Development of calcium titanium oxide coated silicon solar cells for enhanced voltage generation capacity

Calcium titanium oxide (CaTiO₃), belonging to the family of perovskites, was chosen as the AR coating material. It has the general structure of perovskite ABX₃. In this structure, both A and B represent the metal, and X signifies the oxygen molecule. The selected material has unique properties, such as (i) high photoreactivity, (ii) high oxygen ion vacancy tolerance, (iii) no magnetic field produced during charge transfer [8], (iv) stable structure even at a pressure of 120 GPa and temperature up to 2500 K, and (v) superconducting properties even at high temperatures [9].

The chosen AR coating material was received in powder form. However, RF sputter coating system requires coating material in the form of solid target. Hence, the solid target was prepared by pelletization process in which a pellet with dimensions of 50.8 mm diameter and 5 mm thickness was formed by using developed mild steel (MS) die.

For this process, universal testing machine (UTM) was used to make 15 g CaTiO₃ powder into solid target by applying a load of 1.5 GPa on the die. Finally, the compacted solid target was held at a temperature of 100 °C for 10 min in an electric furnace to remove the moisture.

Sputter coating machine (VR SPU-06D) was used to deposit CaTiO₃ film (AR coating) on the top surface of silicon substrate. In this study, both the target and substrate were positioned at a distance of 5 cm by holders which were mounted inside the vacuum chamber. Thin film coating on the substrate was performed at varying time durations between 10 min and 45 min with some constant process parameters during the coating which were as follows: argon pressure of 0.7 GPa, vacuum chamber pressure 6 Pa, and process power of 80 W. When the vacuum pressure reached the preset value, argon gas was introduced into the chamber which acted as a working medium to deposit the target particles over the substrate.

SEM images were taken to assert whether the thin film coating was present on the silicon substrate by applying ICON-Quanta 200 Mark II environmental scanning electron microscope. EDS analysis was carried out on AR coated substrates to find the elemental percentage value and chemical composition of the prepared substrate by using SIGMA HV-Carl Zeiss with Bruker Quantax 200-Z10 EDS detector. FESEM analysis was involved to perform crystallographic study of the developed thin film coating by employing SIGMA HV-Carl Zeiss with Bruker Quantax 200-Z10 equipment.

To evaluate voltage generation capacity for both AR coated and bare solar cell, this analysis was carried out in controlled atmosphere (sodium vapour lamp). Radiation and temperature were controlled to the required value due to periodic fluctuation of radiation in open atmosphere. Pyrheliometer was used to measure direct and diffuse radiation of light energy. Infrared thermometer was utilized to assess the temperature of both bare as well as AR coated solar cells.

SEM and EDS analyses carried out for AR coated substrates demonstrate the CaTiO₃ layer formation (Fig. 1) and presence of Si, Ca, Ti and O elements (Fig. 2) on the prepared substrate.

The coated substrate was examined through EDS and the obtained atomic concentrations have been depicted in Table 2. EDS confirmed the presence of Si, Ca, Ti and O elements and their corresponding atomic concentrations. Since AR coating was deposited at room temperature, the obtained atomic concentration of elements were unbalanced.

Table 2

Amounts of Si, Ca, Ti and O elements on the substrate obtained through EDS analysis.

From the literature, it was evident that sputter coating carried out at high temperature (⩾600 °C) may lead to achievement of balanced atomic concentrations. However, high temperature processing forms anatase phase [10] and also damages the substrate. Anatase phase generates small isolated and sharp crystals which leads to the optical losses [10].

From Fig. 3, it is evident that the crystallite size of the CaTiO₃ varies between 80 nm and 150 nm on the silicon substrate and that the particle size of thin film AR coated solar cell generally coincides with the particle size of raw CaTiO₃ material.

Under controlled atmospheric conditions, the bare and coated solar cells were exposed to radiation level ranging between 200 W/m² and 450 W/m² for a specified time to attain a temperature of 35 °C to 55 °C. The corresponding percentage increment in cell voltage of AR coated silicon solar cells in comparison to the uncoated one is presented in Table 3.

Table 3

Percentage difference in cell voltage generation of AR coated solar cell compared to bare cell under controlled atmosphere conditions.

From Table 3, it is evident that the voltage generation value of AR coated solar cells has been improved compared to the uncoated one. The increase in voltage generation of AR coated solar cell may be due to reduction in the reflection of radiation in the coated region.

A sample graph of radiation versus cell voltage of bare and AR coated solar cell is shown in Fig. 4. The cell voltage of bare and AR coated silicon substrate subsequently increases with gradual increase in radiation level. Comparatively, AR coated solar cell shows higher increment in cell voltage than the bare system.

In line with the above mentioned analysis, a similar study was conducted for solar cells under open atmospheric conditions. The cells (bare and AR coated) were exposed to sun light and the corresponding voltage generation capacities were observed. Voltage generation results of AR coated silicon solar cells were compared with the uncoated ones and the percentage increment in cell voltage is presented in Table 4.

Table 4

Percentage difference in cell voltage generation of AR coated solar cell compared to uncoated cell under open atmosphere conditions.

From Table 4, it is evident that voltage generation of solar cells is enhanced after AR coating. The obtained trend is in accordance with the results generated under controlled atmospheric conditions. Fig. 5 shows a sample comparative plot of radiation versus cell voltage for bare and AR coated solar cell. Fig. 5 provides a clear representation relating to enhancement in cell voltage of AR coated cells compared to the uncoated one with variation in sun radiation.

CaTiO₃ proved to be a potential material to enhance the voltage generation of silicon solar cells. In addition, this wider band gap material substantially reduced the hot-carrier loss and increased the value of generated voltage [11]. Maximum cell voltage was achieved for AR coating obtained in the deposition process lasting 30 min (Table 3 and Table 4). Further increase in coating time, more than 30 min, resulted in the drop in voltage generation capacity. The obtained result is in agreement with the literature on this subject. Excessive deposition of coating material ultimately reduces the velocity of photons emitted from sunlight which leads to a drop in the value of generated cell voltage [12]. Based on the obtained cell voltage results, AR coating deposited for 30 min is the optimal one.