Effects of Na on The Growth of Cu(In, Ga)Se 2 Thin Films and Solar Cells
- PDF / 3,653,876 Bytes
- 12 Pages / 612 x 792 pts (letter) Page_size
- 19 Downloads / 186 Views
B1.9.1
Effects of Na on The Growth of Cu(In,Ga)Se2 Thin Films and Solar Cells D. Rudmanna , A.F. da Cunhaa,b , M. Kaelina , F.-J. Hauga , H. Zogga , A.N. Tiwaria,c a
Thin Film Physics Group, Laboratory for Solid State Physics, ETH Z¨ urich, Technopark, 8005 Z¨ urich, Switzerland b Permanent address: Departamento de F´ısica, Universidade de Aveiro, 3810-193 Aveiro, Portugal c Also at Department of Electronic and Electrical Engineering, Loughborough University, Leicestershire LE11 3TU, UK ABSTRACT Cu(In,Ga)Se2 (CIGS) layers were grown with different vacuum evaporation recipes and the effects of sodium on the grain growth were investigated by scanning electron microscopy. A reduced grain size mainly in the lower part of films grown with the 3-stage process was observed when Na was available during growth. The growth kinetics were found to be influenced during the second stage of the process. In contrast, no clear change in grain size could be observed for CIGS layers grown with constant evaporation rates, with a bi-layer and with an ”extreme” bi-layer process. A mechanism for the action of Na is proposed, which is based on the assumption that Na passivates CIGS grain boundaries and that this results in a surfactant effect. The mechanism explains the different influences of Na on CIGS growth for different evaporation recipes, the occurrence of reduced grain size in the 3-stage process and the surface smoothing effect observed mainly for CIGS grown with the bi-layer process. In-diffusion of Na after CIGS growth is shown to be an alternative Na incorporation method that does not reduce the CIGS grain size. A conversion efficiency improvement from 10.4% to 14.2% was achieved due to the post-deposition treatment on CIGS absorbers grown without Na at a maximum substrate temperature of 450◦ C. INTRODUCTION The incorporation of a small amount of Na into Cu(In,Ga)Se2 (CIGS) absorber layers leads to superiour solar cell performance mainly due to enhanced open circuit voltage and fill factor and therefore it is a necessary requirement for the achievement of high efficiencies. Many studies on electronic or structural effects of Na have been reported during the last decade and a correspondingly large number of models for the role of Na have been proposed, but still no generally applicable mechanism has been accepted. In this paper we focus on the effects of Na on CIGS grain growth. A large grain size of the absorber material is desirable since it indicates a good crystal quality and since grain boundaries are crystal imperfections and therefore potential recombination centres. There is a general agreement that the incorporation of Na at ”high” doses leads to small grain size and porous films and is detrimental for cell performance [1–4]. However, contradicting influences of Na on the CIGS microstructure for ”low” to ”optimum” doses have been reported. The optimum Na dose is often considered to be approximately equal to the
B1.9.2
amount diffused from soda-lime glass during a ”standard” deposition, where no Na compound was addi
Data Loading...
