Supplementary Information
Efficiency limiting factors in Cu(In,Ga)Se2 thin film solar cells prepared by Se-free
rapid thermal annealing of sputter-deposited Cu-In-Ga-Se precursors
Ha Young Park, Dong Gwon Moon, Jae Ho Yun, SeoungKyu Ahn, KyungHoon Yoon, and SeJin
Ahn*
Solar Energy Department, Korea Institute of Energy Research, 152, Gajeong-ro, Yuseong-gu,
Daejeon, 305-343, Korea
S1. Experimental details
Formation of Precursor Films: Cu-In-Ga-Se precursor films with a Se/metal ratio greater
than 1 were deposited on 1 µm-thick Mo-coated soda-lime glass substrates by co-sputtering
Cu52Ga48 (at %), In60Se40 (at %) and Cu50Se50 (at %) targets according to reference [1]. Moon
et al. reported that the Se/metal ratio in sputter-deposited precursor films significantly affects
the degree of Ga segregation in the films after selenization, and CIGS film without any Ga
segregation can be obtained when the ratio is higher than 0.8.[1] In the deposition processes
described above, the Cu-Ga target was sputtered using DC magnetron sputtering, whereas
binary selenide targets (Cu-Se and In-Se) were sputtered using RF magnetron sputtering.
Prior to the precursor deposition, a base pressure of 1x10-6 torr was achieved using a turbomolecular pump. Ar gas then was injected into the chamber to set the working pressure to
3x10-3 torr. The substrate, placed 50 mm from the targets, was rotated at 15 rpm during the
deposition at room temperature to improve film uniformity. The composition of the precursor
films was adjusted to have a Cu/(In+Ga) atomic ratio of approximately 0.8 and a Ga/(In+Ga)
atomic ratio of approximately 0.32 by controlling the sputtering power of each target.
Rapid Thermal Annealing: A schematic of the RTA chamber used in this work is shown in
Fig. S1(a). The chamber is equipped with halogen lamps on its top and bottom sides. The
precursor films were placed on a quartz substrate holder. After loading the samples, a base
pressure of 1x10-6 torr was achieved using a turbo-molecular pump. N2 gas was then injected
into the chamber to set the pressure to 40 torr. The precursor film was thermally annealed
under this N2 gas condition without an additional Se supply. To minimize Se depletion
during the high-temperature process, the gas inlets and outlets were closed during RTA. A
typical temperature profile is also shown in Fig. S1(b); the sample was rapidly heated to 560
o
C in 30 s and then held at that temperature for 15 min. Finally, the sample was naturally
cooled to room temperature.
Solar Cell Fabrication: Solar cells were fabricated according to the conventional
Mo/CIGS/CdS/i-ZnO/n-ZnO/Al structure. A 60-nm-thick CdS buffer layer was deposited on
a CIGS film by chemical bath deposition (CBD) and i-ZnO(50 nm)/Al-doped n-ZnO(500 nm)
were deposited by radio-frequency (rf) magnetron sputtering on the CdS layer. An Al grid of
500 nm in thickness was deposited as a current collector using thermal evaporation. The
active area of the completed cell was 0.44 cm2. No anti-reflection coating was used in this
study.
Characterization: The morphology, composition and crystal structure of the precursor films
and annealed films were investigated using high-resolution scanning electron microscopy
(HRSEM, XL30 SFEG Philips Co., Holland) at 10 kV, energy dispersive spectroscopy (EDS,
EDAX Genesis Apex, acceleration voltage=30 kV, collection time=100 s using a standardless method) and X-ray diffraction (XRD, Rigaku Japan, D/MAX-2500) using CuKα
radiation, respectively. The Raman spectra of the annealed films were obtained in the quasibackscattering geometry using the 514.5 nm line of an Ar-ion laser as the excitation source.
The scattered light was filtered with a holographic edge filter, dispersed by a Spex 0.55-m
spectrometer and detected with a liquid-nitrogen-cooled, back-illuminated charge-coupleddevice (CCD) detector array. The depth compositional profiles of the films were obtained
using Auger electron spectroscopy (AES, Perkin Elmer, SAM 4300). Optical transmission
spectra were obtained by a UV-VIS-NIR spectrophotometer (UV-3101PC, SHIMADZU,
Japan) with a spectral range of 200-1200 nm. Elemental line profiles obtained by secondary
ion mass spectrometry (SIMS: Cameca IMS-7f, CAMECA ADDR.) were examined to verify
Na diffusion from the glass substrate.
Device performance parameters, including the conversion efficiency and EQE (External
Quantum Efficiency), were measured using a class AAA solar simulator (WXS-155S-L2,
WACOM, Japan) and an IPCE (Incident Photon Conversion Efficiency) measurement unit
(PV measurement, Inc., USA), respectively. The capacitance versus voltage characteristics of
the devices were also measured to estimate the net acceptor density of the absorber films
using a precision LCR meter (4284A, Agilent) and a source measurement unit (236,
Keithley) operated at a frequency of 100 kHz.
(a)
halogen lamp
precursor film
Temp.
quartz holder
(b)
560 oC
30 sec
15 min
cooling
time
Fig. S1. (a) Schematic of the RTA chamber and (b) typical temperature profile used in
this work.
S2. XRD pattern of the precursor film
intensity / a.u.
Mo
20
30
40
50
60
70
2theta / degree
S3. General Consideration regarding RTA without Se supply
Because the precursor films are annealed by RTA without Se supply in this work, additional
technical points to be considered arise that are not significant in the conventional selenization
process where the sample is slowly heated with continuous supply of Se vapor; the Se
deficiency of the final CIGS films and the stress build-up in them during the RTA. Therefore,
it is worthy to address these two important aspects in detail.
In supressing the loss of volatile Se during the high temperature process, it is the simplest
way to adjust the working pressure of the chamber. Preliminary experiments revealed that if
the initial chamber pressure, which was adjusted by injecting nitrogen gas, was higher than
40 torr, the Se content of the heat treated CIGS films measured by EDS and AES was almost
the same as the precursor film. This result verifies the feasibility of the non-selenization heat
treatment which may greatly reduce the production cost related to the waste of Se source.
Regarding the stress build-up during the RTA, even though our RTA chamber was designed to
be able to heat the sample up to more than 700 oC in 30 s, the available maximum
temperature was limited by the glass damage and/or the delamination of CIGS film from the
Mo/SLG substrate. In the high temperature range of 620 to 700 oC, wrinkles formed on the
glass surface (just below the Mo layer) due to an unsymmetrical radiative heating.
Unbalanced heating of the sample between the radiation from the sample front, where the
radiation passes through the CIGS and Mo layer, and that from the back side of the sample,
where the radiation passed directly through the glass might be an origin of this unsymmetrical
heating. RTA at intermediate temperatures ranged from 560 to 620 oC caused another issue of
poor adhesion between the CIGS film and the underlying Mo/SLG substrate even though the
glass itself was free of damage. It is notable that in a comparative experiment implying a
slow heating to the same temperatures delamination was not observed, reflecting that the
rapid heating-up stage definitely causes more stress to constituent thin films than the
conventional slow heating process.
Considering both the glass damage and the delamination of CIGS film, it was concluded that
the optimum temperature for the RTA at this stage is around 560 oC.
S4. R-J curves of the CIGSRTA,noSe and CIGSelong,Se devices
10
8
CIGSlong,Se
dV/dJ / cm
2
-2
6
G : 1.72 mScm
RS : 2.41 cm2
A : 1.68
4
CIGSRTA,noSe
-2
2
0
0.00
G : 3.26 mScm
RS : 1.26 cm2
A : 2.67
0.05
2
-1
0.10
(1-G*dV/dJ)/(J+JL+GV) / cm mA
Reference
1. D. G. Moon, J. H. Yun, J. Gwak, S. K. Ahn, A. Cho, K. Shin, K. H. Yoon, S. J. Ahn,
Energy Environ. Sci. 5, 9914-9921 (2012).