Perovskite solar cells (PSCs) have attracted considerable research interest due to their large absorption coefficients, long diffusion lengths, tunable bandgap, and high charge mobility. The power conversion efficiency (PCE) of PSCs has increased from 3.8% in 2009 to 26.08% in 2023. However, their mass-scale production is limited by the inherent instability of the perovskites, which decompose easily during reaction with moisture, oxygen, light and heat. Formamidinium-cesium (FAC) mixed cations perovskites have demonstrated excellent thermal stability and suitable bandgap for solar spectrum absorption. On the other hand, the carrier mobility of Br^- is higher than that of I^-. Therefore, we choose Cs-doped FA_1-xCs_xPbBr₃ (FACsPbBr₃) thin films to study optical properties and construct high-efficient and stable PSCs. However, experimental PCE verification of PSCs is costly and time-consuming. Numerical simulation provides a simple and effective way to evaluate the PSCs performance and explore new possible device architectures. The complex dielectric function is an important optical parameter. Fundamentally, the complex dielectric functions are critical for simulating the external quantum efficiency (EQE) of the PSCs. Furthermore, determining the bandgap from the complex dielectric functions provides information on the band structure and enables the detection of temperature-dependent phase changes. We prepare Cs-doped FA_1-xCs_xPbBr₃(x=0, 0.05, 0.10, 0.15) perovskite thin films and study the corresponding complex dielectric functions by spectroscopic ellipsometry (SE). The resultant complex dielectric functions are then employed to simulate EQE. Meanwhile, the temperature-dependent EQE simulation of FA_0.95Cs_0.05PbBr₃ PSC is also performed. We hope that the basic findings can help design highly efficient and stable PSCs and understand the relationship between the complex dielectric functions and EQE of PSCs.

FACsPbBr₃ thin films with different Cs doping concentrations are prepared by one-step anti-solvent method, and the surface morphology of samples is characterized by atomic force microscopy (AFM). Additionally, the crystal structure of the samples is studied using a D8 Advance X-ray diffractometer, and the effects of Cs-doped concentrations on the surface morphology and crystal structure of the prepared samples are investigated. The optical properties of the samples are analyzed by SE. The resultant complex dielectric functions are adopted to simulate the short-circuit current density (J_sc), open-circuit voltage (V_oc), fill factor (FF), and PCE of the devices. The doping effects on the PSCs performance are discussed in detail. Next, the temperature-dependent ellipsometric measurements (303-423 K) and room temperature absorption measurement of the sample with the highest simulated EQE are performed. Based on the temperature-dependent complex dielectric functions, the influence of temperature and absorber thickness on both the simulated EQE and the short-circuit current density of the device is studied.

The prepared FACsPbBr₃ thin films exhibit smooth and compact surface morphology with pebble stone-like structures, indicating the high quality of the samples (Fig. 1). When the doping concentration increases to 0.1, the appearance of the δ-phase non-perovskite structure is observed in the XRD pattern (Fig. 1). The ellipsometric measurements show that the amplitude of the complex dielectric functions decreases with the increasing doping concentrations (Fig. 2). The EQE simulation shows that Cs doping improves the PCE, but excessive Cs doping degrades PCE of the devices, which might be attributed to the appearance of the δ-phase. The maximum PCE can reach up to 23.47% under the doping concentration of x=0.05 (Table 1). Furthermore, an increase in bandgap with the rising temperature is observed based on the temperature-dependent dielectric functions of FA_0.95Cs_0.05PbBr₃. Additionally, an orthogonal-tetragonal phase transition is observed around 393 K (Fig. 5). The temperature-dependent EQE simulation of FA_0.95Cs_0.05PbBr₃ perovskite solar cell shows that the maximum PCE of the device can stabilize at about 23.47% and exhibits little dependence with temperature. However, there is a rapid EQE decrease in the near-infrared region with the increasing temperature, which reduces the device bandwidth (Fig. 6).

We prepare Cs-doped FA_1-xCs_xPbBr₃ (x=0, 0.05, 0.10, 0.15) perovskite thin films by a one-step anti-solvent method. The complex dielectric functions of FACsPbBr₃ thin films are studied by SE, and the temperature-dependent complex dielectric functions and absorption spectra of FA_0.95Cs_0.05PbBr₃ are researched by spectroscopic ellipsometry and UV-visible spectrophotometer respectively. The optical bandgaps obtained by SE are consistent with that obtained by absorption spectra. The EQE simulation results show that Cs doping can improve the device performance. When the doping concentration is 0.05, the PCE can reach up to 23.47%, but excessive Cs doping concentration will introduce non-perovskite δ-phase, decreasing the device performance. According to temperature-dependent ellipsometric measurements, we find that the bandgap increases with the rising temperature, and there is an obviously orthorhombic-tetragonal phase transition at about 393 K. With the increasing temperature, the device PCE slightly decreases, while the short-circuit current slightly increases. However, the light absorption capability of the device in the NIR region obviously reduces with the increasing temperature. The response bandwidth reduction could be attributed to the increased bandgap. Thus, by considering the performance and stability of the devices, FACsPbBr₃ PSCs with a Cs-doped concentration of 0.05 have the best overall photovoltaic performance.