Design Principles for Organic Small Molecule Hole-Transport Materials for Perovskite Solar Cells: Film Morphology Matters

Abstract

High-efficiency n–i–p perovskite solar cells generally incorporate organic hole-transport layer materials such as spiro-OMeTAD or PTAA, which have intrinsically low charge carrier mobility and therefore require doping to improve transport properties. However, using dopants is known to affect badly the operational stability of perovskite solar cells. Therefore, the development of suitable dopant-free hole-transport materials is the critical issue for realizing perovskite solar cells with high efficiency and long operational lifetimes. Herein, a series of small molecules with triazatruxene, benzodithiophene, triphenylamine, and dithienosilole electron donor core units were designed and explored as solution-processed dopant-free hole-transport materials for perovskite solar cells. The best performance has been obtained using the triazatruxene-based molecule TAT-2T-CNA with terminal alkyl cyanoacetate groups and a 2,2′-bithiophene π-conjugated bridge, which has enabled device efficiency of 20.1% with negligible hysteresis, along with a substantially improved V OC and FF values as compared to the reference devices assembled with PTA as a hole-transport material. The superior performance of TAT-2T-CNA is attributed to the optimal optoelectronic properties of this material and, most importantly, defectless film morphology. Using scanning near-field infrared microscopy (IR-SNOM) technique was shown to be particularly useful for the detection and visualization of defects in thin films of hole-transport materials, which strongly correlate with the device performance. The results obtained in this work are expected to provide new insights facilitating the rational design of efficient dopant-free hole-transport materials for efficient and stable perovskite solar cells.

Novel benzodithiophene-TTBTBTT copolymers: synthesis and investigation in organic and perovskite solar cells

“…32 The AFM and IR s-SNOM images were obtained using a Neaspec system (Germany) as reported previously. 33…”

“…The perovskite solar cells were assembled in an ITO/SnO x /PCBA/perovskite/HTL/VO y /Ag n–i–p configuration following the procedure reported previously. 33 To form a hole-transport layer (HTL), polymer solutions were spin-coated at 1000 rpm for 30 s. More details on the polymer solution preparation and film coating conditions are given in Table S2 (ESI†). The solar cell characterization was performed as reported elsewhere.…”

Recent advances in dopant-free organic hole-transporting materials for efficient, stable and low-cost perovskite solar cells

“…In 2022, a higher PCE of 20.1% was achieved by Frolova's group. 79 They developed a series of D-p-A type star-shaped small molecules (TAT-2T-DCV, TAT-2T-CNA and TPA-3T-DCV) by utilizing triazatruxene and triphenylamine as the central cores (D), 2,2 0 -bithiophene or 2,2 0 :5 0 ,2 00 -terthiophene as p-bridge segments, and alkyl dicyanovinyl or alkyl cyanoacetate as A units as dopant-free HTMs for planar n-i-p PSCs. The device fabricated with TAT-2T-CAN exhibited the best PCE of 20.1% with a negligible hysteresis, along with an excellent light stability.…”

Enhanced photostability of CsPbI2Br-based perovskite solar cells through suppression of phase segregation using a zwitterionic additive

“…The morphology of the perovskite lms and specic features of the Asn distribution in the perovskite material were probed using a scattering-type scanning near-eld optical microscope operating in the mid-infrared spectral range (IR-neaSNOM, Neaspec, Germany, 2020) equipped with a standard metalized cantilever with a tip radius of 25-35 nm as described earlier. 38 Fabrication and characterization of perovskite solar cells…”