Hot spots spontaneously emerging in thin film photovoltaics

• New phenomenon discovered and explained: Spontaneous hot spot formation in photovoltaics.
• Numerical modeling captures time, spatial, and temperature scales of hot spot formation.
• Sufficient substrate thermal conductivity can suppress hot spot formation.
• Simple analytic equations characterize some aspects of the hot spot.

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 °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.

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