Chen Weijie

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.

A carbon‐encapsulated (Cs_0.15FA_0.85)Pb(I_0.9Br_0.1)₃ photocathode with a sandwich‐like structure is prepared and demonstrates state‐of‐the‐art performance for photo‐electrochemical (PEC) CO₂ reduction among organic–inorganic hybrid perovskite‐based PEC devices. A tandem device consisting of this photocathode and a Si photoanode further realizes unbiased PEC CO₂ reduction with an outstanding solar‐to‐CO energy conversion efficiency of 3.34%.

Abstract

Photo‐electrochemical (PEC) carbon dioxide reduction to chemicals or fuels has been regarded as an attractive strategy that can close the anthropogenic carbon cycle. However, identifying a PEC system capable of driving efficient and durable CO₂ conversion remains a critical challenge. Herein, the fabrication of a sandwich‐like organic–inorganic hybrid perovskite‐based photocathode with carbon encapsulation for PEC CO₂ reduction is reported. The carbon encapsulation not only affords protection to the perovskite, but also allows for efficient conductance of photogenerated electrons. When decorated with a cobalt phthalocyanine molecular catalyst, the photocathode shows an onset potential at 0.58 V versus reversible hydrogen electrode (RHE) and a high photocurrent density of −15.5 mA cm⁻² at −0.11 V versus RHE in CO₂‐saturated 0.5 m KHCO₃ under AM 1.5G illumination (100 mW cm⁻²), which represents state‐of‐the‐art performance in this field. Moreover, the photocathode remains stable during a continuous reaction that lasted for 25 h. Unbiased PEC CO₂ reduction is further realized by integrating the photocathode with an amorphous Si photoanode in tandem, delivering a solar‐to‐CO energy conversion efficiency of 3.34% and a total solar‐to‐fuel energy conversion efficiency of 3.85%.

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.

The relationship between structure design, packing arrangement, and molecular property of organic photovoltaic (OPV) acceptors is explored, in which the 3D network packing originating from non‐covalent intermolecular interactions and aggregation states, is found to promote OPV device performance. This review sheds light on charge transport processes in acceptors and provides a guideline for developing new generation OPV materials.

Abstract

The power conversion efficiency of organic solar cell (OSC) devices has surpassed 18% rapidly. In order to further promote OSC development, it is necessary to understand the packing information at the atomic level to help develop acceptor systems with superior performance. The packing arrangements and intermolecular interactions of these acceptors in the solid state, observed by single crystal X‐ray crystallography, are often used to design materials with expected physicochemical properties. In this review, the chemical structures of acceptors revealed by single crystal X‐ray crystallography are summarized, and the relationship between structural design, packing arrangement, and device properties is discussed. In addition, the concept of “3D network packing” in acceptor systems is proposed, which offers better charge transfer properties in reported chlorinated, fluorinated, brominated, and trifluoromethylated systems, an understanding of 3D network transport also provides guidance in high‐performance materials design. Finally, some current issues related to single crystal studies in OSCs are discussed, with an emphasis on the significance of developing acceptors by understanding and adjusting the aggregation states and intermolecular interactions of materials by single crystal analysis.

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.

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.

A bilinkable contact passivation strategy is developed for modifying charge kinetics at the charge transport layer:active layer interface in solar cells. The use of the bifunctional molecule 3‐thiophenecarboxylic acid (TCA) passivates undercoordinated Ti (ETL‐side) and Pb (perovskite‐side), enabling efficient electron extraction through the interface. TCA‐treated films show an increase of PCE of 21.2% compared to 19.8% for reference devices.

Abstract

Charge recombination due to interfacial defects is an important source of loss in perovskite solar cells. Here, a two‐sided passivation strategy is implemented by incorporating a bilinker molecule, thiophene‐based carboxylic acid (TCA), which passivates defects on both the perovskite side and the TiO₂ side of the electron‐extracting heterojunction in perovskite solar cells. Density functional theory and ultrafast charge dynamics reveal a 50% reduction in charge recombination at this interface. Perovskite solar cells made using TCA‐passivated heterojunctions achieve a power conversion efficiency of 21.2% compared to 19.8% for control cells. The TCA‐containing cells retain 96% of initial efficiency following 50 h of UV‐filtered MPP testing.

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.

Bearing high hole mobility and appropriate energy levels, an organic small molecule 2,7‐bis(4‐octylphenyl)naphtho[2,1‐b:6,5‐b 0] difuran (C₈‐DPNDF) is introduced as a dopant‐free hole transporting material in inverted perovskite solar cells. The device with C₈‐DPNDF as HTM shows a decent power conversion efficiency of 17.5% and can keep 92% of its initial value after 30 days in ambient air.

Hole transport material (HTM) is a significant constituent in perovskite solar cells (PSCs). However, HTM generally is not utilized in its pristine form but with dopants (such as lithium salt, tert‐butyl pyridine, F₄‐TCNQ), which accelerates device degradation and leads to poor stability. Therefore, dopant‐free HTM is highly desirable to fabricate stable devices. Herein, a fused furan organic small molecule (C₈‐DPNDF) is introduced as a dopant‐free HTM in inverted PSCs. As a potential HTM candidate, C₈‐DPNDF shows excellent properties, such as high hole mobility, matched energy level with perovskite, and resistance to perovskite precursor solution. As a result, the device based on C₈‐DPNDF as HTM shows a power conversion efficiency (PCE) of 17.5%, compared with 17.1% of the control device based on classic poly(bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine) (PTAA) as the HTM. In addition, the unencapsulated device based on C₈‐DPNDF as HTM keeps 92% of its initial PCE after 30 days of storage in ambient air with a relative humidity of ≈40%. This finding is expected to pave the way toward stable and highly efficient inverted PSCs based on dopant‐free HTMs.

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.

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.

Radio frequency magnetron sputtered titanium‐doped indium oxide (ITiO) films are fabricated in a low‐temperature process. Oxygen flow ratio plays a vital role in the influence of ITiO crystalline growth, as well as optoelectronic properties. It is demonstrated that ITiO films in the front of silicon heterojunction solar cells result in better performance compared with Sn‐doped indium oxide.

One of the challenges in fabricating high‐performance n‐type silicon heterojunction (SHJ) solar cells is developing a high‐quality transparent conductive oxide (TCO) electrode. Herein, the development and application of low‐temperature sputtered titanium‐doped indium oxide (ITiO) in n‐type, rear junction SHJ solar cells as a function of the oxygen flow ratio is presented. The microstructure, morphology, and optoelectronic properties are analyzed. The grain size of ITiO thin films decreases rapidly as the oxygen flow ratio is increased. Compared with an indium tin oxide (ITO) thin film, ITiO shows a superior balance in achieving excellent optoelectronic properties by reducing film resistivity but maintaining weak absorption. Higher fill factor is obtained by substituting ITiO for ITO as the front electrode in SHJ solar cells, which is mainly due to the improved carrier transport. Resistivity contributions of front‐side vertical and lateral carrier transport are disclosed by Quokka3 simulation. A champion cell efficiency of 23.81% with ITiO is achieved, which is so far the highest efficiency among the application of ITiO in SHJ solar cells to the best of our knowledge. The study demonstrates that ITiO is a promising TCO candidate for SHJ solar cells.

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.

Due to the poor morphology and crystallinity of Sn–Pb mixed perovskites, it is found that the addition of potassium thiocyanate (KSCN) can effectively reduce the bulk defects and carrier recombination through optimizing the process of film formation and the perovskite film quality. Therefore, a whole improvement of device performance can be achieved under the optimization effect of KSCN doping.

The organic–inorganic Sn–Pb mixed perovskite has achieved great progress during the last 10 years and is considered as one of the most promising low‐bandgap photovoltaic materials. It has lower toxicity, outstanding optoelectrical properties, and achieved remarkable performance. However, there are still plenty of challenges in controlling the morphology, crystallinity, and defects of the Sn–Pb mixed perovskite film because of the inferior chemical stability of Sn compared with Pb. Herein, it is found that the synergistic effect of potassium thiocyanate (KSCN) in the Sn–Pb mixed perovskites can enlarge the grain size, enhance the crystallization, improve the film morphology, and obtain high‐quality perovskite films which effectively eliminate the bulk defects and smooth carrier transportation of Sn–Pb mixed perovskite solar cells. Through optimizing the concentration of KSCN, a high‐performance MA_0.5FA_0.5Pb_0.5Sn_0.5I₃ solar cell with an efficiency of 15.14% and improved stability is obtained. This work lays a key foundation for the fabrication of efficient and stable Sn‐based or Sn–Pb mixed perovskite solar devices.

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.

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.

The adaptation of the two‐step deposition method is demonstrated, which enables the direct probe into the growth dynamics of perovskites using in situ diagnostics. A detailed view of the effects of solvent, lead halide film solvation, and Br incorporation and alloying on the transformation behavior is presented.

Inorganic−organic hybrid perovskites MAPb(I _x Br_1−x )₃ (0 < x < 1) hold promise for efficient multi‐junction or tandem solar cells due to tunable bandgap and improved long‐term stability. However, the phase transformation from Pb(I _x Br_1−x )₂ precursors to perovskites is not fully understood which hinders further improvement of optoelectronic properties and device performance. Here, adaptation of the two‐step deposition method, which enables the direct probe into the growth dynamics of perovskites using in situ diagnostics, and a detailed view of the effects of solvent, lead halide film solvation, and Br incorporation and alloying on the transformation behavior is presented. The in situ measurements indicate a strong tendency of lead halide solvation prior to crystallization during solution‐casting Pb(I _x Br_1−x )₂ precursor from a dimethyl sulfoxide (DMSO) solvent. Highly‐efficient intramolecular exchange is observed between DMSO molecules and organic cations, leading to room‐temperature conversion of perovskite and high‐quality films with tunable bandgap and superior optoelectronic properties in contrast to that obtained from crystalline Pb(I _x Br_1−x )₂. The improved properties translate to easily tunable and a relatively higher power conversion efficiency of 16.42% based on the mixed‐halide perovskite MAPb(I_0.9Br_0.1)₃. These findings highlight the benefits that solvation of the precursor phases, together with bromide incorporation, can have on the microstructure, morphology, and optoelectronic properties of these films.

The voltage losses due to thermalization of photoexcited charges are a major loss channel in organic solar cells. Herein, it is shown how a composition gradient in the donor:acceptor blend can be used to rectify a significant fraction of these losses, leading to open‐circuit voltage enhancements of up to 0.2 eV.

In virtually all solar cells, including optimized ones that operate close to the Shockley–Queisser (SQ) limit, thermalization losses are a major, efficiency‐limiting factor. In typical bulk heterojunction organic solar cells, the loss of the excess energy of photocreated charge carriers in the disorder‐broadened density of states is a relatively slow process that for commonly encountered disorder values takes longer than the charge extraction time. Herein, it is demonstrated by numerical modeling that this slow relaxation can be rectified by means of a linear gradient in the donor:acceptor ratio between anode and cathode. For experimentally relevant parameters, open‐circuit voltage (VOC) enhancements up to ≈0.2 V in combination with significant enhancements in fill factor as compared to devices without gradient are found. The VOC enhancement can be understood in terms of a simple nonequilibrium effective temperature model. Implications for existing and future organic photovoltaics (OPV) devices are discussed.

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.

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.

Nature Communications, Published online: 09 November 2020; doi:10.1038/s41467-020-19471-9

Strong excitonic effects and spin-orbit coupling in all-inorganic halide perovskite is promising for spintronic application, yet the spin-dependent phenomenon is not well understood. Here, the authors reveal that many-body interactions between spin-polarized excitons act like pseudo-magnetic field, lifting the degeneracy and resulting in circular dichroism.