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Why does exciton dissociation occur at heterojunctions?

While designing heterojunctions for solar cells, I rely on this dissociation process for efficiency. Textbooks state it happens due to energy level offsets, but that feels like a thermodynamic answer. I'm curious about the instantaneous, local force or interaction that overcomes the exciton's binding energy and pulls the charges apart at that specific interface.

 

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By Aarushi Duneja Answered 9 months ago

Having modeled many of these interfaces, I'd say the textbook energy offset is a necessary condition, but the real-time driver is a combination of factors. Primarily, it's the sudden drop in potential energy when the electron senses the lower-lying LUMO of the acceptor material this is an energetic "push." Concurrently, the spatial separation breaks the strong Coulombic binding. I've also seen that local molecular disorder and interfacial dipoles create intense, nanoscale electric fields that actively pull the charges apart once the initial transfer occurs. It's this abrupt change in the electronic environment that does the work.

 

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