bUniversidad de Guanajuato, Guanajuato 36000, México. Abstract: Energy conversion of 3% was achieved in Gratzel-type solar cells. These solar cells.
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Latin America Optics and Photonics Conference (LAOP) © OSA 2014
Quantum Dots Solar Cells of CdS Deposited by Chemical Bath Method Alejandro Martíneza, Issac Zarazuaa, Diego Esparzaa, Andrea Cerdana, Tzarara López-Lukea, Elder de la Rosaa. a
Centro de Investigaciones en Óptica, Guanajuato 37150, México b Universidad de Guanajuato, Guanajuato 36000, México
Abstract: Energy conversion of 3% was achieved in Gratzel-type solar cells. These solar cells were made in a tick TiO2 transparent layer (9 m) sensitized with CdS using the chemical bath method. 1. Introduction Since the report of Grätzel et. al. the investigation of liquid solar cells in base of TiO2 or ZnO like a host material have has been increasing the studies in this type of solar cells [1]. The TiO2 material help with an efficient transport of charge making the solar cells capable of reach high theoretical efficiency, because of that is reported a solar cell using a thick layer of TiO2 capable of host larger quantity of sensitizer material. One inconvenient in the Grätzel solar cells is the high cost of the dye, because of that have raised a branch that study semiconductors materials to decorate the solar cells and help in the electron and energy harvesting. In this case were used CdS because it can photogenerate electrons with quantum efficiencies huger than 60% in the visible region. To deposit the material was used the chemical bath deposition (CBD), these technique has been thoroughly studied to deposit a god quality material [2]. The advantages of this technique are the capacity to deposit any surface in contact with the bath, simple and could be made in near ambient conditions. 2. Experimental procedure The solar cells were made in FTO substrates, previously sonicated for 15 minutes in a solution of isopropyl alcohol and acetone, depositing three layers of TiO2. 1) a thin compact film of TiO2 deposited by spray pyrolysis using solution made of 12 gr. of titanium isopropoxide, 12 gr. of acetyl acetone and 16 gr. of ethanol absolute, that was sprayed over the FTO at 450°C. 2) A transparent layer consisting in 20 nm TiO2 particles that were deposited by doctor blading a commercial TiO2 paste (purchased to Dyesol). And 3) a opaque layer made of 200nm TiO2 particles also deposited by doctor blading dyesol paste. After the deposit, the films were thermally treated at 450°C for 30 min. In the CBD vessel was placed a FTO with three layer of TiO2 to sensitize with CdS. The reagents used to deposit de CdS were 4ml of NH4OH at 30%, 10 ml of Cd acetate dihydrate at 0.05M and 10 ml of thiourea at 0.5 M in a final volume of 50 ml. These reagents were mixed, in the order mentioned, at temperature ambient and the substrates were take off at 1, 6, 9 hrs. The measurements made to characterize the solar cell were: X-ray diffraction (XRD) to determine the crystalline structure of the material, UV-Vis was made to determine absorption of QD’s, curve current-voltage (I-V) was measured to determine the parameters of the solar cell, Incident photon-to-current conversion efficiency (IPCE) help to analyze the response of the solar cell at different wavelengths and scanning electron microscope (SEM). 3. Results In the SEM images were measured the thick of the films, obtaining a transparent layer of 9 m and an opaque layer of 6 m. The figure 1 shows the measurements of XRD of the thick TiO 2 layer, with anatase phase, over FTO (red color) and sensitized with CdS by CBD (black color). Bottom of the figure shows bars in positions of the diffraction peaks of CdS, PDF # 01-089-0440 corresponding to CdS zincblende phase.
LTh4A.37.pdf
Latin America Optics and Photonics Conference (LAOP) © OSA 2014
Intensity (arb. units)
TiO2 + CdS TiO2
20
30
40
50
60
70
80
2 (degrees) Fig. 1 Measure of TiO2 and TiO2 sensitized with CdS by CDB by 6 ours.
The measurements of UV-Vis gave results the absorption edge of CdS with an energy gap of 2.5 eV. In the figure 2 shows the I-V measurements of the solar cells sensitized with CdS by CBD, with these measurements were calculated the Vsc, Joc, F.F., and the efficiency of the solar cell. According to these measurements the cells with more time in the bath have more Jsc, this is associate a mayor quantity of CdS sensitizing the TiO2, at the same time the curve I-V was lost a traditional form associated to superficial defects in the quantum dots deposited. 10 9
2
-3
J, [A/cm x 10 ]
8 7 6 5 4 3 2 1 0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
V,[Volts] Fig. 2 Measure of current-voltage of the solar cells sensitized with CdS by CDB.
In the figure 3 are show the IPCE measurement of the solar cells. The quantum efficiency of the cells have a maximum of electron harvesting in the range of 400 to 500 nm that is the region were the material begin to absorb the light incident due to the bang gap of CdS (2.5 eV). Could be see how the edge of the IPCE, is extended to mayor wavelength indicating a growing of the crystallites deposited in the CBD, this increasing in the crystal size make an Eg near to the bulk CdS.
LTh4A.37.pdf
Latin America Optics and Photonics Conference (LAOP) © OSA 2014
80 70 60
IPCE (%)
50 40 30 20
CBD-1h 10 CBD-6h CBD-9h 0 400 450
500
550 (nm)
600
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700
Fig. 3 Measure of IPCE of the solar cells sensitized with CdS by CDB.
The results obtained in this work indicate that the solar cells fabricated have god response with 3%efficiency, taking account that in some cases have small efficiencies like 0.34 [3], or efficiency of 1.19 in [4]. These was obtained as result of the thick increase of the film that permits large incorporation of QD’s in the tick film, adding to that the crystal size play a roll important increasing the current harvesting electrons at mayor wavelengths, like was observed in the IPCE measurements. 4. Conclusions Were made tick films of TiO2 using doctor blade method. Successfully fabrication of solar cells sensitized with CdS using the chemical bath deposition method. The solar cells deposited have good response reaching a conversion efficiency of 3%. Acknowledgments We acknowledge financial support from CONACYT through grant 134111, the UC-MEXUS program grant 00007, the European Community Seven Framework Program (FP7- 428 NMP-2010-EU-MEXICO), CIO-UGTO 2013 y 2014 and the CEMIE-Solar (04002) consortium. Alejandro Martinez, Diego Esparza, and Andrea Cerdan acknowledge scholarship from CONACYT. 5. References [1] Brian O'Regan and Michael Grätzel, “A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films”, Nature 353, 737 - 740, (1991). [2] Manisree Majumder and Biswanath Mallik, “Influence of annealing on the surface semiconductive and photoconductive properties of nanostructured cadmium sulfide thin films”, J Mater Sci, 47, 5833–5844, (2012): [3] Abdul Razzaq, Jun Young Lee, Bhaskar Bhattacharya, Jung-Ki Park, “Surface treatment properties of CdS quantum dot-sensitized solar cells”, Appl. Nanosci. 4, 745–752, (2014). [4] Na Zhou, Guoping Chen, Xiaolu Zhang, Luyao Cheng, Yanhong Luo, Dongmei Li, “"Highly efficient PbS/CdS co-sensitized solar cells based on photoanodes with hierarchical pore distribution", Electrochemistry Communications 20, 97–100, (2012)
