Lewis acids and bases as molecular dopants for organic semiconductors
Bridges, Colin R.
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Controlling the concentration of charge carriers (mobile electrons and holes) in organic semiconductors is vital to precisely controlling their electronic properties. Significant efforts have gone into understanding how molecular dopants induce charge carriers in organic semiconductors. The most widely used doping mechanisms occur via electron transfer (i.e., oxidation or reduction of the semiconductor) or via reaction with a strong Brønsted acid. Recently, strong Lewis acids have been observed to induce p-type charge carriers in organic semiconductors with greater efficiency than classical dopants. The mechanism of Lewis-acid doping could not easily be unified with either classical doping methods and has been under intense scrutiny over the past 5 years. Very recently, the Lewis-acid doping effects have been shown to be due to water impurities in commercial Lewis acids forming strong Brønsted acids. This means that many studies on doping using Lewis acids may be occurring via a Brønsted-acid doping mechanism. This recent revelation explains some observations in literature, but not all, and there are still unanswered questions. The nature of the Lewis acid and organic semiconductor can significantly impact the doping mechanism and the doping efficiency. Additionally, strong evidence for alternative doping mechanisms using Lewis acids not involving water has been shown. Lewis-acid doping has mostly been studied as a p-type dopant method on Lewis-basic polymers. There is growing literature showing Lewis bases can also act as n-type dopants, excluding Brønsted-acid doping as a possible mechanism. In this tutorial review, we will present a brief overview on molecular doping of organic semiconductors, survey the literature on p-type and n-type Lewis doping, outline several proposed mechanisms, and speculate on some possible mechanisms using literature observations.
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