Recent progress in indoor organic photovoltaics

The exciton dynamics and charge transfer in polymers with the effects of chlorine substituents

Donor-acceptor (D-A) type conjugated polymers, particularly those with electron-withdrawing halogen substituents, have demonstrated high efficiency as donor materials in solar energy conversion. In our previous work, we have successfully synthesized three low-cost D-A type conjugated polymers (designated as PJ-1, PJ-2, and PJ-3) through a gradual chlorination process, of which, devices based on PJ-1 exhibited exceptional power conversion efficiency (15.01%) and figure-of-merit values (45.48). In this study, we further investigated the excited-state dynamics of the three donor polymers by transient absorption spectroscopy to explore the dynamic reasons behind the high power conversion efficiency of PJ-1. Our findings revealed that PJ-1 exhibited pronounced aggregation, which facilitated intermolecular interactions, thereby enhancing charge transport capability and suppressing trap-assisted recombination. Furthermore, the PJ-1-based heterojunction presented efficient exciton dissociation and enhanced hole transfer efficiency. These results underscore the potential of chlorine substitution in improving exciton dissociation and charge transfer via regulating aggregation behavior and energy level, offering a straightforward and effective approach to engineer high-performance conjugated polymer donor materials for photovoltaic applications.

High Thickness Tolerance in All-Polymer-Based Organic Photovoltaics Enables Efficient and Stable In-Door Operation

Organic photovoltaics (OPVs) have great potential to drive low-power consumption electronic devices under indoor light due to their highly tunable optoelectronic properties. Thick devices (>300 nm photo-active junctions) are desirable to maximize photocurrent and to manufacture large-scale modules via solution-processing. However, thick devices usually suffer from severe charge recombination, deteriorating device performances. Herein, the study demonstrates excellent thickness tolerance of all-polymer-based PVs for efficient and stable indoor applications. Under indoor light, device performance is less dependent on photoactive layer thickness, exhibiting the best maximum power output in thick devices (34.7 µW cm-2 in 320-475 nm devices). Thick devices also exhibit much better photostability compared with thin devices. Such high thickness tolerance of all-polymer-based PV devices under indoor operation is attributed to strongly suppressed space-charge effects, leading to reduced bimolecular recombination losses in thick devices. The unbalanced charge carrier mobilities are identified as the main cause for significant space-charge effects, which is confirmed by drift-diffusion simulations. This work suggests that all-polymer-based PVs, even with unbalanced mobilities, are highly desirable for thick, efficient, and stable devices for indoor applications.

Reversible Molecular Conformation Transitions of Smectic Liquid Crystals for Light/Bias-Gated Transistor Memory

In recent years, organic photonic field-effect transistors have made remarkable progress with the rapid development of conjugated polycrystalline materials. Liquid crystals, with their smooth surface, defined layer thickness, and crystalline structures, are commonly used for these advantages. In this work, a series of smectic liquid crystalline molecules, 2,9-didecyl-dinaphtho-thienothiophene (C10-DNTT), 2,7-didecyl-benzothieno-benzothiopene (C10-BTBT), 3,9-didecyl-dinaphtho-thiophene (C10-DNT), and didecyl-sexithiophene (C10-6T), have been used in photonic transistor memory, functioning as both hole-transport channels and electron traps to investigate systematically the reasons and mechanisms behind the memory behavior of smectic liquid crystals. After thermal annealing, C10-BTBT and C10-6T/C10-DNTT are homeotropically aligned from the smectic A and smectic X phases, respectively. The 3D-ordered structure of these smectic-aligned crystals contributed to efficient photowriting and electrical erasing processes. Among them, the device performance of C10-BTBT was particularly significant, with a memory window of 21 V. The memory ratio could reach 1.5 × 106 and maintain a memory ratio of over 3 orders after 10,000 s, contributing to its smectic A structure. Through the research, we confirmed the memory and light/bias-gated behaviors of these smectic liquid crystalline molecules, attributing them to reversible molecular conformation transitions and the inherent structural inhomogeneity inside the polycrystalline channel layer.

Design of Chlorinated Indaceno[1,2-b:5,6-b']dithiophene Acceptors toward Efficient Organic Photovoltaics

Chlorinated modifications have been extensively employed to modulate the optoelectronic properties of π-conjugated materials. Herein, the Cl substitution in designing nonfullerene acceptors (NFAs) with various bandgaps is studied. Four narrow-bandgap electron acceptors (GS-40, GS-41, GS-42, and GS-43) were synthesized by tuning the electrostatic potential distributions of the molecular conjugated backbones. The optical absorption onset of these NFAs ranges from 900 to 1030 nm. Compared to the nonchlorinated analogue, the introduction of Cl atoms on the core of indaceno[1,2-b:5,6-b'] dithiophene (IDT) and π spacer results in an upward shift of the lowest unoccupied molecular orbital levels and induces a blue shift in the absorption spectra of the NFAs. This alteration facilitates achieving appropriate energy-level alignment and favorable bulk heterojunction morphology when blended with the widely used donor PBDB-TF. The PBDB-TF:GS-43-based solar cells show an optimal power conversion efficiency of 13.3%. This work suggests the potential of employing chlorine-modified IDT and thiophene units as fundamental building blocks for developing high-performance photoactive materials.

Self-Powering Sensory Device with Multi-Spectrum Image Realization for Smart Indoor Environments

The development of organic-based optoelectronic technologies for the indoor Internet of Things market, which relies on ambient energy sources, has increased, with organic photovoltaics (OPVs) and photodetectors (OPDs) considered promising candidates for sustainable indoor electronic devices. However, the manufacturing processes of standalone OPVs and OPDs can be complex and costly, resulting in high production costs and limited scalability, thus limiting their use in a wide range of indoor applications. This study uses a multi-component photoactive structure to develop a self-powering dual-functional sensory device with effective energy harvesting and sensing capabilities. The optimized device demonstrates improved free-charge generation yield by quantifying charge carrier dynamics, with a high output power density of over 81 and 76 µW cm-2 for rigid and flexible OPVs under indoor conditions (LED 1000 lx (5200 K)). Furthermore, a single-pixel image sensor is demonstrated as a feasible prototype for practical indoor operating in commercial settings by leveraging the excellent OPD performance with a linear dynamic range of over 130 dB in photovoltaic mode (no external bias). This apparatus with high-performance OPV-OPD characteristics provides a roadmap for further exploration of the potential, which can lead to synergistic effects for practical multifunctional applications in the real world by their mutual relevance.

A Water-Processed Mesoscale Structure Enables 18.5% Efficient Binary Layer-by-Layer Organic Solar Cells

The two-step layer-by-layer (LBL) deposition of donor and acceptor films enables desired vertical phase separation and high performance in organic solar cells (OSCs), which becomes a promising technology for large-scale printing devices. However, limitations including the use of toxic solvents and unpredictable infiltration between donor and acceptor still hinder the commercial production of LBL OSCs. Herein, we developed a water-based nanoparticle (NP) ink containing donor polymer to construct a mesoscale structure that could be infiltrated with an acceptor solution. Using non-halogen o-xylene for acceptor deposition, the LBL strategy with a mesoscale structure delivered outstanding efficiencies of 18.5% for binary PM6:L8-BObased LBL OSCs. Enhanced charge carrier mobility and restricted trap states were observed in the meso-LBL devices with optimized vertical morphology. It is believed that the findings in this work will bring about more research interest and effort on eco-friendly processing in preparation for the industrial production of OSCs.

Progress in organic photovoltaics for indoor application

Organic photovoltaics (OPVs) have recently emerged as feasible alternatives for indoor light harvesting because of their variable optical absorption, high absorption coefficients, and low leakage currents under low lighting circumstances. Extensive research has been performed over the last decade in the quest for highly efficient, ecologically stable, and economically feasible indoor organic photovoltaics (IOPVs). This research covers a wide range of topics, including the development of new donor-acceptor materials, interlayers (such as electron and hole transport layers), energy loss reduction, open-circuit voltage enhancement via material and device engineering, and device architecture optimization. The maximum power conversion efficiency (PCE) of IOPVs has already topped 35% as a consequence of these collaborative efforts. However, further research is needed to improve numerous elements, such as manufacturing costs and device longevity. IOPVs must preserve at least 80% of their initial PCE for more than a decade in order to compete with traditional batteries used in internet of things devices. A thorough examination of this issue is urgently required. We intend to present an overview of recent developments in the evolution of IOPVs.

Efficient and Stable Air-Processed Ternary Organic Solar Cells Incorporating Gallium-Porphyrin as an Electron Cascade Material

Two gallium porphyrins, a tetraphenyl GaCl porphyrin, termed as (TPP)GaCl, and an octaethylporphyrin GaCl porphyrin, termed as (OEP)GaCl, were synthesized to use as an electron cascade in ternary organic bulk heterojunction films. A perfect matching of both gallium porphyrins' energy levels with that of poly(3-hexylthiophene-2,5-diyl) (P3HT) or poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) polymer donor and the 6,6-phenyl C71 butyric acid methyl ester (PCBM) fullerene acceptor, forming an efficient cascade system that could facilitate electron transfer between donor and acceptor, was demonstrated. Therefore, ternary organic solar cells (OSCs) using the two porphyrins in various concentrations were fabricated where a performance enhancement was obtained. In particular, (TPP)GaCl-based ternary OSCs of low concentration (1:0.05 vv%) exhibited a ~17% increase in the power conversion efficiency (PCE) compared with the binary device due to improved exciton dissociation, electron transport and reduced recombination. On the other hand, ternary OSCs with a high concentration of (TPP)GaCl (1:0.1 vv%) and (OEP)GaCl (1:0.05 and 1:0.1 vv%) showed the poorest efficiencies due to very rough nanomorphology and suppressed crystallinity of ternary films when the GaCl porphyrin was introduced to the blend, as revealed from X-ray diffraction (XRD) and atomic force microscopy (AFM). The best performing devices also exhibited improved photostability when exposed to sunlight illumination for a period of 8 h than the binary OSCs, attributed to the suppressed photodegradation of the ternary (TPP)GaCl 1:0.05-based photoactive film.

Spectrum on demand light source (SOLS) for advanced photovoltaic characterization

We report a multi-purpose spectrum-on-demand light source (SOLS), conceived primarily but not exclusively for the multiple and advanced characterization of photovoltaic (PV) materials and devices. The apparatus is a spectral shaper illumination device, providing a tunable and spectrally shaped light beam produced by modulating the intensity and/or wavelength range of a primary light source. SOLS stands out from the state of the art because it produces almost any spectrum on demand and delivers two types of output: a spectrally shaped and spatially homogeneous beam over its cross section for areal illumination or a spatially and spectrally split beam into its wavelength components, a unique capability suited to characterize lateral-tandem (Rainbow) solar cells. The tuneability from broadband to narrowband illumination enables two characterization devices into one, namely, a solar simulator for the determination of the power conversion efficiency and an external quantum efficiency measuring system. We expect the SOLS setup to accelerate material screening, enabling the discovery and optimization of novel multi-component materials and devices, in particular for emergent PV technologies like organic, metal halide perovskites, or multi-junction geometries, as well as novel PV applications such as indoors, building integrated, or agrivoltaics, among others.

High Performance Transparent Silver Grid Electrodes for Organic Photovoltaics Fabricated by Selective Metal Condensation

Silver grid electrodes on glass and flexible plastic substrates with performance that exceeds that of commercial indium-tin oxide (ITO) coated glass are reported and show their suitability as a drop-in replacement for ITO glass in solution-processed organic photovoltaics (OPVs). When supported on flexible plastic substrates these electrodes are stable toward repeated bending through a small radius of curvature over tens of thousands of cycles. The grid electrodes are fabricated by the unconventional approach of condensation coefficient modulation using a perfluorinated polymer shown to be far superior to the other compounds used for this purpose to date. The very narrow line width and small grid pitch that can be achieved also open the door to the possibility of using grid electrodes in OPVs without a conducting poly(3,4-ethylenedioxythiophene-poly(styrenesulfonate) (PEDOT: PSS) layer to span the gaps between grid lines.

Ligand-free template-assisted synthesis of stable perovskite nanocrystals with near-unity photoluminescence quantum yield within the pores of vaterite spheres

Ligand-free methods for the synthesis of halide perovskite nanocrystals are of great interest because of their excellent performance in optoelectronics and photonics. In addition, template-assisted synthesis methods have become a powerful tool for the fabrication of environmentally stable and bright nanocrystals. Here we develop a novel approach for the facile ligand-free template-assisted fabrication of perovskite nanocrystals with a near-unity absolute quantum yield, which involves CaCO₃ vaterite micro- and submicrospheres as templates. We show that the optical properties of the obtained nanocrystals are affected not mainly by the template morphology, but strongly depend on the concentration of precursor solutions, anion and cation ratio, as well as on adding defect-passivating rare-earth dopants. The optimized samples are further tested as infrared radiation visualizers exhibiting promising characteristics comparable to those that are commercially available.

Polymer Solar Cells with Active Layer Thickness Compatible with Scalable Fabrication Processes: A Meta-Analysis

Organic photovoltaics (OPV) has been considered for a long time a promising emerging solar technology. Currently, however, market shares of OPV are practically non-existent. A detailed meta-analysis of the literature published until mid-2021 is presented, focusing on one of the remaining issues that need to be addressed to translate the recent remarkable progress, obtained in devices' performance at lab-scale level, into the requirements able to boost the manufacturing-scale production. Namely, the active layer's thickness is referred to, which, together with device efficiency and stability, represents one of the biggest challenges of this technological research field. Papers describing solar cells containing non-fullerene acceptor (NFA) binary and ternary blends, as well as NFA plus fullerene acceptor (FA) ternary blends are reviewed. The common ground of all analyzed devices is their high-thickness active layers, compatible with large-area deposition techniques. By defining a new figure of merit to discuss the OPV thickness (thickness tolerance, TT), it is found that this parameter is not affected by the chemical family's nature of the active blend components. On the other hand, the analysis suggests that there are promising strategies to improve the TT, which are discussed in the conclusion section.

Indoor photovoltaic energy harvesting based on semiconducting π-conjugated polymers and oligomeric materials toward future IoT applications

With the advancement of artificial intelligence computing systems that can collect, analyze, and utilize metadata from our activities and surrounding environments, establishing self-powered electronic systems/networks supported by energy harvesters is strongly desired. With the lowering of power consumption in contemporary IoT electronics such as wireless sensors, indoor organic photovoltaic devices (iOPVs), which can be driven under ambient indoor light, have recently attracted significant interest as self-sustainable eco-friendly power sources. iOPVs based on organic semiconductors have unique advantages, such as light weight, flexibility, solution processability, and feasibility of low-temperature mass production. Additionally, the spectral tunability and high optical absorptivity of organic semiconductors make iOPVs more effective as energy harvesters in indoor lighting environments. With recent intensive research effort, iOPVs have realized the delivery of high power conversion efficiencies exceeding 25% with output power densities of several tens to a hundred μW cm−2, which are sufficient to drive various low-power electronics compatible with the IoT. This review article focuses on recent progress in iOPVs based on π-conjugated polymers and oligomeric materials and outlines their fundamental principles and characterization techniques.

Small Molecule Based Organic Photo Signal Receiver for High-Speed Optical Wireless Communications

The present work describes the development of an organic photodiode (OPD) receiver for high-speed optical wireless communication. To determine the optimal communication design, two different types of photoelectric conversion layers, bulk heterojunction (BHJ) and planar heterojunction (PHJ), are compared. The BHJ-OPD device has a -3 dB bandwidth of 0.65 MHz (at zero bias) and a maximum of 1.4 MHz (at -4 V bias). A 150 Mbps single-channel visible light communication (VLC) data rate using this device by combining preequalization and machine learning (ML)-based digital signal processing (DSP) is demonstrated. To the best of the authors' knowledge, this is the highest data rate ever achieved on an OPD-based VLC system by a factor of 40 over the previous fastest reported. Additionally, the proposed OPD receiver achieves orders of magnitude higher spectral efficiency than the previously reported organic photovoltaic (OPV)-based receivers.

A Medium-Bandgap Nonfullerene Acceptor Enabling Organic Photovoltaic Cells with 30% Efficiency under Indoor Artificial Light

The correlation between molecular structure and photovoltaic performance is lagging for constructing high-performance indoor organic photovoltaic (OPV) cells. Herein, this relationship is investigated in depth by employing two medium-bandgap nonfullerene acceptors (NFAs). The newly synthesized NFA of FTCCBr exhibits a similar bandgap and molecular energy level, but a much stronger dipole moment and larger average electrostatic potential (ESP) compared with ITCC. After blending with the polymer donor PB2, the PB2:ITCC and PB2:FTCCBr blends exhibit favorable bulk-heterojunction morphologies and the same driving force, but the PB2:FTCCBr blend exhibits a large ESP difference. In OPV cells, the PB2:ITCC-based device produces a power conversion efficiency (PCE) of 11.0%, whereas the PB2:FTCCBr-based device gives an excellent PCE of 14.8% with an open-circuit voltage (V_OC ) of 1.05 V, which is the highest value among OPV cells with V_OC values above 1.0 V. When both acceptor-based devices work under a 1000 lux of 3000 K light-emitting diode, the PB2:ITCC-based 1 cm² device yields a good PCE of 25.4%; in contrast, the PB2:FTCCBr-based 1 cm² device outputs a record PCE of 30.2%. These results suggest that a large ESP offset in photovoltaic materials is important for achieving high-performance OPV cells.

Recent Advances in Nonfullerene Acceptor-Based Layer-by-Layer Organic Solar Cells Using a Solution Process

Recently, sequential layer-by-layer (LbL) organic solar cells (OSCs) have attracted significant attention owing to their favorable p-i-n vertical phase separation, efficient charge transport/extraction, and potential for lab-to-fab large-scale production, achieving high power conversion efficiencies (PCEs) of over 18%. This review first summarizes recent studies on various approaches to obtain ideal vertical D/A phase separation in nonfullerene acceptor (NFAs)-based LbL OSCs by proper solvent selection, processing additives, protecting solvent treatment, ternary blends, etc. Additionally, the longer exciton diffusion length of NFAs compared with fullerene derivatives, which provides a new scope for further improvement in the performance of LbL OSCs, is been discussed. Large-area device/module production by LbL techniques and device stability issues, including thermal and mechanical stability, are also reviewed. Finally, the current challenges and prospects for further progress toward their eventual commercialization are discussed.

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.

Developing the Nondevelopable: Creating Curved-Surface Electronics from Nonstretchable Devices

The incorporation of electronics onto curved surfaces promises to bring new levels of intelligence to the ergonomic, aesthetic, aerodynamic, and optical surfaces that are ever-present in our lives. However, since many of these surfaces have 2D (i.e., nondevelopable) curvature, they cannot be formed from the deformation of a flat, nonstretchable sheet. This means that curved electronics cannot capitalize on the rapid technological advances taking place in the field of ultrathin electronics, since ultrathin devices, though ultraflexible, are not stretchable. In this work, a shrink-based paradigm is presented to apply such thin-film electronics to nondevelopable surfaces, expanding the capabilities of current nondevelopable electronics, and linking future developments in thin-film technology to similar developments in curved devices. The wrinkling of parylene-based devices and the effects of shrinkage on common electrical components are examined, culminating in shrinkable touch sensors and organic photovoltaics, laminated to various nondevelopable surfaces without loss of performance.

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.

All-polymer indoor photovoltaic modules

Indoor photovoltaic (IPV) with power output over 100 μW is promising to power the numerous sensor nodes in the Internet of Things (IoT) ecosystem. All polymer photovoltaic has the advantages of excellent thermal stability and superior mechanical properties. In this work, we fabricate the first all-polymer indoor photovoltaic module with the active area of 10 cm2. The module uses polymer donor CD1 and new polymer acceptor PBN-21 with medium optical band gap of 1.9 eV as the active layer. It is processed with eco-friendly solvent tetrahydrofuran and the morphology can be improved by blade coating at 55°C. Under light emitting diode illumination at 1000 lux, the module exhibits a power conversion efficiency of 12.04% and a power output of 367.2 μW. The sufficient power output, high efficiency, excellent stability, and eco-friendly processing indicate that all-polymer indoor photovoltaic is a promising approach to achieve the self-powered of sensor nodes in the IoT ecosystem.

Perovskite indoor photovoltaics: opportunity and challenges

With the rapid development of the Internet of Things (IoTs), photovoltaics (PVs) has a vast market supply gap of billion dollars. Moreover, it also puts forward new requirements for the development of indoor photovoltaic devices (IPVs). In recent years, PVs represented by organic photovoltaic cells (OPVs), silicon solar cells, dye-sensitized solar cells (DSSCs), etc. considered for use in IoTs mechanisms have also been extensively investigated. However, there are few reports on the indoor applications of perovskite devices, even though it has the advantages of better performance. In fact, perovskite has the advantages of better bandgap adjustability, lower cost, and easier preparation of large-area on flexible substrates, compared with other types of IPVs. This review starts from the development status of IoTs and investigates the cost, technology, and future trends of IPVs. We believe that perovskite photovoltaics is more suitable for indoor applications and review some strategies for fabricating high-performance perovskite indoor photovoltaic devices (IPVs). Finally, we also put forward a perspective for the long-term development of perovskite IPVs.

Hybrid Tetrameric Perylene Diimide Assemblies

Organic photovoltaics have found utility as indoor light recycling devices providing an opportunity for the sustainable powering of IoT sensors and related smart electronics. In the report, two organic π-conjugated molecules consisting of four perylene diimide (PDI) chromophores each are presented and used as non-fullerene acceptors in indoor photovoltaic devices. The new materials consist of a dimeric N-annulated PDI core with single PDIs grafted onto the pyrrolic N-atom positions of the core. Compounds PDI₄ e and PDI₄ i are PDI tetramers and differ with PDI₄ e having the terminal N-annulated PDI with pyrrolic N-atom distal to the core and PDI₄ i having the terminal N-annulated PDI with pyrrolic N-atom proximal to the core. The structural and optoelectronic properties were investigated using NMR spectroscopy, optical absorption and emission spectroscopy, and cyclic voltammetry. The compounds exhibit typical optical signatures for PDIs but notable is that the addition of grafted PDI molecules prevents significant aggregation of the dimeric PDI core, as compared to a reference dimer. Use as non-fullerene acceptors in ternary bulk-heterojunction blends with the polymer FBT and fullerene PC₆₁ BM lead to increased open-circuit voltages and power conversion efficiencies upwards of 13.7 % at 2000 lux light intensity.

Indoor Organic Photovoltaics for Self-Sustaining IoT Devices: Progress, Challenges and Practicalization

Indoor photovoltaics (IPVs) have great potential to provide a self-sustaining power source for Internet-of-Things (IoT) devices. The rapid growth in demand for low-power IoT devices for indoor application not only boosts the development of high-performance IPVs, but also promotes the electronics and semiconductor industry for the design and development of ultra-low-power IoT systems. In this Review, the recent progress in IPV technologies, design rules, market trends, and future prospects for highly efficient indoor photovoltaics are discussed. Special attention is given to the progress and development of organic photovoltaics (OPVs), which demonstrate great possibilities for IPVs, owing to their bandgap tunability, high absorbance coefficient, semitransparency, solution processability, and easy large-area manufacturing on flexible substrates. Highly efficient indoor organic photovoltaics (IOPVs) can be realized through designing efficient donor and acceptor absorber materials that have good spectral responses in the visible region and better energy-aligned interfacial layers, and through modulation of optical properties. Interfacial engineering, photovoltage losses, device stability, and large-area organic photovoltaic modules are surveyed to understand the mechanisms of efficient power conversion and challenges for IOPVs under indoor conditions as a self-sustaining power source for IoT devices. Finally, the prospects for further improve in IOPV device performance and practical aspects of integrating IOPVs in low-power IoT devices are discussed.

An Overview of High-Performance Indoor Organic Photovoltaics

In recent years, indoor organic photovoltaics (IOPVs) have attracted increasing attention because of their ability to power microelectronic devices and sensors, especially for the internet of things (IoT). In contrast with silicon-based indoor PV, the IOPVs exhibit better performance due to their tunable bandgap via molecular design, which could achieve a better spectrum matched with the lighting sources. Based on the simulated power conversion efficiency (PCE) in theory, the maximum value can achieve over 50 % under the white LED illumination, which is much higher than the practical top PCE of 31 %, indicating there is room further to improve the performance of IOPVs by various optimization methods. Based on these benefits, the recent progress in IOPVs with different methods was summarizes, and light was shed on the remaining challenges for achieving practical applications in the future. In the end, some guidelines for the development of IOPVs were proposed.

A New Conjugated Polymer that Enables the Integration of Photovoltaic and Light-Emitting Functions in One Device

Exploring the intriguing bifunctional nature of organic semiconductors and investigating the feasibility of fabricating bifunctional devices are of great significance in realizing various applications with one device. Here, the design of a new wide-bandgap polymer named PBQx-TCl (optical bandgap of 2.05 eV) is reported, and its applications in photovoltaic and light-emitting devices are studied. By fabricating devices with nonfullerene acceptors BTA3 and BTP-eC9, it is shown that the devices exhibit a high power conversion efficiency (PCE) of 18.0% under air mass 1.5G illumination conditions and an outstanding PCE of 28.5% for a 1 cm² device and 26.0% for a 10 cm² device under illumination from a 1000 lux light-emitting diode. In addition, the PBQx-TCl:BTA3-based device also demonstrates a moderate organic light-emitting diode performance with an electroluminescence external quantum efficiency approaching 0.2% and a broad emission range of 630-1000 nm. These results suggest that the polymer PBQx-TCl-based devices exhibit outstanding photovoltaic performance and potential light-emitting functions.

Challenges and prospects of ambient hybrid solar cell applications

The impending implementation of billions of Internet of Things and wireless sensor network devices has the potential to be the next digital revolution, if energy consumption and sustainability constraints can be overcome. Ambient photovoltaics provide vast universal energy that can be used to realise near-perpetual intelligent IoT devices which can directly transform diffused light energy into computational inferences based on artificial neural networks and machine learning. At the same time, a new architecture and energy model needs to be developed for IoT devices to optimize their ability to sense, interact, and anticipate. We address the state-of-the-art materials for indoor photovoltaics, with a particular focus on dye-sensitized solar cells, and their effect on the architecture of next generation IoT devices and sensor networks.

Recent Advances in Non-Fullerene Acceptors of the IDIC/ITIC Families for Bulk-Heterojunction Organic Solar Cells

The introduction of the IDIC/ITIC families of non-fullerene acceptors has boosted the photovoltaic performances of bulk-heterojunction organic solar cells. The fine tuning of the photophysical, morphological and processability properties with the aim of reaching higher and higher photocurrent efficiencies has prompted uninterrupted worldwide research on these peculiar families of organic compounds. The main strategies for the modification of IDIC/ITIC compounds, described in several contributions published in the past few years, can be summarized and classified into core modification strategies and end-capping group modification strategies. In this review, we analyze the more recent advances in this field (last two years), and we focus our attention on the molecular design proposed to increase photovoltaic performance with the aim of rationalizing the general properties of these families of non-fullerene acceptors.

Slot-Die-Coated Ternary Organic Photovoltaics for Indoor Light Recycling

Efficient organic photovoltaics (OPVs) based on slot-die-coated (SD) ternary blends were developed for low-intensity indoor light harvesting. For active layers processed in air and from eco-friendly solvents, our device performances (under 1 sun and low light intensity) are the highest reported values for fluoro-dithiophenyl-benzothiadiazole donor polymer-based OPVs. The N-annulated perylene diimide dimer acceptor was incorporated into a blend of donor polymer (FBT) and fullerene acceptor (PC₆₁BM) to give ternary bulk heterojunction blends. SD ternary-based devices under 1 sun illumination showed enhanced power conversion efficiency (PCE) from 6.8 to 7.7%. We observed enhancement in the short-circuit current density and open-circuit voltage of the devices. Under low light intensity light-emitting device illumination (ca. 2000 lux), the ternary-based devices achieved a PCE of 14.0% and a maximum power density of 79 μW/cm² compared to a PCE of 12.0% and a maximum power density of 68 μW/cm² for binary-based devices. Under the same illumination conditions, the spin-coated (SC) devices showed a PCE of 15.5% and a maximum power density of 88 μW/cm². Collectively, these results demonstrate the exceptional promise of a SD ternary blend system for indoor light harvesting and the need to optimize active layers based on industry-relevant coating approaches toward mini modules.

End-capped group manipulation of indacenodithienothiophene-based non-fullerene small molecule acceptors for efficient organic solar cells

As the key component of organic solar cells (OSCs), the acceptor plays key roles in determining the power conversion efficiency (PCE). Based on the famous non-fullerene acceptor ITIC, a series of acceptors (A1-A5) were designed by introducing fused-ring units (phenanthrene, pyrene, benzopyrazine, dibenzo[a,c]phenazine, and phenanthro[4,5-abc]phenazine) as the end groups. Theoretical calculations showed that A1-A5 display improved solubility and redshifted absorption spectra compared with ITIC. More importantly, the newly designed acceptors exhibit much higher electron mobility, where the electron mobility of A5_h (similar to A5 but with the same hexyl side chain as ITIC) is about four orders of magnitude larger than that of ITIC. The computed binding energies of the donor PBDB-TF with the acceptor ITIC and A5_h are -2.52 eV and -3.75 eV, indicating much stronger interface interactions in PBDB-TF/A5_h. In terms of charge-transfer (CT) mechanism, we found that both PBDB-TF/ITIC and PBDB-TF/A5_h can generate CT states through direct excitation and hot excitons, meanwhile there exist more opportunities of producing CT states via the intermolecular electric field (IEF) mechanism in PBDB-TF/A5_h. Our results not only offer a set of promising ITIC-based acceptors, but also provide new insights into the donor/acceptor interface properties, which are closely related to the PCE of OSCs.

Organic Photovoltaic Cells for Indoor Applications: Opportunities and Challenges

With the growing development of the Internet of Things, organic photovoltaic (OPV) cells are highly desirable for indoor applications because of the unique features of light weight, flexibility, and coloration. Emission spectra of the commonly used indoor light sources are much narrower with lower light intensity as compared to the standard solar spectrum. High tunability in optical absorption, insensitivity to series resistance and the active layer thickness, and mild operating conditions make indoor OPV cells promising as a practically relevant technology. Currently, the OPV module has obtained a power conversion efficiency of over 20%, with excellent stability under indoor conditions. However, at the present stage, the device physics investigations and material design strategies developed in an OPV cell for indoor applications lag behind those for outdoor applications. In particular, the emerging characterizations in photovoltaic measurements have severely affected the reliability of reports. This Spotlight on Applications highlights these opportunities and challenges of OPV cells for indoor applications and reviews the recent progress in indoor OPV cells. In addition, we summarize some studies related to accurate measurement and provide some recommendations.

Polymer Acceptors Containing B←N Units for Organic Photovoltaics

ConspectusOrganic photovoltaics (OPVs), in which blend films of organic or polymer electron donor and electron acceptor are used as the active layer, are a promising photovoltaic technology with the great advantages of solution processing, low cost, and flexibility. The development of small molecular or polymer electron acceptors has boosted power conversion efficiency (PCE) of OPVs from 10% to 18%. Among them, polymer acceptors have the merits of superior morphology stability and excellent mechanical properties. However, owing to the key requirement of very low-lying LUMO/HOMO energy levels for polymer acceptors, very few conjugated polymers can work as polymer acceptors in OPVs. The majority of polymer electron acceptors are based on strong electron-withdrawing imide units or cyano substituents. Since 2015, conjugated polymers containing the boron-nitrogen coordination bond (B←N) have emerged as a new kind of polymer electron acceptor with excellent photovoltaic performance in various kinds of organic photovoltaic devices. In this Account, we summarize our research progress on polymer acceptors containing B←N units.At first, we introduce the principle of B←N to greatly down shift LUMO/HOMO energy levels, which enables B←N to be used to design polymer acceptors. Then we describe the two molecular design strategies for polymer acceptors containing B←N units. For high-efficiency OPVs, polymer acceptors should have wide absorption spectra, proper LUMO/HOMO energy levels, high electron mobility, and good donor/acceptor blend morphology. We discuss how to use molecular design to finely tune the absorption spectra, energy levels, and electron mobility of the B←N-containing polymer acceptors. We also discuss how to improve the phase separation morphology of the blends of these polymer acceptors with small molecular donors or polymer donors. These improvements lead to excellent performance of the polymer acceptors containing B←N units in three kinds of organic photovoltaic devices. The small molecular donor/polymer acceptor type organic solar cells show excellent thermal stability and PCE of 8.0%, which is the highest value reported so far. The all-polymer solar cells exhibit PCE of 10.1%. The all-polymer indoor photovoltaics show PCE as high as 27.4% under fluorescent lamp illumination at 2000 lx. This PCE is fairly comparable to those of the best organic or inorganic indoor photovoltaics. These results provide a solid foundation for future advances. Finally, we propose that great attention should be paid to further PCE enhancement of OPVs and indoor photovoltaic applications of this new emerging kind of polymer acceptor.

Recent advances in non-fullerene organic solar cells: from lab to fab

The key factors for OSC materials toward application mainly include high performance, thickness tolerance, low cost, simple fabrication processing, high stability, and an environmentally-friendly nature.