3.1 Effect of TMAH concentration on Sipa texturation
In order to investigate the effect of TMAH concentration on the pyramid structure of silicon surface, four kinds of samples were prepared at the mass concentration of TMAH of 1%, 2%, 3% and 4% respectively. The IPA concentration and etch time were fixed at 10% and 30 min, respectively. The silicon wafers were placed in etch solution of 80 ℃ for constant temperature reaction, and the samples were characterized by SEM. The detailed experimental conditions were shown in the Table 1.
Figure 2 shows that the SEM diagram of SiPa under different TMAH concentrations. It can be seen that with the increase of TMAH concentration, the size of silicon pyramid has obvious changes. When TMAH concentration is 1%, there are still many parts on the silicon surface that are not etched to form silicon pyramid, and the etched silicon pyramid has different sizes. When the TMAH concentration increases to 2%, the pyramid structure on the silicon surface covers the whole silicon chip, and the size of the pyramid is evenly distributed. When the TMAH concentration increases to 3%, a small number of relatively large silicon pyramid structures will appear, which affects the overall uniformity of the silicon pyramid array. When the concentration of TMAH is further increased to 4%, it can be found that some small silicon pyramids are embedded in the large silicon pyramids.
Figure 3 shows the reflectance spectra of the samples at different TMAH concentrations in the wavelength range of 300~1200 nm. It can be found that the reflectance of the sample decreases with the increase of TMAH concentration. When the TMAH concentration increases to 2%, the reflectance is the lowest, and the average reflectance can reach 12.07%. After increasing the TMAH concentration, the sample reflectivity decreases. This phenomenon can be explained by the etching reaction mechanism of TMAH solution on silicon wafers.
The formula of etching reaction of TMAH solution on silicon wafer is as follows[28]:
(1) TMAH is decomposed into OH- and TMA+((CH3)4N+) ions in solution:
(CH3)4NOH→(CH3)4N++OH- (1)
(2) Redox reaction takes place in the solution, and the silicon wafer is etched by anisotropy, forming the pyramid structure of silicon:
Si+2OH-+H2O→SiO32-+2H2 ↑ (2)
(3) The entire etching reaction of silicon wafer in TMAH solution is:
Si+2(CH3)4NOH+H2O→2(CH3)4N++SiO32-+2H2 ↑ (3)
According to equation (1) of reaction, when the concentration of TMAH increases, a large amount of OH- will be generated, which accelerates the etching rate. When the concentration is higher, TMAH isotropic etching mainly controls the whole reaction process[29]. According to equation (2) of the reaction, with the progress of the reaction, the concentration of OH- gradually decreases, and the generated (CH3)4N+ and SiO32- will be adsorbed on the surface of the silicon chip, thus inhibiting the reaction between OH- and the silicon chip and slowing down the etching rate. In this process, there will be a chemical reaction mechanism of TMAH to enhance the anisotropic etching of silicon wafer and weaken the isotropic etching[30-32]. Therefore, in the whole reaction mechanism, reasonable control of TMAH concentration can obtain the silicon pyramid structure array with uniform size and full of silicon wafers, and the reflectivity is the lowest at this time.
3.2 Effect of IPA concentration on Sipa texturation
In order to explore how the IPA concentration affected the pyramid structures on the silicon surface, four samples were prepared under different IPA concentration of 0%, 5%, 10% and 15%. The TMAH concentration and etch time were respectively fixed as 2% and 30 min (as shown in Table 2). The silicon wafers were placed in etch solution of 80 ℃ for constant temperature reaction. After etching, the samples were directly characterized via SEM.
As a common surfactant, IPA does not directly participate in the etching process of silicon wafers[33]. However, when IPA is contained in TMAH etching solution, it can prevent the H2 bubbles generated by the reaction from adhering to the surface of the silicon wafer, and reduce the surface tension of the silicon wafer, increase the surface wettability, weaken the etching strength of the etching solution, increase the anisotropy of the etching, which is conducive to the nucleation and growth of the silicon pyramid. Figure 4 shows the SEM of SiPa under different IPA concentrations, With the addition of IPA, it can be seen that the uniformity of SiPa has been significantly improved. As shown in figure 4(a), when there is no IPA in the etching solution, the size of silicon pyramids varies greatly, and some silicon wafer surfaces do not have pyramids. This is because the etching rate of the reaction is fast at this time, and a large amount of H2 is generated to attach to the silicon surface with bubbles of different sizes. Therefore, the region etching reaction rates were different and the size of silicon pyramids were greatly different. With the increase of IPA concentration, the size of the silicon pyramid on the surface becomes consistent, covering the whole silicon wafers were showed in figure 4(b)-(c). This is because with the increase of IPA concentration, the wettability of the solution decreases, the H2 bubbles attached to the silicon wafer become smaller, and the etching rate decreases. However, too high concentration of IPA will obviously slow down the etching rate[34]. In a certain period of time, the reaction is not sufficient, resulting in part of the pyramid is not formed was showed figure 4(d).
The reflectivity of silicon etchings at different IPA concentrations was shown in figure 5. It can be seen that without IPA, the reflectivity of silicon wafer is the highest, which is caused by the different sizes of silicon pyramids. There are few pyramids with effective reduction on the surface of silicon wafer, which is not conducive to light absorption. When IPA concentration increases, the uniformity of pyramid increases and it covers the whole silicon chip, which can effectively reflect light multiple times, thus reducing the reflectivity. When IPA concentration is 10%, the reflectivity is the lowest. When the concentration of IPA increases to 15%, the high concentration of IPA inhibits the etching rate, resulting in the unformed positive pyramid in some areas and the increase of reflectivity.
3.3 Effect of etching time on Sipa texturation
In order to explore how the etching time affected the pyramid structures on the silicon surface, four samples were prepared under different etching time of 10 min, 20 min, 30 min and 40 min. The TMAH concentration and IPA concentration were respectively fixed as 2% and 10% (as shown in Table 3). The silicon wafers were placed in etch solution of 80 ℃ for constant temperature reaction. The samples were characterized by SEM.
Compared with SEM images of SiPa at different etching time in figure 6. It can be found that the silicon pyramids were nucleated when etched for 10 min. As the etch time increases to 20 min, the silicon pyramids were grown and formed, forming the silicon pyramid with obvious pyramidal structure and different sizes. When the etch time is increased to 30 min, the size of the silicon pyramids were becomed more uniform and completely covers the entire surface of the silicon chip. However, with the prolongation of etching time (40 min), part of the large silicon pyramids were etched away and became a small pyramid structure, with obvious difference in size, decreased uniformity and pits in some areas.
The reflectance of silicon wafer samples etched at different times was shown in figure 7. The reason for the high reflectivity of silicon wafer after etching for 10 min is that the silicon pyramids were nucleated and had not formed a complete pyramid structure. When the etching time is extended to 20 min to 30 min, the morphology and distribution of silicon pyramids were gradually becomed more uniform. The reflectivity was decreased to the lowest 12.07% when the etching time is 30 min. At this time, the SiPa has a good anti-reflection effect. When the etching time increases to 40 min, the reflectance increases to 16.85%. The main reason is that with the increase of the etching time, the original intact pyramids were etched and appeared pits, which affected the uniformity and integrity of the SiPa and caused the rise of surface reflectance.
3.4 Study on photovoltaic performance of Gr/SiPa Schottky junction solar cells
In order to study the effect of SiPa on the performance of Gr/Si solar cells, the SiPa with excellent anti-reflection performance was prepared by etching with TMAH concentration of 2% and IPA concentration of 10% for 30 min, and the Gr/planar Si solar cells were prepared at the same time. Figure 8(a) shows the J-V characteristic curves of Gr/planar Si solar cells and Gr/SiPa solar cells under illumination. As expected, the SiPa devices showed better photovoltaic performance compared to Gr/planar Si solar cells. The PCE of Gr/Planar Si solar cells is 1.09%,VOC, JSC and fill factor (FF) are 0.43 V, 13.11 mA/cm2 and 19.16%, respectively. However, the Gr/SiPa solar cells prepared on the silicon surface by the SiPa increased PCE by 1.66 times, VOC is 0.45 V, JSC is 23.38 mA/cm2, and FF is 27.40%. It can be seen that VOC, JSC and FF of the device have improved, while JSC has the most significant improvement. The enhancement of JSC is mainly due to the fact that the silicon surface with the pyramid structure can absorb a large amount of incident light. The reflectivity of the SiPa is 12.07% in the entire visible and near infrared region, while the reflectivity of planar silicon is almost over 40%.
Figure 8(b) and (c) show the J-V characteristics and corresponding lnJ-V curves of Gr/Planar Si and Gr/SiPa solar cells in the dark. The J-V characteristic curve in the dark shows that the solar cell prepared by the formation of Schottky junction has good rectification characteristics. In order to provide more insights into the effects of SiPa on Gr/Si solar cell performance, we further analyze their J-V characteristic curves. According to the thermal emission theory, The J-V characteristics of Gr/Si solar cells are mainly determined by the majority of carriers, and most of the (electron) current can be made by
Where n is the ideal factor, k is boltzmann constant, T is the thermodynamic absolute temperature, and q is the elementary charge. Jo is the reverse dark-saturation current density, which is expressed as a function of the Schottky barrier height:
Where A* represents the effective Richardson constant (≈112 A·cm-2·K-2 for n-Si), represents Schottky barrier height.
Gr/planar Si and Gr/SiPa solar cells Schottky barriers height and ideal factors (n) can be extracted in figure8(c). It can be found that Schottky barrier heights are almost equal (see Table 4), which indicates that although the SiPa prepared by TMAH etching increases the specific surface area of the silicon surface, it does not cause relatively large defects on the silicon surface, resulting in serious carrier recombination[35-36]. The n of Gr/planar Si and Gr/SiPa Schottky junctions are 5.51 and 4.16, respectively. It is worth noting that the n of Schottky junction can measure the dominant degree of interface defects in the total recombination. The lower n goes, the closer it gets to 1. It indicates that the device is closer to the ideal junction and the better the cell performance[37]. The n of Gr/SiPa is lower than that of Gr/planar Si, which is mainly attributed to the uniform size of the SiPa prepared by TMAH etching. The contact area between graphene and the SiPa is larger than that of planar silicon, and a good Schottky junction contact is formed[38]. Meanwhile, the SiPa further improves the ability of silicon to collect photogenerated carriers. Better Schottky junction contact between Gr and Si leads to higher VOC, which is closely related to Jsc according to its J-V relationship of:
It can be seen that the value of VOC is tied to JSC in principle, and the increase of VOC of Gr/SiPa solar cells is due to the enhancement of JSC[39]. Figure 8(d) is dV/d(InJ)-J curve, and series resistance (Rs) can be deduced from the slope of the curve by linear fitting. It can be seen that when the SiPa is introduced on the silicon surface, Rs decreases from 7.5 Ω·cm2 to 7.0 Ω·cm2, and the shunt resistance (Rsh) increases from 243.7 Ω·cm2 to 289.4 Ω·cm2, which is conducive to the increase of FF.
Although we took a simple method to fabricate the original Gr/SiPa solar cells, there still exists a major problem of low PCE due to the relatively low electrical conductivity of graphene. To further improve device PCE, graphene was chemically doped. After HNO3 doping, PCE of the Gr/SiPa solar cells increased from 2.90% to 5.67%, FF significantly increased from 27.40% to 52.53%, VOC and JSC increased from 0.45 V and 23.38 mA/cm2 to 0.49 V and 31.07 mA/cm2, respectively (as shown in figure 9). This is mainly attributed to the p-type doping of graphene formed by HNO3, which effectively improves the conductivity of graphene[40]. Finally, the PCE of HNO3/Gr/SiPa Schottky junction solar cells is 5.67%, which is higher than other Gr/Si solar cells with silicon nanowires and porous silicon structure. This work provides a new strategy for introducing SiPa prepared by TMAH etching to improve the performance of Gr/Si solar cells.