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Electron traps at a photooxidised anthracene surface
Volume
33, number
CHEMCAL
2
G. DIETRICH,
WYSLCS LETTERS
1 June 1975
H. PICK
Physikalisches Imtih*t
Teei/ 2. ihliveisitbt
Snrrtgart,
Stziigart,
Germmy
and H.
BAUSER
Received 3 Xlarci 1975
Electron traps at the surface of photooxidised sntlxxene crystz!s were studied by thermally stimnlnted currents. The geczrztrap depth KG c&Mated to be Et = (1.0 k 0.1) eV. Thhcse surfnm txps may nccouni fur the onhonccd optical ids tion observed after photooxidation.
Surface traps for charge carriers in anthracene crysta!s have been postulated for the extrinsic carriergeneration process under irradiaticn of strondy absorbed light [ 1] _ In this process free and trapped carriers of opposite sign xe formed upon interaction of singlet excitons with the surface. Oxidation products, in particular anthraquinone, were suggested to act as surface electron traps since photooxidation gives rise to erihanced hole generation [2]. However, the energy levels of the postulated surface traps have no; been determined so far. For free-hole generation, the energy required for the formation of surfaceelectron traps of depth E, is Et>Ez-E,,
(1)
where Eg and E, arc the band gap and the singletexciton ener=;, respectively. With Es = 3.7 eV 131, and Es = 3.1 eV the trap depth is found ro be Et > 0.6 eV. E,, therefore, is expected to iie within an energy range for which the thermally stimtilated currents (TSC) technique shou!d be appiicable [I]. TSC have widely been used for studying bu!k traps [5]. FOCthe present purpose it was necessary to disctiminate (i) between election and hole traps, and (ii) between surfxe and bulk states. For (I) we used an unsymmetrical arrangement where one electrode injected cutlers of one sign into the crystal and the oth-
er electrode formed a blocking contact. With this zrrangement bath bulk-trapped carriers and carriers trapped at the surface facing the blocking contact could contribute to the TSC (where cs for carriers from surface traps adjacent to the injecting eiectrode the schubweg would be too short for an observable TSC signal). Detectable surface-trapped carriers therefore had to traverse the crystal both for trap ffl!ing and for extraction after release. For the discrirnination of surface states from bu!k states (ii) their TSC signal had to be controlled by surGce tranirnenk. Tile blocking contact consisted of an aluminium electrode covered for insuiaticn with an aIuminn, layer about 0.3 pm thick. Tne other electrode was 2 trn,nsparent chromium iayer evaporated on to the anthracene crystal, which formed a neutral contact in the dark. Upon irradiation using a 200 ‘Y high-pressure mercury lamp charge carriers were swept into the crys:a!, The sign of the cxriers depending on the polarity of the irradiated electrode. The crystal holier was mounted in 2 vacuum cryostat. Further details of the experiment2 setup are described elsewhere [6]. Tie samples were cleaved frown me!t-grown ant&acene crystz!s. EeiCore cc& run the crystz! was kept in vacuum at room temperature for about I5 hours. (Total Fressure was about lo-’ tcrr at room remccrature.) .%!I runs were started with trap emptying by CX-
~~oIIxF.~33, number 2
CHEEICAE
! June 1955
PHYSICS LETrERS
posing the sample to h&t of the merctliy lamp. After cooling down to 83 I(, the traps were Med with carriers of one sign by applying a voltage of 500 V (negative for electron injection) to the illuminated electrode until the current dropred t#~ zero or to a constant leaicing current. After stitching the light off, a three to five times bigber vo!tzge of reverse polarity wzs “pplied. For recording the TSC spectrum the samp!e was heated at a constant rate of 3 K/mm. .Mr was admitted for photooxidation, and the crystal was irradiated with ‘the light of iha mercury lamp for 5 hours at normal pressure and room temperature. After that the cryostat was evacuated, and crystal was allowed to cool zt the same time. The firrsl pressure was reached when the sarqle temprature was arcund 250 I<. Cooling might have caused condensable gases
to deposit at the surface. If present at all, these gases obvio~dy did not have any effect on the TSC signal: changes in the TX curves were observed only after photooxidation, whereas this methcd of cooling alone did not affect
the TSC spectrum.
A typical scties of TSC plots for electrons is shown in fig. 1: curve
a was plotted
ibr a freshly
(-_-_c_ - _...._I
cleaved
crystal, cute
b after photooxidation, and CLIIY~c after the crysta! was again stored in vacuum at room temperature for more than 4 hours. The peak at 225 K has been attributed to z bulk electron trap with El = 0.67 eV [5], Ibis trap is due to an impurity; fig. 2 shorvs a TSC plot for a freshly cleaved crystal ofh@$er uurity where t,Sis peak is absent (to be compared with kg. !a). The important feature tc which the further discussion wti! be confined is the steep rise above 300 K in fig. lb. Heatirrg was terminated at 340 K in order to prevent excessive sublimation while the current was still rising. !n repetition run:: the 300 K rise always built up upon photooxidation and disappeared after extended pumping. The smzll292 K peak (fig. 1c) never disappeared but rather: increased a little each iime. It should be noted tiltit tile 300 K rise in the eleciron TSC sFctrum is obtained only after photooxidaLion irresFctive of whether the admitted air is dry or saturated with water vapour. Neither irradiation with Ii&t iG t.!neabsence of air wthout light caused the obswed change in the T’SC spectrum. Tna 300 # rise was never found when the irradiated electrode was posi:iveQ biased du:ing trazl filling. 258
/L’ 50
3oe
..___I6 -
_. .----..-.__ \\
;y_ ,
zio 200 temperature
150 I”;(1
__-
._. _
4
i
2 100
a
Fig. 1. Therm~~Uy stimuln;ed CUiieniS (SC) SFctra for in an anthzcenc crystal. (a) Freshly clecved crystal, (b) titer phorooaidation, (c) after keeping the crystal under vxuum at room temperature for more than 4 hours. Heating rite: 3 degeesjmin. 4ections
It is concluded from these findings that the steep rise above 300 K is due to a photoosidation product at the anthracene surface. This oxidation product acts as a deep electron trap, and it sublimes (or decomposes) under vacuum.
The trap depth ~5~was evaluated on the basis of the initial rise method [7] which uses the exponential rise in rhe leading tai! of a TSC pea!< be!onging to a single trzp: i(T)
= exp(-E,/kT)
-
The exponential law holds up to about 10% of the Fe.& value, provided that retrapping can Se nsglecied
0)
Fig. 2. TSC spectrum far electrons in XI znthracene crystal of higher purity th2.n that of fig. 1, after photoosidatior.. [7]. This condition is saiisfied at &e field S~~G@IS by tr3napplied (about IOk V/cm), as was confirmed Gent photocurrent mrc ~~sutements using the Swle Sam-
ples. Since the initial rise method can be employed only for a single trap it was necessary tO remove the superimposed 292 K peak, i.e., to empty the corresponding iraps, by “peak cleaning” (81. In fig. 3 (curve 2) the crystal was heated as usual to 300 K, cooled to about 270 EC_,2nd reheated (curve b). By this treatment the srnd!i 292 K p-e32 disappeared, and the “c!eaned” curve b obeyed an exponential law according to eq. (2). The trap depth was found to be E, = (.I& 5 0.1) ev.
This SiLrface irap may give rise to :-he enhanced opt&l hole generation after photooxidation 1,1j: Et saiisfies the condition imposed by eq. {i). Furthermore, phOtOCurrent experiments have dei?lonstraked that the enhanced hole generation wes izrgely redwe(i by keeping the sample under v2cuum for some hours 191 ~;;‘nich is in agreement vA:h “&e repo:ted redtictioe of the 300 K rise. at cannot be decided as yet whether these traps are due to anthraquinone [Zj. In ;1prelininary experimeni anthraquinonz kyers were depcsitcd from ethanol so!u:ioi~ onto anthracene crystals. TIwe samples did not disp!ay ;?I= 300 K bse in their TSC spcctrum. Tnis finding: however, does not ruie out anthraquinone as a pcssible trap in phor.ooddised hampies wkre the ankzquinone moIacu!es may be attached to the antilrzcene surface in a way quite CisSimilai to those deposited from solution. This argument is based on photocurrent-sensitisation espcri[9j. menti with evaporated antiraquinone Ir: principje, tiie maiecular electron 3ffmiQ AZ” of the oxidation product can be determined from the trap depth E, 3116the molecular electron &nit!, 42 of aR!:hracene: hL, CI .$+4;+6+p-, (3) 3 where LIP- is the difference in polarisaiion energy between the l~tlk (Pi) and the surf3ce (P,) A#- = ?g - Ps-_
(4)
Depending on the microscopic environment of the otidised mo[ecu!es, &‘- will range between zero and, say, .$Pb_ Hence, using published dsto [IO! we obtain at least a rough estimsre forA:’ : ii-z‘ = U-2.3, eV. SXTlity Of a!lthr3Values fCi the molecular &CiFO* quinone are reported to range between 0.5 eV 1101 2nd 1.28 eV [II!. Finally it should be noted that our TSC measirrenents did not give evidence of 3 rerersible hole trag due to adsorbed oxygen with E, = 0.77 eV, 3s reported I
2. 0
* 35-o
3w
iw
Fig. 3. ‘Te3k clcmniiq”of TSC spectrum. (4 First tun, (b) second run -&ez coo!Lrq to about 270 K.
Kydd
[i 31. In ax
of two cr~std
batches, however, we found a (permanent) ho!e trap v&h E, = (0.75 5 0.05) eV caused by impurities.
\
250 200 If0 temperature PKJ
by B;ee ad
Volume
33, number
2
CHEMCAL
PHYSICS LETTEXS [5] H. Roh&acher, Thesis, Stuttgzt,(1974). [6] H. Ro-hrbacher and N. 011. Fiws. Stat
References
1975
Scl. (1975), to
P.L. Land, J. NIYS. Ckm. So!i& 30 (1969). P\.A. Cresswe!! 2nd M.X. Pzrlma, J. Appl. Fhy~. 41 (1970) 236.5. P1 G. Dietrich, Thesis, Stgtig.rt
[li L.E. Lyons, J. Cha-o. Phys. 23 (1955) 220. [2] A. 3ree and LE. Lyons, J. CI~ern. Sor (1560) 5179. 13) G. V~~bel and H. Eaesski, pF.y~ Letters 27A (1968) 32s. [4i ii. Kolxdo and W.G. Schzeidcx. J. ~iwn. whys. -10 (196$) 2937.
I lune
(1975), J. Brie&S, Angcw. Chem. 76 (1964) 326. T.L.. kIwii ar.d H. Kurcdz, Tkozt. CnLn. Acti !! (1968) 97. A. Eree 2nd R.A. Kydd, 3. Chem. Finys. 40 (1964) 1775.
33, number
CHEMCAL
2
G. DIETRICH,
WYSLCS LETTERS
1 June 1975
H. PICK
Physikalisches Imtih*t
Teei/ 2. ihliveisitbt
Snrrtgart,
Stziigart,
Germmy
and H.
BAUSER
Received 3 Xlarci 1975
Electron traps at the surface of photooxidised sntlxxene crystz!s were studied by thermally stimnlnted currents. The geczrztrap depth KG c&Mated to be Et = (1.0 k 0.1) eV. Thhcse surfnm txps may nccouni fur the onhonccd optical ids tion observed after photooxidation.
Surface traps for charge carriers in anthracene crysta!s have been postulated for the extrinsic carriergeneration process under irradiaticn of strondy absorbed light [ 1] _ In this process free and trapped carriers of opposite sign xe formed upon interaction of singlet excitons with the surface. Oxidation products, in particular anthraquinone, were suggested to act as surface electron traps since photooxidation gives rise to erihanced hole generation [2]. However, the energy levels of the postulated surface traps have no; been determined so far. For free-hole generation, the energy required for the formation of surfaceelectron traps of depth E, is Et>Ez-E,,
(1)
where Eg and E, arc the band gap and the singletexciton ener=;, respectively. With Es = 3.7 eV 131, and Es = 3.1 eV the trap depth is found ro be Et > 0.6 eV. E,, therefore, is expected to iie within an energy range for which the thermally stimtilated currents (TSC) technique shou!d be appiicable [I]. TSC have widely been used for studying bu!k traps [5]. FOCthe present purpose it was necessary to disctiminate (i) between election and hole traps, and (ii) between surfxe and bulk states. For (I) we used an unsymmetrical arrangement where one electrode injected cutlers of one sign into the crystal and the oth-
er electrode formed a blocking contact. With this zrrangement bath bulk-trapped carriers and carriers trapped at the surface facing the blocking contact could contribute to the TSC (where cs for carriers from surface traps adjacent to the injecting eiectrode the schubweg would be too short for an observable TSC signal). Detectable surface-trapped carriers therefore had to traverse the crystal both for trap ffl!ing and for extraction after release. For the discrirnination of surface states from bu!k states (ii) their TSC signal had to be controlled by surGce tranirnenk. Tile blocking contact consisted of an aluminium electrode covered for insuiaticn with an aIuminn, layer about 0.3 pm thick. Tne other electrode was 2 trn,nsparent chromium iayer evaporated on to the anthracene crystal, which formed a neutral contact in the dark. Upon irradiation using a 200 ‘Y high-pressure mercury lamp charge carriers were swept into the crys:a!, The sign of the cxriers depending on the polarity of the irradiated electrode. The crystal holier was mounted in 2 vacuum cryostat. Further details of the experiment2 setup are described elsewhere [6]. Tie samples were cleaved frown me!t-grown ant&acene crystz!s. EeiCore cc& run the crystz! was kept in vacuum at room temperature for about I5 hours. (Total Fressure was about lo-’ tcrr at room remccrature.) .%!I runs were started with trap emptying by CX-
~~oIIxF.~33, number 2
CHEEICAE
! June 1955
PHYSICS LETrERS
posing the sample to h&t of the merctliy lamp. After cooling down to 83 I(, the traps were Med with carriers of one sign by applying a voltage of 500 V (negative for electron injection) to the illuminated electrode until the current dropred t#~ zero or to a constant leaicing current. After stitching the light off, a three to five times bigber vo!tzge of reverse polarity wzs “pplied. For recording the TSC spectrum the samp!e was heated at a constant rate of 3 K/mm. .Mr was admitted for photooxidation, and the crystal was irradiated with ‘the light of iha mercury lamp for 5 hours at normal pressure and room temperature. After that the cryostat was evacuated, and crystal was allowed to cool zt the same time. The firrsl pressure was reached when the sarqle temprature was arcund 250 I<. Cooling might have caused condensable gases
to deposit at the surface. If present at all, these gases obvio~dy did not have any effect on the TSC signal: changes in the TX curves were observed only after photooxidation, whereas this methcd of cooling alone did not affect
the TSC spectrum.
A typical scties of TSC plots for electrons is shown in fig. 1: curve
a was plotted
ibr a freshly
(-_-_c_ - _...._I
cleaved
crystal, cute
b after photooxidation, and CLIIY~c after the crysta! was again stored in vacuum at room temperature for more than 4 hours. The peak at 225 K has been attributed to z bulk electron trap with El = 0.67 eV [5], Ibis trap is due to an impurity; fig. 2 shorvs a TSC plot for a freshly cleaved crystal ofh@$er uurity where t,Sis peak is absent (to be compared with kg. !a). The important feature tc which the further discussion wti! be confined is the steep rise above 300 K in fig. lb. Heatirrg was terminated at 340 K in order to prevent excessive sublimation while the current was still rising. !n repetition run:: the 300 K rise always built up upon photooxidation and disappeared after extended pumping. The smzll292 K peak (fig. 1c) never disappeared but rather: increased a little each iime. It should be noted tiltit tile 300 K rise in the eleciron TSC sFctrum is obtained only after photooxidaLion irresFctive of whether the admitted air is dry or saturated with water vapour. Neither irradiation with Ii&t iG t.!neabsence of air wthout light caused the obswed change in the T’SC spectrum. Tna 300 # rise was never found when the irradiated electrode was posi:iveQ biased du:ing trazl filling. 258
/L’ 50
3oe
..___I6 -
_. .----..-.__ \\
;y_ ,
zio 200 temperature
150 I”;(1
__-
._. _
4
i
2 100
a
Fig. 1. Therm~~Uy stimuln;ed CUiieniS (SC) SFctra for in an anthzcenc crystal. (a) Freshly clecved crystal, (b) titer phorooaidation, (c) after keeping the crystal under vxuum at room temperature for more than 4 hours. Heating rite: 3 degeesjmin. 4ections
It is concluded from these findings that the steep rise above 300 K is due to a photoosidation product at the anthracene surface. This oxidation product acts as a deep electron trap, and it sublimes (or decomposes) under vacuum.
The trap depth ~5~was evaluated on the basis of the initial rise method [7] which uses the exponential rise in rhe leading tai! of a TSC pea!< be!onging to a single trzp: i(T)
= exp(-E,/kT)
-
The exponential law holds up to about 10% of the Fe.& value, provided that retrapping can Se nsglecied
0)
Fig. 2. TSC spectrum far electrons in XI znthracene crystal of higher purity th2.n that of fig. 1, after photoosidatior.. [7]. This condition is saiisfied at &e field S~~G@IS by tr3napplied (about IOk V/cm), as was confirmed Gent photocurrent mrc ~~sutements using the Swle Sam-
ples. Since the initial rise method can be employed only for a single trap it was necessary tO remove the superimposed 292 K peak, i.e., to empty the corresponding iraps, by “peak cleaning” (81. In fig. 3 (curve 2) the crystal was heated as usual to 300 K, cooled to about 270 EC_,2nd reheated (curve b). By this treatment the srnd!i 292 K p-e32 disappeared, and the “c!eaned” curve b obeyed an exponential law according to eq. (2). The trap depth was found to be E, = (.I& 5 0.1) ev.
This SiLrface irap may give rise to :-he enhanced opt&l hole generation after photooxidation 1,1j: Et saiisfies the condition imposed by eq. {i). Furthermore, phOtOCurrent experiments have dei?lonstraked that the enhanced hole generation wes izrgely redwe(i by keeping the sample under v2cuum for some hours 191 ~;;‘nich is in agreement vA:h “&e repo:ted redtictioe of the 300 K rise. at cannot be decided as yet whether these traps are due to anthraquinone [Zj. In ;1prelininary experimeni anthraquinonz kyers were depcsitcd from ethanol so!u:ioi~ onto anthracene crystals. TIwe samples did not disp!ay ;?I= 300 K bse in their TSC spcctrum. Tnis finding: however, does not ruie out anthraquinone as a pcssible trap in phor.ooddised hampies wkre the ankzquinone moIacu!es may be attached to the antilrzcene surface in a way quite CisSimilai to those deposited from solution. This argument is based on photocurrent-sensitisation espcri[9j. menti with evaporated antiraquinone Ir: principje, tiie maiecular electron 3ffmiQ AZ” of the oxidation product can be determined from the trap depth E, 3116the molecular electron &nit!, 42 of aR!:hracene: hL, CI .$+4;+6+p-, (3) 3 where LIP- is the difference in polarisaiion energy between the l~tlk (Pi) and the surf3ce (P,) A#- = ?g - Ps-_
(4)
Depending on the microscopic environment of the otidised mo[ecu!es, &‘- will range between zero and, say, .$Pb_ Hence, using published dsto [IO! we obtain at least a rough estimsre forA:’ : ii-z‘ = U-2.3, eV. SXTlity Of a!lthr3Values fCi the molecular &CiFO* quinone are reported to range between 0.5 eV 1101 2nd 1.28 eV [II!. Finally it should be noted that our TSC measirrenents did not give evidence of 3 rerersible hole trag due to adsorbed oxygen with E, = 0.77 eV, 3s reported I
2. 0
* 35-o
3w
iw
Fig. 3. ‘Te3k clcmniiq”of TSC spectrum. (4 First tun, (b) second run -&ez coo!Lrq to about 270 K.
Kydd
[i 31. In ax
of two cr~std
batches, however, we found a (permanent) ho!e trap v&h E, = (0.75 5 0.05) eV caused by impurities.
\
250 200 If0 temperature PKJ
by B;ee ad
Volume
33, number
2
CHEMCAL
PHYSICS LETTEXS [5] H. Roh&acher, Thesis, Stuttgzt,(1974). [6] H. Ro-hrbacher and N. 011. Fiws. Stat
References
1975
Scl. (1975), to
P.L. Land, J. NIYS. Ckm. So!i& 30 (1969). P\.A. Cresswe!! 2nd M.X. Pzrlma, J. Appl. Fhy~. 41 (1970) 236.5. P1 G. Dietrich, Thesis, Stgtig.rt
[li L.E. Lyons, J. Cha-o. Phys. 23 (1955) 220. [2] A. 3ree and LE. Lyons, J. CI~ern. Sor (1560) 5179. 13) G. V~~bel and H. Eaesski, pF.y~ Letters 27A (1968) 32s. [4i ii. Kolxdo and W.G. Schzeidcx. J. ~iwn. whys. -10 (196$) 2937.
I lune
(1975), J. Brie&S, Angcw. Chem. 76 (1964) 326. T.L.. kIwii ar.d H. Kurcdz, Tkozt. CnLn. Acti !! (1968) 97. A. Eree 2nd R.A. Kydd, 3. Chem. Finys. 40 (1964) 1775.