MAPLE deposition of polymer multilayer structures: overcoming the limitation of solvent-based techniques

Full text: Polymer multilayer deposition represents a serious challenge for solvent-based techniques like spin coating and drop casting since the deposition of even the second layer profoundly affects the first one. To overcome this problems, orthogonal solvents are generally used. However, this strategy, when applicable,limits the deposition to only two layers. The potentialities of the matrix-assisted pulsed laser evaporation (MAPLE) technique have been already demonstrated in the case of bilayer deposition for solar cell application and nanocomposite films. However, in many cases, the high surface roughness of the films deposited by MAPLE could inhibit the possibility to use it for particular applications, like for example in optoelectronics. In this paper we report the deposition of a trilayer structure consisting of three light emitting conjugated polymers in a well-defined sequence: a first spin-coating deposited layer of the blue emitting polymer Poly[9,9-dioctylfluorenyl-2,7-diyl] (PFO), a second MAPLE-deposited layer of the red emitting polymer Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4- phenylene-vinylene](ADS100RE) and athird MAPLE-deposited layer of the green emitting polymer Poly[(9,9- dioctylfluorenyl-2,7-diyl)-co-(1,4-diphenylene-vinyl- ene-2-methoxy-5-{2-ethylhexyloxy}-benzene)] (ADS125GE). The bottom most layer has been deposited by spin coating because of geometrical constraints of the target holder. The properties of each single layer are reported in terms of surface roughness values and optical/emission properties,investigated by atomic force microscopy (AFM), UV-VIS absorption and photoluminesce (PL) spectroscopy,respectively, together with the behavior of the bi and tri-layer realization. The absorption and photo-emission properties, together with the efficiencies and decay times, of the MAPLE deposited single layers have been compared with that of the films deposited by spin coating obtaining comparable results. By carefully controlling the laser deposition parameters, layers with roughness values compatible with optoelectronic devices have been obtained. In particular, roughness values less than 7 nm have been obtained for the MAPLE-deposited red (ADS100RE) and green (ADS125GE) single layers and for the bilayer PFO/ADS100RE. In the case of thetri-layer structure a mean roughness value of about 10 nm has been recorded over a scan are of 5×5 µm2 and photoluminescence spectrum exhibiting emission by all the layers. Our results are very promising for the realization of a white light polymer-based emitting device. (author)