Facile and low-cost synthesis of a novel dopant-free hole transporting material that rivals Spiro-OMeTAD for high efficiency perovskite solar cells

Abstract

A Spiro fluorene-based dopant-free hole-transporting material denoted as Spiro-IA has been designed and developed from inexpensive starting materials with high yield via a simple synthetic approach for application in perovskite solar cells (PSCs). The unit cost of Spiro-IA can be as low as 1/9^th that of the conventional Spiro-OMeTAD. Moreover, Spiro-IA shows good solubility in different organic solvents, e.g. CHCl₃, acetone, EtOH, and DMF, and showed favorable charge-transport ability and greater photocurrent density compared to Spiro-OMeTAD. The UV absorption/emission spectra of Spiro-IA (λ_max = 430 nm, E_max = 601 nm) are red shifted compared to those of Spiro-OMeTAD (λ_max = 388 nm, E_max = 414 nm) with larger stokes shift values (171 nm) which helps suppress the loss of incident photons absorbed by the HTM and is more beneficial for improving the performance of PSCs. Optical and electrochemical studies show that Spiro-IA fulfilled the basic requirements of the hole transfer and electron regeneration process in the fabricated devices. PSCs fabricated (surface area = 1.02 cm²) with dopant-free Spiro-IA achieved a maximum power conversion efficiency (PCE) of 15.66% (J_SC = 22.14 mA cm⁻², V_OC = 1.042 V, FF = 0.679%), which was comparable to that of the most commonly used Li-doped Spiro-OMeTAD (PCE = 15.93%, J_SC = 20.37 mA cm⁻², V_OC = 1.057 V, FF = 0.74%) and surpassed that of the dopant-free Spiro-OMeTAD (PCE = 9.34%). Additionally, the PSCs based on dopant-free Spiro-IA achieved outstanding long-term stability and favorable conductivity (σ = 2.104 × 10⁻⁴ S cm⁻¹) compared to those based on Spiro-OMeTAD (σ = 9.00 × 10⁻⁸ S cm⁻¹). DFT studies were performed using Gaussian 09 at the B3LYP/6-31G (d/p) level to investigate their electron cloud delocalization in HOMO/LUMO levels. These results showed that Spiro-IA could be a promising candidate for low-cost PSC technology and has a great chance to supersede the expensive Spiro-OMeTAD.