Insufficient lifetimes of organic photovoltaics are manifested in a reduced photovoltaic response, which is a consequence of physical and chemical degradation of the photovoltaic device. To prevent degradation it is vital to gain detailed insight into the degradation mechanisms. This is possible by utilizing state-of-the-art characterization techniques such as TOF-SIMS, XPS, AFM, SEM, interference microscopy and fluorescence microscopy as well as isotopic labeling (18O2 and H218O). By a combination of lateral and vertical analyses of the devices we obtain in-depth and in-plane information on the reactions and changes that take place in the various layers and interfaces. Examples will be presented that describe the advantages and disadvantages of various characterization techniques in relation to obtaining information on the degradation behavior of complete photovoltaic devices.
The fabrication of very large area polymer based solar cell modules with a total aperture area of 1000 cm2 has been accomplished. The substrate was polyethyleneterephthalate (PET) foil with a pre-etched pattern of indium-tin-oxide (ITO) anodes. The module was constructed as a matrix of 91 devices comprising 7 rows connected in parallel with each row having 13 individual cells connected in series. The printing of the organic layer employed screen printing of a chlorobenzene solution of the active material that consisted of either poly-1,4-(2-methoxy-5-ethylhexyloxy) phenylenevinylene (MEH-PPV) on its own or a 1:1 mixture (w/w) of MEH-PPV and [6,6]-phenyl-C61-butanoic acid methyl ester (PCBM). Our first results employed e-beam evaporation of the aluminium cathode directly onto the active layer giving devices with very poor performance that was discouragingly lower than expected by about three orders of magnitude. We found that e-beam radiation leads to a much poorer performance and thermal evaporation of the aluminium using a basket heater improved these values by an order of magnitude in efficiency for the geometry ITO/MEH-PPV/C60/Al. Finally the lifetimes (τ1/2) of the modules were established and were found to improve significantly when a sublimed layer of C60 was included between the polymer and the aluminium electrode. Values for the half life of 150 hours were typically obtained. This short lifetime is linked to reaction between the reactive metal electrode (aluminium) and the constituents of the active layer.
In this paper we would like to address the key role of fabrication in the performance and lifetime of organic photovoltaics. The realization of a complete process line for the construction of large area organic photovoltaics (250 x 400 mm) is described. Among many of the factors that influence organic solar cell lifetime, oxygen and water exposure is the most important. Multiple processes have to be performed under controlled atmosphere and a glove box (or glove boxes), which involves more volume than commercially available glove boxes, needs to house different instruments. The processes housed in the glove boxes were spin coating, evaporation, lamination/sealing and testing, under an inert atmosphere. The main strategy employed multiply connected glove boxes with one load lock. The first glove box was used for spin coating and lamination/sealing, the second will house a screen printer and the third one accommodate an evaporator completely build in house. The evaporator has 2 thermal evaporation sources and 2 e-beams with 4 and 1 crucibles. The process line should allow the entire device realization from substrate coating, to electrode evaporation including the sealing process avoiding air and water exposure. Organic solar cells from small test cells on ITO glass to big modules (250 x 400 mm) of 91 connected cells on ITO PET substrates were fabricated and characterized.