A model for emission yield from planar photocathodes based on photon-enhanced thermionic emission or negative-electron-affinity photoemission

Abstract

A general model is presented for electron emission yield from planar photocathodes that accounts for arbitrary cathode thickness and finite recombination velocities at both front and back surfaces. This treatment is applicable to negative electron affinity emitters as well as positive electron affinity cathodes, which have been predicted to be useful for energy conversion. The emission model is based on a simple one-dimensional steady-state diffusion treatment. The resulting relation for electron yield is used to model emission from thinfilm cathodes with material parameters similar to GaAs. Cathode thickness and recombination at the emissive surface are found to strongly affect emission yield from cathodes, yet the magnitude of the effect greatly depends upon the emission mechanism. A predictable optimal film thickness is found from a balance between optical absorption, surface recombination, and emission rate. Disciplines Mechanical EngineeringComments Sahasrabuddhe, K., Schwede, J., Bargatin, I., Jean, J., Howe, R., Shen, Z., & Melosh, N. (2012). A model for emission yield from planar photocathodes based on photon-enhanced thermionic emission or negativeelectron-affinity photoemission. Journal of Applied Physics, 112(9) A general model is presented for electron emission yield from planar photocathodes that accounts for arbitrary cathode thickness and finite recombination velocities at both front and back surfaces. This treatment is applicable to negative electron affinity emitters as well as positive electron affinity cathodes, which have been predicted to be useful for energy conversion. The emission model is based on a simple one-dimensional steady-state diffusion treatment. The resulting relation for electron yield is used to model emission from thin-film cathodes with material parameters similar to GaAs. Cathode thickness and recombination at the emissive surface are found to strongly affect emission yield from cathodes, yet the magnitude of the effect greatly depends upon the emission mechanism. A predictable optimal film thickness is found from a balance between optical absorption, surface recombination, and emission rate.

Photon-enhanced thermionic emission from heterostructures with low interface recombination

“…This suggests that bulk recombination dominates in this heterostructure 16 . Indeed, the interface recombination velocities S 1 and S 2 could be much lower than 10 4 cm s À 1 and fit experimental data well.…”

“…In this intermediate photon-energy regime, PETE dominates and can be directly compared with theoretical expectations. On the basis of a model we have developed for the PETE process 16 , the rate of PETE can be described using an emission velocity S PETE , analogous to a recombination velocity at the absorber/emitter interface:…”

“…The quantum efficiency is dependent on a simple competition between this PETE velocity (equation 1) and recombination 16 :…”

“…This is a critical comparison for optimizing highefficiency devices, as surface recombination must be lower than bulk recombination not to be the limiting factor in emission from a PETE cathode. Bulk recombination can be reduced in future work by using thinner absorbers and with better light trapping 16 . These improvements to photon management, along with the ultra-low interface recombination velocities possible in heterostructures [25][26][27] , illuminate a path to PETE devices with very high emission efficiencies.…”

Investigation of contact grid geometry for photon-enhanced thermionic emission (PETE) silicon based solar converters

“…Another model accounted only for quantum efficiency and output current, and did not discuss net efficiency (Sahasrabuddhe et al, 2012). The representation of electrical contacts in all one-dimensional models is still not realistic: the contact is a completely transparent layer that covers the entire front surface of the cathode.…”