Eric Colegrove

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...

High-Efficiency Polycrystalline CdS/CdTe Solar Cells on Buffered Commercial TCO-Coated Glass

Multiple polycrystalline CdS/CdTe solar cells with efficiencies greater than 15% were produced on buffered, commercially available Pilkington TEC Glass at EPIR Technologies, Inc. (EPIR, Bolingbrook, IL) and verified by the National Renewable Energy Laboratory (NREL). n-CdS and p-CdTe were grown by chemical bath deposition (CBD) and close space sublimation, respectively. Samples with sputter-deposited CdS were also investigated. Initial results indicate that this is a viable dry-process alternative to CBD for production-scale processing. Published results for polycrystalline CdS/CdTe solar cells with high efficiencies are typically based on cells using research-grade transparent conducting oxides (TCOs) requiring high-temperature processing inconducive to low-cost manufacturing. EPIR’s results for cells on commercial glass were obtained by implementing a high-resistivity SnO2 buffer layer and by optimizing the CdS window layer thickness. The high-resistivity buffer layer prevents the formation of CdTe-TCO junctions, thereby maintaining a high open-circuit voltage and fill factor, whereas using a thin CdS layer reduces absorption losses and improves the short-circuit current density. EPIR’s best device demonstrated an NREL-verified efficiency of 15.3%. The mean efficiency of hundreds of cells produced with a buffer layer between December 2010 and June 2011 is 14.4%. Quantum efficiency results are presented to demonstrate EPIR’s progress toward NREL’s best-published results.

Incorporation and Activation of Arsenic Dopant in Single-Crystal CdTe Grown on Si by Molecular Beam Epitaxy

We report the use of molecular beam epitaxy to achieve p-type doping of CdTe grown on Si(211) substrates, by use of an arsenic cracker and post-growth annealing. A high hole density in CdTe is crucial for high efficiency II–VI-based solar cells. We measured the density of As in single-crystal CdTe by secondary ion mass spectroscopy; this showed that high As incorporation is achieved at low growth temperatures. Progressively higher incorporation was observed during low-temperature growth, presumably because of degradation of crystal quality with incorporation of As at such defect sites as dislocations and defect complexes. After As activation annealing under Hg overpressure, hole concentrations were obtained from Hall measurements. The highest doping level was ∼2.3xa0×xa01016xa0cm−3, and near-1016xa0cm−3 doping was readily reproduced. The activation efficiency was ∼50%, but further optimization of the growth and annealing conditions is likely to improve this value.

Atomic-resolution characterization of the effects of CdCl2 treatment on poly-crystalline CdTe thin films

Poly-crystalline CdTe thin films on glass are used in commercial solar-cell superstrate devices. It is well known that post-deposition annealing of the CdTe thin films in a CdCl2 environment significantly increases the device performance, but a fundamental understanding of the effects of such annealing has not been achieved. In this Letter, we report a change in the stoichiometry across twin boundaries in CdTe and propose that native point defects alone cannot account for this variation. Upon annealing in CdCl2, we find that the stoichiometry is restored. Our experimental measurements using atomic-resolution high-angle annular dark field imaging, electron energy-loss spectroscopy, and energy dispersive X-ray spectroscopy in a scanning transmission electron microscope are supported by first-principles density functional theory calculations.

Simulation of Current Transport in Polycrystalline CdTe Solar Cells

Polycrystalline thin-film CdTe solar cells have demonstrated laboratory efficiency exceeding 17% and are nowadays a commercial technology (albeit with somewhat lower efficiencies). The standard process features a poorly understood recrystallization step, obtained by annealing with a source of chlorine. This study uses two-dimensional numerical modeling to investigate current transport inside the polycrystalline CdTe absorber with and without recrystallization effects [increase of grain size and donor ClTe states at grain boundaries (GBs)]. Solving the Poisson equation and the drift–diffusion model for transport with Fermi statistics, while treating the optical problem by the one-dimensional transfer matrix method and complex refractive indexes, this study shows that: (i) in a columnar absorber (i.e., one where only vertical GBs exist), the presence of ClTe donor traps at GBs results in a dip in the band profiles that effectively serves as an electron collector, significantly increasing the short-circuit current and efficiency compared with nondecorated GBs; (ii) while the same dip acts as a hole barrier and thus can be expected to block holes from flowing when horizontal GBs are present, under illuminated conditions electron collection at GBs reduces the dip enough to allow substantial hole flow, and the cell performance is only moderately affected.

Nitrogen Plasma Doping of Single-Crystal ZnTe and CdZnTe on Si by MBE

We have investigated inxa0situp-type doping of ZnTe and CdZnTe on Si(211) by molecular beam epitaxy using a radiofrequency (RF)-nitrogen plasma source for application to multijunction II–VI-based solar cells. CdZnTe would be used as a wide-gap top cell in a monolithic multijunction device, and ZnTe or CdZnTe could be used for the p-side of tunnel junctions. Highly p-type material is required for producing the high-quality tunnel junctions crucial for maintaining current flow, and p-doping of order 1017xa0cm−3 is required for the generation of a large built-in potential in the absorber region of solar cells. Our uniformly doped films exhibited good Hall characteristics, especially considering the large lattice mismatch between Si and either ZnTe or CdZnTe. Crystal quality was examined by x-ray diffraction. Nitrogen incorporation was examined as a function of the source-gas dilution with argon. A sample with layers of CdZnTe doped using 1% to 100% nitrogen was grown on nominally undoped CdZnTe and analyzed using secondary-ion mass spectrometry. The nitrogen incorporation differed by only a factor of 10, despite the factor of 100 difference in the nitrogen concentration in the plasma, indicating a saturation effect.

Polycrystalline CdTe solar cells on buffered commercial TCO-coated glass with efficiencies above 15%

EPIR Technologies, Inc. reports the production of thin film polycrystalline CdTe devices with National Renewable Energy Laboratory (NREL)-verified efficiencies above 15%. While previous reporting of high efficiency poly-CdTe solar cells utilized high-temperature technical glass, EPIRs cells were produced on commercially-available conductive glass. The devices exhibit fill factors up to 77% and short-circuit current densities around 24 mA/cm2. EPIR developed a robust process for producing thin film CdTe solar cells through implementation of a high resistivity SnO2 buffer layer and optimization of the CdS window layer thickness. The effects of the high resistivity buffer layer on device performance were investigated, demonstrating improved overall performance and yield. To our knowledge, these are among the highest efficiencies yet reported and NREL-verified for a thin film CdTe solar cell fabricated using commercial conductive glass.

Effects of stoichiometry in undoped CdTe heteroepilayers on Si

Crystalline CdTe layers have been grown heteroepitaxially onto crystalline Si substrates to establish material parameters needed for advanced photovoltaic (PV) device development and related simulation. These studies suggest that additional availability of the intrinsic anion (i.e., Te) during molecular beam epitaxy deposition can improve structural and optoelectronic quality of the epilayer and the interface between Si substrate and the epilayer. This is seen most notably for thin CdTe epitaxial films (<; ~10 μm). Although these observations are foundationally important, they are also relevant to envisioned high-performance multijunction II-VI alloy PV devices-where thin layers will be required to achieve production costs aligned with market constraints.

Arsenic doped heteroepitaxial CdTe by MBE for applications in thin-film photovoltaics

Open circuit voltages in CdTe based solar photovoltaics can be improved through increasing the acceptor carrier concentration in the absorber. Arsenic doped heteroepitaxial CdTe layers deposited by MBE are investigated as a means to understand the viability of arsenic as an alternative dopant source without the complication of polycrystalline grain boundaries or high temperature deposition processes. Crystal quality, thickness, and minority carrier lifetimes are correlated with arsenic incorporation and p-type carrier concentrations for both doped and undoped films. Films with carrier concentrations greater than 1015 cm-3 have been produced using both an arsenic cracker source and a Cd3As2 effusion source though incorporation differs drastically between these two. As previous work has found, arsenic incorporation is shown to degrade crystal quality. Despite the lower crystal quality, minority carrier lifetimes greater than 1 ns have been achieved in samples with high carrier concentrations when the Cd3As2 source is used suggesting the benefit of cadmium overpressure. While the feasibility of arsenic doping during high temperature CdTe deposition processes is still not known, arsenic is shown to be a viable dopant source for continued investigation of heteroepitaxial model systems.

Characterization of poly-crystalline CdTe solar cells using aberration-corrected transmission electron microscope

Single CdS/CdTe hetero-junction poly-crystalline photo-voltaic devices are present leaders in thin film solar technology. Commercial success of CdTe-based devices stems from a combination of a nearly ideal direct band gap of the absorber material (~1.5eV), as well as established manufacturing methods and low cost of the modules, which yield fastest return of investment among currently available technologies. Despite substantial progress in the device engineering and fabrication, the conversion efficiency has seen only minor improvements over the last ~20 years with the most recent laboratory record held at ~19.6%. Going beyond 20% efficiency will require breakthrough advances in increasing open circuit voltage, Voc, and minority carrier lifetime. Grain boundaries and intra-grain defects are likely to provide carrier recombination centers but also attract dopants and their influence on a CdTe device performance still needs to be elucidated.