Vacuum and Liquid-Phase Processing of ZnSe Buffer-Layer for Chalcopyrite Absorber Based Photovoltaic Technology

Abstract

ZnSe thin films were processed in-vacuum by EBD and in-solution by CBD for comparative studies of CdS-buffer replacement of ZnO/CdS/Cu(In,Ga)Se 2 /Mo/glass solar-cells. ZnSe films deposited by EBD, at e-beam energies 11keV, on polycrystalline CuGaSe 2 , at 400 • C, were microcrystalline, with average grain-sizes 100nm, and (111)zinc-blende(ZB)-structure. The films exhibited compressive mismatch-stresses σ = −0.9 GPa. The absorption coefficient α = 6.2 × 10 3 cm −1 and the energy band-gap E g = 2.69eV indicated optical transparency suitable for buffer/solar-absorber configurations. In CBD, zinc-sulfate_(ZnSO 4 ) solution with selenourea_(SeC(NH 2 ) 2 ), ammonia_(NH 3 ), hydrazine_(NH 2 NH 2 ), and sodium-sulfite_(Na 2 SO 3 ) were processed at solution-bath temperature 70 • C. The growth-rates of ZnSe on glass_(1.2nm/min), epitaxial_CuGaSe 2 /GaAs(001)_(7.4nm/min), and polycrystalline_Cu(In,Ga)Se 2 /Mo/glass_(17.6nm/min) were strongly facilitated by substrate order, orientation, and conductivity. Mismatch-stresses switched from compressive σ = −0.5GPa of ZnSe_on_epitaxial_CuGaSe 2 /GaAs(001) to tensile σ = 1.1-1.9GPa of ZnSe_on_polycrystalline_Cu(In,Ga)Se 2 /Mo/glass. Film crystallinity was improved by post-growth annealing (300 • C,2h,Ar/N 2 ). The energy band-gap E g = 2.73-2.87eV was broadened with the increase of annealing-time. ZnSe films were deposited by CBD on polycrystalline Cu(In,Ga)Se 2 with ( 101)wurtzite(WZ)-structure. Following process-optimization, high quality ultra-thin films with (ZB)-structure were produced. The quality assessment of ZnSe films processed by cost-effective, easy-applicable, fast-rate, large-area, moderate-temperature CBD competes with that of EBD processed films and opens the perspective for strain-free CIGS TFSCs by lattice-matching of ZB/WZ-ZnSe bilayer to chalcopyrite sub-layer and ZnO over-layer.

Engineering of Optical and Electrical Properties of Electrodeposited Highly Doped Al:ZnO and In:ZnO for Cost-Effective Photovoltaic Device Technology

“…Similar applies to ZnSe (buffer layer of CIGS TFSCs) deposited by electron-beam evaporation (EBD) and characterized by PR (Eg(ZnSe)=2.69 eV) and ZnSe grown by chemical bath deposition (CBD) and characterized by TR (Eg(ZnSe)=2.73 eV) in the author's study cited as Ref. [4]. The surface reflectivity of In:ZnO/ZnO on ZnSe/Mo/glass, in Figure 3a, is significantly lower than the reflectivity of Al:ZnO/ZnO.…”

“…The lattice mismatch between the hexagonal ZnO template (a = b = 3.251 Å) and the cubic Mo substrate (a = 3.146 Å) is ~3%. ZnSe is deposited with wurtzite structure (hexagonal ZnSe) on amorphous glass and polycrystalline substrates [4]. The lattice-and thermalmismatch of the ZnO layer (a = 3.25 Å, α th = 4.31 × 10 −6 K −1 ) and the hexagonal ZnSe buffer (a = 3.98 Å, α th = 7.8 × 10 −6 K −1 ) is ~20%.…”

“…The field of ZnO research and applications has widely been scanned, in the past and at present, as already reported in our recent publications [1][2][3][4]. Our efforts are primarily focused on the engineering of the structural, optical, and electrical properties of chalcopyrite semiconductor based thin-film solar cells (TFSCs) [5][6][7][8][9][10][11][12][13] with Cu(In,Ga)Se 2 (CIGS) absorber [1], ZnSe buffer [4], and ZnO window, front-contact, and antireflective coating (ARC) [2,3] processed by electrochemical deposition (ECD) techniques. Electrodeposition with cost-effective, large-area, moderate-temperature, fast-rate performance can be scaledup to industrial processes.…”

“…Electrodeposition with cost-effective, large-area, moderate-temperature, fast-rate performance can be scaledup to industrial processes. Overall processing of CIGS chalcopyrite selenide absorber [1], ZnSe buffer with alternate cubic/sphalerite and hexagonal/wurtzite structure to relax (elastic) strain/stress [4] in the layer sequence, ZnO window of highly to ultra-highly doped ZnO bilayer [3] to effectively accelerate and collect carriers, and ZnO-Nanorod (ZnO-NR) ARC [2] by ECD is targeted to overcome current process incompatibilities. Common disadvantages of flow-oriented production of commercialized CuInS 2 (CIS) and CIGS TFSCs result mainly from the simultaneous use of moderate temperature (50-70 • C) non-vacuum and high-temperature (500-700 • C) vacuum processes applied to the absorber (multi-step evaporation at elevated temperatures [14,15]), buffer (chemical bath deposition at moderate temperatures [16][17][18][19][20]), and window (sputtering at high temperatures [21][22][23][24][25][26]) layers.…”

“…Electrochemical deposition (ECD) techniques have similar prospects for application in material synthesis and large-volume production, yet distinctly superior rating for costeffective overall processing of efficient chalcopyrite semiconductor based thin-film solar cells (CIS/CIGS TFSCs) extensively analyzed in our previous reports [1][2][3][4][5][6][7][8][9][10][11][12][13] of the List of References . ECD is a reliable method for fabricating, with high reproducibility, excellent-quality n-type ZnO material with high carrier concentration, high carrier mobility, and low resistivity, as demonstrated in our previous reports [2,3] and the current study.…”