Chen Weijie

Publication date: November 2021

Source: Nano Energy, Volume 89, Part B

Author(s): Aleksandra Furasova, Pavel Voroshilov, Mikhail Baranov, Pavel Tonkaev, Anna Nikolaeva, Kirill Voronin, Luigi Vesce, Sergey Makarov, Aldo Di Carlo

Herein, a novel approach, a reactive post-treatment technique using guanidine acetate, to enhance the grain size and introduce a secondary phase simultaneously is applied in a perovskite interface. The enhanced grain size helps to reduce the defect densities at the grain boundaries, while the secondary phases make the perovskite surface more n-type, enhancing the device efficiency and stability.

Organic–inorganic lead halide perovskites (OIHPs) have emerged as promising materials for next-generation photovoltaics. However, performance improvements in the perovskite-based device are still limited due to defects that exist more intensively on the surface as well as grain boundaries (GBs) and mismatching energy levels at the interface. Herein, a reactive post-treatment process (RPP) using guanidine acetate (GA) is adopted to address defects and interfacial energy level matching at the perovskite surface. The RPP with GA (GA-RPP) results in the formation of an improved perovskite layer with large grain size and low GB density, leading to the formation of secondary phases on the perovskite surface with appropriate energy levels, resulting in reduced defect density and charge recombination. Furthermore, density functional theory analysis reveals that the Pb-rich secondary phase could improve the conduction of electrons at the perovskite interface. Therefore, the GA-RPP-based perovskite-based solar cell (PSC) shows enhanced performance with 20.4% efficiency and long-term stability.

A novel series of 1D/2A PBTPBD terpolymers is developed for high-efficiency and long-lived polymer solar cells (PSCs). The PBTPBD-50:IT-4F PSC, processed with a nonhalogenated solvent, maintains 82% of the initial power conversion efficiency even after 204 days at 85 °C, which is the highest thermal stability achieved among PSCs processed with nonhalogenated solvents.

Donor–acceptor (D–A) copolymer-based polymer solar cells (PSCs) processed with nonhalogenated solvents exhibit relatively low power conversion efficiencies (PCE) due to undesirable morphological properties, including high aggregation and unfavorable orientation. Moreover, they show very poor long-term stability owing to excessive molecular aggregation and unfavorable phase separation. Thus, novel p-type polymers are required for high-efficiency and long-lived PSCs that can be processed in ecofriendly nonhalogenated solvents. Herein, a novel series of 1D/2A terpolymers (PBTPBD) composed of 4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDT-F), 1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo-[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD), and 1,3-bis-(4-hexylthiophen-2-yl)-5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (HT-TPD) is synthesized and characterized for high-efficiency and long-lived PSCs. A PBTPBD-50:IT-4F blended film exhibits a favorable face-on orientation and superior hole and electron mobility. Therefore, the corresponding PBTPBD-50:IT-4F PSC, processed with a nonhalogenated solvent, exhibits a high PCE of 13.64%, which is 13% higher than that of the related nonhalogenated solvent-processed PSCs. Furthermore, the PBTPBD-50:IT-4F PSC maintains 82% of the initial PCE even after 204 days at 85 °C, which is the highest thermal stability achieved among PSCs processed with nonhalogenated solvents. The high-efficiency and superior long-term thermal stability of the PBTPBD-50:IT-4F PSC are attributed to the excellent miscibility of PBTPBD-50 and IT-4F and the suppression of the morphological changes in the photoactive layer.

Hole-transport-layer (HTL) free tin perovskite solar cells would solve the stability issue caused by the unstable organic HTL. Formamidinium tin iodide doped with heterogeneous ammonium salts can form an upward band-bending structure to selectively extract the hole in the HTL-free cells. An efficiency of over 10% with reliable light-soaking and thermal stability can be achieved for the cells.

Abstract

Lead-free tin perovskite solar cells (PSCs) have emerged as a promising candidate toward high-performance and eco-friendly photovoltaic technology with great potential for future application. However, tin PSCs with over 10% efficiency usually feature an organic hole transport layer (HTL) at the illumination side that may induce device degradation during long-term operation. Removing the unstable organic HTL is an important way to solve these stability issues, but the efficiency of HTL-free tin PSCs is still much lower than that of the completed cells. Herein, it is demonstrated that formamidinium tin iodide doped with heterogeneous ammonium salts can form an upward band-bending structure to selectively extract the hole in the HTL-free devices. By using this band-bending structure, a promising efficiency of over 10% is first achieved for the lead-free PSCs with a HTL-free structure. More importantly, the optimized cell is highly stable, keeping 95% and 90% of the initial efficiency after continuous light soaking for 40 days and 80 °C annealing for 300 h, respectively. This work paves a route toward the development of efficient, eco-friendly, and highly stable perovskite photovoltaics.

Research on the stability of organic solar cells based on fused-ring electron acceptors (FREAs) is now becoming more urgent. This perspective focuses on discussing the possible degradation mechanisms of FREAs and effective strategies of enhancing their stability reported recently. Also, a conclusion and outlook for the future design of highly efficient and stable FREAs are presented.

Abstract

The power conversion efficiency of organic solar cells (OSCs) has made exceptionally rapid progress in the past five years owing to the emergence of fused-ring electron acceptors (FREAs). To achieve the commercialization, it is urgent to resolve the stability issues of OCSs from materials to devices. In particular, the state-of-the-art FREAs, often synthesized by Knoevenagel condensation, generally contain two exocyclic vinyl groups (CC bond) as the conjugated bridges, which inevitably exhibit an obvious electron-deficient characteristic due to the strong push-pull electronic effect. As a result, these vinyl bridges are vulnerable to nucleophile attacking and/or photooxidation, leading to poor chemical and photochemical stabilities of FREAs that easily cause the degradation of device performance. In this perspective, an in-depth understanding of the degradation mechanism of FREAs is provided, and then effective strategies reported recently are reviewed for improving the chemical and photochemical stabilities of FREAs from interfacial engineering to molecular engineering to additive engineering. Finally, a conclusion and outlook for the future design of highly efficient and stable FREAs are also presented.

Publication date: 15 September 2021

Source: Joule, Volume 5, Issue 9

Author(s): Jin Wook Yoo, Jihun Jang, Unsoo Kim, Yonghui Lee, Sang-Geun Ji, Eunseo Noh, Sungtak Hong, Mansoo Choi, Sang Il Seok

Anhydrous SnCl₄ is substituted with SnCl₄·5H₂O to prepare a Sn⁴⁺ precursor solution in air and two solutions containing Sn⁴⁺ and Sn²⁺ are mixed as the final precursor solution, rendering a feasible scheme to obtain denser films and avoid film cracking. When the ratio of Sn⁴⁺ to Sn²⁺ is 1:1, the best efficiency of 11.1% for CZTSSe solar cells is obtained.

The use of different Sn valence states (such as Sn⁴⁺ and Sn²⁺) in the Cu₂ZnSn(S,Se)₄ (CZTSSe) precursor solution is especially important for the quality of the subsequent growth of the CZTSSe films. The latest study has found that replacing SnCl₂·2H₂O with anhydrous SnCl₄ can remarkably improve the performance of CZTSSe solar cells, but it needs to be operated in the glovebox. Herein, for the precursor solution, SnCl₄·5H₂O powder is used instead of anhydrous SnCl₄ in air environment, and the proportion of Sn⁴⁺ and Sn²⁺ precursor solutions is further systematically studied. When the ratio of Sn⁴⁺ to Sn²⁺ is 1:1, a uniform, compact, and noncracking CZTSSe thin film is obtained, effectively alleviating the interface recombination and reducing the concentration of deep-level defects. In particular, the concentration of Cu_Zn antisite defects is decreased by an order of magnitude, and the carrier recombination and band tail effect are alleviated. When J _SC is maintained, V _OC and FF are considerably improved. Finally, CZTSSe thin-film solar cells are fabricated with an efficiency of over 11%. Herein, the feasibility of controlling the ratio of Sn⁴⁺ to Sn²⁺ in the CZTSSe precursor solution for higher efficiency of CZTSSe thin-film solar cells is demonstrated.

An amorphous small molecule with high molecular weight is first designed and used as a solid additive to regulate the phase separation and molecular packing of the PM6:Y6 blends for efficient and stable nonfullerene organic solar cells.

It is incredibly feasible and effective to adopt a solid additive strategy to optimize the active layer blend films morphology for nonfullerene organic solar cells (OSCs) to achieve high efficiency and stable performance. Herein, a novel amorphous small molecule SJ-IC-M with a comparatively high molecular weight is first designed and used as an efficient solid additive in the OSCs based on PM6:Y6 to enhance the power conversion efficiency (PCE) and long-term stability of the device. After the addition of 0.5 wt% SJ-IC-M into the active layer blends, the PCE can be increased to 16.2% compared with that of the reference device without additive displaying an inferior PCE of 15.0%. Moreover, the device containing the SJ-IC-M additive delivers more excellent long-term stability. The PCE can remain over 90% of its initial value when the unencapsulated device is preserved in a N₂-filled glovebox for a month. Systematic analysis reveals that the introduction of the relatively high molecular weight amorphous SJ-IC-M additive can optimize the crystallinity of Y6. As a result, an improved charge transport, stabilized blend morphology, and enhanced device performance are achieved. Moreover, the current research provides a new strategy which can replace the commonly used solvent additive to fabricate efficient and stable nonfullerene OSCs.

PMMA doping helps prepare a uniform, continuous, and condensed hole-transporting layer (HTL) based on spiro-OMeTAD and improves device efficiency and stability simultaneously. 1) The modification can passivate interface defects, accelerate charge transfer, and retard recombination and thus upgrade device efficiency. 2) The modification provides a strengthened barrier against penetration of “H₂O/O₂/Ag,” thus improving device stability.

Improving efficiency and stability has become an urgent issue in the application of perovskite solar cells (PSCs). Herein, a kind of long-chain polymer or polymethylmethacrylate (PMMA) is added into the spiro-OMeTAD matrix to improve the film formation process and hence the device performance. It is observed that, after modification, the spiro-OMeTAD-based hole-transporting layer becomes uniform, continuous, and condensed. Meanwhile, the power conversion efficiency of the devices is upgraded. Compared with the control device, open-circuit voltage of the modified one (with moderate doping) increases from 1.06 (±0.03) to 1.10 (±0.02) V, fill factor increases from 72.20 (±3.44)% to 75.59 (±3.35)%, and the power conversion efficiency increases from 18.82 (±1.06)% to 20.51 (±0.82)% (highest at 21.78%) under standard test condition (AM 1.5G, 100 mW cm⁻²). Transient photocurrent/photovoltage decay curves, time-resolved photoluminance, and impedance spectroscopy studies show that the modification could accelerate charge transfer and retard interfacial recombination. In addition, the modification improves device stability. Due to the strengthened barrier against penetration of “H₂O/O₂/Ag,” the efficiency of the unsealed device could retain 91.49% (by average) of the initial one after 100 days storage in the dark [relative humidity = 30(±5)%]. This work shows that long-chain polymer doping could simultaneously improve efficiency and stability of spiro-OMeTAD-based PSCs.

Herein, the perovskite solar cells with an investor's eye to identify possibilities to lower the entry barrier and capitalize the current level of achievements for their widespread deployment are visited. Perovskite solar cells are analyzed via a 5S criteria, ie., Stability, Safety, Scalability, Sustainability, and Storage, as a tool for analyzing the opportunities and gaps for commercialization.

Perovskite solar cells (PSCs) have received a large amount of research funds due to their potential as a frontrunner in a new generation of solar cells; consequently, the desire to commercialize this technology is mounting. In this roadmap, the knowledge and the technological gaps between laboratory and industry are critically analyzed from the perspective of 5S criteria (Stability, Safety, Sustainability, Scalability, and Storage). To avoid any favoritism in the arguments toward commercializing this technology, herein, the average parameters of PSCs (photoconversion efficiency, durability, cost, manufacturability, and sustainability) estimated from previous studies are analyzed and discussed. Unique opportunities for PSCs in their current stage of achievements are identified, where application-driven, instead of performance-driven, developments are shown to favor their commercialization. Efforts required to improve the average performance of PSCs to state-of-the-art levels are also identified and discussed.

An organic matrix can assist the crystallization of all-organic perovskites at temperatures lower than the phase transition point of black phase CsPbX₃ perovskite. This low-temperature crystallization takes place through the formation and decomposition of an intermediate state which is reminiscent of an organic–inorganic perovskite matrix.

Abstract

All-inorganic perovskites have attracted increasing attention for applications in perovskite solar cells (PSCs) and optoelectronics, including light-emitting devices (LEDs). Cesium lead halide perovskites with tunable I/Br ratios and a band gap aligning with the sunlight region are promising candidates for PSCs. Although impressive progress has been made to improve device efficiency from the initial 2.9 % with low phase stability to over 20 % with high stability, there are still questions regarding the perovskite crystal growth mechanism, especially at low temperatures. In this Minireview, we summarize recent developments in using an organic matrix, including the addition and use of organic ions, polymers, and solvent molecules, for the crystallization of black phase inorganic perovskites at temperatures lower than the phase transition point. We also discuss possible mechanisms for this low-temperature crystallization and their effect on the stability of black phase perovskites. We conclude with an outlook and perspective for further fabrication of large-scale inorganic perovskites for optoelectronic applications.

Scalable perovskite coating is important to achieve highly efficient perovskite solar modules (PSMs). Materials and methods for high-quality large-area perovskite coatings are described. Both precursor coating solutions and coating processes are found to be equally important in achieving high-efficiency PSMs. This review is expected to provide insights into large-area perovskite coatings toward high-efficiency and stable PSMs.

Beginning with the breakthrough of solid-state perovskite-sensitized solar cells in 2012, the number of studies on photovoltaic devices based on halide perovskite materials has exploded. As of 2021, the certificated record power conversion efficiency (PCE) of small-area perovskite solar cells (≈0.1 cm² active area) is 25.5%, making them very competitive with conventional silicon solar cells (26.7%) in terms of efficiency. For commercialization and large-scale manufacturing purposes, it is necessary to develop high-efficiency large-area perovskite photovoltaics with minimal PCE loss. Herein, the recent progress of perovskite solar modules (PSMs) is reviewed and a research direction toward high-efficiency PSMs is proposed. It is important to carefully examine the materials and methods used for development of high-efficiency scalable perovskite solar cells and modules as the uniformity and quality of large-area perovskite film significantly affect the PCE, more so than in small-area devices. Precursor, additive, and interface engineering are described as well as various coating methodologies for suitable PSMs. Engineering of uniform, pinhole-free, and high-quality large-area perovskite films can contribute to high-efficiency PSMs and pave the way for commercialization. The technologies used for materials and method engineering for high-efficiency PSMs are discussed, with an eye toward commercialization.

A residual charge testing approach is used to investigate the trapping and detrapping process in the perovskite solar cells based on the active layers with different crystallization and the morphology. The results reveal that the residual charge exists widely at the grain boundary, and the residual charge is related to the performance of the perovskite solar cells.

Abstract

The performance of perovskite solar cells is greatly affected by the crystallization of the perovskite active layer. Perovskite crystal grains should neatly arrange and penetrate the entire active layer for an ideal perovskite crystallization. These kinds of crystallized perovskite films exhibit fewer defects and longer carrier lifetime, which is beneficial to enhance the performance of perovskite solar cells. Here, by testing the residual charge of perovskite solar cells with different crystallization conditions, it is demonstrated that the residual charge exists widely at the grain boundary, which is parallel to the device, and the residual charge is related to the performance of the perovskite solar cells. Single crystal grains neatly arranged and penetrate the entire active layer can generate less residual charge and improve device performance of the perovskite solar cells. The results also show that the long decay time of open-circuit voltage comes from the detrapping of trapped carriers. The residual charge testing technology provides a new idea for the investigation of carrier trap and detrap characteristics in photovoltaic devices.

N719 dye molecules effectively link nickel oxide (NiO _x )/perovskite interfaces by facilitating charge transport, concurrently passivating NiO _x and perovskite surface traps, and forming a barrier that prevents undesirable chemical reactions occurring at the interface. The molecule also self-anchors and conformally covers NiO _x films deposited on complex surfaces, enabling fabrication of highly efficient textured monolithic p-i-n perovskite/silicon tandem solar cells.

Abstract

Sputtered nickel oxide (NiO _x ) is an attractive hole-transport layer for efficient, stable, and large-area p-i-n metal-halide perovskite solar cells (PSCs). However, surface traps and undesirable chemical reactions at the NiO _x /perovskite interface are limiting the performance of NiO _x -based PSCs. To address these issues simultaneously, an efficient NiO _x /perovskite interface passivation strategy by using an organometallic dye molecule (N719) is reported. This molecule concurrently passivates NiO _x and perovskite surface traps, and facilitates charge transport. Consequently, the power conversion efficiency (PCE) of single-junction p-i-n PSCs increases from 17.3% to 20.4% (the highest reported value for sputtered-NiO _x based PSCs). Notably, the N719 molecule self-anchors and conformally covers NiO _x films deposited on complex surfaces. This enables highly efficient textured monolithic p-i-n perovskite/silicon tandem solar cells, reaching PCEs up to 26.2% (23.5% without dye passivation) with a high processing yield. The N719 layer also forms a barrier that prevents undesirable chemical reactions at the NiO _x /perovskite interface, significantly improving device stability. These findings provide critical insights for improved passivation of the NiO _x /perovskite interface, and the fabrication of highly efficient, robust, and large-area perovskite-based optoelectronic devices.