Layer In Organic Photovoltaic Devices Research Articles

Synthesis of Organic Semiconductor Nanoparticles with Different Conformations Using the Nanoprecipitation Method.

In recent years, nanoparticulate materials have aroused interest in the field of organic electronics due to their high versatility which increases the efficiency of devices. In this work, four different stable conformations based on the organic semiconductors P3HT and PC71BM were synthesized using the nanoprecipitation method, including blend and core-shell nanoparticles. All nanoparticles were obtained free of surfactants and in aqueous suspensions following the line of ecologically correct routes. The structural and optoelectronic properties of the nanoparticles were investigated by dynamic light scattering (DLS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), UV-visible absorption spectroscopy and UV-visible photoluminescence (PL). Even in aqueous media, the blend and core-shell nanoparticles exhibited a greater light absorption capacity, and these conformations proved to be effective in the process of dissociation of excitons that occurs at the P3HT donor/PC71BM acceptor interface. With all these characteristics and allied to the fact that the nanoparticles are surfactant-free aqueous suspensions, this work paves the way for the use of these colloids as a photoactive layer of organic photovoltaic devices that interface with biological systems.

Multi-fidelity machine learning models for structure–property mapping of organic electronics

Machine learning approaches have been used with significant success in constructing, curating, and exploring relationships between microstructure and property. However, one major limitation of these approaches is the need for a significant amount of training data consisting of microstructure–property pairs. Getting property values associated with a specific microstructure typically requires deploying a detailed physics simulator which becomes resource-intensive. While using a low(er) fidelity property quantifier can offset the cost of creating the training dataset, there is a trade-off in terms of accuracy/fidelity of the estimated property. Here, we leverage the availability of low- and high- fidelity property simulators to construct a multi-fidelity mapping from microstructure to property using deep convolutional neural networks. Starting with a large dataset of morphologies representing the active layer of organic photovoltaic devices, we assimilate data from a rapid graph-based low-fidelity characterization of the morphology with limited data from a high fidelity excitonic drift-diffusion detailed physics simulator. We show that our method provides significant computational savings while maintaining competitive performance. This work can be easily extended to other applications, and we envision it as a basis for accelerated material quantification and discovery.

Review: materials and modelling for organic photovoltaic devices

AbstractThis review gives a simple and pedagogical introduction to the field of materials and their modelling for the active layer in organic photovoltaic devices. It gives a perspective on past work and a summary of the current state of the art. Given the extremely fast changes on‐going in this field, it is hoped that this contribution will serve as both a timely snapshot and a pedagogical entry point to this fascinating subject. Furthermore, an example is given of how modelling can enhance the understanding of the structures and qualities of materials using a leading low‐bandgap polymer donor and non‐fullerene acceptor pair (PM6 and Y6). © 2021 Society of Industrial Chemistry.

Non-radiative energy transfer in aqueously dispersed polymeric nanoparticles for photovoltaic applications

Polymeric nanoparticles dispersed in an aqueous medium were synthesized using the miniemulsion technique. In this study, nanoparticles composed of copolymer F8T2, homopolymer MDMO-PPV and a mixture of both were investigated aiming to unravel the interaction between these materials. By decreasing the distance between the nanostructured materials, higher Förster resonance energy transfer (FRET) was observed from F8T2 to MDMO-PPV. This enhanced energy transfer could generate more free charge carriers, increasing the short circuit current density, when the nanoparticles were applied as an active layer in organic photovoltaic devices, influencing the increase in the efficiency of these devices.

Benzodithiophene-based small molecules for vacuum-processed organic photovoltaic devices

Benzo[1,2-b:4,5-b’]dithiophene (BDT) derivatives were evaluated as donor materials for the first time in vacuum-processed organic photovoltaic (OPV) devices. Simple BDT derivatives coupled with thiophene (M1), 3-methylthiophene (M2), and 3-(2-ethylhexyl)thiophene (M3) were synthesized and characterized as part of this evaluation study. Similar frontier molecular orbitals were determined for the three molecules by DFT calculations, optical, and electrochemical measurements. Therefore, the effect of the alkyl substituents on the film morphology and molecular order was possible to evaluate as well. All molecules exhibited thermal stability for their co-evaporation with C60 to form bulk heterojunction layers in OPV devices. An increase in short-circuit current density was observed for devices with M2. Microscopy images and hole-transport measurements suggested an induced molecular order by the methyl group of M2 on the active layer. Although low photoconversion efficiency values were obtained (~1.2%), this work demonstrates the feasibility to fabricate fully vacuum-processed OPV devices with BDT derivatives as donor materials. Devices with higher photoconversion efficiencies are expected for BDT derivatives with enhanced optoelectronic properties.

Organic heterojunctions of phthalocyanine-reduced graphene oxide above percolation threshold for photovoltaic application

In this article, non-fullerene, cost effective, and highly soluble zinc tetra-tert-butyl phthalocyanine-reduced graphene oxide (ZnTTBPc-RGO) heterojunction have been used as an active layer in organic photovoltaic devices. The RGO content in the active material is varied from 0.25–0.5 volume fractions which are above the percolation threshold of 0.167 volume fraction of RGO. Temperature dependent current density–voltage characteristics show a transition from Ohmic conduction to space charge limited current (SCLC) conduction at higher applied bias voltages (≥1 V). This enables us to extract charge carrier mobility values of the heterojunction at different temperatures using a trap-free SCLC model. A donor–acceptor solution processable photovoltaic device with 0.33 vol.% RGO content of active layer show the highest power conversion efficiency of 1.02%, highest recombination resistance of 48.65 kΩ, and maximum carrier mobility of 4.32 × 10−4 cm2V−1s−1 among the three devices studied. 80 h exposure to ambient conditions see a ∼40% decay of power conversion efficiency. Quantitative analysis on the density of trap states indicates that the best power conversion efficiency and the highest charge carrier mobility correlate well with a minimum of the trap density and a maximum of recombination resistance for these devices.

Catalytic Activity Towards Hydrogen Evolution Dependent of the Degree of Conjugation and Absorption of Six Organic Chromophores.

Conjugated materials can, in many cases, absorb visible light because of their delocalized π electron system. Such materials have been widely used as a photoactive layers in organic photovoltaic devices and as photosensitizers in dye‐sensitized solar cells. Additionally, these materials have been reported for applications in solar fuel production, working as photocatalysts for the hydrogen evolution reaction (HER). The synthesis of three flexible vinyl groups‐containing chromophores is reported. The catalytic activity towards hydrogen evolution of these chromophores has been investigated and compared to their non‐vinyl‐containing analogues. The catalytic effect was confirmed using two different approaches: electrochemical, using the chromophores to modify a working electrode, and photocatalytic, using the chromophores combined with platinum nanoparticles. A relationship between the degree of conjugation and the catalytic activity of the chromophores has been observed with the electrochemical method, while a relationship between the UV absorption in the solid state and the photocatalytic effect with platinum nanoparticles was observed.

Understanding charge transport in donor/acceptor blends from large-scale device simulations based on experimental film morphologies

Simulation of the microscopic processes in the active layers of organic photovoltaic devices directly from experimental film morphologies.

Enhanced Organic Photovoltaic Performance through Modulating Vertical Composition Distribution and Promoting Crystallinity of the Photoactive Layer by Diphenyl Sulfide Additives

To understand the vertical phase separation in the bulk junction active layer of organic photovoltaic devices is essential for controlling the charge transfer behavior and achieving effective charge collection. Here, diphenyl sulfide (DPS) was introduced as a novel additive into the PTB7-Th:PC71BM-based inverted polymer solar cells (PSCs), and the effect of additives on active blend films and photovoltaic characteristics was carefully studied. The results show that DPS could not only modulate the vertical composition distribution but also promote the ordered molecular packing of the photoactive layer, thus effectively improving exciton dissociation, charge transport, and collection, and thus exhibit an excellent power conversion efficiency of 9.7% with an improved fill factor (>70%) after using 3% DPS additive. The results show that the DPS solvent additive can effectively adjust the vertical phase distribution and crystallinity of blend films and improve the photovoltaic performance of the inverted organic photovoltaic devices.

A quantum dot-silica composite as an efficient spectral converter in a luminescent down-shifting layer of organic photovoltaic devices

The power conversion efficiency of organic photovoltaic (OPV) devices with a luminescent down-shifting layer was enhanced by 8.9% compared to pristine OPV devices.

Crystalline Molybdenum Oxide Layers as Efficient and Stable Hole Contacts in Organic Photovoltaic Devices

High work function metal oxides such as molybdenum oxide (MoOx) have demonstrated good hole contact properties in organic photovoltaic (OPV) devices in the past years and have replaced the otherwise conventionally used PEDOT:PSS. In this work, we introduce new crystalline MoOx layers that outperform the otherwise commonly used thermally grown MoOx layers in OPV devices. These hole contact layers are developed from superoxidized MoO3.2 films grown by reactive sputtering, followed by postannealing in high vacuum to induce crystallization of the otherwise amorphous MoOx films. Standard configuration organic solar cell devices based on DBP as electron donor and C70 as electron acceptor were developed on top of the sputtered MoOx, and a large increase in power conversion efficiency as a function of the MoOx annealing temperature was observed, which is in clear contrast to devices grown on thermally deposited MoOx. The crystallization of the sputtered MoOx at elevated temperatures is shown to lead to high work ...

Creation of Au nanoparticles decorated MoO3 nanorods using CoSP and the application as hole transport layer (HTL) in plasmonic-enhanced organic photovoltaic devices

Surfactant free MoO3 nanorods (50 nm–5 µm long and 50–300 nm diameter) with Au nanoparticles with varying nanorod dimensions and Au fraction have been synthesized in-situ by using inexpensive continuous spray pyrolysis (CoSP) technique. In-situ grown gold nanoparticles (∼10–20 nm) with MoO3 nanorods (Au/MoO3) were obtained which proved useful in getting the additional effect due to plasmonics. The CoSP- synthesized Au/MoO3 nanorods have been used as anode buffer layers in organic photovoltaic devices (OPVs). The resulting devices exhibit a remarkable enhancement in power conversion efficiency (from 2.22% to 3.71%) when Au/MoO3 nanorods were used in the device which can be attributed to the strong localized surface plasmon resonance (LSPR) absorption.

Combustion Synthesis of p-Type Transparent Conducting CuCrO2+x and Cu:CrOx Thin Films at 180 °C.

Low-temperature solution processing of p-type transparent conducting oxides (TCOs) will open up new opportunities for applications on flexible substrates that utilize low-cost, large-area manufacturing. Here, we report a facile solution synthesis method that produces two p-type TCO thin films: copper chromium oxide and copper-doped chromium oxide. Using combustion chemistry, both films are solution processed at 180 °C, which is lower than most recent efforts. While adopting the same precursor preparation and annealing temperature, we find that annealing environment (solvent vapor vs open air) dictates the resulting film phase, hence the optoelectronic properties. The effect of annealing environment on the reaction mechanism is discussed. We further characterize the electronic, optical, and transport properties of the two materials, and compare the differences. Their applications in optoelectronic devices are successfully demonstrated in transparent p-n junction diodes and as hole transport layers in organic photovoltaic devices.

Enhanced light absorption and device performances of organic photovoltaic devices with Au tetrahedra nanoparticles

Organic photovoltaic devices (OPVs) have attracted considerable attention because of their advantages of light-weight, low-cost, large-scale manufacturing process and mechanical flexibility. Unfortunately, in order to achieve efficient carrier extraction, the photoactive layer in OPVs must be rather thin (100 nm or less) due to its extremely low carrier mobilities for most of organic/polymer materials (on the order of 10<sup>-4</sup> cm<sup>2</sup>/(V·s)). Such thin photoactive layers lead to a significant loss of incident sunlight, thereby improving a final low light absorption efficiency and power conversion efficiency (PCE). To promote the light absorption and thus enhance PCE of OPVs, Au tetrahedron nanoparticles (NPs) are synthesized in this work and then they are wrapped with poly (sodium 4-styrenesulfonate) (PSS) to form core-shell structure tetrahedron NPs (Au@PSS tetrahedron NPs). They are further incorporated into the interface of hole extraction layer and light photoactive layer to improve PCE of OPVs by enhancing their surface plasmon resonance effect-induced light absorption. The influences of doping concentration and PSS shell thickness of theses Au tetrahedron NPs on device performances are explored. The results indicate that the best performing PCE occurs at 6% concentration of Au@PSS tetrahedron NPs, reaching 3.08%, while it is further improved to 3.65% with an optimized PSS shell thickness of 2.5 nm, showing an enhancement factor of 22.9% compared with that of the control counterpart. The performance improvement of OPVs mainly originates from the promoted light absorption of donor due to the location of the resonant absorption peak of Au@PSS tetrahedron NPs in the absorption region of donor. Simultaneously, the introduction of the PSS shell promotes the dissociation of excitons and charge transfer. All of these contribute to the increasing of short-circuit current, fill factor and PCE of OPVs.

Controlled multiple neutral planes by low elastic modulus adhesive for flexible organic photovoltaics

To protect brittle layers in organic photovoltaic devices, the mechanical neutral plane strategy can be adopted through placing the brittle functional materials close to the neutral plane where stress and strain are zero during bending. However, previous research has been significantly limited in the location and number of materials to protect through using a single neutral plane. In this study, multiple neutral planes are generated using low elastic modulus adhesives and are controlled through quantitative analyses in order to protect the multiple brittle materials at various locations. Moreover, the protection of multiple brittle layers at various locations under both concave and convex bending directions is demonstrated. Multilayer structures that have soft adhesives are further analyzed using the finite element method analysis in order to propose guidelines for structural design when employing multiple neutral planes.

Design and Fabrication of Photonic Structured Organic Solar Cells by Electrospraying

Photonic architectures in optoelectronic devices are comprised of uniform periodic structural arrangement which aids in improved interaction with light. The processing of such photonic arrays generally involves several steps of fabrication. This work proposes single-step fabrication of an equal submicron size porous photonic structure array through electrospraying. This distinctive approach involves a continuous supply of material With simultaneous removal of solvent for fabricating large area samples, compared to other processing techniques. The process optimization is carried out to regulate the pore diameter and spacing. Morphological studies showed the continuity of porous arrays extending to large areas of the order of millimeters. Optical investigations demonstrated that this uniform periodic topography assisted in improved light scattering and a consequent enhancement of light absorption. Further, the incorporation of this array structure in the active layer of organic photovoltaic devices is evaluated through both experiments and modeling. Modeling and simulations suggest that the optimized range of 200-500 mm pore size aids in higher current density. Experiments reveal an improvement of 18% in the photocurrent density at short circuit in the periodic array architecture compared to the planar reference. Therefore, in this study, a facile single-step novel, method of obtaining photonic array structures with unproved device performances is demonstrated.

N‐Type Doping Induced by Electron Transport Layer in Organic Photovoltaic Devices

Doping in an organic photovoltaic (OPV) device can largely impact its performance. In this work, it is discovered that n‐type electrical doping in the poly(3‐hexylthiophene):[6,6]‐phenyl‐C61‐butyric acid methyl ester active layer can be induced by a certain electron transport layer (ETL), particularly branched polyethylenimine (PEI). Consequently, OPV devices with different ETLs exhibit dramatically different current density–voltage behaviors and external quantum efficiency (EQE) spectra. Using drift‐diffusion modeling, Hall effect, and capacitance–voltage measurements, it is shown that the difference in EQE spectra originates from the different background carrier type and concentration in the OPV active layer, dictated by the specific properties of ETL. Factors that influence electrical doping and device behaviors, such as difference between PEI and another polymeric ETL, polyethylenimine ethoxylated, and the active layer thickness and energy of negative integer charge‐transfer state are elucidated. These findings provide insight into material selection and device design for organic electronic devices.

Copper light-catching electrodes for organic photovoltaics

Plasmon-active optically thin copper films catch red light and transfer the energy to the photoactive layer in organic photovoltaic devices.

Suppressing photooxidation of conjugated polymers and their blends with fullerenes through nickel chelates

This work introduces nickel(ii) dibutyldithiocarbamate as a broadband and universal antioxidant for conjugated polymer semiconductors.

Effect of time and deposition method on quality of phosphonic acid modifier self-assembled monolayers on indium zinc oxide

Phosphonic acid (PA) self-assembled monolayers (SAMs) are utilized at critical interfaces between transparent conductive oxides (TCO) and organic active layers in organic photovoltaic devices (OPVs). The effects of PA deposition method and time on the formation of close-packed, high-quality monolayers is investigated here for SAMs fabricated by solution deposition, micro-contact printing, and spray coating. The solution deposition isotherm for pentafluorinated benzylphosphonic acid (F5BnPA) on indium-doped zinc oxide (IZO) is studied using polarization modulation-infrared reflection-absorption spectroscopy (PM-IRRAS) at room temperature as a model PA/IZO system. Fast surface adsorption occurs within the first min; however, well-oriented high-quality SAMs are reached only after ∼48h, presumably through a continual process of molecular adsorption/desorption and monolayer filling accompanied by molecular reorientation. Two other rapid, soak-free deposition techniques, micro-contact printing and spray coating, are also explored. SAM quality is compared for deposition of phenyl phosphonic acid (PPA), F13-octylphosphonic acid (F13OPA), and pentafluorinated benzyl phosphonic acid (F5BnPA) by solution deposition, micro-contact printing and spray coating using PM-IRRAS. In contrast to micro-contact printing and spray coating techniques, 48–168h solution deposition at both room temperature and 70°C result in contamination- and surface etch-free close-packed monolayers with good reproducibility. SAMs fabricated by micro-contact printing and spray coating are much less well ordered.