Silicon Thin Film Solar Cells

Although ZnO has also been applied in so-called amorphous/crystalline heterojunction solar cells consisting of a (doped) silicon wafer and thin doped a-Si H layers to build the p-n junction, we will restrict ourselves here to solar cells and modules with amorphous and/or microcrystalline absorber layers, i.e., real thin film silicon solar cells. For detailed information on the use of ZnO in crystalline silicon wafer based devices, the reader is referred to the literature (see e.g. [23,24]). 

---

High Efficiency Silicon Thin Film Solar Cells.393... 

Fig. 8.1. Schematic energy band diagram of silicon thin film solar cells. The symbols denote the Fermi level Ef, valence band (VB), and conduction band (CB). Note that details of the band behavior at the various interfaces are not included. The light enters from the p-doped side...

Here we describe the layer structure for single junction as well as for tandem solar cells consisting of a-Si H and pc-Si l I. Further, this section will deal with the stability of silicon thin film solar cells and the possibility to reduce degradation by special design. 

In this paragraph we will briefly discuss the stability behavior of silicon thin film solar cells. Two aspects were considered light induced degradation and stability in hot and humid environment. 

Reliable stability data of the p-i-n solar cell itself are not easily obtained, especially for non-encapsulated cells or modules. One of these tests e.g. for EN/IEC 61646 certification of modules is the so-called damp-heat test (85°C, 85% humidity, up to lOOOh). Recent studies were performed by Stiebig et al. [50, 51] exposing different types of cells to harsh conditions. One of the most important results was the excellent stability of silicon thin film solar cells. Remarkably, this is also valid for small area modules even without encapsulation [52]. This is of high interest because costs and efforts for module encapsulation strongly depend on the inherent stability of the solar cells. As a more detailed treatment of this subject is beyond the scope of this chapter, the reader is referred to the original papers [50,51]. 

In this section we present solar cells and modules on different ZnO Al front contacts. Having already established some general requirements for the use of TCO contacts in silicon thin films solar cells (Sect. 8.2.4), we now discuss in detail the relationship between ZnO Al properties and cell performance. Finally, we report the current status of high efficiency solar cells and modules on optimized sputtered and texture etched ZnO Al films. 

---