Polymer Solar Cells
In article number 2000538, Harald Ade, Guillaume Wantz, and co‐workers develop novel, cost‐effective ternary polymer solar cells printed in semi‐industrial conditions from a relatively benign ink, which do not require any further processing. These solar cells show good stability and efficiency due to balanced charge carrier mobilities achieved by optimizing the composition and morphology.
A methylammonium chloride (MACl) additive is used to synthesize FA1–x MA x PbI3 films. The best molar fraction of this additive is determined. The MA content in thin films actually used in solar cells is x = 0.06. This amount is thermodynamically the best for the stabilization of this highly efficient perovskite. The perovskite solar cell achieves a stabilized power conversion efficiency as high as 22.06%.
Nowadays, complex chemistry and precursor solution compositions are developed to stabilize hybrid perovskite films and boost the efficiency of perovskite solar cells (PSCs). In this context, determining the actual composition of these layers, especially in organic cations, and understanding the chemistry behind is challenging. Herein, the introduction of methylammonium (MA+) in formamidinium lead iodide (FAPbI3) 3D perovskite is considered to stabilize the α‐phase, whose quantity must be minimized to reduce the material hydrophilicity and its possible destabilization by degassing. The key effects of methylammonium chloride (MACl) additive on the growth of FA1–x MA x PbI3 perovskite layers are studied. Liquid nuclear magnetic resonance (NMR) is used to analyze the photovoltaic layers. NMR peaks and their origin are identified. The MA and FA content in films actually used in PSCs is reliably measured and prepared over a large additive molar concentration ratio. x is quantified at 0.06 ± 0.01 for pure films, which corresponds to the best entropic compound stabilization. It results in PSCs with a stabilized power conversion efficiency as high as 22.06%. These PSCs are shown to be highly stable under solar irradiation and high moisture.
The optimized ratio of chlorinated organic salt benzyltriethylammonium chloride ([BZTAm]Cl) is helpful for the interfacial defect passivation at the perovskite/PC61BM interface. The corresponding perovskite film treatment produces high‐quality film, suppresses nonradiative recombination, and promotes the energy levels matching, which results in remarkably improved device performance and environmental stability.
In perovskite solar cells (PSCs), the interfaces between perovskite film and charge transport layers have an enormous influence on the device performance and stability. Recently, it has been proven that the surface defect passivation of perovskite layer is an effective strategy to improve the device efficiency. Herein, an organic ammonium salt benzyltriethylammonium chloride ([BZTAm]Cl) is used as an ultra‐thin modification layer in perovskite films in MAPbI3 PSCs for passivating the surface defects. The obtained results demonstrate that the [BZTAm]Cl modifier improves the crystallization/morphology of perovskite film and effectively aligns the energy levels with the corresponding charge‐transporting layers, suppressing the nonradiative recombination and reducing the trap state density. As a result, a champion device efficiency of 20.45% is achieved for optimized concentration of [BZTAm]Cl in comparison with 17.87% for the control device. Moreover, the unencapsulated device presents a good long‐term stability after aging in an ambient environment with 40–50% relative humidity conditions for 30 days.
Exposing metal halide perovskite films to humid air under illumination induces subbandgap defect states and increases trap‐assisted carrier recombination, while in the dark, neither are the carrier dynamics changed nor are subbandgap defect states formed. Thus, light‐activated defect formation is the origin of photovoltage losses, caused by degradation of the perovskite/spiro‐OMeTAD interface in n–i–p metal halide perovskite solar cells.
Metal halide perovskites exhibit outstanding optical and electronic properties, but are very sensitive to humidity and light‐soaking. In this work, the photophysics of perovskites that have been exposed to such conditions are studied and, in this context, the impact of excess lead iodide (PbI2) is revealed. For exposed samples, the formation of subbandgap states and increased trap‐assisted recombination is observed, using highly sensitive absorption and time‐resolved photoluminescence (TRPL) measurements, respectively. It appears that such exposure primarily affects the perovskite surface. Consequently, on n–i–p device level, the spiro‐OMeTAD/perovskite interface is more rapidly affected than its buried electron‐collecting interface. Moreover, both stoichiometric and nonstoichiometric MAPbI3‐based solar cells show reduced device performance mainly due to voltage losses. Overall, this study brings forward key points to consider in engineering perovskite solar cells with improved performance and material stability.
A novel perovskite‐compatible carbon electrode based on low polar alkane solvent decreases the defect at CsPbI2Br/carbon interface and hinders moisture in the atmosphere. The champion device obtains a power conversion efficiency (PCE) of 13.16% and provides outstanding stability with a PCE maintaining 93% of the initial value after 1000 h under a humidity of 30–40% without additional encapsulation.
Carbon electrodes are a promising alternative to metal electrodes in the access of high‐stable and low‐cost perovskite solar cells (PSCs). However, polar components (including cyclohexanone, terpineol, etc.) in commercial carbon pastes for carbon electrodes usually corrode perovskite materials, thereby deteriorating the photovoltaic performance of the resulting solar cells. Therefore, the development of perovskite‐compatible carbon pastes and carbon electrodes is of great significance in obtaining high‐performance carbon‐based PSCs. Herein, carbon pastes based on low polar alkane solvents are developed for perovskite‐compatible carbon electrode (PCCE) in the construction of carbon‐based CsPbI2Br PSCs. The optimized cells based on PCCE offer a champion efficiency of 13.16% (J SC = 14.33 mA cm−2, V OC = 1.22 V, and fill factor (FF) = 0.75), which is remarkably higher than that of commercial carbon paste‐derived counterparts (11.51%). Even without encapsulation, CsPbI2Br PSCs based on PCCE maintain over 93% of their initial efficiency in an air atmosphere with a humidity of 30–40% for over 1000 h.
A modified monomolecular layer strategy (m‐MLS) enables high‐quality perovskite films formation on the hydrophobic polymer hole transporting layer (HTL), and minimizes the ohmic loss induced by the HTL. The perovskite solar cells (PSCs) based on m‐MLS‐modified HTL (F‐PSCs) give a superior reproducibility and a champion efficiency of 19.7% with a fill factor of over 80%.
The hole transport materials that interact with the indium tin oxide (ITO) surface can be processed into monomolecular layers (MLs), which often exhibit different surface and electronic properties than their thin‐film counterparts. Herein, it is found that poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA) films (R‐PTAA) can be easily processed into ML (M‐PTAA) due to the van der Waals interaction between ITO and PTAA. However, compared with R‐PTAA, the work function (WF) and conductivity of M‐PTAA are simultaneously reduced by the charge transfer at the ITO/PTAA interface. To address this issue, a modified monomolecular layer strategy (m‐MLS) is developed, where a small amount of 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) is introduced to enhance the interaction force between ITO and PTAA. PTAA treated by m‐MLS (F‐PTAA) has a hydrophilic physical surface, closely matching electronic energy level with the perovskite layer and smaller bulk resistance. As a result, the efficiency and reproducibility of perovskite solar cells (PSCs) are substantially improved. PSCs based on F‐PTAA demonstrated the highest power conversion efficiency (PCE) of 19.7% with a fill factor of over 80%. This study inspires the development of novel interface modification materials, and provides a simple and convenient direction for the fabrication of high‐performance and reproducible inverted PSCs with high fill factors.
The commercially available pyridinedicarboxylic acid (PDA) molecule with one pyridine and two carboxylic acid groups is used as a passivating agent to cure the defects at both the surfaces and grain boundaries of MAPbI3 perovskites. A champion power conversion efficiency (PCE) approaching 19% with optimized long‐term stability and thermal stability is achieved in PDA‐passivated perovskite solar cells (PSCs).
Electronic defects and grain boundaries of perovskite films will significantly deteriorate both the efficiency and the stability of perovskite solar cells (PSCs), and various methods aimed to reduce these defects are proposed. Herein, an organic solid molecule of pyridinedicarboxylic acid (PDA) with one pyridine and two carboxylic acid groups is used as a passivating agent to cure the defects by regulating the perovskite microstructures in a multiple manner. The defects located at both the surfaces and grain boundaries of polycrystalline MAPbI3 perovskites are simultaneously passivated through the multiple coordination effects between the used functional groups and uncoordinated Pb2+, regardless of the substitution sites of the carboxylic acid and pyridine. Impressively, the PDA‐passivated inverted PSCs achieve remarkably enhanced power conversion efficiencies (PCEs) from 16.43% to nearly 19% and maintain over 90% of its original PCE after 1300 h under an inert environment. These findings indicate that the commercially available PDA molecule emerges as an efficient passivating agent of perovskite defects capable of stimulating the combined effects of the multiple functional groups, which is highly promising for the practical applications of PSCs with both high efficiency and good stability.
Pinhole‐free perovskite films with large grains are fabricated in ambient air by a spinning–bathing–spinning method. The effects of moisture on the formation of I‐dominant grain and Cl‐enriched boundaries and surfaces in the perovskite films are revealed, which enable the air‐processed perovskite solar cells with a high efficiency of more than 20%.
Metallic halide perovskite films are usually fabricated in inert environment due to their high sensitivity to moisture and oxygen. However, the fabrication process in the strictly controlled environment is not economical for mass production. Therefore, the fabrication of high‐quality perovskite films in ambient air is more practical for optoelectronic devices. Herein, a spinning–bathing–spinning (SBS) method is demonstrated to deposit pinhole‐free perovskite films with large grains in ambient air for solar cells. The effect of moisture on the rapid crystallization and grain coarsening can be suppressed using this SBS method. Furthermore, the moisture is found to encourage the halogen separation in the perovskite films when using PbI2–PbCl2 as the lead halide precursor, resulting in the formation of I‐dominant perovskite grains and Cl‐enriched boundaries and surface in the films. The Cl‐enriched grain boundaries and film surface, which mainly originate from the confined methylammonium chloride (MACl), can passivate defects and prevent further damage from moisture and oxygen. This spontaneous inner‐to‐outside passivation enables the air‐processed perovskite solar cells with the high power conversion efficiencies of more than 20% and improved stability.
Ternary organic solar cells (TOSCs) are developed through synergizing small‐molecule donor BPR‐SCl into PM6:Y6 host binary blend, which effectively addresses the trade‐off between photovoltage and photocurrent of regular bulk heterojunction OSCs. An optimal power conversion efficiency of 16.74% is obtained for TOSCs, accounting for 10% and 70% improvements over those of pristine PM6:Y6 and BPR‐SCl:Y6 binary devices, respectively.
Despite the impressive progress that has been achieved for organic solar cells (OSCs) in recent years, challenges remain for OSCs due to the presence of the trade‐off between photovoltage and photocurrent that sets limitation on the performance enhancement of regular bulk heterojunction (BHJ) blends. Herein, a new small‐molecule (SM) donor, BPR‐SCl, with the deep‐lying highest occupied molecular orbital and strong crystallinity has been developed, which, as the third component, is synergized with PM6:Y6 host blend. The introduction of BPR‐SCl enhances molecular packing, exciton dissociation, as well as charge mobilities of ternary blends, yielding simultaneous enhancement of open‐circuit voltage, short‐circuit current density, and fill factor of ternary OSCs (TOSCs). As a result, an optimal power conversion efficiency (PCE) of 16.74% is obtained for TOSCs with 25 wt% BPR‐SCl, accounting for 10% and 70% improvements over those of pristine PM6:Y6 and BPR‐SCl:Y6 binary devices, respectively. Overall, herein, it is demonstrated that the design of SM donor as the third component is effective in achieving high‐performance TOSCs.
An electron transport layer is one of the essential components for most of the efficient perovskite devices. This review focuses on 2D materials as the electron transport layer in perovskite solar cells with tunable work function and high carrier mobility.
Low‐temperature solution‐processed perovskite solar cells (PSCs) based on organic–inorganic hybrid perovskites have emerged as a low‐cost and high‐efficiency thin‐film photovoltaic technology. The reported power conversion efficiency (PCE) of laboratory produced PSCs with an active area of less than 0.1 cm2 has already exceeded 25%, which, however, decreases significantly to about 16% for a large device area of about 100 cm2. Therefore, the scalability has become one of the most significant limits on successful commercialization of perovskite photovoltaics. This includes realizing a homogenous and compact electron transport layer (ETL), facing with issues of defects, energy level mismatch, and high‐temperature annealing requirements. Therefore, an exploration of effective and low‐cost charge transport materials is crucial for scalable fabrication of highly efficient perovskite devices. The 2D materials have drawn wide attention in the PSC community with tunable bandgap and high carrier mobility. So far, the search for a wide range of novel 2D materials for use in PSCs has documented considerable progress; however, a lot remains to be done in this field. This review summarizes recent advancements in the application of emerging 2D materials as effective ETL, thus providing direction for future development toward efficient and large‐scale perovskite devices.
Perovskite solar cells with the 2D passivation layer display excellent photovoltaic performance and superior stability via introducing hydrophobic alkyl molecules with polyfunctional groups.
The charges stuck in trap sites hinder charge transport and lead to V oc below the radiative limit, which seriously restrict the performance and stability of organic–inorganic halide perovskite solar cells (PSCs). Chemical passivation is an effective method to reduce defects and suppress nonradiative recombination. Herein, a new passivation molecule l‐cysteine methyl ester hydrochloride (CME) with thiol and ester groups is designed to modify the interface between the perovskite layer and hole transport layer (HTL). It reveals that thiol possesses outstanding moisture resistance and ester suppresses nonradiative recombination by coordinating with undercoordinated Pb2+. Furthermore, the 2D modified layer at the grain boundaries and surface passivates surface defects and promotes hole extraction. As a result, the CME device achieves the highest PCE of 20.33% with an enhanced open‐circuit voltage (V oc) of 1.11 V. Due to the barrier of highly hydrophobic 2D perovskites, the modified devices show excellent stability while exposed to humidity and high‐temperature environment. A facile and effective strategy to design organic molecular structures with polyfunctional groups to passivate trap‐assisted nonradiative recombination at the surface and grain boundaries is provided.
The defect landscape in metal–halide perovskites is described. This Review highlights the promise of the compounds, explains defects as an outstanding problem, and discusses the background of defects, methods to probe defects, and various passivation strategies used successfully to date.
The rise of hybrid metal–halide perovskites as potential solar energy materials has revolutionized research on next‐generation solar cells. According to recent studies, the rationale behind such success is the rich defect physics of materials. Studies on the origin of different types of prevailing defects, their formation, and mechanism of defect passivation have hence become decisive avenues. Herein, the possible origins of defects and different defect analysis techniques in hybrid halide perovskites are discussed. While initiating the discussion with the archetypal methylammonium lead halide, perovskites beyond the conventional ABX3 structure are included. In this direction, some major advancements to date on defect formation in the bulk of hybrid halide perovskites, at the grains and grain boundaries, are summarized. Numerous effective methods to passivate the defects and the adverse effect of defects on device efficiency are further highlighted. Hence, the prospect of defect engineering in perovskite materials is pointed toward improving the power conversion efficiency and long‐term stability of perovskite solar cells (PSCs). The discussion rightfully addresses that the in‐depth exploration of defect engineering is anticipated to have a gigantic impact toward the achievement of predicted efficiency in metal–halide PSCs.
Defect passivation is an effective strategy to adjust the energy band structure, reduce the density of defect states, and suppress the nonradiative recombination of carriers. Herein, the recent progress in the passivation strategy for perovskite films is summarized and the development direction of passivation strategies to further improve the performance of perovskite solar cells (PSCs) is proposed.
Organic–inorganic halide perovskite photovoltaic devices have advanced rapidly in recent years, and the photoelectric conversion efficiency of perovskite solar cells (PSCs) has exceeded 25%. However, the defects from the crystallization process become nonradiation recombination centers and hinder the performance and the stability of PSCs. Defect passivation by tuning grain size and grain boundary (GB) is an effective strategy to reduce the defects on GBs and film surface. Herein, recent progress in the passivation strategy for perovskite films is summarized, including nonstoichiometric passivation, iodide vacancies filling, dimensional engineering, passivation with crosslink, physical passivation, and other passivation methods. These passivation strategies play an important role in improving the quality of perovskite films, adjusting the energy band structure, reducing the density of defect states, and suppressing the nonradiative recombination of carriers. Finally, this review puts forward the development direction of passivation strategies to further improve the performance of PSCs.
A small molecule of 18C6 is introduced into the perovskite precursor for elongating the antisolvent dripping window from 2 to 20 s, achieving a high‐quality and reproducible perovskite film.
Although perovskite solar cells (PSCs) have exhibited a high‐power conversion efficiency, the reproducibility of high‐quality perovskite films is still a big challenge for large‐scale flexible devices. One reason is the super narrow antisolvent dripping window, the other one is the difficulty in controlling the secondary phases. Herein, 18C6 is introduced into the perovskite precursor to achieve a high‐quality and reproducible large‐scale (7 × 7 cm2) flexible perovskite film by enlarging the antisolvent dripping window from 2 to 20 s, with an average efficiency of 13.33% (best 15.80%). Moreover, from the in situ grazing‐incidence wide‐angle X‐ray scattering result, the 2H phase perovskite is highly suppressed with the additive of 18C6. The generality of the approach is also demonstrated in other antisolvents such as ethyl acetate. This finding provides an innovative solution to the realization of repeatable, large‐scale solution fabrication of PSCs.
A facile and efficient strategy of gradient doping is adopted for optimizing inverted CsPbI2Br‐based perovskite solar cells (PeSCs) using a bicationic iodine salt, namely BFBAI2, as the dopant. The doped PeSCs exhibit significantly improved photovoltaic performance and stability, which is attributed to efficient defect passivation and enhanced electric field upon the gradient doped BFBAI2.
Cesium‐based all‐inorganic perovskites (PVKs) are prized for their high thermal stability and wide bandgap suitable for the top layer of tandem solar cells. To further boost the photovoltaic performance of inorganic PVK solar cells (PeSCs), a variety of strategies aiming to either passivate defects or enhance the electric field are developed. Nevertheless, a double‐aim strategy is less explored. Herein, a facile strategy of gradient doping is adopted for optimizing the inverted CsPbI2Br PeSCs. Particularly, a bicationic iodine salt, namely 2,2′‐bis(trifluoromethyl)‐[1,1′‐biphenyl]‐4,4′‐diamine iodine (BFBAI2), is used to realize gradient doping in PVK and ZnO layers, respectively. As a result, the inverted PeSCs with the doped PVK/ZnO bilayer deliver improves power conversion efficiency (PCE) up to 14.38% along with enhanced device stability under ambient or thermal aging conditions, greatly surpassing the pristine devices. The improvements are attributed principally to the low‐defect PVK layer as well as enhanced electric field across the inverted PeSCs upon gradient doping. This work thus demonstrates an efficient bifunctional strategy toward highly efficient and stable CsPbI2Br PeSCs with inverted configuration.
PbS nanoplatelets (NPLs) used as an external additive with (100) preferential crystal orientation improve a formamidinium‐based perovskite material and solar cell stability. A stable current density of 23 mA cm−2 for 4 months is recorded along with an improved reproducibility, demonstrating the potential of the interaction between the (100) facets of the NPLs and the perovskite α‐phase.
Formamidinium‐based perovskite solar cells (PSCs) present the maximum theoretical efficiency of the lead perovskite family. However, formamidinium perovskite exhibits significant degradation in air. The surface chemistry of PbS has been used to improve the formamidinium black phase stability. Here, the use of PbS nanoplatelets with (100) preferential crystal orientation is reported, to potentiate the repercussion on the crystal growth of perovskite grains and to improve the stability of the material and consequently of the solar cells. As a result, a vertical growth of perovskite grains, a stable current density of 23 mA cm−2, and a stable incident photon to current efficiency in the infrared region of the spectrum for 4 months is obtained, one of the best stability achievements for planar PSCs. Moreover, a better reproducibility than the control device, by optimizing the PbS concentration in the perovskite matrix, is achieved. These outcomes validate the synergistic use of PbS nanoplatelets to improve formamidinium long‐term stability and performance reproducibility, and pave the way for using metastable perovskite active phases preserving their light harvesting capability.
Field‐assisted charge generation upon illumination of neat SubNc sandwiched between charge selective electrodes is shown to be efficient, and results in low voltage losses with high quantum efficiencies. The described effects can play an important role in the current state‐of‐the‐art, high efficiency organic solar cells with low driving force for charge generation.
Efficient charge generation in organic semiconductors usually requires an interface with an energetic gradient between an electron donor and an electron acceptor in order to dissociate the photogenerated excitons. However, single‐component organic solar cells based on chloroboron subnaphthalocyanine (SubNc) have been reported to provide considerable photocurrents despite the absence of an energy gradient at the interface with an acceptor. In this work, it is shown that this is not due to direct free carrier generation upon illumination of SubNc, but due to a field‐assisted exciton dissociation mechanism specific to the device configuration. Subsequently, the implications of this effect in bilayer organic solar cells with SubNc as the donor are demonstrated, showing that the external and internal quantum efficiencies in such cells are independent of the donor‐acceptor interface energetics. This previously unexplored mechanism results in efficient photocurrent generation even though the driving force is minimized and the open‐circuit voltage is maximized.
Herein, a comparison among all candidates to replace indium tin oxide electrodes to fabricate flexible organic solar cells (FOSCs) is discussed, while focusing on the benefits and problems of silver nanowires (AgNWs) electrodes. All progress in addressing the issues that AgNWs present, along with publications regarding the fabrication of highly efficient FOSCs based on AgNWs, are summarized.
In this review, silver nanowires (AgNWs) are introduced, as the primary material to replace indium tin oxide for fabricating cost‐effective flexible organic solar cells (FOSCs), because of their remarkable solution‐processing, flexibility, transparency, and conductivity, along with their enhanced properties in terms of light‐scattering, plasmonic effects, and transmittance in the near infrared region. The drawbacks of AgNWs, particularly their high roughness, low adhesion to substrates, atmospheric corrosion, degradation under UV and visible light, and poor contact at wire‐wire junctions, must be resolved prior to their use in commercial FOSCs applications. Herein, comparisons among all candidates (e.g., graphene, carbon nanotubes, metal grids, and conducting polymers), along with a report of all recent progress in addressing these issues for using AgNWs as flexible transparent conductive electrodes (TCEs), are discussed. In addition, recent publications on the fabrication of highly efficient FOSCs based on AgNWs are summarized. The discussed issues regarding AgNWs‐TCEs apply not only to FOSCs, but can be generalized for other third‐generation solar cells, such as perovskite solar cells and dye‐sensitized solar cells; additionally, they provide insight for other optoelectronic applications, such as organic light‐emitting diodes, liquid crystal displays, smart windows, touch panels, and heaters.
Great progress has been made in the field of inorganic CsPbX3 perovskite solar cells (PSCs), and organic molecule engineering has been playing a vital role in improving device performance. In this review, the roles of organic molecules in inorganic CsPbX3 PSCs are systematically reviewed and discussed, and future research directions are suggested to further improve the performance of inorganic PSCs.
Over 25% efficiencies have been achieved by organic–inorganic hybrid perovskite solar cells (PSCs). However, their practical applications are limited by the instability of the hybrid perovskite materials. Replacing hybrid perovskites with inorganic CsPbX3 perovskites shows great promise to address the above issue and much progress has been made. To achieve high efficiency and stable inorganic CsPbX3 PSCs, organic molecular engineering has been playing a vital role. Herein, the progress of the organic molecular engineering in inorganic CsPbX3 PSCs is systematically reviewed. First, structure evolution induced by organic molecular engineering for inorganic CsPbX3 perovskites is demonstrated. Then, organic molecular engineering in CsPbX3 PSCs is categorized and reviewed (alloying in perovskite structures, as sacrificial agents, forming 2D structures, and modifying surfaces and interfaces). Finally, future research directions are suggested to further improve the performance of inorganic PSCs.
A series of PM6 polymers with different weight‐average molecular weights and polydispersity index are synthesized, and the effects of PM6 polymerization degree on the efficiency and degradation behaviors of the Y6‐based photovoltaic system are systematically studied.
The degree of polymerization can cause significant changes in the blend microstructure and physical mechanism of the active layer of non‐fullerene polymer solar cells, resulting in a huge difference in device performance. However, the diversity of stability issues, including photobleaching stability, storage stability, photostability, thermal stability, and mechanical stability, and more, poses a challenge for the degree of polymerization to comprehensively address the trade‐off between device efficiency and stability and reasonably evaluate the application potential of polymer materials. Herein, a series of PM6 polymers with different weight‐average molecular weights (M w) and polydispersity index (PDI) are synthesized. The effects of the degree of PM6 polymerization on the efficiency and degradation behaviors of the photovoltaic systems based on Y6 as acceptor are investigated systematically. The findings regarding stability issues, together with the trade‐offs in the efficiency‐stability gap, formulate a complete guideline for the material design and performance evaluation in a way that relies much less on trial‐and‐error efforts.
The recent progress in potentially low‐cost polythiophene:nonfullerene‐based solar cells is reviewed from the viewpoints of molecular engineering and morphology control. The molecular design strategies of polythiophenes and nonfullerene acceptors are discussed, followed by the recent achievements in understanding and controlling the morphology of polythiophene:nonfullerene blends. Finally, the future challenges are delineated for advancing the commercial applications of polythiophenes in solar cells.
With the advances in organic photovoltaics (OPVs), the development of low‐cost and easily accessible polymer donors is of vital importance for OPV commercialization. Polythiophene (PT) and its derivatives stand out as the most promising members of the photovoltaic material family for commercial applications, owing to their low cost and high scalability of synthesis. In recent years, PTs, paired with nonfullerene acceptors, have progressed rapidly in photovoltaic performance. This Review gives an overview of the strategies in designing PTs for nonfullerene OPVs from the perspective of energy level modulation. A survey of the typical classes of nonfullerene acceptors designed for pairing with the benchmark PT, i.e., poly(3‐hexylthiophene) (P3HT) is also presented. Furthermore, recent achievements in understanding and controlling the film morphology for PT:nonfullerene blends are discussed in depth. In addition to the effects of molecular weight and blend ratio on film morphology, the crucial roles of miscibility between PT and nonfullerene and processing solvent in determining film microstructure and morphology are highlighted, followed by a discussion on thermal annealing and ternary active layers. Finally, the remaining questions and the prospects of the low‐cost PT:nonfullerene systems are outlined. It is hoped that this review can guide the optimization of PT:nonfullerene blends and advance their commercial applications.
The present work deconvolutes the electronic processes in organic solar cells under short‐circuit conditions by combining readily available experimental methods (current‐voltage characteristics, external quantum efficiency) with optical simulations. The proposed method allows the quantification of geminate recombination, to determine the mobility‐lifetime product, and to quantify extraction. The applicability of this new approach is demonstrated in three different organic photovoltaic systems.
The short‐circuit current (J sc) of organic solar cells is defined by the interplay of exciton photogeneration in the active layer, geminate and non‐geminate recombination losses and free charge carrier extraction. The method proposed in this work allows the quantification of geminate recombination and the determination of the mobility‐lifetime product (µτ) as a single integrated parameter for charge transport and non‐geminate recombination. Furthermore, the extraction efficiency is quantified based on the obtained µτ product. Only readily available experimental methods (current‐voltage characteristics, external quantum efficiency measurements) are employed, which are coupled with an optical transfer matrix method simulation. The required optical properties of common organic photovoltaic (OPV) materials are provided in this work. The new approach is applied to three OPV systems in inverted or conventional device structures, and the results are juxtaposed against the µτ values obtained by an independent method based on the voltage–capacitance spectroscopy technique. Furthermore, it is demonstrated that the new method can accurately predict the optimal active layer thickness.
Novel asymmetric alkoxy and alkyl substitutions on the well‐known nonfullerene acceptor Y6 yield a molecule named Y6‐1O, and its photoelectric properties and photovoltaic performance are systematically compared with the two related symmetric molecules (Y6 and Y6‐2O), which suggests that this design strategy is promising and effective.
In this paper, a strategy of asymmetric alkyl and alkoxy substitution is applied to state‐of‐the‐art Y‐series nonfullerene acceptors (NFAs), and it achieves great performance in organic solar cell (OSC) devices. Since alkoxy groups can have a significant influence on the material properties of NFAs, alkoxy substitution is applied to the Y6 molecule in a symmetric manner. The resulting molecule (named Y6‐2O), despite showing improved open‐circuit voltage (V oc), yields extremely poor performance due to low solubility and excessive aggregation properties, a change that is due to the conformational locking effect of alkoxy groups. In contrast, asymmetric alkyl and alkoxy substitution on Y6, yields a molecule named Y6‐1O that can maintain the positive effect of V oc improvement and obtain reasonably good solubility. The resulting molecule Y6‐1O enables highly efficient nonfullerene OSCs with 17.6% efficiency and the asymmetric side‐chain strategy has the potential to be applied to other NFA‐material systems to further improve their performance.
Metal‐organic frameworks have great potential to be used as photocatalysts due to their ability to combine photosensitizers with catalytic centers within a porous structure. Charge separation and charge carrier mobility are crucial steps controlling the conversion efficiency in photocatalysis. Herein, a computational approach is presented to quantify these two characteristics in terms of charge transfer numbers and effective mass values.
Metal‐organic frameworks (MOFs) are highly versatile materials owing to their vast structural and chemical tunability. These hybrid inorganic–organic crystalline materials offer an ideal platform to incorporate light‐harvesting and catalytic centers and thus, exhibit a great potential to be exploited in solar‐driven photocatalytic processes such as H2 production and CO2 reduction. To be photocatalytically active, UV–visible optical absorption and appropriate band alignment with respect to the target redox potential is required. Despite fulfilling these criteria, the photocatalytic performance of MOFs is still limited by their ability to produce long‐lived electron–hole pairs and long‐range charge transport. Here, a computational strategy is presented to address these two descriptors in MOFs and to translate them into charge transfer numbers and effective mass values. The approach is applied to 15 MOFs from the literature that encompass the main strategies used in the design of efficient photocatalysts including different metals, ligands, and topologies. The results capture the main characteristics previously reported for these MOFs and enable to identify promising candidates. In the quest of novel photocatalytic systems, high‐throughput screening based on charge separation and charge mobility features are envisioned to be applied in large databases of both experimentally and in silico generated MOFs.
The photoinitiator bifunctional bis‐benzophenone is introduced into non‐fullerene solar cells as a multifunctional solid additive for the first time. The doping of this solid additive could not only modify the polymer order and firm morphology of active layer to improve device performance, but also to achieve better reproducibility, thickness insensitivity, and thermal stability for the non‐fullerene solar cells.
Simultaneously improving efficiency and stability is critical for the commercial application of non‐fullerene acceptor polymer solar cells (NFA‐PSCs). Multifunctional solid additives have been considered as a potential route to tune the morphology of the active layer and optimize performance. In this work, photoinitiator bifunctional bis‐benzophenone (BP‐BP) is used as a solid additive, replacing solvent additives, in the PBDB‐T:ITIC NFA system. With the addition of BP‐BP, the intermolecular π–π stacking of PBDB‐T and morphology is improved, leading to more balanced carrier transport and more effective exciton dissociation. Devices fabricated with BP‐BP show a power conversion efficiency (PCE) of 11.89%, with enhanced short‐circuit current (J sc), and fill factor (FF). Devices optimized with BP‐BP show excellent reproducibility, insensitivity to thickness, and an improved thermal stability under atmospheric conditions without encapsulation. This work provides a new strategy for the application of solid additives in NFA‐PSCs.
9.5% semitransparent solar cells with ultrahigh transmission in the visible range (50% AVT) are fabricated via inkjet printing. The effect of different photoactive layer ink solvents on the vertical stratification and performance is explored. The formulation of transport layer inks compatible with highly hydrophobic active layers and with scalable printing processes permits the use of a semitransparent electrode grid.
New polymer donors and nonfullerene acceptors have elevated the performance and stability of solar cells to higher grounds. To achieve their full potential, they require their adaptation to scalable and cost‐effective solution manufacturing techniques for large area deposition. Likewise, formulating scalable solution‐based transport layer inks that are compatible with the photoactive layer is imperative. This manuscript reports the full integration of solution‐based transport layers and electrode alongside a PTB7‐Th:IEICO‐4F bulk heterojunction in inverted architecture through inkjet‐printing, resulting in power conversion efficiencies up to 12.4% opaque devices and 9.5% semitransparent devices with average visible transmittance values of 50.1%, including hole transport layer. The wetting envelope of the highly‐hydrophobic photoactive layer alongside the surface energy of candidate solutions and solvents allows the formulation of thick transport layer inks that are compatible with the drop‐on‐demand inkjet‐printing process and yield uniform and homogenous films. Moreover, the surface energy components of the donor and acceptor serves as a fingerprint to assess the vertical stratification of the photoactive layer with the inclusion of different solvents. This methodology addresses a scale‐up bottleneck of solution‐based transport layers for high‐efficiency organic cells, enabling its adaptation to high‐throughput techniques including slot‐die and roll‐to‐roll coating.
Copper oxide by pulsed‐chemical vapor deposition is introduced as a sputter buffer for semitransparent perovskite solar cells. Semitransparent devices with copper oxide buffers result in stable semitransparent devices, possibly explained by better control of the morphology and stoichiometry with this deposition technique.
In semitransparent perovskite solar cells with n–i–p configuration, thermal evaporation is the common method to deposit the sputter buffer material, such as molybdenum oxide and tungsten oxide. Buffer layers are especially necessary when using organic hole transporting layers, as they are more susceptible to get damaged when sputtering the top transparent conducting oxide. However, there is a limited selection of possible materials and limited control of the materials properties by thermal evaporation, which leads to inefficient protection against sputtering and poor air stability. While there have been well‐established buffer layers by atomic layer deposition, including tin oxide, for p–i–n structured semitransparent perovskite solar cells, this is not the case for n–i–p structured devices. Here, copper oxide is demonstrated by pulsed‐chemical vapor deposition incorporated into perovskite solar cells for the sputter buffer layer, which result in stable encapsulated semitransparent devices maintaining over 95% of the maximum efficiency under AM 1.5 G at maximum power point tracking for 150 h without any temperature control.
The working on the interfacial engineering toward efficient and stable perovskite solar cells (PSCs) is demonstrated. The key role of 2D interface modification for efficient and stable perovskite solar cells is highlighted, especially for the energy band alignment of PSCs. This paper sheds light on the significance of 2D interface modification in PSCs and provides critical guidance for development of highly efficient and stability PSCs.
Interfacial engineering is essential for facilitating carrier separation, charge extraction, and enhancing the stability in organic–inorganic perovskite solar cells (PSCs). Herein, a facile and effective method is demonstrated not only to tune the electronic performance of electron transporting layer (ETL) but also to passivate the defects at the interface between the ETL and perovskite. On the top of the tin(IV) oxide (SnO2) ETL, butylammonium chloride (BACl) and lead(II) iodide (PbI2) are introduced as interface to modify the ETL/perovskite interface. The PSCs with interface modified exhibit a power conversion efficiency (PCE) of 21.15%, compared to 18.33% for the device without interface modified. Such enhancement in efficiency is mainly attributed to a better energy band alignment, and the quality of perovskite films is improved through the interface modification, thus enhancing photogenerated charge extraction and leading to low charge carrier recombination at the interface of ETL/perovskite. Furthermore, the device with interface modified exhibits significant stability. This work provides an alternative strategy on the ETL/perovskite interface to obtain highly stable and efficient PSCs.
A low‐temperature fabrication strategy for TiO2 nanopillars electron transporting layer (ETL) is developed by one‐step glancing angle deposition (GLAD) for efficient and flexible perovskite solar cells. The ETL consisting of a bottom layer of planar TiO2 and an upper layer of TiO2 nanopillars array, which are fabricated both by GLAD consecutively, can significantly improve the device performance and flexibility.
Organometal halide perovskite solar cells (PSCs) are promisingly applied to flexible solar cells because of the high power conversion efficiency (PCE) and intrinsic softness of perovskite materials. In the most efficient PSCs, mesoporous TiO2 generally functions as the electron transporting layer. However, the mesoporous TiO2 is typically generated through high‐temperature thermal annealing that is not suitable for producing flexible PSCs. In this work, TiO2 nanopillar arrays are directly deposited on flexible substrates using glancing angle deposition at low substrate temperature. The TiO2 nanopillars strongly adhere to the flexible substrates, improving light harvesting in the perovskite layers, facilitating electron extraction and transportation, and enhancing the mechanical flexibility of the PSCs. The flexible PSCs hybridized with the TiO2 nanopillars show a PCE as high as 13.3% and excellent photovoltaic stability after 500 cycles of bending at a small radius of curvature.