Relation of open circuit voltage to charge carrier density in organic bulk heterojunction solar cells

TL;DR

This study investigates how open circuit voltage in organic bulk heterojunction solar cells relates to charge carrier density, revealing the influence of contact barriers and the effective bandgap on device performance.

Contribution

It provides experimental and simulation insights into the dependence of Voc on temperature, illumination, and contact barriers in organic solar cells.

Findings

Voc saturates at low temperatures due to contact barriers

Injection barriers affect Voc even at room temperature

Voc is primarily determined by the effective bandgap under typical conditions

Abstract

The open circuit voltage Voc and the corresponding charge carrier density were measured in dependence of temperature and illumination intensity by current-voltage and charge extraction measurements for P3HT:PCBM and P3HT:bisPCBM solar cells. At lower temperatures a saturation of Voc was observed which can be explained by energetic barriers at the contacts (metal-insulator-metal model). Such injection barriers can also influence Voc at room temperature and limit the performance of the working solar cell, as was assured by macroscopic device simulations on temperature-dependent IV characteristics. However, under most conditions - room temperature and low barriers - Voc is given by the effective bandgap.