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Instruments / Technical Area	Publication Details	Published
I07-Surface & interface diffraction	Limiting factors for charge generation in low-offset fullerene-based organic solar cells Anna Jungbluth , Eunkyung Cho , Alberto Privitera , Kaila M. Yallum , Pascal Kaienburg , Andreas E. Lauritzen , Thomas Derrien , Sameer V. Kesava , Irfan Habib , Saied Md Pratik , Natalie Banerji , Jean-Luc Brédas , Veaceslav Coropceanu , Moritz Riede Nature Communications 15 DOI: 10.1038/s41467-024-49432-5 Diamond Proposal Number(s): [26630] Open Access	Jun 2024
I07-Surface & interface diffraction	Probing the energy levels of organic bulk heterojunctions by varying the donor content Anna Jungbluth , Pascal Kaienburg , Andreas E. Lauritzen , Thomas Derrien , Moritz Riede Apl Materials 11 DOI: 10.1063/5.0148191 Diamond Proposal Number(s): [26630] Open Access Show Abstract ▼ Abstract: The performance of organic solar cells is strongly governed by the properties of the photovoltaic active layer. In particular, the energetics at the donor (D)–acceptor (A) interface dictate the properties of charge transfer (CT) states and limit the open-circuit voltage. More generally, energetic landscapes in thin films are affected by intermolecular, e.g., van der Waals, dipole, and quadrupole, interactions that vary with D:A mixing ratio and impact energy levels of free charges (ionization energy, electron affinity) and excitons (singlet, CT states). Disentangling how different intermolecular interactions impact energy levels and support or hinder free charge generation is still a major challenge. In this work, we investigate interface energetics of bulk heterojunctions via sensitive external quantum efficiency measurements and by varying the D:A mixing ratios of ZnPc or its fluorinated derivatives and C60. With increasing donor fluorination, the energetic offset between FxZnPc and C60 reduces. Moving from large to low offset systems, we find qualitatively different trends in device performances with D:C60 mixing ratios. We rationalize the performance trends via changes in the energy levels that govern exciton separation and voltage losses. We do so by carefully analyzing shifts and broadening sEQE spectra on a linear and logarithmic scale. Linking this analysis with molecular properties and device performance, we comment on the impact of charge–quadrupole interactions for CT dissociation and free charge generation in our D:C60 blends. With this, our work (1) demonstrates how relatively accessible characterization techniques can be used to probe energy levels and (2) addresses ongoing discussions on future molecular design and optimal D–A pairing for efficient CT formation and dissociation.	Jun 2023
I07-Surface & interface diffraction	Interfacial rearrangements and strain evolution in the thin film growth of ZnPc on glass Thomas L. Derrien , Andreas E. Lauritzen , Pascal Kaienburg , Ellis Hancox , Chris Nicklin , Moritz Riede Physical Review Materials 6 DOI: 10.1103/PhysRevMaterials.6.033401 Diamond Proposal Number(s): [20426] Open Access Show Abstract ▼ Abstract: We report on the characterization of the growth of vacuum-deposited zinc phthalocyanine (ZnPc) thin films on glass through a combination of in situ grazing incidence x-ray scattering, x-ray reflectivity, and atomic force microscopy. We found that the growth at room temperature proceeds via the formation of two structurally unique substrate-induced interfacial layers, followed by the growth of the γ -ZnPc polymorph thereafter (thickness ≈ 1.0 nm). As the growth of the bulk γ -ZnPc progresses, a substantial out-of-plane lattice strain ( ≈ 15 % relative to γ -ZnPc powder) is continually relaxed during the thin film growth. The rate of strain relaxation was slowed after a thickness of ≈ 13 nm, corresponding to the transition from layer growth to island growth. The findings reveal the real-time microstructural evolution of ZnPc and highlight the importance of substrate-induced strain on thin film growth.	Mar 2022
I07-Surface & interface diffraction	In-situ observations of the growth mode of vacuum deposited α-sexithiophene Thomas L. Derrien , Andreas E. Lauritzen , Pascal Kaienburg , Josue F. M. Hardigree , Christopher Nicklin , Moritz K. Riede The Journal Of Physical Chemistry C DOI: 10.1021/acs.jpcc.0c00447 Diamond Proposal Number(s): [20426, 24871] Open Access Show Abstract ▼ Abstract: The real-time morphological evolution of vacuum deposited α-sexithiophene (α-6T) on a weakly interacting (glass) substrate at ambient temperature is reported. In-situ grazing incidence small angle X-ray scattering (GISAXS) enabled the observation of nanoscale aggregates while in-situ grazing incidence wide angle scattering (GIWAXS) allowed the study of the molecular-scale morphology. The in-situ GISAXS measurements revealed that the α-6T growth proceeds via a Stranski-Krastanov mode, whereby 2-4 complete monolayers are deposited followed by subsequent layers formed via island growth. In-situ GIWAXS also showed the evolution of the polymorph composition during the thin film growth. Initially the disordered β-phase and the low-temperature (LT) phase are deposited in nearly equal proportion until a thickness of 8 nm whereby the LT-phase begins to dominate until a final α-6T thickness of 50 nm where the scattering intensity of the LT-phase is more than double that of the β-phase. The change in polymorph composition coincided with an increase in the LT-phase d-spacing, indicating a lattice strain relief as the thin film moves from surface to bulk mediated growth. The GISAXS findings were confirmed through direct imaging using ex-situ atomic force microscopy (AFM) at various thicknesses revealing the existence of both initial monolayers and intermediate and final island morphologies. The findings reveal the real-time morphological evolution of α-6T across both the molecular scale and the nanoscale and highlight the role of strain in polymorph growth. Due to the importance of thin film microstructure in device performance, it is expected that these results will aid in the development of the structure-property relationships necessary to realise the full potential of organic electronics.	May 2020

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