Multipolymer Interactions in Bulk Heterojunction Photovoltaic Devices

Multipolymer photovoltaics, single layer devices made up of multiple photoactive polymers, can create organic photovoltaics (OPVs) with a wider spectral response than single polymer systems without the difficult fabrication of a tandem. Our group has successfully created multipolymer solar devices with 2% power conversion efficiency. We have analyzed the optical and electrical properties of these devices, and found that it may be possible for polymers to assist each other with charge extraction, though combining polymers disrupts single polymer crystallinity. Introduction and Background. Organic solar, a carbon based technology, is an exciting area of research in the development of renewable energy. The technology has the potential to create photovoltaic energy generation systems that have a lower cost per watt than conventional silicon systems, or even fossil fuels.1 The 580TW of available solar energy dwarfs the projected energy needs of the human race of 16.9TW by 2030, making harnessing it an attractive possibility.2 Solar energy is virtually free once the installations are in place, and the availability of solar energy is enormous, so it would seem that the low cost option of organic solar would make it a naturally widespread technology. Unfortunately, neither efficiency nor device lifetime are high enough to make a commercially viable organic solar device. Our group’s research is focused on improving the efficiency of organic solar cells. The polymer nature of organic photovoltaics (OPVs) causes the active layers of plastic solar cells to degrade in the presence oxygen, water, and ultraviolet light. As such, maintaining device performance over a reasonable lifetime is a challenge that is still being investigated. For OPVs to be accepted by the market, a combination of increased lifetime and efficiency will be required.3 A commercial silicon solar panel will convert light into electricity with an efficiency in the range of 15-20%. Currently, the best Grant Olson • email: [email protected] • Cal Poly, San Luis Obispo ! 1 verified organic system is a tandem device designed by Heliatek with a power conversion efficiency of 10.7%.4 The advancements in the field have moved organics towards a working product, but the technology is not yet ready for widespread use. When creating photovoltaics, the spectrum of light that a cell absorbs plays a large role in the efficiencies that are possible. A material that absorbs a wide range of the solar spectrum will capture more photons, but each photon can only free a single electron, unless it’s an exotic material called a multiple exciton generator.5 The voltage of the cell is determined by the interaction between the energy levels of the active layer and the electrodes, so the cell is ultimately limited in the amount of energy that can be extracted from each photon. This means that the energy of shorter wavelength (higher energy) light will be wasted on such a cell; any energy above the band difference will be lost as heat. It is possible to create a tandem system, like in the record breaking device from Heliatek, which consists of stacking devices on top of each other to give better spectral response.6 This allows each layer to work at different device voltages, so higher energy photons can contribute more of their energy. Unfortunately, high performance tandems require the currents from each layer to be matched, which makes them more difficult to design and produce, and they lose performance when they are put under a spectra that they are not designed for.7 What would be ideal is a system that has the broad absorption capability of a tandem, with the ease of fabrication of a single layer device. The multipolymer bulk heterojunction system has the potential to realize both of these goals by combining multiple photovoltaic polymers with synergistic optical and electrical properties to make a well performing hybrid device.8 In this paper we will explain our methods and results in characterizing our multipolymer system via optical and electrical testing. Theory A bulk heterojunction solar cell is so named because the volume between the electrodes is made up of an interpenetrating layer of dissimilar materials. One of these materials is a polymer, P3HT [poly(3-hexylthiophene)] or PCPDTBT [poly[2,6-(4,4-bisGrant Olson • email: [email protected] • Cal Poly, San Luis Obispo ! 2 (2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b0 ]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] ], which serves as an electron donor to the other material, a fullerene derivative called PC60BM [[6,6]-phenyl-C61-butyric acid methyl ester]. PCPDTBT is also commonly referred to as ZZ50, named after inventor Zhengguo Zhu achieving success on his 50th try . A fullerene is effectively a bucky ball, 60 carbon atoms arranged in a spherical shape with a tail added to the structure to improve solubility, and is used as our electron acceptor. Classical devices (made of a single polymer and PCBM) have a single bulk heterojunction layer made of a blend of polymer and PCBM, so they are relatively easy to fabricate, see Figure (1).9 Tandem devices have multiple polymer layers, with intermediate electrodes separating them. To minimize losses, all active layers of a tandem must be tuned to have the same current output.10 This increases the complexity in design and fabrication of devices, driving up the potential cost. Additionally, because each layer absorbs in a different region of the spectra, devices that are designed to absorb solar light will not convert other light sources efficiently. Our proposed design, the multipolymer bulk heterojunction device, does not require current matching or an intermediate electrode, eliminating the most difficult and costly aspects of design and fabrication of tandems.11 Grant Olson • email: [email protected] • Cal Poly, San Luis Obispo ! 3 Glass Substrate Transparent Conducting Anode