Integrated Quasi-2D Perovskite/Organic Solar Cells with Efficiency over 19% Promoted by Interface Passivation

Integrated perovskite/organic solar cells (IPOSCs) have shown great potential in broadening the light absorption range and improving the photovoltaic performance. However, the severe interface charge recombination and unmatched energy levels between perovskite and organic photoactive layers hinder their performance improvement. Here, an efficient interface passivation strategy for IPOSCs based on a layered Ruddlesden–Popper (RP) perovskite and high photovoltaic performance is successfully demonstrated. It is found that an ultrathin conjugated polymer (PM6) layer could passivate the surface defects of perovskite film, tuning the energy level and suppress the nonradiative recombination loss, leading to efficient interface contact between RP perovskite and organic photoactive layers, boosting the open-circuit voltage from 1.06 to 1.12 V and the efficiency from 17.23% to 19.15%. Importantly, the optimized device shows extended photocurrent response to 930 nm with a peak intensity close to 50% from 800 to 931 nm. The results indicate that interface passivation using a functionalized polymer could be an efficient strategy to improve the photovoltaic performance of integrated devices.     

High‐Performance Phototransistors Based on Single‐Crystalline n‐Channel Organic Nanowires and Photogenerated Charge‐Carrier Behaviors

The photoelectronic characteristics of single‐crystalline nanowire organic phototransistors (NW‐OPTs) are studied using a high‐performance n‐channel organic semiconductor, N,N′‐bis(2‐phenylethyl)‐perylene‐3,4:9,10‐tetracarboxylic diimide (BPE‐PTCDI), as the photoactive layer. The optoelectronic performances of the NW‐OPTs are analyzed by way of their current–voltage (I–V) characteristics on irradiation at different wavelengths, and comparison with corresponding thin‐film organic phototransistors (OPTs). Significant enhancement in the charge‐carrier mobility of NW‐OPTs is observed upon light irradiation as compared with when performed in the dark. A mobility enhancement is observed when the incident optical power density increases and the wavelength of the light source matches the light‐absorption range of the photoactive material. The photoswitching ratio is strongly dependent upon the incident optical power density, whereas the photoresponsivity is more dependent on matching the light‐source wavelength with the maximum absorption range of the photoactive material. BPE‐PTCDI NW‐OPTs exhibit much higher external quantum efficiency (EQE) values (≈7900 times larger) than thin‐film OPTs, with a maximum EQE of 263 000%. This is attributed to the intrinsically defect‐free single‐crystalline nature of the BPE‐PTCDI NWs. In addition, an approach is devised to analyze the charge‐transport behaviors using charge accumulation/release rates from deep traps under on/off switching of external light sources.     

Optimal Structure for High‐Performance and Low‐Contact‐Resistance Organic Field‐Effect Transistors Using Contact‐Doped Coplanar and Pseudo‐Staggered Device Architectures

A low contact resistance achieved on top‐gated organic field‐effect transistors by using coplanar and pseudo‐staggered device architectures, as well as the introduction of a dopant layer, is reported. The top‐gated structure effectively minimizes the access resistance from the contact to the channel region and the charge‐injection barrier is suppressed by doping of iron(III)trichloride at the metal/organic semiconductor interface. Compared with conventional bottom‐gated staggered devices, a remarkably low contact resistance of 0.1–0.2 kΩ cm is extracted from the top‐gated devices by the modified transfer line method. The top‐gated devices using thienoacene compound as a semiconductor exhibit a high average field‐effect mobility of 5.5–5.7 cm² V^?1 s^?1 and an acceptable subthreshold swing of 0.23–0.24 V dec^?1 without degradation in the on/off ratio of ≈10⁹. Based on these experimental achievements, an optimal device structure for a high‐performance organic transistor is proposed.     

Synergistic enhancement of charge generation and transfer in organic solar cells via a separate PbS quantum dot layer

The major impediment to the high photovoltaic performance of organic solar cells (OSCs) involves deficient photon harvesting and ineffective charge transfer from the photoactive layer to the electrodes. To improve these constraints, in this study, a new OSC device architecture is demonstrated by incorporating PbS colloidal quantum dots (QDs) between the organic photoactive layer and the top electrode. PbS QDs were spin-coated on top of an organic blend via a layer-by-layer deposition process, which formed a separate PbS QD layer with high density and uniformity. The PbS QD layer reinforced the optical property of the OSC by harvesting photons that were not absorbed by the underlying organic photoactive layer. In addition, the OSC employing the QD layer showed the enhanced charge transfer and suppressed recombination loss through the hybrid organic-inorganic interfacial contacts. Thus, a significant increase in the efficiency was achieved compared with the OSC with no PbS QD layer (10.12 vs 8.84%). Accompanied with the improved optoelectronic properties, a superior stability of the proposed architecture advances the practical viability of OSCs in various applications.     

Non‐Geminate Recombination as the Primary Determinant of Open‐Circuit Voltage in Polythiophene:Fullerene Blend Solar Cells: an Analysis of the Influence of Device Processing Conditions

The physical origin of the open‐circuit voltage in bulk heterojunction solar cells is still not well understood. While significant evidence exists to indicate that the open‐circuit voltage is limited by the molecular orbital energies of the heterojunction components, it is clear that this picture is not sufficient to explain the significant variations which often occur between cells fabricated from the same heterojunction components. We present here an analysis of the variation in open‐circuit voltage between 0.4–0.65 V observed for a range of P3HT/PCBM solar cells where device deposition conditions, electrode structure, active‐layer thickness and device polarity are varied. The analysis quantifies non‐geminate recombination losses of dissociated carriers in these cells, measured under device operating conditions. It is found that at open‐circuit, losses due to non‐geminate recombination are sufficiently large that other loss pathways may effectively be neglected. Variations in open‐circuit voltage between different devices are shown to arise from differences in the rate coefficient for non‐geminate recombination, and from differences in the charge densities in the photoactive layer of the device. The origin of these differences is discussed, particularly with regard to variations in film microstructure. By separately quantifying these differences in rate coefficient and charge density, and by application of a simple physical model based upon the assumption that open‐circuit is reached when the flux of charge photogeneration is matched by the flux of non‐geminate recombination, we are able to calculate correctly the open‐circuit voltage for all the cells studied to within an accuracy of ±5 mV.     

Heterojunction Ambipolar Organic Transistors Fabricated by a Two‐Step Vacuum‐Deposition Process

Ambipolar organic field‐effect transistors (OFETs) are produced, based on organic heterojunctions fabricated by a two‐step vacuum‐deposition process. Copper phthalocyanine (CuPc) deposited at a high temperature (250 °C) acts as the first (p‐type component) layer, and hexadecafluorophthalocyaninatocopper (F₁₆CuPc) deposited at room temperature (25 °C) acts as the second (n‐type component) layer. A heterojunction with an interpenetrating network is obtained as the active layer for the OFETs. These heterojunction devices display significant ambipolar charge transport with symmetric electron and hole mobilities of the order of 10^–4 cm² V^–1 s^–1 in air. Conductive channels are at the interface between the F₁₆CuPc and CuPc domains in the interpenetrating networks. Electrons are transported in the F₁₆CuPc regions, and holes in the CuPc regions. The molecular arrangement in the heterojunction is well ordered, resulting in a balance of the two carrier densities responsible for the ambipolar electrical characteristics. The thin‐film morphology of the organic heterojunction with its interpenetrating network structure can be controlled well by the vacuum‐deposition process. The structure of interpenetrating networks is similar to that of the bulk heterojunction used in organic photovoltaic cells, therefore, it may be helpful in understanding the process of charge collection in organic photovoltaic cells.     

Trap‐Assisted Recombination via Integer Charge Transfer States in Organic Bulk Heterojunction Photovoltaics

Organic photovoltaics are under intense development and significant focus has been placed on tuning the donor ionization potential and acceptor electron affinity to optimize open circuit voltage. Here, it is shown that for a series of regioregular‐poly(3‐hexylthiophene):fullerene bulk heterojunction (BHJ) organic photovoltaic devices with pinned electrodes, integer charge transfer states present in the dark and created as a consequence of Fermi level equilibrium at BHJ have a profound effect on open circuit voltage. The integer charge transfer state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor–acceptor interface, even though there is a large variation in electron affinity for the fullerene series. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of trap‐assisted recombination via integer charge transfer states. Based on the results, novel design rules for optimizing open circuit voltage and performance of organic bulk heterojunction solar cells are proposed.     

Interface Modification to Improve Hole‐Injection Properties in Organic Electronic Devices

The performance of organic electronic devices is often limited by injection. In this paper, improvement of hole injection in organic electronic devices by conditioning of the interface between the hole‐conducting layer (buffer layer) and the active organic semiconductor layer is demonstrated. The conditioning is performed by spin‐coating poly(9,9‐dioctyl‐fluorene‐co‐N‐ (4‐butylphenyl)‐diphenylamine) (TFB) on top of the poly(3,4‐ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) buffer layer, followed by an organic solvent wash, which results in a TFB residue on the surface of the PEDOT:PSS. Changes in the hole‐injection energy barriers, bulk charge‐transport properties, and current–voltage characteristics observed in a representative PFO‐based (PFO: poly(9,9‐dioctylfluorene)) diode suggest that conditioning of PEDOT:PSS surface with TFB creates a stepped electronic profile that dramatically improves the hole‐injection properties of organic electronic devices.     

An Effective Method for Recovering Nonradiative Recombination Loss in Scalable Organic Solar Cells

Regarded as a critical step in commercial applications, scalable printing technology has become a research frontier in the field of organic solar cells. However, inevitable efficiency loss always occurs in the lab‐to‐manufacturing translation due to the different fabrication processes. In fact, the decline of photovoltaic performance is mainly related to voltage loss, which is mainly affected by the diversity of phase separation morphology and the chemical structures of photoactive materials. Fullerene derivative indene‐C₆₀ bisadduct (ICBA) is introduced into a PBDB‐T‐2F:IT‐4F system to control the active layer morphology during blade‐coating process. Accordingly, as a symmetrical fullerene derivative, ICBA can regulate the crystallization tendency and molecular packing orientation and suppress charge carrier recombination. This ternary strategy overcomes the morphology issues caused by weaker shear impulse in blade‐coating process. Benefiting from the reduced nonradiative recombination loss, 1.05 cm² devices are fabricated by blade coating with a power conversion efficiency of 13.70%. This approach provides an effective support for recovering the voltage loss during scalable printing approaches.     

A Photoresponsive Red‐Hair‐Inspired Polydopamine‐Based Copolymer for Hybrid Photocapacitive Sensors

Inspired by the powerful photosensitizing properties of the red hair pigments pheomelanins, a photoresponsive cysteine‐containing variant of the adhesive biopolymer polydopamine (pDA) is developed via oxidative copolymerization of dopamine (DA) and 5‐S‐cysteinyldopamine (CDA) in variable ratios. Chemical and spectral analysis indicate the presence of benzothiazole/benzothiazine units akin to those of pheomelanins. p(DA/CDA) copolymers display ­impedance properties similar to those of biological materials and a marked photoimpedance response to light stimuli. The use of the p(DA/CDA) copolymer to implement a solution‐processed hybrid photocapacitive/resistive metal‐insulator‐semiconductor (MIS) device disclosed herein is the first example of technological exploitation of photoactive, red‐hair‐inspired biomaterials as soft enhancement layer for silicon in an optoelectronic device. The bio‐inspired materials described herein may provide the active component of new hybrid photocapacitive sensors with a chemically tunable response to visible light.     

Effect of Gate Electrode Work‐Function on Source Charge Injection in Electrolyte‐Gated Organic Field‐Effect Transistors

Systematic investigation of the contact resistance in electrolyte‐gated organic field‐effect transistors (OFETs) demonstrates a dependence of source charge injection versus gate electrode work function. This analysis reveals contact‐limitations at the source metal‐semiconductor interface and shows that the contact resistance increases as low work function metals are used as the gate electrode. These findings are attributed to the establishment of a built‐in potential that is high enough to prevent the Fermi‐level pinning at the metal‐organic interface. This results in an unfavorable energetic alignment of the source electrode with the valence band of the organic semiconductor. Since the operating voltage in the electrolyte‐gated devices is on the same order as the variation of the work functions, it is possible to tune the contact resistance over more than one order of magnitude by varying the gate metal.     

Towards Efficient Integrated Perovskite/Organic Bulk Heterojunction Solar Cells: Interfacial Energetic Requirement to Reduce Charge Carrier Recombination Losses

Integrated perovskite/organic bulk heterojunction (BHJ) solar cells have the potential to enhance the efficiency of perovskite solar cells by a simple one‐step deposition of an organic BHJ blend photoactive layer on top of the perovskite absorber. It is found that inverted structure integrated solar cells show significantly increased short‐circuit current (J_sc) gained from the complementary absorption of the organic BHJ layer compared to the reference perovskite‐only devices. However, this increase in J_sc is not directly reflected as an increase in power conversion efficiency of the devices due to a loss of fill factor. Herein, the origin of this efficiency loss is investigated. It is found that a significant energetic barrier (≈250 meV) exists at the perovskite/organic BHJ interface. This interfacial barrier prevents efficient transport of photogenerated charge carriers (holes) from the BHJ layer to the perovskite layer, leading to charge accumulation at the perovskite/BHJ interface. Such accumulation is found to cause undesirable recombination of charge carriers, lowering surface photovoltage of the photoactive layers and device efficiency via fill factor loss. The results highlight a critical role of the interfacial energetics in such integrated cells and provide useful guidelines for photoactive materials (both perovskite and organic semiconductors) required for high‐performance devices.     

In‐Situ Switching from Barrier‐Limited to Ohmic Anodes for Efficient Organic Optoelectronics

Injection and extraction of charges through ohmic contacts are required for efficient operation of semiconductor devices. Treatment using polar non‐solvents switches polar anode surfaces, including PEDOT:PSS and ITO, from barrier‐limited hole injection and extraction to ohmic behaviour. This is caused by an in‐situ modification of the anode surface that is buried under a layer of organic semiconductor. The exposure to methanol removes polar hydroxyl groups from the buried anode interface, and permanently increases the work function by 0.2–0.3 eV. In the case of ITO/PEDOT:PSS/PBDTTT‐CT:PC71BM/Al photovoltaic devices, the higher work function promotes charge transfer, leading to p‐doping of the organic semiconductor at the interface. This results in a two‐fold increase in hole extraction rates which raises both the fill factor and the open‐circuit voltage, leading to high power conversion efficiency of 7.4%. In ITO/PEDOT:PSS/F8BT/Al polymer light‐emitting diodes, where the organic semiconductor's HOMO level lies deeper than the anode Fermi level, the increased work function enhances hole injection efficiency and luminance intensity by 3 orders of magnitude. In particular, hole injection rates from PEDOT:PSS anodes are equivalent to those achievable using MoO₃. These findings exemplify the importance of work function control as a tool for improved electrode design, and open new routes to device interfacial optimization using facile solvent processing techniques. Such simple, persistent, treatments pave the way towards low cost manufacturing of efficient organic optoelectronic devices.     

Enhanced Structural Stability and Performance Durability of Bulk Heterojunction Photovoltaic Devices Incorporating Metallic Nanoparticles

Despite the progress on organic photovoltaic (OPV) performance, the photoactive layer degradation during prolonged solar illumination is still a major obstacle. In this work, an approach to mitigate the degradation pathway related to structural/morphological changes of the photoactive layer occurring upon continuous illumination in air is presented. It is shown, for the first time, that the incorporation of Ag nanoparticles in poly(3‐hexylthiophene) (P3HT) and [6‐6]‐phenyl‐C61‐butyric acid methyl ester bulk heterojunction (BHJ) leads to improved structural and morphological properties of the composite BHJ solar cells and to better photovoltaic (PV) stability after long periods of continuous illumination. This is evidenced by an original approach based on joint in‐situ time‐resolved X‐ray and atomic force microscopy monitoring. Besides the structural stability improvement and reduced photodegradation rate, it is shown that the composite blends exhibit superior PV performance compared to the pristine BHJs. It can be postulated that the incorporation of metallic nanoparticles in the BHJ leads to a dual enhancement, a plasmon absorption mediated effect, causing improved initial cell efficiency, and a structural stability effect giving rise to reduced degradation rate upon prolonged illumination. The results are in favor of the exploitation of polymer–nanoparticle composites as a promising approach to mitigate the aging effects in OPVs.     

Organic Tandem and Multi‐Junction Solar Cells

The emerging field of stacked layers (double‐ and even multi‐layers) in organic photovoltaic cells is reviewed. Owing to the limited absorption width of organic molecules and polymers, only a small fraction of the solar flux can be harvested by a single‐layer bulk heterojunction photovoltaic cell. Furthermore, the low charge‐carrier mobilities of most organic materials limit the thickness of the active layer. Consequently, only part of the intensity of the incident light at the absorption maximum is absorbed. A tandem or multi‐junction solar cell, consisting of multiple layers each with their specific absorption maximum and width, can overcome these limitations and can cover a larger part of the solar flux. In addition, tandem or multi‐junction solar cells offer the distinct advantage that photon energy is used more efficiently, because the voltage at which charges are collected in each sub‐cell is closer to the energy of the photons absorbed in that cell. Recent developments in both small‐molecule and polymeric photovoltaic cells are discussed, and examples of photovoltaic architectures, geometries, and materials combinations that result in tandem and multi‐junction solar cells are presented.     

On the Role of Bathocuproine in Organic Photovoltaic Cells

The effect of bathocuproine (BCP) on the optical and electrical properties of organic planar heterojunction photovoltaic cells is quantified by current–voltage characterization under 1 sun AM 1.5D simulated solar illumination and spectral response at short‐circuit conditions. By inserting a 10 nm BCP layer in an indium tin oxide (ITO)/subphthalocyanine (SubPc)/buckminsterfullerene (C₆₀)/BCP/Al thin‐film structure, an increase in power‐conversion efficiency from 0.05 to 3.0% is observed, mostly reflected in the enhanced open‐circuit voltage up to 920 mV. Furthermore, the incorporation of a 10‐nm BCP layer in an ITO/C₆₀/BCP/Al structure leads to an increase in built‐in potential from 250 to 850 mV, as demonstrated by electroabsorption. It is argued that BCP passivates C₆₀ such that a 10‐nm layer provides a sufficient buffer layer that prohibits Al contacting the C₆₀ where it would otherwise create donor states.     

Highly Efficient Hole Injection Using Polymeric Anode Materials for Small‐Molecule Organic Light‐Emitting Diodes

A novel, highly efficient hole injection material based on a conducting polymer polythienothiophene (PTT) doped with poly(perfluoroethylene‐perfluoroethersulfonic acid) (PFFSA) in organic light‐emitting diodes (OLEDs) is demonstrated. Both current–voltage and dark‐injection‐current transient data of hole‐only devices demonstrate high hole‐injection efficiency employing PTT:PFFSA polymers with different organic charge‐transporting materials used in fluorescent and phosphorescent organic light‐emitting diodes. It is further demonstrated that PTT:PFFSA polymer formulations applied as the hole injection layer (HIL) in OLEDs reduce operating voltages and increase brightness significantly. Hole injection from PTT:PFFSA is found to be much more efficient than from typical small molecule HILs such as copper phthalocyanine (CuPc) or polymer HILs such as polyethylene dioxythiophene: polystyrene sulfonate (PEDOT‐PSS). OLED devices employing PTT:PFFSA polymer also demonstrate significantly longer lifetime and more stable operating voltages compared to devices using CuPc.     

Rational Design of Small Molecular Donor for Solution‐Processed Organic Photovoltaics with 8.1% Efficiency and High Fill Factor via Multiple Fluorine Substituents and Thiophene Bridge

A series of tetrafluorine‐substituted small molecules with a D₁‐A‐D₂‐A‐D₁ linear framework based on indacenodithiophene and difluorobenzothiadiazole is designed and synthesized for application as donor materials in solution‐processed small‐molecule organic solar cells. The impacts of thiophene π‐bridge and multiple fluorinated modules on the photophysical properties, the energy levels of the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), charge carrier mobility, the morphologies of blend films, and their photovoltaic properties as electron donor material in the photoactive layer are investigated. By incorporating multiple fluorine substituents of benzothiadiazole and inserting two thiophene spacers, the fill factor (FF), open‐circuit voltage, and short‐circuit current density are dramatically improved in comparison with fluorinated‐free materials. With the solvent vapor annealing treatment, further enhancement in charge carrier mobility and power conversion efficiency (PCE) are achieved. Finally, a high PCE of 8.1% with very‐high FF of 0.76 for BIT‐4F‐ T/PC₇₁BM is achieved without additional additive, which is among one of the highest reported for small‐molecules‐based solar cells with PCE over 8%. The results reported here clearly indicate that high PCE in solar cells based small molecules can be significantly increased through careful engineering of the molecular structure and optimization on the morphology of blend films by solvent vapor annealing.     

Exciton‐Charge Annihilation in Organic Semiconductor Films

Time‐resolved optical spectroscopy is used to investigate exciton‐charge annihilation reactions in blended films of organic semiconductors. In donor–acceptor blends where charges are photogenerated via excitons, pulsed optical excitation can deliver a sufficient density of temporally overlapping excitons and charges for them to interact. Transient absorption spectroscopy measurements demonstrate clear signatures of exciton‐charge annihilation reactions at excitation densities of ≈10¹⁸ cm^?3. The strength of exciton‐charge annihilation is consistent with a resonant energy transfer mechanism between fluorescent excitons and resonantly absorbing charges, which is shown to generally be strong in organic semiconductors. The extent of exciton‐charge annihilation is very sensitive not only to fluence but also to blend morphology, becoming notably strong in donor–acceptor blends with nanomorphologies optimized for photovoltaic operation. The results highlight both the value of transient optical spectroscopy to interrogate exciton‐charge annihilation reactions and the need to recognize and account for annihilation reactions in other transient optical investigations of organic semiconductors.     

注入光敏器件是一种新型的光电探测器

注入光敏器件的理论是建立在公认的ｐｎ结光伏公式基础上的，有着可靠根据。严格的理论分析和实验证明抽取受光结的注入电流是可行的。注入光敏三极管与普通光敏三极管比较中可以看出它们之间有许多不同之处并显示出注入光敏器件的优良性能。