Necessary voltage to charge NiMH using Solar Panel
You must have at minimum a battery charge controller because you need a constant current mode. You also need something to make sure you don't overcharge the battery and start a fire. To adequately use a circuit like this you will also need  something to stabilize the voltage from the solar panel called an MPPT tracker to make sure the voltage is constant. 


Source: https://www.mpoweruk.com/chargers.htm