Efficient organic photovoltaics using solution-processed,annealing-free TiO2 nanocrystalline particles as an interface modification layer

A solution-processed, annealing-free TiO₂ nanocrystalline particles (TiO₂ NPs) as an interface modification layer was inserted in organic photovoltaics (OPVs), in which the widely used polymer poly (3-hexyl thiophene) (P3HT), a low band gap alkoxylphenyl substituted [1,2-b:4,5-b′] dithiophene-based polymer (PBDTPO-DTBO), and a soluble small molecule benzodithiophene derivative (TIBDT) were used as the donor material, respectively. The annealing-free TiO₂ NPs could be easily spin-coated upon the surface of organic active layers, and showed comparable properties to thermal-annealed ones. The power conversion efficiencies (PCEs) of OPV devices could be enhanced dramatically with inserting an annealing-free TiO₂ NPs layer. The PCEs of OPV devices based on P3HT:PC₆₁BM, PBDTPO-DTBO:PC₇₁BM and TIBDT:PC₆₁BM bulk heterojunctions were improved by 28%, 15% and 27%, respectively, with an annealing-free TiO₂ NPs layer, in which the highest PCE of 5.76% was achieved in PBDTPO-DTBO:PC₇₁BM OPVs. The solution-processed, annealing-free TiO₂ NPs thin films show great potential applications in the fabrication of large-area OPVs by printing or coating techniques on flexible polymer substrates. In particularly, it would promote to fabricate solution-processed, annealing-free OPV devices with suitable hole transport layer and organic/polymer active materials.     

Enhancing the photovoltaic performance with two similar structure polymers as donors by broadening the absorption spectrum and optimizing the molecular arrangement

Ternary organic photovoltaics (OPVs) were fabricated with two polymers (PM6 and D18) as the donor and the fullerene-free small molecule Y6 as the acceptor in an inverted structure. The blueshifted absorption spectrum of neat D18 relative to neat PM6 can enable harvesting of more short and medium wavelength photons in the ternary photoactive layer, which is beneficial to increasing the short-circuit current density (J_SC). The enhancement of the open-circuit voltage (V_OC) of the ternary OPVs can be explained by the deeper HOMO level of D18 than that of PM6, which is beneficial to broadening the energy bandgap. In addition, the combination of the cascade LUMO levels among D18, PM6 and Y6 and the enhanced crystallinity can lead to more efficient exciton dissociation and charge transport within the ternary films. As a result, the power conversion efficiency of the optimize ternary OPV is 15.85%, which is higher than those of the PM6:Y6- and D18:Y6-base binary OPVs (PCEs of 14.70% and 14.95%, respectively). The results indicate that ternary OPVs with a blend of two similar chemical structure polymers as the donor could achieve high performance by broadening the light spectrum and optimizing the phase separation and crystallinity.     

Improved performance of organic photovoltaic cells with PTB7-Th:PC71 BM by optimized solvent evaporation time in electrospray deposition

As poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b; 4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] has good potential as a low-band gap donor polymer for organic photovoltaic cells (OPVs), we investigated the optimized electrospray deposition condition for realizing suitable polymer ordering and/or crystallite size by controlling the solvent evaporation time. Previous studies on the electrospray process have mainly focused on novel device structure owing to its unique characteristic of small droplet size, which is less than 1 μm. However, in this research, we investigated the spontaneous formation of interpenetrating continuous networks of the donor- and acceptor-rich domains of solvent evaporation during the electrospray process. By evaluating the ultraviolet–visible absorption spectrum, Raman spectroscopy, and direction of polymer ordering, it was shown that the polymer-stacking condition was not influenced by solvent evaporation time, even though poly(3-hexylthiophene-2,5-diyl) along the face-on direction was well stacked under the slow solvent evaporation condition. In contrast, the crystallite size, which was estimated from the full width at half maximum X-ray diffraction pattern, increased as the solvent evaporation time increased. This means that the crystalline grain spontaneously grew in the droplet and that the large crystalline grain was formed during the slow evaporation condition. Furthermore, the photovoltaic performance trend was the same as the performance trend of the crystallite size and were increased with increasing solvent evaporation time for both polymers. Therefore, the crystalline grain size was a dominant factor in determining the photovoltaic performance. Additionally, the crystalline grain size could be controlled by the solvent evaporation time. Finally, by optimizing the active-layer thickness, the highest photoconversion efficiency of 8.6% was achieved. This is the highest value of an electrospray-based device. These results indicate that the solvent evaporation time is an important factor in determining the crystallite size of an organic thin film, which directly affects the photoconversion efficiency of OPVs.     

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.     

Blade Coating of Alloy as Top Electrodes for Efficient All-Printed Organic Photovoltaics

All printing of organic photovoltaics (OPVs) including the top electrode is highly desirable for achieving cost-effective, high-throughput, and large-area photovoltaic manufacturing. Here, the printing of a low-melting-point alloy as top electrodes in OPVs via blade coating is investigated. The Field's metal (FM) with the melting point of 62 °C is adopted for the top electrodes, because FM can be printed under moderate temperatures without harming the active layers while remaining solid state under solar irradiation. The correlations between the processing parameters and properties of the blade-coated electrodes are elucidated. OPVs based on the D18:Y6 active layer and blade-coated FM electrodes achieve a highest power conversion efficiency of 17.28%. The OPVs with FM-electrode demonstrate much higher thermal stability than that of the Ag-electrode devices. All-printed OPVs, in which the FM electrode is blade coated and the other layers are prepared by flexible micro-comb printing, exhibit an efficiency of 16.07%. The results represent the records of evaporation-free and all-printed OPVs, demonstrating that printing FM as OPV electrodes is a cost-effective and time-saving strategy to substitute the vacuum-evaporated metals, as well as a feasible route toward high-performance all-printed OPVs.     

Printable and Large‐Area Organic Solar Cells Enabled by a Ternary Pseudo‐Planar Heterojunction Strategy

Bulk heterojunction (BHJ) processing technology has had an irreplaceable role in the development of organic solar cells (OSCs) in the past decades due to the significant advantages in achieving high‐power conversion efficiency (PCE). However, the difficulty in exploring and regulating morphology makes it inadequate for upscaling large‐area OSCs. In this work, printable high‐performance ternary devices are fabricated by a pseudo‐planar heterojunction (PPHJ) strategy. The fullerene derivative indene‐C₆₀ bisadduct (ICBA) is incorporated into PM6/IT‐4F system to expand the vertical phase separation and facilitate an obvious PPHJ structure. After the addition of ICBA, the IT‐4F enriches on the surface of active layer, while PM6 is accumulated underneath. Furthermore, it increases the crystallinity of PM6, which facilitates exciton dissociation and charge transfer. Accordingly, 1.05 cm² devices are fabricated by blade‐coating with an enhanced PCE of 14.25% as compared to the BHJ devices (13.73%). The ternary PPHJ strategy provides an effective way to optimize the vertical phase separation of organic semiconductor during scalable printing methods.     

Highly Processable Ionogels with Mechanical Robustness

Currently, the increasing needs of conductive ionogels with intricate shapes and high processability by individual requirements of next-generation flexible electronics constitute significant challenges. Here, the design of highly processable ionogels is reported with mechanical robustness by self-assembly of a common triblock copolymer into a precursor in functional mixed ionic liquids (ILs) containing conductivity-enhancing and polymerizable strength-enhancing components. The subsequent in situ polymerization of the precursor forms physical-co-chemical cross-linked networks, in which the entanglement between physical and chemical cross-linked networks and microphase separation give rise to mechanical robustness of as-fabricated ionogel. The viscosity of the self-assembled precursor can be rationally tuned, which makes the fabrication process compatible with diverse technologies including inkjet printing, spray coating, and 3D printing. By virtue of highly processable capability of the designed ionogels, an auxetic-structured ionogel can be easily generated using 3D printing, which exhibits greatly improved sensitivity and thus is able to monitor tiny deformations. This study that relies on designing functional mixed ILs as the dispersion phase rather than focusing on synthesizing new-type polymers establishes a new route for versatile and programmable fabrication of high-performance ionogels for broader applications.     

Compositional and Morphological Studies of Polythiophene/Polyflorene Blends in Inverted Architecture Hybrid Solar Cells

This work investigates the composition and morphology of films of poly(3‐hexylthiophene) (P3HT), polyfluorene co‐polymer poly((9,9‐dioctylfluorene)‐2,7‐diyl‐alt‐[4,7‐bis(3‐hexylthien‐5‐yl)‐2,1,3‐benzothiadiazole]‐2′,2″‐diyl) (F8TBT) and blends thereof that are used in efficient all‐polymer solar cells. Ultraviolet photoemission spectroscopy (UPS) and X‐ray photoemission spectroscopy (XPS) studies on thin polymer and blend films on ZnO substrates reveal the existence of a 1–2 nm thick P3HT layer at the top surface of the blend films. XPS depth profiling studies reveal a density wave (λ ≈ 70 nm) originating from the air interface. As no preferential accumulation is observed at the bottom interface with ZnO, the composition at this interface is consistent with the original composition of the blend solution prior to spin‐coating. The morphology of this buried interface was studied by means of atomic force microscopy (AFM) and revealed that upon annealing the average domain size increases slightly (from 27 nm to 40 nm). It is observed that the photovoltaic performance of such inverted hybrid device improves upon annealing, however we believe this to mostly be a result of increased crystallinity in the P3HT domains leading to improved charge transport in the device, rather than changes in the blend phase separation.     

High Performance Roll‐to‐Roll Produced Fullerene‐Free Organic Photovoltaic Devices via Temperature‐Controlled Slot Die Coating

Solution‐processed organic photovoltaics (OPVs) have continued to show their potential as a low‐cost power generation technology; however, there has been a significant gap between device efficiencies fabricated with lab‐scale techniques—i.e., spin coating—and scalable deposition methods. Herein, temperature‐controlled slot die deposition is developed for the photoactive layer of OPVs. The influence of solution and substrate temperatures on photoactive films and their effects on power conversion efficiency (PCE) in slot die coated OPVs using a 3D printer‐based slot die coater are studied on the basis of device performance, molecular structure, film morphology, and carrier transport behavior. These studies clearly demonstrate that both substrate and solution temperatures during slot die coating can influence device performance, and the combination of hot substrate (120 °C) and hot solution (90 °C) conditions result in mechanically robust films with PCE values up to 10.0% using this scalable deposition method in air. The efficiency is close to that of state‐of‐the‐art devices fabricated by spin coating. The deposition condition is translated to roll‐to‐roll processing without further modification and results in flexible OPVs with PCE values above 7%. The results underscore the promising potential of temperature‐controlled slot die coating for roll‐to‐roll manufacturing of high performance OPVs.     

聚合物光诱导电荷转移光电池的研究进展

综述了以共轭聚合物作为电子施主和 C6 0 及其衍生物作为电子受主的共混与多层器件结构的聚合物光诱导电荷转移光电池的研究进展。对这类新型结构的光电池的基本性能及机制作了介绍。低生产成本、能通过简单甩膜或印刷方式就能制备大面积器件的优势使聚合物光电池在许多实际应用领域具有广阔的前景。对今后进一步提高这类光电池的能量转换效率的研究方向进行了探讨     

On the role of aggregation effects in the performance of perylene-diimide based solar cells

A model bulk-heterojunction of a perylene diimide (PDI) monomeric derivative is studied for interrogating the role of PDI aggregates in the photocurrent generation efficiency (η_PC) of PDI-based organic photovoltaic (OPV) devices. Blend films of the PDI derivative and the poly(indenofluorene) (PIF) polymer annealed between room temperature and 220 °C, are used as the photoactive layers for the fabrication of OPVs. The positive effect of thermal annealing is assigned to the evolution of PDI aggregates in the amorphous PIF matrix. Annealing increases the electron mobility by three orders of magnitude. In contrast, owned to the thermally inert PIF matrix used, hole mobility increases only by a factor of six. High resolution cross-sectional scanning electron microscopy suggests that η_PC in PDI-based OPVs is not limited by the PDI aggregates but by their improper alignment. In situ Raman spectra and density functional theory calculations identify a marker for monitoring the strength of π–π stacking interactions between PDI monomers. It s further demonstrated that the electron-collecting electrode of the PIF:PDI devices dictates their performance. The use of Al is found to impede charge extraction and this is attributed to an unidentified product of the reaction between PDI and Al that leads to the formation of an electron-blocking layer. Device performance rectifies when a Ca/Al electrode is used and the power conversion efficiency is increased by a factor of four.     

Understanding How Processing Additives Tune the Nanoscale Morphology of High Efficiency Organic Photovoltaic Blends: From Casting Solution to Spun‐Cast Thin Film

Adding a small amount of a processing additive to the casting solution of photoactive organic blends has been demonstrated to be an effective method for achieving improved power conversion efficiency (PCE) in organic photovoltaics (OPVs). However, an understanding of the nano‐structural evolution occurring in the transformation from casting solution to thin photoactive films is still lacking. In this report, the effects of the processing additive diiodooctane (DIO) on the morphology of the established blend of PBDTTT‐C‐T polymer and the fullerene derivative PC₇₁BM used for OPVs are investigated, starting in the casting solution and tracing the effects in spun‐cast thin films by using neutron/X‐ray scattering, neutron reflectometry, and other characterization techniques. The results reveal that DIO has no observable effect on the structures of PBDTTT‐C‐T and PC₇₁BM in solution; however, in the spun‐cast films, it significantly promotes their molecular ordering and phase segregation, resulting in improved PCE. Thermodynamic analysis based on Flory‐Huggins theory provides a rationale for the effects of DIO on different characteristics of phase segregation due to changes in concentration resulting from evaporation of the solvent and additive during film formation. Such information may help improve the rational design of ternary blends to more consistently achieve improved PCE for OPVs.     

Key factors to improve the efficiency of roll-to-roll printed organic photovoltaics

In order to achieve the cost-efficient scalability of flexible organic photovoltaics (OPVs), the optimization of key factors related to the materials and roll-to-roll (R2R) processes is necessary. The limited drying during the R2R printing process induces a vertical phase separation leading to the formation of a P3HT-rich top region on the photoactive layer which acts as an electron barrier in normal geometry. We show that the increase of R2R drying time and/or post-annealing can enhance the OPV efficiency by the diffusion of PCBM towards the photoactive layer surface forming an electron transport network. It is estimated that the volume fraction of PCBM at the top region of the films triples from about 9% to 30%. In addition, the direct exposure of PEDOT:PSS to air after printing leads to morphological changes that negatively affect the efficiency. Therefore, the protection of PEDOT:PSS from air in combination to the increase of the R2R drying time enables the significant increase of the R2R printed OPVs efficiency to 1%.     

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 10 min exhibited a power conversion efficiency (PCE) of 2.81% under simulated solar irradiation through an air mass of 1.5 at 100 mW cm⁻². On the other hand, the OPV fabricated using o-dichlorobenzene (boiling point of 181 °C) and annealed at 110 °C for 10 min 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.     

Cover Picture: High Definition Digital Fabrication of Active Organic Devices by Molecular Jet Printing (Adv. Funct. Mater. 15/2007)

A new method for direct patterning of organic optoelectronic/electronic devices using a reconfigurable and scalable printing method is reported by Vladimir Bulovic and co‐workers on p. 2722. The printing technique is applied to the fabrication of high‐resolution printed organic light emitting devices (OLEDs) and organic field effect transistors (OFETs). Remarkably, the final print‐deposited films are evaporated onto the substrate (rather than solvent printed), giving high‐quality, solvent‐free, molecularly flat structures that match the performance of comparable high‐performance unpatterned films. We introduce a high resolution molecular jet (MoJet) printing technique for vacuum deposition of evaporated thin films and apply it to fabrication of 30 μm pixelated (800 ppi) molecular organic light emitting devices (OLEDs) based on aluminum tris(8‐hydroxyquinoline) (Alq₃) and fabrication of narrow channel (15 μm) organic field effect transistors (OFETs) with pentacene channel and silver contacts. Patterned printing of both organic and metal films is demonstrated, with the operating properties of MoJet‐printed OLEDs and OFETs shown to be comparable to the performance of devices fabricated by conventional evaporative deposition through a metal stencil. We show that the MoJet printing technique is reconfigurable for digital fabrication of arbitrary patterns with multiple material sets and high print accuracy (of better than 5 μm), and scalable to fabrication on large area substrates. Analogous to the concept of “drop‐on‐demand” in Inkjet printing technology, MoJet printing is a “flux‐on‐demand” process and we show it capable of fabricating multi‐layer stacked film structures, as needed for engineered organic devices.     

Spray coating methods for polymer solar cells fabrication: A review

The main focus of this review article is the introduction of relevant parameters in spray coating processes to provide better understanding on controlling the morphology of spray coated thin films for producing high performance polymer solar cells (PSC). Three main parameters have been identified as major influences on the spray coating processes. These are nozzle to substrate distance, solvent and mixed solvents effects, and substrate temperature and annealing treatment. Such spray coating techniques show great potential for large scale production, since these methods have no limitation in substrate size and low utilization of polymers which is promising to substitute the conventional spin coating methods. Currently available printing and coating methods are also briefly discussed in this review.     

Eco-friendly fabrication of PBDTTPD:PC71BM solar cells reaching a PCE of 3.8% using water-based nanoparticle dispersions

In this work we report on the eco-friendly processing of PBDTTPD:PC₇₁BM organic solar cells using water-based nanoparticle (NP) dispersions. The polymer:fullerene NPs are prepared using the miniemulsion-solvent evaporation method, despite employing high-boiling solvents. Polymer solar cells are fabricated from these blend NPs and the device characteristics are studied in function of annealing time and temperature. The photoactive layer formation is carefully analyzed using atomic force microscopy (AFM). Annealing for longer times significantly increases the power conversion efficiency (PCE), up to 3.8%, the highest value reported for surfactant based NP solar cells. Our work shows that the low bandgap polymer PBDTTPD has the ability to afford reasonable efficiencies in NP solar cells in combination with PC₇₁BM and paves the way to a truly eco-friendly processing of organic photovoltaics (OPVs).     

Low temperature fabrication of formamidinium based perovskite solar cells with enhanced performance by chlorine incorporation

Recently formamidinium (FA) based perovskite solar cell was demonstrated to show high performance and better stability upon partial substitution of FA with Cs cation. However, the fabrication of device required high-temperature processing on TiO₂ electrode and thus limits the use of flexible polymeric substrates. Here, we present a low temperature approach for the fabrication of p-i-n perovskite solar cells based on Cs_0.15FA_0.85PbI₃. Furthermore, we investigated the effects of chlorine on the morphology and crystallinity of the perovskite films and the corresponding photovoltaic performance. Chlorine incorporation can significantly enlarge the size of grains and improve the crystallinity of perovskite films with full surface coverage. A best power conversion efficiency of 14.5% was realized for planar perovskite solar cells with negligible hysteresis and remarkable reproducibility.     

Lateral Phase Separation Gradients in Spin‐Coated Thin Films of High‐Performance Polymer:Fullerene Photovoltaic Blends

In this study, it is demonstrated that a finer nanostructure produced under a rapid rate of solvent removal significantly improves charge separation in a high‐performance polymer:fullerene bulk‐heterojunction blend. During spin‐coating, variations in solvent evaporation rate give rise to lateral phase separation gradients with the degree of coarseness decreasing away from the center of rotation. As a result, across spin‐coated thin films the photocurrent at the first interference maximum varies as much as 25%, which is much larger than any optical effect. This is investigated by combining information on the surface morphology of the active layer imaged by atomic force microscopy, the 3D nanostructure imaged by electron tomography, film formation during the spin coating process imaged by optical interference and photocurrent generation distribution in devices imaged by a scanning light pulse technique. The observation that the nanostructure of organic photovoltaic blends can strongly vary across spin‐coated thin films will aid the design of solvent mixtures suitable for high molecular‐weight polymers and of coating techniques amenable to large area processing.     

Tuning the viscosity of halogen free bulk heterojunction inks for inkjet printed organic solar cells

For the solution processing of organic photovoltaics on an industrial scale, the exclusion of halogenated solvents is a necessity. However, the limited solubility of most semiconducting polymer/fullerene blends in non-halogenated solvents results in ink formulations with low viscosities which poses limitations to the use of roll-to-roll compatible deposition processes, such as inkjet printing. We propose to add polystyrene as a rheological modifier to increase the viscosity of bulk heterojunction (BHJ) non-halogenated inks. The printing and performance of P3HT/PCBM photoactive layer inks are characterized as a function of polystyrene concentration and three different molecular weights. Addition of 1 wt% polystyrene provided a near two-fold gain in viscosity, with the largest viscosity gains coming from the polymer with the highest molecular weight. However, this coincided with greater viscoelastic behavior, which reduced the jetting performance of the inks. Differences in solvent compatibility of the polystyrene/P3HT/PCBM ternary blend resulted in phase separation upon layer drying, whereby polystyrene segregated to the layer-air interface to form an isolated domain or network like topology. Nevertheless, a 1.7-fold increase in dynamic viscosity was obtained for devices with printed BHJ layers containing polystyrene at the expense of a 20% reduction in OPV performance. The improved viscosity and good printing behavior achieved with small additions of polystyrene demonstrates its potential to overcome the limited viscosity resulting from typical non-halogenated ink formulations for semiconducting polymers. These results offer a step forward to the industrialization of inkjet printing as an effective deposition technique for functional layers of organic electronics.