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Publication date: June 2020

Source: Nano Energy, Volume 72

Author(s): Sora Oh, Nasir Khan, Seon-Mi Jin, Huyen Tran, Namsun Yoon, Chang Eun Song, Hang Ken Lee, Won Suk Shin, Jong-Cheol Lee, Sang-Jin Moon, Eunji Lee, Sang Kyu Lee

A strategy is demonstrated for efficacious regulation of perovskite crystallinity using glycolic acid (GA) against nonvolatile thioglycolic acid (TGA) following dimethyl sulfoxide sublimation, resulting in enhanced device performance. A champion power conversion efficiency as high as 21.32% is achieved for the GA‐based device, which is almost 13% or 20% higher than those of the control device or TGA‐based device.

Abstract

A strategy for efficaciously regulating perovskite crystallinity is proposed by using a volatile solid glycolic acid (HOCH₂COOH, GA) in an FA_0.85MA_0.15PbI₃ (FA: HC(NH₂)₂; MA: CH₃NH₃) perovskite precursor solution that is different from the common additive approach. Accompanied with the first dimethyl sulfoxide sublimation process, the subsequent sublimation of GA before 150 °C in the FA_0.85MA_0.15PbI₃ perovskite film can artfully regulate the perovskite crystallinity without any residual after annealing. The improved film formation upon GA modification induced by the strong interaction between GA and Pb²⁺ delivers a champion power conversion efficiency (PCE) as high as 21.32%. In order to investigate the role of volatility in perovskite solar cells (PSCs), nonvolatile thioglycolic acid (HSCH₂COOH, TGA) with a similar structure to GA is utilized as an additive reference. Large perovskite grains are obtained by TGA modification but with obvious pinholes, which directly leads to an increased defect density accompanied by a decline in PCE. Encouragingly, the champion PCE achieved for GA‐based PSC device (21.32%) is almost 13% or 20% higher than those of the control device or TGA‐based device. In addition, GA‐modified PSCs exhibit the best stability in light‐, thermal‐, and humidity‐based tests due to the improved film formation.

Tin fluoride and methylammonium iodide are employed as precursors for the fabrication of methylammonium tin iodide (MASnI₃) film via an ion exchange/insertion reactions approach, and a highly uniform, pinhole‐free perovskite film is obtained with a high concentration of SnF₂ and a low content of Sn⁴⁺. The corresponding solar cell exhibits the highest power conversion efficiency of 7.78% with high reproducibility and stability.

Abstract

The low toxicity, narrow bandgaps, and high charge‐carrier mobilities make tin perovskites the most promising light absorbers for low‐cost perovskite solar cells (PSCs). However, the development of the Sn‐based PSCs is seriously hampered by the critical issues of poor stability and low power conversion efficiency (PCE) due to the facile oxidation of Sn²⁺ to Sn⁴⁺ and poor film formability of the perovskite films. Herein, a synthetic strategy is developed for the fabrication of methylammonium tin iodide (MASnI₃) film via ion exchange/insertion reactions between solid‐state SnF₂ and gaseous methylammonium iodide. In this way, the nucleation and crystallization of MASnI₃ can be well controlled, and a highly uniform pinhole‐free MASnI₃ perovskite film is obtained. More importantly, the detrimental oxidation can be effectively suppressed in the resulting MASnI₃ film due to the presence of a large amount of remaining SnF₂. This high‐quality perovskite film enables the realization of a PCE of 7.78%, which is among the highest values reported for the MASnI₃‐based solar cells. Moreover, the MASnI₃ solar cells exhibit high reproducibility and good stability. This method provides new opportunities for the fabrication of low‐cost and lead‐free tin‐based halide perovskite solar cells.

This work mainly focuses on materials composition and working mechanism of the hydroiodic acid (HI) hydrolysis‐derived intermediate compound DMAI/DMAPbI _x . Importantly, the main component of the CsPbI₃ film prepared by such precursor is proved to be still inorganic. Finally, the optimized CsPbI₃ film–based device shows significantly enhanced stability in ambient environment with a high power conversion efficiency of 17.32%.

Abstract

Introducing hydroiodic acid (HI) as a hydrolysis‐derived precursor of the intermediate compounds has become an increasingly important issue for fabricating high quality and stable CsPbI₃ perovskite solar cells (PSCs). However, the materials composition of the intermediate compounds and their effects on the device performance remain unclear. Here, a series of high‐quality intermediate compounds are prepared and it is shown that they consist of DMAI/DMAPbI _x . Further characterization of the products show that the main component of this system is still CsPbI₃. Most of the dimethylammonium (DMA⁺) organic component is lost during annealing. Only an ultrasmall amount of DMA⁺ is doped into the CsPbI₃ and its structure is stabilized. Meanwhile, excessive DMA⁺ forms Lewis acid–base adducts and interactions with Pb²⁺ on the CsPbI₃ surface. This process passivates the CsPbI₃ film and decreases the recombination rate. Finally, CsPbI₃ film is fabricated with high crystalline, uniform morphology, and excellent stability. Its corresponding PSC exhibits stable property and improved power conversion efficiency (PCE) up to 17.3%.

A fused‐ring electron acceptor (F10IC2) with solubilizing alkoxy side chains is designed and synthesized, and as‐cast blade‐coated organic solar cells based on PTB7‐Th: F10IC2 blended films are fabricated from chlorobenzene or chlorine‐free o‐xylene as solvents in air without any post‐treatment deliver a PCE of 12.5% and 11.4%, respectively.

The rapid development of organic solar cells (OSCs) based on nonfullerene acceptors has achieved significant breakthroughs in the power conversion efficiency (PCE) of spin‐coated devices. However, the spin‐coating method in a protective atmosphere seems unsuitable for the practical printing of high‐performance solar panels. In addition, the use of highly toxic solvents is also a stumbling block to the commercial application of OSCs. Thus, photoactive materials for scalable coating and ecofriendly manufacturing approaches are necessary to be developed for OSCs. Herein, a fused‐ring electron acceptor named F10IC2 bearing a decacycle core and solubilizing alkoxyl side chains is synthesized and applied in as‐cast blade‐coated OSCs by blending with polymer donor PTB7‐Th. As‐cast OSCs based on PTB7‐Th: F10IC2 blended films fabricated from chlorobenzene or chlorine‐free o‐xylene solvents in air without any posttreatment deliver a PCE of 12.5% and 11.4%, respectively, which are among the highest values reported for as‐cast blade‐coated OSCs. Herein, a strategy of alkoxyl solubilizing to design high‐performance material systems for ecofriendly scalable OSCs is provided, which is suitable for future industrial production.

Based on the previously reported acceptors ITIC2 and ITIC‐SF, using side chains and an end group strategy, a two‐dimension (2D) conjugated acceptor BDTSF‐IC is designed with fluorinated engineering and thiophene end group. The PM6:BDTSF‐IC‐based polymer solar cells achieve a photovoltaic performance of 13.1%, higher than those achieved by devices based on PM6:BDTCH‐IC (10.51%).

The previously reported nonfullerene small molecule ITIC‐SF achieved via side chain tuning, promotes the power conversion efficiency of polymer solar cells (PSCs) with PBDB‐T‐SF as the donor from 10.1% and ITIC2 acceptors up to 12.2% for ITIC‐SF acceptors. To further this research, benzene end groups of molecules are herein substituted with thiophene rings, obtaining two new molecules BDTCH‐IC with alkylthio substituents, and BDTSF‐IC with alkylthio and fluorine substituents on their thiophene‐conjugated side chains. The absorption edges of BDTCH‐IC and BDTSF‐IC are red‐shifted to 824 and 793 nm, respectively. Strengthened molecular crystallinity, promoted charge extraction, and upgraded morphology endorse the advancement of photovoltaic performance of the small molecular acceptors. Using donor PM6, the two small molecule acceptors show good photovoltaic performance, although the highest occupied molecular orbit energy offsets are small between donor and acceptor materials. As a combination of side‐chain and end‐group engineering, the photovoltaic performance of the PSCs is increased to 13.1%, together with the best short‐circuit current (J _SC) and fill factor reported thus far for this series of molecules. The results indicate that the modification of side chain and end groups is an effective way to improve the photovoltaic performance of small molecule acceptors.

Low‐temperature‐processed Zr/F co‐doped SnO₂ is an excellent successor of electron transport layers (ETLs) for high‐efficiency planar perovskite solar cells. Benefiting from an accurate energy level match and enhanced ETL conductivity, the photoelectric conversion efficiency, and hysteresis effect are obviously improved.

The energy band position and conductivity of electron transport layers (ETLs) are essential factors that restrict the efficiency of planar perovskite solar cells (p‐PSCs). Tin oxide (SnO₂) has become a primary material in ETLs due to its mild synthesis condition, but its low conduction band position and limited intrinsic carriers are disadvantageous in electron transport. To solve these problems, this work exquisitely designs a Zr/F co‐doped SnO₂ ETL. The doping of Zr can raise the conduction band of SnO₂, which reduces the energy barrier in electron extraction and inhibits the interface recombination between the ETL and perovskite. The open‐circuit voltage (V _OC) of p‐PSCs consequently increases. F⁻ doping belongs to n‐type doping. Thus, it equips SnO₂ with a large number of free electrons and improves the conductivity of the ETL and short‐circuit current (J _SC). The device based on Zr/F co‐doped ETL achieves a high efficiency of 19.19% and exhibits a reduced hysteresis effect, which is more satisfactory than that of a pristine device (17.35%). Overall, this research successfully adjusts the energy band match and boosts the conductivity of ETL via Zr/F co‐doping. The results provide an effective strategy for fabricating high‐efficiency p‐PSCs.

Incorporation of a 1D PbI₂‐BPy(II) perovskite with a closely rigid‐skeleton structure allows the fabrication of a 1D@3D hybrid perovskite with a fascinating well‐lattice‐matching heterojunction structure, which is experimentally observed to alleviate the ion migration and thereby improve the stability of 3D perovskites. Consequently, the 1D@3D perovskite solar cells demonstrate 21.18% power conversion efficiency and superior long‐term stability.

Abstract

The stability of perovskite solar cells (PSCs) has been identified to be the bottleneck toward their industrialization. With the aim of tackling this challenge, a 1D PbI₂‐bipyridine (BPy)(II) perovskite is fabricated, which is shown to be capable of in situ assembly of a 1D@3D perovskite that is promoted by a PbI₂‐dimethyl sulfoxide complex with a skeletal linear chain structure. The as‐prepared 1D@3D perovskite is observed to demonstrate extremely high stability under external large electric fields in humid environments by means of an in situ characterization technique. This stability is associated with its well lattice‐matching heterojunction structure between 1D and 3D heterojunction domains. Importantly, ion migration is alleviated through blocking of the ion‐migration channels. Accordingly, the 1D@3D hybrid PSC shows a power conversion efficiency of 21.18% maintaining remarkably high long‐term stability in the presence of water, illumination, and external electric fields. This rational design and microstructure study of 1D@3D perovskites provides a new paradigm that may enable higher efficiency and stability of PSCs.

Two hepta‐ring and octa‐ring asymmetric small molecular acceptors IDTP‐4F and IDTTP‐4F are synthesized. S‐shape IDTP‐4F‐based polymer solar cells perform better than their counterparts based on C‐shape IDTTP‐4F, regardless of the polymer donors. The champion efficiency afforded by PM7: IDTP‐4F is as high as 15.2%.

Abstract

The prosperous period of polymer solar cells (PSCs) has witnessed great progress in molecule design methods to promote power conversion efficiency (PCE). Designing asymmetric structures has been proved effective in tuning energy level and morphology, which has drawn strong attention from the PSC community. Two hepta‐ring and octa‐ring asymmetric small molecular acceptors (SMAs) (IDTP‐4F and IDTTP‐4F) with S‐shape and C‐shape confirmations are developed to study the relationship between conformation shapes and PSC efficiencies. The similarity of absorption and energy levels between two SMAs makes the conformation a single variable. Additionally, three wide‐bandgap polymer donors (PM6, S1, and PM7) are chosen to prove the universality of the relationship between conformation and photovoltaic performance. Consequently, the champion PCE afforded by PM7: IDTP‐4F is as high as 15.2% while that of PM7: IDTTP‐4F is 13.8%. Moreover, the S‐shape IDTP‐4F performs obviously better than their IDTTP‐4F counterparts in PSCs regardless of the polymer donors, which confirms that S‐shape conformation performs better than the C‐shape one. This work provides an insight into how conformations of asymmetric SMAs affect PCEs, specific functions of utilizing different polymer donors to finely tune the active‐layer morphology and another possibility to reach an excellent PCE over 15%.

A 2D/3D perovskite heterostructure passivation is employed for double‐cation wide‐bandgap PSCs with engineered bandgap (1.65 eV ≤ E _g ≤ 1.85 eV) which results in improved stabilized PCEs and open‐circuit voltages for opaque and semitransparent perovskite solar cells. Four‐terminal perovskite/c‐Si and perovskite/CIGS tandem solar cells with stabilized PCEs of up to 25.7% and 25.0%, respectively, are demonstrated.

Abstract

Wide‐bandgap perovskite solar cells (PSCs) with optimal bandgap (E _g) and high power conversion efficiency (PCE) are key to high‐performance perovskite‐based tandem photovoltaics. A 2D/3D perovskite heterostructure passivation is employed for double‐cation wide‐bandgap PSCs with engineered bandgap (1.65 eV ≤ E _g ≤ 1.85 eV), which results in improved stabilized PCEs and a strong enhancement in open‐circuit voltages of around 45 mV compared to reference devices for all investigated bandgaps. Making use of this strategy, semitransparent PSCs with engineered bandgap are developed, which show stabilized PCEs of up to 25.7% and 25.0% in four‐terminal perovskite/c‐Si and perovskite/CIGS tandem solar cells, respectively. Moreover, comparable tandem PCEs are observed for a broad range of perovskite bandgaps. For the first time, the robustness of the four‐terminal tandem configuration with respect to variations in the perovskite bandgap for two state‐of‐the‐art bottom solar cells is experimentally validated.

Self‐crystallized multifunctional 2D perovskite (M2P) is formed on top of a 3D perovskite light absorber. The M2P layer performs as a hole‐transfer facilitator and a surface‐trap passivator in perovskite solar cells (PSCs). PSCs using the developed 3D/2D perovskites achieve a power conversion efficiency of 20.79% with highly improved long‐term stability compared to devices without M2P.

Abstract

Recently, perovskite solar cells (PSC) with high power‐conversion efficiency (PCE) and long‐term stability have been achieved by employing 2D perovskite layers on 3D perovskite light absorbers. However, in‐depth studies on the material and the interface between the two perovskite layers are still required to understand the role of the 2D perovskite in PSCs. Self‐crystallization of 2D perovskite is successfully induced by deposition of benzyl ammonium iodide (BnAI) on top of a 3D perovskite light absorber. The self‐crystallized 2D perovskite can perform a multifunctional role in facilitating hole transfer, owing to its random crystalline orientation and passivating traps in the 3D perovskite. The use of the multifunctional 2D perovskite (M2P) leads to improvement in PCE and long‐term stability of PSCs both with and without organic hole transporting material (HTM), 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) compared to the devices without the M2P.

A facile strategy for a vapor‐phase patterning inorganic perovskite single crystal array is demonstrated to fabricate high‐performance micro‐optoelectronic devices. This strategy allows growth of high‐quality inorganic perovskite single crystals with controllable size and location. It provides new opportunities to explore the inorganic perovskite single‐crystal array for various integrated optoelectronics applications.

Abstract

Inorganic perovskite single crystals have emerged as promising vapor‐phase processable structures for optoelectronic devices. However, because of material lattice mismatch and uncontrolled nucleation, vapor‐phase methods have been restricted to random distribution of single crystals that are difficult to perform for integrated device arrays. Herein, an effective strategy to control the vapor‐phase growth of high‐quality cesium lead bromide perovskite (CsPbBr₃) microplate arrays with uniform morphology as well as controlled location and size is reported. By introducing perovskite seeds on substrates, intractable lattice mismatches and random nucleation barriers are surpassed, and the epitaxial growth of perovskite crystals is accurately controlled. It is further demonstrated that CsPbBr₃ microplate arrays can be monolithically integrated on substrates for the fabrication of high‐performance lasers and photodetectors. This strategy provides a facile approach to fabricate high‐quality CsPbBr₃ microplates with controllable size and location, which offers new opportunities for the scalable production of integrated optoelectronic devices.

Publication date: June 2020

Source: Nano Energy, Volume 72

Author(s): Chuanfei Wang, Fabrizio Moro, Shaofei Ni, Qilun Zhang, Guoxing Pan, Jinpeng Yang, Fapei Zhang, Irina A. Buyanova, Weimin M. Chen, Xianjie Liu, Mats Fahlman

Publication date: June 2020

Source: Nano Energy, Volume 72

Author(s): Govindasamy Sathiyan, Ali Asgher Syed, Cheng Chen, Cheng Wu, Li Tao, Xingdong Ding, Yawei Miao, Gongqiang Li, Ming Cheng, Liming Ding

Publication date: 18 March 2020

Source: Joule, Volume 4, Issue 3

Author(s): Hanjian Lai, Qiaoqiao Zhao, Ziyi Chen, Hui Chen, Pengjie Chao, Yulin Zhu, Yongwen Lang, Nan Zhen, Daize Mo, Yuanzhu Zhang, Feng He

A polymerization‐assisted grain growth strategy in the sequential deposition method of perovskite thin films is demonstrated by triggering a polymerization process during PbI₂ film annealing. This strategy effectively passivates undercoordinated lead ions, reduces defect density, and boosts power conversion efficiency up to 23.0%, together with a prolonged lifetime.

Abstract

Intrinsically, detrimental defects accumulating at the surface and grain boundaries limit both the performance and stability of perovskite solar cells. Small molecules and bulkier polymers with functional groups are utilized to passivate these ionic defects but usually suffer from volatility and precipitation issues, respectively. Here, starting from the addition of small monomers in the PbI₂ precursor, a polymerization‐assisted grain growth strategy is introduced in the sequential deposition method. With a polymerization process triggered during the PbI₂ film annealing, the bulkier polymers formed will be adhered to the grain boundaries, retaining the previously established interactions with PbI₂. After perovskite formation, the polymers anchored on the boundaries can effectively passivate undercoordinated lead ions and reduce the defect density. As a result, a champion power conversion efficiency (PCE) of 23.0% is obtained, together with a prolonged lifetime where 85.7% and 91.8% of the initial PCE remain after 504 h continuous illumination and 2208 h shelf storage, respectively.

Addition of the n‐type dopant benzyl viologen (BV) into several best‐in‐class organic bulk‐heterojunctions (BHJ) is shown to consistently improve the power conversion efficiency (PCE) of the resulting solar cells. The presence of BV inside the BHJs increases the absorption coefficient, balances charge transport, and enhances the charge‐carrier density. These synergistic effects result in organic photovoltaics with a maximum PCE of 17.1%.

Abstract

Molecular doping is often used in organic semiconductors to tune their (opto)electronic properties. Despite its versatility, however, its application in organic photovoltaics (OPVs) remains limited and restricted to p‐type dopants. In an effort to control the charge transport within the bulk‐heterojunction (BHJ) of OPVs, the n‐type dopant benzyl viologen (BV) is incorporated in a BHJ composed of the donor polymer PM6 and the small‐molecule acceptor IT‐4F. The power conversion efficiency (PCE) of the cells is found to increase from 13.2% to 14.4% upon addition of 0.004 wt% BV. Analysis of the photoactive materials and devices reveals that BV acts simultaneously as n‐type dopant and microstructure modifier for the BHJ. Under optimal BV concentrations, these synergistic effects result in balanced hole and electron mobilities, higher absorption coefficients and increased charge‐carrier density within the BHJ, while significantly extending the cells' shelf‐lifetime. The n‐type doping strategy is applied to five additional BHJ systems, for which similarly remarkable performance improvements are obtained. OPVs of particular interest are based on the ternary PM6:Y6:PC₇₁BM:BV(0.004 wt%) blend for which a maximum PCE of 17.1%, is obtained. The effectiveness of the n‐doping strategy highlights electron transport in NFA‐based OPVs as being a key issue.

Nontoxic tin‐based perovskite solar cells (PSCs) have attracted attention, but are easily oxidized, which causes their performance and stability to be far behind lead‐based PSCs. Here, strategies to improve the stability of tin‐based PSCs (additive engineering, deoxidizer, partial substitution, and reduced dimensions) are reviewed. Outlooks are also proposed to avoid the shortcoming for fabricating highly efficient and stable PSCs.

Abstract

Although lead‐based perovskite solar cells (PSCs) are highly efficient, the toxicity of lead (Pb) limits its large‐scale commercialization. As such, there is an urgent need to find alternatives. Many studies have examined tin‐based PSCs. However, pure tin‐based perovskites are easily oxidized in the air or just in glovebox with an ultrasmall amount of oxygen. Such a characteristic makes their performance and stability less ideal compared with those of lead‐based perovskites. Herein, how to address the instability of tin‐based perovskites is introduced in detail. First, the crystalline structure, optical properties, and sources of instability of tin‐based perovskites are summarized. Next, the preparation methods of tin‐based perovskite are discussed. Then, various measures for solving the instability problem are explained using four strategies: additive engineering, deoxidizer, partial substitution, and reduced dimensions. Finally, the challenges and prospects are laid out to help researchers develop highly efficient and stable tin‐based perovskites in the future.

The built‐in electric field of a perovskite solar cell is reinforced by introducing electric dipole molecules, and the oriented charge transfer and collection are significantly improved. An efficiency of 21.5% is demonstrated and the average stability of NMFL device retains 95% PCE after storing over 2000 h under ambient conditions.

Abstract

Perovskite solar cells (PSCs) have received great attention due to their outstanding performance and their low processing costs. To boost their performance, one approach is to reinforce the built‐in electric field (BEF) to promote oriented carrier transport. The BEF is maximized by reinforcing the work function difference between cathode and anode (Δμ₁) and increasing the work function difference between lower and upper surfaces of perovskite film (Δμ₂) via introduction of electric dipole molecules, denoted as PTFCN and CF3BACl. The synergistic reinforcement of BEF improves charge transport and collection, and realizes markedly high photovoltaic performances with the best power conversion efficiency (PCE) up to 21.5%, a growth of 15.6% as compared to the control device, which is higher than the superposition of improvements achieved by either raising Δμ₁ or Δμ₂. Importantly, dual‐functional CF3BACl not only supplies dipole effect for tuning the surface potential of perovskite but offers hydrophobic trifluoride group toward the long‐term stable unencapsulated PSCs retaining more than 95% PCE after storing 2000 h under ambient conditions. This work demonstrates the synergistic effect of Δμ₁ and Δμ₂, providing an effective strategy for the further development of PSC in terms of photovoltaic conversion and stability.