Chaudhuri, P

Das, U K

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,lia„ J- Phys. 71A (3), 279-283 (1997)I J P  Aan international journalReduction of microstructures in amorphous silicon by A r  dilution : Influence on stability of solar cellsU. K. D as an d  P. C haudhuriEnergy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, C alcutta 700 032.A b s tra c t:  The hydrogenated amorphous silicon thin films have beendeveloped by Ar dilution in rf plasma enhanced chemical vapour deposi­tion. The change in film microstructure is associated with the metastable Ar bombardment to the growth surface. The microstructural heterogene­ity in the intrinsic amorphous silicon film is liable to cause the light in­duced degradation of electron transport in films as well as in p-i-n solar cells.K ey w o rd s : Microstructure, Light induced degradation, Solar cell P A C S N os. : 81,15.Gh, 81.40.-z, 73.40.Lq1. IntroductionIt, is believed th a t  th e  am orphous silicon based  p-i-n single ju n c tio n  so­lar cell degrades considerab ly  under prolonged light illum ination  owing to  th e  degradation of e lectronic p roperties  of the  in trinsic  (i) am orphous silicon m ate­rial. In th a t co n tex t several m odels have been proposed  for th e  light induced instability in a-Si:II [1,2]. M odel proposed by S tu tzm an n  et a l [2] explains that the dangling  bond  density  in th e  m ateria l increases due to  b reaking  of t hr weak Si-Si bonds in to  two dangling  bonds by tak ing  energy from  non ra d ia ­tive recom bination of p h o to  excited carriers. T h e  hydrogen bonded  in vicinity of the weak bond plays a crucial role in stab iliz ing  th is process. T he increase of hydrogen con ten t in th e  film is associa ted  w ith  th e  increase of m icrostruc- t uro param eter R  which is defined by R  =  > w here / 20O0 and  /2080are the areas u n d er th e  deconvoluted 1R abso rp tio n  sp ec tra  a t wave num bers 2 0 0 0 c m 1 and  2080cm -1 corresponding  to th e  s tre tch in g  m odes of S ill and SiH} respectively [3]. T h e  increase of R  consequently  Enhances th e  light - degradation of th e  m ateria l.In this s tu d y  we have developed in trinsic  am orphous silicon m ateria l us­ing A r  as a  d ilu en t gas in  r f  glow discharge decom position of silane (SiH<i).© 1997 IACS280 U K Das and P ChaudhuriT h e  m ic ro s tru c tu res  in th e  film as e s tim a ted  from  IR  an d  Sm all Angle Jf-ray S ca tte r in g  s tudy , varies w ith  A r d ilu tio n . T h e  lig h t-d eg rad atio n  of electron!, q u a lity  of arSi:H  films an d  perfo rm ance of solar cells seem  to  have some c,)r re la tio n  w ith  th e  m ic ro s tru c tu re  in th e  m ateria l.2. ExperimentalT h e  a -S i:/f  films an d  single ju n c tio n  so lar cells w ere p repared  in a ra- pacitively  coupled r f  (13.56 M Hz) glow discharge cham ber w ith  background! p ressu re  of ~  10-9 T o n . T h e  o th e r deposition  p a ram ete rs  foT a-S i:H  films and th e  i-layer o f p —i — n so lar cells are  th e  r f  power density , 35m W /cm 2; pressure0 .5T orr and  su b s tra te  te m p e ra tu re , 250°G\ T h e  A r d ilu tio n  was varied from I 0% (u n d ilu ted ) to  99%. T h e  films on C orning 7059 glass su b s tra te s  were used for s tu d y in g  d eg rad a tio n  o f p h o to co n d u c tiv ity  un d er w hite  ligh t illumination i (in ten sity  of 1 0 0 m W /rm 2). T h e  single ju n c tio n  p  — i  — n  so lar cell was fain ca ted  w ith  i-layer d ep o sited  by A r d ilu tion  on T C O  coa ted  glass substrata T h e  solar cell efficiency an d  th e  fill facto r was ob ta ined .from  I  — V curve under | a  class 'A '  so lar s im u la to r (W A CO M ) w ith  A M 1.5G inso lation . Iiigli purin A1 foil was used as a  su b s tra te  for s tu d y in g  SAXS. T h e  sam ples on crystalline | Si (111) wafer were used for I R  s tudy. \3. f ResultsT h e  to ta l bonded  hydrogen co n ten t ( C h ) in a-Si J I  m a te ria l was calculated from  th e  in teg ra ted  in tensities for S i—H  wagging m ode cen tered  a t  630cth 1Table 1. Variation of bonded hydrogen content (Ch) and microstructures as rsti mated from IR (R) and SAXS (Q) study.Argondiln(%)C H(at%)R Q .(1022 ru /e rn 3)0 7.0 0.13 6.085 19.6 0.10 2.290 18.7 0.095 . 1.295 22.0 0.07 3.899 35.7 0.17 11in I R  sp ec tra . T h e  value of (7#, m ic ro stru c tu re  p a ram ete r R. and the int|L| g ra ted  SAXS in ten sity  Q  a t different A r  d ilu tions are listed in T a h i t i . I value of Q  is o b ta in ed  from  th e  re la tion  [4],Q = Jqh(q)dq ~Reduction o f microstructures in amorphous silicon by Ar dilution etc 281where q is th e : sca tte rin g  vector (q =  t A is the  wavelength of incident.Y-ray) and  l n {q) is th e  X -ray sca tte ring  in tensity  due to the  nanostruc tu ral features of size range betw een 1 -  30n m  for a particu la r q. T hus th e  to ta l integrated SAXS in tensity  Q  reflects the electron density variation on nano scale. T he  hydrogen content increases significantly w ith the addition of A r  in the p lasm a (from 7at%  for undiluted to  ~  20at% for th e  A r  d iluted a- Si://) (Table-1). T he am ount of m icrostructures as predicted from SAXS (Q) and I R ( R )  exhibit a  gradual decrease w ith A r  dilution up to 90 -  95%. Thus an appreciable im provem ent in s tru c tu ra l heterogeneity (in term s of rnicrostructures) of a-S i:H  films can be achieved by varying A r  to SiH4 ratio. HoweveT, fu rth er increase of A r  d ilu tion Is associated w ith the  increase in ( 'H, R  [5] and Q.D egradation of photoconductiv ity  (<Jph) of a -S i:// films is found to  be very much dependen t on rn icrostructures in the m aterial. T he degradation of o ph and hence th e  device param eters of single junction  p -i-n  solar cell fabricated using th e  m ateria l as an i-layer increases with th e  increase of Q. In figure 1 we have shown changes in the values of photoconductiv ity  (crph), efficiency (rj) and (ill factor (F F ) on lOOOhrs light soaking by th e  ra tio  of the  light soaked values (with ’s ’ w ithin th e  parenthesis) of these param eters to the ir in itial values (with ’i ’ w ithin th e  parenthesis) with respect to  m icrostructu re p aram eter Q. Stability of these param eters for m aterial and solar cell decreases as Q  of the undoped layer increases.to0.90.80.7"OccdF igure  1. The stability parameters of material (<tyi(s)/<tyt(i)) and solar cell {il(s)/n(i) and FF(s)/FF(i)) as a function of Q.282 U K Das and P Chaudhuri4. DiscussionIn th e  p lasm a Ar is fractionally  raised to  th e  excited  s ta te  A r * (3Po 2) and  ionised sa te  A r +  by collision w ith  electrons. R eac tion  ra te s  of A t * and A r+ w ith  S i H t  are 2.6 x 10 10cm 3/s e c  and  1.18 x 10“ u cm 3/s e c  respectively E quilibrium  densities of A t * and A t + in th e  p lasm a depend  upon  S i H 4 to Ar flow ra tio . T he density  of A r*  rises gradually  w ith increase in A r d ilu tion  up to  90% and  beyond th a t  it rises sharp ly  [6J. On the  o th er hand , density of A r j rem ains alm ost constan t over th e  en tire  range of Ar d ilu tion . Bom bard­m ent of th e  growing film by A r*  and also varies in th e  sam e m anner with Ar d ilu tion . It was argued th a t bom bardm ent of A r *  on th e  film surface is m uch softer th an  th a t  of A r+ . A t * im pinges th e  film surface w ith thermal velocities and gives its  excess energy to  the grow th zone while deexcitation. T his energy is absorbed by the  la ttice  and  is used in th e  form ation of more com pact netw ork. On the o ther hand , bom bardm ent by A r+ being assisted by the  coulom bic a ttra c tio n  of the  anode is a  violent process and  creates more defects. T he A r*  bom bardm ent to  the  growth surface reduces th e  num ber of weak Si-Si bonds and consequently  th e  electron density  fluctuations (hetero­geneity) in the m ateria l which decreases th e  value of Q. M oreover th ^  increase of bonded hydrogen for At dilu ted  films ind icate  th a t an appreciably  amount of broken weak Si-Si bonds arc passivated  by hydrogen. T he excess einergy re­leased by A r * may also lead the  weak Si-Si bonds to  reconstruc t in to  stronger Si-Si bonds. Both the  processes resu lt in an u ltim ate  decrease of m icrostrur- tu res (t^ and H) in the film. At high A r  d ilu tion  (>  90%) the enorm ous rise in Ai'* bom bardm ent on to the  grow th surface helps som e regions to grow denser (m ore ordered netw ork w ith strong  Si-Si bonds) which are devoid of hydrogen. However, the  boundary  of these dense regions consists a lot of hydrogen and s tru c tu ra l defects. As a consequence the heterogeneity in the  film and  hence the value of Q  a t high A r  d ilu tion  increases.T he  d etrim en tal effect, of these n an o stru c tu ra l features in a rS i:// m aterial is evident, from fig .l. As the  value of Q  increases the light induced degradation  of pho toconductiv ity  of a-Si:H  m ateria l increases. T he surfaces of nanostructures contain  lot of clustered m onohydride or dihydride which eventually  weaken the nearby Si-Si bonds. T his increase of weak bonds causes an enhanced  instabil­ity of photoconduct ivity of the film in conform ity w ith the  S tu tz m a n n ’s bond breaking model [2]. Thus there appears an ap p a ren t relation  betw een light induced instab ility  in a -S i:i/ film and th e  am ount of m icrostuctu ra l features in th e  film. T he enhanced degradation  of intrinsic a -S i:// layer under light illum ination  effectively reduces the  short, circuit, cu rren t density  and  the fill factor of single ju nc tion  p — i — n solar cell. T hus the stabilised solar cell effi-Reduction o f  microstructures in amorphous silicon by A r dilution etc 283ciency can be significantly improved by reducing the heterogeneity in intrinsic (active) layer.5. Conclusions(1) The m icrostructures in a-Si:H films as determined from Infra Red study and small angle X-ray scattering techniques, can be reduced significantly by using Ar as a diluent in plasma enhanced chemical vapour deposition.(2) The variation of microstructure with Ar dilution can be explained on the basis of variation in Ar*  bombardment on to the growth surface.(3) The reduction of film microstructure leads to less light induced degra­dation in the material properties as well as the solar cell device performance.AcknowledgementsThe authors are grateful to Dr. Don Williamson, ("dorado School of Mines, USA for measuring SAXS of our samples and Prof. A. K. Barua, Kriergv Research Unit, IAOS for constant encouragement, in completing this work. One of the author(UKD) acknowledges the financial support from ("SIR, India.References|1] I). Adler 7. de Phys .  42-C4 3 (1981).[2] M. Stutzinann, W. B. .Jackson and C. C. Tsai Phys.  Rev. B  32 23 (1985).13] B. Bhattacharya and A. II. Malian Appl .  Phys.  Let t .  52 1587 (1988).[4] I). L. Williamson (Mat. Res. Soc. Spring Meeting, USA, 1995)[5] Partha Ohaudhuri and Ujjwal Kumar Das Jpn .  J. Appl .  Phya.  34 3467 (1995). [fi] U. K. Das, P. Ohaudhuri and S. T. Kshirsagar 7. Appl .  Phys.  80 (1 N<>\, 1996).

Reduction of Microstructures in Amorphous Silicon by Ar Dilution : Influence on Stability of Solar Cells

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Reduction of Microstructures in Amorphous Silicon by Ar Dilution : Influence on Stability of Solar Cells

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