Chen Weijie

A small molecule of calix[4]pyrrole (C[4]P) with unique anion-bonding capability is demonstrated to enhance the stability of metal halide perovskites by immobilization of the surface halide. It largely suppresses the halide-diffusion induced electrode corrosion, enabling formamidinium–cesium-based inverted perovskite solar cells (efficiency over 23%) with operational and thermal lifetimes over 2000 h.

Abstract

Halide diffusion across the charge-transporting layer followed by a reaction with metal electrode represents a critical factor limiting the long-term stability of perovskite solar cells (PSCs). In this work, a supramolecular strategy with surface anion complexation is reported for enhancing the light and thermal stability of perovskite films, as well as devices. Calix[4]pyrrole (C[4]P) is demonstrated as a unique anion-binding agent for stabilizing the structure of perovskite by anchoring surface halides, which increases the activation energy for halide migration, thus effectively suppressing the halide–metal electrode reactions. The C[4]P-stabilized perovskite films preserve their initial morphology after ageing at 85 °C or under 1 sun illumination in humid air over 50 h, significantly outperforming the control samples. This strategy radically tackles the halide outward-diffusion issue without sacrificing charge extraction. Inverted-structured PSCs based on C[4]P modified formamidinium–cesium perovskite exhibit a champion power conversion efficiency of over 23%. The lifespans of unsealed PSCs are unprecedentedly prolonged from dozens of hours to over 2000 h under operation (ISOS-L-1) and 85 °C ageing (ISOS-D-2). When subjected to a harsher protocol of ISOS-L-2 with both light and thermal stresses, the C[4]P-based PSCs maintain 87% of original efficiency after ageing for 500 h.

Three diazafluorene-based hole transport materials (HTM) are designed in which different side chains are introduced to regulate solvent resistance. Moreover, the chelating diazafluorene moiety is effective in passivating Pb²⁺ defects. Thus, based on the in situ crosslinked AFL-ENE film with the best solvent resistance, one of the highest device efficiencies (20.8%) among reported crosslinking HTMs for perovskite solar cells has been achieved.

The hole transport materials (HTMs) play important roles in transporting holes and regulating perovskite crystallization in inverted perovskite solar cells (PSCs). Concerning the solubility of small-molecule-type HTMs in perovskite precursor solution during fabrication, the strategies including tailoring and crosslinking have been developed. However, how these strategies will influence the solvent resistance of the resultant hole transport layers (HTLs) and the corresponding device performance have not been systematically evaluated. Herein, upon incorporating tailoring and crosslinking groups into diazafluorene backbones, AFL-COOH and AFL-ENE are designed. Compared to the control HTM (AFL-3) with poor solvent resistance and AFL-COOH, the best solvent resistance of crosslinked AFL-ENE (AFL-ENE-CL) film leads to an HTL with the highest quality covered on electrode, which thus results in the lowest trap density, best surface contact, and hole extraction for devices involving the AFL-ENE-CL type HTL and the best power conversion efficiency of 20.8% (20.0% for AFL-COOH, 18.1% for AFL-3). Furthermore, high reproducibility and stabilities also realize for the AFL-ENE-CL-based PSC.

A rationally designed eco-friendly solvent system comprising of GBL and new additive, MSM, is newly suggested. Highly efficient over 24% of PCE and stable (over 1000 h) PSCs are presented. New solvent system is more suitable for mass production of PSCs, achieving ≈21% PCE in 25 cm² solar minimodule, with high tolerance of processing temperature and humidity conditions.

Abstract

The important but remained issue to be addressed to achieve the mass production of perovskite solar modules include a large-area fabrication of high-quality perovskite film with eco-friendly, viable production methods. Although several efforts are made to achieve large-area fabrication of perovskite, the development of eco-friendly solvent system, which is precisely designed to be fit to scale-up methods are still challenging. Herein, this work develops the eco-friendly solvent/co-solvent system to produce a high-quality perovskite layer with a bathing in eco-friendly antisolvent. The new co-solvent/additive, methylsulfonylmethane (MSM), efficiently improves the overall solubility and has a suitable binding strength to the perovskite precursor, resulting in a high-quality perovskite film with antisolvent bathing method in large area. The resultant perovskite solar cells showed high power conversion efficiency of over 24% (in reverse scan), with a good long-term stability under continuous light illumination or damp-heat condition. MSM is also beneficial to produce a perovskite layer at low-temperature or high-humidity. MSM-based solvent system is finally applied to large-area, resulting in highly efficiency perovskite solar modules with PCE of 19.9% (by aperture) or 21.2% (by active area) in reverse scan. These findings contribute to step forward to a mass production of perovskite solar modules with eco-friendly way.

A vacuum-drying strategy in the two-step method is employed to fabricate low-lead perovskite solar cells (FAPb_0.3Sn_0.7I₃). Compared with the conventional one-step method, the two-step fabricated low-lead perovskite films with the vacuum-drying treatment exhibit a larger grain size, lower trap density, and weaker recombination loss, thus giving rise to a record-high efficiency near 20% with better thermal stability.

Abstract

Lead-tin mixed perovskites are excellent photovoltaic materials that can be used in single- or multi-junction perovskite solar cells (PSCs). However, most high-performance Pb-Sn mixed PSCs reported to date are still Pb-dominant. It is highly demanding to develop environmentally friendly low-lead PSCs, but the poor film quality caused by the uncontrollable crystallization kinetics has been hindering the efficiency improvement of low-lead PSCs. Here, a vacuum-drying strategy in the two-step method to fabricate low-lead PSCs (FAPb_0.3Sn_0.7I₃) with an impressive efficiency of 19.67% is employed. The vacuum treatment induces the formation of low crystalline Pb_0.3Sn_0.7I₂ films containing less solvent, thus facilitating the subsequent FAI penetration and suppressing pinholes. Compared with the conventional one-step method, the two-step fabricated low-lead perovskite films with the vacuum-drying treatment exhibit a larger grain size, lower trap density, and weaker recombination loss, thus giving rise to a record-high efficiency near 20% with better thermal stability.

The rise time of transient photovoltage measurements on halide perovskite solar cells is used to quantify charge extraction. As the decay time of the photovoltage can be used to quantify recombination, the combination of rise and decay provides a complete analysis of recombination and extraction and its impact on photovoltaic performance.

Abstract

The extraction of photogenerated charge carriers and the generation of a photovoltage belong to the fundamental functionalities of any solar cell. These processes happen not instantaneously but rather come with finite time constants, e.g., a time constant related to the rise of the externally measured open circuit voltage following a short light pulse. Herein, a new method to analyze transient photovoltage measurements at different bias light intensities combining rise and decay times of the photovoltage. The approach uses a linearized version of a system of two coupled differential equations that are solved analytically by determining the eigenvalues of a 2 × 2 matrix. By comparison between the eigenvalues and the measured rise and decay times during a transient photovoltage measurement, the rates of carrier recombination and extraction as a function of bias voltage are determined, and establish a simple link between their ratio and the efficiency losses in the perovskite solar cell.

The study discusses the development of stable, efficient, and scalable wide-bandgap perovskite solar cells (WBG-PSCs) using the hybrid evaporation-solution method (HESM), which co-evaporates PbI₂/CsBr layer and coats organic-halide solutions in a green solvent. The study achieves efficiencies of 21.06% in cells and 19.83% in mini-modules. The study concludes that HESM is a promising method for developing stable and high-performance WBG-PSCs .

Abstract

Wide-bandgap perovskite solar cells (WBG-PSCs), when partnered with Si bottom cells in tandem configuration, can provide efficiencies up to 44%; yet, the development of stable, efficient, and scalable WBG-PSCs is required. Here, the utility of the hybrid evaporation-solution method (HESM) is investigated to meet these demanding requirements via its unique advantages including ease of control and reproducibility. A PbI₂/CsBr layer is co-evaporated followed by coating of organic-halide solutions in a green solvent. Bandgaps between 1.55–1.67 eV are systematically screened by varying CsBr and MABr content. Champion efficiencies of 21.06% and 20.35% in cells and 19.83% and 18.73% in mini-modules (16 cm²) for perovskites with 1.64 and 1.67 eV bandgaps are achieved, respectively. Additionally, 18.51%-efficient semi-transparent WBG-PSCs are implemented in 4T perovskite/bifacial silicon configuration, reaching a projected power output of 30.61 mW cm⁻² based on PD IEC TS 60904-1-2 (BiFi200) protocol. Despite similar bandgaps achieved by incorporating Br via MABr solution and/or CsBr evaporation, PSCs having a perovskite layer without MABr addition show significantly higher thermal and moisture stability. This study proves scalable, high-performance, and stable WBG-PSCs are enabled by HESM, hence their use in tandems and in emerging applications such as indoor photovoltaics are now within reach.

The “A-π-D-π-A” type nonfused acceptor pendants have been successfully incorporated into double-cable conjugated polymers, exhibiting a high efficiency of ≈18% in single-component organic solar cells under the illumination of 1000 lux (3000 K).

Herein, three double-cable conjugated polymers are developed for indoor single-component organic solar cells (SCOSCs). The new double-cable conjugated polymers contain pendent nonfused electron acceptors with A-π-D-π-A configuration, in which group A uses 1H-indene-1,3(2H)-dione (ID) as an electron-withdrawing end group. The innovative design enables the double-cable polymer to show absorption spectra in the range of 400–700 nm, which is matched with the spectra of indoor light-emitting diode lights. In addition, the ID units at the acceptor side units and the introduction of halogen atoms make the double-cable polymers show high photovoltage (>1.0 V) in SCOSCs. As a result, SCOSCs based on the new double-cable polymers exhibit a high efficiency of 18% under indoor light illumination. The findings present a novel modular design approach for wide-bandgap double-cable conjugated polymers that can be utilized for indoor photovoltaics.

Publication date: August 2023

Source: Journal of Energy Chemistry, Volume 83

Author(s): Yuanyuan Zhao, Huimin Xiang, Ran Ran, Wei Zhou, Wei Wang, Zongping Shao

Publication date: July 2023

Source: Nano Energy, Volume 112

Author(s): Ming Liu, Xianxian Ge, Xingjian Jiang, Daoyuan Chen, Fengyun Guo, Shiyong Gao, Qiang Peng, Liancheng Zhao, Yong Zhang

A versatile self-assembled molecule (SAM) designed as a hole transport material layer enables impressive efficiencies of 18.63% and 26.24% for wide-bandgap perovskite solar cells and 4-terminal all-perovskite tandem devices with long operational stability. Universality is successfully explored for state-of-the-art organic solar cells, illustrated by 18.84% efficiency for a PM6:BTP-eC9 system. This study is expected to inspire design of new SAMs for broad application prospects.

Abstract

Perovskite solar cells (PSCs) and organic solar cells (OSCs) face device efficiency losses and instability challenges with existing hole transport materials (HTMs). The development of new universal HTMs is in great demand to promote their practical applications. Herein, a versatile self-assembled molecule (SAM) based HTM is designed for record-high efficiency wide-bandgap (WBG, E_g >1.75 eV) PSCs, all-perovskite tandem solar cells (TSCs) and OSCs. The SAM exhibits high transmission and a lower-lying energy level, enabling enhanced interfacial charge transfer and suppressed non-radiative recombination losses. SAM based WBG PSCs deliver a maximum power conversion efficiency (PCE) of 18.63% with over 90% efficiency retention after 250 h continuous work. By stacking the optimal WBG PSC and a narrow-bandgap PSC bottom cell, the 4-terminal all-perovskite TSC achieves a remarkable 26.24% PCE. More importantly, this SAM based HTM exhibits impressive generality in bulk heterojunction OSCs rivalling PEDOT:PSS, with an impressive PCE of 18.84% obtained for PM6:BTP-eC9 based devices. When scaling up the PM6:BTP-eC9 device to 0.5 cm² in area (0.71 cm × 0.71 cm), the SAM based OSCs afford a highest PCE of 16.33%. This work provides a perspective for the design of universal SAM based charge transport materials targeting PSCs and OSCs for facile large-area fabrication.

Disorder in organic solar cells leads to an energetic distribution of charge transfer (CT) states. Modulation electroluminescence spectroscopy shows that higher energy CT states in this distribution respond to a voltage perturbation faster than lower energy CT states, confirming that they do not exist in quasi-equilibrium, even for devices operated in the dark.

Abstract

The notion of quasi-equilibrium is central to most solar cells; however, it has been questioned in organic photovoltaics (OPVs) owing to strong energetic disorder that frustrates efficient relaxation of electrons and holes within their respective density of states (DOS). Here, modulation electroluminescence (EL) spectroscopy is applied to OPVs and it is found that the frequency response of charge transfer (CT) state EL on the high energy side of the spectrum differs from that of the low energy side. This observation confirms that static disorder contributes substantially to the linewidth of the steady-state EL spectrum and is unambiguous proof that the distribution of CT states formed by electrical injection in the dark is not in quasi-equilibrium. These results emphasize the need for caution when analyzing OPV cells on the basis of reciprocity models that assume quasi-equilibrium holds, and highlight a new method to study this unusual aspect of OPV operation.

Single-crystal seeds are embedded in PbI₂ films as nucleation sites to promote the rapid formation of high-quality α-phase formamidinium lead triiodide (α-FAPbI₃). The formamidine lead bromide (FAPbBr₃) seed epitaxial growth crystallization method through a halogen anion exchange process achieves a high-performance perovskite solar cell with prominent power conversion efficiency of 23.84% and impressive open circuit voltage of 1.18 V.

Abstract

α-phase formamidine lead iodine (α-FAPbI₃) as a competitive candidate to pursue high-performance perovskite solar cells (PSCs) suffers from the weakness of random nucleation and disordered growth in chasing high-grade polycrystalline films. Here, single-crystal seeds crystallization (SCSC) method is proposed to obtain high-quality α-FAPbI₃ perovskite films for the first time. Six single-crystals seeds (CsPbBr₃, CsPbI₃, FAPbBr₃, FAPbI₃, MAPbBr₃, and MAPbI₃) are employed to regulate α-FAPbI₃ perovskite crystallization. These seeds act as nucleation sites and allow the perovskite to grow directly. The rapid crystallization significantly improves the crystallinity of perovskite films with uniform morphology, large grain size, and less defects. Of particular note, FAPbBr₃ seed prefers to grow α-FAPbI₃ epitaxially from FAPbBr₃@PbI₂ heterostructure through a halogen anion exchange process. The average grain size of FAPbBr_3-perovskite film is boosted to 2.6 µm, and the trap density reduced by 11 times. The resultant FAPbBr₃-PSCs achieve a maximum power conversion efficiency (PCE) of 23.84% with a high open-circuit voltage of 1.18 V. The unencapsulated PSC retains 91% of the initial PCE after 3000 h of storage at ≈30–50% relative humidity and preserves 98.5% of the original value after 155 h illumination with maximum power point tracking.

Herein, a bithiazole-substituted small molecule donor (ZW1) with high crystallinity is synthesized and employed to fabricate ternary devices. The addition of ZW1 enhances the crystallinity of the blend films. Furthermore, the good miscibility between D18 and ZW1 drives Y6 to separate from its mixed phase with D18, optimizing the phase separation and vertical distribution of the active layer. Combining the regulation of crystallinity and miscibility, charge transport and recombination are improved in ternary devices. As a result, outstanding power conversion efficiencies (PCEs) of 18.50% and 16.67% are achieved for 120 nm-thick and 300 nm-thick ternary organic solar cells (OSCs), respectively. The 16.67% PCE is one of the highest values for thick-film OSCs reported to date.

Abstract

Organic solar cells (OSCs) with thick active layers exhibit great potential for future roll-to-roll mass production. However, increasing the thickness of the active layer generally leads to unfavorable morphology, which decreases the device's performance. Therefore, it is a critical challenge to achieve OSCs with high efficiency and thick film simultaneously. Herein, a small molecular donor, ZW1, incorporating a bithiazole unit along with a thiophene group as a π-bridge is reported. ZW1 with high crystallinity is employed to fabricate D18:ZW1:Y6 ternary devices, which enhances the crystallization, optimizes the morphology, and suppresses bimolecular recombination. Additionally, ZW1 shows better miscibility with D18, resulting in the preferred vertical phase distribution. As a result, an outstanding power conversion efficiency (PCE) of 18.50% is realized in ternary OSCs with 120 nm active layer thickness. Importantly, the thick ternary OSCs attain a high PCE of 16.67% (thickness ≈300 nm), significantly higher than the corresponding binary devices (13.50%). The PCE of 16.67% is one of the highest values for thick-film OSCs reported to date. This work demonstrates that the incorporation of highly crystalline small-molecule donors into ternary OSCs, possessing good miscibility with host materials, presents an effective strategy for fabricating highly efficient thick OSCs.

A pseudo halide-based ionic liquid, methylamine formate, is incorporated into FAPbI₃ perovskite to realize homogeneous and strong compressive strain to stabilize FAPbI₃ perovskite, as well as, enhance the crystallinity, reduce defects density, and prolong the carrier lifetime, which contributes to a record power conversion efficiency of 24.08% for FAPbI₃ inverted perovskite solar cells.

Abstract

Inverted (p-i-n) perovskite solar cells have drawn great attention due to their outstanding stability and low-temperature processibility. However, their power conversion efficiency (PCE) still lags behind conventional (n-i-p) devices mainly due to the lack of strategies to stabilize α-FAPbI₃ without changing the bandgap. In this work, a facile and effective strategy is reported to regulate the residual strain via pseudo halide-based ionic liquids incorporation to stabilize α-FAPbI₃ perovskite in inverted perovskite solar cells (PVSCs). The employment of methylamine formate (MAFa) ionic liquid enables a homogenously stronger compressive strain to restrain the transition of shared-corner PbI₆ octahedron into shared-face δ-FAPbI₃, as well as affecting the dynamic behavior of carriers and defects to achieve a record PCE (24.08%) among the reported inverted FAPbI₃ perovskite solar cells up to now. In addition, the MAFa incorporation results in enhanced device stability, unencapsulated PVSC retains over 90% of its initial efficiency after stored in ambient environment (RH:30 ± 5%) for 1000 h. This work provides an efficient strategy to realize efficient and stable α-FAPbI₃ based inverted PVSCs to further catch up with the conventional ones.

Sodium formate (NaFo) in a CsPbI₂Br perovskite solution as a crystallization agent, which induces the synergetic effect of cation engineering and pseudo-halide anion engineering, is introduced. The NaFo-incorporating CsPbI₂Br PSCs with dopant-free P3HT exhibits a power conversion efficiency of 17.7% with a fill factor (FF) of 84.5%, which is the highest FF value among CsPbI₂Br-based PSCs reported so far.

Abstract

Inorganic CsPbI₂Br perovskite has a substantial potential for triple-junction tandem solar cells as a top subcell, however it exhibits relative instability in the air compared with organic-inorganic perovskites as well as significantly lower efficiency than the theoretical efficiency limit. To further enhance the air-stability and efficiency of CsPbI₂Br-based perovskite solar cells (PSCs), it is vitally crucial to improve the crystallinity and passivate the defects within films that accelerate the phase transformation to the photo-inactive phase in the air. Here, it is reported that crystallization management via incorporating sodium formate (NaFo) in a CsPbI₂Br perovskite solution effectively leads to enlarged grain size and the reduced trap density. The Na⁺ cation and HOOC⁻ anion produce a synergistic effect for engineering the defects by acting as cation and pseudo-halide anion passivators, respectively. As a result, the NaFo-incorporating device shows an improved power conversion efficiency (PCE) of 17.7% with a fill factor (FF) of 84.5%. To the best of the authors' knowledge, this progressive FF value is the highest value among CsPbI₂Br-based PSCs reported thus far. In addition, the NaFo-incorporated device shows improved air stability compared to the control device, retaining over 95% of its initial PCE for 1000 hours under 10% relative humidity at room temperature without any encapsulation.