Structural and electronic properties of pentacene-fullerene heterojunctions

Abstract

In this study the performance differences of layered and bulk-heterojunction based organic solar cells composed of the prototypical p-and n-type organic semiconductors pentacene ͑PEN͒ and fullerene ͑C60͒ are correlated with the physical properties of the heterostructures. The electronic structure of layered and codeposited thin PEN and C60 films on the conducting polymer substrate poly͑ethylenedioxythiophene͒:poly͑styrenesulfonate͒ ͑PEDOT:PSS͒ was investigated with ultraviolet photoelectron spectroscopy. Layered structures of C60 on PEN precovered PEDOT:PSS exhibited an offset of the highest occupied molecular orbital ͑HOMO͒ levels of 1.45 eV. In contrast, codeposited films of PEN and C60 showed a reduced HOMO-level offset of 0.85 eV, which increased to 1.45 eV by precoverage of the substrate with a thin PEN layer. In this case, the PEN-HOMO level was Fermi-level pinned at 0.35 eV binding energy and charge transfer between PEN and PEDOT:PSS decreased the vacuum level by 0.75 eV. In addition, the morphology and crystal structure of the respective systems have been investigated by atomic force microscopy ͑AFM͒, x-ray diffraction ͑XRD͒ and Fourier-transform infrared spectroscopy, which indicated pronounced phase separation of PEN and C60 in the codeposited films. XRD revealed crystalline growth of PEN in all investigated cases forming crystallites that exceeded the nominal film thickness by an order of magnitude, whereas C60 was crystalline only if grown on the PEN precovered substrates. AFM investigations allowed to correlate morphology and structure revealing micro-and nanophase separation between PEN and C60.

Electronic Structures of a Well-Defined Organic Hetero-Interface: C₆₀on Pentacene Single Crystal

“…Accordingly, the present findings of the absence of the vacuum level shift and band bending are consistent with the theoretical predictions. Taking into consideration that the "head-on" orientation is thought to be dominant in the pilot OPV devices with a planar pentacene-C 60 heterojunction [40], which was reported to yield rather better power conversion efficiency than mixed heterojunction OPVs consisting of this materials' combination [15], the present results obtained at a structurally well-defined molecular heterointerface provide realistic insights into electron energetics in the pentacene-C 60 OPV devices.…”

“…They proposed ∆ DA of 1.56 eV and an upward vacuum level shift of 0.36 eV toward the C 60 side. Salzmann and co-workers studied this bi-molecular systems on a conductive polymer film to find the gap width of 1.15 eV and an absence of the vacuum level shift [15]. However, since reported V oc values for this molecular combination is less than 0.5 eV [35], even if an energy conversion loss of 0.3 − 0.5 eV [36,37] due to the binding energy of the electron-hole pairs is taken into account, the ∆ DA values of these previous works may be overestimated to explain the practical systems.…”

“…Interface formation of this couple is interesting also in terms of their distinct molecular symmetries, namely pentacene has a molecular shape of a uniaxial elongation whereas C 60 is of the spherical symmetry [13]. A number of experimental and theoretical studies so far addressed energetics and dynamics of the inter-molecular contact of these two materials [14][15][16][17][18][19][20]. Since calculations have predicted that the electronic energy levels and intermixing tendency strongly depend on the contact orientation i.e.…”

Organic/Organic′ Heterojunctions: Organic Light Emitting Diodes and Organic Photovoltaic Devices

“…Another option, however, is the creation of ''multi-heterojunction'' cells where two or more donors are deposited in contact with an acceptor which provides for exciton dissociation and photocurrent production from two or more O/O 0 heterojunctions in parallel. [136,246,247] A third option being explored is to use thin layers of donors and acceptors arranged to promote both vectoral flow of spectral energy and charge flow, from one donor to the other, while broadening the spectral response of the OPV. Figure 6 shows the first step in this process with the 60 , pentacene/C 60 , and pentacene/TiOPc/C 60 heterojunctions.…”

“…[19,26,34,52,72,134,135] For both bulk heterojunction (BHJ) polymer OPVs, and small molecule PHJ OPVs, it has been observed that the molecular environment can have a significant effect on these energies, helping to control the probability for photocurrent production. [23,136,137] The power conversion efficiency, h, of an OPV is ultimately determined by the product of V OC and the J SC , and by the fill-factor (FF) ( Figure 1C):…”