A SIMPLE COLLOIDAL ROUTE TO NANOCRYSTALLINE ZNO/CUINS2 BILAYERS

Thin semiconductor CuInSe2 and CuInS2 films (CIS) with bandgap values (Eg) 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. Conversion efficiencies of 12±13 % were achieved on large area modules, whereas close to 18 % was achieved with laboratory cells, indicating a large potential for CIS-derived photovoltaic materials. For their preparation, a broad range of physical and electrochemical deposition routes 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 CuInS2 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. By knowledge of the high frequency dielectric constant, e¥, and the reduced effective exciton mass, m = 1/(m e + m ±1 h ), one can calculate the WM-exciton Bohr radius according to RB = (e¥/m) aB, with aB being the Bohr radius of the hydrogen atom. Taking the CIS bulk values of e¥ = 11, me = 0.16 and mh = 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 again later on (build-up and decay of size-quantized cluster±cluster aggregates). There is a crossover concentration value around 50 % S, above which the sol does not self-bleach anymore, retaining a constant orange color. On the further allat-once addition of 50 % S, it takes at least 10 hours for the stoichiometric orange colloid to become deep red. Solvent removal from the above 0.05 M solutions on a rotary evaporator yields 0.5 M stable lacquers which can be used to produce single step coated, 1 mm thick compact CIS-layers with good adhesion to glass or nanocrystalline ZnO films. The preparation of nanocrystalline ZnO films from particulate colloids follows a recently published procedure (see also Experimental). Figure 2 shows the UV± Vis optical spectrum of a 2 mm thick (single step dip-coated, and at 400 C pre-sintered) ZnO film displaying the characteristic absorption edge at around 380 nm. After dip-withdrawing the ZnO film in CIS-colloid, and subsequent sintering in vacuum at 400 C for 20 minutes, a dark brown colored ZnO/CIS bilayer is formed. Its optical absorption exhibits a shoulder at 750 nm indicating that coalescence of small CIS particles to larger nanocrystals took