| 1. |
- Yao, Nannan, 1992-, et al.
(author)
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Solution-Processed Highly Efficient Semitransparent Organic Solar Cells with Low Donor Contents
- 2021
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In: ACS Applied Energy Materials. - : American Chemical Society. - 2574-0962. ; 4:12, s. 14335-14341
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Journal article (peer-reviewed)abstract
- Semitransparent organic solar cells (ST-OSCs) are promising candidates for applications in building-integrated photovoltaics (BIPV) as windows and facades. The challenge to achieve highly efficient ST-OSCs is the trade-off between power conversion efficiency (PCE) and average visible transmittance (AVT). Herein, solution-processed ST-OSCs are demonstrated on the basis a polymer donor, PM6, and a small molecule acceptor, Y6; lowering the visible-absorbing PM6 contents in blends could increase AVT and maintain PCE. Additionally, conductive polymer PEDOT:PSS is used as the top electrode due to its high transparency, good conductivity, and solution processability. Efficient ST-OSCs with 20 wt % PM6 achieve high PCE of 7.46% and AVT of 36.4%. The light utilization efficiency (LUE) of 2.72% is among the best reported values for solution-processed ST-OSCs. This work provides a straightforward approach for solution-processed ST-OSCs by combining a low fraction of visible-wavelength-selective polymer donors with near-infrared nonfullerene acceptors to achieve high PCE and AVT simultaneously.
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| 2. |
- Li, Yaokai, et al.
(author)
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Mechanism study on organic ternary photovoltaics with 18.3% certified efficiency: from molecule to device
- 2022
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In: Energy & Environmental Science. - : ROYAL SOC CHEMISTRY. - 1754-5692 .- 1754-5706. ; 15:2, s. 855-865
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Journal article (peer-reviewed)abstract
- Multi-component organic photovoltaics (OPVs), e.g., ternary blends, are effective for high performance, while the fundamental understanding from the molecular to device level is lacking. To address this issue, we here systematically study the working mechanism of ternary OPVs based on non-fullerene acceptors (NFAs). With both molecular dynamics simulations and morphology characterization, we identify that when adding another larger band gap and highly miscible NFA, namely IT-4F or BTP-S2, into the PBDB-TF:BTP-eC9 blend, the NFAs undergo molecular intermixing selectively with BTP-eC9. This causes the composition-dependent band gap and charge recombination, and hence the composition-dependent V-OC. While the charge recombination still dominantly occurs at the PBDB-TF:BTP-eC9 interface, BTP-S2 or IT-4F plays an auxiliary role in facilitating charge transfer and suppressing non-radiative decay. Interestingly, intermolecular end-group packing in the intermixed blend is improved compared to that in pristine films, leading to higher carrier mobility. These synergistic effects significantly improve the power conversion efficiency of the device to an outstanding value of 18.7% (certified value of 18.3%).
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| 3. |
- Xia, Xinxin, et al.
(author)
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Revealing the crystalline packing structure of Y6 in the active layer of organic solar cells: the critical role of solvent additives
- 2023
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In: Journal of Materials Chemistry A. - : ROYAL SOC CHEMISTRY. - 2050-7488 .- 2050-7496. ; 11:40, s. 21895-21907
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Journal article (peer-reviewed)abstract
- The bulk heterojunction (BHJ) morphology of photovoltaic materials is crucial to the fundamental optoelectronic properties of organic solar cells (OSCs). However, in the photoactive layer, the intrinsic crystalline packing structure of Y6, currently the hallmark molecule among Y-series non-fullerene acceptors (NFAs), has not been unambiguously determined. Here, employing grazing-incidence wide-angle X-ray scattering (GIWAXS), we managed to uncover the intrinsic crystalline packing structure of Y6 in the BHJ active layer of OSCs, which is found to be different from its single-crystal structure reported previously. Moreover, we find that solvent additive 1-chloronaphthalene (CN) can induce highly ordered packing of Y6 in BHJ thin films. With the help of atomistic molecular dynamics simulations, it is revealed that pi-pi interactions generally exist between naphthalene derivatives and IC terminals of Y6 analogues, which would essentially improve their long-range ordering. Our work reveals the intrinsic crystalline packing structure of Y6 in the BHJ active layer as well as its crystallization mechanism in thin films, thus providing direct correlations between this crystalline packing and the device characteristics and photophysical properties.
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| 4. |
- Yao, Nannan, 1992-
(author)
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Fill factor of organic solar cells and applications of dilute donor devices
- 2021
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Doctoral thesis (other academic/artistic)abstract
- Organic solar cells (OSCs) have attracted great attention due to their low cost, flexibility and solution-processibility. In recent years, the development of nonfullerene acceptors (NFAs) has truly promoted the efficiency of OSCs up to 19%, implying high potential for commercial applications. However, the stateof- the-art OSCs still lag behind the Shockley-Queisser limit, besides the intrinsic losses, understanding the extrinsic losses during charge generation, transport and extraction in devices is necessary.The short-circuit current (JSC) and open-circuit voltage (VOC) can be simultaneously optimized in OSCs by tuning the energy levels of NFAs. However, less attention has been paid to the fill factor (FF), a crucial parameter for device efficiency. The FF reflects how the output photocurrent changes for a solar cell with a load from zero to infinity, indicating the charge extraction capability. In this thesis, the roles of energy offset, electric field, disorder and morphology on charge carrier dynamics as well as how these factors influence FF and energy loss are introduced. It is observed that fast and field-insensitive charge extraction is essential for high FF, which can be enabled by balanced transport and reduced bimolecular recombination. Additionally, the correlation between FF and voltage loss are studied based on four NFA systems with different highest occupied molecular orbital (HOMO) offsets. Larger HOMO offset could suppress hole back transfer from donor to acceptor and then lead to a larger FF, but it also induces more voltage loss.The morphology of the active layer governs the charge dynamics and device performance. A comparative study based on all-polymer solar cells processed from chlorobenzene (CB) and o-Xylene has been performed. Film formation process and morphology characteristics demonstrate that CB-cast films exhibit better donor/acceptor miscibility and relatively ordered structure, yielding good device performance. Contrary, in o-Xylene cast devices, electron trapping leads to a smaller FF and more non-radiative recombination.The state-of-the-art OSCs usually require comparable donor/acceptor contents in bulk-heterojunctions. Herein, NFA’s contribution to hole transport is investigated in dilute donor solar cells (10 wt% PM6:Y6). Comparable hole mobilities of PM6 diluted in Y6 and insulators (PS &PMMA) indicate that the hole transport in dilute donor solar cells is still mainly via PM6 phases, although pristine Y6 can support ambipolar transport. Furthermore, impressive performance of the dilute donor solar cells motivate us to explore semitransparent OSCs for building-integrated photovoltaics (BIPV). Decent photovoltaic performance and acceptable visible transparency have been realized in dilute donor solar cells by decreasing visible-absorption and increasing near-infrared absorption.
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