The Chemical Environment about Cd atoms. in Cd Chemical Bath Treated CuInSe 2 and. CuGaSe 2

Transcription

1 The Chemil Environment out C toms in C Chemil Bth Trete CuInSe 2 n CuGSe 2 P. Fons, K. Skuri, A. Ym, K. Mtsur, K. Iwt, T. B, Y. Kimur, H. Nknishi, n S. Niki Ntionl Institute for Avne Inustril Siene n Tehnology, Umezono, Tsuku, JAPAN Tokyo University of Siene, No Chi JAPAN Astrt Reent reports in the literture hve onlue tht hemil th eposite C (CBD-C) hs enefiil effets on Cu(In, G)Se 2 solr ell performne. Before etile mehnisms explining why this is so n e propose, key question tht must e nswere is where in the lttie the C toms resie. We hve use nerege x-ry sorption fine struture (XANES) to seletively investigte the lol tomi environment out C toms in CBD-C trete polyrystlline CuInSe 2 n CuGSe 2 films. CBD-C trete CuInSe 2 n CuGSe 2 lyers grown on Moote glss sustrtes using the three-stge metho were exite y synhrotron rition t the C L 3 -ege (3537 ev) n the resulting XANES spetr were reore. First-priniples lultion using the oe feff 8.10 show tht the XANES spetr re onsistent with C toms eing lote in tion sites in the CuInSe 2 Preprint sumitte to Elsevier Siene 30 My 2003

2 mtrix. From the urrent t it is not possile to efinitively stte to wht extent C resies on the group I or III sites, ut it is ler tht there is ontriution from oth, not from only Cu sites s hs een previously speulte in the literture. This is lso foun to e onsistent with preitions for ion-exhnge retions whih show tht C is more likely to sustitute for (surfe) In thn Cu. Key wors: x-ry sorption, CuInSe 2, CuGSe 2, C, hemil th eposition 1 Introution CS uffer lyers lote etween the p-type Cu(In, G)Se 2 (CIGS) sorer lyer n the n-type ZnO winow lyer hve eome n integrl prt of high-effiieny polyrystlline CIGS solr ells. Experimentlly it hs een foun tht the highest onversion effiienies our in ells with hemilth eposite (CBD) CS uffers. TEM oservtions hve lso shown tht C interiffusion is oservle for istnes on the orer of 10 nm into the Cu(In, G)Se 2 mtrix.(1) Even though the mount of C inorporte into the ells is minute, lrger mounts of C ompouns re use in the proution proess n s onsequene of this, n effort hs een me to unerstn the resons for the effiy of the CDB-CS uffer s premle to fin lterntive, less toxi sustitutes. Severl reent reports hve onlue tht tretment y CBD-C lone, or in omintion with onventionl CBD-CS eposition results in improvement in either ell performne or reprouiility(2; 3; 4). An outstning question tht must e nswere efore etile investigtion of the unerlying mehnism n egin is where in the CIGS mtrix the CBD eposite C resies. We hve use ner-ege x-ry sorption fine struture (XANES) t the C-L 3 ege (3537 ev) to seletively 2

3 investigte the lol tomi environment out C toms in CBD-C trete polyrystlline CuInSe 2 n CuGSe 2 eposite on Mo-ote so-lime glss sustrtes. For the L 3 -ege, the eletron is initilly in p stte n in the ipole exittion pproximtion, the eletron n e exite to sttes of either s or symmetry in the onution n. The ross setion of n soring tom in XANES epens on the sttering of the generte photoeletron from neighoring toms, thus yieling informtion on the lol tomi environment of the soring tom. The XANES spetrum is lso very sensitive to the reltive rrngement of lol toms ue to the strong ontriutions from multiple sttering effets. It is worth noting here tht for the hlopyrite struture, hnges in the seon oorintion shell (12 toms) must e etete to etermine upon whih tion site C tom resies. As in the ilute limit, C sustituting for n In(Cu) site hs eight Cu(In) neighors of twelve totl, mesurle ifferene in the XANES signl n e expete. Experimentl XANES t were ompre with initio lultions using the reltivisti rel-spe sttering oe feff 8.10 to etermine the most likely lol tomi environment of C.(5) 2 Experiment Both the CuInSe 2 n CuGSe 2 smples use in this stuy were eposite y the three-stge metho onto Mo-ote so-lime glss sustrtes. (6) The two enpoint ompositions of the CuIn 1 x G x Se 2 pseuoinry system were hosen to simplify the theoretil interprettion of the XANES t. The s- 3

4 grown smples were ple into CBD-C hemil th kept t temperture of 40 for 40 minutes. The C preursor use ws CI 2 ( mol/l) in n NH 4 OH 0.5 mol/l solution. After eposition, the smples were then introue into vuum hmer t em line BL11B t the Photon Ftory in Tsuku, Jpn. A linerly polrize x-ry em ws then introue with the polriztion vetor perpeniulr to the smples surfe (glning iniene geometry) to llow for exittion of more smple volume. The energy resolution ws 1.8 ev over the energy rnge use in the experiment. The C-L 3 ege (3537 ev) ws exite n oth totl eletron yiel n fluoresene t were tken. No signifint ifferenes in the C L 3 XANES signls were seen etween the totl eletron yiel (espe epth few tens of nnometers) n fluoresene (espe epth out 3 µm) smples implying tht the C tomi environment ws homogeneous. Ientil CuInSe 2 n CuGSe 2 were lso frite into solr ells in the (glss/mo/cuinse 2 /uffer/i-zno/zno/al ontt) onfigurtion. Cells with no CS uffer lyer, onventionl CBD-CS uffer lyer, n only CBD-C tretment were frite n ell onversion effiienies mesure. 3 Results n Disussion Fig. 1 show oth experimentl t for the C-L 3 ege tken for oth CuInSe 2 n CuGSe 2 CBD-C trete films in fluoresene moe. In ition to the experimentl t the results of theoretil lultions one using the initio oe feff 8.10 re shown for C resiing in ifferent ples in the host mtrix s well s C in metlli n oxie form. The feff lultions were performe using self-onsistent fiel pproh s well s full multiple st- 4

5 tering on luster of rius 0.8 nm. In ition to the lultions shown, itionl lultions (not shown) were one for C(OH) 2 s well s C in CuInSe 2 / CuGSe 2 in tetrherl n otherl interstitil positions. These hve een omitte from the plot for spe resons s they re poor mth to the experimentl t n hene hve een rejete s possile struturl moels. As XANES signls from ifferent C sites omine inoherently, it is pproprite to onsier the oserve spetr to e n verge of the XANES signls from iniviul, rystllogrphilly unique C sites. X-ry Asorption (r. units) (i) P 3 e P 4 CO C C on Se site C on Cu site C on In site Experiment (ii) P 2 P 1 P 3 Experiment C on Cu site C on G site C on Se site P 2 P 1 CuInSe 2 :C CuGSe 2 :C Photon Energy (ev) Photon Energy (ev) Fig. 1. Experimentl n simulte C-L 3 XANES spetr for CBD-C trete CuInSe 2 (i) n CuGSe 2 (ii) films It is ler from Fig. 1(i) tht metlli C n CO, o not show goo greement with the CuInSe 2 :CBD-C experimentl t n n e isre s possile struturl moels. For the se of C inorporting sustitutionlly on Se lttie site, the simulte t hs strong resonne t position () whih is not reflete in the experimentl t llowing rejetion of the Se site s possiility for C inorportion. The two remining possiilities, C 5

6 inorporting on Cu or In site, oth show overll greement with the experimentl t. Although the urrent experiments nnot istinguish etween these two possiilities, set of experiments y W, however, show tht Cu is lso present in CBD-C solution fter exposure to Cu(In, G)Se 2 while no signifint in onentrtion ws etete. (7) The presene of Cu ions in the CBD-C solution is ttriute to omplexing with mmoni ( neessry onition). These results, however, were otine for Cu(In, G)Se 2 power smples expose to CBD-C solutions for perios of up to two weeks (presumly to otin mesurle ion onentrtions), mking the well-known lrge moility of Cu in Cu(In, G)Se 2 signifint ftor. For the se of CuGSe 2 :CBD-C, s shown in Fig. 1(ii), similr tren n e seen, with C pprently inorportion on oth tion sites. As erlier works (3; 2; 4) emonstrte tht the use of CBD-C h enefiil effets similr to CBD-CS, ells were frite from freshly prepre CuInSe 2 sorer lyers with CBD-C, CS, n no uffer lyers. Consistent with erlier results, the onversion effiieny of the CBD-C smple ws out two perent versus pproximtely zero perent for the ell frite without uffer lyer, n out 10% for ell me with onventionl CBD-CS uffer lyer. It shoul e mentione here tht s EPMA nlysis of the surfe of the CuInSe 2 n CuGSe 2 smples tken pproximtely six months fter the XANES mesurements showe strong sptil orreltion of O to C onentrtion, the reson for the improvement of the CBD-C trete lyer my lso e srile to the presene of n hyroxie/oxie of C on the uppermost surfe. 6

7 4 Conlusion We hve mesure the lol struturl environment of C in CBD-C trete CuInSe 2 n CuGSe 2 polyrystlline smples using XANES t the C-L 3 ege n ompre the experimentl results to theoretil simultions. Similr results were foun for oth mterils, nmely tht C most likely resies on the tion sites lone. The urrent experiments nnot istinguish etween whih of two tion sites the C ions resie upon. The presene of C on oth tion sites lso implies the ner surfe lyer my e strongly eletrilly ompenste. Referenes [1] T. Nk, A. Kuniok, Diret eviene of C iffusion into Cu(In, G)Se 2 thin films uring the hemil-th eposition proess of CS films, Applie Physis Letters 74 (17) (1999) [2] J. G. K. Rmnthn, R.N. Bhtthry, J. We, D. Niles, M. Contres, H. Wiesner, F. Hsoon,, R. Noufi, Avnes in the CIS reserh t NREL, in: Proeeings of the 26th Photovolti Speilist Conferene, 1997, pp [3] M.A.Contrers, M. J.Romero, B. To, F.Hsoon, R.Noufi, S. r K.Rmnthn, Optimiztion of CBD-CS proess in high-effiieny Cu(In,G)Se 2 se solr ells, Thin Soli Films (2002) [4] T. W, S. Hyshi, Y. Hshimoto, S. Nishiwki, T. Sto, T. Negmi, M. Nishitni, High Effiieny Cu(In, G)Se 2 (CIGS) Solr Cells with Improve CIGS Surfe, in: Proeeings of the 2n Worl Conferene on 7

8 Photovolti Solr Energy Conversion, Vienn, Austri, 1998, pp [5] A. Ankuinov, J. Rehr, Theory of soli stte ontriutions to the x-ry elsti sttering mplitue, Phys. Rev. B 62 (2000) [6] J. Tuttle, J. Wr, A. Du, T. Berens, M. Contrers, K. R. Rmnthn, A. Tennnt, J. Kene, E. Cole, K. Emergy, R. Noufi, The performne of Cu(In, G)Se 2 se solr ells in onventil n onentrtor pplitions, Mt. Res. so. Symp. pro. 426 (1996) 143. [7] T. W, Stuies of CBD-CS uffer lyers in Cu(In, G)Se 2, privte ommunition. 8