Energy-Level Alignment at Metal–Organic and Organic–Organic Interfaces in Bulk-Heterojunction Solar Cells

Abstract

Abstract-Ultraviolet photoelectron spectroscopy measurements in combination with the Integer Charge Transfer model is used to obtain the energy level alignment diagrams for two common types of bulk heterojunction solar cell devices based on poly(3-hexylthiophene) or poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene) as the donor polymer and (6,6)-phenyl-C61-butric-acid as the acceptor molecule. A ground state interface dipole at the donor/acceptor heterojunction is present for both systems but the origin of the interface dipole differs, quadrupole-induced in the case of poly(2-methoxy-5-(3',7'-dimethyl-octyloxy)-1,4-phenylene vinylene) and integer charge transfer state based for poly(3-hexylthiophene). The presence of bound electron-hole charge carriers (charge transfer states) and/or interface dipoles is expected to enhance exciton dissociation into free charge carriers, reducing the probability that the charges become trapped by Coulomb forces at the interface followed by recombination.

Photoelectron spectroscopy and modeling of interface properties related to organic photovoltaic cells

“…Cooling to room temperature does not return all of these chains to their pristine conformation, so the Fermi level remains pinned to the lower value, even though the bulk (and surface) polarons retain their delocalized nature, which is possibly even improved by an annealing-induced increase of the macroscopic order in the films as the IP is slightly lowered. Translating these results to P3HT:PCBM blends, we see that annealing in the rr-P3HT case creates a ground state charge transfer complex at the P3HT:PCBM interface with the negative side of the dipole pointing into the PCBM acceptor layer, as annealing at 110 °C or above introduces chains at the interface with E ICT+ values low enough to promote spontaneous charge transfer to the PCBM (E ICT − = 4.2 eV [23], see Fig. 4), which is not the case for the as prepared RT case (rr-P3HT E ICT+ = 4.4 eV > 4.2 eV, hence vacuum level alignment).…”

“…Furthermore, the charge transfer process described by the ICT model will sample the most easily oxidized polymer chains or chain segments on the P3HT side of the heterojunction, and the most easily reduced PCBM molecules at the other side. In this way, the most tightly bound charge transfer complexes that can be created at the interface are already occupied in the (dark) ground state and are consequently not available to participate in the exciton dissociation process following a photon absorption event [23].…”

“…Though PES and inverse PES derived values for donor h + -polarons and acceptor e --polarons can be used in models to predict V OC , either directly [22] or by substituting cyclic voltammetry based estimates [9] of the same properties, the approach is certainly more expensive and time consuming than the optical spectroscopy (or cyclic voltammetry) approach. Where PES measurements clearly do bring an added value is in mapping out the energy levels (E ICT+,-) that determine the potential step, if any, at interfaces [23,24]. As noted, the electrode interfaces have to be optimized so as to minimize losses to the V OC .…”

Interface Design to Improve the Performance and Stability of Solution‐Processed Small‐Molecule Conventional Solar Cells

“…[ 20,29,30 ] Mats et al successfully applied this integer charge-transfer (ICT) model to derive energy-level alignment diagrams for poly[2-methoxy-5-3(3,7-dimethyloctyloxy)-1-4-phenylene vinylene] (MDMO-PPV): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)-and poly(3-hexylthiophene) (P3HT):PCBM-based bulk-heterojunction solar cells. More specifi cally, it is limited by the positive integer charge-transfer state ( E ICT+ ) of the donor (equivalent to E F,h ) and negative integer charge-transfer state ( E ICT-) of the acceptor (equivalent to E F , e ) due to the corresponding electron and hole transfer from the electrode to the active layer.…”

Intermixing Effect on Electronic Structures of TQ1:PC71BM Bulk Heterojunction in Organic Photovoltaics

“…In this way, the small quantity of spontaneous charge transfer electron‐hole pairs occupies the ground state at interface, however, they are not enough to create a potential step at interface as confirmed by UPS, thus avoiding the trap‐assisted recombination via integer charge transfer states followed by the large V oc loss, which is completely different with the case of regioregular poly(3‐hexylthiophene):[6,6]‐phenyl C61butyric acid methyl ester (rr‐P3HT:PC 61 BM) (E ICT+,rr−P3HT < E ICT−,PC61BM ) featuring a huge V oc loss . Moreover, the electron‐hole pairs at TQ1/PC 71 BM interface would reduce the probability of the free charges trapped by Coulomb force and thus enhance the percentage of excitons converted into free charges …”