  Solar-to-fuel direct conversion devices are a key component to realize a full
transition to a renewable-energy based chemistry and energy, but their limits
and possibilities are still under large debate. In this review article, we
focus on the current density as a fundamental figure of merit to analyse these
aspects and to compare different device configurations and types of solar fuels
produced from small molecules such as H$_2$O, CO$_2$ and N$_2$. Devices with
physical separation of the anodic and cathodic zones, photoelectrochemical-type
(PEC) or with a photovoltaic element integrated in an electrochemical cell
(PV/EC), are analysed. The physico-chemical mechanisms involved in device
operation that affect the current density and relations with device
architecture are first discussed. Aspects relevant to device design in relation
to practical use are also commented on. Then discussion is moved towards the
relevance of these aspects to compare the behaviour in the state-of-the-art of
the conversion of these small molecules, with focus on solar fuels from H$_2$O,
CO$_2$ and N$_2$ conversion, highligthing the gaps and perspectives of such
technologies. The still significant lack of crucial data, notwithstanding the
extensive literature on the topic, has to be remarked on, particularly in terms
of the need to operate these cells in conjunction with sun concentration (in
the 50-100 sun range) which emerges as the necessary direction from this
analysis, with consequent aspects in terms of cell and materials design to
operate in these conditions. The work provides a guide for the optimisation of
the investigated technology and the fixing of their practical limits for
large-scale applications.
