Perovskite photovoltaics have a high light-absorption coefficient (10
4 cm
−1), which results in an impressive photocurrent generation
[103], long carrier diffusion length, and solution processability
[104][105][106]. Perovskite solar cells are cheaper to produce, owing to their inexpensive and naturally abundant materials (lead, iodine, carbon, and hydrogen)
[107]. These solar cells show better defect tolerance than other solar semiconductors on the market
[108], which increases their reusability and lowers production costs. Despite their benefits, the commercialization of perovskite solar cells is hampered by another factor. Water by itself (moisture) and other polar solvents provide a serious problem for perovskite solar cells. These solvents can alter the solvated phases of a perovskite and occasionally hydrate a perovskite to produce a monohydrate phase. Polar solvents can considerably be prevented from harming perovskite solar cells by modifying deposition techniques, according to reports. However, environmental moisture is unavoidable
[109].
4. Dye-Sensitized Solar Cells
Dye-sensitized solar cells (DSSC) are solar cells made of semiconductors that are coated with a dye to increase the efficiency of sunlight
[116]. This type of solar cell was first introduced by Michael Gratzel. DSSCs are composed of a working electrode consisting of fluorine tin oxide (FTO) glass, titanium dioxide (TiO
2), dye, an electrolyte consisting of an I¯/I
3¯ redox pair, and a counter electrode consisting of platinum. DSSCs work in the visible region. DSSC components have undergone various developments over the years to increase their efficiency.
4.1. Advantages and Limitations of Dye-Sensitized Solar Cells
Some of the advantages offered by dye-sensitized solar cells, including their relatively low cost of fabrication, operability under scattered light conditions, and the variable shape of the cell, which can be made opaque or opaque or optically transparent, thereby providing more value from an artistic point of view
[117]. Organic solar cells have dyes derived from organic and synthetic organic materials. Examples of dyes from organic materials are mangosteen, juwet fruit, water henna, nail henna, blueberries, binahong leaves, carrots, kenikir, and mangosteen peel. Synthetic organic dyes such as ruthenium complex (N719) could produce a higher efficiency of 10.4–11.1%
[118][119]. However, the cost of producing ruthenium complex dyes is still relatively high, thus encouraging the development of new dye sensitizers (complexes of osmium, rhenium, iron, and iridium)
[120].
The advantages of using synthetic, organic dyes include their higher conversion efficiency (an efficiency increase of up to 30% in synthetic dyes, with organic dyes being 5%), increased chemical and thermal stability, the color being difficult to degrade compared with organic dyes, and having a higher electron movement than organic dyes. There have been many studies using natural organic dyes and ruthenium dyes with a wide variety of solvents, including research conducted
[121] using nano-particle TiO
2 paste, ruthenium dye (N719), and counter electrodes (platinum) to produce the greatest efficiency of 0.121%. Hardani and co-workers
[122] made DSSC using TiO
2 doping carbon nanotubes with various concentrations and ruthenium (N719) as a dye, and the resulting efficiency was 1.3%. Another research group
[123] used TiO
2 nanoparticles as the active electrode and ruthenium complex (N719) as a dye and the greatest efficiency reached 2.17%.
The solid electrolyte based on PEG polymer gel (polyethylene glycol) containing redox coupling is used (to replace liquid electrolyte) to reduce electrolyte degradation. During the experiment, the doctor blade/slip-casting and spin-coating techniques were used for TiO2 coating. In comparison with other processes, this doctor blade/slip-casting method is incredibly straightforward and uncomplicated. The spin-coating technique is used to deposit homogeneous thin films on a flat substrate. While, the doctor blade/slip-casting technique is a technique for coating the TiO2 suspension on semiconductor glass utilizing a stir rod/spatula by rolling it on the glass surface to flatten the dripping TiO2 suspension. The substrate’s center, which either rotates slowly or not at all, receives a modest amount of coating material. The coating substance is subsequently dispersed throughout the substrate using centrifugal force as the substrate is rotated quickly. The device used for spin coating is called a “coater” or “spinner”. Spin coating is a straightforward TiO2-deposition technology that produces films with regulated thickness and great uniformity by rotating the TiO2 paste at a specific speed.
4.2. Definition and Fundamental of Dye-Sensitized Solar Cells
DSSC generally uses indium tin oxide (ITO) or fluorine-doped tin oxide (FTO) as a substrate. The oxide substrate layer functions as a current collector and the substrate material itself as a sealing layer between the cells in the DSSC and the outside air
[124].
Titanium dioxide (TiO
2) is the material of choice used as a working electrode or photoelectrode in DSSC. The three modifications of TiO
2 include anatase, rutile, and brookite, but only anatase and rutile are stable
[125]. Brookite is difficult to produce and therefore is not considered in DSSC applications
[126]. The particle sizes of anatase and rutile increase
[127] with increasing temperature.
Figure 3, showing different growth rates, shows that rutile has a much higher growth rate than anatase. The growth rate of anatase is flat at 800 °C.
Figure 3. Changes in the particle size of anatase and rutile as a function of annealing temperature
[127].
The absorbance spectrum defines the possibility of a useful semiconductor for photocatalysts having a band gap (E
g) proportional to the photon energy of the visible or ultraviolet spectrum (Eg < 3.5 eV). Most authors have determined that the energy gap of rutile is 3.0 eV and that of anatase is 3.2 eV. A larger active surface area and a more effective photocatalyst
[128] can be observed in the anatase phase.
The photocatalytic process is based on the dual ability of a semiconductor material (TiO
2) to absorb photons and carry out transformation reactions at the material junction simultaneously.
Figure 4 shows several photoexcitation pathways
[129] followed by electron and hole de-excitation. The enlarged portion of
Figure 4 shows electrons excited from the valence band (E
v) to the conduction band (E
c) due to photons with energy (
hv) equal to or greater than the band gap of the semiconductor.
Figure 4. The schematic of photoexcitation followed by de-excitation on the semiconductor’s surface
[129].
From Figure 4, it is assumed that the semiconductor remains intact, and the charge transfer is uniform to the adsorbed organic or inorganic molecules. The excited electron-hole pair can go through several paths as follows: the semiconductor can donate electrons on the surface to reduce the adsorbed acceptor (A/A−), for example, oxygen (lane c). Holes can migrate to the surface and electrons from donors (D/D+) can join holes (c lanes). Competition for electron transfer to adsorbed molecules by the separate recombination of electrons and holes occurs in the volume of the semiconductor particles (band b) or on the semiconductor’s surface (lane a).
Electron paramagnetic resonance spectroscopy (EPR) showed that the detected electrons were either captured as Ti
3+ or as electrons in the conduction band, while the holes were trapped in the O
− free-oxygen center resulting from the O
2− lattice in the valence band. The reaction is written as follows
[130]:
5. Thin-Film Solar Cells
Currently, there are different types of thin-film materials that have been and are employed in solar-cell applications. Technically speaking, these materials have several advantages such as low production cost, being environmentally friendly, and the formation of films can be conducted in various substrates
[131].
5.1. CdTe Film Solar Cells
Cadmium telluride (CdTe) films show high degradation durability, near-optimum band-gap value (1.45 eV), a low production cost, and high direct absorption coefficient (10
4 cm
−1), and can be used to replace silicon-based solar cells. Generally, CdTe-based solar cells consist of glass, transparent conducting oxide (serving as the front contact), a CdS window layer, a CdTe absorber layer, and back contact. However, this type of solar cell has many problems such as glass breakage (during the production process) and poor thermal conductivity (causing very poor performance). The preparation of CdTe films onto flexible metal foils and polymer substrate can solve these problems. The power conversion efficiency was shown to be 11% for devices prepared on polyimide foils
[132], indicating the superstrate and substrate configurations. Several deposition methods have been reported to produce cadmium-tellurium thin films (
Table 1). The photovoltaic properties of the obtained films were studied.
Table 1. The growth of CdTe films using various deposition methods and the photovoltaic properties.
5.2. Cu(In,Ga)Se2 Film Solar Cells
The Cu(In,Ga)Se2 (CIGS) films are already available in the global solar panel market due to their excellent radiation tolerance, high absorption coefficient (105 cm−1), suitable band gap value (1.04 eV to 1.65 eV), and long-term stability. However, these films are very expensive because of the indium and gallium (poor abundance). Several deposition techniques have been reported for the preparation of CIGS films and the photovoltaic parameters were studied as well (Table 2). Thin films could be deposited onto rigid glass substrates, and successfully reached power conversion efficiencies of up to 20.3%. However, the current focus of the production of films onto flexible substrates (metal foils and polyimide films) is because of their significantly lower cost. Polyimide films are highly desirable due to electrical insulation and enabling direct monolithic interconnection.
Table 2. The growth of CIGS films using various deposition methods and the photovoltaic properties.
5.3. Cu2ZnSnS4 Film Solar Cells
Copper–zinc–tin sulfide (Cu2ZnSnS4) films are becoming increasingly prominent for several reasons. These materials could replace indium and gallium in CIGS films, as their constituents are Earth-abundant and do not contain cadmium (toxic material). The experimental results showed that CZTS-based solar cells have an excellent absorption coefficients (more than 104 cm−1), tunable band gap values (1.45 eV to 1.6 eV), and greater stability in the kesterite phase (Figure 5) when compared with stannite and wurtzite. Researchers have reported that vacuum and non-vacuum deposition techniques have been used to produce CZTS films (Table 3). Further, they concluded that high-quality films could be synthesized using the vacuum-deposition method; however, this entails increased production costs.
Figure 5. Crystal structures of (
a) stannite and (
b) kesterite
[165].
Table 3. The growth of CZTS films using various deposition methods and the photovoltaic properties.
5.4. CuInX (X = S, Se, and Te) Film Solar Cells
Several deposition methods have been used to produce CuInSe2, CuInTe2, and CuInS2 films (Table 4). It is noted that the non-vacuum deposition method offers an attractive cost-saving opportunity, higher deposition speed, and less waste of chemicals when compared with the vacuum-deposition technique. The obtained films could be employed for solar-cell applications because of long-term stability under solar radiation, excellent absorption coefficients (105 cm−1) in the visible light portion, and suitable band gap values (1.5 eV).
Table 4. The growth of CuInX (X = S, Se, and Te) films using various deposition methods and the photovoltaic properties.
5.5. Other Metal Chalcogenide Film Solar Cells
Several types of metal sulfide, metal telluride, and metal selenide films have been prepared using different deposition methods. The photovoltaic behavior of these films was studied (Table 5) as reported by many researchers.
Table 5. The growth of different types of thin films using various deposition methods and the photovoltaic properties.