Thanks all for sharing how you're making your PV arrays and antennae! I haven't flown a balloon yet, but I work with solar cells a lot. Here's how I plan to make my first balloon array:
1. I'll dice the cells by digging a trench with a dremel(tm) cutting wheel from the back. Then I'll break the cell. Fracture should cause fewer shorts at the pn junction than a sawed edge will. The pn junction is at the front of the cell.
2. I'll measure the I-V curve of each diced cell under simulated sunlight and reject any with a fill factor (FF) less than 70%. FF is Vmp * Imp / (Voc * Isc) (mp means maximum power point, the pair of I-V points that give the highest I*V product). I'll set the light intensity so that Isc is approximately 40 mA/cm^2 * the cell area for this test.
3. I agree the cells are hard to solder to. What seems to work for me is to apply a tiny bit of flux to the cell, then melt some solder onto my wire (I would use #28 gauge here). Then gently press the blobbed wire to the fluxed spot and touch it with the soldering iron until it yields, very much like what Mikael wrote. It's best to minimize the amount of time the solar cell is hot. I'll test each cell after soldering to see whether the soldering ruined it.
To gather power earlier in the morning and later in the evening, I'm thinking of making a dual array on a folded surface, shaped like a gable roof. I'll measure the I-V curve of each subarray in partial darkness to determine whether the dark array will absorb current from the illuminated one, and if it will, install a schottky series diode with each sub-array to prevent such losses. Has anyone else already gone down this road?