Abstract
Compared to their inorganic counterparts, organic semiconductors suffer from relatively low charge carrier mobilities. Therefore, expressions derived for inorganic solar cells to correlate characteristic performance parameters to material properties are prone to fail when applied to organic devices. This is especially true for the classical Shockley-equation commonly used to describe current-voltage (JV)-curves, as it assumes a high electrical conductivity of the charge transporting material. Here, an analytical expression for the JV-curves of organic solar cells is derived based on a previously published analytical model. This expression, bearing a similar functional dependence as the Shockley-equation, delivers a new figure of merit α to express the balance between free charge recombination and extraction in low mobility photoactive materials. This figure of merit is shown to determine critical device parameters such as the apparent series resistance and the fill factor.
Highlights
Due to intense research in material synthesis and device physics, record power conversion efficiencies of organic solar cells are well exceeding 10%1,2
Though these conditions might appear arbitrary at first glance, they reflect the situation encountered in some state of the art solar cells, where a thin photovoltaic layer is sandwiched between two doped charge transport layers in a p-i-n-type fashion[19,20] or where, because of a large active layer thickness, processes at the contacts are of minor importance[1,21,22]
Having proven that the parameter α determines the shape of the JV-curves of low mobility solar cells, it is meaningful to search for an analytical expression relating the fill factor (FF) to the parameter α
Summary
Photocurrents received: 09 December 2015 accepted: 06 April 2016 Published: 26 April 2016. Expressions derived for inorganic solar cells to correlate characteristic performance parameters to material properties are prone to fail when applied to organic devices This is especially true for the classical Shockley-equation commonly used to describe current-voltage (JV)-curves, as it assumes a high electrical conductivity of the charge transporting material. An analytical expression for the JV-curves of organic solar cells is derived based on a previously published analytical model This expression, bearing a similar functional dependence as the Shockley-equation, delivers a new figure of merit α to express the balance between free charge recombination and extraction in low mobility photoactive materials. Display non-ideal JV-curves with low fill factors This asks for analytical approaches to treat the competition between charge extraction and recombination, and to relate characteristic device parameters such as the ideality factor or the fill factor to relevant material properties. Before discussing in detail how the performance and in particular the FF of organic solar cells is set by the new parameter α , let us briefly consider the physical meaning of Eq 9
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