Non-fullerene Organic Photovoltaics Research Articles

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.

Unveiling structure-performance relationships from multi-scales in non-fullerene organic photovoltaics

Unveiling the correlations among molecular structures, morphological characteristics, macroscopic properties and device performances is crucial for developing better photovoltaic materials and achieving higher efficiencies. To achieve this goal, a comprehensive study is performed based on four state-of-the-art non-fullerene acceptors (NFAs), which allows to systematically examine the above-mentioned correlations from different scales. It’s found that extending conjugation of NFA shows positive effects on charge separation promotion and non-radiative loss reduction, while asymmetric terminals can maximize benefits from both terminals. Another molecular optimization is from alkyl chain tuning. The shortened alkyl side chain results in strengthened terminal packing and decreased π-π distance, which contribute high carrier mobility and finally the high charge collection efficiency. With the most-acquired benefits from molecular structure and macroscopic factors, PM6:BTP-S9-based organic photovoltaics (OPVs) exhibit the optimal efficiency of 17.56% (certified: 17.4%) with a high fill factor of 78.44%, representing the best among asymmetric acceptor based OPVs. This work provides insight into the structure-performance relationships, and paves the way toward high-performance OPVs via molecular design.

Non-fullerene organic photovoltaics based on thienopyrroledione comprising random copolymers; effect of alkyl chains

Two new random donor-acceptor (D-A) copolymers, signed as P1 and P2, were designed and synthesized. Electrochemical and spectroelectrochemical measurements were performed to investigate absorption, energy levels, electronic and optical band gaps for comparison. The polymers were used as donor polymers in the active layer to fabricate non-fullerene, bulk heterojunction (BHJ) organic photovoltaics (OPVs). Investigations were carried out through the conventional BHJ structure; ITO/PEDOT: PSS/Active Layer/LiF/Al, where active layer consists of 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (ITIC) as the acceptor and thienopyrroledione containing donors. The device based on P1:ITIC(1:1) blend with a thickness of 161 nm gave the best performance with a power conversion efficiency (PCE) of 7.94%, an open-circuit voltage (VOC) of 0.86 V, a short-current density (JSC) of 18.45 mA cm−2 and a fill factor (FF) of 50.12%. The highest PCE obtained from P2 based organic solar cell is 1.96%. P2 exhibited low solubility attributed to the lack of alkyl groups enhancing polymer solubility, electronic properties, and photovoltaic performances. The research outputs exhibit that introduction of alkyl chains on the polymer backbone can enhance device performance.

Realizing high-performance organic solar cells through precise control of HOMO driving force based on ternary alloy strategy

A good deal of studies have proven that effective exciton dissociation and fast hole transport can operate efficiently in non-fullerene organic photovoltaics (OPVs) despite nearly zero driving force. Even so, whether such a phenomenon is universal and how small the driving force can realize the best photovoltaic performance still require a thorough understanding. Herein, despite the zero driving force based on PM6:F8IC system, a maximum short-circuit current (Jsc) of 23.0 mA/cm2 and high power conversion efficiency (PCE) of 12.2% can still be achieved. Due to the continuously adjustable energy levels can be realized in organic semiconducting alloys including F8IC:IT-4F and F8IC: Y6, the suitable third components can play the role of energy level regulator. Therefore, the HOMO energy level offset (ΔEHOMO(D‑A)) from zero to 0.07 and 0.06 eV is accomplished in the optimized IT-4F and Y6 ternary devices. Consequently, both ternary devices achieved substantially increased PCE of 13.8% and Jsc of 24.4 and 25.2 mA/cm2, respectively. Besides, pseudo-planar heterojunction (PPHJ) devices based on alloyed acceptors through sequential spin-coating method further improve the photovoltaic performance. Our work puts forward the concept of energy level regulator and prove that the ternary alloy strategy has unique advantages and huge research potential in continuously adjusting the driving force.

Understanding the Critical Role of Sequential Fluorination of Phenylene Units on the Properties of Dicarboxylate Bithiophene-Based Wide-Bandgap Polymer Donors for Non-Fullerene Organic Solar Cells.

Design and development of wide bandgap (WBG) polymer donors with low-lying highest occupied molecular orbitals (HOMOs) are increasingly gaining attention in non-fullerene organic photovoltaics since such donors can synergistically enhance power conversion efficiency (PCE) by simultaneously minimizing photon energy loss (Eloss ) and enhancing the spectral response. In this contribution, three new WBG polymer donors, P1, P2, and P3, are prepared by adding phenylene cores with a different number of fluorine (F) substituents (n=0, 2, and 4, respectively) to dicarboxylate bithiophene-based acceptor units. As predicted, fluorination effectively aides in the lowering of HOMO energy levels, tailoring of the coplanarity and molecular ordering in the polymers. Thus, fluorinated P2 and P3 polymers show higher coplanarity and more intense interchain aggregation than P1, leading to higher charge carrier mobilities and superior phase-separated morphology in the optimized blend films with IT-4F. As a result, both P2:IT-4F and P3:IT-4F realize the best PCEs of 6.89% and 7.03% (vs 0.16% for P1:IT-4F) with lower Eloss values of 0.65 and 0.55eV, respectively. These results signify the importance of using phenylene units with sequential fluorination in polymer backbone for modifying the optoelectronic properties and realizing low Eloss values by synergistically lowering the HOMO energy levels.

Optically Probing Field-Dependent Charge Dynamics in Non-Fullerene Organic Photovoltaics with Small Interfacial Energy Offsets

Current non-fullerene organic photovoltaics (OPVs) can achieve high photocurrent generation yields with small donor/acceptor energy offsets. However, the effect of varying energy offset on the char...

An isomerized alkyl side chain enables efficient nonfullerene organic photovoltaics with good tolerance to pre/post-treatments

An alkyl isomerization strategy is reported to finely modulate the crystallinity of nonfullerene acceptors as well as their photovoltaic responses to post-treatments.

Improved Blend Film Morphology and Free Carrier Generation Provide a High-Performance Ternary Polymer Solar Cell.

Non-fullerene organic photovoltaics (OPVs) have displayed the highest power conversion efficiencies (PCEs) among OPVs. Herein, we describe a two-donor (PM6, TPD-3F)/one-acceptor (Y6) ternary blend having an optimized blend morphology that leads to improved OPV performance. Because TPD-3F has a HOMO energy level deeper than that of PM6, the value of VOC of the corresponding ternary device increased. Good miscibility between PM6 and TPD-3F, in conjunction with device optimization through the use of 1-chloronaphthalene as an additive, provided an optimized ternary blend morphology for efficient exciton dissociation and carrier transport and, therefore, larger PCE. Compared with the preoptimized PM6:Y6 binary device, the ternary device functioned with improvements in its short-circuit current density, value of VOC, and fill factor. As a result, the device PCE improved from 15.5 ± 0.19 to 16.6 ± 0.25% under AM 1.5G (100 mW cm-2) irradiation. The champion cell exhibited a PCE of 17.0%-a value that is one of the highest for a ternary OPV. Furthermore, such devices exhibited outstanding shelf lifetimes, with long-term stability in air (25 °C, 40% humidity) without encapsulation; the performance remained high (at 15.4%) after storage for 820 h.

Efficient Double- and Triple-Junction Nonfullerene Organic Photovoltaics and Design Guidelines for Optimal Cell Performance

The performance of multijunction devices lags behind single-junction organic photovoltaics (OPVs) mainly because of the lack of suitable subcells. Here, we attempt to address this bottleneck and de...

High-Performance Nonfullerene Organic Photovoltaics Applicable for Both Outdoor and Indoor Environments through Directional Photon Energy Transfer.

With the advent of the smart factory and the Internet of Things (IoT) sensors, organic photovoltaics (OPVs) gained attention because of their ability to provide indoor power generation as an off-grid power supply. To satisfy these applications, OPVs must be capable of power generation in both outdoor and indoor at the same time for developing environmentally independent devices. For high performances in indoor irradiation, a strategy that maximizes photon utilization is essential. In this study, graphene quantum dots (GQDs), which have unique emitting properties, are introduced into a ZnO layer for efficient photon utilization of nonfullerene-based OPVs under indoor irradiation. GQDs exhibit high absorption properties in the 350-550 nm region and strong emission properties in the visible region due to down-conversion from lattice vibration. Using these properties, GQDs provide directional photon energy transfer to the bulk-heterojunction (BHJ) layer because the optical properties overlap. Additionally, the GQD-doped ZnO layer enhances shunt resistance (RSh) and forms good interfacial contact with the BHJ layer that results in increased carrier dissociation and transportation. Consequently, the fabricated device based on P(Cl-Cl)(BDD = 0.2) and IT-4F introduces GQDs exhibiting a maximum power conversion efficiency (PCE) of 14.0% with a superior enhanced short circuit current density (JSC) and fill factor (FF). Furthermore, the fabricated device exhibited high PCEs of 19.6 and 17.2% under 1000 and 200 lux indoor irradiation of light emitting diode (LED) lamps, respectively.

Efficient Non-Fullerene Organic Photovoltaics Printed by Electrospray via Solvent Engineering.

Developing scalable processing methods with low material waste is still one of the remaining challenges for organic photovoltaics (OPVs) to become a practical renewable energy source. Here, we report the first study on printing active layers of OPVs containing non-fullerene acceptors (NFAs) by electrospray (ES). The properties of the solvent significantly influence the interfacial morphology of ES-printed organic thin-films, and solvent engineering is essential to facilitate the formation of efficient active-layer films. We introduce low-vapor-pressure non-halogen solvent o-xylene (OXY) into the high vapor pressure solvent of chloroform to form a binary solvent system with appropriate evaporation time, electric conductivity, and solubility. The characteristic times of the ES process using binary solvents are quantified to provide insights into the dynamic formation of thin films. A longer droplet evaporation time with decent solubility collectively decrease the roughness and domain size of the polymer/NFA blend films, thus increase the photocurrent and fill factor of the ES-printed OPV devices. The ES-printed active layers show enhanced crystallinity and phase separation of NFA molecules than the spin-coated films. The champion cell with an ES-printed PTB7-Th:FOIC active layer exhibits a power conversion efficiency of 9.45%, which is on par with the spin-coated cells and is among the highest of spray-deposited organic solar cells to date. This work demonstrates that ES is an effective method to prepare OPVs on NFAs.

Vacuum‐Free, All‐Solution, and All‐Air Processed Organic Photovoltaics with over 11% Efficiency and Promoted Stability Using Layer‐by‐Layer Codoped Polymeric Electrodes

Nonfullerene organic photovoltaics (OPVs) have achieved a breakthrough in pushing the efficiency beyond 15%. Although this sheds light on OPV commercialization, the high cost associated with the scalable device fabrications remains a giant challenge. Herein, a vacuum‐free, all‐solution and all‐air processed OPV is reported that yields 11.12% efficiency with a fill factor of 0.725, due to the usages of high‐merit polymeric electrodes and modified active blends. The design principle toward the high‐merit electrodes is to induce heavy acid doping into the matrices for a raised carrier concentration and mobility, make a large removal of insulating components in the whole matrices rather than surfaces, and restrain the formation of large‐domain aggregates. A unique layer‐by‐layer doping is developed to enable the polymeric electrodes with record‐high trade‐offs between optical transmittance and electrical conductivity. Moreover, solvent vapor annealing is proposed to boost device efficiency and it has the advantages of finely adjusting the active blend morphology and raising the electron mobility. The resulting devices are highly efficient and most (≈91%) of the initial efficiency are maintained in 30 day storage. This work indicates bright future for making cost‐effective all‐solution processed OPVs in air.

Recent progress in wide bandgap conjugated polymer donors for high-performance nonfullerene organic photovoltaics.

In the last five years, the tremendous progress achieved in the field of polymer solar cells (PSCs) has attracted extensive attention to this emerging technology for exploiting renewable energy. Owing to their excellent optoelectronic features and outstanding manufacturability for film deposition, wide bandgap (WBG) polymer donors have become a leading component in bulk heterojunction layers and thus, a correlative review focusing on their molecular design, aggregation behavior and photovoltaic properties is necessary. In this feature article, we summarize our recent efforts in developing WBG polymer donors and understanding the charge separation and non-radiative recombination energy loss in high-performance non-fullerene (NF) PSCs. We also discuss the opportunities and challenges towards the realization of the commercialization of NF PSCs based on WBG polymer donors.

A privileged ternary blend enabling non-fullerene organic photovoltaics with over 14% efficiency

Two non-fullerene small molecule acceptors, NT-4F and NT-4Cl, were designed and synthesized. Power conversion efficiencies of 11.44% and 14.55% were achieved for NT-4Cl-based binary and ternary devices, respectively.

High-Efficiency All-Ambient Air, All-Solution and Low-Temperature Processed Nonfullerene Photovoltaics with Layer-by-Layer Co-Doped Polymeric Anodes

Nonfullerene organic photovoltaics (OPVs) have achieved breakthrough with pushing the power conversion efficiency exceeding 15%. While this shed light on OPV commercialization, the high cost associated to the scalable device fabrications remains a giant challenge. Herein, we reported an all-air, all-solution and low-temperature processed OPV that yielded 12.24% efficiency with impressive fill factor of 0.73, which was mostly attributed to the usages of high-merit polymeric anodes and modified active blends. The design principle toward the high-merit anodes is to induce a heavy acid doping, make a large removal of insulating and hydrophilic components in whole matrices, and restrain the formation of large-domain aggregates. A unique layer-by-layer co-doping was thus developed and it enabled a stable polymeric anode with record-high trade-off between optical transmittance and electrical conductivity. Besides, solvent vapor annealing was proposed to boost device efficiency and it has the advantages of finely adjusting the active blend morphology and raising the electron mobility. As a consequence, the unprecedented device with high efficiency was obtained and it maintained most (~92.3%) of the initial efficiency after 60 day storage. This work gives the bright future for cost-effective all-solution processed OPVs prepared in air ambient at low temperatures.

Fine-tuning of the chemical structure of photoactive materials for highly efficient organic photovoltaics

The performance of organic photovoltaics is largely dependent on the balance of short-circuit current density (JSC) and open-circuit voltage (VOC). For instance, the reduction of the active materials’ optical bandgap, which increases the JSC, would inevitably lead to a concomitant reduction in VOC. Here, we demonstrate that careful tuning of the chemical structure of photoactive materials can enhance both JSC and VOC simultaneously. Non-fullerene organic photovoltaics based on a well-matched materials combination exhibit a certified high power conversion efficiency of 12.25% on a device area of 1 cm2. By combining Fourier-transform photocurrent spectroscopy and electroluminescence, we show the existence of a low but non-negligible charge transfer state as the possible origin of VOC loss. This study highlights that the reduction of the bandgap to improve the efficiency requires a careful materials design to minimize non-radiative VOC losses. Materials design rules play a key role in enabling high performance in organic photovoltaics. Here the authors achieve 12.25% efficiency on 1 cm2 non-fullerene solar cells by tuning the side chains’ branching point and the fluorine substitutions in donor and acceptor materials.

Multiple Roles of a Non-fullerene Acceptor Contribute Synergistically for High-Efficiency Ternary Organic Photovoltaics

Summary Ternary structure is an important design strategy to obtain high-efficiency non-fullerene organic photovoltaics (OPVs). However, the role of the third component to the standard binary system is still unclear. Here, a wide-bandgap small-molecule acceptor, denoted IDT-T, is synthesized and used together with a wide-bandgap donor polymer, PBDB-T, and a low-bandgap acceptor, ITIC, for fullerene-free ternary solar cells. The ternary cell features an enhanced power conversion efficiency (PCE) up to 12.2%, together with improved photocurrent density, open-circuit voltage (VOC), and fill factor. Studies of the thin films indicate that IDT-T functions as an energy-level mediator, a fluorescence resonance energy-transfer donor, an electron acceptor, and a crystallization modulator in the blend, which contribute synergistically in the ternary blend to deliver a higher VOC, more efficient exciton generation, suppressed bimolecular charge recombination and enhanced charge transport, and an overall high photovoltaic performance.

Asymmetric thieno[2,3-b]thiophene-based electron acceptor featuring a seven fused-ring electron donor unit as core for nonfullerene organic photovoltaics

Abstract A n-type organic semiconductor (n-OS) electron acceptor (ITCNTC) based on thieno [2,3-b]thiophene-containing asymmetric seven fused-ring central unit was developed. In comparison with the reported isomer of ITCPTC, ITCNTC exhibits blue-shift absorption spectra and higher LUMO energy level. Organic solar cells (OSCs) based on wide band-gap polymer donor J71 and ITCNTC blends gave power conversion efficiencies (PCEs) of 5.18% without any post-treatment. After thermal annealing (TA), PCE was significantly boosted to 8.52%, with enhanced short circuit current density (JSC) and fill factor (FF). However, the PCE of 8.52% is lower than that of the reported J71:ITCPTC based device (11.63%), which is mainly ascribed to the narrower absorption of ITCNTC and unfavorable morphology. It is noteworthy that the average open circuit voltage (VOC) of the ITCNTC-based device (0.940 V) is 0.06 eV higher than that of the ITCPTC-based device (0.877 V), which is benefitted from the upshifted LUMO energy level of ITCNTC acceptor. These results indicate that the chemical structure of the central fused ring plays a very important role in determining the photovoltaic performance of the n-OS acceptors.

Molecular Engineering for Large Open-Circuit Voltage and Low Energy Loss in Around 10% Non-fullerene Organic Photovoltaics

Recent efforts in organic photovoltaics (OPVs) have been devoted to obtaining low-bandgap non-fullerene acceptors (NFAs) for high photocurrent generation. However, the low-lying lowest unoccupied molecular orbital (LUMO) level in narrow bandgap NFAs typically results in a small energy difference (ΔEDA) between the LUMO of the acceptor and the highest occupied molecular orbital (HOMO) of the donor, leading to low open-circuit voltage (VOC). The trade-off between ΔEDA and photocurrent generation significantly limits the simultaneous enhancement of both VOC and short-circuit current density (JSC). Here, we report a new medium-bandgap NFA, IDTT-T, containing a weakly electron-withdrawing N-ethyl thiabarbituric acid terminal group on each end of the indacenodithienothiophene (IDTT) core. When paired with a benchmark low-bandgap PTB7-th polymer donor, simultaneous enhancement of both ΔEDA and absorption spectral coverage was realized. The OPV devices yield a VOC of 1.01 V, corresponding to a low energy loss of ...

Highly Efficient Non-Fullerene Organic Photovoltaics Processed from o-Xylene without Using Additives

Most efficient organic photovoltaic (OPV) devices are fabricated using halogenated solvents, which are hazardous and environmentally unfriendly. From an industrial perspective, green solvents are necessary for the roll-to-roll production of OPV modules. In this study, we fabricated nonfullerene (NF) OPV devices that are based on the blend films of PTB7-Th and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene (ITIC) by using o-xylene and tetrahydrofuran (THF) as the processing solvents and chlorobenzene (CB) and o-dichlorobenzene as the control solvents. We compared the variations in the ultraviolet visible absorption, atomic force microscopy-derived phase morphologies, space-charge-limited current carrier mobilities, and power conversion efficiency (PCE) of the related OPV devices. The high solubility of ITIC and PTB7-Th in the solvents yielded PCE of 8.11% and 6.79% for the o-xylene- and THF-derived devices, respectively. The PCE of 8.11% is among the highest performance reported to date for NF OPV devices fabricated using a green solvent (without additives or post-treatment). Furthermore, this PCE was suppressed in the CB-based device (PCE: 7.41%) because of the clearly defined morphology and higher and balanced carrier mobility.