The mobility-lifetime product in depleted a-Si:H diodes

]OURNA

Journal of Non-Crystalline Solids 137&138 (1991) 1173-1176 North-Holland

L OF

NON,CRYSLINESOLIDS

THE MOBILITY-LIFETIME PRODUCT IN DEPLETED a-Si:H DIODES R. KONENKAMP Hahn-Meitner Institut Berlin, Glienicker Str. 100, 1000 Berlin 39, FRG S. MURAMATSU, S. MATSUBARA and T. SHIMADA Hitachi Central Research Laboratory, Kokubunji, Tokyo, 185, J a p a n We have performed transient photoconductivity experiments on well-characterized a-Si:H solar cells which show the following: (A) Under depletion conditions recombination is effectively suppressed, leaving only trapping as a loss mechanism in electron transport. (B) Measured trapping times depend on the experimental time scale. (C) Under non-depleted conditions trapping is dependent on the occupancy of the localized states. Our results clarify the meaning and interpretation of experiments related to time-of-flight work, and explain the observed experimental differences in steady state and t r a n s i e n t measurements.

1. INTRODUCTION

T h e r e h a s b e e n an e x t e n d e d d i s c u s s i o n concerning the i n t e r p r e t a t i o n of the mobilitylifetime product in amorphous silicon (a-Si:H). Two different types of experiments are commonly used to determine ~tx: time-of-flight- experiments involving electron transport, and steady-state photoconductivity experiments involving e l e c t r o n s a n d holes u n d e r r e c o m b i n a t i o n conditions. It is well established that the latter type of e x p e r i m e n t yields considerably larger values for ~tx in undoped material. Non-isotropic electron transport, Fermi-level dependent carrier l i f e t i m e s , as well as t h i c k n e s s d e p e n d e n t t r a n s p o r t p a r a m e t e r s have been considered to explain the discrepancy in the e x p e r i m e n t a l r e s u l t s . O t h e r work1, 2 has focussed on the different occupation of localized states in the two types of experiments. Essentially, in the time-offlight experiments, since only one type of carrier is injected, localized states act as traps, while in the p r e s e n c e of both, electrons and holes,

localized states act as recombination centers. It is now believed that this principal difference can well e x p l a i n the e x p e r i m e n t a l l y o b s e r v e d difference in the decay kinetics. This paper deals with some particular aspects of the time-of-flight expeldments. We wish to show, how the trapping kinetics relates to the shallow and deep defect density in a-Si:H, t h a t the e x p e r i m e n t a l time frame is i m p o r t a n t for the determination of a unique ~tx, and finally we will outline how the evaluation scheme has to be modified if thin fihns with non-uniform electric field are to be characterized. 2. EXPERIMENTAL The experiments used a time-of-flight set-up with a 337nm, 200ps light-source and a t r a n s i e n t recorder with sub-ns risetime. The samples were -7000 A thick p-i-n type solar cells with an aSiC:H window and conversion efficiencies between 9 and 12%. More experimental details can be found in ref. (3).

0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.

1174

R. KOnenkarnp et al. / Mobility-lifetime product in depleted a-Si:H diodes

1.2

3. RESULTS A N D D I S C U S S I O N In Fig.

1 we show results

for the

1.0

charge

collection in time-of-flight experiments on these

Q=(negxE/L) (1-exp(-L/(~E))) with E=(V+Vo)/L,

•

0.8!

thin diodes. Curve a) is a s t a n d a r d result obtained without background illumination and d e t e r m i n e d by i n t e g r a t i n g the p h o t o c u r r e n t transient up to 4~ts. Charge collection starts at approximately 0.8V forward bias due to the builtin potential of the solar cell, and approaches unity already at light reverse bias. The solid line is a fit to the simple Hecht equation, giving the collected charge Q as a function of applied potential V and the the mobility lifetime product,

b

0.6. 0.4 0 0

o 0

0.2. 0.0 Applied Voltage (Y)

(1)

where n is the photogenerated carrier area density, e the electron charge , V the applied voltage, and V o the built-in potential. For this equation the electric field and #x are assumed to be constant through the device, the obtained values hence only represent spatial averages. Curves b) and c) are obtained on the same device under modified experimental conditions: b) with an extended integration time of 10ms and c) with 4 0 m W / c m 2 white bias illumination. It is seen t h a t the charge collection is considerably changed, and we now turn to a more detailed investigation of the transient processes. Fig. 2 shows two transients corresponding to different bias voltage conditions. Transient (a), obtained under 2V reverse bias shows a sharp transition from a constant current region to a steeply declining tail region, characteristic for complete carrier sweep-out. The charge in the tail region(Ires< t< 10ms) is found to be only -15% of the total charge under the transient. For transient b), the applied voltage is reduced to 0.4V forward bias, resulting in a smaller amplitude. A monotonous decay starts at ~10ns due to deep trapping which precludes any marked feature in the transient. It is seen that for t
Fig. 1 Collected charge vs. apllied voltage, a) 2 b) integration time 4~s, PXav=5xl0- 9cmTV; integration time 1ms, p%v= 1.2x 107cn}/V; c) with bias illumination, integration time 4~s, ~x=10"acrr~V. t r a n s i e n t (a) is collected, while the tail of transient (b) constitutes more than -40% of the total. Integration of the current transients to 10ms yields equal values within an accuracy of -10%. Thus, although significant charge loss occurs already around 10 to 20 ns, this charge is eventually recovered in a much slower process at times t>lms. This Finding is the characteristics of a t r a p p i n g process, and q u a n t i t a t i v e l y explains the larger collection efficiency when the integration time is increased (Fig. 1). It also indicates that recombination plays no essential role in the time-of-flight type of experiments. The kinetics of the trapping process is well described in a t h e r m a l i z a t i o n model, t h a t includes both, bandtail and deep states. At the outset of the experiment, the carrier packet occupies the large density of tail states, thought to be an exponential distribution. After -15 ns, thermalization moves the carrier packet to a depth of ~0.25eV below the mobility edge. At this point the remaining density of unthermalized

R. KOnenkamp et at/Mobifity-lifetimeproduct in depleted a-Si:H diodes

10-2

~E <

0 lb.

o

,

i

E

l

J

where c~ is the dispersion parameter, N T is the density of deep traps, A is the energy interval covered by the trap level c~1 and c~2 are the

10-4 ~~,\\

capture cross sections for the band tail and deeper states, respectively, and v is the attempt-

10-6

to-escape frequency for the band tail states. Eq. (3) can be used to estimate the deep trap densities from charge collection measurements. Due to the dispersion parameter ~. in eq. (3), the time-offlight g~ products are now also dependent on the

>\\ a \x~\\ b

0

10 .8

x ~ \ . I

10

-8

1175

I

1

i

-6 10

I

10

-4

Time (s) Fig. 2 Electron photocurrent transients obtained at a) 2V reverse bias; b) at 0.4V forward bias. tail states is - 2 x 1015 cm -3, which is of the same magnitude as the midgap density. Hence there are now more traps available at the midgap level than in the band tail. The trapping changes from the tail states into these deeper states. Thermal reemission from these states occurs at a time, t=v-lexp (ET/kT). With v=1012 s -1 and ET=0.5eV from a previous study 4 of deep electron traps in a-Si:H, one finds t=2ms, in basic agreement with the late charge collection in Fig. 2. The nearly featureless t -1 decay of the transient indicates that one deals with a rather wide distribution of deep states, not with a well-defined level. When the transit to the rear electrode is faster than 20ns, as in the case of transient (a) of Fig. 2, the deep trapping process is not effective and a more complete charge collection occurs on the fast time scale, t
by x=v'l((~l kT o No)/(o2 ANT)) 1/a

(2)

shape o f the band tails: narrower band tails giving rise to smaller ~ values. It is surprising that charge collection is still observed at times as long as 10ms in these devices. This value exceeds the expected bulk recombination lifetime indcating that recombination is practically non-existent in depleted p-i-n diodes. Essentially, since only electrons are injected, there are not enough excess holes a v a i l a b l e to complete the recombination process on a ps time scale. It therefore appears unlikely that time-of-flight experiments can easily be used to determine bulk recombination lifetimes. This has also been pointed out by Kakinuma 2 Under recombination conditions when excess holes are also available, as in g a p - t y p e e x p e r i m e n t s , our e x p e r i m e n t s and o t h e r previously reported results 5,6 show that the fast decay at 20ns is not observed. Instead, in p h o t o c o n d u c t i v e decay m e a s u r e m e n t s one observes a gradual decay typically a t ~l~ts. Identifying this time with the recombination l i f e t i m e is c o n s i s t e n t with s t e a d y - s t a t e photoconductivity results ~iving ~tz=10-6cm2/V. It is therefore believed that the recombination lifetime in undoped a-Si:H is -1 Us u n d e r recombination conditions. The decay at 20ns in time-of-flight experiments should then be due to the non- occupation of deep levels. Studies of recombination 5"7 indicate that in undoped a-Si:H, the recombination is a complex interplay between hole and electron densities with trap emission of

1176

R. KOnenkamp et at ~Mobility-lifetimeproduct in depleted a-Si:H diodes

holes determining the recombination rate. The deep electron level is filled with excess electrons and thus rendered ineffective for further capture, in a g r e e m e n t with the long recombination lifetime of ~lgs. Time-of-flight experiments under bias illumination (curve (c) of Fig.l) can mimic the situation of occupied electron trap states. However, since the occupation also depends on the applied voltage, only qualitative c o n c l u s i o n s can be d r a w n from s u c h experiments. We observe an increase in the collection efficiency at low voltages where trap filling is expected to occur; a tentative evaluation of the data in terms of eq. (1) yields gx=10" 8 c m 2 / V , i.e., about twice the value as t h a t obtained in the dark. Finally, we wish to outline the consequences of a non-uniform electric field distribution for the evaluation of the charge collection experiments. As a first approximation one may consider the fitted ~x-values as averages across the bulk of the s a m p l e as we have done here. If ~tx is approximately constant, while the electric field is peaked at the p-i interface 8, one expects a better collection efficiency for forward and low reverse bias. Since the experimental results do not show this, we conclude that the lifetimes in the high field region (i.e. n e a r the interface) are somewhat smaller than in the bulk region. This conclusion is in qualitative a g r e e m e n t with previous investigations. Principally, determination of the electric field profile and a more detailed evaluation of the charge collection experiments g~ves the possibility to determine the profile of the gz -product in these devices 9.

4. CONCLUSION We have presented an analysis of the carrier loss process in time-of-flight type charge collection experiments. This analysis shows that one is dealing with trapping processes, and that recombination is of little influence due to depletion. The deep trapping process can well be studied in experiments on a ~Ls time scale. Extending observation times to the ms time region, however, demonstrates that the deep carriers are eventually emitted and can be collected, thus raising the collection efficiency with time. A simple analysis of the transients can give information about the gap state density in a-Sill. 5. REFERENCES (1) R. S. Crandall and I. Balberg, Appl. Phys. Lett. 58,508 (1991) 2) H. Kakinmna: Phys. Rev. B 39, 10473 (1989) (3) R. KOnenkamp, S. Muramatsu, H. Itoh, S. Matsubara, and T. Shimada, Jap. J. App. Phys. 29, L 2155 (1990) (4) R. Kfnenkamp, Phys. Rev. B 36,2938 (1987) 5) H. Oheda: Philos. Mag. B 52,857 (1985) 6) C. Main, R. Russel, J. Berldn, and J. M. Marshall: Philos. Mag. Lett. 55 , 189 (1987) 7) M. Hack, S. Guha and M. Shur, Phys. Rev. B 30,6991 (1984) (8) R. K~inenkamp, S. Muramatsu, H. Itoh, S. Matsubara and T. Shimada: Appl. Phys. Lett. 57,479 (1990) (9) R. Kfnenkamp, this conference