Ultrasonically sprayed and inkjet printed thin film electrodes for organic solar cells

Thin film pi-conjugated poly(3,4ethylenedioxythiophene): poly(styrenesulphonate) (PEDOT:PSS) as a hole transport layer on indium tin oxide is a key element in some of the most efficient organic photovoltaic and light emitting devices to date. Films are typically deposited by spincoating, which is not readily scalable. In this paper we investigate the critical parameters for both inkjet and ultrasonic spray deposition of PEDOT:PSS thin films on commercial indium tin oxide as a potentially scalable approach to contact formation. Inkjet parameters investigated include drop spacing and substrate temperature. Ultrasonic spray coating parameters investigated include substrate temperature and solution flow rate. We also show that the ink viscosity has a Newtonian character, making it well suited for inkjet printing. Films were characterized via optical profilometry, sheet resistance and atomic force microscopy. Optimized inkjet printed and ultrasonic sprayed PEDOT:PSS films were then compared to spincast layers in a prototypical bulk heterojunction photovoltaic device employing a poly(3-hexylthiophene) and [6,6]-PCBM (6,6-phenylC61-butric acid-methyl ester) blend as the absorber. Practically all three approaches produced devices of comparable efficiency. Efficiencies were 3.6%, 3.5% and 3.3% for spin, spray and inkjet depositions respectively.     

Discriminating between bilayer and bulk heterojunction polymer:fullerene solar cells using the external quantum efficiency

The morphology of the active layer in polymer:fullerene solar cells is a key parameter for the performance. We compare bilayer poly(3-hexylthiophene)/[6,6]-phenyl-C(61)-butyric acid methyl ester (P3HT/PCBM) solar cell devices produced from orthogonal solvents before and after thermal annealing with P3HT:PCBM bulk heterojunction solar cells produced from a single solvent. By comparing the spectral shape and magnitude of the experimental and theoretically modeled EQEs we show that P3HT/PCBM bilayers made via orthogonal solution processing do not lead to bilayers with a sharp interface but that partial intermixing has occurred. Thermal annealing of these diffusive P3HT/PCBM bilayers leads to increased mixing but does not result in the same mixed bulk heterojunction morphology that is obtained when P3HT and PCBM are cast simultaneously from single solution. For thicker layers, the annealed bilayers significantly outperform the bulk heterojunction devices with the same nominal composition and same total thickness.     

Diketopyrrolopyrrole-containing oligothiophene-fullerene triads and their use in organic solar cells

We report the characterization of a series of oligothiophene-diketopyrrolopyrrole-fullerene triads and their use as active materials for solution processed organic solar cells (OSCs). By incorporating the diketopyrrolopyrrole (DPP) core with electron rich oligothiophene units and electron withdrawing fullerene units, multifunctional electronic molecules have been prepared; these molecules show high solubility in common organic solvents, excellent photophysical properties with high extinction coefficients (1 × 10(4) to 1 × 10(5) M(-1) cm(-1)) and broad absorption spectra coverage (250-800 nm), as well as suitable molecular orbital energy levels (HOMO of approximately -5.1 eV, LUMO of approximately -3.7 eV). Solution-processed thin-film organic field effect transistors (OFETs) from these triads revealed good n-type characteristics with electron mobilities up to 1.5 × 10(-3) cm(2) V(-1) s(-1). With these multifunctional triads, single-component OSCs have been fabricated, exhibiting power conversion efficiencies (PCEs) of up to 0.5 % under AM 1.5 G simulated 1 sun solar illumination. Blending these molecules with poly(3-hexylthiophene) (P3HT) afforded bulk heterojunction OSCs with PCEs reaching as high as 2.41%.     

The role of solvent and morphology on miscibility of methanofullerene and poly(3-hexylthiophene)

A bicontinuous, percolating bulk heterojunction morphology is integral to organic polymer solar cells. Understanding the factors affecting the miscibility of photovoltaic polymers with a fullerene electron acceptor molecule is a key to controlling the morphology. Starting from discreet pure phases - a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) bilayer film - the evolution of the P3HT-PCBM interface was studied with particular attention to the role of residual solvent in P3HT on PCBM interdiffusion. This investigation shows that in the bilayer geometry PCBM can rapidly diffuse into amorphous P3HT, but phase separation is maintained if the P3HT layer is cast from a very volatile solvent or if it is annealed prior to casting the PCBM overlayer to complete the bilayer geometry.     

Metallated conjugated polymers as a new avenue towards high-efficiency polymer solar cells

Bulk heterojunction solar cells have been extensively studied owing to their great potential for cost-effective photovoltaic devices. Although recent advances resulted in the fabrication of poly(3-hexylthiophene) (P3HT)/fullerene derivative based solar cells with efficiencies in the range 4.4-5.0%, theoretical calculations predict that the development of novel donor materials with a lower bandgap is required to exceed the power-conversion efficiency of 10%. However, all of the lower bandgap polymers developed so far have failed to reach the efficiency of P3HT-based cells. To address this issue, we synthesized a soluble, intensely coloured platinum metallopolyyne with a low bandgap of 1.85 eV. The solar cells, containing metallopolyyne/fullerene derivative blends as the photoactive material, showed a power-conversion efficiency with an average of 4.1%, without annealing or the use of spacer layers needed to achieve comparable efficiency with P3HT. This clearly demonstrates the potential of metallated conjugated polymers for efficient photovoltaic devices.     

MoO3 buffer layer effect on photovoltaic properties of interpenetrating heterojunction type organic solar cells

Photovoltaic cells, with a conducting polymer/fullerene (C₆₀) interpenetrating heterojunction structure fabricated by spin-coating a conducting polymer onto a C₆₀ thin film, have been investigated and demonstrated a high efficiency as solar cells based on organic materials. The photovoltaic properties of the solar cells with a structure of indium-tin-oxide (ITO)/C₆₀/poly(3-hexylthiophene) (PAT6)/Au have been improved by the insertion of a molybdenum trioxide (VI) (MoO₃) layer as a cathode buffer layer. In the solar cells with the structure of ITO/C₆₀/PAT6/MoO₃/Au, the energy conversion efficiency has been improved to 1.15% under AM1.5 (100 mW/cm²) illumination.     

Modified processing conditions for optimized organic solar cells with inkjet printed P3HT:PC61BM active layers

Inkjet printing can be used to deposit the functional layers of organic solar cells and it offers advantages over spin coating such as the possibility to print films with user-defined patterns. In this study, inkjet printing was utilized to deposit polymer:fullerene solar cell active layers and different drying and annealing conditions were examined in order to optimize device performance. Low fill factors of approximately 30% were found for devices with printed active layers that were dried at 100 °C and a considerable shift in the fill factor of up to 60% was seen after post-annealing at 150 °C. Changes in the fill factor corresponded to an increase in device efficiency from ～1.3% to ～2.4% after post-annealing. An alternative active layer drying procedure was used based on solvent annealing which resulted in high fill factors of 60% and efficiencies of ～2.4% without post-annealing. Blend films were examined with atomic force microscopy, ultra-violet visible spectroscopy and X-ray photoelectron spectroscopy. It was determined that solvent annealed, inkjet printed active layers are considerably rougher and show enhanced organization with respect to films that were dried at 100 °C. Two preparation routes are provided for devices with printed active layers with acceptable efficiencies based on quick drying and post-annealing or slow drying (solvent annealing).     

Polymerizable fullerene-based material for organic solar cells

A polymerizable fullerene derivative was designed and applied as the electron acceptor material in organic bulk heterojunction solar cells in combination with P3HT. This material was easily processed using environmentally friendly solvents such as toluene and xylene instead of conventional chlorobenzene. Moreover, toluene-processed solar cells yielded better performances than devices produced using chlorobenzene as a solvent. Solar cells based on the designed polymerizable fullerene derivative demonstrated highly stable operation even at elevated temperatures.     

Effect of solution-processed niO thin film as a hole transport layer in poly(3-hexylthiophene): [6,6]-phenyl C61-butyric acid methyl ester bulk heterojunction solar cells

In this study, solution-processed nickel oxide (NiO) thin film was investigated as a hole transport layer on anode to improve the performance of bulk heterojunction solar cell based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). We fabricated NiO thin film without any vacuum-related process. Characterization of the NiO film under this study shows that it has maximum transmittance of 93.22% and bandgap of 3.84 eV which are proper for solar cell. Insertion of the NiO layer affords to realize enhanced power conversion efficiency of 1.97% and fill factor of 52.11% showing improvement over existing cells. In addition, NiO suggests one solution of minimizing conventional problems of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) such as interfacial power losses, corrosion of indium tin oxide layer, and degradation of the devices. The value of such hole transporting and electron blocking properties is clearly demonstrated and could be applicable to other organic photovoltaics.     

Utilizing insulating nanoparticles as the spacer in laminated flexible polymer solar cells for improved mechanical stability

Roll-to-roll lamination is one promising technique to produce large-area organic electronic devices such as solar cells with a large through output. One challenge in this process is the frequent electric point shorting of the cathode and anode by the excess or concentrated applied stress from many possible sources. In this paper, we report a method to avoid electric point shorting by incorporating insulating and hard barium titanate (BaTiO(3)) nanoparticles (NPs) into the active layer to work as a spacer. It has been demonstrated that the incorporated BaTiO(3) NPs in poly(3-hexylthiophene):[6,6]-phenyl-c-61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction solar cells cause no deleterious effect to the power conversion process of this type of solar cell. The resulting laminated devices with NPs in the active layer display the same efficiency as the devices without NPs, while the laminated devices with NPs can sustain a ten times higher lamination stress of over 6?MPa. The flexible polymer solar cell device with incorporated NPs shows a much smaller survivable curvature radius of 4?mm, while a regular flexible device can only sustain a bending curvature radius of 8?mm before fracture.     

Hybrid nanostructure heterojunction solar cells fabricated using vertically aligned ZnO nanotubes grown on reduced graphene oxide

Using reduced graphene oxide (rGO) films as the transparent conductive coating, inorganic/organic hybrid nanostructure heterojunction photovoltaic devices have been fabricated through hydrothermal synthesis of vertically aligned ZnO nanorods (ZnO-NRs) and nanotubes (ZnO-NTs) on rGO films followed by the spin casting of a poly(3-hexylthiophene) (P3HT) film. The data show that larger interfacial area in ZnO-NT/P3HT composites improves the exciton dissociation and the higher electrode conductance of rGO films helps the power output. This study offers an alternative to manufacturing nanostructure heterojunction solar cells at low temperatures using potentially low cost materials.     

Coated and Printed Perovskites for Photovoltaic Applications

Hybrid organic–inorganic metal halide perovskite semiconductors provide opportunities and challenges for the fabrication of low‐cost thin‐film photovoltaic devices. The opportunities are clear: the power conversion efficiency (PCE) of small‐area perovskite photovoltaics has surpassed many established thin‐film technologies. However, the large‐scale solution‐based deposition of perovskite layers introduces challenges. To form perovskite layers, precursor solutions are coated or printed and these must then be crystallized into the perovskite structure. The nucleation and crystal growth must be controlled during film formation and subsequent treatments in order to obtain high‐quality, pin‐hole‐free films over large areas. A great deal of understanding regarding material engineering during the perovskite film formation process has been gained through spin‐coating studies. Based on this, significant progress has been made on transferring material engineering strategies to processes capable of scale‐up, such as blade coating, spray coating, inkjet printing, screen printing, relief printing, and gravure printing. Here, an overview is provided of the strategies that led to devices deposited by these scalable techniques with PCEs as high as 21%. Finally, the opportunities to fully close the shrinking gap to record spin‐coated solar cells and to scale these efficiencies to large areas are highlighted.     

The influence of the organic/inorganic interface on the organic-inorganic hybrid solar cells

Organic-inorganic hybrid solar cells based on poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM) hybridized with ZnO nanorods were fabricated by growing vertical ZnO nanorods on indium tin oxide (ITO) substrates and filling with bulk heterojunction polymers (P3HT:PCBM). The interface between the organic and inorganic nanostructures influences the performance of the organic-inorganic hybrid solar cells. In this paper, the influence of the state of the P3HT:PCBM/ZnO interface on the performance of organic-inorganic hybrid solar cells is examined. The solar cell performance was high when the P3HT:PCBM/ZnO junction area was large. The charge separation is effective when the active layer/electron transport layer junction area is large, resulting in increasing photocurrent and a high conversion efficiency. The bulk-heterojunction polymer concentration was kept low to infiltrate into the ZnO nanorods, resulting in a large active layer/electron transport layer junction area.     

喷墨打印技术在功能材料精密器件加工中的应用

喷墨打印技术应用于聚合物成膜或成型,已成为功能聚合物沉积和精密器件加工领域的核心技术之一。介绍了喷墨打印技术在聚合物电致发光器件、有机薄膜晶体管、太阳能电池和传感器等领域的研究和应用进展,包括聚合物墨水的制备、薄膜均匀性、聚合物溶液与打印性能的关系、新型功能材料的研究和开发等问题,并指出了存在的问题和面临的挑战。     

Control of the Surface Morphology of Ceramic/Polymer Composite Inks for Inkjet Printing

The use of organic/inorganic composite inks in the Drop on Demand inkjet printing technology is a promising as well as demanding approach for the fabrication of composite thick films. Therefore, a versatile ceramic/polymer composite ink system for inkjet printing is developed in this study, containing Ba_0.6Sr_0.4TiO₃ (BST) and poly(methyl methacrylate) (PMMA). When developing such inks suitable for a one‐step fabrication, the major challenge is to fulfill the requirements of the inkjet printing technology and to obtain homogeneous surface morphologies after drying. Thus, possible influencing factors like the solvent composition, the solids content, and the ratio of ceramic to polymer are investigated to obtain a detailed knowledge for the general ink development. The fluid mechanical properties, viscosity, density, and surface tension are characterized. The main focus of this study lies on the drying behavior of the different inks, with the interactions of the ceramic particles, and the dissolved polymer molecules being highlighted. Furthermore, the drying behavior depending on the ink composition is shown. This study provides new insights into the possibility of using composite inks for the inkjet printing process and the fabrication of printed composite thick films in a single process step.

     

Vertical concentration gradients in bulk heterojunction solar cells induced by differential material solubility

Highly soluble fullerene derivatives (HSFD) and a low soluble polymer (LSP) were investigated as modifiers of the active layer morphology in conventional P3HT/PCBM bulk heterojunction solar cells. The observed changes in photovoltaic and electrical characteristics of the devices after addition of one or two modifiers suggest that they induced favourable vertical phase separation in the blends simply due to different solubilities of the components. In particular, HSFD is supposed to accumulate at the top of the film serving as a hole-blocking interlayer at the cathode/active layer interface. On the contrary, LSP seems to form electron-blocking buffer layer at the bottom of the device at the active layer/anode interface. Thus, the differential material solubility was suggested as a tool for adjustment of vertical morphology of organic bulk heterojunction solar cells.     

Evolution of vertical phase separation in P3HT:PCBM thin films induced by thermal annealing

The achievement of the desirable morphology at the nanometer scale of bulk heterojunctions consisting of a conjugated polymer with fullerene derivatives is a prerequisite in order to optimize the power conversion efficiency of organic solar cells. The various experimental conditions such as the choice of solvent, drying rates and annealing have been found to significantly affect the blend morphology and the final performance of the photovoltaic device. In this work, we focus on the effects of post deposition thermal annealing at 140 °C on the blend morphology, the optical and structural properties of bulk heterojunctions that consist of poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM). The post thermal annealing modifies the distribution of the P3HT and the PCBM inside the blend films, as it has been found by Spectroscopic Ellipsometry studies in the visible to far-ultraviolet spectral range. Phase separation was identified by AFM and GIXRD as a result of a slow drying process which took place after the spin coating process. The increase of the annealing time resulted to a significant increase of the P3HT crystallinity at the top regions of the blend films.     

Hierarchical nanomorphologies promote exciton dissociation in polymer/fullerene bulk heterojunction solar cells

PTB7 semiconducting copolymer comprising thieno[3,4-b]thiophene and benzodithiophene alternating repeat units set a historic record of solar energy conversion efficiency (7.4%) in polymer/fullerene bulk heterojunction solar cells. To further improve solar cell performance, a thorough understanding of structure-property relationships associated with PTB7/fullerene and related organic photovoltaic (OPV) devices is crucial. Traditionally, OPV active layers are viewed as an interpenetrating network of pure polymers and fullerenes with discrete interfaces. Here we show that the active layer of PTB7/fullerene OPV devices in fact involves hierarchical nanomorphologies ranging from several nanometers of crystallites to tens of nanometers of nanocrystallite aggregates in PTB7-rich and fullerene-rich domains, themselves hundreds of nanometers in size. These hierarchical nanomorphologies are coupled to significantly enhanced exciton dissociation, which consequently contribute to photocurrent, indicating that the nanostructural characteristics at multiple length scales is one of the key factors determining the performance of PTB7 copolymer, and likely most polymer/fullerene systems, in OPV devices.     

High‐Performance Organic Bulk‐Heterojunction Solar Cells Based on Multiple‐Donor or Multiple‐Acceptor Components

Organic solar cells (OSCs) based on bulk heterojunction structures are promising candidates for next‐generation solar cells. However, the narrow absorption bandwidth of organic semiconductors is a critical issue resulting in insufficient usage of the energy from the solar spectrum, and as a result, it hinders performance. Devices based on multiple‐donor or multiple‐acceptor components with complementary absorption spectra provide a solution to address this issue. OSCs based on multiple‐donor or multiple‐acceptor systems have achieved power conversion efficiencies over 12%. Moreover, the introduction of an additional component can further facilitate charge transfer and reduce charge recombination through cascade energy structure and optimized morphology. This progress report provides an overview of the recent progress in OSCs based on multiple‐donor (polymer/polymer, polymer/dye, and polymer/small molecule) or multiple‐acceptor (fullerene/fullerene, fullerene/nonfullerene, and nonfullerene/nonfullerene) components.     

Nanoscale morphology of high-performance polymer solar cells

Transmission electron microscopy and electron diffraction are used to study the changes in morphology of composite films of poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM) in bulk heterojunction solar cells. Thermal annealing produces and stabilizes a nanoscale interpenetrating network with crystalline order for both components. P3HT forms long, thin conducting nanowires in a rather homogeneous, nanocrystalline PCBM film. Both the improved crystalline nature of films and increased but controlled demixing between the two constitutes therein after annealing explains the considerable increase of the power conversion efficiency observed in these devices.