  Modern photovoltaic devices are often based on a heterojunction structure
where two components with different optoelectronic properties are interfaced.
The properties of each side of the junction can be tuned by either utilizing
different materials (e.g. donor/acceptor) or doping (e.g. PN Si junction) or
even varying their dimensionality (e.g. 3D/2D). In this work we demonstrate the
concept of phase heterojunction (PHJ) solar cells by utilizing two polymorphs
of the same material. We demonstrate the approach by forming
$\gamma$-CsPbI3/$\beta$-CsPbI3 perovskite PHJ solar cells. We find that all of
the photovoltaic parameters of the PHJ device significantly surpass those of
each of the single-phase devices, resulting in a maximum power conversion
efficiency of 20.1%. These improvements originate from the efficient
passivation of the $\beta$-CsPbI3 by the larger bandgap $\gamma$-CsPbI3, the
increase in the built-in potential of the PHJ devices enabled by the energetic
alignment between the two phases and the enhanced absorption of light by the
PHJ structure. The approach demonstrated here offers new possibilities for the
development of photovoltaic devices based on polymorphic materials.
