Recent developments in amorphous silicon p-n junction devices

Journal of Non-Crystalline Solids 35 & 36 (1980) 725-730 ©North-Holland Publishing Company

RECENT DEVELOPMENTS

IN AMORPHOUS SILICON p-n JUNCTION DEVICES

R.A. Gibson, W.E. Spear, P.G. Le Comber and A.J. Snell Carnegie Laboratory of Physics, University of Dundee, Dundee, Scotland.

Amorphous Si p-n junctions with various doping profiles have been prepared by the glow discharge process to investigate the effect of the barrier profile on the electrical properties of the diodes. The highest current densities, up to 40A/cm 2, are obtained with n+-~-p + structures. Under AM-I illumination photovoltaic p+-i-n + cells generate open circuit voltages of O.7V and short-circuit currents up to lOmA/cm 2, corresponding to efficiencies between 3 and 4%. The diode quality factors have also been investigated. i. INTRODUCTION The efficient substitutional doping of amorphous (a-) Si [1,2] and the subsequent production of p-n junctions [3] and solar cells [4], has stimulated considerable interest in the possible applications of this material, as the proceedings of this conference will testify. Although the use of a-Si in an insulated-gate field effect transistor has been reported [5], the attention in recent device publications has been concentrated on Schottky [6] and MIS structures [7], particularly for photovoltaic applications [8,9]. This emphasis has arisen because higher solar cell efficiencies were obtained for Schottky barrier devices (5.5% over an area of 0.02 cm 2 [8]) than for the original p+-i-n + junctions (2.4% over 0.005 cm 2 [4]). In this laboratory we have investigated both types of devices, although the present paper will be confined to the recent developments in the field of p-n junctions. We believe that devices based on a volume junction will in the long term have greater applied potential than surface barriers; they are likely to be more stable and also offer the interesting possibility of controlling the barrier profile by variable doping during deposition from the gas phase. This versatility of the amorphous p-n junction may well prove to be an important factor in optimising device performance for particular applications. In this paper we should like to report some of the results obtained in two possible areas of development: high current diodes and photovoltaic cells. 2. DEVICE PREPARATION The properties of a-Si specimens depend critically on the method of preparation and on the precise deposition conditions. The films used in the present work were produced by the r.f. glow discharge decomposition of silane in a capacitively coupled system and contain typically 5-i0 at.% of hydrogen [IO]. They were deposited at a temperature of 55OK onto stainless steel substrates coated with a thin film of chromium. The details of the deposition gear have been described previously[2]. ~le point of interest in the present context is that the specimens, generally 0.7 to l.O~m thick, could be made with either abrupt transitions between regions of doped and undoped material or with continuously graded doping profiles by progressively diluting or concentrating the flow of phosphine- or diborane-doped silane with pure silane gas.

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R.A. Gibson et al. / Recent Developments in a-Si Junction Devices

3. HIGH CURRENT DIODES Recently we have prepared a series of junctions with good rectification ratios and with substantially higher current carrying capacity than previous amorphous diodes.

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Fig.l. J-V characteristics of n+-~-p+ Fig.2. Comparison of J-V characterdiodes showing the effect of different istics for several doping profiles. doping levels of the central v region. Fig. l shows the J-V characteristics of a number of abrupt n+-v-p + devices with Au top electrodes. The nomenclature adopted here lists the doped regions in the order of deposition, so that in the present junction the n + region was the first to be deposited on the substrate. The n + and p+ contacting regions, each approximately 500 ~ thick, are doped with iO4vppm of PH3/SiH4 and B2H6/SiH4 respectively. The doping of the v region for the devices in fig.1 varies from zero (for an n+-i-p + device) up to 300vppm PH3/SiH4 for the most heavily doped junction. The data demonstrate clearly, at least up to ~ = lOOvppm, that the forward currents of these diodes increases with increasing doping of the central v region. For example, at IV and 2V respectively the n+-v-p+ device with v = IOOvppm passes 2.SA/cm2 and iSA/cm2 compared with IOmA/cm2 and 0.7A/cm2 for the n+-i-p + junction. To date, the maximum steady current densities obtained before the onset of thermal runaway have been as high as 4OA/cm2 for device areas of about i/3Ocm2, and even higher currents have been observed under pulsed conditions. It is also encouraging that these characteristics were measured on diodes with rectification ratios as high as 104 to 105 , as can be seen from the reverse data shown in fig. l for two of the devices. At low voltages, J increases exponentially with V over approximately two orders of magnitude in J, and from the general diode equation values of the diode qual~ty factor n of about l.S are obtained. We shall briefly discuss further measurements of n in the next section. Beyond the exponential region the J-V curves are linear for approximately 0.5V with a slope resistance

R.A. Gibson et al. / Recent Developments in a-si Junction Devices

727

corresponding to the bulk conductivity of the central v region. At higher V, the current density increases as V 2 indicating that current injection is taking place. The data in fig.2 compares the performance of th~ n+-v-p + (v = 40vppm) junction with two other structures prepared as p+-~-n + (v = lOOvppm) and p+-i-n +. Even though the higher doping of the central region of the p+-~-n + device enables it to turn on more rapidly at voltages below O.5V, only the n+-~-p + diode passes current densities in excess of iA/cm 2 at IV. This result is found consistently and must mean that the metallic contacts are better in the n+-w-p + samples than in those in which the p+ region is deposited first. Since it is known that both Au [ll] and Cr [12] form an ohmic contact to an n + region this result suggests that the Cr/p + contact is far from ideal. It also implies that in optimising the performance the contacts must be considered as an integral part of the device. ~ i s conclusion is supported by another result: the n values found for n÷-v-p + devices and for their inverted equivalent p+-~-n + junctions are approximately 1.5 and 2.0 respectively. Clearly the contacts must be playing a role in determining the measured n values. This general result underlines the difficulties in interpreting n values for amorphous diodes simply in terms of bulk processes. Further distinguishing features of the dark characteristics of junctions in which the p+ region is deposited first are the absence of a linear region corresponding to a bulk conductivity and a V 3 dependence of J for V > IV. Such a dependence was also observed for V > O.5V in the p+-i-n + structures investigated by the RCA group [4] and was attributed to processes involving a trap distribution decreasing exponentially towards mid-gap, although further work is necessary to distinguish this mechanism from that of double injection [13]. The reverse characteristics of the diodes have also been investigated in some detail. Typical reverse breakdown voltages are of the order of IOV or so, although values as high as 60V have been observed in some cases. Further details of both the forward and reverse results will be published elsewhere. 4. PHOTOVOLTAIC PROPERTIES In this section we discuss the behaviour under illumination of several junctions of varying doping profile. The first a-Si solar cell reported was a p+-i-n + sandwich junction prepared by the RCA group[4]. This device, illuminated through an ITO contact to the p+ region, had an efficiency of 2.4% over an area of 10 5 x IO-3cm 2. We have prepared similar jdevices, but illuminated through a (mA/cm z) semi-transparent gold contact to the -~=93mAk::mz ~ n + region. A typical AM-I photochar8 acteristic is shown in fig.3. The 0.51 AM-I Eff. ~'~"jFF= current density is independent of _ =3-4% ~ __ area over the range studied (0.2 0.5cm2). As a further precaution against edge contributions to Jsc the illuminated area was defined not only hz~ \ by the top contact but also by a surrounding opaque black mask. Voc values obtained were in the range 0.6 0.7 volts, Jsc around iOmA/cm 2 with \ }............... ~ -p =<0.57V fill factors of about 0.5 leading to efficiencies in the 3-4~ range. No -%rainless Steel attempt to provide antireflection coatings has been made at the present stage. [ I [ // I

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The p+-i-n + structure has been the most successful to date for photovoltaic applications, although other profiles with only slightly lower

728

RoA. Gibson et al. / Recent Developments in a-Si Junction Devices

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efficiencies may be capable of further development. Fig.4 shows the spectral response of a p+-~-~-n + junction with an AM-I efficiency of about 3%. The peak response of 52% occurs at 600nm with collection efficiencies greater than 10% throughout the range 450-700nm. A logarithmic dependence of collection efficiency on wavelengt~ is obtained in the range 400-475nm. p+-i-n + junctions give a similar spectral response.

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However, not all profiles have good photovoltaic properties. In general, the open circuit voltage correlates fairly well with the start of the exponential region of the dark characteristic, as is more clearly discernible on

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expanded plots of the low voltage regions of figs. 1 and 2. Evidently from fig.2 p+-i-n + devices are very suitable for generating reasonable values of Voc although high Voc values are also obtained for p+-~-~-n +, p+-~-i-n +, p+-~-i-v-n +, and for most continuously graded junctions. Two examples are given in fig.5 which shows the temperature and intensity dependence of Voc. Of particular interest is the

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of Voc for two junction configurations at a number of

low temperature saturation of Voc observed at higher intensities. In fact, the graded p-n junction approaches saturation values of the order of 1 volt which would be expected from the Fermi level positions on the two sides of the junction. The promising Voc value of O.8V near room temperature given by the p+-~-i-n + configuration is worth noting. High open-circuit voltages alone are of course not sufficient for satisfactory photovoltaic cell performance. A high short circuit current density is also required. Since the photocurrent is carried by minority carriers, the externally collected photocurrent is limited by the carrier with the shorter lifetime. The

R.Ao Gibson et al. / Recent Developments

729

in a-Si Junction Devices

largest Jsc values of about iOmA/cm 2 are found in p÷-i-n + junctions and configurations such as p+-~-v-n + and p+-~-i-n +. On the other hand devices containing a single doped central region (~g. n+-v-p+ or p+-~-n +) produced lower Jsc values. This result would be consistent with the suggestion that the minority carrier lifetime decreases in doped material [12]. SO far continuously graded junctions give Jsc values of only l-2mA/cm 2. A general feature observed for all junctions was the proportionality of Jsc to the light intensity F from 0.O1 - 1 A M - I at all temperatures between 120 and 30OK. Finally we should like to briefly describe some ~ r t h e r investigation of the quality factor n, which characterises junction behaviour. It has been defined by eqn.(1), but can also be determined from a plot of Voc against logJsc according to the well known relation [14],

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e provided that I s c ~ F , and V o c ~ l n F , which means that Voc should lie below its saturation value. For the better photovoltaic devices where Voc tends to saturation~this imposes a severe limitation on the range of intensities which can be investigated. We have therefore ~ p l i e d this approach to the high current devices discussed in section 3 (figs. 1 and 2) which showed a well defined logarithmic dependence of Voc on intensity. Fig.6 shows the results for a p+-~-n+: v= i00 junction. Good agreement with the theoretical ~xpression is obtained over the tempe-

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rature range 120 - 30OK, with n values around l.S. In contrast the n value obtained from the dark characteristfc (eqn.(1)) for the same junction was 2.0. The temperature dependence of the n values determined by the photovoltaic method is summarised in fig.7, for a numb%r of profiles. For those junctions with a Cr/n + bottom contact a room temperature value of n ~ 1.5 was obtained by both methods. However,for those junctions with a Cr/p + contact this was not the case and emphasises the point already made in section 3 concerning the influence of the contact region on the n value. S. CONCLUSIONS In this paper we have reported the effect of various barrier profiles on the properties of a-Si p-n junctions. High current diodes have been prepared from n+-v-p + structures which pass steady currents of 4OA/cm 2 with rectification ratios

730

RoA. Gibson et alo / Recent Developments in a-Si Junction Devices

in excess of 104 . Under AM-I illumination photovoltaic p+-i-n + cells, of areas up to O.5cm 2, generate open circuit voltages of 0.6 to 0.7V, short-circuit current densities up to iOmA/cm 2 and have fill factors of about 0.5, corresponding to efficiencies between 3 and 4%. The quality factors of the diodes at room temperature lie between 1.5 and 2.0 in the dark and around 1.5 under illumination. The results demonstrate clearly the versatility of amorphous p-n junctions, and the possibility of 'tailoring' their barrier profiles for particular applications. ACKNOWLEDGEMENTS The authors are grateful to Stewart Kinmond for sample preparation and for the financial support of the Science Research Council and the Lucas Group Research Centre. REFERENCES

[I]

Spear, W.E. and Le Comber, P.G., Solid St. Commun.

[2]

Spear, W.E. and Le Comber, P.G., Phil. Mag. 33 (1976) 935-949.

17 (1975) 1193-1196.

[3]

Spear, W.E., Le Comber, P.G., Kinmond, Lett. 28 (1976) 105-107.

[4]

Carlson, D.E. and Wronski, C.R., Appl. Phys. Lett. 28 (1976) 671-673.

[5]

Le Comber, 179-181.

[6]

Wronski, C.R., Carlson, 602-605.

S. and Brodsky,

M.H., Appl. Phys.

P.G., Spear, W.E. and Ghaith, A., Electronics Letters 15 (1979) D.E. and Daniel,

R.E., Appl. Phys. Lett. 29 (1976)

[7]

Wilson, J.I.B., McGill, J. and Kinmond,

[8]

Carlson, D.E., IEEE Trans. Electron Devlces,

S., Nature 272 (1978) 152-153.

[9]

For a review of this field, see Wilson, J.I.B., McGill, J. and Weaire, D., Adv. Phys. 27 (1978) 365-385.

ED 24 (1977) 449-453.

[iO]

Jones, D.I., Gibson, R.A., Le Comber, P.G. and Spear, W.E., Solar Energy Materials 1 (1979) at press.

[ii]

Mackenzie, K.D. et al - to be published.

[12]

Wronski, C.R., IEEE Trans Electron Devices,

[13]

See for instance, Lampert, M.A. and Mark, ~: (Academic Press, New York, 1970), p. 25Off.

[14]

Hovel, H.J., Solar Cells: Vol. ii of Semiconductor and Semimetals, Willardson, R.K. and Beer, A.C. (eds) (Academic Press, New York, 1975)p. 58ff.

ED 24 (1977) 351-357. Current Injections in Solids