Titanium-dioxide nanostructures grown by dual DC/HiPIMS for dye-sensitized solar cell applications

Abstract This work reports how the solar conversion efficiency of dye-sensitized solar cells (DSSCs) depends on the crystalline structure of both the compact TiO2 blocking layer (BL) and homoepitaxially grown porous TiO2 mesoporous structure. The films were grown by simultaneous sputtering of titanium targets by DC magnetron and by high-power impulse magnetron systems (HiPIMS). The deposition conditions were managed to produce in situ dense BLs and porous TiO2 films. The only variable was the polarization of the BLs (0 to -200V). The polarization caused phase transformations from pure anatase phase through rutile-anatase mixed phases to rutile phase. The polarization results in decreasing intensity of the anatase (101) peak of the porous layers. The latter promptly decreased linearly the value of the short-current Isc and exponentially the open-circuit voltage Voc of the cells. Another inference is the surface energy of the BLs, which follows an exponential decay as a function of the film polarization. XPS study of the Ti 2p3/2 – Ti 2p1/2 doublet reveals an appearance of a shake-up satellite, whose area exponentially decreases as the polarization potential rises. This phenomenon is discussed and related with other physical aspects of the homo-epitaxially grown films with different texture. The anatase phase content and its purity predefined by the experimental conditions determine the quality of the DSSC, as well as other components such as the dye type, the electrolyte, and the electrode materials.

摘要 本工作系统探究了染料敏化太阳能电池（DSSCs）的太阳能转换效率，与致密TiO₂阻挡层（BL）及同质外延生长的多孔TiO₂介孔结构的晶体结构之间的依赖关系。实验采用直流磁控溅射与高功率脉冲磁控溅射系统（HiPIMS）共同溅射钛靶制备薄膜，通过调控沉积参数原位制备致密阻挡层与多孔TiO₂薄膜，唯一的调控变量为阻挡层的极化电压（0至-200 V）。

极化作用引发了系列相转变：从纯锐钛矿相，历经锐钛矿-金红石混相，最终转变为纯金红石相；同时导致多孔层的锐钛矿(101)晶面衍射峰强度逐渐降低。短路电流Isc随该峰强度呈线性快速衰减，而开路电压Voc则呈指数型下降。

另一项研究推论为：阻挡层的表面能随薄膜极化电压升高呈指数衰减。对Ti 2p3/2–Ti 2p1/2双峰的X射线光电子能谱（XPS）分析显示，出现了shake-up卫星峰，其峰面积随极化电位升高呈指数减小。本文对该现象进行了深入讨论，并将其与不同织构的同质外延生长薄膜的其他物理特性相关联。

实验条件所调控的锐钛矿相含量与相纯度，是决定染料敏化太阳能电池性能的关键因素，该影响与染料种类、电解质以及电极材料等其他组件的作用同等重要。