Green organic solar cells from a water-soluble polymer
The cost of the current generation of inorganic silicon solar cells is very high and, despite over 50 years of development, further breakthroughs in cost and efficiency using traditional materials are looking increasingly less likely. Compared with silicon, next-generation organic solar cells hold the promise of low-cost, liquid-based, large-area fabrication technology at room temperature. But most polymers used in solar cells require organic solvents such as xylene, toluene, chloroform, and chlorobenzene. The toxicity of these solvents to people and the environment makes them complicated and costly to dispose of, and undermines the much-sought-after goal of low-cost, green, renewable energy. To that end, our group has developed a novel organic solar cell based on a water-soluble thiophene polymer, [(sodium poly[2-(3-thienyl)-ethoxy-4-butylsulfonate])] (PTEBS),1 and nanocrystalline TiO2. This system has shown great potential in photovoltaics.
The chemical structure of PTEBS is shown in Figure 1. There are many advantages to using water as the solvent. Water is an environmentally friendly, non-toxic, low-cost alternative that can be processed safely. The TiO2 is suspended in water and the PTEBS dissolves in it, so it is easy to form PTEBS/TiO2 bulk heterojunction devices by blending the two components in the same solution. Moreover, because water is part of the fabrication process, devices made from this polymer are less sensitive to moisture and may well show improved stability under atmospheric conditions.
More interestingly, PTEBS solutions change color depending on their pH value, with a corresponding change in their absorption spectra. Figure 2 shows a fresh acidic solution, an acidic solution that was left exposed to the air for a few days, and a basic solution. The fresh acidic solution was yellow-green, and the exposed liquid turned orange, and the basic solution was dark orange. The absorption spectra of the three solutions are shown in Figure 3. The fresh acidic solution had an absorption peak in the red-IR region that disappeared after the exposure we gave it, whereas the UV band showed little change. Exposure to air had no effect on the absorption spectrum of the basic solution. We also investigated thin films of PTEBS made from acidic and basic solutions, and obtained similar absorption results. Because most polymers absorb only in the blue-green range, the red-IR band absorption opens a possible avenue for increasing device efficiency. Detailed findings have been reported in SPIE proceedings.2
We have developed and explored bilayer heterojunction PTEBS/TiO2 solar cells (shown in Figure 4).3 In these cells, the PTEBS absorbs light and generates excitons. These then dissociate at the PTEBS/TiO2 interface, where electrons are transferred to TiO2 and transported to the anode, and the holes move through the PTEBS to the cathode. An open circuit voltage of 0.81V, a short circuit current density of 0.15mA/cm2, a fill factor of 0.91, and an energy conversion efficiency of 0.15% have been achieved, as shown in Figure 5. These results are comparable to the best polymer/metal-oxide solar cells reported by other groups using organic solvents. We have also examined the characteristics of the PTEBS/TiO2 composite and its behavior in the photovoltaic devices.4
To better understand the fundamental electronic transport physics of PTEBS films, hole mobility, singlet exciton diffusion length, and spectrum response will need to be studied. Research using time-of-flight photocurrent measurements, photoluminescence, and external quantum efficiency is currently under way.
In summary, for the first time, green solar cells have been fabricated from environmentally-friendly water-soluble polymers. The combination of water as solvent and liquid-based processing should ultimately enable the cost of the energy generated by this type of solar cell to approach that of current fossil fuel-based technologies. In addition, the flexible polymer is easy to make and to integrate into different devices.
