Résumé
Interfacial engineering is poised to play a key role in delivering solution-processable organic solar cells that simultaneously feature low cost and high efficiency. Here, we report the strategic design, synthesis and characterisation of phosphonium-functionalised polythiophene homo-(P3HTPMe(3)) and diblock (P3HT-b-P3HTPMe(3)) conjugated polyelectrolytes (CPEs) coupled with either bromide (Br-) or dodecylsulfate (DS-) surfactant counterions, for application as cathodic interlayers in polymer solar cells. The counterion is shown to have a pronounced effect on the properties of the CPEs in solution. Optical studies revealed that the bulkier DS- counterion hinders interchain interactions more effectively, leading to a moderate blue-shift in the absorption and emission maxima. Similarly, small-angle neutron scattering (SANS) studies also indicated that the solution structures, solvent content, and therefore hydrophobicity, were extremely dependent on both the CPE structure and counterion. The effect of the CPE structure on the thermal properties of the CPE-surfactant complexes was also investigated by Rapid Heat-Cool calorimetry (RHC) measurements. CPE-DS complexes were subsequently employed as cathodic interfacial layers and shown to boost the efficiency of PBDTTPD : PC71BM solar cells, leading to enhanced power conversion efficiencies of 8.65% and 8.78% (on average) for P3HTPMe(3), DS and P3HT-b-P3HTPMe(3), DS, respectively. These values are significantly higher (similar to 20%) than those for the corresponding device incorporating a Ca interfacial layer (7.18%), which is attributed to an increase in short-circuit current density. Atomic force microscopy studies revealed distinctions in the adhesion efficiencies of the CPE-DS complexes to the photoactive layer, which is attributed to differences in the relative hydrophobicity of the CPEs in the deposition solution.
This work was supported by CNRS and the Universite de Montpellier. This work was supported in part by Science Foundation Ireland under Grant No. 12/IP/1608. Research in Mons is supported by the FNRS-FRFC and Region Wallonne (OPTI2MAT excellence programme). The authors are also grateful to the National Fund for Scientific Research (F.R.S.-FNRS) in the frame of the FRFC research programme (convention No. 2.4508.12). The University of Mons and Hasselt University co-authors are grateful for financial support by the Science Policy Office of the Belgian Federal Government (BEL-SPO; PAI/IAP 7/05). The HINT COST action MP1202 and French-Irish program "Hubert Curien Ulysses" (31998ZF) are acknowledged for support. This research has been supported by the European Commission under the 7th Framework Programme through the 'Research Infrastructures' action of the 'Capacities' programme. Contract No.: CP-CSA_INFRA-2008-1.1.1 Number 226507-NM13. We thank ISIS and STFC for the allocation of SANS Xpress beamtime.