Hybrid organic-inorganic halide PSCs have been widely used as light-absorbing materials in the past decade, which has seen the power conversion efficiency (PCE) increasing from 3.8% to over 25.5% [1–4]. Due to their rapid development, organic-inorganic halide hybrids have become a formidable rival in the community of thin film solar cells [5, 6]. The benefits of the organic-inorganic halide hybrid perovskite material, such as the extended carrier lifetime and diffusion length [7] and the tunable band gap [8], are among the most significant factors influencing the widespread exploration of PSCs.
Additionally, low-cost raw ingredients and simple preparation techniques enable efficiency. Large-scale production of the PSC materials with the structural formula ABX3, where, A can be a monovalent inorganic cation (Cs + or Rb+) or an organic cation (usually MA = methylammonium, or FA = formamidinium), giving rise to all-inorganic or hybrid organic-inorganic perovskites, B is a divalent inorganic cation (Pb2+). X has characterized a monovalent halide (Cl−, Br−, or I−) [9, 10].
In most cases, they form a three-dimensional (3D) crystal, but when Pb2+ is substituted by a cation charge (Cu+, Sb3+, or Sn4+), the perovskite formula can shift to a two-dimensional (2D) or even a zero-dimensional (0D) form (e.g., A2B2X6, A3B2X9) [4, 10, 11]. The composition can also be altered by changing either the bulk of cation A (causing increased asymmetry in the three-dimensional structure) or the smaller radius of the B atom, which favors 2D [12]. The perovskite layer is typically made by combining the precursor solution, coating the solution onto a substrate, and then subjecting the whole structure to heat treatment, often at low temperatures (100°C). Grain crystallization, film formation, and synthesis are simultaneous processes in such devices [13–15].
The exceptional characteristics of the APbX3 perovskite materials, including their excellent charge carrier mobilities, alterable bandgaps, huge absorption coefficients, and tiny exciton binding energies, are made possible by the lead-based perovskite structure [16]. Unfortunately, the large-scale industrialization of lead-based PSCs is significantly hampered by lead toxicity and the low environmental stability of organo-Pb halides [17]. Therefore, using stable and non-toxic Pb-free perovskites and their analogs is extremely desirable to replace lead-based perovskites. Sn2+, Ge2+, Bi3+, Sb3+, and Cu+ [10, 16] are some of the potential replacement cations for Pb2+. In recent years, there has been increasing interest in stable and non-toxic PSCs [18, 19], such as Bi3+ and Sb3+, which are more environmentally benign than lead-based PSCs despite having similar electronic structures. A-site cation such as methylammonium (MA+) or cesium (Cs+) has been introduced into the bismuth (Bi) halide compound, changing the stable phase from ABX3 to A3B2X9 due to their trivalent cation [10, 20]. However, the low dimension 0D or 2D and bandgaps over 2.1 eV of A3Bi2X9 perovskite materials cause bismuth-based PSCs to have subpar PCE, with the best PCE published so far being around 3.59% and a Voc of 0.89 V for cells in which the perovskite layer was bulk heterojunction bismuth-based PSCs with the perovskite layer comprising Cs3Bi2I9 and Ag3Bi2I9 perovskite [21].
Heterojunction bismuth-based perovskite phase arose from incorporating transition metal cations such as Ag+ into bismuth iodide have also been employed in PSCs, exhibiting the organic-inorganic hybrid architectures [22]. These include AgBiI4, AgBi2I7, and Ag2BiI5, with a general structure AgxBiyIx+3y (x = 1–3, y = 1–2) [22, 23]. Theoretically, the optoelectronic properties of AgxBiyIx+3y are superior to those of low-dimensional Cs3Bi2X9 due to its 3D edge-sharing octahedral structures and smaller optical bandgaps. Most stable photovoltaic PSCs also produced a low PCE of ~ 2.4% due to the low open-circuit voltage (Voc), which is frequently below 0.7 V [24, 25]. This is because the light-harvesting layer itself has a high trap density, leading to energy mismatches at the boundaries [26]. Several silver bismuth iodides (AgI-BiI3) and cesium silver bismuth iodides (CsI-AgI-BiI3) have been researched as possible light absorbers for solar cells [22, 23, 27, 28] and yielded optical bandgap values in the range of 1.38 eV to 1.71eV [27], 1.85 eV [29],1.9 eV [30].
Electron-hole pairs are generated when light strikes a photocurrent. Following exciton generation, electrons and holes should flow toward their respective electrodes. The material’s or devices’ crystalline structures are crucial for efficient charge transmission. Charge carriers can be lost by recombination or trapping if the material is flawed. The films’ roughness causes part of the charge carriers to leak, leading to a weak photocurrent.
Therefore, performance improvement of the Cs3Bi2I9 and Ag2BiI5 composite structure and the quality of the bulk Cs3Bi2I9-AgxBiyIx+3y perovskite has been adjusted sequentially [31]. Nevertheless, unlike lead-based organic-inorganic hybrid PSCs, where significant efficiency improvements can be achieved by enhancing the APbX3 perovskite layer through the solvent, stoichiometry, and additive engineering, such optimizations appear to be less beneficial for bismuth-based photovoltaic cells. This indicates that the photovoltaic characteristics of Cs3Bi2I9/AgxBiyIx+3y are controlled mainly by inherent crystallinity and band arrangements. Because of the inherent limitations of bismuth-based perovskite materials compositions, alternative options must be examined [32].
The bulk heterojunction architecture has subsequently been used in lead-based PSCs. For example, Chiang et al. fabricated bulk heterojunction (BJH) PSCs with the structure CH3NH3PbI3-PCBM (phenyl-C61-butyric acid methyl ester) and concluded that the presence of the PCBM in the BHJ perovskite structure led to more excellent electrical conductivity, ideal electron-hole transport, and long charge absorption properties, advancing the PCE from 11.4–16.0% [33]. Meanwhile, Yang et al. adopted a new heterostructure by doping Sb3+, or In3+, into the Cs0.1FA0.9PbI3 perovskite, resulting in an optimized band structure that enhanced charge transfer and collection, increasing the PCE of PSCs from 14.49–21.04% [34]. These studies show that building bulk heterojunction is useful in improving the efficiencies of PSCs. Moreover, at the time this study was conducted, the blended bulk heterojunction structure PSCs had not yet been published for bismuth-based photovoltaic cells incorporating potassium iodide (KI).
In order to get around these restrictions, previous studies synthesized mixed-cation (Cs+, MA+, FA+) and mixed-halide (I, Br) perovskite combinations, allowing them high generate thermal stability light-harvesting black phases at ambient temperature [22]. Moreover, due to photo-induced halide separation and extra non-radiative loss characterizations, such mixed halide combinations frequently have luminescence quantum yields much below 10% [35]. Different passivation techniques have been investigated to solve these problems, such as adding polymers or bigger cations to the absorber layer, enhancing moisture stability and luminescence quantum yields [36]. It is worth noting that solvent engineering of the precursor solution is largely responsible for most of the significant improvements in the PCE of PSCs [4, 37, 38]. Perovskites exhibit incredible plasticity due to the existence of additives that do not ultimately integrate into the multi-cation perovskite structure [39]. Such configurational modification of the perovskite layer is crucial for enhancing PSCs’ photovoltaic performance. Iodide regulation has been shown to be effective in reducing defects in perovskite thin films more recently, with a verified PCE of 22.1% [40], which was accomplished by adding more iodine to the perovskite precursor solution.
In contrast, potassium was added as a cation to the perovskite structure for other earlier research to generate multi-cation perovskites [41–45]. Bu et al. added potassium (K+) to a triple-cation mixed-halide perovskite composition and yielded an enhanced PCE [46]. Despite the reported improvement in performance, consistent research on triple-cations (Rb+ and MAPbI3) incorporated K+ creates an additional phase separation [43]. Abdi-Jalebi et al. reported KI has a potential for defect passivating additive in triple-cation perovskites, leading to materials with high photoluminescence of over 66%, which decreased photo-induced halide separation significantly [47]. It suggested that the excess iodide KI incorporated in perovskite reduces the defects. In contrast, a potassium-halide reduced photo-induced halide separation and presented grain boundary region [47].
Despite these extremely encouraging results, it is still unclear where the positive benefits of KI incorporation on a triple-cation come from and whether these additions give a triple-cation photovoltaic device long-term operational stability. The process used to deposit the perovskite coating is a vital component of PSC performance. It should be noted that earlier studies on KI-incorporated PSCs focused on durability or operational stability in an inert environment.
The current work is focused on device performance and stability under real-world circumstances such as high humidity and temperatures. It examines the impact of KI incorporation into Cs3Bi2I9/Ag2BiI5 carbon-based PSCs on the perovskite film morphological structure, device stability in ambient and high-humidity environments, and cell optical characteristics, thus complementing existing studies [41, 43–45]. The blended heterojunction bismuth-based Cs3Bi2I9/Ag2BiI5 (K-C-ABI) solar cells incorporating potassium iodide (KI), with the perovskite layer made up of Cs3Bi2I9 (C-ABI) and Ag2BiI5 (A-BI) components are a key innovation of this work.
The present research builds on earlier work by introducing KI into the perovskite precursor solution [47]. In fact, related prior work forms the foundation for the experimental methods adopted in this study and the mechanisms of KI incorporation [30, 41, 44, 47, 48]. The results provided insight into the purpose of researching potassium cations (K+) to enhance the photovoltaic performance of PSCs. In this study, KI incorporation is used to investigate whether adding K+ to the perovskite absorber layer for highly effective PSCs is feasible. The goal is to combine the advantages of excess iodide ions and the advantages of potassium salt.
This study achieved an outstanding performance amongst the bismuth-based PSCs studied with a PCE of 8.82% and an unheard-of Voc of 0.82 V for the K-C-ABI device sky rock the lead-free PSCs generation. Experimental studies and characterizations confirmed the successful synthesis of the K-C-ABI structure.