Transition-metal complexes photosensitize organic solar cells

The search for new sources of renewable energy is a key area of research. Solar energy is one of the most promising potential sources because it is essentially nonexhaustive. Conventional solar cells are based on silicon, an inorganic semiconductor. However, silicon has high production and processing costs, and the weight and rigidity of this type of cell limits its applications in mobile devices. Since the 1980s, researchers have demonstrated that organic compounds can be used as active materials in photovoltaics.^1–3 Large-area organic devices can be produced relatively cheaply because of their flexibility and processing method.

However, organic solar cells still do not perform as well as their silicon counterparts. This is due to several factors, including narrow optical absorption in the visible/near-IR range and low carrier mobilities.⁴ Transition-metal complexes are well known as catalysts in many chemical reactions. In addition, they are promising candidates for light-absorbing species (photosensitizers) because careful ligand design and systematic variation of the metal center can tune optical absorption. They exhibit long-lived excited states and may undergo several reversible redox processes. This trait is important for efficient charge generation and separation. Other researchers have used metal complexes as photosensitizers in dye-sensitized solar cells, which are composed of an inorganic semiconductor coated with organic dye molecules.⁵ We synthesized a series of sublimable rhenium diimine photosensitizing complexes (see Figure 1).^6,7 These can be purified, processed, and deposited onto thin films.

Figure 1. Examples of photosensitizing rhenium complexes.

We fabricated bulk-heterojunction photovoltaic devices, which use indium tin oxide glass as the anode. We then deposited copper phthalocyanine (CuPc), a blend of a rhenium complex (complex 2 in Figure 1) and a fullerene (C₆₀), a fullerene, and an aluminum (Al) cathode (in sequence) by vacuum sublimation onto the anode. To optimize device performance, we carefully controlled the thickness of each layer.

Figure 2. Device operation. Charge separation happens at the interfaces between the complex and C₆₀ (fullerene). LUMO: Lowest unoccupied molecular orbital. HOMO: Highest occupied molecular orbital. e⁻: Electron. h⁺: Hole. hν: Incident light. ITO: Indium tin oxide. CuPc: Copper phthalocyanine.

Figure 2 shows a schematic of a typical device. Upon photoexcitation, the rhenium complex forms an exciton (a loosely bound electron-hole pair), which may dissociate at the complex/fullerene interface. These free electrons and holes then migrate to the cathode and anode via percolation pathways formed by the fullerene and metal-complex molecules, generating photocurrent as a result. We evaluated device performance using the current-voltage curve under irradiation with simulated solar light (see Figure 3). The device fabricated from complex 2 exhibited a modest fill factor of 0.63 and power efficiency of 1.72%.

Figure 3. Current-voltage characteristics of the device under simulated solar light (airmass 1.5). FF: Fill factor. η_p: Power efficiency. J_SC: Short-circuit current. V_OC: Open-circuit voltage.

We confirmed the role of rhenium complexes in photoexcitation by measuring the external quantum efficiency (EQE), i.e., the number of electrons generated per photon absorbed. Figure 4 shows the device's absorption spectrum and EQE as a function of the incident-light wavelength. Complex 2 exhibits a complementary absorption band in the region where CuPc does not absorb (460–500nm), and the EQE over the entire visible range exceeds 10%.

Figure 4. Visible-UV absorption spectrum of the organic active layers in the device and external quantum efficiency (EQE) as a function of the incident-light wavelength. a.u.: Arbitrary units.

In addition to molecular metal complexes, we also developed metal-containing polymers for photovoltaic applications^8–10 to broaden the optical response by incorporating different light-absorption moieties into a polymer molecule. Designing multifunctional polymers is a possible solution to enhance photon harvesting at different wavelengths. However, several scientific and technological problems must be solved to fabricate efficient organic photovoltaic devices. Other than low carrier mobilities and optical absorption, organic materials usually have poor long-term stability under illumination by sun light. Improving device stability offers another challenge to scientists and engineers. In the future, we will optimize performance by modifying the ligand structure so that the absorption of the complexes can be extended to lower energies. We will also study the fundamental photophysical properties of these complexes to better understand the charge-generation and separation processes.

This research was supported by the Research Grants Council of Hong Kong (project numbers HKU7010/05P and HKU7008/05P) and the University Development Fund and Strategic Research Theme (administrated by the University of Hong Kong).