The reduction of charge recombination and performance enhancement by the surface modification of Si quantum dot-sensitized solar cell
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).
References (26)
- et al.
Synthesis and photocatalytic activity of ZnO/ZnO2 composite
Journal of Photochemistry and Photobiology A: Chemistry
(2005) - et al.
Synthesis of nanocrystalline ZnO powder via sol-gel route for dye-sensitized solar cells
Solar Energy Materials and Solar Cells
(2008) - et al.
Fabrication of ZnO films consisting of densely accumulated mesoporous nanosheets and their dye-sensitized solar cell performance
Thin Solid Films
(2008) - et al.
Rapid growth of thick particulate film of crystalline ZnO in an aqueous solution
Thin Solid Films
(2008) - et al.
Efficient polysulfide electrolyte for CdS quantum dot-sensitized solar cells
Journal of Power Sources
(2008) - et al.
Conducting polymer-based counter electrode for a quantum-dot-sensitized solar cell (QDSSC) with a polysulfide electrolyte
Electrochimica Acta
(2011) - et al.
The fabrication of efficiency-improved W-series interconnect type of module by balancing the performance of single cells
Solar Energy
(2009) - et al.
Modeling of an equivalent circuit for dye-sensitized solar cells: improvement of efficiency of dye-sensitized solar cells by reducing internal resistance
Comptes Rendus Chimie
(2006) - et al.
Improvement of efficiency of dye-sensitized solar cells based on analysis of equivalent circuit
Journal of Photochemistry and Photobiology A: Chemistry
(2006) - et al.
Evaluation of treatment effects for high-performance dye-sensitized solar cells using equivalent circuit analysis
Thin Solid Films
(2006)
Highly efficient cdse-sensitized TiO2 photoelectrode for quantum-dot-sensitized solar cell applications
Chemistry of Materials
Colloidal PbS quantum dot solar cells with high fill factor
ACS Nano
Fabrication of InAs/GaAs quantum dot solar cells with enhanced photocurrent and without degradation of open circuit voltage
Applied Physics Letters
Cited by (19)
Performance enhancement of quantum dot-sensitized solar cells based on polymer nano-composite catalyst
2017, Electrochimica ActaCitation Excerpt :Quantum dot-sensitized solar cell (QDSC) is one of promising photovoltaic devices to realize low cost and high efficiency [1–6].
Studies of silicon quantum dots prepared at different substrate temperatures
2017, Superlattices and MicrostructuresCitation Excerpt :Now a days, the research has been diverted to the luminescence properties of silicon quantum dots [15–17] and it has been observed that the optical properties of Si QD’s are affected by quantum confinement. The work on silicon quantum dots for improved solar cell efficiency by Hong-Chen et al. [18], surface modification of Si quantum dot-sensitized solar cell by Seo et al. [19], studies on Si quantum dots by Castaldo et al. [20], effect of different composition to increase the performance of Si quantum solar cells by Seo et al. [21], studies on bifacial n-type silicon solar cells by Rüdiger [22], studies on Si tandem solar cell by Heidarzadeh et al. [23], the photo response study of Si quantum dots by Li et al. [24], the work on Si quantum dot structures and their applications by Shcherbyna et al. [25], the structural and electroluminescent properties of Si quantum dots by Rui et al. [26], the studies on Si nanocrystal polymer solar cells by Ding et al. [27], theoretical modelling of thin-film silicon solar cells by Zeman et al. [28], the analysis of hydrogenated amorphous silicon solar cells by Rodríguez et al. [29], the formation of large defect clusters in multicrystalline silicon solar cells by Kohler et al. [30] are also of high merit. Kesarwani et al. [31] has made an investigation on amorphous and nanocrystalline Si films deposited by filtered Vacuum Arc Technique.
Enhanced photoelectrochemical water splitting from Si quantum dots/TiO<inf>2</inf> nanotube arrays composite electrodes
2015, Materials Research BulletinCitation Excerpt :The band gap of Si particles varies from 1.1 to 5.4 eV depending on their sizes [34], suggesting that its coupling with wide band gap semiconductors, such as TiO2 would allow increasing absorption in the visible light regions of the solar spectrum, thus resulting in better light harvesting and higher efficiency photoelectrodes. Up to now, various methods have been applied to synthesize Si QDs such as multi-hollow discharge plasma chemical vapor deposition, etching or growing from Si substrate [33,37,38]. Among the available methods, chemical etching has been proposed to control well the density and size of Si QDs [39], thus mass production of Si QDs can be achieved easily.
A review of semiconductor materials as sensitizers for quantum dot-sensitized solar cells
2014, Renewable and Sustainable Energy ReviewsCitation Excerpt :In some cases, such as that in CuInS2 QDs, this recombination occurs as a result of band misalignments and of high surface-state density in the heterostructure between TiO2 and QDs [20]. Similarly, the large particle size of Si QDSSCs increases the recombination loss with the redox electrolyte in the Si–TiO2 network [77]. Moreover, low QD loading and the subsequent weak electronic connection between QDs and TiO2 introduce defects in cell performance and subsequently reduce cell efficiency.
The improvement on the performance of quantum dot-sensitized solar cells with functionalized Si
2013, Thin Solid FilmsCitation Excerpt :The characteristics of Si QDs were verified by absorbed photon to current conversion efficiency (APCE). Fig. 2 shows APCE of Si QDSC [30]. For the penetration of short wavelength, Al sputtered quartz substituted for FTO substrate.
Analysis on the effect of polysulfide electrolyte composition for higher performance of Si quantum dot-sensitized solar cells
2013, Electrochimica ActaCitation Excerpt :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. Quantum characteristics of Si QD were already proved in previous researches [7,24–26]. However, there are only few published reports about Si QDSC despite its unique characteristics as QD.