John Moseley

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, 17.5 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...

Physics of grain boundaries in polycrystalline photovoltaic semiconductors

Thin-film solar cells based on polycrystalline Cu(In,Ga)Se2 (CIGS) and CdTe photovoltaic semiconductors have reached remarkable laboratory efficiencies. It is surprising that these thin-film polycrystalline solar cells can reach such high efficiencies despite containing a high density of grain boundaries (GBs), which would seem likely to be nonradiative recombination centers for photo-generated carriers. In this paper, we review our atomistic theoretical understanding of the physics of grain boundaries in CIGS and CdTe absorbers. We show that intrinsic GBs with dislocation cores exhibit deep gap states in both CIGS and CdTe. However, in each solar cell device, the GBs can be chemically modified to improve their photovoltaic properties. In CIGS cells, GBs are found to be Cu-rich and contain O impurities. Density-functional theory calculations reveal that such chemical changes within GBs can remove most of the unwanted gap states. In CdTe cells, GBs are found to contain a high concentration of Cl atoms. Cl atoms donate electrons, creating n-type GBs between p-type CdTe grains, forming local p-n-p junctions along GBs. This leads to enhanced current collections. Therefore, chemical modification of GBs allows for high efficiency polycrystalline CIGS and CdTe thin-film solar cells.

Cathodoluminescence Analysis of Grain Boundaries and Grain Interiors in Thin-Film CdTe

We used low-temperature cathodoluminescence (CL) spectrum imaging (CLSI) with nanoscale spatial resolution to examine charge-carrier recombination and defects at grain boundaries (GBs) and grain interiors (GIs) in as-deposited and CdCl2-treated CdTe thin films. Supporting time-resolved photoluminescence, T = 4 K photoluminescence, secondary ion mass spectrometry, and electron backscatter diffraction measurements were conducted on the same films. Color-coded maps of the luminescence transition energies (photon energy maps) were used to analyze the qualitative characteristics of the CLSI data. We applied an image analysis algorithm to the pixels in grayscale CL intensity images to compare the luminescence intensities and spectra at the GIs and GBs quantitatively and with statistical relevance. Our results show that GBs in as-deposited films are active recombination centers and are thus harmful to solar cell operation. CL GB defect contrast is quantifiably reduced for the CdCl2-treated film, which is direct evidence of passivation of deep GB core states resulting from the treatment. However, the CdCl2 treatment is not a perfect fix for GB recombination, and GB recombination may still be limiting performance in CdCl2-treated devices.

Long carrier lifetimes in large-grain polycrystalline CdTe without CdCl2

For decades, polycrystalline CdTe thin films for solar applications have been restricted to grain sizes of microns or less whereas other semiconductors such as silicon and perovskites have produced devices with grains ranging from less than a micron to more than 1 mm. Because the lifetimes in as-deposited polycrystalline CdTe films are typically limited to less than a few hundred picoseconds, a CdCl2 treatment is generally used to improve the lifetime; but this treatment may limit the achievable hole density by compensation. Here, we establish methods to produce CdTe films with grain sizes ranging from hundreds of nanometers to several hundred microns by close-spaced sublimation at industrial manufacturing growth rates. Two-photon excitation photoluminescence spectroscopy shows a positive correlation of lifetime with grain size. Large-grain, as-deposited CdTe exhibits lifetimes exceeding 10 ns without Cl, S, O, or Cu. This uncompensated material allows dopants such as P to achieve a hole density of 1016 c...

Quantitative determination of grain boundary recombination velocity in CdTe by combination of cathodoluminescence measurements and numerical simulations

We developed a 2D numerical model simulating cathodoluminescence (CL) measurements in CdTe. Using this model we analyze how various material parameters impact the CL contrast and intensity observed in the measured signal, and determine if and when we can accurately determine the value of grain boundary recombination rate. In addition to grain boundary (GB) recombination, the grain size and its ratio to the carrier diffusion length impact the results of the measurement. Holding the grain interior and GB recombination rates constant, we find that as the grain size increases and becomes larger than the diffusion length, the observed CL contrast is larger. In a small grain size material the surface recombination lowers the overall intensity of the signal, but does not impact the observed contrast significantly. In a large grain size material, high surface recombination velocity can lower the observed contrast in a measurement. This model in combination with an experiment is used to quantify the grain boundary recombination velocity in polycrystalline CdTe before and after the CdCl2 treatment.

Explanation of red spectral shifts at CdTe grain boundaries

We use cathodoluminescence spectrum imaging to investigate the nanoscale properties of CdTe thin-films for solar cells deposited by close-spaced sublimation. Luminescence emission is detected (bands) at ∼1.32 eV and ∼1.50 eV, which are consistent with Z- and Y-bands. For the grains in the as-deposited films, there is a significant redshift in the transition energies near the grain boundaries. The high grain boundary recombination velocity and the donor-acceptor pair (DAP) mechanism of the Z-band transition account for the contrast between grain boundaries and the grain interior. By applying DAP theory, we estimate the concentration of the shallow donor species participating in the Z-band transition to be ∼1017 cm−3.

Grain boundary character and recombination properties in CdTe thin films

In this work we present a correlation between the structural and electro-optical properties of grain boundaries in CdTe thin films deposited by vapor transport technique. We were able to identify different types of grain boundaries using electron backscatter diffraction (EBSD), and investigated their recombination properties by cathodoluminescence (CL). The objective is to investigate the existence of “good” and “bad” boundaries in CdTe thin films, which will provide guidance for the growth of better films in the future. The crystallographic orientation, grain size, and relative fraction of different boundaries were determined by EBSD. For the comparison study, the grain boundaries were colored according to their character, and compared to the CL spectra. By applying focused ion beam (FIB) marks, we were able to analyze CL and EBSD maps taken at exactly the same areas. We present a correlation between the types of boundaries with recombination.

Cathodoluminescence spectrum imaging analysis of CdTe thin-film bevels

We conducted T= 6 K cathodoluminescence (CL) spectrum imaging with a nanoscale electron beam on beveled surfaces of CdTe thin films at the critical stages of standard CdTe solar cell fabrication. We find that the through-thickness CL total intensity profiles are consistent with a reduction in grain-boundary recombination due to the CdCl2 treatment. The color-coded CL maps of the near-band-edge transitions indicate significant variations in the defect recombination activity at the micron and sub-micron scales within grains, from grain to grain, throughout the film depth, and between films with different processing histories. We estimated the grain-interior sulfur-alloying fraction in the interdiffused CdTe/CdS region of the CdCl2-treated films from a sample of 35 grains and found that it is not strongly correlated with CL intensity. A kinetic rate-equation model was used to simulate grain-boundary (GB) and grain-interior CL spectra. Simulations indicate that the large reduction in the exciton band intensit...

Resetting the defect chemistry in CdTe

CdTe cell efficiencies have increased from 17% to 21% in the past three years and now rival polycrystalline Si [1]. Research is now targeting 25% to displace Si, attain costs less than 40 cents/W, and reach grid parity. Recent efficiency gains have come largely from greater photocurrent. There is still headroom to lower costs and improve performance by increasing open-circuit voltage (Voc) and fill factor. Record-efficiency CdTe cells have been limited to Voc <; 880 mV, whereas GaAs can attain Voc of 1.10 V with a slightly smaller bandgap [2,3]. To overcome this barrier, we seek to understand and increase lifetime and carrier concentration in CdTe. In polycrystalline structures, lifetime can be limited by interface and grain-boundary recombination, and attaining high carrier concentration is complicated by morphology.

A field evaluation of the potential for creep in thermoplastic encapsulant materials

There has been recent interest in the use of thermoplastic encapsulant materials in photovoltaic modules to replace chemically crosslinked materials, e.g., ethylene-vinyl acetate. The related motivations include the desire to: reduce lamination time or temperature; use less moisture-permeable materials; use materials with better corrosion characteristics or with improved electrical resistance. However, the use of any thermoplastic material in a high-temperature environment raises safety and performance concerns, as the standardized tests currently do not expose the modules to temperatures in excess of 85°C, though fielded modules may experience temperatures above 100°C. Here we constructed eight pairs of crystalline-silicon modules and eight pairs of glass/encapsulation/glass thin-film mock modules using different encapsulant materials of which only two were designed to chemically crosslink. One module set was exposed outdoors with insulation on the back side in Arizona in the summer, and an identical set was exposed in environmental chambers. High precision creep measurements (±20 μm) and performance measurements indicate that despite many of these polymeric materials being in the melt state during outdoor deployment, very little creep was seen because of their high viscosity, temperature heterogeneity across the modules, and the formation of chemical crosslinks in many of the encapsulants as they aged. In the case of the crystalline silicon modules, the physical restraint of the backsheet reduced the creep further.