Hooray for tinnovation! From instructables user mr electro:

This Solar Powered AA Battery Charger was made from vintage parts I had in stock. The solar cell is an early 1970’s hobbyist solar cell put out by International Rectifier. The transistors are RCA 2N404 Germanium transistors which were manufactured in 09/65. These transistors were probably the most popular transistors manufactured in the USA, in the late 50’s to the late 60’s due to their extensive use in the computers of that era. I decided to use germanium transistors because the solar cell only puts out 1 volt (in bright light) and I wanted to get the best efficiency. Germanium transistors are turned on at .25 volts compared to silicon transistors which are turned on at .7 volts. The charger will start working at voltages substantially below 1 volt (lower light conditions) albeit with lower output, but if I used silicon transistors, it would barely work at 1 volt. This circuit can be duplicated using silicon transistors, such as PNP 2N3906 or NPN 2N3903, if two solar cells are used. This circuit can then be fed with higher voltages with the resultant higher output current to charge bigger batteries than the AA type. If the circuit is to be used at higher voltages with silicon transistors, I would change the delicate 1N34 signal diode to a Shottky diode such as an 11DQ09.

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