面向PERC电池背场应用的激光熔覆辅助局域硼扩散机理研究

The main obstacles for further improving the efficiency of passivated emitter and rear contact (PERC) cell are low doped Al junction in the back surface field and high percentage of voids at the rear contact. We use B doped Si paste as doping source, Si film is formed after printing and drying, then the B doped Si cladding layer is formed after picoseconds laser cladding, meanwhile assist local B doping, at last a new structure of local B containing back surface field is prepared. First, we will study the theory of the diffusion mechanism of doping element during laser cladding by molecular dynamic simulations. when the status of local B doping changes, the laws for the structure development of energy band, carrier recombination, and contact barrier in the Si crystal are studied. Second, the phase transition process of the material during laser cladding is analyzed by finite volume method, the interaction mechanism between laser and silicon film will be mastered. After then analyzing the picosecond laser parameters, the influence rule for the structure of the Si cladding layer is illuminated. Third, after using doped Si film by laser cladding in the process of PERC cells, a new method of preparing B contained back surface filed is studied. The effect of minority carrier recombination velocity and fill factor is discussed. The results of this project will provide insights on understanding the theory of laser processing, and also can provide a new method for preparing low cost, high efficiency Si solar cells.

背场掺杂浓度较低及背面孔洞比例较高是目前限制PERC电池性能进一步提升的主要障碍。本项目利用含硼硅浆料作为掺杂源，印刷干燥后形成硅薄膜，采用皮秒激光熔覆形成硼掺杂硅熔覆层结构，辅助局域硼元素扩散，最终制备出一种新型的局域含硼背场结构。首先，采用分子动力学方法分析硅薄膜中硼元素的扩散过程，揭示硼元素的扩散机理。研究局域硼掺杂状况对硅晶体内能带、载流子复合、接触势垒等特性的影响规律，指导扩散条件的选取。其次，运用有限体积法分析激光熔覆过程中材料的相变过程，掌握激光与硅薄膜的相互作用机理。分析并优化皮秒激光参数，探究其对硅熔覆层结构特性的影响规律。再次，研究将硅熔覆层结构兼容到PERC电池工艺中制备新型局域含硼背场结构的方法，探明该背场特性对PERC电池背面孔洞比例、接触电阻、表面复合速率等性能的影响规律。该项目的研究结果将丰富和完善激光加工理论，为制备低成本、高效太阳能电池提供新思路。

背场掺杂浓度较低及背面孔洞比例较高是目前限制PERC电池性能进一步提升的主要障碍。本项目利用含硼硅浆料作为掺杂源，印刷干燥后形成硅薄膜，采用激光熔覆形成硼掺杂硅熔覆层结构，辅助局域硼元素扩散，最终制备出一种新型的局域含硼背场结构。首先，利用三维有限元方法对激光与硅薄膜的相互作用过程中温度场的分布进行了数值模拟，得到了多脉冲激光耦合情况下的温度场变化规律。仿真结果表明：与单脉冲相比，在多脉冲激光作用下，峰值温度增加了3.2%，熔池尺寸扩大了 18.75%，同时热影响区范围也明显增加；激光辐照后，熔覆层表面温度下降，但基体温度仍会继续上升，多脉冲热累积效应为纳米硅薄膜中硼元素扩散提供了有利条件。其次，建立了激光熔覆硅薄膜的实验平台及工艺数据库，掌握了激光工艺条件与硼元素的扩散状况及硅熔覆层结构特性的相互关系，实现了扩散深度为0.6μm，最高扩散浓度为6.2×1019atom/cm3的硼元素掺杂，为局域含硼背场高效、可控制备提供了保障。再次，将硅薄膜的激光熔覆工艺兼容到PERC电池背场工艺中，制备出具有新型局域含硼背场的PERC+电池，电池平均效率最高可提升0.2％以上，最高电池效率提升0.5％以上，电池背面孔洞比例低于30％，电池背面复合速率低于100cm/s，批量电池平均效率超过22.95％。最终，掌握了新型局域含硼背场结构对PERC+电池性能影响的一般规律，为新一代低成本、高效太阳能电池的制备提供了新思路。