ORGANIC SOLAR CELLS
Organic solar cells (OSCs) are currently attracting significant interest due to a number of appealing characteristics such as low cost, low environmental impact, solution-processing ability, conformability, and large-area manufacturing capability. Over the last decade, advances in materials chemistry, device engineering, modeling, and other disciplines resulted in the development of OSC devices with power conversion efficiencies over 13% in single-junction cells.
By exploiting an integrated approach that combines quantum-chemical calculations based on density functional theory, theoretical modeling, and molecular dynamics simulations, our studies focus on establishing chemical structure–electronic structure–morphology–performance relationships. We are currently interested in OSCs where the active layers are based on blends of polymer donors with polymer or small-molecule nonfullerene acceptors. We address a number of fundamental issues related to the description of the photo-excited states and the processes of charge separation and charge recombination at the donor/acceptor interfaces.
While the electronic properties clearly play a key role in the device properties, the morphological and thermal properties of the organic materials are also determining factors. Work in our group addresses the following questions:
How do the polymer chains or molecules pack in the pure domains and in the mixed phases when two or more materials form a blend?
How can we describe the miscibility and phase separation in these blends?
How do the processing conditions impact the packing and morphology of both single-component and multi-component active layers?
How do chemical modifications impact the morphological and thermal properties of the materials?
“Quantitative Relations between Interaction Parameter, Miscibility, and Function in Organic Solar Cells”, L. Ye, H. Hu, M. Ghasemi, T. Wang, B. Collins, J. Kim, K. Jiang, J. Carpenter, H. Li, Z. Li, T. McAfee, J. Zhao, X. Chen, J. Lai, T. Ma, J. L. Brédas, H. Yan, and H. Ade, Nature Mater., DOI: 10.1038/s41563-017-0005-1 (2018).
“Voltage Losses in Organic Solar Cells: Understanding the Contributions of Intramolecular Vibrations to Nonradiative Recombinations”, X. K. Chen, and J. L. Brédas, Adv. Energy Mater., DOI: 10.1002/aenm.201702227 (2017).
“Photovoltaic Concepts Inspired by Coherence Effects in Photosynthetic Systems”, J. L. Brédas, E. H. Sargent, and G.D. Scholes, Nature Mater., 16, 35-44 (2017).
“Molecular Understanding of Fullerene–Electron Donor Interactions in Organic Solar Cells”, S. M. Ryno, M. K. Ravva, X. K. Chen, H. Li, J. L. Brédas, Adv. Energy Mater., 7, 1601370 (2017).
“Suppressing Energy Loss due to Triplet Exciton Formation in Organic Solar Cells: The Role of Chemical Structures and Molecular Packing”, X. K. Chen, T. Wang, J. L. Brédas, Adv. Energy Mater., 7, 1602713 (2017).
“Effect of Solid-State Polarization on Charge-Transfer Excitations and Transport Levels at Organic Interfaces from a Screened Range-Separated Hybrid Functional”, Z. Zheng, D. A. Egger, J. L. Brédas, L. Kronik, and V. Coropceanu, J. Phys. Chem. Lett., 8, 3277-3283 (2017).
ORGANIC LIGHT-EMITTING DIODES
Organic light-emitting diodes (OLEDs) offer tremendous potential for use in full-color displays and solid-state lighting. To fabricate more efficient and less expensive devices that produce strong electro-luminescence, a better understanding of the OLED electronic and optical processes is essential.
In conventional OLEDs based on purely organic emitters, singlet and triplet excited states are produced by charge-recombination events with a probability of ¼ and ¾, respectively. There is currently a major interest in exploiting thermally activated delayed fluorescence (TADF) in order to overcome the 25% internal quantum efficiency limit imposed by spin statistics.Using quantum-mechanical calculations coupled with large-scale molecular dynamics simulations, we are investigating TADF emitters based on both donor-acceptor molecules and donor-acceptor exciplexes, and focus on:
(i) Evaluating the excited-state properties of molecular and exciplex TADF emitters in both singlet and triplet manifolds.
(ii) Understanding the rates of intersystem crossing between triplet and singlet excited state.
(iii) Describing the impact of molecular aggregation and of the surroundings (dielectric medium) on the TADF processes.
(iv) Determining the microscopic parameters and rates for the various intra- and inter-molecular charge-transfer and energy-transfer processes.
(v) Designing new, efficient TADF emitters.
“Up-Conversion Intersystem Crossing Rates in Organic Emitters for Thermally Activated Delayed Fluorescence: Impact of the Nature of Singlet vs Triplet Excited States”, P. K. Samanta, D. Kim, V. Coropceanu, and J. L. Brédas, J. Am. Chem. Soc., 139, 4042 (2017).
“A New Design Strategy for Efficient Thermally Activated Delayed Fluorescence Organic Emitters: From Twisted to Planar Structures”, X. K. Chen, Y. Tsuchiya, Y. Ishikawa, C. Zhong, C. Adachi, and J. L. Brédas, Adv. Mater., 29, 1702767 (2017).
“High-efficiency Electroluminescence and Amplified Spontaneous Emission from a Thermally Activated Delayed Fluorescent Near-infrared Emitter”, D. H. Kim, A. D’Aléo, X. K. Chen, A. S.D. Sandanayaka, D. Yao, L. Zhao, T. Komino, E. Zaborova, G. Canard, Y. Tsuchiya, E. Choi, J. W. Wu, F. Fages, J. L. Brédas, J. C. Ribierre, and C. Adachi, Nature Photon., DOI: 10.1038/s41566-017-0087-y (2018).
TWO-DIMENSIONAL π-CONJUGATED POLYMER NETWORKS
Two-dimensional (2D) covalent organic frameworks (COFs) are a class of porous materials in which monomer cores and linkers organize to form well-defined periodic structures via covalent bonds. The topology derives from the directionality of the covalent linkages, which offers a means to organize chemical functionality with atomic precision over long distances. The vast structural diversity of organic π-conjugated moieties provides great opportunities in terms of designing and tailoring novel properties in 2D polymer networks. These emerging materials show great promise for applications including optoelectronic devices, molecular recognition and sensing, catalysis, or membranes.
Using an integrated theoretical approach including kinetic Monte Carlo (KMC) simulations, molecular dynamics (MD) simulations, density functional theory (DFT) calculations, and tight-binding (TB) modeling, we seek to gain an understanding of the growth mechanism and the electronic, optical, and magnetic properties of 2D COFs both when they are in the form of isolated monolayers or few-layers, and when they interact with various metal or semiconductor surfaces. In particular, we focus on:
- Developing a comprehensive understanding of the relationships among chemical structure, geometric structure, and electronic structure of 2D COFs, in order to design novel materials with tunable electronic band structures. Combining Density functional theory (DFT) calculations with tight-binding (TB) methodologies, we investigate systematically COF structures that can display band structures ranging from completely flat to highly dispersive and to Dirac bands near the Fermi level.
2. Predicting and characterizing the unusual electronic, optical, electrical, and magnetic properties that could be derived from free-standing single-layer and multi-layer 2D COFs, as well as from their interactions with metal surfaces and other inorganic substrates. DFT calculations at advanced levels (such as with range-separated functionals and the GW approximations) are applied to describe the energy-level alignments of a deposited COF layer with respect to the Fermi level of the substrate, which directly correlates with the amount of ground-state charge transfer between the COF and the substrate. As such, reliable and accurate theoretical evaluations of these interfacial properties are critical to the design of new COF structures.
3. Description of the microscopic process involved in the growth of COFs. In order to understand the chemical processes that lead to COF polymerization and crystallization, we use KMC simulations to investigate the dynamics of 2D COF formation from solutions. Our KMC model can successfully reproduce the key features of the experimentally determined kinetic curve of COF-5 growth.
“Nucleation and Growth of Covalent Organic Frameworks from Solution: The Example of COF-5”, H. Y. Li, A. D. Chavez, H. F. Li, H. Li, W. R. Dichtel and J.L. Bredas, Journal of the American Chemical Society, 139, 16310 (2017).
“Local Electronic Structure of a Single-Layer Porphyrin-Containing Covalent Organic Framework”, C. Chen, T. Joshi, H. F. Li, A. D. Chavez, Z. Pedramrazi, P. Liu, H. Li, W. R. Dichtel, J.L. Bredas, and M. F. Crommie, ACS Nano, DOI: 10.1021/acsnano.7b06529.
“Hydrolytic Stability of Boronate Ester-Linked Covalent Organic Frameworks”, H. F. Li, H. Y. Li, Q. Q. Dai, H. Li and J.L. Bredas, Advanced Theory and Simulations, in press.
HYBRID ORGANIC-INORGANIC PEROVSKITOIDS
Perovskite-based organic-inorganic materials with varying dimensions (from 3D to 2D, 1D and 0D) have emerged as a new material platform with applications as thin-film solar cells and light-emitting diodes. These materials combine organic molecular cations with inorganic (BX6) anionic octahedra (where B is usually Pb, Sn, or another metal; and X, a halide), which form corner-, edge-, or face-sharing crystalline structures. The electronic and vibrational couplings between the inorganic and organic moieties are expected to result in hybrid excitonic states with novel properties. Also, the nature of the perovskitoids surfaces and their interfaces with (metal or metal oxide) electrodes and organic hole- and electron-transport materials, is of interest.
Our current computational studies on perovskitoids focus on:
- Understanding the ground and excited-state electronic properties of hybrid systems with organic functional molecules interact with inorganic perovskites layers of various thicknesses.
- Investigating the excitonic and charge-transport properties of semiconducting 2D and 0D perovskitoids.
- Investigating the formation of intrinsic and extrinsic defects in perovskitoids and their impact on the electronic structure.
“Molecular Behavior of Zero-dimensional Perovskites”, Jun Yin, Partha Maity, Michele De Bastiani, Ibrahim Dursun, Osman M Bakr, Jean-Luc Brédas, and Omar F Mohammed, Science Advances, 3, e1701793.
“Excitonic and Polaronic Properties of 2D Hybrid Organic–Inorganic Perovskites”, Jun Yin, Hong Li, Daniele Cortecchia, Cesare Soci, and Jean-Luc Bredas, ACS Energy Letters, 2, 417 (2017).
“Pyridine-Induced Dimensionality Change in Hybrid Perovskite Nanocrystals”, Ghada H. Ahmed, Jun Yin, Riya Bose, Lutfan Sinatra, Erkki Alarousu, Emre Yengel, Noktan M. AlYami, Makhsud I. Saidaminov, Yuhai Zhang, Mohamed N. Hedhili, Osman M. Bakr, Jean-Luc Bredas, and Omar F. Mohammed, Chemistry of Materials, 29, 4393 (2017).
“Characterization of the Valence and Conduction Band Levels of n=1 Two-Dimensional Perovskites: A Combined Experimental and Theoretical Investigation”, Scott Silver, Jun Yin, Hong Li, Jean-Luc Bredas, and Antoine Kahn, Advanced Energy Materials, in press.