PHOTOELECTRIC EFFECTS I N POLYMERS
755
On Photoelectric Effects in Polymers and Their Sensitization by Dopants’
by Helmut Hoegl Battelle Memorial Institute, Carouge-Geneua, Switzerland
(Received December 3. 196&)
Electrostatic imaging techniques have been applied to the study of the chemical aspects of photoconductance in organic compounds. The materials were studied in the form of thin filnis coated on conductive substrates. It was found that poly-9-vinylcarbazole and a variety of other polymers with aromatic or heterocyclic chain units exhibit photo-induced discharge. The photoresponse niay be strongly improved by doping with a wide variety of electron acceptors. The same effects may be observed with aromatic or heterocyclic electron donor-type photoconductors when dispersed in nonphotoconduct ive resin binder and doped with electron acceptors. The reverse case is given when aromatic or heterocyclic electron acceptors are doped with small amounts (0.1-2 mole %) of electron donors and dispersed in a resin binder. These and other findings show that donor (D)-acceptor light
(A) interactions according to DA D+A- play a decisive role for the photo-induced generation of charge carriers. p-Type conduction niay be postulated when D-hosts are doped with A-impurities, which act as electron traps, and if an exchange of positive charges (holes) between excess D molecules takes place. In the reverse case, ti-type conduction with trapping of holes and migration of electrons may be postulated.
Introduction The purpose of this work is to show that the methods used for electrostatic imaging processes (“electros t a t ~ g r a p h y ” ) ~also - - ~ may be used for the study of phenomenological aspects related to photoconduction in organic compounds. The relevant chemical problems are the relationships between chemical structure and photoconductive properties, and the mechanisms of charge carrier formation and transport in a bulk of originally neutral molecules. An objective of more practical importance is to explore whether organic materials can attain a t all the photosensitivity of inorganic photooonductors, e.g., zinc oxide and selenium. Another probleiii was to find better nieans of controlling the photosensitivity of organic photoconductor films. The results presented in this paper have so far appeared only in p a t e ~ i t s . ~ - ’ ~
Experimental Techniques The materials investigated were photoconducting polyniers and low molecular weight compounds embedded in nonphotoconducting polymers. Low molecular weight starting materials were purified by recrystallization or distillation. The polymers were
applied in the form of thin filnis, cast niainly on aluniinum sheets. For coniparisons of results, they were also coated on paper. The film thickness was varied between 2 and 20 p ; however, for measurements it was kept mainly a t about 6 p . The physical principles involved in electrostatog(1) Presented a t the International Conference on Photosensitization in Solids, Chicago, Ill., June 22-24, 1964. (2) (a) J. Dessauer in “Photographic Science, Symposium, Zurich 1961,” T h e Focal Press. London, 1963, p. 263: (h) C. J. Claus, Phot. Sci. Eng., 7, 5 (1963); (c) J. W. Weigl in “Photographic Science, Symposium, Zurich 1961,” p. 345; (d) H. Meier, ”Die I’hotochemie der organischen Farbstoffe,” Springer-Verlag, Berlin, 1963, p. 158. (3) P. M . Cassiers, Ind. Chim. Belge, 2 5 , 127 (1960). (4) C. J. Young and H . G . Greig, B C A Rev., 40, 469 (1951). (5) H . Hoegl, 0. Sus, and W. Neugebauer, German Patent 1,068,115 (1957). (6) H . Hoegl, 0. Siis, and W. Neugebauer, German Patent 1,111,935 (1958). (7) H. Hoegl and W. Neugebauer, Belgian Patent 592,291 (1959). (8) H . Hoegl, E. Lind, and H . Schlesinger, Belgian Patent 592,557 (1959).
(9) H . Hoegl. German Patent 1,133,976 (1959). (10) H . Hoegl and H . Schlesinger, German Patent 1,131,988 (1959). (11) H . Hoegl, German Patent 1,127,218 (1959); see also Belgian Patent 591,164 (1960). (12) H. Hoegl, Belgian Patent 606,574 (1960).
V01zim.e 69,.Vumber S
March 196,5
HELMUT HOEGL
756
raphy are photoconductivity and electrostatics. Typically, a photoconducior plate is grounded at one surface and the other surface is corona-charged to a voltage V,,. The positive or negative charges imparted to the samples in the dark by passing thein under corona wires held a t a potential of a few thousand volts are very probably due to ionized gas molecules (Sz+or 0 2 - ) . After corona-charging, the connection to the ground is switched off. Sow, the electrostatically charged plate may be used either for measuring dark and photoinduced discharge characteristics with an electrostatic voltmeter or for electrostatographic imaging processes. Both techniques have been used in this investigation. 1 . Electrostatic Voltmeter Method. The electrostatically charged plates (positzve in all cases reported) were placed under a wire netting serving as a probe for the electrostatic voltmeter (Type R 1019, Rothschild, Zurich, Switzerland). For coniparison, the dark and light decay characteristics were measured. Usually, a 125-w. mercury high pressure lamp was used as a light source. Since these measurements could be made only rather qualitatively a t this time, they were intended mainly to check results obtained with the imaging methods. 2 . Imaging Methods. In these cases, a latent electrostatic image on the insulating photoconductive surface is obtained by exposure of the uniformly charged layer to light. The light causes neutralization of the charges on the surface. The nonexposed areas remain charged. The latent electrostatic charge patterns are made visible by application of electrically charged particles (toners). The toner particles adhere selectively either to the charged or to the uncharged areas of the photoconductor depending on the polarity of their charge. In all cases presented below, only a “direct” toner was used. Thus all images are direct (“positive”) prints of the originals. These toner iniages were then fixed by melting, i.e., they were not transferred to paper sheets as in xerography with seleniuni plates. Two variations of the same imaging technique have been used. ( a ) D e n s ~ t y Strzp Method. The electrostatically charged plates (negatzue in all cases reported below) were exposed to light under a photographic density strip (Kodak S o . 2 photographic step tablet with the color patches blue, green, and red, seen froin the right end of the strip; 21 steps, uncalibrated; density range from 0 t o 3.0, with the transmittance increasing from step to step by the ratio of 4 5 ) . The light source for all cases of density strip copies shown was a mercury high-pressure lamp (Philips HPR 125 watts, Deutsche Philips GmbH) . Coniparison experiments with other coninierc.ially available light sources were also made. The Journal of Phr/szcal Chemastry
The higher the photosensitivities of the plates, the fewer the dark steps that niay be seen on the copies. ( b ) Exposure T z m e Method. 111this case (simply a variation of the density strip method 2a) the charged plates were exposed to light under any given original (e.g., a drawing or a density strip). The exposure time necessary to obtain background-free copies of the originals after development was estimated. Therefore, the higher the photosensitivity of the plates, the shorter time of exposure mas necessary. For the above methods photosensitzvity is defined as the speed of lightinduced discharge of electrostatically oharged, photoconductive polynier layers by a given light intensity. The methods described gave principally the sanie qualitative results which mere highly reproducible. 3. Optical Measurements. In some cases the optical absorption spectra of the photoconductors were nieasured, e.g., poly-S-vinylcarbazole. They were found to correspond qualitatively with the xerographic act ion spectra. A rough indication of the spectral sciisitivities of the plates is given by the color patches 011 the density strips (see 2a).
Experimental Results Photoconductive Electron Donoi~ T y p e J/aterzals. 1 . Polymers. (a) Poly(-N-vznyl-carbazole) [ P V K ] . Using the aforementioned methods, it was found in 1957 that PVIi behaves like a photoconductor,j which was the first exaiiiple of a polymer with photoconductive properties to be described in the literature. PT’II;
m[Qj-qm ’\
N R’-HC-CHz
/
\
N CH-CH2
,
\
N / CH-CHI-R”
layers can be sensitized (in the bulk) with the same dyes as those which can be used for zinc oxide-silicone layers (e.g., rhodamine B, methyl violet, rose bengal, etc.). Since it is polymeric itself, PVK represents a valuable model for photoconductivity studies with the elecitrostatic imaging methods because it can be cast on conductive substrates without addition of a resin binder. In working with PVK layers, it turned out that all coinniercially available samples (Luvicana samples from the Badische Anilin &. Soda Fabrik, Ludwigshafen a. Rh., Germany) behaved quite differently in their light response. Therefore, the polymer was prepared directly from the monomeric N-vinylcarbazole according to literature specifications. l 3 However, the purest samples obtained by thermal polyiiierization of purified (13) W. Reppe, et al.. Justus Lieb%gs A n n Chem , 601, 132 (19561, German Patent 664,231 (19341, see also ,J Remond, RPZ Prod Chzm , 66. 51.5, 582 (1963)
FVY
DOPANPVK
I
7 DOPANT
NONE
NONE
0.L %
0.L %
0.6 %
06 % 0.8% 0.8 % 1.0 7.
(0 %
I .
i
8.’
~iroiioiiirrgavr thr poorvsi rrsrilts with rrgard i o phoiosciisiiivity. Thr d r g r c ~of polyiiirrizaiioii scwird to cxhiliit iin gwat iiiflucii~~~. I’olyiii(v saiiiplcs ohiained hy risiiip I.wis arids as iniliators (13F3,AICls, ZiiClp, €I,SO,, tctraiiil,roiii(,tliaii(~. ri I..) w r r iiriirh iiinrr saiisitivr. Thus, 1,rwis acids !~rr(>atldril dirrrtly to the st,aiidard I’VK sairiplrs (roiiiiiirrrial I.uvivaii@; t.hcririally polyiiirrieotl sainplrs, or polgiiicm prrpared hy iuii iat ioii with a,a‘-nsniliisoliri~~roniirilr). It was f o r i i i d iliai thr pliotosc,iisitiviiy of the I’VK layers iiierrasrs with iiirr(wiiig I.wis avid roiicciitratioii; ix.,thrrr is a rrlatioiiship lic~twrriii h r 1,cwis wid “dopalit” ~~oiii~c~iiiratioii and t h r light rvspoiisc of the clwtrostaiically rhargrd I’VIi liliiis. Thr Lewis arids wcrr addrd i n iiivreasiiig iiiolar roiirciilratioirshnscd oii S-viiiylrarhazolc rhaiii uiiils. Thr slroirgrsi rffeets on photosciisitivity ww olisrrvrd i i i thr rairgr of about 0.1 to 2 iiiolc yo dopant. I n ihc casr of very stmiig avids sri(.Ir as H2S0,, at highrr eoii~~ciitraiioris the dark 1~oii011i~iivity is iiivrrasrd rather stroiigly. l:urthwiiiorr, it was forind that doping with clecfron acccp/ors quit griirrally iii(wasrs ihr phoiosrnsitiviiy (%
1
l,? %
r
.-
I
7
PVK DOPAN
'
NONE
e' I
I
Figure :i. Ikiaity strip copies: PVK doped wi1.h nitro rompoands ( I mole $;, with reference to vinylcarhazole monomer unit).
aiilhraccne-9-rarhoxylic arid methyl rstcr; halogen conipounds, e.g., tetrahroiiio-p-xyleiie; t riphciiylchloroniethaiic; octachlororiaphthalciie; !)-broiiioaiit,liraerirr; 9,lO-dii.hloroairthraoeiic;quinones, e.g., chloranil; broniaiiil; areiiaphtheirequiiione; phcnarithreiicquiiione; chryseiiequinone-(l,2) ; keto compounds, e.g., pyrene-3aldehyde; henzil; benzoin; 2-1iitroindaiidioiie-(I,3); 9-propionylanthracenc ; xarithorie ; 2,2'-pyrid il ; aza compounds (see I'igure 4). With roiiihiiiatioiis of souie dopaiits and dye scnsitizcrs, PVK layers could heohtainrd which wrre as sriisitivc: as ZnO-siliroiie laycis (sensitized with rhodaiuiuc H),acrordiiig to elertrophotographir iiiiagiiig nrclhods. Thc ropies show effects of iiirrcasing dopant coiirentratious (I:igurcs 1 aiid 2), of diffrrrut dopaiits i n equal eoiicciitrations with thc saiiic suhstitueiils (I"g ' I ures 3-5), and with different suhstiburnts oii one aroinatir ring system (Figures 6-8). I t is iiiiportaiit,to iiote that d e s ~ n ~ i l i ~ a t i oI'VK n o f was observed oii addiiig stroiigcr electron donors (e.g., 4,4'diaminohiphenyl; 2,2'-diiiapht hylaniine; rtc.); scc Figurc 9.
Figure 4. Density strip copies: PVK doped with a m compounds ( I mole Yo)
(b) Olher I'olymric Photoconduclors. The cxperimeiits nisde with I'VK were cxtciided to other poly-
mers eoiitaiiiirrg aroiiiat.ir or heterocyclic chaiii uiiits. The objective was twofold: to test whether polyiiiers with rr-bonded systciiis gcrierally exhibit photoconductive properties wherr iii the form of thiri filius, and whether dopsiit efferts as in I'VK may be observed geuerally. Both cxprclatioiis werr coiifiriried. The polymer filius prepared and souic of the dopalits cxhibitiiig iurrease of photoserisitivitics arc listed iii Table I. Strurtural foriiiulas of the coinpounds arc given i n I:igurcs 10 and 15. Drusity strip ropies of polystyrene and polyaceiiaphthylcuc are sho\vii i n I'igures 11, 12, aiid 13. A series of patents has heeii issued oil xerographic utilization of polyviiiylaroinat ir roiiiporinds,' polyviiiylhrt eroeyclirs,8 polyiiideiie aiid polyacc~iapthyleirc,~aiid derivatives of polyarrylate aird polyiiirtharrylate.'" I t is interestiiig to iiote that sigiiificaut photorurreuts were reported also for glow dischargr-formed polystyrcne filins by Spokas."
PHOTOELECTRIC EFFECTS IX POLYMERS
759
~~~
~
Table I : Some Further Photoconductive Polymers Polymers
KO.
1. I 1. I1
Polystyrene” PolyviriylxyIene*
1. I11
Poly-l-vinylnaphthalene“
1. I V
Pol y-2-vinylnaph thalerie
1. v 1. \-I 1. VI1 1. VI11
Poly-4-vinylbiphenyl” Poly-9-vinylan thracene“ Poly-3-vinylpyrenee Poly-2-vinyIquinoIirie~
1. IX 1. 1. X I
Polyirideneo Polyacenaphthyleneh Poly( 3,3‘-dimethyldiphenylene-4,4’)~ Variocs aromatic and heteroaromatic derivatives of polyacrylamide and polymethacrylamide’
x
1. XI1
Sensitizing dopants
See Figure 11 Tetracyanoethylene (TCNE); dibromomaleic anhydride (DBMA); 3. X I : 3. I X ; 1,3,5-trinitrobenzene (TNB); Z-nitroindandione(1,3) (NIII); 3. 111; 3. X ; 3. IV TCKE; IIBMA; 3. X I ; 3. I X ; TNB; NID; 3.111; 3. X ; 3. I V ; 9,lO-dichloroanthracene (DCA); and 3. XI1 3. X I ; 3. IX; 3. I ; anthraquinone (AQ); 2,4,7-trinitrofluorenone (TNF) No doping experiments made No doping experiments made No doping experiments made All doparits listed above; sensitization was also observed with strong electron donors such as 2,2’-dinaphthylamine T C S E ; 3. X I ; IICA; 3. I See Figures 17 and 18 TCNE 1)B;LIA; 3. XI; 3. I X ; 3. X 3. X I
’ Mostly Polyst,yrol I11 (glasklar) from Badische Anilin & Soda Fabrik (BASF), Germany. * Monomer from L. Light and Co. GmbH, Hamburg, Germany; purified, and polymerized with 0.1% benzoyl peroxide in a tube (48 hr., 110’). ‘ Monomer from L. Light and Co.: polymerized as under b. Synthesized from e-hydroxyethyl naphthalene and polymerized as under b. Monomers prepared according to methods described in the literature; polymerization as under b. PolymerMonomer polymerized as under b. ized with BF3-etherate in methylene (ahloride. Thermally polymerized ( 1lo”, 1 to 2 hr.). ’ Prepared from 4,4’-diiodo-3,3’-dimethylbiphenyl by heating with copper powder amording to W. Kern and 0. Wirth, k’zmststo$e-Plastics, 6 , 1 2 (19.59). For preparation, see German Patent 1,1.31,988(1962).
’
Table I1 : Low 1 1olecular Weight Electron Donor Type Photoconductors XO.
2. 2. 2. 2.
I I1 I11 IV
Donor type photoconductors
Naphthalene Biphenyl Fluorene Anthracene
2. V
Phenanthrene
2. VI 2. VI1 2. VI11 2. I X 2. x 2. XI 2. XI1 2.’XI11 2. XIV 2. XV 2. XVI 2. XVII 2. XVIII
Aceriaphthene Acenaphthylene Chrysene Pyrerie 1,4-l>imethoxybenzene Diphenylamine 2,2’-Dinaphthylamine 1,s-Iliethoxynap hthalene 2-Phenylindole Carbazole Phenothiazine 2,4-13is(4’-diethylaminopheny1)-1,3,4-oxidiazole 2,4-13is(4’-diethylaminophenyl)-1,3,4-triazole
2 . Low Molecular Weight Donor T y p e Photoconductors. Doping experiments were performed also with a
Photosensitizing dopants
3. IV; 3. X I 3. I X ; 3. X I ; picryl chloride (PC); 3. IV; 3. XI1 3. X I ; PC; 3. I X ; 3. X ; 3. IV; 3. X I f Dibromomaleic anhydride ( DBMA); 3,.5-dinitrosalicylic anhydride (DNSA); 3. X ; 3. XI1 TCNE; DBMA; 3. I X ; 1,3,5-trinitrobenzene ( T N B ) ; PC; 3. X ; 3. XII; 3. IV DBMA; 3. X I ; PC; 3. X; 3. IV; 3. XI1 3. X I ; 3. x; 3. I V 3. I X ; IINSA; 3. I X ; PC; 3. X; 3. IT: 3. XI1 TNB; 3. XI1 3. X I ; 3. x; 3. IV 3. X I ; 3. I V IIBMA; 3. X I ; DNSA; PC; 3. I X : 3. X ; 3. IV; 3. XI1 IIBMA; 3. I X ; PC; IINSA; 3. X I : 3. X; 3. IV; 3. XI1 IIBMA; 3. X I ; PC; 3. I X ; 3. X ; 3. IV; 3. XI1 DBMA; 3. XI; P C ; 3. I X ; IINSA; 3. X ; 3. I V 3. XI1 See Figure 17; 3. I X ; 3. X I ; T N B ; 3. X ; 3. IV IlBMA; 3. I X ; IINSA; PC; 3. X ; 3. I\’; 3. XI1
plied only to uniform, continuous films. Therefore, since low niolecular weight compounds of the types
Volume 69, .Vumber 3
March 1,965
d “0.
I
I
CiCY
9‘
listed i i i Tahlc: I1 and show1 iii Figurc: 14 arc riiiahlc to form coiitiiiuoos filiiis, they tririst he ~:i~ihrddcd into a rctsiii hiiider. Tlic hiiidw tirust hr highly iiisulatiiig, iioiipliotoooiiclucting, and should iiot. iiitrraet apprcciahly with tlic: phot ncondiwtor dispcrscd or dissolved iu it.. Ihriiig t.hc (:OIII’SCof tho work dcscrihcd liore, various binders liavr hocii used iii order to ddcriiiiiic their inRiiciirc: e.g., pdlvinyl acelate [llowilit.h@ 50 usually, hut soiirct iiircs also f he lower polyiiicric types llowilit,h@ 40 aiid 30 of I’arhwcrkc Hocctist AG, Ihnkfurt a.lI./ Hiiehsl, Grriiiauy I ; chlorinaled pol!ivin!jl chloride [lthc:tioflcx@ of I>AG-Wc!rk, I~liriiifeldcn,Geriiiariy I. I:or control, also, 1 he following plastic hinders were iisod: chlorinaled ruDber [“l’c.rgut S 400’’ of I’arhonfahrilicn Raycr, I,cvorliiia:ii, G(:riiiaiiy; arid “ParIni@ S 5 cps” of Horculos I’oivdrr Cn., U.S.A. I ; bularlienea/!lrene copol!lmer [“l’liolitc@ S-S~)” of Goodyrar,
u.s..\.1, (:I(!.
For t h v pliotosc.risitivity tvsts, the doiior coiupouiids listd i i i Tablo I 1 wcrc dissolvd i n (.qual parts (hy wiglit) :is t h hiiid(:rs. ICIcvtroir awc~ptorswrre added
iu siiiall wricciitratioiis t o thc. sohitioil (ahoiit 0.1 t o 2 iiiole %) and tho filius \vrrc mist as usual. Thc: cllrcls of dopaiits were iiivcst igatnl hy tlw i i w t hods dc:srrih(d ahovr. 111 all casrs t c s s t ~ lan iticrcasc~of photnsrnsitivitirs iiidriccd hy doping \vas ohsc~rvrd.” ‘I’ahlr 11 lists soiiic of the eoitiporiiids t r s t c d Thr structural foriiiulas of thc: coiiipoiinds arc! givcti in I*‘igrcrrs14 and 15. The cfTcc*tsof dopants arc illustrated in 1:igurcs 11; aiid 17. 3. fi:leclrmz Acceplor 7’jlpe I’holocimduclors. Whcii the iiiiportaiirc of donor-ac:wptor iiitorwt.ions was est.ahlished iu doiior host. systciiis, thc: rctvcrsc case mas studied. Typiral arec:ptor coiiipoimcls \wre doped i i i siiiall niiiouiits with doiior wiiiporuids chosrii froin thosct listcd i n Tahlc 11. The iiiixtiir(: was ctiihcrdded iii a rrsiii hiiidcr as desvrihod uiidcr swtion 2. AII iiiereas(: of photosriisitivity hy dopiiig was f o u r ~ d ’to~ tali(: place with a iiuiiihrr of host iwiiipouiids (Tahlc 111; figtirrs 14 and 15). Is‘igiirc 18 illustratrs thr csffrrts of donor type dopants iipoti photosmsitivity of aii atcwyptor host.
Table 111: h w Alolecular Weight F:lcetron Acceptor Type Photoconducton No.
3. I
3. I1 3. 111 3. I\' 3. \' 3. VI 3. VI1 3. V l I l 3. IX
x
3. 3. X I 3. XI1 3. XI11 3. XI\' 3. X\'
Tetrirehh,rophthalic anhydride" Hexabromonaphthslie anhydride pChloranil 1,ZBeozoaiithr~qiiinone Ijertril I'y rme-3-aldehyde !I-Acetylantlrracene
2. XI11 2. X I I : 2. XVII 2. IV; 'NIX See Figore I8 and HMB; 2. X; 1)1111; NEC; I'VK 2. XI1 PVK: 2. XII: 2. XVII 2. X I i ; 2. XVII 2. IV; 2. I X ; 2. VIII; 2. XII; 2,3,S-tripheiiylpyrole (TIT); N-ethylcarbmole ( N I X ) : 2. IV; 2. I X ; 2. VIII; 2. XII; TPP: N I X Hexamethylbenzene (HMB); 2. X ; 2. I ; 2. \'HI; 2. XII; 2. XIV; dibenmhran (I)BF); 2. XI; PVK NEC HMB: 2. X: 2.1: 2. 1V: 2. VIII: 2. XII: NEC: T P P 2. IV; 2. XII; 2. XVII; PVK PVK; 2. XI1 2. X; HBM; 2. I ; 2. IV; 2. VIII; 2. XII; NEC; TPP 2. X ; HBM; 2. I V ; NEC
In the CIISC of tetrachlorophthalir anhydride, desmiliralion was observed when another acceptor had been added, e.g., 4-bromonitrohcnrene or 4-cyxnnpyridine.
Figure 7. Density strip copies. PVK doped with derivativea of naphthalene, mainly 1,hubstituted ( I mnl? %).
Figure 8. Density strip copies: PVK doped with derivatives of naphthalene, mainly I,&ubstititted ( 1 mole %).
7oltrme 60. Sumber 3
March 1966
HELMUT HOEGL
7tx2
1.v
L VI
LVll
Q I VI11
1.x
I.IX
I
.
f.*;~~x c\N"
. ,
-
..
Figure 9. Density strip copies: PVK doped with increasing amounts of 4,4'diaminobiphenyl.
Discussion of Results The most significant observations made in the course of this study are given in this section. (1) Electrostatic iniaging techniques provide a simple and rapid nieans for the investigation of phenomena associated 1vit.h photoconductivity in organic compounds. They are especially useful for the screening of dopant effects, which would take much more time with the usual conductivity measurements. (2) Photoconductivity is a common feature of organic systems containing conjugated double bonds (aromatic and heteroaromatic, monomeric and high polymeric). (3) One may distinguish clearly between two different types of organic compounds: (a) electron donors (D), s-electronic systems (aromatic, heteroaromatic), especially with electron-repelling groups, such as amino, aklkylamino, alkoxy, acetoxy, alkyl groups, etc.; (b) electron acceplors (A), r-electronic systems with electron-attracting groups, such as nitro, cyano, carbalkoxy, acetyl, carboxylic acid anhydride, halogen groups, etc., and quinones. It should be noted that the dopants
R2
-
Aromlicr H.l"ooromllli..
- R2
l.Xll
Figure 10. Structure formulas of polymeric photoconductam.
must not be photoconductive themselves (e.g., BFa or HISOI, in the case of PVK). (4) The effects of additives upon the photosensitivity of the host compounds are due to charge-transfer (CT) type interactions between host and dopant. This is indicated (a) by structural features, (b) by the appearance of C T bands in the absorption spectra, (e) by the formation of stable CT complexes between the D and A components in many cases, and (d) by electron spin resonance (e.5.r.) measurements.15 (5) There exist mainly the following possibilities for the combination of different D and A compounds, of which the following observations have been made with regard to photosensitivities (see Table IV). However, this classification is useful only for compounds with very pronounced donor or acceptor character. I t has been observed also that a host compound may act either as donor or as acceptor, with respect to different dopants, and that it is sensitized by both of them (e.g., anthracene, poly-2-vinylquinoline). (6) If the same host material is doped with equal molar amounts of different dopants (e.g., PVK wit.h (15) R. P. Kohin. K. A. MUller. and H. Hoegl. Hela. Phve. Acto. 35, 255 (1962).
04 % 2.0 %
FiKiire 1 I . r)cnsity strip copies: polystyrene doped with electron acceptom.
Figure 12. Density strip copies: polyacenaphthyleiie doped with increasing amounts of hexshromonaphthalie anhydride.
Table IV:
the interartion between an electron douor aud ai1 clrctron acceptor is characterized by the equilihriuin
Host-l)opsnt Cumhinations
Phoioeondwlire
m.trix
1)
A I) A
meet
I)""*"L
A I)
U' A'
I,l,"lOC."d"C,iVit~
Sensitization Sensitization I)esensitization Ihensitization
nitro and aza compounds and with naphthalene derivatives; Figures 3 4 , significant relationships between dopaut artivity and chemical structure may be observed. This method provides a means of classifying materials arcording to their relative donor-acceptor streugth.
Interpretation of Results To explain the most significant results, one has to iut rodure several well-founded concepts. (/) Charge Carrier Formation. Accordiug to Weiss,16 Brarkrnan," Mulliken,18 Hriegleb,'Q mid others,"-z3
IiEht
DA S D+AThis equilibrium is influenrcd also by other forms of energy, such as thermal and nierhauiral cuergy (pressure). In the "normal state" (e.g., rooiii temperature, normal pressure, absence of light), the systrni already may coritain signifirant concentrations of the ionized structures depeiiding on the coinpoirents D aud A. Furtherinore, the electron trausfer betweeu adjarent (16) J. Weiss. 3. Chem. Soc.. 245 (1942). (17) W. Bmekman, Rec. 11av. chim.. 68, 147 (1949). (IS) R. 5. Mulliken. 3. Am. Chem. Snc.. 74, 811 (1952): 3. P h w . Chem.. 5 6 , 8 0 1 (1952). (19) G. Briegleh, " E l e k t r o n e n - D o n n t o r A ~ ~ ~Komplexe." ~t~~
Springer-Verlag. Rerlin. 1961. (20) L. J. Andrews. Chem. Ret,., 54, 713 (1954). (21) S. 1'. McGIynn. ihid.. 58, 1113 (1958). (22) J. N. Xlurrell. Quart. Ret. (London). 15, 191 (1981). (23) H . A. StRsh. "EinfBhrung in die theoretirehe orp:mixcl~a Chemie." VWIW Chemie. Weinheim. Oermiuiy. 1959. I). 804.
Vnlemr 0.9. .Yirmber 3
Jlnrch 18/16
HELMUT HOEQL
764
2 VI
2 VI1
2.X
2.XI
2.Xll
X
-
2 XH VI
IO. NHl
2.XVll
2.XVlll
Figure 14. Structure formulas of electmndonor type photownductors. Figitre 18. Ilensity strip copiea: polyacenaphthylene doped with various nitro compounds ('2 mole %).
3.1
0;j CI
3.v
az; 3.Vlll
%:: 0
3:XI
o:-:o 0
ZXlll
a:x>
2.xv H
2.XIV
Cl
2.M
2.VIII
q - 0 CEO
D and A molecules may be more or less complete, 80 that one speaks of interniolecular mesomerism.lP Of importance here is that charged molecules may be formed froin originally neutral molecules by C T in the dark or with light. In the cases of an excess of either D or A molecules (or D and A side groups in polymers) important new consequences arise as discussed below. There is general agreement that the charge carriers formed by the interaction of photons with a photoconductor are electron and hole pairs; see, e.g., Bube" and Rose.p5 If one assumes that C T is essentially electron-hole pair formation, one may deduce niodels for photo-induced discharge. According to the electron-hole pair picture, either one of the charge carriers or both of them may contribute to the photocurrent, i.e., to the discharge of the electrostatically charged plates. The following cases can be distinguished. ( a ) D Host Doped with A Impurity. Migration of positive charges, p t y p e caduction, by exchange between the D-molecules may be postulated from this scheme. Electrons are trapped in the form of A-.
2v
2 IV
a@%)
E5 1 ' I DOPANT
NONE
2 111
2.11
21
0
3.Xlll
Figure 15. Structure formulas of electmnacceptor type photownductors.
765
PHOTOEl.ECTIZIC I
