Investigation of Interfacial Charge Transfer in Solution Processed Cs2SnI6 Thin Films

Cesium tin halide based perovskite Cs2SnI6 has been subjected to in-depth investigations owing to its potentiality toward the realization of environment benign Pb free and stable solar cells. In spite of the fact that Cs2SnI6 has been successfully utilized as an efficient hole transport material owing to its p-type semiconducting nature, however, the nature of the majority carrier is still under debate. Therefore, intrinsic properties of Cs2SnI6 have been investigated in detail to explore its potentiality as light absorber along with facile electron and hole transport. A high absorption coefficient (5 × 104 cm–1) at 700 nm indicates the penetration depth of 700 nm light to be 0.2 μm, which is comparable to conventional Pb based solar cells. Preparation of pure and CsI impurity free dense thin films with controllable thicknesses of Cs2SnI6 by the solution processable method has been reported to be difficult owing to its poor solubility. An amicable solution to circumvent such problems of Cs2SnI6 has been provided utilizing spray-coating in combination with spin-coating. The presence of two emission peaks at 710 and 885 nm in the prepared Cs2SnI6 thin films indicated coexistence of quantum dot and bulk parts which were further supported by transmission electron microscopy (TEM) investigations. Time-resolved photoluminescence (PL) and transient absorption spectroscopy (TAS) were employed to investigate the excitation carrier lifetime, which revealed fast decay kinetics in the picoseconds (ps) to nanoseconds (ns) time domains. Time-resolved microwave photoconductivity decay (MPCD) measurement provided the mobile charge carrier lifetime exceeding 300 ns, which was also in agreement with the nanosecond transient absorption spectroscopy (ns-TAS) indicating slow charge decay lasting up to 20 μs. TA assisted interfacial charge transfer investigations utilizing Cs2SnI6 in combination with n-type PCBM and p-type P3HT exhibited both intrinsic electron and hole transport

Counter electrodes for DSC: Application of functional materials as catalysts

Counter electrodes (CEs) of dye-sensitized solar cells (DSCs) can be prepared with different materials and methods. This review covers recent research on CEs using platinum, graphite, activated carbon, carbon black, single-wall carbon nanotubes, poly( 3,4-ethylenedioxythiophene) (PEDOT), polypyrrole, and polyaniline as catalysts for reduction of triiodide. Moreover, for the ultimate in low-cost counter electrodes, we have prepared a carbon-black-loaded stainless steel electrode for use as a novel CE. This counter electrode exhibits good photovoltaic performance; the efficiency reaches 9.15% (16.3 mA cm(-2) J(sc), 785 mV V-oc, and 71.4% fill factor) with SUS-316 stainless steel, equivalent to the performance with an FTO-glass substrate. (c) 2007 Published by Elsevier B.V

Compositional Design of Spontaneous Heterointerface Modulators for Perovskite Solar Cells Allowing a Broad Process Window

Spontaneous heterointerface modulation techniques have significantly contributed to the rapid development of perovskite solar cells (PSCs), and alkyl-primary-ammonium bis(trifluoromethanesulfonyl)imides (RA–TFSIs), whose archetype is n-octylammonium bis(trifluoromethanesulfonyl)imides (OA–TFSI), have recently emerged as functional additives for hole transport materials (HTMs). RA–TFSIs are designed to allow spontaneous perovskite passivation via the HTM deposition process; leveraging the high reactivity of RA cations toward the perovskite surface, these additives spontaneously and effectively suppress the defects over the perovskite surface and thereby enhance photovoltaic (PV) performance. Moreover, this perovskite passivation negates the need for conventional post-passivation processes, thereby improving the fabrication efficiency of PSCs. Although the advantages of these PSC fabrication processes have been less discussed than methods aimed at enhancing PV performance, they are crucial for further advancement of PSCs, especially in the context of spontaneous heterointerface modulation techniques. A key aspect is the concentration sensitivity of RA–TFSI; excessive OA–TFSI in the HTM solution leads to some OA cations failing to attach to the perovskite surface during spontaneous passivation, remaining in the HTM core and hindering carrier collection. To address this issue, we herein developed RA–TFSIs and synthesized ethylammonium bis(trifluoromethanesulfonyl)imide (EA–TFSI) for the first time. EA–TFSI not only enhanced the PV properties of PSCs but also significantly mitigated the concentration sensitivity owing to its small cation size, reducing the risk of poor carrier collection. In the case of OA–TFSI, increasing its concentration to twice the optimal amount decreased power conversion efficiency (PCE) by 14%, accompanied by drops in fill factor (FF). However, upon EA–TFSI addition, PCE decreased by only 4%, with FF values remaining unchanged (i.e., nearly 100% retention). This research offers insights into designing nascent yet potent spontaneous heterointerface modulators for PSCs, including RA–TFSI, to facilitate a broad process window, which is critical yet rarely discussed aspect. Therefore, this study will contribute to the further development of PSCs

Direct Ion-Exchange Method for Preparing a Solution Allowing Spontaneous Perovskite Passivation via Hole Transport Material Deposition

We propose a direct ion-exchange (DI) method for preparing a hole transport material (HTM) solution undergoing spontaneous perovskite passivation via HTM deposition and verify its applicability for the photovoltaic performance enhancement of perovskite solar cells (PSCs). The simple synthesis of a Spiro-OMeTAD HTM solution based on ion exchange via dissolving and mixing multiple solid materials in a chlorobenzene solution produces an HTM solution similar to that obtained with an n-octylammonium bis(trifluoromethanesulfonyl)imide ionic liquid functioning as a spontaneous perovskite passivator. Using the resulting HTM solution, the power conversion efficiency of PSCs was enhanced up to 23.0% without conventional post-passivation processes

Thermally Stable Phenylethylammonium-based Perovskite Passivation: Spontaneous Passivation with Phenylethylammonium Bis(trifluoromethylsulfonyl)imide under Deposition of PTAA for Enhancing Photovoltaic Performances of Perovskite Solar Cells

Perovskite passivation has been vital for obtaining highly efficient and stable perovskite solar cells (PSCs). Phenylethylammnonium (PEA)-based passivator is one of the most major categories as it is effective in improving PSC performances taking advantages of its large absorption energy over perovskite surface. However, this conventional passivator suffers from thermal stability issues; under thermal stress even at moderate temperature (e.g., 50℃) for several minutes, the overlayer of the perovskite passivated with PEA iodide salt (PEAI) transforms to a two-dimensional perovskite of (PEA)2PbI4, which hampers carrier transfer, thus negating the passivation effects and/or degrading the photovoltaic (PV) performances. Herein, we propose a novel yet simple strategy to address the thermal stability issue using a newly synthesized PEA bis(trifluoromethylsulfonyl)imide (PEA-TFSI) additive for poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) hole transport materials (HTMs). Upon the deposition of the PTAA HTM solution with a PEA-TFSI additive over the perovskite layers, the PEA cations spontaneously passivated the perovskite, forming a monolayer-like passivation overlayer. The resulting PEA-based passivation did not cause crystallization at the elevated temperature of 85℃; hence, it did not cause PV performance drop due to the thermal stress and retained a higher PV performance compared to the samples with the conventional Li-TFSI additive for over 800 h. Besides, compared to PSCs with Li-TFSI, PSCs with PEA-TFSI exhibited an improvement in stability under humid conditions (30℃, 50% relative humidity) for over 600 h, taking advantage of the resulting hydrophobicity. The optimal PEA-TFSI additive enhanced the PV performance, reaching a power conversion efficiency of up to 22.1% (21.2% in the quasi-steady state). These values are relatively high in the PTAA-based PSCs with an n-i-p structure particularly without using lithium species, which is considered to be detrimental for PSCs. These high PV performances were most likely attributed to improved affinity at the interface between PTAA and the perovskite, which is hardly attainable by an aliphatic ammonium passivator, and the PEA passivation effects. These results provide novel insights into the commonly used PEA-based perovskite passivation particularly in combining thermally stable PTAA HTMs and the promising alkylammonium spontaneous passivators, which spurs the further development of PSCs

An Architype-Cation-Based Room Temperature Ionic Liquid: Aliphatic-Primary-Ammonium Bis(Trifluoromethylsulfonyl)Imide as An Additive for Hole Transport Layers in Perovskite Solar Cells Allowing Spontaneous Passivation of The Perovskite Layer

While the architype room temperature ionic liquid (RTIL) based on aliphatic-primary-ammonium was discovered in 1914, it has been undeveloped until 1990s. In this three decades, RTIL composition (i.e., cations and anions) have been developed aiming at electrochemical application and green chemistry. Accordingly, bulky quaternary-ammonium, pyridine, and imidazole have nowadays been the major cations of RTILs. Currently, novel applications using the matured series of RTILs are being highly explored, and one of the promising targets is an additive for hole transport material (HTM) in perovskite solar cells (PSCs). However, although design of RTIL should be modified along each application, RTILs for the PSCs application so far has remained conventional, which limits the function. Herein, a novel RTIL comprising the architype aliphatic-primary-ammonium (i.e., n-octyl-ammonium: OA) cation and a modern Bis(Trifluoromethylsulfonyl)Imide (TFSI) anion, which are designed as an additive for hole transport layer in PSCs, was successfully synthesized for the first time. The OA cation spontaneously and densely passivated perovskite layer taking advantage of the accessibility to its cationic moiety, and simultaneously TFSI anion most likely effectively stabilized cationic radical of HTM owing to the absence of the counter cation in HTM core regime, leading to effective hole correction and thereby power conversion efficiency of 22.9% in the best cell. This work sheds light on importance of RTIL design along each application, even out of the current trend, and will help further development of various material science fields including PSCs

Reactivity Manipulation of Ionic Liquid Based on Alkyl Primary Ammonium: Protonation Control Using Pyridine Additive for Effective Spontaneous Passivation of Perovskite via Hole Transport Material Deposition

Alkyl-primary-ammonium-based room-temperature ionic liquids (RTILs) designed to exhibit specific reactivities allowing the functions that cannot be achieved by the current major RTILs (e.g., pyridine-based RTILs) have recently emerged. The archetype of the reactive RTILs is n-octylammonium bis(trifluoromethanesulfonyl)imide (OA-TFSI), which has promising functions as an additive for the hole transport material (HTM) in perovskite solar cells (PCSs); the high reactivity of the OA cations on the perovskite surface allows effective spontaneous perovskite passivation via HTM deposition, significantly improving the PV performance of the PSC. However, although the reactivity manipulation of the reactive RTILs is instrumental for exploiting their potential functions and exploring their application scope, methods for reactivity control have not been developed. In this study, we propose and demonstrate that the co-addition of a pyridine moiety can effectively manipulate the reactivity of OA-TFSI by controlling the protonation between OA and the 2,2\u27,7,7\u27-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9\u27-spirobifluorene (Spiro-OMeTAD) HTM. The pyridine prevented OA deprotonation presumably via stabilization of the OA cation, thus retaining its ammonium form, which allowed effective spontaneous perovskite passivation. Although the proton being with OA owing to pyridine addition is disadvantageous for Spiro-OMeTAD radical formation via its protonation, which is crucial when conventional RTILs are used, a supportive function of the spontaneous perovskite passivation (i.e., the absence of cationic species in the HTM core) likely facilitated Spiro-OMeTAD radical formation, mitigating the requirement of Spiro-OMeTAD protonation. Therefore, overall, optimal pyridine addition significantly enhanced the PV performance, revealing the preference of protonation in the OA-TFSI system used in this study, which is opposite to that in conventional RTILs and represents the specificity of the reactive RTILs. This study provides valuable guidance for developing spontaneous perovskite passivation techniques, which can lead to further advancement of PCSs. Furthermore, this first proposal of a means in manipulating reactivity of the reactive RTILs will develop the nascent RTILs and contribute to further development of material science