Large area plastic solar cell modules

Preliminary data on the fabrication of 0.1 m2 polymer solar cells are presented. The process employed screen-printing of an active layer onto an indium-tin-oxide (ITO) electrode pattern (50 Ω square−1) on a 200 μm polyethyleneterphthalate (PET) substrate. After the printing, vacuum coating of an optional layer of C60 and the final aluminium electrode was employed to complete the device. The active layer consisted of poly-1,4-(2-methoxy-5-ethylhexyloxy)phenylenevinylene (MEH-PPV). Chlorobenzene was used as solvent for the screen-printing process. The design of the solar cell module was chosen to employ both serial and parallel connection of individual solar cells. Thirteen individual solar cells with an active area of 7.2 cm2 were thus connected in series. The serial connection was chosen to reduce the current density for the large area employed. A step up in voltage is thus preferable to avoid resistive loss. The parallel connection of seven such rows through a screen-printed silver bus gave a solar cell module measuring 40 cm × 25 cm (0.1 m2). The active area was 65% of the total area. The remaining 35% of the area was used for interconnections between cells and for the separation between rows. The 65% active area was chosen to encompass a good margin for prototyping/research and to keep contact resistances between the cells low. In a fully automated process the active area could perhaps reach 90–99% interval but problems with current extraction and interconnections were found to become very critical. There are obvious shortcomings to this approach but the advantage of low current density is believed to be the biggest problem in efficient energy extraction from the module when no simple method for reducing the sheet resistance is available. In the simple geometry ITO/MEH-PPV/aluminium the module gave an open circuit voltage (Voc) of 10.5 V, a short circuit current (Isc) of 5 μA, a fill factor (FF) of 13% and an efficiency (η) of 0.00001% under AM1.5 illumination with an incident light intensity of 1000 W m−2. A geometry employing a sublimed layer of C60 (ITO/MEH-PPV/C60/Al) improved Voc, Isc, FF and η to 3.6 V, 178 μA, 19% and 0.0002%, respectively. The lifetimes (τ½) of the devices defined as the time it takes for the module efficiency to attain half of its maximum value were found to improve significantly when a sublimed layer of C60 was included between the polymer and the aluminium electrode. The modules were laminated with 200 μm polyethyleneterephthalate (PET) foil to mechanically protect the cells. τ½ values of 150 h were typically obtained. This short lifetime is linked to reaction between the reactive metal electrode (aluminium) and the constituents of the active layer. The modules were tested outdoors in different weather condition (wind, high temperature excursion, rain, snow). Tested during a storm the polymer photovoltaic laminate was subject to vibration stress and deformation and delamination in the organic layer was observed with fast bleaching of the active material. Efficient encapsulation with barriers that has very low oxygen and water permeabilities will be needed before future commercialisation can be anticipated.