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Capacitor to Start 3.3V-5V DC Asperating Fan on Solar Radiation Shield

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jumper:
I built a solar radiation shield for my outdoor temp/humidity sensor and installed a 25mm DC fan rated 3.3-5V.  Yet, the fan won't start with 4.2V applied from a solar array. 

Can I "bump" it with a capacitor?  All it takes now is to breath lightly on it or touch it to make it go. 

I'd appreciate a referral spec for a capacitor.

The solar array is comprised of two sets of two 2.1V solar cells with each pair wired in series and with each pair isolated from each other with diodes.  I get 4.2V at the fan when the sun is nearby. (The cells were reclaimed from pathway lights and produce 2.1V each, FWIW. It was fun building the arrays using RTV silicone with paperclips for stability).

The location benefits from some longitudinal  offset of the solar arrays, so it wold be difficult to throw the combined 8.4V at the fan with all of the panels wired in series.

davidmc36:
You have diodes in the circuit?

Where are you measuring voltage?

The diodes will drop voltage by something up to 0.7 volts each ..... depending on type.

jumper:
Diodes, yes.  One IN5819G in the output of each of the two arrays ahead of the fan to prevent voltage loss when either array is shaded.
Voltage is measured at the fan inputs (disconnected)

I see a 0.05V drop across the diode as measured when connected to a 6V deep cycle lead acid battery (6.52V > 6.47V)
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davidmc36:
Some camps at least opine that the diodes would be unnecessary.

"A major advantage of parallel configuration is that the performance of the other panels is not significantly impacted if one panel is shaded or damaged."

Are diodes already included?

jumper:
Yes, the diodes are included as per the diagram. I get 4.2V at the fan even with one shaded array.  The 3.3-5V (advertised) fan just won't start without encouragement.  I used this same approach and the same diodes to add a second, southeasterly facing, solar cell to my Acurite 5-in1. That worked well. 

Maybe I'll remove the diodes, try that, and then try wiring the panels all in series to throw 8+V at the poor fan.  I'd hoped there might a capacitor solution that would kick-start this little induction motor.

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