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ABSTRACT Thermomechanical properties of polymers highly depend on their glass transition temperature (Tg). Differential scanning calorimetry (DSC) is commonly used to measureTgof polymers. However, many conjugated polymers (CPs), especially donor–acceptor CPs (D–A CPs), do not show a clear glass transition when measured by conventional DSC using simple heat and cool scan. In this work, we discuss the origin of the difficulty for measuringTgin such type of polymers. The changes in specific heat capacity (Δcp) atTgwere accurately probed for a series of CPs by DSC. The results showed a significant decrease in Δcpfrom flexible polymer (0.28 J g−1K−1for polystyrene) to rigid CPs (10−3J g−1K−1for a naphthalene diimide‐based D–A CP). When a conjugation breaker unit (flexible unit) is added to the D–A CPs, we observed restoration of the ΔcpatTgby a factor of 10, confirming that backbone rigidity reduces the Δcp. Additionally, an increase in the crystalline fraction of the CPs further reduces Δcp. We conclude that the difficulties of determiningTgfor CPs using DSC are mainly due to rigid backbone and semicrystalline nature. We also demonstrate that physical aging can be used on DSC to help locate and confirm the glass transition for D‐A CPs with weak transition signals. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2019, 57, 1635–1644
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This content will become publicly available on January 30, 2026
Lattice-mismatched and twisted multi-layered materials for efficient solar cells
Abstract We argue that alternating-layer structures of lattice mismatched or misaligned (twisted) atomically-thin layers should be expected to be more efficient absorbers of the broad-spectrum of solar radiation than the bulk material of each individual layer. In such mismatched layer-structures the conduction and valence bands of the bulk material, split into multiple minibands separated by minigaps confined to a small-size emerging Brillouin zone due to band-folding. We extended the Shockley–Queisser approach to calculate the photovoltaic efficiency for a band split into minibands of bandwidth ΔEand mini-gaps δGto model the case when such structures are used as solar cells. We find a significant efficiency enhancement due to impact ionization processes, especially in the limit of small but non-zero δG, and a dramatic increase when fully concentrated Sun-light is used.
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- Award ID(s):
- 2110814
- PAR ID:
- 10626868
- Publisher / Repository:
- IOP Publishing Ltd.
- Date Published:
- Journal Name:
- Journal of Physics: Condensed Matter
- Volume:
- 37
- Issue:
- 12
- ISSN:
- 0953-8984
- Page Range / eLocation ID:
- 125702
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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