Fabrication Of Organic Photovoltaics Research Articles

Toward High Efficiency Water Processed Organic Photovoltaics: Controlling the Nanoparticle Morphology with Surface Energies

AbstractHere efficient organic photovoltaic devices fabricated from water‐based colloidal dispersions with donor:acceptor composite nanoparticles achieving up to 9.98% power conversion efficiency (PCE) are reported. This high efficiency for water processed organic solar cells is attributed to morphology control by surface energy matching between the donor and the acceptor materials. Indeed, due to a low interfacial energy between donor and the acceptor, no large phase separation occurs during the nanoparticle formation process as well as upon thermal annealing. Indeed, synchrotron‐based scanning transmission X‐ray microscopy reveals that the internal morphology of composite nanoparticles is intermixed as well as the active layer morphology after thermal treatment. The PCE of this system reaches 85% that of devices prepared from chlorinated solvent. The gap between water‐based inks and organic solvent‐based inks gets narrower, which is promising for the development of eco‐friendly processing and fabrication of organic photovoltaics.

Surfactant Engineering and Its Role in Determining the Performance of Nanoparticulate Organic Photovoltaic Devices.

The fabrication of organic photovoltaics (OPVs) from non-hazardous nanoparticulate (NP) inks offers considerable promise for the development of eco-friendly large-scale printed solar modules. However, the typical NP core–shell morphology (driven by the different donor/acceptor affinities for the surfactant used in NP synthesis) currently hinders the photovoltaic performance. As such, surfactant engineering offers an elegant approach to synthesizing a more optimal intermixed NP morphology and hence an improved photovoltaic performance. In this work, the morphology of conventional sodium dodecyl sulfate (SDS) and 2-(3-thienyl) ethyloxybutylsulfonate (TEBS)-stabilized poly(3-hexylthiophene) (P3HT) donor:phenyl-C61-butyric acid methyl ester (PC61BM) acceptor NPs is probed using scanning transmission X-ray microscopy, UV–vis spectroscopy, grazing-incidence X-ray diffraction, and scanning electron microscopy. While the SDS-stabilized NPs exhibit a size-independent core–shell morphology, this work reveals that TEBS-stabilized NPs deliver an intermixed morphology, the extent of which depends on the particle size. Consequently, by optimizing the TEBS-stabilized NP size and distribution, NP-OPV devices with a power conversion efficiency that is ∼50% higher on average than that of the corresponding SDS-based NP-OPV devices are produced.

N‐Annulated perylene diimide dimers and tetramer non‐fullerene acceptors: impact of solvent processing additive on their thin film formation behavior

AbstractBACKGROUNDWe aim to determine whether the rationale of designing a tetrameric non‐fullerene acceptor (NFA) to minimize its aggregation compared to its dimer counterparts proves to be a valid strategy in the context of organic photovoltaic (OPV) processing involving solvent additives. We investigate the impact of the molecular design and the processing additive on the thin film formation behavior of four N‐annulated perylene diimide (NPDIx) derivatives. We compare three dimers and a tetramer, processed from a solution containing 1,8‐diiodooctane (DIO), which are used as NFAs and as a processing solvent additive, respectively, in the OPV fabrication.RESULTSWe prepared OPV devices using the PTB7‐Th:NPDIx model system and rationalized the change in their power conversion efficiency (PCE) based on the thin film morphology of the four NDPIx compounds. While the PCE of the OPV devices prepared using the dimers and 1% (v/v) DIO showed a 20–30% drop, those prepared using the tetramer and the same processing conditions showed an 80% drop. Our analysis demonstrates that DIO processing solvent additive‐induced crystallization of the tetramer is the cause of its low performance in OPV devices.CONCLUSIONOur results highlight that relying solely on the tetramer perylene diimide design strategy to prevent the aggregation observed in dimers is not always reliable when processing solvent additives are used in the OPV fabrication. Both the molecular design and the processing solvent additive impact the aggregation behavior of the NFA and need to be taken into account to optimize OPV devices active layer morphology. © 2021 Society of Chemical Industry

Formation and Characterization of mini-emulsion P3HT PCBM Nanoparticles

This report presents the synthesis of nanoparticulate suspensions of P3HT:PCBM for organic photovoltaic active layer through a mini-emulsion process with the used of sodium dodecyl sulphate (SDS) as the surfactant. A series of characterization for the nanoparticles using Dynamic Light Scattering (DLS), Ultraviolet–visible spectroscopy, (UV-VIS), Photolumination (PL), Transmission, Electron Microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) also demonstrated. The P3HT/PCBM nanoparticle performances are comparable to P3HT/PCBM solution in chloroform and met the requirement for use in organic Photovoltaic fabrication

In Situ Studies of Solvent Additive Effects on the Morphology Development during Printing of Bulk Heterojunction Films for Organic Solar Cells

AbstractThe development of polymer morphology and crystallinity of printed bulk heterojunction (BHJ) films doped with the different solvent additives 1,8‐diiodooctane (DIO) or diphenyl ether (DPE) is investigated with in situ grazing‐incidence small/wide‐angle X‐ray scattering. The solvent additives, having different boiling points, lead to a different film drying behavior and morphology growth states in the BHJ films of the benzothiadiazole‐based polymer (PPDT2FBT) and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM). The phase demixing in the printed films is changing over time along with solvent evaporation. Polymer domains start aggregating to form larger domains in the liquid–liquid phase, while phase separation mainly occurs in the liquid–solid phase. The present work provides a profound insight into the morphology development of printed BHJ films doped with different solvent additives, which is particularly important for the large‐scale fabrication of organic photovoltaics.

Functionalized soluble triethylsilylethynyl anthradithiophenes (TESADTs) for organic electronic devices

Four new solution-processable triethylsilylethynyl anthradithiophene-(TESADT) based organic semiconductors, end-capped with phenyl (DP-; 1), thien-2-yl (DT-; 2), thienothien-2-yl (DTT-; 3), and perfluorophenyl (DFP-; 4) groups have been synthesized, characterized, and incorporated in organic thin-film transistors (OTFTs) and organic photovoltaics (OPVs). For the fabrication of solution-processed OTFT, thin films of all four compounds have been fabricated via a solution process of solution-shearing (SS), droplet-pinned crystallization (DPC), and drop-casting (DC). Among various solution-processing methods, solution-shearing produced TFTs the highest electrical performance. Thin films of compound 1 formed via the SS method exhibited p-channel characteristics, with hole mobilities as high as ∼0.034 cm2V−1s−1. The film morphologies and microstructures of these compounds have been characterized by atomic force microscopy and X-ray diffraction to rationalize device performance trends. Furthermore, DP-TESADT (1) and DFP-TESADT (4) has been employed for the fabrication of OPVs. Especially, pseudo-bilayer polymer solar cells based on an underlayer of poly(3-hexylthiophene) (P3HT) doped with 2.4% DP-TESADT and 9.1% DFP-TESADT and an upper layer of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) achieved power conversion efficiency up to 2.1%.

Rapidly Thermal Annealed Si-Doped In2O3 Films for Organic Photovoltaics.

We report the electrical, optical, and structural properties of Si-doped In2O3 (ISO) films prepared using co-sputtering system with multi cathode guns for use in organic photovoltaics (OPVs). We investigated the effect of Si doping power on the electrical, optical, and structural properties of ISO film that was rapidly thermally annealed at a temperature of 400 °C. Due to the high Lewis acid strength (8.096) of the Si dopant, the ISO films showed high mobility and low resistivity despite the low Si doping concentration. Low resistivity of the annealed ISO films indicated that Si(4+) acts as an effective dopant of an In2O3 matrix by substitution with the In(3+) site. At a Si doping power of 50 W, ISO film showed a sheet resistance of 19.7 Ohm/square and optical transparency of 76.7%, which are acceptable values for fabrication of OPVs. Successful operation of OPV cells fabricated on transparent ISO film indicates that ISO is a promising high mobility transparent electrode material and alternative to conventional ITO films.

Digital fabrication of organic solar cells by Inkjet printing using non-halogenated solvents

Inkjet printing offers versatility and flexibility for the sequential deposition of functional layers for the production of large area organic photovoltaics (OPV). Four layers of an OPV cell are inkjet printed, comprising an ITO-free semi-transparent front electrode (a metal current collecting grid, inkjet printed PEDOT:PSS and ZnO nanoparticle layers), and the photo-active layer combined with PEDOT:PSS as hole transport layer. To render the process R2R compatible, large area printing is performed using a 3.5cm wide printhead and non-halogenated ink formulations only. Similar performance is achieved for the inkjet printed cells as for cells with only spin coated layers and ITO. For the P3HT/PCBM bulk-heterojunction, a mixture of non-halogenated solvents ensured good solubility, proper printing behavior and a blend morphology that yields similar performance to a layer spin-coated from chlorinated solvents. The potential of inkjet printing for large area OPV was demonstrated by the fabrication of a module with 92cm2 active area, which showed an efficiency of 0.98%. Losses due to front and back electrode resistances are modeled and used to explain the recorded I-V curve. Combining these functional layers with inkjet printed electrodes lays out the roadmap toward fully roll-to-roll compatible digital fabrication of OPV.

Triphenylamine-based amorphous polymers for bulk-heterojunction photovoltaic cells

In this paper, the recent research progress on triphenylamine (TPA)-based donor-acceptor (D-A) amorphous polymers including our developed polymers is reviewed. TPA has three-dimensional branched structures and can provide D-A polymers containing D and A units in the main chain or side chain. The use of TPA-based amorphous polymers in the fabrication of organic photovoltaics (OPVs) offers great advantages over the use of a polycrystalline film in terms of high reproducibility of the OPV performance. The amorphous polymer design using TPA, therefore, indicates a promising direction for the development of new donor materials in OPVs.

PPy:PSS as alternative to PEDOT:PSS in organic photovoltaics

Polypyrrole-polystyrene sulfonate (PPy:PSS) thin films have been electrodeposited on indium tin oxide (ITO)-coated glass slides and used as hole transport layers (HTLs) in poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT:PCBM)-based organic photovoltaics (OPVs). The OPVs made with PPy:PSS exhibit performance (PCE=2.50%, JSC=9.03mA/cm2, VOC=0.57V, FF=0.49) comparable to that obtained by using standard poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS) spin-coated films (PCE=2.62%, JSC=9.43mA/cm2, VOC=0.54V, FF=0.52). Using XPS, it is shown that electrodeposited PPy:PSS does not etch the ITO electrode, in contrast to the known degradation effect of the PSSH acid excess present in spin-coated PEDOT:PSS. The effects of the film thickness and the rate of electropolymerization of PPy:PSS have been investigated to show that the only main adverse affect on the OPVs performance is the decreased transparency of the films at higher thicknesses. The results reported here indicate that stable electrodeposited PPy:PSS can be successfully used in place of ITO-etchant PEDOT:PSS spin-coated films in the fabrication of OPVs.

Nanostructured layers of a new cross-linkable poly(3-hexylthiophene) in organic photovoltaic cells

We report on the synthesis of a new cross-linkable poly(3-hexylthiophene) copolymer, P3HT-Ox10, consisting on regioregular blocks of 3-hexylthiophene and 10% molar of an oxetane-functionalized thiophene comonomer. This copolymer was used in the fabrication of organic photovoltaics (OPVs) upon combination with 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61 (PCBM), either as blends or in bilayer structures. Bilayer devices with a nanostructured donor/acceptor interface were fabricated using a patterning process based on the phase separation in P3HT-Ox10:polystyrene blends. Columnar-grain films of cross-linked P3HT-Ox10 with diameters of ca. 140nm were thereby obtained. These are the lowest dimensions achieved so far by this approach. Despite the modest power conversion efficiencies achieved, we believe that this easily implemented approach is very effective to prepare OPVs with well-defined morphologies.

Use of benzothiadiazole–triphenylamine amorphous polymer for reproducible performance of polymer–fullerene bulk-heterojunction solar cells

An amorphous polymer, poly(BTD-TPA), which consists of benzothiadiazole and triarylamine units, can be successfully utilized to fabricate bulk heterojunction (BHJ) organic photovoltaics (OPVs), and the OPV performance can be demonstrated to be independent of the casting solvent or thermal annealing temperature. The OPV based on poly(BTD-TPA):PC70BM (1:4) that was fabricated using chloroform (boiling point of 61°C) and annealed at 60°C for 10min exhibited a power conversion efficiency (PCE) of 2.81% under simulated solar irradiation through an air mass of 1.5 at 100mWcm−2. On the other hand, the OPV fabricated using o-dichlorobenzene (boiling point of 181°C) and annealed at 110°C for 10min exhibited a PCE of 2.65%. Almost the same PCEs and incident photon to current conversion efficiencies (IPCEs) were obtained in both OPVs. The use of an amorphous film of poly(BTD-TPA) in the fabrication of OPVs offers great advantages over the use of a polycrystalline film of regioregular poly(3-hexylthiophene) (P3HT) in terms of high reproducibility of the OPV performance.

Plastic-Syringe Induced Silicone Contamination in Organic Photovoltaic Fabrication: Implications for Small-Volume Additives

Herein, the implications of silicone contamination found in solution-processed conjugated polymer solar cells are explored. Similar to a previous work based on molecular cells, we find this contamination as a result of the use of plastic syringes during fabrication. However, in contrast to the molecular case, we find that glass-syringe fabricated devices give superior performance than plastic-syringe fabricated devices in poly(3-hexylthiophene)-based cells. We find that the unintentional silicone addition alters the solution's wettability, which translates to a thinner, less absorbent film on spinning. With many groups studying the effects of small-volume additives, this work should be closely considered as many of these additives may also directly alter the solutions' wettability, or the amount of silicone dissolved off the plastic syringes, or both. Thereby, film thickness, which generally is not reported in detail, can vary significantly from device to device.

Rare solventannealing effective benzo(1,2-b:4,5-b′)dithiophene-based low band-gap polymer for bulk heterojunction organic photovoltaics

A newly synthesized benzo(1,2-b:4,5-b')dithiophene-based low band-gap copolymer pBCN is amenable to solvent annealing in the fabrication of organic photovoltaics, of which power conversion efficiency is greatly improved to 4.2% with PC(61)BM or 4.9% with PC(71)BM.

Designed Bithiophene-Based Interfacial Layer for High-Efficiency Bulk-Heterojunction Organic Photovoltaic Cells. Importance of Interfacial Energy Level Matching

This contribution describes the design, synthesis, characterization, and organic photovoltaic (OPV) device implementation of a novel interfacial layer (IFL) for insertion between the anode and active layer of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk-heterojunction solar cells. The IFL precursor, 5,5′-bis[(p-trichlorosilylpropylphenyl)phenylamino]-2,2′-bithiophene (PABTSi2), covalently anchors to the Sn-doped In2O3 (ITO) surface via the −SiCl3 groups and incorporates a bithiophene unit to align the highest occupied molecular orbital (HOMO) energy with that of P3HT (5.0 eV). The synthesis and subsequent electrochemical analysis of PABTSi2 indicates a HOMO energy of 4.9 eV, while the lowest uoccupied molecular orbital level remains sufficiently high, at 2.2 eV, to effectively block electron leakage to the OPV ITO anode. For the P3HT:PCBM OPV fabrication, PABTSi2 is used as a spin-coated cross-linked (via −SiCl3 hydrolysis and condensation) 1:2 blend with poly[9,9-d...