Synthetic Metals, 28 (1989) C715 C721 C715

I~ACE ~ OF C~GANIC S ~ I ~ ~(]NED WITH METAL ~ ~ C

S ~ 4 I ~ .

Mun-Soo Yun, Chang-Su Huh

Korea Electrotechnology Research Institute

Sungju-dong 28-1, Changwon-City, Gyeongnam, Korea

Abstract

We have studied on the junction characteristics of organic semiconductor-metal

and organic semiconductor - inorganic semiconductor devices. Organic semiconduc-

tor was electro-polymerized on metal electrode by constant current method. Con-

sidering the formation of oxide layer at the interface in manufacturing process

and high resistance of organic semiconductor, we can deduce them as a MIS structure.

Usually, it was known that organic semiconductor have rectifying contact with low

workfunction metal and o~mic contact with high workfunction metal. But by our

experimental results, junction characteristics depend on metal oxide layer rather

than workfunction of metals. This oxide layer largely depends on manufacturing

process of devices. So we studied the effect of an electro-polymerization condi-

tion, film thickness, and doping density in junction characteristics. Also we

investigated the solar energy conversion characteristics of these devices. Photon

have to be reached near the depletion region to increase the conversion efficiency.

We studied on the transparent inorganic semiconductor like CdS as a window material

and interface phenomena.

Introduction

A study on the conducting polymer have been continued for a decade from the

pratical and theoretical aspects. But the conducting polymers applicable to prac-

tical device were restricted to a few kinds, because most of them are sensitive to

air. One of them, PPy is c(mparatively stable in air and easy to make thin ~iha

by electrochemical .ool~aerization. So its appl~catioq sbldy have received tnuch

attention from the pratical point of view due to their unique electric and elec-

tronic properties which can be changed by doping process. In many application

0379-6779/89/$3.50 © Elsevier Sequoia/Printed in The Netherlands

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aspects, using it as a semiconductor device material is bright due to simplicity of

manufacturing process and large size as ccmpared with other inorganic semiconduc-

tors. On the other hand, organic thin film diode and solar cell are most commer-

cially promising application field for deposited semiconducting film by electro-

chemical method. Junction characteristics of organic-inorganic and metal -organic

semiconducter were studied to investigate the effect of insulating layer at the

interface. Also contact resistance problem was studied with changing the process

of fabrication. Specimen was annealed or metal layer was attached by mechanically

to make the oxide layer at the interface. Also PPy could be used as a solar cell

junction material. In some case charateristics of PPy is similar with inorganic

semiconductor except the kinds of carrier and density of state, so efficiency of

its own junction solar cell is worse than that of schottky or heterojunction with

inorganic semiconductor[l]. We tried to make the junction of inorganic semicon-

ducter with PPy. So we studied CdS properties. We investigated the V-I charac-

teristics as a n~thod to appraise device properties.

Experiment

Polypyrrole was prepared by the electrochemical oxidation. The reaction

solvent was acetonitrile and electrolyte was liti~perchlorate. Before the poly-

merization, pure nitrogen bubbled into it for 30rain. in order to remove dissolved

oxgen. Working electrode was 2X3 cm2 gold film which was vacu~ evaporated on

glass or polymer film substrate.

Also (n)Si wafer was used to make heterojunction. Features of samples were

shown in Fig. i. Weused plati~ plate 2X3 cm 2 as a counter electrode. In order

to control the potential of the working electrode, a saturated calomel electrode

was used as a reference electrode. The total amount of charge passed through the

cell was measured by electrometer (Kethly 602) which has integral calculable abi-

lity. The film deposited on the gold electrode was rinsed with large amount of

methylalcohol and dried in air by hair dryer. Undoping treatment was carried out

by discharge with lead. Also we annealed this sample at 80 - ii0 °C for a few

hours. Indi~ was evaporated under 10-5-106 Torr in area of 0.25 ~2 and Ag paste

was used to decrease the series resistance . V-I characteristics was investigated

with electrometer.

Results and Discussion

We conclude the polypyrrole as a p -type semiconducter by many results. The-

oretically, metals with high work function formed an ohmic contact to this polymer

and low work function metal formed blocking contact. We used A1 as a low workfun-

ction metal which assumed to have blocking contact.

Ag paste

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AI203

PPY n-Si

Au electrode

Fig. I -a Structure of ppy/AI heferojunction

Au electrode

SiOz

Ni Ag paste

Fig. I - b Structure of ppy/(n)Si solar cell

The results of V-I characteristics of A1 /PPy junction were shown in Fig. 2.

Line 1 shows of a ohmic contact and line 2 shows a diode characteristics. PPy was

made by electrochemical method on gold electrode and A1 deposited by vacutml evapora-

tion. Because PPy which have conductivity I0- 101J~/ Cm was not annealed, we

suppose that there is no change on the surface of PPy thin film and tunneling takes

place in the junction, and shows ohmic contact. That result is due to high dopant

density of polymer originally which leads to the narrow depleation region[2].

These narrow depletion region made tunneling coefficient 1 ~d made ohmic contact.

So we treat the polymer with discharge process to control doping density. Conduc-

tivity is proportional to the density of charge carrier, so doping density of poly-

pyrrole depends on largely kinds, and sm~ount of electrolyte. Change of characteri-

stics of device was slight with changing condition of polymerization and undoping

process, so we select a kind of electrolyte, liti~ perchlorate and same concentra-

tion.

Annealing effect on junction characteristics was also studied. Usually poly-

pyrrole is stable in air but by heat treatment the junction characteristics are

changed, which might be due to the oxidation of surface, but could not detect.

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mA

.r-"-"

I 2 3 4 V

Fig. 2 V- I characteristic of ppy/AI Junction

0 AI evaporation (sandwich type)

(~) AI mechanical contact (sandwich type)

Sometimes copolymer was used to increase the depletion layer(decrease the resis-

tivity). But we made the junction by mechanical contact to form oxide layer at

interface. Its result was shown in Fig.2. Forward bias is when the nagative

voltage are applied to the In electrode in this device. Usually junction resis-

tance depends on largely polymers resistivity and oxide layer. If we consider

these junction as a schottky barrier, J-V characteristics are expressed by Eq (i)

[3, 4]

J = j exp (qV / nkT ) (Eq. i)

where j is a reverse saturation current.

j = A* T exp (- q 6/kT)

A* ; effective richardson constant

; barrier height

usually we decided the diode factor n by (Eq i). in J Vs V from (Eql) plot give a

straight line under the forward bias above 3kT and n is calculated from the slope.

Also heterojunction characteristics of PPy with Si wafer was studied. It's results

was shown on Fig.3. Junction barrier of 0.8 eV was produced at the interface,

that is depend on largely density of state[5].

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mA

2 ®® I 2 3 4 V

Fig. 3 V - I chQrocteristics of (n)Si/ppy Junction (Vot. sup 0.5V/sec)

~) (n)Si/ppy/Au ~ (n)Si/ppy/Au M M M

The discrepancy of 1 and 2 in Fig.3 is due to Au attaching method which were

evaporated or mechanical attach. Forward voltage drop is large ctmparing the

commercial diode. It is due to contact resistance of electrode and space charge

limiting. Basically organic polymer have wide band gap energy and narrow valence

band and conduction band, So carrier was very limited in room te~perautre. Then

mostly carrier of organic device are from space charge injection from contact metal.

So due to its internal and external resistance, forward voltage drop is larger

than that of inorganic semiconducter devices. Usually when we make the solar cell,

p type semiconductor was selected as a photon absorber. Because electron produced

in p type region has faster mobility than that of hole, so probability to overcome

the junction barrier is higher than that of hole by diffusion and drift field. If

photon energy hv is higher than workfunction of absorber, electron was produced and

drift to the junction by diffusion. While traveling the bulk of p type, a part

of electron was trapped at defect or recombinationed with hole. The other overcome

the junction barrier by drift field. At this moment insulating layer which is

located on metal surface effect on the tunneling coefficient.

Photoconductivity of PPy was studied to use it as a photon absorber, but its

response was very crude ctmpare to inorganic semiconductors[6]. So it was used

C720

mA

|

I 2 5 4 5V

Fig. 4 V - I characteristics of (n)Si/ppy Junction

(Vot. sup 4V/sec)

as a junction material to make potential barrier with inorganic semiconductor

which could be solar energy absorber. Firstly V-I characteristics of ppy / Si

was studied at dark and its result was shown in Fig. 4. Hysteresis loop was

produced when we change applied bias voltage and this result are studied contini-

ously. Because PPy was tried as a window material, it has to be very thin.

But there are alway high possibility of puncture, so junction was short. When

we make the film very thick to eliminate puncture, then all optical energy was

absorbed at the surface and efficiency could not be measured. By the way when

we use CdS as a window material, hazardous problem to make good quality CdS thin

film is exist. Though it need heat treatment, this process change the property of

PPy. So we just evaporated Cc]S on PPy without annealing. When we investigated

the V-I characteristic of it in light, voltage is very low, but current is higher

compared to the other persons results which had short current 3uA[7]. We attained

short current 12uA, still we studied to increase the voltage. This SIS or MIS

type solar cell has smaller Io and higher Vo than those of heterojunction or homo-

junction solar cell. Theoretical analysis about insulating layer was studied by

J.Schewchun and A.K.Ghosh, but still was not explained clearly. Usually insula-

ting layer thickness is 10- 20 A in MIS structure. It has tunneling coefficient

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1 for minority carrier but not for majority carrier, so Io decreased very severely

with increasing insulating layer thickness. MIS and SIS structure was analyzed

by same theory except top layer of SIS has band gap. But in MIS structure,

solar cell have a many problem due to metal layer thickness which have to be very

thin to increase transparency. By the way in SIS structure, CdS has better

transparancy than that of metal, so there are possibility to increase the efficiency

of solar cell. But solar radiation intensity increase, efficiency are decrease due

to space charge limiting current. It is largest problem in practical point of

view in solar cell to solve.

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