J.T. Heath

Bulk and metastable defects in CuIn1−xGaxSe2 thin films using drive-level capacitance profiling

The drive-level capacitance profiling technique has been applied to ZnO/CdS/CuIn1−xGaxSe2/Mo solar cell devices, in order to study properties of defects in the CuIn1−xGaxSe2 film. Properties studied include the spatial uniformity, bulk defect response, carrier density, and light-induced metastable effects. These results indicate that previous estimates of carrier densities, from C–V profiling, may be significantly overestimated. In addition, a defect response previously thought to be located at the interface is observed to exist throughout the bulk material. Finally, an infrared light-soaking treatment is demonstrated to induce metastable changes in the bulk CuIn1−xGaxSe2 film. Hence, the drive-level capacitance profiling technique provides valuable insights into these films. Herein, the technique itself is fully explained, compared to other junction capacitance methods, and its utility is demonstrated using numerical simulation.

Effect of Ga content on defect states in CuIn1−xGaxSe2 photovoltaic devices

Defects in the band gap of CuIn1−xGaxSe2 have been characterized using transient photocapacitance spectroscopy. The measured spectra clearly show response from a band of defects centered around 0.8 eV from the valence band edge as well as an exponential distribution of band tail states. Despite Ga contents ranging from Ga/(In+Ga)=0.0 to 0.8, the defect bandwidth and its position relative to the valence band remain constant. This defect band may act as an important recombination center, contributing to the decrease in device efficiency with increasing Ga content.

Distinguishing metastable changes in bulk CIGS defect densities from interface effects

Abstract The deep defect distributions affecting majority carrier trapping in polycrystalline CuIn 1− x Ga x Se 2 (CIGS) have been studied using drive level capacitance profiling. This technique provides, a spatial and energetic profile of sub-band gap defect transitions in the CIGS layer of working photovoltaic devices, while remaining insensitive to surface states. The bulk response was dominated by a defect which varied between 0.1 and 0.3 eV, according to the Meyer-Neldel rule. Devices grown at reduced substrate temperatures had smaller grain sizes and additional defect response. The effect of a light-soaking treatment, using near-band gap optical excitation, was studied. Both the free carrier density and the density of deeper defects were increased by this treatment. Device quality was degraded, predominantly due to a decreased fill factor.

Near-surface defect distributions in Cu(In, Ga)Se2

The density and distribution of point defects in Cu(In,Ga)Se (CIGS) layers used for solar cell applications is critical to the 2 resulting device performance.These devices are generally thought to be limited by recombination in the space-charge region of the collecting heterojunction.The situation is complicated by the presumed presence of an n-type surface layer on the CIGS absorber.Both the surface inversion and space-charge recombination processes are intimately tied to near-surface point defects. Here, we overview recent results on surface chemistry, transient photocapacitance spectroscopy (TPC) and depth-resolved cathodoluminescence (CL) for polycrystalline device layers from two laboratories, and single crystal epitaxial layers of three orientations.The results are combined with device modeling to provide a picture of the near-surface defect structures in these materials.The TPC results show deep defect levels ;0.7 and 0.9 eV above the valence band. CL shows evidence of subgap radiative recombination, which increases dramatically near the sample surfaces.The results point to a near-surface Cd-containing layer, which could be responsible for the surface carrier type inversion, a near-surface region containing an elevated defect density, possibly near the valence band edge, and deep hole traps near the conduction band.Implementation of the results in a device model provides reasonable fits to the device performances. 2003 Elsevier Science B.V. All rights reserved.

Insights into the mechanisms of light-induced degradation from studies of defects in low Ge fraction a-Si,Ge:H alloys

Abstract The modulated photocurrent (MPC) method was used to study deep defect creation and annealing in low Ge fraction a-Si,Ge:H alloys (2–10 at.% Ge). These measurements reveal two distinct bands of deep defects in these samples which are identified as neutral Si and neutral Ge dangling bonds. Upon thermal annealing in the dark from a strongly light degraded state, these two defects are found to decrease in a manner which indicates a direct competition between the annealing of the Si and Ge dangling bonds, and therefore implies a global reconfiguration mechanism. Comparing purely thermal annealing with light-induced annealing indicates that the relative anneal rate for the two types of defects is different. This therefore tends to rule out models in which the rate limiting step in the annealing process comes from the release of the mediating entity (hydrogen? strain?) from a remote site.

Light-induced Annealing of Deep Defects in Low Ge Fraction a-Si,Ge:H Alloys: Further Insights into the Fundamentals of Light-induced Degradation

Modulated photocurrent (MPC) was employed to study metastable deep defect creation and annealing in low Ge fraction a-Si,Ge:H alloys (Ge fractions below 10at.%). These MPC spectra reveal two distinct bands of deep defects identified as neutral Si and neutral Ge dangling bonds. Upon heating in the dark from a strongly light degraded state, these are reduced in direct competition to each other, therefore implying a global reconfiguration mechanism. We have extended our studies to compare purely thermal annealing with light-induced annealing in these alloys. We have found that the relative anneal rate for the two types of defects differs from the case of purely thermal annealing. This therefore tends to rule out models in which the rate limiting step during annealing comes from the release of the mediating entity from a remote site.

Correlation between deep defect states and device parameters in CuIn/sub 1-x/Ga/sub x/Se/sub 2/ photovoltaic devices

Sub-bandgap defect densities and minority carrier collection properties have been characterized for a series of CuIn/sub 1-x/Ga/sub x/Se/sub 2/ devices with varying device efficiencies. Samples fabricated using reduced substrate temperatures during growth have also been studied, and show additional defect response. Transient photocapacitance and photocurrent spectroscopies were employed to deduce the defect distributions and minority carrier mobilities. Drive-level capacitance profiling was used to determine the spatial and thermal energy distributions of the majority carrier traps. Photovoltaic device parameters were determined for all samples. It appears that the device efficiency is not clearly controlled by any single sub-bandgap defect population.