The effect of activated carbon additives on lead sulphide thin film for solar cell applications
Introduction
Lead chalcogenides are semiconductors with small bandgap of 0.37 eV (PbS), and 0.29 eV (PbSe and PbTe) with large exciton Bohr radius i.e., 5.01 nm (PbS), 13.1 nm (PbSe) and 24.8 nm (PbTe). Nanostructured lead chalcogenides are convenient for industrial and technological applications due to their peculiar structure, opto-electronic, and mechanical properties [1], [2], [3], [4]. These materials are ideal for various infra-red (IR) active and optical applications i.e., (i) solar cells, (ii) IR detectors, (iii) broadband optical amplifiers, (iv) solar control coatings optical fibers, (v) field-effect transistors [5], (vi) thermoelectric cooling materials, and (vii) IR light emitting diodes [6], [7], [8], [9], [10], [11].
Efforts in synthesis of materials with large exciton Bohr radius would offer various tremendous morphologies i.e., nano-cubes [12], nano-pyramid [13], nano-tubes [14], nano-flowers [15], and quantum dots [16]. The synthesized lead chalcogenides below their exciton Bohr radius would exhibit properties in the region of strong quantum confinement effect e.g., expansion of unoccupied energy levels that would favor the occurrence of multiple exciton generation (MEG) upon absorption of one photon with energy higher than the bandgap (Ephoton>>Eg) of quantum confined fluorophore (photon absorber) – theoretically this would increase the photovoltaic conversion efficiency (η) of an excitonic solar cell exceeding 60% [17]. The excited state electron from the lowest unoccupied molecular orbital (LUMO +1), would undergo a radiative relaxation to a lower energy level of LUMO +0; with subsequent photon emission. A neighbouring ground state electron could absorb the emitted photon (e.g., visible light with Ephoton ≥Eg); initiate a secondary electron excitation (viz., HOMO −0 to LUMO +0). Therefore, two excitons would be generated cumulatively upon absorption of a photon in Fig. 1(b).
A ground state electron from valence band of a bulk phase fluorophore would undergo an excitation upon absorption of one photon (Ephoton>>Eg). Subsequently, the excited electron would undergo a non-radiative relaxation from the excited state to the conduction band minimum; would emit heat during the process – which would be dissipated to the surroundings [18] shown in Fig. 1(a). The energy loss due to the dissipated heat could not be utilized to excite a neighbouring ground state electron, therefore only one exciton could be generated per absorbed photon.
Increasing trend of usage of lead chalcogenide as fluorophore in excitonic solar cells could be observed in Fig. 2(a). However, low number of publications which is related to the usage of lead sulphide as a fluorophore in comparison with that of the cumulative usage of lead chalcogenides in excitonic solar cells shown in Fig. 2(b), could be due to low performing yielded-efficiency (0.006–2.12%) of fabricated solar cells using PbS as main light absorber from the year 1970–2019 [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. Two speculations could be made based on the observation i.e., (i) insufficient electronic characterizations of the PbS; which are non-experimentally feasible, that would lead to (ii) energy level misalignment (PbS/electrolyte) – a detrimental effect to the photovoltaic conversion efficiency (η).
The quantum confinement effect could be observed in semiconductors with particle size smaller than its exciton Bohr radius [19]. The opto-electronic properties of a semiconducting particle in the region of strong quantum confinement region would differ from that of the bulk e.g., increment of energy bandgap which due to the expansion and/or contraction of energy level of the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO) [21], [22], [29]. Realistic clusters of quantum confined structures of lead chalcogenides have been established in our previous works to study the occurrence of MEG which inclusive the study of i.e., (i) optical absorption, (ii) excitation energy, and (iii) oscillator strength [31]. The size-dependent opto-electronic properties of the quantum confined lead chalcogenides have widened their light absorption properties ranging from the infra-red to visible light spectrum; which could be targeted to be used as fluorophore in photovoltaic applications.
Various synthesis techniques of lead chalcogenides have been established in the recent years, i.e., (i) sol-gel [24], (ii) microwave [25], (iii) sonochemical [26], (iv) chemical bath deposition [32], (v) electrochemical deposition [33], (vi) hydrothermal [27], (vii) precipitation [34], (viii) green chemical [35], (ix) wet chemical [36], (x) pulse sonochemical [37], (xi) photochemical [38], and (xii) mechanochemical [39], [40]. Vacuum thermal evaporator technique has been used to fabricate thin films due to simple, efficient and low-cost deposition technique; which yielded high quality, uniform, good transparency and dense thin film [41], [42], [43].
Fig. 3 shows an increasing trend of usage of vacuum thermal evaporator in various field from 1970 to 2019. Despite the increasing usage of vacuum thermal evaporator technique in synthesis and fabrication of various nanostructured materials and thin films, it attracted less focus in the synthesis of lead chalcogenides. Few efforts have been demonstrated to synthesize quantum confined lead chalcogenides using vacuum thermal evaporator in the recent years i.e., PbS, PbSe, and PbTe with the size of 3 nm [44], 1.2 nm [45], and 1.2 nm [46] respectively for the application as fluorophore of solar cells which yielded the highest device η ca. 7.4% [37]. Furthermore, only two morphologies of lead chalcogenides i.e., nanoparticles, and quantum dots have been successfully synthesized using the vacuum thermal evaporator technique. However, various morphologies of lead chalcogenides are speculated could be fabricated upon optimizations of experimental parameters i.e., vacuum pressure, voltage, current, speed of evaporation, distance between the source and substrate, and additives.
Lead sulphide with other morphologies e.g., (i) nano-rod [47], (ii) tetrapod-shaped whisker [48], (iii) nano-wire [49], (iv) nano-pyramid [50], and (v) nano-fiber [51] however have been materialized using various techniques. Addition of activated carbon with metal chalcogenide precursors i.e., ZnSe, and ZnO has successfully yielded tetrapods, and nano-rods respectively [52], [53] for photoelectrode applications. The activated carbon could induce a self-seeding mechanism which acts as the first stage of formation of the tetrapods, and nano-rods [54]. Furthermore, the activated carbon also acts as catalyst which lowers the evaporation temperature of the reactants during the fabrication process [55]. Similar method was used in this work in which the activated carbon was added with the lead sulphide precursors. Morphology of the lead sulphide has significant effect to the light absorption properties [46], [56], [57], and the rate of electron transport [13], [58], [59]. The nano-rod morphology would improve the light absorption properties i.e., (i) wider absorption spectra and (ii) higher absorption intensity; would increase the rate of electron transport to three folds in comparison with that of the planar thin film [60]. A solar cell fabricated using fluorophore with efficient excited state electron transport would increase the short circuit current (ISC); therefore, significant increase of η could be expected. The activated carbon has been utilized as an additive during the fabrication of photoelectrode and conductive layer on the counter electrode in dye-sensitized solar cell (DSSC); due to its high surface area [61].
Quantum chemical calculations under the framework of density functional theory (DFT) in the focus of researchers due to its ability to validate and establish realistic cluster of materials. Fig. 4 shows an increasing trend of usage of DFT calculations for photovoltaic research could be speculated due to the accuracy of the calculations at low computational cost [62]. The combination of B3LYP functional, and lanl2dz basis set of the DFT calculations would exhibit accurate description of i.e., (i) binding energy, (ii) surface stabilization, (iii) surface states and (iv) trap states [63], [64].
A complete working solar cell consists of fluorophore (which involves electron excitation mechanism), photoelectrode (electron transportation mechanism), and electrolyte (electron regeneration mechanism). The electron regeneration from electrolyte to fluorophore is a crucial process for completing the cycle of electron flow [65], [66]. The requirement of energy level alignment of LUMOfluorophore>redox potential (Eo)>HOMOfluorophore in order to achieve the efficient electron regeneration. The detail of regeneration mechanism has been explained in our previous work [67].
This research focuses on the effect of addition of activated carbon with various surface area to the yielded morphology of fluorophore. The PbS thin films were fabricated with addition of activated carbon (with three different specific surface areas) i.e., (i) 80 m2/g, (ii) 650 m2/g, and (iii) 1560 m2/g using thermal evaporator at vacuum pressure of 1.0 × 10−5 Torr. The nanosized morphology of PbS was characterized using Field Emission Scanning Electron Microscope (FESEM) and Energy Dispersive X-Ray Spectrometer (EDX), X-ray Diffractometer (XRD), UV-Vis absorption spectrometer (UV-Vis), photoluminescence spectrometer (PL), Micromeritics ASAP 2020 BET (Brunauer-Emmett-Teller) Surface Area and Porosity Analyser, and Bridge Technology 4-point probes (4PP). Realistic models of PbS have been built based on the crystallographic properties of the yielded thin films i.e., (PbS)n; which n=4–80. The bandgap, ground and excited state energy levels of the realistic clusters have been calculated using B3LYP functional and lanl2dz basis set.
Section snippets
Materials
Materials used in the experiment were lead sulphide (Sigma Aldrich; 99%), activated carbon from Zhejiang Forest Energy Technology Co. Ltd and Alfa Aesar with specific surface area of 1560 m2/g and 650 m2/g respectively, and Thermo Fisher Scientific super conductive carbon black (with specific area of 80 m2/g), titanium dioxide (R&M Chemicals), nitric acid (American Chemical Society reagent; 37%), absolute ethanol (Merck; 99.5%), indium tin oxide (ITO; Sigma-Aldrich with dimension of
Crystal structure analysis
The PbS thin films were successfully fabricated on the surface of ITO conducting glass using thermal evaporator technique with addition of activated carbon (AC). Fig. 6 shows the elemental composition in the thin films was validated using EDX spectroscopy. The atomic percentage of Pb and S in the PbS thin films fabricated (a) without the AC (bare PbS) is 44.93% and 55.07%, (b) with addition of AC80 (SBET 80 m2/g) is 40.98% and 59.02%, (c) with addition of AC650 (SBET 650 m2/g) is 42.93% and
Conclusion
This work concluded that PbS, and PbS-AC nano-sheets were formed with and without the addition of AC (SBET of 80 and 1560 m2/g); using vacuum thermal evaporator. PbS nano-tubules were formed upon addition of AC with surface area of 650 m2/g. The opto-electronic properties of the PbS-AC thin films were deviated from that of the bare PbS thin film which correlated to alteration of crystal structure of PbS. Photoluminescence-quenching study of the PbS/TiO2 and PbS-AC/TiO2 thin films revealed that
CRediT authorship contribution statement
Nur Farha Shaafi: Writing - original draft, review & editing, Methodology, Investigation, Software, Validation, Project administration. Saifful Kamaluddin Muzakir: Supervision, Methodology, Conceptualization, Project administration, Writing - original draft, review & editing. Shujahadeen B. Aziz: Term, Writing - review & editing, Validation. Mohd Fakhrul Zamani Kadir: Validation. Suresh Thanakodi: Writing - review & editing.
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.
Acknowledgement
This work is funded by the Research & Innovation Department of Universiti Malaysia Pahang and the Ministry of Education of Malaysia through the Fundamental Research Grant Scheme (RDU 150111) and Postgraduate Research Scheme (PGRS190346). The authors also acknowledged Magna Value Sdn. Bhd. for providing technical supports of the thermal evaporator.
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