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One of the substrate layouts for our organic solar cells
Credit: AFMD Group

Moritz Riede

Professor of Soft Functional Nanomaterials

Research theme

  • Photovoltaics and nanoscience

Sub department

  • Condensed Matter Physics

Research groups

  • Advanced Functional Materials and Devices (AFMD) Group
moritz.riede@physics.ox.ac.uk
Telephone: 01865 (2)72377 (office),01865 (2)82095 (lab)
  • About
  • Research
  • Teaching
  • Publications

Organic solar cells—the path to commercial success

Advanced Energy Materials Wiley 11:1 (2020) 2002653

Authors:

Moritz Riede, Donato Spoltore, Karl Leo

Abstract:

Organic solar cells have the potential to become the cheapest form of electricity, beating even silicon photovoltaics. This article summarizes the state of the art in the field, highlighting research challenges, mainly the need for an efficiency increase as well as an improvement in long‐term stability. It discusses possible current and future applications, such as building integrated photovoltaics or portable electronics. Finally, the environmental footprint of this renewable energy technology is evaluated, highlighting the potential to be the energy generation technology with the lowest carbon footprint of all.
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The role of spin in the degradation of organic photovoltaics

Nature Communications Springer Nature 12:1 (2021) 471

Authors:

Ivan Ramirez, Alberto Privitera, Safakath Karuthedath, Anna Jungbluth, Johannes Benduhn, Andreas Sperlich, Donato Spoltore, Koen Vandewal, Frédéric Laquai, Moritz Riede

Abstract:

Stability is now a critical factor in the commercialization of organic photovoltaic (OPV) devices. Both extrinsic stability to oxygen and water and intrinsic stability to light and heat in inert conditions must be achieved. Triplet states are known to be problematic in both cases, leading to singlet oxygen production or fullerene dimerization. The latter is thought to proceed from unquenched singlet excitons that have undergone intersystem crossing (ISC). Instead, we show that in bulk heterojunction (BHJ) solar cells the photo-degradation of C<sub>60</sub> via photo-oligomerization occurs primarily via back-hole transfer (BHT) from a charge-transfer state to a C<sub>60</sub> excited triplet state. We demonstrate this to be the principal pathway from a combination of steady-state optoelectronic measurements, time-resolved electron paramagnetic resonance, and temperature-dependent transient absorption spectroscopy on model systems. BHT is a much more serious concern than ISC because it cannot be mitigated by improved exciton quenching, obtained for example by a finer BHJ morphology. As BHT is not specific to fullerenes, our results suggest that the role of electron and hole back transfer in the degradation of BHJs should also be carefully considered when designing stable OPV devices.
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Controlling energy levels and Fermi level en route to fully tailored energetics in organic semiconductors

Nature Communications Nature Research 10:1 (2019) 5538

Authors:

R Warren, A Privitera, P Kaienburg, AE Lauritzen, O Thimm, J Nelson, Moritz Riede

Abstract:

Simultaneous control over both the energy levels and Fermi level, a key breakthrough for inorganic electronics, has yet to be shown for organic semiconductors. Here, energy level tuning and molecular doping are combined to demonstrate controlled shifts in ionisation potential and Fermi level of an organic thin film. This is achieved by p-doping a blend of two host molecules, zinc phthalocyanine and its eight-times fluorinated derivative, with tunable energy levels based on mixing ratio. The doping efficiency is found to depend on host mixing ratio, which is explained using a statistical model that includes both shifts of the host's ionisation potentials and, importantly, the electron affinity of the dopant. Therefore, the energy level tuning effect has a crucial impact on the molecular doping process. The practice of comparing host and dopant energy levels must consider the long-range electrostatic shifts to consistently explain the doping mechanism in organic semiconductors.
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Direct visualization of the charge transfer state dynamics in dilute-donor organic photovoltaic blends

Nature Communications Nature Research 15:1 (2024) 9851

Authors:

Gareth John Moore, Florian Günther, Kaila M Yallum, Martina Causa’, Anna Jungbluth, Julien Réhault, Moritz Riede, Frank Ortmann, Natalie Banerji

Abstract:

The interconversion dynamics between charge transfer state charges (CTCs) and separated charges (SCs) is still an unresolved issue in the field of organic photovoltaics. Here, a transient absorption spectroscopy (TAS) study of a thermally evaporated small-molecule:fullerene system (α6T:C60) in different morphologies (dilute intermixed and phase separated) is presented. Spectral decomposition reveals two charge species with distinct absorption characteristics and different dynamics. Using time-dependent density functional theory, these species are identified as CTCs and SCs, where the spectral differences arise from broken symmetry in the charge transfer state that turns forbidden transitions into allowed ones. Based on this assignment, a kinetic model is formulated allowing the characterization of the charge generation, separation, and recombination mechanisms. We find that SCs are either formed directly from excitons within a few picoseconds or more slowly (~30–80 ps) from reversible splitting of CTCs. These findings constitute the first unambiguous observation of spectrally resolved CTCs and SCs.
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From generation to collection – impact of deposition temperature on charge carrier dynamics of high-performance vacuum-processed organic solar cells

Energy & Environmental Science Royal Society of Chemistry (RSC) (2024)

Authors:

Richard Adam Pacalaj, Yifan Dong, Ivan Ramirez, Roderick CI MacKenzie, Seyed Mehrdad Hosseini, Eva Bittrich, Julian Eliah Heger, Pascal Kaienburg, Subhrangsu Mukherjee, Jiaying Wu, Moritz Riede, Harald Ade, Peter Müller-Buschbaum, Martin Pfeiffer, James Robert Durrant

Abstract:

<jats:p>Substrate heating during co-evaporation of bulk heterojunction organic solar cells aids phase separation and improves performance. While recombination remains unaffected, hole transport improves due to more crystalline donor domains.</jats:p>
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