  The spatial collection efficiency portrays the driving forces and loss
mechanisms in photovoltaic and photoelectrochemical devices. It is defined as
the fraction of photogenerated charge carriers created at a specific point
within the device that contribute to the photocurrent. In stratified planar
structures, the spatial collection efficiency can be extracted out of
photocurrent action spectra measurements empirically, with few a priori
assumptions. Although this method was applied to photovoltaic cells made of
well-understood materials, it has never been used to study unconventional
materials such as metal-oxide semiconductors that are often employed in
photoelectrochemical cells. This perspective shows the opportunities that this
method has to offer for investigating new materials and devices with unknown
properties. The relative simplicity of the method, and its applicability to
operando performance characterization, makes it an important tool for analysis
and design of new photovoltaic and photoelectrochemical materials and devices.
