Self-Assembly-Directed Exciton Diffusion in Solution-Processable Metalloporphyrin Thin Films

The study of excited-state energy diffusion has had an important impact in the development and optimization of organic electronics. For instance, optimizing excited-state energy migration in the photoactive layer in an organic solar cell device has been shown to yield efficient solar energy conversion. Despite the crucial role that energy migration plays in molecular electronic device physics, there is still a great deal to be explored to establish how molecular orientation impacts energy diffusion mechanisms. In this work, we have synthesized a new library of solution-processable, Zn (alkoxycarbonyl)phenylporphyrins containing butyl (ZnTCB4PP), hexyl (ZnTCH4PP), 2-ethylhexyl (ZnTCEH4PP), and octyl (ZnTCO4PP) alkoxycarbonyl groups. We establish that, by varying the length of the peripheral alkyl chains on the metalloporphyrin macrocycle, preferential orientation and molecular self-assembly is observed in solution-processed thin films. The resultant arrangement of molecules consequently affects the electronic and photophysical characteristics of the metalloporphyrin thin films. The various molecular arrangements in the porphyrin thin films and their resultant impact were determined using UV-Vis absorption spectroscopy, steady-state and time-resolved fluorescence emission lifetimes, and X-ray diffraction in thin films. The films were doped with C60 quencher molecules and the change in fluorescence was measured to derive a relative quenching efficiency. Using emission decay, relative quenching efficiency, and dopant volume fraction as input, insights on exciton diffusion coefficient and exciton diffusion lengths were obtained from a Monte Carlo simulation. The octyl derivative (ZnTCO4PP) showed the strongest relative fluorescence quenching and, therefore, the highest exciton diffusion coefficient (5.29 × 10−3 cm2 s−1) and longest exciton diffusion length (~81 nm). The octyl derivative also showed the strongest out-of-plane stacking among the metalloporphyrins studied. This work demonstrates how molecular self-assembly can be used to modulate and direct exciton diffusion in solution-processable metalloporphyrin thin films engineered for optoelectronic and photonic applications.

Photophysics of fused ring electron acceptors for photovoltaic applications

<p>Organic photovoltaic cells (OPVs) have received significant interest over the last decade as they offer the potential of cheap renewable energy via direct photon to electron conversion of abundantly available Sun light. Compared to the inorganic equivalents, they offer lightweight, low-cost, and flexibility advantages. Conventional OPVs are typically based on blends of electron-donor materials and fullerene-based electron-acceptor materials that form bulk-heterojunctions (BHJs). But the attention has recently shifted to organic non-fullerene acceptors especially fused ring electron acceptors (FREAs) owing to their attractive properties including flexible energy levels, tunable band gap, crystallinity, and planarity. Thus the power conversion efficiency (PCE) of OPVs has recently attained a record of 16% by synthesizing FREAs with modified chemical structures. Owing to the high crystallinity and packing orientation of acceptors, FREA based OPV systems are characterized by large and pure phases sized 20-50 nm. This is consistent with the observation that charge generation dynamics in these systems lacked the ultrafast component that characterizes most fullerene blends. To test the hypothesis that the optimal phase size can be large due to facile exciton diffusion in FREAs, a planar indacenodithiophene (IDT) based FREA, IDIC is selected as a model system to study the exciton dynamics. Chapter 3 includes the exciton diffusion measurements in IDIC films using transient absorption spectroscopy which resolves a substantially high, quasi-activationless diffusion coefficient that exceeds that of typical organic semiconductors. The study also includes a deep insight of the key factors behind the enhanced exciton diffusion in IDIC and is shown to arise from different molecular and packing factors which enhance the long-range resonant energy transfer. Rapid exciton diffusion in IDIC films introduced the possibility of solution-processed bilayer devices. Chapter 4 comprises both device and photophysics of planar bilayer devices with a PCE of 11.1% which is readily accounted for the material and device design. By pairing a mid-band gap polymer donor with a range of FREAs, it is shown that the combination of high molecular packing densities and absorption coefficients, long exciton diffusion lengths, and efficient, resonant, long-range energy transfer between donor and acceptor layers enable efficient bilayer devices. By designing new materials with these characteristics, along with orthogonal solubility for layer-by-layer deposition of clean bilayers, it is suggested that the clear connection between material design and function in the bilayer structure will accelerate the development of more efficient organic photovoltaic devices. The molecular packing of active layer components has a crucial role in the device performance of OPV devices. Especially for FREA based OPV systems, the longrange structural order induced by end group 𝜋 - 𝜋 stacking is considered as the critical factor for achieving high PCEs. Chapter 5 includes a deep spectroscopic insight into the exciton and charge transport processes in a series of FREA based OPV systems having different molecular packing and ordering. Here the molecular stacking manipulation in FREAs is achieved by changing the length of alkyl side-chains so that the FREA backbone is changed from a 𝜋 -𝜋 stacking mode to a non-stacking mode. Transient absorption spectroscopic analysis of neat FREAs and blends reveals that exciton diffusion and intermolecular charge transfer processes do not necessarily rely on the molecular 𝜋 -𝜋 stacking, while close atom contact can also enable these processes. This work provides new insights into the design of advanced materials for next generation organic photovoltaics considering diverse transport channels formed by close atom interactions. Chapter 6 discusses the advantages of ternary OPV strategy in BHJ world with a 13% efficient ternary device based on a highly efficient FREA, FOIC. The introduction of a mid-band gap small molecule donor TR into the binary blend PTB7- Th:FOIC improves the open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF), and thereby the overall device performance. Transient absorption spectroscopy reveals ultrafast resonant energy transfer from TR to PTB7-Th domains which is consistent with their intermixed morphology. Additionally, the study also includes the rapid long-range energy transfer from PTB7-Th to FOIC phases that</p>

Photophysics of fused ring electron acceptors for photovoltaic applications

<p>Organic photovoltaic cells (OPVs) have received significant interest over the last decade as they offer the potential of cheap renewable energy via direct photon to electron conversion of abundantly available Sun light. Compared to the inorganic equivalents, they offer lightweight, low-cost, and flexibility advantages. Conventional OPVs are typically based on blends of electron-donor materials and fullerene-based electron-acceptor materials that form bulk-heterojunctions (BHJs). But the attention has recently shifted to organic non-fullerene acceptors especially fused ring electron acceptors (FREAs) owing to their attractive properties including flexible energy levels, tunable band gap, crystallinity, and planarity. Thus the power conversion efficiency (PCE) of OPVs has recently attained a record of 16% by synthesizing FREAs with modified chemical structures. Owing to the high crystallinity and packing orientation of acceptors, FREA based OPV systems are characterized by large and pure phases sized 20-50 nm. This is consistent with the observation that charge generation dynamics in these systems lacked the ultrafast component that characterizes most fullerene blends. To test the hypothesis that the optimal phase size can be large due to facile exciton diffusion in FREAs, a planar indacenodithiophene (IDT) based FREA, IDIC is selected as a model system to study the exciton dynamics. Chapter 3 includes the exciton diffusion measurements in IDIC films using transient absorption spectroscopy which resolves a substantially high, quasi-activationless diffusion coefficient that exceeds that of typical organic semiconductors. The study also includes a deep insight of the key factors behind the enhanced exciton diffusion in IDIC and is shown to arise from different molecular and packing factors which enhance the long-range resonant energy transfer. Rapid exciton diffusion in IDIC films introduced the possibility of solution-processed bilayer devices. Chapter 4 comprises both device and photophysics of planar bilayer devices with a PCE of 11.1% which is readily accounted for the material and device design. By pairing a mid-band gap polymer donor with a range of FREAs, it is shown that the combination of high molecular packing densities and absorption coefficients, long exciton diffusion lengths, and efficient, resonant, long-range energy transfer between donor and acceptor layers enable efficient bilayer devices. By designing new materials with these characteristics, along with orthogonal solubility for layer-by-layer deposition of clean bilayers, it is suggested that the clear connection between material design and function in the bilayer structure will accelerate the development of more efficient organic photovoltaic devices. The molecular packing of active layer components has a crucial role in the device performance of OPV devices. Especially for FREA based OPV systems, the longrange structural order induced by end group 𝜋 - 𝜋 stacking is considered as the critical factor for achieving high PCEs. Chapter 5 includes a deep spectroscopic insight into the exciton and charge transport processes in a series of FREA based OPV systems having different molecular packing and ordering. Here the molecular stacking manipulation in FREAs is achieved by changing the length of alkyl side-chains so that the FREA backbone is changed from a 𝜋 -𝜋 stacking mode to a non-stacking mode. Transient absorption spectroscopic analysis of neat FREAs and blends reveals that exciton diffusion and intermolecular charge transfer processes do not necessarily rely on the molecular 𝜋 -𝜋 stacking, while close atom contact can also enable these processes. This work provides new insights into the design of advanced materials for next generation organic photovoltaics considering diverse transport channels formed by close atom interactions. Chapter 6 discusses the advantages of ternary OPV strategy in BHJ world with a 13% efficient ternary device based on a highly efficient FREA, FOIC. The introduction of a mid-band gap small molecule donor TR into the binary blend PTB7- Th:FOIC improves the open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF), and thereby the overall device performance. Transient absorption spectroscopy reveals ultrafast resonant energy transfer from TR to PTB7-Th domains which is consistent with their intermixed morphology. Additionally, the study also includes the rapid long-range energy transfer from PTB7-Th to FOIC phases that</p>

Vertically Optimized Phase Separation with Improved Exciton Diffusion Enables High-Efficiency Organic Solar Cells with Thick Active Layers

The development of high-performance organic solar cells (OSCs) with thick active layers is of crucial importance for the roll-to-roll printing of large-area solar panels. Unfortunately, increasing the active layer thickness usually results in a significant reduction in efficiency. Herein, we fabricated efficient thick-film OSCs with an active layer consisting of one polymer donor and two non-fullerene acceptors. The two acceptors were found to possess enlarged exciton diffusion length in the mixed phase, which is beneficial to exciton generation and dissociation. Additionally, layer by layer approach was employed to optimize the vertical phase separation. Benefiting from the synergetic effects of enlarged exciton diffusion length and graded vertical phase separation, a record high efficiency of 17.31% (certified value of 16.9%) was obtained for the 300 nm-thick OSC, with an unprecedented short-circuit current density of 28.36 mA cm−2, and a high fill factor of 73.0%. Moreover, the device with an active layer thickness of 500 nm also shows a record efficiency of 15.21%. This work provides new insights into the fabrication of high-efficiency OSCs with thick active layers.

Phonon scattering and exciton localization: molding exciton flux in two dimensional disorder energy landscape

Two dimensional excitonic devices are of great potential to overcome the dilemma of response time and integration in current generation of electron or/and photon based systems. The ultrashort diffusion length of exciton arising from ultrafast relaxation and low carrier mobility greatly discounts the performance of excitonic devices. Phonon scattering and exciton localization are crucial to understand the modulation of exciton flux in two dimensional disorder energy landscape, which still remain elusive. Here, we report an optimized scheme for exciton diffusion and relaxation dominated by phonon scattering and disorder potentials in WSe2 monolayers. The effective diffusion coefficient is enhanced by > 200% at 280 K. The excitons tend to be localized by disorder potentials accompanied by the steadily weakening of phonon scattering when temperature drops to 260 K, and the onset of exciton localization brings forward as decreasing temperature. These findings identify that phonon scattering and disorder potentials are of great importance for long-range exciton diffusion and thermal management in exciton based systems, and lay a firm foundation for the development of functional excitonic devices.

Influence of Temperature on Exciton Dynamic Processes in CuPc/C60 Based Solar Cells

Although the effect of high temperature on the performance of organic solar cells has been widely investigated, it is inevitably influenced by the associated annealing effect (which usually leads to film morphology change and variation in electrical properties), which makes the discussion more sophisticated. In this study, we simplified the issue and investigated the influence of low temperatures (from room temperature to 77 K) on the photocurrent and internal/external quantum efficiency of a CuPc/C60 based solar cell. We found that besides the charge dynamic process (charge transport), one or more of the exciton dynamic processes, such as exciton diffusion and exciton dissociation, also play a significant role in affecting the photocurrent of organic solar cells at different temperatures. Additionally, the results showed that the temperature had negligible influence on the absorption of the CuPc film as well as the exciton generation process, but obviously influenced the other two exciton dynamic processes (exciton diffusion and exciton dissociation).