Sodium Hydroxide Assisted Work Function Tuning of Hole Transport Material for Efficient Wide-Bandgap Perovskite Solar Cells

Abstract

Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is widely used as hole transport material (HTM) in perovskite solar cells (PSC) of inverted structure. However, suitable energy level alignment between the Perovskite material and PEDOT.PSS is very important for proper charge transportation. Perovskite materials at various compositions of organic-inorganic compounds usually have different energy levels. Therefore, it is essential to tune the energy level hence the work function of PEDOT:PSS material to fulfill the requirement. In this work, 0.1 M sodium hydroxide (NaOH) is employed to modify the work function (Φ) of PEDOT:PSS HTM in MA0.9Cs0.1Pb(I0.6Br0.4)3 based wide-bandgap (1.82 eV) perovskite solar cells. Kelvin probe force microscopy (KPFM) confirms that Φ of the HTM gradually increases with the addition of very small amount of NaOH. An increase of Φ indicates that highest occupied molecular orbital (HOMO) level of HTM is getting closer to the HOMO of perovskite material which is beneficial for photogenerated hole extraction. Quenching of perovskite emission spectra in steady state photoluminescence (PL) spectroscopy also supports this enhanced hole extraction. Essentially, the optimum amount of NaOH needs to be added as higher amount can adversely affect the electrical conductivity of PEDOT:PSS. Besides, added NaOH provides Na+ ions in PEDOT:PSS which transforms some acidic PSS-H into neutral PSS-Na causing retardation of counter-ion exchange at perovskite and HTM interface. This helps to reduce the potential energy loss in the interface. Consequently, the final power conversion efficiency of the PSC is increased from 10.92% to 12.97% through the improvement of all performance related parameters such as short circuit current density (Jsc ), open circuit voltage (Voc) and fill factor (FF). In addition, similar characteristics X-ray diffraction (XRD) peaks reveal that NaOH treatment in HTM does not affect the regular crystallization of MA0.9Cs0.1Pb(I0.6Br0.4)3 perovskite.