Characterization and Optimization of The Tco/a-Si:H(,B) Interface for Solar Cells by In-Situ Ellipsometry and Sims/XPS Depth Profiling

Abstract

The chemical reactions at the surface of transparent conductive oxides (SnO2, ITO and ZnO) have been studied in silane and hydrogen plasmas by in-situ ellipsometry and by SIMS as well as XPS depth profiling. SIMS and XPS of the interface reveal an increasing amount of metallic phases upon lowering a-Si:H growth rates (controlled by plasma power), indicating that the ion and radical impact is more than compensated by protecting the surface by a rapidly growing a-Si:H film. Hence, optical transmission of TCO films as well as the efficiency of solar cells can be improved if the first few nanometers of the p-layer are grown at higher rates. Comparing a-Si:H deposition on top of different TCOs, reduction effects on ITO and SnO2 have been detected whereas ZnO appeared to be chemically stable. Therefore an additional shielding of the SnO2 surface by a thin ZnO layer has been investigated in greater detail. Small amounts of H are detected close to the ZnO surface by SIMS after hydrogen plasma treatment, but no significant changes occur to the optical and electrical properties. In-situ ellipsometry indicates that a ZnO layer as thin as 20 nm completely protects SnO2 from being reduced to metallic phases. This provides for shielding of textured TCOs, and hence rising solar cell efficiencies, too. Regarding light trapping efficiency we additionally investigated the smoothing of initial TCO texture when growing a-Si:H on top by combining atomic force microscopy and spectroscopie ellipsometry.

Effect of the thickness and hydrogen treatment on the properties of Ga-doped ZnO transparent conductive films

“…Especially, ZnO doped with group III elements (e.g. Al, Ga, B) for improved electrical conductivity is regarded as the promising substitute due to relatively lower processing cost, innoxiousness, abundance of source materials, and thermal stability in hydrogen plasma [6][7][8][9]. While Al-doped ZnO (AZO) has been intensively studied and good transparent and conducting properties have been achieved [10], Ga-doped ZnO (GZO) has drawn attentions recently because it has some advantages over AZO.…”

Influence of substrate temperature and oxygen/argon flow ratio on the electrical and optical properties of Ga‐doped ZnO thin films prepared by rf magnetron sputtering

“…ZnO is nontoxic and much more resistant to hydrogen plasma reduction and can be grown at lower temperatures. Thus, impurity-doped ZnO is more favorable than ITO particularly for amorphous-silicon solar cells fabricated on transparent conducting (TC) substrates, since the TC substrates are exposed to hydrogen plasma [2,3]. Group A elements such as Al, In, Ga, and B have been widely used as n-type dopants for ZnO [4][5][6].…”

Optical properties of Al‐doped ZnO thin films deposited by two different sputtering methods

“…ZnO is nontoxic and much more resistant to hydrogen plasma reduction and can be grown at lower temperatures. Thus, ZnO is more favorable than ITO particularly for amorphous-silicon solar cells fabricated on transparent conducting (TC) substrates, since the TC substrates are exposed to hydrogen plasma, yet impurity-doped ZnO is more appropriate for TCO because the electrical resistivity of undoped ZnO is not low enough to be used as a TCO [2,3]. Group A elements such as Al, In, Ga, and B have been widely used as n-type dopants for ZnO [4,5].…”