Fullerene-based Organic Solar Cells Research Articles

Limiting factors for charge generation in low-offset fullerene-based organic solar cells

Free charge generation after photoexcitation of donor or acceptor molecules in organic solar cells generally proceeds via (1) formation of charge transfer states and (2) their dissociation into charge separated states. Research often either focuses on the first component or the combined effect of both processes. Here, we provide evidence that charge transfer state dissociation rather than formation presents a major bottleneck for free charge generation in fullerene-based blends with low energetic offsets between singlet and charge transfer states. We investigate devices based on dilute donor content blends of (fluorinated) ZnPc:C60 and perform density functional theory calculations, device characterization, transient absorption spectroscopy and time-resolved electron paramagnetic resonance measurements. We draw a comprehensive picture of how energies and transitions between singlet, charge transfer, and charge separated states change upon ZnPc fluorination. We find that a significant reduction in photocurrent can be attributed to increasingly inefficient charge transfer state dissociation. With this, our work highlights potential reasons why low offset fullerene systems do not show the high performance of non-fullerene acceptors.

A Review on Fullerene Derivatives with Reduced Electron Affinity as Acceptor Materials for Organic Solar Cells

Organic solar cells (OSCs) represent a promising emerging photovoltaic technology offering such benefits as light weight, mechanical flexibility, semitransparency, environmental friendliness and aesthetic design of solar panels. Furthermore, organic solar cells can be produced using scalable and high-throughput solution-based printing and coating technologies, which are expected to lead to very low product costs. Fullerene derivatives have been used as acceptor materials in virtually all efficient organic solar cells for more than two decades, following the demonstration of the first proof-of-concept devices in the middle of 1990s. Still, the power conversion efficiencies of fullerene-based organic solar cells became stuck at around 12% due to the suboptimal optoelectronic properties of conventional fullerene acceptors. Therefore, the latest efficiency records (>18%) for organic solar cells were set using different types of non-fullerene acceptor (NFA) materials with tailorable properties. However, NFA materials appeared to be very sensitive to light, thus impairing the operational stability of OSCs. On the contrary, there is growing evidence that rationally designed fullerene-based acceptors enhance the photostability of conjugated polymers and also NFAs, when used in ternary blends. Hence, a renaissance of fullerene-based materials is currently expected in the context of their use in multicomponent organic solar cells (e.g., as stabilizers) and also lead halide perovskite solar cells, where they play an important role of electron transport materials. The success in both of these applications requires the tunability of optoelectronic characteristics of fullerene derivatives. In particular, electron affinity of the fullerene cage has to be reduced in many cases to match the energy levels of other absorber material(s). Herein, we present a systematic review of different strategies implemented to reduce the acceptor strength of the fullerene derivatives and the results of their performance evaluation in OSCs with model conjugated polymers. Particular attention is paid to correlations between the chemical structure of organic addends and their influence on the electronic properties of the fullerene core. We believe this review would be valuable to researchers working on the rational design of new fullerene-based materials with tailored properties for photovoltaic and other electronic applications.

Small-Molecule Acceptors with Asymmetric Thieno[3,2-c]isochromene Bridged Units for Boosting the Efficiency and Stability of Fullerene-Based Organic Solar Cells

Ternary strategy is an effective approach to boost the photovoltaic performance of organic solar cells (OSCs). However, the ternary OSCs using a nonfullerene small-molecule acceptor (SMA) as the third component into the binary fullerene–donor system have not received satisfactory progress. Here, we developed a simple A–D–D′–D–A-type SMA of BDT–2TiC16–2CN, employing benzodithiophene (BDT) as an electron-donating (D′) centra, asymmetric bis(hexadecyl) thieno[3,2-c]isochromene (TiC) as an electron-donating (D) bridged unit, and cyanoindanone (CN) as a strong electron-withdrawing (A) terminal. Its thermal, optical, electrochemical, and photovoltaic properties were systemically studied. It is demonstrated that BDT–2TiC16–2CN exhibits absorption maxima at 703 nm as well as the cascaded HOMO and LUMO energy levels (−5.61/–3.96 eV) with polymers PM6 and PC71BM. Encouragingly, incorporating the BDT–2TiC16–2CN guest into the PM6:PC71BM blend, the ternary OSCs exhibit better photovoltaic properties and higher device stability than the binary OSCs. An improved PCE of 9.39% is obtained in the ternary OSCs, which is increased by 12.2% in comparison with that of the binary OSCs (8.37%). More importantly, 73.71% of the initial PCE is still maintained after 1440 h at room temperature and 67.22% after 1440 min at 65 °C. Our work indicates that BDT–2TiC16–2CN with an asymmetric TiC bridged unit is a potential SMA used as a third component to improve the device efficiency and stability for the ternary fullerene OSCs.

Improved ultraviolet stability of fullerene-based organic solar cells through light-induced enlargement and crystallization of fullerene domains

• Ultraviolet stability of fullerene-based solar cells • Impact of thermal annealing on efficiency time evolution under illumination • Morphological and structural analyses of blend layers • Enhanced crystallinity and size of fullerene enriched domains under illumination Organic solar cells (OSCs) are a promising technology with the potential for low-cost manufacturing. However, to translate to economically viable applications, long-term stability is a fundamental requirement. Amongst intrinsic degradation pathways the sensitivity of OSC to ultraviolet (UV) light severely limits their photostability. Here, we focus on the impact of UV on the stability of solar cells based on well-known fullerene-based blends processed with 1,8-diidooctane (DIO) as additive. The post-annealed devices resulting in DIO-free blends are directly compared to as-cast devices containing residual DIO. After a pronounced initial burn-in, as-cast devices demonstrate a self-healing effect leading to stable solar cells under prolonged exposure to UV light. This initial burn-in can be considerably reduced in annealed devices with a suitable heating process, resulting in very stable solar cells under UV-containing light over a long time period. Under UV-free LED light, solar cells are stable, which implies a direct impact of UV on the performance evolution of devices. Advanced characterization techniques were used for in-depth morphological analyses under light exposure to distinguish the observed UV-related processes in the polymer blends. Our results point thus towards the presence of two processes occurring under UV-light within as-cast devices involving fullerenes, one causing a performance degradation and the other allowing a repair tending towards a performance stability. Due to an improved initial crystal order within annealed devices, the process related to the degradation is in the minority. The UV stability of devices can be attributed to the UV light-induced diffusion of fullerenes, leading jointly to the enlargement of the initial existing fullerene domains and to their crystallization under UV light. These results path the way for a better understanding of the stability of efficient normal OSCs under simulated sunlight.

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 (Eb) 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 Eb 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 (ΔEST) of the narrow-band-gap A-D-A acceptors. Importantly, the intermolecular end-group π-π stacking is demonstrated to effectively decrease the ΔEST 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.

Comparative Study of a Simple Fluorescent Emitter in Ternary Nonfullerene- and Fullerene-Based Organic Solar Cells: The Impact on Energy Transfer and Vertical Phase Distribution

Comparative Study of a Simple Fluorescent Emitter in Ternary Nonfullerene- and Fullerene-Based Organic Solar Cells: The Impact on Energy Transfer and Vertical Phase Distribution

Study on the side chain effect of A2-A1-D-A1-A2 type non-fullerene acceptors matched with P3HT

Because of the advantages of low cost, stability and easy synthesis, poly(3-hexylthiophene) (P3HT) was the most classic photovoltaic polymer and promoted the rapid progress of fullerene-based organic solar cells (OSCs) during 2005–2015. In comparison with A-D-A type non-fullerene acceptors (NFAs), A2-A1-D-A1-A2 type molecules can realize higher open-circuit voltage (VOC) and also match well with P3HT. Hence, here we modified the classic A2-A1-D-A1-A2 type NFA of BTA3 by altering the side chains at three different positions, and synthesized five new materials of BTA3b-3f to investigate the structure-properties relationship. By fine-tuning the side chains at middle D unit, BTA3b, BTA3c and BTA3d show totally different power conversion efficiencies (PCEs) of 5.95, 3.80 and 4.74% respectively, which demonstrate the modification of D unit can significantly influence the intermolecular interaction. On the other hand, with the modulation of side chains at A1 and A2 units, BTA3e and BTA3f show relatively high PCEs of 5.48 and 6.15% respectively. Our results indicate the systematical side chain engineering for A2-A1-D-A1-A2 type NFAs play a vital role to improve the photovoltaic performance of P3HT-based OSCs.

Fluorination on electron-deficient units of benzothiadiazole-based donor-acceptor conjugated polymers for novel fullerene-based organic solar cells

Three new donor-acceptor (D-A) conjugated polymers based on benzo[1,2-b:4,5-b′]dithiophene (BDT) and benzo[c][1,2,5]thiadiazole (BT), namely, PB-BT, PB-BTf and PB-BTPf, were synthesized with different substituents (Hydrogen atoms, Fluorine atoms and fluorophenyl groups) on BT unit. Experimental results and theoretical calculations indicate that molecularly tuning of side chains on BT moeity simultaneously influences the energy levels and intermolecular packing of the resulted polymers by modulating their electron affinity and molecular coplanarity. The polymer solar cells (PSCs) based on a blend of PB-BTf/[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) exhibits the best photovoltaic performance among the three copolymers, with a open-circuit voltage (Voc) of 0.79 V and a power conversion efficiency (PCE) of 8.43%. Furthermore, polymer PB-BTPf containing fluorophenyl groups shows a higher Voc of 0.99 V due to its more low-lying highest occupied molecular orbital (HOMO) compared with other two structural polymeric analogues. The results here provide further fundamental insights into the relationship between the fluorination on electron-withdrawing moiety and the photovoltaic performance for the conjugated polymers applied in fullerene organic solar cells.

Monocyclopropanated fullerene derivatives with decreased electron affinity as promising electron acceptor materials for organic solar cells

Monocyclopropanated fullerene derivatives with decreased electron affinity as compared to standard [60]PCBM acceptor were synthesized, characterized, and investigated as promising electron acceptor materials for fullerene-based organic solar cells. This decrease in electron affinity was attributed to the presence of through-space electronic interactions between the lone electron pairs of oxygen atoms and electron-deficient π-system of the fullerene cage. Organic bulk heterojunction solar cells based on the designed materials blended with conjugated polymers P3HT and PCDTBT have demonstrated spectacular increase in open-circuit voltage by >100 mV and improved light power conversion efficiency as compared to the reference devices comprising [60]PCBM. The application of new fullerene-based acceptor materials in combination with the promising low band gap conjugated copolymer may deliver competitive device performances.

The first application of isoindigo-based polymers in non-fullerene organic solar cells

Although isoindigo (IID)-based polymers can realize high charge mobility, these materials are currently confined to fullerene-based organic solar cells (OSCs). Herein, we designed a pair of alternative D-π-A type copolymers, PE71 and PE72, to expand the application in non-fullerene OSCs, where benzo[1,2-b:4,5-b′]thiophene (BDT), thieno[3,2-b]thiophene (TT) and IID units were used as D, A and π-bridge, respectively. The aim of optimizing the length of alkyl chains on TT bridge is to ensure polymer solubility, crystallinity as well as miscibility with acceptor molecules. We find that PE71 and PE72 exhibit similar optical and electronic properties, but PE71 with shorter hexyl chain tends to aggregate into fiber-like structure. In the end, Y6 is selected as the electron acceptor because of suitable energy levels and complementary absorption spectrum. Finally, PE71:Y6 device realizes a power conversion efficiency (PCE) of 12.03%, which is obviously higher than that of PE72:Y6 device (9.74%) and is also the highest value for IID-based photovoltaic polymers. The charge transport, molecular aggregation, film morphology and energy loss analysis were systematically investigated. The improved photovoltaic performance of PE71:Y6 mainly originates from the better interpenetrating network structure toward facilitating exciton seperation and free charge carrier transportation. Our results indicate that IID-based D-π-A polymers can also be utilized in non-fullerene OSCs and the alkyl chains on the thieno [3,2-b]thiophene π-bridge have a vital effect on the photovoltaic performance.

Effect of Side-Chain Variation on Single-Crystalline Structures for Revealing the Structure–Property Relationships of Organic Solar Cells

The molecular stacking assembly in the active layer plays a significant role in the photovoltaic performance of organic solar cells (OSCs). Here, we report two new small molecular donors with different side chains, FBT-O and FBT-H, and their corresponding fullerene-based OSCs. A slight change in the side chains led to a big difference in the power conversion efficiencies (PCEs). Although the molecular structures of the two donors are similar to each other, PCEs of the devices based on FBT-O were almost three times higher than those of the devices based on FBT-H, with manifold short-circuit current density, fill factor, as well as three orders of magnitude enhancement in the hole mobility. The difference in their single crystal structures was thoroughly investigated, whereby the FBT-O exhibited better planarity leading to appropriate phase separation and domain size. Furthermore, two-dimensional grazing-incidence wide-angle X-ray scattering results of the blend films revealed that the two donors retained a similar stacking structure as compared to the single-crystal structures, thus, establishing a clear relationship between the molecular stacking structure and the device performance.

Polymerizable C70 derivatives with acrylate functionality for efficient and stable solar cells

Fullerene-based organic solar cells are generally suffering from severe microstructure evolution occurring in their bulk heterojunction active layers and thus are extremely stable. To address it, four polymerizable C70 fullerene derivatives, [6,6]-phenyl-C71-ethyl acrylate (PC71EA), [6,6]-phenyl-C71-propyl acrylate (PC71PrA), [6,6]-phenyl-C71-butyl acrylate (PC71BA), and [6,6]-phenyl-C71-pentyl acrylate (PC71PeA), have been designed, synthesized, and investigated. These fullerene compounds have a molecular structure, shape and size very like the conventional C70 fullerene acceptor, [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), and have been found no different in their light absorption, redox potentials, and frontier orbital energy levels. Using these fullerene acrylates individually as acceptor and poly(3-hexylthiophene) as donor, organic solar cells have been fabricated and gave optimal efficiencies ranging from 3.32% to 4.16%, comparable to PC71BM-based reference cells (4.06%). Owing to their acrylate functionality, these fullerene derivatives can turn into insoluble upon heating, and thus endow their solar cell devices much better thermostability than PC71BM-based reference cells. The best one, coming from PC71PeA devices, reported an optimal efficiency of 4.16%, and maintained 91.7% efficiency after heat treatment at 150 °C for 35 h. As a sharp contrast, the PC71BM reference cell dropped its optimal efficiency from 4.06% to 0.48% only after 5 h heat treatment. X-ray diffraction, optical and atomic force microscopy, and space-charge-limited current method have been carried out to understand active layer structure, morphology, and charge mobility change during heat treatment.

Direct Arylation Polycondensation (DAP) Synthesis of Alternating Quaterthiophene−Benzothiadiazole Copolymers for Organic Solar Cell Applications

Poly[(2,1,3-benzothiadiazole-4,7-diyl)-alt-4',3''-difluoro-3,3'''-di(2-octyldodecyl)-2,2';5',2'';5'',2'''-quaterthiophene-5,5'''-diyl)] (PBTff4T-2OD) and poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-3,3'''-di(2-octyldodecyl)-2,2';5',2'';5'',2'''-quaterthiophene-5,5'''-diyl)] (PffBT4T-2OD) for use as the p-donor component of high-efficiency fullerene-based organic solar cells are usually synthesized in established C-C cross-coupling reactions, preferably using the Stille procedure. This report describes how PBTff4T-2OD and PffBT4T-2OD are generated in a direct arylation polycondensation (DAP) approach with molecular weights up to Mn =19.4 kDa and 21.1 kDa, respectively, and how structural defects in the copolymers (e. g., homocoupling defects) show a strong impact on the pre-aggregation behavior. The optimized reaction conditions allow for a distinct reduction of the amount of such defects in the resulting copolymers. When the Stille-type products are used in the active layer of organic solar cells (OCSs) together with fullerene acceptors, high power-conversion efficiencies (PCEs) in the range of 8.6-10.8 % have been reported. The high PCEs are particularly related to the pre-aggregation of the conjugated copolymers prior to film formation. Despite quite similar characterization data, non-optimized OCSs with the DAP polymers as replacement for the Stille products afforded a relatively low power-conversion efficiency of up to 2.4 %.

Efficient and hysteresis-less perovskite and organic solar cells by employing donor-acceptor type π-conjugated polymer

Abstract The development of new and efficient donor-acceptor (D-A)-type π-conjugated polymers has attracted significant attention in the field of organic electronics owing to their fascinating features. Herein, we explore a new and efficient D-A-type π-conjugated polymer, poly[4,8-bis(2-(4-(2-ethylhexyloxy)phenyl)-5-thienyl)benzo[1,2-b:4,5-b′]dithiophene-alt-1,3-bis(6-octylthieno [3,2-b]thiophen-2-yl)-5-(2-hexyldecyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione] (P-TT-TPD) for perovskite solar cells (PSCs) and organic solar cells (OSCs). The well suited energy levels, high mobility, solution processability, high dipole moment difference between the ground and excited states and better passivation of P-TT-TPD delivered a high power conversion efficiency (PCE) of 17.10% and 17.56% in dopant-free and tris(pentafluorophenyl)borane-doped PSCs, respectively, with negligible hysteresis factor. To the best of our knowledge, this is the only polymer hole transporting material (HTM) showing negligible hysteresis factor values in both pristine and doped states. Moreover, P-TT-TPD was also used as a photoactive donor material in fullerene-based OSCs and it displayed a decent PCE of 6.19%. These study results demonstrate that the molecular engineering of a D-A-type π-conjugated polymer is an effective strategy to design multi-functional π-conjugated materials capable of working as both an HTM and a photoactive donor material.

Energy-Gap Law for Photocurrent Generation in Fullerene-Based Organic Solar Cells: The Case of Low-Donor-Content Blends.

The involvement of charge-transfer (CT) states in the photogeneration and recombination of charge carriers has been an important focus of study within the organic photovoltaic community. In this work, we investigate the molecular factors determining the mechanism of photocurrent generation in low-donor-content organic solar cells, where the active layer is composed of vacuum-deposited C60 and small amounts of organic donor molecules. We find a pronounced decline of all photovoltaic parameters with decreasing CT state energy. Using a combination of steady-state photocurrent measurements and time-delayed collection field experiments, we demonstrate that the power conversion efficiency, and more specifically, the fill factor of these devices, is mainly determined by the bias dependence of photocurrent generation. By combining these findings with the results from ultrafast transient absorption spectroscopy, we show that blends with small CT energies perform poorly because of an increased nonradiative CT state decay rate and that this decay obeys an energy-gap law. Our work challenges the common view that a large energy offset at the heterojunction and/or the presence of fullerene clusters guarantee efficient CT dissociation and rather indicates that charge generation benefits from high CT state energies through a slower decay to the ground state.

Loss Mechanisms in Fullerene-Based Low-Donor Content Organic Solar Cells

Low-weight-percent donor content fullerene-based organic solar cells have been reported to have good efficiencies although the exact reason for their performance is still a matter of discussion. We use a solution processable hole-transporting poly(dendrimer) with minimal absorption in the visible region to study the blend-ratio dependency of energy-loss mechanisms in fullerene low-donor content organic solar cells in which [6,6]-phenyl-C71-butyric acid methyl ester is used as the primary absorber. The photocurrent losses for each of the steps that govern the device performance have been assessed, with the optimized device performance achieved when the donor was at a concentration of 6 wt %. The 6 wt % donor device had balanced charge transport, and transient absorption spectra revealed that although the low-donor content devices suffered from a low-exciton-quenching rate, the dissociation efficiency of the resulting charge-transfer states was almost unity. That is, essentially all of the formed charge-tra...

High Mobility Indium Oxide Electron Transport Layer for an Efficient Charge Extraction and Optimized Nanomorphology in Organic Photovoltaics.

The electron transport layer (ETL) plays an important role in determining the device efficiency of organic solar cells (OSCs). A rational design of an ETL for OSCs targets high charge extraction and induction of an optimized active layer morphology. In this Letter, a high mobility In2O3 synthesized via a solution-processed combustion reaction is successfully used as a universal ETL in an organic photovoltaic device. With the modification of a thin layer of polyethylenimine ethoxylated (PEIE), a device based on crystalline In2O3 outperforms its counterpart, ZnO, in both PBDTTT-EFT-based fullerene and nonfullerene systems. As ZnO is replaced by In2O3, the average efficiency increases from 9.5% to 10.5% for PBDTTT-EFT-PC71BM fullerene-based organic solar cells and also increases from 10.8% to 11.5% for PBDTTT-EFT-IEICO-4F nonfullerene-based organic solar cells, respectively. Morphological studies have unraveled the fact that the crystallineIn2O3 ETL with highly aligned nanocrystallites has induced the crystallization of polymer into a preferential molecular packing that favors the charge transport across an active layer. From the photophysical study, it is found that charge extraction in the crystalline In2O3 device is significantly faster than in the ZnO device due to the higher mobility of In2O3 and optimized nanomorphology of the active layer.

Origin of Photocurrent in Fullerene-Based Solar Cells

Fullerene-based organic solar cells with only a minute amount of donor show a substantial photocurrent while maintaining a large open-circuit voltage. At low concentrations the donor is fully dispe...

Branched 2‐Ethylhexyl Substituted Indacenodithieno[3,2‐b]Thiophene Core Enabling Wide‐Bandgap Small Molecule for Fullerene‐Based Organic Solar Cells with 9.15% Efficiency: Effect of Length and Position of Fused Polycyclic Aromatic Units

A pair of 2‐ethylhexyl‐substituted indacenodithieno[3,2‐b]thiophene (EH‐IDTT) based tetrafluorinated substituted wide‐bandgap small molecular donor materials with different electron‐rich terminal units, DT4FIDTT and TT4FIDTT, has been synthesized for high efficiency fullerene‐based organic solar cells. The utilization of the EH‐IDTT center core increased the optical bandgap of DT4FIDTT (Eg of 1.86 eV) and decreased HOMO energy level in thin films in comparison with those of BIT‐4F‐T with indacenodithiophene core and thiophene π bridge. Compared to DT4FIDTT with 2,2′‐bithiophene terminal unit, TT4FIDTT with further fused thieno[3,2‐b]thiophene terminal units exhibited a larger optical bandgap and more straight conformation due to less effective conjugated length of thieno[3,2‐b]thiophene. Devices based on DT4FIDTT/PC71BM exhibited high power conversion efficiency (PCE) up to 9.15%, significantly higher than that of the device based on TT4FIDTT/PC71BM (PCE of 7.35%). The higher Jsc of the device based on DT4FIDTT/PC71BM can be attributed to its better charge transport properties and favorable phase separation features. Such high efficiency of 9.15% is the highest value reported for the fullerene‐based organic solar cells based on wide‐bandgap (Eg > 1.85 eV) small molecular donor, demonstrating the feasibility of branched alkylated‐indacenodithieno[3,2‐b]thiophene core in enhancing overall photovoltaic performance.

Unraveling the efficiency-limiting morphological issues of the perylene diimide-based non-fullerene organic solar cells

Herein we report a comparative morphological analysis of the perylene diimide (PDI)- and fullerene-based organic solar cells (OSCs) to identify the factors responsible for low performance of PDI-based devices. A PDI derivative, bis-PDI, and a fullerene derivative, PC70BM, are mixed with an efficient polymer donor, PffBT4T-2OD. The large disparity in power conversion efficiencies (PCEs) of OSCs composed of PffBT4T-2OD:bis-PDI (PCE = 5.18%) and PffBT4T-2OD:PC70BM (PCE = 10.19%) observed are attributed to differences in the nanostructural motif of bulk heterojunction (BHJ) morphologies of these blend systems. The X-ray scattering and surface energy characterizations revealed that the structurally dissimilar bis-PDI and PC70BM molecules determine the variation in blend film morphologies, and in particular, the molecular packing features of the donor PffBT4T-2OD polymer. In addition, high-resolution transmission electron microscopy (HRTEM) images explore the BHJ morphologies and presence of longer polymer fibrils in PffBT4T-2OD:bis-PDI system, justifying the unbalanced charge transport and high hole mobility. The low performance of PffBT4T-2OD:bis-PDI devices was further investigated by studying charge carrier recombination dynamics by using light-intensity-dependent and transient photovoltage (TPV) experiments. Furthermore, the temperature-dependent experiments showed the photovoltaic properties, including charge recombination losses, are strongly affected by energetic disorder present in bis-PDI-based system.