1. Photoluminescence and Photocurrent in a Blue LED Ben Stroup & Timothy Gfroerer, Davidson College, Davidson, NC Yong Zhang, University of North Carolina at Charlotte, Charlotte, NC Contrast Imaging Motivation: Solar Cells Experimental Setup Laser Semiconductor Camera Spectrometer 4 2 3 1 Low Pass Filter Low Power, Open Circuit High Power, Open Circuit Medium Temperature, 155K High Power, Short Circuit Low Power, Short Circuit Model The high energy portion of the solar spectrum can be converted into electricity most efficiently with a semiconductor that has a bandgap E g in the blue region of the spectrum. Blue LEDs (light emitting diodes) work in reverse: they convert electricity into blue light. We study the behavior of blue LEDs across different temperatures and optical powers to assess their promise as solar cell devices. Mechanisms 1 Absorption 2 Emission 3 Current 4 Heat Conduction Band _ _ _ _ _ _ 3 __ 1 2 4 Defect Level + + + + Valence Band 3 + ++ + Energy EgEg The contrast, defined as the difference in intensity between bright and dark regions, visibly changes with power and temperature. This allows us to analyze the diffusion of charge carriers (which can depend on trapping), as well as the propagation, re- absorption, and re-emission of light throughout the device. 1)A photon with energy greater than E g is absorbed and an electron is excited to the conduction band, leaving a hole in its place. 2)An electron recombines with a hole, producing a photon with energy h ≈ E g (a process called photoluminescence or PL) 3)Electrons and holes can also drift along the built-in electric field of the device producing current. 4)Electrons can become trapped in lower energy defects, producing heat when they recombine with holes. Ammeter 100 μm 7.87 W/cm 2.009 W/cm 2 7.87 W/cm 2.009 W/cm 2 100 μm Spectrum Analysis + _ + _ Semiconductor Edge _ + + _ Side View of LED Device Higher energy photons can be re-absorbed and re-emitted locally. Lower energy photons are less likely to be reabsorbed and travel further, so we see more low energy light from the edge of the device. 1 2 2 The decrease in PL efficiency of the low power, open circuit configuration as temperature increases is due to an increase in non- radiative (heat) recombination. The decrease in efficiency of the high power, closed circuit configuration as temperature increases is due to a decrease in the rate of radiative (light) recombination. 2 4 Conclusions  Spatial inhomogeneity in the PL emission demonstrates the existence of regions of higher and lower quality (i.e. defects) that may be related to LED droop.  Lower energy light recycling largely contributes to the dispersion of light across the LED device, leading to more uniform emission.  Two major factors involved in the temperature-dependent PL efficiency and current generation are radiative (light) and non- radiative (heat) recombination.  For blue solar cells at room temperature: The radiative loss decreases with increasing temperature and should not inhibit current generation. A non-radiative loss mechanism (proportional to the current) will limit the conversion efficiency above a threshold carrier density. Acknowledgements We would like to thank the Faculty Study and Research Committee for supporting this research. Slope: E a ≈ 41 meV Abstract Solar cells are semiconductor devices that convert light from the solar spectrum into electricity. LED lights work in reverse; they convert electricity into light. In this study, we use optical excitation to examine the photoluminescence and photocurrent generated by blue LEDs across a range of temperatures and laser powers. In images of the photoluminescence, we observe contrast related to the localization of charge carriers and the propagation of light through the device. Photoluminescence spectra from different regions of the device demonstrate that propagation depends on the wavelength of emitted light. And our model of the underlying physics helps us identify the mechanisms contributing to changes in photoluminescence and photocurrent with changing temperature and optical power.