1. Photocapacitance measurements on GaP alloys for high efficiency solar cells Dan Hampton and Tim Gfroerer, Davidson College, Davidson, NC Mark Wanlass, National Renewable Energy Lab, Golden, CO Abstract Motivation: Multi-junction solar cells If higher energy photons are absorbed in higher bandgap alloys, the heat loss caused by excess photon energy relative to the gap is reduced. Photocapacitance: Optically Stimulated Escape Deep level transient spectroscopy (DLTS) employs transient capacitance measurements on diodes during and after the application of a bias pulse to monitor the capture/emission of carriers into/out of defect-related traps. The photocapacitance experiment is a modified version of DLTS that uses light (rather than heat) to excite the charge carriers out of traps. Photocapacitance Experimental Setup When a photon is absorbed, an electron is excited into the conduction band, leaving a hole behind in the valence band. Some heat is lost, reducing efficiency. Then an internal electric field sweeps the electrons and holes away, creating electricity. Conduction Band Valence Band Increasing Energy For Holes + Hole Capture Thermal Escape + Trap Depth While stacking materials of different bandgaps will increase the efficiency of solar cells, it will also create defects within the device because of lattice-mismatching. Defects provide energy levels that restrict the movement of charge carriers. This inhibits the production of electricity. Once a carrier becomes trapped, it must be excited out of the defect state either thermally or optically. Acknowledgements We thank Jeff Carapella for growing and processing the test structures. We also thank the Donors of the American Chemical Society – Petroleum Research Fund and the Davidson Research Initiative for supporting this work. Defect Levels Large bandgap materials are an essential component of ultra-high efficiency solar cells. While multi-junction devices harness more of the solar spectrum for electricity production, challenges in the construction of a lattice-mismatched system lead to defects that trap charge carriers and inhibit overall efficiency. Characterization of these defects may enable device engineers to minimize their effects. This work uses photocapacitance measurements to obtain optical escape energies from defect levels in high-bandgap GaInP and GaAsP alloys. We combine these optical results with previous thermal measurements to construct a working model that incorporates a lattice configuration-dependent energy structure. The model explains why the optical escape energy is significantly larger than the thermal capture and escape energies, and in the GaAsP device, it helps explain why the number of escaping carriers depends on the energy of the incident light. N+ P + + Depletion Layer - - - + - - - + + + + - - - - - - - - - - - - - - - - - - - - - - - -- + + + + Depletion without bias 77 K The computer operates the temperature controller and retrieves data from the digital oscilloscope. The pulse generator applies the reverse and pulse biases to the sample while the capacitance meter reads the resulting change in capacitance as a function of time. During this process, the sample is illuminated with a particular wavelength of light from the monochromator. GaInP Results PHOTON + Optical Escape PHONON Optical Escape Working Model for Observed Threshold Energies GaAsP Results The optical escape threshold energy of 0.695 eV is significantly higher than the thermal escape energy of 0.322 eV for GaInP found in previous work. These results support the model described below. In the configuration-dependent model, the energy of the system changes as a function of the local configuration of the lattice. This means that lattice vibrations, or phonons, can push the defect level closer to the valence band, but photons, which carry very little momentum, have no effect on the defect level. As a result, the optical escape energy is much larger than the thermal escape and capture energies. PHOTON Valence Band Defect Levels Thermal Escape (0.394 eV) Thermal Capture (0.370 eV) Enlarged View Optical Cross Sections High Energy Light Low Energy Light As the device cools, charge carriers become frozen into the defects (thermal capture), which extends the depletion region and causes the capacitance to decrease. The charge carriers escape when the device is exposed to light. On the warming cycle the thermal capture occurs at a lower temperature than the thermal escape (the thermal capture energy is smaller than the thermal escape energy). When the energy of the light is reduced, the optical escape rate decreases as expected, but the transient amplitude is also reduced. The reduced amplitude indicates that carriers in some traps are not optically activated by the lower energy light. This result implies the defects in our GaAsP device have a range of energies as shown in the diagram above. For a given defect energy, the optical cross section, which control the optical escape rate, depends on the energy of the incident light. The diagram also shows how low energy light only activates traps in the upper portion of the energy range.