Periodic Reporting for period 4 - SOFTCHARGE (Charge Carrier Transport in Soft Matter: From Fundamentals to High-Performance Materials)
Reporting period: 2021-04-01 to 2022-03-31
Charge transport (CT) in soft condensed matter is at the heart of many exciting and potentially revolutionising technologies ranging from organic photovoltaic cells to nanobioelectronic transistors. Yet, our fundamental understanding of CT in organic and biological semiconductors (OBS) that could rationalise experimental observations and guide further advances in the field is still very limited. These materials are characterised by strong, anharmonic thermal fluctuations and small energy barriers for CT, which renders standard theories such as band theory or activated electron hopping in many cases entirely inadequate. In this project I propose the development of a disruptive computational method‚ based on non-adiabatic molecular dynamics (NAMD), that will open the door for ground-breaking new insight into this problem.
This problem is important for society because organic semiconductors (OS) are an exciting class of materials that have the potential to enable disruptive technologies in the renewable energy sector and in the plastic electronics industry. Leight-weight, flexible and relatively easy to produce from renewable resources, OS combine many desirable materials properties for thin film electronic devices. They have already found first applications in organic photovoltaic devices (OPV), organic light-emitting diodes and flexible displays, and they are envisioned in many other applications as well, such as e-paper and radio-frequency identification tags. Their potential future importance is reflected by the global market for OS forecasted to rise from 20 bn US$ in 2012 to 330 bn US$ in 2027 with OPV devices identified as one of the main areas for growth.
The objectives of the project are
(1) the development of a fast non-adiabatic molecular dynamics (NAMD) method enabling
(2) new fundamental insight into the nature of charge transport in ultrapure single crystalline OS.
The novel simulation methodology will then be used to
(3) establish structure-charge mobility relationships in OS from the bottom-up to understand how disorder, defects and interfaces between crystalline and amorphous domains impact on the quantum mechanical propagation of the excess charge.
The knowledge obtained in (2) and (3) will be used to
(4) guide the development of high dielectric-high mobility hybrid inorganic/organic materials.
Finally, I aim at expanding the methodology to biological systems, specifically to
(5) CT in bacterial nanowire proteins, to identify common principles and differences between man-made and native semiconducting structures.
In conclusion, I consider the fragment orbital-based surface hopping (FOB-SH) non-adiabatic molecular dynamics methodology developed a breakthrough in the computer simulation of non-adiabatic processes in molecular materials. The methodology pushed the state-of-the-art of non-adiabatic molecular dynamics simulation from the molecular to the true nanoscale. Applications of FOB-SH have led to a new understanding of the nature and dynamics of charge carriers in organic molecular materials, which we termed ``transient delocalization mechanism”. The methodology also proofed very useful for establishing design rules for new high-mobility organic semiconductors and for molecular-scale understanding of the impact of microstructure on carrier mobility. The development of the FOB-SH method was planned but it’s efficiency, success and practical usefulness exceeded my expectations.
This problem is important for society because organic semiconductors (OS) are an exciting class of materials that have the potential to enable disruptive technologies in the renewable energy sector and in the plastic electronics industry. Leight-weight, flexible and relatively easy to produce from renewable resources, OS combine many desirable materials properties for thin film electronic devices. They have already found first applications in organic photovoltaic devices (OPV), organic light-emitting diodes and flexible displays, and they are envisioned in many other applications as well, such as e-paper and radio-frequency identification tags. Their potential future importance is reflected by the global market for OS forecasted to rise from 20 bn US$ in 2012 to 330 bn US$ in 2027 with OPV devices identified as one of the main areas for growth.
The objectives of the project are
(1) the development of a fast non-adiabatic molecular dynamics (NAMD) method enabling
(2) new fundamental insight into the nature of charge transport in ultrapure single crystalline OS.
The novel simulation methodology will then be used to
(3) establish structure-charge mobility relationships in OS from the bottom-up to understand how disorder, defects and interfaces between crystalline and amorphous domains impact on the quantum mechanical propagation of the excess charge.
The knowledge obtained in (2) and (3) will be used to
(4) guide the development of high dielectric-high mobility hybrid inorganic/organic materials.
Finally, I aim at expanding the methodology to biological systems, specifically to
(5) CT in bacterial nanowire proteins, to identify common principles and differences between man-made and native semiconducting structures.
In conclusion, I consider the fragment orbital-based surface hopping (FOB-SH) non-adiabatic molecular dynamics methodology developed a breakthrough in the computer simulation of non-adiabatic processes in molecular materials. The methodology pushed the state-of-the-art of non-adiabatic molecular dynamics simulation from the molecular to the true nanoscale. Applications of FOB-SH have led to a new understanding of the nature and dynamics of charge carriers in organic molecular materials, which we termed ``transient delocalization mechanism”. The methodology also proofed very useful for establishing design rules for new high-mobility organic semiconductors and for molecular-scale understanding of the impact of microstructure on carrier mobility. The development of the FOB-SH method was planned but it’s efficiency, success and practical usefulness exceeded my expectations.
We succeeded in developing a computer programme for non-adiabatic molecular dynamics simulation of charge carrier dynamics in molecular materials (Objective I). In the method developed, termed FOB-SH,
both the excess charge and the nuclei of the material are propagated simultaneously in time by solving the time-dependent electronic Schrodinger-equation.
Applying FOB-SH to a series of organic molecular crystals, we characterized the nature of charge carriers in these materials and discovered a novel transport mechanism, which we termed
``transient delocalization mechanism” (Objective II).
We developed structure-function relations for the charge mobility in pentacene (Objective III). FOBSH simulations of electron hole transport in these samples showed a clear correlation between the crystallinity of the sample, the quantum delocalization, and the mobility of the charge carrier.
We have systematically investigated candidate families of side-chain modified tetracene derivatives and perylene diamides (Objective IV). Using FOB-SH we demonstrated that molecular wires of alkylated tetracenes are capable of polaronic hole conduction at room temperature, with high mobility values ranging up to 21 cm2V-1 s-1.
Finally, we made major progress in our understanding of charge transport and the I-V characteristic in multi-heme proteins (Objective V). We implemented the Landauer formalism for calculation of coherent tunnelling currents through very large systems (proteins) and applied the methodology to electron transport across multiheme cytochrome junctions. The I-V characteristic was obtained in excellent agreement with experiment and we uncovered the nature of the electronic conduction channels mediating the current.
The FOB-SH method and the Landauer formalism were implemented in the publicly available CP2K programme package, which has a very wide userbase of several 1000 users worldwide. Our new methodology was disseminated at the international conference (IWOM, June 2021, online) and the Research Seminar we organized (March 2022, in-person) and at several invited lectures and keynote talks (in person and online) at ca. 20 conferences and 20 workshops in the US, UK, Europe, China and Japan.
both the excess charge and the nuclei of the material are propagated simultaneously in time by solving the time-dependent electronic Schrodinger-equation.
Applying FOB-SH to a series of organic molecular crystals, we characterized the nature of charge carriers in these materials and discovered a novel transport mechanism, which we termed
``transient delocalization mechanism” (Objective II).
We developed structure-function relations for the charge mobility in pentacene (Objective III). FOBSH simulations of electron hole transport in these samples showed a clear correlation between the crystallinity of the sample, the quantum delocalization, and the mobility of the charge carrier.
We have systematically investigated candidate families of side-chain modified tetracene derivatives and perylene diamides (Objective IV). Using FOB-SH we demonstrated that molecular wires of alkylated tetracenes are capable of polaronic hole conduction at room temperature, with high mobility values ranging up to 21 cm2V-1 s-1.
Finally, we made major progress in our understanding of charge transport and the I-V characteristic in multi-heme proteins (Objective V). We implemented the Landauer formalism for calculation of coherent tunnelling currents through very large systems (proteins) and applied the methodology to electron transport across multiheme cytochrome junctions. The I-V characteristic was obtained in excellent agreement with experiment and we uncovered the nature of the electronic conduction channels mediating the current.
The FOB-SH method and the Landauer formalism were implemented in the publicly available CP2K programme package, which has a very wide userbase of several 1000 users worldwide. Our new methodology was disseminated at the international conference (IWOM, June 2021, online) and the Research Seminar we organized (March 2022, in-person) and at several invited lectures and keynote talks (in person and online) at ca. 20 conferences and 20 workshops in the US, UK, Europe, China and Japan.
Non-adiabatic molecular dynamics simulation methods have traditionally been applied to photophysical/chemical problems, usually to investigate relaxation processes of single molecules in the gas phase on time scales of 10-100 fs. Owing to our FOB-SH implementation we have greatly expanded the kind of problems that can be investigated with non-adiabatic molecular dynamics: from single molecules to nanomaterials made up of > 1000 molecules and from the fs to the > 10 ps time scale. This advance is based on controlled approximations in the electronic Hamiltonian and efficient implementation of the algorithm with low scaling with respect to number of atoms. Our code can be routinely applied to charge transport problems in molecular materials consisting of up to 1000 molecules with reasonably small overhead compared to classical molecular dynamics simulation of no more than a factor of 10-50, hence ideally suited for high performance computing. This tool will be of great help to investigate structure-function relationships, to aid the design of high mobility organic semiconductors at ambient temperatures where other methods based on band theory or charge carrier hopping are not applicable.