Almost all are buck converters so they cannot boost voltage. Many of the MPPT controllers recommend you hook the
solar cells up in series, this gives them the potential to charge better in low light conditions, but in practice it produces little gain. Hooking the cells in series also creates the downside that there is a disproportionate loss in
power and MPPT point when one panel has partial shade. With a typical
boat installation partial shade on one panel is very common so the losses (from having the cells in series) generally out way the gains (from having a higher solar voltage)