Work function and induced band bending characterization for engineering of selective contact for solar cells

¹Semilab SDI, Tampa, FL 33617, USA

²University of New South Wales (UNSW), Sydney NSW 2052, Australia

³Australian National University (ANU), Canberra ACT 0200, Australia

Adv. Mater. Lett., 2018, 9 (9), pp 629-631

DOI: 10.5185/amlett.2018.2084

Publication Date (Web): Jun 14, 2018

Copyright © IAAM-VBRI Press

E-mail: mwilson@semilabsdi.com

Abstract

This work demonstrates the effectiveness of non-contact Kelvin-probe and surface photovoltage characterization of the work function (WF) induced barriers formed in silicon (Si) by thin 5nm carrier selective contact films of MoO_x, TiO₂ and MgF₂.  The calibrated Kelvin probe in the dark and under strong illumination where used to determine the dark WF of the deposited films and the band bending in the Si, F_BB = WF_Dark – WF_Light.  The ac-surface photo voltage provided an independent measurement of the Si depletion layer width.  Whole wafer mapping of all parameters can be performed.  For n-type Si the high work function oxides MoO_x (WF~5.7eV) and TiO₂ (WF~5.0eV) are found to induce a depletion barrier with the height increasing with WF as F_BB[eV] = 0.23WF – 0.77, i.e. quite similar to the well-known relationship for metal-silicon contacts.  For the low work function MgF₂ film, a depletion barrier was induced only in p-type Si.  For this case, full wafer mapping revealed a lower WF pattern coinciding with larger band bending giving the slope, DF_BB/DWF ~ -0.52.  The slopes of 0.23 and 0.52 for n- and p-type Si deviates significantly from the ideal slope of 1.  This result implies that the barrier formation at the Film-Si heterojunction is limited by the effect of interfacial layers and interface states in analogy to the well-known effects in Metal-Si contacts. It is believed that this demonstrated very fast, preparation-free, non-contact characterization technique can benefit research and engineering of selective contacts for solar cells. 

Keywords

Solar cell, work function, band bending, hole selective contacts, electron selective contacts.