The Photoconversion Mechanism of Excitonic Solar Cells
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The Photoconversion
Mechanism of Excitonic Solar Cells Brian A. Gregg
Abstract Excitonic solar cells (XSCs) function by a mechanism that is different than that of conventional solar cells. They have different limitations on their open circuit photovoltages, and their behavior cannot be interpreted as if they were conventional p–n heterojunctions. Exciton dissociation at the heterojunction produces electrons on one side of the interface already separated from the holes produced on the other side of the interface. This creates a powerful photoinduced interfacial chemical potential energy gradient that drives the photovoltaic effect, even in the absence of a built-in electrical potential. The maximum thermodynamic efficiency achievable in an XSC is shown to be identical to that of a conventional solar cell, with the substitution of the optical bandgap in the XSC for the electronic bandgap in the conventional cell. This article briefly reviews the photovoltaic mechanism of XSCs, the limitations on their photovoltage, and their maximum achievable efficiency. Keywords: efficiency limitations, excitonic mechanism, organic solar cells.
Introduction Beginning with the advent of the dyesensitized solar cell (DSSC) in 1990,1–5 there has been a surge of interest in organic-based solar cells. The DSSC is still the most efficient (up to 10%), but other designs— ranging from the classical planar p–n junction organic solar cells6–8 to the newer bulk heterojunction cells9–13—are the focus of increasing interest and excitement. All of these cells are what I call excitonic solar cells (XSCs): they function by a fundamentally different mechanism than that of conventional solar cells, a mechanism that is primarily controlled by interfacial processes rather than by the bulk processes that mostly control silicon p–n junctions and other conventional cells.14–16 An exciton is a tightly bound electron– hole pair. Excitonic solar cells are a special class of majority carrier devices in which the density of minority carriers is insignificant: that is, electrons are found almost exclusively in one phase (with very few holes) and holes with very few electrons in the other. Excitonic semiconductors are primarily organic materials, but also include a few inorganic materials.15,16 Charge carriers are generated (and simultaneously 20
separated) at the heterointerface by exciton dissociation, and therefore they also recombine at this interface. (Some bulk generation may occur but this is usually deleterious.) This interfacial mode of carrier generation is fundamentally different from the bulk generation occurring in conventional cells and is responsible for many of the unusual features of XSCs.15,16 Interfacial properties are of paramount importance in these devices, while bulk properties are less critical. This allows the use of less pure and therefore less expensive materials. Conventional photovoltaic cells,17,18 on the other hand, are minority carrier devices in which both electrons and holes coexist in the same chemical phase. Their effici
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