Application of Atomic Layer Deposition for Fabrication of Solar Cells

Abstract

Atomic layer deposition (ALD) is exceptional deposition technique because it allows for growth of high quality films on large are substrates at low deposition temperatures. In our contribution ALD was employed for preparation of transparent conducting electrodes and for encapsulation of fabricated solar cells. Al-doped ZnO films were used for transparent conducting electrodes. Deposition was carried out at temperatures between 150 and 250 °C on Si and quartz substrates. Diethyl zinc and trimethyl aluminum were used as precursors and water vapors as reactant. Al-doping was performed by inserting Al2O3 cycles in ZnO growth. Deposition of 1 Al2O3 layer per 7 deposited ZnO layers gave the best results. Resistivity of the Al-doped ZnO films depended on the deposition temperature and film thickness. Transition electron microscopy revealed typical columnar growth of the films with fine grained polycrystalline region close to the substrate. The films with the thickness above 100 nm prepared at 250 °C showed resistivity of 1 mΩcm. This corresponds to the sheet resistance of 40 Ω/square. Hall measurement revealed electron concentration of 4*1020 cm-3 and mobility of 15 cm2Vs-1 for the films with resistivity of 1 mΩcm. Work function of 4.4 eV was measured by Kelvin probe. Optical transmittance of the films was higher than 80% in the wavelength range between 400 and 900 nm. Al-doped ZnO transparent electrodes were used for fabrication of perovskite-type conventional planar solar cell. CH3NH3 PbI3 perovskite was used as absorbing layer, Spiro-OMeTAD as hole transport layer and Au/Ag as top contact layer. Properties of the solar cell with Al-doped ZnO electrode are compared with inverted planar solar cell using indium tin oxide as a transparent electrode. Inorganic-organic hybrid perovskite solar cells suffer from poor stability in ambient atmosphere. Oxygen and moisture are believed to be reason for the solar cell degradation. To protect the cells against ambient atmosphere influence ALD prepared Al2O3 films were employed for encapsulation. Trimethyl aluminum was used as precursor while either water vapors or ozone were applied as reactants. Deposition of the Al2O3 films took place at 50°C. Power conversion efficiency of the solar cell encapsulated by 24 nm Al2O3 thin film deposited using water vapors as reactant decreased to 60% while unprotected solar cell degraded down to 10% of its initial value after 20 days in ambient atmosphere. In our contribution we compare properties of perovskite solar cell encapsulated by ALD Al2O3 films prepared using water vapors or ozone as reactants.