Inverted bulk-heterojunction plastic solar cells
Plastic solar cells, or photovoltaic devices made from organic semiconductors, have demonstrated increasing efficiencies over the last few years.1 Several research groups have claimed devices with ∼5% solar power conversion efficiency. And ∼5% efficiency has been certified at the National Renewable Energy Laboratory (NREL) on a device fabricated by the start-up company Konarka Technologies. While not yet sufficient for commercialization, these devices are approaching the 7–10% efficiency regime necessary for initial photovoltaic applications. As a result, research activity has intensified immensely in an effort to take advantage of the potentially low manufacturing costs and ease of fabrication of these devices.
One concern with plastic solar cells is their longevity. Plastic materials are soft and can suffer sunlight-induced morphology changes or interfacial degradation over their operating lifetime. They can also suffer damage to the back electrode, which is normally made from a low-work-function metal that is reactive and easily oxidized in air. (Work function is the energy required to remove an electron from the surface.) Encouragingly, progress is being made in each of these areas. Organic semiconductors can be made more resistant to oxidation by appropriate tuning of their electronic levels, and they can be made ’harder’ by tuning their glass-transition temperature or with cross-linking. The last issue, electrode degradation, is normally addressed by capping the reactive metal with a less reactive one and by encapsulating the device to protect it from environmental effects during operation. Ideally, however, the device is made more robust by removing the need for the reactive metal in the first place. This can be done by inverting the device geometry such that the holes and electrons generated in the active layer exit the device in the opposite direction as they do in a normal device.
In most organic optoelectronic devices, including organic light-emitting diodes, the front electrode is based on a transparent conducting oxide, such as indium tin oxide (ITO), that serves as the high-work-function, positive electrode. To achieve the built-in electric field needed for most devices to operate, the back electrode must be made from a low-work-function metal that serves as the negative electrode. In the operation of the typical polymer-fullerene blend bulk-heterojunction solar cell, electrons generated in the active layer are collected by the back electrode, and holes are collected by the front. However, this can be reversed by inserting a hole-blocking layer between the ITO and the active layer so that only electrons can reach the ITO, and the back electrode must become the hole-collecting positive electrode (see Figure 1). It can then be made from a high-work-function metal that is more air-stable.
Our group at NREL has shown that inversion of the device can be accomplished by using zinc oxide (ZnO) as the hole-blocking layer. The ZnO layer is fabricated by thermal conversion from a spin-coated film of a chemical precursor solution of zinc acetate. The resulting ZnO layer is nanocrystalline in structure (see Figure 2). Once the conversion is complete, the active layer blend of P3HT:PCBM—poly(3-hexylthiophene):fullerene2,3—is spin-coated, and the back electrode is laid down through vacuum deposition. In our recent work, we used silver as the back electrode.4 Although pristine silver is nominally a low-work-function metal when deposited in an ultrahigh vacuum chamber, it has been found to serve as a good positive electrode.5 This is presumably because the interface between the silver and the organic layer readily oxidizes to form a thin layer of silver oxide (AgxO), which is a p-type semiconductor with a high work function that serves well as a positive electrode.
We have made plastic solar cells using this approach with a certified power conversion efficiency of 2.58% under standard AM1.5 test conditions, which mimic sunlight at the Earth's surface (see Figure 3).2 These devices have external quantum efficiencies exceeding 80% at peak wavelengths. The overall power conversion efficiency of the device is limited to some extent by the photovoltage and the fill factor.
The specific materials we used in this inverted device are clearly not optimal; research into and optimization of each component of the device is needed. For instance, both the ITO and the silver electrodes can probably be replaced by better alternatives, and the P3HT:PCBM blend active layer may not be sufficiently efficient or long-lived for commercial viability. However, these initial results demonstrate a simple fabrication method for more robust inverted plastic solar cells. Additionally, since positive electrodes made of gold have been applied to organic light-emitting diodes using lamination instead of vacuum thermal deposition,6 a similar approach might enable organic solar cell fabrication without vacuum processing. These improvements would enable the plastic solar cell to become a low-cost and scalable photovoltaic technology.
Special thanks to Jao van de Lagemaat at NREL for the image in Figure 2, and to Tom Moriarity, NREL Measurements and Characterization, for the charts in Figure 3.
