Ambipolar organic semiconductor blends for photovoltaic cells

Abstract

One material system of interest for photovoltaic cells is the combination of the p-conducting copper-phthalocyanine (CuPc) and the n-conducting fullerene (C 60 ) as donor and acceptor materials, respectively. Therefore the transport properties for diodes containing neat and blended organic films are analysed in the space charge limited current regime. The charge carrier mobilities are found to decrease upon dilution of the respective conducting phase by the other species. Photovoltaic cells can be realised with bilayered or blended organic donor/acceptor films. The influence of both photo-active layer types on the electronic structure and the open circuit voltage is investigated. From photoelectron spectroscopy a higher open circuit voltage is predicted for bilayered solar cells. Due to mixing of the organic materials the intermolecular gap between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor is reduced. This prediction is proven true by photocurrent measurements.

Molecular semiconductor blends: Microstructure, charge carrier transport, and application in photovoltaic cells

“…Of particular interest will be the difference in the open circuit voltage between the two types of solar cells. As reported previously 48, 12, the intermolecular gap between the HOMO of the donor and the LUMO of the acceptor, which is an important parameter for the open circuit voltage of a solar cell, is reduced in blends of CuPc/C 60 in comparison to the bilayered system. This change is related to the gradual shift of the common vacuum level with the composition of the blend 48.…”

“…The field‐activation parameter γ is very low (or even zero) in both cases and therefore negligible 12. The resulting mobilities for CuPc/C 60 decrease exponentially by dilution of the respective transport material with the other species 12. With the used electrode materials, the transport of both charge carrier types is observed in C 60 as well as CuPc.…”

“… (online color at: http://www.pss-a.com/) Schematic energy diagram for the PHJ (left side) and BHJ (right side) of the material system CuPc/C 60 . The transport gap is 2.3 eV for both neat materials 48, 49, which is also assumed for the mixture 50, 12. The Fermi energy is related to the conductive substrate PEDOT:PSS.…”

“…A number of DA pairs have been implemented in molecular blends for BHJ solar cells. Fullerene C 60 has been used as acceptor with different donor materials such as phthalocyanines 11, 12, pentacene 13, 14, or oligothiophenes 15. Also some perylene derivatives were used as acceptor for solar cells 9.…”

(Photo‐)conduction measurements during the growth of evaporated bulk heterojunctions of a subphthalocyanine donor and a perfluorinated phthalocyanine acceptor

“…Mixed absorber layers of donor–acceptor blends were successfully used also as absorber layers in p‐i‐n‐ cells using wide‐bandgap transport layers as contact materials 24, 25. Evaporated donor–acceptor blends with PcCu and C 60 26 or PcZn and substituted perylene biscarboximids (PTCDI) 27 were studied and proposed as evaporated organic bulk heterojunction cells to provide a unifying approach to the existing very successful strategies.…”