Chemical Bath Deposited ZnS Thin Films for CuIn(S,Se) 2 Solar Cells

Transcription

1 Chemical Bath Deposited ZnS Thin Films for CuIn(S,Se) 2 Solar Cells K. Ernits, M. Danilson, K. Muska, O. Volobujeva, M. Altosaar Chair of Semiconductor Materials Technology Department of Material Science Tallinn University of Technology Estonia Motivation ZnS (zinc sulfide): n-type semiconductor material, wide band gap (3.67 ev * ), environment friendly, cheap (compared to CdS, In 2 S 3 ) CBD (chemical bath deposition): simple and nonvacuum method for growing gthin films good coverage * D.R. Lidle, Handbook of Chemistry and Physics, 78th edition, , p rd Nordic PV Conference 2 1

2 Sequential organization of studies ZnS was deposited from different Zn-sources: Zn(CH 3 COO) 2 ZnSO 4 onto various substrates SLG, Mo, CuInS 2 modifying ph in acidic and basic solutions. CuIn(S,Se) 2 solar cell parameters with ZnS and CdS bufferlayers were compared rd Nordic PV Conference 3 Characterization 1. ZnS thin films Thickness, coverage, morphology (SEM) Composition (XPS, XRD) Optical properties (Spectrophotometry) 2. ZnS/CuIn(S,Se) 2 solar cells Current-Voltage characteristics (I-V) Spectral response rd Nordic PV Conference 4 2

3 Chemical bath solution for ZnS deposition Acidic solution Basic solution ZnSO 4 ZnSO 4 Zn(CH 3 COO) Concentration (mol/l) CH 3 CSNH 2 CS(NH 2 ) H 2 SO NH 4 OH NH 4 OH According to ph Solution temperature Deposition time 85 o C 25 min Stirring Vacuum annealing (0.013 Pa) 220 o C for 10 min rd Nordic PV Conference 5 Chemical route to ZnS deposition ZnS growth through the decomposition of the complexes : 1) 2Zn SC(NH +4NH ) 2 [Zn(SC(NH 2 ) 2 ) 4 ] + [Zn(NH 3 ) 4 ] 2) [Zn(SC(NH 2 ) 2 ) 4 ] OH - ZnS + CN 2 H 2 + 2SC(NH 2 ) 2 + 2H 2 O 3) [Zn(NH 3 ) 4 ] 2+ + SC(NH 2 ) 2 + 2OH - ZnS + 4NH 3 + 2H 2 O + CN 2 H 2 4) Zn OH - Zn(OH) 2 (ph = * ) 5) Zn(OH) +2OH [Zn(OH) 4 ] (ph = * ) 6) Zn(OH) 2 ZnO + H 2 O (t = 220 o C) * Краткий справочник по химий, издательство химия rd Nordic PV Conference 6 3

4 Chemical data Zn complex Zn[L] n Stability constant pk 1 (25 o C) * Compound Solubilty product pk sp (25 o C) * ZnS 26.1 Zn[OH] Zn(OH) Zn[NH 3 ] Zn[SO 4 ] 2.3 ZnSO Zn[CH 3 COO] Zn(Ac) * Справочникр химика III, издательство химия 1964 [ZnSO 4 ] has higher stability constant (pk 1 ) than [ZnNH 3 ] 2+ and Zn[CH 3 COO] +. Does stronger [ZnSO 4 ] complex slow down ZnS deposition rate and affect the film properties? rd Nordic PV Conference 7 CBD-ZnS ph variation Acidic solution (ZnS/CIS) Zn salt is ZnSO 4 ph = 1.7 Thickness d = 24 nm Deposition time t = 20 min ph = 2.5 d = 4 nm t = 20 min rd Nordic PV Conference 8 4

5 CBD-ZnS ph variation Basic solution (ZnS/CIS) Zn salt is ZnSO 4 ph = 10 d = 6nm t = 6 min ph = 11 d = 14 nm t = 18 min ph = 11.5 d = 12 nm t = 22 min With solution ph = 11, the ZnS film was crowing thicker than on ph = 10 and with shorter time than on ph = rd Nordic PV Conference 9 CBD-ZnS zinc source: ZnSO 4 vs Zn(CH 3 COO) 2 on different substrate Zn(Ac) 2 t = 30 min ZnSO 4 t = 30 min ZnS on Mo/SLG ZnS on SLG ZnS on CuInS 2 Magnification for all x rd Nordic PV Conference 10 5

6 ZnS grain size, thickness and composition on Mo and on soda lime glass (SLG) ZnS on Mo ZnS on SLG Zn salt Grain size (nm) Film thickness (nm) Grain size (nm) Zn:S (At%) Zn(Ac) :21 ZnSO Equipment: High-resolution scanning electron microscope (HR-SEM) Zeiss ULTRA 55 Energy dispersive X-ray (EDX) analysis (Röntec EDX Xflash 3001) detector rd Nordic PV Conference 11 Elements ZnS composition study with X-ray photoelectron spectroscopy (XPS) Spike C28 5 (ev) ZnS (at%) ZnS: Ac (at%) Zn 2p 3/ S 2p 3/ O 1s M-O OH ZnS: SO 4 (at%) Equipment: Kratos Axis Ultra DLD XPS (Al Kα mono-chromatized radiation) The atomic ratio for acetic ZnS measured by XPS is ~ 60:40 (Zn:S), additionally Me-O 530 ja -OH 532. Films contain ZnS, ZnO and Zn(OH) 2. ZnS from ZnSO 4 contains more oxygen than ZnS from Zn(Ac) rd Nordic PV Conference 12 6

7 X-ray diffraction study (XRD) 1x10 5 9x10 4 XRD for chemically deposited ZnS 8x10 4 7x10 4 ZnS: (Ac) 2 asgrown Zn salt is Zn(Ac) 2 Intensity (cps) 6x10 4 5x10 4 4x10 4 3x10 4 2x10 4 1x10 4 ZnS: (Ac) 220 o C10 min. vac 2 Equipment: XRD Rigaku Ultima IV theta (degree) The results for XRD measurements show that asgrown and annealed ZnS films are amorphous rd Nordic PV Conference 13 ZnS versus CdS CdS d = 21 nm t = 25 min Cd salt is Cd(Ac) 2 Zn salt is Zn(Ac) 2 2 x ZnS d = 20 nm T = 2 x 18 min rd Nordic PV Conference 14 7

8 Optical properties 100 ZnS and CdS optical transmission Optical Transmissio on (%) ev 2.3 ev ZnS: Ac ZnS: SO 4 CdS: Ac Wavelength (nm) rd Nordic PV Conference 15 Kile Optical properties Thickness (nm) Band gap (ev) ZnS: Ac Optical transmission (%) nm ZnS: SO CdS: Ac Equipment: Jasco V-670 UV-VIS-NIR spectrophotometer with an integrating sphere rd Nordic PV Conference 16 8

9 Spectral response (SR) measurements on MGL CuIn(S,Se) 2 solar cells 1.0 Monograin layer CuIn(S,Se) 2 /buffer solar cells.u) Spectral Response (a ZnS: Ac ZnS: SO 4 CdS: Ac Wavelength (nm) rd Nordic PV Conference 17 Current-voltage (I-V) measurements on MGL CuIn(S,Se) 2 solar cells monograin layer CuIn(S,Se) 2 /buffer J (ma/cm 2 ) ZnS: Ac ZnS: SO 4 CdS: Ac V (V) rd Nordic PV Conference 18 9

10 Current-voltage characteristics of MGL CuIn(S,Se) 2 solar cells V OC FF J SC η Solar cell structure (mv) (%) (ma/cm 2 ) (%) CIS/ZnS: Ac CIS/ZnS: SO CIS/CdS CIS/no buffer rd Nordic PV Conference 19 Results The thickest ZnS film grew at solution ph = 11. Deposited films are amorphous and contain ZnS, ZnO, Zn(OH) 2. Zn(Ac) 2 versus ZnSO 4 : ZnS from ZnSO 4 has smaller grains and thinner films, good coverage of the CIS surface. ZnS from Zn(Ac) 2 has bigger grains and thicker films, the films look dense and cover more continuously, similar to CdS. CISSe/ZnS MGL solar cell efficiency was 5 % when ZnS was deposited from Zn(Ac) 2 and 4.5 % from ZnSO 4 containing bath. ZnS versus CdS: Optical transmission is higher for ZnS than for CdS in short wavelength area. Spectral response is higher in short wavelenght area for ZnS/CISSe solar cells. ZnS/CISSe MGL solar cell currents, fill factors and efficiencies are higher than solar cells with CdS in this experiment rd Nordic PV Conference 20 10

11 Conclusions CBD ZnS films can be beneficially used as buffer layers for CuIn(S,Se) 2 solar cells. Zn(Ac) 2 as a zinc source results in higher ZnS growth rate, thicker ZnS films with bigger grains and lower oxygen content compared with ZnS from ZnSO 4 source. The higher stability of [ZnSO 4 ] complex in comparision with [ZnNH 3 ] 2+ complex in CBD solution can limit the growth rate of ZnS from ZnSO 4 containing bath rd Nordic PV Conference 21 Thank you very much! rd Nordic PV Conference 22 11