Role of ZnO nanostructured layer spray deposited under an electric field in stability of inverted organic solar cells

Spray process under an electric field was used for the deposition of zinc oxide (ZnO) electron transport layer by applying a DC voltage to the nozzle in order to make the layer smoother, highly crystalline and defect free having good electrical and optical properties. The nanostructure formed differed from that in the spray deposited ZnO layer without applying any voltage. The photovoltaic performance of the inverted organic solar cells (IOSC - ITO/ZnO/P3HT:PCBM/Ag) made using the nanostructured layer (deposited with the applied voltage = 1000 V) and the cell stability were found to improve in comparison with the devices made using the nanostructured layer with the applied voltage = 0 V. A change in the surface morphology of ZnO layer occurred with time and its effect on the cell performance was investigated. Photoluminescence (PL) spectra as well as XPS spectra showed that defects related mainly to oxygen vacancies were reduced in the spray deposited ZnO layer with the applied voltage = 1000 V. This resulted in a higher IOSC efficiency (~2.7%) and good stability for a long duration (~105 days) with only 20% degradation in PCE. On the other hand, there was a 46% degradation in PCE of IOSC using spray deposited ZnO layer without applying voltage.