Experimental Approach for Determining Semiconductor/liquid Junction Energetics by Operando Ambient-Pressure X-ray Photoelectron Spectroscopy

Summary

We present a method for the determination of the energy relations of semiconductor/liquid junctions, which are the basis for the successful operation of such renewable solar energy converting systems.

Abstract

Operando Ambient Pressure X-ray photoelectron spectroscopy (operando AP-XPS) investigation of semiconductor/liquid junctions provides quantitative understanding of the energy bands in these photoelectrochemical solar cells. Liquid junction photoelectrochemical cells allow a uniform contact between the light-absorbing semiconductor and its contacting electrolyte phase. Standard Ultra High Vacuum (UHV) based X-ray photoelectron spectroscopy (XPS) has been used to analyze the electronic energy band relations in solid-state photovoltaics. We demonstrate how operando AP-XPS may be used to determine these relationships for semiconductor/liquid systems. The use of "tender" X-ray synchrotron radiation produces photoelectrons with enough energy to escape through a thin electrolyte overlayer; these photoelectrons provide information regarding the chemical and electronic nature of the top ~10 nm of the electrode as well as of the electrolyte. The data can be analyzed to determine the energy relationship between the electronic energy bands in the semiconductor electrode and the redox levels in the solution. These relationships are critical to the operation of the photoelectrochemical cell and for understanding such processes as photoelectrode corrosion or passivation. Through the approach described herein, the major conditions for semiconductor-electrolyte contacts including accumulation, depletion, and Fermi-level pinning are observed, and the so-called flat-band energy can be determined.

Introduction

Semiconductor/liquid junctions have long been investigated due to their simplicity of construction and economical possibility of fuel generation ^1-4, with some such systems obtaining efficiencies over 17%.⁵ These systems operate based on the formation of a rectifying junction at the interface between the semiconductor electrode and the electrolyte. The energetics of semiconductor/liquid junctions are similar to those of a semiconductor/metal, Schottky, junction ³ where an electrolyte assumes the role of the metal. The semiconductor Fermi level, E_F, is the electrochemical potential of the electron in the semiconductor and is ....

Protocol

1. Preparation of Semiconductor for Analysis

1. Clean a p⁺, (100)-oriented boron doped Czochralski-grown Si wafer with a resistivity of ρ < 0.005 Ωcm. First soak for 2 min in a 3:1 (by volume) "piranha" solution of concentrated H₂SO₄ (98%) to 30% H₂O_2(aq).
2. Etch for 10 s in a 10% (by volume) solution of HF_(aq).
3. Immediately after step 1.2, etch in a 5:1:1 (by volume) solution of H₂O, 36% hydrochlo.......

Discussion

The most critical steps in the technique for data collection are the application of voltage and the collection of the XPS data. The semiconductor preparation is necessarily crucial but can be generalized to any system where the semiconductor/liquid junction is stable enough to be investigated. However, for the choice of electrolyte, a number of experimental parameters must be considered. First, there must be sufficient interaction (hydrophilic or hydrophobic) between the solid electrode and the electrolyte in order to fo.......

Materials

Name	Company	Catalog Number	Comments
p+-Si(100)	Addison	3P-111	Resistivity < 0.005 Ω - cm
H2SO4	Sigma Aldrich	339741	99.999%
H2O2	Sigma Aldrich	216763	30%
HF	Sigma Aldrich	339261	99.99%
millipore H2O	EMDMillipore	Milli-Q® Advantage A10	18.2 MΩ
HCl	Sigma Aldrich	320331	ACS Reagent, 37%
Tetrakis(dimethylamido)titanium(IV) (TDMAT)	Sigma Aldrich	469858	99.999%
N2	Praxair	NI 6.0 RS	>99.9999%
Ni target	AJA International	7440-02-0	>99.99%
In/Ga	Sigma Aldrich	495425	>99.99%
Hysol 9460	Ellsworth Adhesives	83128	Dual cartridge
KOH	Sigma Aldrich	306568	Semiconductor grade, 99.99%
Liquid Nitrogen	Praxair	NI 5.0
Gold foil	Sigma Aldrich	326496	99.99%
HNO3	Sigma Aldrich	438073	ACS Reagent, 70%
1-sided copper tape	adafruit	1128	For electrode construction
glass microscope slides	VWR	48300-025	For electrode construction
Ag/AgCl reference electrode	eDaq	ET072-1
Platinum foil	Sigma Aldrich	349348	99.99%
SP-300 Biologic Potentiostat	Biologic	SP-300
Scienta r4000 HiPP-2 Detector APPES	Scienta	HiPP-2