Simultaneous plasma and thermal treatments of ITO surfaces for organic solar cells

Organic solar cells' power conversion efficiency was improved by combined thermal and CF4 plasma surface treatments of indium tin oxide (ITO). This was further enhanced by adding O2 to the CF4 plasma, which increased atomic fluorine concentration. Organic solar cells were produced with layered structures of ITO, spin-cast thin film of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) mixture (100 nm), and Al (100 nm) as top electrode. While CF4 plasma surface treatment alone significantly improved power conversion efficiency (PCE), from 0.34% to 1.99%, further enhancements occurred with the combined thermal treatment, up to 2.24%. 20% O2 addition to the CF4 plasma resulted in greatest improvement (up to 2.34%) due to optimized surface fluorination. Measured work functions of surface treated ITO increased from 4.80 to 5.17 eV with the plasma and thermal treatments. Such treatments reduce the energy barrier for hole injection at the ITO surface by increasing ITO's work function.