We present data exhibiting the spontaneous emergence of hot spots in forward biased thin film photovoltaics based on a-Si:H technology. These spots evolve over time, shrinking in their diameter and increasing in temperature up to approximately 300 degrees C above that of the surrounding area. Our numerical approach explores a system of many identical diodes in parallel connected through the resistive electrode and through thermal connectors, a model which couples electric and thermal processes. The modeling results show that hot spots emerge from a rather large area of nonuniform temperature, then collapse to local entities. Finally, we present a simplified analytical treatment establishing relations between the hot spot parameters. The technological importance of our findings is that they open a venue to improving the large area device performance and reliability by properly scaling the device thickness, substrate material, and thermal insulation. (C) 2014 Elsevier Ltd. All rights reserved.