Hello, A new project on the LCRC cluster has been requested. Please forward the information on to the LCRC Allocation sub-committee. Applicant's name: Fatih Sen Applicant's institution: ANL Applicant's division: NST Project Name: grbdft Project title: Atomistic modeling of grain boundaries in CdTe Associated funding: LDRD Other Systems: NERSC (500,000 core hours) Stampede (50,000 core hours) Science: The efficiency of polycrystalline CdTe solar cells has been below their theoretical limits [1-3] due to three primary factors: Shockley-Read-Hall (SRH) [4, 5] recombination at grain boundaries and within grains, difficulty in doping, and difficulty in forming ohmic and transparent contacts. A major uncertainty in poly-CdTe is a careful atomistic understanding of grain boundaries and their effects. Much insight on the role of grain boundaries can be gained by combining DFT calculations with experiments as to why certain behaviors are observed in poly-CdTe and why cells are far from their theoretical limit. With underlying mechanisms revealed, new techniques can be developed which may eliminate recombination centers and allow for high-performance CdTe devices. The goal of this project is to determine defect and impurity thermodynamics and their effects on electronic structures at grain boundaries in CdTe. Atomic structure models of grain boundaries in CdTe will be built based on STEM images, and structural relaxation and energetics analysis will be performed using density functional theory (DFT) calculations. 1. C. Ferekides and J. Britt, Solar Energy Materials and Solar Cells 35, 255 (1994). 2. X. Wu, Solar Energy 77, 803 (2004). 3. W. Shockley and H. J. Queisser, Journal of Applied Physics 32, 510 (1961). 4. W. Shockley and W. T. Read, Jr., Physical Review 87, 835 (1952). 5. R. N. Hall, Physical Review 87, 387 (1952). Project description: The proposed research aims to investigate simplified model structures of representative grain boundaries in CdTe, towards a detailed understanding of complete grain boundary networks present in poly-CdTe solar cells. We are going to screen the created grain boundaries based on their interface energies and will carry out detailed electronic structure analysis for the best candidate grain boundary structures within different categories. The electronic structures of the lowest-energy models will be also investigated using hybrid functionals, which give more accurate values of the band gaps and band edges than local and semilocal functionals. Site- and angular-momentum projected DOS analysis will allow us to determine the atomic orbitals from which mid-gap states arise, and suggest strategies for passivation. For possible passivants, the thermodynamics of incorporation in grain boundary structures, and effects on mid-gap states, will also be studied. In this way preferred, GB orientations can be found, as well as passivation techniques for given GB orientations. This will allow passivation techniques to be developed to exploit the behavior and encourage doping while reducing recombination. All DFT calculations will be carried out using a parallel plane wave code Vienna Ab Initio Simulation Package (VASP), which is already available in LCRC resources. Accordingly, our proposed work can be listed as: i) DFT relaxations on grain boundaries to find lowest energy structures. For 4.8º tilt (110)||(110) grain boundary, each full atomic relaxation does approximately take 20 hours using 512 processors. We are estimating to carry out 5 such calculations for three types of dislocation cores, which would take 20 hours *512 processors * 5 trials * 3 cores = 153,600 core hours. ii) For 2.0º tilt (111)||(111) grain boundary, each atomic relaxation takes approximately 20 hours using 128 processors. We estimate to do 20 trials to obtain the accurate atomic structure at the interface, such calculations, which would take 20 trials * 128 processors * 20 hours=51,200 SUs. iii) Possible passivants at 4.8º tilt (110)||(110) grain boundary will be considered for both the three types of dislocation cores. We are planning to try 5 different dopants at each dislocation core. Each atomic relaxation and electronic structure calculation is expected to take 20 hours using 512 processors. For each dopant we will also consider two different charged states. As a result the time requested for these calculations will be: 5 dopants * 3 dislocation cores * 3 charge states * 10 hours *512 processors = 230,400 SUs. iv) Hybrid functional calculations of selected systems. 5 system * 100 hours * 128 cores = 64,000 core hours. Total computation time requested: 450,000 core hours. Industry partnership: Project URL: Requested allocation: 450000 Q1: 112500 Q2: 112500 Q3: 112500 Q4: 112500 Justification: Storage requirements: 1 TB The requester has used undetermined amount hours of their initial startup project. In addition to approving an initial amount, please specify a Category and Subcategory for this project. For a list of the current selection of approved categories, please see: https://wiki.lcrc.anl.gov/wiki/Processes/Categories Once the Allocation committee has approved the project, please go to the Project Management page to create it: https://accounts.lcrc.anl.gov/projects.php Thank You, The LCRC Accounts System