Communication
pubs.acs.org/cm
Ordered Vacancy Compound Formation by Controlling Element
Redistribution in Molecular-Level Precursor Solution Processed
CuInSe2 Thin Films
Choong-Heui Chung,*,† Ki-Ha Hong,† Doh-Kwon Lee,*,‡ Jae Ho Yun,§ and Yang Yang*,∥
†
Department of Materials Science and Engineering, Hanbat National University, Daejeon 305-719, Republic of Korea
Photoelectronic Hybrid Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea
§
Photovoltaic Research Center, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea
∥
Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California 90095, United States
‡
CIS thin films were prepared by spin-coating CIS-precursor
solutions, and then present phases in the prepared films were
identified using Raman spectroscopy (Figure 1). Preparation of
I
t has been reported that the formation of a thin layer of
ordered vacancy compound (OVC) on a chalcopyrite
absorber layer would provide a high quality p−n junction
owing to similarity in their crystal structures as well as a better
band alignment for carrier transport in thin film solar cells.1−5
Most studies on the formation of the OVC phase on the
chalcopyrite phase have been carried out for vacuum processed
Cu(In,Ga)Se2 (CIGS) related thin films. Although the use of
high vacuum deposition techniques for the preparation of CIGS
absorbers allowed researchers to fabricate thin film solar cells
with power conversion efficiencies (PCE) over 20%,6−8 for
CIGS thin film solar cells to be more competitive in the
photovoltaic market, it will be necessary to reduce processing
cost as well as initial investment cost. In the effort of reducing
production cost of CIGS solar panels, various kinds of
nonvacuum processes have been developed.9−11
Nonvacuum approaches to CIGS layer deposition can be
roughly divided into three different categories: electrochemical
process, particulate process, and molecular-level precursor
solution process.9 The PCE of nonvacuum processed CIGS
thin films solar cells have steadily increased.9,10 However,
research on the formation of the OVC phase on and/or in
chalcopyrite films prepared by nonvacuum processes have not
been extensively carrier out yet. A better understanding on the
formation mechanism of the OVC phase in solution processed
CIGS related thin films could provide an opportunity for
researchers to improve further performances of the solution
processed thin film solar cells.
Here, we report the formation of the OVC phase
preferentially on the top surface region of the CuInSe2 (CIS)
thin films prepared by a hydrazine solution process. It was also
possible to form the OVC phase relatively uniformly through
the bulk of CIS films. Although the prepared films are not pure
CuInSe2, we here noted the films as CuInSe2 because the films
are devoid of sulfur, less than 5 at. %, despite the addition of
excess sulfur to the precursor solutions. The hydrazine solution
process used in this work has demonstrated the highest PCE
among the molecular-level precursor solution processed thin
film solar cells.12 In a vacuum process, each atomic flux
reaching to substrates can be controlled in real time, so that
desired phases can be relatively easily formed in desired regions.
In contrast, those works are relatively difficult in solution
processes. We here show that adjustment of the degree of
decomposition of molecular precursors during film preparation
will make those works possible even in solution processes.
© 2015 American Chemical Society
Figure 1. Process flow for the preparation and the Raman
characterization of CuInSe2 thin films.
the precursor solutions and the CIS thin films were performed
inside a N2 filled glovebox. All of the precursor solutions were
prepared by dissolving desired powders into hydrazine.
Caution: hydrazine is highly toxic and should be handled with
appropriate protecting equipment to prevent contact with either the
vapors or liquid. The CIS-precursor solutions were prepared by
simply combining Cu-precursor solutions and In-precursor
solutions. The Cu-precursor solutions were prepared by
dissolving equal molar amounts of Cu2S and sulfur powder
into hydrazine. The In-precursor solutions were prepared by
dissolving In2Se3 and Se powder with equal molar ratios into
hydrazine. To dissolve completely the employed powders, Cuand In-precursor solutions were stirred for several days. CIS
thin films were deposited on Mo-coated soda lime glass
substrates by spin-casting the prepared CIS-precursor solutions
at a spin speed of 3000 rpm for 30 s. Each CIS layer thickness
Received: September 30, 2015
Revised: October 25, 2015
Published: October 27, 2015
7244
DOI: 10.1021/acs.chemmater.5b03841
Chem. Mater. 2015, 27, 7244−7247
Communication
Chemistry of Materials
was about 150 nm. To prepare thicker CIS films, the coating
step was repeated multiple times. Between each coating step,
the samples were annealed on a preheated hot plate at a
temperature of 220−290 °C for a few minutes, and finally
annealed on a hot plate at a temperature of 400 °C for 30 min.
Raman analysis was performed on the prepared CIS thin
films to identify the present phases in a confocal backscattering
configuration at room temperature with unpolarized light using
a Renishaw inVia Raman system equipped with a 514.5 nm Ar
laser. The laser beam size was approximately 2 μm. Penetration
depth into CIS films of the Ar laser (514.5 nm) used in this
Raman spectroscopy is estimated to be approximately 100
nm.13 In other words, depth resolution of this measurement is
about 100 nm.
Raman spectroscopy has been known to be one of the best
tools to identify clearly both the chalcopyrite phase and OVC
phase simultaneously.14−17 The A1-mode is corresponding to
selenium atom vibration with respect to their neighboring
atoms.14,15 In the chalcopyrite CIS phase, each Se atom is
bonded with two indium atoms and two copper atoms. In the
OVC phase, some fraction of copper atoms is replaced by
vacancies and indium atoms, so that the A1-mode in the OVC
phase exhibits a lower value of corresponding peak position
compared to that in the CIS chalcopyrite phase.16,17
Raman spectra were obtained from the top surface, the back
surface, and the cross section of a CIS thin film to identify
present phases at each position (Figure 2). To probe the
OVC peak, similar to the spectrum obtained from the back
surface. On the basis of the measured three Raman spectra, we
reasonably reach a temporary conclusion that the OVC phase is
formed preferentially in the topmost layer of the prepared CIS
films.
To understand how the OVC phase is present preferentially
in the topmost layer of the solution processed multilayered CIS
thin films, we traced the intensity of the OVC phase peak
obtained from the top surface of the films with different
number of layers (Figure 3). With increasing the number of
Figure 3. (a) Raman spectra and (b) OVC/CIS peak height ratio
obtained from the top surface of CuInSe2 thin films with different
number of layers.
coating layers from two to eight, the peak height ratio of OVC/
CIS was almost linearly increased to the number of layers from
0.16 to 0.61, indicating that the content of the OVC phase on
the top surface layer was gradually increased. Because the OVC
phase has more indium content than the CIS phase, it is thus
evident that indium content on the top surface layer was
progressively increased with repeating spin-casting of CISprecursor solutions.
A potential mechanism for this observation is as follows.
[Cu6S4]2− and [In2Se4]2− are two major molecular complexes
in hydrazine CIS-precursor solutions.18,19 [Cu6S4]2− ions
convert to Cu7S4 sheets by losing sulfur while drying at room
temperature and completely convert insoluble Cu2S phase at
temperatures below 120 °C.18,20 In contrast, readily soluble
[In2Se4]2− related species are fairly thermally stable even after
casted on substrates, so higher temperature annealing above
350 °C is required to convert them completely to an insoluble
solid phase.21 Note that the samples were annealed at
temperature in the range of 220−290 °C between each coating
step. By considering the thermal stability of two major
complexes, [Cu6S4]2− and [In2Se4]2−, and annealing temperature between each coating step, casting CIS-precursor
solutions for the deposition of the following upper layer
partially redissolves [In2Se4]2− related species from the under
layer. As a result, redissolved indium species are redeposited in
the following upper layer, which then becomes more indium
rich than the casted CIS-precursor solution as well as the under
Figure 2. Raman spectra obtained from the top surface, the back
surface, and the cross section of a CuInSe2 thin film. Each position was
marked on the cross-sectional scanning electron microscopy image of a
CuInSe2 thin film.
present phase at the back surface, the film was mechanically
exfoliated and flipped over for Raman spectroscopic analysis. A
spectrum obtained from the top surface showed two distinct
peaks located at 175 and 152 cm−1 coming from the CIS phase
and OVC phase, respectively.14−17 The peak height ratio of
OVC/CIS was approximately 0.55. In contrast, a Raman
spectrum obtained from the bottom surface showed a strong
CIS peak but a negligibly weak OVC peak. To explore present
phases in the bulk of CIS films, we obtained Raman spectra
from the cross section of the film. It should be noted that
Raman spectrum obtained from the cross section represents the
microstructural properties of whole bulk of the film because Ar
laser beam size (∼2 μm) fully covers the thickness of the film
(∼1 μm). Raman spectrum obtained from the cross section of
the film also showed a strong CIS peak but a negligibly weak
7245
DOI: 10.1021/acs.chemmater.5b03841
Chem. Mater. 2015, 27, 7244−7247
Communication
Chemistry of Materials
molecular-level precursor solution processes by understanding
the thermal properties of molecular complexes present in each
solution.
layer. By repeating this procedure, indium content is gradually
increased in the topmost layer. This high indium content thus
causes the formation of the OVC phase preferentially in the
topmost layer.
On the basis of our suggested mechanism for this
experimental observation, high temperature annealing above
350 °C between each coating step will completely convert
indium precursor complexes to an insoluble solid phase. In
other words, redissolving indium complex species will be highly
suppressed by thermal annealing at temperatures above 350 °C.
Thus, an OVC peak will not be significant even in the topmost
layer.
Designed experimental work was thus performed, in which
the annealing temperature between each coating step was
simply raised up to 390 °C. Figure 4 shows Raman signals
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected] (C.-H. Chung).
*E-mail: [email protected] (D.-K. Lee).
*E-mail: [email protected] (Y. Yang).
Author Contributions
C.-H.C. conceived the idea, wrote the paper, and did the
experiment. K.-H.H., D.-W.L., and J.H.Y. did interpretation of
data and wrote the paper. Y.Y. conceived the idea and guided
research.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This research was supported by Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education (Grant No. NRF2014R1A1A2059181), and in-house Research and Development Program of the Korea Institute of Energy Research
(KIER) (B5-2605).
■
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By selectively redissolving indium species during casting
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Chemistry of Materials
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DOI: 10.1021/acs.chemmater.5b03841
Chem. Mater. 2015, 27, 7244−7247