Impact of Phosphorescent Sensitizers and Morphology on the Photovoltaic Performance in Organic Solar Cells

Popp, Johannes; Kaiser, Walter; Gagliardi, Alessio

doi:10.1002/adts.201800114

Cited by 7 publications

(14 citation statements)

References 99 publications

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“…The electronic coupling for spin-triplet excitations in purely organic materials is dominated by exchange interaction (Dexter-type transfer), which depends on the wavefunction overlap [20]. Thus, the understanding developed in the process of investigating TTA can be utilized to control the triplet diffusion in order to optimize the efficiency of organic semiconductor devices extending from next-generation thermally activated delayedfluorescence OLEDs [21] to phosphorescent-sensitizeremployed OSCs [22].…”

Section: Introductionmentioning

confidence: 99%

Kinetic Monte Carlo Study of Triplet-Triplet Annihilation in Conjugated Luminescent Materials

et al. 2020

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It is well known that in organic solids the collision of two excitons can give rise to delayed fluorescence (DF). Revived interest in this topic is stimulated by the current endeavor towards the development of efficient organic optoelectronic devices such as organic light-emitting diodes (OLEDs) and solar cells, or sensitizers used in photodynamic therapy. In such devices, triplet excitations are ubiquitously present but their annihilations can be either detrimental, e.g., giving rise to a roll-off of intensity in an OLED, or mandatory, e.g., if the sensitizer relies on up-conversion of long-lived low-energy triplet excitations. Since the employed materials are usually noncrystalline, optical excitations migrate via incoherent hopping. Here, we employ kinetic Monte Carlo simulations (KMC) to study the complex interplay of triplet-triplet annihilation (TTA) and quenching of the triplet excitations by impurities in a single-component system featuring a Gaussian energy landscape and variable system parameters such as the length of the hopping sites, i.e., a conjugated oligomer, the morphology of the system, the degree of disorder (σ ), the concentration of triplet excitations, and temperature. We also explore the effect of polaronic contributions to the hopping rates. A key conclusion is that the DF features a maximum at a temperature that scales with σ/k B T. This is related to disorder-induced filamentary currents and thus locally enhanced triplet densities. We predict that a maximum for the TTA process near room temperature or above requires typically a disorder parameter of at least 70 meV.

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Section: Introductionmentioning

confidence: 99%

Kinetic Monte Carlo Study of Triplet-Triplet Annihilation in Conjugated Luminescent Materials

et al. 2020

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“…These results agree with numerical studies of engineered morphologies. 26,27 An increase in energetic disorder can substantially reduce the V oc due to the trapping behavior of the disorder-induced tail states. 25 In addition, rising carrier densities and increased illumination rates lead to a higher V oc .…”

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confidence: 99%

Kinetic Monte Carlo Study of the Role of the Energetic Disorder on the Open-Circuit Voltage in Polymer/Fullerene Solar Cells

Kaiser

Gagliardi

2019

J. Phys. Chem. Lett.

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One major factor limiting the efficiency in organic solar cells (OSCs) is the low open-circuit voltage (V oc). Existing theoretical studies link the V oc with the charge transfer (CT) state and nonradiative recombination. However, also morphology and energetic disorder can have a strong impact on the V oc within realistic bulk-heterojunction OSCs. In this work, we present a kinetic Monte Carlo study on the role of the energetic disorder on the maximum V oc. We compute the quasi-Fermi level splitting for different energetic disorder and analyze the impact of the energetic disorder at the donor–acceptor interface as well as correlations in the site energies on the V oc. Our results show that the interface strongly controls the maximum V oc. For a higher interface disorder, charge densities and nongeminate recombination increase and the V oc is reduced. Furthermore, the correlated morphologies show an increase in the maximum V oc and a reduced impact of the energetic disorder.

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“…[ 12 ] High V oc are attributed to the formation of a Schottky barrier between the fullerene matrix and a high work function anode. [ 15 ] In addition, reduced recombination [ 16 ] and ideal morphology at the extraction layers [ 17 ] can further reduce the V oc losses in dilute donor OSCs.…”

Section: Introductionmentioning

confidence: 99%

Origin of Hole Transport in Small Molecule Dilute Donor Solar Cells

Kaiser

Murthy

Chung

et al. 2021

Adv Energy and Sustain Res

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Dilute donor organic solar cells (OSCs) are a promising technology to circumvent the trade‐off between open‐circuit voltage (Voc) and short‐circuit current density (Jsc). The origin of hole transport in OSCs with donor concentrations below the percolation threshold is diversely discussed in the community. Herein, both hole back transfer and long‐range hopping (tunneling) are analyzed as possible mechanisms of photocurrent in small molecule dilute donor OSCs using kinetic Monte Carlo (kMC) simulations. In contrast to previous kMC studies, the driving force for exciton dissociation is accounted for. As a study system, nitrogen‐bridged terthiophene (NBTT) molecules in a [6,6]‐phenyl‐C70‐butyric acid methyl ester (PC71BM) matrix are investigated. The simulations show that hole back transfer from the small molecule donor to the fullerene matrix explains the measured concentration dependences of the photocurrents as well as the Jsc dependence on the light intensity for donor concentrations below 5 wt%. For 5 wt%, distances between NBTT molecules decrease to reasonable ranges that long‐range hopping or tunneling cannot be discounted. Compared with polymer donors, larger hole localization is observed. The results emphasize that the barrier for hole back transfer is not only due to the highest occupied molecular orbital (HOMO) offset, but also by hole localization.

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Impact of Phosphorescent Sensitizers and Morphology on the Photovoltaic Performance in Organic Solar Cells

Cited by 7 publications

References 99 publications

Kinetic Monte Carlo Study of Triplet-Triplet Annihilation in Conjugated Luminescent Materials

Kinetic Monte Carlo Study of Triplet-Triplet Annihilation in Conjugated Luminescent Materials

Kinetic Monte Carlo Study of the Role of the Energetic Disorder on the Open-Circuit Voltage in Polymer/Fullerene Solar Cells

Origin of Hole Transport in Small Molecule Dilute Donor Solar Cells

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