Field Of Organic Photovoltaics Research Articles

Multi-dimensional interfacial engineering for a practical large-area transparent flexible organic photovoltaics

Although the efficiency improvement by the research of interlayer and photo-active layer in the field of organic photovoltaics has been achieved, the practical research for commercialization to be applied in a large area, flexible and transparent module is still rare and lagging behind. Herein, we present a flexible and semitransparent large area photovoltaic with a high-performance by engineering the charge transport with a molecular linker as an organic interfacial layer. The insertion of a molecular linker between the electron transporting layer and the organic active layer led to a considerable increase in power conversion efficiency as well as an improvement in stability and flexibility. We also compared the device performance after 100 days, there is an 80% decrease in photovoltaic efficiency. However, after the inclusion of the molecular linker, there is only a 10% decrease. The molecular linker can bond well the electron transporting layer and photo-active layer to improve mechanical durability. We also tested with various photo-active layer conditions (thickness) and module types (area and number of cells). After charge transport engineering, we obtained improved results for all conditions. Finally, with this charge transport engineering, we obtained a remarkably high figure of merit for transparent flexible organic photovoltaic mini-modules which has 41.45% transmittance with an efficiency of 8.22% for sub-(50 mm × 70 mm) and 6.43% for sub-(100 mm × 100 mm) modules. This FOM is higher than the reported studies. For application we made a convergence module (100 mm × 100 mm) which composed electrochromic and photovoltaic. As a result, there is a self-powered EC-PV convergence module well-operated under the light. It could potentially be applied to smart window systems.

Layer-by-Layer Processed Ternary Organic Photovoltaics with Efficiency over 18.

Obtaining a finely tuned morphology of the active layer to facilitate both charge generation and charge extraction has long been the goal in the field of organic photovoltaics (OPVs). Here, a solution to resolve the above challenge via synergistically combining the layer-by-layer (LbL) procedure and the ternary strategy is proposed and demonstrated. By adding an asymmetric electron acceptor, BTP-S2, with lower miscibility to the binary donor:acceptor host of PM6:BO-4Cl, vertical phase distribution can be formed with donor-enrichment at the anode and acceptor-enrichment at the cathode in OPV devices during the LbL processing. In contrast, LbL-type binary OPVs based on PM6:BO-4Cl still show bulk-heterojunction like morphology. The formation of the vertical phase distribution can not only reduce charge recombination but also promote charge collection, thus enhancing the photocurrent and fill factor in LbL-type ternary OPVs. Consequently, LbL-type ternary OPVs exhibit the best efficiency of 18.16% (certified: 17.8%), which is among the highest values reported to date for OPVs. The work provides a facile and effective approach for achieving high-efficiency OPVs with expected morphologies, and demonstrates the LbL-type ternary strategy as being a promising procedure in fabricating OPV devices from the present laboratory study to future industrial production.

A theoretical exploration of charge transfer dynamics in PTB7Ir/PC71BM triplet-material-based organic photovoltaics

In the field of organic photovoltaics (OPVs), triplet materials are expected to be good candidates for improving power conversion efficiency (PCE) due to their long-lifetime and energy-tunable triplet excitons. A comparative photophysical analysis (including absorption spectra, exciton diffusion lengths, singlet and triplet charge transfer dynamics and so on) of PTB7/PC71BM and PTB7Ir/PC71BM cell devices was investigated, by DFT and TD-DFT methods, to explore why PTB7Ir/PC71BM device has obvious higher PCE (8.71%) than PTB7/PC71BM device (6.64%) in the same condition. The results manifest that the enhanced PCE of the PTB7Ir/PC71BM device is mainly attributed to the improving exciton diffusion lengths owing to the longer lifetime and increasing diffusivity of triplet excitons. Besides, to reveal the structure-property relationships of the polymer donor materials, we calculated the values of driving force for charge recombination to the triplet (CRT) with the changes of torsional angles between electron donating and electron withdrawing units of PTB7 and PTB7Ir. It is found that suitably increasing the angle and number of torsional angles of polymer donors to limit conjugation extent will prevent the CRT process at donor/acceptor interface and thus improve photovoltaic performance. This work would provide theoretical guidelines for the molecular design of high-efficiency triplet-material-based OPVs.

Unraveling the Complex Nanomorphology of Ternary Organic Solar Cells with Multimodal Analytical Transmission Electron Microscopy

Elucidating the complex materials distribution in the active layers of ternary organic solar cells is one of the greatest challenges in the field of organic photovoltaics. Knowledge of the nanomorphology is key to understanding photophysical processes (e.g., charge separation, adjustment of the recombination mechanism, and suppression of the radiationless and energetic losses) and thus improving the device performance. Herein, for the first time, the successful discrimination and spatial mapping of the active layer components of a ternary organic solar cell are demonstrated using analytical transmission electron microscopy. The material distribution of all three organic components is successfully visualized by multimodal imaging using complementary electron energy loss signals. A complete picture of the morphological aspects can be gained by studying the lateral and cross‐sectional morphology as well as the morphology evolution as a function of the mixing ratio of the polymers. Finally, a correlation between the morphology, photophysical processes, and device performance of the ternary and the reference binary system is achieved, explaining the differences of the power conversion efficiency between the two systems.

Indeno[1,2-b]thiophene End-capped Perylene Diimide: Should the 1,6-Regioisomers be systematically considered as a byproduct?

Usually considered as a byproduct, the 1,6-dibrominated PDI has rarely been functionalized for the preparation of electro-active conjugated molecules, particularly in the field of organic photovoltaics. In light of the literature, one can ask oneself: Does a 1,7-isomer based functional molecule systematically perform better than its 1,6-analogue? To answer this question, we report herein the synthesis and direct comparison of two indeno[1,2-b]thiophene (IDT) end-capped perylene diimide regioisomers (PDI) (1,6 and 1,7) used as non-fullerene acceptors in organic solar cells. It turned out that in our case, ie, when blended with the well-known PTB7-Th donor polymer, higher performance was reached for devices made with the 1,6-analogue.

A hybrid P3HT-Graphene interface for efficient photostimulation of neurons

Graphene conductive properties have been long exploited in the field of organic photovoltaics and optoelectronics by the scientific community worldwide. We engineered and characterized a hybrid biointerface in which graphene is coupled with photosensitive polymers, and tested its ability to elicit light-triggered neural activity modulation in primary neurons and blind retina explants. We designed such a graphene-based device by modifying a photoactive P3HT-based retinal interface, previously reported to rescue light sensitivity in blind rodents, with a CVD graphene layer replacing the conductive PEDOT:PSS layer to enhance charge separation. The new graphene-based device was characterized for its electrochemical features and for the ability to photostimulate primary neurons and blind retina explants, while preserving biocompatibility. Light-triggered responses, recorded by patch-clamp in vitro or MEA ex vivo, show a stronger light-transduction efficiency when the neurons are interfaced with the graphene-based device with respect to the PEDOT:PSS-based one. The possibility to ameliorate flexible photo-stimulating devices via the insertion of graphene, paves the way for potential biomedical applications of graphene-based neuronal interfaces in the context of retinal implants.

Design of novel thiazolothiazole-containing conjugated polymers for organic solar cells and modules

One of the major challenges in the field of organic photovoltaics is associated with high-throughput manufacturing of efficient and stable organic solar cells. Practical realization of technologies for production of large-area organic solar cells requires the development of novel materials with a defined combination of properties ensuring sufficient reliability and scalability of the process in addition to good efficiency and operation stability of the devices. In this work, we designed two novel polymers comprising thiazolothiazole units and investigated their performance as absorber materials for organic solar cells and modules. Optimized small-area solar cells based on P1/[70]PCBM ([6,6]-phenyl-C71-butyric acid methyl ester) blends exhibited promising power conversion efficiency (PCE) of 7.5%, while larger area modules fabricated using slot die coating showed encouraging PCE of 4.2%. Additionally, the fabricated devices showed promising outdoor stability maintaining 60–70% of the initial efficiency after 20 sun days being exposed to natural sunlight at the Negev desert. The obtained results feature the designed polymer P1 as a promising absorber material for a large-scale production of organic solar cells under ambient conditions.

Significant influence of doping effect on photovoltaic performance of efficient fullerene-free polymer solar cells

Abstract The modification mechanism of the water/alcohol cathode interlayer is one of the most complicated problems in the field of organic photovoltaics, which has not been clearly elucidated yet; this greatly restricts the further enhancement of the PCE for polymer solar cells. Herein, we clarified the different effects of PFN and its derivatives, namely, poly[(9,9-bis(3′-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN-Br) in modifying fullerene-free PSCs. It is found for the first time that doping on IT-4F by the amino group of PFN leads to the unfavorable charge accumulation, and hence, forms a dense layer of electronegative molecule due to the poor electron transport capacity of the non-fullerene acceptor IT-4F. The electronegative molecular layer can block the electron transfer from the active layer to the interlayer and cause serious charge recombination at the active layer/cathode interface. This mechanism could be verified by the ESR measurement and electron-only devices. By replacing PFN with PFN-Br, the excessive doping effect between the cathode interlayer and IT-4F is eliminated, by which the charge transport and collection can be greatly improved. As a result, a high PCE of 13.5% was achieved in the fullerene-free PSCs.

Resonance Raman Spectroscopy and Imaging of Franck-Condon Vibrational Activity and Morphology in Conjugated Polymers for Solar Cells.

Vibrational reorganization influences photophysical outcomes in conjugated polymers used as active materials for optoelectronic devices. Excited state geometric rearrangements typically involve many displaced vibrations, yet most materials design schemes rely solely on pure electronic models with limited predictive capability. Although the coupling of vibrational motions to electronic processes occurs over a broad range of time scales, resolving structural displacements immediately following photon absorption can be particularly insightful for understanding the intrinsic stabilities of excited states. These Franck-Condon vibrational relaxation processes occur on time scales of <1 ps in polymers and mainly involve high-frequency skeletal motions. Establishing correlations between Franck-Condon vibrational reorganization and steady-state material properties could generate new avenues for informing materials design, which is especially important in the fast-paced field of organic photovoltaics (OPV) where seemingly elegant strategies often fail but molecular-level insights are usually lacking. The goal of this Account is to highlight relationships between molecular structure, packing, and vibrational reorganization in OPV systems, such as blends of conjugated polymers with fullerenes. Resonance Raman spectroscopy (RRS) is a sensitive probe of Franck-Condon activity in OPV materials, and signals are bolstered by large resonance enhancements and low backgrounds from quantitative fluorescence quenching. Our group has undertaken extensive RRS investigations of heterogeneous OPV materials in functioning device environments to uncover new insights of the multidimensional excited state potential energy landscape and fluctuations with local morphology. Time-dependent quantum mechanical approaches facilitate this effort by providing an intuitive theoretical framework to access dynamical perspectives of Raman transitions. Moreover, dynamics regimes of Franck-Condon excited state structural evolution can be selected simply by tuning excitation energies. This excitation detuning approach also reveals structurally and electronically distinct conformers with unique Franck-Condon signatures typically concealed under inhomogeneously broadened absorption line shapes. Interestingly, long and rich progressions of overtone and combination transitions-rare for large molecules with multiple displaced modes-are frequently resolved that exhibit strong sensitivity to the local chromophore environment. These harmonic features encode useful dynamics information by serving as internal "clocks" of Franck-Condon vibrational activity in addition to enabling quantitative estimates of mode-specific displacements. RRS attributes may be further exploited to perform noninvasive imaging of functioning OPV devices in concert with variable frequency electrical imaging probes. This approach generates direct spatial correlations between morphology-dependent Franck-Condon vibrational activity and material performance metrics (e.g., photocurrent generation) on submicrometer size scales.

Isomer Effects of Fullerene Derivatives on Organic Photovoltaics and Perovskite Solar Cells.

Solar energy conversion is one of the most important issues for creating and maintaining a future sustainable society. In this regard, photovoltaic technologies have attracted much attention because of their potential to solve energy and environmental issues. In particular, thin-film solar cells, such as organic photovoltaics (OPVs) and perovskite solar cells (PSCs), are highly promising owing to their flexibility, light weight, and low-cost production. One of the most important factors used to evaluate solar-cell performance is the power conversion efficiency (PCE), which is the ratio of the output electric power divided by the input light power. The PCEs of PSCs have become comparable to those of multicrystalline silicon solar cells in a laboratory level, but the PCEs of OPVs have yet to catch up with them and still need to be improved. The insufficient durability of PSCs and OPVs is also a challenge that needs to be addressed. Fullerene derivatives have been utilized as electron acceptors and electron-transport materials in OPVs and PSCs. However, the use of fullerene derivatives requires attention to their isomers if they are multiadducts or even monoadducts produced from fullerenes with low symmetry. Their nonuniform structures and electronic properties may exert a negative effect on photovoltaic properties. However, most researchers in the field of OPVs and PSCs have been unaware of the importance of the isomerism. Even the most prevalent, high-performance fullerene acceptor, [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM), has been used as an isomer mixture. In this Account, we summarize recent studies on the effects of isomer separation of fullerene derivatives on the device performances of OPVs and PSCs. Largely, fullerene derivatives containing various isomers are categorized into [60]fullerene bisadducts, [70]fullerene bisadducts, and [70]fullerene monoadducts. In all cases, the difference in isomerism was found to have a large impact on PCEs. The miscibility with polymer donors and film-forming property of fullerene derivatives were affected by the isomer separations, which exert the most potent influence on device performances. Although the disorders in energy levels among isomers are not definitely influencing on photovoltaic properties of isomer mixtures, the molecular packing structures of fullerene derivatives make a significant effect on their photovoltaic properties. Notably, isomerically pure fullerene derivatives often-but not always-exhibit higher PCEs than the isomer mixture. The search for the best isomers of fullerene derivatives and their optimal compositional ratios, which extensively depend on their roles and the combined materials, will be an indispensable step to achieving consistently higher device performances for OPVs and PSCs.

Light Management in Organic Photovoltaics Processed in Ambient Conditions Using ZnO Nanowire and Antireflection Layer with Nanocone Array.

Low carrier mobility and lifetime in semiconductor polymers are some of the main challenges facing the field of organic photovoltaics (OPV) in the quest for efficient devices with high current density. Finding novel strategies such as device structure engineering is a key pathway toward addressing this issue. In this work, the light absorption and carrier collection of OPV devices are improved by employment of ZnO nanowire (NW) arrays with an optimum NW length (50 nm) and antireflection (AR) film with nanocone structure. The optical characterization results show that ZnO NW increases the transmittance of the electron transporting layer as well as the absorption of the polymer blend. Moreover, the as-deposited polymer blend on the ZnO NW array shows better charge transfer as compared to the planar sample. By employing PC70BM:PV2000 as a promising air-stable active-layer, power conversion efficiencies of 9.8% and 10.1% are achieved for NW devices without and with an AR film, indicating 22.5% and 26.2% enhancement in PCE as compared to that of planar device. Moreover, it is shown that the AR film enhances the water-repellent ability of the OPV device.

Outdoor Stability of Chloro–(Chloro)n–Boron Subnaphthalocyanine and Chloro–Boron Subphthalocyanine as Electron Acceptors in Bilayer and Trilayer Organic Photovoltaics

In the field of organic photovoltaics (OPVs), outdoor stability research has lagged behind material development and device engineering. Testing protocols established at the International Summit of OPV Stability (ISOS) have stimulated some stability research, but these studies are almost exclusively limited to already-refined devices made with already-commercialized materials. If OPV materials were tested outdoors during small-scale stages, stability issues could be detected earlier in the development cycle. Chloro–(chloro)n–boron subnaphthalocyanine (Cl–ClnBsubNc) is a material with high OPV performance but has not previously been tested outdoors. An OPV power conversion efficiency of 8.4% has been previously demonstrated for a trilayer stack containing α-sexithiophene, Cl–ClnBsubNc, and chloro–boron subphthalocyanine (Cl–BsubPc). Building on the most advanced ISOS outdoor testing protocols (ISOS-O3), we assess the outdoor stability of small-scale bilayer and trilayer OPVs while establishing an improved stability screening method for future derivatives. The outdoor stability of Cl–ClnBsubNc is determined to be comparable to that of Cl–BsubPc.

Improving The Efficiencies Of Conducting Conjugated Polymers In The Field Of Organic Photovoltaics

Improving The Efficiencies Of Conducting Conjugated Polymers In The Field Of Organic Photovoltaics

Next-generation organic photovoltaics based on non-fullerene acceptors

Over the past three years, a particularly exciting and active area of research within the field of organic photovoltaics has been the use of non-fullerene acceptors (NFAs). Compared with fullerene acceptors, NFAs possess significant advantages including tunability of bandgaps, energy levels, planarity and crystallinity. To date, NFA solar cells have not only achieved impressive power conversion efficiencies of ~13–14%, but have also shown excellent stability compared with traditional fullerene acceptor solar cells. This Review highlights recent progress on single-junction and tandem NFA solar cells and research directions to achieve even higher efficiencies of 15–20% using NFA-based organic photovoltaics are also proposed.

Morphology Characterization of Bulk Heterojunction Solar Cells

AbstractRevealing the nanostructure of bulk‐heterojunction (BHJ) photovoltaic blends is a critical task in the field of organic photovoltaics. The complicated morphology, ranging from binary blends to ternary mixtures, shows quite varied structural details that need to be fully characterized in terms of correlating them with device performance. The nonequilibrium nature of the BHJ thin‐film morphology makes the structure optimization difficult, and fundamental principles that relate to chemical structure, processing methods, and device performances have not yet been fully understood. On the other hand, various high‐powered techniques ranging from in‐house instrumentation to synchrotron techniques have been applied to this tremendously difficult research topic. More structural details, including length scale of phase separation, structural ordering, chain/molecular orientation, surface topology, etc., can be obtained in a quantitative manner, which finally allows the structure–property relationships to be determined. Here, the most important characterization techniques in understanding the morphology of BHJ solar cells are summarized, targeting chemists and materials scientists.

The Impact of Local Morphology on Organic Donor/Acceptor Charge Transfer States

AbstractA major breakthrough in the field of organic photovoltaics (OPVs) was the development of the donor/acceptor heterojunction that aids in separating Coulombically bound excitons that are generated upon photoabsorption. Additionally, bound charge transfer (CT) states that result from the exchange of charge carriers across the donor/acceptor interface are believed to play an important role in charge generation. Though organic thin films are often disordered, enhancements to the local structural order at the donor/acceptor interface have recently been shown to greatly influence CT state energetics and the charge generation process. In this progress report, recent efforts to understand the role that donor/acceptor morphology plays in the behavior of CT states and the resulting implications on OPV function are presented. It is aimed to provide a survey of different experimental approaches and to present a balanced examination of current interpretations of key results, and to offer best practices for the fabrication and study of morphologically tunable donor/acceptor CT states.

Tunable Electron Donating and Accepting Properties Achieved by Modulating the Steric Hindrance of Side Chains in A-D-A Small-Molecule Photovoltaic Materials

Modulation of the electron donating and accepting properties of organic semiconductors is an important topic in the field of organic photovoltaics. Although the small-molecule semiconductors (SMs) with an A-D-A structure can act as either electron donor or acceptor in organic photovoltaic (OPV) devices, the reason why molecules with similar conjugated structures play different roles remains unclear. In this work, we designed and synthesized two A-D-A SMs named BTCN-O and BTCN-M, which have an identical backbone and differ in the alkyl substitution position. BTCN-O and BTCN-M demonstrate similar optical absorption spectra in solution and molecular energy levels in a solid film. BTCN-O forms an ordered lamellar packed structure with compact π–π stacking, whereas BTCN-M demonstrates only a weak π–π stacking effect in solid film. We also investigated their photovoltaic properties by blending each with a polymer donor, PBDB-T, and a fullerene acceptor, PC71BM, and found that the electron donating and accepting...

Fine-tuning the side-chains of non-fullerene small molecule acceptors to match with appropriate polymer donors

Side-chain engineering of donor and acceptor materials is an important topic in the field of organic photovoltaics.

A series of bowl-shaped PDI dimers designed for organic photovoltaic cells through engineering N-annulated bridge towards potential alternatives of PDI bridged dimer acceptors

Perylene diimides (PDIs) dimer-based non-fullerene acceptors have made significant progress in the field of organic photovoltaics (OPVs) recently. To further render PDI derivatives soluble and tailor self-assembly, the incorporation of N-annulations in PDI dimers becomes an effective strategy in latest researches. Here, we focus on engineering PDI dimers with N-annulated bridges to construct bowl-shaped PDI dimers with the aim of improving efficiency, stereoscopic dimensions and the electron transfer abilities by introducing different functional linkers in N-annulated bridges. Besides some properties relevant to photovoltaic performance of these investigated acceptors were characterized based on density functional theory (DFT) and (time dependent density functional theory) TD-DFT calculations, Marcus semi-classical model was also employed to further estimate the exciton dissociation ability at D/A interface. Our results demonstrate that, compared to planar N-annulated PDI dimers, bowl-shaped PDI dimers possess higher LUMO energy levels, larger open circuit voltage (VOC), complementary optical absorption with the donor PBDTTT-C-T, and more effective charge separation at the interface. Moreover, we highlight that introducing SF and NDT linkers in N-annulated bridges would be more favorable to optical performance. We expect that these modified N-annulated PDI dimers could become potential acceptor materials in OPVs.

Molecular Interlayers in Hybrid Perovskite Solar Cells

AbstractOrganic–inorganic hybrid perovskite solar cells (PSC) are promising third‐generation solar cells. They exhibit good power conversion efficiencies and in principle they can be fabricated with lower energy consumption than many more established technologies. To improve the efficiency and long‐term stability of PSC, organic molecules are frequently used as “interlayers.” Interlayers are thin layers or monolayers of organic molecules that modify a specific interface in the solar cell. Here, the latest progress in the use of interlayers to optimize the performance of PSC is reviewed. Where appropriate interesting examples from the field of organic photovoltaics (OPV) are also presented as there are many similarities in the types of interlayers that are used in PSC and OPV. The review is organized into three parts. The first part focuses on why organic molecule interlayers improve the performance of the solar cells. The second section discusses commonly used molecular interlayers. In the last part, different approaches to make thin and uniform interlayers are discussed.