Markus Gloeckler

CdTe Solar Cells at the Threshold to 20% Efficiency

After a long period of stagnancy for record cell efficiency and several years of growth of the industry, CdTe solar cell efficiency has been rapidly increasing recently. First Solar (FSLR) fabricated an 18.7% NREL certified cell at the end of 2012 and reports an increase to a certified 19.0% in this paper. Although the improvements were dominated by increases in short-circuit current and fill factor, there is now evidence that the open-circuit voltage of polycrystalline CdTe is not fundamentally limited to ~850 mV, and first devices exceeding 900 mV have been demonstrated. In-depth device characterization indicates that the responsible third-level metric is the increase of carrier lifetime, which can be characterized by transient photoluminescence. The progress at hand suggests that the near-term achievable target for CdTe solar cells should be raised from 19% to 22%. A detailed numerical model is used to translate cell results into predicted module efficiency. These simulations are in agreement with FSLRs recently certified 16.1% total-area module efficiency record. Forward-looking simulations show that with already demonstrated technology components, a module efficiency exceeding 17% is attainable. Disruptive changes and implementation of new device architectures can provide further room for improvement for cell efficiency beyond 22%.

Recombination Analysis in Cadmium Telluride Photovoltaic Solar Cells With Photoluminescence Spectroscopy

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 <;100 K, lifetimes in devices were similar to those in single crystals and ranged from 300-400 ns. Strong lifetime temperature dependence at 100-300 K suggests the presence of a relatively shallow recombination center. Therefore, it is possible that in CdTe solar cells, lifetime is limited by relatively shallow defects.

The impact of Cu on recombination in high voltage CdTe solar cells

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.

Measurement of Chlorine Concentrations at CdTe Grain Boundaries

We measure the chlorine concentrations at grain boundaries in CdTe thin films using Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) and find that over 90% of the total Cl in the film is concentrated on the grain boundary. We describe a method for quantifying the grain boundary and intragrain chlorine concentrations. Using this method, the average saturation chlorine concentration in the grain boundary for vapor deposited CdTe with a mixed (1 1 1) and (0 0 1) texture is 2.4 × 1014 atoms/cm2. When the chlorine concentration at the grain boundary is well below the saturation level, solar photovoltaic device performance degrades. We also find that twin boundaries in CdTe with (1 1 1)/〈1 1 1〉 orientations (CSL sigma 3) do not have Cl decorating the boundary above the ToF-SIMS detection limit of 1012 atoms/cm2. However, we find multiple instances, where the (1 0 1)/〈1 1 1〉 twins (CSL sigma 9) are often strongly decorated with Cl at levels comparable with average grain boundaries. Total Cl concentration in the films is strongly correlated with the grain boundary density in the analysis area.