How the Energetic Landscape in the Mixed Phase of Organic Bulk Heterojunction Solar Cells Evolves with Fullerene Content

Sweetnam, Sean; Prasanna, Rohit; Burke, Timothy M.; Bartelt, Jonathan A.; McGehee, Michael D.

doi:10.1021/acs.jpcc.6b00753

Cited by 22 publications

(29 citation statements)

References 24 publications

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“…The energy level of the CT state is determined by its local environment such that states in intimately mixed phases are lower in energy . This implies that the interface between donor and acceptor domains need not be sharp—in fact, a concentration gradient between the two components at an interface has been suggested to create an energetic gradient that promotes efficient charge separation . Optimal solar cell performance can be achieved by balancing ordered and pure domains of each component with an interfacial mixed phase to promote exciton dissociation into free charges .…”

Section: A Brief History Of Solvent Additives In Organic Photovoltaicsmentioning

confidence: 99%

“…The slower drying of ODT promotes the formation of three phases: 1) an aggregated polymer phase formed by the rapid evaporation of the deposition solvent; 2) a mixed polymer–fullerene phase; and 3) fullerene solvated by ODT, which becomes fullerene aggregates as the additive evaporates. Again, this “three‐phase” morphology is believed to drive efficient exciton dissociation . Lee et al proposed that processing additives should be selected based on two primary criteria: 1) selective solubility of the fullerene component; and 2) having a higher boiling point (and thus lower vapor pressure) than the deposition solvent …”

Section: A Brief History Of Solvent Additives In Organic Photovoltaicsmentioning

confidence: 99%

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Solvent Additives: Key Morphology‐Directing Agents for Solution‐Processed Organic Solar Cells

et al. 2018

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Organic photovoltaics (OPV) have the advantage of possible fabrication by energy-efficient and cost-effective deposition methods, such as solution processing. Solvent additives can provide fine control of the active layer morphology of OPVs by influencing film formation during solution processing. As such, solvent additives form a versatile method of experimental control for improving organic solar cell device performance. This review provides a brief history of solution-processed bulk heterojunction OPVs and the advent of solvent additives, putting them into context with other methods available for morphology control. It presents the current understanding of how solvent additives impact various mechanisms of phase separation, enabled by recent advances in in situ morphology characterization. Indeed, understanding solvent additives' effects on film formation has allowed them to be applied and combined effectively and synergistically to boost OPV performance. Their success as a morphology control strategy has also prompted the use of solvent additives in related organic semiconductor technologies. Finally, the role of solvent additives in the development of next-generation OPV active layers is discussed. Despite concerns over their environmental toxicity and role in device instability, solvent additives remain relevant morphological directing agents as research interests evolve toward nonfullerene acceptors, ternary blends, and environmentally sustainable solvents.

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Section: A Brief History Of Solvent Additives In Organic Photovoltaicsmentioning

confidence: 99%

Section: A Brief History Of Solvent Additives In Organic Photovoltaicsmentioning

confidence: 99%

Solvent Additives: Key Morphology‐Directing Agents for Solution‐Processed Organic Solar Cells

et al. 2018

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“…Equation (2) suggests that variations in the donor ionization potential (IP D ) would induce variations in the E CT . [6,9,27,[46][47][48][49][50] When C 60 is used as the acceptor, the effect of acceptor orientation is minimal thanks to its spherical symmetry. [6,9,27,[46][47][48][49][50] When C 60 is used as the acceptor, the effect of acceptor orientation is minimal thanks to its spherical symmetry.…”

Section: First Principle Calculations To Understand the Origin Of Ct mentioning

confidence: 99%

Open‐Circuit Voltage in Organic Solar Cells: The Impacts of Donor Semicrystallinity and Coexistence of Multiple Interfacial Charge‐Transfer Bands

Ndjawa

Graham

Mollinger

et al. 2017

Advanced Energy Materials

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excited states are referred to as chargetransfer (CT) states and are notable for determining the mechanism of free carrier generation and recombination processes. [8] The CT state energy (E CT ), defined as the energy required to promote the D-A complex from its ground state to its vibrationally relaxed first excited state, has been found to strongly depend on structural and conformational factors, such as the aggregation state of the donor and/or acceptor near the D/A junction, and the relative conformation of D and A molecules forming the CT complex. [9][10][11][12][13][14] Recent studies have revealed a strong dichroic absorption of CT complexes at planar interfaces in bilayers with crystalline donors, thus providing experimental evidence for oriented CT complexes. [15] Several models have related E CT to the open-circuit voltage (V OC ) in organic solar cells. [2,[16][17][18] The fundamental and striking feature that emerges from these models is that V OC depends linearly on E CT and only logarithmically on other factors such as the number, oscillator strength, and lifetime of the CT states.Because of this linear dependence of V OC on E CT , strategies to improve V OC are often oriented toward increasing E CT . E CT can be roughly viewed as the energy difference between the donor ionization potential (IP) and the acceptor electron affinity (EA), corrected to the binding energy between a hole in D molecule(s) and an electron on adjacent A molecule(s) within their respective environments, as highlighted schematically in Figure 1. Because the energy levels of both the donor and acceptor depend strongly on morphological factors such as

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“…The exact numerical values of the molecular energy levels of the mixed-composition phase and their impact on device performance is an area of active research. [53][54][55] Our goal in this work is not to include every possible detail in our simulated OPV devices, but rather to choose a consistent, realistic model for which we can controllably tune the degree of mixing to see how the resulting device behavior changes. Nonetheless, we have also simulated the morphological impact of the components' energy levels on device performance by implementation of a hopping model for charge transport across internal interfaces, [56,57] and then studied the effects of mixing on top of the variableenergy hopping.…”

Section: Mapping Morphology To Simulated Device Parametersmentioning

confidence: 99%

Drift-Diffusion Studies of Compositional Morphology in Bulk Heterojunctions: The Role of the Mixed Phase in Photovoltaic Performance

Finck

Schwartz

2016

Phys. Rev. Applied

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The active layers of most OPV devices are constructed from a blend of two organic compounds. The two materials spontaneously segregate into pure component phases during device fabrication, creating a bicontinuous network of conduction pathways that are selective for electron or hole charge carriers. The morphological distribution of these materials within the active layer has long been known to influence charge transport and resulting device performance. In addition to the two purecomponent phases present in these devices, a third, mixed-composition phase exists at the interface between the two pure phases. The exact effects of this mixed-composition phase on OPV device performance are not well understood, although it has been argued that the presence of a mixed phase is necessary for optimal device operation. In this work, we probe the effects of having a mixed-composition, interfacial phase on the performance and charge transport characteristics of organic photovoltaic (OPV) devices through a series of drift-diffusion model simulations. We start with set of model morphologies with only pure component phases and then introduce an interfacial, mixed-phase in a controllable fashion. Our simulations show that a modest amount of mixing initially improves device efficiency by reducing the tortuosity of the device's conduction pathways and easing morphological traps. However, an excessive amount of mixing can actually degrade highconductivity pathways, reducing photovoltaic performance. The point at which mixing switches from being beneficial to instead detrimental to OPV performance differs depending on the average domain size of a device's morphology. Devices with smaller feature sizes are more susceptible to the debilitating effects of overmixing, so that the presence of a mixed-phase may either raise power conversion efficiency by as much as 100% or lower it by as much as 50%, depending on the average domain size and the extent of mixing. This suggests that variations in the amount of mixed-composition phase with different processing conditions is one of the key factors that makes optimizing bulk heterojunction OPV devices difficult.

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How the Energetic Landscape in the Mixed Phase of Organic Bulk Heterojunction Solar Cells Evolves with Fullerene Content

Cited by 22 publications

References 24 publications

Solvent Additives: Key Morphology‐Directing Agents for Solution‐Processed Organic Solar Cells

Solvent Additives: Key Morphology‐Directing Agents for Solution‐Processed Organic Solar Cells

Open‐Circuit Voltage in Organic Solar Cells: The Impacts of Donor Semicrystallinity and Coexistence of Multiple Interfacial Charge‐Transfer Bands

Drift-Diffusion Studies of Compositional Morphology in Bulk Heterojunctions: The Role of the Mixed Phase in Photovoltaic Performance

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