JIALINGBO

A newly conceived n‐type small molecule (Y‐Th2) is incorporated as an efficient additive in perovskite solar cells, achieving simultaneous improvements in device performance and stability. Y‐Th2 effectively passivates defects in perovskite crystals by Lewis acid–base interactions and intermolecular hydrogen bonds, obtaining high‐quality perovskite film. The inverted structure device exhibits a power conversion efficiency of 21.5% with notably enhanced operational stability.

Abstract

Significant efforts have been devoted to modulating the grain size and improving the film quality of perovskite in perovskite solar cells (PSCs). Adding materials to the perovskite is especially promising for high‐performance PSCs, because the additives effectively control the crystal structure. Although the additive engineering approach has substantially boosted the efficiency of PSCs, instability of the perovskite film has remained a primary bottleneck for the commercialization of PSCs. Herein, a newly conceived bithiophene‐based n‐type conjugated small molecule (Y‐Th2) is introduced to PSCs, which simultaneously enhances the performance and stability of the cell. The Y‐Th2 effectively passivates the defect states in perovskite through Lewis acid–base interactions, increasing the grain size and quality of the perovskite absorber. An inverted PSC containing the Y‐Th2 additive achieves a power conversion efficiency of 21.5%, versus 18.3% in the reference device. The operational stability is also considerably enhanced by the improved hydrophobicity and intermolecular hydrogen bonds in the perovskite.

Novel interface polarization induced field‐effect passivation based on amorphous transition metal oxide is developed for efficient and ambient‐air‐stable perovskite solar cells. Comprehensive insights into the interaction between the field‐effect passivation, interface polarities, and the performance of the device have been elucidated in detail.

Abstract

Organolead halide hybrid perovskite solar cells (PSCs) have become a shining star in the renewable devices field due to the sharp growth of power conversion efficiency; however, interfacial recombination and carrier‐extraction losses at heterointerfaces between the perovskite active layer and the carrier transport layers remain the two main obstacles to further improve the power conversion efficiency. Here, novel field‐effect passivation has been successfully induced to effectively suppress the interfacial recombination and improve interfacial charge transfer by incorporating interfacial polarization via inserting a high work function interlayer between perovskite and holes transport layer. The charge dynamics within the device and the mechanism of the field‐effect passivation are elucidated in detail. The unique interfacial dipoles reinforce the built‐in field and prevent the photogenerated charges from recombining, resulting in power conversion efficiency up to 21.7% with negligible hysteresis. Furthermore, the hydrophobic interlayer also suppresses the perovskite decomposition by preventing the moisture penetration, thereby improving the humidity stability of the PSCs (>91% of the initial power conversion efficiency (PCE) after 30 d in 65 ± 5% humidity). Finally, several promising research perspectives based on field‐effect passivation are also suggested for further conversion efficiency improvements and photovoltaic applications.

Nature Energy, Published online: 05 October 2020; doi:10.1038/s41560-020-00705-5

Publication date: January 2021

Source: Nano Energy, Volume 79

Author(s): Shaobing Xiong, Ying Dai, Jianming Yang, Wei Xiao, Danqin Li, Xianjie Liu, Liming Ding, Pingping Gao, Mats Fahlman, Qinye Bao

Publication date: January 2021

Source: Nano Energy, Volume 79

Author(s): Ming-Hua Li, Tian-Ge Sun, Jiang-Yang Shao, Yu-Duan Wang, Jin-Song Hu, Yu-Wu Zhong

A low‐cost industrial organic pigment, quinacridone (QA), was applied as surface passivation agent for perovskite solar cells (PSCs) by solution processing of a soluble QA derivative followed by thermal annealing to convert it into insoluble QA. Passivation with strong interactions between QA molecules and metal halides, together with the hydrophobicity of QA coating, enabled highly efficient PSCs with remarkable stability.

Abstract

Surface passivation of perovskite solar cells (PSCs) using a low‐cost industrial organic pigment quinacridone (QA) is presented. The procedure involves solution processing a soluble derivative of QA, N,N‐bis(tert‐butyloxycarbonyl)‐quinacridone (TBOC‐QA), followed by thermal annealing to convert TBOC‐QA into insoluble QA. With halide perovskite thin films coated by QA, PSCs based on methylammonium lead iodide (MAPbI₃) showed significantly improved performance with remarkable stability. A PCE of 21.1 % was achieved, which is much higher than 18.9 % recorded for the unmodified devices. The QA coating with exceptional insolubility and hydrophobicity also led to greatly enhanced contact angle from 35.6° for the pristine MAPbI₃ thin films to 77.2° for QA coated MAPbI₃ thin films. The stability of QA passivated MAPbI₃ perovskite thin films and PSCs were significantly enhanced, retaining about 90 % of the initial efficiencies after more than 1000 hours storage under ambient conditions.

A terpolymer donor PM6‐Tz20 was developed by incorporating the third 5,5′‐dithienyl‐2,2′‐bithiazole (DTBTz) unit into the PM6 backbone. The introduction of DTBTz can tailor the molecular ordering, orientation, and aggregation properties, and then optimize the morphology and electrical properties of devices, ultimately improving fill factor (0.77) and thus device efficiency (17.6 %).

Abstract

Regulating molecular structure to optimize the active layer morphology is of considerable significance for improving the power conversion efficiencies (PCEs) in organic solar cells (OSCs). Herein, we demonstrated a simple ternary copolymerization approach to develop a terpolymer donor PM6‐Tz20 by incorporating the 5,5′‐dithienyl‐2,2′‐bithiazole (DTBTz, 20 mol%) unit into the backbone of PM6 (PM6‐Tz00). This method can effectively tailor the molecular orientation and aggregation of the polymer, and then optimize the active layer morphology and the corresponding physical processes of devices, ultimately boosting FF and then PCE. Hence, the PM6‐Tz20: Y6‐based OSCs achieved a PCE of up to 17.1% with a significantly enhanced FF of 0.77. Using Ag (220 nm) instead of Al (100 nm) as cathode, the champion PCE was further improved to 17.6%. This work provides a simple and effective molecular design strategy to optimize the active layer morphology of OSCs for improving photovoltaic performance.

Publication date: January 2021

Source: Nano Energy, Volume 79

Author(s): Tahmineh Mahmoudi, Yousheng Wang, Yoon-Bong Hahn

Quinoxaline derivatives, featuring with rotatable and chemically fixed thienyl substitutes, are introduced as the core for constructing dopant‐free hole transporting materials (HTMs). The coplanar π‐extended quinoxaline‐based HTM TQ4 achieves the best photovoltaic performance (exceed 21 %) among planar n‐i‐p structured dopant‐free perovskite solar cells.

Abstract

Developing dopant‐free hole transporting materials (HTMs) is of vital importance for addressing the notorious stability issue of perovskite solar cells (PSCs). However, efficient dopant‐free HTMs are scarce. Herein, we improve the performance of dopant‐free HTMs featuring with a quinoxaline core via rational π‐extension. Upon incorporating rotatable or chemically fixed thienyl substitutes on the pyrazine ring, the resulting molecular HTMs TQ3 and TQ4 show completely different molecular arrangement as well as charge transporting capabilities. Comparing with TQ3, the coplanar π‐extended quinoxaline based TQ4 endows enriched intermolecular interactions and stronger π–π stacking, thus achieving a higher hole mobility of 2.08×10⁻⁴ cm² V⁻¹ s⁻¹. It also shows matched energy levels and high thermal stability for application in PSCs. Planar n‐i‐p structured PSCs employing dopant‐free TQ4 as HTM exhibits power conversion efficiency (PCE) over 21 % with excellent long‐term stability.

The self‐assembled monolayer (SAM) has emerged as a powerful nanomaterial for improving performance of perovskite solar cells (PSCs). This review article covers recent studies that demonstrate direct benefits of SAM‐based interfacial engineering on the mechanistic understanding of the electronic functions of PSCs and their power conversion efficiency.

Abstract

Self‐assembled monolayers (SAMs), owing to their unique and versatile abilities to manipulate chemical and physical interfacial properties, have emerged as powerful nanomaterials for improving the performance of perovskite solar cells (PSCs). Indeed, in the last six years, a collection of studies has shown that the application of SAMs to PSCs boosts the performance of devices compared to the pristine PSCs. This review describes recent studies that demonstrate the direct advantages of SAM‐based interfacial engineering to power conversion efficiency (PCE) of PSCs. This review includes 1) a brief introduction on SAMs as interfacial engineering nanomaterials; 2) a thorough survey of molecules used in SAM‐engineered PSCs and analysis of chemical structures; 3) an extensive discussion on how SAMs affect the morphology of perovskite film and the electronic function of devices; and 4) a comprehensive summary of various types of approaches for producing SAM‐engineered PSCs. This review provides an insightful perspective to stimulate new ideas and innovation in the development of PSCs for the next‐generation photovoltaics and beyond.

Hole‐Transporting Materials

In article number 2000327, Aung Ko Ko Kyaw, Haibo Ma, Gongqiang Li, and co‐workers synthesize and systemically characterize saddle‐shaped small molecules α, β‐COTh‐Ph‐OMeTAD and β, β‐COTh‐Ph‐OMeTAD as dopant‐free hole‐transporting materials (HTMs) in inverted perovskite solar cells (i‐PSCs). High power conversion efficiencies (PCEs) (17.59% and 18.53%) and stable‐enhanced PSCs devices are achieved, and more than 80% of the maximum PCE is retained after storing in glove box for 150 days.

Herein, a one‐step solution‐processing of [MA_0.9Cs_0.1Pb(I_0.6Br_0.4)₃] wide‐bandgap perovskite using phenylhydrazine iodide with amino groups to successfully passivate the trap density within grain boundaries and increase the perovskite grain size is demonstrated. The reinforced morphology and grain boundaries treatment considerably enhance the power conversion efficiency from 12.16% for pristine to 14.63% for the treated devices.

Due to the attraction of fabricating highly efficient tandem solar cells, wide‐bandgap perovskite solar cells (PSCs) have attracted substantial interest in recent years. However, polycrystalline perovskite thin‐films show the existence of trap states at grain boundaries which diminish the optoelectronic properties of the perovskite and thus remains a challenge. Here, a one‐step solution‐processing of [ MA0.9Cs0.1Pb(I0.6Br0.4)3] wide‐bandgap perovskite using phenylhydrazine iodide with amino groups is demonstrated to successfully passivate the trap density within grain boundaries and increase the perovskite grain size. The reinforced morphology and grain boundaries treatment considerably enhanced the power conversion efficiency (PCE) from 12.16% for pristine to 14.63% for the treated devices. This strategy can be easily adopted to other perovskites and help realize highly efficient perovskite solar cells.

Perovskite solar cells (PSCs) and mini‐modules (PSM) scaling‐up strategies based on the design of their active areas and geometrical shapes lead to relevant power conversion efficiency (PCE) improvements. Optimized thermally co‐evaporated PSM with 6.4 cm² active area and geometrical fill factor of ≈91% reach a PCE of 18.4% with just 0.7% absolute losses as compared to 40 times smaller PSCs.

Perovskite solar cells (PSCs) have emerged as a promising technology for next‐generation photovoltaics thanks to their high power‐conversion‐efficiency (PCE). Scaling up PSCs using industrially compatible processes is a key requirement to make them suitable for a variety of applications. Herein, large‐area PSCs and perovskite solar modules (PSMs) are developed based on co‐evaporated MAPbI₃ using optimized structures and active area designs to enhance PCEs and geometrical fill factors (GFFs). Small‐area co‐evaporated PSCs (0.16 cm²) achieve PCE over 19%. When the PSCs are scaled‐up, the thin films high quality allows them to maintain consistent V _oc and J _sc, while their fill factors (FF), which depend on the substrate sheet resistance, are substantially compromised. However, PSCs with active areas from 1.4 to 7 cm² show a substantially improved FF when rectangular designs with optimized length to width ratios are used. Reasoning these results in the PSM design with optimal subcell size and for specific dead areas, a 6.4 cm² PSM is demonstrated with a record 18.4% PCE and a GFF of ≈91%. Combining the high uniformity of the co‐evaporation deposition with active areas design, it is possible to scale up 40 times the PSCs with PCE losses smaller than 0.7% (absolute value).

Mixtures of cations or halides with FAPbI₃ (where FA is formamidinium) lead to high efficiency in perovskite solar cells (PSCs) but also to blue-shifted absorption and long-term stability issues caused by loss of volatile methylammonium (MA) and phase segregation. We report a deposition method using MA thiocyanate (MASCN) or FASCN vapor treatment to convert yellow -FAPbI₃ perovskite films to the desired pure α-phase. NMR quantifies MA incorporation into the framework. Molecular dynamics simulations show that SCN^– anions promote the formation and stabilization of α-FAPbI₃ below the thermodynamic phase-transition temperature. We used these low-defect-density α-FAPbI₃ films to make PSCs with >23% power-conversion efficiency and long-term operational and thermal stability, as well as a low (330 millivolts) open-circuit voltage loss and a low (0.75 volt) turn-on voltage of electroluminescence.

Polymer additives in perovskite solar cells are found to act as barriers at the perovskite grain boundaries and hinder the ion migration, improving the device stability under both light irradiation and electrical stressing. The polymer incorporated perovskite solar cells have significantly increased electrical‐field tolerance with an increase in breakdown voltage from −0.4 to −2 V.

Abstract

Passivation of organometal halide perovskites with polar molecules has been recently demonstrated to improve the photovoltaic device efficiency and stability. However, the mechanism is still elusive. Here, it is found that both polymers with large and small dipole moment of 3.7 D and 0.6 D have negligible defect passivation effect on the MAPbI₃ perovskite films as evidenced by photothermal deflection spectroscopy. The photovoltaic devices with and without the polymer additives also have comparable power conversion efficiencies around 19%. However, devices with the additives have noticeable improvement in stability under continuous light irradiation. It is found that although the initial mobile ion concentrations are comparable in both devices with and without the additives, the additives can strongly suppress the ion migration during the device operation. This contributes to the significantly enhanced electrical‐field stress tolerance of the perovskite solar cells (PVSCs). The PVSCs with polymer additives can operate up to −2 V reverse voltage bias which is much larger than the breakdown voltage of −0.5 V that has been commonly observed. This study provides insight into the role of additives in perovskites and the corresponding device degradation mechanism.

Polymer additives in perovskite solar cells are found to act as barriers at the perovskite grain boundaries and hinder the ion migration, improving the device stability under both light irradiation and electrical stressing. The polymer incorporated perovskite solar cells have significantly increased electrical‐field tolerance with an increase in breakdown voltage from −0.4 to −2 V.

Abstract

Passivation of organometal halide perovskites with polar molecules has been recently demonstrated to improve the photovoltaic device efficiency and stability. However, the mechanism is still elusive. Here, it is found that both polymers with large and small dipole moment of 3.7 D and 0.6 D have negligible defect passivation effect on the MAPbI₃ perovskite films as evidenced by photothermal deflection spectroscopy. The photovoltaic devices with and without the polymer additives also have comparable power conversion efficiencies around 19%. However, devices with the additives have noticeable improvement in stability under continuous light irradiation. It is found that although the initial mobile ion concentrations are comparable in both devices with and without the additives, the additives can strongly suppress the ion migration during the device operation. This contributes to the significantly enhanced electrical‐field stress tolerance of the perovskite solar cells (PVSCs). The PVSCs with polymer additives can operate up to −2 V reverse voltage bias which is much larger than the breakdown voltage of −0.5 V that has been commonly observed. This study provides insight into the role of additives in perovskites and the corresponding device degradation mechanism.

Thermal annealing of 2D/3D perovskite heterostructures leads to beneficial diffusion passivation; however, it also causes lattice expansion of the 2D perovskite. Here a novel preparation strategy, simultaneously inhibiting lattice expansion, compensating the large tensile stress of 2D perovskite, and inducing diffusion passivation, is introduced. As a result, a certified efficiency of 20.22% is obtained.

Abstract

Lattice matching and passivation are generally seen as the main beneficial effects in 2D/3D perovskite heterostructured solar cells, but the understanding of the mechanisms involved is still incomplete. In this work, it is shown that 2D/3D heterostructure are unstable under common thermal processing conditions, caused by the lattice expansion of strained 2D perovskite. Therefore an innovative fabrication technology involving a compressively strained PEA₂PbI₄ layer is proposed to compensate the internal tensile strain and stabilize the 2D/3D heterostructure. Moreover, a small amount of PEA⁺ diffusing into the grain boundaries of 3D perovskite forms 2D perovskite and passivates the defects there. Combining the effects of strain compensation and diffusion passivation, the stabilized 2D/3D perovskite solar cells deliver a reproducible and robust laboratory power conversion efficiency (PCE) of 21.31% for the p‐i‐n type devices, along with a high V _OC of 1.18 V. A certified PCE of 20.22% is confirmed by an independent national metrology institute.

Aiming at stable and efficient perovskite light‐emitting diodes (PeLEDs), this work proposes an all‐inorganic strategy involving an insulator–perovskite–insulator device structure and cascade ZnS‐ZnSe electron transport layers, which improve charge‐injection efficiency and suppress the electric‐field‐induced ion migration channels. The findings provide an addressable approach access to future commercialization of PeLEDs.

Abstract

Stability issue is one of the major concerns that limit emergent perovskite light‐emitting diodes (PeLEDs) techniques. Generally, ion migration is considered as the most important origin of PeLEDs degradation. In this work, an all‐inorganic device architecture, LiF/perovskite/LiF/ZnS/ZnSe, is proposed to address this imperative problem. The inorganic (Cs_{1− x} Rb _x )_{1− y} K _y PbBr₃ perovskite is optimized with achieving a photoluminescence quantum yield of 67%. Depth profile analysis of X‐ray photoelectron spectroscopy indicates that the LiF/perovskite/LiF structure and the ZnS/ZnSe cascade electron transport layers significantly suppress the electric‐field‐induced ion migrations of the perovskite layers, and impede the diffusion of metallic atoms from cathode into perovskites. The as‐prepared PeLEDs display excellent shelf stability (maintaining 90% of the initial external quantum efficiency [EQE] after 264 h) and operational stability (half‐lifetime of about 255 h at an initial luminance of 120 cd m⁻²). The devices also exhibit a maximum brightness of 15 6155 cd m⁻² and an EQE of 11.05%.

B‐site doping provides a new approach to improve the optoelectronic properties and stability of CsPbX₃ inorganic perovskite. By judiciously selecting B‐site dopants and optimizing their concentration, B‐site doping strategy remarkably enhances the stability, tunes the bandgap, reduces the defects of CsPbX₃ inorganic perovskites, and thereby improves the photovoltaic performance of inorganic perovskite solar cells.

CsPbX₃ (X = I, Br) inorganic perovskite solar cells (PSCs) have been considered as one of the most appealing research topics in the fields of photovoltaic technologies in the past several years due to their excellent thermal stability and booming conversion efficiency. Nevertheless, there are still a large number of critical challenges and issues for inorganic PSCs, such as unstable phase structure of I‐rich inorganic perovskites at ambient condition, the wide bandgap of Br‐rich inorganic perovskites, and serious defect traps, hindering further development of inorganic PSCs. Recently, partially substituting Pb²⁺ with other metal ions has been shown to enhance the stability, tune the bandgap, reduce the defects of CsPbX₃ inorganic perovskites, and thereby improve the photovoltaic performance of inorganic PSCs. Herein, the recent progress in improving the photovoltaic performance of inorganic PSCs through the B‐site doping strategy is summarized, and the influence of the alternative metal ions on the stability and optoelectronic properties of inorganic perovskites and photovoltaic characteristics of CsPbX₃‐based PSCs is discussed. Finally, the issues that need to be understood in more detail are presented. It is believed that B‐site doping offers a practical strategy to gain high‐performance perovskite photovoltaic devices.

A new type of methylammonium‐free formamidinium (FA) based perovskites is reported. The low‐dimensional perovskite films are obtained in the presence of the FACl additive, and the role of Cl is investigated through grazing‐incidence X‐ray diffraction. Solar cell devices based on (PDA)(FA)₃Pb₄I₁₃ films show extremely high thermal stability and a remarkable PCE of 13.8 %.

Abstract

Currently, most two‐dimensional (2D) metal halide perovskites are of the Ruddlesden–Popper type and contain the thermally unstable methylammonium (MA) molecules, which leads to inferior photovoltaic performance and mild stability. Here we report a new type of MA‐free formamidinium (FA) based low‐dimensional perovskites, featuring a general formula of (PDA)(FA) _n−1Pb_nI_3n+1 with propane‐1,3‐diammonium (PDA) as the organic spacer cation. The perovskite films with well‐oriented crystal grains are attained under the assistance of the FACl additive, where the role of Cl is investigated through the grazing‐incidence X‐ray diffraction technique. The photovoltaic device based on the optimized (PDA)(FA)₃Pb₄I₁₃ film demonstrates a remarkable power conversion efficiency of 13.8 %, the highest record for the FA‐based 2D perovskite solar cells. In addition, compared to (PDA)(MA)₃Pb₄I₁₃, the MA‐containing analogue and a renowned stable 2D perovskite, both the (PDA)(FA)₃Pb₄I₁₃ films and their derived devices exhibit exceedingly higher thermal stability.