For very small solar cells, the MAX20361 is the best option as it has a very low input voltage range (225mV to 2.5V) but most importantly it can start up and operate with solar cells that output between 15μW and 300mW of power: MAX20361 Datasheet and Product Info | Analog Devices
For small solar cells with a higher output power rating than 300mW, the LTC3105 is a good option: LTC3105 Datasheet and Product Info | Analog Devices