Impacts of alkaline on the defects property and crystallization kinetics in perovskite solar cells

Abstract

Further minimizing the defect state density in the semiconducting absorber is vital to boost the power conversion efficiency of solar cells approaching Shockley-Queisser limit. However, it lacks a general strategy to control the precursor chemistry for defects density reduction in the family of iodine based perovskite. Here the alkaline environment in precursor solution is carefully investigated as an effective parameter to suppress the incident iodine and affects the crystallization kinetics during film fabrication, via rationale adjustment of the alkalinity of additives. Especially, a 'residual free' weak alkaline is proposed not only to shrink the bandgap of the absorber by modulating the stoichiometry of organic cation, but also to improve the open circuit voltage in the resultant device. Consequently, the certified efficiency of 20.87% (Newport) is achieved with one of the smallest voltage deficits of 413 mV in the planar heterojunction perovskite solar cell.

Fig. 1

Suppression of I₂ impurity via alkaline additives. UV-vis absorption spectra of the organic cation solution, which is used to prepare the perovskite films, with different amounts of (a) FAAc, (b) CH₃COONa, (c) NaHCO₃, (d) NaOH, and (e) KOH ethanol solution. f Normalized absorbance for organic cation solution with different additives as a function of the volume

Fig. 2

Morphology, phase, and carrier lifetime analysis. SEM images for the perovskite films deposited on SnO₂-coated ITO glass prepared from the precursor solution with (a) no additive, (b) FAAc, (c) CH₃COONa, and (d) NaOH ethanol solution, with a scale bar of 1 μm. e X-ray diffraction (XRD) pattern for the perovskite films deposited on SnO₂-coated ITO glass prepared from the precursor solution with different alkaline additives. f XRD pattern for the perovskite films with NO additive, MA, and FAAc ethanol solution. g Time-resolved photoluminescence (TRPL) spectra for the perovskite films deposited on glass prepared from the precursor solution with different alkaline additives

Fig. 3

Crystallization kinetics control of perovskite films. a Real-time UV-vis absorption spectra for the perovskite films without annealing prepared under different alkalinity environments. In situ absorption measurement for perovskite films with addition of (b) FAAc or (c) NaOH. The inset figures are derived from the change of absorption intensity (∆I) for each sample at different times and the y-axis is the right one (Time). d Schematic illustration for the impacts of alkalinity on the crystallization kinetics of perovskite films. e Activation energies of the α- and δ-phase of FAPbI₃ in the dependence of OH⁻ concentration

Fig. 4

Weak alkaline additives to reduce defect state density. Color plots of normalized steady-state PL spectra for (a) PVSK and (b) PVSK–FA at different temperatures from 78 to 350 K. c Corresponding steady-state PL spectra for both films at various temperatures from 250 to 350 K. d The PL quantum efficiency of PVSK and PVSK–FA films as a function of excitation power. The admittance spectra of the (e) PVSK and (f) PVSK–FA devices measured at a temperature from 250 to 350 K with a step of 10 K. g The corresponding Arrhenius plots of the characteristic frequencies to extract the defect activation energy (E_a) for both devices. h Trap density of states (tDOS) for both devices obtained by thermal admittance spectroscopy at room temperature

Fig. 5

Photovoltaic device performance. a Current density–voltage curves for PVSK and PVSK–FA devices. b Left part: the UV-vis and PL spectra of for the PVSK and PVSK–FA films. Right part: the histogram of V_OC for the PVSK and PVSK–FA devices and the curve represents Gaussian function fit to the statistics data. c The champion device performance under reverse scan (1.2 to –0.1 V) and forward scan (–0.1 to 1.2 V), with a scan rate of 40 mV s^–1. d Steady-state current density and power conversion efficiency holding the voltage at the maximum power point (0.95 V) for the PVSK–FA device. All these photovoltage characterizations were carried out under standard AM 1.5 radiation at ambient condition. e The transient photovoltage decay curves and (f) V_OC as a function of light intensity for the PVSK and PVSK–FA devices