The power bank is usually a bank of individual LiIon or LiPo cells in series (or parallel and series depending on the current they output). They have a specific charging profile with specific current and voltage values needed for charging. Without this specific profile in your charge circuitry you could damage the battery. There are specific chips available for this such as this one from Adafruit.
What you need is something like this between the power bank and the solar panel. Even though the solar panel outputs 5W, that maybe a best case estimate and you should do your own tests in varying amounts of sunlight. While the panel may give 18V, you should see if the charger circuitry can accept 18V. If you are looking for sophisticated technology to track and extract the maximum power the panel can output, TI sells specialized chips for charging from solar panels such as this one.
