Engineering topics

Cheng, Christina

Publications and source records attributed to Cheng, Christina.

5 records

DOE OSTI2024

Impact of Dilute DIO Additive on Local Microstructure of Fluorinated, pNDI‐Based Polymer Solar Cells

The performance of all‐polymer solar cells is often enhanced by incorporating solvent additives during solution processing. Here, in particular, blends based on the model all‐polymer system PBDBT:N2200 have been shown to have increased short‐circuit current and fill factor when processed with dilute diiodooctane (DIO). However, the morphological mechanism that drives the increase in performance is often not well understood due to limitations in common characterization techniques. In this study, it is shown that a combination of X‐ray techniques with cryogenic high‐resolution transmission electron microscopy (HRTEM) analysis can provide a quantitative and spatially resolved picture of polymer chain orientation and alignment in all‐polymer blends. It is found that DIO induces vertical phase separation in PBDBT‐2F:F‐N2200 and increases donor crystallite thickness in the pi‐stacking direction leading to an acceptor‐rich film surface. However, it is also shown that DIO does not disrupt the formation of face‐on donor–acceptor interfaces. These findings suggest that dilute DIO primarily affects crystalline domain formation in single component regions as opposed to mixed regions; thus, dilute DIO can impact vertical charge transport pathways without sacrificing donor–acceptor interfacial connectivity.

Cheng, Christina↗

DOE OSTI2024

Using Molecular Structure to Tune Intrachain and Interchain Charge Transport in Indacenodithiophene-Based Copolymers

Here, in this work, we compare two structurally near-amorphous rigid-rod polymers-poly(indacenodithiophene-co-benzothiadiazole), p(IDT-BT), and poly(indacenodithiophene-co-benzopyrollodione), p(IDT-BPD)-with orders of magnitude different mobilities to understand the effect charge carrier intrachain delocalization has on electronic transport. Quantum chemical calculations show that p(IDT-BPD) has a barrier to torsion that is significantly lower than that of p(IDT-BT) and is thus more likely to have reduced conjugation lengths. We utilize absorption and photoluminescence spectroscopy to characterize energetic disorder and show that p(IDT-BPD) has higher energetic disorder. Charge modulation spectroscopy (CMS) and model calculations are used to show that charge carriers are substantially delocalized in p(IDT-BT) and occupy near-uniform energetic environments. We find that mobility activated hopping barriers are similar in these two materials. Electronic structure calculations show that both intrachain and interchain couplings of monomer units are poor enough in p(IDT-BPD) that charge carriers collapse to single IDT units and transport via a through-space tunneling mechanism. This work highlights the remarkable charge transport properties of p(IDT-BT) by showing that high mobilities are achievable on device-relevant length scales with only 1D carrier delocalization.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Tuning polymer-backbone coplanarity and conformational order to achieve high-performance printed all-polymer solar cells

Abstract All-polymer solar cells (all-PSCs) offer improved morphological and mechanical stability compared with those containing small-molecule-acceptors (SMAs). They can be processed with a broader range of conditions, making them desirable for printing techniques. In this study, we report a high-performance polymer acceptor design based on bithiazole linker (PY-BTz) that are on par with SMAs. We demonstrate that bithiazole induces a more coplanar and ordered conformation compared to bithiophene due to the synergistic effect of non-covalent backbone planarization and reduced steric encumbrances. As a result, PY-BTz shows a significantly higher efficiency of 16.4% in comparison to the polymer acceptors based on commonly used thiophene-based linkers (i.e., PY-2T, 9.8%). Detailed analyses reveal that this improvement is associated with enhanced conjugation along the backbone and closer interchain π-stacking, resulting in higher charge mobilities, suppressed charge recombination, and reduced energetic disorder. Remarkably, an efficiency of 14.7% is realized for all-PSCs that are solution-sheared in ambient conditions, which is among the highest for devices prepared under conditions relevant to scalable printing techniques. This work uncovers a strategy for promoting backbone conjugation and planarization in emerging polymer acceptors that can lead to superior all-PSCs.

14 SOLAR ENERGY↗

DOE OSTI2023

Linking Phase Behavior to Performance Parameters in Non-Fullerene Acceptor Solar Cells

Performance of nonfullerene-based organic solar cells is largely dependent on the donor:acceptor blend morphology, which is initially tuned by composition. However, morphological design rules are difficult to identify due to the complex relationship between composition-dependent phase behavior and performance properties. In this study, the authors are able to establish a direct link between PM6:Y6 film morphology and device properties by combining grazing-incidence wide-angle X-ray scattering with highly sensitive Fourier transform photocurrent spectroscopy. By analyzing properties across the full composition range, interfacial microstructure and the corresponding charge transfer (CT) states are identified and direct structure–property relationships are established. Here the results indicate that open-circuit voltage is controlled by the CT state at amorphous-PM6:crystalline-Y6 interfaces. Crystalline Y6 is found to increase both transport and non-radiative recombination, suggesting that morphological tradeoffs may be necessary for overall device optimization.

14 SOLAR ENERGY↗

DOE OSTI2021

Ion Pair Uptake in Ion Gel Devices Based on Organic Mixed Ionic–Electronic Conductors

Abstract In organic mixed ionic–electronic conductors (OMIECs), it is critical to understand the motion of ions in the electrolyte and OMIEC. Generally, the focus is on the movement of net charge during gating, and the motion of neutral anion–cation pairs is seldom considered. Uptake of mobile ion pairs by the semiconductor before electrochemical gating (passive uptake) can be advantageous as this can improve device speed, and both ions can participate in charge compensation during gating. Here, such passive ion pair uptake in high‐speed solid‐state devices is demonstrated using an ion gel electrolyte. This is compared to a polymerized ionic liquid (PIL) electrolyte to understand how ion pair uptake affects device characteristics. Using X‐ray photoelectron spectroscopy, the passive uptake of ion pairs from the ion gel into the OMIEC is detected, whereas no uptake is observed with a PIL electrolyte. This is corroborated by X‐ray scattering, which reveals morphological changes to the OMIEC from the uptake of ion pairs. With in situ Raman, a reorganization of both anions and cations is then observed during gating. Finally, the speed and retention of OMIEC‐based neuromorphic devices are tuned by controlling the freedom of charge motion in the electrolyte.

Quill, Tyler J.↗