A New Figure of Merit for Organic Solar Cells with Transport-limited Photocurrents

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.

A Shockley‐Type Polymer: Fullerene Solar Cell

“…[6,7,26,27] The mobility and carrier lifetime product have been extensively used as a "figure of merit" (FoM) to quantify the quality of the charge collection in (organic) solar cells. [6,7,26,27] The mobility and carrier lifetime product have been extensively used as a "figure of merit" (FoM) to quantify the quality of the charge collection in (organic) solar cells.…”

“…[6,7,26,27] The mobility and carrier lifetime product have been extensively used as a "figure of merit" (FoM) to quantify the quality of the charge collection in (organic) solar cells. Recently, a modified Shockley equation was introduced by Neher et al [26] based on their previous efforts to correlate the FF with transport and recombination properties. This is because the carrier lifetime of one type of carrier is dependent on the density of its counterpart and thereby on the light intensity.…”

“…This is because the carrier lifetime of one type of carrier is dependent on the density of its counterpart and thereby on the light intensity. The work of Neher et al [26] shows that depending on parameters such as thickness of the photoactive layer, light intensity, electron and hole mobilities, and bimolecular recombination rate constant, the operation of the organic solar cells can either be transport limited or Shockley type, in which the device is not affected by charge collection loss. [7,28] However, this FoM is not a device FoM and does not account for the active layer thickness and light intensity regime and does not yield the fill factor.…”

Charge Generation and Recombination in an Organic Solar Cell with Low Energetic Offsets

“…[5][6][7] The FF is a parameter used to describe the efficiency of charge generation and collection across the operating voltages of the solar cell. [12][13][14][15] In recent years, there have been some reports on organic solar cells that achieve very impressive FF values, approaching 80%. However, in OPVs, charge generation and extraction are known to be hindered by geminate and/or nongeminate (specifically, bimolecular) charge recombination losses, and thus the FF of OPVs is often lower.…”

Sensitivity of the Drift-Diffusion Approach in Estimating the Power Conversion Efficiency of Bulk Heterojunction Polymer Solar Cells

“…Alignment of the Fermi levels at the semiconductor/contact interfaces modeled by Mott-Schottky [13], and the thermionic injection model for charge carrier transfer at the metal-organic interface described through the kinetic rate model has been generalized by Scott and Malliaras [14] for the disordered materials. One of the major material properties which significantly influences the performance of OSCs is charge carrier mobility [15,16]. In fact, charge carrier mobility in organic materials is a function of hopping rates between donor (or acceptor) molecules that depend on the material structure and morphology of the active layer [17,18].…”