Optimizing organic photovoltaics using tailored heterojunctions: A photoinduced absorption study of oligothiophenes with low band gaps

Abstract

A power conversion efficiency of 3.4% with an open-circuit voltage of 1 V was recently demonstrated in a thin film solar cell utilizing fullerene C 60 as acceptor and a new acceptor-substituted oligothiophene with an optical gap of 1.77 eV as donor ͓K. Schulze et al., Adv. Mater. ͑Weinheim, Ger.͒ 18, 2872 ͑2006͔͒. This prompted us to systematically study the energy-and electron transfer processes at the oligothiophene:fullerene heterojunction for a homologous series of these oligothiophenes. Cyclic voltammetry and ultraviolet photoelectron spectroscopy data show that the heterojunction is modified due to tuning of the highest occupied molecular orbital energy for different oligothiophene chain lengths, while the lowest unoccupied molecular orbital energy remains essentially fixed due to the presence of electron-withdrawing end groups ͑dicyanovinyl͒ attached to the oligothiophene. Use of photoinduced absorption ͑PA͒ allows the study of the electron transfer process at the heterojunction to C 60. Quantum-chemical calculations performed at the density functional theory and/or time-dependent density functional theory level and cation absorption spectra of diluted DCVnT provide an unambiguous identification of the transitions observed in the PA spectra. Upon increasing the effective energy gap of the donor-acceptor pair by increasing the ionization energy of the donor, photoinduced electron transfer is eventually replaced with energy transfer, which alters the photovoltaic operation conditions. The optimum open-circuit voltage of a solar cell is thus a trade-off between efficient charge separation at the interface and maximized effective gap. It appears that the open-circuit voltages of 1.0-1.1 V in our solar cell devices have reached an optimum since higher voltages result in a loss in charge separation efficiency.

Computationally connecting organic photovoltaic performance to atomistic arrangements and bulk morphology

“…Here, we use fine-graining to inform electronic property calculations, which complement the aforementioned structural analysis. Quantum chemical calculations (QCCs), such as density functional theory, can be used to to calculate important electronic properties for a molecular system such as the ground-state electronic structure and molecular orbital energies [9][10][11][12][13][14]. These calculations are therefore a prerequisite for quantifying the charge transport characteristics of a system.…”

“…Computational chemistry allows the electronic properties of an active layer to be calculated by way of the electronic structure calculations of each constituent molecule [9][10][11][12][13][14]. Combining computational chemistry with modeled morphologies offers a feasible path to inform the design of low-cost, high-efficiency OPVs.…”

Gewald reaction: synthesis, properties and applications of substituted 2-aminothiophenes

“…153 Since the initial discovery of organic compounds showing metallic conductivity, for which 2000 Nobel prize in chemistry was awarded, [154][155][156] oligo-and polythiophenes have attracted much attention as advanced molecules with practical use in electronic devices [157][158][159][160] and their potential application in field-effect transistors, 161 photovoltaic devices 162 and organic electroluminescent devices. 163 The employment of substituted 2-aminothiophenes in such areas represents the latest discovery showing a great promise in materials chemistry for the generation of novel oligo-and polythiophene structures.…”

Fullerene-Based Electron Transport Layers for Semi-Transparent MAPbBr3 Perovskite Films in Planar Perovskite Solar Cells

“…The optimum device exhibited an outstanding performance, with short-circuit current density = 7.23 mA/cm 2 , open-circuit voltage = 1.2 V, fill factor = 0.42, and a power conversion efficiency = 4.19%. The lowest unoccupied molecular orbital (LUMO) of the MAPbBr 3 , PCBM, and C 60 films are -3.36 [24], -3.80 [25], and -3.9 [26], respectively. When the PCBM/C 60 bi-layer is used as an electron transport layer, step jumping makes it easier for electrons to transport into the Ag electrode, which leads to an increased current density and open voltage, as shown in Figure 6d.…”