Aqueous synthesis of colloidal CdSe_xTe_1-x – CdS core–shell nanocrystals and effect of shell formation parameters on the efficiency of corresponding quantum dot sensitized solar cells

Highlights

• CdSeTe and CdSeTe-CdS core–shell NCs were synthesized through an aqueous approach.

• The synthesis and shell-making conditions were controlled for proper optimizations.

• The fabricated QDSC with TiO2/CdSeTe/CdS photoanode showed a maximum PCE of 4.2%.

• The efficiency was enhanced to 4.6% by application of the optimized CdSeTe-CdS NCs.

• This PCE was 10% and 74% improved compared to the CdSeTe/CdS and CdS sensitized cells.

Abstract

In this research CdSe_0.4Te_0.6 NCs were synthesized in aqueous solution through a cheap, simple, and modified hot injection chemical precipitation method. The reflux time was altered in a wide range for the synthesis of these NCs with different size and bandgap energies. Then they were applied as a co-sensitizing layer together with CdS NCs film in the photoanod of quantum dot sensitized solar cells (QDSCs). It was shown that the QDSC with TiO₂ NCs/CdSeTe(7 h)/CdS/ZnS photoelectrode demonstrated the maximum power conversion efficiency (PCE) of 4.2% in AM1.5 solar irradiation. The selected CdSeTe NCs prepared in 7 h of the reflux time were utilized in a second growth process to form core–shell CdSeTe-CdS NCs. The shell formation time was widely altered and NCs were applied in the photoanod of corresponding QDSCs. It was demonstrated that the cell with TiO₂ NCs/CdSeTe(7 h)-CdS(100 min)/CdS/ZnS photoelectrode revealed the best efficiency of 4.6% in the experiments. This was due to the surface passivation of the sensitizing NCs and better charge collection efficiency.

Introduction

Quantum dot sensitized solar cells (QDSCs) have been under investigation in two recent decades (Nideep et al., 2019). The researches in this area started with the article published by Vogel in 1991 which reported a novel TiO₂/CdS/electrolyte/Pt structure for the photovoltaic (PV) applications. The different components of this kind of nanostructured solar cells were quite similar to those of the well-known dye sensitized solar cells (DSCs) (Ahmad et al., 2017, Ahmad et al., 2017, Ahmed et al., 2018, Grätzel, 2003). Nevertheless the dye molecules were substituted with semiconductor quantum dots (QD) which were utilized as the efficient light sensitizers. These QDs could be easily synthesized and deposited on TiO₂ mesoporous scaffold by a variety of low-cost methods (Marandi et al., 2017, Wang et al., 2016, Bo et al., 2014, Kouhnavard et al., 2014, Halim, 2013, Mahdy and El Zawawi, 2016). Besides, they own a considerable absorption in a wider range of solar spectrum compared to the well-known, mostly utilized dye sensitizing molecules (Lai et al., 2014, Cheng and Yeh, 2012, Yu et al., 2003). The multiple exciton generation (impact ionization) is another advantage of these semiconductor QDs. As a result, the high energy photons could generate more than one electron-hole pair through the absorption. Consequently, the light absorption is more efficient and photon to current conversion efficiency (IPCE) can be well-increased (Kumara et al., 2017, Zhao and Rosei, 2017). The bandgap energy of the semiconductor QDs is also tunable with corresponding dimension (Quantum Dot, 2015, Shionoya and Yen, 2013, Bockelmann, 1992). As a result, the band edge positions are also changing and could be tuned for better electron/hole transport inside the cells (Chand et al., 2019, Badawi, 2018, Kamat et al., 2010, Albero et al., 2014). This could create a possibility for optimization of light absorption and charge carriers transport in this photovoltaic devices (Kumara et al., 2017, Kamat, 2018, Nozik et al., 2010).

Several researches have been carried out on different components of QDSCs to improve the efficiency. Application of different nanostructures in wide bandgap scaffold (Wu et al., 2014, Huang et al., 2013, Du et al., 2016, Tian et al., 2013), utilizing various light scattering components in the photoanode (Ahmad et al., 2017, Marandi et al., 2017, Marandi et al., 2018, Marandi and Bayat, 2018, Shen et al., 2015), co-sensitization of photoelectrode with several semiconductor QDs (Kim et al., 2014, Marandi and Mirahmadi, 2019), application of different recombination passivating layers on the photoanode (Marandi and Mirahmadi, 2019, Marandi and Mirahmadi, 2019, Liua et al., 2020), fabrication of more efficient

electrolyte compositions (Esparza et al., 2017, Ma et al., 2018, Yu et al., 2017) and various nanostructured counter electrodes (Chang and Lee, 2007, Jun et al., 2013, Naresh Kumar et al., 2016, Tachan et al., 2011, Ahmed et al., 2018, Quanxin et al., 2010) have been vastly investigated.

The light absorption in QDSCs is mainly performed by one or several successive, different QDs layers (Zhengji et al., 2012, Liua et al., 2020, Emin et al., 2011, Esparza et al., 2015, Gonzalez-Pedro et al., 2010), CdS (Zhang et al., 2012, Marandi et al., 2019, Toyoda et al., 2013), CdSe (Kim et al., 2014, Marandi and Mirahmadi, 2019, Liua et al., 2020, Marandi et al., 2019, Toyoda et al., 2013, Shalom et al., 2009), PbS (Liu and Kamat, 1993, Sh Pan et al., 2017, Ahangarani Farahani and Marandi, 2017), ZnSe (Xinga et al., 2020, Asgari Fard and Dehghani, 2019), CdTe (Marandi and Mirahmadi, 2019, Marandi et al., 2019, Huang et al., 2016, Mobedi et al., 2014) and other QDs have been explored for sensitization/co-sensitization of the photoelectrodes. The power conversion efficiencies were achieved in the range of 0.53%–12% (Nideep et al., 2019, Liu and Yu, 2009, Gopi et al., 2016, Gopi et al., 2015, Yang et al., 2015) and 2.8%–10% (Sh Pan et al., 2017, Marandi and Mirahmadi, 2019, Liua et al., 2020, Chen et al., 2014, Chen et al., 2014, Esparza et al., 2017, Zhou et al., 2016, Yuan et al., 2016, Mu et al., 2013) for the single QDs sensitization or co-application of several QDs layers, respectively. In this area, the application of CdSe QDs layer together with CdS sensitizing film demonstrated the highest PCEs and better PV performances (Asgari Fard and Dehghani, 2019). This is due to the lower bandgap energy of CdSe and successful deposition method of this quantum dots layer (Marandi and Mirahmadi, 2019, Xinga et al., 2020, Li et al., 2012, Justin Raj et al., 2014, Chen et al., 2009, Jina et al., 2019). Besides, the bands edge positions are quite favored for the photoexited electrons to be transferred to the CdS and TiO₂ layer (Kamat, 2018, Xinga et al., 2020). The hole charge carries could also be simply moved from the CdS/CdSe layers to the electrolyte redox level and were transported (Kamat, 2018, Aung Kyaw et al., 2019, Peng et al., 2018). The application of some electron blocking layer (EBL) like ZnS or SiO₂ was even effective on considerable efficiency enhancement (Pawara et al., 2016, Ruhle et al., 2012, Tachan et al., 2013, Shen et al., 2008).

Meanwhile the absorption area of the QDs sensitizing films could be extended to the near infrared region (Fuente et al., 2013, Pan et al., 2013, Mussa Farkhani and Valizadeh, 2012, Brus, 1984). This was a great achievement and was carried out based on using ternary CdSe_xTe_1-x, (x = 0–1) compositions (Zhang et al., 2003, Michelle and Han, 2010, Korgel and Monbouquette, 2000, Harrison et al., 2000, Zheng et al., 2007, Zhong et al., 2003, Liu et al., 2018). The bandgap energy of this material is not calculated as the “X E_g (CdSe) + (1-X) E_g (CdTe)” and there is the third term of “–bX (1-X)”which lowers this energy (Robert et al., 2003, Özdemir, 2009, Gao and Gaoa, 2013, Lemos et al., 1986). This can result in a higher light absorption and better power conversion efficiency for corresponding cells (Fuente et al., 2013, Weia et al., 2000). The conventional approach for the synthesis of CdSe_xTe_1-x Nanocrystals (NCs) is the well-known high- temperature organometallic method (Gopi et al., 2015, Fuente et al., 2013, Robert et al., 2003, Wei et al., 2015). This carried out due to the high crystalinity of the as-prepared QDs and controllable X value (Liu et al., 2018, Robert et al., 2003, Özdemir, 2009, Wan et al., 2011, Lianga and Zhu, 2015). Besides, the growth can be well-progressed in high temperature and create the absorption edges above 750 nm for these NPs (Fuente et al., 2013, Liu et al., 2018, Robert et al., 2003, Özdemir, 2009, Wei et al., 2015, Wan et al., 2011, Lianga and Zhu, 2015, Bailey and Nie, 2003). These NCs are finally ligand exchanged and deposited on TiO₂ mesoporous layer by drop-casting method (Lianga and Zhu, 2015, Bailey and Nie, 2003). The PCE of fabricated cells with this sensitizing layer was reported in the high range of 4.0%−9.0% in the literature (Gopi et al., 2015, Fuente et al., 2013, Weia et al., 2000, Wei et al., 2015, Wan et al., 2011, Lianga and Zhu, 2015, Bailey and Nie, 2003). Nevertheless, the synthesis method is not that easy and applied precursors are quite expensive and toxic (Weia et al., 2000, Wei et al., 2015, Wan et al., 2011, Lianga and Zhu, 2015, Bailey and Nie, 2003).

Consequently, the aqueous synthesis of CdSe_xTe_1-x QDs is an easy and cheap alternative for fabrication of corresponding QDSCs (Robert et al., 2003, Wan et al., 2011, Herrera et al., 2018). There are some reports on the synthesis of these alloyed QDs in aqueous solution (Robert et al., 2003, Lewis, 2004, Piven et al., 2008, Fan et al., 2016, Luo et al., 2014). Nevertheless, there are a few published researches on the application of CdSeTe QDs synthesized in water, in the photoelectrode of the QDSCs (Wan et al., 2011, Tang et al., 2015). The recorded efficiencies are also in the range of 2.1%−5.0% which are lower than those of the cells with NCs prepared in high temperature, non-aqueous solutions (Fan et al., 2016). Meanwhile the aqueous synthesis is still attractive due to the mentioned advantages and could be more improved (Fan et al., 2016).

In this research, we have synthesized the CdSe_xTe_1-x NCs in aqueous solution through a modified, hot injection chemical precipitation method. The refluxing time was altered in a wide range of 1–9 h for the synthesis of NCs with different size. These alloyed NCs were applied as a light sensitizing layer in a TiO₂ NCs/CdSeTe(Xh)/CdS/ZnS photoanode structure of the quantum dot sensitizing solar cells. According to the results the optimized QDSC with CdSeTe QDs prepared in 7 h of the reflux time demonstrated the maximum power conversion efficiency of 4.2%. In the second stage, the CdSeTe(7 h) NCs were utilized to form core–shell CdSeTe/CdS NCs for the sensitization. The refluxing time in the shell making process was also changed in the time range of 0–120 min in the experiments. These core–shell NCs were applied as the co-sensitizers and photovoltaic performance of corresponding QDSCs were investigated. It was shown that the cell with TiO₂ NCs/CdSeTe-CdS(100 min)/CdS/ZnS photoanode represented the best PV parameters of J_sc = 13.93 mA/cm², V_oc = 638 mV, FF = 0.51 and PCE of 4.6%. This efficiency was improved about 10% compared to that of the cell with including CdSeTe NCs as the light sensitizing layer.