James R. Sites

Diode quality factor determination for thin-film solar cells

Abstract A technique is proposed for accurate determination of the diode quality factor for non-ideal photodiodes. It is applied to thin-film polycrystalline solar cells at varying temperatures and intensities. The procedure is to measure the slope of d V/ d J vs. (J + J L ) −1 , with correction for shunting effects when necessary. This technique allows one (1) to deal with large deviations from superimposition, (2) to separate unambiguously the illuminated diode quality factor and series resistance, and (3) to recognize easily the onset of current shunting or limiting effects. For reasonably well-behaved polycrystalline cells the diode quality factor uncertainty is ±0.04 .

Grain-boundary recombination in Cu(In,Ga)Se2 solar cells

Two-dimensional simulations are performed to investigate the impact of grain boundaries (GBs) on Cu(In,Ga)Se2 (CIGS) solar-cell performance. Charged defect levels and compositional variations at GBs are considered. Neutral grain boundaries in the CIGS layer are predicted to be most detrimental if they are parallel to the main junction and located within the depletion region. For columnar GBs with a grain size near 1μm, the effective grain-boundary recombination velocity must be less than 104cm∕s to allow for record-efficiency devices. The majority-hole repulsion (additional donors at the GB) and the resulting band bending have a small effect on current collection but substantially lower the open-circuit voltage, and the combined effect is generally a lowering of the solar-cell efficiency. Minority-electron repulsion (additional acceptors at the GB) will partially mitigate GB recombination. A downshift of the valence-band energy, as predicted by the observed Cu depletion at CIGS GBs, can effectively block ...

Efficient photovoltaic heterojunctions of indium tin oxides on silicon

Heterojunction diodes of indium tin oxide films sputtered onto p‐silicon using ion beam techniques display significant photovoltaic effects when exposed to sunlight. Galvanomagnetic and optical measurments confirm that the oxide films are highly degenerate transparent semiconductors. At a tin oxide concentration of 10%, we observed an open‐circuit voltage of 0.51 V, short‐circuit current of 32 mA/cm2, fill factor of 0.70, and conversion efficiency of 12%. As the concentration was raised to 70%, the voltage remained steady, the current fell to 27 mA/cm2, and the fill factor fell to 0.60.

Effects of oxygen content on the optical properties of tantalum oxide films deposited by ion-beam sputtering.

Ion-beam sputter deposition of tantalum oxide films was investigated for possible optical coating applications. Optical properties of such films were found to be a sensitive function of oxygen-to-argon ratio in the ion beam. Refractive index and absorption coefficient were determined in the 250–2000-nm wavelength range by spectrophotometric transmissivity. The different bonding states of the tantalum atoms were revealed by x-ray photoelectron spectroscopy. The visible wavelength refractive index was found to be 2.18 and optical band gap 4.3 eV, so long as the films did not contain inclusions of metallic tantalum. Films with an admixture of oxygen deficient suboxide components had a low-energy tail of increasing magnitude in the absorption spectrum.

15.4% CuIn1−xGaxSe2-based photovoltaic cells from solution-based precursor films

We have fabricated 15.4%- and 12.4%-efficient CuIn1-XGaXSe2 (CIGS)-based photovoltaic devices from solution-based electrodeposition (ED) and electroless-deposition (EL) precursors. As-deposited precursors are Cu-rich CIGS. Additional In, Ga, and Se are added to the ED and EL precursor films by physical vapor deposition (PVD) to adjust the final film composition to CuIn1-XGaXSe2. The ED and EL device parameters are compared with those of a recent world record, an 18.8%-efficient PVD device. The tools used for comparison are current voltage, capacitance voltage, and spectral response characteristics.

Band offset of GaAs/In0.48Ga0.52P measured under hydrostatic pressure

Low‐temperature photoluminescence spectra of an In0.48Ga0.52P alloy and a p‐type GaAs/In0.48Ga0.52P multiple quantum well, both grown by molecular beam epitaxy, have been obtained under hydrostatic pressures from 0 to 6 GPa. The zero‐pressure extrapolation of the InGaP(X) to GaAs(Γ) transitions yields a 0.40±0.02 valence‐band offset, and hence only a small, 0.06 ± 0.02 eV, conduction‐band offset. These offset values are in agreement with measured values of the confinement energy versus well width.

Blue-photon modification of nonstandard diode barrier in CuInSe2 solar cells

Abstract The current–voltage curves of many ZnO/CdS/CuInSe 2 solar cells display significant distortion when only red light illumination is employed. This distortion generally disappears or partially disappears for a period of time following illumination with blue light. Similarly, the dark diode curve shortly after illumination containing blue light is shifted significantly from the equilibrium dark curve. This effect is more common than generally realized and indicates a mechanism that is potentially detrimental to photovoltaic efficiency. A model is presented that is based on a low free-electron concentration and a high concentration of deep levels in the CdS window layer. This model is consistent with observed variations in current–voltage, capacitance, and laser scan data with illumination wavelength and history.

New metastable phases of silicon

Abstract It has been known since 1963 that a metastable phase (III,BC-8) results from the decompression of silicon from its high pressure metallic phases. However recent theoretical studies suggest the possibility of several metastable phases with similar total energies. Upon rapid release of pressure from the metallic state, two new, metastable, phases of Si have been discovered, with tetragonal structures. X-ray diffraction data on these new phases are presented.

Secondary barriers in CdS-CuIn1-xGaxSe2 solar cells

Previous work on CdS–CuInSe2 (CIS) solar cells, which reported distortions of their current-voltage (J–V) curves under red illumination, is expanded in this work to include CdS–CuIn1−xGaxSe2 cells with variable Ga and CIS cells with variable CdS thickness. Different amounts of J–V distortion were observed in these cells under red light. The details are in good agreement with predictions of a photodiode model, in which a secondary barrier caused by the positive conduction-band discontinuity (spike) at the buffer–absorber interface is responsible for the current limitation. The illumination of the cell with high-energy photons lowers the barrier due to buffer photoconductivity, and thus removes the J–V distortion.

Thermally induced crystallization of amorphous-titania films

Three types of amorphous‐titania films prepared by ion‐ and electron‐beam techniques have been annealed thermally. An amorphous‐crystalline transformation is found in each of these film types at around 350 °C. Its resulting microcrystalline structure and the exact transition temperature appear to be dictated by the rutile microcrystalline seed present in the as‐deposited films under different deposition conditions. An amorphous film with a weak rutile seed crystallizes at a lower temperature into the anatase structure, while a film with a relatively strong rutile base crystallizes into the rutile structure at a somewhat higher temperature. It is demonstrated that Raman spectroscopy is a simple and effective tool for characterization of these submicron‐thick amorphous films and for the dynamical study of such a phase transformation. Accompanying this amorphous‐crystalline transformation, a two‐order increase in elastic light scattering is noted implying optical degradation associated with microcrystalline ...