E. Adrián Martín Tovar and R. Castro-Rodríguez Centro de Investigación y de Estudios Avanzados Unidad Mérida “ZnO thin films as buffer and antireflective layer in glass/ITO/ZnO/CdS structures” E. Adrián Martín Tovar and R. Castro-Rodríguez September 26, 2013

Introduction Zinc Oxide (ZnO) is a II-VI semiconductor material with the following properties: A 3.37 eV direct band gap Thin film resistivity in the range of 10-4-1013 -cm Refractive index of 2.03 High transparency in the visible region of the electromagnetic spectrum Crystallizes in three different phases : Hexagonal wurtzite. Zincblende Rocksalt

Thickness of 80-150 nm for wavelenghts in a range of 650- 1200 nm Goals To deposit ZnO thin films using PLD technique for their use as a buffer and anti-reflective layer in Glass/ITO/ZnO/CdS structures using two types of targets. For a buffer layer high resistivities are required 102-103 -cm As an anti-reflective layer: Thickness of 80-150 nm for wavelenghts in a range of 650- 1200 nm

Experimental Procedures Target-substrate distance Vacuum of 10-5 Torr What’s PLD? Laser pulse Nd:YAG =1064 nm ~2 J/(cm2) per shot 5 Hz Number of shots: 3000 Substrate heating Ts = 300° C Target-substrate distance 50 mm

Target (type A) ZnO sintered powder, 99.99% Area 50.3 mm2 Force = 10 Ton T = 600° C (Heating) Time = 2 hours

Target (Type B) Cured powder ZnO, 99.99% Mixed with cyanoacrylate (C5H5NO2) Proportion of 70:30 in weight (70% ZnO, 30% cyanoacrylate) Dried at room temperature for three days

Results and discussions

80-150 nm for wavelenghts in a range of 650 a1200 nm Thickness 80-150 nm for wavelenghts in a range of 650 a1200 nm Sample Thickness (nm) Deposition time (min) Type A 111.5 10 Type B 101.1

Morphological Properties Type A Sample RMS (nm) Type A 17.8 Type B 6.50 Type B

Structural Properties 36-1451 ICCD crystallographic letter associated with ZnO. Hexagonal wurtzite structure. Preferential growth along the c-axis (002) Displacements of diffraction peaks (002) to the left FWHM (2) Type A

Structural Properties 36-1451 ICCD crystallographic letter associated with ZnO. Preferential growth along (100) and (101) planes Hexagonal wurtzite structure. Displacements of diffraction peaks (002) to the right FWHM (2) Type B

Structurales Properties Nano-estructured thin films D for type A suggests a quantum confinement effect A negative sign indicates that the unit cell of type A has a smaller volume than the standard unit cell A positive sign indicates that the unit of type B has a greater volume than the standard unit cell Sample e (%) D (nm) Type A -2.03 9.60 Type B 0.213 38.95 Williamson Hall equation:

Structurales Properties Sample a (Å) c (Å) Volume (Å3) Volume difference (%) Unit cell stress e (%) Standard ZnO 3.2495 5.2069 47.615 Type A 3.1937 5.2384 46.272 -2.82 -2.03 Type B 3.2548 5.198 47.689 0.155 0.213 c b a a

Optical Properties Transmitance 85% Abortion edge isn’t defined UV VIS IR

Band gap Values Sample Experimental Gap (eV) Grain size (nm) Calculated gap (eV) Standard ZnO 3.37 ---------- -------- Type A 4.08 9.60 4.00 Type B 3.32 38.95

Photocurrent Calculations Structure Jsc total (mA/cm2) Loss (mA/cm2) Glass 23.23 2.49 Glass/ITO 21.80 3.92 Glass/ITO/CdS 18.57 7.15 Glass/ITO/ZnOtipoA/CdS 19.04 6.67 Glass/ITO/ZnOtipoB/CdS 19.09 6.63 Elimination of 7.52% of losses Maximum value of 25.72 mA/cm2

Resitivity Measures Measuare at RT Sample Resistence (M) Resistivity (-cm) Type A 10.52 + 0.300 1.17 x102 Type B 12.80 + 0.300 1.30 x102 Measuare at RT For CdS/CdTe Solar cells: 102 -103 -cm

Conclusions ZnO thin films were obtained through PLD by using two types targets. Eg for the films A and B were 4.08 and 3.32 eV respectively Eg sample for type A is the result of quantum confinement effects.

Conclusions The samples were suitable as anti-reflective coating, the presence of ZnO resulted in an increase of about 10% of transmittance in glass/ITO/ZnO/CdS structures. Photocurrent was obtained to be 19.04 and 19.09 mA/cm2 for samples A and B respectively, with a 7.52% elimination of photocurrent losses. The type B film has the best results for an application as buffer layer and anti-reflective.

Thank you!