Organic Photovoltaic Performance Research Articles

Systematic Analysis of Polymer Molecular Weight Influence on the Organic Photovoltaic Performance

The molecular weight of an electron donor-conjugated polymer is as essential as other well-known parameters in the chemical structure of the polymer, such as length and the nature of any side groups (alkyl chains) positioned on the polymeric backbone, as well as their placement, relative strength, the ratio of the donor and acceptor moieties in the backbone of donor-acceptor (D-A)-conjugated polymers, and the arrangement of their energy levels for organic photovoltaic performance. Finding the "optimal" molecular weight for a specific conjugated polymer is an important aspect for the development of novel photovoltaic polymers. Therefore, it is evident that the chemistry of functional conjugated polymers faces major challenges and materials have to adopt a broad range of specifications in order to be established for high photovoltaic performance. In this review, the approaches followed for enhancing the molecular weight of electron-donor polymers are presented in detail, as well as how this influences the optoelectronic properties, charge transport properties, structural conformation, morphology, and the photovoltaic performance of the active layer.

Systematic Variation of Fluorinated Diketopyrrolopyrrole Low Bandgap Conjugated Polymers: Synthesis by Direct Arylation Polymerization and Characterization and Performance in Organic Photovoltaics and Organic Field-Effect Transistors

The synthesis of four different diketopyrrolopyrrole (DPP) low bandgap polymers by direct arylation polymerization (DArP) is reported. These materials were designed for use in organic photovoltaic (OPV) and organic field-effect transistor (OFET) devices. While the DPP conjugated unit was held constant for each of the materials, the alternating unit of the copolymer was varied from thiophene (TTT), to phenyl (TPT), to 3,4-difluorothiophene (TTfT), to 2,5-difluorophenyl (TPfT) creating a series of DPP materials that can be used to study structure–property-performance relationships. Molecular weights (Mw) of 17–110 kg/mol were achieved by DArP and the resulting polymers displayed excellent optical and electrical properties, comparable to previous reports of similar materials synthesized by Stille or Suzuki polycondensation. The fluorinated TTfT and TPfT materials had similar absorption profiles, but exhibited reduced Ehomo levels (by 0.1–0.2 eV) relative to TTT and TPT, which is due to the incorporation of t...

ITO surface modification for inverted organic photovoltaics

The work function (WF) of indium-tin-oxide (ITO) substrates plays an important role on the inverted organic photovoltaic device performance. And electrode engineering has been a useful method to facilitate carrier extraction or charge collection to enhance organic photovoltaic (OPV) performance. By using self-assembly technique, we have deposited poly(dimethyl diallylammonium chloride) (PDDA) layers onto ITO coated glass substrates. The results indicate that the surface WF of ITO is reduced by about 0.3 eV after PDDA modification, which is attributed to the modulation in electron affinity. In addition, the surface roughness of ITO substrate became smaller after PDDA modification. These modified ITO substrates can be applied to fabricate inverted OPVs, in which ITO works as the cathode to collect electrons. As a result, the photovoltaic performance of inverted OPV is substantially improved, mainly reflecting on the increase of short circuit current density.

Electron Energy-Loss Spectroscopy of Organic Photovoltaics

Advances in organic photovoltaic (OPV) based solar cell device technology have increased power conversion efficiencies (PCE) beyond 11% [1], pushing flexible architecture OPV devices closer to being a viable low-cost, environmentally friendly alternative to contemporary inorganic based solar cells [2, 3]. Extending OPV performance beyond this limit is a critical challenge that requires better understanding of the PCE limiting processes. Since only photo-generated excitons that diffuse to the interface between electron donor and acceptor materials can dissociate into holes and electrons, understanding the chemistry and molecular structure of this interface is critical to identifying and mitigating these limitations. Other factors such as the amount of light absorption, efficiency of photogeneration of electrons and holes, and their collection efficiency at the respective electrodes must also be optimized in order to improve the device PCE. Electron energy-loss spectroscopy (EELS) is an extremely useful tool that can be used to probe the nature and structure of these interfaces and further the understanding of processes that occur there.

Effect of a cathode buffer layer on the stability of organic solar cells

We present the effect of a cathode buffer layer on the performance and stability of organic photovoltaics (OPVs) based on a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Six kinds of cathode buffer layers, i.e. lithium fluoride, sodium chloride, NaCl/Mg, tris-(8-hydroxy-quinoline) aluminum, bathocuproine and 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene, were inserted between the photoactive layer and an Al cathode, which played a dominant role in the device’s performance. Devices with the cathode buffer layers above exhibited improved performance. The degradation of these devices with encapsulation was further investigated in an inert atmosphere. The results indicated that devices with inorganic cathode buffer layers exhibited better stability than those with organic cathode buffer layers.

Density of organic thin films in organic photovoltaics

A practical parameter, the volume density of organic thin films, found to affect the electronic properties and in turn the performance of organic photovoltaics (OPVs), is investigated in order to benefit the polymer synthesis and thin film preparation in OPVs. To establish the correlation between film density and device performance, the density of organic thin films with various treatments was obtained, by two-dimensional X-ray diffraction measurement using the density mapping with respect to the crystallinity of thin films. Our results suggest that the OPV of higher performance has a denser photoactive layer, which may hopefully provide a solution to the question of whether the film density matters in organic electronics, and help to benefit the OPV industry in terms of better polymer design, standardized production, and quality control with less expenditure.

Photooxidation studies on indene-C60 adducts

Photooxidation of indene-C60 bisadduct (ICBA) and indene-C60 monoadduct (ICMA) was studied using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), IR spectroscopy, and density functional theory (DFT) calculations. MALDI-TOF MS suggested that the oxidation occurred on the fullerene cage of ICBA or ICMA and that the oxidation of ICBA proceeded faster than that of ICMA. The IR study and DFT calculation indicated that carbonyl substituents were generated upon photoirradiation. The effects of the oxidized ICMA were examined by measuring the device properties of organic photovoltaic (OPV) cells. The increase in energy levels of fullerene derivatives affects the stability against photooxidation negatively, while it increases the open circuit voltage of OPV devices. These results indicate that it is important to balance the OPV performance with stability against photooxidation.

Study of Nano-Filtration and Solvent Effects for Improving Efficacy of Organic Photovoltaic

Conductive polymer such as poly (3,4-ethylenedioxythiophene) and polystyrene sulfonate acid (PEDOT:PSS) plays an important role for improving performance of organic photovoltaics (OPVs). In this study, we investigated the effect of filtrating the buffer layer between the indium tin oxide (ITO) and bulk heterojunction (BHJ); poly(3-hexylthiophene) and [6,6]-phenylC71 butyric acid methyl ester (P3HT:PC71BM) used in our organic photovoltaics (OPVs) as shown in Figure 1. This buffer layer consists of poly (3,4-ethylenedioxythiophene) and polystyrene sulfonate acid (PEDOT:PSS) doping with different solvents [isopropyl alcohol (IPA), ethanol, and methanol]. For comparison, we also studied the OPV devices without filtering the buffer layers. The OPV devices fabricated with filtered buffer layer show better performance than OPV devices without buffer layer filtration. The OPV devices with buffer layers; PEDOT:PSS/IPA, PEDOT:PSS/Ethanol, and PEDOT:PSS/Methanol after filtration showed 24 %, 16 %, and 11 % improvement in short circuit current density (Jsc) , respectively compared to the same OPV devices without filtration. The maximum exciton generation rates (Gmax), and the exciton dissociation probabilities [P (E, T)] were calculated for all fabricated OPV devices to explain the performance enhancement. The data showed that the performance enhancement of OPVs devices after filtering the buffer layers originate from the large increase of Jsc. Filtering the buffer layers enhanced light absorption of BHJs, increased the surface roughness, and decreased the sheet resistance of buffer layer after filtration. Figure 1. A schematic diagram of ITO/PEDOT:PSS:different organic solvents/P3HT: PC71BM/Al Figure 1

에너지 준위 접합 최적화를 통한 유기태양전지 효율 향상법

Organic photovoltaics (OPVs) have attracted significant interest in an interdisciplinary research field for the decades as a next-generation photovoltaic device due to their unique advantages. One of requirements for OPVs having high power conversion efficiency is the favorable energy level alignment between the electrode/organic and organic/organic interfaces to manage the exciton dissociation and improve the charge transport. In this review, strategies to enhance the OPV performance by controlling the energy level alignment are discussed. The insertion of an exciton blocking layer leads to the efficient dissociation of photogenerated excitons at the donor/acceptor interface enhancing the short-circuit current density. The choice of a donor having a high ionization energy and an acceptor having a low electron affinity increases the open-circuit voltage. The insertion of an appropriate work function modifier which reduces the charge injection barrier removes the S-kink in current density-voltage characteristics of OPVs and improves the fill factor. This review would give a valuable guide to design the efficient OPV structure.

Amorphous oxide alloys as interfacial layers with broadly tunable electronic structures for organic photovoltaic cells

In diverse classes of organic optoelectronic devices, controlling charge injection, extraction, and blocking across organic semiconductor-inorganic electrode interfaces is crucial for enhancing quantum efficiency and output voltage. To this end, the strategy of inserting engineered interfacial layers (IFLs) between electrical contacts and organic semiconductors has significantly advanced organic light-emitting diode and organic thin film transistor performance. For organic photovoltaic (OPV) devices, an electronically flexible IFL design strategy to incrementally tune energy level matching between the inorganic electrode system and the organic photoactive components without varying the surface chemistry would permit OPV cells to adapt to ever-changing generations of photoactive materials. Here we report the implementation of chemically/environmentally robust, low-temperature solution-processed amorphous transparent semiconducting oxide alloys, In-Ga-O and Ga-Zn-Sn-O, as IFLs for inverted OPVs. Continuous variation of the IFL compositions tunes the conduction band minima over a broad range, affording optimized OPV power conversion efficiencies for multiple classes of organic active layer materials and establishing clear correlations between IFL/photoactive layer energetics and device performance.

Rapid synthesis of ultra-long silver nanowires for tailor-made transparent conductive electrodes: proof of concept in organic solar cells

Rapid synthesis of ultralong silver nanowires (AgNWs) has been obtained using a one-pot polyol-mediated synthetic procedure. The AgNWs have been prepared from the base materials in less than one hour with nanowire lengths reaching 195 μm, which represents the quickest synthesis and one of the highest reported aspect ratios to date. These results have been achieved through a joint analysis of all reaction parameters, which represents a clear progress beyond the state of the art. Dispersions of the AgNWs have been used to prepare thin, flexible, transparent and conducting films using spray coating. Due to the higher aspect ratio, an improved electrical percolation network is observed. This allows a low sheet resistance (RS = 20.2 Ω/sq), whilst maintaining high optical film transparency (T = 94.7%), driving to the highest reported figure-of-merit (FoM = 338). Owing to the light-scattering influence of the AgNWs, the density of the AgNW network can also be varied to enable controllability of the optical haze through the sample. Based on the identification of the optimal haze value, organic photovoltaics (OPVs) have been fabricated using the AgNWs as the transparent electrode and have been benchmarked against indium tin oxide (ITO) electrodes. Overall, the performance of OPVs made using AgNWs sees a small decrease in power conversion efficiency (PCE), primarily due to a fall in open-circuit voltage (50 mV). This work indicates that AgNWs can provide a low cost, rapid and roll-to-roll compatible alternative to ITO in OPVs, with only a small compromise in PCE needed.

Performance improvement of organic solar cells with the introduction of branched zinc oxide nanorods

Zinc oxide (ZnO) nanorods with branches are synthesised through facile hydrothermal approach at relatively low temperature. With the prepared ZnO nanostructures as the hole-blocking layer, organic solar cells based on poly(3-hexylthiophene): 1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61 are fabricated. For comparison, devices with planar ZnO thin film and bare ZnO nanorods are manufactured as well. J-V curves show that the best device performance is achieved by the branched nanorod-based solar cell. Owing to the decreased charge carrier travelling length, the nanorod-based device shows higher short circuit current (Jsc) than that of the device with planar ZnO layer. Moreover, the solar cell with branched nanorods shows larger open-circuit voltage (Voc) than that with bare nanorods. Therefore, the performance of organic photovoltaic is enhanced with the introduction of branched ZnO nanorods. Finally, the mechanism of device performance improvement upon the introduction of branched nanorods is discussed. c 2015 The Institution of Engineering and Technology.

The Effect of Processing Additives on Energetic Disorder in Highly Efficient Organic Photovoltaics: A Case Study on PBDTTT-C-T:PC71 BM.

Energetic disorder, an important parameter affecting the performance of organic photovoltaics, is significantly decreased upon the addition of processing additives in a highly efficient benzodithiophene-based copolymer blend (PBDTTT-C-T:PC71 BM). Wide-angle and small-angle X-ray scattering measurements suggest that the origin of this reduced energetic disorder is due to increased aggregation and a larger average fullerene domain size together with purer phases.

Influence of Molecular Orientation on Charge-Transfer Processes at Phthalocyanine/Metal Oxide Interfaces and Relationship to Organic Photovoltaic Performance

The effect of the molecular orientation distribution of the first monolayer of donor molecules at the hole-harvesting contact in an organic photovoltaic (OPV) on device efficiency was investigated. Two zinc phthalocyanine (ZnPc) phosphonic acids (PA) deposited on indium tin oxide (ITO) electrodes are compared: ZnPc(PA)4 contains PA linkers in all four quadrants, and ZnPcPA contains a PA linker in one quadrant. ZnPcPA monolayers exhibited a broad distribution of molecular orientations whereas ZnPc(PA)4 adsorption produced a monolayer with a narrower orientation distribution with the molecular plane more parallel to the ITO surface. We used potential-modulated attenuated total reflectance spectroelectrochemistry (PM-ATR) to characterize the charge-transfer kinetics of these films and show that the highest rate constants correspond to ZnPc subpopulations that are oriented more parallel to the ITO surface plane. For ZnPc(PA)4, rate constants exceeded 104 s–1 and are among the highest ever reported for a surface-confined redox couple, which is attributable to both its orientation and the small ZnPc–electrode separation distance. The performance of OPVs with ITO hole-harvesting contacts modified with ZnPc(PA)4 was comparable to that achieved with highly activated bare ITO contacts, whereas for ZnPcPA-modified contacts, the OPV performance was similar to that observed with (hole-blocking) alkyl-PA modifiers. These results demonstrate the synergism between molecular structure, energetics, and dynamics at interfaces in OPVs.

Effect of thermal annealing on the performance of ternary organic photovoltaics based on PTB7:PC71BM:F8BT

The effect of thermal annealing on the inverted PTB7:PC71BM:F8BT ternary organic photovoltaics (OPVs) was systematically investigated, which was the first study about the effect of thermal annealing on the performance of PTB7-based OPVs. The results showed that the devices thermal annealed at 120 °C leading to 60 % increase of power conversion efficiency (PCE) compared with the as-spun ternary OPVs and 3 % enhancement compared with the reference PTB7:PC71BM binary OPVs coated from chlorobenzene. The performance improvement was mainly attributed to formation of fine nanomorphology of the ternary blend film which could secure both charge separation and charge percolation path. However, the PCE decreased to 1.90 % with temperature higher than 140 °C, which was due to the disruptive morphology change and low quantum yield. The results also showed that the thermal annealing process could not only maintain optimized self-assembled morphology of the ternary blend film, but also influence the quantum yield and improve charge transfer efficiency further.

Enhanced photovoltaic performance of inverted hybrid bulk-heterojunction solar cells using TiO 2 /reduced graphene oxide films as electron transport layers

In this study, we investigated inverted hybrid bulk-heterojunction solar cells with the following configuration: fluorine-doped tin oxide (FTO) |TiO2/RGO|P3HT:PC61BM|V2O5 or PEDOT:PSS|Ag. The TiO2/GO dispersions were prepared by sol-gel method, employing titanium isopropoxide and graphene oxide (GO) as starting materials. The GO concentration was varied from 0.1 to 4.0 wt%. The corresponding dispersions were spin-coated onto FTO substrates and a thermal treatment was performed to remove organic materials and to reduce GO to reduced graphene oxide (RGO). The TiO2/RGO films were characterized by x-ray diffraction, Raman spectroscopy, and microscopy techniques. Atomic force microscopy (AFM) images showed that the addition of RGO significantly changes the morphology of the TiO2 films, with loss of uniformity and increase in surface roughness. Independent of the use of V2O5 or PEDOT: PSS films as the hole transport layer, the incorporation of 2.0 wt% of RGO into TiO2 films was the optimal concentration for the best organic photovoltaic performance. The solar cells based on TiO2/RGO (2.0 wt%) electrode exhibited a ∼22.3% and ∼28.9% short circuit current density (Jsc) and a power conversion efficiency enhancement, respectively, if compared with the devices based on pure TiO2 films. Kelvin probe force microscopy images suggest that the incorporation of RGO into TiO2 films can promote the appearance of regions with different charge dissipation capacities.

General method to synthesize ultrasmall metal oxide nanoparticle suspensions for hole contact layers in organic photovoltaic devices

Abstract

Optically-Enhanced Polymer Bulk Heterojunction Solar Cells by Addition of Gold Nanostructures

We investigate the effects of Au nanoparticles (Au NPs) on poly [N-9′′-hepta-decanyl-2,14-carbazole-alt-5,5-(4′,14′-di-2-thienyl-2′,1,3′-benzothiadiazole)]:ph-enyl-C61-butyric acid methyl ester (PCDTBT:PCBM) based organic photovoltaic (OPV) devices by thermal evaporating Au NPs onto polytetrafluoroethylene (PTFE) layer which is based on indium-tin-oxide (ITO) glass substrate.Significant improvement in terms of short-circuit current density (Jsc) by 33.6%, fill factor (FF) by 0.9%, and thereby commensurate power conversion efficiency (PCE) by 40.7% were achieved compared to devices without Au NPs. The OPVs performance enhancement is attributed to the formation of Au NPs-induced surface plasmons that increases the rate of exciton generation, and the probability of exciton dissociation.

Methods for Improving the Lifetime Performance of Organic Photovoltaics with Low‐Costing Encapsulation

Recent years have seen considerable advances in organic photovoltaics (OPVs), most notably a significant increase in their efficiency, from around 4 % to over 10 %. The stability of these devices, however, continues to remain an issue that needs to be resolved to enable their commercialization. This review discusses the main degradation processes of OPVs and recent methods that help to increase device stability and lifetime. One of the most effective steps that can be taken to increase the lifetime of OPVs is their encapsulation, which protects them from atmospheric degradation. Efficient encapsulation is essential for long-term device performance, but it is equally important for the commercialization of OPVs to strike a balance between achieving the maximum device protection possible and using low-cost processing for their encapsulation. Various encapsulation techniques are discussed herein, with emphasis on their cost effectiveness and their overall suitability for commercial applications.

Development of Novel n-Type Materials Based on Benzothiadiazole Derivatives for Organic Photovoltaics: Effects of Acceptor Terminal Substituents

Abstract Novel n-type materials of BTD-CN (1) and BTD-CF3 (2) were synthesized for organic photovoltaics. Both materials show approximately the same HOMO–LUMO level and position of absorption peaks. However, the organic photovoltaic performance of a device with 1 is clearly higher than that with 2. In this paper, we discuss the effects of terminal substituents on film morphology, crystallinity, and photovoltaic performance. The results suggest that cyano-modified benzothiadiazole is useful because of its crystallinity, carrier recombination, and series resistance.