The solar cell device is classified based on materials, namely silicon-based solar cells, organic-based semiconductor solar cells. In a planar structure, solar cells have the disadvantages of a high level of impurities and crystalline defects. In nanowire-based solar cells, it has many optical and electrical advantages, such as beam optics, light tapping absorption, short diffusion lengths for the high aspect ratio of the minority carrier.
To combine the advantages of compound semiconductors and nanowires, many researchers have studied solar cells based on III-V semiconductor nanowires. Consequently, we fabricated a solar cell device using InAs0.75P0.25/InP nanowires on silicon wafers grown as an induced strain method. We find that solar cells using densely grown InAs0.75P0.25/InP nanowires have low conversion efficiency for the agglomeration effect.
Furthermore, we find that solar cells using InAs0.75P0.25/InP nanowires have the best efficiency at a density of 45 × 106/cm2. We also construct a process to fabricate solar cells based on InAs0.75P0.25/InP nanowires for high conversion efficiency.
Introduction
Overview of photovoltaic industries
Classification of Solar Cells
Because of that, most solar cells that have been commercialized are those based on silicon[24]. Moreover, the band gap of Si is relatively small (1.1 eV), therefore the theoretical limit for Si-based solar cells is 33.7% (Shockley-Queisser limit)[26]. Organic semiconductors based on the solution process have attracted considerable interest as an alternative to conventional inorganic solar cells.
The following main advantages of organic solar cells have been identified: light weight and flexibility of solar cells, semi-transparency, easy integration into other products, lower production costs compared to conventional inorganic solar cells[27]. Indeed, CIGS has proven to be the most successful I-III-VI compound semiconductor solar cell[28]. CdTe, a II-VI compound semiconductor, solar cells are manufactured by a low-cost process and have high efficiency.
These solar cells such as CIGS or CdTe are commonly called thin film solar cells for low cost solar cells. Because when compared to crystalline Si-based solar cells, they require semiconductor materials that are only a few micrometers thick. And also, Te is one of the rarest stable solid elements in the earth's crust and Cd affects the human body toxically.
These weaknesses of planar wafer-based crystal silicon solar cells lead to a reduction in the minority carrier diffusion length, consequently reduced conversion efficiency of solar cells. Furthermore, planar wafer-based crystalline silicon solar cells have less optical properties than nanowire-based solar cells[30]. Nanowire-based solar cells have a very short diffusion distance and require fewer materials than a plane-based solar cell.
And also, nanowire geometries offer the advantages of reduced light reflection, extreme light trapping, which offers the opportunity to improve efficiency compared to planar wafer-based crystalline silicon[30].
Principles of solar cells
Typical voltage-current characteristics, known as the J-V curve, of a diode without irradiation are shown in the dark in Figure 1.7. The curve shows the turn-on and build-up of the forward bias current in the diode. When sunlight enters, the J-V curve shifts and indicates that external current flows from the solar cell to a passive load.
However, when the voltage has a forward bias (V > 0), the solar cell has a much larger current than the reverse bias (V < 0). The total current and voltage of the solar cells can be approximated as the sum of the short-circuit current and the dark current. 𝑆𝐶𝑉𝑂𝐶 (1.6) The conversion efficiency of solar cells is the power density delivered at the point of operation as a fraction of the power density of incident light, Ps.
When calculating the conversion efficiency of solar cells, contact resistance and leakage currents are also taken into account throughout the device. These effects are electrically equivalent to two parasitic resistances in series (Rs) and (Rsh) with the solar cell. When current flows in the solar cell device, the resistance of the device material leads to increased series resistance, especially through the front surface to the contacts and from the resistive contacts.
Leakage of current through solar cells leads to increased shunt resistance, around the edges of the device and between contacts of different polarity.
Experiments
Spin on dopant (SOD) process
Growth of InAs 0.75 P 0.25 /InP nanowires
Fabrication of solar cells
Results and discussion
Morphological distribution of InAs 0.75 P 0.25 /InP nanowires
InAs0.75P0.25/InP nanowire arrays using strain-induced growth mechanisms have varying morphology across the wafer due to the inhomogeneous strain and large compositional distribution.[32] First, we checked the SEM image of InAs0.75P0.25/InP nanowires grown on a p-Si (111) wafer. We also observed that the InAs0.75P0.25/InP nanowires on the p-Si (111) wafer have different lengths and diameters with each density. However, we observed that nanowires tend to coalesce after the BCB coating process at densities greater than 75×106/cm2.
The assembled nanowire arrays have poor optical properties and conversion efficiency of solar cell. Therefore, we fabricated a solar cell using InAs0.75P0.25/InP nanowire, which has a density of 45.
ITO deposition by e-beam evaporator
The uniformity of the ITO layer occurred from the self-shadowing effect and the angular distribution of the deposition rate that follows the cosine distribution for oblique angle deposition (OAD). From the morphological changes, the effective surface area of ITO films clearly depends on the tilt angle [33]. For the inclined columnar structures, the morphological characteristics certainly have an influence on the optical properties of the ITO film.
The step coverage of evaporated films is poor due to the directional nature of the evaporated material. To solve the step coverage problem, we shook the substrate on the e-beam evaporator with shaken and rotated. When we uniformly deposit an ITO layer on nanowires, we observe that the ITO layer is uniformly deposited at an OAD of 40.
An ITO layer is uniformly deposited on the nanowires under an OAD of 40° to reduce the shadow effect. As a result, solar cell devices deposited at an ITO OAD of 40° have the highest Jsc, i.e. the highest efficiency (6.4. When the ITO layer is too thin below 400 n, this results in increased series resistance.
Otherwise, when the ITO layer is too thick, above 400 nm, the decrease in conversion efficiency is due to the increase of light absorption in the ITO layer.
Reactive ion etching (RIE) treatment for exposing nanowires
There is a method of effective surface passivation, which is a built-in electric field on the back side as a low-high junction, which improves not only the Jsc current, but also the Voc of a solar cell. For these reasons, a boron-diffused back surface field (BSF) is more suitable than an aluminum alloy back surface field (BSF). The back surface field (BSF) layer generates a surface field that repels the minority carriers away from the back surface.
In solar cell technology, the improvement of the conversion efficiency depends not only on the structure and materials, but also on the optimization of the front finger pattern design. If the grid line spacing is too narrow or too wide, it can cause large power losses as the current density generated by the solar cell is too high. The conversion efficiency of the solar cells was monitored using two types of grid pattern having finger spacings (F/D) of 940 μm and 440 μm with a constant tapered guide of 300-60 μm.
We produced grid patterns on InAs0.75P0.25 / InP nanowire-based solar cells in the size 2cm×2cm. We optimized this solar cell fabrication process towards high efficiency using an InAs0.75P0.25/InP nanowires grown by strain-induced method. It leads to reduce open circuit voltage (Voc), thus reducing the conversion efficiency of solar cells.
Otherwise, when the ITO layer is too thick above 400 nm, the decrease in conversion efficiency is due to the increase of light absorption on the ITO layer. The optimal condition of the ITO layer is a tilt angle of 40 °, a deposition temperature of 240 ℃, a thickness of 400 nm. We note that the most exposed tip of the nanowire does not necessarily have the best performance due to the disconnected ITO layer.
The back surface field (BSF) of the p-type layer generates a surface field that repels minority carriers from the back surface. Characterization of large surface area cadmium telluride films and solar cells deposited on near sublimation moving substrates (Doctoral Dissertation, University of South Florida), 2003. Optical and Electrical Effects of Gold Nanoparticles in the Active Layer of Solar Cells polymer.
Back surface filed (BSF) effects
Grid pattern design for top metal contact
If the spacing is too narrow, the grid shadow loss will be greater, whereas if the spacing is too wide, the series resistance loss will be greater. Plate resistivity, the power loss due to the emitter resistance can be calculated as a function of the finger distance in the top contact. The current can be collected from the base close to the finger and therefore has only a short distance to flow to the finger or alternatively if the current enters the emitter between the fingers then the length of the resistive path seen by such a carrier is half of the the grid spacing.
Conclusion
View-angle deposited ITO films for efficiency improvement of a-Si:H/μc-Si:H tandem thin-film solar cells, OPTICS EXPRESS p.
