Elucidating Different Mass Flow Direction Induced Polyaniline–Ionic Liquid Interface Properties: Insight Gained from DC Voltammetry and Impedance Spectroscopy

This work describes the use of direct current (DC) cyclic voltammetry (CV) and alternating current (AC) electrochemical impedance spectroscopy (EIS) as a means to monitor an electrochemical interface of different mass flow direction induced polyaniline (PANI) film in IL (BmimPF₆). Observed by SEM, vertical mass flow (VMF) and horizontal mass flow (HMF) induce porous nanorod and compact granular morphology of PANI, respectively. The present work explores in detail analysis of double layer capacitance, polarization resistance, diffusion mechanism, as well as other electrochemical features associated with the PANI–IL interface. A comparatively higher value of capacitance obtained for VMF PANI film from CV measurement confirms the higher electroactivity at the VMF electrode than the HMF film. Impedance spectroscopy, using a small amplitude perturbation, confirms the CV result. Impedance measurement gives a value of capacitance larger than that from CV where the amplitude of the perturbation is much larger. The implications of these results for its potential application in energy storage devices are discussed

Role of Heterocyclic Organic Compounds on the Optoelectronic Properties of Halide Perovskite Single Crystals

Single crystals (SCs) of metal halide perovskites (MHPs) are known to possess superior properties. However, the surface of the SCs possesses a higher defect density, which can deteriorate the optoelectronic properties of SCs. Herein, we have engineered the growth of methylammonium lead tribromide SCs using (R)-3-aminopiperidine dihydrochloride (API). The modified crystals also showed improved photoresponse validated by the increased photocurrent. The modified crystal showed a highest photoresistivity of 0.47 mA/W and a detectivity of 3.7 × 1011 Jones at an applied bias of 2 V. On the other hand, the control crystal showed a resistivity of 0.32 mA/W and a detectivity of 1.0 × 1011 Jones. This shows that the API additive improves the charge collection efficiency and reduces the recombination. The charge accumulation and ion migration were further studied using electrochemical impedance spectroscopy and capacitance–frequency measurement. Thus, this study presents systematic investigation of the electrical response of additive-modified crystals

Reduction in the Interfacial Trap Density of Mechanochemically Synthesized MAPbI₃

Organo-lead halide perovskites have emerged as promising light harvesting materials for solar cells. The ability to prepare high quality films with a low concentration of defects is essential for obtaining high device performance. Here, we advance the procedure for the fabrication of efficient perovskite solar cells (PSCs) based on mechanochemically synthesized MAPbI₃. The use of mechano-perovskite for the thin film formation provides a high degree of control of the stoichiometry and allows for the growth of relatively large crystalline grains. The best device achieved a maximum PCE of 17.5% from a current–voltage scan (<i>J–V</i>), which stabilized at 16.8% after 60 s of maximum power point tracking. Strikingly, PSCs based on MAPbI₃ mechanoperovskite exhibit lower “hysteretic” behavior in comparison to that comprising MAPbI₃ obtained from the conventional solvothermal reaction between PbI₂ and MAI. To gain a better understanding of the difference in <i>J–V</i> hysteresis, we analyze the charge/ion accumulation mechanism and identify the defect energy distribution in the resulting MAPbI₃ based devices. These results indicate that the use of mechanochemically synthesized perovskites provides a promising strategy for the formation of crystalline films demonstrating slow charge recombination and low trap density

Donor–Acceptor-Type <i>S</i>,<i>N</i>‑Heteroacene-Based Hole-Transporting Materials for Efficient Perovskite Solar Cells

Two new donor–acceptor (D–A)-substituted <i>S</i>,<i>N</i>-heteroacene-based molecules were developed and investigated as hole-transporting material (HTM) for perovskite solar cells (PSCs). Optical and electrochemical characterization brought out that the energy levels of both HTMs are suitable for their use in PSCs. Consequently, a power-conversion efficiency of 17.7% and 16.1% was achieved from PSCs involving the HTM<b>-1</b> and HTM-<b>2</b>, respectively. The optoelectronic properties in terms of series resistance, conductivity, and charge carrier recombination were further examined to unfold the potential of these new HTMs. Time-resolved photoluminescence spectroscopy brought out that the hole injection from the valence band of perovskite into HTMs follows the trend, which is in accordance with the position of the highest occupied molecular orbital. Overall, our findings underline the potential of <i>S</i>,<i>N</i>-heteroacene co-oligomers as promising HTM candidates for PSCs

Unraveling the Impact of Rubidium Incorporation on the Transport-Recombination Mechanisms in Highly Efficient Perovskite Solar Cells by Small-Perturbation Techniques

We applied intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) techniques to explore the effect of rubidium (Rb) incorporation into lead halide perovskite films on the photovoltaic parameters of perovskite solar cells (PSC). IMPS responses revealed the transport mechanisms at the TiO₂/perovskite interface and inside the perovskite absorber films. For recombination time constants, IMVS showed that the two perovskite solar cells differ in terms of trap densities that are responsible for recombination loss. Impedance spectroscopy carried out under illumination at open circuit for a range of intensities showed that the cell capacitance was dominated by the geometric capacitance of the perovskite layer. Our systematic studies revealed that Rb containing PSCs exhibit enhanced charge transport, slower charge recombination, faster photocurrent transient response, and lower capacitance than the Rb-free samples

Elucidation of Charge Recombination and Accumulation Mechanism in Mixed Perovskite Solar Cells

Organic–inorganic perovskite solar cells (PSCs) have gained considerable attention owing to their impressive photovoltaic properties and simple device manufacturing. In general, PSC employs a perovskite absorber material sandwiched between an electron and hole selective transport layer optimized with respect to optimal band alignment, efficient charge collection, and low interfacial recombination. The interfaces between the perovskite absorber and respective selective contacts play a crucial role in determining photovoltaic performance and stability of PSCs. However, a fundamental understanding is lacking, and there is poor understanding in controlling the physical processes at the interfaces. Herein, we investigate the interfacial characteristics of PSCs with both planar and mesoporous architecture that provide a deeper insight into the charge recombination and accumulation mechanism and the origin of open-circuit voltage (<i>V</i>_oc). The effect of electron- and hole-selective contacts in the final cell performance of PSCs has been analyzed by impedance spectroscopy and capacitance–frequency analysis. This study demonstrates that the excess of charge accumulation under illumination in planar-based devices is responsible for the origin of <i>V</i>_oc and hysteresis phenomena

Formation of Stable Mixed Guanidinium–Methylammonium Phases with Exceptionally Long Carrier Lifetimes for High-Efficiency Lead Iodide-Based Perovskite Photovoltaics

Methylammonium (MA)- and formamidinium (FA)-based organic–inorganic lead halide perovskites provide outstanding performance as photovoltaic materials, due to their versatility of fabrication and their power conversion efficiencies reaching over 22%. The proposition of guanidinium (GUA)-doped perovskite materials generated considerable interest due to their potential to increase carrier lifetimes and open-circuit voltages as compared to pure MAPbI₃. However, simple size considerations based on the Goldschmidt tolerance factor suggest that guanidinium is too big to completely replace methylammonium as an A cation in the APbI₃ perovskite lattice, and its effect was thus ascribed to passivation of surface trap states at grain boundaries. As guanidinium was not thought to incorporate into the MAPbI₃ lattice, interest waned since it appeared unlikely that it could be used to modify the intrinsic perovskite properties. Here, using solid-state NMR, we provide for the first time atomic-level evidence that GUA is directly incorporated into the MAPbI₃ and FAPbI₃ lattices, forming pure GUA_<i>x</i>MA_1–<i>x</i>PbI₃ or GUA_<i>x</i>FA_1–<i>x</i>PbI₃ phases, and that it reorients on the picosecond time scale within the perovskite lattice, which explains its superior charge carrier stabilization capacity. Our findings establish a fundamental link between charge carrier lifetimes observed in photovoltaic perovskites and the A cation structure in ABX₃-type metal halide perovskites

Molecular Engineering of Azahomofullerene-based Electron Transporting Materials for Efficient and Stable Perovskite Solar Cells

The rational molecular design of fullerene-based molecules with exceptional physical and electrical properties is in high demand to ensure efficient charge transport at the perovskite/electron transport layer interface. In this work, novel azahomofullerene (AHF) is designed, synthesized, and introduced as the interlayer between the SnO2/perovskite interface in planar n–i–p heterojunction perovskite solar cells (PSCs). The AHF molecule (denoted as AHF-4) is proven to enhance charge transfer capability compared to the commonly used fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) due to its superior coordination interaction and electronic coupling with the SnO2 surface. In addition, the AHF-4 interlayer concurrently improves the quality of the perovskite film and reduces charge recombination in PSCs. The resultant AHF-4-based device exhibits a maximum efficiency of 21.43% with lower hysteresis compared to the PCBM device (18.56%). Benefiting from the enhanced stability of the AHF-4 film toward light soaking and elevated temperature, the AHF-4-based devices show improved stability under continuous 1 sun illumination at the maximum power point and 45 °C. Our work opens a new direction to the design of AHF derivatives with favorable physical and electrical properties as an interlayer material to improve both the performance and stability of PSCs