Fluorination on cyclopentadithiophene-based hole-transport materials for high-performance perovskite solar cells.

A new class of fluorinated cyclopenta[2,1-b:3,4-b']dithiophene (CPDT)-based small molecules, namely YC-oF, YC-mF, and YC-H, are demonstrated as hole-transporting materials (HTMs) for high-performance perovskite solar cells (PSCs). PSCs employing YC-oF as the HTM delivered an excellent efficiency of 22.41% with encouraging long-term stability.

Effect of Fullerene Passivation on the Charging and Discharging Behavior of Perovskite Solar Cells: Reduction of Bound Charges and Ion Accumulation.

Ion accumulation of organometal halide perovskites (OHPs) induced by electrode polarization of perovskite solar cells (PSCs) under illumination has been intensely studied and associated with a widely observed current-voltage hysteresis behavior. This work is dedicated to the investigation of the behavior of charged species at the compact TiO2/OHP interface with respect to electrode polarization in PSC devices. By providing a comprehensive discussion of open-circuit voltage ( VOC) buildup and VOC decay under illumination and in the dark for the PSCs modified with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) at the TiO2/OHP interface and their corresponding electrochemical impedance spectroscopies (EISs), a justified mechanism is proposed attempting to elucidate the dynamics of interfacial species with respect to the time and frequency domains. Our results demonstrate that the retarded VOC buildup and decay observed in PSC devices are related to the formation of bound charges in TiO2, which is essential to neutralize the oppositely charged ions accumulating at the OHP side. Besides, inserting a thicker PCBM at the TiO2/OHP interface as a passivation layer can alleviate the electrode polarization more efficiently as verified by the low dielectric constant measured from EIS. Moreover, photoluminescence measurements indicate that PCBM at the TiO2/OHP interface is capable of passivating a trap state and improving charge transfer. However, with respect to the time scale investigated in this work, the reduction of the hysteresis behavior on a millisecond scale is more likely due to less bound charge formation at the interface rather than shallow trap-state passivation by PCBM. After all, this work comprehensively demonstrates the interfacial properties of PSCs associated with PCBM passivation and helps to further understand its impact on charging/discharging as well as device performance.

Amino-Acid-Induced Preferential Orientation of Perovskite Crystals for Enhancing Interfacial Charge Transfer and Photovoltaic Performance.

Interfacial engineering of perovskite solar cells (PSCs) is attracting intensive attention owing to the charge transfer efficiency at an interface, which greatly influences the photovoltaic performance. This study demonstrates the modification of a TiO2 electron-transporting layer with various amino acids, which affects charge transfer efficiency at the TiO2 /CH3 NH3 PbI3 interface in PSC, among which the l-alanine-modified cell exhibits the best power conversion efficiency with 30% enhancement. This study also shows that the (110) plane of perovskite crystallites tends to align in the direction perpendicular to the amino-acid-modified TiO2 as observed in grazing-incidence wide-angle X-ray scattering of thin CH3 NH3 PbI3 perovskite film. Electrochemical impedance spectroscopy reveals less charge transfer resistance at the TiO2 /CH3 NH3 PbI3 interface after being modified with amino acids, which is also supported by the lower intensity of steady-state photoluminescence (PL) and the reduced PL lifetime of perovskite. In addition, based on the PL measurement with excitation from different side of the sample, amino-acid-modified samples show less surface trapping effect compared to the sample without modification, which may also facilitate charge transfer efficiency at the interface. The results suggest that appropriate orientation of perovskite crystallites at the interface and trap-passivation are the niche for better photovoltaic performance.

Near-Infrared-Absorbing and Dopant-Free Heterocyclic Quinoid-Based Hole-Transporting Materials for Efficient Perovskite Solar Cells.

New heterocyclic quinoid-based hole transporting materials (HTMs) with a rigid quinoid core [3,6-di(2H-imidazol-2-ylidene)cyclohexa-1,4-diene] have been synthesized. The new HTMs have good hole mobility (>10-4  cm2 V-1 s-1 ) and very intense absorption in the near-infrared region extending to >800 nm. High performance perovskite solar cells can be fabricated using these HTMs without dopant. The best cell efficiency under simulated AM 1.5 G illumination reaches 12.22 %, which is comparable with that (12.58 %) using doped 2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD) as the HTM.