Acceptor Bulk Heterojunction Research Articles

Validating the Novel Electron Transport Layer with the Use of Experimentally Studied D18:Y6 Bulk Heterojunction Solar Cell

AbstractOrganic solar cells (OSC) are showing steady efficiency improvement due to the development in the materials synthesis, sophisticated characterization techniques, in‐depth understanding of materials and devices. In the recent years, bulk heterojunction OSC with a non‐fullerene acceptor /polymer acceptor shows significant enhancement in efficiency (≈19%). Efficiency of the polymer acceptor OSCs is much higher than the fullerene derivative‐based acceptors. In this work, OSC simulations are done using D18 donor and Y6 acceptor bulk heterojunction as a photoactive layer. As a first step, validity of the experimental results for ITO/PEDOT:PSS/D18:Y6/PDIN/Ag structure is done. To investigate efficiency, 2,8,15‐trifluoro‐3,9,14‐tris(heptylsulfonyl)diquinoxalino[2,3‐a:2′,3′‐c]phenazine (HATNASO2C7‐Cs) electron transport layer is validated in place of PDIN in the following device structure, ITO/PEDOT:PSS/D18:Y6/HATNASO2C7‐Cs/Ag. Energy level matching of the HATNASO2C7‐Cs is well aligned compared with PDIN at the cathode interface. Device simulation optimization are carried out for various photoactive layer, ETL and HTL condition. Highest efficiency of 20.99% is obtained for ITO/PEDOT:PSS/D18:Y6/HATNASO2C7‐Cs/Ag when the HATNASO2C7‐Cs thickness, bandgap, electron affinity, carrier mobility, and defect density is matched for ≈30 nm, ≈2.8 eV, ≈4.16 eV, ≈2 × 10−3 cm2 V−1 s−1, and 1014 cm−3 respectively. Obtained results are discussed in details and results will be helpful for preliminary understanding of the system.

Understanding the Nonradiative Charge Recombination in Organic Photovoltaics: From Molecule to Device

AbstractOrganic photovoltaics (OPVs) have made significant strides with efficiencies now exceeding 20%, positioning them as potential competitors to inorganic solar technologies. One of the most critical challenges toward this goal is the severe open‐circuit voltage (Voc) loss caused by the nonradiative charge recombination (NRCR). Herein, this review comprehensively summarizes the NRCR mechanisms and suppression techniques of OPVs across various scales from molecule to device. Specifically, the origins of NRCR in a single molecule are first summarized, and molecular design principles toward high photoluminescence quantum yield are reviewed following the Marcus theory. Next, the effect of aggregation on NRCR is reviewed, as well as the molecular and processing strategies to modulate the film packing for NRCR suppression. Furthermore, the progresses in the avoidance of nonradiative loss pathways mediated by charge transfer states and triplet states in donor:acceptor bulk heterojunctions are tracked. Besides, the interfacial optimization and device structure design to maximize the electroluminescent quantum efficiency are presented. Finally, several potential pathways toward curtailing NRCR for high‐performance OPVs are outlined. Therefore, this review shows an insightful perspective to understand and mitigate the NRCR at multi‐scales, and is poised to provide a clear roadmap for the next breakthrough of OPVs.

Dark Current in Broadband Perovskite-Organic Heterojunction Photodetectors Controlled by Interfacial Energy Band Offset.

Lead halide perovskite and organic semiconductors are promising classes of materials for photodetector (PD) applications. State-of-the-art perovskite PDs have performance metrics exceeding silicon PDs in the visible. While organic semiconductors offer bandgap tunability due to their chemical design with detection extended into the near-infrared (NIR), perovskites are limited to the visible band and the first fraction of the NIR spectrum. In this work, perovskite-organic heterojunction (POH) PDs with absorption up to 950nm are designed by the dual contribution of perovskite and the donor:acceptor bulk-heterojunction (BHJ), without any intermediate layer. The effect of the energetics of the donor materials is systematically studied on the dark current (Jd) of the device by using the PBDB-T polymer family. Combining the experimental results with drift-diffusion simulations, it is shown that Jd in POH devices is limited by thermal generation via deep trap states in the BHJ. Thus, the best performance is obtained for the PM7-based POH, which delivers an ultra-low noise current of 2×10-14AHz-1/2 and high specific detectivity of 4.7×1012Jones in the NIR. Last, the application of the PM7-based POH devices as NIR pulse oximeter with high-accuracy heartbeat monitoring at long-distance of 2 meters is demonstrated.

Visualization of Tens of Nanometers Spaced Donor: Acceptor Bulk Heterojunctions across Submicrometer-Square Cross Sections of Organic Photovoltaic Cells

Visualization of Tens of Nanometers Spaced Donor: Acceptor Bulk Heterojunctions across Submicrometer-Square Cross Sections of Organic Photovoltaic Cells

Long-Range Charge Separation Enabled by Intramoiety Delocalized Excitations in Copolymer Donors in Organic Photovoltaic Blends.

For over two decades, most high-performance organic photovoltaics (OPVs) have been made with donor:acceptor bulk heterojunctions with domain sizes limited by exciton diffusion, where charge separation mostly takes place through the dissociation of the interfacial charge-transfer (xCT) excitons. Recently, nonfullerene acceptor (NFA)-based OPVs have shown excellent compatibility to device structures with large domains in active layers. However, it remains elusive how the excitations that are distant from the interfaces are converted into free charges. Here, we report the identification of a new charge separation channel in model copolymer/NFA blends mediated by intra-moiety delocalized excitations in both planar heterojunctions and donor-enriched bulk heterojunctions. The delocalized excitations induced by interchromophore electronic interactions in copolymer donors mediate the long-range charge separation and dissociate into free charges without forming the bound xCT states first, releasing the constraints associated with the short exciton diffusion length in organic materials. The long-range charge separation mechanism uncovered in this work, in cooperation with the short-range xCT-mediated pathway, holds the potential to further optimize OPVs with diverse device structures.

Organic Semiconductor-Based Photoelectrochemical Cells for Efficient Solar-to-Chemical Conversion

Organic semiconductor-based photoelectrodes are gaining significant attention in photoelectrochemical (PEC) value-added chemical production systems, which are promising architectures for solar energy harvesting. Organic semiconductors consisting of conjugated carbon–carbon bonds provide several advantages for PEC cells, including improved charge transfer, tunable band positions and band gaps, low cost, and facile fabrication using organic solvents. This review gives an overview of the recent advances in emerging single organic semiconductor-based photoelectrodes for PEC water splitting and the various strategies for enhancing their performance and stability. It highlights the importance of photoelectrodes based on donor–acceptor bulk heterojunction (BHJ) systems for fabricating efficient organic semiconductor-based solar energy-harvesting devices. Furthermore, it evaluates the recent progress in BHJ organic base photoelectrodes for producing highly efficient PEC value-added chemicals, such as hydrogen and hydrogen peroxide. Finally, this review highlights the potential of organic-based photoelectrodes for bias-free solar-to-chemical production, which is the ultimate goal of PEC systems and a step toward achieving reliable commercial technology.

Photomultiplication‐Type Organic Photodetectors with High EQE‐Bandwidth Product by Introducing a Perovskite Quantum Dot Interlayer

AbstractA photomultiplication (PM)‐type organic photodetector (OPD) that exploits the ionic motion in CsPbI3 perovskite quantum dots (QDs) is demonstrated. The device uses a QD monolayer as a PM‐inducing interlayer and a donor–acceptor bulk heterojunction (BHJ) layer as a photoactive layer. When the device is illuminated, negative ions in the CsPbI3 QD migrate and accumulate near the interface between the QDs and the electrode; these processes induce hole injection from the electrode and yield the PM phenomenon with an external quantum efficiency (EQE) >2000% at a 3 V applied bias. It is confirmed that the ionic motion of the CsPbI3 QDs can induce a shift in the work function of the QD/electrode interface and that the dynamics of ionic motion determines the response speed of the device. The PM OPD showed a large EQE‐bandwidth product >106 Hz with a −3 dB frequency of 125 kHz at 3 V, which is one of the highest response speeds reported for a PM OPD. The PM‐inducing strategy that exploits ionic motion of the interlayer is a potential approach to achieving high‐efficiency PM OPDs.

Photovoltaics literature survey (No. 181)

Photovoltaics literature survey (No. 181)

An organic semiconductor obtained with a low-temperature process for light-addressable potentiometric sensors

In this study, the first trial of donor and acceptor bulk heterojunctions (BHJs) is proposed for light-addressable potentiometric sensors (LAPSs) due to organic conjugated molecules with the advantages of energy level/band gap modification and high absorption of visible light. The benzodithiophene-based polymer PTB7-Th and fullerene PC71BM are selected for integration with an Al2O3 sensing membrane using atomic layer deposition at temperatures lower than 100 ℃ for pH sensing LAPS. In the stability performance, the hysteresis width of the best condition with a mixing weight ratio of PC71BM to PTB7-Th of 1.5 is 4.4 mV, which is inferior to that of the conventional inorganic semiconductor-based LAPSs. By means of an additional spin-coated zinc oxide (ZnO) layer, the signal-to-noise ratio of the photocurrent of this fabricated LAPS can be maintained at 3.2 after 170 min of measurement with an acceptable sensitivity, linearity, drift coefficient and hysteresis of 50.2 mV/pH, 99.9%, − 3.1 mV/h and 1.4 mV, respectively. With the proper adjustment of illumination with low power and large spots, chemical imaging with a scanning step of 25 µm can be conducted to indicate the feasibility of this low-temperature process developed for spin-coated organic semiconductor-based LAPSs.

Impact of the bilayer electron transport layer in the donor acceptor bulk heterojunctions for improved inverted organic photovoltaic performance

Functionalization at interface of the electron transport layer and the organic polymeric donor and non-fullerene acceptor bulk heterojunction layer in photovoltaic devices that have an inverted structure, is a unique strategy for improving performance. Here we demonstrate improved interfacial nanomorphology of the well-known PTB7-Th donor and IEICO-4F acceptor bulk heterojunction (BHJ) on an atomic layer deposited (ALD) ultrathin titanium oxide (TiO2) passivated sol–gel grown zinc oxide (ZnO) electron transport bilayer and highlight how this is correlated to the overall organic photovoltaic device performance. Synchrotron-based grazing incidence wide-angle X-ray scattering (GIWAXS) has been used to demonstrate an enhanced face-on conformation within the BHJ with highly crystalline ZnO/TiO2/BHJ layers and this is greatest for the very thinnest of TiO2 layers on ZnO (ZnT1), from just two deposition cycles, relative to both four (ZnT2) and eight (ZnT3) cycles. Photovoltaic devices with BHJs grown over ZnT1, showed an overall average increase of 30.3 ± 0.5 % in the power conversion efficiency (PCE 10.3 ± 0.5 %), a 14.3 ± 1 % increase in the fill-factor, FF (60.1 ± 1 %), 2.9 ± 0.05 % improvement in Voc (0.70 ± 0.05 mV) and 13.6 ± 0.5 % higher JSc (24.2 ± 0.5 mA/cm2), compared to the control device,with a best efficiency of 11.1 ± 0.5 % PCE, 61.9 ± 1 % FF, 0.70 ± 0.5 mV Voc and 25.0 ± 0.5 mA/cm2Jsc. We conclude that the molecular crystal conformation of the organic donor acceptor BHJ layer can be partly controlled through the ETL along with an enhanced organic inorganic interface which strongly influences the OPV performance.

Device modeling of two-dimensional hole transport materials for boosting the performance of non-fullerene acceptor bulk heterojunction organic solar cells

Simple and low-cost two-dimensional (2D) materials are of great interest considering their wide use in various device applications. Here, we present the application of 2D materials, i.e., graphene oxide (GO), molybdenum disulfide (MoS2), and tungsten disulfide (WS2) as hole transport layer (HTL) in non-fullerene acceptors (NFAs) organic photovoltaics (OPVs) using device simulation. The numerical simulation was carried out using SCAPS-1D and specifically focused on the effect of thickness and defect density at the active layer on the photovoltaic characteristics of the devices. In addition, the influence of defect density at the HTL/active layer interface on the device performance was also studied. After conducting a series of optimization, an optimum power conversion efficiency (PCE) of 15.89%, 20.05%, and 23.55% was achieved for NFA-based OPV with GO, MoS2, and WS2, respectively. This work provides a practical explanation of the feasible performance enhancement and critical design parameters for OPV devices. The results showcase the potential application of 2D materials as low-cost HTL for high-efficiency organic solar cells.

Polymer/non-fullerene acceptor bulk heterojunction nanoparticles for efficient photocatalytic hydrogen production from water

Bulk heterojunction nanoparticles (NPs) composed of p-type polymer donor and non-fullerene acceptor are emerging photocatalysts for water splitting. Herein, bulk heterojunction NPs consisting of donor PBDB-T and non-fullerene acceptor ITIC are produced by micro-emulsion method, and are demonstrated as efficient photocatalyst to generate hydrogen from water splitting upon full spectrum illumination, with ascorbic acid as the sacrificial agent. The photocatalytic efficiency is determined by the nanoscale morphology of these bulk heterojunction NPs, which can be regulated by donor/acceptor ratio, binary solvent volume fraction, as well as surfactant content. The best-performing bulk heterojunction NPs show an outstanding hydrogen evolution rate of 257 mmol h−1 g−1, benefiting from the ideal phase separation, reasonable particle size and efficient charge separation and transfer. This work provides a rational guide for producing bulk heterojunction NPs made of organic semiconductors to reach high photocatalytic efficiency.

Efficiency Limits in Wide‐Bandgap Ge‐Containing Donor Polymer:Nonfullerene Acceptor Bulk Heterojunction Solar Cells

A precise picture of the photophysics that determine the efficiency of nonfullerene acceptor (NFA) bulk heterojunction (BHJ) organic solar cells is still missing, yet needed to mitigate the remaining loss channels. Herein, charge carrier generation and recombination dynamics in blends of a novel wide‐bandgap germanium‐containing donor polymer, namely, PEHGeNDT‐BT, paired with either O‐IDTBR or O‐IDTBCN as NFA in BHJ solar cells are investigated by (ultrafast) transient spectroscopy and time‐delayed collection field (TDCF) experiments. Photovoltaic devices yield moderate power conversion efficiencies (PCEs) of 5.3% when using O‐IDTBCN as the acceptor, and only about 2% when using O‐IDTBR as the acceptor, the latter severely limited by its low photocurrent and moderate fill factor (FF) of ≈43%. Time‐resolved photoluminescence experiments reveal limited exciton quenching as one loss channel in O‐IDTBR‐based blends, accompanied by significant geminate recombination further reducing the photocurrent, while field‐dependent charge generation is identified as the origin of the low FF. Geminate recombination is less in the O‐IDTBCN blend and charge generation is field‐independent, leading to improved photocurrent and FF. Carrier drift‐diffusion simulations of the devices’ current–voltage (J–V) characteristics confirm that the experimentally determined kinetic parameters and process yields can reproduce the measured J–V curves under steady‐state solar illumination.

(Invited) Conjugated Semiconductors for Photoelectrochemical and Photocatalytic Hydrogen Production Via Water Splitting

The application of pi-conjugated polymeric semiconductors toward direct solar-to-fuel energy conversion by photoelectrochemical (PEC) or heterogeneous photocatalytic (PC) approaches is an emerging trend.[1] While these materials offer impressive tunability of energy levels and light absorption, their long-term robustness under the harsh conditions of PEC or PC water splitting is a major concern.[2] In this presentation critical factors that influence the operational stability of donor:acceptor bulk heterojunction photoelectrodes[3] for solar hydrogen production are discussed and routes to significantly advance their performance under long term operation are presented. An outlook on employing organic based semiconductor systems in comparison to their inorganic counterparts will be presented with respect to the possible economic viability of this technology.[1] Adv. Energy Mater. 2018, 8, 1802585.[2] ACS Energy Lett. 2020, 5, 1970.[3] J. Am. Chem. Soc. 2020 , 142, 7795.

Recent progress of ultra-narrow-bandgap polymer donors for NIR-absorbing organic solar cells.

Solution-processed near-infrared (NIR)-absorbing organic solar cells (OSCs) have been explored worldwide because of their potential as donor:acceptor bulk heterojunction (BHJ) blends. In addition, NIR-absorbing OSCs have attracted attention as high specialty equipment in next-generation optoelectronic devices, such as semitransparent solar cells and NIR photodetectors, owing to their feasibility for real-time commercial application in industry. With the introduction of NIR-absorbing non-fullerene acceptors (NFAs), the value of OSCs has been increasing while organic donor materials capable of absorbing light in the NIR region have not been actively studied yet compared to NIR-absorbing acceptor materials. Therefore, we present an overall understanding of NIR donors.

Vertical Miscibility of Bulk Heterojunction Films Contributes to High Photovoltaic Performance

AbstractThe power conversion efficiencies (PCEs) of organic photovoltaic cells employing donor (polymer):acceptor (small molecule) bulk heterojunction (BHJ) are being rapidly improved. The phase segregation along vertical (film‐depth) direction of the active layer, referring to vertical immiscibility between donor and acceptor, leads to vertically varied and nonlinearly distributed composition, optical and electronic properties. However, the correlation between vertical miscibility and photovoltaic performance is still confusing. Here, it is semi‐empirically found that such vertical variations induced by vertical immiscibility deteriorate PCE. Subsequently, using PM6:Y6‐based binary and ternary BHJ as examples, a combined statistical, experimental, and numerical investigation on the dependence of photovoltaic performance on vertical miscibility is reported. The vertical phase evolution of BHJ significantly depends on solvents and processing methods. As compared with other donor:acceptor systems, polymer:Y6 deposited from appropriate solvents could have the best vertical miscibility which is nearly independent on film‐depth, leading to a higher PCE. PM6:Y6:fullerene ternary blends also have a good and uniform vertical miscibility, forming spatially well‐mixed ternary BHJ. Consequently, under this design guideline, the optimized film‐depth‐dependent miscibility contributes to optimized vertical distribution of optical and electronic properties, leading to an optimized PCE 17.1% with a low sensitivity to fluctuation of film thickness.

Unifying Charge Generation, Recombination, and Extraction in Low‐Offset Non‐Fullerene Acceptor Organic Solar Cells

AbstractEven though significant breakthroughs with over 18% power conversion efficiencies (PCEs) in polymer:non‐fullerene acceptor (NFA) bulk heterojunction organic solar cells (OSCs) have been achieved, not many studies have focused on acquiring a comprehensive understanding of the underlying mechanisms governing these systems. This is because it can be challenging to delineate device photophysics in polymer:NFA blends comprehensively, and even more complicated to trace the origins of the differences in device photophysics to the subtle differences in energetics and morphology. Here, a systematic study of a series of polymer:NFA blends is conducted to unify and correlate the cumulative effects of i) voltage losses, ii) charge generation efficiencies, iii) non‐geminate recombination and extraction dynamics, and iv) nuanced morphological differences with device performances. Most importantly, a deconvolution of the major loss processes in polymer:NFA blends and their connections to the complex BHJ morphology and energetics are established. An extension to advanced morphological techniques, such as solid‐state NMR (for atomic level insights on the local ordering and donor:acceptor ππ interactions) and resonant soft X‐ray scattering (for donor and acceptor interfacial area and domain spacings), provide detailed insights on how efficient charge generation, transport, and extraction processes can outweigh increased voltage losses to yield high PCEs.

(Invited) Photoelectrochemical Solar Fuel Production Using Organic Semiconductors

Pi-conjugated carbon-based semiconductors are well-established as high-performance and easily-processed materials for inexpensive and scalable solar energy conversion in organic photovoltaic (OPV) devices, but their promise in the economic conversion of solar energy into chemical energy (solar fuels) has only recently been recognized. In this presentation, I will discuss the main approaches for employing organic semiconductor-based devices towards solar H2 generation via water splitting and review the state-of-the-art.[1] OPV-biased water electrolysis has advanced significantly, now exceeding 6% solar-to-hydrogen conversion efficiency. However, the direct water splitting by an organic semiconductor in a photoelectrochemical cell, which has attractive advantages, remains limited by the intrinsic stability of organic semiconductors when placed in direct junction with aqueous electrolyte. I will further present experimental insights into the operational stability of donor:acceptor bulk heterojunction photocathodes and photoanodes[2] with an organic/water interface. Using sacrificial redox couples and various electrochemical analysis including impedance spectroscopy we identify and quantify the key factors that govern long-term photoactivity (>12 h). These insights can be extended to overall water splitting with appropriate catalyst overlayer design. Overall our advances with organic semiconductor photoelectrodes open a new path for economically feasible solar fuel production.[1] L. Yao, A. Rahmanudin, N. Guijarro, K. Sivula, Adv. Energy Mater. 2018, 1802585.[2] P. Bornoz, M. S. Prévot, X. Yu, N. Guijarro, K. Sivula, J. Am. Chem. Soc. 2015, 137, 15338.

Quantification of Photophysical Processes in All‐Polymer Bulk Heterojunction Solar Cells

All‐polymer solar cells lag behind the state‐of‐the‐art in small molecule nonfullerene acceptor (NFA) bulk heterojunction (BHJ) organic solar cells (OSCs) for reasons still unclear. Herein, the efficiency‐limiting processes in all‐polymer solar cells are investigated using blends of the common donor polymer PBDT‐TS1 with different acceptor polymers, namely P2TPD[2F]T and P2TPDBT[2F]T. Combining data from steady‐state optical spectroscopy and time‐resolved photoluminescence, transient absorption, and time‐delayed collection field experiments, provides not only a concise but also quantitative assessment of the losses due to limited photon absorption, geminate and nongeminate charge carrier recombination, field‐dependent charge generation, and inefficient carrier extraction. Although both systems exhibit a similar charge separation efficiency in the absence of external bias, charge separation is significantly enhanced in P2TPDBT[2F]T‐based blends when biased. Kinetic parameters obtained via pulsed laser spectroscopy are used to reproduce the experimentally measured device current–voltage (J–V) characteristics and indicate that low fill factors originate either from nongeminate recombination competing with charge extraction, or from a pronounced field dependence of charge generation, depending on the acceptor polymer. The methodology presented here is generic and can be used to quantify the loss processes in BHJ OSCs including both all‐polymer and small molecule NFA systems.

Establishing Stability in Organic Semiconductor Photocathodes for Solar Hydrogen Production.

As organic semiconductors attract increasing attention to application in the fields of bioelectronics and artificial photosynthesis, understanding the factors that determine their robust operation in direct contact with aqueous electrolytes becomes a critical task. Herein we uncover critical factors that influence the operational stability of donor:acceptor bulk heterojunction photocathodes for solar hydrogen production and significantly advance their performance under operational conditions. First, using the direct photoelectrochemical reduction of aqueous Eu3+ and impedance spectroscopy, we determine that replacing the commonly used fullerene-based electron acceptor with a perylene diimide-based polymer drastically increases operational stability and identify that limiting the photogenerated electron accumulation at the organic/water interface to values of ca. 100 nC cm-2 is required for stable operation (>12 h). These insights are extended to solar-driven hydrogen production using MoS3, MoP, or RuO2 water reduction catalyst overlayers where it is found that the catalyst morphology strongly affects performance due to differences in charge extraction. Optimized performance of bulk heterojunction photocathodes coated with a MoS3:MoP composite gave 1 Sun photocurrent density up to 8.7 mA cm-2 at 0 V vs RHE (pH 1). However, increased stability was gained with RuO2 where initial photocurrent density (>8 mA cm-2) deceased only 15% or 33% during continuous operation for 8 or 20 h, respectively, thus demonstrating unprecedented robustness without a protection layer. This performance represents a new benchmark for organic semiconductor photocathodes for solar fuel production and advances the understanding of stability criteria for organic semiconductor/water-junction-based devices.