Optimized multilayer indium-free electrodes for organic photovoltaics

Flexible multilayer electrodes that combine high transparency, high conductivity, and efficient charge extraction have been deposited, characterised and used as the anode in organic solar cells. The anode consists of an AZO/Ag/AZO stack plus a very thin oxide interlayer whose ionization potential is fine-tuned by manipulating its gap state density to optimise charge transfer with the bulk heterojunction active layer consisting of poly(n-3- hexylthiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester (P3HT:BC61BM). The deposition method for the stack was compatible with the low temperatures required for polymer substrates. Optimisation of the electrode stack was achieved by modelling the optical and electrical properties of the device and a power conversion efficiency of 2.9% under AM1.5 illumination compared to 3.0% with an ITO-only anode and 3.5% for an ITO:PEDOT electrode. Dark I-V reverse bias characteristics indicate very low densities of occupied buffer states close to the HOMO level of the hole conductor, despite observed ionization potential being high enough. Their elimination should raise efficiency to that with ITO:PEDOT.

P-n junction organic photovoltaics fabricated by all solution processing

We have prepared p-n junction organic photovoltaic cells using an all solution processing method with poly(3-hexylthiophene) (P3HT) as the donor and phenyl-C 61-butyric acid methyl ester (PCBM) as the acceptor. Interdigitated donor/acceptor interface morphology was observed in the device processed with the lowest boiling point solvent for PCBM used in this study. The influences of different solvents on donor/acceptor morphology and respective device performance were investigated simultaneously. The best device obtained had characteristically rough interface morphology with a peak to valley value ∼15 nm. The device displayed a power conversion efficiency of 1.78%, an open circuit voltage (V oc) 0.44 V, a short circuit current density (J sc) 9.4 mA/cm 2 and a fill factor 43%.

Room-temperature tilted-target sputtering deposition of highly transparent and low sheet resistance Al doped ZnO electrodes

Target-tilted room temperature sputtering of aluminium doped zinc oxide (AZO) provides transparent conducting electrodes with sheet resistances of <10 Ω □-1 and average transmittance in the visible region of up to 84%. The properties of the AZO electrode are found to be strongly dependent on the target-tilting angle and film thickness. The AZO electrodes showed comparable performance to commercial indium tin oxide (ITO) electrodes in organic photovoltaic (OPV) devices. OPV devices containing a bulk heterojunction active layer comprised of poly(3-n-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) and an AZO transparent conducting electrode had a power conversion efficiency (PCE) of up to 2.5% with those containing ITO giving a PCE of 2.6%. These results demonstrate that AZO films are a good alternative to ITO for transparent conducting electrodes.

Stable organic photovoltaics using Ag thin film anodes

The interaction at the interface between a metal electrode and photoactive polymer is crucial for overall performance and stability of organic photovoltaics (OPVs). In this article, we report a comparative study of the stability of thin film Ag and indium tin oxide (ITO) as electrodes when used in conjunction with an interfacial PEDOT:PSS layer for P3HT:PCBM blend OPV devices. XPS measurements were taken for Ag and ITO/PEDOT:PSS layered samples with different exposure times to ambient conditions (∼25 °C, ∼50% relative humidity) to investigate the migration of Ag and In into the PEDOT:PSS layer. The change in efficiency of OPVs with a longer exposure time and degree of migration is explained by the analysis of XPS results. We propose the mechanism behind the interactions occurring at the interfaces. The efficiency of the ITO electrode OPVs continuously decreased to below 10% of the initial efficiency. However, the Ag devices displayed a slower degradation and maintained 50% of the initial efficiency for the same period of time.