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.
Light harvesting and energy transfer in two oligomer-dye assemblies has been investigated. In both cases the oligomer was a poly(terphenylenecyanovinylene) derivative while two different dyes was used, a porphyrin and an ionic dye. It is well known that the efficiency of solar cells consisting of a single homopolymer is limited. To increase overall efficiency different strategies have been used. One possible strategy aims at covalently linking different domains. With careful design, this type of assemblies is envisaged to show improved charge separation and charge transport properties. We have shown how photophysical measurements can be used to determine what happens to an exciton formed on any of the domains. From fluorescence and absorption measurements on the assemblies, along with model compounds, it was possible to quantify the number of excitons that are emitted (fluorescence), transferred between domains or lost in internal transfer processes. Both steady state and lifetime measurements were performed in solution and on solid films. The effect of acid was investigated in the cases of the oligomer-porphyrin assembly. We found that in solution the effect of acid was an increase in the time of energy transfer, probably due to acid induced structural change of the porphyrin moiety. It was possible to make LB-films of the ionic dye-assembly, which made it possible to investigate a monolayer of the assembly.
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.
The design of light harvesting systems based on zinc-porphyrin linked polyphenyleneethynylene systems is demonstrated through two different synthetic procedures. The use of the Sonogashira cross-coupling reaction was found to be problematic in several aspects and it was only possible for one of the synthetic strategies to incorporate a single porphyrin molecule in each polymer chain. The polymer materials were separated into fractions using preparative SEC and subjected to photophysical studies in both the solution and in the solid state. In the solid state light harvesting by the pendent polyphenyleneethynylene and energy transfer to the zinc-porphyrin was found to be high whereas it was low in solution. This observation was confirmed when making electroactive devices. I/V characteristics were measured on fabricated solar cells of the polymers. Material properties for the native polyphenyleneethynylene were determined using ultra violet photoelectron spectroscopy on thin films and carrier mobility studies were performed using pulse radiolysis time resolved microwave conductivity.