Doubling the power conversion efficiency in CdS/CdSe quantum dot sensitized solar cells with a ZnSe passivation layer
Introduction
Developing low cost and high performance solar cells for harvesting and converting solar energy to electricity is one of the most promising technologies to meet the imperative societal need for sustainable clean energy with minimal or no detrimental environmental impact [1]. Over the decades, many new technologies and materials have been investigated to meet the demand for low cost and highly efficient solar power conversion, and tremendous progress has been made [2], [3], [4], [5]. For example, organic photovoltaics have demonstrated a power conversion efficiency of 12% [6] and the perovskite solar cells reached a conversion efficiency of ~20% [7]. However, they both suffer from poor stability in ambient environment [8], [9]. Dye sensitized solar cells (DSSCs) have received intensive research over the past 25 years, due mainly to its insensitivity of impurities, inexpensive and simple fabrication process, and relatively high power conversion efficiency [10], [11], [12]. But the further development of DSSCs is limited by the light harvesting and the challenge of synthesis of new and low cost dyes [13]. Narrow band gap quantum dots (QDs) such as CdS [14], CdSe [15], [16], [17], PbS [18], [19], CdTe [20] and Ag2S [21] can be used as solar harvesters to replace dyes, due to their tunable band gaps, larger extinction coefficients, multiple exciton generation (MEG) effect with single-photon absorption, and higher stability in the water and air [22], [23]. Quantum dot sensitized solar cells (QDSSCs) can be regarded as a derivative of dye sensitized solar cells and are promising in being next generation of photovoltaics for wide spread applications.
In spite of many efforts made on the QDSSCs with much high theoretical conversion efficiency in the past, the best power conversion efficiency remains obviously lower than that of DSSCs [11]. There are many factors that influence the performance of QDSSCs, including the electrocatalytic activity of the counter electrode, the efficiency of redox couples in electrolyte, the type of QDs and the structure of mesoporous film for photoanode. However, the relatively lower efficiency was primarily attributed to high charge recombination, mainly including the recombination between the electrons in QDs and the oxidized form of the redox couples and between the electrons in the oxide (for example, TiO2) and the oxidized form of redox couples [3], [24]. The latter is due to the fact that it is very difficult to get a full and conformal coverage of QDs on the inner surface of mesoporous photoanode, so part of the bare photoanode (TiO2, for example) would have direct contact with the electrolyte permitting a leakage of electrons to the electrolyte [15], [25]. When in-situ growth methods including chemical bath deposition (CBD) and successive ionic layer absorption and reaction (SILAR) have been widely used to deposit QDs directly onto photoanode to ensure the intimate contact between photoanode and QDs and the homogeneous distribution of QDs. However, the resulting QDs, particular the surface states of the QDs are far from perfect due to the low temperature processing. Such the surface states of QDs exert an appreciable but detrimental role leading to pronounced charge recombination [26], Therefore, surface passivation of the QDs as well as the mesoporous oxide electrode with organic and/or inorganic materials has been a critically important aspect in the QDSSCs research [24], [27].
Among all the QDs, CdS and CdSe have attracted much research attention. CdS has a higher conduction band edge than the commonly used oxides such as TiO2 and ZnO that forming the photoanode. The high conduction band edge of CdS may facilitate the photo excited electrons injecting into photoanode, however its wide band gap (2.25 eV in bulk) leads to limited light absorption [15]. Compared with CdS, CdSe has a narrow band gap (1.7 eV in bulk) corresponding to the wavelength of 720 nm in visible-light region [15]. Therefore, co-sensitization of the oxide photoanode with CdS and CdSe quantum dots would allow both broad light absorption and effective charge injection, and the deposition of CdS and CdSe quantum dots can be readily achieved by SILAR, CBD [28], [29], link assisted binding of the pre-synthesized colloidal QDs [30], photodeposition, and electrophoresis methods [31]. CdS/CdSe co-sensitized solar cells have been extensively studied. So far the highest PCE of CdS/CdSe co-sensitization system with mesoporous TiO2 films as photoanodes has reached 6% [32], [33], [34], where typically a wide-band-gap semiconductor ZnS is coated to function as a passivation layer.
As for the passivation layer in QDSSCs, ZnS has been the most frequently studied inorganic passivation agent to reduce and prevent interfacial charge recombination [26], [35], [36]; while other inorganic materials such as SiO2 [37], TiO2 [38], PbClx [39] have also been reported. In addition, organic materials such as thiols [40], amines [41], and carboxylic acids [42] have also been studied to passivate the surface states of QDs by acting as electron or hole traps. All these works demonstrated that the passivation treatment is an effective way to improve the power conversion efficiency of QDSSCs. Recently, Zhong's group applied a ZnS/SiO2 barrier coating on the CdSexTe1−x QD-sensitized photoanode to inhibit interfacial charge recombination as well as to improve the cell stability, with the highest power conversion efficiency of 8.55% achieved. Their result further illustrated that the suppression of charge recombination at the oxide/electrolyte interface is a very vital aspect to improve photovoltage and photocurrent of QDSSCs [43].
ZnS used for passivation layer is because it has a more negative minimum conduction band edge (Ecb) compared to CdS/CdSe QDs, and can thus prevent electron transfering from the QDs and oxide to the electrolyte. However, since the maximum valence band edge of ZnS is more positive than those of CdS/CdSe QDs, the use of ZnS for passivation layer would undesirably retard the hole transfer from QDs to electrolyte, causing reduced charge separation in QDs [36]. In order to circumvent the detrimental impact on the charge separation and injection, alternate passivation material with appropriate electronic structure is worth being explored. ZnSe is a good candidate, in view of its Ecb and Evb are higher than those of CdS/CdSe QDs; such band alignment would not only prevent the electron back transfer to the electrolyte but may also facilitate the desired hole transport from the QDs to the electrolyte. Soni et al. [44] reported a thick ZnSe layer in CdSe/CdS/ZnSe type II core/intermediate/shell (C/I/S) structures facilitates electron-hole pair separation. Ahmed et al. [45] constructed CdSe/CdS/ZnSe type II core/shell structures in QDSSCs and demonstrated the power conversion efficiency increased from 1.86% to 3.99% with the introduction of a ZnSe layer. Zhou et al. [46] introduced a thin ZnSe layer between CdSe QDs and ZnS layer, resulting in a significantly increased photocurrent and a large enhancement in solar energy conversion efficiency. It was explained that the introduction of ZnSe could reduce the lattice mismatch between CdSe and ZnS, which led to the suppression of defect formation at the CdSe/ZnS interface and thus facilitated the growth of ZnS with enhanced quality and improved the stability of CdSe QDSSC.
Although ZnSe as a passivation layer has a favorable electronic configuration and has demonstrated effects on improving the power conversion efficiency in QDSSCs, the efficiency remains lower than 5% at present [45], [47], [48]. In this work, we demonstrated the effects of ZnSe passivation layer with appropriate coverage and thickness on the photovoltaic performance of TiO2/CdS/CdSe QDSSCs in terms of light absorption, charge transport, and charge recombination. It was found that the PCE of a solar cell with three SILAR cycles deposition of ZnSe passivation layer increased from 3.4–6.4% (Voc=0.58 V, Jsc=20.11 mA cm−2, FF=0.55), an almost 87% enhancement compared to the solar cells without ZnSe passivating layer. For the comparison, the CdS/CdSe co-sensitized solar cells with optimum ZnS passivation layer was also fabricated, which generated a power conversion efficiency of 4.9%, much lower than 6.4% of ZnSe passivated QDSSCs. This work demonstrated that ZnSe would be a good alternative to ZnS as a passivation material.
Section snippets
Chemicals and material
Titanium oxide (TiO2, Degussa, P25), α−terpineol (C10H8O, 96%, Sigma Aldrich), ethyl cellulose ([C6H7O2(OC2H5)3]n, 48.0−49.5% (w/w) as ethoxyl, Sigma Aldrich), zinc acetate dihydrate (Zn(AC)2·2 H2O, AR, 98.0%), cadmium acetate dihydrate (Cd(AC)2·2 H2O, AR, 98.0%), cadmium nitrate tetrahydrate (Cd(NO3)2·4 H2O, AR, 98.0%), sodium sulfide (Na2S, AR, 98.0%), sodium borohydride (NaBH4, AR, 98.0%), selenium powder (Se, −200 mesh, 99.9%), and sulfur (S, Reagent grade). brass foil (alloy 260, 0.3 mm thick,
Results and discussion
Fig. 1 is the SEM images representing the top view of the photoanodes. Fig. 1(a) and (b) show the surface morphology of TiO2/CdS/CdSe photoanodes without ZnSe passivation layer, while Fig. 1(c) and (d) are the surface morphology of TiO2/CdS/CdSe photoanodes with three SILAR cycle deposition of ZnSe passivation layer. There is no apparent change in the morphology, except a little increase in the particle size with the ZnSe passivation layer deposition, however, the exact increase in size is
Conclusions
Introduction of ZnSe passivation layer to TiO2/CdS/CdSe QDSSCs by a SILAR method has been demonstrated to be an efficient and promising approach to significantly improve the power conversion efficiency. The EDX composition mapping confirmed the molar ratio of CdS, CdSe and ZnSe and homogeneous distribution of elemental cadmium, selenium, zinc and sulfur. The UV–vis, EIS and IPCE investigation of the TiO2/CdS/CdSe QDSSCs with varying numbers of ZnSe SILAR cycles showed: (a) enhanced light
Acknowledgments
This work was financially supported by the National Science Foundation (NSF, DMR 1505902) and Fei Huang would also like to acknowledge the scholarship by China Scholarship Council for the scholarship. This work was also supported by National Natural Science Foundations of China (Nos. 21377023 and 51362026).
Fei Huang is a Ph.D. candidate under the supervision of Prof. Jianshe Liu at Donghua University, China. She is currently a visiting student under the supervision of Prof. Guozhong Cao at University of Washington, Seattle. Her recent research mainly focuses on interface modification, photoanode material synthesis of quantum dot sensitized solar cells.
References (59)
- et al.
Renew. Sustain. Energy Rev.
(2015) - et al.
Nano Energy
(2015) - et al.
Electrochem. Commun.
(2010) - et al.
Nano Energy
(2015) - et al.
Nano Energy
(2015) - et al.
Electrochim. Acta
(2013) - et al.
Electrochim. Acta
(2002) Science
(2007)- et al.
J. Phys. Chem. Lett.
(2015) - et al.
Adv. Funct. Mater.
(2013)
Nature
Prog. Photovolt.: Res. Appl.
Nature
J. Mater. Chem. A
Adv. Mater.
Adv. Mater.
Nat. Chem.
Inorg. Chem.
Chem. Eur. J.
J. Am. Chem. Soc.
Adv. Funct. Mater.
J. Mater. Chem. A
ACS Nano
ACS Nano
ACS Nano
ACS Nano
ACS Nano
Acc. Chem. Res.
J. Phys. Chem. C
Cited by (117)
MXene-based hybrid nanomaterials in photocatalysis
2024, MXene-Based Hybrid Nano-Architectures for Environmental Remediation and Sensor Applications: From Design to ApplicationsQuantum dot-sensitized solar cells: A review on interfacial engineering strategies for boosting efficiency
2023, Journal of Industrial and Engineering ChemistrySynthesis, structural properties, and applications of cadmium sulfide quantum dots
2023, Quantum Dots: Emerging Materials for Versatile ApplicationsQuantum dots synthesis for photovoltaic cells
2023, Quantum Dots: Emerging Materials for Versatile Applications
Fei Huang is a Ph.D. candidate under the supervision of Prof. Jianshe Liu at Donghua University, China. She is currently a visiting student under the supervision of Prof. Guozhong Cao at University of Washington, Seattle. Her recent research mainly focuses on interface modification, photoanode material synthesis of quantum dot sensitized solar cells.
Juan Hou earned her Ph.D. degree from University of Chinese Academy of Sciences in 2003. Currently she is an assistant professor in college of science at Shihezi University, China. Her current research focuses on quantum dots and their application in photovoltaic conversion devices.
Qifeng Zhang is currently working at University of Washington as a Research Assistant Professor. His research interests involve engineering applications of nano-structured materials on electrical devices including solar cells, UV light-emitting diodes (LEDs), eld-effect transistors (FETs), and gas sensors. His current research focuses on the synthesis of nanomaterials and the application of nanomaterials in electronic and optoelectronic devices, such as dye-sensitized solar cells (DSCs) and organic/inorganic hybrid solar cells.
Yuan Wang is working for her Ph.D. degree in state key laboratory of organic-inorganic composites, Beijing University of Chemical Technology. Her current research is focused on development of visible-light-responsive photocatalysts and Quantum dot sensitized solar cells.
Robert C. Massé received his B.Sc. degree from the University of Wisconsin-Madison. He is currently a Ph.D. student at the University of Washington under the supervision of Prof. Guozhong Cao. His research interests include the electrochemistry and characterization of energy storage materials for alkali-ion batteries.
Shanglong Peng is an associate professor of school of physical science and technology at Lanzhou University. His current research is focused mainly on new energy materials and devices including Si-based inorganic-organic hybrid solar cells, quantum dot sensitized solar cells and supercapacitor. And he has published over 50 papers.
Huanli Wang is now working as a Lecturer in Qingdao Technological University. She obtained her PhD degree from Donghua University, in 2015. From 2013 to 2014, she worked in Professor Nicholas A. Kotov's group as a visiting student at University of Michigan, USA. Her current research concentrates on the synthesis and properties of nanostructured photocatalytic materials.
Jianshe Liu is Dean of the college of environmental science and engineering, and Professor of environmental science at Donghua University. He has published more than 100 peer-reviewed articles. His recent research mainly focuses on environmental biotechnology, advanced oxidation technology, functional nanostructured materials and their applications in photocatalysis and solar cells.
Guozhong Cao is Boeing-Steiner Professor of materials science and engineering, professor of chemical engineering, and adjunct professor of mechanical engineering at the University of Washington, and also a professor at Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences and Dalian University of Technology. His current research is focused on chemical processing of nanomaterials for energy related applications including solar cells, rechargeable batteries, supercapacitors, and hydrogen storage.