Elsevier

Thin Solid Films

Volume 544, 1 October 2013, Pages 93-98
Thin Solid Films

Characteristics of photocurrent generation in the near-ultraviolet region in Si quantum-dot sensitized solar cells

https://doi.org/10.1016/j.tsf.2013.04.111Get rights and content

Highlights

  • We have developed on Si quantum-dot sensitized solar cells using Si particles.

  • Current of solar cells increases by surface-termination of Si particles.

  • Incident photo-to-current conversion efficiency increases below 300 nm.

Abstract

We have studied photocurrent generation in Si quantum-dot (QD) sensitized solar cells, where QD thin films composed of Si nanoparticles were deposited using the double multi-hollow discharge plasma chemical vapor deposition process in an SiH4/H2 and CH4 or N2 gas mixture. The short-circuit current density of the Si QD sensitized solar cells increases by a factor of 2.5 by using Si nanoparticles prepared by irradiation of CH4 or N2 plasma onto the Si nanoparticle surface. We have measured incident photon-to-current conversion efficiency (IPCE) in the near-ultraviolet range using quartz-glass front panels of the QD sensitized solar cells. With decreasing the wavelength of irradiation light, IPCE gradually increases upon light irradiation in a wavelength range less than about 600 nm, and then steeply increases below 300 nm, corresponding to 2.2 times the optical band-gap energy of Si QD film.

Introduction

The pressing need for massively scalable carbon-free energy sources has focused attention on both increasing the efficiency and decreasing the cost of solar cells. Quantum-dot (QD) solar cells employing multiple exciton generation (MEG) have attracted much attention as a candidate for the third generation solar cells [1], [2], [3], [4], [5], [6], since MEG represents a promising route to increased solar conversion efficiencies of up to about 44% in a single junction [7]. MEG is a process wherein multiple electron–hole pairs are produced upon absorption of a single incident photon, where the excess energy is used to excite a second electron across the band-gap instead of being dissipated as heat through sequential phonon emission.

QD sensitized solar cells, which are assembled by replacing dye molecules with semiconductor nanoparticles, have been developed over the past two decades as low-cost solar cells [8], [9], [10], [11], [12], [13]. For this type of solar cell, small band-gap semiconductors such as CdS, PbS, CdSe, InP, and InAs serve as sensitizers because electrons photogenerated within them can be transferred to large band-gap semiconductors such as TiO2 under light excitation. MEG has been reported in colloidal PbSe, PbS, PbTe, CdSe, CdTe, Si and InAs QD nanoparticles [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. Recently, Sambur et al. clearly demonstrated the MEG effect in PbS QD sensitized solar cells, where they observed over 100% internal quantum yield [24].

Our interest has been motivated by QD sensitized solar cells using Si nanoparticles [25], [26], [27], because the technologically important and abundant Si poses no environmental problems regarding toxicity, and is the backbone of the current electronics and solar cell industries. MEG yields in colloidal Si nanoparticles have been reported by Beard et al., and they found the threshold photon energy for MEG in 9.5 nm diameter of Si nanoparticles to be 2.4 times the band-gap energy (Eg) of Si nanoparticles using ultrafast transient absorption spectroscopy [20]. Su et al. have theoretically identified the MEG threshold energy to be in the range of 2.2–3.1 times Eg depending on the Si dot radius [28].

In this study, we measured the quantum yield in Si QD sensitized solar cells in the near-ultraviolet (near-UV) region, which was found to be more than twice the Eg of Si nanoparticles. QD solar cells using Si nanoparticles have already been reported by some groups [29], [30], [31]. In particular, Liu et al. have shown power conversion efficiencies as high as 1.47% in organic/inorganic hybrid solar cells based on mixtures of Si nanoparticles and polymers such as P3HT. Also, we fabricated hybrid sensitized solar cells using Si nanoparticles and ruthenium dye, and observed an increase of conversion efficiency up to 3% [26]. The main purpose of this study was to discuss the characteristics of the quantum yield in view of the MEG effect, and the focus was on the analysis of the quantum yield under short incident light. Our measurements show that the quantum yield drastically increases below 300 nm corresponding to 2.2 Eg of Si nanoparticle films, and we achieved a high quantum yield of about 60% at shorter wavelengths. In the next section, we describe deposition of core–shell Si nanoparticle films using SiH4/H2 and CH4 or N2 double multi-hollow discharge plasma chemical vapor deposition (CVD). Then, in the Section 3, we present performance of Si QD sensitized solar cells.

Section snippets

Deposition of Si nanoparticle films

The production of Si nanoparticles and surface treatment by hydrocarbon or nitrogen radicals were carried out using a multi-hollow discharge plasma CVD method, shown in Fig. 1 [32], [33], [34], [35], wherein two discharge plasmas of SiH4/H2 (plasma 1) and CH4 or N2 (plasma 2) are independently generated in a vacuum chamber. The multi-hollow electrode consisted of a powered electrode and two grounded electrodes 30 mm in diameter. The discharges were sustained in eight small holes of 5 mm diameter.

Performance of Si quantum dot solar cells

The schematic geometry of the Si QD sensitized cell in this study is depicted in Fig. 4(a). F-doped tin oxide (FTO) was used as a transparent conducting oxide substrate; FTO glass has low transmittance in the near-UV region less than 350 nm wavelength. TiO2 paste was coated on an FTO electrode by a screen printing method and baked in air. The size of TiO2 nanoparticles was about 20 nm, and the thickness of the TiO2 films was about 6 μm. Si QD layers were fabricated by the double multi-hollow

Conclusions

We developed a Si QD sensitized solar cell using Si nanoparticle films deposited by a multi-hollow discharge plasma CVD method. Here, dangling bonds on the Si nanoparticle surfaces are terminated by C with CH4 plasma irradiation, and N with N2 plasma irradiation. APCE of Si QD sensitized solar cell steeply increased below 300 nm corresponding to 2.2 Eg, and achieved a high value of about 60% in the near-UV region.

Acknowledgments

This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO).

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