Fullerene derivatives are commonly used as acceptor materials in organic solar c
ID: 1004429 • Letter: F
Question
Fullerene derivatives are commonly used as acceptor materials in organic solar cells. Compared to most polymer acceptor materials, fullerenes have significantly larger electron mobility in bulk heterojunction devices. a) Explain why this is the case. The optimal ratio that is found in bulk heterojunction polymer-polymer solar cells is often 1:1, while for polymer-fullerene cells this is 1:2, 1:3 or even 1:4. b) Which one of the next reasons is absolutely not valid? Explain your answer. • The size of the polymer is much larger, so you need more fullerene to make a 1:1 volume mixture. • The hole transport in the donor polymer is higher than the electron transport in the fullerene and to balance that more fullerene is needed. • The fullerene can be intercalated in between the side chains of the polymers,which makes that more fullerenes are needed to create a closed network to transport the electrons to the electrodes.Explanation / Answer
a) Fullerene derivatives have an energetically deep lying LUMO (lowest unoccupied molecular orbitals) than the other organic compounds, as a result of which they act as very good electron acceptors.
b) The hole transport in the donar polymer is higher than the lectron transport in fullerene and to balance that out more fullerene is added.
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