Interface energetics and engineering of organic heterostructures in organic photovoltaic cells

The reliable information about interface energetics of organic materials, especially the energy level alignment at organic heterostructures is of pronounced importance for unraveling the photon harvesting and charge separation process in organic photovoltaic(OPV) cells. This article provides an overview of interface energetics at typical planar and mixed donor-acceptor heterostructures, perovskite/organic hybrid interfaces, and their contact interfaces with charge collection layers. The substrate effect on energy level offsets at organic heterostructures and the processes that control and limit the OPV operation are presented. Recent efforts on interface engineering with electrical doping are also discussed.     

Influence of intermolecular orientation on the photoinduced charge transfer kinetics in self-assembled aggregates of donor-acceptor arrays

The kinetics of photoinduced charge transfer reactions in covalently linked donor-acceptor molecules often undergoes dramatic changes when these molecules self-assemble from a molecular dissolved state into a nanoaggregate. Frequently, the origin of these changes is only partially understood. In this paper, we describe the intermolecular spatial organization of three homologous arrays, consisting of a central perylene bisimide (PERY) acceptor moiety and two oligo(p-phenylene vinylene) (OPV) donor units, in nanoaggregates and identify both face-to-face (H-type) and slipped (J-type) stacking of the OPV and PERY chromophores. For the J-type aggregates, short intermolecular OPV-PERY distances are created that give rise to a charge-transfer absorption band. The proximity of the donor and acceptor groups in the J-type aggregates enables a highly efficient photoinduced charge separation with a rate (k(cs) > 10(12) s(-1)) that significantly exceeds the rate of the intramolecular charge transfer of the same compounds when molecularly dissolved, even in the most polar media. In the H-type aggregates, on the other hand, the intermolecular OPV-PERY distance is not reduced compared to the intramolecular separation, and hence, the rates of the electron transfer reactions are not significantly affected compared to the molecular dissolved state. Similar to the forward electron transfer, the kinetics of the charge recombination in the aggregated state can be understood by considering the different interchromophoric distances that occur in the H- and J-type aggregates. These results provide the first consistent rationalization of the remarkable differences that are observed for photoinduced charge-transfer reactions of donor-acceptor compounds in molecularly dissolved versus aggregated states.     

Tuning the Hybridization of Local Exciton and Charge-Transfer States in Highly Efficient Organic Photovoltaic Cells

Decreasing the energy loss is one of the most feasible ways to improve the efficiencies of organic photovoltaic (OPV) cells. Recent studies have suggested that non-radiative energy loss ( ) is the dominant factor that hinders further improvements in state-of-the-art OPV cells. However, there is no rational molecular design strategy for OPV materials with suppressed . Herein, taking molecular surface electrostatic potential (ESP) as a quantitative parameter, we establish a general relationship between chemical structure and intermolecular interactions. The results reveal that increasing the ESP difference between donor and acceptor will enhance the intermolecular interaction. In the OPV cells, the enhanced intermolecular interaction will increase the charge-transfer (CT) state ratio in its hybridization with the local exciton state to facilitate charge generation, but simultaneously result in a larger . These results suggest that finely tuning the ESP of OPV materials is a feasible method to further improve the efficiencies of OPV cells.     

Tuning the Hybridization of Local Exciton and Charge‐Transfer States in Highly Efficient Organic Photovoltaic Cells

Decreasing the energy loss is one of the most feasible ways to improve the efficiencies of organic photovoltaic (OPV) cells. Recent studies have suggested that non‐radiative energy loss ( ) is the dominant factor that hinders further improvements in state‐of‐the‐art OPV cells. However, there is no rational molecular design strategy for OPV materials with suppressed . Herein, taking molecular surface electrostatic potential (ESP) as a quantitative parameter, we establish a general relationship between chemical structure and intermolecular interactions. The results reveal that increasing the ESP difference between donor and acceptor will enhance the intermolecular interaction. In the OPV cells, the enhanced intermolecular interaction will increase the charge‐transfer (CT) state ratio in its hybridization with the local exciton state to facilitate charge generation, but simultaneously result in a larger . These results suggest that finely tuning the ESP of OPV materials is a feasible method to further improve the efficiencies of OPV cells.     

Competition between singlet fission and charge separation in solution-processed blend films of 6,13-bis(triisopropylsilylethynyl)pentacene with sterically-encumbered perylene-3,4:9,10-bis(dicarboximide)s

The photophysics and morphology of thin films of N,N-bis(2,6-diisopropylphenyl)perylene-3,4:9,10-bis(dicarboximide) (1) and the 1,7-diphenyl (2) and 1,7-bis(3,5-di-tert-butylphenyl) (3) derivatives blended with 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) were studied for their potential use as photoactive layers in organic photovoltaic (OPV) devices. Increasing the steric bulk of the 1,7-substituents of the perylene-3,4:9,10-bis(dicarboximide) (PDI) impedes aggregation in the solid state. Film characterization data using both atomic force microscopy and X-ray diffraction showed that decreasing the PDI aggregation by increasing the steric bulk in the order 1 < 2 < 3 correlates with a decrease in the density/size of crystalline TIPS-Pn domains. Transient absorption spectroscopy was performed on ~100 nm solution-processed TIPS-Pn:PDI blend films to characterize the charge separation dynamics. These results showed that selective excitation of the TIPS-Pn results in competition between ultrafast singlet fission ((1*)TIPS-Pn + TIPS-Pn → 2 (3*)TIPS-Pn) and charge transfer from (1*)TIPS-Pn to PDIs 1-3. As the blend films become more homogeneous across the series TIPS-Pn:PDI 1 → 2 → 3, charge separation becomes competitive with singlet fission. Ultrafast charge separation forms the geminate radical ion pair state (1)(TIPS-Pn(+?)-PDI(-?)) that undergoes radical pair intersystem crossing to form (3)(TIPS-Pn(+?)-PDI(-?)), which then undergoes charge recombination to yield either (3*)PDI or (3*)TIPS-Pn. Energy transfer from (3*)PDI to TIPS-Pn also yields (3*)TIPS-Pn. These results show that multiple pathways produce the (3*)TIPS-Pn state, so that OPV design strategies based on this system must utilize this triplet state for charge separation.     

结晶动力学策略在非富勒烯体系太阳能电池形貌调控领域的应用

非富勒烯体系太阳能电池具有吸收范围宽、半透明及可大面积溶液加工等优势,已在清洁能源领域占据重要地位。在高性能材料开发、活性层形貌及器件工艺优化的推动下,器件能量转换效率已经突破19%。非富勒烯体系太阳能电池的基本结构包括阴极、阳极、相应的界面层及活性层,其中活性层形貌对器件性能有着重要影响。然而,由于活性层中给体与受体分子均为半晶性分子,在成膜过程中两者结晶存在竞争耦合;此外,活性层的结晶和相分离往往同时发生,导致形貌可控性差。针对上述问题,本专论系统总结了通过控制共混体系结晶动力学,精细调控活性层形貌的相关进展,详细介绍了共混体系中分子扩散速率、成核与晶粒生长相对速率、结晶顺序等动力学行为对活性层相分离结构、相区尺寸、结晶度及分子取向等的影响,建立了活性层多层次结构与器件光物理过程间的构效关系,为制备高性能有机太阳能电池器件奠定了基础。     

Energy and electron transfer in bifunctional non-conjugated dendrimers

Nonconjugated dendrimers, which are capable of funneling energy from the periphery to the core followed by a charge-transfer process from the core to the periphery, have been synthesized. The energy and electron donors involve a diarylaminopyrene unit and are incorporated at the periphery of these dendrimers. The energy and electron acceptor is at the core of the dendrimer, which involves a chromophore based on a benzthiadiazole moiety. The backbone of the dendrimers is benzyl ether based. A direct electron-transfer quenching of the excited state of the periphery or a sequential energy transfer-electron-transfer pathway are the two limiting mechanisms of the observed photophysical properties. We find that the latter mechanism is prevalent in these dendrimers. The energy transfer occurs on a picosecond time scale, while the charge-transfer process occurs on a nanosecond time scale. The lifetime of the charge separated species was found to be in the range of microseconds. Energy transfer efficiencies ranging from 80% to 90% were determined using both steady-state and time-resolved measurements, while charge-transfer efficiencies ranging from 70% to 80% were deduced from fluorescence quenching of the core chromophore. The dependence of the energy and charge-transfer processes on dendrimer generation is analyzed in terms of the backfolding of the flexible benzyl ether backbone, which leads to a weaker dependence of the energy and charge-transfer efficiencies on dendrimer size than would be expected for a rigid system.     

面向非富勒烯型有机光伏电池的聚合物给体材料设计

可溶液加工的有机光伏电池(OPV)是一种具有重要应用潜力的新型光伏技术。在OPV技术的发展过程中,富勒烯衍生物作为电子受体材料占据了相当长时间的统治地位,因此聚合物给体材料设计中对如何与富勒烯受体材料相互匹配考虑较多。最近几年来,基于聚合物给体和非富勒烯有机受体的OPV电池,简称为非富勒烯型NF-OPV,得到了十分快速的发展。在此类电池中,聚合物电子给体和非富勒烯型电子受体材料均起到了十分重要的作用。相比于较为经典的富勒烯型OPV,NF-OPV对聚合物给体的光电特性和聚集态结构提出了新的要求。因此,本文针对NF-OPV的特点,重点介绍NF-OPV对聚合物给体材料的吸收光谱、分子能级以及聚集态结构等特征的新要求,总结最近几年来的相关进展,并在此基础上进一步讨论聚合物电子给体材料面临的挑战和展望。     

Alternating oligo(p-phenylene vinylene)--perylene bisimide copolymers: synthesis,photophysics, and photovoltaic properties of a new class of donor--acceptor materials

A Suzuki polycondensation reaction has been used to synthesize two copolymers consisting of alternating oligo(p-phenylene vinylene) (OPV) donor and perylene bisimide (PERY) acceptor chromophores. The copolymers differ by the length of the saturated spacer that connects the OPV and PERY units. Photoinduced singlet energy transfer and photoinduced charge separation in these polychromophores have been studied in solution and in the solid state via photoluminescence and femtosecond pump-probe spectroscopy. In both polymers a photoinduced electron transfer occurs within a few picoseconds after excitation of the OPV or the PERY chromophore. The electron transfer from OPV excited state competes with a singlet energy transfer state to the PERY chromophore. The differences in rate constants for the electron- and energy-transfer processes are discussed on the basis of correlated quantum-chemical calculations and in terms of conformational preferences and folding of the two polymers. In solution, the lifetime of the charge-separated state is longer than in the films where geminate recombination is much faster. However, in the films some charges are able to escape from geminate recombination and diffuse away and can be collected at the electrodes when the polymers are incorporated in a photovoltaic device.     

The importance of nanoscopic ordering on the kinetics of photoinduced charge transfer in aggregated pi-conjugated hydrogen-bonded donor-acceptor systems

Aggregated complexes of diaminotriazine oligo(p-phenylene vinylene) (OPV) units hydrogen bonded to different complementary perylene bisimide (PERY) compounds have been investigated by means of absorption, circular dichroism, photoluminescence, and photoinduced absorption spectroscopy. These studies reveal that in the aggregated state an ultrafast photoinduced charge separation occurs via an intermolecular pathway in the J-type stack of hydrogen-bonded OPV-PERY arrays. The subsequent charge recombination reaction strongly depends on small structural differences within the J-type geometry as revealed by comparison of stacked supramolecular dimers, trimers, and covalently OPV-PERY linked systems. A coupled oscillator model is used to analyze absorption and circular dichroism spectra and to identify intermolecular arrangements that are consistent with the experimental spectra and the charge-transfer kinetics.     

Solvent dependence of the charge-transfer properties of a quaterthiophene–anthraquinone dyad

An electron donor–acceptor dyad (quaterthiophene–anthraquinone) mediates ultrafast intramolecular photoinduced charge separation and consequent charge recombination when in polar or moderately polar solvents. Alternatively, non-polar media completely impedes the initial photoinduced electron transfer by causing enough destabilization of the charge-transfer state and shifting its energy above the energy of the lowest locally excited singlet state. Furthermore, femtosecond transient-absorption spectroscopy reveals that for the solvents mediating the initial photoinduced electron-transfer process, the charge recombination rates were slower than the rates of charge separation. This behavior of donor–acceptor systems is essential for solar-energy-conversion applications. For the donor–acceptor dyad described in this study, the electron-transfer driving force and reorganization energy place the charge-recombination processes in the Marcus inverted region.     

Efficient phosphorescent polymer light-emitting diodes by suppressing triplet energy back transfer

Phosphorescent polymer light-emitting diodes (PhPLEDs) are promising devices in flat panel displays and solid state lighting sources since they can combine the advantages of the high efficiency of electrophosphorescence and low-cost, large-scale manufacture by using a solution process. However, their efficiencies are generally much lower than those of small-molecule-based devices fabricated by using a thermal deposition approach. One of the major reasons for their low efficiency is that energy is lost by back transfer to a polymer host. This tutorial review gives a brief introduction to the fundamentals of PhPLEDs, and then highlights recent progress in the main approaches to suppress triplet energy back transfer from the phosphor to the polymer host towards realizing highly efficient PhPLEDs. The suppressing mechanisms are discussed, and the achievement of high device efficiencies are demonstrated. Emphasis is placed on the relationships between molecular structure, the extent of suppressing triplet energy back transfer, and device performance.     

Multifunctional triphenylamine polymers synthesized via direct (hetero) arylation polymerization

The design rules for creating multifunctional organic electronic materials are currently limited. By copolymerizing twisted triphenylamine (TPA) and electron rich dioxythiophene (XDOT) monomers via Direct (Hetero) Arylation Polymerization (DHAP), a set of polymers are obtained that perform as yellow to transmissive electrochromic (EC) films with up to 45% contrast, as well as in electroluminescent (EL) applications, achieving a luminance of ∼450 cd/m² in yellow‐green polymer light‐emitting diodes (PLEDs). In addition, polymerizing TPA with a donor‐acceptor‐donor monomer results in a low‐bandgap polymer that achieves power conversion efficiencies up to 2.5% when blended with PC₇₁BM in conventional organic photovoltaic (OPV) devices. Incorporation of TPA units into the polymer backbone largely breaks any aggregation and ordering in the solid‐state, leading to highly soluble materials that form smooth, reproducible thin films. The TPA unit also serves to break conjugation throughout the polymer backbone, providing precise control over optical and electronic properties through choice of comonomer. These results suggest that TPA copolymers can be useful for achieving multi‐functionality without sacrificing facile solution processability, making them promising candidates for multifunctional devices like dual EC/EL displays. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 147–153     

Black Phosphorus Quantum Dots Used for Boosting Light Harvesting in Organic Photovoltaics

Although organic photovoltaic devices (OPVs) have been investigated for more than two decades, the power conversion efficiencies of OPVs are much lower than those of inorganic or perovskite solar cells. One effective approach to improve the efficiency of OPVs is to introduce additives to enhance light harvesting as well as charge transportation in the devices. Here, black phosphorus quantum dots (BPQDs) are introduced in OPVs as an additive. By adding 0.055 wt % BPQDs relative to the polymer donors in the OPVs, the device efficiencies can be dramatically improved for more than 10 %. The weight percentage is much lower than that of any other additive used in OPVs before, which is mainly due to the two‐dimentional structure as well as the strong broadband light absorption and scattering of the BPQDs. This work paves a way for using two‐dimentional quantum dots in OPVs as a cost‐effective approach to enhance device efficiencies.     

Supramolecular control over donor-acceptor photoinduced charge separation

A novel donor-bridge-acceptor system has been synthesized by covalently linking a p-phenylene vinylene oligomer (OPV) and a perylene diimid (PERY) at opposite ends of a m-phenylene ethynylene oligomer (FOLD) of twelve phenyl rings, containing nonpolar (S)-3,7-dimethyl-1-octanoxy side chains. For comparison, model compounds have been prepared in which either the donor or acceptor is absent. In chloroform, the oligomeric bridge is in a random coil conformation. Upon addition of an apolar solvent (heptane) the oligomeric bridge first folds into a helical stack and subsequently intermolecular self-assembly of the stacks into columnar architectures occurs. Photoexcitation in the random coil conformation, where the interaction between the donor and acceptor chromophores is small, results only in long-range intramolecular energy transfer in which the OPV singlet-excited state is transformed into the PERY singlet-excited state. In the folded conformation of the bridge, donor and acceptor are closer and their enhanced interaction favors the formation the OPV(*)(+)-FOLD-PERY(*)(-) charge-separated state upon photoexcitation. As a result, the extent of photoinduced charge separation depends on the degree of folding of the bridge between donor and acceptor and therefore on the apolar nature of the medium. As a consequence, and contrary to conventional photoinduced charge separation processes, the formation of the OPV(*)(+)-FOLD-PERY(*)(-) charge-separated state is more favored in apolar media.     

Transforming Ionene Polymers into Efficient Cathode Interlayers with Pendent Fullerenes

A new and highly efficient cathode interlayer material for organic photovoltaics (OPVs) was produced by integrating C₆₀ fullerene monomers into ionene polymers. The power of these novel “C₆₀‐ionenes” for interface modification enables the use of numerous high work‐function metals (e.g., silver, copper, and gold) as the cathode in efficient OPV devices. C₆₀‐ionene boosted power conversion efficiencies (PCEs) of solar cells, fabricated with silver cathodes, from 2.79 % to 10.51 % for devices with a fullerene acceptor in the active layer, and from 3.89 % to 11.04 % for devices with a non‐fullerene acceptor in the active layer, demonstrating the versatility of this interfacial layer. The introduction of fullerene moieties dramatically improved the conductivity of ionene polymers, affording devices with high efficiency by reducing charge accumulation at the cathode/active layer interface. The power of C₆₀‐ionene to improve electron injection and extraction between metal electrodes and organic semiconductors highlights its promise to overcome energy barriers at the hard‐soft materials interface to the benefit of organic electronics.     

Hole transfer in a C-shaped molecule: conformational freedom versus solvent-mediated coupling

The electronic coupling matrix elements attending the charge separation reactions of a C-shaped molecule containing an excited pyrene as the electron acceptor and a dimethylaniline as the donor are determined in aromatic, ether, and ester solvents. Band shape analyses of the charge-transfer emission spectra (CT --> S(0)) provide values of the reaction free energy, the solvent reorganization energy, and the vibrational reorganization energy in each solvent. The free energy for charge separation in benzene and toluene solvents is independently determined from the excited state equilibrium established between the locally excited pyrene S(1) state and the charge-transfer state. Analyses of the charge separation kinetics using the spectroscopically determined reorganization energies and reaction free energies indicate that the electronic coupling is solvent independent, despite the presence of a cleft between the donor and acceptor. Hence, solvent molecules are not involved in the coupling pathway. The orientations of the donor and acceptor units, relative to the spacer, are not rigidly constrained, and their torsional motions decrease solvent access to the cleft. Generalized Mulliken-Hush calculations show that rotation of the pyrene group about the bond connecting it to the spacer greatly modulates the magnitude of through-space coupling between the S(1) and CT states. The relationship between the torsional dynamics and the electron-transfer dynamics is discussed.     

Suppressing trap states and energy loss by optimizing vertical phase distribution through ternary strategy in organic solar cells

Suppressing the trap-state density and the energy loss via ternary strategy was demonstrated.Favorable vertical phase distribution with donors(acceptors)accumulated(depleted)at the interface of active layer and charge extraction layer can be obtained by introducing appropriate amount of polymer acceptor N2200 into the systems of PBDB-T:IT-M and PBDB-TF:Y6.In addition,N2200 is gradiently distributed in the vertical direction in the ternary blend film.Various measurements were carried out to study the effects of N2200 on the binary systems.It was found that the optimized morphology especially in vertical direction can significantly decrease the trap state density of the binary blend films,which is beneficial for the charge transport and collection.All these features enable an obvious decrease in charge recombination in both PBDB-T:IT-M and PBDB-TF:Y6 based organic solar cells(OSCs),and power conversion efficiencies(PCEs)of 12.5%and 16.42%were obtained for the ternary OSCs,respectively.This work indicates that it is an effective method to suppress the trap state density and thus improve the device performance through ternary strategy.     

Ultrafast exciton dissociation followed by nongeminate charge recombination in PCDTBT:PCBM photovoltaic blends

The precise mechanism and dynamics of charge generation and recombination in bulk heterojunction polymer:fullerene blend films typically used in organic photovoltaic devices have been intensively studied by many research groups, but nonetheless remain debated. In particular the role of interfacial charge-transfer (CT) states in the generation of free charge carriers, an important step for the understanding of device function, is still under active discussion. In this article we present direct optical probes of the exciton dynamics in pristine films of a prototypic polycarbazole-based photovoltaic donor polymer, namely poly[N-11'-henicosanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT), as well as the charge generation and recombination dynamics in as-cast and annealed photovoltaic blend films using methanofullerene (PC(61)BM) as electron acceptor. In contrast to earlier studies we use broadband (500-1100 nm) transient absorption spectroscopy including the previously unobserved but very important time range between 2 ns and 1 ms, which allows us not only to observe the entire charge carrier recombination dynamics but also to quantify the existing decay channels. We determine that ultrafast exciton dissociation occurs in blends and leads to two separate pools of products, namely Coulombically bound charge-transfer (CT) states and unbound (free) charge carriers. The recombination dynamics are analyzed within the framework of a previously reported model for poly(3-hexylthiophene):PCBM (Howard, I. A. J. Am. Chem. Soc. 2010, 132, 14866) based on concomitant geminate recombination of CT states and nongeminate recombination of free charge carriers. The results reveal that only ~11% of the initial photoexcitations generate interfacial CT states that recombine exclusively by fast nanosecond geminate recombination and thus do not contribute to the photocurrent, whereas ~89% of excitons create free charge carriers on an ultrafast time scale that then contribute to the extracted photocurrent. Despite the high yield of free charges the power conversion efficiency of devices remains moderate at about 3.0%. This is largely a consequence of the low fill factor of devices. We relate the low fill factor to significant energetic disorder present in the pristine polymer and in the polymer:fullerene blends. In the former we observed a significant spectral relaxation of exciton emission (fluorescence) and in the latter of the polaron-induced ground-state bleaching, implying that the density of states (DOS) for both excitons and charge carriers is significantly broadened by energetic disorder in pristine PCDTBT and in its blend with PCBM. This disorder leads to charge trapping in solar cells, which in turn causes higher carrier concentrations and more significant nongeminate recombination. The nongeminate recombination has a significant impact on the IV curves of devices, namely its competition with charge carrier extraction causes a stronger bias dependence of the photocurrent of devices, in turn leading to the poor device fill factor. In addition our results demonstrate the importance of ultrafast free carrier generation and suppression of interfacial CT-state formation and question the applicability of the often used Braun-Onsager model to describe the bias dependence of the photocurrent in polymer:fullerene organic photovoltaic devices.     

杂化太阳电池中异质结界面的修饰及其对电池光电性能的影响

有机-无机杂化太阳电池综合了有机、无机材料的优点,成本低、理论效率高,受到人们的广泛关注.杂化太阳电池的光活性层由无机半导体和有机共轭聚合物复合而成.当光照射到活性层上时,共轭聚合物吸收光子产生激子(电子-空穴对);激子迁移到有机给体-无机受体的异质结界面处发生解离而产生自由电子和空穴;自由电子和空穴分别向无机半导体和有机聚合物传输,从而实现电荷的分离和传导.激子在有机-无机异质结界面处的分离效率是影响电池性能的一个重要因素.有机、无机两相材料往往因为接触面积小以及相容性差使此两相材料接触不佳,激子迁移到此界面不能有效分离,从而严重影响了杂化太阳电池的效率.这个问题可以通过此界面的修饰加以改善.本文即综述了有机-无机异质结界面修饰的方法、作用和意义,并展望了杂化太阳电池未来的发展趋势和应用前景.