JIANG ZHI XUAN

The self-doping effect on the light stability of perovskite solar cells (PSCs) is systemically investigated through various opto-electrical characterizations. Although both PSCs with Pb-rich and Pb-deficient conditions exhibit similar initial performance, the Pb-rich PSC degrades relatively quickly under light illumination even without H₂O and O₂, resulting in the shift of the defect state associated with the formation of deep-level defects.

Abstract

Although there have been significant advances in the stability of perovskite solar cells through encapsulation techniques to remove extrinsic degradation factors, such as moisture and oxygen, irreversible photo-degradation originating from intrinsic defects is still challenging and remains elusive. Herein, the photo-aging mechanism due to intrinsic defects is investigated in nitrogen-filled conditions, excluding extrinsic degradation factors. Devices with similar power conversion efficiencies (PCE) of 21%, but with different Fermi levels in the perovskite films, via controlling the self-doping effect, have been investigated. Opto-electronic investigations and depth profiles of the elemental constituents show that after photo-aging, strain relaxation in the perovskite lattice and a Fermi level shift towards conduction band edge are observed, implying the formation of new defect states in Pb-rich devices. Furthermore, thermal admittance spectroscopy measurement of the devices suggests that the formation of the deep-traps in the perovskite leads to irreversible degradation. Thin-film solar cells that are relatively Pb-deficient (FA-rich) exhibit improved long-term stability, retaining over 90% of their initial PCE during 500 h of continuous 1-Sun illumination. This study suggests passivation of the Pb-I related antisite defects near the grain boundaries and the interface is crucial for the fabrication of solar cells with enhanced long-term stability.

Replace Pb²⁺ cations in the lead halide perovskite with other suitable environmentally friendly metal cations can address the toxicity of lead and the structure‐induced intrinsic instability of lead halide perovskites. Moreover, it can maintain the perovskite crystal structure and excellent photoelectric properties. Herein, a systematic review summarizes recent progress of the lead‐free halide perovskite photodetectors.

Abstract

Lead halide perovskite (LHP) has been widely researched in the photovoltaic field due to its highly attractive optoelectronic properties. Among the LHP‐based devices, the detectivity of the photodetector is as high as 10¹⁵ Jones. However, their practical application is limited by the toxicity of lead in perovskite and the inherent instability induced by the perovskite structure against moisture, heat, and light. To address these issues, tremendous efforts have been made to replace Pb²⁺ with other environmentally friendly metal cations such as Sn²⁺, Bi³⁺, Cu²⁺, Sb³⁺, and Ge²⁺. Thus, considerable breakthroughs in device performance and stability using lead‐free metal halide perovskite (LFMHP) have been made in recent years. In this review, the synthesis methods and strategies are focused for enhancing the material quality and photoelectric properties of LFMHPs and the recent research progress of LFMHP‐based photodetectors is summarized. This research provides some promising perspectives for high‐performance LFMHP photodetectors to achieve a broader range of practical applications in the future.

The perovskite single crystals (PVSK SCs) without detrimental grain boundaries and defects possess orders of magnitude larger diffusion length, carrier lifetime, and lower trap density in comparison with PVSK polycrystalline film. In this review, the recent progress of synthesis method of PVSK SCs and their application in optoelectronic devices is tried to be summarized.

Abstract

Recently, lead halide perovskite (PVSK) polycrystalline films have drawn much attention as photoactive material and scored tremendous achievements in solar cells, photodetectors, light‐emitting diodes, and lasers owing to their engrossing optoelectronic properties and facile solution‐processed fabrication. However, large amounts of grain boundaries unfavorably induce ion migration, surface defect, and poor stability, impeding PVSK polycrystalline film‐based optoelectronic devices from practical application. In comparison with the polycrystalline counterparts, PVSK single crystals (SCs) with lower trap density serve as a better platform for not only fundamental research but also device applications. In light of this, the idea of using PVSK single crystals (SCs) to construct the optoelectronic devices is then proposed. Since then, a series of synthesis methods of PVSK SCs have emerged. In this review, recent progress of synthesis method of PVSK SCs is tried to be summarized and their advantages and limitations are analyzed. And then, the optoelectronic properties including carrier dynamic, defects, ion migration, and instability issues in these 3D and 2D PVSK SCs are overviewed and accordingly the proper device configurations of corresponding solar cells, photodetectors, X‐ray, γ‐ray detectors, etc., are proposed. It is believed that this review can provide the guidance for the further development of PVSK SCs and their applications.

The detailed crystallization pathways of perovskite film manipulated by Lewis base additives are investigated using in situ grazing‐incidence wide‐angle scattering. The modulated crystallization process can be attributed to the intermolecular interaction between Lewis base molecules and perovskite precursors, which results in reduced defect density and ameliorative carrier behavior in high‐crystalline perovskite film as well as the elevated photovoltaic performance.

Abstract

The Lewis acid–base adduct approach has been widely used to form high‐crystalline perovskite films, but the complicated crystallization pathway and underlying film formation mechanism are still ambiguous. Here, the detailed crystallization process of perovskites manipulated by Lewis base additives has been revealed by in situ X‐ray scattering measurements. Through monitoring the film formation process, two distinct crystal growth stages have been definitely recognized: i) an intermediate phase‐dominated stage; and ii) a phase transformation stage from intermediates to crystalline perovskite phase. Incorporating Lewis base additives significantly prolongs the duration of stage 1 and induces a postponed phase transformation pathway, which could be responsible for retardant crystallization kinetics. Based on a series of experimental results and theoretical calculations, it is indicated that the manipulation of perovskite crystallization pathway is a result of the modulated molecular interactions between Lewis base additives and solution precursors. Owing to the retardant crystallization kinetics, enhanced‐quality perovskite films with reduced defect density and improved optoelectronic properties, as well as optimized photovoltaic performance have been demonstrated. This work provides in‐depth understanding with respect to perovskite crystallization pathway modulated by Lewis base additives and perceptive guidelines for precise regulation of crystallization kinetics of perovskite film toward high performance.

In article number 2007131, Yong Soo Cho, Da Bin Kim, and Jung Won Lee propose an in‐situ strain engineering method to substantially enhance the bending fracture behavior of flexible halide perovskite films. The strain‐dependent mechanical properties with structural simulations are extensively useful for halides‐based electronic and optoelectronic devices.

An unassisted overall solar‐to‐hydrogen efficiency of 8.54% is achieved on a monolithic integration of perovskite solar cells with low‐cost earth‐abundant co‐catalysts. The effective encapsulation of the perovskite solar cells and engineering of the co‐catalysts interfaces results in robust monolithic photoelectrodes, demonstrating continuous stable operation over 13 h. The excellent stability and good performance demonstrate the potential for efficient direct solar H₂ production.

Abstract

Metal halide perovskite solar cells have an appropriate bandgap (1.5–1.6 eV), and thus output voltage (>1 V), to directly drive solar water splitting. Despite significant progress, their moisture sensitivity still hampers their application for integrated monolithic devices. Furthermore, the prevalence of the use of noble metals as co‐catalysts for existing perovskite‐based devices undermines their use for low‐cost H₂ production. Here, a monolithic architecture for stable perovskite‐based devices with earth‐abundant co‐catalysts is reported, demonstrating an unassisted overall solar‐to‐hydrogen efficiency of 8.54%. The device layout consists of two monolithically encapsulated perovskite (FA_0.80MA_0.15Cs_0.05PbI_2.55Br_0.45) solar cells with low‐cost earth‐abundant CoP and FeNi(OH) _x co‐catalysts as the photocathode and photoanode, respectively. The CoP‐based photocathode demonstrates more than 17 h of continuous operation, with a photocurrent density of 12.4 mA cm⁻² at 0 V and an onset potential as positive as ≈1 V versus reversible hydrogen electrode (RHE). The FeNi(OH) _x ‐based photoanode achieves a photocurrent of 11 mA cm⁻² at 1.23 V versus RHE for more than 13 h continuous operation. These excellent stability and performance demonstrate the potential for monolithic integration of perovskite solar cells and low‐cost earth‐abundant co‐catalysts for efficient direct solar H₂ production.

Nature Reviews Materials, Published online: 19 January 2021; doi:10.1038/s41578-021-00280-5

An article in Nature Photonics reports a one-dopant alloying strategy for the synthesis of monodisperse perovskite nanoparticles for high-efficiency light-emitting diodes.

A facile yet effective thioamides passivation strategy is proposed to suppress defects at the surface and grain boundary of CsSnI₃ perovskite, which reduces the deep level trap density from undercoordinated Sn²⁺ and Sn²⁺ oxidation. The surface passivated CsSnI₃ perovskite solar cell (PSC) delivers a efficiency of 8.20% which is the highest among all lead‐free all‐inorganic PSCs.

Abstract

Despite remarkable progress in hybrid perovskite solar cells (PSCs), the concern of toxic lead ions remains a major hurdle in the path towards PSC's commercialization; tin (Sn)‐based PSCs outperform the reported Pb‐free perovskites in terms of photovoltaic performance. However, it is of a particularly great challenge to develop effective passivation strategies to suppress Sn(II) induced defect densities and oxidation for attaining high‐performance all‐inorganic CsSnI₃ PSCs. Herein, a facile yet effective thioamides passivation strategy to modulate defect state density at surfaces and grain boundaries in CsSnI₃ perovskites is reported. The thiosemicarbazide (TSC) with SCN functional groups can make strong coordination interaction with charge defects, leading to enhanced electron cloud density around defects and increased vacancy formation energies. Importantly, the surface passivation can reduce the deep level trap state defect density originated from undercoordinated Sn²⁺ ion and Sn²⁺ oxidation, significantly restraining nonradiative recombination and elongating the carrier lifetime of TSC treated CsSnI₃ PSCs. The surface passivated all‐inorganic CsSnI₃ PSCs based on an inverted configuration delivers a champion power conversion efficiency (PCE) of 8.20%, with a prolonged lifetime over 90% of initial PCE, after 500 h of continuous illumination. The present strategy sheds light on surface defect passivation for achieving highly efficient all‐inorganic lead‐free Sn‐based PSCs.

Nature Materials, Published online: 04 January 2021; doi:10.1038/s41563-020-00865-5

Diffuse X-ray scattering with femtosecond resolution shows the formation and relaxation of polaronic distortions in halide perovskites. These structural changes are also quantified and correlated to transient changes in carrier effective mass.

The perovskite single crystals (PVSK SCs) without detrimental grain boundaries and defects possess orders of magnitude larger diffusion length, carrier lifetime, and lower trap density in comparison with PVSK polycrystalline film. In this review, the recent progress of synthesis method of PVSK SCs and their application in optoelectronic devices is tried to be summarized.

Abstract

Recently, lead halide perovskite (PVSK) polycrystalline films have drawn much attention as photoactive material and scored tremendous achievements in solar cells, photodetectors, light‐emitting diodes, and lasers owing to their engrossing optoelectronic properties and facile solution‐processed fabrication. However, large amounts of grain boundaries unfavorably induce ion migration, surface defect, and poor stability, impeding PVSK polycrystalline film‐based optoelectronic devices from practical application. In comparison with the polycrystalline counterparts, PVSK single crystals (SCs) with lower trap density serve as a better platform for not only fundamental research but also device applications. In light of this, the idea of using PVSK single crystals (SCs) to construct the optoelectronic devices is then proposed. Since then, a series of synthesis methods of PVSK SCs have emerged. In this review, recent progress of synthesis method of PVSK SCs is tried to be summarized and their advantages and limitations are analyzed. And then, the optoelectronic properties including carrier dynamic, defects, ion migration, and instability issues in these 3D and 2D PVSK SCs are overviewed and accordingly the proper device configurations of corresponding solar cells, photodetectors, X‐ray, γ‐ray detectors, etc., are proposed. It is believed that this review can provide the guidance for the further development of PVSK SCs and their applications.

Replace Pb²⁺ cations in the lead halide perovskite with other suitable environmentally friendly metal cations can address the toxicity of lead and the structure‐induced intrinsic instability of lead halide perovskites. Moreover, it can maintain the perovskite crystal structure and excellent photoelectric properties. Herein, a systematic review summarizes recent progress of the lead‐free halide perovskite photodetectors.

Abstract

Lead halide perovskite (LHP) has been widely researched in the photovoltaic field due to its highly attractive optoelectronic properties. Among the LHP‐based devices, the detectivity of the photodetector is as high as 10¹⁵ Jones. However, their practical application is limited by the toxicity of lead in perovskite and the inherent instability induced by the perovskite structure against moisture, heat, and light. To address these issues, tremendous efforts have been made to replace Pb²⁺ with other environmentally friendly metal cations such as Sn²⁺, Bi³⁺, Cu²⁺, Sb³⁺, and Ge²⁺. Thus, considerable breakthroughs in device performance and stability using lead‐free metal halide perovskite (LFMHP) have been made in recent years. In this review, the synthesis methods and strategies are focused for enhancing the material quality and photoelectric properties of LFMHPs and the recent research progress of LFMHP‐based photodetectors is summarized. This research provides some promising perspectives for high‐performance LFMHP photodetectors to achieve a broader range of practical applications in the future.

Flexible organic solar cells (OSCs) come to the forefront of organic electronics. It's critical to develop high‐merit flexible transparent electrodes (FTEs). The work covers the frontier progress of PEDOT:PSS, graphene, metallic nanostructures, metal oxide/metal/metal oxide, Mxene and hybrid electrodes. It raises the awareness for the importance of developing the FTEs and reveals the critical role in flexible OSCs.

Abstract

Substantial effort has been devoted to both chemical doping and design of flexible transparent electrodes (FTEs) for flexible organic solar cells (OSCs) in the past decade. Poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate), graphene, metal nanostructures, metal oxide/ultrathin metal/metal oxide, Mxene, and their hybrid electrodes emerge to be the most promising flexible conducting materials over indium tin oxide. The FTE fabrications play a critical role in flexible OSCs. This feature review article summarizes the current status on the researches of the FTEs including various approaches and strategies to boost the conductivity, work function, mechanical flexibility, wettability, etc, which directly affect the performances of the flexible OSCs. The most cutting edge progresses on both FTEs and flexible OSCs are highlighted along the line. Advantages and plausible issues are pointed out. Perspectives are provided that can advance the developments of the flexible OSCs. This review raises the awareness for the importance of developing plenty of FTEs and reveals their critical role in flexible OSCs.

Tandem solar cells that pair silicon with a metal halide perovskite are a promising option for surpassing the single-cell efficiency limit. We report a monolithic perovskite/silicon tandem with a certified power conversion efficiency of 29.15%. The perovskite absorber, with a bandgap of 1.68 electron volts, remained phase-stable under illumination through a combination of fast hole extraction and minimized nonradiative recombination at the hole-selective interface. These features were made possible by a self-assembled, methyl-substituted carbazole monolayer as the hole-selective layer in the perovskite cell. The accelerated hole extraction was linked to a low ideality factor of 1.26 and single-junction fill factors of up to 84%, while enabling a tandem open-circuit voltage of as high as 1.92 volts. In air, without encapsulation, a tandem retained 95% of its initial efficiency after 300 hours of operation.

The moisture instability and unscalable fabrication protocols are still unsolved and blocking FACs‐based perovskite solar cells’ further applications. Here, high‐quality FACsPbI₃ films are fabricated by crown ether tailoring (which chelated with Cs⁺/Pb²⁺ ions) to inhibit the moisture invasion and stabilize the α‐phase FACsPbI₃, producing large‐area perovskite films and improving solar module performance.

Abstract

FACs‐based (FA⁺, formamidinium and Cs⁺, cesium) perovskite solar cells have gained great attention due to their remarkable light and thermal stabilities toward practical application of perovskite modules. However, the moisture instability and difficulty in scalable fabrication are still the main obstacles blocking their photovoltaic applications in current status. Here, the employment of novel interaction between crown ether with metal cations is introduced to tailor the uniform growth and inhibit moisture invasion during the crystallization of α‐phase FACsPbI₃, yielding the successful synthesis of high‐quality perovskite films in a large scale. Consequently, perovskite solar cells (PSC) modules in the total area of 4 × 4 and 10 × 10 cm² are readily fabricated with respective champion efficiencies of 16.69% and 13.84% and excellent stability over 1000 h. This facile scaling‐up strategy assisted by crown ether has shown great promise for pursuing efficient and highly stable large‐area PSC modules.

Half‐mixed Pb‐Sn perovskite solar cells with significantly improved performance and stability are prepared by introducing an ionic imidazolium tetrafluoroborate additive. The synergistic effects of IM cation and tetrafluoroborate anion enable efficient defect passivation at grain boundaries, reducing leakage current, and enlargement in grain size with relaxed lattice strain simultaneously, thereby exerting a remarkable impact on device performance and stability.

Abstract

Narrow‐bandgap mixed Pb‐Sn perovskite solar cells (PSCs) have great feasibility for constructing efficient all‐perovskite tandem solar cells, in combination with wide‐bandgap lead halide PSCs. However, the power conversion efficiency of mixed Pb‐Sn PSCs still lags behind lead‐based counterparts. Here, additive engineering using ionic imidazolium tetrafluoroborate (IMBF₄) is proposed, where the imidazolium (IM) cation and tetrafluoroborate (BF₄) anion efficiently passivate defects at grain boundaries and improve crystallinity, simultaneously relaxing lattice strain, respectively. Defect passivation is achieved by the chemical interaction between the IM cation and the positively charged under‐coordinated Pb²⁺ or Sn²⁺ ions, and lattice strain relaxation is realized by lattice expansion with the intercalation of BF₄ anions into the perovskite lattice. As a result, the synergistic effects of the cation and anion in the IMBF₄ additive greatly enhance the optoelectronic performance of half‐mixed Pb‐Sn perovskites, leading to much longer carrier lifetimes. The best‐performing half‐mixed Pb‐Sn PSC shows an efficiency above 19% with negligible hysteresis, while retaining over 90% of its initial efficiency after 1000 h in a nitrogen‐filled glovebox and showing a lifetime to 80% degradation of 53.5 h under continuous illumination.

In the structure of perovskite solar cells, N‐type semiconductor AgBiS₂ and dimethyl sulfoxide solvent mixed polyethylene glycol are used for perovskite film treatment. Finally, the perovskite solar cells with dual‐interfacial modification exhibite a remarkable improvement of power conversion efficiency from 18.58% to 21.19%, as well as show the excellent long‐term and moisture stability.

Although the research on perovskite solar cells (PSCs) has achieved rapid progress, its efficiency and stability still need to be further improved to meet the industrial requirements. The defects located inside the cells, on the surfaces, interfaces, or grain boundaries, will primarily affect carrier transportation through the formation of nonradiative recombination centers and hinder the further enhancement of the power conversion efficiency (PCE). Herein, a straightforward and simple defect passivation method is developed to increase the PCE and stability of PSCs. In the device, the N‐type semiconductor AgBiS₂ is introduced by thermal evaporation as a modified layer between the perovskite films and electron transport layer, which can improve the charge transport characteristic and bandgap optimization of PSCs. Simultaneously, dimethyl sulfoxide (DMSO) solvent mixed polyethylene glycol (PEG) is used for solvent annealing treatment, which can further improve the quality of perovskite film and reduce the trap density by increasing grain size and enhancing the crystallinity. As a result, the PSCs with dual‐interfacial modification exhibit a remarkable improvement in PCE from 18.58% to 21.19% with exceptional long‐term and moisture stability. This work provides an innovative insight for fabricating the stable and efficient PSCs toward the industrialization.

Scaling up perovskite solar cells (PSCs) to fabricate efficient perovskite solar modules (PSMs) is the fundamental for application. To make a goal of PSC commercialization achievable, device architecture designs, scalable deposition methods, perovskite morphology modulation, charge transport materials, electrode materials, and encapsulation methods are all important to fabricate stable, low‐cost, and high‐efficiency PSMs.

Abstract

Power conversion efficiency of perovskite solar cells (PSCs) has been boosted to 25.5% among the highest efficiency for single‐junction solar cells, making PSCs extremely promising to realize industrial production and commercialization. Scaling up PSCs to fabricate efficient perovskite solar modules (PSMs) is the fundamental for applications. Here, present progresses on scaling up PSCs are reviewed. The structure design for PSMs is discussed. Various scalable methods and related morphology control strategies for large‐area uniform perovskite films are summarized. Potential charge transport materials and electrode materials together with their scalable methods for low‐cost, efficient, and stable PSMs are also summarized. Besides, current attempts on encapsulation for improving stability and reducing lead leakage are introduced, and the calculated cost and environment influence of PSMs are also outlined.

The T2‐CNORH organic passivation layer (OPL) is used to obtain low energy loss organic photovoltaics. The T2‐CNORH‐deposited PM6:Y6 device exhibits a power conversion efficiency (PCE) of 15.5% with low non‐radiative energy loss (0.203 eV). Furthermore, the OPL improves various photoactive layer systems with a best PCE of 16.4% for the PM6:Y7 system.

Abstract

Despite the tremendous development of various high‐performing photoactive layers in organic photovoltaic (OPVs) cells, improving their performance remains the most important challenge in the field. Here, an effective and compatible strategy (i.e., the concept of vacuum deposition of an organic passivation layer (OPL) on the photoactive layer) is presented to enhance the efficiency of the state‐of‐the‐art photoactive systems, where easy‐deposition processable T2‐ORH and T2‐CNORH OPLs are used. After the deposition process, T2‐ORH forms 2D‐like edge‐on crystalline structure, and the 3D‐like face‐on crystalline growth is induced in T2‐CNORH. Resulting from its relatively higher crystalline features and increased wettability with the cathode interfacial material, the performance of T2‐CNORH‐deposited OPVs with both small and the scaled‐up areas surpass devices without OPL and with T2‐ORH. Experimental studies are conducted linking conductivity, electroluminescence quantum efficiency, carrier transport, and recombination dynamics to find the reasons for the performance difference. Furthermore, by applying the T2‐CNORH to other photoactive platforms, the efficiencies are enhanced by 4.4–9.0% relative to those of the corresponding control devices; an optimal 16.4% efficiency is achieved, which validates its great applicability for photoactive layers that will be developed in the near future.

A comprehensive review of the advances in poly(3,4‐ethylenedioxythiophene) (PEDOT) fundamentals (synthesis and doping/dedoping), properties’ tuning (transmittance, conductivity, work function, chemical reactivity), film fabrication methods to versatile photovoltaic applications (single‐junction, tandem, semitransparent, colorful, flexible and ultraflexible, fully printed solar cells) is presented.

Abstract

Poly(3,4‐ethylenedioxythiophene) (PEDOT) is a very unique polymer. It can be very conductive, highly transparent, and environmentally stable. It is highly switchable between its oxidation state and neutral state. It forms a micellar complex with polystyrene sulfonate in aqueous conditions, and then can be solution‐processible and printable. Based on these advantages, PEDOT has been widely used as conductors and transparent electrodes in electronics and optoelectronics. The device performance is highly correlated with the structure and properties of PEDOT. In this review, advances in the synthesis, optoelectronic and chemical properties are comprehensively described and analyzed, as well as the strategies for tuning these properties to fulfill the requirement for device applications. Film processing techniques (printing and transfer printing) for the conducting polymer are also presented. Then, the applications of PEDOT as conductors for versatile organic and perovskite solar cells (single‐junction, tandem, semitransparent, colorful, flexible and ultraflexible, fully printed solar cells) are summarized. Finally future study directions for PEDOT in terms of conductivity enhancement and application‐oriented formulations are discussed.

Three new dithieno[2,3‐d;2ʹ,3ʹ‐dʹ]benzo[1,2‐b;4,5‐bʹ]dithiophene based small‐molecule donors with different branching points for alkyl side chains are designed and synthesized for all small molecular organic solar cells. Modifying the branching points tunes the properties in the aggregation state, and an optimal nanofiber‐based hierarchical morphology for efficient charge separation and transport is successfully demonstrated.

Abstract

The optimization of bulk heterojunction morphology is one of the most challenging topics in all‐small‐molecule organic solar cells. Herein, three small molecular donors based on dithieno[2,3‐d;2′,3′‐d′]benzo[1,2‐b;4,5‐b′]dithiophene (DTBDT) unit by systematically moving the branching point of the alkyl chain have been designed, synthesized, and applied in organic solar cells. Modifying the branching points enables the properties of the aggregation state to be tuned, and an efficient nanofiber‐based hierarchical morphology is successfully demonstrated by combining with different nonfullerene acceptors. The molecules with far branching points can form nanofibers in active layers, and theses nanofibers help the charge separation and charge transport in a large donor‐rich or acceptor‐rich domain of approximately 100 nm. Using nonfullerrene Y6 as an acceptor, the highest power conversion efficiency of 14.78% is obtained, which is one of the highest efficiencies in all‐small‐molecule organic solar cells. The strategy of modification of alkyl side chain branching points can be a practical way to actualize crystallinity control and active layer morphology for improving the performance of all‐small‐molecule organic solar cells.

Three benzo[1,2‐b:4,5‐b']dithiophene‐thienothiophene π‐bridged N‐octylthieno[3,4‐c]pyrrole‐4,6‐dione‐based polymer donors named as PBDT‐X (X=H, F, Cl) are developed. While a planar accepting unit helps improve the crystallinity, all three photovoltaic parameters are simultaneously increased with the introduction of halogen atoms. PBDT‐Cl:Y6‐based devices yield an efficiency of 15.63%, attributed to the enhanced crystallinity, hole mobility, and domain purity.

Abstract

In this work, a new series of polymer donors consisting of thienothiophene π‐bridged N‐octylthieno[3,4‐c]pyrrole‐4,6‐dione (8ttTPD) and benzo[1,2‐b:4,5‐b']dithiophene (BDT) units for producing highly efficient organic solar cells (OSCs) paired with a Y6 acceptor is developed. The incorporation of the highly planar 8ttTPD unit enhances crystalline properties as well as hole mobilities of the BDT‐based polymers that typically have amorphous features. Further, the 2D side chains with halogen atoms (fluorine and chlorine) are designed as another handle to control the crystallinity and energy levels of the BDT‐based polymer donors: PBDT‐X (X = H, F, or Cl). Synergistic effects of incorporated 8ttTPD unit and the halogenated 2D side chain generate significantly enhanced charge transport and recombination properties of the OSCs, which is mainly attributed to optimized crystallinity and hole mobility of the polymer donors. Therefore, the PBDT‐Cl:Y6‐based OSCs exhibit the highest power conversion efficiency (PCE) of 15.63% with simultaneous improvements of open‐circuit voltage, short‐circuit current density, and fill factor, which outperforms the PCEs of PBDT‐H:Y6 (11.84%) and PBDT‐F:Y6 (14.86%).

A PCE of 16.17% is achieved in the doctor‐bladed PM6:Y6‐2Cl device with CF:CB co‐solvent, which is much higher than those of CF‐ and CB‐processed devices. Of note is that the use of this co‐solvent approach in the other two high‐performance photovoltaic systems is also confirmed, demonstrating its good generality of employing in the printing organic solar cells.

Abstract

Studies of the relationship between blend microstructure and photovoltaic performance are becoming more common, which is a prerequisite for rationally improving device performance. Non‐fullerene acceptors usually have planar backbone conformation and strong intermolecular packing, normally resulting in excessive phase separation. Herein, an effective co‐solvent blending strategy to turn the molecular organization of a chlorinated small molecule acceptor Y6‐2Cl and phase separation of the corresponding active layer with PM6 as donor is demonstrated. The in situ photoluminescence measurements and relevant morphological characterizations illustrate that the film‐forming process is fine‐turned when using the mixtures of chloroform (CF) and chlorobenzene (CB) solvents, and the blend showed high domain purity with suitable phase‐separated networks. Thus, better exciton dissociation and charge generation, more balanced charge transport, and less recombination loss are obtained in the co‐solvent blade‐coated devices. As a result, a maximum power conversion efficiency (PCE) of 16.17% is achieved, which is much higher than those of CF‐ and CB‐bladed devices (14.08% and 11.44%, respectively). Of note is that the use of this co‐solvent approach in the other two high‐performance photovoltaic systems is also confirmed, demonstrating its good generality of employing in the printing organic solar cells.

In the past decade, the perovskite solar cell (PSC) has attracted tremendous attention. The electron transport layer (ETL) is one of the most important functional layers in PSCs. This review provides an up‐to‐date summary of the developments in inorganic electron transport materials (ETMs) for PSCs. Strategies to optimize ETL, an outlook on current challenges and further development are discussed.

Abstract

In the past decade, the perovskite solar cell (PSC) has attracted tremendous attention thanks to the substantial efforts in improving the power conversion efficiency from 3.8% to 25.5% for single‐junction devices and even perovskite‐silicon tandems have reached 29.15%. This is a result of improvement in composition, solvent, interface, and dimensionality engineering. Furthermore, the long‐term stability of PSCs has also been significantly improved. Such rapid developments have made PSCs a competitive candidate for next‐generation photovoltaics. The electron transport layer (ETL) is one of the most important functional layers in PSCs, due to its crucial role in contributing to the overall performance of devices. This review provides an up‐to‐date summary of the developments in inorganic electron transport materials (ETMs) for PSCs. The three most prevalent inorganic ETMs (TiO₂, SnO_2, and ZnO) are examined with a focus on the effects of synthesis and preparation methods, as well as an introduction to their application in tandem devices. The emerging trends in inorganic ETMs used for PSC research are also reviewed. Finally, strategies to optimize the performance of ETL in PSCs, effects the ETL has on J–V hysteresis phenomenon and long‐term stability with an outlook on current challenges and further development are discussed.

Rb₄SnSb₂Br₁₂ can unexpectedly possess fertile low formation‐energy polymorphs holding van de Walls layered structures, exhibiting a wide range of bandgap covering the visible spectrum, thus potentially working in all‐perovskite multiple‐junction tandem solar cells. It may also be possible to achieve both type‐I and type‐II band alignment in single‐compound Rb₄SnSb₂Br₁₂ heterojunctions.

Abstract

Discovering new types of layered perovskites has great importance for designing novel optoelectronic devices. In this article, combining first‐principle calculations with global structure searching, it is found that Rb₄SnSb₂Br₁₂, a typical halide double perovskite, can unexpectedly possess fertile low formation‐energy polymorphs holding van de Walls (vdW) layered structures. Consequently, these polymorphs can be effectively classified into 12 types according to their local octahedral motifs, exhibiting a wide range of bandgap covering the visible spectrum. Interestingly, the structure‐dependent bandgap in these polymorphs can be well understood by developing a simple machine learning model. Moreover, as a layered system, the optoelectronic properties of Rb₄SnSb₂Br₁₂ can be effectively tuned by the layer thickness, and both type‐I and type‐II band alignment can be achieved in single‐compound Rb₄SnSb₂Br₁₂ heterojunctions. Finally, it is suggested that the Sn‐moderate condition can be considered to grow intrinsic p‐type Rb₄SnSb₂Br₁₂ with lower defect density. Those findings not only provide a promising material system for designing the vdW tandem solar cell, but also offer a new opportunity to achieve exotic optoelectronic applications in a single‐phase layered perovskite compound.

9.5% semitransparent solar cells with ultrahigh transmission in the visible range (50% AVT) are fabricated via inkjet printing. The effect of different photoactive layer ink solvents on the vertical stratification and performance is explored. The formulation of transport layer inks compatible with highly hydrophobic active layers and with scalable printing processes permits the use of a semitransparent electrode grid.

Abstract

New polymer donors and nonfullerene acceptors have elevated the performance and stability of solar cells to higher grounds. To achieve their full potential, they require their adaptation to scalable and cost‐effective solution manufacturing techniques for large area deposition. Likewise, formulating scalable solution‐based transport layer inks that are compatible with the photoactive layer is imperative. This manuscript reports the full integration of solution‐based transport layers and electrode alongside a PTB7‐Th:IEICO‐4F bulk heterojunction in inverted architecture through inkjet‐printing, resulting in power conversion efficiencies up to 12.4% opaque devices and 9.5% semitransparent devices with average visible transmittance values of 50.1%, including hole transport layer. The wetting envelope of the highly‐hydrophobic photoactive layer alongside the surface energy of candidate solutions and solvents allows the formulation of thick transport layer inks that are compatible with the drop‐on‐demand inkjet‐printing process and yield uniform and homogenous films. Moreover, the surface energy components of the donor and acceptor serves as a fingerprint to assess the vertical stratification of the photoactive layer with the inclusion of different solvents. This methodology addresses a scale‐up bottleneck of solution‐based transport layers for high‐efficiency organic cells, enabling its adaptation to high‐throughput techniques including slot‐die and roll‐to‐roll coating.

Using time‐resolved time‐of‐flight secondary ion mass spectrometry (tr‐ToF‐SIMS), electric field and light induced ion migration in hybrid organic‐inorganic perovskites are directly observed, revealing the migration behavior of methylammonium and halides. It is found that light‐induced methylammonium migration is more significant. In addition, the light with sub‐bandgap energy cannot induce ion migration.

Abstract

Unique optoelectronic, electronic, and sensing properties of hybrid organic–inorganic perovskites (HOIPs) are underpinned by the complex interactions between electronic and ionic states. Here, the photoinduced field ion migration in HOIPs is directly observed. Using newly developed local probe time‐resolved techniques, more significant CH₃NH₃ ⁺ migration than I⁻/Br⁻ migration in HOIPs is unveiled. It is found that light illumination only induces CH₃NH₃ ⁺ migration but not I⁻/Br⁻ migration. By directly observing temporal changes in bias‐induced and photoinduced ion migration in device conditions, it is revealed that light illumination suppresses the bias‐induced ion redistribution in the lateral device. These findings, being a necessary compensation of previous understandings of ion migration in HOIPs based on simulations and static and/or indirect measurements, offer advanced insights into the distinct light effects on the migration of organic cation and halides in HOIPs, which are expected to be helpful for improving the performance and the long‐term stability of HOIPs optoelectronics.

Comprehensive high‐pressure experiments show that the structural and optoelectronic properties of methylammonium lead iodide perovskite (MAPbI₃) will be drastically improved when hydrogen is replaced with deuterium in organic cation. The improved lattice stability boosts the photoluminescence intensity of MAPbI₃ by threefold. The pressure‐treated CD₃ND₃PbI₃ exhibits a nearly reversible emission properties, demonstrating its superior mechanical robustness. CD₃ND₃PbI₃‐based device also exhibits slower degradation of photovoltaic performance.

Abstract

The soft nature of organic–inorganic halide perovskites renders their lattice particularly tunable to external stimuli such as pressure, undoubtedly offering an effective way to modify their structure for extraordinary optoelectronic properties. Here, using the methylammonium lead iodide as a representative exploratory platform, it is observed that the pressure‐driven lattice disorder can be significantly suppressed via hydrogen isotope effect, which is crucial for better optical and mechanical properties previously unattainable. By a comprehensive in situ neutron/synchrotron‐based analysis and optical characterizations, a remarkable photoluminescence (PL) enhancement by threefold is convinced in deuterated CD₃ND₃PbI₃, which also shows much greater structural robustness with retainable PL after high peak‐pressure compression–decompression cycle. With the first‐principles calculations, an atomic level understanding of the strong correlation among the organic sublattice and lead iodide octahedral framework and structural photonics is proposed, where the less dynamic CD₃ND₃ ⁺ cations are vital to maintain the long‐range crystalline order through steric and Coulombic interactions. These results also show that CD₃ND₃PbI₃‐based solar cell has comparable photovoltaic performance as CH₃NH₃PbI₃‐based device but exhibits considerably slower degradation behavior, thus representing a paradigm by suggesting isotope‐functionalized perovskite materials for better materials‐by‐design and more stable photovoltaic application.

Novel asymmetric alkoxy and alkyl substitutions on the well‐known nonfullerene acceptor Y6 yield a molecule named Y6‐1O, and its photoelectric properties and photovoltaic performance are systematically compared with the two related symmetric molecules (Y6 and Y6‐2O), which suggests that this design strategy is promising and effective.

Abstract

In this paper, a strategy of asymmetric alkyl and alkoxy substitution is applied to state‐of‐the‐art Y‐series nonfullerene acceptors (NFAs), and it achieves great performance in organic solar cell (OSC) devices. Since alkoxy groups can have a significant influence on the material properties of NFAs, alkoxy substitution is applied to the Y6 molecule in a symmetric manner. The resulting molecule (named Y6‐2O), despite showing improved open‐circuit voltage (V _oc), yields extremely poor performance due to low solubility and excessive aggregation properties, a change that is due to the conformational locking effect of alkoxy groups. In contrast, asymmetric alkyl and alkoxy substitution on Y6, yields a molecule named Y6‐1O that can maintain the positive effect of V _oc improvement and obtain reasonably good solubility. The resulting molecule Y6‐1O enables highly efficient nonfullerene OSCs with 17.6% efficiency and the asymmetric side‐chain strategy has the potential to be applied to other NFA‐material systems to further improve their performance.