Wangyikai

The detailed crystallization pathways of perovskite film manipulated by Lewis base additives are investigated using in situ grazing‐incidence wide‐angle scattering. The modulated crystallization process can be attributed to the intermolecular interaction between Lewis base molecules and perovskite precursors, which results in reduced defect density and ameliorative carrier behavior in high‐crystalline perovskite film as well as the elevated photovoltaic performance.

Abstract

The Lewis acid–base adduct approach has been widely used to form high‐crystalline perovskite films, but the complicated crystallization pathway and underlying film formation mechanism are still ambiguous. Here, the detailed crystallization process of perovskites manipulated by Lewis base additives has been revealed by in situ X‐ray scattering measurements. Through monitoring the film formation process, two distinct crystal growth stages have been definitely recognized: i) an intermediate phase‐dominated stage; and ii) a phase transformation stage from intermediates to crystalline perovskite phase. Incorporating Lewis base additives significantly prolongs the duration of stage 1 and induces a postponed phase transformation pathway, which could be responsible for retardant crystallization kinetics. Based on a series of experimental results and theoretical calculations, it is indicated that the manipulation of perovskite crystallization pathway is a result of the modulated molecular interactions between Lewis base additives and solution precursors. Owing to the retardant crystallization kinetics, enhanced‐quality perovskite films with reduced defect density and improved optoelectronic properties, as well as optimized photovoltaic performance have been demonstrated. This work provides in‐depth understanding with respect to perovskite crystallization pathway modulated by Lewis base additives and perceptive guidelines for precise regulation of crystallization kinetics of perovskite film toward high performance.

Passive attack: The α‐FAPbI₃ perovskite layer in a solar cell is stabilized without deteriorating the spectral features by passivating with 2,3,4,5,6‐pentafluorobenzyl phosphonic acid (PFBPA). High‐quality perovskite solar cells with an improved efficiency of 22.25 % was achieved with excellent moisture stability maintaining >90 % of its initial efficiency at high humidity levels.

Abstract

Perovskite solar cells (PSCs) have shown great promise for photovoltaic applications, owing to their low‐cost assembly, exceptional performance, and low‐temperature solution processing. However, the advancement of PSCs towards commercialization requires improvements in efficiency and long‐term stability. The surface and grain boundaries of perovskite layer, as well as interfaces, are critical factors in determining the performance of the assembled cells. Defects, which are mainly located at perovskite surfaces, can trigger hysteresis, carrier recombination, and degradation, which diminish the power conversion efficiencies (PCEs) of the resultant cells. This study concerns the stabilization of the α‐FAPbI₃ perovskite phase without negatively affecting the spectral features by using 2,3,4,5,6‐pentafluorobenzyl phosphonic acid (PFBPA) as a passivation agent. Accordingly, high‐quality PSCs are attained with an improved PCE of 22.25 % and respectable cell parameters compared to the pristine cells without the passivation layer. The thin PFBPA passivation layer effectively protects the perovskite layer from moisture, resulting in better long‐term stability for unsealed PSCs, which maintain >90 % of the original efficiency under different humidity levels (40–75 %) after 600 h. PFBPA passivation is found to have a considerable impact in obtaining high‐quality and stable FAPbI₃ films to benefit both the efficiency and the stability of PSCs.

The different aggregation forms of hole‐transporting materials (HTMs) affect intermolecular charge transfer and hole transporting in achieving highly efficient dopant‐free perovskite solar cells. The combination of twisted periphery groups with planar core units shows an efficient approach to regulate the state of molecular aggregation after a systematical investigation of 6,12‐dihydroindeno[1,2‐b]fluorine (IDF)‐HTMs with the same IDF core and the different periphery groups.

Abstract

Although several hole‐transporting materials (HTMs) have been designed to obtain perovskite solar cells (PSCs) devices with high performance, the dopant‐free HTMs for efficient and stable PSCs remain rare. Herein, a rigid planar 6,12‐dihydroindeno[1,2‐b]fluorine (IDF) core with different numbers of bulky periphery groups to construct dopant‐free HTMs of IDF‐SFXPh, IDF‐DiDPA, and IDF‐TeDPA is modified. Thanks to the contributions of the planar IDF core and the twisted SFX periphery groups, the dopant‐free IDF‐SFXPh‐based PSCs device achieves a device performance of 17.6%, comparable to the doped 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD)‐based device (17.6%), with much enhanced device stability under glovebox and ambient conditions.

Publication date: 1 September 2019

Source: Solar Energy Materials and Solar Cells, Volume 199

Author(s): Rajendra Kumar Gunasekaran, Prem Jyoti Singh Rana, Sung Heum Park, Vellaiappillai Tamilavan, Senthil Karuppanan, Hee-Je Kim, Kandasamy Prabakar

Herein, three polyfluorene copolymers (TFB, PFB, and PFO) are investigated as hole‐transport materials (HTMs) for the construction of inverted perovskite solar cells. The photovoltaic performance of the device is found to be closely correlated with the electronic properties of HTMs. The TFB‐based device exhibits the highest efficiency of 18.48% due to its high mobility and favored energy‐level alignment.

Inverted perovskite solar cells (PSCs) that can be entirely processed at low temperatures have attracted growing attention due to their cost‐effective production. Hole‐transport materials (HTMs) play an essential role in achieving efficient inverted PSCs, as they determine the effectiveness of charge extraction and recombination at interfaces. Herein, three polyfluorene copolymers (TFB, PFB, and PFO) are investigated as HTMs for construction of inverted PSCs. It is found that the photovoltaic performance of the solar cells is closely correlated with the electronic properties of the HTMs. Due to its high mobility along with the favored energy‐level alignment with perovskite, TFB shows superior charge extraction and suppressed interfacial recombination than PFB‐ and PFO‐based devices, which delivers a high efficiency of 18.48% with an open‐circuit voltage (V _OC) of up to 1.1 V. In contrast, the presence of a large energy barrier in the PFO‐based devices results in substantial losses in both V _OC and photocurrent. These results demonstrate that TFB can serve as a superior HTM for inverted PSCs. Moreover, it is anticipated that the performance of the three HTMs identified here might guide the molecular design of novel HTMs for the manufacture of highly efficient inverted PSCs.

Herein, it is demonstrated that solution‐processed dopant‐free spiro molecules can serve as superior hole‐transport materials (HTMs) to fabricate efficient inverted (p‐i‐n) perovskite solar cells. An entirely solution process is achieved by rational choice of orthogonal solvent, which allows to deposit uniform and pinhole‐free perovskite films without compromising the hole‐extraction capability of the spiro interlayers.

Spiro‐linked compounds have been used as benchmark hole‐transport materials (HTMs) for the construction of efficient normal architecture (n‐i‐p) perovskite solar cells (PSCs). However, the heavy reliance on the use of dopants not only complicates the device fabrication but imposes long‐term stability concern of the devices. Herein, it is reported that solution‐processed dopant‐free spiro molecules can serve as superior HTMs to fabricate efficient inverted (p‐i‐n) PSCs. Rational choice of orthogonal solvent allows us to solution deposit uniform and pinhole‐free perovskite films without compromising the hole‐extraction capability of the spiro‐based interface layers. To illustrate the generality of the strategy, three spiro‐linked molecules are investigated side by side as HTMs in one‐step solution‐processed CH₃NH₃PbI₃ PSCs. Due to the favored energy‐level alignment and high hole mobility, solar cells based on the HTM of spiro‐TTB yield a high efficiency of 18.38% with open‐circuit voltages (V _OC) up to 1.09 V. These results suggest that small molecular HTMs commonly developed for normal structure devices can be of great potential to fabricate cost‐effective and highly efficient inverted PSCs.

Hole transport material (HTM) plays important roles in n–i–p type perovskite solar cells. It affects both efficiency and the stability. After the recognition of its importance, a number of HTMs have been developed. This review summarizes various types of HTMs and discusses their development.

Abstract

With the application of organic–inorganic hybrid perovskites to liquid‐type solar cells, the unprecedented development of perovskite solar cells (Per‐SCs) has been boosted by the introduction of solid‐state hole transport materials (HTMs). The removal of liquid electrolyte has lead to improved efficiency and stability. Supported by high‐quality perovskite films, the certified efficiency of Per‐SCs has reached 25.2%. For Per‐SCs assembled in a conventional structure (n–i–p), the hole transport layer (HTL) plays an extra role in preventing the perovskite layer from external stimuli. In summary, the successful design and fabrication of the HTL must meet various requirements in terms of solubility, hole transport, recombination prevention, stability, and reproducibility, to name but a few. Many research strategies are focused on the development of high‐performance HTMs to meet such requirements. Such strategies for the development of HTMs employed in conventional n–i–p solar cells are reviewed herein. A vision of the future HTMs is proposed in this review based on the already proposed solutions and current trends.

The excited state characteristics of organic hole transport materials in perovskite photovoltaics (PVs), such as transition dipole moment, is confirmed to be a critical factor in improving the built‐in potential of devices for efficient charge extraction along with reduced carrier recombination. Moreover, the aggregation property of the organic semiconductor can have a synergistic effect with its excited state property for high‐efficiency perovskite PVs.

Abstract

Intrinsic characteristics of organic semiconductor‐based hole transport materials (HTMs) such as facile synthesizability, energy level tunability, and charge transport capability have been highlighted as crucial factors determining the performances of perovskite photovoltaic (PV) cells. However, their properties in the excited state have not been actively studied, although PVs are operated under solar illumination. Here, the characteristics of organic HTMs in their excited state such as transition dipole moment can be a decisive factor that can improve built‐in potential of PVs, consequently enhancing their charge extraction property as well as reducing carrier recombination. Moreover, the aggregation property of organic semiconductors, which has been an essential factor for high‐performance organic HTMs to improve their carrier transport property, can induce a synergistic effect with their excited state property for the high‐efficiency perovskite PVs. Additionally, it is also confirmed that their optical bandgaps, manipulated to have their absorption in the UV region, are beneficial to block UV light that degrades the quality of perovskite, consequently improving the stability of perovskite PV in p–i–n configuration. As a proof‐of‐concept, a model system, composed of triarylamine and imidazole‐based organic HTMs, is designed, and it is believed that this strategy paves a way toward high‐performance and stable perovskite PV devices.

A series of Donor–π–Acceptor porphyrins coded as CS0, CS1, and CS2 that can effectively passivate the perovskite surface, increase V _OC and FF, reduce the hysteresis effect, enhance power conversion efficiency to be higher than 22%, and improve the device stability have been developed.

Abstract

In recent years, hybrid perovskite solar cells (PSCs) have attracted much attention owing to their low cost, easy fabrication, and high photoelectric conversion efficiency. Nevertheless, solution‐processed perovskite films usually show substantial structural disorders, resulting in ion defects on the surface of lattice and grain boundaries. Herein, a series of D–π–A porphyrins coded as CS0, CS1, and CS2 that can effectively passivate the perovskite surface, increase V _OC and FF, reduce the hysteresis effect, enhance power conversion efficiency to be higher than 22%, and improve the device stability is developed. The results in this study demonstrated that the donor–π–acceptor type porphyrin derivatives are promising passivators that can improve the cell performance of PSCs.

The novel use of cheap copper‐based corrole as hole transporting material in perovskite solar cells is shown by improving the device thermal stability of n–i–p mesoscopic architecture under prolonged 85 °C stress conditions. Corrole‐based devices show a remarkable power conversion efficiency above 16% by retaining more than 65% of the initial power conversion efficiency after 1000 h of thermal stress.

Abstract

Perovskite solar cells (PSCs) represent nowadays a promising starting point to develop a new efficient and low‐cost photovoltaic technology due to the demonstrated power conversion efficiency (PCE) exceeding 25% on small area devices. However, best reported devices suffer from stability issue under real working conditions thus slowing down the race for the commercialization. In particular, the hole transporting material commonly employed in mesoscopic n–i–p PSCs (nip‐mPSCs), namely spiro‐OMeTAD, is strongly corrupted when subjected to temperatures above 70 °C due to intrinsic thermal instability and because of the dopant employed to improve the hole mobility. In this work, the novel use of a copper‐based corrole as HTM is proposed to improve the device thermal stability of nip‐mPSCs under prolonged 85 °C stress conditions. Corrole‐based devices show remarkable PCE above 16% by retaining more than 65% of the initial PCE after 1000 h of thermal stress, while spiro‐OMeTAD cells abruptly lose more than 60% after the first 40 h. Once scaled‐up to large area modules, the proposed device structure can truly represent a possible way to pass thermal stress tests proposed by IEC‐61646 standards and, not less importantly, the high temperature required by the lamination process for panel production.

Using time‐resolved time‐of‐flight secondary ion mass spectrometry (tr‐ToF‐SIMS), electric field and light induced ion migration in hybrid organic‐inorganic perovskites are directly observed, revealing the migration behavior of methylammonium and halides. It is found that light‐induced methylammonium migration is more significant. In addition, the light with sub‐bandgap energy cannot induce ion migration.

Abstract

Unique optoelectronic, electronic, and sensing properties of hybrid organic–inorganic perovskites (HOIPs) are underpinned by the complex interactions between electronic and ionic states. Here, the photoinduced field ion migration in HOIPs is directly observed. Using newly developed local probe time‐resolved techniques, more significant CH₃NH₃ ⁺ migration than I⁻/Br⁻ migration in HOIPs is unveiled. It is found that light illumination only induces CH₃NH₃ ⁺ migration but not I⁻/Br⁻ migration. By directly observing temporal changes in bias‐induced and photoinduced ion migration in device conditions, it is revealed that light illumination suppresses the bias‐induced ion redistribution in the lateral device. These findings, being a necessary compensation of previous understandings of ion migration in HOIPs based on simulations and static and/or indirect measurements, offer advanced insights into the distinct light effects on the migration of organic cation and halides in HOIPs, which are expected to be helpful for improving the performance and the long‐term stability of HOIPs optoelectronics.

The perovskite single crystals (PVSK SCs) without detrimental grain boundaries and defects possess orders of magnitude larger diffusion length, carrier lifetime, and lower trap density in comparison with PVSK polycrystalline film. In this review, the recent progress of synthesis method of PVSK SCs and their application in optoelectronic devices is tried to be summarized.

Abstract

Recently, lead halide perovskite (PVSK) polycrystalline films have drawn much attention as photoactive material and scored tremendous achievements in solar cells, photodetectors, light‐emitting diodes, and lasers owing to their engrossing optoelectronic properties and facile solution‐processed fabrication. However, large amounts of grain boundaries unfavorably induce ion migration, surface defect, and poor stability, impeding PVSK polycrystalline film‐based optoelectronic devices from practical application. In comparison with the polycrystalline counterparts, PVSK single crystals (SCs) with lower trap density serve as a better platform for not only fundamental research but also device applications. In light of this, the idea of using PVSK single crystals (SCs) to construct the optoelectronic devices is then proposed. Since then, a series of synthesis methods of PVSK SCs have emerged. In this review, recent progress of synthesis method of PVSK SCs is tried to be summarized and their advantages and limitations are analyzed. And then, the optoelectronic properties including carrier dynamic, defects, ion migration, and instability issues in these 3D and 2D PVSK SCs are overviewed and accordingly the proper device configurations of corresponding solar cells, photodetectors, X‐ray, γ‐ray detectors, etc., are proposed. It is believed that this review can provide the guidance for the further development of PVSK SCs and their applications.