Degradation processes in encapsulated ZnS: Cu powder electroluminescent phosphors
In this paper we present experimental results of the studying degradation processes in electroluminescent panels, prepared from encapsulated ZnS:Cu powder phosphors and theoretical simulation of energy parameters for the phosphor. Energy band diagrams ZnS, Cu₂S, ZnS-Cu₂₋xS heterojunction and Cu-Z...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2007
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| Zitieren: | Degradation processes in encapsulated ZnS: Cu powder electroluminescent phosphors / K.O. Popovych // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2007. — Т. 10, № 4. — С. 77-80. — Бібліогр.: 12 назв. — англ. |
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irk-123456789-1183372017-05-30T03:05:42Z Degradation processes in encapsulated ZnS: Cu powder electroluminescent phosphors Popovych, K.O. In this paper we present experimental results of the studying degradation processes in electroluminescent panels, prepared from encapsulated ZnS:Cu powder phosphors and theoretical simulation of energy parameters for the phosphor. Energy band diagrams ZnS, Cu₂S, ZnS-Cu₂₋xS heterojunction and Cu-ZnS metal-semiconductor junction have been constructed and cohesive energies for Zn-S, Cu-S, Zn-O, Cu-O and Zn-Cu bonds have been calculated by the method based on a linear combination of atomic orbitals and pseudo-potential. Time dependences of brightness have been found to adequately fit a two-component exponential dependence. The first part of the exponential curve has been attributed to the diffusion processes taking place in Cu₂₋xS, and the second one to the diffusion of Cu in ZnS matrix. 2007 Article Degradation processes in encapsulated ZnS: Cu powder electroluminescent phosphors / K.O. Popovych // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2007. — Т. 10, № 4. — С. 77-80. — Бібліогр.: 12 назв. — англ. 1560-8034 PACS 78.60.Fi, 71.15.Fv, 71.55.Gs http://dspace.nbuv.gov.ua/handle/123456789/118337 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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In this paper we present experimental results of the studying degradation
processes in electroluminescent panels, prepared from encapsulated ZnS:Cu powder
phosphors and theoretical simulation of energy parameters for the phosphor. Energy band
diagrams ZnS, Cu₂S, ZnS-Cu₂₋xS heterojunction and Cu-ZnS metal-semiconductor
junction have been constructed and cohesive energies for Zn-S, Cu-S, Zn-O, Cu-O and
Zn-Cu bonds have been calculated by the method based on a linear combination of
atomic orbitals and pseudo-potential. Time dependences of brightness have been found to
adequately fit a two-component exponential dependence. The first part of the exponential
curve has been attributed to the diffusion processes taking place in Cu₂₋xS, and the second
one to the diffusion of Cu in ZnS matrix. |
| format |
Article |
| author |
Popovych, K.O. |
| spellingShingle |
Popovych, K.O. Degradation processes in encapsulated ZnS: Cu powder electroluminescent phosphors Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Popovych, K.O. |
| author_sort |
Popovych, K.O. |
| title |
Degradation processes in encapsulated ZnS: Cu powder electroluminescent phosphors |
| title_short |
Degradation processes in encapsulated ZnS: Cu powder electroluminescent phosphors |
| title_full |
Degradation processes in encapsulated ZnS: Cu powder electroluminescent phosphors |
| title_fullStr |
Degradation processes in encapsulated ZnS: Cu powder electroluminescent phosphors |
| title_full_unstemmed |
Degradation processes in encapsulated ZnS: Cu powder electroluminescent phosphors |
| title_sort |
degradation processes in encapsulated zns: cu powder electroluminescent phosphors |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2007 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/118337 |
| citation_txt |
Degradation processes in encapsulated ZnS: Cu powder electroluminescent phosphors / K.O. Popovych // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2007. — Т. 10, № 4. — С. 77-80. — Бібліогр.: 12 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
AT popovychko degradationprocessesinencapsulatedznscupowderelectroluminescentphosphors |
| first_indexed |
2025-07-08T13:50:17Z |
| last_indexed |
2025-07-08T13:50:17Z |
| _version_ |
1837086926275870720 |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 4. P.77-80 .
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
77
PACS 78.60.Fi, 71.15.Fv, 71.55.Gs
Degradation processes in encapsulated ZnS:Cu powder
electroluminescent phosphors
K.O. Popovych
Uzhgorod National University, 46, Pidhirna str., 88000 Uzhgorod, Ukraine
E-mail: atr@mail.uzhgorod.ua
Abstract. In this paper we present experimental results of the studying degradation
processes in electroluminescent panels, prepared from encapsulated ZnS:Cu powder
phosphors and theoretical simulation of energy parameters for the phosphor. Energy band
diagrams ZnS, Cu2S, ZnS-Cu2-xS heterojunction and Cu-ZnS metal-semiconductor
junction have been constructed and cohesive energies for Zn-S, Cu-S, Zn-O, Cu-O and
Zn-Cu bonds have been calculated by the method based on a linear combination of
atomic orbitals and pseudo-potential. Time dependences of brightness have been found to
adequately fit a two-component exponential dependence. The first part of the exponential
curve has been attributed to the diffusion processes taking place in Cu2-xS, and the second
one to the diffusion of Cu in ZnS matrix.
Keywords: electroluminescence, ZnS:Cu powder phosphor, degradation, heterostructure,
electronic structure, cohesive energy.
Manuscript received 03.12.07; accepted for publication 19.12.07; published online 20.02.08.
1. Introduction
The main problem arising in application of electro-
luminescent (EL) panels prepared from ZnS:Cu powder
phosphors is loss in the emission brightness. The
respective degradation mechanism in luminescent ZnS
phosphors has been the focus of attention for a long time
and it is still not clearly understood [1-6]. Different
mechanisms of electroluminescence depending on the
energy level structure and local field strength, e.g.,
injection and excitation mechanisms are assumed and
confirmed by the experimental studies in ZnS phosphors
[7]. The process of brightness decay is thought to be
partially related to the structural relaxation at ZnS-
Cu2−xS and/or Cu-ZnS interfaces promoting copper
diffusion, especially in the presence of sulphur vacancies
[6].
The purpose of this work is to determine general
relationships for time changes in brightness during
operation of EL panels prepared from encapsulated ZnS:
Cu powder phosphors, and to gain an understanding of
the degradation mechanism. For the latter, we have
performed additional theoretical calculations of the
energy band diagram for ZnS-Cu2−xS heterostructure,
Cu-ZnS metal-semiconductor junction, and also
cohesive energy for Zn-S and Cu-S bonds.
2. Experimental and computational details
Degradation processes were investigated on EL panels
prepared from ZnS: Cu powder of the following trade
marks: ANE, Durel, GG and GGL provided by different
manufacturers.
The brightness changes of EL panels have been
studied during continuous operation at 400 Hz frequency
with voltage of 115 V in the time range from 0 to 2000
hours at intervals of 0.5 hour by using the tailor-made
device [8].
Theoretical calculation of the energy band diagram
for ZnS-Cu2−xS heterojunction has been performed by the
method based on the linear combination of atomic orbitals
and pseudo potential [9] with the atomic terms calculated
within Hartree-Fock approximation. Details of the
computation procedure can be found in our previous paper
where we have presented results of calculations of the
energy band diagram and gap states for ZnS: Cu, Cl [10].
Energy band diagrams for heterostructures and metal-
semiconductor junctions have been computed following
the procedure given in Ref. [11].
Cohesive energy was determined as:
( ) ( )
,
8
92 pro
2
3
2
2
2
1
2
1
4
2
3
2
2
2
coh E
VV
VV
VVE c
c −
+
+α
++α−= −+ (1)
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 4. P.77-80 .
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
78
0 500 1000 1500 2000
t, hours
0
0.5
1
B
/B
0,
r
el
. u
n.
1
2
3
Fig. 1. Time changes in brightness for electroluminescent
panel in continuous operation (400 Hz, 115 V) (crosses).
Theoretical curves 1, 2 and 3 are the first second, and the sum
of 1 and 2 terms in Eq. (2), respectively.
with the promotion energy Epro = (εp+ − εp−)/2 + V1+ + V1−;
αc – covalency; V1, V2 and V3 – matrix elements
characterizing the metallic energy, covalent energy, and
polar energy [9].
3. Results
Fig. 1 shows relative changes in brightness (B/B0) with
time (t) for one of the EL panel types (GG43 pigment).
Our graphic analysis of relaxation curves has shown that
they can be approximately described by an empiric
formula containing at least two components
),/exp()/exp(/ 210 τ−β+τ−α= ttBB (2)
with α and β – numerical coefficients (α + β = 1), τ1 and
τ2 – relaxation times. These parameters for different
types of EL panels are given in Table l along with τ0.5 –
time interval of the decrease by half of the initial
brightness (B0) for EL panel.
Fig. 2 shows the formation of the energy band from
the atomic terms Zn 4p, Zn 4s, S 3p for ZnS (a) and S 3s
(Cu s, S p) for Cu2S (b). Experimental data for the
photoemission threshold and optical band gap (in
electron-volts) are given in parentheses for comparison.
The results of energy band diagram calculations for
ZnS and Cu2S compounds enabled construction of the
energy band diagrams for ZnS-Cu2S heterostructure and
Cu-ZnS metal-semiconductor junction shown in Figures
3a and b, respectively.
-25
-20
-15
-10
-5
0
E, eV
V1
σ
6.29 (6.2)3.61 (3.6-3.8)
σ*
σ
Ev
Zn 4p
S 3p
Ec
U/2
V1
σ∗
(V2
2+V3
2)1/2
∆ Es-o
Zn 4s
S 3s
EF
Es
S h
Zn h
-25
-20
-15
-10
-5
0
5.86
1.82 (1.7-2.0)
E, eV
σ*
σ
S p (V2
2+V3
2)1/2
Cu s
Ec
Ev
V1
σ ∗
V1
σ
Fig. 2. Formation of the of energy band from the atomic terms
for ZnS (a) and Cu2S (b): Zn 4p, Zn 4s, S 3p and S 3s (Cu s,
S p), – atomic terms; bonding σ and antibonding σ* states;
spin-orbital splitting ∆Es−o; the metallicity energy over bonding
V1
σ and antibonding V1
σ* states; intra-atomic Coulomb
repulsion U/2. Calculated and experimental (given in
parentheses) data for the photoemission threshold and optical
band gap are given in electron-volts.
Table 1. Parameters of electroluminescent panels (CM - capsule material, d1 – phosphor thickness, d2 – thickness of
dielectric, C0 – initial capacity, B0 – initial brightness) and relaxation parameters according to Eq. (2).
Parameters of EL panels Relaxation parameters
Pigment
CM d1 (µm) d2 (µm) C0 (nf) B0 (Cd/m2) α β τ1 (hours) τ2 (hours) τ0.5 (hours)
GG43 Al2O3 44 20 4.27 129 0.238 0.762 174 3100 1300
GGL43 Al2O3 36 19 4.13 82 0.133 0.867 87 4100 2300
Durel (Ti-Si)O2 45 20 4.20 105 0.262 0.738 145 3100 1200
ANE AlN 33 22 4.31 79 0.191 0.809 217 3100 1500
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 4. P.77-80 .
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
79
0
-5E
ne
rg
y,
e
V
Cu2SZnS
Ec
EF
Ev
∆ Ec
Eh
s
0
-5E
ne
rg
y,
e
V
ZnSCu
ϕ m − ϕ s= 1.62 eV
Ec
EF
Ev
Fig. 3. Energy band diagrams for ZnS-Cu2S heterostructure (a)
and Cu-ZnS metal-semiconductor junction (b).
The results of calculation of the cohesive energy
are listed in the Table 2, where summarized are the
interatomic distance (d), promotion energy (Epro),
covalency (αc), level splitting energy ((V2
2+V3
2)1/2),
components of cohesive energy and its value for most
probable compounds and bondings in the phosphors
under investigation. For the Zn-S bond cohesive energy
has been found to be 1.14 eV, while the energy of the
cohesive bond Cu-S was about 0.77 eV. Therefore, the
first part of the relaxation curve (Eq. (2)) is attributed to
the diffusion processes taking place in Cu2−xS, when the
second one could be attributed to the diffusion of Cu in
ZnS matrix.
4. Discussion
Let us discuss the problem of brightness degradation
under the assumption that in ZnS: Cu crystals there are
linear dislocations arising at the boundary between
crystallites with the hexagonal and cubic structures
where Cu2−xS inclusions can be found.
It is known that there is an elastic stress field
around dislocations in crystal interacting with stress
fields arising around impurity atoms [2]. The energy of
this interaction is determined by the following
expression:
Table 2. Interatomic distance (d), promotion energy (Epro),
covalency (αc), level splitting energy ((V2
2+V3
2)1/2),
components of cohesive energy and its value (Ecoh).
Compound Parameters
ZnS Cu2S ZnO CuO ZnCu
d, nm 0. 235 0.240 0.198 0.201 0.266
Epro, eV 7.91 8.05 11.70 11.84 2.13
αc 0.654 0.610 0.588 0.557 0.992
(V2
2+V3
2)1/2, eV 5.04 5.23 7.91 8.10 2.59
(2 − αc
2)(V2
2+V3
2)1/2,
eV
7.92 8.51 13.09 13.69 2.63
9αc
4
(V1+
2
+V1−
2
)/
/8(V2
2
+V3
2
)
1/2
, eV
0.43 0.31 0.33 0.26 0.68
Ecoh, eV 1.14 0.77 1.72 2.11 1.19
,sin
1
)1(
3
4 3
r
bRGE ϑ
γ−
γ+ε
−= (3)
with G – shear modulus, R(1 +ε) – impurity atom radius,
R – average radius of solvent atoms, b - Burgers vector,
γ – Poisson’s ratio, r – distance between a dislocation
and impurity, θ – polar angle between the direction of
sliding and radius-vector. Since the size of the Cu
impurity atom is smaller than that for Zn atom (ε < 0),
the bonding energy is positive, and these atoms are
attracted into the contraction region in the elastic stress
field of an edge dislocation. Therefore, in our case we
should account both for diffusion flow of Cu atoms
towards the dislocation as binding boundary and drift
flow in the elastic stress field with the same direction as
the diffusion one.
Allowing for diffusion and drift flows, we derive
the differential equation describing the impurity
concentration (N) changes in the volume of semi-
conductor when impurity is condensated on dislocations
as:
( ) ( ) ( )( ),,,11,1
322
2
trN
r
L
r
trN
rrt
trN
t
N
D
−
∂
∂
⎟
⎠
⎞
⎜
⎝
⎛ ++
∂
∂
=
∂
∂ (4)
with D – diffusion coefficient and L= Er / kT. The first
term of this equation describes the diffusion flow, the
other two describe the drift one.
An approximate solution of the Eq. (4) with long
times (t → ∞) and allowance for the probability of Cu
impurity atom extraction from dislocation and their
turning back into the semiconductor can be described by
the expression [12]:
( )[ ]kTEtt
t eeeNN /
0 1 −α−α− −−= ,
(5)
where ,2 DN Dπ=α ND is the dislocation density, N0
and Nt are impurity concentrations in semiconductor,
initial and in t moment, respectively.
The given relationship adequately describes the
observed experimental time and temperature
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 4. P.77-80 .
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
80
dependences for brightness degradation of EL panels
(Fig. 1). In terms of this model, at the initial stage of
degradation when Cu atoms are settled at dislocations
the binding energy between a Cu atom and dislocation is
high [exp(−E/kT) << 1] and concentration decrease is
described only by the first term of Eq. (5). Then, as Cu
atoms are precipitated farther and farther from the
dislocation centre (r increases) the value of E decreases
and the degradation rate goes down. This results in at
least two exponential parts in dependences of brightness
on the operation time (Fig. 1). To obtain more precise
interpretation of the experimental result, it is necessary
to take into consideration the changes in concentration of
the emitting centres at the vacancy associations, some
complexes, etc.
5. Conclusions
Degradation processes in electroluminescent panels
prepared from encapsulated ZnS: Cu powder phosphors
in continuous operation (400 Hz, 115 V) in the time
range from 0 to 2000 hours have been studied. It has
been found that operation time dependences of
brightness adequately fit a two-component exponential
dependence. Calculated energy band diagrams and
cohesive energy allowed inference that the first part of
the relaxation curve may be attributed to the diffusion
processes taking place in Cu2−xS, and the second one to
the diffusion of Cu inside the ZnS matrix.
References
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