Materials for Interfaces in Organic Solar Cells and Photodetectors

Hetero-donors-adorned anthraquinones with near infra-red absorption for solution-processable photodetectors

We introduce a donors-acceptor-based molecular design strategy of organic heteroaromatic compounds with enhanced photosensitivity in the ultraviolet (UV)/visible/near-infrared (NIR) regions. Three organic dyes are meticulously designed and synthesized, involving various donor (D) moieties and the anthraquinone acceptor (A) unit, following the so-called quasi-orthogonal A-D-D' architecture. The target compounds are synthesized via the Buchwald-Hartwig cross-coupling reactions, with the yields of up to 54 %. The molecular structure of the synthesized compounds is confirmed by a combination of experimental and theoretical methods. Density functional theory (DFT) calculations are performed for geometry optimization and analysis of the vibrational normal modes. The time-dependent calculations (TD-DFT) reveal a manifold of intramolecular charge-transfer (ICT) states with various degrees of the central D donor involvement and the local excitation (LE) admixtures. The lowest S1 state of the ICT nature (D' → A) provides the weak absorption in the near IR region. The TD-DFT calculation affords interpretation of the UV-visible-NIR absorption spectra as well as photoconductivity of the fabricated diodes, which includes ICT complexes of the studied compounds with the known triphenylamine derivative (TCTA). Implementation of the semiconductor monolayer of the molecular mixture of A-D-D' type compound and TCTA allows to obtain the high-performance near-infrared organic photodetectors.

An n-Doped Organic Cross-Linked Electron Transport Layer with High Electrical Conductivity for High-Efficiency Tandem Organic Photovoltaics

With merits of good solution processability, intrinsic flexibility, etc, organic/organic interconnecting layers (ICLs) are highly desirable for tandem organic photovoltaics (OPVs). Herein, an n-doped cross-linked organic electron transport layer (ETL), named c-NDI-Br:PEI is developed, via a simple in situ quaternization reaction between bromopentyl-substituted naphthalene diimide derivative (NDI-Br) and polyethylenimine (PEI). Due to strong self-doping, c-NDI-Br:PEI films exhibit a high electrical conductivity (0.06 S cm-1), which is important for efficient hole and electron reombination in ICL of tandem OPVs. In addition, the cross-linked ETLs show strong work function modulation ability, and good solvent-resistance. The above features enable c-NDI-Br:PEI to function as an efficient ETL not only for single-junction OPVs, but also for tandem devices without any metal layer in ICL. Under solar radiation, the single-junction device with c-NDI-Br:PEI as ETL achieves a power conversion efficiency (PCE) of 18.18%, surpassing the ZnO-based device (17.09%). The homo- and hetero-tandem devices with m-PEDOT:PSS:c-NDI-Br:PEI as ICL exhibit remarkable PCEs of 19.06% and 20.06%, respectively. Under 808 nm laser radiation with a photon flux of 57 mW cm-2, the homo-tandem device presents a superior PCE of 38.5%. This study provides a new ETL for constructing all-solution-processed organic/organic ICL, which can be integrated in flexible and wearable devices.

Sulfur-Doped ZnO as Cathode Interlayer for Efficient Inverted Organic Solar Cells

Bulk heterojunction (BHJ) organic solar cells (OSCs) represent a promising technology due to their cost-effectiveness, lightweight design and potential for flexible manufacturing. However, achieving a high power conversion efficiency (PCE) and long-term stability necessitates optimizing the interfacial layers. Zinc oxide (ZnO), commonly used as an electron extraction layer (EEL) in inverted OSCs, suffers from surface defects that hinder device performance. Furthermore, the active control of its optoelectronic properties is highly desirable as the interfacial electron transport and extraction, exciton dissociation and non-radiative recombination are crucial for optimum solar cell operation. In this regard, this study investigates the sulfur doping of ZnO as a facile method to effectively increase ZnO conductivity, improve the interfacial electron transfer and, overall, enhance solar cell performance. ZnO films were sulfur-treated under various annealing temperatures, with the optimal condition found at 250 °C. Devices incorporating sulfur-doped ZnO (S-ZnO) exhibited a significant PCE improvement from 2.11% for the device with the pristine ZnO to 3.14% for the OSC based on the S-ZnO annealed at 250 °C, attributed to an enhanced short-circuit current density (Jsc) and fill factor (FF). Optical and structural analyses revealed that the sulfur treatment led to a small enhancement of the ZnO film crystallite size and an increased n-type transport capability. Additionally, the sulfurization of ZnO enhanced its electron extraction efficiency, exciton dissociation at the ZnO/photoactive layer interface and exciton/charge generation rate without altering the film morphology. These findings highlight the potential of sulfur doping as an easily implemented, straightforward approach to improving the performance of inverted OSCs.

Effects of Self-Assembled Polymer-Based Hole Transport Monolayer on Organic Photovoltaics

In this study, the first attempt is made to implement conjugated polymer-based self-assembled monolayer (SAM), poly[3-(6-carboxyhexyl) thiophene-2,5-diyl] (P3HT-COOH), is implemented as the hole transport layer (HTL) in fabricatiing organic photovoltaics (OPVs). The scanning tunneling microscopy (STM) results show that those P3HT-COOH molecules with periodic carboxylic acid anchoring groups pack periodically on the indium tin oxide (ITO) surface and form a monolayer. Further, this monolayer is smooth and dense with a polar feature that minimizes defects, forms an excellent interface with the photoactive layer, and tunes the work function of ITO beneficial for hole extraction. OPVs with this P3HT-COOH polar monolayer as HTL exhibit an improved exciton dissociation rate, enhanced polymer crystallinity of the photoactive layer with increased hole mobility for more balanced charge transport, reduced trap state density, and weaker bimolecular recombination with larger recombination resistance. The improved charge transport properties lead to a ≈9% increment in power conversion efficiency (PCE) of OPVs relative to those using well-known PEDOT:PSS as HTLs. Additionally, the hydrophobic feature of P3HT-COOH SAM stabilizes the OPVs with residual PCE of ≈80% even after 5.3 months. The proposed approach is very useful and timely and provides a key step for developing optoelectronic devices with high-efficiency, high stability, and cost-effective production.

Thiophene Copolymer Donors Containing Ester-Substituted Thiazole for Organic Solar Cells

Organic solar cells have seen significant progress in the past 2 decades with power conversion efficiencies (PCEs) exceeding 20% but mostly based on high-cost photovoltaic materials. Polythiophenes (PTs) without a fused-ring structure are good candidates as low-cost donor materials, deserving more attention for studying. In this work, ester-substituted thiazole (E-Tz) was explored as the electron-withdrawing unit to design PTs, and further optimization on the fluorinated/nonfluorinated donor segment contents via copolymerization strategy was simultaneously performed, yielding polymer donors of PTETz-100F, PTETz-80F, and PTETz-0F. Suitable temperature-dependent aggregation for reasonable phase separation and compact molecular packing for improved charge transport were achieved in the PTETz-80F-based system, resulting in higher exciton dissociation probability and charge collection probability. Thereby, devices based on PTETz-80F:L8-BO exhibited the best photovoltaic performance with a PCE of 12.69%. In addition, the synthetic complexity of PTETz-XF polymers is 46.05%, which is significantly lower than those of other representative high-performance polymer donors. This work demonstrates the feasibility of designing PTs with an E-Tz unit and the effectiveness of the copolymerization strategy on material property and device performance optimization.

Perspective on Flexible Organic Solar Cells for Self-Powered Wearable Applications

The growing advancement of wearable technologies and sophisticated sensors has driven the need for environmentally friendly and reliable energy sources with robust mechanical stability. Flexible organic solar cells (OSCs) have become promising substitutes for traditional energy solutions thanks to their remarkable mechanical flexibility and high power conversion efficiency (PCE). These unique properties allow flexible OSCs to seamlessly integrate with diverse devices and substrates, making them an excellent choice for powering various electronic devices by efficiently harvesting solar energy. This review summarizes recent achievements in flexible OSCs from the perspective of self-powered wearable applications. It discusses advancements in materials, including substrates and transparent electrodes, evaluates performance criteria, and compares the PCEs of flexible OSCs to their rigid counterparts. Subsequently, novel applications of flexible OSCs in self-powered wearable applications are explored. Finally, a summary and perspectives on the current challenges and obstacles facing flexible OSCs and their applications in self-powered wearables are provided, aiming to inspire further research toward practical implementations.

Construction of organic/GaN heterostructures for DUV-to-NIR broadband photodetection

Herein, a broadband photodetector (BPD) is constructed with consistent and stable detection abilities for deep ultraviolet to near-infrared spectral range. The BPD integrates the GaN template with a hybrid organic semiconductor, PM6:Y6, via the spin-coating process, and is fabricated in the form of asymmetric metal-semiconductor-metal structure. Under an optimal voltage, the device shows consistent photoresponse within 254 to 850 nm, featuring high responsivity (10 to 60 A/W), photo-to-dark-current ratio over 103, and fast response time. These results show the potential of such organic/GaN heterojunctions as a simple and effective strategy to build BPDs for a reliable photo-sensing application in the future.

Stabilizing Non-Fullerene Organic Photodiodes through Interface Engineering Enabled by a Tin Ion-Chelated Polymer

The recent emergence of non-fullerene acceptors (NFAs) has energized the field of organic photodiodes (OPDs) and made major breakthroughs in their critical photoelectric characteristics. Yet, stabilizing inverted NF-OPDs remains challenging because of the intrinsic degradation induced by improper interfaces. Herein, a tin ion-chelated polyethyleneimine ethoxylated (denoted as PEIE-Sn) is proposed as a generic cathode interfacial layer (CIL) of NF-OPDs. The chelation between tin ions and nitrogen/oxygen atoms in PEIE-Sn contributes to the interface compatibility with efficient NFAs. The PEIE-Sn can effectively endow the devices with optimized cascade alignment and reduced interface defects. Consequently, the PEIE-Sn-OPD exhibits properties of anti-environmental interference, suppressed dark current, and accelerated interfacial electron extraction and transmission. As a result, the unencapsulated PEIE-Sn-OPD delivers high specific detection and fast response speed and shows only slight attenuation in photoelectric performance after exposure to air, light, and heat. Its superior performance outperforms the incumbent typical counterparts (ZnO, SnO2 , and PEIE as the CILs) from metrics of both stability and photoelectric characteristics. This finding suggests a promising strategy for stabilizing NF-OPDs by designing appropriate interface layers.

Biological Interfacial Materials for Organic Light-Emitting Diodes

Organic optoelectronic devices have received appreciable attention due to their low cost, mechanical flexibility, band-gap engineering, lightness, and solution processability over a broad area. Specifically, realizing sustainability in organic optoelectronics, especially in solar cells and light-emitting devices, is a crucial milestone in the evolution of green electronics. Recently, the utilization of biological materials has appeared as an efficient means to alter the interfacial properties, and hence improve the performance, lifetime and stability of organic light-emitting diodes (OLEDs). Biological materials can be known as essential renewable bio-resources obtained from plants, animals and microorganisms. The application of biological interfacial materials (BIMs) in OLEDs is still in its early phase compared to the conventional synthetic interfacial materials; however, their fascinating features (such as their eco-friendly nature, biodegradability, easy modification, sustainability, biocompatibility, versatile structures, proton conductivity and rich functional groups) are compelling researchers around the world to construct innovative devices with enhanced efficiency. In this regard, we provide an extensive review of BIMs and their significance in the evolution of next-generation OLED devices. We highlight the electrical and physical properties of different BIMs, and address how such characteristics have been recently exploited to make efficient OLED devices. Biological materials such as ampicillin, deoxyribonucleic acid (DNA), nucleobases (NBs) and lignin derivatives have demonstrated significant potential as hole/electron transport layers as well as hole/electron blocking layers for OLED devices. Biological materials capable of generating a strong interfacial dipole can be considered as a promising prospect for alternative interlayer materials for OLED applications.

Enhancement of electronic effects at a biomolecule-inorganic interface by multivalent interactions

The binding of peptides and proteins through multiple weak interactions is ubiquitous in nature. Biopanning has been used to "hijack" this multivalent binding for the functionalization of surfaces. For practical applications it is important to understand how multivalency influences the binding interactions and the resulting behaviour of the surface. Considering the importance of optimization of the electronic properties of surfaces in diverse electronic and optoelectronic applications, we study here the relation between the multivalency effect and the resulting modulation of the surface work function. We use 12-mer peptides, which were found to strongly bind to oxide surfaces, to functionalize indium tin oxide (ITO) surfaces. We show that the affinity of the peptides for the ITO surface, and concurrently the effect on the ITO work function, are linearly affected by the number of basic residues in the sequence. The multivalent binding interactions lead to a peptide crowding effect, and a stronger modulation of the work function for adodecapeptide than for a single basic amino acid functionalization. The bioderived molecular platform presented herein can pave the way to a novel approach to improve the performance of optoelectronic devices in an eco-friendly manner.

Annealing-Insensitive, Alcohol-Processed MoOx Hole Transport Layer for Universally Enabling High-Performance Conventional and Inverted Organic Solar Cells

At present, most solution-processed molybdenum oxide (s-MoO_x) hole transport layers (HTLs) are still mainly used in conventional organic solar cells (OSCs) but unsuitable for inverted OSCs. Herein, we demonstrate for the first time an annealing-insensitive, alcohol-processed MoO_x HTL that can universally enable high-performance conventional and inverted OSCs. The s-MoO_x HTL is spin-coated from the MoO_x nanoparticle dispersion in alcohol, where the MoO_x nanoparticles are synthesized by simple nonaqueous pyrolysis conversion of MoO₂(acac)₂. The MoO_x nanoparticles possess uniform and very small sizes of less than 5 nm and can be well dispersed in alcohol, so the s-MoO_x HTLs on ITO and active layer both show an overall uniform and smooth surface, suitable for conventional and inverted OSCs. In addition, the s-MoO_x HTL possesses decent optical transmittance and appropriate work function. Utilizing the s-MoO_x HTL annealed between room temperature and 110 °C and PM6:Y6 active layer, the conventional OSCs show an excellent power conversion efficiency (PCE) of 16.64-17.09% and the inverted OSCs also show an excellent PCE of 15.74-16.28%, which indicate that the s-MoO_x HTL could be annealing-insensitive and universal for conventional and inverted OSCs. Moreover, conventional and inverted OSCs with the s-MoO_x HTLs annealed at 80 °C both exhibit optimal PCEs of 17.09 and 16.28%, respectively, which are separately superior than that of the PEDOT:PSS-based conventional OSCs (16.94%) and the thermally evaporated MoO₃ (e-MoO₃)-based inverted OSCs (16.03%). Under light soaking and storage aging in air, the unencapsulated inverted OSCs based on the s-MoO_x HTL show similarly excellent ambient stability compared to the e-MoO_x-based devices. In addition, the s-MoO_x HTL also shows a universal function in conventional and inverted OSCs with PBDB-T:ITIC and PM6:L8-BO active layers. Notably, the s-MoO_x-based conventional and inverted OSCs with the PM6:L8-BO active layer exhibit very excellent PCEs of 18.21 and 17.12%, respectively, which are slightly higher than those of the corresponding PEDOT:PSS-based device (18.17%) and e-MoO₃-based device (17.00%). The annealing-insensitive, alcohol-processed MoO_x HTL may be very promising for flexible and large-scale processing conventional/inverted OSCs.

Constructing a High-Quality t-Se/Si Interface Via In Situ Light Annealing of a-Se for a Self-Powered Image Sensor

The quality of the interface, e.g., the semiconductor-semiconductor or metal-semiconductor interface, is the main factor restricting the photodetection performance of a heterojunction. In this study, a high-quality Se/Si interface is constructed via in situ directional transformation of amorphous Se (a-Se) into crystalline Se (t-Se) on a Si substrate via light annealing. Benefitting from the high-quality interface and appropriate energy band between Si and Se, the t-Se/Si heterojunction exhibits an extremely high responsivity and detectivity of 583.33 mA W^-1 and 8.52 × 10¹² Jones at 760 nm, respectively. In addition, the device exhibits an ultrafast rise time of 183 µs and a decay time of 405 µs. Furthermore, an image sensor fabricated via local light annealing successfully recognizes patterns of "N," "P," and "U." This study provides valuable guidance for the construction of high-quality interfaces and the design of self-powered image sensors.

Solution-Processed Organic Photodetectors with Renewable Materials

An organic photodetector prepared by a simple solution method based on renewable citrus pectin with an optimized concentration of aluminum nitrate (AlC05) is introduced herein. The effects of different concentrations of aluminum nitrate on the morphology and optical properties were investigated through various characterization methods. An AlC concentration of 0.5 mg/mL was found to provide the highest on/off ratio and acceptable rise and decay times. Also, the optimized device (Al/AlC0.5/ITO) exhibited good stability and repeatability at a 0.1 V bias under 440 nm visible light. Based on these results, citrus pectin materials were successfully used to fabricate an organic photodetector with a simple and cost-efficient fabrication process, while taking into account environmental commitments.

Flexible organic optoelectronic devices on paper

Paper substrate has many advantages, such as low cost, bendable, foldable, printable, and environmentally friendly recycling. Nowadays, paper has been further extended as a flexible platform to deliver electronic information with the integration of organic optoelectronic devices, such as organic thin-film transistor, organic solar cell, organic electrochromic device, and organic light-emitting device. It has great potential to become the new generation of flexible substrate. Given rough surface and porous of paper, many efforts have been underway in recent years to enable the compatibility between optoelectronics and paper substrate. In this review, we present the development history of paper and its physicochemical properties, and summarize the current development of paper-based organic optoelectronic devices. We also discuss the challenges that need to be addressed before practical uses of paper-based organic optoelectronic devices.

Tin Oxide Electron Transport Layers for Air-/Solution-Processed Conventional Organic Solar Cells

Commercialization of organic solar cells (OSC) is imminent. Interlayers between the photoactive film and the electrodes are critical for high device efficiency and stability. Here, the applicability of SnO₂ nanoparticles (SnO₂ NPs) as the electron transport layer (ETL) in conventional OSCs is evaluated. A commercial SnO₂ NPs solution in butanol is mixed with ethanol (EtOH) as a processing co-solvent to improve film formation for spin and slot-die coating deposition procedures. When processed with 200% v/v EtOH, the SnO₂ NPs film presents uniform film quality and low photoactive layer degradation. The optimized SnO₂ NPs ink is coated, in air, on top of two polymer:fullerene-based systems and a nonfullerene system, to form an efficient ETL film. In every case, addition of SnO₂ NPs film significantly enhances photovoltaic performance, from 3.4 and 3.7% without the ETL to 6.0 and 5.7% when coated on top of PBDB-T:PC₆₁BM and PPDT2FBT:PC₆₁BM, respectively, and from 3.7 to 7.1% when applied on top of the PTQ10:IDIC system. Flexible, all slot-die-coated devices, in air, are also fabricated and tested, demonstrating the versatility of the SnO₂ NPs ink for efficient ETL formation on top of organic photoactive layers, processed under ambient condition, ideal for practical large-scale production of OSCs.

Molecular bixbyite-like In12-oxo clusters with tunable functionalization sites for lithography patterning applications

Indium oxides have been widely applied in many technological areas, but their utilization in lithography has not been developed. Herein, we illustrated a family of unprecedented In₁₂-oxo clusters with a general formula [In₁₂(μ₄-O)₄(μ₂-OH)₂(OCH₂CH₂NHCH₂CH₂O)₈(OR)₄X₄]X₂ (where X = Cl or Br; R = CH₃, C₆H₄NO₂ or C₆H₄F), which not only present the largest size record in the family of indium-oxo clusters (InOCs), but also feature the first molecular model of bixbyite-type In₂O₃. Moreover, through the labile coordination sites of the robust diethanolamine-stabilized In₁₂-oxo core, these InOCs can be accurately functionalized with different halides and alcohol or phenol derivatives, producing tunable solubility. Based on the high solution stability as confirmed by ESI-MS analysis, homogeneous films can be fabricated using these In₁₂-oxo clusters by the spin-coating method, which can be further used for electron beam lithography (EBL) patterning studies. Accordingly, the above structural regulations have significantly influenced their corresponding film quality and patterning performance, with bromide or p-nitrophenol functionalized In₁₂-oxo clusters displaying better performance of sub-50 nm lines. Thus, the here developed bixbyite-type In₁₂-oxo cluster starts the research on indium-based patterning materials and provides a new platform for future lithography radiation mechanism studies.

Bixbyite-like In₁₂-oxo clusters with labile coordination sites show tunable solubility, varying film quality and distinct lithography patterning performance.

Enhanced Static and Dynamic Properties of Highly Miscible Fullerene-Free Green-Selective Organic Photodetectors

We developed p-n junction organic photodetectors (OPDs) composed of a polymer donor and a nonfullerene acceptor (NFA) to increase both the responsivity (R) and detectivity (D*) while maintaining a narrow wavelength selectivity. The selection of the polymer donor and NFA with similar green (G) absorption is important for achieving G-wavelength selectivity in these OPDs, which differentiates them from current fullerene-based OPDs and NFA-based panchromatic absorption OPDs. In addition, mixing the polymer donor and asymmetric NFA was efficient toward increasing the miscibility and decreasing the interfacial energy difference of the blended films, resulting in the formation of a uniform and well-mixed nanomorphology in the photoconductive layer. Two-dimensional (2D) grazing incidence X-ray diffraction and Fourier-transform infrared spectroscopy revealed that the lamellar ordering of the polymer donor was enhanced in the blend film prepared with an asymmetric NFA, whereas the aggregation of a symmetric NFA in the blend films did not increase the lamellar ordering of the polymer donor. Consequently, we achieved an R value of 0.31 A/W and D* value of 2.0 × 10¹³ Jones with a full width at half-maximum value of 230 nm at -2 V and fast response time of 27 μs without any external bias in the asymmetric NFA-based OPDs. The enhancement in the lamellar ordering and miscibility of the blended films are crucial toward increasing the static and dynamic properties of OPDs.

Carrier Blocking Layer Materials and Application in Organic Photodetectors

As a promising candidate for next-generation photodetectors, organic photodetectors (OPDs) have gained increasing interest as they offer cost-effective fabrication methods using solution processes and a tunable spectral response range, making them particularly attractive for large area image sensors on lightweight flexible substrates. Carrier blocking layers engineering is very important to the high performance of OPDs that can select a certain charge carriers (holes or electrons) to be collected and suppress another carrier. Carrier blocking layers of OPDs play a critical role in reducing dark current, boosting their efficiency and long-time stability. This Review summarizes various materials for carrier blocking layers and some of the latest progress in OPDs. This provides the reader with guidelines to improve the OPD performance via carrier blocking layers engineering.

Benzo[1,2-b:4,5-b']difuran Polymer-Based Non-Fullerene Organic Solar Cells: The Roles of Non-Fullerene Acceptors and Molybdenum Oxide on Their Ambient Stabilities and Processabilities

The ambient stability and processability of organic solar cells (OSCs) are important factors for their commercialization. Herein, we selected four benzo[1,2-b:4,5-b']difuran (BDF) polymers and two electron acceptors to examine the role of photovoltaic materials in the ambient stability. The investigations revealed that the MoO_x layer is the detrimental factor for the ambient stabilities. The penetration of MoO_x into the active layer and their interactions will strengthen the interface and form a favorable contact, hence leading to the increased photovoltaic performance, in which the efficiency loss induced by air was balanced out. As such, these BDF polymer-based non-fullerene (NF) OSCs possessed very promising ambient stabilities even after ∼1000 h with the almost maintained power conversion efficiencies (PCEs). These results drive us to further investigate the ambient processability of these NF-OSCs. The PCEs from the devices processed under ambient condition only possessed 0.3-2% loss compared to those devices under inert conditions, which suggest the significant potentials of BDF polymers to develop highly efficient and stable NF-OSCs for the practical applications.

Cross-Linkable and Alcohol-Soluble Pyridine-Incorporated Polyfluorene Derivative as a Cathode Interface Layer for High-Efficiency and Stable Organic Solar Cells

Device performance and commercialization of organic solar cells (OSCs) are strongly influenced by the characteristics of the interface layers. Cross-linked polymer interface layers with solvent-resistant properties are very compatible with large-area solution-processing methods of OSCs and may be beneficial to the environmental stability of OSCs due to the viscoelastic and cross-linked characteristics of the cross-linked polymer. In this work, a novel cross-linkable and alcohol-soluble pyridine-incorporated polyfluorene derivative, denoted as PFOPy, is synthesized and used as a cathode interface layer (CIL) in OSCs. For PFOPy, the pendant epoxy group can be effectively cross linked through cationic polymerization under thermal treatment and the pendant pyridine group can offer good alcohol solubility. Optical absorption tests of PFOPy films before/after washing by chloroform demonstrate the excellent solvent-resistance property for the cross-linked PFOPy film. Compared with the typical ZnO CIL, the cross-linked PFOPy CIL can also substantially reduce ITO's work function and form a better interface contact with the active layer. Utilizing an inverted device structure and a typical active layer of PM6:Y6, ZnO-based OSCs display an optimal power conversion efficiency (PCE) of 15.83% while PFOPy-based OSCs exhibit superior photovoltaic performance with an optimal PCE of 16.20%. Moreover, ZnO-based and PFOPy-based OSCs separately maintain 89% and 90% of the corresponding initial PCE after 12 h of illumination, indicating similarly excellent photostability. More importantly, after 26 complete thermal cycles, ZnO-based OSCs only maintain 81% of the initial PCE while PFOPy-based OSCs retain 92% of the initial PCE and exhibit obviously better thermal cycling stability, indicating that the cross-linked PFOPy CIL should offer stronger interface robustness against thermal cycling stress due to the viscoelastic and cross-linked characteristics of PFOPy. The impressive results indicate that the cross-linked PFOPy CIL would be a very promising CIL in OSCs.

Copper Thiocyanate as an Anode Interfacial Layer for Efficient Near-Infrared Organic Photodetector

Interfacial modification between the electrode and the overlying organic layer has significant effects on the charge injection and collection and thus the device performance of organic photodetectors. Here, we used copper(I) thiocyanate (CuSCN) as the anode interfacial layer for organic photodetector, which was inserted between the anode and an organic light-sensitive layer. The CuSCN layer processed with ethyl sulfide solution presented similar optical properties to the extensively used anode interlayer of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), while the relatively shallow conduction band of CuSCN resulted in a much higher electron-injection barrier from the anode and shunt resistance than those of PEDOT:PSS. Moreover, the CuSCN-based device also exhibited an increased depletion width for the PEDOT:PSS-based device, as indicated by the Mott-Schottky analysis. These features lead to the dramatically reduced dark current density of 2.7 × 10^-10 A cm^-2 and an impressively high specific detectivity of 4.4 × 10¹³ cm Hz^1/2 W^-1 under -0.1 V bias and a working wavelength of 870 nm. These findings demonstrated the great potential of using CuSCN as an anode interfacial layer for developing high-performance near-infrared organic photodetectors.

Development of low bandgap polymers for red and near-infrared fullerene-free organic photodetectors

Two low bandgap donor polymers, PDTPTT and PCPDTTT, were synthesized and their photodetecting properties were investigated under a 680 nm red LED.

Synthesis and Photovoltaics of Novel 2,3,4,5-Tetrathienylthiophene-co-poly(3-hexylthiophene-2,5-diyl) Donor Polymer for Organic Solar Cell

This report focuses on the synthesis of novel 2,3,4,5-tetrathienylthiophene-co-poly(3-hexylthiophene-2,5-diyl) (TTT-co-P3HT) as a donor material for organic solar cells (OSCs). The properties of the synthesized TTT-co-P3HT were compared with those of poly(3-hexylthiophene-2,5-diyl (P3HT). The structure of TTT-co-P3HT was studied using nuclear magnetic resonance spectroscopy (NMR) and Fourier-transform infrared spectroscopy (FTIR). It was seen that TTT-co-P3HT possessed a broader electrochemical and optical band-gap as compared to P3HT. Cyclic voltammetry (CV) was used to determine lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy gaps of TTT-co-P3HT and P3HT were found to be 2.19 and 1.97 eV, respectively. Photoluminescence revealed that TTT-co-P3HT:PC71BM have insufficient electron/hole separation and charge transfer when compared to P3HT:PC71BM. All devices were fabricated outside a glovebox. Power conversion efficiency (PCE) of 1.15% was obtained for P3HT:PC71BM device and 0.14% was obtained for TTT-co-P3HT:PC71BM device. Further studies were done on fabricated OSCs during this work using electrochemical methods. The studies revealed that the presence of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) on the surface of indium tin oxide (ITO) causes a reduction in cyclic voltammogram oxidation/reduction peak current and increases the charge transfer resistance in comparison with a bare ITO. We also examined the ITO/PEDOT:PSS electrode coated with TTT-co-P3HT:PC71BM, TTT-co-P3HT:PC71BM/ZnO, P3HT:PC71BM and P3HT:PC71BM/ZnO. The study revealed that PEDOT:PSS does not completely block electrons from active layer to reach the ITO electrode.

High-Detectivity Non-Fullerene Organic Photodetectors Enabled by a Cross-Linkable Electron Blocking Layer

The anode interlayer plays a critical role in the performance of organic photodetectors, which requires sufficient electron-blocking ability to simultaneously attain a high photocurrent and low dark current. Here, we developed two cross-linkable polymers, which can be deposited on the top of the widely used poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and form a robust layer that can effectively suppress the electron injection from the anode under reverse bias. The optimized device with the resulting cross-linkable XP2 exhibited the lowest dark current density of 5.81 × 10^-9 A cm^-2 at -0.1 V, which is about 2 orders of magnitude lower than the control devices. A remarkable responsivity of 0.5 A W^-1 and a detectivity of >1 × 10¹³ Jones at a near-infrared wavelength of 800 nm were achieved. Of particular importance is that the resulting device exhibited a linear dynamic range of >135 dB associated with a high working frequency that is shorter than typical commercial digital imagers. The planar heterojunction devices demonstrate that the dark current is closely correlated to the charge generation, which relied on the highest occupied molecular orbital energy levels of the developed cross-linked interlays. The Mott-Schottky analysis revealed that the optimized cross-linked interlayer increased the depletion width of the devices.

Cationic Polyelectrolytes with Alkylsulfonate Counterions as a Cathode Interface Layer for High-Performance Polymer Solar Cells

Three cationic polyelectrolytes polyethyleneimine ethoxylate (PEIE)-1,4-butanediol dimethylsulfonate (MSB), PEIE-1,4-butanediol diethylsulfonate (ESB), and PEIE-1,4-butanediol dibenzylsulfonate (BSB), containing methylsulfonate, ethylsulfonate, and benzylsulfonate, respectively, were prepared for cathode interface layers (CILs) via a one-step reaction with 1,4-butanediol dialkylsulfonate and PEIE as the reactants. The results indicate that PEIE-MSB and PEIE-ESB with smaller counterions possess more efficient electron extraction, higher electron mobilities, and better photovoltaic performance than PEIE-BSB with larger counterions. The PTB7-Th:PC₇₁BM-based single junction bulk heterojunction polymer solar cells (PSCs) with PEIE-ESB as the CIL showed power conversion efficiencies (PCEs) of 10.44 and 9.23% under the thickness conditions of 8 and 30 nm, respectively. The PM6:Y6-based PSCs displayed a high PCE of 15.69%. The study provides not only new high-performance CILs but also a new strategy to construct light-soaking-free PSCs via tuning alkylsulfonate counterions.

Composite Interlayer Consisting of Alcohol-Soluble Polyfluorene and Carbon Nanotubes for Efficient Polymer Solar Cells

We report the synthesis of composite interlayers using alcohol-soluble polyfluorene (ASP)-wrapped single-walled carbon nanotubes (SWNTs) and their application as electron-transport layers for efficient organic solar cells. The ASP enables the individual dispersion of SWNTs in solution. The ASP-wrapped SWNT solutions are stable for 54 days without any aggregation or precipitation, indicating their very high dispersion stability. Using the ASP-wrapped SWNTs as a cathode interlayer on zinc oxide nanoparticles (ZnO NPs), a power conversion efficiency of 9.45% is obtained in PTB7-th:PC₇₁BM-based organic solar cells, which is mainly attributed to the improvement in the short circuit current. Performance enhancements of 18 and 17% are achieved compared to those of pure ZnO NPs and ASP on ZnO NPs, respectively. In addition, the composite interlayer is applied to non-fullerene-based photovoltaics with PM6:Y6, resulting in a power conversion efficiency of up to 14.37%. The type of SWNT (e.g., in terms of diameter range and length) is not critical to the improvement in the charge-transport properties. A low density of SWNTs in the film (∼1 SWNTs/μm² for ASP-wrapped SWNTs) has a significant influence on the charge transport in solar cells. The improvement in the performance of the solar cell is attributed to the increased internal quantum efficiency, balanced mobility between electrons and holes, and minimized charge recombination.