Enhanced Light Absorption in Bifacial Solar Cells Suhaila Sepeai, M.Y.Sulaiman, Saleem H.Zaidi, Kamaruzzaman Sopian

Solar Energy Research Institute (SERI) Universiti Kebangsaan Malaysia (UKM) 43600 UKM Bangi, Selangor, Malaysia

Email: suhailas@ukm.my/ suhaila_sepeai@yahoo.com

Abstract- Solar cell is a semiconductor device that converts sunlight into electricity. Bifacial solar cell is a specially designed solar cell for the production of electricity from both sides of the solar cell. Bifacial solar cell became an active field of research making photovoltaic (PV) more competitive together with current efforts to increase the efficiency and lower material costs. In silicon (Si) solar cell, the inability to absorb all the incident sunlight fundamentally limits the Si solar cell performance. As the wafer is efficiently used by fabricate a bifacial solar cell to save on Si costs, efficiencies will become even lower due to incomplete optical absorption. Subwavelength surface texturing helps offset some of the absorption loss. To trapped more lights, anti reflecting coating (ARC) was studied. There are three type of texturing methods and two type of ARCs that has been studied in this research. According to optical properties of the textured Si wafer and ARC thin film, it was found that wet texturing and SiN are the best methods to improve the light absorption in bifacial solar cell. The efficiency obtained from the bifacial solar cell is 9.62% for front side and 4.5% for back surface.

I. INTRODUCTION

Currently, close to 90% of the global photovoltaic (PV) production is based on crystalline silicon (Si) solar cell. In spite of the expensive manufacturing, the crystalline silicon solar cell still dominates the market due to the stable technology, abundant supply of silicon as a raw material, low ecological impact and no degradation in crystalline form [1]. Incomplete optical absorption is one of the key factors that fundamentally limit the Si solar cell performance [2]. In this research, we address to minimizing the reflection losses and improve the light absorption by deposition of anti reflective coating (ARC) and textured Si wafer.

Texturing of crystalline silicon (c-Si) has been agreed for the development of high efficiency c-Si solar cells. In PV device, texturing is used to enhance the amount of light absorbed into devices by reduce the light reflectance on the silicon surface. Beside that, texturing is used in order to increase the light trapping, and therefore increase the short circuit current and the efficiency in the solar cells. The geometry of the texture determines how the photons absorb in solar cell. There are a few geometry has been reported, for instance pyramid [3], inverted pyramids [4], microgroove and honeycomb [5].

For crystalline silicon, some sophisticated technique has been used for texturing, such as plasma etching [6,7], Reactive Ion Etching (RIE) [8]. The photolithographic patterning and isotropic etching technique is used to generate a square matrix of holes through a masking oxide in a square array [5]. Traditional method for texturing is anisotropic chemical etching. For pores with a size from 2 to 20 microns, the silicon was anodized in hydrofluoric acids [9]. This technique is widely used due to their dependence on silicon surface crystallographic orientation and can maintain wafer lifetime [10]. Electrochemical method is better than chemical method due to its current density, but the reflectance of silicon surface subjected to chemical texturing is higher than reflectance of inverted pyramid and microgroove [9]. Isotropic etching with acidic solution includes the formation of meso- and macro-porous structures on mc-Si that helps to minimize the grain-boundary delineation [3] and also remove metallic contamination [10].

Anti Reflective Coating (ARC) is the most essential layer in silicon solar cells. The main purpose of ARC is anti-reflection. This layer allows more photon absorption and reduces the light reflection. The second role of ARC is as surface passivation to solve the defect and impurities problem in solar cells. It is important to note that those defects and impurities act as recombination centers for the electrons and the holes created by external photons and degrade the efficiency of silicon solar cell [11]. The capability of ARC to reduce the recombination of charge carriers at the Si surface is well known for more than 20 years [12]. The third function is hydrogenation of the wafer if the layer undergoes a short thermal treatment after deposition. This hydrogenation induces passivation of defects and impurities in the bulk Si especially at the grain boundaries and can extensively increase the lifetime of minority charge carriers in the bulk of crystalline wafers [13].

In this paper, we discussed our approach to reduce the reflection losses in bifacial solar cell. For texturing, we had investigated three methods of texturing, namely chemically-etch, vapor etch and wet texturing. Meanwhile, we studied two types of ARC had been investigated in this research, namely Silicon Nitride (SiN) and Silicon Dioxide (SiO2). SiN is deposited by plasma enhanced chemical vapor deposition

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(PECVD) while for SiO2, we used two types of deposition method, namely thermally grown oxide and PECVD.

II. EXPERIMENTAL

Bifacial solar cells with a configuration of n+pp+ with Aluminium Back Surface Field (Al-BSF) were designed. The cell has screen-printed front surface Ag and back surface Ag/Al contacts. Figure 1 is a schematic of the basic bifacial solar cell structure.

Fig. 1. Bifacial solar cell with n+pp+ structure

A p-type <100> Si wafer with a sheet resistivity ranging between 1 ohm/cm and 10 ohm/cm was used. The Si wafer was initially cleaned by dipping into solution of hydrofluoric acid (HF) and nitric acid (HNO3) in a ratio of 1:100 for 10 minutes. After rinsing with deionized water, it was then dipped into HF and water (H2O) in a ratio of 1:50 for 1 minute. The wafers were then subjected to the texturing process. In this research, we used three types of texturing process, namely chemical etching, vapor etching, and wet texturing. Chemical and vapor etching used the same solution, that is HF:HNO3. For chemical etching, the Si wafer was immersed in that solution for 3 hours, while for vapor etching, the Si wafer was exposed to the vapor of the solution for 24 hours. For a wet texturing process, the solution of texturing process is KOH:IPA:H2O, where IPA is iso-propil alcohol (IPA) in the ratio of 1:5:125. The texturing temperature was set at 70˚C for 30 minutes. For the characterization of the the texturing, cross section and top view images by Scanning Electron Microscope (SEM), surface photovoltage measurement and reflectance measurement checked the outcome from that experiment.

After the texturing process, the wafers were subjected to the n-type diffusion procedure using gas-source phosphorous oxychloride (POCl3) at a temperature of 908 °C. The edges of the Si wafers were then mechanically diced to achieve edge isolation. For bifacial solar cells with Al- BSF, Al pastes were screen-printed onto the back side of the Si wafer. The paste was annealed at 150˚C for 10 minutes prior to firing at a temperature of 830˚C in a rapid thermal annealing (RTA) furnace to form Al-diffused p+ layer. Excess Al was removed by soaking in 100% hydrochloric acid (HCl) solution. Thus, n+pp+ structure was successfully fabricated.

Next, to determine an appropriate Anti Reflective Coating (ARC) to be used in bifacial solar cell, we had performed the characterization studies thermally grown Silicon Dioxide (SiO2) and Silicon Nitride (SiN). SiN is deposited by plasma enhanced chemical vapor deposition (PECVD) at the temperature of 150 °C, while for SiO2, we used two types of deposition method, namely thermally grown oxide and PECVD- SiO2. The ARC thin film has been characterized by surface photovoltage (SPV) measurement system. Finally, the metallization processes were carried out using screen printing of Ag and Ag/Al pastes by employing identical grid masks on the front and back surfaces, respectively. Screen-printed contacts were fired at 830˚C to form ohmic front and back contacts. The finished solar cells were experimentally analyzed using light Current-Voltage (LIV) Measurement System.

III. RESULT AND DISCUSSION

We had performed characterization analyses after texturing process before proceed to device fabrication. Figure 2 shows the Scanning Electron Microscopy (SEM) images of (a) chemically-etched, (b) vapour etched and (c) wet texturing of Si wafer with a magnification of 10 000 and an operating voltage of 3 kV. From Figure 2 (a) and (b), we can see that the HF:HNO3 that were used as chemical and vapour etch solution is not uniformly textured the Si wafer. This is due to the etched thickness that in a range of 1.7 µm to 3.0 µm. The texture pattern is not able to determine too. Compared to the wet texturing, it is clearly shown that this technique produces a uniform pyramid-like structure. The diameter of the pyramid is in nano size.

Fig. 2 SEM images of (a) chemically-etched, (b) vapor-etched and (c) wet texturing.

In order to justify the best texturing method, we study the reflectance measurement (Figure 3) of chemically-etched, vapor-etched and wet texturing. We compare the textured Si wafer with planar or un-textured Si wafer. From the figure, it is

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clearly shown that textured wafer reflect less light than un-textured or planar wafer in a visible light (range from 400-700 nm). The planar wafer shows the highest reflectance followed by vapor-etched, chemically-etched and wet texturing. We can conclude that wet texturing is shows the best results with least reflection of light as well as enhance the light absorption in solar cells.

Fig. 3. I-V curve of solar cell with a variation on firing temperature, x = 650 – 880 °C

Figure 4 shows the surface photovoltage (SPV) meaurement of PECVD-SiN, thermally grown SiO2 and PECVD-SiO2. All measurements are done in an operating voltage of 50 µV. From Figure 4, it can be seen that all ARC increases the SPV for the visible light range (400 nm – 700 nm). This means that blue light absorbs more compared to red light since small number of carriers are collected in the range of 400-500 nm. It is good to compare between thermally grown SiO2 and PECVD-SiO2. In visible light range, both of them show similar performance, but they differ in longer wavelength response. The thermally grown SiO2 has a faster decay rate in 700 – 800 nm compared to PECVD- SiO2. The thermally grown process consists of the transportation of oxygen to the surface, diffusion of the oxygen through the already grown oxide and finally the reaction of the oxygen with the silicon at the interface between silicon and silicon oxide. With growing oxide thickness, the growing rate slows down because the time of the diffusion through the oxide depends on its thickness. This process makes the degradation of thermally grown SiO2 faster than PECVD-SiO2. Meanwhile, SiN has shows the highest response amplitude compared to thermally grown SiO2 and PECVD-SiO2, means that a lot of minority carriers are collected on SiN surface. This indicates that SiN is a good passivation compared to SiO2. Based on SPV performance, we decided to choose SiN as a surface passivation for bifacial solar cell.

Fig. 4. Surface photovoltage (SPV) measurement of various anti reflecting coating (ARC).

After performing the texturing and ARC characterization on thin film, we applied the best result of texturing and ARC, namely wet texturing and SiN in our n+pp+ bifacial solar cells. Figure 5 shows the current-voltage (I-V) curve of bifacial solar cell for a front and back surface. Open circuit voltage (Voc) obtained from the device was 580 mV and 560 mV for illumination from front and back sides respectively. The short circuit current (Isc) are 0.47 A and 0.23 A for the front and rear sides; respectively. The efficiency obtained was 9.62% and 4.5% for front and back surface, respectively. It was found that the efficiency of back side is approximately half from the front side. The poor performance could be due to the insufficient effect of back surface field (BSF). Removing the fired Al from the Si wafer using hydrochloric acid (HCl) is a messy and hazardous process and therefore we think that this approach is not a good practice to obtain BSF.

Fig. 5. I-V curve of bifacial solar cell for (a) front (a) and back (b) surfaces.

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IV. CONCLUSION

Bifacial solar cells performance with a structure of n+pp+ had been investigated. The pyramid pattern from wet texturing and SiN has been choose for improve the light absorption in bifacial solar cell. The efficiency for front surface is 9.62%. Meanwhile 4.5% of efficiency is occurred from back surface of bifacial solar cell. As our intuition on the poor performance of back surface may be due to lack of BSF, for our future research, we plan to developed a simulation model of bifacial solar cells with a structure of n+pp+ using PC1D software where the simulated parameters are obtained from experiment. The study will increase understanding of the parameters that influence the bifacial solar cell performance. We also interested to develop a dry process for texturing and improve the device performance by double anti-reflective coating, namely SiN/SiO2 stack.

ACKNOWLEDGMENT

This work has been carried out with the support of the Malaysia Ministry of Science, Technology and Innovation (MOSTI).

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