Evidence for an accumulation layer at the donor-acceptor interface in organic solar cells

Expected band bending at typical donor-acceptor interfaces in solar cells suggests an accumulation layer could develop for sufficiently high bias, though this is rarely discussed in the photovoltaic literature. There is evidence from bilayer organic field effect transistors as well as organic light emitting transistors that high mobility interfacial accumulation layers exist [1], as well as a report on unexpectedly long diffusion lengths in a solar cell [2]. Our model of dark current in solar cells, which carefully considers multiple physical contributions to the total current, suggests the role of an accumulation layer in supporting observed high current densities. Here we present bulk electrical measurements on bilayer transistors, along with scanning Kelvin probe force microscopy capable of measuring band bending and density of states. Materials studied include CuPC/NTCDA bilayer transistors.

[1] H. Wang and D. Yan (2010), “Organic heterostructures in organic field-effect transistors,” NPG Asia Materials, 2(2), 69-78.
[2] Q. Burlingame et al. (2018), “Centimetre-scale electron diffusion in photoactive organic heterostructures,” Nature, 554, 77-80.