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半导体材料对于激光的吸收机制主要为本征吸收,电子吸收光子能量产生跃迁,形成光生载流子,载流子也会吸收一部分激光能量,并通过弛豫碰撞进行能量交换,最终将能量传导到晶格,晶格温度升高,这种载流子系统与晶格的能量弛豫时间在皮秒量级,所以激光脉冲宽度在皮秒量级及以下时需要考虑载流子通过弛豫碰撞的能量交换时间。皮秒激光作用下,半导体材料晶格的温度升高在载流子温度升高之后,即到皮秒脉冲激光结束辐照后,激光能量还大部分处于载流子系统中。
实验中皮秒激光的脉冲宽度仅为15 ps,脉冲激光辐照结束后,激光辐照的大部分能量仍然在载流子系统中,通过弛豫碰撞将能量以热传导的形式传导到晶格,从而导致电池温度的升高。实验中所用激光器单脉冲能量较低,激光辐照结束后电池温度上升幅度较小,但是在高重频作用下,短时间内成千上万个脉冲的能量沉积使电池局部温度过高,发生熔融烧蚀损伤。
表1和2分别为不同功率皮秒激光辐照三结GaInP2/GaAs/Ge电池栅线与非栅线部位后,电池的最大功率下降情况。根据表中的数据对比发现,相比激光辐照非栅线部位,当激光辐照三结GaInP2/GaAs/Ge电池栅线时,仅0.15 mW的激光功率便导致三结GaInP2/GaAs/Ge电池产生损伤,而辐照非栅线部位时激光功率需要达到1.5 mW才能产生类似的损伤;当激光功率为150 mW时,辐照栅线电极后太阳能电池最大功率下降幅度达到94.8%,而辐照非栅线部位的下降幅度仅为27.6%。
辐照非栅线部位的表面形貌测量显示,随着激光功率的增加,激光光束中心辐照区域形成一个逐渐增大的烧蚀坑,由于激光脉宽为皮秒量级,因此形成的烧蚀坑轮廓清晰,烧蚀坑周围区域由于温度低于光电材料熔点而发生氧化还原反应,形成环状致密氧化层。电池电致发光图像显示,尽管激光光斑较小,但电池内部损伤面积随着激光功率的增加而增大。辐照栅线部位时,由于栅线电极受到激光辐照导致熔断,表面形貌测量显示,损伤面积随着激光功率的增加而变大,主要是因为栅线部位受热熔断会影响太阳能电池对载流子的吸收,降低电池的光电转换能力。因此在真空环境下,皮秒激光辐照三结GaInP2/GaAs/Ge电池栅线具有更好的损伤效果。
Laser power/mW 1.5 15 150 1500 3750 15000 Reduction of battery power 3.4% 17.2% 27.6% 27.6% 37.9% 48.3% Table 1. Maximum power reduction of three-junction GaInP2/GaAs/Ge cell irradiated by picosecond laser at the non-grid line area
Laser power/mW 0.15 0.75 1.5 15 150 Reduction of battery power 3.4% 65.5% 75.9% 94.1% 94.8% Table 2. Maximum power reduction of three-junction GaInP2/GaAs/Ge cell irradiated by picosecond laser at the grid line area
Damage characteristics of solar cells irradiated by picosecond pulsed lasers (invited)
doi: 10.3788/IRLA20210870
- Received Date: 2022-10-13
- Rev Recd Date: 2022-12-14
- Publish Date: 2023-02-25
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Key words:
- laser irradiation /
- picosecond pulsed laser /
- solar cell /
- voltage-current characteristic /
- surface morphology damage /
- electroluminescent image
Abstract: Based on the background that picosecond pulse width laser with multi-pulses damaging the solar cells, we use three methods that are the surface morphology, voltammetry characteristics and electrolumi-nescence of solar cells to obtain the damage characteristics of solar cells with the laser before and after laser ablation. A three-junction GaAs solar cell with pulse width of 15 ps and wavelength of 1 064 nm is irradiated by picosecond pulsed laser. By changing the laser irradiation power through repetition frequency regulation, the damage characteristics of grid line and non-grid line of solar cell under laser irradiation are analyzed. The experimental results show that, although the laser spot is small, the material inside the battery has been damaged. It's mainly because the damage of the ordered structure of the material inside the battery is gradually increased. When the laser power is higher, the internal damage area is larger. When the gate line is irradiated by laser, the fusion of the gate line will greatly affect the absorption of the carriers by the solar cell, thus reducing the photoelectric conversion ability of the solar cell, and then affecting the electrical performance of the solar cell, so that the damage effect of the gate line is stronger than that of the non-gate line.