Fabrication and TCAD validation of ambient air-processed ZnO NRs/CH₃NH₃PbI₃/spiro-OMeTAD solar cells

Highlights

• PSCs with 10.18% PCE fabricated under ambient-air atmosphere.

• PSCs for undoped and doped HTL validated using SetFos™ TCAD simulation.

• ZnO NRs grown by solvothermal process upon ZnO QDs sheet layer coated on FTO.

Abstract

This paper reports the fabrication, characterization and simulation of hybrid perovskite solar cells (PSCs) in ambient condition. The proposed PSC structures use a CH₃NH₃PbI₃ hybrid perovskite based active layer sandwiched between a ZnO nanorods (NRs) electron transport layer (ETL) and a spiro-OMeTAD (undoped and doped) hole transport layer (HTL). The ZnO NRs are grown using low-cost solvothermal process at relatively low temperature. The performance of fabricated PSCs are analyzed for both the undoped and doped (with TBP and LiTFSI) spiro-OMeTAD based HTLs. All the solar parameters namely, short circuit current density (J_SC), open circuit voltage (V_OC), fill factor (FF), power conversion efficiency (PCE) and external quantum efficiency (EQE) are calculated from experimentally measured current density versus voltage (J-V) and wavelength transient characteristics in ambient condition. The maximum PCE of 10.18% is obtained for the doped HTL whereas 9.51% for undoped HTL. The improved performance due to HTL doping is attributed to the enhanced charge transportation of the HTL. The experimental results obtained from the fabricated PSCs are also compared with the SetFos™ TCAD simulation data using drift-diffusion model. The simulated results are observed to be well matched to the experimental data.

Introduction

Hybrid perovskite material based photovoltaic devices have gained immense interest among the research community due to their direct energy band gap with suitable values, high absorbing coefficient, long diffusion constant and solution processed low-cost fabrication methods [[1], [2], [3], [4], [5]]. The perovskite solar cells (PSCs) are commonly fabricated in PIN diode structures consisting of a p-type hole transport layer (HTL), an undoped perovskite based active layer, and an n-type electron transport layer (ETL) [6,7]. Each of the layers has significant importance in the performance of solar cell. The first PSC with power conversion efficiency (PCE) of 2.2% is reported in 2006 by Miyaska et al. [8]. In 2011, Park's group have reported PCE of 6.5% by using CH₃NH₃PbI₃ quantum dot as photoactive material [9]. Kim et al. [10] have achieved the PCE to 9.7% using a spiro-OMeTAD based HTL in the CH₃NH₃PbI₃ perovskite based PSC. There are continuous efforts from the researchers to improve the PCE close to the Shockley–Queisser limit (i.e. the maximum theoretical limit of PCE) [11]. New synthesis process for the perovskite material and new materials with doping engineering in the HTL have explored for achieving highly efficient, stable and environment-friendly PSCs [[12], [13], [14], [15]].

PEDOT: PSS and P3HT are the commonly used materials for HTL in the PSCs. However, PEDOT:PSS has poor stability on perovskite materials due to its hygroscopic and acidic nature [16]. On the other hand, P3HT suffers from proper band alignment with the perovskite materials [17]. The spiro-OMeTAD based HTL shows better performance over PEDOT:PSS and P3HT in the PSCs [18] due to its wide band gap, relatively good stability in ambient-air conditions, and high conductivity [19]. Further, the mobility/conductivity of spiro-OMeTAD can be increased further by appropriate doping. The common p-type dopants to the spiro-OMeTAD are bis(trifluoromethane) sulfonamide lithium salt (Li-TFSI) and 4-tert-butylpyridine (TBP) [12]. It is observed that lithium salt helps in oxidation process of spiro-OMeTAD whereas TBP improves conductivity of active layer by dissolving in perovskite layer [19].

ZnO is widely used as ETL material due to its high mobility, easy synthesis, abundant availability and high stability [[20], [21], [22], [23], [24]]. In general, ZnO nanostructures are preferred in the ETL over its bulk counterpart due to larger surface-to-volume ratio. The low temperature processing requirement ZnO nanorods (NRs) based ETL is widely explored for large-area PSCs [20]. Son et al. [25] have reported the maximum PCE of 14.35% using (NH₄)₂TiF₆ treated ZnO NRs based ETL in the PSCs [25]. Xu et al. [26] have obtained the PCE of 9.15% using ZnO NR arrays synthesized by modified solvothermal method.

Experimental verifications of various effects of device parameters on the PSCs are very difficult and time consuming due to the requirement of a larger number of device fabrication and characterization. The suitable numerical simulation approaches can be adopted for the performance optimization of solar cells [27,28]. The commercial TCAD tool, SetFos™, is a powerful tool for analyzing the characteristics of organic semiconductors, perovskites and quantum-dots based organic solar cells [29]. This commercially available TCAD tool can be effectively used to design the high efficiency solar cells and/or to fit parameters for experimental data.

In this work, we have investigated the performance of a ZnO NRs/CH₃NH₃PbI₃/spiro-OMeTAD based hybrid PSC using solution processing method under ambient environments. The ZnO NRs based ETL is developed on transparent fluorine doped tin oxide (FTO) coated glass substrates covered by ZnO quantum dots (QDs) using low-cost modified solvothermal process. Various parameters such as the short circuit current density (J_SC), open circuit voltage (V_OC), fill factor (FF), and power conversion efficiency (PCE) of PSCs have been studied. The effect of HTL doping on the performance of the fabricated and simulated PSCs have also been investigated. The experimental results have been compared with the commercially available SetFos™ TCAD simulation data for their validation.