• PDF / 2,134,134 Bytes
• 6 Pages / 414.72 x 648 pts Page_size
• 66 Downloads / 201 Views

DOWNLOAD

REPORT

---

ABSTRACT A cross sectional TEM study has been conducted into the structure and morphology of p- and i-type a-Si:H layers of device quality deposited on textured TCO on glass. The layer thickness over peaks is shown to be equivalent to that for flat regions, while defective regions are found in narrow valleys, initiating from the pit of the valleys. These regions may act as regions of excessive recombination and/or shunting regions, thus leading to a reduced Voc and fill factor in thin solar cells. A cosine relationship was found between the deposited thickness and the facet angles of the surface TCO crystals. 11:is concluded that for best performance of the deposited layer, the deposition has to be completely isotropic, and that the preferred surface morphology of textured TCO be sharp peaks with wider valleys.

INTRODUCTION One of the techniques used to increase the efficiency of hydrogenated amorphous silicon solar cells (a-Si:H) is using a textured transparent conductive oxide layer (TCO). This has the effect to scatter incident light into the thin layers and effectively increase the light path through the layers, especially the i-layer thus also increasing the absorption of light. The morphology of PECVD a-Si:H thin layers deposited on such textured transparent conductive oxide (TCO) material on glass, plays a major role in the electrical characteristics of thin film a-Si solar cells. When deposited on smooth surfaces the characteristics of these a-Si:H layers (as used for solar cells) are well known and homogeneous and could be measured with a variety of techniques. However when deposited on the rough surface of the textured TCO, the structure and characteristics of such layers are less known and becomes more difficult to determine with the standard techniues. An important issue in the optimization of a-Si:H solar cells is that a reduction in Vc is frequently observed when substrates with enhanced light scattering capability are used. Previous reports mentioned especially three aspects than can drastic change the output characteristics of thin film solar cells manufactured on textured TCO. lida [1] proposed that at sharp peaks and edges the deposited film might be non-uniform (thinner than on flat surfaces) such that the p-layer becomes too thin locally, leading to a reduction in the open circuit voltage V., and fill factor (FF). Reference is also made to defective regions found in the a-Si:H layer in valleys between Sn0 2 grains [2], also leading to reduction of V,. This white stripe region is suggested to form where facets from 3 grains meet, starting a distance away into the layer from the pit of' the valley. A third effect reported on is the difference in thickness of the deposited layer on different angled slopes

597 Mat. Res. Soc. Symp. Proc. Vol. 377 0 1995 Materials Research Society

of the TCO crystals, leading to a different Vo, for each slope [3]. All these effects were studied by cross sectional transmission electron microscopy (XTEM), to determine when and if they do occur in device quality hydr