Effect of doping on performance of organic solar cells

TL;DR

This study models how doping affects the performance of organic solar cells, revealing that doping can either decrease or improve efficiency depending on cell type, material properties, and contact quality.

Contribution

The paper extends existing models by analyzing doping effects on both planar and bulk heterojunction organic solar cells, highlighting conditions under which doping enhances or reduces performance.

Findings

Doping decreases JSC, FF, and efficiency in optimized bulk heterojunction cells.

Doping can increase efficiency in unoptimized bulk heterojunction cells with low mobilities.

Doping improves performance in acceptor-doped planar heterojunction cells, especially with non-ohmic contacts.

Abstract

Conventional models of planar and bulk heterojunction organic solar cells have been extended by introducing doping in the active layer. We have studied the performance of organic solar cells as a function of dopant concentration. For bulk heterojunction cells, the modeling shows that for the most studied material pair (poly-3-hexylthiophene, P3HT, and phenyl-C61-butyric acid methyl ester, PCBM) doping decreases the short-circuit current density (JSC), fill factor (FF) and efficiency. However, if bulk heterojunction cells are not optimized, namely, at low charge carrier mobilities, unbalanced mobilities or non-ohmic contacts, the efficiency can be increased by doping. For planar heterojunction cells, the modeling shows that if the acceptor layer is n doped, and the donor layer is p doped, the open-circuit voltage, JSC, FF and hence the efficiency can be increased by doping. Inversely, when the acceptor is p doped, and the donor is n doped; FF decreases rapidly with increasing dopant concentrations so that the current-voltage curve becomes S shaped. We also show that the detrimental effect of nonohmic contacts on the performance of the planar heterojunction cell can be strongly weakened by doping.