Improved photovoltaic performance of silicon nanowires/conjugated polymer hybrid solar cells
Introduction
Polymer solar cells have been considered as attractive candidates for renewable energy sources due to their several and multiple advantages of environmental friendliness, low cost and large area processing [1], [2], [3]. The bulk heterojunction structure composed of interpenetrating network of polymer donor and electron acceptor has deeply promoted the development of polymer solar cells [4], [5]. Both inorganic and organic electron acceptors are investigated; the latter are mainly fullerene derivatives [6], [7], [8]. Although power conversion efficiency (PCE) of polymer solar cells with these fullerene derivatives electron acceptors reached up to 9%, the complicated synthesis process and easy oxidation of fullerene in ambient still hinder their practical investigation [9], [10]. Alternatively, a multitude of concepts have been demonstrated by combining p-type donor polymers with n-type acceptor inorganic nanostructures such as CdSe [11], TiO2 [12] and ZnO [13]. One dimensional (1-D) inorganic semiconductor nanostructures are among some of the most attractive nanomaterials for solar cell devices because they provide a direct path for charge transport. Other advantages include high carrier mobility, solution processability, thermal and ambient stability, and high electron affinity necessary for charge injection from the complementary organic donor material. Silicon nanowires (SiNWs) are an example of this class of materials that have been used for hybrid solar cells [14], [15], [16], [17], [18]. The particular structure of SiNWs can provide high-mobility pathway from the active interface to the electrodes for carriers. They can significantly reduce the reflection and induce strong light trapping between nanowires, resulting in strong absorption. Whilst their volume is low, SiWNs participate to the increase of the contact area between the two materials and hence restrain the back charge recombination and consequently improve the charge transfer at polymer/SiNWs interfaces. From the relationship between structure and performance, the morphology changes of the photoactive layer are derived from a combination of dopant and conjugated polymer. It has been shown that the power conversion efficiency of bulk heterojunction solar cells can be improved drastically by controlling the morphology of the blend.
Herein, we report enhanced performance of bulk heterojunction hybrid solar cells based on poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) as electron donor and Si nanowires as electron acceptor. In order to evaluate the performance of the SiNWs effect, we have prepared different compositions of polymer:SiNWs (1:0.25; 1:0.5; 1:1; 1:2 and 1:4). Hybrid solar cells with the structure of ITO/PEDOT:PSS/MEH-PPV:SiNWs/Al were fabricated and characterized with optical, morphological and electrical tools.
Section snippets
Materials
MEH-PPV was purchased from Aldrich (weight average molecular weight (Mw) = 86,000 g/mol). Silicon nanowires (SiNWs) were grown using Vapor–Liquid–Solid (VLS) mechanism on n-doped Si (1 1 1) wafer. The growth step was performed in a horizontal low pressure chemical vapor deposition (LPCVD) reactor at 600 °C growth temperature and 3 Torr total pressure. Gold colloids were used as catalyst, hydrogen (H2) was used as the carrier gas and silane (SiH4) was used as Si precursor [19]. The average diameter and
Absorption and photoluminescence study
To elucidate the role of SiNWs in the elaborated devices, we measure the photo-absorption spectra of the pure MEH-PPV and MEH-PPV/SiNWs thin films. The normalized absorption spectra of MEH-PPV:SiNWs for different contents of SiNWs are displayed in Fig. 2. The pure MEH-PPV thin film shows a well-ordered absorption behavior: the π–π* transition was observed at 500 nm wavelengths. Upon blending with SiNWs, the whole absorption spectra show a light red-shift as the content of SiNWs increases. These
Conclusion
The effects of SiNWs incorporation on optical behavior of hybrid nanocomposites based on MEH-PPV:SiNWs are investigated. Different amounts of SiNWs have been introduced into MEH-PPV polymer matrix. It was found that SiNWs have contributed to the improvement of the absorption, the transfer charge and the transport charge processes within the device, with an optimum composition for MEH-PPV:SiNWs (1:2). With progressive incorporation of one dimensional SiNWs, more interfacial area is present to
Acknowledgment
Nadia Chehata gratefully acknowledges Dr. Gisèle Boiteux (Polymer Engineering Materials Laboratory, Lyon 1, France) for her support and encouragement.
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