The backing layer dependence of open circuit voltage in ZnO/polymer composite solar cells

Different layered ZnO/MEH:PPV composite solar cells have been fabricated to assess the role of the ZnO backing layer on the open circuit voltage of nanowire composite solar cells. Comparisons between cells using a ZnO layer prepared by spray pyrolysis and by sputtering and oxidising a Zn layer are compared. Cells with a sputtered Zn layer which is then oxidised show a significantly improved open circuit voltage compared to cells with a blocking layer prepared by spray pyrolysis. The thickness dependence of the blocking sputtered blocking layer is investigated. A 130 nm ZnO layer gives in a cell configuration with MEH PPV an open circuit voltage of 0.41 V, which decreases with thicker ZnO layers to 0.28 V at 650 nm. Simultaneously the EQE reduces from 7.2% at 130 nm ZnO to 2% for the 650 nm thick films. The increase in open circuit voltage of sputtered and oxidised layers of Zn compared to spray pyrolysis ZnO layers is attributed to a dense and pinhole free film for sputtered films, whilst the reduction in EQE is attributed to charge carrier reductions in the thicker films. The sputter ZnO/MEH:PPV devices have been shown to have reproducible I V characteristics over many pixels indicating the high quality of the sputtered ZnO films.