Chen Weijie

A novel nickel oxide hole transport layer top-down synthesis route via electrochemical anodization is developed. A-NiO shows outstanding optoelectrical properties such as uniform film thickness, enhanced transmittance, low trap density, and high hole extraction ability. The A-NiO-based inverted perovskite solar cell shows improved power conversion efficiency of 21.9% and long-term stability.

The hole transport layer (HTL) plays a key role in inverted perovskite solar cells (PSCs), and nickel oxide has been widely adopted for HTL. However, a conventional solution-processed bottom-up approach for NiO _x (S-NiO) HTL fabrication shows several drawbacks, such as poor coverage, irregular film thickness, numerous defect sites, and inefficient hole extraction from the perovskite layer. To address these issues, herein, a novel NiO _x HTL top-down synthesis route via electrochemical anodization is developed. The basicity of the electrolyte used in anodization considerably influences electrochemical reactions and results in the structure of the anodized NiO _x (A-NiO). The optimized A-NiO provides outstanding optoelectrical properties, including uniform film thickness, enhanced transmittance, deep-lying valance band, low trap density, and better hole extraction ability from the perovskite. Owing to these advantages, the A-NiO-based inverted PSC exhibits an improved power conversion efficiency of 21.9% compared with 19.1% for the S-NiO-based device. In addition, the A-NiO device shows a higher inlet and long-term ambient stability than the S-NiO device due to the superior hole transfer ability of A-NiO, which suppresses charge accumulation between NiO _x and the perovskite interface.

Herein, the role of ionic liquids in perovskite solar cells as additives, solvents, interface engineering, and charge transport layer is discussed. This review guides the researchers in understanding bulk doping and interface engineering for fabricating efficient, stable, and eco-friendly perovskite solar cells.

Although the power conversion efficiency of perovskite solar cells (PSCs) has reached 25.7%, there is still great potential for improvement in their performance and stability. In the past few years, ionic liquids (ILs) have been extensively investigated and demonstrated to enhance the efficiency and stability of devices substantially. Herein, the role of ILs in PSCs as additives, solvents, interface engineering, and charge transport layer is reviewed. Also, this review will guide the researchers in understanding bulk doping and interface engineering for efficient and stable PSCs.

The addition of 3,5-dichlorobromobenzene can effectively tune the phase separation of PBQx-TF:eC9-2Cl during film formation, results in a favorable phase separation and a reinforced molecular packing. Consequently, the binary organic solar cells exhibit 19.2% efficiency and superior photostability.

Abstract

Morphology optimization is critical for achieving high efficiency and stable bulk-heterojunction (BHJ) organic solar cells (OSCs). Herein, the use of 3,5-dichlorobromobenzene (DCBB) with high volatility and low cost to manipulate evolution of the BHJ morphology and improve the operability and photostability of OSCs is proposed. Systematic simulations reveal the charge distribution of DCBB and its non-covalent interaction with the active layer materials. The addition of DCBB can effectively tune the aggregation of PBQx-TF:eC9-2Cl during film formation, resulting in a favorable phase separation and a reinforced molecular packing. As a result, a power conversion efficiency of 19.2% (certified as 19.0% by the National Institute of Metrology) for DCBB-processed PBQx-TF:eC9-2Cl-based OSCs, which is the highest reported value for binary OSCs, is obtained. Importantly, the DCBB-processed devices exhibit superior photostability and have thus considerable application potential in the printing of large-area devices, demonstrating outstanding universality in various BHJ systems. The study provides a facile approach to control the BHJ morphology and enhances the photovoltaic performance of OSCs.

We doped yitterbium ions into perovskite nanocrystal hosts for extending their electroluminescence wavelength towards 1000 nm. The synergy of halide-stoichiometry control and surface passivation enables us to achieve an efficient near-infrared light-emitting diode with a peak external quantum efficiency of 7.7 %, representing the highest efficiency among organic LEDs and perovskite LEDs with peak wavelengths beyond 850 nm to date.

Abstract

Perovskite nanocrystals (PeNCs) deliver size- and composition-tunable luminescence of high efficiency and color purity in the visible range. However, attaining efficient electroluminescence (EL) in the near-infrared (NIR) region from PeNCs is challenging, limiting their potential applications. Here we demonstrate a highly efficient NIR light-emitting diode (LED) by doping ytterbium ions into a PeNCs host (Yb³⁺ : PeNCs), extending the EL wavelengths toward 1000 nm, which is achieved through a direct sensitization of Yb³⁺ ions by the PeNC host. Efficient quantum-cutting processes enable high photoluminescence quantum yields (PLQYs) of up to 126 % from the Yb³⁺ : PeNCs. Through halide-composition engineering and surface passivation to improve both PLQY and charge-transport balance, we demonstrate an efficient NIR LED with a peak external quantum efficiency of 7.7 % at a central wavelength of 990 nm, representing the most efficient perovskite-based LEDs with emission wavelengths beyond 850 nm.

Dopant-free non-stoichiometric nickel oxide nanocrystals (NiO _x NCs) have been extensively studied as a low-cost and effective hole transport material in perovskite optoelectronics. In this minireview, Zhang et al. summarize the synthesis and surface-functionalization methods of NiO _x NCs and their applications in different perovskite optoelectronic devices.

Abstract

Advancing inverted (p-i-n) perovskite solar cells (PSCs) is critical for commercial applications given their compatibility with different bottom cells for tandem photovoltaics, low-temperature processability (≤100 °C), and promising operational stability. Although inverted PSCs have achieved an efficiency of over 25 % using doped or expensive organic hole transport materials (HTMs), their synthesis cost and stability still cannot meet the requirements for their commercialization. Recently, dopant-free and low-cost non-stoichiometric nickel oxide nanocrystals (NiO _x NCs) have been extensively studied as a low-cost and effective HTM in perovskite optoelectronics. In this minireview, we summarize the synthesis and surface-functionalization methods of NiO _x NCs. Then, the applications of NiO _x NCs in other perovskite optoelectronics beyond photovoltaics are discussed. Finally, we provide a perspective for the future development of NiO _x NCs for the commercialization of perovskite optoelectronics.

Two BTP-eC9 analogs, BTP-eC9-EH and BTP-eC9-HD, are synthesized to investigate the effect of alkyl chain length on the solubility and photovoltaic properties systematically. Surprisingly, the PM6:BTP-eC9-HD based device exhibited an outstanding power conversion efficiency of 17.5% and a remarkable fill factor of 79% in the toluene processing blend system, which can be applied in achieving high-performance large-scale organic solar cells processed from non-halogenated solvents.

Most cutting-edge performance binary bulk heterojunction organic solar cells (OSCs) with high power conversion efficiency (PCE) over 18% generally use harmful halogenated solvent systems to guarantee the optimal morphology of active layers, urging more investigations on the non-halogenated solvent processed OSCs and their further large-scale devices fabrication. Herein, two BTP-eC9 analogs, BTP-eC9-EH and BTP-eC9-HD, were synthesized to investigate the effect of alkyl chain length on the solubility and photovoltaic properties systematically. Despite the subtle side-chain engineering, the PM6: BTP-eC9-HD based device exhibited an outstanding PCE of 17.5% and a remarkable fill factor of 79% in the toluene processing binary blend system, which precedes the toluene processing PM6: BTP-eC9 based binary blend system (17.1%). The combined investigation provides a valuable insight into the modulation of branched alkyl chains, which can serve as an effective approach for adjusting the aggregation and molecular packing of A-DA′DA-type Y-series based non-fullerene acceptors to balance the materials solubility and solvents selection, thereby achieving high-performance large-scale OSCs processed from non-halogenated solvents.

Herein, high-performance inverted perovskite solar cells with distinctively improved performance are reported by introducing carbazole analog 3,6-dibromocarbazole (36-DBC) into the perovskite precursor. By introducing 36-DBC into the perovskite film the band structure was regulated and the surface morphology modified. As a consequence, a champion power conversion efficiency of 22.25% is achieved.

Perovskite solar cells (PSCs) have aroused vast attention and achieved unprecedented development. However, commercial utilization requires better reliability while maintaining high efficiency. Typically, the uncoordinated band energy configuration and structural defects led to recombination, these bottlenecks have seriously damaged the device performance and limited the further development of PSCs. In addressing those questions, researchers have provided various countermeasures including doping to trim the band structure and solvent engineering to modify the crystallization process and introducing reduced dimensional perovskite to maintain long-term stability. Herein, a convenient carbazole analog doping strategy is reported by adding a certain amount of bromide-substituted carbazole analog in the perovskite precursor which succeeded in modifying the surface morphology and optimizing the bandgap structure of the film, additionally maintaining considerable stability. As a result, a champion power conversion energy (PCE) of 22.25% is reached and preserved 85% of the initial PCE while stored in ambient air for more than 1200 h (around 30% relative humidity), and with low hysteresis. A simple but practical doping strategy to fabricate efficient and stable perovskite films in ambient air is provided.

In this work, a novel strategy of applying polymerized small molecular acceptors to a high-efficiency inverted perovskite solar cell is proposed and proven effective by the material PY-IT. Via the bi-passivation effect and additional electron transport channels, a 23.57% power conversion efficiency is obtained, as well as a decent operational stability.

Abstract

Optimizing the interface between the perovskite and transport layers is an efficient approach to promote the photovoltaic performance of inverted perovskite solar cells (IPSCs). Given decades of advances in bulk materials optimization, the performance of IPSCs has been pushed to its limits by interface engineering with a power conversion efficiency (PCE) over 25% and excellent stability. Herein, an n-type polymeric semiconducting material, PY-IT, that has shown remarkable performance in organic photovoltaics, is introduced as an interface regulator between perovskite and ETL. Encouragingly, this polymerized small molecular acceptor (PSMA) exhibits significant effectiveness in both passivation defects and electron transfer facilitation properties with the merits of strong planarity and rotatable linkers, which significantly optimizes perovskite grain growth orientation and added charge transport channels. As a result, the PSMA-treated IPSC devices obtain an optimal efficiency of 23.57% with a fill factor of 84%, among the highest efficiency among PSMA-based IPSCs. Meanwhile, the photo-stability of PSMA devices is eye-catching, maintaining ≈80% of its initial PCE after 1000 h of simulated 1-sun illumination under maximal power point tracking. This work combines the achievements of polymer science and IPSC device engineering to provide a new insight into interface regulation of efficient and stable devices.

A dimer acceptor with flexible linker was designed and synthesized, which not only acts as the third component to enhance the intermolecular packing, but also stabilizes the morphology by suppressing the molecular diffusion. As a result, the PM6 : Y6 : dT9TBO based organic solar cells exhibited a high power conversion efficiency of 18.41 % with excellent thermal/photo stability and mechanical performance.

Abstract

Organic solar cells (OSCs) have advanced rapidly due to the development of new photovoltaic materials. However, the long-term stability of OSCs still poses a severe challenge for their commercial deployment. To address this issue, a dimer acceptor (dT9TBO) with flexible linker is developed for incorporation into small-molecule acceptors to form molecular alloy with enhanced intermolecular packing and suppressed molecular diffusion to stabilize active layer morphology. Consequently, the PM6 : Y6 : dT9TBO-based device displays an improved power conversion efficiency (PCE) of 18.41 % with excellent thermal stability and negligible decay after being aged at 65 °C for 1800 h. Moreover, the PM6 : Y6 : dT9TBO-based flexible OSC also exhibits excellent mechanical durability, maintaining 95 % of its initial PCE after being bended repetitively for 1500 cycles. This work provides a simple and effective way to fine-tune the molecular packing with stabilized morphology to overcome the trade-off between OSC efficiency and stability.

Nature Materials, Published online: 23 March 2023; doi:10.1038/s41563-023-01477-5

Nature Photonics, Published online: 23 March 2023; doi:10.1038/s41566-023-01173-5

Publication date: 15 June 2023

Source: Nano Energy, Volume 111

Author(s): Haoxin Wang, Wei Zhang, Biyi Wang, Zheng Yan, Cheng Chen, Yong Hua, Tai Wu, Linqin Wang, Hui Xu, Ming Cheng

An N-heterocyclic olefin-type ionic liquid IdMe with strong Lewis acid-base interactions is demonstrated to be an effective additive in both quasi-2D (Q-2D) and 3D perovskite solar cells. IdMe regulates the crystallization of Q-2D perovskites (n = 4), boosting the power conversion efficiency (PCE) from 14.03% to 17.68%, and passivates 3D perovskite defects for attaining higher PCE and stability.

Abstract

In optimizing perovskites with ionic liquid (IL), the comparative study on Lewis acid-base (LAB) and hydrogen-bonding (HB) interactions between IL and perovskite is lacking. Herein, methyl is substituted for hydrogen on 2-position of imidazolium ring of N-heterocyclic carbene (NHC) type IL IdH to weaken HB interactions, and the resulting N-heterocyclic olefin (NHO) type IL IdMe with softer Lewis base character is studied in both hybrid quasi-2D (Q-2D) and 3D perovskites. It is revealed that IdMe participates in constructing high-quality Q-2D perovskite (n = 4) and provides stronger passivation for 3D perovskite compared with IdH. Power conversion efficiency (PCE) of Q-2D PEA₂MA₃Pb₄I₁₃ perovskite solar cells (PVSCs) is boosted to 17.68% from 14.03%. PCE and device stability of 3D PVSCs enhances simultaneously. Both theoretical simulations and experimental results show that LAB interactions between NHO and Pb²⁺ take the primary optimization effects on perovskite. The success of engineering LAB interactions also offers inspiration to develop novel ILs for high-performance PVSCs.

A strategy of 4-fluorobenzamide (FBAD) molecular controlled perovskite crystallization is proposed to suppress the fast reaction between the FAI and PbI₂ for high-quality α-FAPbI₃ films. The optimized solar cell achieves a champion device power conversion efficiency of 24.08% with excellent stability. This study offers an efficient approach to incorporate the only additive of FBAD to produce high-quality perovskite films for high-performance solar cells.

Abstract

The two-step sequentially deposition strategy has been widely used to produce high-performance FAPbI₃-based solar cells. However, due to the rapid reaction between PbI₂ and FAI, a dense perovskite film forms on top of the PbI₂ layer immediately and blocks the FAI diffusion into the bottom of the PbI₂ film for a complete reaction, which results in a low-efficiency and limited reproducibility of perovskite solar cells (PSCs). Here, high-quality α-FAPbI₃ perovskite films by crystal growth regulation with 4-fluorobenzamide additives is fabricated. The additives can interact with FAI to suppress the fast reaction between the FAI and PbI₂ and effectively passivate the under-coordinated Pb²⁺ or I^- defects. As a result, α-FAPbI₃ perovskite films with low trap density and large grain size are prepared. The modified PSCs present a high-power conversion efficiency of 24.08%, maintaining 90% of their initial efficiency after 1400 h in high humidity. This study provides an efficient strategy of synergistic crystallization and passivation to form high-quality α-FAPbI₃ films for high-performance PSCs.

A fast solidification and slow growth strategy is developed to obtain high-quality quasi-2D perovskite film. As a result, the efficiency of the upscaled quasi-2D perovskite solar cells (PVSCs) is significantly increased from 15.08% to 19.08% with an area of 1 cm², which is among the highest values for large-area quasi-2D PVSCs (≥1 cm²).

Abstract

Smooth perovskite film with high crystallinity and vertical orientation is highly favored for high-performance quasi-2D perovskite solar cells (PVSCs), yet limited by the critical balance between nucleation and crystal growth. To address this issue, here a fast solidification and slow growth (FSSG) strategy is developed to effectively optimize the film morphology. The fast solidification enables a smooth, pinhole-free film, while slow growth allows it to further ripen into high-crystallinity film. This process is enabled by the low-boiling point solvent system, that is, acetonitrile, with high-boiling point additives, that is, NH₄SCN and CH₃NH₃Cl. Compared to the traditional method, uniform film with a larger grain/crystallite size as well as better crystallinity can be easily fabricated through this FSSG strategy. As a result, the quasi-2D PVSCs based on (GA)₂(MA)₄Pb₅I₁₆ processed with the FSSG strategy show a champion power conversion efficiency of 20.44%, as well as 19.08% for large-area (1 cm²) devices, which suggest the capability of the FSSG strategy for up-scaled PVSC fabrication. Therefore, this work opens a new avenue toward morphology control of PVSCs for practical applications.

Dimethyl sulfoxide (DMSO) is found to cause Sn(IV) species and unbalanced complexation in a Sn–Pb perovskite precursor. Chemically stable N,N′-dimethylpropylene urea with a strong coordination ability is used to replace DMSO to finely regulate the crystalline quality of perovskite film. A new performance record of 22.41% is obtained for corresponding devices.

Abstract

Harmful Sn(IV) vacancies and uncontrolled fast crystallization commonly occur in tin–lead alloyed perovskite films. The typical dimethyl sulfoxide (DMSO) processing solvent is suggested to be the primary source of problems. Here, a DMSO-free solvent strategy is demonstrated to obtain high-performance Cs_0.25FA_0.75Pb_0.5Sn_0.5I₃ solar cells. A rational solvent selection process via Hansen solubility parameters and Gutmann's donor number shows that N,N′-dimethylpropyleneurea (DMPU) has a strong coordinate ability to form complete complexation with organic (formamidinium iodide) and inorganic (CsI, PbI₂, and SnI₂) components. This treatment suppresses the iodoplumbate (PbI _n ^{2- n} ) or iodostannate (SnI _n ^{2- n} ) preformed in precursor solution, thereby promoting pure intermediate complexes and retarding crystallization, realizing enlarged grain size, and improved film crystallinity. Additionally, it is demonstrated that DMPU-based solvent system can further inhibit the oxidation of Sn(II) and reduced Sn(IV) content by nearly 75% due to its superior thermal and chemical stability. This DMSO-free strategy generates a record efficiency of 22.41%, with a V _oc of 0.88 V and a FF of 82.72% for the MA-free Sn–Pb alloyed device. The unencapsulated devices display much-improved humidity stability at 30 ± 5% relative humidity in air for 240 h, impressive thermal stability at 85 °C for 500 h, and promote continuous operation stability at maximum power point for 150 h.

It is shown that using an enantiomerically pure chiral fullerene as the electron transport layer (ETL) in a perovskite solar cell (PSC) gives an enhanced power conversion efficiency and improved stability, over the racemic material. This provides strong evidence that single isomer ETLs can improve PSC performance and positions chiral fullerenes as an exciting material class moving forward.

Abstract

The rapidly advancing improvements in perovskite solar cells (PSCs) are driven, in part, by the inclusion of suitable electron transport layers (ETLs) in high performance devices. Fullerene derivatives are particularly useful ETLs in PSCs, but many of the utilized fullerenes are present as isomeric mixtures. The opportunities presented by single-isomer, single-enantiomer fullerenes in PSCs are poorly understood. Here, inverted PSCs are prepared using bis[60]phenyl-C61-butyric acid methyl ester derivative (anti)16,17-bis[60]PCBM, comparing the performance of enantiomerically pure material to the corresponding racemate. The single enantiomer devices are found to have an improved performance, giving a power conversion efficiency (PCE) of 23.2%, compared to 20.1% PCE for the racemate. It is also shown that enantiomerically pure PSC modules can be prepared with a state-of-the-art PCE of 20.1%. Such excellent performance for the single enantiomer devices is accompanied by enhanced operational stability. This study thus provides strong evidence that single isomer ETLs can provide important improvements in PSC performance and it positions chiral fullerenes as an exciting material class moving forward.

With tailored conjugated ligand design, the 2D/3D heterostructure built in perovskite solar cells enhances the interface adhesion between the polymeric hole-transporting layer—PTAA and perovskite, and at the same time, realizes a uniform growth of 2D structure on top of 3D perovskite. Therefore, the as-fabricated device yields a 23.7% power conversion efficiency, along with excellent stability.

Abstract

Perovskite solar cells (PSCs) have delivered a power conversion efficiency (PCE) of more than 25% and incorporating polymers as hole-transporting layers (HTLs) can further enhance the stability of devices toward the goal of commercialization. Among the various polymeric hole-transporting materials, poly(triaryl amine) (PTAA) is one of the promising HTL candidates with good stability; however, the hydrophobicity of PTAA causes problematic interfacial contact with the perovskite, limiting the device performance. Using molecular side-chain engineering, a uniform 2D perovskite interlayer with conjugated ligands, between 3D perovskites and PTAA is successfully constructed. Further, employing conjugated ligands as cohesive elements, perovskite/PTAA interfacial adhesion is significantly improved. As a result, the thin and lateral extended 2D/3D heterostructure enables as-fabricated PTAA-based PSCs to achieve a PCE of 23.7%, improved from the 18% of reference devices. Owing to the increased ion-migration energy barrier and conformal 2D coating, unencapsulated devices with the new ligands exhibit both superior thermal stability under 60 °C heating and moisture stability in ambient conditions.

A hydrophobic all-organic salt is reported to modify the top surface of large-area slot-die-coated methylammonium (MA)-free halide perovskite layers to achieve an active area efficiency of 19.28% (58.5 cm²) with the retention of ≈80% efficiency after 7500 h (313 days).

Abstract

The power conversion efficiency (PCE) of the state-of-the-art large-area slot-die-coated perovskite solar cells (PSCs) is now over 19%, but issues with their stability persist owing to significant intrinsic point defects and a mass of surface imperfections introduced during the fabrication process. Herein, the utilization of a hydrophobic all-organic salt is reported to modify the top surface of large-area slot-die-coated methylammonium (MA)-free halide perovskite layers. Bearing two molecules, each of which is endowed with anchoring groups capable of exhibiting secondary interactions with the perovskite surfaces, the organic salt acts as a molecular lock by effectively binding to both anion and cation vacancies, substantially enhancing the materials’ intrinsic stability against different stimuli. It not only reduces the ingression of external species such as oxygen and moisture, but also suppresses the egress of volatile organic components during the thermal stability testing. The treated PSCs demonstrate efficiency of 19.28% (active area of 58.5 cm²) and 17.62% (aperture area of 64 cm²) for the corresponding mini-module. More importantly, unencapsulated slot-die-coated mini-modules incorporating the all-organic surface modifier show ≈80% efficiency retention after 7500 h (313 days) of storage under 30% relative humidity (RH). They also remarkably retain more than 90% of the initial efficiency for over 850 h while being measured continuously.