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.     

Additive-free organic solar cells with enhanced efficiency enabled by unidirectional printing flow of high shear rate

High-performance OSCs prepared by scalable techniques without additives are highly desirable because residual additives may cause gradual deterioration of the photoactive-layer morphology and device performance. Printing flows with high shear rate have the potential to replace additives by inducing higher degree of ordered stacking and crystallinity of organic molecules, as well as favorable phase separation. Here, PTQ10:Y6 organic solar cells (OSCs) without any additives were fabricated by a scalable and robust processing approach termed as soft porous blade printing (SPBP). The fluid flow and drying process of the wet films made by SPBP, blade coating and spin coating are visualized by high speed imaging, which reveals that the blade coating and SPBP introduce unidirectional flow while the wet film interference pattern of spin coating is irregular and random. The simplified flow model of SPBP suggests that the shear rate could be as high as ~1000 s⁻¹. The additive-free SPBP produces photoactive-layer with adequate morphology, which could be attributed to three intrinsic properties of SPBP: very high shear rate, flow assisted crystallizations induced by microstructures of the soft porous blade, and numerous nucleation sites generated as the liquid contact line follows the motion of the blade. The additive-free SPBP device demonstrates weaker charge recombination, higher and more balanced charge transport, and consequently better device performance than the spin-coated and blade-coated devices with 0.25 vol% 1,8-diioctane (DIO). SPBP achieved power conversion efficiency (PCE) of 16.45%, which is higher than those of spin-coated and blade-coated counterparts doped with DIO.     

Control of Crystal Growth toward Scalable Fabrication of Perovskite Solar Cells

With the impressive record power conversion efficiency (PCE) of perovskite solar cells exceeding 23%, research focus now shifts onto issues closely related to commercialization. One of the critical hurdles is to minimize the cell‐to‐module PCE loss while the device is being developed on a large scale. Since a solution‐based spin‐coating process is limited to scalability, establishment of a scalable deposition process of perovskite layers is a prerequisite for large‐area perovskite solar modules. Herein, this paper reports on the recent progress of large‐area perovskite solar cells. A deeper understanding of the crystallization of perovskite films is indeed essential for large‐area perovskite film formation. Various large‐area coating methods are proposed including blade, slot‐die, evaporation, and post‐treatment, where blade‐coating and gas post‐treatment have so far demonstrated better PCEs for an area larger than 10 cm². However, PCE loss rate is estimated to be 1.4 × 10^?2% cm^?2, which is 82 and 3.5 times higher than crystalline Si (1.7 × 10^?4% cm^?2) and thin film technologies (≈4 × 10^?3% cm^?2) respectively. Therefore, minimizing PCE loss upon scaling‐up is expected to lead to PCE over 20% in case of cell efficiency of >23%.     

Enhancing Photovoltaic Performance Using an All‐Conjugated Random Copolymer to Tailor Bulk and Interfacial Morphology of the P3HT:ICBA Active Layer

Bulk heterojunction (BHJ) solar cells are fabricated using active material blends of poly(3‐hexylthiophene) (P3HT) donor, indene‐C₆₀ bisadduct (ICBA) acceptor, and an all‐conjugated random copolymer (RCP) additive. By optimizing RCP loading, power conversion efficiencies (PCEs) up to 20% higher than those of a binary P3HT:ICBA mixture are achieved. The improved device characteristics are rationalized in terms of the differences between the photoactive thin film morphologies. Energy‐filtered transmission electron micro­scopy reveals that incorporation of the RCP improves the degree of structural order of the BHJ fibrillar network and increases the extent of microphase separation between P3HT and ICBA. Additionally, a combination of atomic force microscopy and X‐ray photoelectron spectroscopy analysis indicates segregation of the RCP at the free interface, leading to a shift in the surface potentials measured by Kelvin probe force microscopy. These changes, both in the bulk morphology and in the interfacial composition/energetics, are correlated to improved carrier collection efficiency due to a reduction of non‐geminate recombination, which is measured by charge extraction of photo­generated carriers by linearly increasing voltage.     

Sequential Deposition of High‐Quality Photovoltaic Perovskite Layers via Scalable Printing Methods

Sequential deposition is demonstrated as an effective technology for preparation of high‐performance perovskite solar cells based on lab‐scale spin coating. However, devices fabricated by scalable methods are lagging far behind their state‐of‐the‐art spin‐coated counterparts, largely due to the difficulty in obtaining high‐quality thin films of perovskites crystallized from printed precursors. Here, a generic strategy that allows sequential deposition of dense and uniform perovskite films via two‐step blade coating is reported. The rational selection of solvent combined with a mild vacuum extraction process enables us to produce uniform lead iodide (PbI₂) films over large areas. Significantly, the resulting PbI₂ films possess a mesoporous structure that is highly beneficial for the insertion reaction with methylammonium iodide (MAI). It is further identified that the deposition temperature of MAI plays an important role in determining the morphology and crystallinity of the perovskite films. Solar cells using these sequentially bladed perovskite layers yield efficiencies over 16% with high fill factors up to 78%. These results represent important progress toward the large‐scale deposition of perovskite thin films for practical applications.     

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.     

P3HT based solution-processed pseudo bi-layer organic solar cell with enhanced performance

We present a solution-processed pseudo bi-layer organic solar cell with poly(3-hexyl thiophene) (P3HT) as donor and indene-C₆₀ bisadduct (ICBA) as acceptor. The devices were fabricated by sequential processing of the active components followed by a thermal annealing treatment. An efficiency of 5.9% was achieved under AM 1.5G irradiation (1000 W/m²). The obtained efficiency is attributed to an enhanced nanomorphology that arises from the inter-diffusion of the ICBA molecules into a layer of pre-organised polymer (P3HT) and also due to the subsequent crystallisation of the ICBA molecules. These processes facilitate efficient charge generation and extraction. Time of flight-secondary ion mass spectroscopy (TOF-SIMS) depth profiling was carried out for different thermal annealing treatments of these pseudo bi-layer devices, which reveals full inter-diffusion of ICBA into the polymer P3HT. Photo-CELIV (charge extraction by linearly increasing voltage) studies elucidates that the thermal annealing imparts crystallinity to the fullerene phase which results in the improvement of charge carrier mobility.     

Elimination of Drying-Dependent Component Deviation Using a Composite Solvent Strategy Enables High-Performance Inkjet-Printed Organic Solar Cells with Efficiency Approaching 16%

Inkjet printing (IJP) is a roll-to-roll (R2R) compatible fabrication method for large-area organic solar cells (OSCs). Unlike the coating process, the films are formed through droplet leveling and merging during IJP, and the pre-deposited droplets are partly dissolved by the subsequent droplets. Such a process yields undesired printing pattern lines, especially in large-area printed films. This study reveals that such a temperature-dependent “drying lines-related” phase separation morphology has caused component variation in the organic blend films, which leads to an obvious inhomogeneity of photocurrent in the printed OSCs. Such a phenomenon is attributed to the solubility difference between organic donor and acceptor molecules in the main printing solvent. A composite solvent strategy of ortho-dichlorobenzene (oDCB)/trimethylbenzene (TMB) and tetralin (THN) is developed to solve this problem. The introduction of THN suppresses the formation of printing drying lines during high-temperature printing due to the preferential miscibility of acceptor in THN, leading to the efficiency improvement to 13.96% and 15.78% for the binary and ternary devices. In addition, the 1 cm² device with a disruptive pattern gives an efficiency of 12.80% and a certificated efficiency of 12.18%.     

Room‐Temperature Meniscus Coating of >20% Perovskite Solar Cells: A Film Formation Mechanism Investigation

Perovskite solar cells (PSCs) are ideally fabricated entirely via a scalable solution process at low temperatures to realize the promise of simple manufacturing, low‐cost processing, compatibility with flexible substrates, and perovskite‐based tandem solar cells. However, high‐quality photoactive perovskite thin films under those processing conditions is a challenge. Here, a laminar air‐knife‐assisted room‐temperature meniscus coating approach that enables one to control the drying kinetics during the solidification process and achieve high‐quality perovskite films and solar cells is devised. Moreover, this approach offers a solid model platform for in situ UV–vis and microscopic investigation of the perovskite film drying kinetics, which provide rich insights correlating the degree of supersaturation, the nucleation, and growth rate during the kinetic drying process, and ultimately, the film morphology and performance of the solar cell devices. Manufacturing friendly, antisolvent‐free room‐temperature coating of hysteresis‐free PSCs with a power conversion efficiency of 20.26% for 0.06 cm² and 18.76% for 1 cm² devices is demonstrated.     

Morphology of a Ternary Blend Solar Cell Based on Small Molecule:Conjugated Polymer:Fullerene Fabricated by Blade Coating

Here, conjugated polymer is added as third component to tune the solution viscosity, morphology, and function of small molecule (SM) based bulk‐heterojunction (BHJ) solar cells, which are fabricated using blade coating. Novel information about the effect of blade coating speed on the nanoscale morphology and function of ternary blend solar cells is provided. The crystal sizes increase with an increase of coating speed for both binary and ternary blends, while the addition of the third component tends to favor smaller SM crystal grains and improves the connectivity of SM crystals. Small angle neutron scattering experiments provide the first clear experimental evidence that the addition of the third component would significantly impact the fullerene phase separation, which is crucial for bimolecular recombination and charge transport. It shows that for both binary and ternary blends, the concentration and sizes of [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) aggregates increase with an increase of coating speed, while addition of third component does not affect the volume fraction of PCBM aggregates but impacts the size of PCBM aggregates. It is demonstrated that the judicious selection of blade coating speed and addition of conjugated polymer optimize the morphology of SM‐BHJ, providing guidelines for high performance SM‐BHJs from roll‐to‐roll production.     

A Scalable Free‐Standing V2O5/CNT Film Electrode for Supercapacitors with a Wide Operation Voltage (1.6 V) in an Aqueous Electrolyte

While vanadium oxides have many attractive pseudocapacitive features for energy storage, their applications are severely limited by the poor electronic conductivity and low specific surface area. To overcome these limitations, a scalable, free‐standing film electrode composed of intertwined V₂O₅ nanowires and carbon nanotubes (CNTs) using a blade coating process has been prepared. The unique architecture of this hybrid electrode greatly facilitates electronic transport along CNTs while maintaining rapid ion diffusion within V₂O₅ nanowires and fast electron transfer across the V₂O₅/CNTs interfaces. When tested in a neutral aqueous electrolyte, this hybrid film electrode demonstrates a volumetric capacitance of ≈460 F cm^?3. Moreover, a symmetric capacitor based on two identical film electrodes displays a wide operation voltage window of 1.6 V, delivering a volumetric energy density as high as 41 Wh L^?1.     

Revealing the effect of donor/acceptor intermolecular arrangement on organic solar cells performance based on two-dimensional conjugated small molecule as electron donor

A series of solution processed organic solar cells (OSCs) were fabricated with a two-dimensional conjugated small molecule SMPV1 as electron donor and fullerene derivatives PC₇₁BM or ICBA as electron acceptor. The champion power conversion efficiency (PCE) of OSCs arrives to 7.05% for the cells with PC₇₁BM as electron acceptor. A relatively large open circuit voltage (V_OC) of 1.15 V is obtained from cells using ICBA as electron acceptor with an acceptable PCE of 2.54%. The fill factor (FF) of OSCs is 72% or 61% for the cells with PC₇₁BM or ICBA as electron acceptor, which is relatively high value for small molecule OSCs. The relatively low performance of OSCs with ICBA as electron acceptor indicates that ICBA cannot play positive role in photoelectric conversion processes, which is very similar to the phenomenon observed from the OSCs with high efficient narrow band gap polymers other than P3HT as electron donor, the underlying reason is still in debate. The SMPV1 has strong self-assemble ability to form an ordered two dimensional lamellar structure, which provides an effective platform to investigate the effect of electron acceptor chemical structure on the performance of OSCs. Experimental results exhibit that ICBA molecules may prefer to vertical cross-intercalation among side chains of SMPV1, PC₇₁BM molecules may have better miscibility with SMPV1 in the active layer. The different donor/acceptor (D/A) intermolecular arrangement strongly influences photon harvesting, exciton dissociation and charge carrier transport, which may provide a new sight on performance improvement of OSCs by adjusting D/A intermolecular arrangements.     

Electrodeposited zinc grid as low‐cost solar cell front contact

This paper presents an innovative low‐cost electrodeposition process to grow metallic zinc grids as a front contact for Cu(In,Ga)(Se,S)₂ (CIGS) and silicon heterojunction solar cells as an alternative to complex and expensive monolithic integration and silver screen printing techniques respectively. Morphological and electrical properties of the grid have been investigated and compared with a reference evaporated one. High quality and conformal zinc grids have been deposited showing very high growth rates up to 3.3 µm min⁻¹. Zinc grid is successfully deposited as front electrode for CIGS solar cells that are fabricated by a variety of deposition processes. Efficiency (16.3%) is achieved without antireflection coating on a 0.5 cm² co‐evaporated absorber and 14.8% on an electrodeposited one. Using electrodeposition for the growth of the doped ZnO film as well, a 14.1% efficiency is demonstrated on an all‐wet solar cell only composed of layers deposited by atmospheric methods—from absorber to metallic grid. The process is then applied to a 4.2 cm² cell as a first step toward large‐scale application. Finally, a zinc grid is deposited on a 0.5 cm² silicon heterojunction showing a promising 17% efficiency. Copyright © 2016 John Wiley & Sons, Ltd.     

High-performance alloy-like ternary organic solar cells with two compatible non-fullerene acceptors

Ternary blending is one of the effective strategies to modulate the blend film morphology for achieving high efficiency organic solar cells (OSCs). In this work, high-performance ternary OSCs are fabricated by introducing a non-fullerene acceptor, namely IDTP-4F into the PM6:Y6 binary system to enhance the device performance. Detailed investigations indicate that IDTP-4F can form an alloy phase with Y6, resulting in the optimized morphology, which can facilitate the charge transport and reduce recombination, leading to enhanced open-circuit voltage (V_oc) and fill factor (FF) simultaneously. Consequently, the optimized ternary OSCs exhibit an excellent power conversion efficiency (PCE) of 17.1%, which is much higher than that of PM6:Y6 binary OSCs (15.9%). These results indicate that combining two compatible non-fullerene acceptors is an effective strategy to fabricate high efficiency ternary OSCs.     

Thermal oxidation for crystalline silicon solar cells exceeding 19% efficiency applying industrially feasible process technology

Thermal oxides are commonly used for the surface passivation of high‐efficiency silicon solar cells from mono‐ and multicrystalline silicon and have led to the highest conversion efficiencies reported so far. In order to improve the cost‐effectiveness of the oxidation process, a wet oxidation in steam ambience is applied and experimentally compared to a standard dry oxidation. The processes yield identical physical properties of the oxide. The front contact is created using a screen‐printing process of a hotmelt silver paste in combination with light‐induced silver plating. The contact formation on the front requires a short high‐temperature firing process, therefore the thermal stability of the rear surface passivation is very important. The surface recombination velocity of the fired oxide is experimentally determined to be below S ≤ 38 cm/s after annealing with a thin layer of evaporated aluminium on top. Monocrystalline solar cells are produced and 19·3% efficiency is obtained as best value on 4 cm² cell area. Simulations show the potential of the developed process to approach 20% efficiency. Copyright © 2008 John Wiley & Sons, Ltd.     

Distinguishing the Importance of Fullerene Phase Separation from Polymer Ordering in the Performance of Low Band Gap Polymer:Bis‐Fullerene Heterojunctions

One way to improve power conversion efficiency (PCE) of polymer based bulk‐heterojunction (BHJ) photovoltaic cells is to increase the open circuit voltage (V _oc). Replacing PCBM with bis‐adduct fullerenes significantly improves V _oc and the PCE in devices based on the conjugated polymer poly(3‐hexyl thiophene) (P3HT). However, for the most promising low band‐gap polymer (LBP) system, replacing PCBM with ICBA results in poor short‐circuit current (J _sc) and PCE although V _oc is significantly improved. The optimization of the morphology of as‐cast LBP/bis‐fullerene BHJ photovoltaics is attempted by adding a co‐solvent to the polymer/fullerene solution prior to film deposition. Varying the solubility of polymer and fullerene in the co‐solvent, bulk heterojunctions are fabricated with no change of polymer ordering, but with changes in fullerene phase separation. The morphologies of the as‐cast samples are characterized by small angle neutron scattering and neutron reflectometry. A homogenous dispersion of ICBA in LBP is found in the samples where the co‐solvent is selective to the polymer, giving poor device performance. Aggregates of ICBA are formed in samples where the co‐solvent is selective to ICBA. The resultant morphology improves PCE by up to 246%. A quantitative analysis of the neutron data shows that the interfacial area between ICBA aggregates and its surrounding matrix is improved, facilitating charge transport and improving the PCE.     

Achieving a Record Fill Factor for Silicon–Organic Hybrid Heterojunction Solar Cells by Using a Full‐Area Metal Polymer Nanocomposite Top Electrode

Carrier collection in conventional n‐type Si (n‐Si)/organic hybrid heterojunction solar cells (HHSCs) is mainly limited by the nonoptimized top grid‐electrode and inadequate work function (WF) of the PH1000‐type poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Here, a novel modified metal polymer nanocomposite top electrode (M‐MPNTE) is designed to achieve a full‐area carrier collection in n‐Si/PEDOT:PSS HHSCs. The carrier collection in both lateral and vertical directions is significantly improved by the introduction of an ultrathin Au/MoO_x modified layer between 6 nm ultrathin Ag film and AI4083‐type PEDOT:PSS layer. In addition, the carrier separation is boosted by the enhanced built‐in potential owing to a high WF of M‐MPNTE, which also suppresses the carrier recombination at the surface of n‐Si. Due to these collaborative improvements, a record fill factor of 80.21% is obtained, which is even comparable to the best value of the traditional Si‐based solar cells. With the addition of a MoO_x antireflective coating layer on top of M‐MPNTE, the short‐circuit current density and open‐circuit voltage are finally increased to 23.13 mA cm^?2 and 621.07 mV, respectively, yielding a power conversion efficiency of 10.82%. The finding suggests a novel strategy for the development of highly efficient HHSCs with ideal carrier transport mechanism.     

Identifying a Threshold Impurity Level for Organic Solar Cells: Enhanced First‐Order Recombination Via Well‐Defined PC84BM Traps in Organic Bulk Heterojunction Solar Cells

Small amounts of impurity, even one part in one thousand, in polymer bulk heterojunction solar cells can alter the electronic properties of the device, including reducing the open circuit voltage, the short circuit current and the fill factor. Steady state studies show a dramatic increase in the trap‐assisted recombination rate when [6,6]‐phenyl C₈₄ butyric acid methyl ester (PC₈₄BM) is introduced as a trap site in polymer bulk heterojunction solar cells made of a blend of the copolymer poly[N‐9″‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and the fullerene derivative [6,6]‐phenyl C₆₁ butyric acid methyl ester (PC₆₀BM). The trap density dependent recombination studied here can be described as a combination of bimolecular and Shockley–Read–Hall recombination; the latter is dramatically enhanced by the addition of the PC₈₄BM traps. This study reveals the importance of impurities in limiting the efficiency of organic solar cell devices and gives insight into the mechanism of the trap‐induced recombination loss.     

Highly Improved Photocurrent Density and Efficiency of Perovskite Solar Cells via Inclined Fluorine Sputtering Process (Adv. Funct. Mater. 25/2023)

Increase in incident light and surface modification of the charge transport layer are powerful routes to achieve high-performance efficiency of perovskite solar cells (PSCs) by improving the short-circuit current density (J_SC) and charge transport characteristics, respectively. However, few techniques are studied to reduce reflection loss and simultaneously improve the electrical performance of the electron transport layer (ETL). Herein, an inclined fluorine (F) sputtering process to fabricate high-performance PSCs is proposed. The proposed process simultaneously implements the antireflection effect of F coating and the effect of F doping on a TiO₂ ETL, which increases the amount of light transmitted into the PSC due to the extremely low refractive index (≈1.39) and drastically improves the electrical properties of TiO₂. Consequently, the J_SC of the F coating and doping perovskite solar cell (F-PSC) increased from 25.05 to 26.01 mA cm⁻², and the power conversion efficiency increased from 24.17% to 25.30%. The unencapsulated F-PSC exhibits enhanced air stability after 900 h of exposure to ambient environment atmosphere (30% relative humidity, 25 °C under dark condition). The inclined F sputtering process in this study can become a universal method for PSCs from the development stage to commercialization in the future.     

Efficient Perovskite Hybrid Solar Cells via Ionomer Interfacial Engineering

The surface of the solution‐processed methylammonium lead tri‐iodide (CH₃NH₃PbI₃) perovskite layer in perovskite hybrid solar cells (pero‐HSCs) tends to become rough during operation, which inevitably leads to deterioration of the contact between the perovskite layer and the charge‐extraction layers. Moreover, the low electrical conductivity of the electron extraction layer (EEL) gives rises to low electron collection efficiency and severe charge carrier recombination, resulting in energy loss during the charge‐extraction and ‐transport processes, lowering the efficiency of pero‐HSCs. To circumvent these problems, we utilize a solution‐processed ultrathin layer of a ionomer, 4‐lithium styrenesulfonic acid/styrene copolymer (LiSPS), to re‐engineer the interface of CH₃NH₃PbI₃ in planar heterojunction (PHJ) pero‐HSCs. As a result, PHJ pero‐HSCs are achieved with an increased photocurrent density of 20.90 mA cm^?2, an enlarged fill factor of 77.80%, a corresponding enhanced power conversion efficiency of 13.83%, high reproducibility, and low photocurrent hysteresis. Further investigation into the optical and electrical properties and the thin‐film morphologies of CH₃NH₃PbI₃ with and without LiSPS, and the photophysics of the pero‐HSCs with and without LiSPS are shown. These demonstrate that the high performance of the pero‐HSCs incorporated with LiSPS can be attributed to the reduction in both the charge carrier recombination and leakage current, as well as more efficient charge carrier collection, filling of the perforations in CH₃NH₃PbI_3, and a higher electrical conductivity of the LiSPS thin layer. These results demonstrate that our method provides a simple way to boost the efficiency of pero‐HSCs.