Work Function Measurements for Energy Applications
J. Dulski * and C. Burrows (Supervisor)
Department of Physics, University of Warwick, Coventry, CV4 7AL, United Kingdom.
* Corresponding author (e-mail: J.W.Dulski@warwick.ac.uk)
Introduction
Equipment
 The work function (WF) is the energy required to eject an
electron from the surface of a material to the outside world. The
lower the WF is the more efficient it is to generate a current [1].
 Applications of low WF materials, such as Silicon and
Germanium as well as Copper Printed Circuit Boards (THGEM),
range from photocathodes to photovoltaic cells [2].
 By alterating their surface via adsorption of organic polymers,
for example polyethylenimine (PEI), and salts such as Caesium
Iodide (CsI) a reduction in WF can be achieved [3].
Experimental details
 Samples cleaved to size from Si(001) and
THGEM sample
Ge(001) base. THGEMs manufactured on
Copper PCBs via chemical etching and drilling.
 Si and Ge samples cleaned in IPA & Acetone
ultrasonic bath (1:1), rinsed in methanol and
nitrogen dried (SCP [4]). THGEMs not cleaned.
 Branched PEI dissolved in H2O and/or CH3OH.
 Systematic approach used by only varying certain conditions:
spin coating speed/acceleration, annealing time/temperature,
solution concentrations/solvents, oxidisation thickness.
 Use of Atomic Force Microscopy (AFM) and X-Ray Photoelectron
Spectroscopy (XPS) techniques to analyse coated surfaces.
Kelvin Probe
 Used for work function measurements
on the surface of materials.
 High resolution of 1-3 meV.
 Off-null data linearization capabilities.
Spin Coater
 Solutions deposition instrument.
 Top speed of 6800 RPM.
 Used to coat samples with PEI and CsI.
Plasma Coater
 Surface preparation and cleaning
system.
 High power UV light and oxidation
method, for treating THGEMs.
 Additional O, N, Ar gases use
capabilities.
Results and analysis
WF change after PEI deposition on Si and Ge samples
Annealing Test
4.6
4.6
4.5
4.5
4.4
Work Function (eV)
Work Function (eV)
Concentration Test
Baseline Si(001)
0.4 wt% H2O
0.8 wt% H2O
1.6 wt% H2O
3.2 wt% H2O
4.3
4.2
4.1
AFM surface imaging of PEI layer
Baseline Si(001)
160 oC - 10 min.
130 oC - 20 min.
100 oC - 30 min.
4.4
4.3
4.2
4.1
4.0
4.0
3.9
3.9
0
Measurement Index
0
1000
Measurement Index
1000
Effects of oxidation on THGEMs coated with CsI and PEI
Cs 3d
Max: 0.5 Min: 0.0(a)
0.2
201 um
0.0
0.0
Max: 0.1 Min: 0.0
I 3d
 (a) Ridge of oxidised THGEM.
Max: 0.0 Min: 0.0
Observed by XPS analysis, a
0.0
deposition ratio of 1:3:15 of I, Cs
and Cu respectively.
 Considerably higher decrease in WF
for CuO samples over Cu and Cu2O.
0.0
 A thicker PEI layer is needed for
0.0
similar results to CsI due to surface
roughness.
Cu 2s
 WF reduction directly proportional to the
amount of PEI deposited, up to a few nm.
 Longer annealing time is more favourable;
high temperature indicates PEI breakdown.
 AFM analysis indicates presence of PEI with
layer thickness of a few nanometres.
 Similar results observed on Ge samples.
 Methanol solutions resulted in no tangible
difference (within error) compared to H2O.
Conclusions
 Uniform layer of PEI results in a less volatile WF reduction.
 Performing the Standard Cleaning Procedure (SCP) increases
wettability and thus adsorption of PEI on Si/Ge.
 Oxide layer does so for THGEMs with best result on CuO.
 Complete solvent evaporation (through annealing) produces
stable and lower WFs than with solvent present.
 Best WF reduction with PEI coating on Si: -0.71 eV (3.96 eV).
 With PEI deposited on Ge: -0.50 eV (3.95 eV).
 With CsI coating on THGEMs: -0.88 eV (4.02 eV).
References
Acknowledgements
1.
2.
3.
4.
Rob Johnston and Dr Gavin Bell are thanked for technical support during this investigation.
Dr Marc Walker has provided HOPG for calibration purposes and performed XPS analysis
which we are very grateful for.
Tom Berry and Dr Yorck Ramachers are also thanked for collaborating on the project and
providing THGEM samples.
A. Suzuki et al., J. Vac. Soc. Jpn. 57 (2014) 197.
A. J. Breeze et al., Sol. En. Mat. & Sol. Cel. 83 (2004) 263.
L. V. Malyar et al., J. El. Mat. 41 (2012) 12.
W. Kern et al., RCA Rev. 31 (1970) 187.