I doubt Smartphones will make serious in-roads to the African market soon. Do not forget that most of the phones used in Africa are not new devices and have had several owners. Given most modern smartphones are rather fragile items and sensitive to moisture & dust, I doubt they will have that much longevity. (Think of how indestructible feature phones were a decade ago and mostly are today) Having a non-replaceable battery would rule out a lot of current smart phone devices too.
Also, the top "features" for a phone in Africa are Torch and Radio, neither of which are top on the list of smartphone features.
You are correct, power is a big problem with smartphones. "Power" comes from people who ride a bike (or walk) into a city and purchase a car battery that is charged. Or connect to the village's car battery connected to wind/solar electricity sources. The people peddling around with car batteries is also where top-up credit is purchased from.
Nokia's moniker for this market was "Internet for the Next Billion", trying to raise the level of device in use beyond basic GSM handsets. Nokia were well aware that the relationship with this market was one that was at arm's length. Perhaps a village could pool resources to purchase a phone (Nokia sell kits for longer range external antennas, solar/car battery charging points, etc) but that was about as direct a relationship Nokia had.