a-Si:H application to Solar Cells Jonathon Mitchell Semiconductors and Solar Cells

Overview  Fundamentals  Process  Where we’re at…

Fundamentals  Photovoltaic effect results from incident light on some materials  PV effect promotes electrons into higher energy conduction bands, leaving holes behind  Separation of carriers, electrons (-ve) and holes (+ve) important to solar cells  +ve and –ve carriers transported through material in all directions Surface recombination Bulk recombination

Fundamentals  Recombination of these carriers occurs in a variety of ways  End Result: Nothing useful  Need to separate these charges  Recombination occurs at the surface as well due to free opposing charges (defects)  Passivation of this surface is necessary to solar cells

Surface Passivation  High density of defects at surface of c-Si  Defect passivating layers: SiO x, SiN x, a-Si:H SiO x Best results Good insulator Temperatures >900 o C High risk of impurity contamination SiN x Very good results Good insulator Temperatures  400 o C Industrial BSF used Risk of impurities a-Si:H Equal or better than others Good conductor Temperatures <250 o C Doped layers Heterojunctions possible Thin layers <10nm Expensive Cheaper Cheapest/Easiest

Process  c-Si wafers etched, RCA cleaned  a-Si:H deposited by plasma enhanced chemical vapour deposition (PECVD) Non-homogenous Deposition difficult to control Lower quality layer homogenous layer quality and deposition conditions improved

PECVD  Deposition initiates reactions at surface  Desorption/abstraction  Absorption HF

QSSPC ? ? ? ? ?  Carrier lifetime measured within materials  Quasi-Steady State Photoconductance (QSSPC)  Transient Photoconductance (PCD)

Where we’re at…  Post-deposition thermal anneal greatly improves passivation quality of a-Si:H layer  Carrier lifetimes equivalent or better than those reported by other groups  Non-diffusion process defined for surface passivation

Where we’re at…  Post-deposition thermal anneal  Thermal annealing near deposition temperature significantly improved results  Thermally stable once saturation reached  Optimal a-Si:H layer thickness ~10-20nm  Other thicknesses work well

Where we’re at…  Non-diffusion surface passivation process measured and defined  Surface passivation believed to occur from hydrogen diffusing from within these thin layers towards the surface  Less energy needed for surface passivation than for diffusion  A re-configuration of the surface fits these results Energy ~1.5eV Surface Passivation Activation Energy ~ 0.69 ± 0.1eV

Conclusion  a-Si:H thin film layers provide excellent surface passivation for c-Si solar cells  Ultra-clean, state of the art, high power systems aren’t necessary for these results  Bulk diffusion of hydrogen is insufficient to explain surface passivation 1.5eV  Non-diffusion surface passivation reactions suggest surface reconfiguration is the underlying process 0.69 ± 0.1eV  Thermal annealing improves the surface passivation provided by the deposited a-Si:H  Solar cell are possible with the work that has been done so far

Acknowledgements  ARC for providing funding  Murdoch University for use of PECVD Questions?