Efficiency and stability enhancement of perovskite solar cells by introducing CsPbI3 quantum dots as an interface engineering layer

 

Abstract

Although great efforts have been devoted to enhancing the efficiency and stability of perovskite solar cells (PSCs), the performance of PSCs has been far lower than anticipated. Interface engineering is helpful for obtaining high efficiency and stability through control of the interfacial charge transfer in PSCs. This paper demonstrates that the efficiency and stability of PSCs can be enhanced by introducing stable α-CsPbI₃ quantum dots (QDs) as an interface layer between the perovskite film and the hole transport material (HTM) layer. By synergistically controlling the valence band position (VBP) of the perovskite and the interface layer, an interface engineering strategy was successfully used to increase the efficiency of hole transfer from the perovskite to the HTM layer, resulting in the power conversion efficiency increasing from 15.17 to 18.56%. In addition, the enhancement of the stability of PSCs can be attributed to coating inorganic CsPbI₃ QDs onto the perovskite layer, which have a high moisture stability and result in long-term stability of the PSCs in ambient air.

Introduction

Research to achieve high-efficiency solar cells is currently very active^{1, 2}. As one of the most quickly developing solar cells, perovskite solar cells (PSCs) have become a hot research topic because of their high solar conversion efficiencies and low cost^3,4,5. PSCs have made impressive progress in just a few years with record power conversion efficiencies (PCEs) evolving from 3.8% in 2009 to a certified 22.1% in 2016 ^6,7,8,9,10. However, the development of PSCs is still restricted by the instability of the organic composition of PSCs in the presence of water and ambient moisture. To improve the stability of PSC devices, some recent papers have reported that composition engineering by doping with cations is effective for obtaining intrinsically stable perovskites with good properties. Seok et al. first found that the mixed type (FAPbI₃)_0.85(MAPbBr₃)_0.15 perovskite exhibits an excellent efficiency and stability¹¹. Inorganic cations have also been incorporated into multiple cation perovskites, such as Cs⁺ and Rb⁺, to improve the stability of PSC devices^{12, 13}. Although remarkable progress has been achieved, the crystal structure and energy band compatibility of the materials have largely hindered further improvement using this mixed perovskite strategy. Recently, another effective strategy was reported for promoting the stability of PSCs, i.e., coating barrier layers on the surface of the perovskite layer, and this strategy has been shown to be effective for improving device stability in ambient air¹⁴. Yang et al. utilized hydrophobic ammonium ions to modify the surface of a perovskite layer and improved the moisture stability¹⁵. Wang et al. utilized a hydrophobic fluorosilane layer as a water-resistant layer, resulting in greater stability of the perovskite film in water¹⁶. Nevertheless, these studies have ignored the band position matching factor (BPMF) between the barrier layer and the perovskite layer. On the other hand, a large number of studies have demonstrated that the BPMF greatly affects the charge transport efficiencies (CTEs) between the perovskite layer and the interface layer, which can substantially change the PCEs of PSCs^17,18,19. These results inspire us to design a new interface engineering layer to improve both the stability and efficiency of PSCs at the same time. To achieve this strategy, the interface engineering layer should possess two important properties, a suitable band position and high moisture resistance. Unfortunately, until now, few materials have been recorded to possess these two properties.
Recently, all-inorganic perovskite materials have exhibited better environmental durability than organic−inorganic hybrid perovskite materials²⁰. Furthermore, the band position of inorganic perovskite quantum dot (QD) materials is tunable by adjusting the components of the QDs²¹. However, it is difficult to deposit a dense and uniform QD film. Therefore, to date, it is still a challenge to achieve high PCEs with PSCs fabricated using inorganic perovskite materials as the light-absorption layer²². Considering their advantages and disadvantages, we believe that inorganic perovskite QDs are appropriate for use as an interface engineering layer in PSCs due to their superior stability and tunable band gap position.
Among all-inorganic perovskite materials, cubic CsPbI₃ may be the most appropriate candidate as an interface engineering material due to its high valence band position (VBP) and high moisture stability. In addition, the synthesis of CsPbI₃ QDs can effectively improve the stability of the cubic phase due to the generous contribution of the surface energy²³. In this work, we propose an interface engineering method to enhance both the efficiency and stability of PSCs by introducing stable CsPbI₃ QDs between the perovskite layer and the hole-transporting material (HTM) layer. Furthermore, we used mixed-type perovskite MA_0.17FA_0.83Pb(I_0.83Br_0.17)₃ (marked as FAMAPbI₃) as the light-absorption layer to further fulfill the VBP matching requirement between the perovskite layer and the CsPbI₃ QD layer. Our results demonstrated that PSCs containing CsPbI₃ QDs have a much higher PCE and better stability in ambient air than those without CsPbI₃ QDs. The results indicate that this is a new strategy to achieve both high efficiency and stable PSCs by utilizing inorganic perovskite QDs.

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