Voltage-dependent excitation dynamics in UV-absorbing organic photovoltaics with efficient charge transfer exciton emission. Issue 4 (17th March 2023)

• Record Type: Journal Article
• Authors: Burlingame, Quinn C.; Liu, Xiao; Ball, Melissa L.; Rand, Barry P.; Loo, Yueh-Lin
• Is Part Of: Energy & environmental science. Volume 16:Issue 4(2023)
• Page Start: 1742
• Page End: 1751
• Publication Date: 2023-03-17
• Subjects: Energy conversion -- Periodicals; Fuel switching -- Periodicals; Environmental sciences -- Periodicals; Environmental chemistry -- Periodicals; 333.79
• DOI: 10.1039/d2ee03548k
• Languages: English
• ISSNs: 1754-5692

Abstract

Abstract : Voltage-dependent characterizations of organic solar cells with brightly-emitting charge-transfer excitons reveal excitation dynamics and trends as a function of donor molecule. Abstract : Intermolecular charge-transfer excitons play a central role in determining the performance of organic solar cells as their voltage-dependent formation, dissociation, and recombination dynamics contribute to photocurrent generation, radiative/nonradiative voltage losses, and photovoltaic fill factor. Here, we explore the properties of brightly-emitting wide energy gap (>2 eV) charge transfer excitons by measuring the voltage-dependent photoluminescence, photocurrent, and ultrafast pump–probe transient absorption spectra of organic solar cells employing five UV-absorbing donor molecules that differ only by the length of the oligophenylene or acene group at their core. We find that organic solar cells with a strong correlation between their voltage-dependent photocurrent and charge-transfer exciton photoluminescence have low photovoltaic fill factors as they require voltage to facilitate efficient charge-transfer exciton dissociation. In contrast, solar cells that are efficient can readily generate charges without an applied field and have a separate population of tightly-bound charge-transfer excitons that are responsible for emission. Considering that the sum of all excitation loss rates ( i.e., recombination and charge extraction) must be equal to the excitation generation rate in the steady state, these voltage-dependent data allow us to solve for the voltage-dependent fate of all excitations in the solar cells and estimate upper and lower bounds for geminate and non-geminate recombination, respectively.