Enhanced efficiency of dye co-sensitized solar cells based on pulsed-laser-synthesized cadmium-selenide quantum dots
Graphical abstract
Introduction
The harvesting of solar energy based on photovoltaic cells allows for a decrease in the consumption of non-renewable energy resources. Moreover, the rapid depletion of non-renewable resources (fossil fuels) has a considerable environmental impact. The effective utilization of solar energy by the photovoltaic (PV) process has attracted extensive research and development interest for several decades; however, solar energy only accounts for a small amount of total global energy production. Climatically, most regions on earth receive sufficient sunshine, which guarantees that PV technology can be a potentially significant contributor to the global energy demand (O’Regan and Grätzel, 1991). In addition, progress in PV technology is required to decrease the wastage of fossil fuels, which are rapidly being depleted and have a significant impact on the environment (Lee et al., 2012). Due to the high cost of first-generation silicon-based solar cells and the scarcity of second-generation rare-element-loaded semiconducting thin film-based solar cells, dye-sensitized solar cells (DSSCs) were founded as third-generation low-cost solar cells by O’Regan and Grätzel (1991). The main advantages of DSSC over silicon based solar cell are (i) fabrication process is simple and cost effective; (ii) shows a good performance under low light condition, as DSSC can maintain its achievable efficiency even at low light condition due to the excellent optical absorbance of the dye; (ii) unlike Si based solar cells, DSSC does not need a high crystallographic perfection of the material. The photocurrent in the conventional silicon based solar cell is due to the diffusion of electrons and holes in the space charge region and hence, its performance is quite sensitive to the crystallographic defects in silicon, and therefore expensive defect free silicon material is required to enhance the efficiency of solar cells. On the other hand, in DSSC photons absorbed by the organic dye/QD gives rise to electrons, which are transferred into the CB of inexpensive semiconductors and hence, the crystallographic defects in the material is not detrimental for the generation of photocurrent. The operating principle of the DSSC is based on an electrochemical process, involving electron transfer between several interfaces. TiO2 is the most commonly used semiconducting material in DSSCs due to its high stability and mesoporosity, thereby providing a larger surface area and excellent electron transport properties (Gupta and Tripathi, 2012, Shakeel Ahmad et al., 2017). Hence, to utilize the effective visible light region of solar radiation, TiO2 particles are coated with organic dyes with significantly broad and efficient spectral absorption regions. The organic dye in DSSCs acts as a light absorber/sensitizer, and the photoexcited electrons in the excited state of the dye are transferred to the conduction band (CB) of TiO2, which transports electrons through the external circuit to the platinum counter electrode (Lelii et al., 2014). The redox couple (the electrolytic solution) between the electrodes regenerates the oxidized dye by reducing it, and the redox couple itself is reduced at the counter electrode by the incoming electrons in the circuit (Yeoh and Chan, 2017).
The efficiency and the figures of merit of a DSSC are dependent on the light absorption of the dye-coated semiconducting material, which indicates that with an increase in the dye concentration, the number of photogenerated charge carriers injected into the CB of TiO2 increases, resulting in a high photocurrent (Sun et al., 2017). There are many spontaneous chemical pathways such as the electron recombination pathway in the CB of TiO2 with the cations of the dye and the redox couple, which have a negative impact on the performance of DSSCs. Thus, to achieve higher efficiencies, the dye in the photoanode should have high visible-light absorption, and the energy levels of the dye and CB of the semiconductor should be significantly compatible for rapid electron transfer at the excitation of the dye. This kinetic process should be more rapid than the relaxation of the dye (Listorti et al., 2011). Moreover, the energy state between the redox couple and dye should allow for sufficiently rapid reduction of the oxidized dye to avoid reduction by the photogenerated electrons in TiO2 (Wang et al., 2005). In several studies on PVs (Dissanayake et al., 2020, Powar et al., 2020, Yuan et al., 2019), quantum dots (QDs) were used as a substitute for the organic dye or as a co-sensitizer in conjunction with the dye, due to their (i) size-dependent tenability over a wide spectral range; (ii) higher extinction coefficient than most organic dyes; (iii) high chemical stability in aqueous environments; (iv) low cost and simple synthesis; (v) multiple excitons per photon absorption, and large intrinsic dipole moment (Hagfeldt et al., 2010). After the absorption of light of an appropriate wavelength, electron–hole pairs are generated, and electrons are injected into the CB of the n-type semiconducting material, i.e., TiO2 in the case of quantum dot-sensitized solar cells (QDSSCs), or the excited state of the organic dye in the case of QD co-sensitized solar cells; on the other hand, the photogenerated holes are scavenged by the hole transport material, i.e., the electrolyte. Chalcogenides such as CdS and CdSe QDs are common semiconductor QDs that have been employed as co-sensitizers, in conjunction with organic dyes, in DSSCs (Kim et al., 2011, Tian et al., 2012, Marandi and Mirahmadi, 2019). Guijarro et. al loaded CdSe QDs onto the surface of TiO2 and found that the efficiency was higher due to the direct loading of the QDs when compared with the use of a molecular linker (Guijarro et al., 2009). Another research group enhanced the efficiency of DSSCs by incorporating CdS QDs and silver nanoparticles photoanode (Amiri et al., 2016). Yaman et. al. fabricated chemically-modified CdSe-TiO2 photoanodes with 3.35% solar cell efficiency (Yaman et al., 2017).
There have been several methods employed for the synthesis of QDs, which mostly involve colloidal QDs, such as the hydrothermal method (Zhu et al., 2006), chemical bath deposition (CBD) (Chang and Lee, 2007), solvothermal method (Golobostanfard and Abdizadeh, 2014), successive ionic layer absorption reaction (SILAR) (Mohamed Mustakim et al., 2018, Kishore Kumar et al., 2020) and pulse laser ablation in liquid (PLAL) (Moqbel et al., 2018a, Mostafa et al., 2020).
The synthesis of QDs is characterized by rapid nucleation and slow growth (Lee et al., 2016). The rapid nucleation is controlled by the solute concentration, temperature, and interfacial tension; and the slow growth is mediated by several passivation reagents in the steric barrier between the bulky organic surfactants and the other semiconducting materials, which all present practical limitations and contaminations. Another method employed for the synthesis of QDs is plasma synthesis due to its simplicity; nevertheless, colloidal synthesis is the most versatile and low-cost method. PLAL is a type of hybrid method between the colloidal and plasma synthesis methods, where the local plasma is developed in the organic colloidal medium. Basically in the PLAL method of synthesis, the interaction between the high energy pulsed laser beam and the solid particles creates the plasma plume in the liquid medium. During the temporal evolution of the plasma plume, the cavitation bubbles created in the liquid medium get bigger in size and explode after a certain time (in microseconds), resulting in the generation of a ultrasonic shockwave in the liquid medium, which is instrumental for the generation of quantum dots through the fragmentation of the nanoparticles (Xiao et al., 2017). Moreover, this method of QD synthesis has many advantages, i.e., it is facile, rapid, and does not involve any foreign chemicals or severe experimental conditions. In addition, it allows for the realization of high-purity end products, for which further purification is not required (Moqbel et al., 2018b). PLAL involves many experimental and laser parameters that have an influence on the sizes and shapes of the QDs, which determine the spectral response of the QDs (Gondal et al., 2016).
In this study, active CdSe-QDs in the visible-light spectrum were synthesized using PLAL, and were used as a co-sensitizer, in conjunction with N719 organic dye, in conventional TiO2-based DSSCs. The novelty of the configuration of the DSSC presented in this work lies on (i) a facile PLAL method was employed for the synthesis of CdSe-QDs, the material used as a co-sensitizer in the photoanode of our DSSC configuration; (ii) CdSe-QD co-sensitization brought about a significant reduction of the recombination of the photo generated electrons and the oxidized dye/electrolyte; (iii) the improved recombination resistance coupled with a better electron transport contributed positively to the enhancement of all the figures of merits of DSSC in general, and photovoltaic conversion efficiency around 40% in particular.
Section snippets
Materials
Conductive glass substrates fluorine doped tin oxide (FTO): SnO2/F, (thickness 2.2 mm, 7 Ω/sq), TiO2 paste (Ti-Nanoxide T/SP), blocking layer (Ti-Nanoxide BL/SC, Solaronix), electrolyte (Iodolyte Hi-30), Ruthenium dye N719 and Pt solution (plastisol T/SP, Solaronix) were purchased from Solaronix, Switzerland. CdSe powder (99.9% purity) was purchased from Sigma Aldrich.
Synthesis of CdSe quantum dots
The schematic diagram of the PLAL synthesis of CdSe QDs is depicted in Fig. 1. The second harmonic of the Q-switched Nd-YAG
Elemental and morphological analysis of best performing photoanode
In order to verify the proper anchoring of CdSe QDs on TiO2 surface, we present an elemental analysis by carrying out X-ray photoelectron spectroscopy (XPS) of the pure TiO2 photoanode and the TiO2 photoanode immersed in CdSe QDs (TiO2/CdSe QDs) and their XPS survey spectra are depicted in Fig. 2. In comparison, the spectrum of the CdSe-QD coated photoanode clearly exhibits elemental peaks of Cd and Se, in addition to Ti and O peaks, indicating that the CdSe QDs were securely anchored onto the
Conclusions
In summary, CdSe QDs were synthesized by PLAL, and the fabricated CdSe-QDs were employed as a co-sensitizer of TiO2 in conjunction with N719 dye in DSSCs. The advantage of this synthesis route with respect to CdSe QDs is that the method is simple, rapid, contamination-free, and requires no post synthesis purification. Among the three different concentrations of the CdSe QDs, that with a concentration of 1 mg/15 ml of ethanol exhibited superior light absorption in the visible region with the
CRediT authorship contribution statement
Jwaher M. AlGhamdi: Conceptualization, Supervision, Investigation, Formal analysis, Writing - original draft. Shorooq AlOmar: Data curation, Formal analysis, Writing - original draft. Mohammed A. Gondal: Conceptualization, Supervision, Funding acquisition, Investigation, Validation, Writing - original draft, Writing - review & editing. Redhwan Moqbel: Data curation, Writing - review & editing, Formal analysis. Mohamed Abdulkader Dastageer: Formal analysis, Validation, Formal analysis, Writing -
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge Imam Abdulrahman Bin Faisal University (IAU) for funding through project # 2016-134-Sci. The authors also acknowledge King Fahd University of Petroleum & Minerals (KFUPM) to support this work under project # RG181002.
References (57)
- Amiri, O., Salavati-Niasari, M., Bagheri, S., Yousefi, A.T., 2016. Enhanced DSSCs efficiency via Cooperate...
- et al.
An all carbon dye sensitized solar cell: a sustainable and low-cost design for metal free wearable solar cell devices
J. Colloid Interface Sci.
(2020) - et al.
Crystal phase engineering on photocatalytic materials for energy and environmental applications
Nano Res.
(2019) - Birman, J.L., Nazmitdinov, R.G., Yukalov, V.I., 2013. Effects of symmetry breaking in finite quantum systems. Phys....
- et al.
Carbon dots as cosensitizers in dye-sensitized solar cells and fluorescence chemosensors for 2,4,6-trinitrophenol detection
Ind. Eng. Chem. Res.
(2019) - et al.
Recent developments on the synthesis, structural and optical properties of chalcogenide quantum dots
Sol. Energy Mater. Sol. Cells
(2017) - et al.
Chemical bath deposition of CdS quantum dots onto mesoscopic TiO 2 films for application in quantum-dot-sensitized solar cells
Appl. Phys. Lett. DOI
(2007) - et al.
A femtosecond study of the anomaly in electron injection for dye-sensitized solar cells: the influence of isomerization employing Ru(ii) sensitizers with anthracene and phenanthrene ancillary ligands
Phys. Chem. Chem. Phys.
(2015) - et al.
CdS/CdSe co-sensitized solar cells based on hierarchically structured SnO2/TiO2 hybrid films
Nanoscale Res. Lett.
(2016) - et al.
Ultrathin highly luminescent two-monolayer colloidal CdSe nanoplatelets
Adv. Funct. Mater.
(2019)