Organic and nano-structured composite photovoltaics: An overview
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Organic and nano-structured composite photovoltaics: An overview Sophie E. Gledhill,a) Brian Scott, and Brian A. Gregg National Renewable Energy Laboratory, Golden, Colorado 80401 (Received 13 April 2005; accepted 22 June 2005)
Organic photovoltaic devices are poised to fill the low-cost, low power niche in the solar cell market. Recently measured efficiencies of solid-state organic cells are nudging 5% while Grätzel’s more established dye-sensitized solar cell technology is more than double this. A fundamental understanding of the excitonic nature of organic materials is an essential backbone for device engineering. Bound electron-hole pairs, “excitons,” are formed in organic semiconductors on photo-absorption. In the organic solar cell, the exciton must diffuse to the donor–accepter interface for simultaneous charge generation and separation. This interface is critical as the concentration of charge carriers is high and recombination here is higher than in the bulk. Nanostructured engineering of the interface has been utilized to maximize organic materials properties, namely to compensate the poor exciton diffusion lengths and lower mobilities. Excitonic solar cells have different limitations on their open-circuit photo-voltages due to these high interfacial charge carrier concentrations, and their behavior cannot be interpreted as if they were conventional solar cells. This article briefly reviews some of the differences between excitonic organic solar cells and conventional inorganic solar cells and highlights some of the technical strategies used in this rapidly progressing field, whose ultimate aim is for organic solar cells to be a commercial reality. I. INTRODUCTION
Inorganic crystalline silicon (c-Si) wafer solar cell technology dominates the current photovoltaic (PV) market, yet c-Si is encumbered by high cost and energyconsuming production,1 and it is thus imperative to seek alternative materials if solar cells are to become an economically viable energy resource. Organic materials offering low production costs, potentially low-energy production, and the option of flexible substrates appear to fit our future PV material’s requirement criterion to the letter. Despite organic semiconductors being less stable and having lower conductivities than their inorganic rivals, recent rapid advances have been made in organic electronics,2–5 which have paved the way for the recent surge of interest and promise in organic photovoltaic cells. Organic PV cells include the dye-sensitized solar cell (DSSC), and cells made from small organic molecules and semiconducting polymers. They are all what we classify as “excitonic solar cells.”6–9 Light absorption in excitonic solar cells leads to the production of excitons
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0407
(bound electron-hole pairs) while in most conventional inorganic solar cells, it leads to direct creation of free electron–hole pairs (it should be noted a few inorganic materials7,10 have been observed t
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