Exciton diffusion plays a decisive role in determining the recent rise in charge-generation yield (CGY) and powerconversion efficiency brought about by non-fullerene acceptor (NFA) solar cells. In this presentation a technique, named pulsed-PLQY, is introduced to measure the exciton diffusion length in organic semiconductors through exciton-exciton annihilation (EEA) without the need for temporal measurements. Pulsed-PLQY is validated by comparing to established EEA techniques using both simulated and experimental results. It is found that Pulsed-PLQY conserves the validity of established EEA techniques while the implementation is faster, easier, significantly simplifies the equipment required, and is less sensitive to experimental conditions than traditional EEA techniques. Utilising pulsed-PLQY, it is found that NFA’s have increased diffusion lengths, compared to fullerene acceptors, and that this increase is driven by increases in diffusivity
Ultrabithorax (Ubx) is a Hox gene transcription factor regulating the growth of wings and limbs in Drosophila melanogaster. However, the protein can also be produced recombinantly, and can self-assemble to form a film at the air-water interface. Materials drawn from this film, including fibres and thin coatings, are elastic, bio- and cyto-compatible. Moreover, Ubx can be functionalised with other biomolecules to form protein fusions. In this work, the physical properties of Ubx fibres and electrospun mats containing Ubx were investigated. An electrical conductivity comparable to semiconductors was discovered in enhanced green fluorescent protein- Ubx (EGFP-Ubx) fibres. The photoluminescence properties of pure Ubx, EGFP-Ubx, and electrospun poly(ethylene oxide)/EGFP-Ubx complexes were compared and the fluorescence emission peaks were found at 420nm for poly(ethylene oxide), 442nm for Ubx, and 512nm for EGFP. Moreover, the effect of material production method on the fluorescence lifetime was investigated and revealed differences between self-assembled microfibres and electrospun mats with fibre diameters below 1 μm. Finally, Ubx fibres were functionalised with DNA aptamers, and E.coli binding was increased using three different aptamers compared to pure fibres. The increase was 15% increase for the P12 aptamer, 92% for the STC12 aptamer, and 167% for the Antibac2 aptamer. Developments in large-scale material formation could support the functional Ubx materials in becoming a platform material for biosensing and tissue regeneration.
The spectral shape near the energy gap determines the radiative limit of the open-circuit voltage in organic photovoltaic devices. In this work, we employ ultrasensitive photocurrent measurements and detect sub-gap states with energies far below gap in a large number of different donor-acceptor blends. We provide evidence that these low-energy sub-gap states are associated with radiative mid-gap trap states, generating photocurrent via an optical release process. To account for the radiative mid-gap states, we implement a two-diode model which accurately describes both the dark current and the open-circuit voltage in organic solar cells. These findings provide important insights for our current understanding of organic photovoltaic devices.
Probing the photovoltaic external quantum efficiency (EQE) at photon energies well below the semiconductor bandgap is an important tool for achieving a better understanding of the contribution of trap and tail states involved in charge generation processes in photovoltaic devices, notably solar cells. In this work, we present an electrical and optical noise-reduced EQE apparatus achieving 100 dB dynamic range. We carefully identify and study several device- and EQE apparatus-related factors limiting the EQE measurement sensitivity. Minimizing these factors allows us to detect photocurrents smaller than a fA, corresponding to EQE signals as small as -100 dB. We use these ultra-sensitive EQE measurements to probe weak sub-bandgap absorption features in organic, inorganic and perovskite semiconductors. In this regard, we directly observe photocurrent-contributing sub-gab trap states in organic solar cells significantly lower in energy than the corresponding charge-transfer state.
Recent advances in organic solar cell material development based around non-fullerene electron acceptors in bulk heterojunctions have propelled power conversion efficiencies to >18%, with 20% on the horizon and 25% predicted. These efficiencies are close to traditional inorganic semiconductor photovoltaics and thus focus is now turning to manufacturability and creating a viable solar cell technology. In this presentation we report the highest efficiency to date (16% with a Fill Factor >70%) in a thick junction binary organic solar cell based upon PM6:BTP-eC9. Using a very accurate approach based upon temperature dependent ultra-sensitive EQE measurements we find that this system (and a similar one based upon PM6:Y6) have near unity charge generation yields (CGY > 99%). In this regime, we observe that a small increase in CGY of only 0.5% leads to a 2.5 times more reduction in bimolecular recombination relative to the Langevin limit enabling high efficiency thick junction solar cells.
Non-radiative losses to the open-circuit voltage are a primary factor in limiting the power conversion efficiency of organic photovoltaic solar cells. The dominate non-radiative loss is intrinsic to the active layer which, along with the thermodynamic limit to the open-circuit voltage, define the quasi-Fermi level splitting (QFLS). Quantification of the QFLS in organic photovoltaic devices is challenging due to the excitonic nature of photoexcitation and device-related non-radiative losses. In this presentation I will outline an experimental approach based on electro-modulated photoluminescence to quantify the QFLS in organic solar cells. Drift-diffusion simulations are used to verify the accuracy of the method, while state-of-the art PM6:Y6 solar cells are created with varying non-radiative losses. This method quantifies the QFLS in organic photovoltaics, fully characterizing the magnitude of different contributions to the non-radiative losses of the open-circuit voltage.
Interest in the hybrid organic-inorganic perovskite semiconductors has skyrocketed in recent years due to unprecedented high performance as solution-processable absorber layers in solar cells [1]. These materials also have potential for applications in semiconductor spintronics due to their large spin-orbit interaction. Theoretical studies predict a large Rashba spin splitting [2], and recent experiments have revealed a large photoinduced magnetization [3-5] and spin-dependent optical Stark effect [6], yet still relatively little is known about the spin-related properties of these materials. Here we report polarization-dependent pump probe studies of the 2D perovskite butylammonium methylammonium lead iodide. Our experiments indicate a strong influence of the Rashba spin splitting on the carrier kinetics in this system, consistent with our recent four-wave mixing studies of bulk CH3NH3PbI3.
[1] https://www.nrel.gov/pv/assets/images/efficiency_chart.jpg.
[2] M. Kepenekian and J. Even, J. Phys. Chem. Lett. 8, 3362 (2017).
[3] D. Giovanni et al. Nano Lett. 15, 1553 (2015).
[4] C. Zhang et al. Nat. Phys. 11, 427 (2015).
[5] P. Odenthal et al. Nat. Phys. 13, 894 (2017).
[6] D. Giovanni et al. Science Advances 2, e1600477 (2016).
The lead halide hybrid perovskites have gained considerable attention in recent years due to their stellar performance as absorber layers in solution-processed solar cells, with efficiencies recently reaching over 22 percent [1]. Owing to their large spin-orbit coupling, these materials are also of interest for spintronic applications, in which the presence of lead may be less of an impediment to their adoption [2]. Measurements of spin dynamics in bulk CH3NH3PbI3-xClx have been reported in recent years [3,4,5], the spin-dependent optical Stark effect was demonstrated in 4F-PEPI [6], and a large Rashba effect has been predicted in both bulk and 2D perovskites [2], highlighting the need for further studies of the spin-related properties of these materials. Here we report spin-dependent measurements of carrier kinetics in butylammonium methylammonium lead iodide 2D perovskite and measurements of the coherent carrier response in 3D CH3NH3PbI3. Both experiments provide direct evidence of the impact of Rashba on the carrier kinetics in these systems, further supporting the potential for developing spin-optoelectronic devices using these materials.
[1] https://www.nrel.gov/pv/assets/images/efficiency_chart.jpg.
[2] M. Kepenekian and J. Even, J. Phys. Chem. Lett. 8, 3362 (2017).
[3] D. Giovanni et al. Nano Lett. 15, 1553 (2015).
[4] C. Zhang et al. Nat. Phys. 11, 427 (2015).
[5] P. Odenthal et al. Nat. Phys. 13, 894 (2017).
[6] D. Giovanni et al. Science Advances 2, e1600477 (2016).
KEYWORDS: Solar cells, Four wave mixing, Semiconductors, Ultrafast phenomena, Thin film solar cells, Thin films, Excitons, Perovskite, Signal detection, Absorption, Spectroscopes, Dielectrics
The interest in perovskite-based solar cell absorber materials has skyrocketed in recent years due to the rapid rise in solar cell efficiency and the potential for cost reductions tied to solution-processed device fabrication. Due to complications associated with the presence of strong static and dynamic disorder in these organic-inorganic materials, the fundamental photophysical behavior of photo-excited charge carriers remains unclear. We apply four-wave mixing spectroscopy to study the charge carrier dynamics in CH3NH3PbI3 thin films. Our experiments reveal two discrete optical transitions below the band gap of the semiconductor with binding energies of 13 meV and 29 meV, attributed to free and defect-bound excitons respectively.
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