Armantas Melianas received his Ph.D. in Applied Physics from Linköping University (Sweden) in 2017 for his work on time-resolved charge transport in organic solar cells in the groups of Prof. Olle Inganäs and Prof. Martijn Kemerink. He joined Stanford on October 2017 as a recipient of the Wallenberg Foundation Postdoctoral Fellowship and is currently developing organic electronic materials and devices for applications ranging from solar energy harvesting to brain-inspired (neuromorphic) computing.
Honors & Awards
Wallenberg Postdoctoral Fellowship, The Knut and Alice Wallenberg Foundation (2017)
Doctor of Philosophy, Linkoping University (2017)
MSc, Vilnius University, Materials Technology (2012)
BSc, Vilnius University, Physics (2010)
Alberto Salleo, Postdoctoral Faculty Sponsor
Nonequilibrium site distribution governs charge-transfer electroluminescence at disordered organic heterointerfaces.
Proceedings of the National Academy of Sciences of the United States of America
The interface between electron-donating (D) and electron-accepting (A) materials in organic photovoltaic (OPV) devices is commonly probed by charge-transfer (CT) electroluminescence (EL) measurements to estimate the CT energy, which critically relates to device open-circuit voltage. It is generally assumed that during CT-EL injected charges recombine at close-to-equilibrium energies in their respective density of states (DOS). Here, we explicitly quantify that CT-EL instead originates from higher-energy DOS site distributions significantly above DOS equilibrium energies. To demonstrate this, we have developed a quantitative and experimentally calibrated model for CT-EL at organic D/A heterointerfaces, which simultaneously accounts for the charge transport physics in an energetically disordered DOS and the Franck-Condon broadening. The 0-0 CT-EL transition lineshape is numerically calculated using measured energetic disorder values as input to 3-dimensional kinetic Monte Carlo simulations. We account for vibrational CT-EL overtones by selectively measuring the dominant vibrational phonon-mode energy governing CT luminescence at the D/A interface using fluorescence line-narrowing spectroscopy. Our model numerically reproduces the measured CT-EL spectra and their bias dependence and reveals the higher-lying manifold of DOS sites responsible for CT-EL. Lowest-energy CT states are situated 180 to 570 meV below the 0-0 CT-EL transition, enabling photogenerated carrier thermalization to these low-lying DOS sites when the OPV device is operated as a solar cell rather than as a light-emitting diode. Nonequilibrium site distribution rationalizes the experimentally observed weak current-density dependence of CT-EL and poses fundamental questions on reciprocity relations relating light emission to photovoltaic action and regarding minimal attainable photovoltaic energy conversion losses in OPV devices.
View details for DOI 10.1073/pnas.1908776116
View details for PubMedID 31690666
- Parallel programming of an ionic floating-gate memory array for scalable neuromorphic computing SCIENCE 2019; 364 (6440): 570-+
Photogenerated Charge Transport in Organic Electronic Materials: Experiments Confirmed by Simulations.
Advanced materials (Deerfield Beach, Fla.)
The performance of organic optoelectronic devices, such as organic photovoltaic (OPV) cells, is to a large extent dictated by their ability to transport the photogenerated charge, with relevant processes spanning a wide temporal (fs-s) and spatial (1-100 nm) range. However, time-resolved techniques can access only a limited temporal window, and often contradict steady-state measurements. Here, commonly employed steady-state and time-resolved techniques are unified over an exceptionally wide temporal range (fs-s) in a consistent physical picture. Experimental evidence confirmed by numerical simulations shows that, although various techniques probe different time scales, they are mutually consistent as they probe the same physical mechanisms governing charge motion in disordered media-carrier hopping and thermalization in a disorder-broadened density of states (DOS). The generality of this framework is highlighted by time-resolved experimental data obtained on polymer:fullerene, polymer:polymer, and small-molecule blends with varying morphology, including recent experiments revealing that low donor content OPV devices operate by long-range hole tunneling between non-nearest-neighbor molecules. The importance of nonequilibrium processes in organic electronic materials is reviewed, with a particular focus on experimental data and understanding charge transport physics in terms of material DOS.
View details for DOI 10.1002/adma.201806004
View details for PubMedID 30719756
Organic Electronics for Neuromorphic Computing
2018; 1 (7): 386-397
View details for DOI 10.1038/s41928-018-0103-3
Charge Transport in Pure and Mixed Phases in Organic Solar Cells
ADVANCED ENERGY MATERIALS
2017; 7 (20): 1700888
View details for DOI 10.1002/aenm.201700888
Photogenerated Carrier Mobility Significantly Exceeds Injected Carrier Mobility in Organic Solar Cells
ADVANCED ENERGY MATERIALS
2017; 7 (9): 1602143
View details for DOI 10.1002/aenm.201602143
Photo-generated carriers lose energy during extraction from polymer-fullerene solar cells
In photovoltaic devices, the photo-generated charge carriers are typically assumed to be in thermal equilibrium with the lattice. In conventional materials, this assumption is experimentally justified as carrier thermalization completes before any significant carrier transport has occurred. Here, we demonstrate by unifying time-resolved optical and electrical experiments and Monte Carlo simulations over an exceptionally wide dynamic range that in the case of organic photovoltaic devices, this assumption is invalid. As the photo-generated carriers are transported to the electrodes, a substantial amount of their energy is lost by continuous thermalization in the disorder broadened density of states. Since thermalization occurs downward in energy, carrier motion is boosted by this process, leading to a time-dependent carrier mobility as confirmed by direct experiments. We identify the time and distance scales relevant for carrier extraction and show that the photo-generated carriers are extracted from the operating device before reaching thermal equilibrium.
View details for DOI 10.1038/ncomms9778
View details for Web of Science ID 000366294700004
View details for PubMedID 26537357
View details for PubMedCentralID PMC4659933
- Dispersion-Dominated Photocurrent in Polymer: Fullerene Solar Cells ADVANCED FUNCTIONAL MATERIALS 2014; 24 (28): 4507-4514
Equilibrated Charge Carrier Populations Govern Steady-State Nongeminate Recombination in Disordered Organic Solar Cells.
The journal of physical chemistry letters
We employed bias-assisted charge extraction techniques to investigate the transient and steady-state recombination of photogenerated charge carriers in complete devices of a disordered polymer-fullerene blend. Charge recombination is shown to be dispersive, with a significant slowdown of the recombination rate over time, consistent with the results from kinetic Monte Carlo simulations. Surprisingly, our experiments reveal little to no contributions from early time recombination of nonequilibrated charge carriers to the steady-state recombination properties. We conclude that energetic relaxation of photogenerated carriers outpaces any significant nongeminate recombination under application-relevant illumination conditions. With equilibrated charges dominating the steady-state recombination, quasi-equilibrium conceptsappear suited for describing the open-circuit voltage of organic solar cells despite pronounced energetic disorder.
View details for DOI 10.1021/acs.jpclett.9b00516
View details for PubMedID 30829040
- Mechanisms for Enhanced State Retention and Stability in Redox-Gated Organic Neuromorphic Devices ADVANCED ELECTRONIC MATERIALS 2019; 5 (2)
- Comment on "Charge Carrier Extraction in Organic Solar Cells Governed by Steady-State Mobilities" ADVANCED ENERGY MATERIALS 2018; 8 (36)
- Automated open-source software for charge transport analysis in single-carrier organic semiconductor diodes ORGANIC ELECTRONICS 2018; 61: 318–28
- Dead Ends Limit Charge Carrier Extraction from All-Polymer Bulk Heterojunction Solar Cells ADVANCED ELECTRONIC MATERIALS 2018; 4 (8)
- Relating open-circuit voltage losses to the active layer morphology and contact selectivity in organic solar cells JOURNAL OF MATERIALS CHEMISTRY A 2018; 6 (26): 12574–81
- Optimized pulsed write schemes improve linearity and write speed for low-power organic neuromorphic devices JOURNAL OF PHYSICS D-APPLIED PHYSICS 2018; 51 (22)
- Thermal annealing reduces geminate recombination in TQ1:N2200 all-polymer solar cells JOURNAL OF MATERIALS CHEMISTRY A 2018; 6 (17): 7428–38
- A fullerene alloy based photovoltaic blend with a glass transition temperature above 200 degrees C JOURNAL OF MATERIALS CHEMISTRY A 2017; 5 (8): 4156-4162
Role of coherence and delocalization in photo-induced electron transfer at organic interfaces
Photo-induced charge transfer at molecular heterojunctions has gained particular interest due to the development of organic solar cells (OSC) based on blends of electron donating and accepting materials. While charge transfer between donor and acceptor molecules can be described by Marcus theory, additional carrier delocalization and coherent propagation might play the dominant role. Here, we describe ultrafast charge separation at the interface of a conjugated polymer and an aggregate of the fullerene derivative PCBM using the stochastic Schrödinger equation (SSE) and reveal the complex time evolution of electron transfer, mediated by electronic coherence and delocalization. By fitting the model to ultrafast charge separation experiments, we estimate the extent of electron delocalization and establish the transition from coherent electron propagation to incoherent hopping. Our results indicate that even a relatively weak coupling between PCBM molecules is sufficient to facilitate electron delocalization and efficient charge separation at organic interfaces.
View details for DOI 10.1038/srep32914
View details for Web of Science ID 000382648800001
View details for PubMedID 27605035
View details for PubMedCentralID PMC5015064
- New method for lateral mapping of bimolecular recombination in thin-film organic solar cells PROGRESS IN PHOTOVOLTAICS 2016; 24 (8): 1096-1108
- Nonequilibrium drift-diffusion model for organic semiconductor devices PHYSICAL REVIEW B 2016; 94 (3)
High-Entropy Mixtures of Pristine Fullerenes for Solution-Processed Transistors and Solar Cells
2015; 27 (45): 7325-?
The solubility of pristine fullerenes can be enhanced by mixing C60 and C70 due to the associated increase in configurational entropy. This "entropic dissolution" allows the preparation of field-effect transistors with an electron mobility of 1 cm(2) V(-1) s(-1) and polymer solar cells with a highly reproducible power-conversion efficiency of 6%, as well as a thermally stable active layer.
View details for DOI 10.1002/adma.201503530
View details for Web of Science ID 000367833200008
View details for PubMedID 26460821
- A New Fullerene-Free Bulk-Heterojunction System for Efficient High-Voltage and High-Fill Factor Solution-Processed Organic Photovoltaics ADVANCED MATERIALS 2015; 27 (11): 1900-?
- Fully-solution-processed organic solar cells with a highly efficient paper-based light trapping element JOURNAL OF MATERIALS CHEMISTRY A 2015; 3 (48): 24289-24296
- Comparison of selenophene and thienothiophene incorporation into pentacyclic lactam-based conjugated polymers for organic solar cells POLYMER CHEMISTRY 2015; 6 (42): 7402-7409
- Origin of Reduced Bimolecular Recombination in Blends of Conjugated Polymers and Fullerenes ADVANCED FUNCTIONAL MATERIALS 2013; 23 (34): 4262-4268
Unified Study of Recombination in Polymer:Fullerene Solar Cells Using Transient Absorption and Charge-Extraction Measurements
JOURNAL OF PHYSICAL CHEMISTRY LETTERS
2013; 4 (12): 2069-2072
Recombination in the well-performing bulk heterojunction solar cell blend between the conjugated polymer TQ-1 and the substituted fullerene PCBM has been investigated with pump-probe transient absorption and charge extraction of photogenerated carriers (photo-CELIV). Both methods are shown to generate identical and overlapping data under appropriate experimental conditions. The dominant type of recombination is bimolecular with a rate constant of 7 × 10(-12) cm(-3) s(-1). This recombination rate is shown to be fully consistent with solar cell performance. Deviations from an ideal bimolecular recombination process, in this material system only observable at high pump fluences, are explained with a time-dependent charge-carrier mobility, and the implications of such a behavior for device development are discussed.
View details for DOI 10.1021/jz4009745
View details for Web of Science ID 000320979400014
View details for PubMedID 26283254