Additive engineering and interface engineering for high-quality perovskite films toward efficient and stable perovskite solar cells

Abstract

Perovskite solar cells (Pero-SCs) have emerged as one of the research hotspots due to the rapidly increasing power conversion efficiency (PCE) from 3.8% to 27.3%, a simple and environmentally friendly preparation process and great commercialization potential. Despite significant progress, Pero-SCs still face considerable challenges in achieving commercialization, particularly in further enhancing their efficiency and long-term stability. Perovskite layers play critical roles in determining device performance, governing exciton absorption, charge transport and recombination dynamics, and overall device stability. However, during the fabrication of perovskite layers, it is challenging to completely suppress the formation of defects and non-radiative recombination centers, which significantly impact charge carrier dynamics. Moreover, the inherent soft-lattice nature of perovskite materials renders them highly sensitive to environmental factors, accelerating degradation and ultimately compromising device performance and long-term stability. To obtain stable high-quality perovskites, additive engineering has emerged as a highly effective strategy. Furthermore, the growth substrates underlying the perovskite layers play a critical role in governing perovskite crystallization kinetics and film morphology, such that interface engineering is also receiving significant attention. Among various modification materials, inorganic compounds are widely adopted due to their superior stability and semiconducting properties, zwitterionic molecules offer additional advantages owing to their multifunctional groups, and perovskite A-site cation halides can facilitate structural modulation of the perovskite lattice. Consequently, these three categories of additives have been investigated for performance enhancement in Pero-SCs. In this thesis, inorganic copper sulfide (CuS) nanomaterials, the zwitterionic molecule soybean lecithin (SL), guanidium iodide (GAI) and cesium iodide (CsI) are selected as additives for the hole transport layer or perovskite layers to improve the PCE and stability. The research is presented as four studies： 1) Although poly[bis(4-phenyl) (2,4,6-trimethylphenyl) amine] (PTAA) is a widely used hole transport layer (HTL) in Pero-SCs, its poor conductivity and the mismatched energy levels between PTAA and formamidinium-based perovskites increase interfacial carrier recombination and charge-transport resistance. To solve these problems, inorganic CuS nanosheets were synthesized and applied as additives in PTAA for the first time. After the addition of CuS, the conductivity of the HTL improves, and the energy levels are better aligned. In addition, perovskite growth is controlled through the interaction between CuS and PbI2, thus improving the quality of the perovskite films, which reduces nonradiative recombination. The addition of CuS into PTAA improved the PCE from 21.99 % to 22.92 %. Moreover, both the thermal and humidity stability improved. 2) Although excess PbI2 can passivate perovskite boundaries and improve the PCE, under continuous illumination, the decomposition of PbI2 will introduce non-radiative recombination centers and destroy the device stability. To mitigate the side effects of PbI2, CuS nanoparticles were synthesized and incorporated into the PbI2 solution. The interaction between PbI2 and the CuS nanoparticles inhibited the PbI2 crystallization and decreased the PbI2 particle size. With the addition of CuS nanoparticles, more porous PbI2 films were obtained and the reaction between PbI2 and ammonium salts was facilitated due to smoother diffusion of formamidinium iodide (FAI). In addition, CuS nanoparticles can replace PbI2 to prevent defects. As a result, the PCE of Pero-SCs increased from 23.21% to 24.31% with improved N2, humidity and light stability. 3) The low affinity caused by the mismatched surface energies of the perovskite precursor solution and the underlayer is the main reason for the poor coverage of perovskite films, which is also responsible for the pinholes in the perovskite films. To solve this problem, amphiphilic SL, which has two long aliphatic chains, is applied as an additive in the perovskite precursor solution. The amphiphilic nature of SL improves the coverage of perovskite films on hydrophobic PTAA, which is conducive to the fabrication of large-scale devices. In addition, the C=O, P=O, and quaternary ammonium groups in the zwitterion segment can passivate charged defects, thus decreasing the defect density of perovskite films. Notably, the PCE of the corresponding Pero-SCs with an active area of 0.1 cm2 increased from 20.11% to 22.93%. Furthermore, the SL-modified devices with an active area of 1.1 cm2 demonstrated a PCE of 18.32%. The SL-modified Pero-SCs also showed better humidity stability than the pristine Pero-SCs. 4) GAI and CsI have been demonstrated as effective functional additives to FAI and PbI2, respectively, significantly enhancing the PCE and stability of perovskite solar cells. It has been observed that the introduction of GAI into the PbI2 lattice forms a long-range hydrogen-bonding network within the [PbI6]4- octahedra. However, the large GA⁺ induces lattice distortion. To address this, this study innovatively introduces Cs⁺ ions, which have smaller atomic radii, to synergistically regulate crystal growth kinetics and successfully achieve lattice stress balance. Experimental results show that the synergistic effect of GAI and CsI significantly reduces the defect-state density in the perovskite thin film (from 3.0 ×1016 to 2.3 ×1016 cm-3). A n-i-p structured device based on this approach achieves an efficiency of 24.29% (compared to 22.66% for the control group) and exhibits excellent operational stability under 80 ± 5% relative humidity at room temperature — retaining 86% of its initial efficiency after 1000 hours of storage. This study provides a new technological pathway for improving perovskite crystal quality and device performance through cation-size-engineering strategies. In summary, in order to solve various problems existing in perovskite films, SL, CuS nanomaterials, GAI and CsI with different properties and functions were utilized in additive engineering and interface engineering. These strategies passivated the defects in perovskite, regulate the growth of perovskite, adjust the content of PbI2, thereby reducing non-radiative recombination, promoting charge transfer, and improving device efficiency and stability.