低温制备窄带隙纳米锗超晶格材料及其光伏应用

The long wavelengths absorption of silicon thin film solar cells is limited to 1100nm due to the band gap of 1.1eV for hydrogenated microcrystalline silicon materials, which are unable to absorb long-wave radiation in the solar spectrum at 1100-1800nm. The key points in improving the efficiency of silicon-based thin-film solar cells are expanding the long-wave spectral absorption range greater than 1100nm. Compared to microcrystalline silicon, a spectral response extending to 1200 nm was achieved by using narrow band gap microcrystalline silicon-germanium alloy and microcrystalline germanium. Unfortunately, a monotonic decrease of structural and carrier transport properties were generally found, which leads to a degradation of photocarrier collection and an unsatisfactory IR response at the longer-wavelength. Based on the new concepts and new theories of the low-dimensional nano-materials and the third-generation solar cells, a low cost PECVD method at low temperatures is proposed for fabricating periodical nanocrystalline germanium superlattices thin films, which could leads to an ultra narrow band gap (0.67 eV) closed to the crystal germanium. Thus, an opportune passivation of structural defects during germanium crystal growth process can be achieved through the multilayer film structure. The effective band gap of superlattice can be tailored by the strain in the SiGex layer. On the other hand, an improving of mobility and an increasing of photocurrent gain can be achieved by the continuous minibands and resonant tunneling effects of superlattice structure. Then, ultra narrow band gap photovoltaic materials with superior photoelectronic properties can be obtained. In the application of the devices, with the combination of the conventional fabrication process of multi-junction silicon-based thin-film solar cells, a novel design and preparation is achieved for four terminal wide spectrum multi-junction solar cells based on the IV family semiconductor materials. Thus, the solar spectrum in the long wavelengths range can be effective absorbed and used.

带隙为1.1eV 的微晶硅材料，使硅基薄膜太阳电池无法利用太阳光谱在1100~1800nm的长波段辐射，拓展长波光谱吸收范围是提高效率的关键。现有的高锗含量微晶硅锗乃至微晶锗材料因结构缺陷多、光电性能差，仅能使电池长波响应拓展至1200nm，窄带隙优势并不明显。本项目借鉴低维纳米材料以及第三代太阳电池的新理论，提出低温下采用PECVD法制备周期性交替生长的纳米锗超晶格材料，获得与晶体锗接近的超窄带隙(0.67 eV)；通过多层膜结构及时钝化锗晶粒生长过程中的结构缺陷，并利用超晶格应变调节能带分布，利用微带、载流子共振隧穿效应提高迁移率、实现更大的光电流增益，最终获得光电性能优良的窄带隙光伏材料。在器件应用方面，与传统的硅基薄膜叠层电池相结合，设计并制备出一种新颖的基于Ⅳ族薄膜材料的宽光谱四端叠层太阳电池，充分发挥窄带隙锗在拓展光谱响应方面的优势，达到充分利用太阳光谱、实现效率突破的目的。

国家自然科学基金青年项目“低温制备窄带隙纳米锗超晶格材料及其光伏应用”（编号：61404073），研究目标为借鉴低维纳米材料以及第三代太阳电池的新理论，制备周期性交替生长的纳米锗超晶格材料，获得与晶体锗接近的、光电性能优良的窄带隙光伏材料。充分发挥窄带隙锗在拓展光谱响应方面的优势，达到充分利用太阳光谱、实现效率突破的目的。.研究的主要内容包括（1）窄带隙纳米锗薄膜吸收层材料制备的关键技术、结构特征和光电性能研究及其在薄膜太阳电池中的应用；（2）通过制备薄膜场效应晶体管表征纳米锗薄膜的电学性能；（3）退火过程中纳米硅/纳米锗界面的分子动力学模拟研究；（4）新型纳米锗太阳电池的构建以及电池长波响应的拓展；（5）纳米锗材料从薄膜相到颗粒相的转变机制研究。.取得的主要研究成果为：（1）获得了制备超薄纳米锗薄膜材料的关键技术，建立起工艺参数、等离子体状态、纳米锗结晶机制三者之间的有机联系，使纳米锗材料结构和光电性能的调控成为可能；（2）以纳米锗薄膜以及纳米锗颗粒薄膜为基础，构建了两种新型纳米锗太阳电池，采用纳米Ge/纳米Si周期性多层膜结构作为吸收层，首次实现了低温制备（不高于200℃）长波光谱响应拓展至1450 nm的纳米Ge太阳电池。从关键技术和原理方面突破了现有技术存在的晶界缺陷和界面缺陷等难点，使电池长波响应得到拓展的同时，对纳米硅/纳米锗界面间的原子互扩散机制有了较为深入的认识。