Realtime Observation of Carrier Trapping and Recombination in a Bulk Heterojunction Film by Simultaneous Optical and Electrical Detection

Abstract

Since the photocharge generation efficiency has reached nearly 100% for recent organic photovoltaic film materials, clarification of loss mechanisms becomes of increasing importance. Here, we report a new time-resolved measurement method that enables simultaneous detection of the optical absorption and electric current due to photogenerated carriers in organic semiconductor films. Microsecond-order decays of both the number density (recombination) and the averaged drift mobility (trapping) of photocarriers are quantitatively observed for an identical device of a regioregular poly[3-hexylthiophene-2,5-diyl]:(6,6)-phenyl C 61 butyric acid methyl ester bulk heterojunction film. The decay feature of the timedependent mobility is highly dispersive especially at lower temperatures. It can be explained by fast thermalization of "hot" photocarriers into lower energy sites, which is followed by thermally activated hopping. This quantitative time-resolved observation method can be a powerful tool for characterization and optimization of photovoltaic materials.

Microscopic Structures, Dynamics, and Spin Configuration of the Charge Carriers in Organic Photovoltaic Solar Cells Studied by Advanced Time-Resolved Spectroscopic Methods

“…Hence, the free carriers in the transport should be observed to clarify the carrier dynamics. In this section, we show a new experimental technique for studying carrier transport in a BHJ film, which is the time-resolved simultaneous optical and electrical detection (SOED) of photogenerated carriers . This time-resolved SOED allows us to visualize the dispersive mobility of carriers due to trapping and recombination in structural and energetic disorders of the BHJ film.…”

“…The broken curves are fittings with the exponential function. Reconstructed from ref . Copyright 2021 American Chemical Society.…”

“…Second, we present our recent efforts to obtain molecular insights into the spin configuration and interaction of electrons and holes in polymer-based BHJ thin films and OPV cells by using the time-resolved electron paramagnetic resonance (EPR) method, − time-resolved simultaneous optical and electrical detection (SOED), and measurement of the magneto­conductance (MC) effect. − The charges generated by the electron transfer of the exciton have unpaired electron spins accompanied by magnetic moments. Magnetic coupling between the spins of the two photogenerated opposite charges, i.e., the electron and hole, is sensitive to the molecular geometry, i.e., the separation and direction of the two charges.…”

“…Time-resolved EPR using a pulsed laser for photoexcitation is employed to observe the magnetic resonances between spin sublevels for the transient spin pair with microwaves under an external magnetic field. − From the analysis of time-resolved EPR spectra, one can obtain detailed information on the molecular geometries, electronic coupling, and molecular motions of the transient spin pairs. On the other hand, the time-resolved SOED can chase the carrier dynamics after complete separation of the transient spin pair and permit visualization of the time dependence of the carrier mobility . The electron spin also affects the exciton and charge carrier dynamics at the donor/acceptor interface.…”

“…26 Here, we focus on the dependence of the alkyl chain length on the charge carrier recombination dynamics in BHJ OPVs based on DPP-BP. 27 Second, we present our recent efforts to obtain molecular insights into the spin configuration and interaction of electrons and holes in polymer-based BHJ thin films and OPV cells by using the time-resolved electron paramagnetic resonance (EPR) method, 28−31 time-resolved simultaneous optical and electrical detection (SOED), 32 and measurement of the magnetoconductance (MC) effect. 33−35 The charges generated by the electron transfer of the exciton have unpaired electron spins accompanied by magnetic moments.…”