Jonathan H. Boyle

Structural and optical properties of (Ag,Cu)(In,Ga)Se2 polycrystalline thin film alloys

The structural and optical properties of pentenary alloy (Ag,Cu)(In,Ga)Se2 polycrystalline thin films were characterized over the entire compositional range at a fixed (Cu + Ag)/(In + Ga) ratio. Films deposited at 550 °C on bare and molybdenum coated soda-lime glass by elemental co-evaporation in a single-stage process with constant incident fluxes exhibit single phase chalcopyrite structure, corresponding to 122 spacegroup (I-42d) over the entire compositional space. Unit cell refinement of the diffraction patterns show that increasing Ag substitution for Cu, the refined ao lattice constant, (Ag,Cu)-Se bond length, and anion displacement increase in accordance with the theoretical model proposed by Jaffe, Wei, and Zunger. However, the refined co lattice constant and (In,Ga)-Se bond length deviated from theoretical expectations for films with mid-range Ag and Ga compositions and are attributed to influences from crystallographic bond chain ordering or cation electronegativity. The optical band gap, derive...

Characterization and device performance of (AgCu)(InGa)Se 2 absorber layers

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%.

Device characterization of (AgCu)(InGa)Se 2 solar cells

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.

An Investigation of the Surface Properties of (Ag,Cu)(In,Ga)Se

(Ag,Cu)(In,Ga)Se2 alloy absorber layers with various Ga/(Ga+In) and Ag/(Ag+Cu) ratios were deposited using multisource elemental evaporation and analyzed by glancing incidence X-ray diffraction and energy dispersive X-ray spectroscopy. All films exhibit chalcopyrite reflections in the X-ray diffraction pattern and films with 0.5 ≤ Ga/(Ga+In) <; 1 and Ag/(Ag+Cu) >; 0.5 have additional reflections consistent with an ordered defect phase which is limited to the near-surface region of the film. X-ray photoelectron spectroscopy measurements show that all films studied have low (Ag+Cu)/Se and (Ag+Cu)/(Ga+In) ratios near the surface relative to the bulk composition, consistent with an ordered defect compound identified as (Ag,Cu)(In,Ga)5Se8. Additionally, the near-surface region of (Ag,Cu)(In,Ga)Se2 films contains a higher Ag/(Ag+Cu) ratio than the bulk and the Ag(In,Ga)Se2 film contains excess Ag near the surface.

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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.

Thin Films

(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.

Wide-bandgap (AgCu)(InGa)Se 2 absorber layers deposited by three-stage co-evaporation

AgCu(InGa)Se<inf>2</inf> alloy absorber layers with various Ga/(Ga+In) and Ag/(Ag+Cu) ratios were deposited using multi-source elemental evaporation and analyzed by glancing incidence x-ray diffraction and energy dispersive x-ray spectroscopy. All films exhibit satellite chalcopyrite reflections in the x-ray diffraction pattern and films with 0.5 ≤ Ga < 1 and Ag > 0.5 have additional reflections consistent with an ordered defect phase which is limited to the near-surface region of the film. X-ray photoelectron spectroscopy results show that all films have low (Ag+Cu)/Se ratios near the surface, consistent with an ordered defect compound. Films with 0 < w < 1 have (Ag+Cu)/Se and (Ag+Cu)/(Ga+In) ratios at the surface close to the (AgCu)(InGa)<inf>5</inf>Se<inf>8</inf> ordered defect phases. Additionally the near-surface region of (AgCu)(InGa)Se<inf>2</inf> films contains a higher Ag/(Ag+Cu) ratio than the bulk and the Ag(InGa)Se<inf>2</inf> film contains excess Ag near the surface.