Optimization of the Ag/PCBM interface by a rhodamine interlayer to enhance the efficiency and stability of perovskite solar cells

Abstract

Effective control of the interface between the metal cathode and the electron transport layer (ETL) is critical for achieving high performance p-i-n planar heterojunction perovskite solar cells (PSCs). Several organic molecules have been explored as interlayers between the silver (Ag) electrode and the ETL for the improvement in the photovoltaic conversion efficiency (PCE) of p-i-n planar PSCs. However, the role of these organic molecules in the charge transfer at the metal/ETL interface and the chemical degradation processes of PSCs has not yet been fully understood. In this work, we systematically explore the effects of the interfacial modification of the Ag/ETL interface on PSCs using rhodamine 101 as a model molecule. By the insertion of rhodamine 101 as an interlayer between Ag and fullerene derivatives (PC60BM and PC70BM) ETLs improve the PCE as well as the stability of p-i-n planar PSCs. Atomic force microscopy (AFM) characterization reveals that rhodamine passivates the defects at the PCBM layer and reduces the band bending at the PCBM surface. In consequence, charge transfer from the PCBM towards the Ag electrode is enhanced leading to an increased fill factor (FF) resulting in a PCE up to 16.6%. Moreover, rhodamine acts as a permeation barrier hindering the penetration of moisture towards the perovskite layer as well as preventing the chemical interaction of perovskite with the Ag electrode. Interestingly, the work function of the metal cathode remains more stable due to the rhodamine incorporation. Consequently, a better alignment between the quasi-Fermi level of PCBM and the Ag work function is achieved minimizing the energy barrier for charge extraction. This work contributes to reveal the relevance of proper interfacial engineering at the metal-cathode/organic-semiconductor interface.

Impact of Interfacial Layers in Perovskite Solar Cells

“…Bathocuproine( BCP) is one of the well-knowni nterfacial materials between C 60 derivatives and metal electrodes. As electrode interfacialm aterials, organic molecules used widely for organic electronics were proposed for perovskite solar cells including BCP, [109] 4,7-diphenyl-1,10-phenanthroline (Bphen), [110] Rhodamine 101, [111] and metal acetylacetonates [112] (Figure 18). As expected, FF was commonly improved by using the electrode interfacial layer.B esides the organic interlayer,i norganic materials were proved to work well as the electrode interlayer, for example, tin oxide [113] and aluminum-doped ZnO.…”

8‐Hydroquinolatolithium as a Highly Effective Solution‐Processable Cathode Interfacial Material in Inverted Perovskite Solar Cells with an Efficiency Over 19%

“…Currently, the reported PCE of inverted PSCs has been over 20% . In inverted PSCs, [6,6]‐phenyl‐C 61 ‐butyric acid methyl ester (PCBM) is one of the most studied electron‐transport materials due to its appropriate energy band alignment with perovskite, good electron mobility, and low‐temperature solution processability . Unfortunately, there is a high energy mismatch at the interface between PCBM and the commonly used metal cathodes such as silver (Ag) or aluminum (Al), which would severely restrict the electron extraction and the device built‐in potential ( V bi ), and thus lead to relatively poor device performance .…”

“…However, the involved high‐vacuum deposition process is demanding, costly and undesirable for low‐cost large‐scale industrial production. Alternatively, several reports have focused on exploration and utilization of low‐temperature solution‐processed CILs, such as amino‐functionalized polymers, fullerene derivatives, perylene‐diimide derivative, metal acetylacetonate series, and inorganic materials . Nevertheless, the performance of most of the PSCs based on these CILs is not satisfactory enough, and the related device fabrication process is relatively complicated.…”

Molecular Interlayers in Hybrid Perovskite Solar Cells

“…As mentioned above hydrophobic interlayers have been investigated in all four interlayer positions indicated in Scheme 2, including immediately adjacent to the metal cathode. [64,65] For instance, Ciro et al used Rhodamine 101 as a cathode interlayer to optimize the Ag/PCBM interface in a p-i-n structured perovskite solar cell. [64] They showed that device stability was significantly improved as the Rhodamine molecular interlayer appeared to act as a permeation barrier to slow the ingress of moisture from the atmosphere.…”