Chen Weijie

A simple and generally applicable method to fabricate efficient and stable Pb‐Sn binary perovskite solar cells (PVSCs) based on a galvanic displacement reaction (GDR) is demonstrated. With optimizing the ratio of Pb and Sn, high PCEs of 15.85% and 18.21% are achieved for Pb‐Sn binary based PVSCs. Moreover, these Pb‐Sn based PVSCs exhibit improved stability with encapsulation.

Abstract

Here, a simple and generally applicable method of fabricating efficient and stable Pb‐Sn binary perovskite solar cells (PVSCs) based on a galvanic displacement reaction (GDR) is demonstrated. Different from the commonly used conventional approaches to form perovskite precursor solutions by mixing metal halides and organic halides such as PbI₂, SnI₂, MAI, FAI, etc., together, the precursor solutions are formulated by reacting pure Pb‐based perovskite precursor solutions with fine Sn metal powders. After the ratios between Pb and Sn are optimized, high PCEs of 15.85% and 18.21% can be achieved for MAPb_0.4Sn_0.6I₃ and (FAPb_0.6Sn_0.4I₃)_0.85(MAPb_0.6Sn_0.4Br₃)_0.15 based PVSCs, which are the highest PCEs among all values reported to date for Pb‐Sn binary PVSCs. Moreover, the GDR perovskite‐based PVSCs exhibit significantly improved ambient and thermal stability with encapsulation, which can retain more than 90% of their initial PCEs after being stored in ambient (relative humidity (RH) ≈50%) for 1000 h or being thermal annealed at 80 °C for more than 120 h in ambient conditions. These results demonstrate the advantage of using GDR to prepare tunable bandgap binary perovskites for devices with greatly improved performance and stability.

Interfaces between the photoactive layer and electrodes play a critical role in ultimate device behaviors in organic bulk heterojunction solar cells (OSCs) and hybrid halide perovskite solar cells (PSCs). Here, recent progress in interface modification for OSCs and PSCs aimed to improve interfacial charge extraction and mitigate surface recombination, and to enhance trap passivation and device stability is presented.

Abstract

Organic bulk heterojunction solar cells (OSCs) and hybrid halide perovskite solar cells (PSCs) are two promising photovoltaic techniques for next‐generation energy conversion devices. The rapid increase in the power conversion efficiency (PCE) in OSCs and PSCs has profited from synergetic progresses in rational material synthesis for photoactive layers, device processing, and interface engineering. Interface properties in these two types of devices play a critical role in dictating the processes of charge extraction, surface trap passivation, and interfacial recombination. Therefore, there have been great efforts directed to improving the solar cell performance and device stability in terms of interface modification. Here, recent progress in interfacial doping with biopolymers and ionic salts to modulate the cathode interface properties in OSCs is reviewed. For the anode interface modification, recent strategies of improving the surface properties in widely used PEDOT:PSS for narrowband OSCs or replacing it by novel organic conjugated materials will be touched upon. Several recent approaches are also in focus to deal with interfacial traps and surface passivation in emerging PSCs. Finally, the current challenges and possible directions for the efforts toward further boosts of PCEs and stability via interface engineering are discussed.

The role of ligands in perovskite photovoltaics is discussed from the perspective of film formation, passivation, and stability. Ligands can be used to improve the morphology and film quality by modulating the crystallization kinetics of perovskite precursors and passivating the defects via coordinate bonding. Besides, the stability of perovskites can be enhanced by ligand engineering.

Abstract

Due to their outstanding optoelectronic properties, metal halide perovskites have been intensively studied in recent years. The latest certificated efficiency of 23.3% recently achieved in perovskite solar cells (PVSCs) enables them to be used as a very promising candidate for next‐generation photovoltaics. The morphology, defect density, and water resistance of perovskite films have an enormous impact on the performance and stability of PVSCs. Ligands, with coordinating capability, have been widely developed to improve the quality and stability of perovskite materials significantly. In the first section of this review, the role of ligands in fabricating perovskite films by different methods (one‐step, two‐step, and postdeposition treatment) is discussed. In the second section, the progress on ligand‐passivated perovskites via post‐treatment, in situ passivation during perovskite formation, and modifying the substrates before perovskite formation is reviewed. In the third section, a discussion of ligand‐stabilized perovskite films from the perspectives of crystal crosslinking, dimensionality engineering, and interfacial modification is presented. Finally, a summary and an outlook are given.

A low‐temperature solution‐processed TFB is demonstrated as an ideal hole‐transporting layer to push the PCE of the inverted perovskite solar cells (PVSCs) up to 20.2%. Moreover, this TFB‐based inverted PVSC exhibits good stability, retaining 90% of its original efficiency after storage for 30 days in ambient air.

Abstract

Low‐temperature‐processed inverted perovskite solar cells (PVSCs) attract increasing attention because they can be fabricated on both rigid and flexible substrates. For these devices, hole‐transporting layers (HTLs) play an important role in achieving efficient and stable inverted PVSCs by adjusting the anodic work function, hole extraction, and interfacial charge recombination. Here, the use of a low‐temperature (≤150 °C) solution‐processed ultrathin film of poly[(9,9‐dioctyl‐fluorenyl‐2,7‐diyl)‐co‐(4,4′‐(N‐(4‐secbutylphenyl) diphenylamine)] (TFB) is reported as an HTL in one‐step‐processed CH₃NH₃PbI₃ (MAPbI₃)‐based inverted PVSCs. The fabricated device exhibits power conversion efficiency (PCE) as high as 20.2% when measured under AM 1.5 G illumination. This PCE makes them one of the MAPbI₃‐based inverted PVSCs that have the highest efficiency reported to date. Moreover, this inverted PVSC also shows good stability, which can retain 90% of its original efficiency after 30 days of storage in ambient air.

By introducing phenylethylammonium iodide (PhEtNH₃I) treatment can significantly enhance film and device stability under light and oxygen stress. These observations are consistent with the iodide salt treatment reducing iodide vacancies and therefore lowering the yield of superoxide formation and improving stability.

Organic–inorganic halide perovskite materials have emerged as attractive alternatives to conventional solar cells, but device stability remains a concern. Recent research has demonstrated that the formation of superoxide species under exposure of the perovskite to light and oxygen leads to the degradation of CH₃NH₃PbI₃ perovskites. In particular, it has been revealed that iodide vacancies in the perovskite are key sites in facilitating superoxide formation from oxygen. This paper shows that passivation of CH₃NH₃PbI₃ films with an iodide salt, namely phenylethylammonium iodide (PhEtNH₃I) can significantly enhance film and device stability under light and oxygen stress, without compromising power conversion efficiency. These observations are consistent with the iodide salt treatment reducing iodide vacancies and therefore lowers the yield of superoxide formation and improves stability. The present study elucidates a pathway to the future design and optimization of perovskite solar cells with greater stability.

The surface treatment of the electron transport layer, PCBM, is done using stearic acid. The treated surface consists of perpendicularly aligned monolayers of stearic acid which repel water, creating a hydrophobic film on top of PCBM. Amide linkages which crosslink stearic acid and the methyl ester group of PCBM, act as a barrier by preventing iodine ions which migrate from the active layer, reacting with the aluminum electrode.

Having achieved power conversion efficiencies higher than 22%, perovskite solar cells (PSCs) look set to be game changers in the field of photovoltaics. Their instability in humid environments, however, reduces their potential for commercialization. In this study, the role chemical degradation plays in moisture‐affected devices is investigated, and, based on this concept, a technique that enhances the device stability of p‐i‐n PSCs is developed. By surface treatment of the [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) layer with hydrophobic stearic acid and ethylenediamine, increased moisture resistivity of PCBM is achieved. The treated surface of the PCBM layer improves hydrophobicity, with a contact angle of 108°, and also prevents water ingress in the perovskite layer longer than non‐treated surfaces. In addition, interfacial stability is enhanced by the suppressed interaction between the ions and the electrodes, resulting in treated devices exhibiting improved stability in their photovoltaic parameters compared to non‐treated devices when exposed to a dark environment with a relative humidity of 45%.

Formamidinium iodide (FAI) post‐treatment is used on MAPbI₃ surfaces to obtain high‐quality perovskite films, which leads to traps‐states or defects reduction, thus reducing hysteresis. The FAI post‐treated solar cells show improved device performance, with the average efficiency increasing from 16.86% to 18.40%, and the power conversion efficiency reaching 20.25%.

Abstract

Organolead trihalide perovskite films with a large grain size and excellent surface morphology are favored to good‐performance solar cells. However, interstitial and antisite defects related trap‐states are originated unavoidably on the surfaces of the perovskite films prepared by the solution deposition procedures. The development of post‐growth treatment of defective films is an attractive method to reduce the defects to form good‐quality perovskite layers. Herein, a post‐treatment tactic is developed to optimize the perovskite crystallization by treating the surface of the one‐step deposited CH₃NH₃PbI₃ (MAPbI₃) using formamidinium iodide (FAI). Charge carrier kinetics investigated via time‐resolved photoluminescent, open‐circuit photovoltage decay, and time‐resolved charge extraction indicate that FAI post‐treatment will boost the perovskite crystalline quality, and further result in the reduction of the defects or trap‐states in the perovskite films. The photovoltaic devices by FAI treatment show much improved performance in comparison to the controlled solar cell. As a result, a champion solar cell with the best power conversion efficiency of 20.25% is obtained due to a noticeable improvement in fill factor. This finding exhibits a simple procedure to passivate the perovskite layer via regulating the crystallization and decreasing defect density.

Incorporation of indium(III) chloride is directly shown to induce the structural reconstruction of CsPbI₂Br perovskite at the microscopic level, which allows the stabilization of the α‐phase perovskite by means of increasing the structure tolerance factor and decreasing the grain size. Consequently, the square‐centimeter all‐inorganic InCl₃:CsPbI₂Br perovskite solar cells yield a power conversion efficiency of 11.4% with high stability.

Abstract

Although all‐inorganic perovskite solar cells (PSCs) demonstrate high thermal stability, cesium‐lead halide perovskites with high iodine content suffer from poor stability of the black phase (α‐phase). In this study, it is demonstrated that incorporating InCl₃ into the host perovskite lattice helps to inhibit the formation of yellow phase (δ‐phase) perovskite and thereby enhances the long‐term ambient stability. The enhanced stability is achieved by a strategy for the structural reconstruction of CsPbI₂Br perovskite by means of In³⁺ and Cl⁻ codoping, which gives rise to a significant improvement in the overall spatial symmetry with a closely packed atom arrangement due to the crystal structure transformation from orthorhombic (Pnma) to cubic (Pm‐3m). In addition, a novel thermal radiation heating method that further improves the uniformity of the perovskite thin films is presented. This approach enables the construction of all‐inorganic InCl₃:CsPbI₂Br PSCs with a champion power conversion efficiency of 13.74% for a small‐area device (0.09 cm²) and 11.4% for a large‐area device (1.00 cm²).

Multijunction organic solar cells provide higher power conversion efficiencies than the corresponding single junction solar cells by reducing thermalization and transmission losses and are fabricated by sequential layer deposition from solution. In recent years, important progress has been made in terms of novel materials and device design and the most salient advances are discussed.

Abstract

The efficiency of organic solar cells can benefit from multijunction device architectures, in which energy losses are substantially reduced. Herein, recent developments in the field of solution‐processed multijunction organic solar cells are described. Recently, various strategies have been investigated and implemented to improve the performance of these devices. Next to developing new materials and processing methods for the photoactive and interconnecting layers, specific layers or stacks are designed to increase light absorption and improve the photocurrent by utilizing optical interference effects. These activities have resulted in power conversion efficiencies that approach those of modern thin film photovoltaic technologies. Multijunction cells require more elaborate and intricate characterization procedures to establish their efficiency correctly and a critical view on the results and new insights in this matter are discussed. Application of multijunction cells in photoelectrochemical water splitting and upscaling toward a commercial technology is briefly addressed.

Publication date: 20 March 2019

Source: Joule, Volume 3, Issue 3

Author(s): Sajjad Ahmad, Ping Fu, Shuwen Yu, Qing Yang, Xuan Liu, Xuchao Wang, Xiuli Wang, Xin Guo, Can Li

Context & Scale

Summary

Graphical Abstract

N‐type Sb³⁺ heterovalent doping is conducted to regulate the polarity of perovskite and to optimize the interface band structure of ETL‐free PSCs. With doping, the band bending at both FTO/perovskite and perovskite/spiro‐MeOTAD interfaces ensures efficient collection of majority carrier and blocking of minority carrier, contributing to 12.62% efficient ETL‐free device with a Schottky/p‐n cascade heterojunction, pointing out new insights into the understanding of such devices.

Electron transport layer (ETL)‐free perovskite solar cells (PSCs) are getting much more attention with their simpler structure and potentially low cost as well as higher stability. However, the elimination of ETL (such as TiO₂) with intrinsically deep valance band level leads to the absence of the hole blocking mechanism and thus serious charge recombination at the FTO/perovskite interface compared with ETL‐based devices. An interface band bending associated built‐in electric field is an essential driving force of charge separation. Here, by intentional polarity tailoring of perovskite via incorporation of Sb as a shallow donor, ETL‐free PSCs with optimized energy level alignment both at the front and rear interfaces are constructed, resulting in an enhanced built‐in electric field and thus efficient majority carrier collection and minority blocking as well as reduced interfacial charge recombination at both interfaces. Device simulation calculations also confirm the importance of polarity control for device performance improvement. The effect of doping on the perovskite films properties and device performance are systematically demonstrated. Correspondingly, ETL‐free PSCs with a champion power conversion efficiency of 12.62% is achieved.

A simple antipolar route is proposed to prepare hierarchical electron transporting layers for boosting the efficiency of dopant‐free perovskite solar cells (PSCs). The photovoltaic performance of PSCs is enhanced owing to the enhanced light‐scattering, the improved Ostwald ripening process, and the promoted photo‐generated electron extraction.

Abstract

The breakthrough of organometal halide perovskite solar cells (PSCs) based on mesostructured composites is regarded as a viable member of next generation photovoltaics. In high efficiency PSCs, it is crucial to finely optimize the charge dynamics and optical properties matching between the perovskites and electron transporting materials to relax the trade‐off between the optical and electrical requirements. Here, a simple antipolar route with H₂O as the additive is proposed to prepare hierarchical electron transporting layers to boost the efficiency of dopant‐free PSCs. The photovoltaic performance of the PSCs is enhanced owing to increased light‐scattering, improved Ostwald ripening, and photo‐generated electron extraction. Optimization of the H₂O addition enables a valid power conversion efficiency of 19.9% (reverse scan: 20.02%) to be achieved. The device can retain more than 90% of its initial performance after storage in air more than 30 days. These results are inspiring in that they present that a mesoporous transporting layer could be easily re‐constructed to hierarchical architecture by the antipolar method to further improve the performance of PSCs.

Caught in a trap: A series of new double perovskite nanocrystals Cs₂AgSb_1−y Bi _y X₆ (X: Br, Cl; 0≤y≤1) is synthesized. Intrinsic self‐trapping (ca. 1–2 ps) arising from giant carrier–phonon scattering and surface‐defect trapping (ca. 50–100 ps) are revealed. Slow hot‐carrier cooling is observed at high pump fluence.

Abstract

A series of lead‐free double perovskite nanocrystals (NCs) Cs₂AgSb_1−y Bi _y X₆ (X: Br, Cl; 0≤y≤1) is synthesized. In particular, the Cs₂AgSbBr₆ NCs is a new double perovskite material that has not been reported for the bulk form. Mixed Ag–Sb/Bi NCs exhibit enhanced stability in colloidal solution compared to Ag–Bi or Ag–Sb NCs. Femtosecond transient absorption studies indicate the presence of two prominent fast trapping processes in the charge‐carrier relaxation. The two fast trapping processes are dominated by intrinsic self‐trapping (ca. 1–2 ps) arising from giant exciton–phonon coupling and surface‐defect trapping (ca. 50–100 ps). Slow hot‐carrier relaxation is observed at high pump fluence, and the possible mechanisms for the slow hot‐carrier relaxation are also discussed.