Donor-acceptor Interface Research Articles

Liberation of Charge Carriers by Optical Pumping Excitons in Poly(3-hexylthiophene) Aggregates

In conjugated polymers used in photovoltaics, charges may be produced on ultrafast time scales without requiring exciton diffusion to a donor-acceptor interface. To investigate the role of high-energy, delocalized exciton states in charge generation within polymer domains, we apply a pump-push-probe transient absorption technique to pristine poly(3-hexylthiophene) nanoparticles. The near-infrared push pulse induces exciton dissociation through the S-3 <- S-1 electronic transition, which is predicted to show intra-molecular charge-transfer character. We suggest that the spatial extent of the high-energy exciton, which induces electron-hole separation, is sufficient to overcome the intrinsic Coulombic attraction of the electron-hole pair. We observe that similar to 10% of the pushed excitons undergo dissociation to form free charges. The kinetics of charge recombination indicate that the electron and hole are separated by a distance of similar to 3 nm across the polymer domains.

Chlorophyll-based organic solar cells with improved power conversion efficiency

Abstract Chlorophylls (Chls), and associated chlorophyll derivatives, are one of the oldest, most versatile organic semiconductors found in nature. Herein, we present two easily semi-synthesized chlorophyll derivatives, namely, chlorin e6 trimethyl ester (Ce6Me3) and its copper complex (Cu–Ce6Me3), as the p-type dopants for organic semiconductors and their impact in organic solar cells (OSCs). In our study, both Chls showed intense Soret and Qy bands in the UV-visible spectra, leading to an effect means for capturing solar light and energy. Chls also exhibited high carrier mobility owing to the partial formation of aggregates through the spin-coating process. Using Chls, we fabricated OSCs in both planar-heterojunction (PHJ) and bulk-heterojunction (BHJ) solar cell configurations, together with C70/PC70BM as electron acceptors. In PHJ solar cells, we received solar power conversion efficiencies (PCEs) of only 0.85% and 0.93% for Cu–Ce6Me3- and Ce6Me3-based devices, respectively, with the thickness of the donor layer at 5 nm. In BHJ cells, we achieved much higher PCEs of 1.53% and 2.05% for Cu–Ce6Me3:PC70BM and Ce6Me3:PC70BM respectively, where both blending ratios were set to 1:8. The improvement on PCE in BHJ cells may be attributed to the better charge separation increase at the donor–acceptor interface.

Singlet/triplet exciton dissociation and charge recombination in donor-acceptor ThQs-C60 /PDIxCN2 complexes.

The donor-acceptor interface plays a critically important role in determining the power conversion efficiency of organic solar cells via controlling charge separation (CS) and recombination (CR) processes. Here, we combine the electronic structure calculations with electron transfer rate theory to clarify the CS and CR processes in ThQs-C60 /PDIxCN2 donor-acceptor complexes. The results reveal that in ThQs-PDIxCN2 the CS comes from both the dissociations of photo-induced singlet exciton and singlet fission-induced triplet exciton with a high efficiency, whereas in ThQs-C60 only the singlet exciton dissociation can take place because the triplet exciton lies below the charge-transfer exciton. However, very high CR rates in ThQs-PDIxCN2 obliterate the benefit of fast CS, inversely leading to the ThQ-C60 complex with a better cell efficiency. The present results are consistent with experimental observation and may furnish a possible patten to improve the overall conversion efficiency. © 2018 Wiley Periodicals, Inc.

Nanomorphology in A–D–A type small molecular acceptors-based bulk heterojunction polymer solar cells

Abstract Recent developments in acceptor–donor–acceptor (A– D –A) type non-fullerene acceptors have led to substantial improvements in bulk-heterojunction polymer solar cells efficiency. The device performance strongly depends on photoactive layer morphology, as the molecular packing, donor–acceptor interface and phase separation significantly affect the charge-transfer states and charge carrier dynamics. In this review, we start with a brief introduction of the techniques most effectively utilized to characterize multiphase morphology. Then, we summarize recent progress in A– D –A type acceptors, with the emphasis on understanding the molecular structure–morphology–performance relationships. Finally, an outlook on correlating morphological characteristics with photovoltage losses is presented for further improving device performance.

Surface States and Interface States: Two Fundamental Sources of Photodegradation in Organic Bulk Heterojunction Devices

In this paper we present a systematic study of evolution of surface and interface states in organic bulk heterojunction devices under prolonged illumination. Identical solar cells were prepared with and without calcium as cathode buffer layer (CBL) to study the effect of anaerobic photodegradation. The device with only Al (without CBL) underwent significant degradation in V oc (∼35%) and I sc (∼26%) during the initial illumination time of 2000 min. Electrical characterization such as dark current, capacitance spectroscopy, and bias modulated photocurrent measurements suggested that in the absence of any CBL, the surface state density at organic–metal cathode interface increased by ∼5× leading to increase in reverse saturation current and consequently significant drop in V oc and I sc. It was experimentally demonstrated that increase in both surface state density (metal-organic interface by ∼5×) and interface state density (donor–acceptor interface by ∼3×) contributes to the overall increase in reverse saturation current by ∼15× times in devices without CBL. Presence of Ca as surface passivating layer greatly reduces the amount of photodegradation under inert atmosphere.

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 constantsthat governthe 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 concludethat 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.

Overcoming the trade-off between exciton dissociation and charge recombination in organic photovoltaic cells

The electron donor-acceptor (D-A) interface is an essential component for realizing efficient exciton dissociation and charge generation in organic photovoltaic cells (OPVs). It can also however enable rapid charge recombination due to the close spatial proximity of electrons and holes. To frustrate recombination losses, attempts have been made to separate charge carriers by introducing an insulating blocking interlayer at the D-A interface. It is challenging to realize increased efficiency using this approach as the relative similarity of interlayer optical and transport energy gaps may also frustrate exciton harvesting and charge generation. To overcome this trade-off, the interlayer must block charge carriers while continuing to permit exciton migration to the dissociating interface. In this work, we demonstrate this configuration in archetypical copper phthalocyanine (CuPc)-C60 planar OPVs containing a rubrene interlayer to frustrate charge recombination. Critically, the similarity in triplet exciton energy levels between rubrene and CuPc allows the interlayer to be permeable to excitons. Devices containing an interlayer show a reduction in the charge transfer state binding energy and non-geminate recombination rate with increasing interlayer thickness. For thin interlayers, geminate recombination is also suppressed. Thus, devices containing an exciton permeable interlayer show a simultaneous increase in open-circuit voltage, short-circuit current, and power efficiency.

Understanding Structure-Property Relationships in All-Small-Molecule Solar Cells Incorporating a Fullerene or Nonfullerene Acceptor.

To investigate the influence of donor molecule crystallinity on photovoltaic performance in all-small-molecule solar cells, two dithieno[2,3- d:2',3'- d']-benzo[1,2- b:4,5- b']dithiophene (DTBDT)-based small molecules, denoted as DTBDT-Rho and DTBDT-S-Rho and incorporating different side chains, are synthesized and characterized. The photovoltaic properties of solar cells made of these DTBDT-based donor molecules are systemically studied with the [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) fullerene acceptor and the O-IDTBR nonfullerene acceptor to study the aggregation behavior and crystallinity of the donor molecules in both blends. Morphological analyses and a charge carrier dynamics study are carried out simultaneously to derive structure-property relationships and address the requirements of all-small-molecule solar cells. This study reveals exciton decay loss driven by large-scale phase separation of the DTBDT molecules to be a crucial factor limiting photocurrent generation in the all-small-molecule solar cells incorporating O-IDTBR. In the all-small-molecule blends, DTBDT domains with dimensions greater than 100 nm limit the exciton migration to the donor-acceptor interface, whereas blends with PC71BM exhibit homogeneous phase separation with smaller domains than in the O-IDTBR blends. The significant energy losses in nonfullerene-based devices lead to decreased Jsc and fill factor values and unusual decrease in Voc values. These results indicate the modulation of phase separation to be important for improving the photovoltaic performances of all-small-molecule blends. In addition, the enhanced molecular aggregation of DTBDT-S-Rho with the alkylthio side chain leads to higher degrees of phase separation and unfavorable charge transfer, which are mainly responsible for the relatively low photocurrent when using DTBDT-S-Rho compared with that when using DTBDT-Rho. On the other hand, this enhanced molecular aggregation improves the crystallinity of DTBDT-S-Rho and results in its increased hole mobility.

Does Electron Delocalization Influence Charge Separation at Donor–Acceptor Interfaces in Organic Photovoltaic Cells?

We use bilayer devices with a series of three fullerene acceptors differing in order and intermolecular coupling to systematically explore the influence of electron delocalization in the acceptor phase on the dissociation efficiency of charge transfer states. Structural information from GIWAXS measurements is combined with the results of optical and electrical characterization as well as theoretical modeling. Our results indicate that an increase in CT-dissociation efficiency is directly coupled to an enhancement in electron delocalization that is particularly prominent for C60 which forms crystalline domains. Therefore, our results substantiate the concept of delocalization of electrons taking a positive role in the charge separation process, and of acceptor crystallinity being crucial in this respect.

Modulation of the Molecular Orientation at the Bulk Heterojunction Interface via Tuning the Small Molecular Donor-Nonfullerene Acceptor Interactions.

The interfacial molecular packing orientation of the nonfullerene systems at the donor-acceptor interface is considered as one of the key parameters in fabricating high-performance devices because of the anisotropic molecular characteristics of conjugated donors (D) and nonfullerene acceptors (A). However, regulating the interfacial orientation for the nonfullerene systems is still scarcely studied. Herein, modulation of the interfacial molecular packing orientation of bulk heterojunction layer is successfully realized via tuning the D-A interactions. The results indicate that the molecule with relatively shorter alkyl side chain (2F-C4C6) because of weak D-A interactions is unable to influence the molecular orientation of the active layer, as compared to their longer alkyl side-chain counterpart (2F-C6C8), which demonstrates strong D-A interactions and thus efficiently modulates the overall packing orientation. The power conversion efficiencies of 6.41 and 8.23% are obtained for the relatively short and long alkyl side-chain donors with IDIC acceptor, respectively. Hence strong D-A interactions because of long enough alkyl side chain on a donor small molecule can modify the interfacial molecular packing orientation of the system, leading to a better performing device.

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.

(Invited) Photocarrier Generation in Single-Absorber Organic Solar Cells

Power Conversion efficiency of organic photovoltaic devices has exceeded 10% by taking advantage of bulk heterojunction (BHJ) structure, where donor and acceptor molecules are microscopically mixed. A remained problem is photovoltage loss from absorbed photon energy. In the BHJ system, LUMO offset is required to promote photodissociation at the donor-acceptor interface, causing serious photovoltage loss. To solve this problem, we are approaching it from single absorber cells, where there is no donor-acceptor interface. Generally, photogeneration efficiency in organic film bulk is quite low because of large exciton binding energy of Frenkel exciton. On the other hand, photogeneration from CT excitons occurs almost spontaneously in the BHJ system. If we could generate charge-transfer (CT) excitons in the single-component film directly, photovoltage loss would be reduced. Recently, we found intramolecular charge-transfer molecules including donor and acceptor units within one molecule show considerable high performance in the single-absorber device. In particular, DTDCPB having triphenyl amine unit (donor) and benzothiadiazole unit (acceptor) was a promising material for the single-absorber system. In this presentation, photogeneration process in the single-component film of CT molecules was investigated from several viewpoints. Molecular design for intramolecular charge-transfer absorption is discussed from the viewpoint of quantum chemical calculation and organic synthesis. The photocarrier dynamics in the single-absorber solar cells is investigated by several techniques such as intensity-modulated photovoltage/photocurrent (IMVS/IMPS) and time delayed collection field (TDCF). Exciton binding energy, that is a key factor for photodissociation in the film bulk, was estimated by Onsager-Braun analysis by using a planar device. Figure 1

Synthesis of Novel Intramolecular Charge-Transfer Molecules Bearing Triphenylamine and Benzothiadiazole Moieties, and Application to Single-Absorber Organic Solar Cells

Organic solar cells (OSCs) have been extensively studied due to their potential for flexible, printable, and low-cost devices. Recently, power conversion efficiency (PCE) has reached around 10% by using bulkhetero junction (BHJ) system which possesses miscible interface between two kinds of materials, electron donor and acceptor. In the BHJ system, LUMO offset is required to promote charge separation at donor-acceptor interface, causing serious photovoltage loss. In order to overcome this problem, we have pursued the possibility of single-absorber organic solar cells, whose active organic layer consists of one component. Generally, photogeneration efficiency in organic bulk film is quite low because of the large exciton binding energy (EBE). In previous work, we found that molecules with intramolecular charge transfer photoabsorption are suitable for the single-absorber OSC. A donor-acceptor (D-A) linked molecule DTDCPB (Fig. 1) was applied to the single absorber OSC, and comparatively good performance was obtained. In this study, we synthesized novel donor-acceptor linked molecules bearing triphenylamine and benzothiadiazole moieties by derivatizing DTDCPB (Fig. 1). DTDCPB was extended by insertion of thienothiophene at the donor-acceptor linkage part to form DTDCPB-TT. DTERPB and DTRCNPB bear an acceptor moiety of 3-ethyl rhodanine and 2-(3-ethyl-4-oxothiazolidin-2-ylidene)malononitrile, respectively, instead of dicyanovinyl moiety. EBEs in the solid state were estimated with quantum chemical calculation to design a suitable molecule for lower EBE. The D-A molecules were applied to a single absorber OSC composed of ITO/PEDOT:PSS/organic/Ca/Al. In the DTDCPB-TT device, FF and hole mobility increased compared with those in the DTDCPB device because the film crystallinity was improved due to thienothiophene. In the DTERPB and DTRCNPB devices, the photovoltage loss was only 0.60 V, that was less than that of the typical BHJ system. The ratio of the voltage loss to the absorbed energy was 33% for DTERPB and 35% for DTRCNPB, which were rather small in comparison with the typical BHJ system, e.g. 65% for P3HT:PC61BM and 56% for PTB7:PC71BM. Figure 1

Naphtho[1,2-c:5,6-c']bis[1,2,5]thiadiazole-Based Nonfullerene Acceptors: Effect of Substituents on the Thiophene Unit on Properties and Photovoltaic Characteristics.

The development of new electron-accepting π-conjugated systems for application as nonfullerene acceptors in organic solar cells (OSCs) is urgently needed. Although π-conjugated systems based on naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole (NTz) and naphthalimide (Np) as central and terminal units, respectively, represent possible candidates for nonfullerene acceptors, our knowledge of the structure–property–device performance relationship of these compounds remains limited. We report herein on an investigation of the effect of the substituents on the thiophene (T) linker between NTz and Np on the properties and photovoltaic performance. The photophysical and physicochemical measurements showed that the absorption behavior as well as frontier-orbital energy levels can be fine-tuned by the choice of the substituent on the thiophene rings. Bulk-heterojunction-type OSCs based on these acceptors under blending with poly(3-hexylthiophene) as a donor showed various power conversion efficiencies, ranging from 0.26 to 2.14%. The substituents on the thiophene rings also have a significant influence on the blend film properties, which explain the differences in the short-circuit current densities and fill factors in the OSCs. These results indicate the importance of molecular design in preparing nonfullerene acceptors with NTz and Np units in terms of tuning both the molecular properties of the materials and donor–acceptor interface engineering in the blended films.

Variable charge transfer state energies at nanostructured pentacene/C60 interfaces

While it has recently been recognized that organic donor-acceptor charge transfer (CT) state energies can vary substantially under different interfacial morphologies, this behavior is under-appreciated in the context of organic singlet fission solar cells where a specific alignment between the triplet state of the fission material and the CT state of the donor-acceptor interface is necessary to the function of the device. In this work, we demonstrate that the CT state energy of a prototypical pentacene-C60 singlet fission system is around 1 eV in most systems, but can vary over 300 meV depending on the composition and morphology of the interface. Moreover, we show that the inclusion of a poly(3-hexylthiophene-2,5-diyl) underlayer, which commonly serves as a triplet blocker and hole collector in pentacene/C60 solar cells, helps promote active layer morphologies with stabilized, low energy CT states. These trends in the interfacial energetics are correlated with structural characterization of the films by atomic force microscopy and x-ray diffraction.

An insight into the relationship between morphology and open circuit voltage/electronic absorption spectrum at donor-acceptor interface in boron subphthalocyanine chloride/C70 solar cell: A DFT/TDDFT exploration

Abstract SubPC/C70 is a promising solar cell and great efforts have been made to explore its properties and improve its power conversion efficiency. However, it is still an open question how molecular arrangement and orientation influence its physical and optical properties at the subPC/C70 interface. By modeling different subPC/C70 configurations in condensed phase, our calculated results, obtained with long range corrected density functional plus integral equation formalism variant polarizable continuum model, show the change of Voc can be up to 0.45 V and the absorption strength in visible region can be doubled due to the modification of configuration. We find that B-P and B-V configurations are clearly superior with respect to photon absorption and Voc, which both significantly influence the power conversion efficiency of subPC/C70 solar cell, among the modeled configurations. In addition, our results show that the most stable configuration U-V is a bad choice considering the strength of photon absorption and open circuit voltage in subPC/C70 solar cell.

Overcoming the efficiency limitations of SnS2 nanoparticle-based bulk heterojunction solar cells

This study examined the effects of heat treatment, the electron transport layer, and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) incorporation on the performance of hybrid bulk heterojunction (BHJ) solar cells composed of tin disulfide (SnS2) nanoparticles (NPs) and low band gap energy polymers 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) or poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PBT7). Inserting an electron transport layer (ETL) (i.e., ZnO) on the top of the photoactive layer improved the surface morphology of the photoactive layer, which led to an improvement in charge transport. Moreover, adding a suitable amount of PCBM to the SnS2/polymer active layer enhanced the device performance, such as short circuit current density (Jsc) and power conversion efficiency (PCE). In particular, adding 0.5 mg of PCBM to the composite solution led to a 25% and 1.5% improvement in the Jsc value and PCE, respectively. The enhanced performance was due mainly to the improvements in the surface morphology of the photoactive layer, charge carrier mobility within the donor–acceptor interface, and carrier collection efficiency at the cathode.

Graphene Field-Effect Transistor as a High-Throughput Platform to Probe Charge Separation at Donor-Acceptor Interfaces.

In organic and low-dimensional materials, electrons and holes are bound together to form excitons. Effective exciton dissociation at interfaces is essential for applications such as photovoltaics and photosensing. Here, we present an interface-sensitive, time-resolved method that utilizes graphene field effect transistor as an electric-field sensor to measure the charge separation dynamics and yield at donor-acceptor interfaces. Compared to other interface-sensitive spectroscopy techniques, our method has a much reduced measurement time and can be easily adapted to different material interfaces. Hence, it can be used as a high throughput screening tool to evaluate the charge separation efficiency in a large number of systems. By using zinc phthalocyanine/fullerene interface, we demonstrate how this method can be used to quantify the charge separation dynamics and yield at a typical organic donor-acceptor interface.

Ultraviolet photoelectron spectroscopy reveals energy-band dispersion for π-stacked 7,8,15,16-tetraazaterrylene thin films in a donor–acceptor bulk heterojunction

7,8,15,16-tetraazaterrylene (TAT) thin films grown on highly oriented pyrolytic graphite (HOPG) substrates were studied extensively with regard to their intrinsic and interfacial electronic properties by means of ultraviolet photoelectron spectroscopy (UPS). Merely weak substrate–adsorbate interaction occurs at the TAT/HOPG interface, with interface energetics being only little affected by the nominal film thickness. Photon energy-dependent UPS performed perpendicular to the molecular planes of TAT multilayer films at room temperature clearly reveals band-like intermolecular dispersion of the TAT highest occupied molecular orbital (HOMO) energy. Based on a comparison with a tight-binding model, a relatively narrow bandwidth of 54 meV is derived, which points to the presence of an intermediate regime between hopping and band-like hole transport. Upon additional deposition of 2,2′:5′,2″:5″,2″′-quaterthiophene (4T), a 4T:TAT donor–acceptor bulk heterojunction with a considerable HOMO-level offset at the donor–acceptor interface is formed. The 4T:TAT bulk heterojunction likewise exhibits intermolecular dispersion of the TAT HOMO energy, yet with a significant decreased bandwidth.

On the Molecular Origin of Charge Separation at the Donor–Acceptor Interface

AbstractFullerene‐based acceptors have dominated organic solar cells for almost two decades. It is only within the last few years that alternative acceptors rival their dominance, introducing much more flexibility in the optoelectronic properties of these material blends. However, a fundamental physical understanding of the processes that drive charge separation at organic heterojunctions is still missing, but urgently needed to direct further material improvements. Here a combined experimental and theoretical approach is used to understand the intimate mechanisms by which molecular structure contributes to exciton dissociation, charge separation, and charge recombination at the donor–acceptor (D–A) interface. Model systems comprised of polythiophene‐based donor and rylene diimide‐based acceptor polymers are used and a detailed density functional theory (DFT) investigation is performed. The results point to the roles that geometric deformations and direct‐contact intermolecular polarization play in establishing a driving force (energy gradient) for the optoelectronic processes taking place at the interface. A substantial impact for this driving force is found to stem from polymer deformations at the interface, a finding that can clearly lead to new design approaches in the development of the next generation of conjugated polymers and small molecules.