Built-in potential and validity of Mott--Schottky analysis in organic bulk heterojunction solar cells

TL;DR

This study critically evaluates the applicability of Mott-Schottky analysis in organic BHJ solar cells, revealing its limitations and proposing pulsed photocurrent measurements as a more reliable method for assessing flat band conditions.

Contribution

The paper demonstrates that Mott-Schottky analysis is not suitable for organic BHJ solar cells and introduces pulsed photocurrent measurements as a better alternative for determining flat band potentials.

Findings

Mott-Schottky analysis depends on active layer thickness in BHJ cells.

Pulsed photocurrent measurements reveal flat band conditions are below the built-in potential.

Flat band potential from pulsed photocurrent is independent of active layer thickness.

Abstract

We investigated poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid methyl ester bulk heterojunction (BHJ) solar cells by means of pulsed photocurrent, temperature dependent current-voltage and capacitance-voltage measurements. We show that a direct transfer of Mott-Schottky (MS) analysis from inorganic devices to organic BHJ solar cells is not generally appropriate to determine the built-in potential, since the resulting potential depends on the active layer thickness. Pulsed photocurrent measurements enabled us to directly study the case of quasi flat bands (QFB) in the bulk of the solar cell. It is well below the built-in potential and differs by diffusion-induced band-bending at the contacts. In contrast to MS analysis the corresponding potential is independent on the active layer thickness and therefore a better measure for flat band conditions in the bulk of a BHJ solar cell as compared to MS analysis.