Alkoxybenzothiadiazole-Based Fullerene and Nonfullerene Polymer Solar Cells with High Shunt Resistance for Indoor Photovoltaic Applications

Cathode Interlayer Engineering for Efficient Organic Solar Cells under Solar Illumination and Light-Emitting Diode Lamp

Organic solar cells (OSCs) have become a potential energy source for indoor light harvesting in recent years as they have witnessed a record power conversion efficiency (PCE) of over 30% under indoor lights. Among various strategies, interlayer engineering is one of the important factors in improving the performance of OSCs. Here, we reported an efficient OSC based on PM6:Y6 photoactive layer showing an excellent PCE of ~22% and ~14% under light-emitting diode (LED, 1000-lx) and 1-sun (AM1.5 G) conditions, respectively. The performance of OSCs was optimized by systematically investigating the optical, electrochemical, and morphological characteristics of three different cathode interlayers (CILs) named as: PEIE, ZnO, and ZnO/PEIE (bilayer). The high transmittance (~90%), suitable work function (~4.1 eV), and improved surface morphology (RMS: 2.61 nm) of the bilayer CIL contributes in improving the performance of OSCs. In addition, the suppressed charge recombination and improved charge carrier transport are attributed to high shunt resistance and appropriate energy levels alignment between photoactive layer and bilayer CIL. The findings in the study might provide guidelines for designing novel interlayers in the development of efficient OSCs for different illumination conditions.

Revisiting the Classical Wide-Bandgap Homo- and Random Copolymers for Indoor Artificial Light Photovoltaics

Organic indoor photovoltaics (IPVs) are attractive energy harvesting devices for low-power consumption electronic devices and the Internet of Things (IoTs) owing to their properties such as lightweight, semi-transparency, multi-coloring capability, and flexibility. It is important to match the absorption range of photoactive materials with the emission spectra of indoor light sources that have a visible range of 400-700 nm for IPVs to provide sustainable, high-power density. To this end, we synthesize benzo[1,2-b:4,5-b']dithiophene-based homopolymer (PBDTT) as a polymer donor, which is a classical material that has a wide bandgap with a deep highest occupied molecular orbitals (HOMO) level, and a series of random copolymers by incorporating thieno[3,4-c]pyrrole-4,6,-dione (TPD) as a weak electron acceptor unit in PBDTT. We vary the composition of the TPD unit to fine tune the absorption range of the polymers; the polymer containing 70% TPD (B30T70) perfectly covers the entire range of indoor lamps such as LED and FL. Consequently, B30T70 shows a dramatic enhancement of the power conversion efficiency (PCE) from 1-sun (PCE: 6.0%) to the indoor environment (PCE: 18.3%) when fabricating organic IPVs by blending with PC₇₁ BM. We suggest simple, easy molecular design guidelines to develop photoactive materials for efficient organic IPVs. This article is protected by copyright. All rights reserved.

Different Morphology Dependence for Efficient Indoor Organic Photovoltaics: The Role of the Leakage Current and Recombination Losses

Efficient indoor organic photovoltaics (OPVs) have attracted strong attention for their application in indoor electronic devices. However, the route to optimal photoactive film morphology toward high-performance indoor devices has remained obscure. The leakage current dominated by morphology exerts distinguishing influence on the performance under different illuminations. We have demonstrated that morphology reoptimization plays an important role in indoor OPVs, and their optimal structural features are different from what we laid out for outdoor devices. For indoor OPVs, in order to facilitate low leakage current, it is essential to enhance the crystallinity, phase separation, and domain purity, as well as keeping small surface roughness of the active layer. Furthermore, considering the reduced bimolecular recombination at low light intensity, we have shown that PM6:M36-based indoor devices can work effectively with a large ratio of the donor and acceptor. Our work correlating structure-performance relation and the route to optimal morphology outlines the control over device leakage current and recombination losses boosting the progress of efficient indoor OPVs.

Orthogonal Printable Reduced Graphene Oxide 2D Materials as Hole Transport Layers for High-Performance Inverted Polymer Solar Cells: Sheet Size Effect on Photovoltaic Properties

Creating an orthogonal printable hole-transporting layer (HTL) without damaging the underlying layer is still a major challenge in fabricating large-area printed inverted polymer solar cells (PSCs). In this study, we prepared orthogonal-processable fluorine-functionalized reduced graphene oxide (FrGO) series with various two-dimensional sheet sizes such as large-sized FrGO (1.1 μm), medium-sized FrGO (0.7 μm), and small-sized FrGO (0.3 μm) and systematically investigated the size effect of FrGOs on the hole transport properties of PSCs. The FrGOs exhibit highly stable dispersion without change over 90 days in 2-propanol solvent, indicating very high dispersion stability. Decreasing the sheet size of FrGOs enhanced hole-transporting properties, resulting in power conversion efficiencies (PCEs) of 9.27 and 9.02% for PTB7-Th:EH-IDTBR- and PTB7-Th:PC₇₁BM-based PSCs, respectively. Compared to devices with solution-processed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), a 14% enhancement of PCEs was achieved. Interestingly, the PCEs of devices with the smallest FrGO sheet are higher than the PCE of 8.77% of a device with vacuum-deposited MoO₃. The enhancement in the performance of PSCs is attributed to the enhanced charge collection efficiency, decreased leakage current, internal resistance, and minimized charge recombination. Finally, small-sized FrGO HTLs were successfully coated on the photoactive layer using the spray coating method, and they also exhibited PCEs of 9.22 and 13.26% for PTB7-Th:EH-IDTBR- and PM6:Y6-based inverted PSCs, respectively.

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 (R_Sh) 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 (J_SC) 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.

Transparent Thin-Film Silicon Solar Cells for Indoor Light Harvesting with Conversion Efficiencies of 36% without Photodegradation

With the development of the Internet of Things (IoT), indoor photovoltaics are attracting considerable interest owing to their potential to benefit various IoT-related fields. Therefore, this study investigates the use of transparent hydrogenated amorphous silicon (a-Si:H) solar cells for a broad range of applications, including indoor light harvesting. High-gap triple layers were employed in the a-Si:H solar cells to obtain a high shunt resistance and high short-circuit current, J_SC, and open-circuit voltage, V_OC, under indoor illumination. Additionally, multiple color-adjusting layers were added without noticeable costs to the conversion efficiency. The maximum efficiency of 36.0% was obtained at a transmittance of 20.44% under white LED light (3000 lx and 0.92 mW cm^-2). Furthermore, the fabricated transparent solar cells show excellent long-term performance, sustaining over 99% of original efficiency under continuous indoor light illumination for 200 h. These cells could accelerate the progress of energy harvesting in IoT applications and facilitate the construction of integrated photovoltaics.

Solar Cells for Indoor Applications: Progress and Development

The Internet of things (IoT) has been rapidly growing in the past few years. IoT connects numerous devices, such as wireless sensors, actuators, and wearable devices, to optimize and monitor daily activities. Most of these devices require power in the microwatt range and operate indoors. To this end, a self-sustainable power source, such as a photovoltaic (PV) cell, which can harvest low-intensity indoor light, is appropriate. Recently, the development of highly efficient PV cells for indoor applications has attracted tremendous attention. Therefore, different types of PV materials, such as inorganic, dye-sensitized, organic, and perovskite materials, have been employed for harvesting low-intensity indoor light energy. Although considerable efforts have been made by researchers to develop low-cost, stable, and efficient PV cells for indoor applications, Extensive investigation is necessary to resolve some critical issues concerning PV cells, such as environmental stability, lifetime, large-area fabrication, mechanical flexibility, and production cost. To address these issues, a systematic review of these aspects will be highly useful to the research community. This study discusses the current status of the development of indoor PV cells based on previous reports. First, we have provided relevant background information. Then, we have described the different indoor light sources, and subsequently critically reviewed previous reports regarding indoor solar cells based on different active materials such as inorganic, dye-sensitized, organic, and perovskite. Finally, we have placed an attempt to provide insight into factors needed to further improve the feasibility of PV technology for indoor applications.

Recent progress in indoor organic photovoltaics

Among various potential applications of organic photovoltaics (OPVs), indoor power generation has great potential because of several advantages over outdoor light harvesting under 1 sun conditions. Commonly used indoor light sources have narrower emission spectra with lower intensity (by 3 orders of magnitude) as compared to the solar spectrum. Highly tunable optical absorption, large absorption coefficients, and small leakage currents under dim lighting conditions make OPVs promising candidates for indoor applications. For optimizing indoor photovoltaic materials and devices, several key issues (different from those under 1 sun conditions), such as developing new indoor photovoltaic materials and devices with suitable absorption spectra, large open-circuit voltages with low energy loss, minimized trap-mediated charge recombination and leakage currents, and device stability under indoor conditions, should be considered carefully. In this review, the recent progress in optimization of indoor photovoltaic materials and devices, and the key strategies to optimize the indoor photovoltaic characteristics will be summarized and discussed.

Photo-Carrier-Guiding Behavior of Vertically Grown MoS2 and MoSe2 in Highly Efficient Low-Light Transparent Photovoltaic Devices on Large-Area Rough Substrates

Two-dimensional MoX₂ (X = S, Se) films were vertically grown on highly rough transparent conducting F-doped SnO₂ glass substrates for the first time and successfully used as photogenerated carrier-guiding layers (CGLs) in transparent hydrogenated amorphous silicon (a-Si:H) thin film solar cells (TFSCs). The MoSe₂ CGL layers could be grown at 530 °C using thermally cracked small Se-molecules on transparent FTO glass substrates and significantly improved cell performance. A transparent cell transmitting 26.0% of visible light with a 20 nm-thick vertically grown MoSe₂ CGL showed an outstanding power conversion efficiency of 27.1% at a light intensity of 0.16 mW cm^-2 (500 lx; corresponding to normal indoor irradiation). The shunt resistance (R_sh) of the TFSCs reached 32,000 Ω at a light intensity of 7 mW cm^-2. An R_sh value this large is essential for low-light photovoltaic (PV) devices to prevent the dissipation of photogenerated carriers. These results strongly demonstrate that transparent a-Si:H-TFSCs with vertically grown MoX₂ films should find wide use in building-integrated PV windows or indoor PV applications, as they can generate power even in very low-light environments.

Flexible energy generation and storage devices: focus on key role of heterocyclic solid-state organic ionic conductors

This review addresses the vital role of solid-state electrolytes to develop highly efficient, customizable flexible energy generation and storage devices.

Understanding of Fluorination Dependence on Electron Mobility and Stability of Naphthalenediimide-Based Polymer Transistors in Environment with 100% Relative Humidity

A family of copolymers (P(NDIOD-T2Fx)) based on naphthalenediimide (NDI) and 2,2'-bithiophene (T2) units with different amounts of 3,3'-difluoro-2,2'-bithiophene (T2F) decoration were synthesized, characterized, and used in n-type organic field-effect transistors (OFETs). With increasing T2F content in the backbone, we observe increased melting and crystallization transitions, blue-shifted absorptions, and deeper-lying highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels, together with improved hydrophobicity. The highest electron mobility of 4.48 × 10^-1 cm² V^-1 s^-1 was obtained for P(NDIOD-T2F0) without a T2F unit, which is attributed to the larger domain grains and crystallites, as well as a more tightly packed and oriented crystalline structure, as evidenced from the morphological study. In contrast, P(NDIOD-T2F100) with the highest T2F content has superior air stability, showing greater than 25% electron mobility retention after 30 days in wet conditions of 100% relative humidity without encapsulation. Even P(NDIOD-T2F100) is able to operate normally after 30 min of immersion in water, which is due to the synergistic contributions from the deep HOMO/LUMO levels and improved hydrophobicity. This study advances our fundamental understanding of how the morphology/crystallinity, device performance, and device stability of n-type copolymers are tuned by incorporating different concentrations of T2F in the backbone, shedding light on an important modification for air- and water-stable n-type materials for future OFET applications.

Highly Efficient Indoor Organic Solar Cells by Voltage Loss Minimization through Fine-Tuning of Polymer Structures

Herein, we report a detailed study on the optoelectronic properties, photovoltaic performance, structural conformation, morphology variation, charge carrier mobility, and recombination dynamics in bulk heterojunction solar cells comprising a series of donor-acceptor conjugated polymers as electron donors based on benzodithiophene (BDT) and 5,8-bis(5-bromothiophen-2-yl)-6,7-difluoro-2,3-bis(3-(octyloxy)phenyl)quinoxaline as a function of the BDT's thienyl substitution (alkyl (WF3), alkylthio (WF3S), and fluoro (WF3F)). The synergistic positive effects of the fluorine substituents on the minimization of the bimolecular recombination losses, the reduction of the series resistances (R_S), the increment of the shunt resistances (R_Sh), the suppression of the trap-assisted recombination losses, the balanced charge transport, the finer nanoscale morphology, and the deeper highest occupied molecular orbital (E_HOMO) are manifested versus the alkyl and alkylthio substituents. According to these findings, the WF3F:[6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM)-based organic photovoltaic device is a rare example that features a high power conversion efficiency (PCE) of 17.34% under 500 lx indoor light-emitting diode light source with a high open-circuit voltage (V_OC) of 0.69 V, due to the suppression of the voltage losses, and a PCE of 9.44% at 1 sun (100 mW/cm²) conditions, simultaneously.

Two Well-Compatible Acceptors with Efficient Energy Transfer Enable Ternary Organic Photovoltaics Exhibiting a 13.36% Efficiency

Organic photovoltaics (OPVs) are fabricated with PM6 as donor and T6Me, IT-2F, or their mixture as acceptor. A 13.36% power conversion efficiency (PCE) is achieved from the optimized ternary OPVs with 50 wt% IT-2F in acceptors, which is attributed to the enhanced photon harvesting of ternary active layers and improved exciton utilization efficiency through energy transfer from IT-2F to T6Me. The efficient energy transfer from IT-2F to T6Me can be confirmed from the photoluminescence spectra of neat and blend films, which may provide additional channels to enhance exciton utilization efficiency for achieving short-circuit current density (J_SC ) improvement of ternary OPVs. It should be highlighted that the fill factor (FF) of ternary OPVs can be monotonously increased along with the incorporation of IT-2F, indicating the gradually optimized phase separation degree of ternary active layers. The third component IT-2F plays a key role in optimizing phase separation as a morphology regulator. Over 8% PCE improvement is achieved in the optimized ternary OPVs compared with the over 12% PCEs of the corresponding binary OPVs, respectively. This work indicates that the performance of ternary OPVs can be well optimized by carefully picking materials with good compatibility and complementary absorption spectra, as well as the appropriate energy levels.

Non-halogenated diphenyl-chalcogenide solvent processing additives for high-performance polymer bulk-heterojunction solar cells

The ability to control the morphologies of active layers is a critical factor in the successful development of polymer solar cells (PSCs), and solvent processing additives offer a simple and effective way to accomplish this. In particular, diphenyl ether (DPE) is one of the most effective solvent additives but analogous additives based on this structure have not yet been extensively investigated. In this work, we have fabricated PSCs and investigated photovoltaic device characteristics using the series of non-halogenated, diphenyl-chalcogen solvent additives; DPE, diphenyl sulfide (DPS) and diphenyl selenide (DPSe). DPS devices showed optimal power conversion efficiencies (PCEs) of up to 9.08%, and DPE devices also showed similarly high PCEs of up to 8.85%. In contrast, DPSe devices showed relatively low PCEs (5.45% at best) which we attribute to significant surface recombination and high series resistance, which led to limited open-circuit voltage (V_OC). In the case of DPS, fast, field-independent photocurrent saturation with negligible bimolecular recombination led to efficient charge separation and collection, which resulted in the highest PCEs. Additionally, using 1,2,4-trimethylbenzene and DPS as an entirely non-halogenated solvent/additive system, we successfully demonstrated device fabrication with comparably high PCEs of up to 8.4%. This work elucidates the effects of diphenyl-based solvent additives in PSCs and suggests a great potential of DPS as an effective non-halogenated solvent additive.

This work provides a detailed, structure–function analysis of the topology of diphenyl-chalcogenides to photovoltaic device performances and polymer bulk-heterojunction films.