Chen Weijie

Publication date: December 2024

Source: Journal of Energy Chemistry, Volume 99

Author(s): Danqing Ma, Dongmei He, Qing Zhu, Xinxing Liu, Yue Yu, Xuxia Shai, Zhengfu Zhang, Sam Zhang, Jing Feng, Jianhong Yi, Jiangzhao Chen

Dimensional engineering for the application of 2D/3D perovskite heterojunction is an excellent strategy for efficient and stable inverted perovskite solar cells (PSCs), which can effectively passivate defects, release residual tensile stress, strengthen structural stability, ameliorate carrier transport and extraction, and so on. This method makes a great contribution to increasing the power conversion efficiency and operational stability of inverted PSCs.

Perovskite solar cells (PSCs) have attracted much attention in the field of photovoltaics, due to their high power conversion efficiency (PCE) and low cost. In recent years, inverted PSCs have achieved significant advancements in PCE and operational stability. Among the strategies for optimizing PCE and lifespan of inverted PSCs, dimensional engineering plays a critical role and garners increasing attention due to its versatile functions of passivating defects, releasing residual tensile stress, strengthening structural stability, ameliorating carrier transport and extraction, and so on. Considering the importance of dimensional engineering, a comprehensive and deep understanding of 2D perovskites and 2D/3D heterojunction is definitely necessary. In this review, first, the progress of low-dimensional perovskite light-harvesting materials in inverted PSCs is summarized. Subsequently, the advances in constructing 2D/3D perovskite heterojunctions, including 2D/3D bulk heterojunction within perovskite materials, 2D/3D interfacial heterojunction at the interface between perovskite film and carrier transport layer, and bottom-up 2D/3D perovskite heterojunction are discussed. The simultaneous construction of 2D/3D heterojunction at dual interfaces is highlighted. Finally, the legitimate outlook on the further development of dimensional engineering is proposed to advance the commercialization of inverted photovoltaic technology.

HCOO⁻ is introduced to effectively passivate halogen vacancy defects in perovskite film and enhance the crystalline quality. Its strong coordination with Pb²⁺ slows down crystal growth, enlarges grains, relieves lattice stress, strengthens ion migration barriers, and reduces the risk of lead leakage. The strategy improves the power conversion efficiency of device from 22.15% to 24.32%.

Abstract

Formamidinium lead iodide (FAPbI₃) perovskite has lately surfaced as the preferred contender for highly proficient and robust perovskite solar cells (PSCs), owing to its favorable bandgap and superior thermal stability. Nevertheless, volatilization and migration of iodide ions (I⁻) result in non-radiating recombination centers, and the presence of large formamidine (FA) cations tends to cause lattice strain, thereby reducing the power conversion efficiency (PCE) and stability of PSCs. To solve these problems, the lead formate (PbFa) is added into the perovskite solution, which effectively mitigates the halogen vacancy and provides tensile strain outside the perovskite lattice, thereby enhancing its properties. The strong coordination between the C═O of HCOO⁻ and Pb–I backbones effectively immobilizes anions, significantly increases the energy barrier for anion vacancy formation and migration, and reduces the risk of lead ion (Pb²⁺) leakage, thereby improving the operation and environmental safety of the device. Consequently, the champion PCE of devices with Ag electrodes can be increased from 22.15% to 24.32%. The unencapsulated PSCs can still maintain 90% of the original PCE even be stored in an N₂ atmosphere for 1440 h. Moreover, the target devices have significantly improved performance in terms of light exposure, heat, or humidity.

The introduction of the strong electron-deficient TPD group with a high dipole moment into the PM7 backbone can suppress entropy increase and enhance the crystallization tendency of the donor terpolymer, achieving high power conversion efficiencies (PCEs) of 18.26% and 19.40% in binary and ternary blends in polymer solar cells, respectively.

Abstract

The ternary copolymerization strategy has emerged as a promising strategy for developing high-efficiency donor polymers in polymer solar cells (PSCs). Terpolymers based on the star polymer PM6 have already realized good photovoltaic performance. However, challenges such as the intricate synthesis of fluorine-substituted benzodithiophene (F-BDT) unit of PM6 and entropy increase induced by backbone disorder have hindered the construction of high-performance donor terpolymers. In this work, these challenges are addressed by opting for the cost-effective chlorinated-substituted benzodithiophene unit (Cl-BDT) as an alternative to F-BDT and incorporating the large dipole moment and electron-deficient TPD group as the third component into the high-performance donor polymer of PM7. As expected, this approach effectively suppresses terpolymer backbone disorder while enhancing crystallinity, thereby optimizing morphology and improving charge generation and transport. Remarkably, the PM7-TPD-10-based device with 10% TPD replacement achieves a champion power conversion efficiency (PCE) of 18.26%. After introducing PM7-TPD-10 as the third component into D18:L8-BO blend, a dual mechanism for improving the efficiency to 19.40% is realized. This work demonstrates that the high dipole moiety as the third component to construct terpolymers is an important strategy to suppress the backbone disorder and increase the crystallinity, facilitating the optimization of morphology and device performance.

Eutectic gallium–indium liquid metal is used as the rear electrode for perovskite solar cells and their interfacial properties are explored. It is proposed that the oxide interlayer of the liquid metal serves two beneficial functions: A barrier against metal diffusion and a tunnel for enhancing charge transfer. The liquid metal electrode is readily recollected through a straightforward acid treatment.

Abstract

In this study, eutectic gallium–indium alloy (EGaIn) liquid metal is used as the rear electrode for perovskite solar cells (PSCs), where the interfacial properties of the device, particularly the beneficial roles of the surface oxide of the liquid metal, are explored. The findings demonstrate that the native oxide of the EGaIn electrode significantly affects the stability of photovoltaic performance and impedance characteristics including series and shunt resistances. Based on the results, the following hypothesis is formulated: the oxide interlayer serves two crucial functions of a barrier against metal diffusion and a tunnel for enhancing charge extraction and transfer. The results of elemental mapping and trap density calculation support the former function of the hypothesis that the oxide film can effectively prevent metal penetration into the perovskite layer. Furthermore, measurements involving capacitance−voltage and time-resolved photoluminescence confirm that the oxide film on the liquid metal eliminates the interfacial Schottky barrier, promoting efficient charge extraction and transfer processes. Finally, the investigation is extended to develop flexible PSCs using the EGaIn electrode, which consistently exhibits stable performance during repeated bending cycles. Notably, the EGaIn rear electrode can be readily removed and collected through a straightforward acid treatment, offering a promising avenue for efficient cell recycling.

First of its type, here this work illustrates the impact of π-conjugated molecular modifier to modulate the buried interface on the photovoltaic performance in regular planar perovskite solar cells. Benefiting from the high voltage measured from the fabricated perovskite cells, this work further reaches efficiency of 11.76% for the reduction of CO₂ to CO driven by simulated sunlight.

Abstract

This work proposes a methodology to increase the open-circuit voltage of perovskite solar cells via modulating the buried interface using π-conjugated molecules, featuring a push-pull electronic structure configuration. In the planar perovskite solar cells using tin oxide nanocrystal as an electron transport layer, the 2-methyl-1-aminobenzene derivatives with 4-(Heptafluoropropan)-2-methylaniline notable not only reduce the interfacial energy barrier but also passivate the defects at the buried interface. This modulation enhances the open circuit voltage of Cs_0.05(FA_0.85MA_0.15)_0.95Pb(I_0.85Br_0.15)₃ (bandgap ≈1.60 eV) perovskite solar cell to a high value of 1.241 V and thus the power conversion efficiency to 24.16% under standard testing condition. An even higher efficiency of 25.11% can be achieved when employing in the Cs_0.05MA_0.05FA_0.9PbI₃ (bandgap ≈1.54 eV) perovskite solar cell. The open circuit voltage (1.241 V) is among the highest in triple-cation perovskite solar cells which reaches 95% Shockley–Queisser limit. A solar-to-CO conversion efficiency of 11.76% can be achieved in the fabricated perovskite solar minimodule driven carbon dioxide electrolyzer. This demonstrates the potential of utilizing perovskite solar cells for CO₂ conversion as a clean and green energy environment.

Lattice-matched coordination of Sn-Pb alloying perovskites by cobweb-like quadrangular macrocyclic porphyrin chelators beneficially regulates the surface metal abundance for versatile construction of high-efficiency single-junction and tandem solar cells with extended lifespan and minimized toxic Pb leakage.

Abstract

Narrow-bandgap Sn-Pb alloying perovskites showcased great potential in constructing multiple-junction perovskite solar cells (PSCs) with efficiencies approaching or exceeding the Shockley-Queisser limit. However, the uncontrollable surface metal abundance (Sn²⁺ and Pb²⁺ ions) hinders their efficiency and versatility in different device structures. Additionally, the undesired Pb distribution mainly at the buried interface accelerates the Pb leakage when devices are damaged. In this work, a novel strategy is presented to modulate crystallization kinetics and surface metal abundance of Sn-Pb perovskites using a cobweb-like quadrangular macrocyclic porphyrin material, which features a molecular size compatible with the perovskite lattice and robustly coordinates with Pb²⁺ ions, thus immobilizing them and increasing surface Pb abundance by 61%. This modulation reduces toxic Pb leakage rates by 24-fold, with only ∼23 ppb Pb in water after severely damaged PSCs are immersed in water for 150 h.This strategy can also enhance chemical homogeneity, reduce trap density, release tensile strain and optimize carrier dynamics of Sn-Pb perovskites and relevant devices. Encouragingly, the power conversion efficiency (PCEs) of 23.28% for single-junction, full-stack devices and 21.34% for hole transport layer-free Sn-Pb PSCs are achieved.Notably, the related monolithic all-perovskite tandem solar cell also achieves a PCE of 27.03% with outstanding photostability.

Nature Energy, Published online: 07 August 2024; doi:10.1038/s41560-024-01600-z

Additives derived from polycyclic aromatic hydrocarbons, particularly dibenzofuran (DBF), refine the crystallinity and free volume fraction of within photoactive layers, optimizing the efficiency and stability of organic solar cells.

Abstract

Volatile solid additives have emerged as a promising strategy for enhancing film morphology and promoting the power conversion efficiency (PCE) of organic solar cells (OSCs). Herein, a series of novel polycyclic aromatic additives with analogous chemical structures, including fluorene (FL), dibenzothiophene (DBT), and dibenzofuran (DBF) derived from crude oils, are presented and incorporated into OSCs. All these additives exhibit strong interactions with the electron-deficient terminal groups of L8-BO within the bulk-heterojunction OSCs. Moreover, they demonstrate significant sublimation during thermal annealing, leading to increase free volumes for the rearrangement and recrystallization of L8-BO. This phenomenon leads to an improved film morphology and an elevated glass-transition temperature of the photoactive layers. Consequently, the PCE of the PM6:L8-BO blend has been boosted from 16.60% to 18.60% with 40 wt% DBF additives, with a champion PCE of 19.11% achieved for ternary PM6:L8-BO:BTP-eC9 OSCs. Furthermore, the prolonged shelf and thermal stability have been observed in OSCs with these additives. This study emphasizes the synergic effect of volatile solid additives on the performance and thermal stability of OSCs, highlighting their potential for advancing the field of photovoltaics.

CBD-SnO₂ buried interface has been modified by a multifunctional molecule, MES-K, to passivate interfacial defects, tailor perovskite crystallization, release residual stress, and tune energy bands as well. A high PCE of 25.14% on 0.09 cm² device and 24.45% on 1 cm² device, with negligible hysteresis, are achieved with excellent storage and light illumination stability.

Abstract

Tin oxide (SnO₂) based on chemical bath deposition method (CBD-SnO₂) is considered as an ideal electron transporting layer for perovskite solar cells (PSCs), however, the research on precise regulation toward CBD-SnO₂ layer is lacking. Here, the study introduces a multifunctional molecule, potassium 2-(N-morpholino)ethanesulfonate (MES-K), on the surface of CBD-SnO₂ layer to synergistically modify the buried interface between the SnO₂ and perovskite. It is found that MES-K introduction can passivate interfacial defects, favor the perovskite crystal growth, and improve carrier transportation. 25.14% efficiency of small-size perovskite solar cells has been achieved with 24.45% efficiency of 1 cm² perovskite devices, with negligible hysteresis. Besides, unencapsulated devices based on CBD-SnO₂ can maintain > 95% of its initial efficiency after 2000 h storage at ambient environment, and > 90% after 1000 h LED illumination.

Featuring numerous carbonyl groups, a supramolecular cucurbit[5]uril is introduced to effectively passivate defects in SnO₂ and perovskite via coordination bonding interactions to afford devices with high efficiency and outstanding operational stability.

Abstract

Reducing non-radiative recombination caused by defects at buried interfaces is crucial to the development of efficient and stable perovskite solar cells (PSCs). Herein, supramolecular cucurbit[5]uril (CB[5]) is introduced into the SnO₂ layer, where it engages in host–guest interactions to suppress oxygen vacancies in SnO₂, prevent particle aggregation, and enhance the electron mobility of SnO₂. By serving as a bridging agent at the buried interface between SnO₂ and the perovskite layer, CB[5] reduces the defect density and improves the carrier extraction efficiency. It also enhanced the surface energy of the SnO₂ substrate, facilitates the formation of large grains in the perovskite film, alleviates residual lattice stresses, and enhances the film quality. Consequently, the PSC with CB[5] shows a champion power conversion efficiency of 24.83%. Moreover, an unencapsulated device incorporating CB[5] retains more than 87% of its initial PCE under continuous illumination at the maximum power point tracking for 1000 h. This study pioneers the utilization of cucurbiturils in PSCs and provides insights into how supramolecular compounds can regulate buried interfaces.

Regulating the buried interface is crucial for perovskite crystallization and charge transport. An innovative approach, using hydroxylamine salts for interface modification precisely anchors perovskite ions, promoting controlled crystal growth. This approach significantly enhances the efficiency and long-term stability of Sn−Pb perovskite solar cells, critical for future development of all-perovskite tandems.

Abstract

Mixed tin-lead perovskite solar cells can reach band gaps as low as 1.2 eV, offering high theoretical efficiency and serving as base materials for all-perovskite tandem solar cells. However, instability and high defect densities at the interfaces, particularly the buried surface, have limited performance improvements. In this work, we present the modification of the bottom perovskite interface with multifunctional hydroxylamine salts. These salts can effectively coordinate the different perovskite components, having critical influences in regulating the crystallization process and passivating defects of varying nature. The surface modification reduced traps at the interface and prevented the formation of excessive lead iodide, enhancing the quality of the films. The modified devices presented fill factors reaching 81 % and efficiencies of up to 23.8 %. The unencapsulated modified devices maintained over 95 % of their initial efficiency after 2000 h of shelf storage.

Z-W-03, a quasi-planar spiro-type hole-transporting materials, has been developed to enhance the stability and efficiency of perovskite solar cells. This improvement is achieved through defect passivation, Fermi-level splitting, and intermolecular stacking. An undoped Z-W-03 device with interface modification can achieve an efficiency of 22.92 %.

Abstract

Hole-transporting material (HTMs) are crucial for obtaining the stability and high efficiency of perovskite solar cells (PSCs). However, the current state-of-the-art n-i-p PSCs relied on the use of 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) exhibit inferior intrinsic and ambient stability due to the p-dopant and hydrophilic Li-TFSI additive. In this study, a new spiro-type HTM with a critical quasi-planar core (Z-W-03) is developed to improve both the thermal and ambient stability of PSCs. The results suggest that the planar carbazole structure effectively passivates the trap states compared to the triphenylamine with a propeller-like conformation in spiro-OMeTAD. This passivation effect leads to the shallower trap states when the quasi-planar HTMs interact with the Pb-dimer. Consequently, the device using Z-W-03 achieves a higher V _oc of 1.178 V compared to the spiro-OMeTAD's 1.155 V, resulting in an enhanced efficiency of 24.02 %. In addition, the double-column π–π stacking of Z-W-03 results in high hole mobility (~10⁻⁴ cm² V⁻¹ s⁻¹) even without p-dopant. Moreover, when the surface interface is modified, the undoped Z-W-03 device can achieve an efficiency of nearly 23 %. Compared to the PSCs using spiro-OMeTAD, those with Z-W-03 exhibit enhanced stability under N₂ and ambient conditions. This superior performance is attributed to the quasi-planar core structure and the presence of multiple CH/π and π–π intermolecular stacking in Z-W-03. The multiple CH/π and π–π intermolecular contacts of HTMs can improve the hole hopping transport. Therefore, it is imperative to focus on further molecular structure design and optimization of spiro-type HTMs incorporating quasi-planar cores and carbazole moieties for the commercialization of PSCs.

Nature Energy, Published online: 05 August 2024; doi:10.1038/s41560-024-01623-6

A fast, non-invasive method assesses luminescent coupling between subcells in monolithic perovskite/silicon tandem solar cells, showing over 85% of photons from the perovskite top cell contribute additional current to the silicon bottom cell. Findings confirm the ratio of photons escaping remains constant under investigated conditions, confirming the method's applicability for outdoor conditions.

Abstract

Luminescent coupling (LC) is a key phenomenon in monolithic tandem solar cells. This study presents a nondestructive technique to quantitatively evaluate the LC effect, addressing a gap in the existing predictions made by optical modeling. The method involves measuring the ratio of photons emitted from the high bandgap top cell that escape through the rear, contributing additional current to the bottom cell, and to those escaping from the front side of top cell. The findings indicate that in the analyzed monolithic perovskite/silicon tandem solar cells, more than 85% of the emitted photons escaping from the perovskite top cell are used to generate additional current in the bottom cell. This process notably reduces the mismatch in the generated current between each subcell, particularly when the current is limited by the low bandgap subcell. The presented method is applicable to a variety of monolithic tandem structures, providing vital information for subcell characterization, providing vital information for predicting energy output and optimization for outdoor applications.

Two pyrazinyl polymer donors of PPy1 and PPy2 are constructed, demonstrating the optical bandgaps over 2.0 eV. Due to stronger self-aggregation property and enhanced dielectric feature, PPy2:F-ThCl based organic solar cells show a good efficiency of 14.50%. In light of its large open-circuit voltage of 1.07 V, tandem devices are further fabricated and exhibit an impressive PCE of 19.35%.

Abstract

In series-connected tandem organic solar cells (TOSCs), various light-harvesting molecules with complementary absorptions are explored with the aim of collaboratively utilizing solar light to the maximum extent. In sharp contrast to the small molecular acceptors that possessing almost the successively tunable bandgaps, high-performance wide-bandgap (WBG) polymer donors in TOSCs are quite scarce, with only PM6 (optical bandgaps, E _g ^opt = 1.80 eV) and D18 (E _g ^opt = 1.98 eV) being widely used. Herein, to develop WBG polymer donors with large open-circuit voltages (V _OC) and high-energy photon absorption, two pyrazinyl polymer donors, PPy1 and PPy2, are synthesized with branched 2-butyloctyl and n-dodecyl chains on polymeric backbones, respectively, demonstrating the downshifted highest occupied molecular orbital energy levels of ≈−5.60 eV and thus afford E _g ^opt over 2.0 eV. Consequently, when blending with a WBG acceptor F-ThCl, PPy2:F-ThCl-based devices exhibit a higher power conversion efficiency (PCE) of 14.50% and fill factor of 77.66%. In light of its large V _OC of 1.07 V, TOSCs based on PPy2 are further fabricated and exhibit an impressive PCE of 19.35% by using a narrow bandgap blend of PM6:CH1007:F-2F as a rear cell. This work demonstrates the great potential of pyrazine units in constructing WBG polymer donors for achieving record-breaking TOSCs.

Sodium gluconate (SG) is used to disperse tin oxide (SnO₂) nanoparticles (NPs) in order to create a uniform SG-SnO₂ film and control the buried interface. This allows for the deposition of perovskite films with nearly pinhole-free at the bottom contact interface in ambient air. The SG-SnO₂ PSCs achieved an impressive power conversion efficiency (PCE) of 25.34% (certified as 25.17%). Additionally, these PSCs retained around 90% of their initial PCE after 1000 h of operation (T₉₀ = 1000 h). Microstructure analysis revealed that light-induced degradation primarily occurred at the buried holes, so SG-SnO₂ PSCs enhance light stability by suppressing the formation of holes at the buried interface.

Abstract

The bottom contact in perovskite solar cells (PSCs) is easy to cause deep trap states and severe instability issues, especially under maximum power point tracking (MPPT). In this study, sodium gluconate (SG) is employed to disperse tin oxide (SnO₂) nanoparticles (NPs) and regulate the interface contact at the buried interface. The SG-SnO₂ electron transfer layer (ETL) enabled the deposition of pinhole-free perovskite films in ambient air and improved interface contact by bridging effect. SG-SnO₂ PSCs achieved an impressive power conversion efficiency (PCE) of 25.34% (certified as 25.17%) with a high open-circuit voltage (V _OC) exceeding 1.19 V. The V _OC loss is less than 0.34 V relative to the 1.53 eV bandgap, and the fill factor (FF) loss is only 2.02% due to the improved contact. The SG-SnO₂ PSCs retained around 90% of their initial PCEs after 1000 h operation (T₉₀ = 1000 h), higher than T₈₀ = 1000 h for the control SnO₂ PSC. Microstructure analysis revealed that light-induced degradation primarily occurred at the buried holes and grain boundaries and highlighted the importance of bottom-contact engineering.

Publication date: October 2024

Source: Nano Energy, Volume 129, Part B

Author(s): Qingduan Li, Xiaolan Liao, Ziling Yang, Sixue Zhang, Rouren Chen, Li-Ming Wang, Xiaozhi Zhan, Songyang Yuan, Tao Jia, Yilong Meng, Yue-Peng Cai, Hongfang Zhu, Yuang Fu, Guilong Cai, Shengjian Liu

Reduced hole extraction as a result of evaporation of 4-tBP dopant molecules from the organic hole transport layer of spiro-OMeTAD at a high temperature is found to be one source of the thermal degradation in perovskite solar cells. To suppress the dopant evaporation, perovskite solar cells are coated with a fluoro-polymer CYTOP layer, greatly improving the high-temperature durability.

Halide perovskites are promising as the light absorbers of solar cells with efficient solar power conversion. However, why the degradation of perovskite solar cells (PSCs), especially at high temperatures, happens has not been completely understood to date. Herein, it is shown that evaporation of 4-tert-butylpyridine (4-tBP) from the hole transport layer (HTL) of 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD) is one of possible degradation mechanisms in PSCs at a high temperature of 85 °C. In fresh PSCs, the chemical doping of the spiro-OMeTAD HTL with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is not so efficient because of the formation of a LiTFSI:4-tBP complex in the HTL. When PSCs are placed at 85 °C, 4-tBP gradually evaporates from the HTL, resulting in the dissociation of the LiTFSI:4-tBP complex. This 4-tBP evaporation enhances the chemical doping of spiro-OMeTAD by LiTFSI and makes the hole transport level of the spiro-OMeTAD HTL deeper, thereby impeding hole extraction at the perovskite/spiro-OMeTAD/Au interfaces. Herein, the 4-tBP evaporation by covering PSCs with a fluoro-polymer CYTOP layer, significantly improving the high-temperature durability of PSCs, is suppressed. The basic understanding obtained in this study would help promote the spread of more thermally durable PSC products in the future.

The review summarizes the advancements in composition engineering, additive engineering, and interface engineering of wide-bandgap (WBG) perovskite solar cells (PSCs). Furthermore, the applications of WBG PSCs in various tandem solar cells and their development are discussed. Finally, future prospects for the development of WBG PSCs are outlined.

The exceptional optoelectronic performance and cost-effectiveness of manufacturing have propelled organic–inorganic hybrid perovskite solar cells (PSCs) into the spotlight within the photovoltaic community. Currently, the single-junction PSCs have achieved a certified power conversion efficiency surpassing 26%, edging closer to the illustrious Shockley–Queisser theoretical limit. To further enhance device performance, researchers are currently directing their attention toward the integration of wide-bandgap (WBG) perovskites (Eg > 1.60 eV) as top subcells in conjunction with narrow-bandgap materials, such as perovskite, crystalline silicon, and copper indium gallium selenium, to construct multijunction tandem devices that maximize solar spectral utilization and minimize thermal losses. However, WBG perovskites encounter challenges associated with suboptimal crystal quality, high defect density, and severe phase separation, leading to significant voltage losses and inferior performance. In this regard, extensive research has been conducted, yielding significant findings. This review article summarizes the advancements in composition engineering, additive engineering, and interface engineering of WBG PSCs. Furthermore, the applications of WBG PSCs in various tandem solar cells and their development are discussed. Finally, future prospects for the development of WBG PSCs are outlined.

An facile interfacial bridging strategy by using functional graphene quantum dots to comprehensively solve SnO₂/perovskite buried interface issues. The champion FF reaches 85.24% by synergistic effects of enhanced conductivity of SnO₂, preferable energy alignment at the buried interface and improved perovskite crystal orientation. The champion PCE reaches 24.86% in the FACs-based devices and 24.44% in the flexible devices.

Abstract

The power conversion efficiency (PCE) of perovskite solar cells (PSCs) is approaching their Shockley-Queisser (S-Q) limit through numerous efforts in key parameters improvement. To further approaching the limit, it is important to facilitate the fill factor (FF), a parameter closely related to carrier transport and nonradiative recombination. Herein, an interfacial bridging strategy is proposed to improve FF, which utilizes functional graphene quantum dots at the tin oxide (SnO₂)/perovskite buried interface. As a result, synergistic effects of enhanced conductivity of SnO₂, preferable energy alignment at the buried interface and improved perovskite crystal orientation are realized. The champion FF reaches 85.24% in formamidinium lead iodide (FAPbI₃) based PSCs, which ranks among the highest in the n-i-p structure. Such strategy is also proven successful in other perovskite systems, where the champion PCE reaches 24.86% in the formamidinium-cesium (FACs)-based devices and 24.44% in the flexible devices. Therefore, this work provides a practical design rule for pursuing high FF of PSCs with carbon materials.

Harnessing dipole-dipole interactions, NFREA 412-6F with halogen atoms exhibit exceptional crystallinity and improved charge transport. The D18 : 412-6F blend in OSCs shows enhanced efficiency over 18 %. 412-6F′s lower MOC compared to Y6 underscores its cost-effectiveness. This innovative approach in non-covalent forces yields a high-performance electron acceptor for superior photovoltaic outcomes.

Abstract

This study successfully designed and synthesized two nonfused ring electron acceptors, 412-6F and 412-6Cl, modified with fluorine and chlorine substituents, respectively. Single-crystal analysis revealed that 412-6F possesses a planar molecular backbone and exhibits pronounced dipole-dipole interactions between the fluorine atoms on the lateral phenyl groups and the carbonyl oxygen atoms on the end groups. This specific interaction promotes dense end-group stacking, leading to a reduced interlayer spacing. Improved crystallinity and coherence length are observed in the D18 : 412-6F blend film. Conversely, 412-6Cl adopts a more distorted configuration and lacks these interactions. As a result, the organic solar cell (OSC) based on D18 : 412-6F achieved a remarkable power conversion efficiency of 18.03 %, surpassing the performance of the D18 : 412-6Cl OSC. This underscores the importance of designing novel acceptors with beneficial intermolecular interactions to enhance OSC efficiency, thus providing a new direction for organic photovoltaic advancement.

Formamidinium perovskite layers with ultra-long and controlled crystallization are fabricated through single-source magnetron sputtering and subsequent post-annealing. The presence of MABr is crucial in the formation of perovskite film, as evidenced by a distinct solid-state phase transformation process. Perovskite solar cells fabricated with the optimized films display a substantial increase in the power conversion efficiency (20.1%) and negligible hysteresis effect.

Abstract

Perovskite solar cells (PSCs) based on formamidinium lead iodide (FAPbI₃) have demonstrated the highest power conversion efficiencies. Typically fabricated through solution-processing, FAPbI₃ films necessitate intricate crystallization control to avoid the formation of a more stable non-perovskite phase. Magnetron sputtering, an effective solvent-free technique, has gained attention for producing large-area perovskite films. Here, a novel approach to create high-quality FAPbI₃-based perovskite thin films by employing magnetron sputtering is presented. Specifically, a mechanosynthesized blend of (FA_{1- x} MA _x )Pb(I_{1- x} Br _x )₃ (MA = methylammonium) is used as a single-source target for sputtering, and a post-annealing transforms various phases in the initial deposited film into coherent FAPbI₃-based films with an ultra-long and controlled crystallization process. Despite minimal residual MABr after post-annealing, its presence is crucial in the formation of perovskite film, as evidenced by a distinct solid-state phase transformation process is presented. A comprehensive investigation of the films' structural, compositional, and optical properties, with respect to varying MABr doping ratios, reveals details of perovskite phase development and the pivotal influence of MABr. PSCs fabricated with the optimized films display a substantial increase in the power conversion efficiency, reaching 20.1%. These results underscore the potential of combining magnetron sputtering and post-annealing in the scalable production of high-efficiency PSCs.

V_I, Pb_I, and I_Pb defects in Lead-based (Pb) perovskites is difficult to solve. It is found that small molecules (M4) with multiple active sites can coordinate with defects and enhance the carrier transport rate. This work provides a new idea for simultaneous passivation of defects in perovskites.

Abstract

The inherent defects (lead iodide inversion and iodine vacancy) in perovskites cause non-radiative recombination and there is also ion migration, decreasing the efficiency and stability of perovskite devices. Eliminating these inherent defects is critical for achieving high-efficiency perovskite solar cells. Herein, an organic molecule with multiple active sites (4,7-bromo-5,6-fluoro-2,1,3-phenylpropyl thiadiazole, M4) is introduced to modify the upper interface of perovskites. When M4 interacts with the perovskite surface, the active bromine (Br) site interacts with lead (Pb) at the surface to repair iodine atomic vacancy defects. The fluorine (F) site of M4 interacts with Pb to correct octahedral crystal lattice distortions and eliminate Pb_I defects. Additionally, sulfur–iodine (S–I) interactions reduce I–I dimerization and eliminate I_Pb defects. It is also calculated that the energy level of M4 aligns with the band gap, promoting charge transfer. As a result, the perovskite devices achieve an efficiency of 25.1%, a stabilized power output (SPO) of 25.0%, a voltage of 1.19 V, and a fill factor of 85.2%. The device retains 95% of its initial efficiency after 2000 h of ageing in a nitrogen atmosphere. Thus, multi-point cooperative passivation of surface defects provides an effective method to improve the efficiency and stability of perovskite solar cells.