ZnSe passivation layer for the efficiency enhancement of CuInS2 quantum dots sensitized solar cells
Introduction
In order to effectively relieve the environmental problem and energy crisis, solar cells have been widely investigated by many researchers in recent years [1]. Since dye-sensitized solar cells are introduced, many investigations have focused on the modification of dye sensitizers, photo-anodes and counter electrodes [2], [3], [4]. However, many significant factors are needed to be considered, such as photovoltaic power efficiency, cost, environmental pollution and devices stability. To our knowledge, the semiconductor quantum dots with narrow-band-gap (QDs), such as CdS, CdSe, PbS and CuInS2, have been employed as the sensitizers to fabricate the solar cells due to the unique properties of quantum size effect, large extinction coefficients, ultrafast electron transfer and multiple exciton generation [5], [6], [7], [8], [9], [10], [11]. Among these semiconductor QDs, the CuInS2 QDs with excellent full solar spectra absorption has been widely studied for their preparation and application. It had been gradually considered as the photoactive layer or sensitizers to try to fabricate the high efficiency solar cells in recent years [12], [13], [14]. However, the photovoltaic performance is still needed to be improved.
In recently years, the passivation layers had been introduced to improve the photovoltaic power efficiency of the quantum dot sensitized solar cells (QDSSCs). Due to the wide-band-gap and chemical stability, ZnS had been widely investigated as the passivation layer for the QDSSCs. It can prevent leakage of current from the QDs to electrolyte, which can enhance the performance of solar cells. It is well known that ZnS has significant effect as the passivation layer of the binary semiconductor QDs and QDSSCs. In the latest research works, employing ZnS as the shell of binary alloyed core/shell QDs can effectively improve the PL quantum yield from 42% to 63% [15]. And employing ZnS as the passivation layer, the photocurrent density of both ZnO/CdTe/ZnS and TiO2 nanorod/CdS/ZnS QDSSCs have nearly twice improved from 6.5 mA/cm2 to 13.8 mA/cm2 [16], [17]. With the development of the ternary QDSSCs, the ZnS is also investigated as the passivation layer. The PL lifetime can effectively reduce for the CuInS2/ZnS core/shell linking onto TiO2 nanoparticles [18]. The photovoltaic efficiency of TiO2/CuInS2/CdSe QDSSCs had improved to some extent with the coating of the ZnS passivation layer [19]. In our previous report, the CuInS2 QDs have been coated with ZnS layer can improve the photovoltaic efficiency of solar cells [20]. However, the effect of ZnS passivation layer for the photovoltaic enhancement of CuInS2 QDSSCs is not great as CdS QDSSCs. To our knowledge, the lattice constants of ZnSe are closely matched with that of CuInS2. The minimal lattice mismatch (2%) between CuInS2 and ZnSe will favor to relieve the interfacial strain and reduce the interfacial defects [21], [22]. Some investigations about employing ZnSe as the passivation layer of CuInS2 system QDSSCs have been initially studied [19]. Therefore, it is very necessary for the systematical research for employing ZnSe instead of ZnS as the passivation layer of CuInS2 QDSSCs to obtain the higher photovoltaic performance.
In the present work, we present an approach to investigate the effect of the ZnSe passivation layer in the efficiency enhancement of the CuInS2 QDSSCs. Here, CuInS2 QDs are firstly adsorbed on TiO2 nanoparticle films by using linking assembly method. Then the ZnSe passivation layers are coated onto the TiO2/CuInS2 electrodes by successive layer ionic absorption and reaction (SILAR) method. For comparison, the ZnS passivation layers are also investigated. The UV–vis spectra are employed to analyze the effect of optical properties. Under solar light illumination, the electrochemical properties and photovoltaic performance of the ZnSe passivation layer coated CuInS2 QDSSCs were studied.
Section snippets
Preparation of MPA-capped CuInS2 QDs
CuInS2 QDs were prepared by the solvothermal method as previous report [23]. In typical, 0.1 mmol cuprous chloride (CuCl) and 0.1 mmol indium chloride (InCl3) were dissolved in oleylamine (OA), and maintain at 120 °C for 1 h under vigorous stirring. Then this solution was immediately injected into the Teflon-lined stainless steel autoclave containing 100 mL hexane with another OA solution, inside which 10 mmol sulfur (S) was dissolved. The solvothermal preparation of 3.5 nm CuInS2 QDs was conducted at
Results and discussion
The controllable solvothermal method and ligand-exchange process is employed to prepare the desired CuInS2 QDs. Fig. 1a shows the TEM image, which indicates the uniform size of about 3.5 nm for the QDs. The lattice fringe of the QDs is about 0.196 nm corresponding to the (2 0 4) crystal plane of CuInS2. In order to further identify the size of CuInS2 QDs, the UV–vis absorption spectrum and PL spectrum of the sample have been investigated in Fig. 1b. It can be found that the UV–vis absorption edge
Conclusions
In summary, the CuInS2 QDs sensitized TiO2 solar cells have been fabricated through assembly linking technique, followed by SILAR technique to deposit ZnS or ZnSe passivation layers on the surface. The optical absorption edge and PL peak have small red-shift of 10–20 nm with the passivation layer coating. The CuInS2 QDSSCs with ZnSe passivation layer coating have the better photovoltaic performance and IPCE peak response than that of pure CuInS2 QDSSCs or with ZnS passivation layer coating,
Acknowledgements
This work is supported by the National Basic Research Program of China (No. 2009CB939704), Defense Industrial Technology Development Program (No. B1420110168), the National Nature Science Foundation of China (No. 51072152, and the A3 Foresight Program – No. 51161140399), Science-Technology Chenguang Foundation for Young Scientist of Wuhan (201150431087), and the Fundamental Research Funds for the Central Universities (2012-IV-007).
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These authors contributed equally to this study and share first authorship.