M.F.A.M. van Hest

Titanium-doped indium oxide: A high-mobility transparent conductor

We report on the effects of titanium doping (0–7at.%) on the optical and electrical properties of In2O3 using combinatorial deposition and analysis techniques. Maximum mobilities are observed at Ti concentrations of 1.5–2.5at.% and are >80cm2∕Vs in sputtered films. The carrier concentration increased with titanium content to a high of 8.0×1020cm−3. Data show that one carrier is generated per added Ti between 1 and 3at.%. Conductivities up to 6260Ω−1cm−1 were observed. These remained very high >5000Ω−1cm−1 across a wide compositional range. The optical transparency is high (>85%) in a wide spectral range from 400nm to at least 1750nm. The work function of titanium-doped indium oxide varies substantially over the studied compositional range.

Multi-Layer Inkjet Printed Contacts for Silicon Solar Cells

Ag, Cu, and Ni metallizations were inkjet printed with near vacuum deposition quality. The approach developed can be easily extended to other conductors such as Pt, Pd, Au, etc. Thick highly conducting lines of Ag and Cu demonstrating good adhesion to glass, Si, and printed circuit board (PCB) have been printed at 100-200degC in air and N2 respectively. Ag grids were inkjet-printed on Si solar cells and fired through the silicon nitride AR layer at 850degC, resulting in 8% cells. Next generation inks, including an ink that etches silicon nitride, have now been developed. Multi-layer inkjet printing of the etching ink followed by Ag ink produced contacts under milder conditions and gave solar cells with efficiencies as high as 12%

Direct write contacts for solar cells

Ag, Cu and Ni metallizations were inkjet printed with near vacuum deposition quality. The approach developed can be easily extended to other conductors such as Pt, Pd, Au etc. Thick highly conducting lines of Ag and Cu demonstrating good adhesion to glass, Si and PCB have been printed at 100-200 /spl deg/C in air and N/sub 2/ respectively. Ag grids were inkjet-printed on Si solar cells and fired through the silicon nitride AR layer at 850 /spl deg/C resulting in 8% cells. Next generation multicomponent inks (including etching agents) have also been developed with improved fire through contacts leading to higher cell efficiencies. PEDOT-PSS polymer based conductors were inkjet printed with conductivity as good or better than that of spin-coated films.

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.

Combinatorial study of reactively sputtered Cr-Ti-N

Abstract A combinatorial study of ternary Cr–Ti–N libraries is reported. Sixteen libraries were grown with a range of metallic composition gradients and different nitrogen contents. X-ray photoelectron spectroscopy (XPS) and electron microprobe analysis (EPMA) show that these libraries encompass a large portion of the ternary phase space. Optical transmission and reflection measurements from near ultra-violet to infra-red wavelengths suggest that both the color and infra-red reflectivity can be optimized for this ternary system.

Combinatorial optimization of transparent conducting oxides (TCOs) for PV

Transparent conducting oxides (TCOs) can serve a variety of important functions in thin film photovoltaics such as transparent electrical contacts, antireflection coatings and chemical barriers. Two areas of particular interest are TCOs that can be deposited at low temperatures and TCOs with high carrier mobilities. We have employed combinatorial high-throughput approaches to investigate both these areas. Conductivities of /spl sigma/ = 2500 /spl Omega//sup -1/-cm/sup -1/ have been obtained for In-Zn-O (IZO) films deposited at 100/spl deg/C and /spl sigma/ > 5000 /spl Omega//sup -1/-cm/sup -1/ for In-Ti-O (ITiO) and In-Mo-O (IMO) films deposited at 550/spl deg/C. The highest mobility obtained was 83 cm/sup 2//V-sec for ITiO deposited at 550/spl deg/C.

Optical absorption and electrical conductivity in amorphous In-Zn-O: A new TCO for CIGS PV

We report the coupled optical and electrical properties of a-IZO films as a function of both the metals composition and the %O2 in the Ar/O2 sputter gas mix. In particular, a-IZO films with conductivity δ ≫ 2000 S/cm can be grown for a broad range of metals compositions (60 to 85 at.% In) as long as the corresponding optimal oxygen level is used in the deposition. The amount of oxygen required increases with increasing indium content. When too much oxygen is used, the a-IZO remains clear but the conductivity is reduced due to a decreased carrier concentration. Whereas, when too little oxygen is used, the conductivity is reduced due to decreased electron mobility and there is a concurrent increase in the optical absorption from 400 to 1000 nm which renders the oxygen deficient a-IZO samples gray.

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.

Laser nucleation and solid phase crystallization of a-Si:H

The ability to grow large-area, large-grained polycrystalline silicon on inexpensive substrates is becoming increasingly important for photovoltaic (PV) devices. With large-grained (grain size <10 μm) 10 μm thick films it is possible with light trapping to achieve PV efficiencies exceeding 15%. If crystallites could be nucleated and grown for longer times before native nucleation occurs, then potentially these much larger grain, thin film silicon material could be produced.The interaction of sub-crystallization threshold laser fluence with hydrogenated amorphous silicon (a-Si:H) has been demonstrated on a macroscopic scale to shorten the incubation time in subsequently thermally annealed films. Further examination of crystallite laser nucleation, found that nucleation was suppressed around PECVD a-Si:H thin film(50-100nm) sample edges, and scratches, in addition to laser-ablated areas, extending as much as 100-200 μm laterally from these features. Optical microscopy and stepwise high temperature thermal annealing were used to investigate this behavior for the a-Si:H films deposited on glass substrates. The nucleation rates were measured in the treated and untreated regions. The data suggests that these features (edges, scratches, and laser ablated areas) provide stress relief by interrupting the surface connectivity. We confirm the existence of stress and stress relief by μ-Raman measurements of the crystallite transverse optical peak position relative to that of c-Si. PECVD films were annealed at temperatures between 540-600C, to enable a determination of rn at each anneal temperature. The temperature dependent measurements enabled the determination of the nucleation rate activation energies (EA), and how they are affected by film stress.

Transparent conducting oxide development for electronics applications

We have employed both combinatorial composition spread and conventional single composition approaches to determine the relative roles of metals and oxygen stoichiometries on the opto-electronic properties of amorphous In-Zn-O (a-IZO) thin film transparent conductors. Two major results were found. First, that the optimization of conductivity in a-IZO is a coupled process with the best metals composition depending upon the oxygen content of the sputter gas. Second, that the electron mobility as a function of carrier concentration is given by a common curve for all a-IZO films independent of the metals composition.