Lead free CH₃NH₃SnI₃ based perovskite solar cell using ZnTe nano flowers as hole transport layer

Highlights

• Investigation of ZnTe as a hole transport layer for lead free CH₃NH₃SnI₃ perovskite solar cell.

• ZnTe can effectively replace Spiro-OMeTAD which is very expensive material.

• Both simulation and fabrication is carried out and power conversion efficiencies are obtained as 22.96% and 8.41% respectively.

• The nano flower structure of ZnTe has enhanced the absorption capacity of the modeled solar cell.

Abstract

The potential impact of thin film perovskite solar cell is metamorphic as it can reduce the cost associated with manufacturing of solar cells. Perovskite materials have impressive optoelectronic properties such as high mobility, high absorption and long diffusion length of charge carriers. The power conversion efficiency of lead contained metal halide perovskite has reached up to 23%, but lead is a highly toxic element thereby harmful for environment. In this work Sn based perovskite (CH₃NH₃SnI₃) has been proposed and investigated using hole transport layer. Along with absorption of light by active layer, charge separation and extraction also plays a crucial role in controlling the device performance. The potential of ZnTe as hole collector has been explored. Here we report modeling, simulation and fabrication of lead-free CH₃NH₃SnI₃ perovskite solar cell processed on ZnTe as hole transport layer. Remarkably, we achieve efficiency of 22.96% in simulation and 8.41% in fabrication.

Introduction

In the new emerging world of mankind, electricity has been placed in the list of essential ingredient for survival. With the extraordinary growth in number of people, need of energy is also increasing. There is a rising alarm about the energy produced from fossil fuels due to its adverse effect to the nature. Solar energy is becoming a leading actor for obtaining ‘green’ energy in last decades. Photovoltaic cells are known for direct conversion of solar radiation into electricity without disturbing the environment. Researchers from all over the world are in search for finding a better alternative for silicon solar cell which can be cost effective and efficient. Perovskite solar cells can become one of those alternative as it has certain advantages of being able to be prepared by cost effective simple solution based processing [13]. Perovskite solar cell has shown a significant improvement in power conversion efficiency in last few years. There are several perovskite materials but, CH₃NH₃PbI₃ is the most used perovskite material in solar cells as an active layer. Since it contains lead (Pb) which is very toxic to the human health and it degrades very easily when it comes in contact with water, a suitable alternative of CH₃NH₃PbI₃ is being researched across the world [[3], [4], [5], [6]]. There has been many articles reported the viability of Sn based perovskite solar cell. It has better stability, wider range of coverage of visible spectrum and low recombination rate than Lead perovskite solar cell. The diffusion length in Sn perovskite solar cell is much shorter leading to electron generation closer to electron collector layer. The generation of electrons near ETL helps in electron collection significantly and reduces recombination rate [[12], [14], [15], [16], [17], [18], [19], [20], [21]]. CH₃NH₃SnI₃ has arisen as a feasible replacement of CH₃NH₃PbI₃ due to its low band gap (almost 1.3 eV) and high absorption coefficient. CH₃NH₃SnI₃ based perovskite solar cell (TCO/TiO₂/CH₃NH₃SnI₃/spiro-MeOTAD) have already achieved significant power conversion efficiency of 23.76% [21]. However, despite the rapid increase in efficiency of different types of perovskite solar cells, Hole Transport Layers (HTL) were mainly limited to organic compounds such as spiro-MeOTAD and other conducting polymers. But these types of organic hole-transporting materials are quite expensive compared to perovskite materials or n-type semiconducting materials used in the perovskite solar cells as Electron Transport Layer, due to complicated synthetic procedure or high-purity requirement [1]. In recent times, the focus of the researchers has been on finding an effective inorganic material as hole transport layer to overcome these challenges. In this work, p-doped Zinc Telluride (ZnTe), an inorganic group II-VI material is proposed as the hole transport layer for lead free CH₃NH₃SnI₃ based perovskite solar cells. The proposed structure is “metal contact/ZnTe(p)/CH₃NH₃SnI₃/TiO₂(n)/TCO/metal contact” and the structure is modeled using SCAPS simulating tool [9] and the result is almost equal to spiro-MeOTAD.

ZnTe as Hole transport layer: As we know that after absorption of photon by active layer an exciton is generated. Electron and holes are transported to circuit through electron and hole collector material that are deposited at front and back side of an active layer. Hole transport layer (HTL) material should possess properties like HUMO level of perovskite material should be lower than the valence band of the HTL material. HTL material should have very high hole mobility and band gap should be large. ZnTe hold all the above properties as it has wide band gap (~2.26 eV) with very high hole mobility of 80–100 cm²/V which is almost 100 times more than that of CuSCN and 1000 times more than that of spiro-MeOTAD [8,10]. The wide band diagram of ZnTe is shown (Fig. 4). The relation between current density, Jsc and mobility is:

$$J_{SC}=qG\left(\sqrt{{\mu }_p}+\sqrt{{\mu }_n}\right)\left(\sqrt{\frac{KT}{q}}\tau \right)$$

From the given equation it implies that if the mobility of the carriers increase it will certainly lead to improve the current density and hence the performance of solar cell. This is our inspiration behind the study of ZnTe as hole transport layer for ${\text{CH}}_3{\text{NH}}_3{\text{SnI}}_3$ CH₃NH₃SnI₃ based perovskite solar cells.