Optimizing Small Molecule Organic Solar Cells
An Alternative to Traditional Solar Power
With the ever-growing energy demands of our modern world and an increased awareness of the environmental impacts of traditional fossil fuels, there is a great need for an abundant supply of clean, alternative energy. Every day the sun provides us with a nearly limitless supply of solar energy, however, commercially available solar panels still remain too expensive to viably compete with fossil fuels on a large scale. As an alternative to the traditional solar panels, which utilize expensive single crystalline inorganic materials, organic solar cells utilize semiconducting plastics as the active layers, which have extremely good absorption using very thin layers and can be easily modified through chemical synthesis. Furthermore, these materials can be deposited from solution, leading to the potential for ink-jet printing or roll-to-roll coating, making the technology scalable and low-cost.
Due to the intrinsic properties of organic materials, a single material cannot be used in an organic solar cell. Instead, two organic semiconductors, an electron donor material and electron are co-deposited from a volatile solvent, leaving a thin film of interpenetrating phases termed a bulk heterojunction. Integral to improving efficiency is controlling the three-dimensional self-assembly of this bicontinuous network during the drying process, either through processing conditions such as solvent choice or molecular design. Ideally, one must maximize the donor-acceptor interface, while still maintaining good percolation of the two-phases for improved charge extraction.
In a recent study (referenced below) we developed a new small molecule organic semiconductor for use as a donor material in organic solar cells. These molecular donor materials are more easily purified than their more commonly used polymeric counterparts, which helps improve reproducibility. During the optimization of the device fabrication procedure we found that performance was affected by processing conditions; use of additives in the casting solvent or thermal treatment of the film both led to improvements in efficiency. Structural characterization via transmission electron microscopy and x-ray scattering revealed that it was crystallization of the donor material in the blend film which was paramount to device performance. By changing the solvent or thermal treatment we were able to manipulate this crystallization. In the optimum device, using a small amount of solvent additive, we were able to precisely control the size of the crystals, which led to a new record in efficiency of 7% for small molecule organic photovoltaic materials.
Figure 1: Organic Semiconductor Devices
Where We Are Headed
While organic solar is yet to achieve commercial relevance, record efficiencies have continued to improve over the last several years which make it a promising technology for the near future. Moving forward we will continue to develop new materials systems and processing methods, as well as expand to large-area printing methods to help bring this technology from bench top to roof top.
Author: Jack Love, December 2013
Department of Chemistry, UC Santa Barbara
To read the full paper click here.