Engineering of Conductive Polymer Using Simple Chemical Treatment in Silicon Nanowire-Based Hybrid Solar Cells
Po-Hsuan Hsiao, Ilham Ramadhan Putra, and Chia-Yun Chen
National Cheng Rung University
Introduction
The demand for clean energy continues to be followed up by the development of renewable energy coping with the environmentally friendly, cheap, and efficient routes. Solar energy is one of the energy sources offering clean, sustainable, and stable energy. In 2018, the worldwide solar photovoltaic (PV) power was expected to generate approximately 2.8% of the worldwide electricity [1]. In recent years, silicon (Si)-based solar cells have been considered the most widely used type of PV devices. Unfortunately, the manufacturing of this type of solar cells requires a fairly complex process. In this regard, the various types of solar cells followed by the development of alternative materials essentially turn out to be a prospective field on both academic research and industrial applications. The emerging PV structures such as dye-sensitized, organic-based, and inorganic thin film-based solar cells are widely being escalated and utilizing various materials. Moreover, one more type of “layer-by-layer” solar cells that possess the potential impact on energy-related applications is hybrid-solar cells.
The “hybrid” term is typically known as the sequence of organic-inorganic materials applied for the device with active junction carrying on the charge separation of photogenerated electrons and holes. Further completed device or cell system development utilizing current collector layers as the top and bottom electrodes becomes the main point in various developments of solar cells. The main focus on the device engineering is to build up remarkable performance and efficiency by considering several factors especially the charge transport and separation. These include materials selection, p- and n-type materials, nanostructural approaches, passivation layer, interface contact improvement, and physical engineering techniques such as junction thickness determination. Moreover, several studies spotlighted to other parts, for example, are the electrode materials selection as current collectors and hole (h+) or electron (e~) selective layer applied between the active layer and current collectors. In the organic-inorganic junction, the structure consists of transparent conductive polymer in combination with the inorganic semiconductor, contributing to an adequate transport and separation of photogenerated carriers. From various materials acquired for the cell construction, the arrangement of PEDOT:PSS polymer and n-type crystalline Si was proposed by considering both superiorities. This type of hybrid solar cells has attracted worldwide attention since it was proposed in 2010 and further developed as the potential solar cells at present. Compared with ion-based batteries as the energy-storage system, hybrid solar cells turn out be the efficient way for supplying the electric energy in a stable and reliable way. By combing hybrid solar cells with battery system, this can be an ecofriendly and renewable platform for supplying energy required in use. Thus, this book can be served as the bridge for the readers to fully understand the related technology of energy generation and storage.
Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOTPSS) is a conductive polymer that offers numerous superiorities such as tunable electrical conductivity by uncomplicated treatments, facile fabrication to form thin film, acceptable thermal stability, material flexibility, and superior transparency in the visible range. In contrast, crystalline Si as the n-type material was considered to offer several advantages such as higher minority carrier lifetime, adjustable doping type, and concentrations, thus making it a preferable candidate for creating p-n junction with p-type organic materials. In this architecture of hybrid solar cells, PEDOT:PSS/pla- nar n-type Si junctions have led to achieving cells efficiency by up to 12%. Later, many approaches to improve the performance of these hybrid solar cells were discovered including nanostructural technique [2], passivation layer at the interface between PEDOT:PSS/n-type Si, and additional charge selective layer configuration [3].
Interestingly, the PEDOT:PSS material as the p-type layer in this type of cell arrangement surely has an important impact in the junction section, and the study of this organic is substantial to be conducted. Specifically, this polymer in its commercial product has a low applicable electrical conductivity around 0.2-10 S cm-1. This value, of course, becomes a problem when applied it to hybrid solar cells that require a preferable high electrical conductivity. Many studies on the improvement of electrical conductivity in PEDOT:PSS have been widely carried out, such as applying physical treatments by heat or light and chemical treatment including cosolvents, surfactants, acids, and ionic liquids, known to increase its conductivity up to 103 orders for many applications. Furthermore, these chemical utilizations conducted by different combinations of pretreatment, direct addition, and post-treatment may benefit the desired applications based on treated PEDOT:PSS. Specifically, the underlying mechanism regarding the conductivity improvement correlates with the structural and morphological changes. However, in some cases of the post-treatment, the process may influence the thin-film uniformity or even damage the prepared thin film [4]. For these reasons, an easy-and-efficient approach to overcome some of these issues needs to be studied.
The use of cosolvents and surfactants is certainly an interesting consideration because it offers simplicity and reliability in the fabrication process. Of the various uses of chemicals, the addition of cosolvents using ethylene glycol (EG) is known to increase the conductivity of PEDOT:PSS to around 620 S cm-1 [5]. In contrast, the addition of surfactants such as fluorosurfactant provides superior wetting properties of the PEDOT:PSS solution, which enables the facile deposition on a variety of hydro- phobic substrates [6]. By combining both organic solutions and additional annealing processes, the improvement of both conductivity and uniformity in PEDOT:PSS thin film can actually benefit the deposition of functional PEDOT:PSS on the nanostruc- tured n-type Si. For example, the application of emerging PEDOT:PSS/planar Si for the cell design has been carried out and proven to improve the photovoltaic performance [7,8]. However, the research for combining PEDOT:PSS thin film with nanowires (SiNWs) is still limited. In fact, the SiNWs have various advantages, and one of the well-known applications in solar cells is being able to provide a lighttrapping effect so as to enable the optimization of incoming light transmitted from the transparent PEDOT:PSS layer [9]. Therefore, PEDOT:PSS/SiNW has been considered an efficient way for the construction of high-performance hybrid solar cells.
