Heterojunction-Based Hybrid Silicon Nanowires Solar Cell

*Riam Abu Much, Prakash Natarajan, Awad Shalabny, Sumesh Sadhujan, Sherina Harilal and Muhammad Y. Bashouti*

## **Abstract**

It is known that defect-free, i.e., oxide-free, Si nanowires (Si NWs) exhibit lower defect density emissions than unmodified Si NWs. This is successfully established by grafting organic molecules on the surface. Here we show that by using a two-step chlorination/alkylation process, we are able to graft organic molecules on Si NWs for solar cell applications. Afterward, we show the electronic properties of the molecular surface (such as work function and band bending). Finally, we correlate these properties to the solar cell performance.

**Keywords:** silicon nanowire, defect-free surface, oxide-free silicon, chlorination/alkylation process, hybrid solar cell, oxidation resistance, photoemission, heterojunction

## **1. Introduction**

Recently, many one-dimensional (1D) nanostructures have been realized by different methods [1, 2]. One-dimensional nanostructures such as nanowires (NWs) are considered a promising material for various applications in electronics [3], optoelectronics [4], photovoltaics [2, 5–15], and sensing [16–18].

Specifically, silicon nanowires (Si NWs) received a considerable attention since it can be integrated in the microelectronic industry. Therefore, Si NWs revealed their potential to become the mainstream building blocks of future nanodevices such as field effect transistors (FETs) [19–21] and solar cells [21–26], thus reducing process redesign costs. However, before such applications, we need at first to control the growth of the Si NWs and to understand its electronic properties. Here, we show a promising growth method and robust characterization method, i.e., the vapor-liquidsolid (VLS) method and the X-ray photoelectron spectroscopy (XPS), respectively.

Moreover, studies show that the electronic properties of Si NWs can be tuned through attachment of molecules at the surface. The high ratio between the surface and the volume of NWs makes the electronic properties highly sensitive to surface properties. To this end, grafting the surface (through dangling bonds) with organic molecules is expected to have a significant impact on the final physical and chemical properties of Si NWs. The resulting surface is known as "hybrid Si NWs" [27, 28].

However, for many applications the presence of oxide (mainly native oxide) at the Si surfaces introduces defects and decreases the device performance. Native oxide grows at the Si surface after exposure to air or/and to humidity. The defects form an undesirable layer of oxide with high impurity levels, which can result in uncontrolled

oxide/silicon interfaces. Thus, to obtain efficient Si NWs, we need to protect the surface against oxidation. For example, hydrogenated Si NWs, i.e., Si▬H bonds, exhibit low surface recombination velocities [29]. However, the Si▬H bonds tend to oxidize within a few minutes. Another method to functionalize the Si NW is through different bonds such as Si▬C bonds, which may increase the oxidation resistance from several minutes to a few hundred hours or even months [27, 28]. Another advantage of the Si▬C bonds (rather than stability and tuning the electronic properties) is their being selectively sensitive to the environment. For instance, gas sensors based on Si NWs can tune their functionality by adding special molecules at the surface with specific reactivity with the target gas [30–32]. In this article, we, first, explain how to grow the Si NWs, then how we functionalize their surface through chlorination/ alkylation process, electronic properties, and finally, their application in solar cells.
