1. Fig 2a: Illustration of macroscopic defects Diffusion lengths are calculated by the equation where μ is the mobility of electron with literature value of 8500 cm 2 V -1 s -1 k is the Boltzmann constant and q is the electron charge Note: Initial densities for theoretical calculations over-estimate experimental steady-state conditions A Recombination Model for GaAs Solar Cells Keyuan Zhou and Tim Gfroerer, Davidson College Yong Zhang, UNC Charlotte Abstract Solar cells convert sunlight into electricity. But defects in solar cells are one of the major factors inhibiting conversion efficiency. Defects allow for the recombination of charge carriers, so they fail to contribute to the electrical output. Measurements and preliminary analysis by Ashley Finger (’14) show illumination and temperature-dependent trends in GaAs that help clarify the role of defects. My research aims to develop a new way to analyze the data. In particular, I seek to improve the model describing the recombination process under the influence of defects in the semiconductor material. What is a GaAs Solar Cell? What are Defects? Occur during the manufacturing process Statistically, defects are unavoidable Microscopic or bulk defects are misplaced or alternative atoms in the crystal Macroscopic defects are extended mismatch features in the crystalline structure Why do defects matter? Energy from the incident light is dissipated as heat Reduces electrical output Heat generation can also damage the cell Previous Experiments Our focus is the recombination process of the electrons and holes around a macroscopic defect. Previous work included measurements of response under different illumination at different temperatures ranging from 77K-295K We have constructed a physical model and we compare theoretical curves with the experimental results Late this summer, we discovered a new method that uses the differential equation model to fit the data directly without any assumptions/simplifications Working Model and Future Work Carrier Generation and Recombination A device that converts light energy directly into electricity GaAs is a crystalline compound of elements gallium and arsenic High efficiency but high cost! Energy Valence Band Conduction Band ElectronsHoles Photon in Photon out Electrons absorb the energy from a photon and jump to a higher energy level The vacancies left behind will have an effective positive charge – these vacancies are called holes This absorption process is called carrier generation Fig 3a: Illustration of Carrier generation and recombination When the electrons fall down to the lower energy level, a photon may be re-emitted The electron fills the hole (a process called recombination) and the time between generation and recombination is called the lifetime Our Study and Preliminary Model GaInP GaAs Upper Confinement Layer Lower Confinement Layer Active Layer Bulk defect region Interface (macroscopic defect) region Fig 5a: GaAs sample diagram Fig 5b: Experimental setup diagram from thesis file of Ashley Finger (’14) Ashley Finger (’14) and Dr. Gfroerer made prior measurements on a GaAs sample By shining a laser on the sample, electron-hole pairs are generated, and a camera and oscilloscope are used to study the recombination process Carriers can diffuse before recombining, and the distance traveled is called the diffusion length Fig 3b: Illustration of diffusion and laser excitation Fig 4a: Thermal picture of a solar panel by Dr. Gfroerer Fig 4b: Diagram of defect-related loss Fig 1: Picture of a solar cell and diagram of its working mechanism Fig 2a: Illustration of microscopic defects Fig 6a: Radiative efficiency analysis Fig 6b: Transient analysis of carrier lifetime Fig 6c: Diffusion length analysis, open symbols are theoretical results MODEL Fig 7: Comparison of the transient data fits with the preliminary model (red and green) and working model (dark green) at 239K