= Solution
A suitable donor-acceptor heterojunction provides an energetically favourable route from a bound excitation to an interfacial charge-transfer state and then separated carriers. The electron is preferentially transported in the acceptor and the hole in the donor. The interface facilitates dissociation that is inefficient in a single strongly excitonic material; it must also avoid excessive <energy> loss and rapid interfacial <recombination>.
\Image[/past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2017/paper-2-organic-junction.png]
{title=An interpenetrating donor-acceptor structure balances short exciton travel with continuous charge-extraction paths}
{source=paper-2-organic-junction.py}
An idealized <bulk heterojunction> places most generated <excitons> within roughly a <diffusion> length of an interface while maintaining continuous donor paths to the hole contact and acceptor paths to the electron contact. Domain dimensions should therefore be comparable to the material's <exciton> transport scale, not arbitrarily large. The total absorbing thickness should capture substantial light yet remain within the charge-collection capability set by mobility, field and <recombination>. Excessive mixing can impair transport or increase <recombination>; very coarse domains waste <excitons> before they reach an interface.<Tailored exciton diffusion in organic photovoltaic cells by S. Matthew Menke and others>[<Exciton> <diffusion> as a constraint on planar and <bulk heterojunction> dimensions]{ref}
Back to article page
