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We investigate the degree of ordering present in GaInP layers grown by dynamic hydride vapor phase epitaxy (D-HVPE) as a function of various growth conditions. We assess order parameter from both transmission electron and x-ray diffraction and compare these trends to the order parameter estimated by the photoluminescence-determined band gap in the GaInP layers. The degree of ordering increases with deposition temperature and the ratio of gas-phase group V to group III precursors in the reactor. We also investigate the effect of growth rate, demonstrating the capability of HVPE to suppress order through rapid growth rates. Such ordering dependences are consistent with mechanisms established for ordering observed in GaInP grown by organometallic vapor phase epitaxy (OMVPE), but with weaker ordering than GaInP grown by that method.

Using a combination of optical and electrical measurements, we develop a model for metastable defects in Ag-alloyed Cu(In,Ga)Se-2, one of the leading thin film photovoltaic materials. By controlling the pre-selenization conditions of the back contact prior to the growth of polycrystalline (Ag,Cu)(In,Ga)Se-2 absorbers and subsequently exposing them to various stresses (light soaking and dark-heat), we explore the nature and role of metastable defects on the electro-optical and photovoltaic performance of high-efficiency solar cell materials and devices. Positron annihilation spectroscopy indicates that dark-heat exposure results in an increase in the concentration of the selenium-copper divacancy complex (V-Se-V-Cu), attributed to depassivation of donor defects. Deep-level optical spectroscopy finds a corresponding increase of a defect at E-v+0.98eV, and deep-level transient spectroscopy suggests that this increase is accompanied by a decrease in the concentration of mid-bandgap recombination centers. Time-resolved photoluminescence excitation spectroscopy data are consistent with the presence of the V-Se-V-Cu divacancy complex, which may act as a shallow trap for the minority carriers. Light-soaking experiments are consistent with the V-Se-V-Cu optical cycle proposed by Lany and Zunger, resulting in the conversion of shallow traps into recombination states that limit the effective minority carrier recombination time (and the associated carrier diffusion length) and an increase in the doping density that limits carrier extraction in photovoltaic devices.

Commercial CdTe PV modules have polycrystalline thin films deposited on glass, and devices made in this format have exceeded 22% efficiency. Devices made by the authors with a magnesium zinc oxide window layer and tellurium back contact have achieved efficiency over 18%, but these cells still suffer from an open-circuit voltage far below ideal values. Oxide passivation layers made by sputter deposition have the potential to increase voltage by reducing interface recombination. CdTe devices with these passivation layers were studied with photoluminescence (PL) emission spectroscopy and time-resolved photoluminescence (TRPL) to detect an increase in minority carrier lifetime. Because these oxide materials exhibit barriers to carrier collection, micropatterning was used to expose small point contacts while still allowing interface passivation. TRPL decay lifetimes have been greatly enhanced for thin polycrystalline absorber films with interface passivation. Device performance was measured and current collection was mapped spatially by light-beam-induced current.

In this work, we demonstrate that the surface layers of single-crystal layered-2D Bi2Se3 can be converted to layered-2D rhombohedral β-In2Se3 by annealing under a trimethylindium (TMIn) flux. Samples were prepared in a metalorganic chemical vapor deposition (MOCVD) chamber, then transferred under vacuum to a surface analysis chamber for analysis with low-energy electron diffraction (LEED) and Auger electron spectroscopy. Additional ex situ characterization included x-ray diffraction, transmission-electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS) elemental mapping, and Raman spectroscopy. The resulting single-crystal β-In2Se3 adopts the rhombohedral crystal structure (space group R-3m) and orientation of the underlying Bi2Se3, and the excess Bi atoms generated by this process create an underlying region of Bi-rich BixSey. Due to the difference in bandgap between Bi2Se3 and In2Se3, this conversion reaction presents a pathway to lateral heterojunctions if only selected regions are converted by masking the surface to spatially define the TMIn exposure. The conversion may also have implications for heteroepitaxy, because the in-plane lattice constants of Bi2Se3 and In2Se3 (0001) surfaces match those of InP and GaAs (1 1 1), respectively, and the natural cleavage planes of a layered-2D crystal facilitate substrate removal and reuse.

The use of InGaAsP/InGaP quantum well structures is a promising approach for subcells in next generation multi-junction devices due to their tunable bandgap (1.50–1.80 eV) and for being aluminum-free. Despite these potentials, the accumulation of stress during the growth of these structures and high background doping in the quantum well region have previously limited the maximum number of quantum wells and barriers that can be included in the intrinsic region and the sub-bandgap external quantum efficiency to less than 30.0%. In this paper, we report on the use of in-situ curvature monitoring by multi-beam optical stress (MOS) sensor measurements during the growth of this quantum well structure to monitor the stress evolution in these thin films. A series of In0.32Ga0.68AsP/In0.49Ga0.51P quantum wells with various arsine to phosphine ratios have been analyzed by in-situ curvature monitoring and X-ray diffraction (XRD) to obtain nearly strain-free lattice matched structures. Sharp interfaces, as indicated by the XRD fringes, have been achieved by using triethyl-gallium and trimethyl-gallium as gallium precursors in InGaAsP and InGaP, respectively, with constant flows of trimethyl-indium and phosphine through the entire quantum well structure. The effect of the substrate miscut on quantum well growth was compared and analyzed using XRD, photoluminescence and time resolved photoluminescence. A 100 period quantum well device was successfully grown with minimal stress and approximately flat in-situ curvature.

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

Minority carrier lifetime, τB, is one of the key metrics for polycrystalline solar cell absorbers. Based on different spatial carrier-generation profiles obtained using one-photon and two-photon excitation (1PE and 2PE, respectively), we developed a new approach to determine τB in polycrystalline thin films. By comparing time-resolved photoluminescence data measured with 1PE and 2PE, we extract τB and surface recombination velocity S, and resolve charge separation at the pn junction. For coevaporated kesterite (Cu2ZnSnSe4) absorbers, we find S=(0.8–2.1)×104 cm s−1 and τB=7.0±0.5 ns. For corresponding photovoltaic devices, charge separation occurs in ≤2 ns.

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 (>10 5 cm/s) typical for single-crystal CdTe was reduced by an order of magnitude to ≈ 1×10 4 cm/s.

To advance the state-of-the-art in III-V multijunction solar cells towards high concentration efficiencies approaching 50%, development of a high-quality ∼1.7 eV second junction solar cell is of key interest for integration in five or more junction devices. Quaternary GalnAsP solar cells grown lattice-matched on GaAs allows bandgap tunability in the range from 1.42 to 1.92 eV and offers an attractive Al-free alternative to conventional AlGaAs solar cells. In this work, we investigate the role of growth temperature towards understanding the optimal growth window for realizing high-quality GalnAsP alloys. We demonstrate bandgap tunability from 1.6 to 1.8 eV in GalnAsP alloys for compositions close to the miscibility gap, while still maintaining lattice-matched condition to GaAs. We perform an in-depth investigation to understand the impact of varying base thickness and doping concentration on the carrier collection and performance of these 1.7 eV GalnAsP solar cells. The photo-response of these cells is found to be very sensitive to p-type zinc dopant incorporation in the base layer. We demonstrate prototype 1.7 eV GalnAsP solar cell designs that leverage enhanced depletion width as an effective method to overcome this issue and boost long-wavelength carrier collection. Short-circuit current density (J sc ) measured in field-aided devices were as high as 17.25 m A/cm2. The best GalnAsP solar cell in this study achieved an efficiency of 17.2% with a J sc of 17 m A/cm2 and a fill-factor of 86.4%. The corresponding open-circuit voltage (V oc ) 1.7 eV measured on this cell represents the highest V oc reported for a 1.7 eV GalnAsP solar cell. These initial cell results are encouraging and highlight the potential of Al-free GalnAsP solar cells for integration in the next generation of III-V multijunction solar cells.

The use of vacuum co-evaporation to produce Cu2ZnSnSe4 photovoltaic devices with 9.15% total-area efficiency is described. These new results suggest that the early success of the atmospheric techniques for kesterite photovoltaics may be related to the ease with which one can control film composition and volatile phases, rather than a fundamental benefit of atmospheric conditions for film properties. The co-evaporation growth recipe is documented, as is the motivation for various features of the recipe. Characteristics of the resulting kesterite films and devices are shown in scanning electron micrographs, including photoluminescence, current-voltage, and quantum efficiency. Current-voltage curves demonstrate low series resistance without the light-dark cross-over seen in many devices in the literature. Band gap indicated by quantum efficiency and photoluminescence is roughly consistent with that expected from first principles calculation.

Reducing recombination in polycrystalline solar cells by orders of magnitude is currently one of the greatest challenges for increasing thin-film solar cell efficiency to theoretical limits. The question of how to do this has been a challenge for the thin-film community for decades. This work indicates that effective interface passivation is critical. Here, polycrystalline Al2O3/CdSeTe/Al2O3/glass heterostructures are grown, and a combination of spectroscopic, microscopic, and time-resolved electro-optical measurements demonstrates that the interface recombination velocity at alumina/thin-film interfaces can be less than 100 cm/s. This is three orders of magnitude less than typical CdTe interfaces without passivation, commensurate with single-crystal epitaxial CdMgSeTe/CdSeTe/CdMgSeTe double heterostructures, and enables minority-carrier lifetimes in polycrystalline CdSeTe well above 100 ns. Microscopic interfacial electric-field measurements identify the field effect as a potential mechanism for polycrystalline Al2O3/CdSeTe interface passivation. The results provide guidance for modeling and interface passivation in devices and indicate future paths to realize highly efficient thin-film solar cells.Reducing recombination in polycrystalline solar cells by orders of magnitude is currently one of the greatest challenges for increasing thin-film solar cell efficiency to theoretical limits. The question of how to do this has been a challenge for the thin-film community for decades. This work indicates that effective interface passivation is critical. Here, polycrystalline Al2O3/CdSeTe/Al2O3/glass heterostructures are grown, and a combination of spectroscopic, microscopic, and time-resolved electro-optical measurements demonstrates that the interface recombination velocity at alumina/thin-film interfaces can be less than 100 cm/s. This is three orders of magnitude less than typical CdTe interfaces without passivation, commensurate with single-crystal epitaxial CdMgSeTe/CdSeTe/CdMgSeTe double heterostructures, and enables minority-carrier lifetimes in polycrystalline CdSeTe well above 100 ns. Microscopic interfacial electric-field measurements identify the field effect as a potential mechanism for polycrys...

Low-temperature photoluminescence (LTPL) and time-resolved photoluminescence (TRPL) were used to study bulk material and interface properties of MBE-grown CdTe. CdTe and CdTe ternary-alloy double heterostructures (DH) grown on CdTe and InSb substrates show LTPL emission from excitons, dislocations, and other defects. Photoluminescence spectra changed with material composition and quality, with near-band exciton emissions increasing and emissions related to extended and point defects decreasing as defect density decreased and interfaces improved. Measured lifetimes from TRPL decay curves also reflected the quality of the DHs. Data showed that overall DH quality depends more upon buffer thicknesses than absorber layer thicknesses. CdTe/CdMgTe DHs grown on InSb substrates had the highest near-band PL and lowest defect emission as seen in low-temperature spectral emission and highest lifetimes in TRPL data.

Thin films deposited on crystalline silicon surfaces are critical components of many silicon-based devices, including high-efficiency photovoltaic cells. Knowledge of their optical and electronic properties is important in designing and fabricating efficient solar cells, especially as the device complexity increases and includes micron-scale features. Here, a camera-based photoluminescence (PL) technique is developed to image the thickness of specific passivating films on silicon wafers. The technique is entirely contactless and nondestructive, requires minimal sample preparation, and provides a large dynamic range in terms of field of view, with micron-scale spatial resolution possible. It retains the advantages of PL imaging, a mainstream technique for photovoltaic material and device characterization, and it has some extra capabilities similar to those of ellipsometry. This newly developed technique is demonstrated on hydrogenated amorphous silicon and silicon nitride films, two important passivation technologies for high-efficiency silicon solar cells.

Heterostructures with CdTe and CdTe1-xSex (x ∼ 0.01) absorbers between two wider-band-gap Cd1-xMgxTe barriers (x ∼ 0.25–0.3) 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 1.460 eV 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 ∼6 μm, suggesting that recombinati...

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, CdCl 2 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.

Minority-carrier lifetime in silicon directly relates to defect- and impurity-related recombination, and thus gives a measure of material quality. Lifetime measurements are useful in research laboratories and commercial production environments as an indicator for process development and quality control. While photoconductivity (PCD) techniques for measuring lifetime are commercially available, there has recently been interest in using photoluminescence (PL) to characterize lifetime in silicon because of the measurement speed to image an entire wafer and higher mapping resolution. The intensity of band-to-band PL is theoretically proportional to the effective bulk lifetime in low-injection conditions if carrier diffusion and re-absorption are neglected, surface recombination is small, and silicon properties, such as carrier concentration and the radiative recombination coefficient, are constant. We show data that compare lifetimes from PCD techniques to PL intensity for varying-resistivity, single-crystal silicon. Surface conditions are also varied (native oxide, thermal oxide, and HF etch/methyl-iodine solution), and the measured lifetimes are compared to corresponding PL intensity.

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 150 meV 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.

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 x ≈ 0.3) as 22 ± 2 cm2(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 ...

Polycrystalline thin-film CdTe is one of the leading materials used in photovoltaic solar cells. One way to improve device performance and stability is through understanding how various process steps alter defect states in the CdTe layer. Low-temperature photoluminescence (PL) studies show a 1.456-eV PL peak in single-crystal CdTe that is likely due to a Cui-OTe defect complex. A similar peak, observed in as-deposited glass/SnO2:F/CdS/CdTe thin-film structures, strongly suggests a common origin. The 1.456-eV peak is also seen in a thin-film sample after performing the CdCl2 treatment needed for high efficiencies.

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, 17.5 μm 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 650 cm2 (Vs)−1 and diffusion coefficient D of 17 cm2 s−1. We find limiting recombination at the epitaxial film surface (surface recombination velocity Ssurface = (2.8 ± 0.3) × 105 cm s−1) and at the heteroepitaxial interface (interface recombination velocity Sinterface = (4.8 ± 0.5) × 105 cm s−1). The results demonstrate that reducing surface and interface recombination velocity is critical for photovoltaic solar cells and electronic d...

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 (325 °C) and high (475 °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 475 °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...

We examined the Cu(In,Ga)Se2 thin-film growth from (In,Ga)2Se3 precursors and found that the evolution of the microstructure and intrinsic native defects depends on the compositional changes that occur as the film transitions from being Cu rich to In(Ga) rich.

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