Pat Dippo

Minority-Carrier Lifetime and Surface Recombination Velocity in Single-Crystal CdTe

We apply an earlier developed method [Kuciauskas et al., IEEE J. Photovoltaics , vol. 3, p. 1319, 2013] to analyze surface and bulk recombination in high-quality undoped single-crystal CdTe. By using two-photon excitation time-resolved photoluminescence, we determined a room-temperature minority-carrier lifetime of 360 ns. This lifetime has weak temperature dependence at 202-295 K. We also show that the high surface recombination velocity (>105 cm/s) typical for single-crystal CdTe was reduced by an order of magnitude to ≈ 1×104 cm/s.

Charge-carrier transport and recombination in heteroepitaxial CdTe

We analyze charge-carrier dynamics using time-resolved spectroscopy and varying epitaxial CdTe thickness in undoped heteroepitaxial CdTe/ZnTe/Si. By employing one-photon and nonlinear two-photon excitation, we assess surface, interface, and bulk recombination. Two-photon excitation with a focused laser beam enables characterization of recombination velocity at the buried epilayer/substrate interface, 17.5u2009μm from the sample surface. Measurements with a focused two-photon excitation beam also indicate a fast diffusion component, from which we estimate an electron mobility of 650u2009cm2 (Vs)−1 and diffusion coefficient D of 17u2009cm2u2009s−1. We find limiting recombination at the epitaxial film surface (surface recombination velocity Ssurfaceu2009=u2009(2.8u2009±u20090.3)u2009×u2009105u2009cmu2009s−1) and at the heteroepitaxial interface (interface recombination velocity Sinterfaceu2009=u2009(4.8u2009±u20090.5)u2009×u2009105u2009cmu2009s−1). The results demonstrate that reducing surface and interface recombination velocity is critical for photovoltaic solar cells and electronic d...

Investigation of combinatorial coevaporated thin film Cu2ZnSnS4. I. Temperature effect, crystalline phases, morphology, and photoluminescence

Cu2ZnSnS4 is a promising low-cost, nontoxic, earth-abundant absorber material for thin-film solar cell applications. In this study, combinatorial coevaporation was used to synthesize individual thin-film samples spanning a wide range of compositions at low (325u2009°C) and high (475u2009°C) temperatures. Film composition, grain morphology, crystalline-phase and photo-excitation information have been characterized by x-ray fluorescence, scanning electron microscopy, x-ray diffraction, Raman spectroscopy, and photoluminescence imaging and mapping. Highly textured columnar grain morphology is observed for film compositions along the ZnS-Cu2ZnSnS4-Cu2SnS3 tie line in the quasi-ternary Cu2S-ZnS-SnS2 phase system, and this effect is attributed to structural similarity between the Cu2ZnSnS4, Cu2SnS3, and ZnS crystalline phases. At 475u2009°C growth temperature, Sn-S phases cannot condense because of their high vapor pressures. As a result, regions that received excess Sn flux during growth produced compositions falling alon...

Spectrally and time resolved photoluminescence analysis of the CdS/CdTe interface in thin-film photovoltaic solar cells

Light absorption and charge separation in thin-film polycrystalline cadmium telluride (CdTe) photovoltaic (PV) solar cells largely occur in the vicinity of the CdS/CdTe interface. Sulfur alloying at this interface to form CdSxTe1-x and doping with Cu appear to be important for efficient PV devices. Based on the different band gaps of CdSxTe1-x and CdTe, we apply spectroscopic and computational photoluminescence (PL) analysis to characterize this interface. We find that Cu concentration changes the dynamics of charge separation and PL emission intensities from the CdSxTe1-x and CdTe regions. We have determined charge separation lifetime and minority carrier lifetime, and we have estimated minority carrier mobility as <100u2009cm2u2009V−1u2009s−1.

Recombination Analysis in Cadmium Telluride Photovoltaic Solar Cells With Photoluminescence Spectroscopy

We used photoluminescence (PL) spectroscopy to study recombination in polycrystalline CdS/CdTe photovoltaic solar cells with open-circuit voltages in the range of 0.899-0.895 V. From PL emission spectra, we identified defects with an activation energy of 0.11-0.12 eV in the junction and back-contact regions. At <;100 K, lifetimes in devices were similar to those in single crystals and ranged from 300-400 ns. Strong lifetime temperature dependence at 100-300 K suggests the presence of a relatively shallow recombination center. Therefore, it is possible that in CdTe solar cells, lifetime is limited by relatively shallow defects.

The impact of Cu on recombination in high voltage CdTe solar cells

Using photoluminescence spectroscopy, we construct a recombination model for state-of-the-art CdTe solar cells doped with Cu. We observe that Cu on Cd sites form a dominant acceptor state about 150u2009meV from the valence band. Although it is intuitive that this state can increase hole density, we also find that this relatively shallow dopant can also limit lifetime. Consequently, CdTe solar cells doped with Cu could have a lifetime limitation inversely proportional to the hole concentration.

Impact of extended defects on recombination in CdTe heterostructures grown by molecular beam epitaxy

Heterostructures with CdTe and CdTe1-xSex (xu2009∼u20090.01) absorbers between two wider-band-gap Cd1-xMgxTe barriers (xu2009∼u20090.25–0.3) were grown by molecular beam epitaxy to study carrier generation and recombination in bulk materials with passivated interfaces. Using a combination of confocal photoluminescence (PL), time-resolved PL, and low-temperature PL emission spectroscopy, two extended defect types were identified and the impact of these defects on charge-carrier recombination was analyzed. The dominant defects identified by confocal PL were dislocations in samples grown on (211)B CdTe substrates and crystallographic twinning-related defects in samples on (100)-oriented InSb substrates. Low-temperature PL shows that twin-related defects have a zero-phonon energy of 1.460u2009eV and a Huang-Rhys factor of 1.50, while dislocation-dominated samples have a 1.473-eV zero-phonon energy and a Huang-Rhys factor of 1.22. The charge carrier diffusion length near both types of defects is ∼6u2009μm, suggesting that recombinati...

Optical-fiber-based, time-resolved photoluminescence spectrometer for thin-film absorber characterization and analysis of TRPL data for CdS/CdTe interface

We describe the design of a time resolved photoluminescence (TRPL) spectrometer for rapid semiconductor absorber characterization. Simplicity and flexibility is achieved by using single optical fiber to deliver laser pulses and to collect photoluminescence. We apply TRPL for characterization of CdS/CdTe absorbers after deposition, CdCl2 treatment, Cu doping, and back contact formation. Data suggest this method could be applied in various stages of PV device processing. Finally, we show how to analyze TRPL data for CdS/CdTe absorbers by considering laser light absorption depth and intermixing at CdS/CdTe interface.

Influence of Na on the electro-optical properties of Cu(In,Ga)Se/sub 2/

Polycrystalline Cu(In,Ga)Se/sub 2/ (CIGS) thin films, grown by coevaporation of the constituent elements with different amounts of sodium (Na), were investigated. In some devices, an increased Na content was achieved through the incorporation of a thin layer of NaF deposited on the substrate prior to the growth of CIGS. The effect of Na on the electro-optical properties was addressed through characterization of the finished devices using photoluminescence (PL) and capacitance techniques. Results indicate the beneficial effect of Na as evidenced by increases in the device efficiency, open-circuit voltage, and PL intensity. Furthermore, these measurements provide evidence that Na (1) increases the net carrier concentration, and (2) reduces the number of gap states including those that act as minority-carrier traps.

Charge-carrier dynamics in polycrystalline thin-film CuIn1-xGaxSe2 photovoltaic devices after pulsed laser excitation: Interface and space-charge region analysis

We used time-resolved photoluminescence (TRPL) spectroscopy to analyze time-domain and spectral-domain charge-carrier dynamics in CuIn1−xGaxSe2 (CIGS) photovoltaic (PV) devices. This new approach allowed detailed characterization for the CIGS/CdS buffer interface and for the space-charge region. We find that dynamics at the interface is dominated by diffusion, where the diffusion rate is several times greater than the thermionic emission or interface recombination rate. In the space-charge region, the electric field of the pn junction has the largest effect on the carrier dynamics. Based on the minority-carrier (electron) drift-rate dependence on the electric field strength, we estimated drift mobility in compensated CuIn1−xGaxSe2 (with xu2009≈u20090.3) as 22u2009±u20092u2009cm2(Vs)−1. Analysis developed in this study could be applied to evaluate interface and junction properties of PV and other electronic devices. For CIGS PV devices, TRPL spectroscopy could contribute to understanding effects due to absorber compositional ...