Transparent photovoltaic cells and self-powered photodetectors by TiO2/NiO heterojunction
Graphical abstract
Introduction
The ever-growing technological advances significantly depend on the energy sources. Most of the energy demand has been mostly fulfilled by fossil fuels. The addiction on that not only causes environmental hazards but also results into depletion of the fundamental resources of the Earth. The feasible and valuable solution is the utilization of renewable or green energy sources to overcome the rapidly growing energy demand. In this context, solar energy sources emerge as the most reliable, abundant, efficient, sustainable, inexhaustible and clean energy source [[1], [2], [3], [4]]. Solar energy is frequently used in our daily life including photocatalysis, heating water and generating electricity [5]. The application and development of photovoltaic devices and solar cells are the most promising choices to convert solar power into electricity [6,7]. These devices can be used for wide range of applications including medical purposes, military defences, self-powered electronics, and energy effective environment, such as clean energy vehicles or hydrogen energy production [[5], [6], [7], [8]].
The current focus of research community is to design and fabricate cost effective and reliable photoelectric devices. The performance of solar cell is governed by photo-generated electron-hole pairs in the semiconductor material by absorbing photon energy [7,9]. The use of opaque surface of conventional solar panels is a critical issue to hinder the wide utilization in the human life. To overcome this problem, the transparent photovoltaic cells (TPCs) are the promising approach, because they ideally needs no extra space for installation as transparent power generators [5,6,10]. Moreover, the transparent photovoltaic solar cell is not the vision barrier to human eyes, and thus it can be the invisible energy source to be applied as power windows in mobile electronics, displays, vehicles, and buildings. The key requirement of the semiconductor material for TPCs can pass the visible wavelength light to be transparent to human eyes, while absorbing the invisible light of ultraviolet or infrared range.
Till date various wide bandgap semiconductors, like metal-oxide and chalcogenides have been developed and used for solar cells [[11], [12], [13], [14]]. The metal-oxide materials are potentially regarded as promising partners or alternatives of conventional silicon based solar cells. Moreover, the non-toxic metal-oxide entities may induce the friendly uses in human electronics. Some metal-oxide materials such as, TiO2, ZnO, CuO, and NiO have been applied for various types of photovoltaic cells [7,[15], [16], [17]]. Among all, TiO2 is one of the most desirable materials for photovoltaic devices because it exhibits excellent optoelectronic properties, wide bandgap, and eco-friendly in nature [18,19]. TiO2 has been widely studied for various photovoltaic structure such as dye sensitized, perovskite and polymer solar cells which clearly shows its vital role for developing the third generation of solar cells [20,21]. Apparently, the quest for rapid, large-scale, and reliable fabricating methods to obtain high quality TiO2 plays a key step in realizing the practical application of photovoltaic cell.
Developing heterostructure with TiO2 for photovoltaic cell has gained huge research interest. Due to the stable formation of electric field at the interface, photo-generated carriers can be collected more efficiently, resulting in device performance improvement. Different semiconductors can combine with TiO2 to establish a stable junction, such as SnO2 [22], BiOCl [23], CdS [24], and MoS2 [25]. Among them, p-type NiO has been recently considered for the good counterpart of n-TiO2 to form the efficient metal-oxide p/n junction due to the high mobility [26], optical transparency [27], natural availability properties.
The potential of NiO/TiO2 heterostructure was demonstrated in various research subjects, including photo-detection, electrochromic application and photocatalyst [18,26,[28], [29], [30]]. In the regard of photon-electric conversion application, K. Khun et al. first time utilized NiO/TiO2 heterostructure for UV photodetector based on TiO2 nanorods, achieved by simple hydrothermal method [26]. The study presented the advantage of built-in electric field at NiO/TiO2 heterojunction for self-powered TiO2 based UV photodetector. Yanyan Gao et al. further improved the UV detecting capability of NiO/TiO2 heterostructure through controlling growth of TiO2 nanorods which resulted in enhancement of built-in electric field [18]. The aforementioned researches demonstrated the importance of NiO/TiO2 heterojunction for efficient UV utilizing device based TiO2. Nevertheless the great potential, no or less researches have been reported on the NiO/TiO2 transparent photovoltaics.
Herein, the solid-state NiO/TiO2 heterojunction was fabricated for transparent photovoltaic cells. The photovoltaic effect is demonstrated for different polymorphs of TiO2 of Anatase and Rutile structures. Both TiO2 structures were synthesised by rapid and large-scale methods. Above n-TiO2 layer, p-type metal oxide of NiO is deposited to form the transparent p/n heterojunction. The FTO and silver nanowires (AgNWs) act as the bottom and the top electrodes, respectively. Devices possess the excellent UV responses under the photovoltaic mode (0.23 A W−1 for Rutile-TiO2 and 0.19 A W−1 for Anatase-TiO2) with high transparency in visible light region (about 57%). Furthermore, the transparent photovoltaic NiO/TiO2 cells exhibit reasonable power conversion efficiency of UV light (2.1% for Rutile-TiO2 device and 0.9% for Anatase-TiO2). This work indicates the promising functional use of metal oxides NiO/TiO2 heterostructure for transparent solar photovoltaics.
Section snippets
Device fabrication
Prior to the depositing process, FTO/glass substrates (735,159 Aldrich, sheet resistance 7 Ω/□) were cleaned with a sequence of acetone, methanol and deionized water under ultra-sonication for 10 min and then dried by flowing nitrogen gas. Rutile TiO2 layer was prepared by two steps including DC sputtering of Ti metal and oxidation of Ti layer by rapid thermal processing (RTP) method. Ti was deposited by DC sputtering (SNTEK, Korea) using Ti target (iTASCO, purity 99.99%). The Ar flow rate of
Result and discussions
Fig. 1a represents the schematic of TiO2-based transparent photovoltaic device. Two types of n-type TiO2 (Anatase or Rutile polymorphs) structures were prepared and investigated as the light absorber. The cross-section SEM image of both TiO2 devices are shown in Fig. S1 (Supporting Information). This result shows average thicknesses of TiO2 films (Anatase and Rutile) are ~350 nm. The p-type semiconductor NiO serves as the heterojunction counterpart to n-type TiO2. The FTO and silver nanowires
Conclusions
The low cost and functional transparent photovoltaic devices have been realized by p-NiO/n-TiO2 heterostructure with different polymorphs of Anatase and Rutile phase of TiO2. The transparent AgNW/NiO/TiO2/FTO device can operate as a self-powered photodetector and a power generator (solar cell), attributed to the established built-in potential at the interface of NiO/TiO2 staggered gap heterojunction and efficient photon-excited carrier collection. The transparent solar cell (AgNW/NiO/Rutile-TiO2
CRediT authorship contribution statement
Thanh Tai Nguyen: Methodology, Formal analysis, Data curation, Writing - original draft. Malkeshkumar Patel: Data curation, Writing - original draft. Sangho Kim: Formal analysis. Rameez Ahmad Mir: Visualization. Junsin Yi: Supervision, Writing - original draft, Writing - review & editing. Vinh-Ai Dao: Supervision, Writing - original draft, Writing - review & editing. Joondong Kim: Supervision, Funding acquisition, Writing - original draft, 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.
Acknowledgements
The authors acknowledge the financial support of High Risk, High Return research program (2019) in Incheon National University and the Korea Institute of Energy Technology Evaluation and Planning (KETEP-20203030010310) of the Republic of Korea.
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