IntroductionThe inverted inorganic perovskite solar cells (IPSCs) with a bandgap of 1.7 eV are prospective candidates for the next generation of photovoltaic cells due to their elemental composition and unparalleled light and thermal stability. However, the higher defect state density and energy level mismatch at the interface result in an inferior photovoltaic performance to that of organic-inorganic hybrid perovskite solar cells (HPSCs) with the same bandgap. In this paper, tetrabutylammonium iodide (TBAI) was used to treat the surface of the optical absorption layer of the inverted device. The interface engineering technique can effectively reduce the density of defect states in the inverted device, suppress non-radiative recombination, and decrease the carrier accumulation at the interface. Finally, the photoelectric conversion efficiency (J) of IPSCs prepared in air is increased from 17.10% to 19.76%, and the PCE is increased by 15.56%. In addition, the stability of the device is also considerably improved.MethodThe NiO_x solution was spun on the surface of FTO glass and annealed at 130 ℃ for 20 min. After cooling to room temperature, the P3CT-N solution was spun coated on the surface of the annealed NiO_x and annealed in air at 130 ℃ for 10 min. Afterwards, the prepared perovskite precursor liquid was spun coated on the surface of P3CT-N preheated at 70 ℃，and annealing on a hot plate at 190 ℃ for 5–8 min. The annealed films were transferred to a N₂ atmosphere glove box for dynamic spin coating of TBAI (IPA) solution, and then annealed at 80 ℃ for 5 min. MgF₂ was thermally evaporated on the surface of TBAI by an evaporation equipment. PC₆₁BM (CB) was spun as the electron transport layer, and BCP (IPA) was spun as the hole barrier layer. Finally, the evaporation device vaporized Ag with the thickness of 80.0 nm on the surface of the BCP as a metal electrode.Results and discussionThe SEM images show that the particles of TBAI treated films are fuller and the holes significantly reduce. These holes are the center of non-radiative recombination, seriously restricting the improvement of photovoltaic performance of devices. The results of UV-absorption test indicate that TBAI post-treatment does not change the bandgap of inorganic perovskite CsPbI_2.85Br_0.15. The results of TRPL test show that the TBAI-treated films have a longer carrier lifetime, indicating that the non-radiative recombination is inhibited. Based on the results of the Mott-Schottky test, the TBAI-treated device demonstrates a built-in electric field of 1.196 V, which is much higher than that of the reference group (i.e., 1.038 V), and a larger built-in electric field means a more intense carrier separation drive, corresponding to a higher V_OC. The steady-state power output (SPO) efficiency of the TBAI treated device is tested for 300 s, and the average SPO efficiency of the device is 19.36% for over 400 s. A pure hole device is constructed to analyze the defect state density of the two films. The V_TFL values of the control and TBAI treated devices are 1 V and 0.95 V, respectively, and the corresponding defect state densities are 1.68×10¹⁶ and 1.56×10¹⁶ cm^–3, respectively. The TBAI post-treatment reduces the defect state density of the films, inhibiting non-radiative recombination. The humidity stability of the two films is monitored under specific conditions. The decomposition the films in the control group appears when the films in the control group are in humid air for 72 h, while that in the experimental group almost constancy.ConclusionsThe defect state density at the interface of IPSCs was high, and the non-radiation recombination loss was serious, thus restricting the further improvement of the photovoltaic performance of the device. In this paper, TBAI was used to post-treated the surface of CsPbI_2.85Br_0.15, which improved the crystal quality of the film, effectively reduced the density of defect states, inhibited non-radiative recombination, and reduced unnecessary carrier accumulation at the interface. IPSCs prepared in air treated by this interface engineering achieved a champion PCE of 19.76% with a V_OC of 1.200 V. This work could provide a method for preparing highly efficient and stable IPSCs in air, thus promoting the practical process.