awn

Herein, all‐vacuum‐processed perovskite solar cells are reported in an inverted device architecture using copper (II) phthalocyanine (CuPC) as the hole transporting layer (HTL), and power conversion efficiencies (PCEs) of 20.3% and 18.68% on rigid and flexible substrates are achieved, respectively.

The fabrication of efficient perovskite solar cells (PSCs) using all‐vacuum processing is still challenging due to the limitations in the vacuum deposition of the hole transporting layer (HTL). Herein, inverted PSCs using copper (II) phthalocyanine (CuPC) as an ideal alternative HTL for vacuum processing are fabricated. After proper optimization, a PSC with a power conversion efficiency (PCE) of 20.3% is achieved, which is much better than the PCEs (16.8%) of devices with solution‐based CuPC. As it takes a long time to dissolve CuPC in the solution‐based device, the evaporation approach has better advantage in terms of fast processing. In addition, the device with the evaporated CuPC HTL indicates an excellent operational stability, showing only 9% PCE loss under continuous illumination after 100 h, better than its counterpart device. Interestingly, the device shows negligible hysteresis. As all fabrication processes are conducted at low temperatures, flexible PSCs are also fabricated on ITO/PET substrates and a PCE of 18.68% is obtained. After 200 bending cycles, the flexible device retains 87.5% of its initial PCE value, indicating its great flexibility. Herein, the role of a suitable HTL for the fabrication of all‐vacuum‐processing PSCs with great efficiency and stability is highlighted.

A series of novel n‐doped conjugated polyelectrolytes are designed and synthesized via highly efficient aldol condensation polymerization. The designed conjugated polyelectrolytes possess an electron‐deficient and rigid conjugated backbone, resulting in easy charge delocalization, enhanced n‐doping behaviors, and high conductivity. These conjugated polyelectrolytes can be used as electron transport materials to enable high‐performance nonfullerene polymer solar cells.

Doping provides an efficient strategy to control the electronic properties of organic semiconductors. However, compared with the widely reported p‐type doping protocols, n‐type doping of organic semiconductors still remains a challenge. Herein, a series of novel n‐doped conjugated polyelectrolytes (CPEs) with high doping levels and conductivity are designed. These CPEs are synthesized via a facile, metal‐free, and high‐yield aldol condensation protocol from bis‐isatin and bis‐oxindole monomers. The designed CPEs possess a n‐type electron‐deficient and rigid conjugated backbone, resulting in easy charge delocalization, enhanced n‐doping behaviors, and high conductivity. The evolution on the counterions of these CPEs further alters their n‐doping behaviors, charge transporting properties, and work function tunability, etc. By using these CPEs as electron transport materials (ETMs) for nonfullerene polymer solar cells (NF–PSCs), high power conversion efficiencies (PCEs) over 16% can be achieved when PM6:Y6 is used as the active component. Moreover, these CPEs can enable efficient NF–PSCs even if their thicknesses are up to 60 nm, indicating the potential of these CPEs as thickness‐insensitive ETMs for the fabrication of large‐area NF–PSCs.

This article introduces sodium benzenesulfonate (SBS) to modify the poly (3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) layer to improve hole extraction capacity and work function for better energy‐level alignment. As a result, the power conversion efficiency of inverted perovskite solar cells (PSCs) achieves 19.41% and maintains ≈95% after 20 days, with a V _oc up to 1.08 V.

The p‐type conducting polymer poly (3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is widely utilized as the hole transport layer (HTL) for solution‐processed planar perovskite solar cells (PSCs) with a p‐i‐n structure. However, the inverted PSCs based on PEDOT:PSS HTL suffer from deficient open‐circuit voltage (V _OC) and poor stability issues, because of the high electron affinity and acid nature of PEDOT:PSS. Herein, sodium benzenesulfonate (C₆H₅SO₃Na) (SBS) is applied to modify the PEDOT:PSS (SBS–PEDOT:PSS) layer to form a smoother surface and better energy‐level alignment with the perovskite layer. In addition, the SBS–PEDOT:PSS layer with improved hole extraction capacity and suppressed charge recombination, significantly increases the grain size and crystallinity of the MA_0.8FA_0.2PbI_3‐x Cl _x perovskite film. Consequently, the power conversion efficiency (PCE) and V _OC of the inverted PSCs are improved from 18.07% and 1.04 V to 19.41% and 1.08 V, respectively, after SBS modification. Moreover, the efficiency of the unencapsulated PSCs based on SBS–PEDOT:PSS remains above 90% after storing in ambient condition for 20 days. This research provides an accessible route to surmount the intrinsic imperfection of PEDOT:PSS and ameliorate the performance of PSCs.

A gentle butyl acrylate additive is introduced into CH₃NH₃PbI₃‐based perovskite solar cells to enhance the efficiency and stability by improving the perovskite film quality, constructing a suitable energy‐level alignment, and increasing charge carrier lifetime. As a result, the device with butyl acrylate additive delivers a superior power conversion efficiency of 20.0% and excellent moisture/thermal stability under ambient conditions.

Organic–inorganic halide perovskite solar cells (PSCs) are a new class of photovoltaic devices, which have attracted increasing interests due to high efficiency, low cost, and simple fabrication process. Although a remarkable enhancement in power conversion efficiency (PCE) has been achieved in the last decade, the stability issue, including high water/heat sensitivity and UV light‐induced degradation, has become one of the main obstacles for practical applications of PSCs. Herein, hydrophobic butyl acrylate with CO and CC groups is reported as a new additive to enhance the PCE and moisture/thermal stability of CH₃NH₃PbI₃‐based PSCs, which improves the quality of the perovskite film by promoting defect passivation, constructing a suitable energy‐level alignment, and increasing the charge carrier lifetime in PSCs. Such an additive is easily introduced into PSCs without the need for thermal treatment, which is typically required by other additives, causing a negative effect on the efficiency. The CH₃NH₃PbI₃‐based PSC with butyl acrylate additive shows a superior PCE of 20.0%, 19% higher than that of the unmodified device (16.8%), as well as much improved moisture/thermal stability. Herein, a facile way is provided to enhance the PCE and stability of CH₃NH₃PbI₃‐based PSCs simultaneously, which facilitates the commercialization of PSCs.

Organic Solar Cells

In article number 2000460, Fernando Langa, Ganesh D. Sharma, and co‐workers construct a simple and efficient ternary organic solar cell based on two well‐defined conjugated small molecules donors FG3 and FG4, and Y6 as a well‐known nonfullerene small molecule acceptor. This results in best‐performance in comparison with the binary counter parts, showing an excellent power conversion efficiency of 14.31% with a high fill factor and short‐circuit current density.

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.

Perovskite Solar Cells

In article number 2000481, Runfeng Chen and co‐workers employ commercially available pyridinedicarboxylic acid (PDA) as a novel passivating agent to cure various defects of perovskite microstructures. Benefiting from the multiple coordination feature of PDA molecules, high power conversion efficiency and good long‐term and thermal stabilities are achieved in the PDA‐passivated inverted perovskite solar cells.

The spontaneous black‐to‐yellow phase transition of cesium lead halides (CsPbX₃) after long‐time storage hinders their development in solar cells despite ever‐growing efficiencies. This review focuses on the current advances from recognizing phase transition behaviors to addressing phase instability issue of CsPbX₃ and provides potential avenues for further enhancing stability of CsPbX₃ based on current understandings.

Cesium lead halide (CsPbX₃) perovskite solar cells have gained considerable attention for their rapid evolution to over 19% power conversion efficiency. Despite high chemical stability, the spontaneous phase transition from desired black phase to nonperovskite yellow phase after long‐time storage or under attack of extrinsic factors significantly hinders their development and application. This review summarizes the current advances in recognizing phase transition behaviors of cesium lead halides, especially cesium lead tri‐iodide, and addressing phase instability issues. Advancing strategies that are used for phase stabilization, including compositional engineering, grain size reduction, modification of surface termination, and strain engineering, are highlighted as well as their present limitations. Also, existing scientific debates on phase transition and stability, origin of these arguments, and possible solutions are presented and discussed. Finally, some potential avenues for further enhancing stability of cesium lead halides are proposed based on current understandings.

Quasi‐Fermi level splitting (QFLS), which sets the maximum value of the open‐circuit voltage (V _OC) of a photovoltaic device, in state‐of‐the‐art organic solar cells is evaluated using spectroscopic and semiconductor device physics approaches.

The power conversion efficiency (PCE) of state‐of‐the‐art organic solar cells is still limited by significant open‐circuit voltage (V _OC) losses, partly due to the excitonic nature of organic materials and partly due to ill‐designed architectures. Thus, quantifying different contributions of the V _OC losses is of importance to enable further improvements in the performance of organic solar cells. Herein, the spectroscopic and semiconductor device physics approaches are combined to identify and quantify losses from surface recombination and bulk recombination. Several state‐of‐the‐art systems that demonstrate different V _OC losses in their performance are presented. By evaluating the quasi‐Fermi level splitting (QFLS) and the V _OC as a function of the excitation fluence in nonfullerene‐based PM6:Y6, PM6:Y11, and fullerene‐based PPDT2FBT:PCBM devices with different architectures, the voltage losses due to different recombination processes occurring in the active layers, the transport layers, and at the interfaces are assessed. It is found that surface recombination at interfaces in the studied solar cells is negligible, and thus, suppressing the non‐radiative recombination in the active layers is the key factor to enhance the PCE of these devices. This study provides a universal tool to explain and further improve the performance of recently demonstrated high‐open‐circuit‐voltage organic solar cells.

A novel structural design of donor–acceptor dyads, in which an oligothiophene donor and fullerene acceptor are covalently linked by flexible spacer of variable length, is presented. Favorable optoelectronic, charge transport, and self‐organization properties of dyads are the basis for reaching power conversion efficiencies of 4.26% in single‐material organic solar cells, which promise advantages for printed large‐area solar foils.

Single‐material organic solar cells (SMOSCs) promise several advantages with respect to prospective applications in printed large‐area solar foils. Only one photoactive material has to be processed and the impressive thermal and photochemical long‐term stability of the devices is achieved. Herein, a novel structural design of oligomeric donor–acceptor (D–A) dyads 1–3 is established, in which an oligothiophene donor and fullerene acceptor are covalently linked by a flexible spacer of variable length. Favorable optoelectronic, charge transport, and self‐organization properties of the D–A dyads are the basis for reaching power conversion efficiencies up to 4.26% in SMOSCs. The dependence of photovoltaic and charge transport parameters in these ambipolar semiconductors on the specific molecular structure is investigated before and after post‐treatment by solvent vapor annealing. The inner nanomorphology of the photoactive films of the dyads is analyzed with transmission electron microscopy (TEM) and grazing‐incidence wide‐angle X‐ray scattering (GIWAXS). Combined theoretical calculations result in a lamellar supramolecular order of the dyads with a D–A phase separation smaller than 2 nm. The molecular design and the precise distance between donor and acceptor moieties ensure the fundamental physical processes operative in organic solar cells and provide stabilization of D–A interfaces.

Significant progress has been made in non‐fullerene organic solar cells (OSCs) in recent years, including in materials development, device engineering, and mechanistic understanding. This review summarizes progress and offers some reflections on the emerging methods for enabling high efficiency and improved stability for non‐fullerene OSCs.

Abstract

The past three years have witnessed rapid growth in the field of organic solar cells (OSCs) based on non‐fullerene acceptors (NFAs), with intensive efforts being devoted to material development, device engineering, and understanding of device physics. The power conversion efficiency of single‐junction OSCs has now reached high values of over 18%. The boost in efficiency results from a combination of promising features in NFA OSCs, including efficient charge generation, good charge transport, and small voltage losses. In addition to efficiency, stability, which is another critical parameter for the commercialization of NFA OSCs, has also been investigated. This review summarizes recent advances in the field, highlights approaches for enhancing the efficiency and stability of NFA OSCs, and discusses possible strategies for further advances of NFA OSCs.

The chemical composition modulation and electric field‐induced ion migration of organic‐inorganic hybrid perovskites are utilized to fabricate performance‐enhanced triboelectric nanogenerators (TENGs). The chemical composition modulation induced conductive type conversion and electric field‐induced self‐doping on the surfaces enable controlled performance of the TENGs.

Abstract

In this paper, new strategies are proposed to design high‐performance organic–inorganic hybrid perovskite (PVK)‐based triboelectric nanogenerators (TENGs) via both chemical composition modulation and electric field‐induced ion migration in the films. Both composition variation and ion migration under electric field are found to change the type of conductivity of the perovskite films, then modify their surface potentials and electron affinities. These are utilized to fabricate PVK‐based TENGs in pairs with poly‐tetrafluoroethylene (PTFE) or nylon films, respectively. Results show that PVK films are able to work as either a positive or a negative tribo‐material depending on the tribo‐material pair used; the optimal performances are obtained for PTFE/PVK TENGs using a PVK film with a MAI/PbI₂ ratio of 2 and forward polarization, and for nylon/PVK TENGs using a PVK film with a MAI/PbI₂ ratio of 0.4 and reverse polarization, respectively. The maximum output voltage and peak power density of PTFE/PVK TENGs are about 979 V and 24 W m⁻², 2.5 and 6.5 times higher than those of TENGs with nonoptimal composition ratio or that are poorly polarized. This work provides a new material design method for high‐performance TENGs and a novel polarization strategy for TENG performance enhancement.

The self‐assembled monolayer (SAM) has emerged as a powerful nanomaterial for improving performance of perovskite solar cells (PSCs). This review article covers recent studies that demonstrate direct benefits of SAM‐based interfacial engineering on the mechanistic understanding of the electronic functions of PSCs and their power conversion efficiency.

Abstract

Self‐assembled monolayers (SAMs), owing to their unique and versatile abilities to manipulate chemical and physical interfacial properties, have emerged as powerful nanomaterials for improving the performance of perovskite solar cells (PSCs). Indeed, in the last six years, a collection of studies has shown that the application of SAMs to PSCs boosts the performance of devices compared to the pristine PSCs. This review describes recent studies that demonstrate the direct advantages of SAM‐based interfacial engineering to power conversion efficiency (PCE) of PSCs. This review includes 1) a brief introduction on SAMs as interfacial engineering nanomaterials; 2) a thorough survey of molecules used in SAM‐engineered PSCs and analysis of chemical structures; 3) an extensive discussion on how SAMs affect the morphology of perovskite film and the electronic function of devices; and 4) a comprehensive summary of various types of approaches for producing SAM‐engineered PSCs. This review provides an insightful perspective to stimulate new ideas and innovation in the development of PSCs for the next‐generation photovoltaics and beyond.

Three isomeric small‐molecule acceptors (SMAs) are developed by altering the substitution site of Cl and Br on the benzene‐fused end group, namely, BTP‐ClBr, BTP‐ClBr1, and BTP‐ClBr2, and the effects of substitution position in the SMAs on the photoelectric properties and photovoltaic performance are systematically investigated.

Abstract

It is widely recognized that subtle changes in the chemical structure of organic semiconductors can induce dramatic variations in their optoelectronic properties and device performance, especially for the nonfullerene small‐molecule acceptors (SMAs). For instance, halogenation of the end groups in the acceptor–donor–acceptor‐type SMAs is an effective strategy to modulate the properties of the end group and thus the entire SMA. While previous position modulations have focused on only one substituent, this study shows the development of three isomeric SMAs (BTP‐ClBr, BTP‐ClBr1, and BTP‐ClBr2) via manipulating the position of two halogen substituents (chlorine and bromine) on the terminal unit. BTP‐ClBr exhibits a blueshifted absorption, a shallower lowest unoccupied molecular orbital energy level, and a weaker crystallization tendency relative to BTP‐ClBr1 and BTP‐ClBr2. A power conversion efficiency (16.82%) and an excellent fill factor (FF) (0.79) are realized in the optimal PM6:BTP‐ClBr organic solar cell device. The higher FF is consistent with the results of the characterization including a longer charge recombination lifetime, a faster photocurrent decay, a weaker bimolecular recombination, and a more favorable domain size for PM6:BTP‐ClBr, which all originate from a subtle change in the substitution sites that strongly influences the physicochemical properties of the SMA.

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.

Abstract

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.

Herein, novel Ruddlesden–Popper Cs₂PbI₂Cl₂ nanosheets are synthesized and creatively employed as a multifunctional interface optimization material to improve the performance of CsPbI₂Br solar cells. Based on the heterostructured NSs/CsPbI₂Br/NSs inorganic film, an efficiency of 16.65% is obtained, which is one of the best reported for CsPbI₂Br solar cells, along with much‐enhanced UV, air, and thermal stabilities.

Abstract

Inorganic CsPbI₂Br perovskite solar cells (PSCs) have gained enormous research interest due to their excellent thermal and light stabilities. However, their unsatisfactory power‐conversion efficiency and poor intrinsic phase stability remain roadblocks to their further development. Herein, Cs₂PbI₂Cl₂ nanosheets (NSs) with the Ruddlesden–Popper (RP) structure are synthesized, and an NSs/CsPbI₂Br/NSs heterostructure is employed to enhance both the stability and efficiency of CsPbI₂Br solar cells. The novel Cs₂PbI₂Cl₂ NSs can not only passivate the top and bottom surfaces of the perovskite film and top surface of the TiO₂ film but also enhance the stability of the perovskite film. Based on the heterostructured NSs/CsPbI₂Br/NSs inorganic perovskite film, the efficiency of the CsPbI₂Br PSCs is improved from 15.02% to 16.65%. Moreover, the unencapsulated CsPbI₂Br devices with the NSs/CsPbI₂Br/NSs heterostructure sustain over 90% of their original efficiencies after being exposed to ambient conditions (≈25 °C and ≈35% RH) for 648 h. Both the UV‐light‐soaking stability (100 mW cm⁻¹ 365 nm UV light) and thermal stability (T = 85 °C) of the optimized devices are dramatically improved in comparison with their counterparts with only a 3D active layer. Therefore, this work promotes the application of RP inorganic perovskite nanocrystals in a range of perovskite optoelectronic devices.

Nonfullerene acceptors offer new opportunities for high efficiencies in organic solar cells, but the suppression of photodegradation of the materials in the presence of dioxygen is essential. The complex behavior of the reactive oxygen species superoxide and singlet oxygen in the degradation of the donor polymer poly(3‐hexylthiophene), the small molecule acceptor 5,5′‐[(9,9‐dioctyl‐9H‐fluorene‐2,7‐diyl)bis(2,1,3‐benzothiadiazole‐7,4‐diylmethylidyne)]bis[3‐ethyl‐2‐thioxo‐4‐thiazolidinone], and their blends is unraveled in detail.

Abstract

With rapid advances in material synthesis and device performance, the long‐term stability of organic solar cells has become the main remaining challenge toward commercialization. An investigation of photodegradation in blend films of the donor polymer poly(3‐hexylthiophene) (P3HT) and the rhodanine‐flanked small molecule acceptor 5,5′‐[(9,9‐dioctyl‐9H‐fluorene‐2,7‐diyl)bis(2,1,3‐benzothiadiazole‐7,4‐diylmethylidyne)]bis[3‐ethyl‐2‐thioxo‐4‐thiazolidinone] (FBR) is presented in an ambient atmosphere. The photobleaching kinetics of the pure materials and their blends is correlated with the generation of radicals and triplet excitons using optical and magnetic resonance techniques. In addition, spin‐trapping methods are employed to identify reactive oxygen species (ROS). In films of P3HT, FBR, and the P3HT:FBR blend, superoxide is generated by electron transfer to molecular oxygen. However, it is found that the generation of singlet oxygen by energy transfer from the FBR triplet state is responsible for the poor stability of FBR and for the accelerated photodegradation at later times of the P3HT:FBR blend. In the early stage of degradation of the neat blend, it is protected from singlet oxygen by the fast donor–acceptor charge transfer, which competes with triplet exciton formation. These results provide initial input for a rational design of donor and acceptor materials through tuning the molecular singlet and triplet energy levels to prevent ROS‐related photodegradation.

A newly conceived n‐type small molecule (Y‐Th2) is incorporated as an efficient additive in perovskite solar cells, achieving simultaneous improvements in device performance and stability. Y‐Th2 effectively passivates defects in perovskite crystals by Lewis acid–base interactions and intermolecular hydrogen bonds, obtaining high‐quality perovskite film. The inverted structure device exhibits a power conversion efficiency of 21.5% with notably enhanced operational stability.

Abstract

Significant efforts have been devoted to modulating the grain size and improving the film quality of perovskite in perovskite solar cells (PSCs). Adding materials to the perovskite is especially promising for high‐performance PSCs, because the additives effectively control the crystal structure. Although the additive engineering approach has substantially boosted the efficiency of PSCs, instability of the perovskite film has remained a primary bottleneck for the commercialization of PSCs. Herein, a newly conceived bithiophene‐based n‐type conjugated small molecule (Y‐Th2) is introduced to PSCs, which simultaneously enhances the performance and stability of the cell. The Y‐Th2 effectively passivates the defect states in perovskite through Lewis acid–base interactions, increasing the grain size and quality of the perovskite absorber. An inverted PSC containing the Y‐Th2 additive achieves a power conversion efficiency of 21.5%, versus 18.3% in the reference device. The operational stability is also considerably enhanced by the improved hydrophobicity and intermolecular hydrogen bonds in the perovskite.

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.

Abstract

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.

Abstract

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.

Significant progress has been made in non‐fullerene organic solar cells (OSCs) in recent years, including in materials development, device engineering, and mechanistic understanding. This review summarizes progress and offers some reflections on the emerging methods for enabling high efficiency and improved stability for non‐fullerene OSCs.

Abstract

The past three years have witnessed rapid growth in the field of organic solar cells (OSCs) based on non‐fullerene acceptors (NFAs), with intensive efforts being devoted to material development, device engineering, and understanding of device physics. The power conversion efficiency of single‐junction OSCs has now reached high values of over 18%. The boost in efficiency results from a combination of promising features in NFA OSCs, including efficient charge generation, good charge transport, and small voltage losses. In addition to efficiency, stability, which is another critical parameter for the commercialization of NFA OSCs, has also been investigated. This review summarizes recent advances in the field, highlights approaches for enhancing the efficiency and stability of NFA OSCs, and discusses possible strategies for further advances of NFA OSCs.

Solution‐processed fullerene derivative, [6,6]‐phenyl‐C61 butyric acid n‐hexyl ester, is reported as an effective electron transport material in perovskite solar cells. It allows smooth capping of the perovskite surface, resulting in high efficiencies, reaching 18.4% for large‐area, flexible devices. Furthermore, compared to other fullerenes, it shows reduced recombination losses at the interface with perovskite and facile scalability with the ink‐jet printing technique.

Abstract

Metal halide perovskites have raised huge excitement in the field of emerging photovoltaic technologies. The possibility of fabricating perovskite solar cells (PSCs) on lightweight, flexible substrates, with facile processing methods, provides very attractive commercial possibilities. Nevertheless, efficiency values for flexible devices reported in the literature typically fall short in comparison to rigid, glass‐based architectures. Here, a solution‐processable fullerene derivative, [6,6]‐phenyl‐C61 butyric acid n‐hexyl ester (PCBC6), is reported as a highly efficient alternative to the commonly used n‐type materials in perovskite solar cells. The cells with the PCBC6 layer deliver a power conversion efficiency of 18.4%, fabricated on a polymer foil, with an active area of 1 cm². Compared to the phenyl‐C61‐butyric acid methyl ester benchmark, significantly enhanced photovoltaic performance is obtained, which is primarily attributed to the improved layer morphology. It results in a better charge extraction and reduced nonradiative recombination at the perovskite/electron transporting material interface. Solution‐processed PCBC6 films are uniform, smooth and displayed conformal capping of perovskite layer. Additionally, a scalable processing of PCBC6 layers is demonstrated with an ink‐jet printing technique, producing flexible PSCs with efficiencies exceeding 17%, which highlights the prospects of using this material in an industrial process.

An N‐type semiconductor material, (CH₃)₂Sn(COOH)₂ (CSCO), is prepared for the first time as an electron transport layer for n‐i‐p planar perovskite solar cells, which leads to one of the highest power conversion efficiencies of 22.21%, and to remarkable stability, retaining over 83% of its initial power conversion efficiency without encapsulation after 130 days of storage in ambient conditions.

Abstract

The electron transport layer (ETL) has an important influence on the power conversion efficiency (PCE) and stability of n‐i‐p planar perovskite solar cells (PSCs). This paper presents an N‐type semiconductor material, (CH₃)₂Sn(COOH)₂ (abbreviated as CSCO) that is synthesized and prepared for the first time as an ETL for n‐i‐p planar PSCs, which leads to a high PCE of 22.21% after KCl treatment, one of the highest PCEs of n‐i‐p planar PSCs to date. Further analysis reveals that the high PCE is attributed to the excellent conductivity of CSCO because of its more delocalized electron cloud distribution due to its unique −O=C−O− group, and to the defect passivation of the Cs_0.05(FA_0.85MA_0.15)_0.95Pb(I_0.85Br_0.15)₃ (denoted as CsFAMA) perovskite through the interaction between the O (Sn) atoms of CSCO and the Pb (halogen) atoms of CsFAMA at CSCO/CsFAMA interface, while the traditional ETL materials such as SnO₂ film lack this function. In addition to the high PCE, the optimal PSCs using CSCO as ETL show remarkable stability, retaining over 83% of its initial PCE without encapsulation after 130 days of storage in ambient conditions (≈25 °C at ≈40% humidity), much better than the traditional SnO₂‐based n‐i‐p PSCs.

An electrospray printing technique is developed to continuously print the TiO₂ electron transport layer, perovskite layer, and carbon layer, enabling a cost‐effective device. The electrospray technique is capable of printing uniform, compact, and high adhesion layers with controllable dimensions and patterns. This work demonstrates a fully printed low‐cost solar cell and provides a feasible process for perovskite solar cells to initial industrialization.

Abstract

With the power conversion efficiencies of perovskite solar cells (PSCs) exceeding 25%, the PSCs are a step closer to initial industrialization. Prior to transferring from laboratory fabrication to industrial manufacturing, issues such as scalability, material cost, and production line compatibility that significantly impact the manufacturing remain to be addressed. Here, breakthroughs on all these fronts are reported. Carbon‐based PSCs with architecture fluorine doped tin oxide (FTO)/electron transport layer/perovskite/carbon, that eliminate the need for the hole transport layer and noble metal electrode, provide ultralow‐cost configuration. This PSC architecture is manufactured using a scalable and industrially compatible electrospray (ES) technique, which enables continuous printing of all the cell layers. The ES deposited electron transport layer and perovskite layer exhibit properties comparable to that of the laboratory‐scale spin coating method. The ES deposited carbon electrode layer exhibits superior conductivity and interfacial microstructure in comparison to films synthesized using the conventional doctor blading technique. As a result, the fully ES printed carbon‐based PSCs show a record 14.41% power conversion efficiency, rivaling the state‐of‐the‐art hole transporter‐free PSCs. These results will immediately have an impact on the scalable production of PSCs.

An in‐situ formed polymeric interlayer enables enhanced photovoltaic performance of the methylammonium‐, cesium‐, and bromide‐free perovskite solar cells with superior photo‐ and thermal‐stability. The polymeric interlayer promotes growth of perovskite crystals with reduced defect density and improves the contact between the perovskite and hole transporting layers to assists in photo‐excited charge extraction.

Abstract

The vast majority of high‐performance perovskite solar cells (PSCs) are based on multi‐cation mixed‐anion compositions incorporating methylammonium (MA) and bromide (Br). Nevertheless, the thermal instability of MA and the tendency of mixed halide compositions to phase segregate limit the long‐term stability of PSCs. However, reports of MA‐free and/or Br‐free compositions are rare in the community since their performance is generally inferior. Here, a strategy is presented to achieve highly efficient and stable PSCs that are altogether cesium (Cs)‐free, MA‐free and Br‐free. An antisolvent quenching process is used to in‐situ deposit a polymeric interlayer to promote the growth of phase‐pure formamidinium lead tri‐iodide perovskite crystals with reduced defect density and to assist in photo‐excited charge extraction. The PSCs developed are among the best‐performing reported for such compositions. Moreover, the PSCs show superior stability under continuous exposure to both illumination and 85 °C heat.

Aided by theoretical calculation, a multifunctional 2,2‐difluoropropanediamide (DFPDA) molecule that bears carbonyl, amino, and fluorine groups is first introduced into the perovskite precursor, serving as a crystal growth mitigator, grain boundaries passivator, and surface protection material. With the help of the combined effects of multifunctional groups in DFPDA, the perovskite cells deliver an efficiency of 22.21% and improved stability.

Abstract

With a certified efficiency as high as 25.2%, perovskite has taken the crown as the highest efficiency thin film solar cell material. Unfortunately, serious instability issues must be resolved before perovskite solar cells (PSCs) are commercialized. Aided by theoretical calculation, an appropriate multifunctional molecule, 2,2‐difluoropropanediamide (DFPDA), is selected to ameliorate all the instability issues. Specifically, the carbonyl groups in DFPDA form chemical bonds with Pb²⁺ and passivate under‐coordinated Pb²⁺ defects. Consequently, the perovskite crystallization rate is reduced and high‐quality films are produced with fewer defects. The amino groups not only bind with iodide to suppress ion migration but also increase the electron density on the carbonyl groups to further enhance their passivation effect. Furthermore, the fluorine groups in DFPDA form both an effective barrier on the perovskite to improve its moisture stability and a bridge between the perovskite and HTL for effective charge transport. In addition, they show an effective doping effect in the HTL to improve its carrier mobility. With the help of the combined effects of these groups in DFPDA, the PSCs with DFPDA additive achieve a champion efficiency of 22.21% and a substantially improved stability against moisture, heat, and light.

Two well‐regular polymer acceptors (PY‐IT and PY‐OT) with different polymerization sites are developed. For comparison, a random ternary copolymer (PY‐IOT) with the same ratio of the two acceptors is synthesized. All‐polymer solar cells (PSCs) based on PM6:PY‐IT achieve an excellent PCE of 15.05%, which is significantly higher than those based on PY‐OT (10.04%) and PY‐IOT (12.12%).

Abstract

Recent advances in the development of polymerized A–D–A‐type small‐molecule acceptors (SMAs) have promoted the power conversion efficiency (PCE) of all‐polymer solar cells (all‐PSCs) over 13%. However, the monomer of an SMA typically consists of a mixture of three isomers due to the regio‐isomeric brominated end groups (IC‐Br(in) and IC‐Br(out)). In this work, the two isomeric end groups are successfully separated, the regioisomeric issue is solved, and three polymer acceptors, named PY‐IT, PY‐OT, and PY‐IOT, are developed, where PY‐IOT is a random terpolymer with the same ratio of the two acceptors. Interestingly, from PY‐OT, PY‐IOT to PY‐IT, the absorption edge gradually redshifts and electron mobility progressively increases. Theory calculation indicates that the LUMOs are distributed on the entire molecular backbone of PY‐IT, contributing to the enhanced electron transport. Consequently, the PM6:PY‐IT system achieves an excellent PCE of 15.05%, significantly higher than those for PY‐OT (10.04%) and PY‐IOT (12.12%). Morphological and device characterization reveals that the highest PCE for the PY‐IT‐based device is the fruit of enhanced absorption, more balanced charge transport, and favorable morphology. This work demonstrates that the site of polymerization on SMAs strongly affects device performance, offering insights into the development of efficient polymer acceptors for all‐PSCs.

Molecular packing and thermodynamic properties of D18‐based fullerene‐free organic solar cells are studied. The D18 polymer exhibits strong chain extension in films, which is beneficial to charge transport. Miscibility and other characterizations explain the disparate performance of three systems and the processing procedures.

Abstract

Organic solar cells (OSCs) based on D18:Y6 have recently exhibited a record power conversion efficiency of over 18%. The initial work is extended and the device performance of D18‐based OSCs is compared with three non‐fullerene acceptors, Y6, IT‐4F, and IEICO‐4Cl, and their molecular packing characteristics and miscibility are studied. The D18 polymer shows unusually strong chain extension and excellent backbone ordering in all films, which likely contributes to the excellent hole‐transporting properties. Thermodynamic characterization indicates a room‐temperature miscibility for D18:Y6 and D18:IT‐4F near the percolation threshold. This corresponds to an ideal quench depth and explains the use of solvent vapor annealing rather than thermal annealing. In contrast, D18:IEICO‐4Cl is a low‐miscibility system with a deep quench depth during casting and poor morphology control and low performance. A failure of ternary blends with PC₇₁BM is likely due to the near‐ideal miscibility of Y6 to begin with and indicates that strategies for developing successful ternary or quaternary solar cells are likely very different for D18 than for other high‐performing donors. This work reveals several unique property–performance relations of D18‐based photovoltaic devices and helps guide design or fabrication of yet higher efficiency OSCs.

Through investigation of the underlying thermodynamic and kinetic aspects of non‐fullerene acceptor crystallization, the importance of diffusion coefficients and melting enthalpies in controlling the crystal growth rates is demonstrated, and it is revealed and that differences in halogenation can drastically change crystallization kinetics and device stability.

Abstract

With power conversion efficiency now over 17%, a long operational lifetime is essential for the successful application of organic solar cells. However, most non‐fullerene acceptors can crystallize and destroy devices, yet the fundamental underlying thermodynamic and kinetic aspects of acceptor crystallization have received limited attention. Here, room‐temperature (RT) diffusion coefficients of 3.4 × 10⁻²³ and 2.0 × 10⁻²² are measured for ITIC‐2Cl and ITIC‐2F, two state‐of‐the‐art non‐fullerene acceptors. The low coefficients are enough to provide for kinetic stabilization of the morphology against demixing at RT. Additionally profound differences in crystallization characteristics are discovered between ITIC‐2F and ITIC‐2Cl. The differences as observed by secondary‐ion mass spectrometry, differential scanning calorimetry (DSC), grazing‐incidence wide‐angle X‐ray scattering, and microscopy can be related directly to device degradation and are attributed to the significantly different nucleation and growth rates, with a difference in the growth rate of a factor of 12 at RT. ITIC‐4F and ITIC‐4Cl exhibit similar characteristics. The results reveal the importance of diffusion coefficients and melting enthalpies in controlling the growth rates, and that differences in halogenation can drastically change crystallization kinetics and device stability. It is furthermore delineated how low nucleation density and large growth rates can be inferred from DSC and microscopy experiments which could be used to guide molecular design for stability.

With synergistic optimization of the active layer morphology, flexible substrate properties, and processing temperature, large‐area flexible organic solar cells with high performance are achieved by the slot‐die coating process. The 1 cm² flexible devices produce an excellent power conversion efficiency (PCE) of 12.16%, and, for modules with an area of 25 cm², an extraordinary PCE of 10.09% is observed.

Abstract

Slot‐die coating is generally regarded as the most effective large‐scale methodology for the fabrication of organic solar cells (OSCs). However, the corresponding device performance significantly lags behind spin‐coated devices. Herein, the active layer morphology, flexible substrate properties, and the processing temperature are optimized synergistically to obtain high power conversion efficiency (PCE) for both the flexible single cells and the modules. As a result, the 1 cm² flexible devices produce an excellent PCE of 12.16% as compared to 12.37% for the spin‐coated small‐area (0.04 cm²) rigid devices. Likewise, for modules with an area of 25 cm², an extraordinary PCE of 10.09% is observed. Hence, efficiency losses associated with the upscaling are significantly reduced by the synergistic optimization. Moreover, after 1000 bending cycles at a bending radius of 10 mm, the flexible devices still produce over 99% of their initial PCE, whereas after being stored for over 6000 h in a glove box, the PCE reaches 103% of its initial value, indicating excellent device flexibility as well as superior shelf stability. These results, thus, are a promising confirmation the great potential for upscaling of large‐area OSCs in the near future.