Photovoltaic Characterization of Porphyrin and Porphyrin-Fullerene Electropolymer Films

The success of porphyrin-fullerene systems at producing long-lived photoinduced charge separated states^1,2 suggests that these type of systems may be useful for solar energy conversion.  We have begun photovoltaic characterization of the porphyrin-fullerene dyad electropolymer, poly2, and the analogous porphyrin electropolymer, poly1, shown at the top of the Figure.^3,4   Our results show a greater than 10-fold increase in photocurrent and a greater than 3-fold increase in photovoltage for poly2 when compared with poly1.

Our results also demonstrate the importance of interfacial effects on photovoltaic performance.  In our system with electropolymer films deposited on fluorine doped tin oxide (FTO) and contacted on the bottom with a mercury drop, the direction of photocurrent was unpredictable until a suitable surface treatment was found for the mercury drop electrode.  When alkanethiols are used as surface treatments for the mercury drop electrode there is a trade-off between photovoltage and photocurrent, longer alkane chains result in higher photovoltages, but lower photocurrents.  

Treating the mercury drop with hexanethiol consistently produces photocurrents in which electrons flow through the external circuit from the mercury to the FTO.  The dipole moment of the hexanethiol reduces the effective work function of the mercury drop thereby creating a work function difference between the electrodes.  Under short-circuit conditions, this difference in work functions produces an electric field which directs electrons away from the FTO and toward the mercury electrode.^5,6

The incident photon to current efficiency (IPCE) is not proportional to the light harvesting efficiency (LHE) at all wavelengths (see the lower portion of the Figure).  For thicker films of both polymers the photocurrent decreases for wavelengths where light absorption is highest.  This is consistent with photocurrent generation at the polymer-mercury interface.  The films were illuminated through the FTO so thicker films only serve to decrease the number of photons absorbed near the polymer-mercury interface.  Light absorption in organic semiconductors produces Frenkel excitons rather than free electron-hole pairs and the presence of the dominant photocurrent generating mechanism at the negative terminal suggests that either excitons are split more efficiently at this interface or that the electron mobility in both polymer films is significantly lower than the hole mobility.  

References

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[2]  Imahori, H.; Guldi, D. M.; Tamaki, K.; Yoshida, Y.; Luo, C.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 6617–6628. 

[3]  Liddell, P. A.; Gervaldo, M.; Bridgewater, J. W.; Keirstead, A. E.; Lin, S.; Moore, T. A.; Moore, A. L.; Gust, D. Chem. Mater. 2008, 20, 135–142. 

[4]  Gervaldo, M.; Liddell, P. A.; Kodis, G.; Brennan, B. J.; Johnson, C. R.; Bridgewater, J. W.; Moore, A. L.; Moore, T. A.; Gust, D. Photoch. Photobio. Sci. 2010, 9, 890–900. 

[5]  Goh, C.; Scully, S. R.; McGehee, M. D. J. Appl. Phys. 2007, 101, 114503. 

[6]  Yip, H.-L.; Hau, S. K.; Baek, N. S.; Ma, H.; Jen, A. K. Y. Adv. Mater. 2008, 20, 2376–2382.