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Solar & Alternative Energy

Solution-processed titanium oxide increases the efficiency of polymer solar cells

An architectural innovation that reallocates light intensity within plastic solar cells results in better-performing devices.
28 November 2006, SPIE Newsroom. DOI: 10.1117/2.1200611.0496

Photovoltaic cells based on polymer-fullerene composites are promising potential sources of renewable energy.1–4 Because they offer the advantages of large area, light weight, and flexibility at a low cost of fabrication, efficient ‘plastic’ solar cells would have a major impact on the energy industry. However, the performance of plastic solar cells is still inadequate for commercial use.

One of the factors that limits plastic solar cell efficiency is optical interference between the incident and back-reflected light at the metallic electrode, such that the light intensity is extremely low for a relatively large fraction near the electrode, as shown in Figure 1(a).5–7 This effect also causes more electron-hole pairs to be produced near the hole-collecting electrode, a distribution known to reduce photovoltaic conversion efficiency.8,9 Optical interference is especially problematic in thin-film structures, in which layer thicknesses are comparable to the absorption depth and wavelength of the incident light, as is the case for photovoltaic cells fabricated from semiconducting polymers.

Figure 1. Distribution of the squared optical electric field strength |E|2 inside the photovoltaic devices with a structure consisting of ITO/PEDOT/active-layer/Al (a) and ITO/PEDOT/active-layer/optical spacer/Al (b). The shaded region in (a) denotes the dead zone, as explained in the text. ITO: Indium-tin oxide. PEDOT: Poly(3,4-ethylenedioxythiophene).

One approach to this issue is to change the architecture to redistribute the light intensity inside the device by introducing an optical spacer between the active layer and the Al electrode: see Figure 1(b).7 Although this revised architecture would appear to solve the problem, the prerequisites for an ideal optical spacer limit the choice of materials: the layer must be made from a material that is a good electron acceptor and that has a conduction band edge that is lower in energy than the lowest unoccupied molecular orbital (LUMO) of C60; the LUMO must be above (or close to) the Fermi energy of the collecting metal electrode; and it must be transparent to light with wavelengths within the solar spectrum.

We have used a solution-based titanium oxide (TiOx) layer as an optical spacer fabricated on top of the polymer-fullerene active layer: see Figure 2(a). Dense TiOx films were prepared using a TiOx precursor solution.10 The resulting TiOx films are transparent and smooth with surface features smaller than a few nanometers. We then fabricated donor/acceptor composite photovoltaic cells using a phase-separated bulk heterojunction material comprising poly(3-hexylthiophene) (P3HT) as the electron donor and the fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) as the acceptor.

Figure 2. (a) The TiOx optical spacer layer is inserted between the active layer and an Al electrode. (b) The component energy levels of this photovoltaic cell exhibit excellent band matching for cascading charge transfer.

Figure 3(a) compares the incident photon to current collection efficiency spectrum (IPCE) of devices fabricated with and without the TiOx optical spacer. The IPCE is defined as the number of photogenerated charge carriers that contribute to the photocurrent per incident photon. The conventional device (without the TiOx layer) shows a typical spectral response for P3HT:PCBM composites with a maximum IPCE of ∼60% at 500nm, consistent with previous results.11 Results for the device with the TiOx optical spacer demonstrate substantial enhancement of ∼40% in the IPCE over the entire excitation spectral range. The corresponding data obtained under AM1.5 illumination from a calibrated solar simulator also shows an ∼50% increase in short circuit current with power conversion efficiency of around 5%; see Figure 3(b).

Figure 3. (a) External quantum efficiency (or incident monochromatic photon to current collection efficiency, IPCE) spectra for devices with and without a TiOx optical spacer layer. (b) The current density-voltage characteristics of polymer solar cells with and without TiOx optical spacer under AM1.5 illumination from a calibrated solar simulator with an intensity of 90mW/cm2.

We attribute this enhancement to increased absorption in the bulk heterojunction layer as a result of the TiOx optical spacer. The TiOx layer increases the efficiency ∼50% by modifying the spatial distribution of the light intensity inside the device, thereby creating more photogenerated charge carriers in the bulk heterojunction layer.

Kwanghee Lee
Center for Polymers and Organic Solids, University of California at Santa Barbara
Santa Barbara, CA, 
Department of Physics, Pusan National University
Busan, South Korea