The work reported here explores the impact of polymer morphology on the physics and performance of perylene benzimidazole/poly(3-hexylthiophene) bilayer photovoltaic devices. By varying both the annealing temperature and the solvent used for polymer deposition, we demonstrate control of the polymer chain morphology. An increase in the relative ordering of the polymer chain conformation is observed through a shift in the absorption onset and absorption spectral shape, and results in improved photovoltaic performance.
We have explored the use of polymer / small molecule organic composites in the form of a polymer / perylene diimide heterojunction bilayer in order to combine the advantageous properties of both materials. Using the electron transporting perylene benzimidazole (PBI) and the hole conducting polymer poly[2,5-dimethoxy-1,4-phenylene-1,2-ethenylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-1,2-ethenylene (M3EH-PPV), we have achieved increased power conversion efficiencies for a planar device of up to 0.71% under 80 mW/cm2 white illumination. By varying the order of the photoactive layers, we have probed the mechanisms creating the photovoltage and found that the photovoltage is not determined by the difference in electrode work functions alone. In addition to the internal field, the interfacial chemical potential gradient, caused by exciton dissociation at the polymer / perylene diimide interface, appears to contribute to the photovoltage. We also discuss why, contrary to some expectations, the polymer / perylene diimide devices are more efficient than the analogous pure small molecule perylene diimide / phthalocyanine cells.
Interfaces are important in all solar cells, but they are especially significant in organic-based photovoltaic (OPV) cells such as organic semiconductor PV cells and dye-sensitized solar cells. In OPV cells, charge generation, charge separation and charge recombination processes often take place at interfaces, while in conventional PV cells these crucial processes occur mainly in the bulk. Interfacial exciton dissociation can lead to photovoltages that exceed the built-in potential of the cell, thus Voc is not necessarily a measure of the band bending. Photovoltages up to ~1V have been achieved without band bending. Most OPV cells are majority carrier devices, and thus are fundamentally different than conventional p-n junction cells. The interfacial potential induced by the photogeneration of charge carriers may, at high light intensities, overwhelm the equilibrium potential and hinder charge separation. It is often advantageous to increase the exciton-dissociating surface area by structuring the interface, however this also increases the area over which carriers can recombine. In dye-sensitized solar cells, the surface area is so high, and recombination so rapid, that only a single redox couple with ultra-slow kinetics is viable. We describe a method of passivating the interfacial recombination sites in these cells that permits for the first time the use of kinetically fast redox couples and may facilitate the development of solid-state dye cells. Finally, we describe a UV treatment of dye cells that alters the interfacial energetics and dramatically increases the efficiency in some cases.
This paper summarizes efforts at the National Renewable Energy Laboratory to develop self-powered electrochromic window coatings that could be used for economical retrofit to existing building windows. The self-power is provided in either of two ways in separate approaches to the electrochromic (EC) design: by very thin, nearly transparent a-silicon PV cells or by dye-sensitized titania half-cells built into the multi-layer structure of the window coating. The goal of both designs is the ability to incorporate the self-powered EC coating into a laminated flexible polymer film. This film could then be retrofitted to a building window by bonding it to the interior surface and attaching it to local electronic controls that also derive their power from the windows that they control. Laboratory-scale prototype self-powered EC window coatings have been fabricated using both approaches. These are described and the remaining development challenges are discussed.
Electrochromic films have been combined with photoelectrochemical electrodes to make a self-powered smart window. These 'photoelectrochromic' smart windows combine the advantages of photochromic films, namely that they are self- powered by the incident light, with the advantages of electrochromic windows, such as the ability to control the state of coloration externally when desired. When the windows are short-circuited, the observable process is photochromism, but the mechanism is unique and has several potential advantages over conventional photochromic films. The light absorbing process is physically separate from the coloration process, allowing each to be individually optimized. The materials constraints are greatly relaxed compared to single- component photochromic films in which one material must meet all criteria (color change, switching speed, photostability, etc.). Furthermore, since the coloration process in a PEC cell requires an external electrical current between the two electrodes, a particular state (transparent, absorbing, or imaged) can either be stored when the electrodes are at open circuit, or can be changed when the electrodes are connected. The light-absorbing function in the PEC cell is performed by a dye-sensitized semiconductor electrode that produces a photovoltage sufficient to color the electrochromic film deposited on the counterelectrode. We describe the dye sensitization process, its advantages over conventional photovoltaic devices in applications such as smart windows, and recent developments in photoelectrochromic smart windows.
Conference Committee Involvement (3)
Organic Photovoltaics V
4 August 2004 | Denver, Colorado, United States
Organic Photovoltaics IV
7 August 2003 | San Diego, California, United States