CdS quantum dots pre-deposition for efficiency enhancement of quantum dot-sensitized solar cells

CdS quantum dots pre-deposition for efficiency enhancement of quantum dot-sensitized solar cells

Solar Energy 188 (2019) 825–830 Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener CdS quantu...

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Solar Energy 188 (2019) 825–830

Contents lists available at ScienceDirect

Solar Energy journal homepage: www.elsevier.com/locate/solener

CdS quantum dots pre-deposition for efficiency enhancement of quantum dot-sensitized solar cells ⁎

T



Mahmoud Samadpoura, , Hieng Kiat Junb, , Parisa Paranda, M.N. Najafic a

Physics Department, K. N. Toosi University of Technology, Tehran, Iran Department of Mechanical and Material Engineering, Universiti Tunku Abdul Rahman, Sungai Long Campus, Bandar Sg. Long, 43000 Kajang, Malaysia c Department of Physics, University of Mohaghegh Ardabili, P.O. Box 179, Ardabil, Iran b

A R T I C LE I N FO

A B S T R A C T

Keywords: Solar cell Quantum dot Sensitizer Life time Impedance spectroscopy

Here we report a general study on the effect of various semiconductor sensitizers i.e. CdS, CdSe, CdS/CdSe, and PbS/CdS on the performance of quantum dot sensitized solar cells (QDSCs). The electron life time in the cells is investigated by applied bias voltage decay method. We clearly indicate that the electron life time could be considerably enhanced by CdS pre-deposition in CdS/CdSe QD sensitized cells. The charge transfer properties of cells are investigated by impedance spectroscopy and it is shown that a clear downward shift in TiO2 conduction band take place through CdS QDs sensitizer pre-deposition. According to the results, type and the sequence of QDs deposition has a clear effect on the recombination resistance in cells. Here the efficiency of solar cells was improved to 3.63% by CdS pre-deposition which is more than 25% improvement in comparison to CdSe sensitized cells (2.84%). This general study indicates that tuning the sensitizer type can be systematically used in QDSCs to increase the electron life time and consequently the cells’ performance in a simple and cost effective way.

1. Introduction Recently, environmentally clean energies have been considered noticeably in order to fulfill the growing demand of the renewable energy. Among the various energy sources, photovoltaic solar power is one of the most promising ones which has been considerably developed during the past decades. In the last few years, third-generation photovoltaic devices like dye and quantum dot sensitized solar cells have attracted substantial attention (Mathew et al., 2014; Hagfeldt et al., 2010; Sherafati-Tabarestani and Samadpour, 2019; Hod et al., 2011; Samadpour and Arabzade, 2017; Pishdar and Samadpour, 2017; Arabzade et al., 2015; Samadpour et al., 2012). This is mainly attributed to the unique properties of QDs like high extinction coefficients, low cost and especially the tunable opto-electronical properties by controlling their shape and size (Bhattacharyya et al., 2000; Kongkanand et al., 2008; Bailey and Nie, 2003; Govorov and Kalameitsev, 2005). In QDSCs, semiconductor QDs, are employed as light harvesting materials. After light absorption, the photogenerated electrons are injected into the TiO2 conduction band and subsequently transported to the external circuit. In spite of the interesting properties of QDs, the real performance of QDSCs is far below the theoretical thermodynamic



predictions. Literature review indicates that, considerable recombination in QDSCs is a key parameter which hinders the further efficiency improvements and need to be considered for further developments (Barea et al., 2010; Samadpour, 2018; Zhao et al., 2016; Samadpour et al., 2015; Mora-Sero et al., 2009; Samadpour and Molaei, 2014; Samadpour et al., 2013). It is known that a considerable density of deep/surface trap states is presented in QD sensitizers which enhances the charge recombination and consequently reduces the cells’ photovoltaic properties (Mora-Sero et al., 2009; Kim et al., 2011; Guijarro et al., 2011; Yang et al., 2011). Until now various methods are utilized to improve the electron life time in QDSCs. For example the QD surface ligands were modified in order to enhance the recombination resistance in the cells (Zarghami et al., 2010; Tang et al., 2011; Ip et al., 2012). The photovoltaic device architecture has been modified by various researchers in order to enhance the electron life-time in QDSCs. For instance, it was shown that inverse opal structures could enhance the electron life time in QDSCs (Diguna et al., 2007; Samadpour et al., 2012). One dimensional structures have also been extensively investigated in order to improve the charge transport in the cells and reduce the recombination chances (Seol et al., 2010; Sudhagar et al., 2009). It is shown that a thin layer of amorphous TiO2 could reduce the charge recombination in a clear way (Shalom et al., 2009).

Corresponding authors. E-mail addresses: [email protected] (M. Samadpour), [email protected] (H.K. Jun).

https://doi.org/10.1016/j.solener.2019.06.078 Received 22 April 2019; Received in revised form 5 June 2019; Accepted 28 June 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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deposition (Lee et al., 2009). For each SILAR cycle, TiO2 substrates were dipped into the Cd2+ and Se2− precursors, respectively. The time of dip coating for CdSe deposition was 30 s for both cadmium and selenide precursor bathes. After each Cd2+/Se2− precursor bath, photoanodes were rinsed by ethanol and subsequently dried by an Ar/N2 gun. Here various type and sequence of QDs containing CdS, CdSe, PbS/CdS and CdS/CdSe were deposited on TiO2 electrodes. CdS and CdSe were deposited by 5 and 7 SILAR cycles respectively. PbS/CdS structures were made by 2 and 5 SILAR cycles of PbS and CdS respectively. CdS/CdSe structures were made by 2 and 5 SILAR cycles for CdS and CdSe respectively. In order to enhance the stability of cells, all sensitized electrodes were coated by ZnS as explained before (Samadpour, 2017). Here 0.1 M Zn(CH3COO)2 and Na2S solutions were utilized as Zn2+ and S2− precursors. CuS/PbS cathode electrodes were prepared by the sequential deposition of CuS and PbS layers on the FTO coated glass substrates according to the method that we explained recently (Arabzade et al., 2015). Polysulfide electrolyte was prepared by dissolving 2 M Na2S, 2 M S, and 0.1 M NaOH solution in Milli-Q ultrapure water under stirring at room temperature. Some samples were used to be sensitized by N719 dye molecules in order to make DSSCs and comparing their electron life time with QDSCs. In these samples, TiO2 electrodes were dipped in an ethanolic solution of N719 dye for 24 h. After that, substrates were rinsed by ethanol and dried by Ar pressure.

Considerable improvement in open circuit voltage is obtained through reducing the recombination by simple silica treatment (Zhao et al., 2015). Surface passivation by ZnS treatment is generally accepted as an effective method for improving the stability and charge recombination resistance of the cells (Guijarro et al., 2011; Shen et al., 2008; Samadpour, 2017). It is mentioned that the redox electrolyte chemical composition could determine the electron life time in QDSCs (Li et al., 2011; Lee and Chang, 2008). It is observed that the thickness of the CdSe sensitized layer has a crucial role on the recombination rate in QDSCs (Kongkanand et al., 2008). QD surface passivation through the use of halide ions, led to a noticeable improvements in power conversion efficiencies of QDSCs (Ip et al., 2012; Yuan et al., 2013) which was explained by the reduced density of trapped charge carriers. Until now, various type of semiconductor QDs with different optical properties are utilized in order to enhance the light harvesting in cells. Improving the light harvesting, enhances the rate of the photogenerated electrons in cells, and consequently efficiency increases. Literature review indicates that in spite of the various studies, the effects of type and the sequence of QDs deposition on the recombination haven’t been investigated in a clear way. Here a systematic investigation on the effects of the type and the sequence of QDs deposition on electron life time was performed. A general investigation was performed by using various QD light absorbing materials: CdS, CdSe, PbS/CdS and CdS/CdSe. Electron life time was also investigated in dye sensitized solar cells (DSSCs) for comparison. The charge transfer properties of the cells were investigated by impedance spectroscopy method and it showed that a clear downward shift in TiO2 conduction band took place through dye/QDs sensitizer deposition. A significant improvement in photovoltaic properties was obtained, by engineering the sequence of the sensitizer deposition. The results presented here highlight a key finding which is the capability of tuning the photo electrochemical properties of cells by adjusting QD sensitizers without any further modifications. Especially it is shown that CdS pre-deposition has a clear effect on enhancing the performance of CdS/CdSe sensitized cells which is originated from the reduced charge recombination. As a general conclusion, it was shown that tuning the sensitizer’s type can be systematically used in QDSCs in order to enhance the cell performance in a simple and cost effective way.

2.2. QDSC preparation and characterization QDSCs were prepared by sealing the cathode and working electrodes by a 50 µm surlyn spacer. Electrolyte was injected through the cells from the hole that was made on the cathode. After injection, the hole was sealed by surlyn thermoplastic and a piece of thin glass. The area of cells was 0.3 cm2. The optical transmission spectrum of the photoanodes was recorded by an Avantes spectrometer. Current-voltage properties of cells, and impedance spectroscopy were measured by PGSTAT-30 from Autolab Company. Open circuit voltage decay measurements were performed by applying a bias voltage to the cells and monitoring the subsequent decay in the dark. The applied bias voltage decay (ABVD) analysis were also performed with PGSTAT-30 potentiostat from Autolab Company. Cells were illuminated utilizing a solar simulator from Sharif Solar Company. Measurements were performed at AM1.5 G under light intensity of 100 mW/cm2. Impedance spectroscopy (IS) measurement, were carried out at different applied biases at dark condition. For impedance measurements, frequency was scanned between 300 kHz and 0.1 Hz.

2. Experimental methods 2.1. Photoanode and cathode preparation Photoanode electrodes are prepared by the deposition of two TiO2 layers on the fluorine doped tin oxide (FTO) glass substrates. TiO2 layers were prepared by T20 pastes from Sharif Solar Company. After TiO2 deposition, photoanodes were annealed in 450 °C for one hour. All semiconductor sensitizers were grown on the nanostructured TiO2 layers by successive ionic layer adsorption and reaction (SILAR) deposition method. For CdS deposition, an ethanolic 0.05 M solution of Cd (NO3)2 × 4H20 and a 0.05 M solution of Na2S × 9 H2O were utilized as Cd2+ and S2− sources, respectively. For each CdS SILAR cycle, TiO2 electrodes were dipped for one minute in Cd2+ and S2− precursors respectively. Between each precursor bath, samples were rinsed by ethanol and were dried by Ar/N2 gas pressure. PbS QDs were deposited from a 0.02 M methanolic solution of Pb (NO3)2·4H2O as a Pb2+ source and a 0.02 M solution of Na2S × 9 H2O in methanol/water (50/50 V/V) as a S2− sources. A single SILAR cycle for PbS consisted of one minute dipping of the TiO2 electrode into the Pb2+ precursor and subsequently into the sulfide solution. For CdSe deposition an ethanolic Cd(NO3)2 solution (0.03 M) was utilized as Cd2+ precursor. Also a 0.03 M Se2− was utilized as the Se2− precursor. In order to prepare the Se2− precursor solution, SeO2 was reduced by NaBH4 in ethanol under Ar/N2 atmosphere. The solution was then transferred into a glove box in order to perform the SILAR

3. Results and discussion Fig. 1 indicates the SEM micrograph from the layers which are made by the TiO2 paste. Based on Fig. 1, size of the TiO2 nanoparticles is around 30 nm which prepares enough surface area for adsorption of dye/QD sensitizers. Fig. 2 indicates the current-voltage properties of cells which are sensitized by CdS, CdSe, PbS/CdS, and CdS/CdSe QDs. All measurements were performed under 100 mW/cm2 light intensity at standard AM 1.5G condition. The photovoltaic parameters which were derived from Fig. 2 are presented in Table 1: photocurrent jsc, open circuit voltage Voc, fill factor FF, and efficiency E. According to this Table, the efficiencies are below the maximum efficiencies which are explained before in the literature (Zhao et al., 2016; Wang et al., 2019; Zhang et al., 2017). It is important to note that the champion efficiency cells, usually utilize various non-simple strategies like sensitization with pre synthesized semiconductor QDs, performing various surface engineering and etc. (Wang et al., 2019; Zhang et al., 2017), which are not general among researchers and not investigated here. According to Table 1, a low current density is observed for CdS 826

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Fig. 3. Transmission spectrum of CdS, PbS/CdS, CdSe and CdS/CdSe sensitized cells.

enhancing the light absorption in the photoanode (Fig. 3). More than 10 mA/cm2 current density is obtained in CdSe QDSCs which is raised from superior light absorption properties of CdSe in comparison to CdS QDs (Fig. 3). On the other hand while the current density and open circuit voltage of CdSe and CdS/CdSe sensitized cells are almost same (Table 1 and Fig. 2), considerably higher fill factors are obtained in CdS/CdSe sensitized cells (0.62) in comparison to CdSe sensitized cells (0.47) which will be investigated in more details later. Electron life time of CdS, CdSe, PbS/CdS, and CdS/CdSe sensitized cells is presented in Fig. 4. From this figure higher electron life time is observed for CdS sensitized cells in comparison to CdSe sensitized cells. This could be explained by the higher recombination resistance in CdS QDs in comparison to CdSe QDs which will explain in Fig. 8. Comparison of electron life time between CdSe and CdS/CdSe sensitized cells indicates that CdS pre-deposition has an effective role on improving the electron life time in co sensitized CdS/CdSe cells. This result is in good agreement with the superior current voltage properties (especially fill factor) of the CdS/CdSe sensitized cells in comparison with the CdSe sensitized cells as discussed before (Table 1). Based on Fig. 4, it is observed that the type of QD sensitizer has a clear effect on

Fig. 1. SEM micrograph from the layers which are made by TiO2 paste.

Fig. 2. Current-voltage properties of cells which are sensitized by various QD sensitizers: CdS, CdSe, PbS/CdS and CdS/CdSe.

Table 1 Photovoltaic parameters of the QDSCs: photocurrent jsc, open circuit voltage Voc, fill factor FF, and efficiency E, as a function of the sensitizer type. Sensitizer Type

Voc (mV)

Jsc (mA/cm2)

FF

E (%)

CdS PbS/CdS CdSe CdS/CdSe

493 459 556 556

4.34 8.55 10.86 10.52

0.53 0.47 0.47 0.62

1.13 1.84 2.84 3.63

sensitized cells. This is attributed to the low photogenerated electron efficiency in cells due to the low light absorption by CdS QDs as it is clearly shown in the transmission spectrum of cells (Fig. 3). A clear improvement in the cells’ current density is observed in PbS/CdS sensitized cells compared to the CdS sensitized ones which is expected from

Fig. 4. Electron life time of CdS, PbS/CdS, CdSe and CdS/CdSe sensitized cells. 827

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Fig. 5. The electron life time for the cells which are made by the bare TiO2 in polyiodide/polysulfide electrolyte (no sensitizer), CdS sensitized and dye sensitized solar cells.

the electron life time in QDSCs. This could be originated from the different band structure and/or the density of surface trap states in semiconductor QDs. It is known that QDs have a considerable density of deep and surface trap states with different energy levels (Barea et al., 2010; Rühle et al., 2010; Hod et al., 2011). In comparison to QDs, dye molecules have a clear HUMO and LUMO energy levels. Consequently they are utilized in DSSCs without concerning about the trap states (Kim et al., 2013; Kumara et al., 2017; Matsui et al., 2017; Tian et al., 2010). In order to have a clear discussion, here also the effect of organic dye sensitizers on the electron life time was investigated in comparison to semiconductor QDs. Fig. 5 indicates the electron life time of the cells which are made by the bare TiO2 in polyiodide/polysulfide electrolyte (no sensitizer), CdS sensitized, and dye sensitized solar cells. According to Fig. 5, more recombination is observed at the TiO2/ polysulfide interface in comparison to the TiO2/polyiodide interface. This result proves the superior properties of typical polyiodide electrolytes compared to the polysulfide ones which are generally utilized in QDSCs. Consequently the full coverage of TiO2 surface with semiconductor sensitizers is more critical in comparison to dye sensitized cells. According to Fig. 5, sensitizing the photoanodes by dye molecules has improved the electron life time in comparison to the bare TiO2/ polyiodide electrolyte in a clear way. Here the electron life time in dye sensitized cells is more than CdS sensitized cells which could be explained by the high density of trap states in QDs in comparison to the dye molecules with clear HUMO and LUMO energy levels (Rühle et al., 2010; Hod et al., 2011; Kim et al., 2013; Kumara et al., 2017; Matsui et al., 2017; Tian et al., 2010). In order to understand the effect of sensitizer type on the electron life time in dye/QD sensitized cells, impedance spectroscopy measurements is carried out on dye/QD sensitized cells under various applied biases. Fig. 6 indicates the typical Nyquist plots and the corresponding fitting results at 0.55 and 0.60 V bias voltages for the cells with no sensitizer and polyiodide electrolyte (Bare TiO2). Fitting model is presented in Fig. 6b (González-Pedro et al., 2010). According to Fig. 6a, the experimental data are fitted by the proposed model (Fig. 6b) in a clear manner. The equivalent circuit model, shown in Fig. 6b, consists Rs, Rtr, Rre, Rcathode, Cµ, and Ccathode elements which are the series resistance, electron transport resistance in the photoanode, electron recombination resistance at the photoanode/electrolyte interface, charge transfer resistance at the cathode/electrolyte interface, chemical capacitance of

Fig. 6. (a) Typical Nyquist plots/Fitting results at 0.55 and 0.60 V bias voltages for cells with no sensitizer and polyiodide electrolyte (Bare TiO2). (b) Fitting model; Rs, Rtr, Rre, Cµ, Rcathode, and Ccathode are the series resistance, electron transport resistance, electron recombination resistance, chemical capacitance of the photoanode, charge transfer resistance at the cathode/electrolyte interface and the cathode chemical capacitance, respectively.

the photoanode, and the cathode chemical capacitance, respectively. Such Nyquist plots were examined and fitted for various cells at 0, 0.05, 0.1, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, and 0.70 V bias voltages (All curves are not shown here). From the fitting results, chemical capacitance and recombination resistance were measured and plotted in Fig. 7a and b respectively. Cµ is plotted against the photoanode voltage, VF. VF was obtained by subtracting the voltage drop at series resistance from the applied bias voltage in the IS measurements. According to the Fig. 7a more chemical capacitances are obtained for dye sensitized cells at the same Fermi voltage (VF) which corresponds to a downward shift of TiO2 CB band (González-Pedro et al., 2010; Fabregat-Santiago et al., 2005). In order to remove the effect of different TiO2 CB levels between the samples, chemical capacitance was plotted against the voltage drop in a common equivalent conduction band, Vecb, in the Fig. 7b (FabregatSantiago et al., 2005). Rrec is plotted against the Vecb in Fig. 7c in order to compare the recombination for the same density of electron in cells (Fabregat-Santiago et al., 2005). Vecb is obtained by overlapping the chemical capacitances in Fig. 7a as it was explained by more details previously (González-Pedro et al., 2010; Fabregat-Santiago et al., 2005). From the results in Fig. 7c, dye sensitized samples present higher recombination resistance (lower recombination) than bare TiO2 samples. This is in good agreement with considerably higher electron life times in dye sensitized cells in comparison to the cells with no sensitizer (Fig. 5). The Nyquist plots were also obtained for CdS, PbS/CdS, CdSe and CdS/CdSe sensitized cells at various forward biases. Here also the results were fitted by the model that was explained before in Fig. 7b. Fig. 8a indicates the chemical capacitance of cells which are sensitized by CdS, PbS/CdS, CdSe, and CdS/CdSe QDs. According to Fig. 8a more chemical capacitances are obtained for CdS sensitized cells at the same Fermi voltage (Vf). This enhancement in the chemical capacitance corresponds to a downward shift of TiO2 CB 828

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Fig. 7. (a) Chemical capacitance versus the photoanode voltage, VF (b) Chemical capacitance against the voltage drop in a common equivalent conduction band, Vecb, (c) Recombination resistance, Rrec is plotted against the Vecb.

Fig. 8. (a) Chemical capacitance versus the photoanode voltage, VF (b) Chemical capacitance against the voltage drop in a common equivalent conduction band, Vecb, (c) Recombination resistance, Rrec is plotted against the Vecb.

band in CdS sensitized cells in comparison to the PbS/CdS, CdSe, and CdS/PbS sensitized cells. Fig. 8b and c indicate the chemical capacitance and recombination resistance versus the Vecb respectively. Based on Fig. 8c, a clear enhancement in the recombination resistance is observed in the CdS sensitized cells in comparison to the PbS/CdS, CdSe and CdS/PbS sensitized cells. This result indicates the crucial role of the type of QDs in determination of the both light harvesting (Fig. 3) and charge transfer

properties of cells. From the results in the Fig. 8c, higher electron life times in CdS sensitized cells could be expected as it was shown in Fig. 5. This result indicates that effective role of QDs type on the charge transfer properties in the cells. As a general conclusion it is shown here that CdS could potentially improve the life time in QDSCs.

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4. Conclusions

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