Chen Weijie

The surface cation ratio between CH₃NH₃ ⁺ and CH₃ ⁺, for which the CH₃ ⁺‐type defects result from CH₃NH₃I dissociation, has a clear impact on the work function of CH₃NH₃PbI₃ perovskite films fabricated by different methods, and controls the energy‐level alignment with charge transport layers.

Abstract

CH₃NH₃PbI₃ thin films are fabricated using several representative synthesis methods such as spin‐coating, evaporation, and a combination of the two. These methods, which frequently occur in reported literatures, use the same precursors PbI₂ and CH₃NH₃I but differ in how the two are mixed. It is found that the latter plays a vital role in determining the surface morphology, composition, and grain size of the films, even when the same stoichiometric ratio of the precursors is used. X‐ray photoelectron spectroscopy reveals that the amount of CH₃ ⁺‐type defects, which results from CH₃NH₃I dissociation, is sensitive to both the physical state of CH₃NH₃I and the order of mixing sequence. The variation of the CH₃NH₃ ⁺:CH₃ ⁺ ratio also affects the valence band and the work function of the corresponding films, as revealed by ultraviolet photoelectron spectroscopy. Furthermore, the energy‐level alignment between the perovskite film and a model hole transport layer, N,N′‐di(1‐naphthyl)‐N,N′‐diphenylbenzidine (NPB) is examined. It is found that the CH₃NH₃ ⁺:CH₃ ⁺ ratio correlates with the offsets between the valence band maximum of perovskite film and the highest occupied molecular orbital of NPB as well, and the energy‐level alignment with the dual‐source, coevaporated CH₃NH₃PbI₃ film is most suitable for efficient hole transport.

Publication date: April 2019

Source: Nano Energy, Volume 58

Author(s): Jianmin Li, Lan Huang, Jie Hou, Xiao Wu, Jiabin Niu, Guilin Chen, Junbo Gong, Yifan Kong, Xudong Xiao

Abstract

Graphical abstract

As a colloid aggregation process, chemical bath deposited (CBD) Zn(O,S) thin films always consist of many particles or clusters on the surface and their removal is a long-standing problem. The mode of ‘Inclined substrate in static solution’ could deposit smooth and uniform Zn(O,S) layer with minimal particles/clusters adsorbed and with the best quality, which in turn lead to the best performance of Zn(O,S)-based CIGS solar cell devices. In contrast, disturbing the CBD solution by stirring or rocking would in general introduce large sized particles/clusters to form/adsorb on the Zn(O,S) films and resulted in poor quality of junctions and strong light soaking effect, in addition to the poor device performance.

Publication date: April 2019

Source: Nano Energy, Volume 58

Author(s): Sang-Chul Shin, Chang Woo Koh, Premkumar Vincent, Ji Soo Goo, Jin-Hyuk Bae, Jae-Joon Lee, Changhwan Shin, Hyeok Kim, Han Young Woo, Jae Won Shim

Abstract

Graphical abstract

The semi-crystalline polymer (PPDT2FBT) and a fullerene derivative (PC₇₀BM) were used as a photoactive layer of organic photovoltaics. By varying the thickness of the photoactive layer (120–870 nm), we compared the performance under 1-sun and the indoor lights (LED, fluorescent and halogen). Unlike 1-sun condition, short-circuit current density and fill factor remained high under indoor illumination even as the active layer thickness was increased from 280 to 870 nm.

Solution‐processed layers of PEDOT:PSS and SnO₂ nanoparticles serve as an interconnecting layer (ICL) for solution‐processed tandem polymer solar cells in p‐i‐n and n‐i‐p configurations, providing power conversion efficiencies over 10%. The resilience of SnO₂ against acidic PEDOT:PSS dispersions enables fabricating p‐i‐n tandem cells with negligible loss in an open‐circuit voltage, giving it a distinct advantage compared to ubiquitously used ZnO.

Tin oxide nanoparticles are employed as an electron transporting layer in solution‐processed polymer solar cells. Tin oxide based devices yield excellent performance and can interchangeably be used in conventional and inverted device configurations. In combination with poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as a hole transporting layer, tin oxide forms an effective interconnecting layer (ICL) for tandem solar cells. Conventional and inverted tandem cells with this ICL provide efficiencies up to 10.4% in good agreement with optical‐electrical modeling simulations. The critical advantage of tin oxide in an ICL in a conventional tandem structure over the commonly used zinc oxide is that the latter requires the use of a pH‐neutral formulation of PEDOT:PSS to fabricate the ICL, limiting the open‐circuit voltage (V _OC) because of its low work function. The SnO₂/PEDOT:PSS ICL, on the other hand, provides a nearly loss‐free V _OC.

Methoxy‐functionalized triphenylamine‐imidazole derivatives, simultaneously working as hole transport materials and bifacial interface‐modifiers passivating defects in the perovskite and NiO _x layers, are developed for high‐performance and stable perovskite solar cell. They are advantageous to improve charge‐extraction kinetics of devices and significantly enhance the stability of devices under constant UV illumination in air.

Abstract

Methoxy‐functionalized triphenylamine‐imidazole derivatives that can simultaneously work as hole transport materials (HTMs) and interface‐modifiers are designed for high‐performance and stable perovskite solar cells (PSCs). Satisfying the fundamental electrical and optical properties as HTMs of p‐i‐n planar PSCs, their energy levels can be further tuned by the number of methoxy units for better alignment with those of perovskite, leading to efficient hole extraction. Moreover, when they are introduced between perovskite photoabsorber and low‐temperature solution‐processed NiO _x interlayer, widely featured as an inorganic HTM but known to be vulnerable to interfacial defect generation and poor contact formation with perovskite, nitrogen and oxygen atoms in those organic molecules are found to work as Lewis bases that can passivate undercoordinated ion‐induced defects in the perovskite and NiO _x layers inducing carrier recombination, and the improved interfaces are also beneficial to enhance the crystallinity of perovskite. The formation of Lewis adducts is directly observed by IR, Raman, and X‐ray photoelectron spectroscopy, and improved charge extraction and reduced recombination kinetics are confirmed by time‐resolved photoluminescence and transient photovoltage experiments. Moreover, UV‐blocking ability of the organic HTMs, the ameliorated interfacial property, and the improved crystallinity of perovskite significantly enhance the stability of PSCs under constant UV illumination in air without encapsulation.

Multijunction solar cells have demonstrated the capability to overcome the Shockley–Quiesser limit for sing‐junction solar cells. This work reviews the recent progress of the different types of multijunction solar cells based on perovskites, conceives a triple‐junction solar cell, and outlooks their applications in emerging markets such as in portable electrons, Internet of Things, etc.

Abstract

Up to now, multijunction cell design is the only successful way demonstrated to overcome the Shockley–Quiesser limit for single solar cells. Perovskite materials have been attracting ever‐increasing attention owing to their large absorption coefficient, tunable bandgap, low cost, and easy fabrication process. With their rapidly increased power conversion efficiency, organic–inorganic metal halide perovskite‐based solar cells have demonstrated themselves as the most promising candidates for next‐generation photovoltaic applications. In fact, it is a dream come true for researchers to finally find a perfect top‐cell candidate in tandem device design in commercially developed solar cells like single‐crystalline silicon and CIGS cells used as the bottom component cells. Here, the recent progress of multijunction solar cells is reviewed, including perovskite/silicon, perovskite/CIGS, perovskite/perovskite, and perovskite/polymer multijunction cells. In addition, some perspectives on using these solar cells in emerging markets such as in portable devices, Internet of Things, etc., as well as an outlook for perovskite‐based multijunction solar cells are discussed.

Two isomeric perylene diimide based acceptors (BPT‐Se and BPT‐Se1) are developed. The BPT‐Se1‐based device exhibits higher power conversion efficiency of 9.54% with excellent fill factor of 73.2% relative to the BPT‐Se based control device.

Abstract

A strategy that employs the central‐core regiochemistry to develop two isomeric perylene diimide (PDI)‐based small molecular acceptors (SMAs), BPT‐Se and BPT‐Se1, is introduced, and the effect of the central‐core regiochemistry on the optical, electronic, charge‐transport, photovoltaic, and morphological properties of the molecules and their devices is investigated. The PDBT‐T1:BPT‐Se1‐based device delivers a power conversion efficiency (PCE) of 9.54% with an excellent fill factor (FF) of 73.2%, while the BPT‐Se‐based device yields a PCE of 7.78%. The large improvement of PCE upon isomerization of BPT‐Se should be ascribed to the concurrent enhancement of FF, short circuit current ( J _SC), and open circuit voltage (V _OC) of the PDBT‐T1:BPT‐Se1 devices. The higher FF of the organic solar cells (OSCs) based on PDBT‐T1:BPT‐Se1 can be attributed to the higher charge dissociation and charge collection efficiency, less bimolecular combination, more balanced µ _h/µ _e, better molecular packing and a more favorable morphology. It is worth mentioning that the FF of 73.2% is the highest value for PDI‐based SMAs OSCs to date. The result shows that regiochemistry of the central core in PDI‐based SMAs greatly affects the physicochemical properties and photovoltaic performance. The success of the isomerization strategy offers exciting prospects for the molecular design of PDI‐based SMAs.

Wide bandgap donor polymers employing a simultaneous fluorination and alkylation are demonstrated to be an effective technique to design the donor materials in application of polymer solar cells (PSCs). Specifically, P(fTh‐2DBDT)‐C6 shows a high efficiency and stability when introducing eco‐friendly solvent in nonfullerene PSCs.

Abstract

Nonfullerene organic solar cells (NFOSCs) are attracting increasing academic and industrial interest due to their potential uses for flexible and lightweight products using low‐cost roll‐to‐roll technology. In this work, two wide bandgap (WBG) polymers, namely P(fTh‐BDT)‐C6 and P(fTh‐2DBDT)‐C6, are designed and synthesized using benzodithiophene (BDT) derivatives. Good oxidation stability and high solubility are achieved by simultaneously introducing fluorine and alkyl chains to a single thiophene (Th) unit. Solid P(fTh‐2DBDT)‐C6 films present WBG optical absorption, suitable frontier orbital levels, and strong π–π stacking effects. In addition, P(fTh‐2DBDT)‐C6 exhibits good solubility in both halogenated and nonhalogenated solvents, suggesting its suitability as donor polymer for NFOSCs. The P(fTh‐2DBDT)‐C6:3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(5‐hexylthienyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (ITIC‐Th) based device processed using chlorobenzene/1,8‐diiodooctane (CB/DIO) exhibits a remarkably high power conversion efficiency (PCE) of 11.1%. Moreover, P(fTh‐2DBDT)‐C6:ITIC‐Th reaches a high PCE of 10.9% when processed using eco‐friendly solvents, such as o‐xylene/diphenyl ether (DPE). The cell processed using CB/DIO maintains 100% efficiency after 1272 h, while that processed using o‐xylene/DPE presents a 101% increase in efficiency after 768 h and excellent long‐term stability. The results of this study demonstrate that simultaneous fluorination and alkylation are effective methods for designing donor polymers appropriate for high‐performance NFOSCs.

This paper investigates the formation mechanism of Cs_0.15(MA_0.7FA_0.3)_0.85PbI₃ perovskite fabricated by a dynamic sequential solution process. It is revealed that motion dispense for the 2nd deposition step “suspends” the dimethyl sulfoxide (DMSO)‐complex necessary for the intermediate phase i) promoting intercalation between precursors and ii) slowing down perovskite crystallization for full conversion, resulting in film with higher uniformity and better electrical properties.

Abstract

This paper provides deep understanding of the formation mechanism of perovskite film fabricated by sequential solution‐based methods. It compares two sequential spin‐coating methods for Cs_0.15(MA_0.7FA_0.3)_0.85PbI₃ perovskite. First is the “static process,” with a stoppage between the two spin‐coating steps (1st PbI₂‐CsI‐dimethyl sulfoxide (DMSO)‐dimethylformamide (DMF) and 2nd methylammonium iodide (MAI)‐formamidinium iodide (FAI)‐isopropyl alcohol). Second is the “dynamic process,” where the 2nd precursor is dispensed while the substrate is still spinning from the 1st step. For the first time, such a dynamic process is used for Cs_0.15(MA_0.7FA_0.3)_0.85PbI₃ perovskite. Characterizations reveal improved film formation with the dynamic process due to the “retainment” of DMSO‐complex necessary for the intermediate phase which i) promotes intercalation between precursors and ii) slows down perovskite crystallization for full conversion. The comparison on as‐deposited perovskite before annealing indicates a more ordered film using this dynamic process. This results in a thicker, more uniform film with higher degree of preferred crystal orientation and higher carrier lifetime after annealing. Therefore, dynamic‐processed devices present better performance repeatability, achieving a higher average efficiency of 17.0% compared to static ones (15.0%). The new insights provided by this work are important for perovskite solar cells processed sequentially as the process has greater flexibility in resolving solvent incompatibility, allowing separate optimizations and allowing different deposition methods.

A new asymmetric, terminally tetrafluorinated nonfullerene acceptor, namely ITIF, was prepared for ternary solar cells based on PBDB‐T:ITIF:ITIC blends. Owning to the unique structure, ITIF is promised to work efficiently in ternary blends, simultaneously boosting the devices performance para‐meters. Therefore, the power conversion efficiencies of the ternary solar cells are boosted from 9.2% to 10.5%.

Abstract

Fabricating ternary solar cells is a pivotal strategy to improve the power conversion efficiencies (PCEs) of organic photovoltaic devices. However, it is still a challenge to simultaneously improve the performance parameters of ternary devices. Therefore, the third ingredient in ternary blends should be precisely designed or selected. Herein, a new medium‐bandgap small‐molecule acceptor, namely, 3,9‐bis(2‐methylene‐(3‐(1‐(3,5‐dimethylphenyl)‐1cyanomethylene)indanone))‐5,5,11,11‐tetrakis‐(4‐hexylphenyl)dithieno[2,3‐d:2′,3′‐d′]‐sindaceno[1,2‐b:5,6‐b′]dithiophene (ITIF), is synthesized by end‐capping with a new fluorinated, asymmetric terminal group, (Z)‐2‐(3,5‐difluorophenyl)‐2‐(3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene) acetonitrile. Replacing the CN substituent with the asymmetric 3,5‐difluorophenyl substituent obviously up‐shifts the lowest unoccupied molecular orbital (LUMO) level of ITIF to −3.78 eV, enlarges the bandgap to 1.82 eV, and improves the absorption coefficient to ≈50% higher than that of 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)indanone))‐5,5,11,11‐tetrakis‐(4‐hexylphenyl)dithieno[2,3‐d:2′,3′‐d′]‐sindaceno[1,2‐b:5,6‐b′]dithiophene (ITIC). Due to the similar structures, ITIF and ITIC can combine as an alloyed acceptor, which makes it convenient to tune the morphology and optical and electrochemical properties of ternary blends. The enhanced absorption coefficient of ITIF and the rapid fluorescence resonance energy transfer from ITIF to ITIC remarkably improve the absorption of the ternary blend film, hence compensating for the external quantum efficiency (EQE) curves. When ITIF is introduced into ternary solar cells based on poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione)] (PBDB‐T):ITIF:ITIC blends, the PCEs of the ternary devices are increased from 9.2% to 10.5%, and the short‐circuit currents, open‐circuit voltages, and fill factors are simultaneously improved.

The application of formamidinium (FA)‐based perovskite solar cells application has largely been hindered by phase transition from the dark cubic phase to yellow orthorhombic phase. Here, a highly efficient and phase stable FA‐based perovskite solar cell is fabricated by using NbF₅ as a novel additive. NbF₅ can improve the quality of perovskite films and effectively suppress the formation of the yellow δ‐phase.

Abstract

The HC(NH₂)₂ ⁺(FA⁺) is a well‐known substitute to CH₃NH₃ ⁺(MA⁺) for its capability to extend light utilization for improved power conversion efficiency for perovskite solar cells; unfortunately, the dark cubic phase (α‐phase) can easily transition to the yellow orthorhombic phase (δ‐phase) at room temperature, an issue that prevents its commercial application. In this report, an inorganic material (NbF₅) is developed to stabilize the desired α‐phase perovskite material by incorporating NbF₅ additive into the perovskite films. It is found that the NbF₅ additive effectively suppresses the formation of the yellow δ‐phase in the perovskite synthesis and aging process, thus enhancing the humidity and light‐soaking stability of the perovskite film. As a result, the perovskite solar cells with the NbF₅ additive exhibit improved air stability by tenfold, retaining nearly 80% of their initial efficiency after aging in air for 50 d. In addition, under full‐sun AM 1.5 G illumination of a xenon lamp without any UV‐reduction, the perovskite solar cells with the NbF₅ additive also show fivefold improved illumination stability than the control devices without NbF₅.

Tin‐based reduced‐dimensional perovskites: NH₄Cl additive is introduced in the preparation of AVA₂FA _{n −1}Sn _n I_{3 n +1} (<n> = 5) perovskite, leading to highly vertically oriented tin‐based reduced‐dimensional perovskite films with enhanced efficiency and stability. Herein, under the effects of NH₄Cl additive, the optimized power conversion efficiency of tin‐based quasi‐2D perovskite solar cells increases from 4.19% to 8.71% with an enhanced stability.

Abstract

Tin‐based perovskites have exhibited high potential for efficient photovoltaics application due to their outstanding optoelectrical properties. However, the extremely undesired instabilities significantly hinders their development and further commercialization process. A novel tin‐based reduced‐dimensional (quasi‐2D) perovskites is reported here by using 5‐ammoniumvaleric acid (5‐AVA⁺) as the organic spacer. It is demonstrated that by introducing appropriate amount of ammonium chloride (NH₄Cl) as additive, highly vertically oriented tin‐based quasi‐2D perovskite films are obtained, which is proved through the grazing incidence wide‐angle X‐ray scattering characterization. In particular, this approach is confirmed to be a universal method to deliver highly vertically oriented tin‐based quasi‐2D perovskites with various spacers. The highly ordered vertically oriented perovskite films significantly improve the charge collection efficiency between two electrodes. With the optimized NH₄Cl concentration, the solar cells employing quasi‐2D perovskite, AVA₂FA _{n −1}Sn _n I_{3 n +1} (<n> = 5), as light absorbers deliver a power conversion efficiency up to 8.71%. The work paves the way for further employing highly vertically oriented tin‐based quasi‐2D perovskite films for highly efficient and stable photovoltaics.

An innovative interfacial modifier, namely, 3‐phenyl‐2‐propen‐1‐amine (t‐PPEA) is developed for perovskite solar cells to overcome the dilemma of the trade‐off between transport and stability of the device, with unique “quasi‐coplanar” rigid geometrical configuration and distinct electron delocalization characteristic.

Abstract

Interfacial ligand passivation engineering has recently been recognized as a promising avenue, contributing simultaneously to the optoelectronic characteristics and moisture/operation tolerance of perovskite solar cells. To further achieve a win‐win situation of both performance and stability, an innovative conjugated aniline modifier (3‐phenyl‐2‐propen‐1‐amine; PPEA) is explored to moderately tailor organolead halide perovskites films. Here, the conjugated PPEA presents both “quasi‐coplanar” rigid geometrical configuration and distinct electron delocalization characteristics. After a moderate treatment, a stronger dipole capping layer can be formed at the perovskite/transporting interface to achieve favorable banding alignment, thus enlarging the built‐in potential and promoting charge extraction. Meanwhile, a conjugated cation coordinated to the surface of the perovskite grains/units can form preferably ordered overlapping, not only passivating the surface defects but also providing a fast path for charge exchange. Benefiting from this, a ≈21% efficiency of the PPEA‐modified solar cell can be obtained, accompanied by long‐term stability (maintaining 90.2% of initial power conversion efficiency after 1000 h testing, 25 °C, and 40 ± 10 humidity). This innovative conjugated molecule “bridge” can also perform on a larger scale, with a performance of 18.43% at an area of 1.96 cm².

In article number 1803438 by Hin‐Lap Yip and co‐workers, spectral engineering and ternary blend approaches are employed to demonstrate an efficient semitransparent polymer solar cell (ST‐PSC) tailored for greenhouse photovoltaic applications. The ST‐PSC transmits mainly blue and red light, which are important for photosynthesis in plants. Such optimal sunlight harvesting for both photovoltaic and photosynthesis will be beneficial for future self‐powered greenhouse applications.

A highly transparent nickel oxide hole transport layers prepared by oxygen‐assisted electron beam evaporation for perovskite‐based photovoltaics is reported. Using these layers in perovskite solar cells, efficient devices with stable power conversion efficiencies up to 18.5% for inkjet‐printed absorbers and 15.4% for co‐evaporated absorbers are demonstrated. In addition, good stability at elevated temperature and under ultraviolet radiation is shown.

Abstract

High‐quality charge carrier transport materials are of key importance for stable and efficient perovskite‐based photovoltaics. This work reports on electron‐beam‐evaporated nickel oxide (NiO _x ) layers, resulting in stable power conversion efficiencies (PCEs) of up to 18.5% when integrated into solar cells employing inkjet‐printed perovskite absorbers. By adding oxygen as a process gas and optimizing the layer thickness, transparent and efficient NiO _x hole transport layers (HTLs) are fabricated, exhibiting an average absorptance of only 1%. The versatility of the material is demonstrated for different absorber compositions and deposition techniques. As another highlight of this work, all‐evaporated perovskite solar cells employing an inorganic NiO _x HTL are presented, achieving stable PCEs of up to 15.4%. Along with good PCEs, devices with electron‐beam‐evaporated NiO _x show improved stability under realistic operating conditions with negligible degradation after 40 h of maximum power point tracking at 75 °C. Additionally, a strong improvement in device stability under ultraviolet radiation is found if compared to conventional perovskite solar cell architectures employing other metal oxide charge transport layers (e.g., titanium dioxide). Finally, an all‐evaporated perovskite solar mini‐module with a NiO _x HTL is presented, reaching a PCE of 12.4% on an active device area of 2.3 cm².

An n‐doping of SnO₂ is successfully realized through the use of the triphenylphosphine‐oxide molecule, where electrons are revealed to be transferred from the R₃P⁺O⁻ σ‐bond to the peripheral tin atoms and delocalized. That novel effect enlarges the built‐in‐field from 0.01 to 0.07 eV and declines the energy‐barrier from 0.55 to 0.39 eV at the SnO₂–perovskite interface enabling a device conversion‐efficiency from 19.01% to 20.69%.

Abstract

Molecular doping of inorganic semiconductors is a rising topic in the field of organic/inorganic hybrid electronics. However, it is difficult to find dopant molecules which simultaneously exhibit strong reducibility and stability in ambient atmosphere, which are needed for n‐type doping of oxide semiconductors. Herein, successful n‐type doping of SnO₂ is demonstrated by a simple, air‐robust, and cost‐effective triphenylphosphine oxide molecule. Strikingly, it is discovered that electrons are transferred from the R3P⁺O⁻σ‐bond to the peripheral tin atoms other than the directly interacted ones at the surface. That means those electrons are delocalized. The course is verified by multi‐photophysical characterizations. This doping effect accounts for the enhancement of conductivity and the decline of work function of SnO₂, which enlarges the built‐in field from 0.01 to 0.07 eV and decreases the energy barrier from 0.55 to 0.39 eV at the SnO₂/perovskite interface enabling an increase in the conversion efficiency of perovskite solar cells from 19.01% to 20.69%.

The halide composition of Pb²⁺‐based perovskite materials provides great control over their physical properties. Yet the range of X ions explored in the APbX₃ perovskite motif has so far been generally confined to the series of halides Cl⁻, Br⁻, and I⁻. The possibility of polyatomic pseudohalide anions in Pb²⁺‐based perovskites is explored.

Abstract

The emerging class of lead halide perovskite (LHP) semiconductors offers a surprising combination of low cost, ease of preparation, outstanding material properties, and performance in optoelectronic devices that has not yet been observed in any other class of material. Considering their general ABX₃ formula, the halide (X) composition in LHP compositions has proven to be one of the best handles to control the material characteristics such as bandgap, morphology, and electronic properties. However, compared to the amount of effort that has been expended to discover new A cations and B cations, relatively few reports have dealt with the subject of discovering new X anions outside of the series of halides (Cl⁻, Br⁻, I⁻). In principal, a much wider range of anions with a −1 charge (pseudohalides) may form the ABX₃ stoichiometry with Pb²⁺, yet the general ability of polyatomic pseudohalides to form semiconducting perovskite crystal phases with Pb²⁺ remains an open question. Herein, the prospect of using polyatomic pseudohalide anions in LHP semiconductors is addressed.

In contrast to conjugated donaor–acceptor (D–A) alternating copolymers, incorporating a third component, either D′‐ or A′‐unit, to their D–A type polymer backbones can improve their light absorption, and tune energy levels and interchain packing synergistically. Moreover, the well‐controlled stoichiometry for these components in terpolymers also provides further access to fine‐tune these factors, thus resulting in high photovoltaic performance in polymer solar cells.

Abstract

The development of conjugated alternating donor–acceptor (D–A) copolymers with various electron‐rich and electron‐deficient units in polymer backbones has boosted the power conversion efficiency (PCE) over 17% for polymer solar cells (PSCs) over the past two decades. However, further enhancements in PCEs for PSCs are still imperative to compensate their imperfect stability for fulfilling practical applications. Meanwhile development of these alternating D–A copolymers is highly demanding in creative design and syntheses of novel D and/or A monomers. In this regard, when being possible to adopt an existing monomer unit as a third component from its libraries, either a D′ unit or an A′ moiety, to the parent D–A type polymer backbones to afford conjugated D–A terpolymers, it will give a facile and cost‐effective method to improve their light absorption and tune energy levels and also interchain packing synergistically. Moreover, the rationally controlled stoichiometry for these components in such terpolymers also provides access for further fine‐tuning these factors, thus resulting in high‐performance PSCs. Herein, based on their unique features, the recent progress of conjugated D–A terpolymers for efficient PSCs is reviewed and it is discussed how these factors influence their photovoltaic performance, for providing useful guidelines to design new terpolymers toward high‐efficiency PSCs.

An effective but simple approach to rationally tune the crystallinity and miscibility of small molecular acceptors is reported. With a phenyl introduced at the tail of alkyl side chains, the morphology and molecular orientations of heterojunction are greatly improved. Outstanding efficiencies of 13.23% and 14.04% are detected from the as‐cast and annealed devices, promoted by the fairly high fill factors.

Abstract

Research on fused‐ring small‐molecular‐acceptors (SMAs) has deeply advanced the development of organic solar cells (OSCs). Compared to fruitful studies of ladder‐type cores and end‐caps of SMAs, the exploration of side chains is monotonous. The widely utilized alkyl and aryl side chains usually produce a conflicting association between SMAs' crystallinity and miscibility. Herein, a fresh idea about the modification of side chains is reported to explore the subtle balance between the crystallinity and miscibility. Specifically, a phenyl is introduced to the tail of the alkyl side chain whereby a new acceptor IDIC‐C4Ph is reported. Moderately weakened crystallinity is observed, while maintaining preferred absorption profiles and face‐on orientations. Concurrently, remarkably improved heterojunction morphologies and stacking orientations are detected. PBDB‐T:IDIC‐C4Ph devices exhibit greater efficiency of 11.50% than devices from alky and aryl modified acceptors. Notably, the as‐cast OSCs of PBDB‐TF:IDIC‐C4Ph reveal outstanding FF over 76% with the best efficiency up to 13.23%. The annealed devices reveal further increased efficiency exceeding 14% with the state of the art FF of 78.32%. Overall, an effective but easily navigable approach is demonstrated to modulate the crystallinity of SMAs toward synergistically improved morphologies and molecular orientations of bulk heterojunction enabling highly efficient OSCs.

Building blocks: Semi‐locked tetrathienylethene (TTE), featuring a hybrid planar and orthogonal molecular conformation, is introduced as the core for constructing state‐of‐the‐art hole‐transporting materials (HTMs). The resulting TTE achieves the best photovoltaic performance among dopant‐free HTM‐based planar n‐i‐p structured perovskite solar cells.

Abstract

The construction of state‐of‐the‐art hole‐transporting materials (HTMs) is challenging regarding the appropriate molecular configuration for simultaneously achieving high morphology uniformity and charge mobility, especially because of the lack of appropriate building blocks. Herein a semi‐locked tetrathienylethene (TTE) serves as a promising building block for HTMs by fine‐tuning molecular planarity. Upon incorporation of four triphenylamine groups, the resulting TTE represents the first hybrid orthogonal and planar conformation, thus leading to the desirable electronic and morphological properties in perovskite solar cells (PSCs). Owing to its high hole mobility, deep lying HOMO level, and excellent thin film quality, the dopant‐free TTE‐based PSCs exhibit a very promising efficiency of over 20 % with long‐term stability, achieving to date the best performances among dopant‐free HTM‐based planar n‐i‐p structured PSCs.

Planar p–n homojunction perovskite solar cells with efficiency exceeding 21.3%

Planar p–n homojunction perovskite solar cells with efficiency exceeding 21.3%, Published online: 04 February 2019; doi:10.1038/s41560-018-0324-8