Jose Juan Chavez

Defect formation dynamics during CdTe overlayer growth

The presence of atomic-scale defects at multilayer interfaces significantly degrades performance in CdTe-based photovoltaic technologies. The ability to accurately predict and understand defect formation mechanisms during overlayer growth is, therefore, a rational approach for improving the efficiencies of CdTe materials. In this work, we utilize a recently developed CdTe bond-order potential (BOP) to enable accurate molecular dynamics (MD) simulations for predicting defect formation during multilayer growth. A detailed comparison of our MD simulations to high-resolution transmission electron microscopy experiments verifies the accuracy and predictive power of our approach. Our simulations further indicate that island growth can reduce the lattice mismatch induced defects. These results highlight the use of predictive MD simulations to gain new insight into defect reduction in CdTe overlayers, which directly addresses efforts to improve these materials.

High-fidelity simulations of CdTe vapor deposition from a bond-order potential-based molecular dynamics method

CdTe has been a special semiconductor for constructing the lowest-cost solar cells and the CdTe-based Cd1-xZnxTe alloy has been the leading semiconductor for radiation detection applications. The performance currently achieved for the materials, however, is still far below the theoretical expectations. This is because the property-limiting nanoscale defects that are easily formed during the growth of CdTe crystals are difficult to explore in experiments. Here we demonstrate the capability of a bond order potential-based molecular dynamics method for predicting the crystalline growth of CdTe films during vapor deposition simulations. Such a method may begin to enable defects generated during vapor deposition of CdTe crystals to be accurately explored.

Understanding misfit strain releasing mechanisms via molecular dynamics simulations of CdTe growth on {112}zinc-blende CdS

Molecular dynamics simulations have been used to analyse microstructures of CdTe films grown on {112} surfaces of zinc-blende CdS. Interestingly, CdTe films grow in ⟨331⟩ orientations as opposed to ⟨112⟩ epitaxial orientations. At the CdTe-{331}/CdS-{112} interface, however, there exists an axis that is parallel to the ⟨110⟩ orientation of both CdS and CdTe. It is the direction orthogonal to this ⟨110⟩ that becomes different, being ⟨116⟩ for CdTe and ⟨111⟩ for CdS, respectively. Missing CdTe-{110} planes are found along the ⟨110⟩ axis, suggesting that the misfit strain is released by the conventional misfit dislocation mechanism along this axis. In the orthogonal axis, the misfit strain is found to be more effectively released by the new grain orientation mechanism. Our finding is supported by literature experimental observations of the change of growth direction when Cd0.96Zn0.04Te films are deposited on GaAs. Analyses of energetics clearly demonstrate the cause for the formation of the new orientation, and the insights gained from our studies can help understand the grain structures experimentally observed in lattice mismatched systems.

Nanopatterning and bandgap grading to reduce defects in CdTe solar cells

We present simulation and experimental results proving the feasibility of a novel concept to increase efficiency of CdTe based solar cells. In order to achieve

Molecular dynamics simulations of ZnTe/Cu back contacts for CdTe solar cells

0.50/W price in CdTe based modules, higher efficiencies need to be attained. The high defect density due to lattice-mismatch between CdS and CdTe reduces lifetime, voltage, and efficiency of the cells. We propose the use of a graded composition structure and a patterned substrate to reduce defects, increase lifetime, and efficiency of the cells. Innovative simulations using high-fidelity molecular dynamics predict that defect-free films are possible if the CdTe film is graded with Zn and is constructed as nano-islands with sizes below 90 nm. Both graded structure and nano-islands reduce the lattice-mismatch stresses. Also, the graded composition creates a back surface field and an enhanced ohmic contact. We have attempted to grow ZnTe and CdTe films on CdS substrates using a template of micro and nano-islands. Selective growths on patterned substrates have shown fewer grain boundaries when the island size decreases below 300 nm. Also, larger grain sizes were obtained using a CdTe/ZnTe stack when compared to a single layer CdTe. The simulation and experimental results demonstrate for the first time the ability to use nanopatterned substrates to enhance uniformity in thin film solar cells.

A molecular dynamics study on defect reduction in thin film Cd 1−x Zn x Te/CdS solar cells

Molecular dynamics (MD) simulations have been applied to study the growth of ZnTe/Cu/CdTe layers on CdTe substrates. Our studies show that Cu forms pure clusters and ejects Cd atoms within the CdTe layer out towards the films surface. Elemental concentration plots indicate that the amount of Cu added to the growth plays an important role on the intermixing between ZnTe and CdTe layers and the doping of CdTe. These results provide useful insight to the development of effective and reliable back contacts used in CdTe solar cells.

Towards model-guided defect reduction in Cd 1−x Zn x Te/CdS solar cells: Development of molecular dynamics models

Recently developed molecular dynamics models have been applied to study the formation of defects during growth of ZnTe-on-CdS multilayers. Our studies indicated that misfit dislocations are formed during growth, and the dislocation density can be reduced if the ZnTe layer is grown in a nano island configuration as opposed to a continuous film. These results highlight the use of molecular dynamics methods in providing valuable defect formation mechanism insight and guiding experimental efforts to produce high efficiency Cd1-xZnxTe solar cells.

Record breaking solar cells : ZnxCd(1-x)Te graded bandgap nanoarrays.

Cd1-xZnxTe/CdS solar cells are currently limited by material defects. While nano-structuring promises further defect reductions, the materials synthesis and characterization become more challenging. Molecular dynamics models capable of growth simulations enable defects to be explored without assumptions, and can therefore guide nanoscale experiments. Such models are difficult to develop, and are not routinely available in literature for semiconductor compounds. To fill this gap, we have developed growth simulation enabling Stillinger-Weber and bond-order potentials. These new models begin to enable molecular dynamics to be used to explore nano-structured Cd1-xZnxTe/CdS solar cells with reduced defects.

A prediction of dislocation-free CdTe/CdS photovoltaic multilayers via nano-patterning and composition grading

CdTe is the leading material for thin-film solar cells due to ease of processing and reduced cost. However, low conversion efficiencies due to defects in the material are still a problem in these devices. We propose implementing micro and nano-enabled pseudomorphic growth of ZnCdTe to dramatically increase the efficiency of CdTe solar cells. Simulations predict that defect-free films are possible in graded ZnCdTe nanoislands below 90 nm as well as 24 % efficient solar cells with the use of ZnCdTe grading. Selective growth of CdTe showed single grains when the island sizes decreased below 300 nm. The simulation and experimental results demonstrate for the first time the ability to use nanopatterned substrates to enhance uniformity and efficiency in CdTe thin film solar cells. More than 20 reports, presentations, and papers were produced as part of this work and the results from this project enabled UTEP and Sandia to secure to grants.