DavidLaFehrEEResearchProject
- 1. Carrier Selective Contacts for Ultra-Thin-Film c-Si Solar Cells David LaFehr Department of Electrical Engineering, Stanford University Motivation Background Fabrication Fabrication Acknowledgements CharacterizationCharacterization Minority Carrier Lifetime Contact Resistivity How can we make the economics work? u Conventional Si solar cells: ~$1.5/W u ~$1/W to be competitive with other sources u Reduce price • Use less material • Increase efficiency u Proposed solution: Ultra-thin-film crystalline silicon solar cells with carrier selective contacts Light is incident upon the solar cell and generates electrons and holes. Ideally, metal contacts collect all of these carriers. But metal-semiconductor interface defects permit the recombination of electrons and holes, rendering them useless. My research investigates titanium dioxide (TiO2) as an electron-selective material that reduces recombination and thereby increases solar cell efficiency. Two parameters that give insight into the effectiveness of a contact are minority carrier lifetime and contact resistivity. 1. Intrinsic Si wafer 2. Atomic layer deposition (ALD) of TiO2 (one wafer only) 3. Thin film deposition (TFD) of Ti 4. Manual coating of PMMA The quasi-steady-state photoconductance method was used to measure minority carrier lifetime. Before the measurement, a dip in hydrofluoric acid was performed to remove silicon dioxide. During the measurement, the device was submerged in quinhydrone in methanol, which passivated the backside of the wafer. 1. Slightly p-type Si wafer 2. Epitaxial growth of n-type Si 3. Coat 4. Lithography 5. Develop 6. Etch 7. ALD of TiO2 8. Repeat 2-6 9. TFD of Ti and Al 10. Lift-off The transfer length method was used to measure contact resistivity. For each column of metal platforms, the resistance between adjacent columns was determined from an I-V sweep. The contact resistivity was gleaned from the resistance vs distance graph. Want to learn more? u Visit PVEducation.org u Take EE116 (Semiconductor Device Physics) u Yusi Chen, for his guidance, support, and enthusiasm u Yi Liu, for her theoretical knowledge and data analysis u Dr. Yangsen Kang, for his PhD dissertation on ultra-thin-film c-Si solar cells u Professor James S. Harris and the solar group RT = 2RC + Rsheet L W ρC = RCWLtransfer ResultsResults Future Work The group will continue to investigate carrier selective contacts through minority carrier lifetime and contact resistivity. Variations will include: • Doping level • Annealing conditions • Hole-selective NiO in place of electron-selective TiO2 Only samples that underwent RTA at 500°C with forming gas followed the expected trend. The result for one such sample is shown below. This resistivity value is impressive given that the emitter doping level (1017 cm-3) is low. RC =138.99Ω ρC =14.6mΩ⋅cm2 Sample Lifetime (µs) Ti 16.7 TiO2/Ti 317.4 The average minority carrier survives 19 times as long in a TiO2/Ti device as in a Ti device. So, a TiO2 layer significantly reduces recombination. In just one hour, enough sunlight reaches the Earth to power human civilization for an entire year. The goal of my research is to expand on Yangsen Kang’s PhD dissertation on ultra-thin-film c-Si solar cells by investigating carrier selective contacts. Light strikes a semiconductor, generating electrons and holes that accelerate in opposite directions. Incorporating carrier selective contacts into solar cells reduces recombination and increases efficiency. Yusi Chen, Yi Liu, Max Vilgalys, Daniel DeWitt, Jieyang Jia, Huiyang Deng, Li Zhao, Dr. Yijie Huo, Dr. Yangsen Kang, Professor James S. Harris h"p://www.renewablegreenenergypower.com/solar-‐ energy-‐facts-‐solar-‐energy-‐alone-‐can-‐power-‐the-‐world/ http://pvinsights.com/Knowledge/Principle.php http://tuttle.merc.iastate.edu/ee432/ topics/metals/tlm_measurements.pdf Resistance vs Distance Resistance vs Distance ρC