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43075-AEF
Plastic Solar Cells Based on Block Copolymer-Carbon Nanotube Composites
Polymer-based solar cells have gained growing attention as a potentially low cost, lightweight, and solution processable energy conversion platform. Polymer-fullerene composite solar cells are the state-of-the-art in this class of photovoltaics, with power conversion efficiencies approaching 5%. The development of solar cells based on composites of organic polymers and carbon nanotubes is another promising route toward flexible, lightweight photovoltaics that can be processed in a cost-effective manner using such solution process techniques as inkjet printing. Carbon nanotubes are sought as an alternative to the fullerene-based electron acceptors and offer several potential advantages based on their more favorable aspect ratio for generating percolated networks at low weight percentage and the possibility of improved mechanical strength in polymer composites. Relatively little work has been published on nanotube-based solar cells due to the inherent insolubility and sample heterogeneity of carbon nanotubes, which hampers device fabrication. The primary goal of this project is to develop conjugated polymers that are capable of solubilizing carbon nanotubes for the realization of homogenous, bicontinuous composites, but the underlying goal is to gain a deeper understanding of the role of polymer structure in the development of effective organic composite solar cells.
The first stage of research involved the synthesis of new classes of conjugated polymers designed specifically for an enhanced miscibility with carbon nanotubes. In the last year however, work focused on the much more fundamental goal of understanding the role of polymer structure in controlling the morphology of composite organic solar cells. For this goal, polythiophene-fullerne composite solar cells were investigated, based on the improved solubility and sample homogeneity in fullerene acceptors relative to carbon nanotubes. In this way efforts were focused directly on the effects of polythiophene structure on solar cell performance and stability.
This work involved the synthesis of a variety of poly(3-alkylthiophene) polymers and the correlation of the regioregularity (RR) and sequence distribution of the solubilizing alkyl chains with fullerene-composite morphology, photovoltaic performance, and thermal stability in solar cells. The first portion of the project was focused on determining the effect of polymer RR (the percentage of head-to-tail linkages in the polymer) on the thermal stability of polymer-fullerene composites. It was found that, contrary to the accepted picture in the literature, a RR greater than 95% is not necessary for achieving high solar cell efficiencies, and that higher RR values lead to decreased thermal stability in fullerene-composites. It is observed that macrophase separation results upon annealing blends with a 96% RR polymer, but not with a 91% RR polymer. This can be attributed to polymer crystallization-induced phase separation as a primary cause of thermal instability in these composite films, as the highly RR polymer is more prone to crystallize.
The second portion of this project focused on the effect of alkyl chain distribution as a means of probing the effects of long range order in polythiophenes. Here two polymers, PQT-DD and P3DDT-co-T, which both contain 50% dodecylthiophene and 50% thiophene repeat units, but differ in the sequence distribution of repeat units. The regiosymmetric PQT-DD and the random copolymer P3DDT-co-T were used as model polymers, as both polymers have identical chemical composition and electronic structures. The regiosymmetric polymer, PQT-DD, was found to be highly crystalline by XRD, while the random copolymer was found to be amorphous. Importantly, PQT-DD has been previously investigated and found to crystallize in a three-dimensional structure aided by alkyl chain interdigitation. It was found that the photovoltaic performance of P3DDT-co-T was superior to PQT-DD under all investigated conditions. The important point is that PQT-DD is not capable of achieving an effective morphology in fullerene composites due to the strong driving force for polymer crystallization. For P3DDT-co-T, the random backbone structure inhibits the driving force for crystallization and allows the generation of an effective morphlogy.
The conclusions drawn from these two sets of results point to the strong effect of alkyl chain RR and distribution on photovoltaic performance, where subtle changes that do not affect the polymer electronic structure can influence the photovoltaic performance over an entire order of magnitude from 0.5-5% among polymers with identical chemical compositions and electronic structures.
The work done over the past two years has greatly increased my interest in solar energy and the development of a more robust organic photovoltaics platform. Such goals will be pursued in my future research endeavors as an independent academic investigator.
