Applied Science and Convergence Technology 2022; 31(1): 1-8
Published online January 31, 2022
Copyright © The Korean Vacuum Society.
aPhotoelectric and Energy Device Application Lab (PEDAL) and Multidisciplinary Core Institute for Future Energies (MCIFE), Incheon National University, Incheon 22012, Republic of Korea
bDepartment of Electrical Engineering, Incheon National University, Incheon 22012, Republic of Korea
cSolarLight Ltd, 119 Academy Rd., Incheon 22012, Republic of Korea
†These authors equally contributed to this work.
Transparent photovoltaics (TPVs) are a crucial energy platform for harvesting solar energy in windows, enabling onsite power generation for widespread applications in buildings, vehicles, displays, sensors, and the Internet of things. TPV devices are fabricated using eco-friendly processing methods and materials, and must perform stably for an adequate societal impact. This review article is focused on the emerging TPV devices made of inorganic materials, including oxides and two-dimensional sulfides. Herein, we briefly review the wide-bandgap inorganic TPVs and their performances. Specifically, the sputtering method is considered for the large-scale and eco-friendly preparation of inorganic heterostructures.
Keywords: Transparent photovoltaic, Inorganic thin film, Onsite power generation, Large-scale fabrication
The two outcomes of the industrial development in the modern era, viz. a high energy demand and global warming, have compelled the technologists to turn to naturally available and environment-friendly sources of renewable energy. Figure 1(a) indicates a growth in global electricity production, using renewable energy sources, from 24.5 (20 16) to 26.3% (2018) . Among the sources of renewable energy, electricity production from wind and solar energy showed a remarkable increase from 4.0 to 5.5% and 1.5 to 2.4%, respectively, between 2016 and 2018, indicating the focus that these technologies have received in the recent years. Photovoltaic technology has the capacity to meet the requirements of the future energy demand of humankind with a reduced environmental impact. The amount of energy that the Sun delivers to Earth per hour is approximately equal to the total energy consumption of humanity per year (~5 × 1020 J in 2018) [2–5]. In the recent years, photovoltaic technology has undergone commendable improvements, resulting in the development of highly efficient, lowcost, and environment-friendly solar cells. These achievements have been realized by modifying both the fabrication processes and materials used. The current renewable energy trends indicate that the renewable electricity production will exceed the energy production from fossil fuels by 2026. Likewise, in 2040, two-thirds of the global electricity supply is expected to be derived from renewable sources. Expectedly, solar and wind power would produce 40% of the energy used, and the other renewable energy sources, including hydropower and bioenergy, would account for 25% of the total energy production .
At present, 95% of the photovoltaic device market is controlled by Si-wafer-based solar cells, which are first-generation products. To improve the efficiency–cost ratio, second-generation cells, based on the thin-film technology, have been introduced. Thin-film solar cells made of cadmium telluride, copper indium gallium sulfide, and copper zinc tin sulphur selenide show efficiencies of 22.1, 22.6, and 12.6%, respectively [18–22]. The emerging photovoltaic techniques are included in the third generation of solar cells, mainly perovskite, tandem, dye-sensitized, organic, and quantum-dot solar cells with power conversion efficiencies (PCEs) of 25.2, 29.1, 12.3, 17.4, and 16.6%, respectively [18, 23, 24].
However, the large surface area, required for the installation of these opaque photovoltaics, is a hindering factor for the energy requirements of humankind. Thus, transparent photovoltaic (TPV) cells are attracting significant interest in photovoltaic research because of their wide application as building-integrated, building-applied, and vehicle-integrated photovoltaic devices [25, 26]. Hence, TPV technology could aid in the conversion of the largest energy consumers into the largest power plants. In conventional solar cells, the absorption of visible light leads to the generation of electron–hole pairs that produce current. In contrast, TPVs allow the visible light to pass through and generate current by absorbing ultraviolet (UV) light photons. Therefore, TPV devices meet our high energy demands as well as block the hazardous part of the electromagnetic spectrum. The Shockley– Queisser maximum theoretical efficiency limit for a TPV is ~20%, whereas for the conventional photovoltaics (CPVs), this limit is ~32% [26,27]. Although the TPVs exhibit low PCEs than do the CPVs, their features like transparency, onsite power production, protection from high-energy photons like UV, and high flexibility paved a new pathway for the widespread application of TPVs.
Table I shows a comparison of the photovoltaic performance of different types of TPVs along with their average visible-light transmittance (AVT) values. Figure 1(b) compares the reported highest PCE and AVT values for different types of TPVs (AVT > 40%) such as screen-printed dye-sensitized solar cells (SP-DSSC), near-infraredorganic solar cells (NIR-OSC), polymer solar cells (PSCs), perovskite solar cells (PVSC), electrophoretic deposition dye-sensitized solar cells (EPD-DSSC), and dip coated-dye sensitized solar cells (DC-DSSC) [6, 7, 10, 11, 13, 14, 28, 29]. Among these, the highest PCE, i.e., 12.7% (AVT of 77%), is exhibited by the perovskite solar cells with a device configuration of TiO2/MAPbI3/spiro-OMeTAD/AgNW (where NW represents nanowire). However, the use of conventional organic spiro-OMeTAD in the device configuration as a hole transport layer hinders its further development owing to the complex synthesis procedure, high cost, and instability. The commercially available spiro- OMeTAD is nearly 23 times more expensive than Au and Pt, by its weight percentage . Hence, although these devices exhibit high PCEs (> 4%), their toxicity and high costs, along with the low stability of their organic molecules, are the major limitations of these devices. Consequently, the recent investigations in the field of TPV technology have been focused either on overcoming these hurdles by introducing suitable alternatives or moving toward inorganic TPV technology. Thus, inorganic TPV technology has become more attractive because of the incorporation of wide-band-gap metal oxides as the active UV layer.
Wide-band metal oxides are promising candidates in the search for naturally available and acceptable semiconductor materials, in the field of inorganic TPVs [31–33]. Figure 2 presents a schematic diagram of an inorganic TPV. It consists of top and bottom transparent electrodes, separated by a surface field layer, and wide-band-gap metal oxides with n-type conductivity, such as ZnO, TiO2, and so forth, and those with p-type conductivity, such as NiO, CuAlO2, and Cu2O. The wide band gaps of these materials allow visible-light transmittance and UV-light absorbance, which are the underlying principles of TPV applications. Incident UV photons with energies higher than the band gap of the materials result in the formation of electron–hole pairs. Further, charge separation is achieved using built-in electric fields generated by the p–n heterojunction. The separate extraction of these charge carriers to the external circuit results in the generation of electricity. The surface field layer prevents the recombination of the minority carriers.
A literature review indicated that inorganic TPVs have attracted considerable research interest since 2013 because of the improvements in photovoltaic performance, facilitated by the incorporation of various mechanisms, to overcome the factors that limit device efficiency [15, 33–43]. Over the past few years, several research groups have reviewed the technologies underlying TPVs, semi-transparent thin-film TPVs, and organic TPVs [6,26,44–46]. However, no reports are available on inorganic TPVs. Thus, considering the promising aspects of inorganic TPVs, in this review article, we focus on the fabrication and mechanisms of inorganic TPVs, along with the improvements in their performance (AVT < 40%), which may provide support to the future investigations for attaining commercially acceptable cost– efficiency ratios. In the TPVs, the p–n heterojunction is typically prepared using the reactive sputtering technique, and consequently, this review article highlights the optoelectrical properties of the reactivesputtered wide-band-gap metal-oxide films.
Some widely used wide-band-gap metal oxide materials are NiO, Cu2O, and Cu delafossite (CuGaO2, CuCrO2, CuFeO2, and CuFeO2), which are p-type materials, and ZnO, AZO, TiO2, SnO2, and Zn2SnO4, which are n-type materials. Table II compares the optical and electrical properties of these metal oxides along with their deposition parameters [37,39,47–59]. Figure 3 presents an energy-level diagram of both the p-type and n-type metal oxides [33, 60–66].
NiO has a rock salt crystal structure with octahedral Ni2+ and O2− sites. It exhibits a wide band gap in the range of 3.5–4 eV, allowing a higher transmission of visible light, p-type conductivity, large absorption coefficients (105 cm−1), high stability, and high work function (~5.0 eV) [30, 68]. Due to its low electron affinity (1.33–1.85 eV), a thin NiO film induces the effect of electron blocking with a high carrier mobility (~1–10 cm2/Vs), i.e., an efficient hole-transporting property [52, 53]. The high exciton-binding energy (110 meV) of the NiO films enables a lower thermalization loss of the exciton . These excellent optical and electrical properties make it a suitable material for a wide range of applications, such as an anode buffer layer in organic solar cells, a hole transport layer in PSC, radiation detectors, laser materials, thermoelectric devices, and so forth. Further tuning of the electrical properties (carrier concentration, hole mobility, and work function) is attained through doping with Cu+, Ag+, Cs+, Li+, and Mg2+, which can improve the reported PCE of the PSCs [30,70,71]. Although NiO thin films have been prepared using various wet chemical routes, chemical vapor deposition, and physical vapor deposition, TPV technology is mainly focused on large-scale preparation methods like reactive sputtering [33,39,68]. Sputtering techniques have the advantage of realizing higher uniformity over a large surface area, a higher film density, and stronger adhesion. Warasawa
Cuprous oxide (Cu2O) is a well-known p-type semiconductor with a comparatively high band gap of 2.38–2.51 eV, high carrier mobility, high diffusion length, and high work function (~5.0 eV). It has a cubic crystal structure, and each unit cell consists of four Cu (at the fcc position) and two O atoms (at tetrahedral sites). The p-type conductivity of Cu2O is due to the presence of Cu vacancies (VCu) [60, 72].
Cu delafossite metal oxides, such as CuGaO2, CuCrO2, CuAlO2, and CuFeO2, have recently attracted significant attention as suitable materials for hole transport layers in PSC. The delafossite metal oxides have an A+B3+O2 structure, in which a sheet of monovalent A ions are stacked between the edge-shared octahedral BO6 ions. The Cu delafossite metal oxides are p-type semiconductors with low electron affinities (~2.1 eV), high carrier mobilities (~100 cm2/Vs), and large diffusion lengths . These materials exhibit a direct energy band gap in the range of 3.2 to 3.6 eV, with a high visible-light transmittance (~80%), which makes them a suitable choice for TPVs. The p-type conductivity of these materials is correlated with the formation of Cu defects and O interstitials . Yu
Table II shows a comparatively high reaction temperature for CuCrO2 and CuGaO2, and such a high temperature is disadvantageous for the real-life applications of TPVs. In contrast, Zhang
ZnO has garnered significant attention as an n-type material for a wide range of applications. ZnO has a wurtzite hexagonal crystal structure with a direct energy band gap of ~3.2 eV, high electron mobility, and large exciton binding energy (60 meV), rendering it feasible for the fabrication of p–n heterojunctions . The tetrahedral coordination in ZnO results in a non-symmetric structure with piezoelectric and pyroelectric properties. Patel
TiO2 is another widely used n-type polymorphic semiconductor with anatase, brookite, and rutile crystal structures. Among these, the rutile structure is highly stable at macroscopic sizes, whereas the anatase occurs only in nanoscopic sizes. Brookite is the rarest naturally occurring form of TiO2. Anantase-TiO2 exhibits a direct energy band gap of 3.2 eV with an electrical resistivity of ~0.1 Ω·cm, a donor concentration of ~1017 cm−3, and a bulk mobility in the range of 0.1–4.0 cm2/Vs . Patel
Sn-based metal oxides are attractive owing to their excellent optoelectronic properties, which are either similar to or better than those of TiO2. The low electron mobility of TiO2 causes inadequate charge separation at the heterojunction. SnO2 is a wide-band-gap (3.6–4.0 eV) n-type semiconductor with a rutile crystal structure. The highly efficient charge extraction property of SnO2 is mainly due to its deep conduction band and high carrier mobility (100–200 cm2/Vs) . Zn2SnO4 (ZTO) is another well-known n-type semiconductor with a wide direct-energy band gap of 3.8 eV and refractive index of ~2, which allows a high visible-light transmittance . The conductionband edge position of ZTO is similar to that of TiO2, and it exhibits a high electron mobility of ~10–15 cm2/Vs, which allows ~10 times faster electron transport with a higher charge collection efficiency than that of TiO2. The O vacancy (VO) in ZTO acts as a double-ionized donor that contributes two electrons to the electrical conduction, such that the carrier concentration can be improved by annealing under a reducing atmosphere . Although these Sn-based transparent conducting oxides have gained notable attention as electron-transportlayer materials for PSCs, they have not been used in TPVs until recently.
To improve the PCE further, Nguyen
This study was performed to investigate the potential of inorganic TPVs to meet the global energy requirements with a reduced environmental impact. Inorganic TPV technology has the ability to produce cost-effective and highly stable solar cells with higher efficiency to convert the largest energy consumers, such as buildings and vehicles, into the largest energy producers. In the recent years, the semiconductor technology has been advanced with the introduction of various wide-band-gap metal oxides that have excellent optoelectrical properties and can be used in various photovoltaic applications. Among these materials, NiO/ZnO heterojunctions with AVT and PCE values of 70.00 and 0.56%, respectively, or Cu2O/ZnO heterojunctions with AVT and PCE values of 80.00 and 0.46%, respectively, have become the focus of inorganic TPV research. The addition of an AZO layer as the surface field layer plays a significant role in improving the photovoltaic performance of both the NiO- and Cu2O-based TPVs. Further, the introduction of a surface field layer of TiO2 in NiO/ZnO-based TPVs yields
This study was supported by the Basic Science Research Program through the National Research Foundation (2020R1A2C1009480 and 2020R1I1A1A01068573), funded by the Ministry of Education of Korea, Brain Pool Program funded by the Ministry of Science and ICT (NRF-2020H1D3A2A02085884), and Korea Institute of Energy Technology Evaluation and Planning (KETEP-20203030010310) of Korea.
The authors declare no conflicts of interest.