Abstract
Many technologies deposit thin films on inexpensive substrates, resulting in small grains due to classic nucleation and grain growth theory. For example, state-of-the-art solar cells are made by depositing CdSeTe and CdTe layers on inexpensive glass coated with nanocrystalline transparent conducting oxides (TCOs), like SnO2. Characteristically, the grain size of these films is on the order of the film thickness, i.e. a few microns. CdTe small-grain films have poor electro-optical properties and require CdCl2 passivation which fails to fully passivate grain boundaries, causes carrier compensation, and prevents implementing other II–VI alloys and materials to improve performance. Here, we present a method to increase grain size to 1 mm in CdSexTe1−x thin films deposited on glass/TCO substrates without CdCl2 treatment. The colossal grain growth is driven by mechanisms distinct from classic nucleation, grain growth, and Ostwald ripening and only occurs at low selenium content (x ∼ 0.1). We also demonstrate how these films can serve as templates for subsequent large-grain epitaxy of other compositions like CdTe, again without exposure to CdCl2. The results open new paths for thin film solar cell technology, and thin film devices in general.
Highlights
The colossal grain growth is driven by mechanisms distinct from classic nucleation, grain growth, and Ostwald ripening and only occurs at low selenium content (x ∼ 0.1). We demonstrate how these films can serve as templates for subsequent large-grain epitaxy of other compositions like CdTe, again without exposure to CdCl2
The single crystal nature of these large grains is shown in the Electron back scatter diffraction (EBSD) image of figure 2(c)
All EBSD images are inverse-pole figures (IPFs) where the orientation of grains in the image normal to the substrate can be determined by using the accompanying ‘color triangle’
Summary
CdTe solar cell technology Polycrystalline (pX) thin films for solar cell applications can be deposited very quickly using high throughput deposition techniques and inexpensive substrates such as glass and metal foils. It offers a different paradigm than traditional melt-solidification of single-crystal Si wafers over the course of multiple steps and days of growth, or epitaxial growth at the rate of ∼1 μm h−1 on expensive single crystals. With a passivated absorber layer, CdTe solar cell performance has recently improved by using more transparent buffer layers like (Mg,Zn)O [5,6,7,8] and bandgap engineering at the junction using Se, e.g. CdSe or Cd(Se,Te) alloys [9,10,11,12,13,14]. A further advantage of Se is that when combined with CdCl2, GB passivation is improved relative to CdCl2 use alone [12, 15, 16]
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