Mahui

Perovskite Solar Cells

In article number 2000001, Jingjing Chang and co‐workers demonstrate a novel approach where a short‐period deep‐ultraviolet (DUV) photoactivation process is employed to modify SnO₂ electron transport layers to achieve all‐inorganic perovskite solar cells with high efficiency exceeding 15% with good stability. The DUV treatment induces better energy level alignment, more ordered crystal growth, and reduces interface stress related defects.

Perovskite Solar Cells

In article number 2000093, Ming‐Chung Wu, Kun‐Mu Lee, and co‐workers use polyethylene glycol (PEG) as an additive for the perovskite active layer in lead‐reduced perovskite solar cells. The PEG can effectively control the surface morphology of the perovskite film and improve the charge carrier transport. For 10% lead‐reduced perovskite solar cells, a champion power conversion effi ciency of 16.1% is obtained without signifi cant hysteresis.

The surface and bulk defects of perovskite films are simultaneously passivated through the treatment of CsBr/methanol solution, in which the methanol helps CsBr penetrate the depth of the perovskite and reconstruct high‐quality films. This strategy can effectively improve the photovoltaic performance and operational stability of the resultant devices.

It is challenging to passivate defects that are buried in the depth of perovskite films; most of the reported passivation methods cannot reach the deep defects. Herein, methanol is adopted as a dual‐functional reagent to not only act as a solvent but also help the dissolved ions penetrate the depth of perovskite films. By treating the as‐prepared perovskite films with CsBr/methanol solution, Br⁻ ions can react with the undercoordinated Pb²⁺, and Cs⁺ ions can fill in the cation vacancies. This strategy enables surface and bulk defects passivation to be achieved simultaneously. The nonradiative recombination of the double‐passivated devices is significantly suppressed and the migration of charged defects is remarkably hindered. As a result, an improved power conversion efficiency of 19.5% and an open‐circuit voltage of 1.183 V is achieved. Moreover, the passivated device can retain ≈80% of the initial efficiency after working for 500 h at maximum power point under 1‐sun illumination, whereas the pristine device reaches 80% of the initial efficiency after only 90 h. This work demonstrates that surface and bulk defects passivation is critical to improve the efficiency and long‐term operational stability of the perovskite solar cells.

The role of energy offset between the optical bandgap and charge transfer (CT) state energies in nonradiative voltage loss ΔV _nr in organic solar cells is discussed. It is found that the ΔV _nr reduces considerably down to 0.185 V, when local excited and CT states are remarkably close in energy.

The voltage loss incurred by nonradiative charge recombination should be reduced to further improve the power conversion efficiency of organic solar cells (OSCs). This work discusses the nonradiative voltage loss in OSCs with systematically controlled energy offset between optical bandgap and charge transfer (CT) states. It is demonstrated that the nonradiative voltage loss is a function of the energy offset; it drops sharply with decreasing energy offset. By measuring the quantum yields of electroluminescence from OSCs and decay kinetics of CT states, it is found that the radiative decay rate of CT states becomes larger when the energy offset is negligible compared with those in conventional OSCs with sufficient energy offset. This behavior is rationalized by hybridization between CT and local excited states, resulting in a considerable enhancement of the oscillator strength of CT states. Based on a trend observed in this study, the precise mechanism by which the energy offset affects the nonradiative voltage loss is discussed.

Three novel donor–π‐bridge–acceptor (D–π–A)‐type small organic molecules are designed and synthesized as dopant‐free hole transport materials for perovskite solar cells. Combination of triazatruxene donor, terthiophene π‐bridge, and dicyanovinylene N‐ethyl rhodanine electron‐accepting unit as CI‐B3 creates well‐ordered edge‐on aggregated π–π stacking. Solar cell performance and long‐term stability are significantly improved.

Three donor–π‐bridge–acceptor (D–π–A)‐type organic small molecules coded CI‐B1, CI‐B2, and CI‐B3 are designed, synthesized, and used as dopant‐free hole transporting materials (HTMs) for perovskite solar cells (PSCs). The strong electron‐donating triazatruxene central core (D), terthiophene conjugated arms (π), and three different strong electron‐accepting units (A) provide high intramolecular charge transfer nature and eliminate the need of dopants during the fabrication of PSCs. HTMs are investigated to understand the effect of terminal functional groups on the PSC performance. Interestingly, due to the change of end‐capping, three different organizations of self‐assembly with π–π stacking are observed in the solid thin films. Dopant‐free CI‐B1, CI‐B2, CI‐B3, and spiro‐OMeTAD with dopants are used with triple cation perovskite composition Cs_0.1(MA_0.15FA_0.85)_0.9Pb(I_0.85Br_0.15)₃ (MA: CH₃NH₃ ⁺, FA: NHCHNH₃ ⁺) in n‐i‐p architecture. The cells prepared with CI‐B3 not only exhibits a comparable power conversion efficiency (PCE) of 17.54% to the state‐of‐art of spiro‐OMeTAD with dopants (18.02%), but also demonstrates improved long‐term stability, maintaining 88% of its original PCE after 1000 h of illumination. The superior photovoltaic performance, synthetic simplicity, dopant‐free nature, high durability, and edge‐on molecular orientation of CI‐B3 show its great promise as a HTM candidate for efficient and stable PSCs.

A synthetic polyhalide ligand (2‐picolyl)amine triiodide as a molecular glue is used to passivate halide vacancies at grain boundaries directionally and throughout grain bulk of perovskites. The inverted perovskite solar cells after passivation are allowed to be more efficient, and are profoundly stabilized in both ambient air and light‐soaking circumstances.

The fundamental instability of hybrid perovskite solar cells originates from the considerable halide vacancies. Furthermore, the local roles of halide vacancies between grain boundaries and grain bulk generally conflict, thus inhibiting complete passivation. To overcome this obstacle, a rational polyhalide ligand, di‐(2‐picolyl)amine triiodide, is designed as a molecular “glue” to achieve comprehensive passivation. Unlike a monohalide ligand, this ligand has multiple iodide ions and a quasiplanar tridentate chelation capability, contributing to directional passivation along the grain boundaries and overall passivation throughout the grain bulk. Using this molecular glue passivation, the best inverted solar cell yields an efficiency of 20.02%. Moreover, the relative stability of this cell in ambient air (≈40% humidity, 800 h aging) and under light‐soaking conditions (500 h aging) is profoundly enhanced by 33.33% and 22.26%, respectively. Herein, important insights into the design of passivating molecules to achieve low‐defect perovskites toward the development of multifunctional devices are provided.

n‐Type polymer semiconductors with a broad absorption and ultranarrow bandgap down to 1.28 eV are synthesized. When applied as electron acceptor materials, a power conversion efficiency of over 10% with a photoresponse reaching 950 nm is realized for all‐polymer solar cells.

Abstract

Compared to organic solar cells based on narrow‐bandgap nonfullerene small‐molecule acceptors, the performance of all‐polymer solar cells (all‐PSCs) lags much behind due to the lack of high‐performance n‐type polymers, which should have low‐aligned frontier molecular orbital levels and narrow bandgap with broad and intense absorption extended to the near‐infrared region. Herein, two novel polymer acceptors, DCNBT‐TPC and DCNBT‐TPIC, are synthesized with ultranarrow bandgaps (ultra‐NBG) of 1.38 and 1.28 eV, respectively. When applied in transistors, both polymers show efficient charge transport with a highest electron mobility of 1.72 cm² V⁻¹ s⁻¹ obtained for DCNBT‐TPC. Blended with a polymer donor, PBDTTT‐E‐T, the resultant all‐PSCs based on DCNBT‐TPC and DCNBT‐TPIC achieve remarkable power conversion efficiencies (PCEs) of 9.26% and 10.22% with short‐circuit currents up to 19.44 and 22.52 mA cm⁻², respectively. This is the first example that a PCE of over 10% can be achieved using ultra‐NBG polymer acceptors with a photoresponse reaching 950 nm in all‐PSCs. These results demonstrate that ultra‐NBG polymer acceptors, in line with nonfullerene small‐molecule acceptors, are also available as a highly promising class of electron acceptors for maximizing device performance in all‐PSCs.

A multi‐technique in situ structural and optoelectronic characterization on planar perovskite solar cells reveals perovskite amorphization and phase segregation as the crucial degradation mechanisms due to ion migration on a daily timescale. The degradation has a severe negative impact on the charge collection, which reduces the photocurrent and the power conversion efficiency. The mechanism is partially reversible after rest in the dark.

Abstract

The operation of halide perovskite optoelectronic devices, including solar cells and LEDs, is strongly influenced by the mobility of ions comprising the crystal structure. This peculiarity is particularly true when considering the long‐term stability of devices. A detailed understanding of the ion migration‐driven degradation pathways is critical to design effective stabilization strategies. Nonetheless, despite substantial research in this first decade of perovskite photovoltaics, the long‐term effects of ion migration remain elusive due to the complex chemistry of lead halide perovskites. By linking materials chemistry to device optoelectronics, this study highlights that electrical bias‐induced perovskite amorphization and phase segregation is a crucial degradation mechanism in planar mixed halide perovskite solar cells. Depending on the biasing potential and the injected charge, halide segregation occurs, forming crystalline iodide‐rich domains, which govern light emission and participate in light absorption and photocurrent generation. Additionally, the loss of crystallinity limits charge collection efficiency and eventually degrades the device performance.

Formamidinium methylammonium (FAMA)‐perovskite‐Cu:NiO and Al₂O₃/Cu:NiO composites are developed for highly stable and efficient perovskite solar cells. The composites not only improve the perovskite film quality but also suppress charge recombination with substantial reduction of trap density. The composites based devices yielded power conversion efficiency of 20.7% with fill factor of 80.5%. More importantly, unencapsulated cells showed significant enhancement of air‐stability, thermal‐ and photo‐stability with retaining 97% of PCE over 240 days under ambient conditions.

Abstract

To solve critical issues related to device stability and performance of perovskite solar cells (PSCs), FA_0.026MA_0.974PbI_{3− y} Cl _y ‐Cu:NiO (formamidinium methylammonium (FAMA)‐perovskite‐Cu:NiO) and Al₂O₃/Cu:NiO composites are developed and utilized for fabrication of highly stable and efficient PSCs through fully‐ambient‐air processes. The FAMA‐perovskite‐Cu:NiO composite crystals prepared without using any antisolvents not only improve the perovskite film quality with large‐size crystals and less grain boundaries but also tailor optical and electronic properties and suppress charge recombination with reduction of trap density. A champion device based on the composites as light absorber and Al₂O₃/Cu:NiO interfacial layer between electron transport layer and active layer yields power conversion efficiency (PCE) of 20.67% with V _OC of 1.047 V, J _SC of 24.51 mA cm⁻², and fill factor of 80.54%. More importantly, such composite‐based PSCs without encapsulation show significant enhancement in long‐term air‐stability, thermal‐ and photostability with retaining 97% of PCE over 240 d under ambient conditions (25–30 °C, 45–55% humidity).

A simple, generic, and effective concentration‐induced morphology manipulation approach is demonstrated to prompt the state‐of‐the‐art all‐small‐molecule (ASM) BTR‐Cl:Y6 and BTR:PC₇₁BM organic solar cells (OSCs) to a record level. This approach provides a promising way to delicately control the morphology toward high‐performance ASM OSCs.

Abstract

Morphology is a critical factor to determine the photovoltaic performance of organic solar cells (OSCs). However, delicately fine‐tuning the morphology involving only small molecules is an extremely challenging task. Herein, a simple, generic, and effective concentration‐induced morphology manipulation approach is demonstrated to prompt both the state‐of‐the‐art thin‐film BTR‐Cl:Y6 and thick‐film BTR:PC₇₁BM all‐small‐molecule (ASM) OSCs to a record level. The morphology is delicately controlled by subtly altering the prepared solution concentration but maintaining the identical active layer thickness. The remarkable performance enhancement achieved by this approach mainly results from the enhanced absorption, reduced trap‐assistant recombination, increased crystallinity, and optimized phase‐separated network. These findings demonstrate that a concentration‐induced morphology manipulation strategy can further propel the reported best‐performing ASM OSCs to a brand‐new level, and provide a promising way to delicately control the morphology towards high‐performance ASM OSCs.

Intensity‐dependent absolute photoluminescence studies on perovskite neat materials and partial cell stacks highlight how interface recombination can account for ideality factors between 1 and 2, commonly observed in perovskite devices. The findings are rationalized via a recombination model which details how interface recombination can lead to ideality factors of unity, in this case, not representative of a better device.

Abstract

The measurement of the ideality factor (n _id) is a popular tool to infer the dominant recombination type in perovskite solar cells (PSC). However, the true meaning of its values is often misinterpreted in complex multilayered devices such as PSC. In this work, the effects of bulk and interface recombination on the n _id are investigated experimentally and theoretically. By coupling intensity‐dependent quasi‐Fermi level splitting measurements with drift diffusion simulations of complete devices and partial cell stacks, it is shown that interfacial recombination leads to a lower n _id compared to Shockley–Read–Hall (SRH) recombination in the bulk. As such, the strongest recombination channel determines the n _id of the complete cell. An analytical approach is used to rationalize that n _id values between 1 and 2 can originate exclusively from a single recombination process. By expanding the study over a wide range of the interfacial energy offsets and interfacial recombination velocities, it is shown that an ideality factor of nearly 1 is usually indicative of strong first‐order non‐radiative interface recombination and that it correlates with a lower device performance. It is only when interface recombination is largely suppressed and bulk SRH recombination dominates that a small n _id is again desirable.

The nonionic polymer with multiple amino groups is introduced to passivate both metal‐ and halide‐induced defects of all‐inorganic CsPbI₂Br perovskite by coordination and hydrogen bonds, simultaneously. Consequently, a well‐controlled grain size, reduced defects, and reinforced phase structure of CsPbI₂Br film are achieved, which boosts the efficiency of perovskite solar cells up to 15.48% with excellent humidity stability.

The all‐inorganic CsPbI₂Br perovskite with superior thermal durability faces challenges of low‐phase stability and high moisture sensitivity. Herein, a nonionic additive of polyethyleneimine (PEI) with multiple amino groups is introduced to form hydrogen bond with I⁻/Br⁻ ions and coordinate with Pb²⁺/Cs⁺ ions simultaneously. The strong interplay between PEI and CsPbI₂Br achieves a well‐controlled grain size, reduced defects, and reinforced phase structure of CsPbI₂Br film, which boosts the power conversion efficiency (PCE) of perovskite solar cells to 15.48%. The hydrophobic long alkyl chain of PEI greatly improves the humidity resistance, retaining 81.9% of initial PCE of zjr unsealed device under 20 ± 5% relative humidity (RH) for 500 h. Remarkably, a PCE of 13.37% is achieved by the device based on CsPbI₂Br–PEI film processed under ambient condition (≈22% RH, ≈25 °C).

Post‐treating the mesoporous TiO₂/ZrO₂/carbon triple layer by alkali metal sulfonate compounds enables a significantly enhanced photovoltage for hole‐conductor‐free printable mesoscopic perovskite solar cells. The devices demonstrate high operational stability, retaining 91.7% of their initial efficiency after 1000 h continuous operation at the maximum power point under 1 sun illumination.

Triple‐mesoscopic perovskite solar cells (PSCs) based on TiO₂/ZrO₂/carbon architecture have attracted much attention due to their excellent long‐term stability and screen‐printing technique‐based fabrication process. However, the relatively low open‐circuit voltage (V _OC) limits the further improvement of power conversion efficiency (PCE) for triple‐mesoscopic PSCs. Herein, 2‐phenyl‐5‐benzimidazole sulfonate‐Na to post‐treat the triple‐mesoscopic structured scaffold is introduced. The conduction band of the mesoporous TiO₂ layer (electron transport layer [ETL]) is significantly shifted up from −4.22 to −4.11 eV, which favors the electron transfer from the perovskite absorber to the ETL. At the same time, the recombination at the interface of ETL/perovskite is effectively suppressed. Correspondingly, the V _OC and fill factor (FF) of the devices are enhanced without sacrificing the photocurrent density (J _SC). With optimal post‐treatment conditions, the champion device delivers a V _OC of 1.02 V and an FF of 0.70 with J _SC of 23.06 mA cm⁻², showing an overall PCE of 16.51%. After 1000 h continuous operation at the maximum power point under AM1.5G 1 sun illumination, the devices can maintain 91.7% of the initial efficiency. This simple procedure and significant photovoltage enhancement render this method promising for fabricating efficient PSCs based on mesoporous charge transport layers.

An effective intramolecular locking strategy is designed by introducing the central electron‐deficient quinoid to unfused ring A₁–D–A₂–D–A₁‐type nonfullerene small molecule acceptors (NF‐SMAs). The polymer solar cells (PSCs) based on BT2FIDT‐4Cl with difluorobenzothiadiazole central unit show a power conversion efficiency (PCE) of 12.5% with V _oc of near 1 V. This is the best result for nonfused ring NF‐SMAs with electron‐deficient A₂ unit in binary PSCs.

Abstract

Here, a pair of A₁–D–A₂–D–A₁ unfused ring core‐based nonfullerene small molecule acceptors (NF‐SMAs), BO2FIDT‐4Cl and BT2FIDT‐4Cl is synthesized, which possess the same terminals (A₁) and indacenodithiophene unit (D), coupling with different fluorinated electron‐deficient central unit (difluorobenzoxadiazole or difluorobenzothiadiazole) (A₂). BT2FIDT‐4Cl exhibits a slightly smaller optical bandgap of 1.56 eV, upshifted highest occupied molecular orbital energy levels, much higher electron mobility, and slightly enhanced molecular packing order in neat thin films than that of BO2FIDT‐4Cl . The polymer solar cells (PSCs) based on BT2FIDT‐4Cl:PM7 yield the best power conversion efficiency (PCE) of 12.5% with a V _oc of 0.97 V, which is higher than that of BO2FIDT‐4Cl ‐based devices (PCE of 10.4%). The results demonstrate that the subtle modification of A₂ unit would result in lower trap‐assisted recombination, more favorable morphology features, and more balanced electron and hole mobility in the PM7:BT2FIDT‐4Cl blend films. It is worth mentioning that the PCE of 12.5% is the highest value in nonfused ring NF‐SMA‐based binary PSCs with high V _oc over 0.90 V. These results suggest that appropriate modulation of the quinoid electron‐deficient central unit is an effective approach to construct highly efficient unfused ring NF‐SMAs to boost PCE and V _oc simultaneously.

Operationally stable and high‐efficiency all‐inorganic CsPbI_2.5Br_0.5 mixed‐halide perovskite solar cells are achieved for the first time, by introducing the different amount of PbI₂ in the all‐inorganic perovskite precursor. The 1.02‐PbI₂ devices maintain 76% of their initial efficiency (17.1%) after continuous power output at the maximum power point for 420 h under continuous full‐sun, AM 1.5G illumination (100 mW cm⁻²).

Abstract

Cesium‐based inorganic perovskites have recently attracted great research focus due to their excellent optoelectronic properties and thermal stability. However, the operational instability of all‐inorganic perovskites is still a main hindrance for the commercialization. Herein, a facile approach is reported to simultaneously enhance both the efficiency and long‐term stability for all‐inorganic CsPbI_2.5Br_0.5 perovskite solar cells via inducing excess lead iodide (PbI₂) into the precursors. Comprehensive film and device characterizations are conducted to study the influences of excess PbI₂ on the crystal quality, passivation effect, charge dynamics, and photovoltaic performance. It is found that excess PbI₂ improves the crystallization process, producing high‐quality CsPbI_2.5Br_0.5 films with enlarged grain sizes, enhanced crystal orientation, and unchanged phase composition. The residual PbI₂ at the grain boundaries also provides a passivation effect, which improves the optoelectronic properties and charge collection property in optimized devices, leading to a power conversion efficiency up to 17.1% with a high open‐circuit voltage of 1.25 V. More importantly, a remarkable long‐term operational stability is also achieved for the optimized CsPbI_2.5Br_0.5 solar cells, with less than 24% degradation drop at the maximum power point under continuous illumination for 420 h.

A secondary bond‐constructed isotropic electron transfer 3D‐network is fabricated based on biomass‐derived demethylated kraft lignin (D_MeKL). Secondary bonds successfully modify the contact of the perylene diiminde/active layer and conjugate‐blocked linkages in D_MeKL, to overcome anisotropy‐aroused electron transfer barriers at the cathode interface. The enhancement of cross/vertical‐sectional electron transfer performance and well‐matched energy levels yields the highest power conversion efficiency reported among biomaterial‐based organic solar cells.

Abstract

Fabricating high‐efficient electron transporting interfacial layers (ETLs) with isotropic features is highly desired for all‐directional electron transfer/collection from an anisotropic active layer, achieving excellent power conversion efficiency (PCEs) on nonfullerene acceptor (NFA) organic solar cells (OSCs). The complicated synthesis and cost‐consumption in exploring versatile materials arouse great interest in the development of binary‐doping interlayers without phase separation and flexible manipulation. Herein, for the first time, a novel cathode interfacial layer based on biomass‐derived demethylated kraft lignin (D_MeKL) is proposed. Features of multiple phenolic‐hydroxyl (PhOH) and uniform‐distributed render D_MeKL to exhibit an excellent bonding capacity with amino terminal substituted perylene diiminde (PDIN), and successfully form a high‐efficient isotropic electron transfer 3D network. Synchronously, secondary bonds completely modify conjugate‐blocked linkages of D_MeKL, significantly enhance the electron transporting performance on cross‐section and vertical‐sections, and repair the contact of PDIN with active layer. The D_MeKL/PDIN‐based 3D‐network exhibits well‐matched work function (WF) (–4.34 eV) with cathode (–4.30 eV) and energy level of electron acceptor (–4.11 eV). D_MeKL/PDIN‐based NFAs‐OSC shows excellent short‐circuit current density (26.61 mA cm^–2) and PCE (16.02%) beyond the classic PDIN‐based NFA‐OSC (25.64 mA cm^–2, 15.41%), which is the highest PCEs among biomaterials interlayers. The results supply a novel method to achieve high‐efficient cathode interlayer for NFAs‐OSCs.

An effective strategy is demostrated to create a double barrier that not only blocks the invasion of the moisture but also takes advantage of the permeated moisture to increase the moisture durability of perovskite films, which results in an n–i–p perovskite solar cell with moisture stability over 115 days in a relative humidity of 70% and a champion efficiency up to 21.34%.

Abstract

The rapid growth in the device efficiency of perovskite solar cells (PSCs) has raised great demands for tackling their long‐term stability upon external environmental stimuli that restricts the commercialization of PSCs, in which the instability upon exposure to moisture has been one of the major obstacles. Herein, an effective way of building up double barriers for moisture degradation of the perovskite films is demonstrated by modifying them with rationally selected hydrolyzable hydrophobic molecules (1H,1H,2H,2H‐perfluorooctyl trichlorosilane, PFTS). The layer of oligomer derived from the hydrolyzed PFTS at the surface that increases the hydrophobicity of perovskite film could serve as an efficient wall preventing the moisture invasion. The long‐term exposure of the film upon moisture allows for the formation of a secondary wall that employs the hydrolyzation of PFTS at grain boundaries, favoring defects passivation to further improve the humidity stability. Such gradual hydrolyzation is encouragingly helpful for the enhancement of the open‐circuit voltage of the PSCs from the original 1.136 up to 1.205 V. The PSCs constructed with the double barriers demonstrate excellent stability upon moisture and improved thermal and light stabilities, as well as a champion power conversion efficiency up to 21.34%.

A massive flux of environmental problems has impelled enormous research focus on photo(electro)catalysis due to its utilization of free and sustainable solar energy. Halide perovskites have witnessed burgeoning development from the pioneering TiO₂ and conventional oxide perovskites to Pb‐based halide perovskites. In this review, all aspects of halide perovskites are reviewed, from the crystal dimensions to light‐driven applications, toxicity, and stability.

Abstract

Photo(electro)catalysis has triggered ripples of excitement in environmental protection and energy conversion due to its potential applications in the degradation of organic pollutants, evolution of H₂ and O₂ from H₂O splitting, and reduction of CO₂ by utilizing solar energy. Over the past three years, halide perovskites, which render extraordinary charge transport capability in solar cells, have witnessed a burgeoning development in photocatalysis over the conventional oxide perovskites. This type of perovskite demonstrates a small surface area, limited light utilization, and high carrier recombination, resulting in inadequate reactant contact on catalyst surfaces and decreased catalytic activity. In this review, the progress of halide perovskites is presented starting from fundamental properties (i.e., synthesis and structure) to applications in light‐driven reactions with the focus on crystal dimensions, toxicity, and stability. In addition, computational studies on halide perovskites from electronic properties to catalytic mechanisms are presented to lay a foundation for future research and advancement in this field. Last, critical insights are provided into the existing limitations and favorable prospects for halide perovskites.

Operationally stable and high‐efficiency all‐inorganic CsPbI_2.5Br_0.5 mixed‐halide perovskite solar cells are achieved for the first time, by introducing the different amount of PbI₂ in the all‐inorganic perovskite precursor. The 1.02‐PbI₂ devices maintain 76% of their initial efficiency (17.1%) after continuous power output at the maximum power point for 420 h under continuous full‐sun, AM 1.5G illumination (100 mW cm⁻²).

Abstract

Cesium‐based inorganic perovskites have recently attracted great research focus due to their excellent optoelectronic properties and thermal stability. However, the operational instability of all‐inorganic perovskites is still a main hindrance for the commercialization. Herein, a facile approach is reported to simultaneously enhance both the efficiency and long‐term stability for all‐inorganic CsPbI_2.5Br_0.5 perovskite solar cells via inducing excess lead iodide (PbI₂) into the precursors. Comprehensive film and device characterizations are conducted to study the influences of excess PbI₂ on the crystal quality, passivation effect, charge dynamics, and photovoltaic performance. It is found that excess PbI₂ improves the crystallization process, producing high‐quality CsPbI_2.5Br_0.5 films with enlarged grain sizes, enhanced crystal orientation, and unchanged phase composition. The residual PbI₂ at the grain boundaries also provides a passivation effect, which improves the optoelectronic properties and charge collection property in optimized devices, leading to a power conversion efficiency up to 17.1% with a high open‐circuit voltage of 1.25 V. More importantly, a remarkable long‐term operational stability is also achieved for the optimized CsPbI_2.5Br_0.5 solar cells, with less than 24% degradation drop at the maximum power point under continuous illumination for 420 h.

Publication date: September 2020

Source: Nano Energy, Volume 75

Author(s): Xin Ke, Lingxian Meng, Xiangjian Wan, Yao Cai, Huan-Huan Gao, Yuan-Qiu-Qiang Yi, Ziqi Guo, Hongtao Zhang, Chenxi Li, Yongsheng Chen

This work highlights several pure‐bromide perovskites with ideal bandgaps for top‐cell absorbers in photovoltaic tandems. This is achieved by taking advantage of the bandgap‐bowing behavior of mixed tin–lead perovskites. Nontemplated samples as well as thin films are prepared by several routes including solvent‐free mechanochemical synthesis, spin‐coating, and single‐source thermal vacuum deposition.

Abstract

Herein the mechanochemical synthesis of inorganic as well as hybrid organic–inorganic monohalide perovskites with tunable bandgaps is reported. It is shown that the bandgap bowing known for iodide mixed Sn–Pb perovskites is also present in the pure bromide analogous. This results in technologically very interesting materials with bandgaps in the range of 1.7–1.9 eV. Similar bandgap perovskites are typically achieved by mixing two halides that are prone to segregate over time. This limits the achievable open circuit voltage. For monohalide perovskites this problem is eliminated, making these materials especially promising wide bandgap absorbers for tandem solar cells.

Morphology of an efficient ternary polymer blend is manipulated and the impacts on photovoltaic properties are explored. The morphology of ternary all‐polymer blending is determined by the interplay of the heterogeneous components from solution to solid state. Morphology with nanosized crystallite fibers in a mixing matrix assists in enhancing the solar cell performances.

Morphology control in multiblend all‐polymer solar cells is crucial for improving charge generation processes. Herein, it is demonstrated that the film morphology of the light‐harvesting layer of all‐polymer solar cells can be manipulated by incorporating a copolymer as the compatibilizer. Through in situ grazing‐incidence wide‐angle X‐ray scattering characterization, the insights of the crystallization kinetics of the polymer blends from solution to thin‐film state are provided. Of particular importance is that by kinetic and thermodynamic control of the film‐processing conditions, an optimal morphology with appropriate nanoscale fibrillar structure in a well‐mixed matrix is achieved. These findings indicate that the interplay between the crystalline regions and weakly/noncrystalline regions that are induced by the compatibilizer polymer plays a critical role in determining the morphology in multicomponent blend all‐polymer solar cells.

An environmentally stable acetamidinium (Aa⁺)‐incorporated MAPbI₃ film is successfully fabricated via hot casting in air. The large Aa⁺ immobilizes ions and improves crystal structure of MAPbI₃ through strong coordination bonds. The corresponding Aa–MAPbI₃ device shows 20.68% power conversion efficiency. Its 80% is maintained after 1300 h testing at 85 °C and 85 relative humidity (RH)%.

Ion migration in organometal halide perovskite solar cell (OHPSC) and crystal structure evolution of organometal halide perovskites (OHPVSKs) in air are considered as one of the critical factors for unstable performance and of the urgent issues for the reliability of OHPSCs. Herein, a novel cation of acetamidinium (Aa⁺) with stronger coordinated bond with I⁻ than methylammonium is induced into OHPVSK to stabilize its crystal structure. By incorporating Aa⁺ ions into OHPVSKs, the power conversion efficiency (PCE) of OHPSC without an encapsulation can maintain higher than 75% of its initial PCE after a 200 h humidity (60–80% relative humidity (RH) in air) or a 24 h thermal stress test (85 °C in dry N₂). The Aa–MAPbI₃ device exhibits an outstanding efficiency of 20.68%, and over 80% of initial PCE is maintained after a 1300 h damp heat as encapsulated. This novel cation can be easily incorporated into OHPVSK via a hot casting process in air with a high environmental tolerance as compared with that from the conventional coating process, which suffers from sophisticated crystallization steps and a strict processing atmosphere. It extends processing windows for OHPVSK fabrication and provides a promising path toward mass production and further commercialization.

This work introduces an ultra‐thin MgO layer between SnO₂ electron transport layer and CsPbIBr₂ perovskite layer to reduce the interface and nonradiative recombination. Meanwhile, the MgO provides better substrate for pure α‐phase perovskite growth. As a result, it achieves power conversion efficiency of 11.04% and maintains ≈90% after 1250 hours, with an open‐circuit voltage up to 1.36 V.

Although the power conversion efficiency (PCE) of thermally stable inorganic CsPbIBr₂ perovskite solar cells (PSCs) is over 10%, the severe interfacial and nonradiative recombination deteriorates the open‐circuit voltage (V _oc). Herein, an ultrathin wideband MgO is mediated between the SnO₂ electron transport layer (ETL) and the CsPbIBr₂ photoabsorber to passivate the undesirable recombination, thereby enhancing the V _oc. Meanwhile, the δ‐phase perovskite located at the interface between SnO₂ ETL and CsPbIBr₂ film is reduced after MgO modification, because the MgO layer provides a substrate for perovskite growth and reduces vacancy. Moreover, the tunneling effect and better band alignment effectively block holes and accelerate electrons to the electrode. Consequently, for optimal MgO‐modified devices, a shining improvement of V _oc from 1.25 to 1.36 V without short‐circuit current losses boosts the champion CsPbIBr₂ PSCs to obtain a PCE of 11.04%, which is the highest value among the pure‐CsPbIBr₂ PSCs. However, the MgO layer significantly reduces severe hysteresis and increases device stability.

The degradation of printed organic solar cells based on polymer PBDB‐T‐SF and small molecule IT‐4F is studied in operando for two different donor:acceptor ratios. Grazing incidence small angle X‐ray scattering, simultaneous current–voltage measurements and a theoretical model give insight into morphological changes during operation correlated with a decline of short‐circuit current.

Understanding the degradation mechanisms of printed bulk‐heterojunction (BHJ) organic solar cells during operation is essential to achieve long‐term stability and realize real‐world applications of organic photovoltaics. Herein, the degradation of printed organic solar cells based on the conjugated benzodithiophene polymer PBDB‐T‐SF and the nonfullerene small molecule acceptor IT‐4F with 0.25 vol% 1,8‐diiodooctane (DIO) solvent additive is studied in operando for two different donor:acceptor ratios. The inner nano‐morphology is analyzed with grazing incidence small angle X‐ray scattering (GISAXS), and current–voltage (I–V) characteristics are probed simultaneously. Irrespective of the mixing ratio, degradation occurs by the same degradation mechanism. A decrease in the short‐circuit current density (J _SC) is identified to be the determining factor for the decline of the power conversion efficiency. The decrease in J _SC is induced by a reduction of the relative interface area between the conjugated polymer and the small molecule acceptor in the BHJ structure, resembling the morphological degradation of the active layer.

A multifunctional conjugated benzene ammonium halide is introduced to enhance phase purity, reduce trap‐state density, and suppress nonradiative charge recombination. Blade‐coated solar cells based on stabilized formamidinium‐dominant perovskite compositions deliver an impressive efficiency of 22.0% and an improved operational stability.

Abstract

Currently, blade‐coated perovskite solar cells (PSCs) with high power conversion efficiencies (PCEs), that is, greater than 20%, normally employ methylammonium lead tri‐iodide with a sub‐optimal bandgap. Alloyed perovskites with formamidinium (FA) cation have narrower bandgap and thus enhance device photocurrent. However, FA‐alloyed perovskites show low phase stability and high moisture sensitivity. Here, it is reported that incorporating 0.83 molar percent organic halide salts (OHs) into perovskite inks enables phase‐pure, highly crystalline FA‐alloyed perovskites with extraordinary optoelectronic properties. The OH molecules modulate the crystal growth, enhance the phase stability, passivate ionic defects at the surface and/or grain boundaries, and enhance the moisture stability of the perovskite film. A high efficiency of 22.0% under 1 sun illumination for blade‐coated PSCs is demonstrated with an open‐circuit voltage of 1.18 V, corresponding to a very small voltage deficit of 0.33 V, and significantly improved operational stability with 96% of the initial efficiency retained under one sun illumination for 500 h.