Chen Weijie

Herein, the device efficiency is improved from 18.13% to 22.51% using NiO _x prepared by chemical bath deposition (CBD) as the hole-transport layer through seed-assisted growth and Cu ion doping. The overall conversion efficiency of 19.29% is achieved in the 36 cm² module. This work establishes an efficient CBD route for preparing planar NiO _x films for perovskite solar cells.

P-type NiO _x films are widely used as hole-transport layers (HTLs) in p–i–n perovskite solar cells (PSCs) owing to their wide bandgap, stability, and optical transmittance. Chemical bath deposition (CBD) is an effective method for growing metal oxide HTLs. However, NiO _x films prepared by the CBD method have pinholes because of their small grain size, which makes it difficult to cover the substrate in all directions, leading to severe carrier recombination at the interface between NiO _x and perovskite. Herein, the device efficiency is improved from 18.13% to 22.51% using NiO _x prepared by CBD with seed-assisted growth and Cu-ion doping as the HTL. The addition of crystal seeds significantly enhances the grain size, resulting in better substrate coverage by the prepared NiO _x films. Cu-ion doping improves the conductivity of the film and enhances its ability to extract holes. In addition, the results confirm that this method is suitable for the manufacturing of large-area modules and has good reproducibility. This research demonstrates an effective CBD method for creating NiO _x films for use in PSCs and offers a new approach for preparing inorganic HTLs using CBD.

Herein, a stable (>2 months in storage) low-molarity (0.8 m) nontoxic solvent based perovskite precursor ink is demonstrated to provide the fabrication of highly-efficient and stable all-printed annealing-free perovskite photovoltaic devices under ambient atmosphere. The prototype carbon-based hole transport material free perovskite mini-module of 100 cm² aperture-area shows >10% efficiency (stabilized), outstanding stability (T ₉₅ = 1000 h under ISOS-D-1 conditions) and low upscaling losses (8.3%rel dec⁻¹ per upscaled active area).

Abstract

Inkjet-printing is considered an emerging manufacturing process for developing perovskite solar cells (PSCs) with low material wastes and high production throughput. Up-to-now, all case studies on inkjet-printed PSCs are based on the exploitation of toxic solvents and/or high-molarity perovskite precursor inks that are known to enable the development of high-efficiency photovoltaics (PVs). The present study provides a new insight for developing lower-toxicity, high performance and stable (for more than 2 months) inkjet-printable perovskite precursor inks for fully ambient air processed PSCs. Using an ink composed of a green low vapor pressure noncoordinating solvent and only 0.8 m of perovskite precursors, the feasibility of fabricating high-quality and with minimum coffee-ring defects, annealing-free perovskite absorbent layers under ambient atmosphere is demonstrated. Noteworthily, the PSCs fabricated using the industry-compatible carbon-based hole transport material free architecture and the proposed ink present an efficiency >13% that is considered on the performance records for the under-consideration PV architecture employing an inkjet-printed active layer. Outstanding is also found the stability of the devices under the conditions determined by the ISOS-D-1 protocol (T ₉₅ = 1000 h). Finally, the perspective of upscaling PSCs to the mini-module level (100 cm² aperture area) is demonstrated, with the upscaling losses to be as low as 8.3%rel dec⁻¹ per upscaled active area.

Two dimeric-type acceptors, DIBP3F-Se and DIBP3F-S were synthesized with selenophone and thiophene as a bridge, respectively. Both exhibit O-shape conformation as determined by NMR and DFT simulations, which is driven by the robust π–π interaction of intramolecular terminal groups. The PM6 : DIBP3F-Se-based solar cell demonstrated an efficiency of 18.09 %.

Abstract

The determination of molecular conformations of oligomeric acceptors (OAs) and their impact on molecular packing are crucial for understanding the photovoltaic performance of their resulting polymer solar cells (PSCs) but have not been well studied yet. Herein, we synthesized two dimeric acceptor materials, DIBP3F-Se and DIBP3F-S, which bridged two segments of Y6-derivatives by selenophene and thiophene, respectively. Theoretical simulation and experimental 1D and 2D NMR spectroscopic studies prove that both dimers exhibit O-shaped conformations other than S- or U-shaped counter-ones. Notably, this O-shaped conformation is likely governed by a distinctive “conformational lock” mechanism, arising from the intensified intramolecular π–π interactions among their two terminal groups within the dimers. PSCs based on DIBP3F-Se deliver a maximum efficiency of 18.09 %, outperforming DIBP3F-S-based cells (16.11 %) and ranking among the highest efficiencies for OA-based PSCs. This work demonstrates a facile method to obtain OA conformations and highlights the potential of dimeric acceptors for high-performance PSCs.

Electron Beam Annealing

In article number 2300178, Heo and co-workers improved the power conversion efficiency and stability of organic photovoltaics by applying an electron beam annealing process to solution-based ZnO as an electron transport layer. The electrons extracted from the Ar plasma were irradiated to contribute to the improvement of efficiency and stability by removing impurities and improving the density of the ZnO thin film.

An origin of the instability of narrow-bandgap perovskite solar cells is found to be the unstable interface between acidic PEDOT:PSS and basic additive SnF₂ in the perovskite. The above interface is stabilized by modifying the acidity of PEDOT:PSS layer with NH₃∙H₂O. The derived all-perovskite tandem solar cells exhibit high efficiency of 25.3% with excellent stability.

Abstract

Developing all-perovskite tandem solar cells has been proved to be an effective approach to boost the efficiency beyond the Shockley–Queisser limit. However, the Sn-based narrow-bandgap (NBG) perovskite solar cells (PSCs) suffer from the relatively low photostability, which limits their further application in all-perovskite tandem solar cells. In this work, the instability of NBG PSCs is found to come from the commonly used acidic hole transporting material PEDOT:PSS, which reacts with the indispensable basic additive SnF₂ in the perovskite layer. By acidity control of PEDOT:PSS via aqueous ammonia, the NBG PSCs yield an efficiency of 22.0% with much improved photostability, which can maintain 91.3% of the initial value after 800 h illumination under AM 1.5G. As an application, the corresponding all-perovskite tandem cells exhibit a stabilized efficiency of 25.3% with 92% remaining after 560 h illumination. This work reveals an origin of instability of NBG PSCs and provides an effective approach to enhance the device stability, which can promote the development of all-perovskite tandem solar cells.

This work presents the use of a highly crystalline solid additive, phenoxathiin, for high-performance organic solar cells. Phenoxathiin additive has good miscibility with the acceptor and, therefore, extends the time for the active layer to form from the solution state to the film state, providing sufficient time for acceptor aggregation, allowing fine control during morphology development for enhancing photovoltaic performance.

Abstract

Volatile solid additives are an effective strategy for optimizing morphology and improving the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Much research has been conducted to understand the role of solid additives in active layer morphology. However, it is crucial to delve deeper and understand how solid additives affect the entire morphology evolution process, from the solution state to the film state and the thermal annealing stage, which remains unclear. Herein, the use of a highly crystalline solid additive, phenoxathiin (Ph), in D18-Cl:N3-based OSCs and study its impact on morphology formation and photovoltaic performance is presented. Owing to its good miscibility with the acceptor N3, Ph additive can not only extend the time for the active layer to form from the solution state to the film state, but also provide sufficient time for acceptor aggregation. After thermal annealing, Ph solid additive volatilizes better aligned the N3 molecules and formed a favorable hybrid morphology. Consequently, the D18-Cl:N3–based OSC exhibited an outstanding PCE of 18.47%, with an enhanced short-circuit current of 27.50 mA cm⁻² and a fill factor of 77.82%. This research is spurring the development of high-performance OSCs using solid additives that allow fine control during morphology development.

An ion-diffusion management strategy using dual passivating reagents of octylammonium iodide (OAI) and guanidinium chloride is demonstrated to realize all-interface passivation via one-step post-treatment, outperforming the traditional passivation with OAI. This strategy delivers a superior efficiency of 25.43% and an open-circuit voltage (V _OC) of 1.202 V, representing 94.05% of the Shockley–Queisser limit V _OC for the 1.55-eV perovskite absorber.

Abstract

Perovskite solar cells (PSCs) have demonstrated over 25% power conversion efficiency (PCE) via efficient surface passivation. Unfortunately, state-of-the-art perovskite post-treatment strategies can solely heal the top interface defects. Herein, an ion-diffusion management strategy is proposed to concurrently modulate the top interfaces, buried interfaces, and bulk interfaces (i.e., grain boundaries) of perovskite film, enabling all-interface defect passivation. Specifically, this method is enabled by applying double interactive salts of octylammonium iodide (OAI) and guanidinium chloride (GACl) onto the 3D perovskite surface. It is revealed that the hydrogen-bonding interaction between OA⁺ and GA⁺ decelerates the OA⁺ diffusion and therefore forms a dimensionally broadened 2D capping layer. Additionally, the diffusion of GA⁺ and Cl⁻ determines the composition of the bulk and buried interface of PSCs. As a result, n–inter–i–inter–p, i.e., five-layer structured PSCs can be obtained with a champion PCE of 25.43% (certified 24.4%). This approach also enables the substantially improved operational stability of perovskite solar cells.

A bridge link strategy for buried surface with perovskite provides superior defect passivation and energy alignment by employing the rationally selected mediator of glycocyamine. The interface enables slower crystal growth with enlarged grain size and absence of pinholes. The modified planar perovskite solar cell exhibits a champion power conversion efficiency of 24.70 % with an open circuit voltage of 1.194 V and retains 89 % of its initial efficiency after heating at 85 °C for 800 h.

Abstract

The interface of perovskite solar cells (PSCs) is significantly important for charge transfer and device stability, while the buried interface with the impact on perovskite film growth has been paid less attention. Herein, we use a molecular modifier, glycocyamine (GDA) to build a molecular bridge on the buried interface of SnO₂/perovskite, resulting in superior interfacial contact. This is achieved through the strongly interaction between GDA and SnO₂, which also appreciably modulates the energy level. Moreover, GDA can regulate the perovskite crystal growth, yielding perovskite film with enlarged grain size and absence of pinholes, exhibiting substantially reduced defect density. Consequently, PSCs with GDA modification demonstrate significant improvement of open circuit voltage (close to 1.2 V) and fill factor, leading to an improved power conversion efficiency from 22.60 % to 24.70 %. Additionally, stabilities of GDA devices under maximum power point and 85 °C heat both perform better than the control devices.

Imidazole-modified graphene quantum dots are introduced at SnO₂/perovskite interface as a dual bridge for fast electron transport and chemical linking, while absorbing detrimental ultraviolet (UV) light and re-emitting visible light through down-conversion, which enables increased efficiency of 24.11%, as well as improved long-term UV stability (300 h, 365 nm, 20 mW cm⁻²) and humidity stability (1650 h, 20–30 °C, RH 45–55%).

Abstract

Organic–inorganic hybrid perovskite solar cells (PVSCs) have achieved stunning progress during the past decade, which has inspired great potential for future commercialization. However, tin dioxide (SnO₂) as a commonly used electron transport layer with varied defects and energy level mismatch with perovskite contributes to the energy loss and limitation of charge extraction. Herein, imidazole-modified graphene quantum dots (IGQDs) are introduced as the interlayer, which plays a significant role in three aspects: 1) dually passivating the defects of SnO₂ and buried interface of perovskite by first-principles calculations; 2) accelerating the carrier extraction and transfer owing to ideal band alignment; and 3) improving light utilization through down-conversion proved by light intensity measurement. Consequently, the devices based on IGQDs/SnO₂ not only exhibit the champion power conversion efficiency (PCE) of 24.11%, but display a significantly enhanced ultraviolet (UV) stability retaining about 81% of their initial PCEs after continuous UV irradiation (365 nm, 20 mW cm⁻²) for 300 h. Moreover, the unencapsulated modified device remains 82% after storing for 1650 h in air (20–30 °C, RH 45–55%). This work furnishes a novel method for the combination of interfacial passivation and photon management, which holds out for the prospect of employment in other optoelectronic applications.

Strain regulation and nonradiative recombination suppression by 2D ligands in Pb/Sn-based narrow-bandgap perovskite solar cells (PSCs) are comprehensively understood. It is found that the mixture of electroneutral cation with long alkyl chain and iodate with short alkyl chain balances the tensile strain throughout perovskite film, which contributes to minimizing the energy losses from bulk and interfaces in PSCs.

Abstract

Mixed lead and tin (Pb/Sn) hybrid perovskites exhibit a great potential in fabricating all-perovskite tandem devices due to their easily tunable bandgaps. However, the energy deficit and instability in Pb/Sn perovskite solar cells (PSCs) constrain their practical applications, which renders defect passivation engineering indispensable to develop highly efficient and long-term stable PSCs. Herein, the mechanisms of strain tailoring and defect passivation in Pb/Sn PSCs by 2D ligands are investigated. The 2D ligands include electroneutral cations with long alkyl chain (LAC), iodates with relatively short alkyl chain (SAC) and their mixtures. This study reveals that LAC ligands facilitate the relaxation of tensile strain in perovskite films while SAC ligands cause strain buildup. By mixing LAC/SAC ligands, tensile strain in perovskite films can be balanced which improves solar cell performance. PSCs with admixed β-guanidinopropionic acid (GUA)/phenethylammonium iodide (PEAI) exhibit enhanced open circuit voltage and fill factor, which is attributed to reduced nonradiative recombination losses in the bulk and at the interfaces. Furthermore, the operational stability of PSCs is slightly improved by the mixed 2D ligands. This work reveals the mechanisms of 2D ligands in strain tailoring and defect passivation toward efficient and stable narrow-bandgap PSCs.

The hydrophobicity of low-loss self-assembled monolayer, [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz), impacts device reproducibility. Herein, the use of Al₂O₃ nanoparticles to pin the perovskite is demonstrated, which leads to improved wettability of the perovskite on Me-4PACz. This modification also leads to an enhanced charge carrier lifetime of the perovskite resulting in a champion power conversion efficiency approaching 20%.

The development of perovskite solar cells (PSCs) with low recombination losses at low processing temperatures is an area of growing research interest as it enables compatibility with roll-to-roll processing on flexible substrates as well as with tandem solar cells. The inverted or p–i–n device architecture has emerged as the most promising PSC configuration due to the possibility of using low-temperature processable organic hole-transport layers and more recently, self-assembled monolayers such as [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz). However, devices incorporating these interlayers suffer from poor wettability of the precursor leading to pin hole formation and poor device yield. Herein, the use of alumina nanoparticles (Al₂O₃ nanoparticles (NPs)) for pinning the perovskite precursor on Me-4PACz is demonstrated, thereby improving the device yield. While similar wettability enhancements can also be achieved by using poly[(9,9-bis(3′-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]dibromide (PFN-Br), a widely employed surface modifier, the incorporation of Al₂O₃ NPs results in significantly enhanced Shockley–Read–Hall recombination lifetimes exceeding 3 μs, which is higher than those on films coated directly on Me-4PACz and on PFN-Br-modified Me-4PACz. This translates to a champion power conversion efficiency of 19.9% for PSCs fabricated on Me-4PACz modified with Al₂O₃, which is a ≈20% improvement compared to the champion device fabricated on PFN-Br-modified Me-4PACz.

Poor-lead all-inorganic perovskite solar cells (PSCs) are considered as potential photovoltaic cells for practical application due to their low toxicity and high thermal stability. This review summarizes the recent efficiency progress of kinds of poor-lead inorganic PSCs and discusses effective strategies for further efficiency enhancement. Moreover, challenges and future perspectives in this promising field are proposed.

Abstract

Organic–inorganic hybrid perovskite solar cells (PSCs) have achieved an impressive certified efficiency of 25.7%, which is comparatively higher than that of commercial silicon solar cells (23.3%), showing great potential toward commercialization. However, the low stability and high toxicity due to the presence of volatile organic components and toxic metal lead in the perovskites pose significant challenges. To obtain robust and low-toxicity PSCs, substituting organic cations with pure inorganic cations, and partially or fully replacing the toxic Pb with environmentally benign metals, is one of the promising methods. To date, continuous efforts have been made toward the construction of highly performed low-toxicity inorganic PSCs with astonishing breakthroughs. This review article provides an overview of recent progress in inorganic PSCs in terms of lead-reduced and lead-free compositions. The physical properties of poor-lead all-inorganic perovskites are discussed to unveil the major challenges in this field. Then, it reports notable achievements for the experimental studies to date to figure out feasible methods for efficient and stable poor-lead all-inorganic PSCs. Finally, a discussion of the challenges and prospects for poor-lead all-inorganic PSCs in the future is presented.

Application of solution-processed doped Spiro-OMeTAD hole transport layers (HTLs) is being held back by poor stability and unsatisfied scalability. Herein, a versatile molecular implantation-assisted sequential doping approach is developed to improve the spatial doping uniformity and fabricate all-evaporated HTL. The resultant devices achieve a record efficiency of 23.4%, and exhibit impressive stability both in ambient and working conditions.

Abstract

Although hole transport layers (HTLs) based on solution-processed doped Spiro-OMeTAD are extremely popular and effective for their remarkable performance in n-i-p perovskite solar cells (PSCs), their scalable application is still being held back by poor chemical stability and unsatisfied scalability. Essentially, the volatile components and hygroscopic nature of ionic salts often cause morphological deformation that deteriorate both device efficiency and stability. Herein, a simple and effective molecular implantation-assisted sequential doping (MISD) approach is strategically introduced to modulate spatial doping uniformity of organic films and fabricate all evaporated Spiro-OMeTAD layer in which phase-segregation free HTL is achieved accompanied with high molecular density, uniform doping composition, and superior optoelectronic characteristics. The resultant MISD-based devices attain a record power conversion efficiency (PCE) of 23.4%, which represents the highest reported value among all the PSCs with evaporated HTLs. Simultaneously, the unencapsulated devices realize considerably enhanced stability by maintaining over 90% of their initial PCEs in the air for 5200 h and after working at maximum power point under illumination for 3000 h. This method provides a facile way to fabricate robust and reliable HTLs toward developing efficient and stable perovskite solar cells.

Publication date: 19 July 2023

Source: Joule, Volume 7, Issue 7

Author(s): Salma Siddika, Zhengxing Peng, Nrup Balar, Xinyun Dong, Xiaowei Zhong, Wei You, Harald Ade, Brendan T. O’Connor

Herein, the illustration of strategies for morphology controlling in all-small-molecule organic solar cells from donor materials design to device engineering is presented. The material design includes four aspects: central-core engineering, side-chain engineering, π-bridge engineering, and end-group engineering. The device engineering includes ternary strategy, posttreatment, additive strategy, and interface engineering.

Compared to polymer-based organic solar cells, all-small-molecule organic solar cells (ASM-OSCs) have garnered significant attention due to their well-defined chemical structures, lower batch-to-batch variation, straightforward synthesis and purification procedures, and easy to modulate properties. Recent developments in small molecule donors have enabled ASM-OSCs to achieve power conversion efficiencies in excess of 17%, gradually approaching those of polymer-based devices, and demonstrating considerable potential for commercialization. However, structural and morphological features in the all-small-molecule blend films, including crystallization behavior, phase separation, and molecular arrangement, play a crucial role in the photoelectric performance. This review systematically introduces and discusses recent advancements in ASM-OSCs in terms of design strategies for novel small molecule donors and device engineering. Additionally, the correlation between active layer morphology and structure and device performance is analyzed. Finally, the challenges and prospects of ASM-OSCs are discussed.

The simple replacement of flat blades (F) with V-shaped blades (V1) for manufacturing large-area organic solar cells (OSCs) facilitates the alignment of donor and acceptor molecules, optimizing the internal and external morphology of bulk-heterojunction films. The maximum device performance of OSCs fabricated using V1 exhibits an improved power conversion efficiency (PCE) of 15.97% relative to a PCE of 14.31% (F).

Organic solar cells (OSCs) have been studied widely as renewable energy resources for a few decades owing to their technological advantages, including low cost, light weight, flexibility, and the potential of large area printing. To upgrade the active layers of OSCs into an optimized form, a simple and straightforward strategy is demonstrated by introducing two types of size-controlled chevron (V)-shaped blades to delicately control PM6:Y6-based bulk heterojunction (BHJ) films under ambient conditions. A power conversion efficiency of 15.97% is achieved from the blade-coating methods using a small-sized V-shaped blade (V1), compared with 14.31% using a conventional flat blade (F). Using blade coating based on V1 improves the phase separation and molecular crystallinity in the active layer, which lead to decreased trap-assisted recombination and leakage current, finally increasing the short circuit current and fill factor. In addition, following these morphological changes in the optimal BHJ structure, the devices printed with V1 exhibit higher stability in the long-term shelf-life test (320 h storage in ambient conditions). Notably, strategy is a facile layer fabrication method that adopts V-shaped blades to achieve high device performance and stability for the practical use of large-area OSCs.

Herein, interface and bulk passivation strategies applied to p–i–n metal halide perovskite solar cells under accelerated testing at 70 °C and illumination are compared. This work finds that the hole-transport layer (HTL)/perovskite interface has the largest stability impact at elevated temperature and motivates the development of high-performance HTLs.

Metal halide perovskite photovoltaic performance required for commercial technology encompasses both efficiency and stability. Advances in both these parameters have recently been reported; however, these strategies are often difficult to directly compare due to differences in perovskite composition, device architecture, fabrication methods, and accelerated stressors applied in stability tests. In particular, it is found that there is a distinct lack of elevated temperature, operational (light and bias) stability data. Furthermore, significant testing is required to understand the interactions when combinations are used (e.g., additives used with posttreatments). Herein, individual and combined additive, posttreatment, and contact layer strategies from recent literature reports under standardized operational stability tests of p–i–n CsMAFA perovskites at 70 °C are evaluated. Through analysis of over 1000 devices, it is concluded that the hole-transport layer (HTL) is the most significant component impacting elevated temperature operational stability. This analysis motivates future development of high-performance HTLs.

The energy offsets are found to play an essential impact in charge photogeneration and recombination for Y-series molecule-based polymer solar cells. The energy transfer for donor exciton reduces with the increase ΔE _LUMO. After photoexciting, the Y-series molecule acceptor, exciton dissociates efficiency enhances with the increasing ΔE _HOMO. Besides, the higher the energy offsets, the lower the charge recombination rate in the ultrafast timescale.

Recent research has revealed that low-energy offset polymer solar cells (PSCs) are capable of a power conversion efficiency of over 19%. However, it is unclear how energy offsets and the charge photogeneration process are correlated. Herein, the effect of energy offsets on charge photogeneration dynamics for Y-series molecules (Y5, Y6, Y10, and BTP-4F-12)-based PSCs with the variations of the lowest unoccupied molecular orbital energy offsets (ΔE _LUMO) of 0.11–0.42 eV and the highest occupied molecular orbital energy offsets (ΔE _HOMO) of 0.08–0.23 eV utilizing steady-state and time-resolved spectroscopies is studied. The steady-state measurement shows that the probability of photoluminescence quenching via energy transfer for the donor exciton reduces with the increasing ΔE _LUMO. It is found that even in PM6:Y6 with the highest ΔE _LUMO, ≈18% of PM6 exciton dissociated via the path of “energy transfer first and then hole transfer,” manifesting the energy transfer also plays a vital role in the process of exciton dissociation. Furthermore, it is found that the PM6 exciton can efficiently dissociate under the ΔE _LUMO of 0.11 eV. After photoexcitation of the Y-series molecule acceptors, the exciton dissociation efficiency enhances with the increase of ΔE _HOMO. Besides, the higher energy offsets, the lower charge recombination rate in the ultrafast timescale has been found from the transient absorption measurement. These findings reveal that energy offsets are important for charge photogeneration and recombination in an ultrafast timescale for Y-series molecule-based PSCs, which may shed light on the design of high-performance PSCs.

This study represents a significant advancement in the field of perovskite solar cells (PSCs) by providing a comprehensive assessment of the mechanistic roles of different alkali cations in enhancing PSC performance. The mechanistic findings motivated the development of a dual-cation treatment strategy using CsF and RbF simultaneously, resulting in a remarkable increase in the power conversion efficiency and stability of the PSCs.

Abstract

The facile synthesis and beneficial properties of tin oxide have driven the development of efficient planar perovskite solar cells (PSCs). To increase the PSC performance, alkali salts are used to treat the SnO₂ surface to minimize the defect states. However, the underlying mechanism of alkali cations' role in the PSCs needs further exploration. Herein the effect of alkali fluoride salts (KF, RbF, and CsF) on the properties of SnO₂ and PSC performance is investigated. The results show different alkali have significant roles depending on their nature. Larger cations Cs⁺ preferably locate at the SnO₂ film surface to passivate surface defects and enhance conductivity, while smaller cations like Rb⁺ or K⁺ cations tend to diffuse into the perovskite layer to reduce trap density of the material. The former effect leads to enhanced fill factor while the latter effect increases the open circuit voltage of the device. It is then demonstrated that a dual cation post-treatment of the SnO₂ layer with RbF and CsF achieves PSC with a significantly higher power conversion efficiency (PCE) of 21.66% compared to pristine PSC with a PCE of 19.71%. This highlights the significance of defect engineering of SnO₂ using selective multiple alkali treatment to improve PSC performance.

By comparing and analyzing nicotinamide (NA) and its derivative 6-methylnicotinamide (CNA), the effects of molecular dipole and electronic configuration on perovskite defect passivation and photovoltaic performance of perovskite solar cells are systematically studied. CNA with its high-density electron cloud distribution improves thepower conversion efficiency to 24.33%, and improves the environment, thermal and optical stability of devices.

Abstract

The design of additives mainly involves selection of functional groups with coordination relationships with defects in perovskite materials. However, it is particularly important to further adjust the geometrical configuration and electronic structure of an additive. Here, the nicotinamide (NA) and its derivative 6-Methylnicotinamide (CNA) with electron-donor functional groups are comparatively analyzed to investigate the effect of molecular dipole and electronic configuration on the defect passivation of perovskite absorbers and the photovoltaic properties of perovskite solar cells (PSCs). Theoretical calculations demonstrate that the CNA molecule with its large molecular dipole combine with the undercoordinated Pb²⁺ ions in perovskite to form a higher binding energy, which is beneficial to improve the formation energy of Pb-related defects. Experimental characterization confirms that the CNA molecule significantly enhances the coordination effect between acylamino and undercoordinated defective Pb²⁺ cations, which is conducive to obtain high-quality, low-defect density of state, large grain size, and smooth surface perovskite absorbers. Thanks to the electronic configuration and electronic cloud distribution of CNA molecules, the PSCs yield impressive efficiency as high as 24.33% with excellent environmental storage, heat, and light stabilities. This research provides a research basis for designing additives with steric-charge-dependence to assist perovskite photovoltaics.