High-efficiency Organic Photovoltaics Research Articles

Shamrock-shaped Non-fullerene Acceptors Enable High-Efficiency and High-Voltage Organic Photovoltaics

Minimizing energy loss to increase open-circuit voltage (VOC) is an essential topic to further improve the power conversion efficiency (PCE) in organic photovoltaics (OPVs). Though various molecular strategies have been...

Delayed Crystallization Kinetics Allowing High-Efficiency All-Polymer Photovoltaics with Superior Upscaled Manufacturing.

Though encouraging performance is achieved in small-area organic photovoltaics (OPVs), reducing efficiency loss when evoluted to large-area modules is an important but unsolved issue. Considering that polymer materials show benefits in film-forming processability and mechanical robustness, a high-efficiency all-polymer OPV module is demonstrated in this work. First, a ternary blend consisting of two polymer donors, PM6 and PBQx-TCl, and one polymer acceptor, PY-IT, is developed, with which triplet state recombination is suppressed for a reduced energy loss, thus allowing a higher voltage; and donor-acceptor miscibility is compromised for enhanced charge transport, thus resulting in improved photocurrent and fill factor; all these contribute to a champion efficiency of 19% for all-polymer OPVs. Second, the delayed crystallization kinetics from solution to film solidification is achieved that gives a longer operation time window for optimized blend morphology in large-area module, thus relieving the loss of fill factor and allowing a record efficiency of 16.26% on an upscaled module with an area of 19.3 cm2 . Besides, this all-polymer system also shows excellent mechanical stability. This work demonstrates that all-polymer ternary systems are capable of solving the upscaled manufacturing issue, thereby enabling high-efficiency OPV modules.

Interfacial modification of LiF-incorporated MnO2 mixed P3HT:PC60BM-based organic photoactive layer

Interfacial charge transfer in the photoactive layer plays an essential role in determining device performance in organic photovoltaics. Recently, numerous studies have reported interfacial modification by incorporating inorganic nanostructures into the bulk heterojunction photoactive layer, with the aim of increasing the interfacial area to improve charge transfer efficiency. However, there is limited understanding of in-depth defect analysis and the charge transfer mechanism. In this report, different weight percentages of LiF were incorporated into MnO2 (LMO) mixed photoactive layers. The robust relationship between charge transport dynamics and defect density of the device in the presence of LiF is elucidated. The results show that the photoactive layer with 10.0 wt% LMO exhibited a 41% improvement in device performance compared to that of pristine P3HT:PC60BM, with a power conversion efficiency (PCE), open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF) of 2.43%, 0.52 V, 9.77 mA/cm2, and 0.48, respectively. The photovoltaic performance improvement can be attributed to (i) the efficient charge transfer properties in the 10.0 wt% LMO, as evidenced by the suppressed S 2p and intensified O=C in X-ray photoelectron spectroscopy (XPS) analysis, and (ii) the low carrier recombination due to the success of LiF as a surface passivation agent, as indicated by 53% photoluminescence quenching. This report provides a fundamental understanding and offers an alternative solution for designing high-efficiency organic photovoltaics in the near future.

Spontaneous vertical phase distribution of multi-acceptors system enables high-efficiency organic photovoltaics in non-halogenated solvent and large-area module application

Building a vertical phase distribution morphology with a simple method is the critical key for high-efficiency organic photovoltaics (OPVs) and their industrial application. Here, OPVs based on a quaternary system with one donor PM6 and three acceptors (L8-BO, BTP-S8, and BTP-S2) were fabricated. The L8-BO-based blend was demonstrated to show unique vertical phase distribution (acceptors enriched on the top surface), leading to an exquisite morphology and superior carrier behaviors, thus power conversion efficiencies (PCEs) of 19.19% (certified as 19.18%) and 19.04% were achieved for chloroform-processed and toluene-processed devices, respectively. The high efficiency obtained from greener solvent is encouraging for commercialization of OPVs. Notably, the device processed with toluene without any additives shows a PCE of 17.28% with good stability, and the upscaled large-area module with a total area of 72.25 cm2 processed in air presents an outstanding PCE of 12.78%. This work provides valuable insights into the fabrication of greener solvent and large-area module OPVs.

Impact of Polarization Effect on Exciton Binding Energies and Charge Transport for the Crystals of Chlorinated ITIC Derivatives

Exciton binding energy (Eb) and carrier mobility are the key parameters to determine organic photovoltaic performance. Here, the impact of polarization effect on the Eb and carrier mobility in the crystals of different end-group-chlorinated ITIC isomers have been investigated by the self-consistent quantum mechanics/embedded charge method. In contrast to large values and small fluctuation for the gas phase Eb, the Eb in the solid crystals is substantially decreased and ranges from 0.08 to 0.36 eV due to important charge polarization and distinct molecular packing. Because of the counteraction between the electrostatic effect of the hole and electron, a good linear relationship is found between the Eb and the charge induction term. Accordingly, the nonequivalent molecules in each crystal have similar Eb. However, their ionization potential and electron affinity can be much changed due to different electrostatic effects, leading to appreciable site energy differences. Thus, the carrier mobility is decreased to varying degrees, up to 3 orders of magnitude. Remarkably, benefiting from a three-dimensional dense packing, α-ITIC-2Cl possesses the smallest Eb and the highest electron mobility. These results underline that chlorination is an effective way to tune molecular packing to simultaneously reduce Eb and improve carrier mobility toward high-efficiency organic photovoltaics.

Achieving High-Efficiency Organic Photovoltaics from a New Completely Amorphous Donor Polymer

Achieving High-Efficiency Organic Photovoltaics from a New Completely Amorphous Donor Polymer

Rational tuning of intermolecular and intramolecular interactions enabling high-efficiency indoor organic photovoltaics

Indoor organic photovoltaics (IOPVs) are one of promising candidates for transferring artificial illumination to power the Internet of Things (IoT). However, their power conversion efficiencies (PCE) are limited by the fact that only a few efficient non-fullerene acceptors are available for IOPVs. Herein, new strategy is adopted to tune the intramolecular charge transfer (intra-CT) and intermolecular charge transfer (inter-CT) to achieve high PCEs under indoor environment in the combination of changing skeleton from symmetry to asymmetry and end group from chlorination to non-chlorination. The study reveals that, in comparison with symmetric molecule BTP-2ThCl, asymmetric structure TB-SCl possesses a smaller intra-CT resulting in higher VOC and strengthened inter-CT for neighboring acceptors resulting in more efficient carrier collection and thus higher FF. Moreover, the non-chlorine molecule TB-S exhibits smaller intra and inter-CT leading to further promoting VOC but smaller FF compared with TB-SCl under 1-sun illumination. When the illumination is switched to artificial light, the mitigated trap state, minimized leakage current, and suppressed non-radiative energy loss (∆Enonrad) enable a novel acceptor TB-S yielding high-efficiency rigid PCE of 23.3 % which surpasses TB-SCl and flexible PCE of 20.02% which is one of the best PCE for flexible IOPVs. Our observation provides a new insight into molecule structure modification to promote the indoor photovoltaic performance.

Important role of alloyed polymer acceptor for high efficiency and stable large-area organic photovoltaics

We developed high performance PM6:N3 bulk-heterojunctions by synthesizing polymers as the second acceptor. The acceptor PY-P2 where a biphenyl linker makes the polymer chain to have high dihedral angles forms intricate alloy-like composites in N3 domains whereas stiff polymer chain of PY-T2 exists separated from the acceptor domain. Introduction of PY-P2 results in much longer lifetimes and diffusion lengths of excitons generated in N3 domain of the PM6:N3:PY-P2 blend compared to those of the excitons in PM6:N3 as well as PM6:N3:PY-T2. Consequently, the PM6:N3:PY-P2 based OPV devices show improved exciton dissociation and change transport with reduced charge recombination. The PM6:N3:PY-P2 organic photovoltaic (OPV) devices prepared with blade-coating at 1 cm2 active area achieved efficiency of 15.2% compared with 12.9% of the PM6:N3 control device; whereas, the corresponding OPV device using PY-T2 shows decreased efficiency of 11.7%. OPV mini-module (active area 5.4 cm2) with PY-P2 achieves high efficiency of 14.7% compared with 11.9% of the PM6:N3 devices. Furthermore, the alloyed PY-P2 acceptor effectively improves OPV thermal stability under 85 °C heating for 1000 h, compared with PY-T2 and control devices. We demonstrated importance of the second polymer acceptor for achieving high-performance large-area OPV, which is significantly affected by its chemical structure.

Desired open-circuit voltage increase enables efficiencies approaching 19% in symmetric-asymmetric molecule ternary organic photovoltaics

<h2>Summary</h2> The large open-circuit voltage (<i>V</i>_oc) loss is currently the main obstacle for pursuing the highly efficient organic photovoltaics (OPVs). To address this issue, we construct the ternary OPVs (TOPVs) consisting of symmetric BTP-eC9 and asymmetric BTP-S9 acceptors with similar absorption profiles and demonstrate it as a feasible way to increase <i>V</i>_oc via reducing the energetic disorder and non-radiative decay. This strategy enables a desired higher voltage in TOPVs than those in both binary OPVs, without sacrificing absorbing range. Besides, the ternary blend exhibits enhanced charge transport property due to stronger π-π stacking than binary films, resulting in the enhanced fill factor (FF). Consequently, the optimal TOPV exhibits a champion efficiency of 18.8% (certified as 18.7%) with a high voltage of 0.861 V and a high FF of 79.34%, representing the best among certified single-junction OPVs. This work demonstrates an effective approach to increase <i>V</i>_oc, paving the way toward high-efficiency OPVs.

Toward High-Efficiency Organic Photovoltaics: Perspectives on the Origin and Role of Energetic Disorder.

The power conversion efficiency of organic photovoltaics is strongly limited by relatively large energy loss, which is partially due to the disordered nature of organic semiconductors. This disordered nature not only hinders the rational design of molecules with excellent photophysical properties but also prevents a more thorough understanding of the inherent link between microscopic parameters and physical phenomena. In this Perspective, we demonstrate that the injection-dependent emission line-shape in organic semiconductors is primarily associated with a state-filling effect, where the extent of spectral blue-shift can be a strong indicator for energetic disorder. Molecular geometry with rigidity and coplanarity not only promotes preferential face-on stacking that narrows the energetic distribution of subgap states but also impedes torsional deformations of the conjugated backbone away from planarity, thereby facilitating larger π-electron delocalization. These structural characteristics explain the seemingly contradictory high radiative efficiency of low-bandgap nonfullerene molecules, providing promising molecular design strategies to realize high-efficiency organic photovoltaics.

Design Rules of the Mixing Phase and Impacts on Device Performance in High-Efficiency Organic Photovoltaics.

In nonfullerene acceptor- (NFA-) based solar cells, the exciton splitting takes place at both domain interface and donor/acceptor mixture, which brings in the state of mixing phase into focus. The energetics and morphology are key parameters dictating the charge generation, diffusion, and recombination. It is revealed that tailoringthe electronic properties of the mixing region by doping with larger-bandgap components could reduce the density of state but elevate the filling state level, leading to improved open-circuit voltage (VOC) and reduced recombination. The monomolecular and bimolecular recombinations are shown to be intercorrelated, which show a Gaussian-like relationship with VOC and linear relationship with short-circuit current density (JSC) and fill factor (FF). The kinetics of hole transfer and exciton diffusion scale with JSC similarly, indicating the carrier generation in mixing region and crystalline domain are equally important. From the morphology perspective, the crystalline order could contribute to VOC improvement, and the fibrillar structure strongly affects the FF. These observations highlight the importance of the mixing region and its connection with crystalline domains and point out the design rules to optimize the mixing phase structure, which is an effective approach to further improve device performance.

Haze-enhanced ZnO/Ag/ZnO nanomesh electrode for flexible, high-efficiency indoor organic photovoltaics

An oxide/metal/oxide multilayer electrode is employed to improve the mechanical flexibility as well as power conversion efficiency of organic photovoltaics. However, its performance needs to be further improved to provide a higher conversion efficiency and better environmental compatibility with curved indoor electronics. In this study, ZnO/Ag/ZnO nanomesh electrodes are incorporated into the inverted non-fullerene organic photovoltaics to enhance the efficiency under indoor and outdoor lighting illumination in a flexible mode. The opto-electrical properties of the perforated ZnO/Ag/ZnO nanomesh electrode with different hole sizes are compared with those of the planar ZnO/Ag/ZnO and indium tin oxide electrodes. The micro-cavity effect and haze effect, which plays a crucial role in determining the performance of the organic photovoltaics, are directly related to the hole diameter. Despite higher transmittance of indium tin oxide, organic photovoltaics using ZnO/Ag/ZnO nanomesh electrodes with a hole diameter of 350 nm exhibits an average conversion efficiency of 15.7% under a 1000 lux light-emitting diode lamp; this efficiency is 45.3% and 27.6% greater than those of organic photovoltaics using indium tin oxide and planar ZnO/Ag/ZnO electrodes, respectively. Furthermore, all ZnO/Ag/ZnO nanomesh-based organic photovoltaics show much higher mechanical flexible properties than those of the planar ZnO/Ag/ZnO-based organic photovoltaics. • ZnO/Ag/ZnO nanomesh is employed as a cathode for flexible organic photovoltaics. • Opto-electrical properties of the ZnO/Ag/ZnO nanomesh electrode are optimized. • As the hole size decreases, optical haze and micro-cavity effect are found to increase. • Fabricated photovoltaic shows a 15.7% efficiency under 1000 lux LED light. • Fabricated photovoltaic also exhibits an excellent mechanical flexibility.

Micropower energy harvesting using high-efficiency indoor organic photovoltaics for self-powered sensor systems

Micropower energy harvesting using high-efficiency indoor organic photovoltaics for self-powered sensor systems

Enhanced Efficiency and Excellent Thermostability in Organic Photovoltaics via Ternary Strategy with Twisted Conjugated Compound.

The thermal stability of high-efficiency organic photovoltaics (OPVs) is critical to the manufacturing of this technology. Therefore, targeted strategies and approaches shall be developed to improve efficiency and stability, simultaneously. Herein, a seleno twisted benzodiperylenediimides (TBD-PDI-Se) acceptor-doping strategy is taken advantage of to demonstrate the ternary bulk heterojunction OPVs with excellent stability, achieving outstanding power conversion efficiency of 14.43% and 17.25% based on PM6:IT-4F and PM6:Y6 ternary blend devices, respectively, which are superior to the corresponding binary devices. As evidenced by the active layer morphology, exciton dynamic study and the characterizations of the enabled device, the ternary blend device keeps nearly 90% original efficiency (t= 1000 h) under continuous constant heating at 140°C. Furthermore, the application of acceptor as the third component in PBDB-T:ITIC, J71:ITIC, and PBDB-T:PC71 BM systems is also verified, proving the good universality of acceptor-doping ternary strategy.

Reduced non-radiative charge recombination enables organic photovoltaic cell approaching 19% efficiency

<h2>Summary</h2> Reducing non-radiative charge recombination is of critical importance to achieving high-performance organic photovoltaic (OPV) cells. The correlation between the exciton behaviors and non-radiative charge recombination is rarely studied. In this work, we achieved an increase in the exciton diffusion length (<i>L</i>_D) in the acceptor phase via introducing HDO-4Cl to the PBDB-TF:eC9-based system. Compared with the eC9-based film, the exciton <i>L</i>_D in the HDO-4Cl:eC9-based film is increased from 12.2 to 16.3 nm. The enlarged exciton <i>L</i>_D can obviously decrease the non-radiative charge recombination and increase the efficiency of photon utilization in the PBDB-TF:eC9-based OPV cell. Finally, we not only obtained an outstanding power conversion efficiency (PCE) of 18.86% but also demonstrated the correlations between the non-radiative energy loss and exciton behaviors. The results show that regulating the exciton behaviors is an effective way to reduce the non-radiative energy loss and realize high-efficiency OPV cells.

Rational Molecular Design of Nonfullerene Acceptors for Bulk Heterojunction Solar Cells

Organic photovoltaics (OPVs) have received considerable attention as clean energy conversion devices because of their lightweightness, cost effectiveness, flexibility, and suitability for roll-to-roll printing. In the past few decades, the active layer of OPV devices has usually consisted of conjugated polymers as donors and fullerene derivatives as acceptors, which typically form a bulk heterojunction (BHJ) structure.1,2 The power conversion efficiency (PCE) of OPVs based on fullerene derivative acceptors has reached over 10%. However, it is difficult to further increase the PCE of fullerene-based OPVs due to their inherent limitations including arduous energy-level tunability and low absorption profile. To enhance the photovoltaic performance of OPVs, nonfullerene acceptors (NFAs) with aromatic fused-ring structures have been extensively developed in recent years to fabricate high-efficiency OPVs. Compared to traditional fullerene acceptors, NFAs possess several advantages, such as facile synthesis and high absorption profile. Most high-performance NFAs possess acceptor-donor-acceptor (A-D-A) type structures, in which fused multi-ring ladder structures are used as the D unit and 1,1-dicyanomethylene-3-indanone (IC) derivatives as the A unit.In this talk I will give an overview of our recent studies on novel nonfullerene acceptors. In paticular, several nonfullerene acceptors possessing different D units have been designed and synthesized to address the relationship between the structure and the photophysical and photovoltaic properties of the A-D-A type nonfullerene acceptors.3,4 [1] T. Umeyama and H. Imahori, J. Mater. Chem. A (Feature Article), 2, 11545-11560 (2014).[2] T. Umeyama and H. Imahori, Acc. Chem. Res. 52, 2046-2055 (2019).[3] T. Umeyama, K. Igarashi, D. Sasada, Y. Tamai, K. Ishida, T. Koganezawa, S. Ohtani, K. Tanaka, H. Ohkita, and H. Imahori, Chem. Sci., 11, 3250-3257 (2020).[4] T. Umeyama, K. Igarashi, D. Sasada, K. Ishida,T. Koganezawa, S. Ohtani, K. Tanaka, and H. Imahori, ACS Appl. Mater. Interfaces, 12, 39236-39244 (2020).

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.

Optimization of the Bulk Heterojunction of All-Small-Molecule Organic Photovoltaics Using Design of Experiment and Machine Learning Approaches

All-small-molecule organic photovoltaic (OPV) cells based upon the small-molecule donor, DRCN5T, and nonfullerene acceptors, ITIC, IT-M, and IT-4F, were optimized using Design of Experiments (DOE) and machine learning (ML) approaches. This combination enables rational sampling of large parameter spaces in a sparse but mathematically deliberate fashion and promises economies of precious resources and time. This work focused upon the optimization of the core layer of the OPV device, the bulk heterojunction (BHJ). Many experimental processing parameters play critical roles in the overall efficiency of a given device and are often correlated and thus are difficult to parse individually. DOE was applied to the (i) solution concentration of the donor and acceptor ink used for spin-coating, (ii) the donor fraction, (iii) the temperature, and (iv) duration of the annealing of these films. The ML-based approach was then used to derive maps of the power conversion efficiencies (PCE) landscape for the first and second rounds of optimization to be used as guides to determine the optimal values of experimental processing parameters with respect to PCE. This work shows that with little knowledge of a potential combination of components for a given BHJ, a large parameter space can be effectively screened and investigated to rapidly determine its potential for high-efficiency OPVs.

High-Efficiency Organic Photovoltaic Cells With an Antimony Quantum Sheet Modified Hole Extraction Layer

2D materials with unique opto-electronic properties have shown immense potential for application in organic photovoltaics (OPVs). In this article, we reported a facile and efficient strategy to enhance the device performance of OPVs by doping 2D material of antimonene quantum sheets (AMQSs) into PEDOT:PSS emulsion. The interfacial properties between the hole extraction layer (HEL) and PBDB-T:ITIC-based active layer were properly meliorated, achieving boosted power conversion efficiency ( PCE ). Results indicate that the introduced AMQSs could help suppress the recombination loss and improve charge extraction efficiency. Over 8.09% enhancement of PCE is observed compared to that of the control cell with pure PEDOT:PSS. This work provides a simple and valid strategy of HEL modification to realize high efficiency OPVs.

High-Efficiency Indoor Organic Photovoltaics with a Band-Aligned Interlayer

Summary The emergence of indoor electronic devices for internet of things (IoT) has motivated the scientific community to develop photovoltaic devices that can efficiently convert indoor light into electricity. In this paper, we report high-efficiency non-fullerene organic photovoltaic (OPV) cells with over 30% power conversion efficiency (PCE) under indoor conditions. Our results show that the choice of electron-transporting layer (ETL) is critically important to enable such performance. The use of an ETL (named PDI-NO) with a deep highest occupied molecular orbital (HOMO) level can effectively suppress leakage current and reduce trap-assisted recombination of the devices. Thus, using this ETL, we achieve record PCE of 31% by utilizing a low-band-gap acceptor in the bulk-heterojunction (BHJ) blend. Whereas, in another case, by employing a large-band-gap acceptor, a PCE of 26.7% with over 1V is achieved. Our study paves the way toward high-performance indoor OPV devices for powering IoT electronics.