Non-fullerene Organic Photovoltaics Research Articles

(Invited) Molecular Mechanisms That Enhance Charge Separation at Donor-Acceptor Interfaces in Non-Fullerene Organic Photovoltaics

We analyze molecular mechanisms that enhance charge separation at donor-acceptor interfaces in non-fullerene organic photovoltaics by means of molecular dynamics calculations, quantum chemistry, and QM/MM approach. We consider interfaces of π-conjugated polymers (PM6) and non-fullerene acceptors (Y6 and ITIC) as a model system. We clarified the effect of molecular disorders at the interface on the charge separation efficiency.

Repairing Interfacial Defects in Self-Assembled Monolayers for High-Efficiency Perovskite Solar Cells and Organic Photovoltaics through the SAM@Pseudo-Planar Monolayer Strategy.

Lately, carbazole-based self-assembled monolayers (SAMs) are widely employed as effective hole-selective layers (HSLs) in inverted perovskite solar cells (PSCs). Nevertheless, these SAMs tend to aggregate in solvents due to their amphiphilic nature, hindering the formation of a monolayer on the ITO substrate and impeding effective passivation of deep defects in the perovskites. In this study, a series of new SAMs including DPA-B-PY, CBZ-B-PY, POZ-B-PY, POZ-PY, POZ-T-PY, and POZ-BT-PY are synthesized, which are employed as interfacial repairers and coated atop CNph SAM to form a robust CNph SAM@pseudo-planar monolayer as HSL in efficient inverted PSCs. The CNph SAM@pseudo-planar monolayer strategy enables a well-aligned interface with perovskites, synergistically promoting perovskite crystal growth, improving charge extraction/transport, and minimizing nonradiative interfacial recombination loss. As a result, the POZ-BT-PY-modified PSC realizes an impressively enhanced solar efficiency of up to 24.45% together with a fill factor of 82.63%. Furthermore, a wide bandgap PSC achieving over 19% efficiency. Upon treatment with the CNph SAM@pseudo-planar monolayer, also demonstrates a non-fullerene organic photovoltaics (OPVs) based on the PM6:BTP-eC9 blend, which achieves an efficiency of 17.07%. Importantly, these modified PSCs and OPVs all show remarkably improved stability under various testing conditions compared to their control counterparts.

Device engineering of non-fullerene organic photovoltaics with extrapolated operational T80 lifetime over 45,000 h in air

Device engineering of non-fullerene organic photovoltaics with extrapolated operational T80 lifetime over 45,000 h in air

The role of isomeric alkyl chains of polymer donors on charge-transfer processes in non-fullerene organic photovoltaics: a theoretical investigation

The role of isomeric alkyl chains of polymer donors on charge-transfer processes in non-fullerene organic photovoltaics: a theoretical investigation

Physical insights into non-fullerene organic photovoltaics

Physical insights into non-fullerene organic photovoltaics

Optimization of Non-fullerene Organic Photovoltaics Through Interface Engineering with Graphene Oxide: A Numerical Simulation

Optimization of Non-fullerene Organic Photovoltaics Through Interface Engineering with Graphene Oxide: A Numerical Simulation

Simple π-Conjugated Polymers Based on Bithiazole for Nonfullerene Organic Photovoltaics.

Thiazole, as a family of five-membered heteroaromatic rings, is an interesting building unit that can play a role in coplanarizing the backbone as well as deepening the HOMO energy level, which is beneficial for the design of π-conjugated polymers for the photoactive materials in organic photovoltaics (OPVs). Here, we designed and synthesized π-conjugated polymers with simple chemical structures, which consist of 2,2'-bithiazole or 5,5'-bithiazole and alkylthiophenes as the polymer backbone. In fact, the polymers can be easily synthesized in much fewer steps compared to the typical high-performance polymers based on fused heteroaromatic rings. Interestingly, PTN5 exhibited a markedly higher ordered structure than PTN2. This was likely ascribed to the more coplanar and rigid backbone of PTN5 than that of PTN2 originating in the effectively arranged S···N interaction. As a result, the nonfullerene photovoltaic cell based on PTN5 showed a PCE of 12.2%, which was much higher than the cell based on PTN2 (4.3%) and was high for the polymers consisting of only nonfused rings. These results demonstrate that thiazole-based polymers are promising photoactive materials for OPVs and emphasize the importance of careful molecular design utilizing noncovalent interactions.

Ordered π-conjugated polymer backbone in amorphous blend for high efficiency nonfullerene organic photovoltaics

In π-conjugated polymers, the amorphous region absent from π–π stacking order typically limits polymer functions compared to the crystalline region with high π–π stacking order. Here, we show that a benzodithiophene–thiazolothiazole copolymer containing tripropylsilyl groups (PSTz2) has a greater coplanar backbone structure when the π–π stacking order is absent, such as in solution and in a film blended with a nonfullerene acceptor, than when it is present in a neat film. The excitons and charge carriers generated in PSTz2 are more highly delocalized in the blend film than in the neat film, presumably through the backbone. The unconventional structural feature of PSTz2 shows higher photovoltaic performance in nonfullerene-based cells compared to its alkyl-functionalized counterpart. Our results show that it is possible to develop π-conjugated polymers that perform well in amorphous blends due to the ordered backbone structure.

Effect of terminal fluorination of indenoindene-cored non-fullerene molecular acceptors for photovoltaic application

Two new non-fullerene molecule acceptors (NFAs), IDTIC and IDTIC-4F, based on a 5,10-dihydroindeno[2,1-a]indene core and 4H-cyclopenta[1,2-b:5,4-b']dithiophene (CPDT) bridges, were designed and synthesized. The fluorine-terminated IDTIC-4F shows a larger bathochromic shift than the non-fluorinated IDTIC. When blending with a PBDB-T polymer donor to fabricate organic solar cells, the IDTIC and IDTIC-4F acceptors respectively showed power conversion efficiencies (PCEs) of 5.60% and 6.77%. The PBDB-T:IDTIC-4F device performed better on Jsc and FF, which was attributed to the morphological evolution due to terminal fluorinated design of NFAs. Comparatively, the terminal fluorinated IDTIC-4F contributes to the formation of a more uniform film with less traps, which improves the charge transport and reduces the bimolecular recombination within the device. The work demonstrates that terminal fluorination of NFAs is an efficient design to modifying the morphology and performance of non-fullerene organic photovoltaics.

High performance non-fullerene organic photovoltaics under implant light illumination region

Implantable biomedical electronics, such as pacemakers, drug pumps, cochlear implants, cardioverter-defibrillators, and neurological stimulators, help humans to overcome various diseases. Currently, the power supply for these devices relies on small-size batteries, and replacement of the battery is required after running for a period of time. Recharging the battery could be a way to prolong the replacement cycle. Organic photovoltaics (OPVs) are a class of emerging photovoltaics, which are now becoming more practical with recently developed device and material engineering. The absorption of OPVs using a non-fullerene acceptor (NFA) could be extended to the near-infrared (NIR) region to cover the transmission window of human skin between 650 and 1000 nm. Motivated by this, we conducted a study of NFA-based OPVs under light irradiation of wavelengths of 650–1000 nm for implants. The devices using donor (PTB7-Th) and NFA (IEICO-4F) as the active material have strong absorption in the NIR region and obtained a promising power conversion efficiency (PCE) of 14.3% under the implant light illumination, compared to 8.11% when using a benchmark fullerene derivative-based acceptor (PC71BM). Importantly, the PCE and power density of the NFA-based OPVs are significantly higher than the previously reported fullerene-based OPVs devices. This study shows that NFA-based OPVs have high potential for future applications in powering implants, e.g., through charging batteries.

Selective fluorination on donor and acceptor for management of efficiency and energy loss in non-fullerene organic photovoltaics

Selective fluorination on donor and acceptor for management of efficiency and energy loss in non-fullerene organic photovoltaics

Real‐Time Probing and Unraveling the Morphology Formation of Blade‐Coated Ternary Nonfullerene Organic Photovoltaics with In Situ X‐Ray Scattering

AbstractCharacterizing the bulk heterojunction (BHJ) morphology of the active layer is essential for optimizing blade‐coated organic solar cells (OSCs). Here, the morphology evolution of a highly efficient ternary polymer:nonfullerene blend PM6:N3:N2200 under different blade coating conditions is probed in real‐time by in situ synchrotron X‐ray scattering and in situ ultraviolet‐visible (UV‐vis) spectroscopy. Besides, the morphology of blade‐coated blend films at different conditions is detailed by ex situ X‐ray scattering and microscopic imaging. The ternary blend film exhibited optimized morphology, such as superior molecular stacking structure and appropriate phase separation structure, and boosted photovoltaic performance of the binary blend, as adding a second polymer component to the host polymer:nonfullerene system can balance nucleation and crystallization of polymers and small molecules, facilitating molecular rearrangement to perfect crystallization. Both binary and ternary blends obtained optimized morphology and photovoltaic properties at medium coating speed, mainly attributed to the movement of the polymer and small molecules at the long crystallization and aggregation stage. These findings help understand morphology formation under film drying and provide guidance for optimizing the morphology in blade‐coated OSCs.

Energy differences as descriptors for the correlation between JSC and VOC in nonfullerene organic photovoltaics.

ITIC-series nonfullerene organic photovoltaics (NF OPVs) have realized the simultaneous increases of the short-circuit current density (JSC) and open-circuit voltage (VOC), called the positive correlation between JSC and VOC, which could improve the power conversion efficiency (PCE). However, it is complicated to predict the formation of positive correlation in devices through simple calculations of single molecules due to their dimensional differences. Here, a series of symmetrical NF acceptors blended with the PBDB-T donor were chosen to establish an association framework between the molecular modification strategy and positive correlation. It can be found that the positive correlation is modification site-dependent following the energy variation at the different levels. Furthermore, to illustrate a positive correlation, the energy gap differences (ΔEg) and the energy level differences of the lowest unoccupied molecular orbitals (ΔELUMO) between the two changed acceptors were proposed as two molecular descriptors. Combined with the machine learning model, the accuracy of the proposed descriptor is more than 70% for predicting the correlation, which verifies the reliability of the prediction model. This work establishes the relative relationship between two molecular descriptors with different molecular modification sites and realizes the prediction of the trend of efficiency. Therefore, future research should focus on the simultaneous enhancement of photovoltaic parameters for high-performance NF OPVs.

Thermal Activation of PEDOT:PSS/PM6:Y7 Based Films Leads to Unprecedent High Short‐Circuit Current Density in Nonfullerene Organic Photovoltaics

AbstractFinding an effective approach to suppress trap formation is a potential route for enhancing the performance of nonfullerene organic photovoltaic (NF‐OPVs) devices. Here, an extraordinary short‐circuit current density (JSC) value of 32.65 mA cm‐2 is achieved, higher than the state‐of‐the art NF‐OPVs reported, reaching a high power conversion efficiency (PCE) of 17.92%. This remarkable enhancement is exhibited through the fine‐tuning of PEDOT:PSS/PM6:Y7 films and interface morphologies via applying the prethermal treatment approach (Pre‐TT) to the devices, which exhibit JSC and PCE enhancement of 21% and 8%, respectively, compared to the pristine devices. Accordingly, the dependence of the JSC upon the Pre‐TT approach through a range of morphological, optical, electrical, and advanced transient measurements is investigated. The Pre‐TT‐based films are found to possess optimal smooth blend morphology with better dispersity owing to reduced domain size. Moreover, the measurements show that the optimized treated devices present higher exciton dissociation probabilities and generation rate of the free charge carriers, showing an ideal balanced electron/hole mobility that reveals the JSC and PCE enhancement. Hence, Pre‐TT approach provides a facile passivation strategy that reduces the trap state density of the blend film, improves interface charge transfer, allows balanced electron/hole mobility, and thus promotes device performance.

Quantum Mechanical Assessment of Optimal Photovoltaic Conditions in Organic Solar Cells.

Recombination losses contribute to reducing JSC, VOC, and the fill factor of organic solar cells. Recent advances in non-fullerene organic photovoltaics have shown, nonetheless, that efficient charge generation can occur under small energetic driving forces (ΔEDA) and low recombination losses. To shed light on this issue, we set up a coarse-grained open quantum mechanical model for investigating the charge generation dynamics subject to various energy loss mechanisms. The influences of energetic driving force, Coulomb interaction, vibrational disorder, geminate recombination, temperature, and external bias are included in the analysis of the optimal photovoltaic conditions for charge carrier generation. The assessment reveals that the overall energy losses are not only minimized when ΔEDA approaches the effective reorganization energy at the interface but also become insensitive to temperature and electric field variations. It is also observed that a moderate reverse bias reduces geminate recombination losses significantly at vanishing driving forces, where the charge generation is strongly affected by temperature.

Revealing conflicting effects of solvent additives on morphology and performance of non-fullerene organic photovoltaics under different illuminance conditions

The use of solvent additives is effective for controlling the morphology and efficiency of non-fullerene bulk-heterojunction (BHJ) organic photovoltaics (OPVs) under both indoor and outdoor conditions. In this study, we examined the effects of solvent additives on the morphology and photovoltaic operation of non-fullerene OPVs under different illuminance conditions. Solvent additives (e.g., 1,8-diiodooctane) improved molecular packing and charge transport in non-fullerene BHJ layers, increasing the power conversion efficiency (PCE) under 1-sun illumination. However, incorporating the solvent additives promoted the formation of a crystallization-induced roughened surface and an isolated-non-fullerene domain in the intermixing region. These unexpected morphological features increased the degree of trap-assisted charge recombination and leakage current, significantly reducing the PCE of non-fullerene OPVs at low light levels corresponding to indoor lighting. Understanding the effect of solvent additives inspires us to further control the morphological features of the non-fullerene OPVs by fine-tuning the solvent-additive concentration beyond the globally accepted level. This was effective for optimizing the indoor PCE of non-fullerene OPVs (21.71%), which exceeded that of the conventionally accepted reference (18.02%). This study provides guidelines for understanding the critical roles of solvent additives and optimizing the morphology of non-fullerene BHJs for high-performance indoor OPV applications.

Promoting the Efficiency and Stability of Nonfullerene Organic Photovoltaics by Incorporating Open-Cage [60]Fullerenes in the Nonfullerene Nanocrystallites.

The device efficiency of PM6:Y6-based nonfullerene organic solar cells is fast advanced recently. To maintain organic solar cells (OSCs) with high power conversion efficiency over 16% in long-term operation, however, remains a challenge. Here, a novel non-volatile additive, an open-cage [60]fullerene (8OC60Me), is incorporated into PM6:Y6-based OSCs for high-performance with high durability. With optimized addition of 1.0 wt % 8OC60Me, the PCE value of PM6:Y6/8OC60Me OSCs can be promoted to 16.5% from 15.0%. Most strikingly, such a high PCE performance can maintain nearly 100% for over 500 h at room temperature; at an elevated operation temperature of 80 °C, the PCE can be stabilized above 15.0% after 45 h of operation. Grazing incidence small- and wide- angle X-ray scattering studies reveal improved orientation and crystallinity of Y6 in a fractal-like network structure of PM6 in PM6:Y6/8OC60Me films under in situ annealing, parallel to the enhanced electron mobility. Analysis of charge distributions lines up possible van der Waals interaction between the thienyl/carbonyl moiety of 8OC60Me and difluorophenyl-based FIC-end groups of Y6. This result is of great contrast to those devices with the best-selling PC61BM as the additives─8OC60Me might be of interest to be incorporated into future Y6-based OSCs for concomitantly improved PCE and excellent stability.

PS-b-PMMA templated ITO electrodes for improving the performance of non-fullerene organic photovoltaics

We have developed a block copolymer (BCP)-based patterned electrode with a periodic structure of 80 nm and investigated their application in organic photovoltaics. The indium tin oxide (ITO) electrodes with the thicknesses of 30, 50, 70, 90, and 120 nm were sputtered onto BCP-patterned templates and the morphology and optoelectronic properties were investigated. Using the finite difference in time domain method, the effect of different thickness electrodes on the light absorption of the active layer was studied. We found that the ITO thickness of 70 nm has an absorption enhancement effect in the visible wavelength band. Finally, we prepared organic photovoltaic devices based on the non-fullerene PBDB-T:ITIC system, the BCP-patterned templates enhance greatly the light absorption of the active layer through both light diffraction and coupling to surface plasmon polariton modes achieving a 12.9% increase in power conversion efficiency. • ITO electrodes with 80 nm periodic structure were developed by BCP self-assembly. • Patterned ITO greatly enhance the light absorption of the PBDB-T:ITIC active layer. • OPVs with patterned ITO achieve a 12.9% increase in power conversion efficiency.

Machine Learning-Assisted Polymer Design for Improving the Performance of Non-Fullerene Organic Solar Cells.

Despite the progress in machine learning (ML) in terms of prediction of power conversion efficiency (PCE) in organic photovoltaics (OPV), the effectiveness of ML in practical applications is still lacking owing to the complex structure-property relationship. Therefore, verifying the potential of ML through experiments can amplify the use of ML models. Herein, we developed a new series of π-conjugated polymers comprising benzodithiophene and thiazolothiazole with fluorination and alkylthio chains (PBDTTzBO, PFSBDTTzBO, and PFBDTTzBO) for non-fullerene (NF) acceptors based on our random-forest ML model for OPVs. Notably, the order of the ML-predicted PCEs of these polymers with IT-4F (9.93, 11.35, and 11.47%) was in good agreement with their experimental PCEs (5.24, 7.35, and 10.30%). In contrast, an inverse correlation was observed between the predicted (9.20, 12.29, and 12.20%) and experimental (11.98, 1.57, and 6.53%) PCEs with Y6. Both the findings are interpreted in terms of surface morphology, transient photoconductivity, charge carrier mobility, polymer orientation, and miscibility, quantified by the Flory-Huggins parameters. Herein, we present an ML-assisted polymer design for high-performance non-fullerene organic photovoltaics (NFOPVs) and elucidate the importance of the subtle alterations in the morphology of NFOPVs.

Slow Hole Transfer Kinetics Lead to High Blend Photoluminescence of Unfused A–D–A′–D–A‐Type Acceptors with Unfavorable Highest Occupied Molecular Orbitals Offset

Nonfullerene organic photovoltaics (OPVs) with small energetic offset between donor and acceptor can provide a much‐reduced voltage loss and power conversion efficiencies over 18%, challenging the previous understanding of the charge generation mechanisms. Herein, a study is presented on nonfullerene OPVs with negative energetic offsets, by investigating a model system which exhibits poor photoluminescence quenching (PLQ, ≈17%) yet reasonably high external quantum efficiency (EQE, ≈50%). It is found that the discrepancy between the poor PLQ and relatively high EQE is dependent on the measurement conditions, namely, short or open circuit, under which the devices exhibit differences in charge generation. Transient absorption spectroscopy shows surprisingly slow hole transfer on nanosecond timescale. It is revealed that the poor PLQ/high radiative recombination originates from an increased back transfer from free carriers to the singlet excitons under open circuit. It is made possible by the slow charge‐transfer (CT) kinetics, long‐lived CT states, and emissive excitons. Despite the negative offset, the charge generation can be relatively efficient under short circuit. The results suggest that the PLQ under open circuit is not necessarily a good indicator of charge separation efficiency and provide new insight to efficient charge generation of OPVs with unfavorable energy offsets.