The effect of charge transfer state on the open-circuit voltage of small-molecular organic photovoltaic devices: A comparison between the planar and bulk heterojunctions using electroluminescence characterization

• Open-circuit voltage of small-molecular organic photovoltaic device was studied.
• Variation in open-circuit voltage was related to the properties of charge transfer states.
• Electroluminescence measurement was used to characterize the properties of charge transfer states.
• Red shift of charge transfer was observed under different bulk volumes.
• Increased degree of interaction between donor and acceptor reduced nonradiative energetic loss.

This work studies the open-circuit voltage (VOC) of planar and bulk heterojunction organic photovoltaic (OPV) devices by means of electroluminescent (EL) technique to resolve the charge transfer (CT) states between donor and acceptor. The OPV devices containing a small part of bulk heterojunction increases the VOC as a result of the enhanced CT process as compared with a complete planar structure. Red shift of the CT charge transfer was observed by increasing the bulk volume, which indicates the increased degree of interaction between both molecules and excitons. By characterizing the EL spectra of OPV devices and relating them to the CT absorption, the interfacial property between the donor and acceptor is shown to be crucial for determining the VOC in small-molecule OPV devices. Detailed analysis of the energetic loss was also used to interpret the VOC under the effect of CT states.

Observation of bulk-volume induced open-circuit voltage variation by characterizing the charge transfer state using reduced electroluminescence and external quantum efficiency spectra.Figure optionsDownload as PowerPoint slide