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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).

This review summarizes recent advances in the application of inorganic materials such as copper‐based compounds, with an emphasis on copper iodide and copper thiocyanate, transition metal chalcogenides, carbides, and nitrides as well as hybrid materials including copper compounds as hole and electron transport layers in organic and perovskite solar cells.

Abstract

As organic solar cells (OSCs) and perovskite solar cells (PVSCs) move closer to commercialization, further efforts toward optimizing both cell efficiency and stability are needed. As interfaces strongly affect device performance and degradation processes, interfacial engineering by employing various materials as hole transport layers (HTLs) and electron transport layers (ETLs) has been a very active field of research in OSCs and PVSCs. Among them, inorganic materials exhibit significant advantages in promoting device performance due to their excellent charge transporting properties and intrinsic thermal and chemical robustness. In this review, an extensive overview is provided of inorganic semiconductors such as copper‐based ones with emphasis on copper iodide and copper thiocyanate, transition metal chalcogenides, nitrides and carbides as well as hybrid materials based on these inorganic compounds that have been recently employed as HTLs and ETLs in OSCs and PVSCs. Following a short discussion of the main optoelectronic and physical properties that interfacial materials used as HTLs and ETLs should possess, the functionalities of the aforementioned materials as interfacial, charge transport, layers in OSCs and PVSCs are discussed in depth. It is concluded by providing guidelines for further developments that could significantly extend the implementation of these materials in solar cells.

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.

Ultrathin 2D titanium‐carbide MXenes (Ti₃C₂T_x quantum dots) and Cu_1.8S nanocrystals are simultaneously introduced to enhance the device performance of perovskite solar cells, achieving a remarkable hysteresis‐free power conversion efficiency of 21.64% with high long‐term air stability and light stability. The findings show that Ti₃C₂ and Cu_1.8S can act as superfast electron and hole tunnel for optoelectronic devices.

Abstract

The performance of perovskite solar cells (PSCs) strongly depends on the electron transport layer (ETL), perovskite absorber, hole transport layer (HTL), and their interfaces. Herein, the first approach to utilize ultrathin 2D titanium‐carbide MXenes (Ti₃C₂T _x quantum dots, TQD) by engineering the perovskite/TiO₂ ETL interface and perovskite absorber and introducing Cu_1.8S nanocrystals to perfect the Spiro‐OMeTAD HTL is represented. A significant hysteresis‐free power conversion efficiency improvement from 18.31% to 21.64% of PSCs is achieved after modifications with the enhanced short‐circuit current density, open‐circuit voltages, and fill factor. Various advanced characterizations, including femtosecond transient absorption spectroscopy, electrochemical impedance spectroscopy, and ultraviolet photoelectron spectroscopy, elucidate that the TQD/Cu_1.8S significantly contribute to the improved crystalline quality of the perovskite film with its large grain size and improved electron/holes extraction efficiencies at perovskite/ETL and perovskite/HTL interfaces. Furthermore, the long‐time ambient and light stability of PSCs are largely boosted through the TQD and/or Cu_1.8S nanocrystals doping, originating from the better crystallization of perovskite, suppressing the film aggregation and crystallization of HTL, and inhibiting the ultraviolet‐induced photocatalysis of the ETL. The findings highlight the TQD and Cu_1.8S can act as a superfast electrons and holes tunnel for the optoelectronic devices.

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.

Gigantic morphological transformation, from a continuous film to an assembly of well‐defined islands, is discovered when depositing LiBr on CsPbBr₃. The LiBr surface coating improves Cs/Pb chemical stoichiometry, reduces Br vacancies, and introduces n‐type doping. These effects collectively lead to a much enhanced (by a factor 11) and narrow (≈16 nm FWHM) light emission.

Abstract

Halide perovskites have been shown to be promising materials in making light‐emitting diodes. At present, almost all of perovskite materials are made by solution‐based synthesis. There are very limited reports on fabricating perovskite LEDs by vapor‐phase deposition (VPD), a method that can be easily scaled up for commercial production. In this paper, dual‐source VPD is used to fabricate stable CsPbBr₃ perovskite thin films with excellent luminescent properties. Scanning electron microscope and atomic force microscope studies show that CsPbBr₃ films, when coated with a thin LiBr overlayer, demonstrate an extraordinary mass transport at room temperature to re‐assemble into well‐defined islands. LiBr is also shown to passivate nonradiative defects and boost photoluminescence performance of the CsPbBr₃, improving the intensity by a factor of 11 for a nominal 18 nm perovskite film and leading to extremely narrow photoluminescence peaks (16 nm FWHM). This self‐assembled perovskite LED shows major improvement in the electroluminescence performance, almost tripling the brightness of reference devices. X‐ray photoelectron spectroscopy measurement shows that surface LiBr improves Cs/Pb chemical stoichiometry, reduces Br vacancies, and shift the Fermi energy level toward conduction band minimum.

This article reports the interfacial modification of mixed‐cation (Cs) _x (FA)_{1− x} PbI₃ perovskite films with guanidinium hydroiodide salt, which results in the formation of a low‐dimensional δ‐FAPbI₃‐like phase on the 3D perovskite surface. The presence of this thin layer facilitates charge transfer at interfaces and reduces charge carrier recombination pathways. Consequently, the performance and moisture stability of the control devices is improved compared to the unmodified device.

Abstract

Interfacial engineering between the perovskite and hole transport layers has emerged as an effective way to improve perovskite solar cell (PSC) performance. A variety of organic halide salts are developed to passivate the traps and enhance the charge carrier transport. Here, the use of guanidinium iodide (GuaI) for interfacial modification of mixed‐cation (Cs) _x (FA)_{1− x} PbI₃ perovskite films, which results in the formation of a low‐dimensional δ‐FAPbI₃‐like phase on the 3D perovskite surface, is reported. The presence of this thin layer facilitates charge transfer at interfaces and reduces charge carrier recombination pathways as evidenced by enhanced carrier lifetimes and favorable interfacial band alignment. As a result, the power conversion efficiency of the control device is boosted from 19.22% to 20.07%, mainly due to improved open‐circuit voltage (V _oc) and fill factor. Furthermore, the post‐treatment with GuaI improves the moisture stability of perovskite polycrystalline films and ambient stability of PSCs.

The Lewis base poly(ethylene glycol) (PEG) is introduced to assist the crystallization and growth of CsPbIBr₂ perovskite, yielding a pinhole‐free perovskite film with reduced defect states and favorable band energy level. Finally, the optimal device achieves a superior power conversion efficiency of 11.10% and great long‐term stability.

Abstract

Cesium lead mixed‐halide perovskite (CsPbIBr₂), as one of the all‐inorganic perovskites, has attracted great attention owing to its great ambient stability and suitable bandgap. Unfortunately, due to its low film coverage, high density of defects and unfavorable band energy level, the CsPbIBr₂ based solar cells suffer from low efficiency. In this work, the Lewis base poly(ethylene glycol) (PEG) is adopted as additive to modify the pure CsPbIBr₂. By optimizing the molecular weight and dosage of PEG, the resultant PEG:CsPbIBr₂ film possesses suppressed non‐radiative electron–hole recombination, a favorable energy band structure and a weaker sensitive to the moisture. As a result, the device based on the PEG:CsPbIBr₂ yields a champion power conversion efficiency (PCE) of 11.10%, with a open‐circuit voltage of 1.21 V, a short‐circuit current of 12.25 mA cm⁻², and a fill factor of 74.82%, which is 44.3% higher than its counterpart without PEG. Moreover, the PEG modified device shows excellent long‐term stability, retaining over 90% of the initial efficiency after 600 h storage in ambient condition without encapsulation. In comparison, the device without PEG shows an inferior stability with PCE sharply dropping to 0% within 50 h.

The 2D/3D perovskite film shows excellent stability in the ambient‐air condition and thermal condition, but its stability does not increase with the adding of (PEA)₂PbI₄ perovskites. Comparing different concentrations of 2D/3D perovskite solutions, it is found that at high concentrations the PEA⁺ can damage the crystal structure of 3D grain, which leads to accelerate its degradation and reduces its stability.

Abstract

The recent studies widely believe that the stability of (PEA)₂PbI₄/MAPbI₃ perovskite solar cells (PSCs) can be progressively increased with the addition of phenethylammonium ion (PEA⁺). Herein, it is reported on the stability variations after the addition of PEA⁺ into the MAPbI₃ film and it is demonstrated that excess PEA⁺ has negative effect on the stability of (PEA)₂PbI₄/MAPbI₃ perovskite film. X‐ray photoelectron spectroscopy, nuclear magnetic resonance, and X‐ray absorption fine structure spectroscopy are utilized to testify that the excess PEA⁺ damages the MAPbI₃ crystal structure and generates defects due to the strong coordination between NH₃ ⁺ from PEA⁺ and I⁻ from [PbI₆]⁴⁻. This work reveals that the stability of (PEA)₂PbI₄/MAPbI₃ perovskite films is not proportional to the addition of PEA⁺, which is of interest for the developing of stable 2D/3D PSCs.

Publication date: September 2020

Source: Nano Energy, Volume 75

Author(s): Ignasi Burgués-Ceballos, Yongjie Wang, M. Zafer Akgul, Gerasimos Konstantatos

Publication date: August 2020

Source: Nano Energy, Volume 74

Author(s): Jungrak Choi, Donguk Kwon, Byeongsu Kim, Kyungnam Kang, Jimin Gu, Jihwan Jo, Kwangmin Na, Junseong Ahn, Dionisio Del Orbe, Kyuyoung Kim, Jaeho Park, Jongmin Shim, Jung-Yong Lee, Inkyu Park

Publication date: September 2020

Source: Nano Energy, Volume 75

Author(s): Ravi D. Raninga, Robert A. Jagt, Solène Béchu, Tahmida N. Huq, Weiwei Li, Mark Nikolka, Yen-Hung Lin, Mengyao Sun, Zewei Li, Wen Li, Muriel Bouttemy, Mathieu Frégnaux, Henry J. Snaith, Philip Schulz, Judith L. MacManus-Driscoll, Robert L.Z. Hoye

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

Lawful behavior : A wide range of monodisperse lead perovskite nanocrystals with different cation and anion compositions and varying sizes were synthesized and their biexciton Auger recombination lifetimes measured by ultrafast spectroscopy (see picture). Volume scaling laws for the Auger lifetime of the nanocrystals were determined, thus enabling facile estimation of Auger rates, which are key parameters for perovskite‐nanocrystal‐based devices.

Abstract

Lead halide perovskite nanocrystals (NCs) hold strong promise for a variety of light‐harvesting, emitting, and detecting applications, all of which, however, could be complicated by multicarrier Auger recombination. Therefore, complete documentation of the size‐ and composition‐dependent Auger recombination rates of these NCs is highly desirable, as it can guide system design in many applications. Herein we report the synthesis and Auger measurements of monodisperse APbX₃ (A=Cs and FA; X=Cl, Br, and I) NCs in an extensive size range (ca. 3–9 nm). The biexciton Auger lifetime of all the NCs scales linearly with the NC volume. The scaling coefficient is virtually independent of the cation but rather depends sensitively on the anion, and is 0.035, 0.085, and 0.142 ps nm⁻³ for Cl, Br, and I, respectively. In all of these nanocrystals the Auger recombination is much faster than in standard CdSe and PbSe NCs (ca. 1 ps nm⁻³).

Rubidium‐incorporated air‐stable Cs_{(1 −  x )}Rb_xPbI₂Br perovskite solar cells are fabricated through a surface passivation strategy. The resulting devices under optimal conditions yield an efficiency of over 15% with excellent long‐term thermal as well as light‐soaking stability in ambient atmosphere.

Inorganic CsPbI₂Br perovskite has gained growing attention due to its potential for improving device performance and stability. However, the notorious phase transition from the photoactive to photoinctive phase in ambient atmosphere hinders its further development. Herein, air‐stable rubidium (Rb)‐incorporated Cs_{(1 −  x )}Rb_xPbI₂Br perovskite with guanidinium bromide (GABr) post‐treatment is demonstrated. The incorporation of smaller monovalent Rb cation contributes to a contraction of the perovskite crystal, leading to an improvement in structure stability. In addition, GABr modification induces a 2D/3D heterostructure perovskite with high crystallinity, appropriate surface morphology, favorable electronic properties, and significantly reduced trap‐state density. Consequently, the fabricated perovskite solar cells deliver a power conversion efficiency (PCE) of 15.6%, which is much higher than the 12.9% reported for reference CsPbI₂Br‐based devices. Meanwhile, the significantly enhanced long‐term (88% of initial PCE after 60 days), thermal (76% of initial PCE after 30 days) as well as light soaking (90% of initial PCE after 300 min) stability in ambient atmosphere is demonstrated.

The A‐site incorporation in the all‐inorganic cesium lead mixed halide (CsPbI₂Br) perovskite facilitates thermodynamic stability. The Rb cation‐incorporated Cs_1−x M _x PbI₂Br (M = Rb)‐based perovskite absorber layer processed by hot air method under ambient conditions with additives doped poly(3‐hexylthiophene‐2,5‐diyl) as a hole‐transporting layer produces a power conversion efficiency of more than 17%.

Due to its excellent thermal stability and high performance, inorganic cesium lead mixed halide (ABX₃, where A  = Cs, B = Pb, and X = I/Br) all‐inorganic perovskite solar cells (IPVSCs) have attracted much interest in optoelectronic applications. However, the film quality, enough absorption by desired film thickness, and nature of partial replacement of cations determine the stability of the CsPbI₂Br perovskite films. Herein, a hot air method is used to control the thickness and morphology of the CsPbI₂Br perovskite thin film, and the A‐site (herein, Cs⁺) cation is partially incorporated by rubidium (Rb⁺) cations for making the stable black phase under ambient conditions. The Rb cation‐incorporated Cs_1−x Rb _x PbI₂Br (x  = 0–0.03) perovskite thin films exhibit high crystallinity, uniform grains, extremely dense, and pinhole‐free morphology. The fabricated device with its Cs_0.99Rb_0.01PbI₂Br perovskite composition with poly(3‐hexylthiophene‐2,5‐diyl) as a hole‐transporting layer exhibits a power conversion efficiency (PCE) of 17.16%, which is much higher than that of CsPbI₂Br‐based IPVSCs. The fabricated Cs_0.99Rb_0.01PbI₂Br‐based IPVSC devices retain >90% of the initial efficiency over 120 h at 65 °C thermal stress, which is much higher than that of CsPbI₂Br samples.

The power conversion efficiency of inorganic perovskite solar cells with compositions CsPbI_1.8Br_1.2, CsPbI_2.0Br_1.0, and CsPbI_2.2Br_0.8 exhibits a high dependence on the initial annealing step that is found to significantly affect the crystallization and texture behavior of the final perovskite film. This work brings new thoughts on the critical factors that lead to high efficiency in inorganic perovskite solar cells.

Inorganic perovskites with cesium (Cs⁺) as the cation have great potential as photovoltaic materials if their phase purity and stability can be addressed. Herein, a series of inorganic perovskites is studied, and it is found that the power conversion efficiency of solar cells with compositions CsPbI_1.8Br_1.2, CsPbI_2.0Br_1.0, and CsPbI_2.2Br_0.8 exhibits a high dependence on the initial annealing step that is found to significantly affect the crystallization and texture behavior of the final perovskite film. At its optimized annealing temperature, CsPbI_1.8Br_1.2 exhibits a pure orthorhombic phase and only one crystal orientation of the (110) plane. Consequently, this allows for the best efficiency of up to 14.6% and the longest operational lifetime, T _S80, of ≈300 h, averaged of over six solar cells, during the maximum power point tracking measurement under continuous light illumination and nitrogen atmosphere. This work provides essential progress on the enhancement of photovoltaic performance and stability of CsPbI_3 − x Br _x perovskite solar cells.

A simple dual‐source vapor transport deposition (VTD) process is used to fabricate V‐shaped bandgap Sb₂(S,Se)₃ solar cells in one step. The improved efficiency of 7.27% with V _OC (0.46 V) and J _SC (29.6 mA cm⁻²) being synergistically improved is a new efficiency record of Sb₂(S,Se)₃ solar cells based on vacuum method over the previous record of 6.3%.

Antimony chalcogenides (including Sb₂S₃, Sb₂Se₃, and Sb₂(S,Se)₃ alloy) have emerged as promising solar absorber materials. Notably, the Sb₂(S,Se)₃ alloy possesses continuously tunable bandgap from 1.1 to 1.7 eV, which covers the ideal bandgap for single‐junction photovoltaics governed by the Shockley–Queisser theory. Moreover, the bandgap gradient provides effective ways for photogenerated carriers collection and has the potential for high‐efficient Sb₂(S,Se)₃ alloy solar cells. Herein, a V‐shaped distributional bandgap in Sb₂(S,Se)₃ solar cells is reported through a simple dual‐source vapor transport deposition process, enabling the synergetic increase of the open‐circuit voltage (V _OC) and short‐circuit current (J _SC). Through careful optimization, a power conversion efficiency of 7.27% under AM1.5G illumination is obtained, with V _OC and J _SC of 0.46 V and 29.6 mA cm⁻², respectively. This V‐shaped bandgap engineering provides an effective method to enhance the device performance and can be extended to other chalcogenide thin‐film solar cells such as Sn–X, Ge–X, Cu–Sb–X (X = S and Se), and so on.