Enhanced Quantum Efficiencies and Short-Circuit Photocurrents in Solid Porphyrin Thin Film Cells by Internal Electric Fields

We describe here dramatic increases in the observed photocurrents in cells composed of a thin (∼1-2 μm) solid film of zinc octakis(â-decoxyethyl)porphyrin (ZnODEP) sandwiched between two pieces of indium-tin oxide (ITO)-coated glass by applying an electric field to (i.e., poling) the ZnODEP during cell preparation from the molten material. Photovoltaic cells composed of organic molecular crystals, such as porphyrins, phthalocyanines, and dye molecules, generally show low (,1%) quantum efficiencies (Φ, electrons per incident photon) even though these materials have high optical absorption coefficients (a ≈ 105).1 This low efficiency, compared to cells of inorganic materials, can be traced to inefficient dissociation of photogenerated excitons into electron/hole pairs and to the lower mobility of the carriers. We have found, however, that an internal electric field can be produced within the organic crystal film by solidifying it from the molten state in the presence of an applied electric field thus freezing-in trapped space charges or ions and aligning polar molecules and that this field promotes charge separation and transport. The short-circuit photocurrent (Isc) with poled samples was increased by >1 order of magnitude and Φ values of up to 8.4% were attained. Photovoltaic cells of ITO/ZnODEP/ITO were first described by Gregg et al.2 and are prepared in an oven by capillary filling of ZnODEP in its molten state into the empty ITO cells separated by about 1.5-2.5 μm followed by a slow cooling to room temperature. Detailed procedures for the cell fabrication were reported previously.2,3 The steady-state Isc observed with ZnODEP cells occurs by the preferential photoinjection of electrons at the illuminated interface into the ITO.2 The measured Φ for the cells produced in this study by this procedure was 0.5-0.7% under 400 nm irradiation with an intensity of about 1013 photon cm-2 s-1, close to values obtained previously with porphyrins with shorter hydrocarbon chains.2 When a bias voltage was applied between the two ITO electrodes during the cell preparation, the Φ of Isc increased substantially.4 Since cells with different organic layer thicknesses produced different photocurrent levels, the same cell was used in comparative measurements with and without poling. For example, a 1.5 μm thick cell prepared without an electric field yielded an Φ of 0.7% (irradiation at 400 nm at an intensity of 4.9 μW/cm2). However, when this cell was reheated to its molten state and held at 160 °C for about 20 min under a bias of 0.3 V, corresponding to an electric field of about 2000 V/cm, its Φ increased 12 times and reached 8.4% measured under identical conditions (Figure 1). In these measurements, the cell was short-circuited in the dark for 30 s before the irradiation and no current was seen in the dark state when the light was chopped. Cells of different thickness showed similar enhancements in Φ upon poling as shown in Table 1. As seen in previous studies,2,5 Φ increased with decreasing cell thickness. Many heating and cooling cycles with a number of different cells following the same procedure have further confirmed that Φ increased by ∼1 order of magnitude with an electric field of ∼2000 V/cm. Φ might be expected to increase even more with a larger field. However, the cells often became shorted under a larger bias. These shorted cells looked the same as unshorted ones under an optical microscope and could be repaired in many cases by heating again to their molten states in the absence of a field. Cells could be repaired a number of times after shorting without apparent damage. This suggests that the shorts occurred only at isolated points probably caused by impurities or dust rather than by breakdown of the ZnODEP, leading to chemical change and irreversible damage. The observed effect of the electric field can be attributed to several factors. Impurity ions, almost inevitably present in these materials, could be displaced within the organic layer in the presence of the applied bias, causing the buildup of an internal electric field which would increase the probability of light-induced charge separation and charge movement. Polar molecules could also be oriented by the applied field and contribute to an internal one. ZnODEP is a symmetric molecule without a dipole moment and thus its orientation and crystal growth should not be affected by the field. In fact, no morphological difference was seen under an optical microscope between cells prepared with and without an electric field. However, dipoles of impurity molecules could (1) Forrest, S. R. Chem. ReV. 1997, 97, 1793 and references therein. (2) Gregg, B. A.; Fox, M. A.; Bard, A. J. J. Phys. Chem. 1990, 94, 1586. (3) Liu, C.-Y.; Pan, H.-L.; Tang, H.; Fox, M. A.; Bard, A. J. J. Phys. Chem. 1995, 99, 7632. (4) Some preliminary experiments of this type were performed, but the large enhancements shown here were not found. Gregg, B. A. Ph.D. Dissertation, University of Texas, 1988; p 181. (5) Liu, C.-Y.; Bard, A. J. Chem. Mater. 1998, 10, 840. Figure 1. Comparison of quantum efficiency (left scale) and short-circuit photocurrent (right scale) as a function of time for an ITO/ZnODEP/ ITO symmetrical sandwich cell (9 mm2 and 1.5 μm thick) prepared with and without an electric field (2000 V/cm). Calculated quantum efficiency based on incident light intensity, 4.9 μW/cm2.