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Title: Origin of Hole Transport in Small Molecule Dilute Donor Solar Cells
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Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
Advanced Energy and Sustainability Research
Medium: X
Sponsoring Org:
National Science Foundation
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  1. Abstract

    The nanoscale interpenetrating electron donor–acceptor network in organic bulk heterojunction (BHJ) solar cells results in efficient charge photogeneration but creates complex 3D pathways for charge transport. At present, little is known about the extent to which out‐of‐plane charge flow relies on lateral electrical connectivity. In this work, a procedure, based on conductive atomic force microscopy, is introduced to quantify lateral current spreading during out‐of‐plane charge transport. Using the developed approach, the dependence of lateral spreading on BHJ phase separation, composition, and molecule type (small molecule vs polymer) is studied. In the small‐molecule BHJ, 7,7′‐(4,4‐bis(2‐ethylhexyl)‐4H‐silolo[3,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)bis(6‐fluoro‐4‐(5′‐hexyl‐[2,2′‐bithiophen]‐5‐yl)benzo[c]‐[1,2,5]thiadiazole):(6,6)‐Phenyl‐C71‐butyric acid methyl ester (p‐DTS(FBTTh2)2:PC71BM), an increase is observed in lateral hole current spreading as the population of donor crystallites, bearing an edge‐on molecular orientation, is increased. When integrated into BHJs, the polymer donor poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) leads to greater lateral hole current spreading and more spatially uniform charge transport than the small‐molecule donor, owing to in‐plane charge transport along the polymer backbone. Through the newly introduced electrical characterization scheme, these experiments bring to light the role of lateral electrical connectivity in assisting charge navigation across BHJs.

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    Although tip‐enhanced tribo‐tunneling in metal/semiconductor point nanocontact is capable of producing DC with high current density, scaling up the process for power harvesting for practical applications is challenging due to the complexity of tip array fabrication and insufficient voltage output. Here, it is demonstrated that mechanical contact between a carbon aerogel and silicon (SiO2/Si) interface naturally forms multiple nanocontacts for tribo‐tunneling current generation with an open‐circuit voltage output (VOC) reaching 2 V, and short‐circuit DC current output (ISC) of ≈15 µA. It has a theoretical current density ( J*) on the order of 100 A m−2. Molecular dynamics simulation and atomistic field theory show that a strong localized electronic excitation can be induced at a dynamic carbon/SiO2sliding interface, which is in good agreement with the experimental results. The DC power output is enhanced by the intense local pressure at the presence of nanocontacts, as well as the increased sliding velocityv. To demonstrate the method for practical applications, light‐emitting diodes (LEDs) with different colors are successfully lighted by a single‐carbon aerogel monolith/SiO2sliding unit, and the DC electricity is stored in a capacitor without an additional rectification circuit.

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  3. Abstract

    Organic solar cells (OSCs) based on bulk heterojunction structures are promising candidates for next‐generation solar cells. However, the narrow absorption bandwidth of organic semiconductors is a critical issue resulting in insufficient usage of the energy from the solar spectrum, and as a result, it hinders performance. Devices based on multiple‐donor or multiple‐acceptor components with complementary absorption spectra provide a solution to address this issue. OSCs based on multiple‐donor or multiple‐acceptor systems have achieved power conversion efficiencies over 12%. Moreover, the introduction of an additional component can further facilitate charge transfer and reduce charge recombination through cascade energy structure and optimized morphology. This progress report provides an overview of the recent progress in OSCs based on multiple‐donor (polymer/polymer, polymer/dye, and polymer/small molecule) or multiple‐acceptor (fullerene/fullerene, fullerene/nonfullerene, and nonfullerene/nonfullerene) components.

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