A study of electron regeneration efficiency in fluorophore

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Abstract

Archetypical excitonic solar cell consists of fluorophore (main light absorber), photoelectrode (electron transportation), and conducting polymer (electron regeneration). Fluorophore generates excited state electron upon absorption of light with sufficient energy. Electron in the highest occupied molecular orbitals (HOMO) would undergo an excitation to the lowest unoccupied molecular orbitals (LUMO) during the light absorption process. Therefore an electron vacancy in the HOMO of fluorophore is expected; need to be replenished for a continuous process of a photovoltaic mechanism. However the quantum of research on electron regeneration efficiency is still low due to limited computational facility. Two parameters are hypothesized to have significant impact on the electron regeneration process i.e., (i) conductivity (σ), and (ii) redox potential (Eo) of the conducting polymer. This study aims to establish a correlation between the stated parameters with the photovoltaic conversion efficiency, η. Two conducting polymer were used in this work i.e., (i) alginate, and (ii) a mixture of 60 wt% of carboxymethyl cellulose (CMC) and 40 wt% of polyvinyl alcohol (PVA). The conductivity of the conducting polymer was calculated based on the measured bulk resistance using Electrical Impedance Spectrometer (EIS); showed that σalginate > σCMC/PVA. The redox potentials were calculated using quantum chemical calculations under the framework of density functional theory (DFT) at the level of b3lyp/lanl2dz. The lead sulphide thin film (fluorophore) was deposited using thermal evaporator on a pre-fabricated TiO2 layer on indium-doped tin oxide (ITO) conducting glass. The CMC/PVA-based cell yielded the highest η of 0.0015% under one-sun condition; showed higher η than that of the alginate conducting polymer. Therefore concluded that the conductivity would only determine the speed of the electrons during the regeneration. Nonetheless the efficiency of the regeneration process could be determined by the compatibility analysis of the conducting polymer and fluorophore. The compatibility analysis was carried out based on the energy level alignment between the Eo of the conducting polymer, and the HOMO energy level of the fluorophore. The calculated Eo of the conducting polymer used i.e., CMC/PVA is −3.144 eV, and alginate is −1.908 eV; incompatible to be paired with the fluorophore (PbS), which the HOMO, and LUMO energy levels are −5.100 eV, and −4.000 eV respectively. The low η of the CMC/PVA, and alginate-based cells however is speculated could also due to energy loss which is equivalent to 1.956 eV, and 3.912 eV energy offset respectively.

Introduction

Electron regeneration in the HOMO of fluorophore is a crucial process which completes the cycle of electron flow during a photovoltaic mechanism. A device structure and an ideal photovoltaic mechanism in a typical excitonic solar cell is illustrated in Fig. 1. The device consists of three main components which the function is stated in the parentheses i.e., (i) fluorophore (electron excitation), (ii) photoelectrode (electron transportation), and (iii) conducting polymer (electron regeneration). The fluorophore acts as a reactor for electron excitation upon absorption of photon with sufficient energy (Ephoton > bandgap, Eg-fluorophore). The jump of an electron from the HOMOfluorophore to the LUMOfluorophore could be used as an analogy of the excitation, therefore leaving a vacancy (hole) in the HOMOfluorophore. The photoelectrode (typical example e.g., metal oxide semiconductors) functions as a medium for transportation of the excited state electron from the fluorophore to the external circuit. The electron vacancy in the HOMOfluorophore could be replenished from the third process (electron regeneration) by the conducting polymer. Redox potential is the key that partially influences the efficiency of the regeneration. The conducting polymer would receive electron from the external circuit (reduction), and regenerate electron in the HOMOfluorophore (oxidation). Redox potential, Eo of the conducting polymer plays important role that determines the compatibility of the conducting polymer-fluorophore pair (which the LUMOfluorophore > Eo > HOMOfluorophore).

Quantum chemical calculations under the density functional theory (DFT) framework could be employed to calculate the Eo [1] with high accuracy. A comparison between the theoretical calculations and experimental results of the Eo of Cu-protein complexes has been made by Yan et al. (2016), which yielded insignificant error and standard deviation [2].

The effective mass of electron, me is a unique property of material; could be calculated using similar procedure to that of the Eo, where the details are discussed elsewhere. The me is correlated with the speed of electron movement, υd during the regeneration process, and mobility as depicted in the following equations:υd=ħ3ηπ21/3meμ=e×τ/mewhere η is the concentration of electron, τ is time of relaxation between two electron scattering incidents in the conducting polymer, which originated from defect and impurity [3], [4]. The electron mobility is related with conductivity of a material, which represented by equation:σ=η×e×μwhere σ is conductivity, and e is charge of electron [5]. Therefore, the σ could be used as reference that described the speed of electron movement during the redox process. The conducting polymers are used in the recent efforts of fabrication of solar cell due to advancements made from the perspective of their conductivity. The CMC/PVA, and alginate conducting polymer showed achievement ofσ ∼ 9.12 × 10−6 S/cm [6], and 1.97 × 10−4 S/cm [7] at room temperature respectively.

Section snippets

Materials

Lead (II) sulfide (Aldrich 99.9%), nitric acid (ACS reagent, 37%), titanium dioxide (R&M chemicals), absolute ethanol (Merck, 99.5%), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA) and alginate powder (Sigma Aldrich).

Sample preparation

Indium-doped tin oxide (ITO) glass was cut into 2.5 cm × 2.5 cm. The working electrode of solar cell was prepared using the following methods. Titanium dioxide paste was prepared by dissolving 1.5 g TiO2 powder with 1 ml of concentrated nitric acid and 2 ml of ethanol. The

Energy level alignment analysis

An ideal energy level alignment of components of an excitonic solar cells (e.g., conducting polymer, fluorophore, and photoelectrode) is presented in Fig. 1 (b); would favor the following processes i.e., (i) electron regeneration from conducting polymer to HOMOPbS, and (ii) electron injection from LUMOPbS to conduction band of photoelectrode. Therefore an efficient working mechanism of a solar cell could be achieved.

Through the quantum chemical calculations, the energy levels of fluorophore,

Conclusion

The redox potential of the CMC/PVA, and alginate-based conducting polymer are incompatible with the energy levels of PbS i.e., Eo > LUMOPbS > HOMOPbS; therefore resulted inefficient electron regeneration. The Eo is higher than that of the LUMOPbS, would favor unnecessary electron injection from the conducting polymer to the LUMOPbS. Furthermore, the energy offset between the Eo and HOMOPbS is extremely large that would cause high energy loss during the electron regeneration i.e., offset

Acknowledgements

This work if 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).

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