C-doped TiO2 nanotubes with pulsed laser deposited Bi2O3 films for photovoltaic application
Graphical abstract
Introduction
The attempt to surpass the world energy crisis is a high priority for the scientific community. Studies focusing on the conversion of solar energy to electricity, face many challenges. The foremost outset is to choose materials that are not only cheap and abundant but easy to process as well. Titanium dioxide (TiO2) is in this sense the epitome compound, already extensively deployed in many commercial applications, such as in the well-known Gratzel solar cells [1]. Some alternative improvements have been proposed in an attempt to enhance TiO2 photovoltaic (PV) performances. Replacing nanoparticles with nanotubular (NT) structures is known to enhance the transfer of electrons [2,3]. Another approach entails the direct modification of TiO2 to fine-tune the absorption of higher wavelength photons (>387 nm [4]) to generate more electron-hole pairs. This can be achieved by doping either with a cation or an anion or even via co-doping [[5], [6], [7]]. It was shown that some types of doping, for example with carbon, not only extend the light absorption of TiO2 to the visible region but also increase the lifetime of the photogenerated electron–hole pairs [8]. Additionally, electron-hole recombination can be prevented by applying the hole transport material on TiO2 nanotubular film [9]. We propose bismuth oxide (Bi2O3), relying on its previously reported suitability in junctions with TiO2 for a wide range of applications such as photocatalytic water splitting [10] or degradation of the methyl blue dye [11] and toluene [12]. Furthermore, it has been shown that the photocurrent density measured in a three-electrode cell with the Bi2O3–TiO2 NT composite film as the working electrode, in a 0.5 M Na2SO4 solution under white light illumination (of 190–1100 nm) is four times higher than of the bare TiO2 film. This points to a significantly enhanced photoelectrochemical performance of the composite film [13]. Moreover, opposite to the zero-contiguous dark current measured for the bare TiO2 film, the asymmetry of the forward and reverse currents depicts the typical diode behaviour of the Bi2O3–TiO2 NT film. This proves the formation of the p-n heterojunction in areas with a transfer of electrons from the n-type TiO2 to the p-type Bi2O3, accompanied by the simultaneous movement of holes in the opposite sense to preserve the system's equilibrium. The carriers diffusion results in the advent of an internal electrostatic field inside the p-n heterojunction. The illumination of the Bi2O3–TiO2 NT film boosts TiO2 to absorb the UV light, which in turn triggers the excitation of electrons from the valence (VB) to the conduction band (CB). Concurrently, there is a migration of the photogenerated holes from the TiO2 VB to the Bi2O3 VB via an electrostatic driving force in the p-n heterojunction. The narrow band-gap of Bi2O3 (2.8 eV [11]) facilitates the selective absorption of visible light along with the UV, with a direct impact on the photoelectrons generated in the CB of the excited Bi2O3. The photoelectrons flow from the Bi2O3 CB to the TiO2 CB is supported by the driving force of the inner electrostatics, ultimately leading to an effective separation of the photogenerated electron-hole pairs in the Bi2O3–TiO2 composite film. The above-described mechanism is applied in this study for solar cells by growing and structuring TiO2 NTs as a transparent film on top of a conductive glass, such as the FTO (F–SnO2) glass.
The study aimed to investigate the effect of C-doping on the absorption properties of the TiO2 NTs films and how it can influence the PV performances of the TiO2 NTs photoanode in junction with Bi2O3. Bi2O3 was deposited onto the undoped and the C-doped TiO2 NTs films by pulsed laser deposition (PLD) method [14] and the effect of the in situ and ex situ post-deposition thermal treatments of Bi2O3 on the PV performances of the solar cells was evaluated.
Section snippets
Synthesis of pure and C-doped TiO2 NTs films
Pure titanium (Ti) thin films were deposited by radio frequency magnetron sputtering (RF-MS) onto FTO glass substrates (PI-KEM Ltd, 200 nm FTO film, 12–14 Ω/cm2) using a titanium target (Alfa Aesar GmbH). The FTO substrates were cleaned successively in acetone, ethanol and deionized water for 10 min in an ultrasonic bath before being mechanically fixed inside the deposition chamber, at a 40 mm target-substrate separation distance. Before deposition, a shadow mask was applied on the FTO glass
Results and discussion
The anodization of Ti sputtered on FTO glass was applied for obtaining TiO2 NTs, as it is a feasible and reproducible method for the controlled synthesis of highly ordered TiO2 NTs arrays [18]. Since the nanotubes obtained by anodization are amorphous, they should be annealed to become crystalline and so, more conductive [19]. The changes in surface morphology due to annealing either in air or in CH4, were studied by comparing the micrographs taken before and after the annealing procedure. Fig.
Conclusion
We introduce a successful method for solar cell production, incorporating a p-n heterojunction that consists of Bi2O3 and anodized TiO2, built directly on FTO glass. Two alternative material compositions and several approaches were tested for improving the structural properties and ultimately, the photovoltaic performances of the final cell. It is shown that annealing the TiO2 anodized films in the CH4 atmosphere, at 500 °C, induces an effective carbon-doping which in turn shifts the absorption
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 Serbian authors acknowledge with thanks the financial support of the Ministry of Education, Science and Technological Development, the Republic of Serbia through Projects III 45019 and Contracts No. 451-03-68/2021-14/200287 and 451-03-68/2021-14/200135. The XPS and TOF-ERDA analyses were accomplished thanks to the financial support of the CERIC (20177018 proposal) and of the Czech Ministry of Education (grant LM2018116).
The National Institute for Lasers, Plasma, and Radiation Physics
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Present address: C2N - Centre for Nanoscience and Nanotechnology, University of Paris-Saclay, 10 boulevard Thomas Gobert, 91120 Palaiseau, France.