Abstract
In recent years it has been shown that graphene oxide (GO) can be used to passivate silicon surfaces resulting in increased photocurrents in metal-insulator-semiconductor (MIS) tunneling diodes, and in improved efficiencies in Schottky-barrier solar cells with either metal or graphene barriers, however, the source of this passivation is still unclear. The suggested mechanisms responsible for the enhanced device performance include the dangling bond saturation at the surface by the diverse functional groups decorating the GO sheets which reduce the recombination sites, or field effect passivation produced by intrinsic negative surface charge of GO. In this work through a series of measurements of minority carrier lifetime with the microwave photo-conductance decay (µPCD) technique, infrared absorption spectra, and surface potential with Kelvin probe force microscopy (KPFM) we show that there is no evidence of significant chemical passivation coming from the GO films but rather negative field effect passivation. We also discuss the stability of GO's passivation and the flexibility of this material for its application as temporary passivation layer for bulk lifetime measurements, or as a potential cheap alternative to current passivation materials used in solar cell fabrication.
Highlights
It is well known that one of the most effective approaches to improve the efficiency of silicon-based solar cells, whilst maintaining low cost, is to increase the lifetime of photo-generated carriers by reducing recombination at the surface and in the bulk of low cost materials [1]
We have carried out transient photoconductance mapping to compare the level of surface passivation achieved by graphene oxide (GO) in various sets of samples, here we present the comparison of GO's passivation on an electronic grade float zone (FZ) material to that of a solar grade Cz material
These values are comparable to the surface recombination velocities of some existent surface passivation materials [3,4]
Summary
It is well known that one of the most effective approaches to improve the efficiency of silicon-based solar cells, whilst maintaining low cost, is to increase the lifetime of photo-generated carriers by reducing recombination at the surface and in the bulk of low cost materials [1]. For surface passivation materials we need to consider three material property aspects: the characteristics of the material to be passivated (doping type and resistivity); the physical properties of the passivating materials (optical, chemical and electrical), to determine the type of passivation that the material will provide including bond saturation, the field effect control of carriers, refractive index, and stability; and processing requirements like surface cleaning and synthesis methods With this in mind, in this work we have studied the passivation capabilities of graphene oxide which we know fulfils some key requirements for surface passivation including: high transmittance [5], fixed surface negative charge [6,7] and high refractive index [8,9]. It has been recently shown that for a GO derivative dispersion with an optimal dilution, it is possible to obtain uniform coverage even on textured silicon surfaces for solar cells [10]
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