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Development of Earth-Abundant and Non-Toxic
Thin-Film Solar Cells
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Park, Helen Hejin. 2015. Development of Earth-Abundant and Non-Toxic Thin-Film Solar Cells.
Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
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Development of Earth-Abundant and Non-Toxic
Thin-Film Solar Cells

A dissertation presented
by
Helen Hejin Park
to
The School of Engineering and Applied Sciences
in partial fulfillment of the requirements
for the degree of
Doctor of Philosophy
in the subject of
Applied Physics

Harvard University
Cambridge, Massachusetts
August 2014
(cid:164) 2014 by Helen Hejin Park
All right reserved.
Roy G. Gordon Helen Hejin Park
Development of Earth-Abundant and Non-Toxic
Thin-Film Solar Cells

Abstract
Although solar energy is the most abundant energy resource available,
photovoltaic solar cells must consist of sufficiently abundant and environmentally
friendly elements, for scalable low-cost production to provide a major amount of
the  world’s  energy  supply.    However,  scalability  is  limited  in  current  thin-film
solar cell technologies based on Cu(In,Ga)(S,Se) and CdTe due to scarce,
2
expensive, and toxic elements. Thin-film solar cells consisting of earth-abundant
and non-toxic materials were made from pulsed chemical vapor deposition
(pulsed-CVD) of SnS as the p-type absorber layer and atomic layer deposition
(ALD) of Zn(O,S) as the n-type buffer layer. Solar cells with a structure of
Mo/SnS/Zn(O,S)/ZnO/ITO were studied by varying the synthesis conditions of
the SnS and Zn(O,S) layers. Annealing SnS in hydrogen sulfide increased the
mobility by more than one order of magnitude, and improved the power
conversion efficiency of the solar cell devices.
Solar cell performance can be further optimized by adjusting the
stoichiometry of Zn(O,S), and by tuning the electrical properties of Zn(O,S)
through various in situ or post-annealing treatments. Zn(O,S) can be post-
iii
annealed in oxygen atmosphere or doped with nitrogen, by ammonium hydroxide
or ammonia gas, during the ALD growth to reduce the carrier concentration,
which can be critical for reducing interface recombination at the p-n junction.
High carrier concentration buffer layers can be critical for reducing contact
resistance with the ITO layer. Zn(O,S) can also be incorporated with aluminum
by trimethylaluminum (TMA) doses to either increase or decrease the carrier
concentration based on the stoichiometry of Zn(O,S).

iv
Table of Contents
Abstract iii
Table of Contents v
List of Figures vii
List of Tables x
Acknowledgements xi

Chapter 1 Introduction .......................................................................................1
1.1 Solar Cells ...........................................................................................1
1.2 Buffer Layers ......................................................................................2
1.3 Atomic Layer Deposition and Pulsed Chemical Vapor Deposition ..7
1.4 References ...........................................................................................9

Chapter 2 Co-Optimization of SnS Absorber and Zn(O,S) Buffer Materials
for Improved Solar Cells ...............................................................10
2.1 Chapter Abstract ...............................................................................10
2.2 Introduction .......................................................................................11
2.3 Experiments ......................................................................................12
2.4 Results and Discussion .....................................................................16
2.5 Conclusions .......................................................................................26
2.6 References .........................................................................................26

Published in Prog. Photovolt: Res. Appl. (2014); DOI: 10.1002/pip.2504; Helen
Hejin Park, Rachel Heasley, Leizhi Sun, Vera Steinmann, R. Jaramillo, Katy
Hartman, Rupak Chakraborty, Prasert Sinsermsuksakul, Danny Chua, Tonio
Buonassisi, and Roy G. Gordon,  “Co-optimization of SnS Absorber and Zn(O,S)
Buffer Materials for Improved Solar Cells”

v
Chapter 3 Post-Annealing Oxygen Treatment of Zinc Oxysulfide by
Atomic Layer Deposition ............................................................30
3.1 Chapter Abstract ............................................................................30
3.2 Introduction ....................................................................................31
3.3 Experiments ...................................................................................33
3.4 Results and Discussion ..................................................................35
3.5 Conclusions ....................................................................................46
3.6 References ......................................................................................46
Published in Appl. Phys. Lett. 102, 132110 (2013); Helen Hejin Park, Rachel
Heasley,  and  Roy  G.  Gordon,  “Atomic  Layer  Deposition  of  Zn(O,S)  Thin  Films  
with  Tunable  Electrical  Properties  by  Oxygen  Annealing”

Chapter 4 In Situ Nitrogen-Doping or Aluminum-Incorporation of Zinc
Oxysulfide by Atomic Layer Deposition ...................................50
4.1 Chapter Abstract ............................................................................50
4.2 Introduction ....................................................................................51
4.3 Experiments ...................................................................................52
4.4 Results and Discussion ..................................................................54
4.4.1 Nitrogen-Doped Zn(O,S) ...................................................54
4.4.2 Aluminum-Incorporated Zn(O,S) ......................................58
4.5 Conclusions ....................................................................................66
4.6 References .....................................................................................67
Published in Appl. Phys. Lett. 105, 202101 (2014); Helen Hejin Park, Ashwin
Jayaraman, Rachel Heasley, Chuanxi Yang, Lauren Hartle, Ravin Mankad,
Richard Haight, David B. Mitzi, Oki Gunawan, and  Roy  G.  Gordon,  “Atomic  
Layer Deposition of Al-Incorporated Zn(O,S) Thin Films with Tunable Electrical
Properties”

Chapter 5 Conclusion ....................................................................................68

Appendix A Diagram of Deposition System ...................................................71
Appendix B Collaborations ..............................................................................72
vi
List of Figures

1.2.1 Conventional device structure for Cu(In,Ga)(Se,S) (CIGS) based solar
2
cells. [Source: National Renewable Energy Laboratory (NREL)]
1.2.2 Energy band diagram in conventional CIGS-based solar cells.
1.2.3 Conventional device structure for CdTe-based solar cells. [Source: NREL]
1.2.4 Current-Voltage (J-V) characteristics of a solar cell with and without
illumination. The short-circuit current (J ), open-circuit voltage (V ),
SC OC
and maximum power point (P ) are defined.
max
1.3.1 Schematic representation of the process of atomic layer deposition (ALD).
1.3.2 Schematic representation of the process of pulsed chemical vapor
deposition (pulsed-CVD).

2.3.1 (a) Schematic diagram of the solar-cell device stack under short-circuit
conditions and (b) structural formula for the Sn precursor. SnS/Zn(O,S)
band-alignment was drawn in accordance with the ultraviolet
photoelectron spectroscopy (UPS) measurements from previous study by
L. Sun et al. [23].
2.4.1 Plot of resistivity (green), hole carrier concentration (blue), and hole
mobility (red) vs. annealing temperature in H S, for SnS films deposited at
2
70°C (dotted) and 120°C (solid). Resistivity and carrier concentration are
plotted on a semilog scale, whereas mobility is plotted on a linear scale.
2.4.2 (a) Cross-sectional (with 12° tilt) and (b) plan-view FESEM images of as-
deposited SnS grown at 70°C and 120°C, and SnS post-annealed at 400°C
in H S.
2
2.4.3 (a) Current density vs. voltage (J-V) plots under dark (dotted) and 1 Sun
illumination (solid) and (b) semilog J-V plots under dark for devices with
vii
the SnS layer annealed at different temperatures having Zn(O,S), S/Zn =
0.37 as the buffer layer.
2.4.4 (a, c) J-V characteristics under dark (dotted) and 1 Sun illumination
(solid) and (b, d) semilog J-V characteristics under dark for devices with
varied stoichiometry of the Zn(O,S) buffer layer. Comparison of the SnS
layer annealed in H S at 300°C (a, b) and 400°C (c, d).
2
2.4.5 X-ray diffraction scans of the as-deposited SnS grown at 70°C and 120°C,
and SnS films grown at 120°C annealed in pure H S at various
2
temperatures. Vertical lines represent orthorhombic SnS (o-SnS), cubic
SnS (c-SnS), and Mo diffraction peak positions listed by JCPDS No. 00-
039-0354, 04-004-8426, and 00-004-0809, respectively.
2.4.6 External quantum efficiency (EQE) of the various solar cell devices.

3.4.1 Depth profile of Zn(O,S) with S/Zn = 0.37, up to the Si substrate.
3.4.2 Plots of (a) resistivity, (b) electron carrier concentration, and (c) electron
mobility vs. S/Zn, for the as-deposited Zn(O,S) and Zn(O,S) samples
annealed at 200°C, 230°C, 260°C, and 290°C in O . Measurements were
2
repeated for films after ~1 yr with S/Zn = 0.37, as shown on the right side
of the plots.
3.4.3 Plots of (a) α2 vs. hν and (b) bandgap energy vs. O annealing temperature
2
for Zn(O,S) with S/Zn = 0.37.
3.4.4 X-ray diffraction for the as-deposited ZnO, as-deposited Zn(O,S) with
S/Zn = 0.37, and Zn(O,S) with S/Zn = 0.37, annealed in O at various
2
temperatures on quartz substrates.
3.4.5 Plots of (a) lattice constant, (b) grain size, and (c) local strain vs. O
2
annealing temperature for the as-deposited ZnO, as-deposited Zn(O,S)
with S/Zn = 0.37, and Zn(O,S) with S/Zn = 0.37, oxygen annealed at
different temperatures on quartz substrates.
3.4.6 X-ray diffraction for the as-deposited ZnO, as-deposited Zn(O,S) with
S/Zn = 0.37, and Zn(O,S) with S/Zn = 0.37, annealed in O at various
2
temperatures on Si substrates.
viii
3.4.7 Plots of (a) lattice constant, (b) grain size, and (c) local strain vs. O
2
annealing temperature for the as-deposited ZnO, as-deposited Zn(O,S)
with S/Zn = 0.37, and Zn(O,S) with S/Zn = 0.37, oxygen annealed at
different temperatures on Si substrates.
3.4.8 Cross-sectional and plan-view FESEM images for the as-deposited ZnO,
as-deposited Zn(O,S) with S/Zn = 0.37, and Zn(O,S) with S/Zn = 0.37,
oxygen annealed at 200°C, 230°C, 260°C, and 290°C.

4.4.1.1 Plots of (a) resistivity, (b) electron carrier concentration, and (c) electron
mobility vs. ~S/Zn, for the undoped Zn(O,S) and N-doped Zn(O,S)
using different NH OH solution concentrations.
4
4.4.1.2 Plots of (a) resistivity, (b) electron carrier concentration, and (c) electron
mobility vs. ~S/Zn, for the undoped Zn(O,S) and N-doped Zn(O,S)
using NH gas.
3
4.4.2.1 Plots of O/(Zn+Al) (black), S/(Zn+Al) (red), and Al/(Zn+Al) (blue) vs.
(the number of H S pulses)/(the number of DEZ pulses) for Zn(O,S)
2
films (a) without and (b) with Al incorporation.
4.4.2.2 X-ray diffraction of Zn(O,S) films (a) without and (b) with Al
incorporation for various S/(Zn+Al) and Al/(Zn+Al) ratios. Vertical
lines are for the hexagonal ZnO (JCPDS No. 01-079-2204).
4.4.2.3 Plots of (a) resistivity, (b) electron carrier concentration, and (c) carrier
mobility vs. S/(Zn+Al) for Zn(O,S) (black) and Zn(O,S):Al (red) films.
4.4.2.4 Plots of α2 vs. hν for (a) Zn(O,S) and (b) Zn(O,S):Al films for various
S/(Zn+Al) ratios.
4.4.2.5 Bandgap, as measured using absorption data (see text), vs. S/(Zn+Al) for
Zn(O,S) (black) and Zn(O,S):Al (red) films.
ix