A Simple Colloidal Route to Nanocrystalline ZnO/CuInS2 Bilayers

Abstract

Thin semiconductor CuInSe 2 and CuInS 2 films (CIS) with bandgap values (E g ) of around 1.04 eV (for selenide) and 1.5 eV (for sulfide) represent an important class of the currently developed light absorbers for solar energy harvesting. [1,2] Conversion efficiencies of 12±13 % were achieved on large area modules, [1] whereas close to 18 % was achieved with laboratory cells, [3] indicating a large potential for CIS-derived photovoltaic materials. For their preparation, a broad range of physical [1±3] and electrochemical deposition routes [4] are available. Typically, CIS films are created via a rapid thermal sintering of elemental Cu, In and Se layers evaporated on Mo-coated glass substrates. The photovoltaic cell is then completed by overcoating the CIS-macrograins with a thin CdS buffer layer and a metal± organic chemical vapor deposition derived, transparent Al/ ZnO window electrode. In this contribution, we address a low cost colloidal route to nanocrystalline ZnO/CIS bilayers on indium tin oxide (ITO) glass. For the film deposition, concentrated coating colloids, with size-quantized CuInS 2 particles were developed. It is well-established that size quantization in semiconductors (i.e. increasing bandgap energy with decreasing semiconductor dimension) takes place at particle dimensions smaller than the Wannier±Mott (WM) exciton of the corresponding macroscopic bulk phase.[5] By knowledge of the high frequency dielectric constant, e ¥ , and the reduced effective exciton mass, m = 1/(m ±1 e + m ±1 h ), one can calculate the WM-exciton Bohr radius according to R B = (e ¥ /m)´a B , with a B being the Bohr radius of the hydrogen atom. Taking the CIS bulk values [6] of e ¥ = 11, m e = 0.16 and m h = 1.3, we calculated the WM-exciton size to be 8.1 nm, which predicts a blue shift in the optical absorption threshold (below 826 nm = 1240/1.5 eV) for CIS-particle sizes below 8 nm. Figure 1 shows changes in the optical absorption spectrum during the CIS condensation. Condensation was induced on addition of bis(trimethylsilyl)sulfide to a mixture of Cu(I)±P(OPh) 3 and In(III)±P(OPh) 3 complexes (Cu/ In = 1) in Ar saturated acetonitrile (for details see Experimental).At sulfide concentrations~25 % (with respect to the present metals), the absorption spectrum exhibits a shoulder located at 370 nm that is strongly blue-shifted with respect to the bulk crystals (a gap energy difference of more than 2 eV). On further addition of the sulfide source (50 %), the absorption shoulder shifts from 370 nm to 400 nm, and the optical density rises due to increasing particle concentration. Under stoichiometric conditions (100 % S corresponds to the Cu:In:S stoichiometry of 1:1:2), a steep tail is observed with the absorption onset located near 580 nm.A remarkable dynamic color change accompanies this condensation process which can be seen with the naked eye. On each dropwise addition of the sulfide source, the color of the reacting solution rapidly changes from colorless to yellow to orange to red and becomes colorless or yellow agai...

A new green synthesis method of CuInS2 and CuInSe2 nanoparticles and their integration into thin films

“…These diffraction patterns are similar to those reported in the literature (Czekelius et al 1999). Three relatively broad diffraction peaks are detected around 2h (2-theta) = 28°,4 7°, and 55°.…”

“…Three relatively broad diffraction peaks are detected around 2h (2-theta) = 28°,4 7°, and 55°. These three (112), (204)/(220), and (116)/ 312 planes of the tetragonal structure, respectively (Czekelius et al 1999;Akaki et al 2003).…”

“…Various synthesis approaches have been recently developed specifically to produce sub-micron CIGS particles (Gurin 1998;Azad-Malik et al 1999;Czekelius et al 1999;Grisaru et al 2003;Castro et al 2004;Chun et al 2005;Roh et al 2005;Wei and Mu 2005;Dutta and Sharma 2006;Nakamura et al 2006). There are very few cases where monodispersed chalcopyrite nanoparticles with a diameter around 5 nm or less are obtained.…”

Synthesis and characterisation of CuInGaS2nano-ink for photovoltaic applications

“…This leads to a slower reaction [5] as well as growth rate leading to the formation of uniform nanoparticles of diameter 5 nm. For CIGS films, because Ga replaces In, Cu:In ratio 4 1:1 (Figure 4).…”

Ultra-Lightweight Hybrid Thin-Film Solar Cells: A Survey of Enabling Technologies for Space Power Applications

“…O'Brien and co-workers reported the synthesis of CuInSe 2 nanoparticles from the chlorides and trioctylphosphine oxides (TOPO) and related compounds to form nearly spherical crystals with a diameter of approximately 4 nm. 49 Czekelius et al reported the formation of elongated nanocrystallites of 7 nm CuInS 2 by treatment of a Cu I -P(OPh) 3 /In III -P(OPh) 3 mixture in acetonitrile with (TMS) 2 S. 50 Gurin reported the formation of nanocrystals of CuInS 2 or CuInSe 2 by treatment of an aqueous mixture of CuCl and In 2 (SO 4 ) 3 with hydrogen sulfide or hydrogen selenide, respectively. 51 In each case, the authors observed blue shifts of the absorption onset of the material with respect to the bulk values, indicative of size-induced quantum confinement.…”