Achieving 17.4% Efficiency of Ternary Organic Photovoltaics with Two Well‐Compatible Nonfullerene Acceptors for Minimizing Energy Loss

A power conversion efficiency (PCE) of 16.2% is achieved in PM6:BTP‐4F‐12 based organic photovoltaics (OPVs). On the basis of efficient binary OPVs, a series of ternary OPVs are constructed by incorporating MeIC as the third component. The open circuit voltages (V_OCs) of ternary OPVs can be gradually increased along with the incorporation of MeIC, suggesting the formation of an alloy state between BTP‐4F‐12 and MeIC with good compatibility. The energy loss (E_loss) of ternary OPVs can be decreased compared with that of two binary OPVs, contributing to the V_OC improvement of ternary OPVs. The short circuit current density (J_SC) and fill factor (FF) of ternary OPVs can also be simultaneously enhanced with MeIC content up to 10 wt% in acceptors, leading to 17.4% PCE of the optimized ternary OPVs. The J_SC and FF improvement of ternary OPVs is thought to result from the optimized ternary active layers with more efficient photon harvesting, exciton dissociation and charge transport. The 17.4% PCE and 79.2% FF is among the top values of ternary OPVs. This work indicates that a ternary strategy is an emerging method to simultaneously minimize E_loss and optimize photon harvesting as well as improve the morphology of active layers for realizing performance improvement for OPVs.     

NDI‐Based Small Molecule as Promising Nonfullerene Acceptor for Solution‐Processed Organic Photovoltaics

A novel naphthalene diimide (NDI)‐based small molecule (BiNDI) is designed and synthesized by linking two NDI monomers via a vinyl donor moiety. The electronic structure of BiNDI is carefully investigated by ultraviolet photoelectron spectroscopy (UPS). Density functional theory (DFT) sheds further light on the molecular configuration and energy level distribution. Thin film transistors (TFT) based on BiNDI show a highest electron mobility of 0.365 cm² V^?1 s^?1 in ambient atmosphere. Organic photovoltaics (OPVs) by using BiNDI as the acceptor show a highest power conversion efficency (PCE) of 2.41%, which is the best result for NDI‐based small molecular acceptors. Transmission electron microscopy (TEM), atomic force microscopy (AFM), grazing incidence wide‐angle X‐ray diffraction (GIXD), and X‐ray photo­electron spectroscopy (XPS) characterization to understand the morphology and structure order of the bulk heterojunction film are performed. It is found that small amount of 1,8‐diiodooctane (DIO) (i.e., 0.5%) in the blended film facilitates the crystallization of BiNDI into fibrillar crystals, which is beneficial for the improvement of device performance.     

Microstructural and Electronic Origins of Open‐Circuit Voltage Tuning in Organic Solar Cells Based on Ternary Blends

Organic ternary heterojunction photovoltaic blends are sometimes observed to undergo a gradual evolution in open‐circuit voltage (V_oc) with increasing amounts of a second donor or an acceptor. The V_oc is strongly correlated with the energy of the charge transfer state in the blend, but this value depends on both local and mesoscopic orders. In this work, the behavior of V_oc in the presence of a wide range of interfacial electronic states is investigated. The key charge transfer state interfaces responsible for V_oc in several model systems with varying morphology are identified. Systems consisting of one donor with two fullerene molecules and of one acceptor with a donor polymer of varying regio‐regularity are used. The effects from the changing energetic disorder in the material and from the variation due to a law of simple mixtures are quantified. It has been found that populating the higher‐energy charge transfer states is not responsible for the observed change in V_oc upon the addition of a third component. Aggregating polymers and miscible fullerenes are compared, and it has been concluded that in both cases charge delocalization, aggregation, and local polarization effects shift the lowest‐energy charge transfer state distribution.     

Impact of Noncovalent Sulfur–Fluorine Interaction Position on Properties,Structures, and Photovoltaic Performance in Naphthobisthiadiazole‐Based Semiconducting Polymers

Controlling the energetics and backbone order of semiconducting polymers is essential for the performance improvement of polymer‐based solar cells. The use of fluorine as the substituent for the backbone is known to effectively deepen the molecular orbital energy levels and coplanarize the backbone by noncovalent interactions with sulfur of the thiophene ring. In this work, novel semiconducting polymers are designed and synthesized based on difluoronaphthobisthiadiazole (FNTz) as a new family of naphthobisthiadiazole (NTz)–quaterthiophene copolymer systems, which are one of the highest performing polymers in solar cells. The effect of the fluorination position on the energetics and backbone order is systematically studied. It is found that the dependence of the solar cell fill factor on the active layer thickness is very sensitive to the fluorination position. It is thus further investigated and discussed how the structural features of the polymers influence the photovoltaic parameters as well as the diode characteristics and bimolecular recombination. Further, the polymer with fluorine on both the naphthobisthiadiazole and quaterthiophene moieties exhibits a quite high power conversion efficiency of 10.8% in solar cells in combination with a fullerene. It is believed that the results would offer new insights into the development of semiconducting polymers.     

Constructing High‐Performance Organic Photovoltaics via Emerging Non‐Fullerene Acceptors and Tandem‐Junction Structure

In consideration of the unique advantages of new non‐fullerene acceptors and the tandem‐junction structure, organic photovoltaics (OPVs) based on them are very promising. Studies related to this emerging area began in 2016 with achieved power conversion efficiencies (PCEs) of 8–10%, which have now been boosted to 17%. In this essay, the construction of high‐performance OPVs is discussed, with a focus on combining the advantages of new non‐fullerene acceptors and the tandem‐junction structure. In order to achieve higher PCEs, methods to enable high short‐circuit current density, open‐circuit voltage, and fill factor are discussed. In addition, the stability and reproducibility of high‐efficiency OPVs are also addressed. Herein, it is forecast that the new non‐fullerene acceptors‐based tandem‐junction OPVs will become the next big wave in the field and achieve high PCEs over 20% in the near future. Some promising research directions on this emerging hot topic are proposed which may further push the field into the 25% high efficiency era and considerably advance the technology beyond laboratory research.     

A Systematic Approach to the Design Optimization of Light‐Absorbing Indenofluorene Polymers for Organic Photovoltaics

The synthesis and optimization of new photovoltaic donor polymers is a time‐consuming process. Computer‐based molecular simulations can narrow the scope of materials choice to the most promising ones, by identifying materials with desirable energy levels and absorption energies. In this paper, such a retrospective analysis is presented of a series of fused aromatic push–pull copolymers. It is demonstrated that molecular calculations do indeed provide good estimates of the absorption energies measured by UV–vis spectroscopy and of the ionization potentials measured by photoelectron spectroscopy in air. Comparing measured photovoltaic performance of the polymer series to the trend in efficiencies predicted by computation confirms the validity of this approach.     

Electrical and Photo‐Induced Degradation of ZnO Layers in Organic Photovoltaics

We present the case of degradation of organic solar cells by sunlight concentrated to a moderate level (～4 suns). This concentration level is not enough for sufficient acceleration of the photobleaching or trap‐generation in the photoactive layer and therefore such short treatment (100 minutes) does not affect the short‐circuit current of the device. However, a significant degradation of V_OC and FF has been recorded by measurements of the cell current‐voltage curves with a variation of light intensity, for the devices before and after the treatment. The same degradation was found to occur after short application of forward voltage biases in the dark. This kind of degradation is found to be repairable, and could even be prevented by simple electrical treatment (short pulses of the reverse bias). Moreover, even the fresh cells can be improved by the same process. Generation and degeneration of shunts in ZnO hole‐blocking layer as underlying physical mechanisms for the cell degradation and restoration, respectively, can explain the results.     

Charge Photogeneration in Organic Photovoltaics: Role of Hot versus Cold Charge‐Transfer Excitons

The role of excess excitation energy on long‐range charge separation in organic donor/acceptor bulk heterojunctions (BHJs) continues to be unclear. While ultrafast spectroscopy results argue for efficient charge separation through high‐energy charge‐transfer (CT) states within the first picosecond (ps) of excitation, charge collection measurements suggest excess photon energy does not increase the current density in BHJ devices. Here, the population dynamics of charge‐separated polarons upon excitation of high‐energy polymer states and low‐energy interfacial CT states in two polymer/fullerene blends from ps to nanosecond time scales are studied. It is observed that the charge‐separation dynamics do not show significant dependence on excitation energy. These results confirm that excess exciton energy is not necessary for the effective generation of charges.     

Disorder‐Induced Open‐Circuit Voltage Losses in Organic Solar Cells During Photoinduced Burn‐In

The photoinduced open‐circuit voltage (V_oc) loss commonly observed in bulk heterojunction organic solar cells made from amorphous polymers is investigated. It is observed that the total charge carrier density and, importantly, the recombination dynamics are unchanged by photoinduced burn‐in. Charge extraction is used to monitor changes in the density of states (DOS) during degradation of the solar cells, and a broadening over time is observed. It is proposed that the V_oc losses observed during burn‐in are caused by a redistribution of charge carriers in a broader DOS. The temperature and light intensity dependence of the V_oc losses can be described with an analytical model that contains the amount of disorder broadening in a Gaussian DOS as the only fit parameter. Finally, the V_oc loss in solar cells made from amorphous and crystalline polymers is compared and an increased stability observed in crystalline polymer solar cells is investigated. It is found that solar cells made from crystalline materials have a considerably higher charge carrier density than those with amorphous materials. The effects of a DOS broadening upon aging are suppressed in solar cells with crystalline materials due to their higher carrier density, making crystalline materials more stable against V_oc losses during burn‐in.     

Solution‐Processed Hydrogen Molybdenum Bronzes as Highly Conductive Anode Interlayers in Efficient Organic Photovoltaics

Highly efficient and stable organic photovoltaic (OPV) cells are demonstrated by incorporating solution‐processed hydrogen molybdenum bronzes as anode interlayers. The bronzes are synthesized using a sol‐gel method with the critical step being the partial oxide reduction/hydrogenation using an alcohol‐based solvent. Their composition, stoichiometry, and electronic properties strongly correlate with the annealing process to which the films are subjected after spin coating. Hydrogen molybdenum bronzes with moderate degree of reduction are found to be highly advantageous when used as anode interlayers in OPVs, as they maintain a high work function similar to the fully stoichiometric metal oxide, whereas they exhibit a high density of occupied gap states, which are beneficial for charge transport. Enhanced short‐circuit current, open‐circuit voltage and, fill factor, relative to reference devices incorporating either PEDOT‐PSS or a solution processed stoichiometric molybdenum oxide, are obtained for a variety of bulk heterojunction mixtures based on different polymeric donors and fullerene acceptors. In particular, high power conversion efficiencies are obtained in devices that employed the s‐H_xMoO_2.75 as the hole extraction layer.     

In‐situ Crosslinking and n‐Doping of Semiconducting Polymers and Their Application as Efficient Electron‐Transporting Materials in Inverted Polymer Solar Cells

In this study, we demonstrate in‐situ n‐doping and crosslinking of semiconducting polymers as efficient electron‐transporting materials for inverted configuration polymer solar cells. The semiconducting polymers were crosslinked with bis(perfluorophenyl) azide (bis‐PFPA) to form a robust solvent‐resistant film, thereby preventing solvent‐induced erosion during subsequent solution‐based device processing. In addition, chemical n‐doping of semiconducting polymers with (4‐(1,3‐dimethyl‐2,3‐dihydro‐1H‐benzoimidazol‐2‐yl)phenyl)dimethylamine (N‐DMBI) substantially improved the power conversion efficiency of solar cells from 0.69% to 3.42%. These results open the way for progress on generally applicable polymeric interface materials, providing not only high device performance but also an effective fabrication method for solution‐processed multilayer solar cell devices.     

Breaking the 10% Efficiency Barrier in Organic Photovoltaics: Morphology and Device Optimization of Well‐Known PBDTTT Polymers

With the advances in organic photovoltaics (OPVs), the invention of model polymers with superior properties and wide applicability is of vital importance to both the academic and industrial communities. The recent inspiring advances in OPV research have included the emergence of poly(benzodithiophene‐co‐thieno[3,4‐b]thiophene) (PBDTTT)‐based materials. Through the combined efforts on PBDTTT polymers, over 10% efficiencies have been realized recently in various types of OPV devices. This review attempts to critically summarize the recent advances with respect to five well‐known PBDTTT polymers and their design considerations, basic properties, photovoltaic performance, as well as device application in conventional, inverted, tandem solar cells. These PBDTTT polymers also make great contributions to the rapid advances in the field of emerging ternary blends and fullerene‐free OPVs with top performances. Addtionally, new challenges in developing novel photovoltaic polymers with more superior properties are prospected. More importantly, the research of highly efficient PBDTTT‐based polymers provides useful insights and builds fundamentals for new types of OPV applications with various architectures.     

Work Function Control of Interfacial Buffer Layers for Efficient and Air‐Stable Inverted Low‐Bandgap Organic Photovoltaics

A water‐soluble cationic polythiophene derivative, poly[3‐(6‐{4‐tert‐butylpyridiniumyl}‐hexyl)thiophene‐2,5‐diyl] [P3(TBP)HT], is combined with anionic poly(3,4‐ethylenedioxythiophene):poly(p‐styrenesulfonate) (PEDOT:PSS) on indium tin oxide (ITO) substrates via electrostatic layer‐by‐layer (eLbL) assembly. By varying the number of eLbL layers, the electrode's work function is precisely controlled from 4.6 to 3.8 eV. These polymeric coatings are used as cathodic interfacial modifiers for inverted‐mode organic photovoltaics that incorporate a photoactive layer composed of either poly[(3‐hexylthiophene)‐2,5‐diyl] (P3HT) and the fullerene acceptor [6,6‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) or the low bandgap polymer [poly({4,8‐di(2‐ethylhexyloxyl)benzo[1,2‐b:4,5‐b′]dithiophene}‐2,6‐diyl)‐alt‐({5‐octylthieno[3,4‐c]pyrrole‐4,6‐dione}‐1,3‐diyl) (PBDTTPD)] and the electron acceptor [6,6‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM)]. The power conversion efficiency (PCE) of the resulting photovoltaic device is dependent on the composition of the eLbL‐assembled interface and permits the fabrication of devices with efficiencies of 3.8% and 5.6% for P3HT and PBDTTPD donor polymers, respectively. Notably, these devices demonstrate significant stability with a P3HT:PC₆₁BM system maintaining 83% of its original PCE after 1 year of storage and a PBDTTPD:PC₇₁BM system maintaining 97% of its original PCE after over 1000 h of storage in air, according to the ISOS‐D‐1 shelf protocol.     

Evidencing Excellent Thermal‐ and Photostability for Single‐Component Organic Solar Cells with Inherently Built‐In Microstructure

Solution‐processed organic solar cells (OSCs) are promising low‐cost, flexible, portable renewable sources for future energy supply. The state‐of‐the‐art OSCs are typically fabricated from a bulk‐heterojunction (BHJ) active layer containing well‐mixed donor and acceptor molecules in the nanometer regime. However, BHJ solar cells suffer from stability problems caused by the severe morphological changes upon thermal or illumination stress. In comparison, single‐component organic solar cells (SCOSCs) based on a double‐cable conjugated polymer with a covalently stabilized microstructure is suggested to be a key strategy for superior long‐term stability. Here, the thermal‐ and photostability of SCOSCs based on a model double‐cable polymer is systematically investigated. It is encouraging to find that under 90 °C & 1 sun illumination, the performance of SCOSCs remains substantially stable. Transport measurements show that charge generation and recombination (lifetime and recombination order) hardly change during the aging process. Particularly, the SCOSCs exhibit ultrahigh long‐term thermal stability with 100% PCE remaining after heating at temperature up to 160 °C for over 400 h, indicating an excellent candidate for extremely rugged applications.     

Trade‐Off between Trap Filling,Trap Creation,and Charge Recombination Results in Performance Increase at Ultralow Doping Levels in Bulk Heterojunction Solar Cells

Doping of organic bulk heterojunction solar cells has the potential to improve their power conversion efficiency (PCE). Deconvoluting the effect of doping on charge transport, recombination, and energetic disorder remains challenging. It is demonstrated that molecular doping has two competing effects: on one hand, dopant ions create additional traps while on the other hand free dopant‐induced charges fill deep states possibly leading to V _OC and mobility increases. It is shown that molar dopant concentrations as low as a few parts per million can improve the PCE of organic bulk heterojunctions. Higher concentrations degrade the performance of the cells. In doped cells where PCE is observed to increase, such improvement cannot be attributed to better charge transport. Instead, the V _OC increase in unannealed P3HT:PCBM cells upon doping is indeed due to trap filling, while for annealed P3HT:PCBM cells the change in V _OC is related to morphology changes and dopant segregation. In PCDTBT:PC70BM cells, the enhanced PCE upon doping is explained by changes in the thickness of the active layer. This study highlights the complexity of bulk doping in organic solar cells due to the generally low doping efficiency and the constraint on doping concentrations to avoid carrier recombination and adverse morphology changes.     

All‐Organic Battery Composed of Thianthrene‐ and TCAQ‐Based Polymers

An all‐organic battery consisting of two redox‐polymers, namely poly(2‐vinylthianthrene) and poly(2‐methacrylamide‐TCAQ) is assembled. This all‐organic battery shows excellent performance characteristics, namely flat discharge plateaus, an output voltage exceeding 1.3 V, and theoretical capacities of both electrodes higher than 100 mA h g^?1. Both organic electrode materials are synthesized in two respective three synthetic steps using the free‐radical polymerization technique. Li‐organic batteries manufactured from these polymers prove their suitability as organic electrode materials. The cathode material poly(2‐vinylthianthrene) (3) displays a discharging plateau at 3.95 V versus Li⁺/Li and a discharge capacity of 105 mA h g^?1, corresponding to a specific energy of about 415 mW h g^?1. The anode material poly(2‐methacrylamide‐TCAQ) (7) exhibits an initial discharge capacity of 130 mA h g^?1, corresponding to 94% material activity. The combination of both materials results in an all‐organic battery with a discharge voltage of 1.35 V and an initial discharge capacity of 105 mA h g^?1 (95% material activity).     

High‐Performance and Stable All‐Polymer Solar Cells Using Donor and Acceptor Polymers with Complementary Absorption

To explore the advantages of emerging all‐polymer solar cells (all‐PSCs), growing efforts have been devoted to developing matched donor and acceptor polymers to outperform fullerene‐based PSCs. In this work, a detailed characterization and comparison of all‐PSCs using a set of donor and acceptor polymers with both conventional and inverted device structures is performed. A simple method to quantify the actual composition and light harvesting contributions from the individual donor and acceptor is described. Detailed study on the exciton dissociation and charge recombination is carried out by a set of measurements to understand the photocurrent loss. It is unraveled that fine‐tuned crystallinity of the acceptor, matched donor and acceptor with complementary absorption and desired energy levels, and device architecture engineering can synergistically boost the performance of all‐PSCs. As expected, the PBDTTS‐FTAZ:PNDI‐T10 all‐PSC attains a high and stable power conversion efficiency of 6.9% without obvious efficiency decay in 60 d. This work demonstrates that PNDI‐T10 can be a potential alternative acceptor polymer to the widely used acceptor N2200 for high‐performance and stable all‐PSCs.