Interplay of Orientation and Blending: Synergistic Enhancement of Field Effect Mobility in Thiophene-Based Conjugated Polymers

Trade-off between mechanical flexibility due to amorphicity and highly facile charge transport emanating from molecular crystallinity demands the orientation of conjugated polymers (CPs) for their utilization as active semiconducting material for flexible organic electronics. We have already demonstrated that it is rather easy to orient nonregiocontrolled poly­(3-hexylthiophenes) (NR-P3HT) as compared to their highly regioregular counterparts due to very high alkyl chain interdigitation. To provide an amicable solution, efforts have been directed to orient blends of two CPs such as NR-P3HT (amorphous and flexible) and poly­(2,5-bis­(3-tetradecylthiophen-2-yl)­thieno­[3,2-<i>b</i>]­thiophene) (PBTTT) (crystalline and facile charge transport) using a solution-based procedure floating film and transfer method (FTM). FTM-processed thin films of this blend system exhibited very high field effect transistor (FET) mobility reaching up to 0.1 cm²/V s, which is much higher than the corresponding individual CPs. In spite of only 10% incorporation of PBTTT in blend of NR-P3HT and PBTTT, there was a synergistically enhanced optical dichroic ratio (4.6 to 7.2) and FET mobility (8-fold) as compared to pristine NR-P3HT. At the same time, there was a 5-fold enhancement of FET mobility when 20% NR-P3HT was added in PBTTT as compared to that of PBTTT. This synergistic enhancement of charge carrier transport in the blend system has been explained by formation of oriented self-assembled fibrous domains of NR-P3HT and facile interdomain transport in crystalline PBTTT

Study To Observe the Effect of PbI₂ Passivation on Carbon Electrode for Perovskite Solar Cells by Quartz Crystal Microbalance System

A perovskite solar cell (PSC) utilizing a carbon electrode is a potential candidate for industrially viable, low-cost and highly stable photovoltaics. Therefore, it is important to understand the interface between perovskite layer and carbon electrode to achieve the improved performance of PSCs. We demonstrate an improvised two-step perovskite MAPbI₃ (methylammonium lead iodide) deposition method, involving a pretreatment of PbI₂ on the porous structure of TiO₂/ZrO₂/Carbon, which led to the difference in performance. A PbI₂ passivation layer at the interface between carbon electrode and perovskite resulted in the improved power conversion efficiency (PCE) of 7.30% from 2.21% compared to a one-step perovskite deposition with no pretreatment of PbI₂. This study further explores that an enhanced PCE of 6.55% can be achieved with one-step fabrication while keeping the same perovskite. A fascinating methodology, utilizing quartz crystal microbalance (QCM), which involves the adsorption of PbI₂ on the carbon surface, was employed to unravel this difference. QCM monitored adsorbed mass in real time and revealed that the mass of PbI₂ on carbon layer increased with the increase in concentration of PbI₂ in dimethylformamide (DMF). It was noticed that PbI₂ was still adsorbed on the carbon surface even after rinsing with DMF, suggesting strong bonding of PbI₂ with carbon. PbI₂ presence after rinsing was also verified by X-ray photoelectron spectroscopy (XPS), which indicates that part of the Pb–I reacts with the −OH on the carbon surface forming C–O–Pb linkages. Our study demonstrates that a carbon electrode passivated with PbI₂ could reduce carrier recombination and improve performance of PSCs

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