Advanced selective emitter structures by laser opening technique for industrial mc-Si solar cells

Abstract

A laser opening technique is employed as the photolithography process to form selective emitter (SE) structures on multi-crystalline silicon (mc-Si) substrates for the large-area (156 × 156 mm 2 ) solar-cell industry. The best efficiency of 16.35% is obtained with the developed SE structure after a damage removal process with optimisation of heavily and lightly doped dopants, which yields a gain of 0.88% absolute compared with that of a reference cell. Significantly, the SE mc-Si solar cell without the damage removal process can also reach a gain of 0.48% absolute. The developed SE process has simplicity, reliability, is fast, cost-effective, and could be effectively applied to mass production in industrial applications.Introduction: The common crystalline-silicon (C-Si) solar cell for commercial application is about 40 -50 V/sq with homogeneous doping in the emitter region. This method can reduce contact resistance in the metal -semiconductor interface. However, it would increase surface recombination velocity, thus decreasing the performance of cells [1]. To overcome these disadvantages, a trade-off compromise could be obtained by a selective emitter (SE) structure [2, 3], which has heavily doped dopants (HDDs) underneath the contact metal, and lightly doped dopants (LDDs) in the illuminated area. This leads to reduced contact resistance as well as lower surface recombination velocity, thus resulting in the improved performance of cells.Recently, much effort has been made to prepare large-area SE solar cells by many approaches, such as etching paste [2], laser doping [3] and etching back [4]. In the case of etching paste, it has the advantages of being fast, simple and of reliable process. Furthermore, it is possible to achieve a low-cost production concept. But the etching paste method has some disadvantages, such as dilation of line-width in the opening region, and leads to an increase of the HDD region, thus increasing the surface recombination velocity. In the case of laser doping, the remaining phosphosilicate glass (PSG) on the wafer serves as a dopant source. The laser selectively melts the Si and locally increases the amount of phosphorus (P) in the emitter as well as driving it deeper into the wafer, thus lowering the emitter sheet resistance underneath the metal contact. However, this laser doping process leads to reduction of open-circuit voltage (Voc) and filled factor (FF) owing to laser-induced damage. In addition, etch-back techniques are based on the selective etching of the homogeneous emitter using the metallisation scheme to form an SE. The SE was obtained by etching-back nonprotected regions in an acidic etch bath of HF/HNO 3 until it reached the appropriate sheet resistance. This drawback is that the etching solution is hard to set accurately, thus lowering the reproducibility in industrial applications. Laser processing as a photolithography step is an important tool for C-Si solar cell fabrication. Recently, many processing technologies have utilised lasers in C-Si solar cell fabrication,...

Performance Improvements of Selective Emitters by Laser Openings on Large-Area Multicrystalline Si Solar Cells

“…For the fabrication of SE mc-Si solar cells as described previously [7,8], an Nd:YAG laser ( = 532 nm and average power ( avg ) ≧ 3W) with a pulse duration of 10-25 ns (@25 kHz) melts the wafer surface. In the first fabrication step of Figure 1(a), texturization of the mc-Si is produced using an acid solution (a mixture of HNO 3 and HF).…”

“…A silicon dioxide (SiO 2 , approximately 10 nm in thickness) is grown through oxidation as the diffusion barrier layer. The procedures of mc-Si texturization and the growth of the diffusion barrier layer are described in a previous study [7]. Laser ablations (at a frequency of 25 kHz, a pulse duration of 10 ns, and under stage speed of 400 mm/s) were next used to open a region in the barrier layer, which acts as a mask, taking advantage of avoiding the photolithography step.…”

“…Laser processing [6], proving suitable for large-area solar cells, has the advantage of highly localized steps and meets the requirements of SE formation. An SE mc-Si solar cell is proposed in this study to overcome the drawbacks of the four aforementioned methods by means of the laser opening process avoiding the photolithography step [7]. In order to improve the performance of the proposed SE mc-Si solar cell, various laser fluences were investigated in the laser opening process.…”

Effects of Sheet Resistance on mc-Si Selective Emitter Solar Cells Using Laser Opening and One-Step Diffusion

“…For the fabrication of mc-Si solar cells as in our previous studies [9,17], p-type (100) oriented Si with a resistivity of 1 Ω-cm and dimensions of 156 × 156 mm 2 was used as the substrate. This wafer had a thickness of approximately 200 m. An Nd:YAG laser ( = 532 nm) with a pulse duration of 30 ns melted the barrier layer on the wafer surface to accommodate metal contacts.…”

“…Laser opening can be successfully used in SE solar cell fabrication, avoiding the standard photolithography step. A detailed analysis and further discussion of the laser opening process can be found in [9]. The SE structure can be implemented using double diffusion [10] or one-step diffusion together with wet backetching of the cell area between the fingers [11].…”