of promising non-fullerene acceptors with different degrees of asymmetry due to halogenations of terminal groups

Over the past few years, the strategy of asymmetric modification has become popular for designing new photovoltaic materials because it can effectively improve optoelectronic performance and morphology, therefore power conversion efficiency (PCE). However, how the halogenations (to further change asymmetry) of terminal groups (TGs) of an asymmetric small-molecule non-fullerene acceptor (Asy-SM-NFA) influence optoelectronic properties is still not very clear. In this work, we have selected a promising Asy-SM-NFA IDTBF (the OSC based on it has a PCE of 10.43 %), exacerbated the asymmetry through fluorinations of TGs, and finally designed six new molecules. Based on density functional theory (DFT) and time-dependent DFT calculations, we systematically examine how the changed asymmetry impacts the optoelectronic properties. We find that the halogenations of TGs may significantly affect the molecular planarity, dipole moment, electrostatic potential, exciton binding energy, energy loss, and absorption spectrum. And the results show that newly designed BR-F1 and IM-mF (m = 1,3, and 4) are potential Asy-SM-NFAs because they all have enhanced absorption spectra in the visible region. Therefore, we provide a meaningful direction for the design of asymmetric NFA.

The principles, design and applications of fused-ring electron acceptors

Fused-ring electron acceptors (FREAs) have a donor-acceptor-donor structure comprising an electron-donating fused-ring core, electron-accepting end groups, π-bridges and side chains. FREAs possess beneficial features, such as feasibility to tailor their structures, high property tunability, strong visible and near-infrared light absorption and excellent n-type semiconducting characteristics. FREAs have initiated a revolution to the field of organic solar cells in recent years. FREA-based organic solar cells have achieved unprecedented efficiencies, over 20%, which breaks the theoretical efficiency limit of traditional fullerene acceptors (~13%), and boast potential operational lifetimes approaching 10 years. Based on the original studies of FREAs, a variety of new structures, mechanisms and applications have flourished. In this Review, we introduce the fundamental principles of FREAs, including their structures and inherent electronic and physical properties. Next, we discuss the way in which the properties of FREAs can be modulated through variations to the electronic structure or molecular packing. We then present the current applications and consider the future areas that may benefit from developments in FREAs. Finally, we conclude with the position of FREA chemistry, reflecting on the challenges and opportunities that may arise in the future of this burgeoning field.

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.

Designing Potential Donor Materials Based on DRCN5T with Halogen Substitutions: A DFT/TDDFT Study

Experimental researchers have found that the organic solar cell (OSC) based on DRCN5T (an oligothiophene) possesses excellent power conversion efficiency (PCE) of 10.1%. However, to date, there have been few studies about halogenation of DRCN5T, and its effects on photovoltaic properties of halogenated DRCN5T are still not clear. In the present work, we first perform benchmark calculations and effectively reproduce experimental results. Then, eight halogenated DRCN5T molecules are designed and investigated theoretically by using density functional theory (DFT) and time-dependent DFT. The dipole moments, frontier molecular orbital energies, absorption spectra, exciton binding energy (E_b), singlet–triplet energy gap (ΔE_ST), and electrostatic potential (ESP) of these molecules, and the estimated open circuit voltages (V_OCs) of the OSCs with PC₇₁BM as acceptor are presented. We find that (1) generally, halogen substitutions would increase V_OC; (2) E_b rises with more fluorine substitutions, but for Cl and Br substitutions, E_b increases firstly and then drops; (3) ΔE_ST keeps increasing with more halogen substitutions; (4) except for Br substitutions, the averaged ESP arises along with more halogen substitutions; (5) the absorption strength of UV–Vis spectra of DRCN5T2F, DRCN5T4F, DRCN5T6F, and DRCN5T2Cl in the visible region is enhanced with respect to DRCN5T. Based on these results, overall, DRCN5T2Cl, DRCN5T4F, and DRCN5T6F may be promising donors.

Direct Plasmonic Solar Cell Efficiency Dependence on Spiro-OMeTAD Li-TFSI Content

The proliferation of the internet of things (IoT) and other low-power devices demands the development of energy harvesting solutions to alleviate IoT hardware dependence on single-use batteries, making their deployment more sustainable. The propagation of energy harvesting solutions is strongly associated with technical performance, cost and aesthetics, with the latter often being the driver of adoption. The general abundance of light in the vicinity of IoT devices under their main operation window enables the use of indoor and outdoor photovoltaics as energy harvesters. From those, highly transparent solar cells allow an increased possibility to place a sustainable power source close to the sensors without significant visual appearance. Herein, we report the effect of hole transport layer Li-TFSI dopant content on semi-transparent, direct plasmonic solar cells (DPSC) with a transparency of more than 80% in the 450-800 nm region. The findings revealed that the amount of oxidized spiro-OMeTAD (spiro⁺TFSI^-) significantly modulates the transparency, effective conductance and conditions of device performance, with an optimal performance reached at around 33% relative concentration of Li-TFSI concerning spiro-OMeTAD. The Li-TFSI content did not affect the immediate charge extraction, as revealed by an analysis of electron-phonon lifetime. Hot electrons and holes were injected into the respective layers within 150 fs, suggesting simultaneous injection, as supported by the absence of hysteresis in the I-V curves. The spiro-OMeTAD layer reduces the Au nanoparticles' reflection/backscattering, which improves the overall cell transparency. The results show that the system can be made highly transparent by precise tuning of the doping level of the spiro-OMeTAD layer with retained plasmonics, large optical cross-sections and the ultrathin nature of the devices.

The role of charge recombination to triplet excitons in organic solar cells

The use of non-fullerene acceptors (NFAs) in organic solar cells has led to power conversion efficiencies as high as 18%¹. However, organic solar cells are still less efficient than inorganic solar cells, which typically have power conversion efficiencies of more than 20%². A key reason for this difference is that organic solar cells have low open-circuit voltages relative to their optical bandgaps³, owing to non-radiative recombination⁴. For organic solar cells to compete with inorganic solar cells in terms of efficiency, non-radiative loss pathways must be identified and suppressed. Here we show that in most organic solar cells that use NFAs, the majority of charge recombination under open-circuit conditions proceeds via the formation of non-emissive NFA triplet excitons; in the benchmark PM6:Y6 blend⁵, this fraction reaches 90%, reducing the open-circuit voltage by 60 mV. We prevent recombination via this non-radiative channel by engineering substantial hybridization between the NFA triplet excitons and the spin-triplet charge-transfer excitons. Modelling suggests that the rate of back charge transfer from spin-triplet charge-transfer excitons to molecular triplet excitons may be reduced by an order of magnitude, enabling re-dissociation of the spin-triplet charge-transfer exciton. We demonstrate NFA systems in which the formation of triplet excitons is suppressed. This work thus provides a design pathway for organic solar cells with power conversion efficiencies of 20% or more.

Factors That Prevent Spin-Triplet Recombination in Non-fullerene Organic Photovoltaics

Managing the dynamics of spin-triplet electronic states is crucial for achieving high-performance organic photovoltaics. Here we show that the replacement of fullerene with non-fullerene acceptor (NFA) molecules leads to suppression of triplet recombination and thus more efficient charge generation. This indicates that the relaxation of charges to the local triplet exciton state, although energetically allowed, is outcompeted by the thermally activated separation of interfacial charge-transfer excitons (CTEs) in the NFA-based system. By rationalizing our results with Marcus theory, we propose that triplet recombination in the fullerene system is driven by the small energy difference and strong electronic couplings between the CTE state and the lowest-lying triplet exciton state (T₁) of fullerene acceptor molecules. In contrast, the large energy difference and small electronic couplings between these states in the NFA-based blends lead to sufficiently slow triplet relaxation rate compared to the charge separation rate (≪10¹⁰ s^-1), thus preventing triplet recombination.

Heptacyclic S,N-Heteroacene-Based Near-Infrared Nonfullerene Acceptor Enables High-Performance Organic Solar Cells with Small Highest Occupied Molecular Orbital Offsets

The reduction of energy offsets between donors and acceptors is a direct way to improve the open-circuit voltage (V_OC) and overall performance of organic solar cells (OSCs). In this work, two nonfullerene acceptors (NFAs) (BDTBO-4F and BDTBO-4Cl) were synthesized, which were composed of a heptacyclic S,N-heteroacene core and terminal units with halogen atoms, where the latter modulates the energy level of the frontier molecular orbital. Consequently, BDTBO-4Cl exhibited a deeper highest occupied molecular orbital level (E_HOMO) and lowest unoccupied molecular orbital level (E_LUMO) than BDTBO-4F. Moreover, these two NFAs exhibited high electron mobility and strong absorption at 700-900 nm. The polymer donor PM6 was combined with BDTBO-4F and BDTBO-4Cl, and the resulting OSCs exhibited outstanding power conversion efficiencies of 14.83% for the PM6:BDTBO-4F device and 13.87% for the PM6:BDTBO-4Cl device. More encouragingly, these OSCs exhibited efficient hole transfer from NFAs to PM6, despite small ΔE_HOMO(D-A) values (<0.10 eV). These results prove that modulation of E_HOMO of acceptors to decrease ΔE_HOMO(D-A) is an efficient strategy for high-performance OSCs.

Migration of Charge-Transfer States at Organic Semiconductor Heterojunctions

Charge-transfer (CT) states formed at organic donor-acceptor (D-A) semiconductor heterojunctions play a critical role in optoelectronic devices. While mobile, their migration has not been extensively characterized. In addition, the factors impacting the CT state diffusion length (L_D) have not been elucidated. Here, CT state L_D is measured by using photoluminescence quenching for several D-A mixtures, with migration occurring along the bulk heterojunction. All D-A pairings considered yield a similar L_D ∼ 5 nm in equal mixtures despite variations in the CT state energy and the constituent molecular structures. The CT state L_D varies strongly with mixture composition and is well-correlated to the slowest charge carrier mobility, suggesting a direct method to tune CT state transport. These findings may be applied to elucidate the role of CT state migration in organic photovoltaic and light-emitting devices as well as to broadly explain the transport of interfacial excited states along inorganic and hybrid organic-inorganic heterojunctions.

Reducing the Singlet-Triplet Energy Gap by End-Group π-π Stacking Toward High-Efficiency Organic Photovoltaics

To improve the power conversion efficiencies for organic solar cells, it is necessary to enhance light absorption and reduce energy loss simultaneously. Both the lowest singlet (S1) and triplet (T1) excited states need to energertically approach the charge-transfer state to reduce the energy loss in exciton dissociation and by triplet recombination. Meanwhile, the S1 energy needs to be decreased to broaden light absorption. Therefore, it is imperative to reduce the singlet-triplet energy gap (ΔE_ST ), particularly for the narrow-bandgap materials that determine the device T1 energy. Although maximizing intramolecular push-pull effect can drastically decrease ΔE_ST , it inevitably results in weak oscillator strength and light absorption. Herein, large oscillator strength (≈3) and a moderate ΔE_ST (0.4-0.5 eV) are found for state-of-the-art A-D-A small-molecule acceptors (ITIC, IT-4F, and Y6) owing to modest push-pull effect. Importantly, end-group π-π stacking commonly in the films can substantially decrease the S1 energy by nearly 0.1 eV, but the T1 energy is hardly changed. The obtained reduction of ΔE_ST is crucial to effectively suppress triplet recombination and acquire small exciton dissociation driving force. Thus, end-group π-π stacking is an effective way to achieve both small energy loss and efficient light absorption for high-efficiency organic photovoltaics.

Buildup of Triplet-State Population in Operating TQ1:PC71BM Devices Does Not Limit Their Performance

Triplet generation in organic solar cells has been considered a major loss channel. Determining the density of the triplet-state population in an operating device is challenging. Here, we employ transient absorption (TA) spectroscopy on the quinoxaline-thiophene copolymer TQ1 blended with PC₇₁BM, quantify the transient charge and triplet-state densities, and parametrize their generation and recombination dynamics. The charge recombination parameters reproduce the experimentally measured current-voltage characteristics in charge carrier drift-diffusion simulations, and they yield the steady-state charge densities. We demonstrate that triplets are formed by both geminate and nongeminate recombination of charge carriers and decay primarily by triplet-triplet annihilation. Using the charge densities in the rate equations describing triplet-state dynamics, we find that triplet-state densities in devices are in the range of charge carrier densities. Despite this substantial triplet-state buildup, TQ1:PC₇₁BM devices exhibit only moderate geminate recombination and significantly reduced nongeminate charge recombination, with reduction factors between 10^-4 and 10^-3 compared to Langevin recombination.

High Efficiency Polymer Solar Cells with Efficient Hole Transfer at Zero Highest Occupied Molecular Orbital Offset between Methylated Polymer Donor and Brominated Acceptor

Achieving efficient charge transfer at small frontier molecular orbital offsets between donor and acceptor is crucial for high performance polymer solar cells (PSCs). Here we synthesize a new wide band gap polymer donor, PTQ11, and a new low band gap acceptor, TPT10, and report a high power conversion efficiency (PCE) PSC (PCE = 16.32%) based on PTQ11-TPT10 with zero HOMO (the highest occupied molecular orbital) offset (ΔE_HOMO(D-A)). TPT10 is a derivative of Y6 with monobromine instead of bifluorine substitution, and possesses upshifted lowest unoccupied molecular orbital energy level (E_LUMO) of -3.99 eV and E_HOMO of -5.52 eV than Y6. PTQ11 is a derivative of low cost polymer donor PTQ10 with methyl substituent on its quinoxaline unit and shows upshifted E_HOMO of -5.52 eV, stronger molecular crystallization, and better hole transport capability in comparison with PTQ10. The PSC based on PTQ11-TPT10 shows highly efficient exciton dissociation and hole transfer, so that it demonstrates a high PCE of 16.32% with a higher V_oc of 0.88 V, a large J_sc of 24.79 mA cm^-2, and a high FF of 74.8%, despite the zero ΔE_HOMO(D-A) value between donor PTQ11 and acceptor TPT10. The PCE of 16.32% is one of the highest efficiencies in the PSCs. The results prove the feasibility of efficient hole transfer and high efficiency for the PSCs with zero ΔE_HOMO(D-A), which is highly valuable for understanding the charge transfer process and achieving high PCE of PSCs.

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

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

Spin Selectivity in Photoinduced Charge-Transfer Mediated by Chiral Molecules

Optical control and readout of electron spin and spin currents in thin films and nanostructures have remained attractive yet challenging goals for emerging technologies designed for applications in information processing and storage. Recent advances in room-temperature spin polarization using nanometric chiral molecular assemblies suggest that chemically modified surfaces or interfaces can be used for optical spin conversion by exploiting photoinduced charge separation and injection from well-coupled organic chromophores or quantum dots. Using light to drive photoexcited charge-transfer processes mediated by molecules with central or helical chirality enables indirect measurements of spin polarization attributed to the chiral-induced spin selectivity effect and of the efficiency of spin-dependent electron transfer relative to competitive relaxation pathways. Herein, we highlight recent approaches used to detect and to analyze spin selectivity in photoinduced charge transfer including spin-transfer torque for local magnetization, nanoscale charge separation and polarization, and soft ferromagnetic substrate magnetization- and chirality-dependent photoluminescence. Building on these methods through systematic investigation of molecular and environmental parameters that influence spin filtering should elucidate means to manipulate electron spins and photoexcited states for room-temperature optoelectronic and photospintronic applications.

Assessing the nature of the charge-transfer electronic states in organic solar cells

The charge-transfer electronic states appearing at the donor-acceptor interfaces in organic solar cells mediate exciton dissociation, charge generation, and charge recombination. To date, the characterization of their nature has been carried out on the basis of models that only involve the charge-transfer state and the ground state. Here, we demonstrate that it is essential to go beyond such a two-state model and to consider explicitly as well the electronic and vibrational couplings with the local absorbing state on the donor and/or acceptor. We have thus developed a three-state vibronic model that allows us: to provide a reliable description of the optical absorption features related to the charge-transfer states; to underline the erroneous interpretations stemming from the application of the semi-classical two-state model; and to rationalize how the hybridization between the local-excitation state and charge-transfer state can lead to lower non-radiative voltage losses and higher power conversion efficiencies.

Previous descriptions of the charge-transfer absorptions in organic solar cells only involve the charge transfer state and the ground state. Here Chen et al. underline that a third state, i.e., the local absorbing state on the donor and/or acceptor, needs to be considered.

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.

Transient Magnetic Field Effect of Photoexcitations in Donor-Acceptor Organic Semiconductors

We report transient photoinduced absorption (t-PA) and magnetic field ( B)-dependent t-PA (t-MPA( B)) in a pristine low band gap π-conjugated copolymer composed of donor and acceptor moieties, namely, the poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thi-eno[3,4- b]thiophenediyl]]) (or PTB7) used in photovoltaic applications. Unlike traditional π-conjugated polymers in which the primary photoexcitations are singlet excitons (SE), in pristine PTB7 we find at short times coexistence of two primary photoexcitation species, namely, SE and triplet-triplet (TT) pair. Both species are photogenerated directly from the ground state and are spin-correlated. Although the TT pair decomposes into two separate triplet excitons (TEs) in ∼100 ps, the separated TE spins are still entangled up to ∼6 μs. At longer times, the t-MPA( B) response of the surviving TEs shows transient narrowing effect, which is attributed to a distribution of the TE size.

Convenient fabrication of conjugated polymer semiconductor nanotubes and their application in organic electronics

Organic heterojunction is indispensable in organic electronic devices, such as organic solar cells, organic light-emitting diodes and so on. Fabrication of core-shell nanostructure provides a feasible and novel way to prepare organic heterojunction, which is beneficial for miniaturization and integration of organic electronic devices. Fabrication of nanotubes which constitute the core-shell structure in large quantity is the key for the realization of application. In this work, a simple and convenient method to prepare nanotubes using conjugated copolymer of perylene diimide and dithienothiophene (P(PDI-DTT)) was demonstrated. The relationship between preparation conditions (solvent atmosphere, solution concentration and pore diameter of templates) and morphology of nanostructure was studied systematically. P(PDI-DTT) nanotubes could be fabricated in regular shape and large quantity by preparing the solution with appropriate concentration and placing anodic aluminium oxide template with nanopore diameter of 200 nm in the solvent atmosphere. The tubular structure was confirmed by scanning electron microscopy. P(PDI-DTT) nanotubes exhibited electron mobility of 0.02 cm2 V-1 s-1 in field-effect transistors under ambient condition. Light-emitting nanostructures were successfully fabricated by incorporating tetraphenylethylene into polymer nanotubes.

Design rules for minimizing voltage losses in high-efficiency organic solar cells

The open-circuit voltage of organic solar cells is usually lower than the values achieved in inorganic or perovskite photovoltaic devices with comparable bandgaps. Energy losses during charge separation at the donor-acceptor interface and non-radiative recombination are among the main causes of such voltage losses. Here we combine spectroscopic and quantum-chemistry approaches to identify key rules for minimizing voltage losses: (1) a low energy offset between donor and acceptor molecular states and (2) high photoluminescence yield of the low-gap material in the blend. Following these rules, we present a range of existing and new donor-acceptor systems that combine efficient photocurrent generation with electroluminescence yield up to 0.03%, leading to non-radiative voltage losses as small as 0.21 V. This study provides a rationale to explain and further improve the performance of recently demonstrated high-open-circuit-voltage organic solar cells.

Increased Exciton Delocalization of Polymer upon Blending with Fullerene

Interfaces between donor and acceptor in a polymer solar cell play a crucial role in exciton dissociation and charge photogeneration. While the importance of charge transfer (CT) excitons for free carrier generation is intensively studied, the effect of blending on the nature of the polymer excitons in relation to the blend nanomorphology remains largely unexplored. In this work, electroabsorption (EA) spectroscopy is used to study the excited-state polarizability of polymer excitons in several polymer:fullerene blend systems, and it is found that excited-state polarizability of polymer excitons in the blends is a strong function of blend nanomorphology. The increase in excited-state polarizability with decreased domain size indicates that intermixing of states at the interface between the donor polymers and fullerene increases the exciton delocalization, resulting in an increase in exciton dissociation efficiency. This conclusion is further supported by transient absorption spectroscopy and time-resolved photoluminescence measurements, along with the results from time-dependent density functional theory calculations. These findings indicate that polymer excited-state polarizability is a key parameter for efficient free carrier generation and should be considered in the design and development of high-performance polymer solar cells.

Balanced Partnership between Donor and Acceptor Components in Nonfullerene Organic Solar Cells with >12% Efficiency

Relative to electron donors for bulk heterojunction organic solar cells (OSCs), electron acceptors that absorb strongly in the visible and even near-infrared region are less well developed, which hinders the further development of OSCs. Fullerenes as traditional electron acceptors have relatively weak visible absorption and limited electronic tunability, which constrains the optical and electronic properties required of the donor. Here, high-performance fullerene-free OSCs based on a combination of a medium-bandgap polymer donor (FTAZ) and a narrow-bandgap nonfullerene acceptor (IDIC), which exhibit complementary absorption, matched energy levels, and blend with pure phases on the exciton diffusion length scale, are reported. The single-junction OSCs based on the FTAZ:IDIC blend exhibit power conversion efficiencies up to 12.5% with a certified value of 12.14%. Transient absorption spectroscopy reveals that exciting either the donor or the acceptor component efficiently generates mobile charges, which do not suffer from recombination to triplet states. Balancing photocurrent generation between the donor and nonfullerene acceptor removes undesirable constraints on the donor imposed by fullerene derivatives, opening a new avenue toward even higher efficiency for OSCs.

Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells

The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a solution-processed organic photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene derivative have been the most commonly used electron donor and electron acceptor, respectively. While most advances of the device performance come from the design of new polymer donors, fullerene derivatives have almost been exclusively used as electron acceptors in organic photovoltaics. Recently, nonfullerene acceptor materials, particularly small molecules and oligomers, have emerged as a promising alternative to replace fullerene derivatives. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chemical, thermal, and photostability. The facile functionalization of these molecules affords excellent tunability to their optoelectronic and electrochemical properties. Within the past five years, there have been over 100 nonfullerene acceptor molecules synthesized, and the power conversion efficiency of nonfullerene organic solar cells has increased dramatically, from ∼2% in 2012 to >13% in 2017. This review summarizes this progress, aiming to describe the molecular design strategy, to provide insight into the structure-property relationship, and to highlight the challenges the field is facing, with emphasis placed on most recent nonfullerene acceptors that demonstrated top-of-the-line photovoltaic performances. We also provide perspectives from a device point of view, wherein topics including ternary blend device, multijunction device, device stability, active layer morphology, and device physics are discussed.

Organic solar cells based on non-fullerene acceptors

Organic solar cells (OSCs) have been dominated by donor:acceptor blends based on fullerene acceptors for over two decades. This situation has changed recently, with non-fullerene (NF) OSCs developing very quickly. The power conversion efficiencies of NF OSCs have now reached a value of over 13%, which is higher than the best fullerene-based OSCs. NF acceptors show great tunability in absorption spectra and electron energy levels, providing a wide range of new opportunities. The coexistence of low voltage losses and high current generation indicates that new regimes of device physics and photophysics are reached in these systems. This Review highlights these opportunities made possible by NF acceptors, and also discuss the challenges facing the development of NF OSCs for practical applications.

Order enables efficient electron-hole separation at an organic heterojunction with a small energy loss

Donor-acceptor organic solar cells often show low open-circuit voltages (V _OC) relative to their optical energy gap (E _g) that limit power conversion efficiencies to ~12%. This energy loss is partly attributed to the offset between E _g and that of intermolecular charge transfer (CT) states at the donor-acceptor interface. Here we study charge generation occurring in PIPCP:PC₆₁BM, a system with a very low driving energy for initial charge separation (E _g-E _CT ~ 50 meV) and a high internal quantum efficiency (η _IQE ~ 80%). We track the strength of the electric field generated between the separating electron-hole pair by following the transient electroabsorption optical response, and find that while localised CT states are formed rapidly (<100 fs) after photoexcitation, free charges are not generated until 5 ps after photogeneration. In PIPCP:PC₆₁BM, electronic disorder is low (Urbach energy <27 meV) and we consider that free charge separation is able to outcompete trap-assisted non-radiative recombination of the CT state.

Fine-Tuning the Energy Levels of a Nonfullerene Small-Molecule Acceptor to Achieve a High Short-Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells

Organic solar cell optimization requires careful balancing of current-voltage output of the materials system. Here, such optimization using ultrafast spectroscopy as a tool to optimize the material bandgap without altering ultrafast photophysics is reported. A new acceptor-donor-acceptor (A-D-A)-type small-molecule acceptor NCBDT is designed by modification of the D and A units of NFBDT. Compared to NFBDT, NCBDT exhibits upshifted highest occupied molecular orbital (HOMO) energy level mainly due to the additional octyl on the D unit and downshifted lowest unoccupied molecular orbital (LUMO) energy level due to the fluorination of A units. NCBDT has a low optical bandgap of 1.45 eV which extends the absorption range toward near-IR region, down to ≈860 nm. However, the 60 meV lowered LUMO level of NCBDT hardly changes the V_oc level, and the elevation of the NCBDT HOMO does not have a substantial influence on the photophysics of the materials. Thus, for both NCBDT- and NFBDT-based systems, an unusually slow (≈400 ps) but ultimately efficient charge generation mediated by interfacial charge-pair states is observed, followed by effective charge extraction. As a result, the PBDB-T:NCBDT devices demonstrate an impressive power conversion efficiency over 12%-among the best for solution-processed organic solar cells.

Ultrafast Long-Range Charge Separation in Nonfullerene Organic Solar Cells

Rapid, long-range charge separation in polymer-fullerene organic solar cells (OSCs) enables electrons and holes to move beyond their Coulomb capture radius and overcome geminate recombination. Understanding the nature of charge generation and recombination mechanisms in efficient, nonfullerene-acceptor-based OSCs are critical to further improve device performance. Here we report charge dynamics in an OSC using a perylene diimide (PDI) dimer acceptor. We use transient absorption spectroscopy to track the time evolution of electroabsorption caused by the dipolar electric field generated between electron-hole pairs as they separate after ionization at the donor-acceptor interface. We show that charges separate rapidly (<1 ps) and that free charge carriers are generated very efficiently (∼90% quantum yield). However, in the PDI-based OSC, external charge extraction is impaired by faster nongeminate decay to the ground state and to lower-lying triplet states.

Charge-Transfer Dynamics in the Lowest Excited State of a Pentacene-Fullerene Complex: Implications for Organic Solar Cells

We characterize the dynamic nature of the lowest excited state in a pentacene/C₆₀ complex on the femtosecond time scale, via a combination of ab initio molecular dynamics and time-dependent density functional theory. We analyze the correlations between the molecular vibrations of the complex and the oscillations in the electron-transfer character of its lowest excited state, which point to vibration-induced coherences between the (pentacene-based) local-excitation (LE) state and the complex charge-transfer (CT) state. We discuss the implications of our results on this model system for the exciton-dissociation process in organic solar cells.