Photoconversion Mechanism at the pn-Homojunction Interface in Single Organic Semiconductor.

Ji-Hyun Lee,Seiichiro Izawa,Masahiro Hiramoto,Armand Perrot

doi:10.3390/ma13071727

Abstract

Clarifying critical differences in free charge generation and recombination processes between inorganic and organic semiconductors is important for developing efficient organic photoconversion devices such as solar cells (SCs) and photodetector. In this study, we analyzed the dependence of doping concentration on the photoconversion process at the organic pn-homojunction interface in a single organic semiconductor using the temperature dependence of J–V characteristics and energy structure measurements. Even though the organic pn-homojunction SC devices were fabricated using a single host material and the doping technique resembling an inorganic pn-homojunction, the charge generation and recombination mechanisms are similar to that of conventional donor/acceptor (D/A) type organic SCs; that is, the charge separation happens from localized exciton and charge transfer (CT) state being separated by the energy offset between adjacent molecules, and the recombination happens from localized charge carrier at two adjacent molecules. The determining factor for photoconversion processes is the localized nature of charges in organic semiconductors. The results demonstrated that controlling the delocalization of the charges is important to realize efficient organic photoconversion devices.

Highlights

Free charge generation processes by light absorption in inorganic and organic semiconductors are completely different
The energy structure measurement revealed that the energy offset between difference in the Ea induced by doping is attributed to the charge separation process and not to the exciton diffusion, nor the bimolecular recombination process
The temperature dependence of J–V characteristics and energy structure measurement revealed that the increase in JSC and decrease in VOC with an increase in doping concentration in the organic pn-homojunction solar cells (SCs) devices was the result of the acceleration of charge separation and the change in energy of the recombination center

Summary

Introduction

Free charge generation processes by light absorption in inorganic and organic semiconductors are completely different. Inorganic semiconductors have a large dielectric constant; free charges are directly formed after the light absorption in a single semiconductor material [1]. The question is whether free charge formation is possible in single organic semiconductor material by light absorption, similar to that in inorganic semiconductors. The measurement gives information about thermal activation for the charge generation and process in organic pn-homojunction SC devices mainly by a temperature dependence measurement. Upon combining an energy structure measurement near the pnThe measurement gives information about thermal activation for the charge generation and recombination homojunction, we revealed detailed mechanism of photoconversion at the single organic pathways [5,6]. The detailed mechanism of photoconversion at the single organic semiconductor interface

Findings

Results

Conclusions