Chen Weijie

Publication date: 18 December 2019

Source: Joule, Volume 3, Issue 12

Author(s): Jing Gao, Florent Sahli, Chenjuan Liu, Dan Ren, Xueyi Guo, Jérémie Werner, Quentin Jeangros, Shaik Mohammed Zakeeruddin, Christophe Ballif, Michael Grätzel, Jingshan Luo

Context & Scale

Summary

Graphical Abstract

A two‐step annealing method is developed for studying the water effect on different kinds of perovskites. It is demonstrated that 60 °C is favorable to the formation of hydrate phase which leads to a reconstruction process in the second annealing stage. The corresponding water effects highly depend on the cations of the perovskite itself.

Water effect on perovskite solar cells has received growing interest in recent years. A widely accepted view is that moderate water content induces the formation of hydrate phase which enhances the recrystallization of the perovskite. However, the underlying factors which influence the formation of hydrate phase are yet to be investigated. Herein, by controlling the annealing temperature, it is demonstrated that 60 °C is the most suitable temperature for the formation of hydrated perovskite. After further annealing at 120 °C, the resulting perovskite film reveals enhanced crystallinity with a more uniform morphology, contributing to device efficiency above 20%. In addition, the water effect on different types of perovskites is studied and it is concluded that the formation of hydrated perovskite is mainly determined by the cations of the perovskite itself. The findings in this work elucidate the conditions for the formation of hydrated perovskite, contributing to the fabrication of highly efficient perovskite solar cells.

Film fabrication environment and anti‐solvent properties strongly influence the microstructure evolution of perovskite films. An ambient fabrication environment induces anisotropies in crystallization. The choice of antisolvent is critical to alleviating these anisotropies. The key is to induce uniform spherulitic crystallization to achieve robust pinhole‐free films possessing grains, crystallinity, crystallographic phases, and crystallite orientations unaffected by the processing environment.

Abstract

The influence of precursor solution properties, fabrication environment, and antisolvent properties on the microstructural evolution of perovskite films is reported. First, the impact of fabrication environment on the morphology of methyl ammonium lead iodide (MAPbI₃) perovskite films with various Lewis‐base additives is reported. Second, the influence of antisolvent properties on perovskite film microstructure is investigated using antisolvents ranging from nonpolar heptane to highly polar water. This study shows an ambient environment that accelerates crystal growth at the expense of nucleation and introduces anisotropies in crystal morphology. The use of antisolvents enhances nucleation but also influences ambient moisture interaction with the precursor solution, resulting in different crystal morphology (shape, size, dispersity) in different antisolvents. Crystal morphology, in turn, dictates film quality. A homogenous spherulitic crystallization results in pinhole‐free films with similar microstructure irrespective of processing environment. This study further demonstrates propyl acetate, an environmentally benign antisolvent, which can induce spherulitic crystallization under ambient environment (52% relative humidity, 25 °C). With this, planar perovskite solar cells with ≈17.78% stabilized power conversion efficiency are achieved. Finally, a simple precipitation test and in situ crystallization imaging under an optical microscope that can enable a facile a priori screening of antisolvents is shown.

A dopant‐free hole transporting material (HTM) named DMZ, is synthesized and applied in inverted planar perovskite solar cells (PSCs). High power conversion efficiency (PCE) (18.61%) and stable‐enhanced PSCs devices are achieved and after storage for nearly 560 h, 90% of the maximum PCE is retained in air with a relative humidity ≈ 45%–50% without any encapsulation.

Abstract

A new hole transporting material (HTM) named DMZ is synthesized and employed as a dopant‐free HTM in inverted planar perovskite solar cells (PSCs). Systematic studies demonstrate that the thickness of the hole transporting layer can effectively enhance the morphology and crystallinity of the perovskite layer, leading to low series resistance and less defects in the crystal. As a result, the champion power conversion efficiency (PCE) of 18.61% with J _SC = 22.62 mA cm⁻², V _OC = 1.02 V, and FF = 81.05% (an average one is 17.62%) is achieved with a thickness of ≈13 nm of DMZ (2 mg mL⁻¹) under standard global AM 1.5 illumination, which is ≈1.5 times higher than that of devices based on poly(3,4‐ethylenedioxythiophene)/poly(styrene sulfonic acid) (PEDOT:PSS). More importantly, the devices based on DMZ exhibit a much better stability (90% of maximum PCE retained after more than 556 h in air (relative humidity ≈ 45%–50%) without any encapsulation) than that of devices based on PEDOT:PSS (only 36% of initial PCE retained after 77 h in same conditions). Therefore, the cost‐effective and facile material named DMZ offers an appealing alternative to PEDOT:PSS or polytriarylamine for highly efficient and stable inverted planar PSCs.

Lead-based organic-inorganic hybrid perovskite (OIHP) solar cells can attain efficiencies over 20%. However, the impact of ion mobility and/or organic depletion, structural changes, and segregation under operating conditions urge for decisive and more accurate investigations. Hence, the development of analytical tools for accessing the grain-to-grain OIHP chemistry is of great relevance. Here, we used synchrotron infrared nanospectroscopy (nano-FTIR) to map individual nanograins in OIHP films. Our results reveal a spatial heterogeneity of the vibrational activity associated to the nanoscale chemical diversity of isolated grains. It was possible to map the chemistry of individual grains in CsFAMA [Cs_0.05FA_0.79MA_0.16Pb(I_0.83Br_0.17)₃] and FAMA [FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃] films, with information on their local composition. Nanograins with stronger nano-FTIR activity in CsFAMA and FAMA films can be assigned to PbI₂ and hexagonal polytype phases, respectively. The analysis herein can be extended to any OIHP films where organic cation depletion/accumulation can be used as a chemical label to study composition.

Oxidants will happen: The effective oxidation of spiro‐OMeTAD is carried out by a hybrid polyoxometalate@metal–organic framework (POM@MOF) POM@Cu‐BTC which can be used as the hole transport material (HTL). This composite oxidant contributes to enhanced efficiency, as well as improved long‐time stability of perovskite solar cells (PSCs).

Abstract

The controllable oxidation of spiro‐OMeTAD and improving the stability of hole‐transport materials (HTMs) layer are crucial for good performance and stability of perovskite solar cells (PSCs). Herein, we report an efficient hybrid polyoxometalate@metal–organic framework (POM@MOF) material, [Cu₂(BTC)_4/3(H₂O)₂]₆[H₃PMo₁₂O₄₀]₂ or POM@Cu‐BTC, for the oxidation of spiro‐OMeTAD with Li‐TFSI and TBP. When POM@Cu‐BTC is introduced to the HTM layer as a dopant, the PSCs achieve a superior fill factor of 0.80 and enhanced power conversion efficiency 21.44 %, as well as improved long‐term stability in an ambient atmosphere without encapsulation. The enhanced performance is attributed to the oxidation activity of POM anions and solid‐state nanoparticles. Therefore, this research presents a facile way by using hybrid porous materials to accelerate oxidation of spiro‐OMeTAD, further improving the efficiency and stability of PSCs.

Oxidants will happen: The effective oxidation of spiro‐OMeTAD is carried out by a hybrid polyoxometalate@metal–organic framework (POM@MOF) POM@Cu‐BTC which can be used as the hole transport material (HTL). This composite oxidant contributes to enhanced efficiency, as well as improved long‐time stability of perovskite solar cells (PSCs).

Abstract

The controllable oxidation of spiro‐OMeTAD and improving the stability of hole‐transport materials (HTMs) layer are crucial for good performance and stability of perovskite solar cells (PSCs). Herein, we report an efficient hybrid polyoxometalate@metal–organic framework (POM@MOF) material, [Cu₂(BTC)_4/3(H₂O)₂]₆[H₃PMo₁₂O₄₀]₂ or POM@Cu‐BTC, for the oxidation of spiro‐OMeTAD with Li‐TFSI and TBP. When POM@Cu‐BTC is introduced to the HTM layer as a dopant, the PSCs achieve a superior fill factor of 0.80 and enhanced power conversion efficiency 21.44 %, as well as improved long‐term stability in an ambient atmosphere without encapsulation. The enhanced performance is attributed to the oxidation activity of POM anions and solid‐state nanoparticles. Therefore, this research presents a facile way by using hybrid porous materials to accelerate oxidation of spiro‐OMeTAD, further improving the efficiency and stability of PSCs.

This study reports a systematic study in terms of efficiency and stability by post‐treatment with alkyl ammonium iodides of different alkyl lengths on (FAPbI₃)_0.95(MAPbBr₃)_0.05 perovskite surface. As the length of the alkyl chain increases, the electron‐blocking ability and humidity stability increase, but the highest efficiency is obtained at the optimal alkyl length.

Abstract

Recently, two‐dimensional (2D) structure on three‐dimensional (3D) perovskites (graded 2D/3D) has been reported to be effective in significantly improving both efficiency and stability. However, the electrical properties of the 2D structure as a passivation layer on the 3D perovskite thin film and resistance to the penetration of moisture may vary depending on the length of the alkyl chain. In addition, the surface defects of the 2D itself on the 3D layer may also be affected by the correlation between the 2D structure and the hole conductive material. Therefore, systematic interfacial study with the alkyl chain length of long‐chained alkylammonium iodide forming a 2D structure is necessary. Herein, the 2D interfacial layers formed are compared with butylammonium iodide (BAI), octylammonium iodide (OAI), and dodecylammonium iodide (DAI) iodide on a 3D (FAPbI₃)_0.95(MAPbBr₃)_0.05 perovskite thin film in terms of the PCE and humidity stability. As the length of the alkyl chain increased from BA to OA to DA, the electron‐blocking ability and humidity resistance increase significantly, but the difference between OA and DA is not large. The PSC post‐treated with OAI has slightly higher PCE than those treated with BAI and DAI, achieving a certified stabilized efficiency of 22.9%.

Gap states present a new approach to develop multi‐photon upconversion light emission in quasi‐2D perovskite films under continuous‐wave infrared excitation. Magneto‐photoluminescence (PL) and polarization‐dependent PL reveal that the gap states are essentially spatially extended states involved in orbit–orbit interaction toward generating multi‐photon excitation in quasi‐2D perovskite films.

Abstract

A new approach to generate a two‐photon up‐conversion photoluminescence (PL) by directly exciting the gap states with continuous‐wave (CW) infrared photoexcitation in solution‐processing quasi‐2D perovskite films [(PEA)₂(MA)₄Pb₅Br₁₆ with n = 5] is reported. Specifically, a visible PL peaked at 520 nm is observed with the quadratic power dependence by exciting the gap states with CW 980 nm laser excitation, indicating a two‐photon up‐conversion PL occurring in quasi‐2D perovskite films. Decreasing the gap states by reducing the n value leads to a dramatic decrease in the two‐photon up‐conversion PL signal. This confirms that the gap states are indeed responsible for generating the two‐photon up‐conversion PL in quasi‐2D perovskites. Furthermore, mechanical scratching indicates that the different‐n‐value nanoplates are essentially uniformly formed in the quasi‐2D perovskite films toward generating multi‐photon up‐conversion light emission. More importantly, the two‐photon up‐conversion PL is found to be sensitive to an external magnetic field, indicating that the gap states are essentially formed as spatially extended states ready for multi‐photon excitation. Polarization‐dependent up‐conversion PL studies reveal that the gap states experience the orbit–orbit interaction through Coulomb polarization to form spatially extended states toward developing multi‐photon up‐conversion light emission in quasi‐2D perovskites.

Publication date: 18 December 2019

Source: Joule, Volume 3, Issue 12

Author(s): Jia Zhang, Jiajun Qin, Miaosheng Wang, Yujie Bai, Han Zou, Jong Kahk Keum, Runming Tao, Hengxing Xu, Haomiao Yu, Stefan Haacke, Bin Hu

Context & Scale

Summary

Graphical Abstract

Via an energy‐transfer mechanism, ternary organic solar cells based on a wide‐bandgap donor (PBDB‐T‐2Cl), a middle‐bandgap acceptor (ITC‐2Cl), and an ultranarrow‐bandgap acceptor (IOIC‐2Cl) achieve a champion power conversion efficiency of 14.75% with a low energy loss of 0.48 eV, outcompeting PBDB‐T‐2Cl: ITC‐2Cl (13.66%) and PBDB‐T‐2Cl: IOIC‐2Cl (11.60%) binary devices.

An effective way to improve the power conversion efficiency of organic solar cells (OSCs) is to use the ternary architecture consisting of a donor, an acceptor, and a third component. Identifying the proper third component for successful ternary OSCs, however, is not an easy task. Herein, it is demonstrated that a middle‐bandgap acceptor, ITC‐2Cl, functions as a successful third component for several wide‐bandgap donor: ultranarrow bandgap acceptor binary systems (PBDB‐T‐2F: F8IC, PBDB‐T‐2F: IOIC‐2Cl, and PBDB‐T‐2Cl: IOIC‐2Cl). Photovoltaic parameters, including V _OC, J _SC, fill factor (FF), and power conversion efficiency (PCE), are effectively improved by incorporating ITC‐2Cl, which lies in the complementary absorption of ITC‐2Cl and host binary system, high‐lying LUMO level of ITC‐2Cl, and the inhibition of bimolecular recombination. The ternary device based on PBDB‐T‐2Cl: ITC‐2Cl: IOIC‐2Cl achieves a champion PCE of 14.75% (certified as 13.78%) with a very low energy loss of 0.48 eV. These results provide critical insight into the ternary strategy and encourage re‐evaluation and restudy of the photoactive materials previously reported with moderate performance.

A hybrid electron transport layer (ETL) of SnO₂ and carbon nanotubes (CNTs) is designed by simple thermal decomposition of a mixed solution of SnCl₄·5H₂O and pretreated CNTs. Based on the hybrid ETL, a high efficiency of 20.33% is achieved in the hysteresis‐free perovskite solar cell, which shows 13.58% enhancement compared with the conventional device (power conversion efficiency = 17.90%).

Tin oxide (SnO₂) has recently received increasing attention as an electron transport layer (ETL) in planar perovskite solar cells (PSCs) and is considered a possible alternative to titanium oxide (TiO₂). However, planar devices based on pure solution‐processed SnO₂ ETL still have hysteresis, which greatly limits the application of SnO₂ in high‐efficiency solar cells. Herein, to address this issue, a hybrid ETL of SnO₂ and carbon nanotubes (CNTs) is fabricated by a simple thermal decomposing of a mixed solution of SnCl₄·5H₂O and pretreated CNTs (termed SnO₂–CNT). The addition of CNTs can significantly improve the conductivity of SnO₂ films and reduce the trap‐state density of SnO₂ films, which benefit carrier transfer from the perovskite layer to the cathode. As a result, a high efficiency of 20.33% is achieved in the hysteresis‐free PSCs based on SnO₂–CNT ETL, which shows 13.58% enhancement compared with the conventional device (power conversion efficiency = 17.90%).

A method of combined electron transporting bilayer is reported to reduce energy loss and inhibit defects in the perovskite solar cells (PSCs) by combining the commercially accessible SnO₂ and home‐made TiO₂ nanoparticles. Consequently, the PSCs devices acquire a high efficiency of 20.50%, which is superior to that based on SnO₂ layers with a efficiency of 18.09%.

Herein, commercially accessible SnO₂ and home‐made TiO₂ nanoparticles as a combined electron transporting bilayer (ETBL) are applied to achieve highly efficient planar perovskite solar cells (PSCs). With the formed cascade‐aligned energy levels from the proper stacking of SnO₂ and TiO₂ layers and the excellent defect‐passivation ability of TiO₂, SnO₂/TiO₂ ETBLs effectively reduce energy loss and inhibit defects formation both at the electron transporting layers (ETL)/perovskite interfaces and within the bulk of perovskite layer as revealed by a comprehensive analysis of photoelectric characteristic analysis, including ultraviolet photoelectron spectroscopy, photoluminescence, and electrochemical impedance spectroscopy. Consequently, the PSC devices acquired a power conversion efficiency (PCE) of 20.50% with a V _oc of 1.10 V, a J _sc of 24.2 mA cm⁻² and an fill factor of 77%, which are superior to the values of the control device based on single SnO₂ layer with a PCE of 18.09% (a 13.3% boosting on PCE). Moreover, there was no degradation after 49 days, indicating the great stability after adding TiO₂ layer. Herein, it is demonstrated that the cascaded alignment of energy levels between the electrode and perovskite layer by ETBLs could be an effective approach to improve the photovoltaic performance of the PSCs with excellent long‐term stability.

The power conversion efficiency of inorganic perovskite solar cells (PSCs) is still low compared with hybrid PSCs. The use of lithium fluoride on SnO₂ and PbCl₂ additive in perovskite is reported for reducing the charge recombination; 18.64% efficiency of CsPbI_3–x Br _x solar cells is demonstrated; and the devices show over than 1000 h light soaking stability.

Abstract

Cesium‐based inorganic perovskite solar cells (PSCs) are promising due to their potential for improving device stability. However, the power conversion efficiency of the inorganic PSCs is still low compared with the hybrid PSCs due to the large open‐circuit voltage (V _OC) loss possibly caused by charge recombination. The use of an insulated shunt‐blocking layer lithium fluoride on electron transport layer SnO₂ for better energy level alignment with the conduction band minimum of the CsPbI_{3‐ x} Br _x and also for interface defect passivation is reported. In addition, by incorporating lead chloride in CsPbI_{3‐ x} Br _x precursor, the perovskite film crystallinity is significantly enhanced and the charge recombination in perovksite is suppressed. As a result, optimized CsPbI_{3‐ x} Br _x PSCs with a band gap of 1.77 eV exhibit excellent performance with the best V _OC as high as 1.25 V and an efficiency of 18.64%. Meanwhile, a high photostability with a less than 6% efficiency drop is achieved for CsPbI_{3‐ x} Br _x PSCs under continuous 1 sun equivalent illumination over 1000 h.