J. Leisch

Amorphous transparent conductors for PV applications

Transparent conducting oxides (TCOs) with qualitatively better resistance to humidity than ZnO, the traditional Cu(In,Ga)Se2 (CIGS) TCO, are needed to reduce the water-induced degradation of CIGS photovoltaics (PV). Amorphous In-Zn-O (a-IZO) is found both to act as a water vapor transport barrier and to be essentially inert in damp heat testing at 85°C, 85%RH (85/85). In particular, no significant reduction in conductivity or transparency was observed after 40 days at 85/85. In initial PV application testing, a-IZO-finished CIGS solar cells have demonstrated 16.4% efficiency, essentially equal to equivalent CIGS cells finished with Al-doped ZnO.

Stoichiometric analysis of compositionally graded combinatorial amorphous thin film oxides using laser-induced breakdown spectroscopy.

Laser-induced breakdown spectroscopy (LIBS) is a recently developed locally destructive elemental analysis technique that can be used to analyze solid, liquid, and gaseous samples. In the system explored here, a neodymium-doped yttrium aluminum garnet laser ablates a small amount of the sample and spectral emission from the plume is analyzed using a set of synchronized spectrometers. We explore the use of LIBS to map the stoichiometry of compositionally graded amorphous indium zinc oxide thin-film libraries. After optimization of the experimental parameters (distance between lens and samples, spot size on the samples, etc.), the LIBS system was calibrated against inductively coupled plasma atomic emission spectroscopy which resulted in a very consistent LIBS-based elemental analysis. Various parameters that need to be watched closely in order to produce consistent results are discussed. We also compare LIBS and x-ray fluorescence as techniques for the compositional mapping of libraries.

Combinatorial study of thin-film Cu 2 ZnSnS 4 synthesis via metal precursor sulfurization

We report on studies related to the synthesis of thin-film Cu2ZnSnS4 via sulfurization of metal-precursor thin films. Combinatorially graded thin-film Cu-Zn-Sn library samples spanning various regions of the ternary Cu-Zn-Sn phase diagram were deposited at temperatures below 375 K and subsequently sulfurized in a high-vacuum system equipped with a sulfur valved-cracking source at temperatures from 600 K to 675 K. Comparisons of x-ray fluorescence and x-ray diffraction data from pre- and post-sulfurization films have revealed correlations between processing conditions, film composition, and the crystalline phases present. We have also performed cathodoluminescence and photoluminescence measurements and identified emission features consistent with the formation of the Cu2ZnSnS4.

Spray deposition of high quality CuInSe 2 and CdTe films

A number of different ink and deposition approaches have been used for the deposition of CuInSe2 (CIS), Cu(In,Ga)Se2 (CIGS), and CdTe films. For CIS and CIGS, soluble precursors containing Cu, In, and Ga have been developed and used in two ways to produce CIS films. In the first, In-containing precursor films were sprayed on Mo-coated glass substrates and converted by rapid thermal processing (RTP) to In2Se3. Then a Cu-containing film was sprayed down on top of the In2Se3 and the stacked films were again thermally processed to give CIS. In the second approach, the Cu-, In-, and Ga-containing inks were combined in the proper ratio to produce a mixed Cu-In-Ga ink that was sprayed on substrates and thermally processed to give CIGS films directly. For CdTe deposition, ink consisting of CdTe nanoparticles dispersed in methanol was prepared and used to spray precursor films. Annealing these precursor films in the presence of CdCl2 produced large-grained CdTe films. The films were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM). Optimized spray and processing conditions are crucial to obtain dense, crystalline films.

Advanced superimposed sputtering of amorphous Indium Zinc Oxide transparent conductors for Energy applications

In this work, we investigate the use of RF, DC, and mixed RF/DC magnetron sputtering and its effect on the optoelectronic properties of amorphous Indium Zinc Oxide (IZO) films sputtered from an 87/13 wt% ceramic target. The effect of oxygen concentration in the sputter gas is examined at several RF/DC power ratios and a variety of total powers to help optimize conductivity. We find that higher conductivities can be achieved at 50% of the typical DC sputtering oxygen concentration when duplicated via mixed RF/DC sputtering. By combining the two, mixed RF/DC sputtering allows for the high deposition rates of typical DC sputter deposition, while obtaining the greater conductivities and transparencies assisted by RF sputtering.

Low cost copper indium gallium selenide by the FASST® process

Low cost manufacturing of Cu(In,Ga)Se2 (CIGS) films for high efficiency PV devices by the innovative Field-Assisted Simultaneous Synthesis and Transfer (FASST®) process is reported. The FASST® process is a two-stage reactive transfer printing method relying on chemical reaction between two separate precursor films to form CIGS, one deposited on the substrate and the other on a printing plate in the first stage. In the second stage these precursors are brought into intimate contact and rapidly reacted under pressure in the presence of an applied electrostatic field. The method utilizes physical mechanisms characteristic of anodic wafer bonding and rapid thermal annealing, effectively creating a sealed micro-reactor that insures high material utilization efficiency, direct control of reaction pressure, and low thermal budget. The use of two independent precursors provides the benefits of independent composition and flexible deposition technique optimization, and eliminates pre-reaction prior to the second stage FASST® synthesis of CIGS. High quality CIGS with large grains on the order of several microns are formed in just several minutes based on compositional and structural analysis by XRF, SIMS, SEM and XRD. Cell efficiencies of 12.2% have been achieved using this method.