High Fidelity Polycrystalline CdTe/CdS Heterostructures via Molecular Dynamics
- PDF / 1,148,264 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 52 Downloads / 146 Views
High Fidelity Polycrystalline CdTe/CdS Heterostructures via Molecular Dynamics Rodolfo Aguirre1, Jose J. Chavez2, Xiaowang Zhou2 and David Zubia1 1 The University of Texas at El Paso El Paso, Texas 79968, U.S.A 2 Sandia National Laboratories Livermore, CA 94550, U.S.A ABSTRACT Molecular dynamics simulations of polycrystalline growth of CdTe/CdS heterostructures have been performed. First, CdS was deposited on an amorphous CdS substrate, forming a polycrystalline film. Subsequently, CdTe was deposited on top of the polycrystalline CdS film. Cross-sectional images show grain formation at early stages of the CdS growth. During CdTe deposition, the CdS structure remains almost unchanged. Concurrently, CdTe grain boundary motion was detected after the first 24.4 nanoseconds of CdTe deposition. With the elapse of time, this grain boundary pins along the CdS/CdTe interface, leaving only a small region of epitaxial growth. CdTe grains are larger than CdS grains in agreement with experimental observations in the literature. Crystal phase analysis shows that zinc blende structure dominates over the wurtzite structure inside both CdS and CdTe grains. Composition analysis shows Te and S diffusion to the CdS and CdTe films, respectively. These simulated results may stimulate new ideas for studying and improving CdTe solar cell efficiency. INTRODUCTION CdTe is an ideal material for the development of high efficiency solar cells due to its ideal band gap [1] and excellent absorption coefficient [2]. Its theoretical efficiency limit is ~30% [3] and the highest laboratory efficiency obtained up to date is 22.1% [4], thus giving ~8% room for improvement. Obtaining greater efficiency can be achieved by increasing the open-circuit voltage (Voc) to above 1 volt without compromising short-circuit current (Jsc) and fill factor (FF). Higher Voc can be obtained by doping the CdTe to values higher than 1015 cm-3 [5]. Intrinsic defects in the CdTe such as point defects, dislocations and grain boundaries have demonstrated to impact in the dopability of the CdTe [6, 7, 8]. Surprisingly, the highest efficiency CdTe solar cells are made out of polycrystalline material [9]. Therefore, there should be some defects that are detrimental for the overall performance of the solar cell and other defects that are beneficial. For example, recent studies show that Lomer dislocations exhibit midgap states that increase electron-hole pair recombination rates, thus decreasing the solar cell efficiency [10]. On the other hand, in a different study it was found that intragrain Shockley partial dislocations cores do not introduce midgap states but they induce band bending, thus encouraging greater separation of carriers and improving cell efficiency [7]. Thus, it is important to study the formation and evolution of these defects in detail. Experimental techniques to study the structure of these defects such as Transmission Electron Microscopy (TEM) and Atomic Probe Tomography (APT) are expensive and time consuming. On the other hand, Molecular Dynamics (MD) offer
Data Loading...
