  We show that thin film diode structures, such as photovoltaics and light
emitting arrays, can undergo zero threshold localized thermal runaway leading
to thermal and electrical nonuniformities spontaneously emerging in originally
uniform systems. The linear stability analysis is developed for a system of
thermally and electrically coupled two discrete diodes, and for a distributed
system. These results are verified with numerical modeling that is not limited
to small fluctuations. The discovered instability negatively affects the device
performance and reliability. It follows that these problems can be mitigated by
properly designing the device geometry and thermal insulation.
