Indian Journal of ChemistryVol. 23A, April 1984. pp, 295-299

Electroluminescence of Manganese Doped Zinc Pyrophosphate Phosphors

S N DESHMUKH & D S AMBARDEKAR *Department of Chemistry, Shivaji University, Kolhapur 416004

Received 24 January 1983; reuised and accepted 14 Norember 1983

Preparative conditions have been established to obtain a new formulation for red emitting electroluminescent (EL)phosphor with zinc pyrophosphate base. Activation by manganese under controlled conditions gives intense red emission withi·m., at 6850 A. Studies on voltage and frequency dependence of EL emission reveal that the EL brightness (B) varies with theapplied voltage ( n according to the equation.

-hB = a.exp 01.;

at constant frequency. The brightness (B) increases with frequency upto 700 Hz. attains a broad maximum between 700 and750 Hz, and then falls off rapidly upto 1.5 kHz. Frequency dependence of EL emission has been explained on the basis ofmechanisms with different relaxation times.

A review of the work on electro luminescence showsthat very little work has been done on zincpyrophosphate phosphors activated by manganese,Jones ' has reported that Zn1PzO,-Mn phosphors andsome other oxide-rich base matrices like zinc silicate-Mn, zinc tungstate-Mn, zinc orthophosphate-Mn, etc.are good electroluminescent materials. However, nodetails have been given regarding preparativeconditions. spectral energy distribution (SED). andvoltage frequency dependence of EL emission.Moreover, earlier observations+:' on the voltage-frequency dependence of emittance in the oxide-richphosphors are quite different from those in the cases ofthe ZnS and ZnO based phosphors. This necessitates afresh experimentation on these phosphor systems.

An attempt has, therefore, been made in the presentstudy to establish optimum preparative conditions fora good EL phosphor based on zinc pyrophosphatematrix with manganese as an activator. The voltageand frequency dependences of EL intensity have alsobeen studied and some new observations differentfrom the earlier+:' ones have been obtained. Thecorrelation between the EL emission and crystalstructure of the base matrix is given and a suitablemechanism for the EL process is suggested.

Materials and Methods

Phosphor synthesisZinc pyrophosphate and manganese pyrophosphate

were used as the basic materials in the synthesis. Thesewere prepared by the well known analytical methods asused for the estimation of zinc and manganese?".Thus. zinc ammonium orthophosphate (ZnNH4P04)and manganese ammonium orthophosphate(MnNH4P04) were separately precipitated from

solution In the presence of NH4CJ andNH40H (PH = 8 to 10) by the addition ofdiammonium hydrogen orthophosphate(NH4)2HP04' The precipitate was dried at TWC andthe product was then heated at 800QC for 4 hr. Thistemperature and duration of heating were found tosuffice for the complete conversion ofZnNH4P04 intoZnZP107 according to the equation.

2 ZnNH4P04 ¢ Zn2P207 + 2 NH3 + H20Purity of the compound so obtained was found to be99.72 ~() as given by its P20S analysis?". All thechemicals used during preparation were of AR grade.

For the synthesis of the phosphor, the appropriateamounts of zinc pyrophosphate and manganesepyrophosphate were intimately mixed in absoluteethanol and the slurry was dried at 70-'C. The mixturewas then heated at elevated temperature (700-960°C) insilica crucibles in a specially constructed silica tubefurnace" under the atmospheres of various gases (N2•

N 2 + CO and air). The phosphor synthesised underthe optimum conditions was used in the present study.

Intensity measurementsFor intensity measurements a sandwich type EL cell

was used 5. Thus. a thin layer of the phosphor powderwas sandwiched between a clean aluminium electrodeand a conducting glass electrode. A thin sheet of micawas introduced between the two electrodes to avoidelectric break-down. No binder (solid or liquid) wasused. The cell parameters such as cell spacing, area ofillumination etc. were kept under close control toachieve reproducible results.

The EL cell was kept in front of a constant deviationspectrometer and it was excited by sinusoidal voltagesat different frequencies. The intensity of dispersed lightwas recorded in the form of current by a sensitive spot

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INDIAN J. CHEM., VOL. 23A, APRIL 1984

reflecting galvanometer (sensitivity 2 x 10-9 ampereper mm deflection). A photomultiplier tube (EMIE6097 B peak sensitivity at about 6200 A) was used as adetector. The SED curves were recorded at constantvoltage and frequency (2500 V, 50 Hz) for differentconcentrations of manganese. However, for a selectedphosphor (5 x 10 -3 g atom Mn/mol Zn2P207 fired at960"C in N2 for 3 hr) the measurements were carriedout over a wide range of voltages and frequencies.

X-ray analysis of the phosphorsThe phase compositions of the phosphors were

determined with the help of a Philips X-raydiffractometer (PW 1051, TIFR, Bombay) employing,CuK~ radiation. The intensity of the diffracted beamwas recorded using a Geiger counter at thedistinguishing peaks for different phases present in thesample. A direct relationship between the intensity ofthe beam at the peak and the concentration of thecorresponding phase was assumed as an approxi-mation, and it is not likely to introduce any seriouserror" . The calculations were based on comparison ofintensities (counts/see) of pure phases with those of themixture.

Results and Discussion

Effect of temperature and duration of firingA phosphor containing 5 x 10 -3 g at Mn/mol

Zn2P 207 was heated at different temperatures and fordifferent durations of firing, under N2 atmosphere. Itwas observed that EL intensity of the phosphorincreased with temperature (700-960°C) and also withincrease in duration of firing. However, withprolonged heating (> 3 hr), the colour of EL emissionbecame slightly orange. Thus, the phosphor exhibitedgood EL when fired at 960'C for 3 hr. When thephosphor was fired in air, the EL emission greatlydeteriorated. EL emission of a phosphor fired underslightly reducing atmosphere (N2 + CO) was not verydifferent from that of the phosphor fired under N2

atmosphere. Hence, to determine the optimumconcentration of Mn, the phosphors with varyingconcentrations of Mn were heated at 900 C for 3 hrunder Nz atmosphere.

Effect 0/ manganese on crystal structure and ELemission

The manganese concentration was varied from 0 to5 x lO -2 g at Mn/mol Zn2P207' The results arepresented in Table I, whereas the spectral energydistribution (SED) curves are shown in Fig. I. Therelative proportions of various phases present in thephosphor (as obtained by X-ray analysis) are alsogiven. It is seen from Fig. I that variation in Mncontent has no specific effect on the shape of the SEDcurves. Thus, the curve shows i.max at 6850 A and tailsup slowly to 5500 A. The intensity of the 6850 A peakincreases with Mn concentration upto 5 x 10 ·3 g atMn/mol Zn2P207 and afterwards falls to a low value.

The X-ray analysis indicates that {I-Zn2P207 andZnO are absent (or they are below detectable limit i.e. 30,,). However, percentage of z-phase increases with Mnconcentration upto the optimum level (5 x 10 -3 gat/mol Zn2PZ07) and again decreases sharply.

It may be noted that manganese in itself does notplay any significant role in the phase transition. This isobvious because the inversion of high temperature {3-form to :x-Zn2P207 form is a rapidly reversibleprocess 7 (inversion temperature I32"C). Also,manganese variation does not affect i·max, though itinfluence the peak intensity. It is seen from the data inTable 1 that the variation in relative proportions ofdifferent phases in the phosphors does not affect thespectral distribution or the peak wavelength. Thus, it isfound that the EL emission is independent of crystalstructure. Since Mn is divalent and has more or less thesame atomic size as Zn, it forms a solid solution withthe base matrix and hence no problem of chargecompensation is involved. Thus, it may be concludedthat the radiative transitions take place between theatomic levels of Mn2 + (6S ~ 4G) and not between the._-_._-_._---------- ._---

Table 1--- Effect of Variation of Mn on EI Emission of ZnzPz07-Mn Phosphor[Atmosphere: N2 gas, temperature = 960 C, duration = 3 hr]

Phosphor Mn Phosphor composition Visual colour Intensity at(g at/mol --------_._---- -- -- --~------ of EI emission 6850 A

of Zn2P207) IX-phase fJ-phase Disordered (Relative mt.) (50 Hz 2500 V)phase (Arb. units)

0 No EI5 x 10 4 60 < 3 40 Red (w) 0.642 x 10 3 70 < 3 30 Red(m) 0.855 x 10 .1 90 < 3 10 Red (v.s.) 1.77

1.52xl0 2 68 < 3 32 Red (s) 1.405.2 x 10 2 35 < 3 65 Red (w) 0.52

- -- - ..- --- ---------- -- -- -- --- -- ------ - -- -- ----------- - -----

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DESHMUKH & AMBARDEKAR: ZINC PYROPHOSPHATE PHOSPHORS

Mn Conc~ntrQtion

-0---0- (M, I 5"O.,<f' 9m otom 01 Mnlmol~ 01-3 IZIt PoP.

~(M212-0X'0 " »

-9--4IM315'Ox103,. _IM411.52x,62 "

.•....•.. CMS1S.2xlO-2 '1

-'W

0·5

I I

6000 6500

WAVELENGTH, A7000

Fig. 1- Spectral distribution of EL. emission (Zn ,P ,0,: Mnphosphor. M n-variation)

perturbed discrete levels within the energy gap of thebase matrix. These atomic transitions correspond toenergy difference of 3.32 eV (3730 A). However, in theZn2P20, - Mn phosphor. Mn? + is coordinated tooxygen atoms and this oxygen environment perturbsthe ground and excited states 01" Mn~' , reducing theenergy of transitions. Hence, wavelengths of emissionare shifted to higher values.

According to Linwood-Weyl hypothesis". theperturbation of Mn2+ levels is minimum in fourcoordinate configuration and the emission should bein blue-green region. In six coordinate configurationthe perturbation is more, so as to give spectral emissionfrom orange to red region. This postulate has beenstrengthened by a number of workers q .

In the case of :x-Zn1P107, which has a monoclinicsymmetry I ') (S.G.l1/C), the coordination number ofZn2 + is either 5 (ZnO, site) or six (ZnO" site). TheM n 2 + ion in the six coordinat ion configuration hasgreater perturbation and gives rise to a.band at 6850 A.The penta coordination should give a band around6100 A, but its intensity is usually so low compared tothe band at 6850 A that it is not observed in ELemission. However, its contribution to EL emissionrenders the major peak (6850 A) unsymmetrical onlower wavelength side, decaying slowly upto 5500 A.

Voltage undfrcquencv dependence of EL intensityThe SED curves of a typical red-emitting Zn2P 207'

Mn phosphor at different voltages and frequencies areshown in Fig. 2. It is seen that the general shape of the

-0-0- 50 Hzs 1600 V

~ 200 " "-0--0- 700 " II

............50 " 1800 v

--:-3·0 ~ 200 1/ ":::> ~ 700 v "<l:2000 v~ 50 II

>~200 " "f-- -(/) .•.....•... 700 " J)

Z~ 2·0zw>f--<l:-'W0:: 1·0

0.055~0~~~~~60~0~~==~=6~5:0:~--":~7iooJ

WAVELENGTH, AFig. 2·-Spectral distribution of EL. emission for a typical red EL

Zn,P,07:Mn phosphor as a function of frequency and voltage

SED curve is not appreciably altered due to variationsin applied voltage or frequency, although the ELintensity at 6850 A is greatly affected.

Voltage dependenceIt is seen from Fig. 2 that intensity at i·max increases

slowly at low voltages and rather rapidly at highervoltages. Although anum ber of equa tions for voltage-brightness relationship have been proposed in the past,the present data seem to fit the equation ll.12 •

B = a. exp r f;~JA plot of log B against V 0.' at different frequencies islinear thus confirming the validity of the aboveequation. It is evident that values of b (slope) areslightly dependent on the frequency while the values ofa (intercept) increase with frequency, attain asaturation around 700-800 Hz and then decreaseslowly. Similar observations have been recorded bymany workers for ZnS based phosphors I .1 .

Such a type of voltage dependence of EL brightnesshas been explained in the case of Cu-activated zincsulphide phosphors on the basis of existence of a Molt-Schottky type potential barrier at the interface of zincsulphide conductor. The electrons are transported bythe field leaving behind a positive space charge due towhich the field strength at the sensitive spot isenhanced and other electrons penetrate the potentialbarrier or are released from surface donor levels and

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INDIAN J. CHEM., VOL. 23A, APRIL 1984

4·0-<>-<>- 2400 Volts~ 2200 "~ 2000~ 1800

3·0 .......... 1600

3 -- 1400-b--b- 1200oCt+---+ 1000

CO

tf)tf) 2·0UJZl-I00:CO--.iUJ

°O~----~5~OO~----~10~!OO~-~~~O~--~2~OOOFREQUENCY, (Hz s )

Fig. ~ - Frequency dependence of EL emission for a typical red ELZn,P,O-:Mn phosphor at different voltages

produce secondaries enlarging the space charge, thusstarling an avalanche until the voltage-drop acrosssuch a spot reaches maximum.

Frcqucncv dcpcndcuc«The variation ofEL intensity with applied frequency

at various voltages is shown in Fig. 3. It is seen that athigh voltages. brightness increases linearly withfrequency. However, at moderate and low voltages.intensity increases with frequency upto 700 Hz andthen falls rapidly upto I Hz. and slowly upto 1.5 kill.Thus, it is evident that FL intensity attains an optimumvalue at about 700 HI for the voltages studied.However. Fig. 2 shows that peak position shiftsslightly to lower wavelength side at higher frequencies.above 2000 V. Such a spectra I shift can be explained onthe basis of the postulate.' .• that EL emission takesplace via two mechanisms with different relaxationtimes. The emission corresponding to the mechanismwith low relaxation time is enhanced more than theemission due to mechanism with high relaxation timewhen the frequency of excitation is increased. Thisleads to a shift in the SED curve to lower wavelengthside. However. in the present investigation, it seemstha t the two relaxation times are not very differentfrom each other and hence the observed spectral shift issmall. The crystal structure of Zn2P20, with two sitesfor Zn (ZnO, and ZnOh) as mentioned earlier alsosupports the postulate of two relaxation times.

It is seen from Fig. 3 that EL brightness varieslinearly with frequency at low frequencies. as is

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observed for most of the EL phosphors. The ELemission takes place as a result of pulse excitation. Asthe number of excitation pulses (frequency) increases,the time-averaged EL brightness also should increaselinearly. However, as the frequency continues toincrease, successive excitation pulses begin to interactand hence the linear relation cannot continue further.This leads to saturation in EL brightness. Thefrequency range in which such saturation is set willevidently depend upon the relaxation time (r) for theprocess. An inverse relation between -: and saturationfrequency should exist.

It is seen from Fig. 3 thaI EL brightness saturates inthe frequency range 700-g00 Hz. In ZnS phosphors.the saturation is observed at comparatively higherfrequencies 1.\ (1000 Hz for green band and 2000 Hz forblue band). This suggests that the value of T should behigher for Zn2P20.-Mn phosphor.

The decrease in EL brightness with frequency aftersaturation IS still unexplained. Henisch 12 hasattributed this decrease to the change in spectralcomposition of the EL emission with frequency.However. in the present study frequency dependenceof EL brightness was recorded at a constantwavelength (in"" = 6X50 A). and hence, there is nopossibility of changes in spectral composition of theemission. It seems more likely that the impedance ofthe Fl. cell increases rapidly in this frequency range asa result of which actual voltage across the phosphorgrains is decreased and it directly affects the ELbrightness.

This type of frequency dependence is similar to theresults obtained for typical ZnS phosphors. However,Kolomi tsev and coworkers 2 and Pallila and Rinkevic'observed that in the case of oxide-rich phosphors, ELbrightness decreases with frequency. They haveattributed this decrease to the presence of a UVcomponent present in the field induced luminescence.which seems to be absent in the Zn2P10,-Mnphosphors studied here. The fact that Zn1P207-Mnphosphors are not photoluminescent under UVirradiation also supports this view.

The Zn2P20· ...Mn phosphor synthesised in thepresent study is a good ELemitting phosphor with i.ma,

at 6850 A. The position of the emission peak isindependent of activator concentration and of crystalstructure of the matrix as discussed above.

It is also concluded that the voltage and frequencydependence of EL emission in such oxide richphosphors is similar to those in the typical ZnSphosphors and not far different from some othermentioned earlier:' ..\ .

References1 Jones S . .I electrochem Soc. 111 (1964) 307.

DESHMUKH & AMBARDEKAR: ZI IC PYROPHOSPHATE PHOSPHORS

2 Kolomotseve Fl. Isotov V P & Staur E V. Optics & Spectroscopv,12 (1962) 64.

3 Pallila F C & Rinckevics M. J electrochem Soc. 110 (1963) 750.4 Vogel A I. A text book of quantitatire inorganic analvsis

(Longmans. London), 1964 (a) 532-36. (b) 477-78.5 Kulkarni V S & Ambardekar D S. Indian J pure appl Phvs. 12

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10 Robertson B E & Calvo C. J solid state Chell I. I II 970) 120.

II Luyckz A & Stokkink A J. Brit J appl Phis. 6 (1955) 557.

12 Henisch H K. Electroluminescence ill International series of'monographs on semiconductors (Pergamon. l.ondon). 1962,239.

13 Waymouth J F & Bitter F. l'hvs Rei. 95 (1954) 941.

14 Alfrey C F & Taylor j B. Pro" Phv» Soc. 868 (1955) 775.

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