Elsevier

Electrochimica Acta

Volume 87, 1 January 2013, Pages 213-217
Electrochimica Acta

The reduction of charge recombination and performance enhancement by the surface modification of Si quantum dot-sensitized solar cell

https://doi.org/10.1016/j.electacta.2012.09.087Get rights and content

Abstract

Multiple exciton generation solar cell based on quantum dots has higher theoretical efficiency than single exciton generation solar cell. In this work, Si quantum dots with the diameter of 10 nm were fabricated by the multi-hollow discharge plasma chemical vapor deposition and applied to the quantum dot-sensitized solar cell. In this cell, there was considerable electron recombination with redox electrolyte in the Si–TiO2 network because of large Si particle size. For the reduction of recombination and the enhancement of performance, a barrier layer was introduced. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and zinc acetate dihydrate (Zn(CH3COO)2·H2O) were employed as precursors for surface modification. Consequently, short circuit current and open circuit voltage of the cells were increased by the surface modification with both precursors. The improvement was ascribed to the inhibition of electrons back transfer from TiO2 to the electrolyte by the barrier layer. This result clearly demonstrated that the surface modification with ZnO was advantageous for the performance enhancement.

Introduction

Photochemical solar cells such as dye-sensitized and organic solar cells have much attracted in the research fields because of the low manufacturing cost and simple fabrication process. However, they have obvious limitation in the performance because of their characteristics of single exciton generation (SEG) which means only one electron generation by one incident photon. Therefore, quantum dot-sensitized solar cells (QDSCs) using narrow band gap semiconductors such as CdS, CdSe, PbS and PbSe have attracted considerate attention recently [1], [2], [3], [4], [5], [6], [7]. The unique characteristics such as tunable optical property and multiple exciton generation (MEG) are expected to enhance the efficiency over Shockley and Queisser limit of 33% [8]. MEG is able to produce multiple excitons by one incident photon in semiconductor nano-crystals and represents a promising route to increase solar conversion efficiency. Hanna and Nozik already proved the possibility of performance enhancement in MEG solar cell [9]. Its theoretical efficiency is 11% higher than that of SEG solar cell. There are many kinds of QDs developed so far and Cd compounds-sensitized solar cell has the best conversion efficiency [10]. However, they have some disadvantages such as toxicity and scarcity. Therefore, we focused on Si QD as the alternative to conventional QDs. Si is one of good QD materials and has abundance and absence of toxicity as a dominant material in the photovoltaics. In addition, Si QD has high stability against light soaking as compared with a-Si:H films and a high optical absorption coefficient due to its quantum size effect as compared with μc-Si films. However, there are few reports about Si QDSC despite its unique characteristics because the collection of Si QDs is too difficult contrary to easy film deposition. In our previous researches, this problem was solved by multi-hollow discharge plasma chemical vapor deposition (CVD) and carrier generation was proved in the wavelength range of less than 550 nm in Si QDSCs [11], [12]. However, there was considerable electron recombination in Si QDSC because of its large particle size.

For its improvement, the surface modification was investigated using a barrier layer in this work. The core–shell structure was introduced because it prevents the charge recombination at the TiO2/electrolyte interface and increases photocurrent. It consists of a nano-porous TiO2 covered with a shell of ZnO as shown in Fig. 1. ZnO exhibits very similar photoelectrochemical properties as TiO2. Its band gap and electron injection efficiency are also analogous to those of TiO2. It has additional advantages: a higher flat-band potential to achieve higher photovoltage and suitability for the fabrication of nano-structures with outstanding transport and optical properties [13], [14]. Zinc acetate dihydrate (Zn(CH3COO)2·H2O) [15], [16] and zinc nitrate hexahydrate (Zn(NO3)2·6H2O) [17], [18] were used for a barrier layer. In order to verify the effect of a barrier layer and the enhancement of the performance, the structural characteristics of deposited film, photovoltaic performance and internal electrochemical impedance of completed Si QDSC with a barrier layer were examined. Consequently, Si QDSC with effective ZnO barrier layer had the improvement on the overall performance.

Section snippets

Synthesis of Si nano-particles

Si nano-particles were synthesized by multi-hollow discharge plasma CVD. Schematic of the deposition method is shown in Fig. 2. Hydrogen (H2) diluted silane (SiH4) gas was introduced from the bottom of the reactor. It flows through the hollows of the electrodes and pumped out from the top of the reactor. SiH4 was converted to high-ordered silane with SiH2 and ionized. Then, crystalline Si nano-particles were nucleated and grown in the discharge plasma region. Eq. (1) simplified this process.

Results and discussion

The characteristics of Si QDs were confirmed with the rapid increase of absorbed photon to current conversion efficiency (APCE) in the range of short wavelength. Fig. 4 shows APCE of Si QDSC. In this case, FTO substrate of the photo electrode was substituted by Al sputtered quartz for the penetration of short wavelength. APCE was increased from 400 nm and rapidly climbed at 2 Eg point (280 nm). Although the measurement in the range of below 250 nm was impossible by the limitation of equipment, the

Conclusions

In this work, Si nano-particles synthesized by multi-hollow discharge plasma CVD with the diameter of 10 nm were used for the fabrication of Si QDSCs. There was considerable electron recombination because of its large particle size. To prevent the recombination in the Si–TiO2 nano-porous network, ZnO barrier layer was applied to the photo electrode as the surface modification. Zn(NO3)2·6H2O and Zn(CH3COO)2·H2O) were employed as precursor to surface-modify. As a result, IPCE and internal

Acknowledgment

This work was partially supported by TEMCO Memorial Foundation and New Energy and Industrial Technology Development Organization (NEDO).

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