Richard K. Ahrenkiel

Properties of the Mo‐CuInSe2 interface

Mo has been suggested and used as an ohmic back contact for CdS/p‐CuInSe2 solar cells. The Mo‐ p‐CuInSe2 interface has been studied for both polycrystalline and single‐ crystal CuInSe2, using electron beam induced current and capacitance‐voltage techniques. The interface is found to form a Schottky barrier, thereby limiting the attainable voltage of a solar cell with Mo back contact. Au is the only known ohmic contact to p‐CuInSe2.

Electron mobility in p‐GaAs by time of flight

The minority‐carrier mobility of electrons in metalorganic chemical vapor deposition grown p‐GaAs has been measured by a diffusion time‐of‐flight technique. Doping levels of 1×1017 and 2×1018 cm−3 were investigated. The measured mobilities were about 2900 and 1300 cm2/V s, respectively. The minority‐carrier mobilities are lower than the expected majority‐carrier mobilities at the same doping levels. The lower mobility is caused by heavy‐hole scattering.

Photoluminescence lifetime in heterojunctions

Abstract It is desirable to obtain minority carrier lifetimes on real photovoltaic structures. However, the photoluminescence (PL) lifetime in such devices is less than the real bulk lifetime because of collection of carriers at the p-n junction. A transient analysis of the PL decay produces lifetime as a parameter. By fitting the data to the model, we can estimate the bulk lifetime in window-absorber heterojunction structures. For ITO/InP devices, there was a strong correlation between such lifetime values and the device efficiency. Similar results were found for n-Al0.9Ga0.1As/p-Al0.37Ga0.63As heterostructures.

Recombination by grain-boundary type in CdTe

We conducted cathodoluminescence (CL) spectrum imaging and electron backscatter diffraction on the same microscopic areas of CdTe thin films to correlate grain-boundary (GB) recombination by GB “type.” We examined misorientation-based GB types, including coincident site lattice (CSL) Σ = 3, other-CSL (Σ = 5–49), and general GBs (Σ > 49), which make up ∼47%–48%, ∼6%–8%, and ∼44%–47%, respectively, of the GB length at the film back surfaces. Statistically averaged CL total intensities were calculated for each GB type from sample sizes of ≥97 GBs per type and were compared to the average grain-interior CL intensity. We find that only ∼16%–18% of Σ = 3 GBs are active non-radiative recombination centers. In contrast, all other-CSL and general GBs are observed to be strong non-radiative centers and, interestingly, these GB types have about the same CL intensity. Both as-deposited and CdCl2-treated films were studied. The CdCl2 treatment reduces non-radiative recombination at both other-CSL and general GBs, but ...

Imaging characterization techniques applied to Cu(In,Ga)Se2 solar cells

The authors present examples of imaging characterization on Cu(In,Ga)Se2 (CIGS) solar cell devices. These imaging techniques include photoluminescence imaging, electroluminescence imaging, illuminated lock-in thermography, and forward- and reverse-bias dark lock-in thermographies. Images were collected on CIGS devices deposited at the National Renewable Energy Laboratory with intentional spatial inhomogeneities of the material parameters. Photoluminescence imaging shows brightness variations, which correlate to the device open-circuit voltage. Photoluminescence and electroluminescence imaging on CIGS solar cells show dark spots that correspond to bright spots on images from illuminated and forward-bias lock-in thermography. These image-detected defect areas are weak diodes that conduct current under solar cell operating conditions. Shunt defects are imaged using reverse-bias lock-in thermography. The authors show how imaging can be used to detect structural defects detrimental to solar cell performance. T...

Minority‐carrier lifetime in n‐Al0.38Ga0.62As

The minority-carrier lifetime in n-Al/sub 0.38/Ga/sub 0.62/As has been investigated by laser-induced photoluminescence. A variety of device structures were used to reduce interface recombination effects, including double heterostructures. Bulk lifetimes of about 18 ns were seen at doping levels of 1 x 10/sup 16/ cm/sup -3/ or less. These data suggest that minority-carrier devices are feasible in high aluminum AlGaAs, contrary to the suggestion of earlier work

Minority-carrier lifetime in n-Al/sub 0. 38/Ga/sub 0. 62/As

The minority-carrier lifetime in n-Al/sub 0.38/Ga/sub 0.62/As has been investigated by laser-induced photoluminescence. A variety of device structures were used to reduce interface recombination effects, including double heterostructures. Bulk lifetimes of about 18 ns were seen at doping levels of 1 x 10/sup 16/ cm/sup -3/ or less. These data suggest that minority-carrier devices are feasible in high aluminum AlGaAs, contrary to the suggestion of earlier work

Heterojunction formation in (CdZn)S/CuInSe2 ternary solar cells

The electrical properties of (CdZn)S/CuInSe2 solar cells have been investigated by combining electron beam induced current measurements and capacitance‐voltage measurements on the same device. In the as‐grown device, the CuInSe2 is lightly doped n type. After baking to about 225 °C in vacuum, the CuInSe2 converts to p type forming the heterojunction. Oxygen does not appear to be necessary for type conversion to occur.

A new method to analyze multiexponential transients for deep‐level transient spectroscopy

A new technique is introduced to analyze digitally recorded capacitive transients in order to determine the properties of deep states. Using a nonlinear double exponential fitting routine, it is shown that a two‐trap model can be applied to the transient data. We determine the individual trap concentrations and produce two Arrhenius plots. The latter yields the thermal activation energies and capture cross sections of closely spaced traps. The excellent agreement between the new technique and the standard rate window technique is shown via a simulation deep‐level transient spectroscopy spectrum. The new method is applied to Se‐doped AlxGa1−xAs (x=0.19 and 0.27) grown by metal‐organic chemical vapor deposition. The measured results for all deep states including the DX centers agree well with the values published in the literature.

Deep-level transient spectroscopy of AlGaAs and CuInSe2

Abstract A new technique is used to obtain the activation energies and capture cross-sections of closely spaced traps. It is applied to single-crystal p-type CuInSe2 prepared by the vertical Bridgeman method and selenium-doped Al x Ga 1−x As (x = 0.27) grown by metal-organic chemical vapor deposition. Deep centers, which are active in the same temperature range, usually yield overlapping peaks in conventional deep-level transient spectroscopy rate-window spectra. The digitally recorded capacitive transients are analyzed directly by fitting a double exponential to the data. This fitting produces two Arrhenius plots which yield the trap depths of the two defects. This technique revealed two hole traps for CuInSe2 at 423 and 489 meV. Likewise, for AlGaAs, electron traps were discovered at 435, 455 and 368 meV. The first two energy levels were obtained by applying the double exponential fit in the temperature range 214–246 K. The energy level of the third state was the result of a single exponential fitting in the temperature range from 184 to 214 K after subtracting the two known traps from the transient. It will also be shown that the agreement between the new technique and the standard technique is excellent, and considerably more information about the traps is obtained.