Gregory M. Hanket

Incongruent reaction of Cu–(InGa) intermetallic precursors in H2Se and H2S

The reaction pathways to form Cu(InGa)Se2 or Cu(InGa)S2 films at 450°C from metallic precursors were evaluated by reacting Cu–In–Ga films in H2Se or H2S for 10, 30, or 90min and characterizing the phase composition of the resulting films. A starting composition comprising Cu9(In0.64Ga0.36)4 and In phases was detected by x-ray diffraction in Cu–Ga–In precursors annealed at 450°C in an Ar atmosphere. When the precursors were reacted in H2Se, a graded Cu(InGa)Se2 film was formed with a Ga-rich composition and residual Cu–Ga intermetallics at the interface with the Mo back contact. The intermetallic compounds were observed to evolve from Cu9(In0.64Ga0.36)4 to Cu9Ga4 with increasing selenization time. Reaction in H2S formed inhomogeneous Cu(InGa)S2 with Cu–In intermetallics. The results are consistent with thermochemical predictions of the preferential reaction of In with Se, and Ga with S. These reaction preferences can explain the formation of a graded Cu(InGa)Se2 film during reaction in H2Se and provide a r...

Three-step H2Se/Ar/H2S reaction of Cu-In-Ga precursors for controlled composition and adhesion of Cu(In,Ga)(Se,S)2 thin films

Control of the through-film composition and adhesion are critical issues for Cu(In,Ga)Se2 (CIGS) and/or Cu(In,Ga)(Se,S)2 (CIGSS) films formed by the reaction of Cu–In–Ga metal precursor films in H2Se or H2S. In this work, CIGSS films with homogenous Ga distribution and good adhesion were formed using a three-step reaction involving: (1) selenization in H2Se at 400 °C for 60 min, (2) temperature ramp-up to 550 °C and annealing in Ar for 20 min, and (3) sulfization in H2S at 550 °C for 10 min. The 1st selenization step led to fine grain microstructure with Ga accumulation near the Mo back contact, primarily in a Cu9(In1−xGax)4 phase. The 2nd Ar anneal step produces significant grain growth with homogenous through-film Ga distribution and the formation of an InSe binary compound near the Mo back contact. The 3rd sulfization step did not result in any additional change in Ga distribution or film microstructure but a small S incorporation near the CIGSS film surface and complete reaction of InSe to form CIGSS ...

Phase behavior in the CdTe-CdS pseudobinary system

Polycrystalline thin films deposited by coevaporation of CdTe and CdS form metastable single-phase CdTe–CdS alloys. Subsequent heating in a kinetic-enhancing ambient segregates CdTe1−xSx and CdS1−yTey phases from the original alloy phase. The equilibrium miscibility gap between the resulting CdTe1−xSx and CdS1−yTey alloy phases is determined for CdTe1−xSx films with initial x ∼ 0.4 treated from 360 to 700 °C. At 625 °C, the equilibrium compositions correspond to published results for mixed crystals. Below 625 °C the miscibility gap widens asymmetrically due to different mixing free energies for S in CdTe and Te in CdS. The solubility thermodynamics are modeled with an excess mixing free energy to account for nonideal mixing behavior.

Design of a vapor transport deposition process for thin film materials

A vapor transport process for continuous deposition of elemental and compound thin film materials is presented. The process saturates a carrier gas with a vapor from a subliming source. The saturated mixture is directed over a substrate at lower temperature, resulting in a supersaturation condition and subsequent film growth. The process geometry, comprising the dimensions of the saturation and deposition zones, carrier gas pressure and flow rate, and saturation zone temperature are determined by calculating worst-case characteristic times and simply insuring that the residence time of the carrier gas sufficiently exceeds these times. A model was used to design a system, which is currently being used to deposit 1–10μm thick CdTe films on a 10×10cm2 translating substrate. The process produces film thickness uniformity to within ±5% in the translation direction and across the deposition zone, with a material utilization of 50%. Linear translation speed of 12.5cm∕min has been demonstrated in depositing a 4.5...

Effect of Reduced Cu(InGa)(SeS)

Cu(In,Ga)(Se,S)2 (CIGSS) absorbers with thicknesses from 1.9 to 0.25 μm have been grown using a three-step selenization/Ar-anneal/sulfization reaction of Cu-In-Ga metal precursors. Material characterization revealed changes in orientation, apparent grain size, and formation of voids at the Mo/CIGSS interface with reduced thickness. Even with absorber thickness decreased to 0.25 μm and lateral compositional nonuniformity, VOC and fill factor were nearly sustained, while JSC decreased due to incomplete absorption. With the 0.25-μm-thick absorber layer, an efficiency of 9.1% (without AR coating) with VOC = 612 mV, JSC = 21.0 mA/cm2, and FF = 71.1% was obtained.

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The study of (AgCu)(InGa)Se<inf>2</inf> absorber layers is of interest in that Ag-chalcopyrites exhibit both wider bandgaps and lower melting points than their Cu counterparts. (AgCu)(InGa)Se<inf>2</inf> absorber layers were deposited over the composition range 0 ≪ Ag/(Ag+Cu) ≪ 1 and 0.3 ≪ Ga/(In+Ga) ≪ 1.0 using a variety of elemental co-evaporation processes. Films were found to be single-phase over the entire composition range, in contrast to prior studies. Devices with Ga content 0.3 ≪ Ga/(In+Ga) ≪ 0.5 tolerated Ag incorporation up to Ag/(Ag+Cu) = 0.5 without appreciable performance loss. Ag-containing films with Ga/(In+Ga) = 0.8 showed improved device characteristics over Cu-only control samples, in particular a 30–40% increase in short-circuit current. An absorber layer with composition Ag/(Ag+Cu) = 0.75 and Ga/(In+Ga) = 0.8 yielded a device with V<inf>OC</inf> = 890 mV, J<inf>SC</inf> = 20.5 mA/cm², fill factor = 71.3%, and η = 13.0%.

Thickness Using Three-Step H

Ag-alloying of Cu(InGa)Se<inf>2</inf> thin films presents the possibility to increase the bandgap with improved structural properties as a result of a lower melting temperature. (AgCu)(InGa)Se<inf>2</inf> films were deposited by elemental co-evaporation and the resulting solar cell behavior was characterized. While the bandgap in the highest efficiency Cu(InGa)Se<inf>2</inf> cells is ∼1.15 eV, Ag alloying allows the bandgap to be increased to 1.3 eV with an increase in V<inf>OC</inf>, no loss in device efficiency, and fill factors up to 80%. With high Ga content to increase bandgap > 1.5 eV, Ag alloying improves solar cell efficiency. Analysis of the device behavior shows that the basic mechanisms controlling (AgCu)(InGa)Se<inf>2</inf> solar cells and limiting performance with wide bandgap are comparable to those with Cu(InGa)Se<inf>2</inf>. Finally the effect of Na in (AgCu)(InGa)Se<inf>2</inf> devices is shown to be comparable to that with Cu(InGa)Se<inf>2</inf> including a decrease in V<inf>OC</inf> attributed to interface recombination with insufficient Na.

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Wide-bandgap (AgCu)(InGa)Se<inf>2</inf> absorber layers with Ga/(In+Ga) = 0.8 were deposited by a three-stage co-evaporation process using varying Se incident flux and stage-one substrate temperature. Films exhibited preferential (204)/(220) orientation and a Ga-deficient notch near the surface, both characteristics analogous to previously reported Cu(InGa)Se<inf>2</inf> films deposited using the same process. Increasing Se-to-metals molar flux ratio from Se/M ≈ 5 to Se/M ≈ 20 reduced process variability, but did not result in an overall improvement in device performance. Reducing stage-one substrate temperature from T<inf>SS</inf> = 550 to T<inf>SS</inf> = 400 °C also did not affect device performance. Consistent with earlier results, Ag incorporation improved wide bandgap device efficiencies from η ≈ 8% with no Ag to η ≈ 12% with Ag/(Ag+Cu) = 0.75.

Se/Ar/H

(Ag,Cu)(In,Ga)Se2 thin films have been deposited by elemental co-evaporation over a wide range of compositions and their optical properties characterized by transmission and reflection measurements and by relative shift analysis of quantum efficiency device measurements. The optical bandgaps were determined by performing linear fits of (ahv)2 vs. hv, and the quantum efficiency bandgaps were determined by relative shift analysis of device curves with fixed Ga/(In+Ga) composition, but varying Ag/(Cu+Ag) composition. The determined experimental optical bandgap ranges of the Ga/(In+Ga) = 0.31, 0.52, and 0.82 groups, with Ag/(Cu+Ag) ranging from 0 to 1, respectively. The optical bowing parameter of the different Ga/(In+Ga) groups was also determined.

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A variety of junction capacitance-based characterization methods were used to investigate alloys of Ag into Cu(In 1-x Ga x )Se 2 photovoltaic solar cells over a broad range of compositions. Alloys show encouraging trends of increasing V OC with increasing Ag content, opening the possibility of wide-gap cells for use in tandem device applications. Drive level capacitance profiling (DLCP) has shown very low free carrier concentrations for all Ag-alloyed devices, in some cases less than 10 14 cm −3 , which is roughly an order of magnitude lower than that of CIGS devices. Transient photocapacitance spectroscopy has revealed very steep Urbach edges, with energies between 10 meV and 20 meV, in the Ag-alloyed samples. This is in general lower than the Urbach edges measured for standard CIGS samples and suggests a significantly lower degree of structural disorder.