Bulk heterojunctions of conducting polymers and fullerenes (as phenyl-C61-butyric acid methyl ester (PCBM)) have been particularly successful as active layers in organic photovoltaics, however, the currently employed randomly oriented heterojunction morphology has to be reevaluated if organic photovoltaics are to be commercialized for widespread use. To that end, electropolymerization has been used to create dense composites of poly(3-alkylthiophene)s (P3ATs) with carbon nanotubes and PCBM to facilitate exploration of the properties of new bulk heterojunctions with preferential alignment of the thiophene rings parallel to the electrode surface. In conventional, solution-processed P3ATs, the thiophene ring alignment is governed by the interactions between the alkyl side chain and the substrate, which directs charge mobility along the substrate plane. On the other hand, electrochemically grown polymer chains exhibit in-plane orientation of their monomer units. This orientation with monomers stacked normal to the electrode surface can facilitate broad absorbance and promote charge transport to the active layer/electrode interface.
The optical properties of the e-P3HT films were tuned by modifying the stacking orientation of the electrodeposited monomers. Crystals oriented in the (100) direction (thiophene rings perpendicular to substrate) were found in e-P3HT and regioregular, solution cast P3HT, however, orientation in the (010) direction (thiophene rings parallel to substrate) was only present in e-P3HT. This in-plane stacking of the e-P3HT resulted in a broader optical absorbance; however, the lack of long-range conjugation caused a blueshift in the absorbance maximum compared to rr-P3HT. Raman spectroscopy revealed that despite these stacking and conjugation differences, π-π interactions in e-P3HT were comparable to those in regioregular P3HT and significantly higher than in regiorandom P3HT.
The degree of stacking in e-P3HT depended on the applied polymerization potential as well as polymerization time, and an increase in either of these deposition parameters was detrimental to the degree of stacking and the electrical properties of the films. At high polymerization potentials, the faster rate of polymerization does not allow for adequate orientation of the thiophene ring at the growing surface, resulting in a film with more disorder than a film that is deposited slowly. Thick films that were grown for long times showed increased disorder compared to thin films grown under the same conditions. Films that were electrodeposited slowly (at 1 mA cm-2) for a short amount of time (90 s) resulted in a stronger (010) x-ray diffraction peak and lower photoluminescence intensity compared to films that were polymerized at higher current density (fast deposition).
Electropolymerization affords the deposition of thin polymer films that are normally insoluble as chemically synthesized polymers, eliminating the need of solubility enhancers, such as alkyl side chains, that do not contribute to the performance of the polymers. As such, the optical and electrical properties of poly(3-methylthiophene) (e-P3MT) were observed to be superior to those of e-P3HT, showing improvements by at least an order of magnitude in conductivity and by as much as three-orders in mobility compared to films of e-P3HT. Furthermore, the absorptivity and field-effect mobility of e-P3MT was much higher than that of e-P3HT, and similar to that of rr-P3HT. With the added benefit of a facile fabrication method over solution processed polymers, e-P3MT was further considered as a replacement for rr-P3HT in bulk heterojunction active layers.
Finally, thiophene monomers were electrodeposited within carbon nanotube mats on fluorine-doped tin oxide (FTO) to form dense e-P3HT/CNT composites. Scanning electron microscopy and Raman spectroscopy revealed that the polymer was infused throughout the thickness of the CNT mat, resulting in a composite film with a highly dense CNT network. The e-P3HT/CNT composites exhibited photoluminescence quenching with increasing CNT density, providing evidence of charge transfer from the polymer phase to the CNT phase. Higher conductivity and mobilities were also observed in the composite films. Binary and tertiary composites of e-P3MT, carbon nanotubes, and fullerenes were electrochemically deposited into photovoltaic geometries to test their photovoltaic response, demonstrating the promise of this method as a cost and time saving way for device fabrication.