• Energy Research
• natural sciences close

We report a detailed study on the operating principles of silver nanowire (Ag NW) networks as a transparent conducting electrode. Current limitations to achieve high conversion efficiencies are overcome with the combination of Ag NW and specifically designed ZnO nanoparticles (NP). The new transparent electrode presents improved optoelectronic properties with respect to the most widely used ITO—indium tin oxide. We demonstrate that there are non-conducting void spaces left between neighboring nanowires onto the holding substrate as a consequence of processing the Ag NW film from a solvent dispersion. Moreover, the fabrication of efficient ITO-free organic solar cells (OSC) based on Ag NW relies on filling the resulting void spaces with a highly selective and conductive supporting thin layer. This slight modification of the Ag NW film enables the great majority of photogenerated charge carriers – that would otherwise be recombined at voids – to be efficiently collected by the nanowires. In this way, we report – to our knowledge – one of the highest ITO-free OSC (3.8%) also fabricated onto flexible substrates (3.2%) with commercially available Poly (3-Hexylthiophene) (P3HT) and Phenyl-C61-butyric acid methyl ester (PCBM). This breakthrough could once and for all bring the potential of flexible low-cost organic photovoltaics into reality.

We report organic photovoltaic devices in which the standard ITO transparent electric contact has been substituted by lower cost ultrathin metallic electrodes. Solution and vacuum processable n and p-type semiconductors provide the electrode with the rectifying behavior of the diode. We are in this way able to invert the built-in electric field at wish and make the device deliberately either top or bottom sensitive with the same efficiency depending on the application. Taking advantage of these new generation electrodes we furthermore report devices with fill factors over 70%—to our knowledge, the largest published to date for an organic photovoltaic cell—and power conversion efficiencies over the state-of-art with 3.5% in inverted P3HT:PCBM devices, ITO free designs over 2.5% and (semi)transparent photovoltaic devices with conversion efficiencies close to 2.6%. This breakthrough could once and for all trigger the fabrication of organic tandem solar cells and photovoltaic windows.

-

Abstract We present a study of dark air-exposure degradation of organic solar cells based on photoactive blends of the conjugated polymer, poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Photovoltaic devices were fabricated on indium tin oxide (ITO) glass with or without a layer of poly (3,4-ethylenedioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS), and were studied without encapsulation. Photovoltaic performance characteristics were measured as a function of time for different ambient conditions (under white light irradiation and in the dark, and under air, dry oxygen and humid nitrogen atmospheres). It was found that a key cause of degradation under air exposure is light independent and results from water adsorption by the hygroscopic PEDOT:PSS layer. Measurements of the charge mobility and hole injection after air exposure showed that the degradation increases the resistance of the PEDOT:PSS/blend layer interface.