Prof. Wei You Profile

Prof. Wei You

Professor at Univ of North Carolina at Chapel Hill

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Author

Publications (9)

Proceedings Article | 15 March 2023 Presentation

Probing carrier transport in MAPbI3-based photovoltaic cells with nonlinear photocurrent spectroscopy

Zhenyu Ouyang, Liang Yan, Wei You, Andrew Moran

Proceedings Volume PC12419, (2023) https://doi.org/10.1117/12.2649182

KEYWORDS: Solar cells, Spectroscopes, Pulsed laser operation, Spectroscopy, Signal generators, Picosecond phenomena, Nonlinear response, Laser development, Instrument modeling

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Proceedings Article | 3 October 2022 Presentation

Coherent phonon-induced modulation of charge transfer in two-dimensional hybrid perovskites

Dovletgeldi Seyitliyev, Kenan Gundogdu, Volker Blum, David Mitzi, Wei You, Svenja Janke, Manoj Jana, Xixi Qin

Proceedings Volume PC12228, PC1222805 (2022) https://doi.org/10.1117/12.2633700

KEYWORDS: Perovskite, Modulation, Phonons, Ultrafast phenomena, Optoelectronics, Electrons, Crystals, Absorption spectroscopy

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Proceedings Article | 3 October 2022 Presentation

A universal conundrum of determination of Frenkel exciton binding energy

Somayeh Kashani, Jeromy Rech, Tuo Liu, Kyle Baustert, Wei You, Kenneth Graham, Carlos Silva, Natalie Stingelin, Gitti Frey, Harald Ade

Proceedings Volume PC12209, PC1220903 (2022) https://doi.org/10.1117/12.2633814

KEYWORDS: Excitons, Photoemission spectroscopy, Ultraviolet radiation, Solid state electronics, Solar energy, Solar cells, Quasiparticles, Polymers, Photovoltaics, Organic semiconductors

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The absorption of a photon by Organic Semiconductors (OSCs) results in the formation of a bound electron-hole pair quasiparticle or Frenkel Exciton. The energy required to separate an exciton into noninteracting electron and hole, Exciton Binding Energy (Eb), is a critical parameter for the purpose of improving the efficiency of optoelectronic devices such as solar cells and light emitting diodes. In the last two decade there have been many efforts to measure the Eb of OSCs using different techniques. However, there are discrepancies in the literature and the reported values are scattered over a large range between a few meV to 1.5eV, even for a specific material. Eb of Frenkel excitons can be estimated as the difference between the transport (Eg) and the optical gap (Eopt), a traditional definition borrowed from the language of Wannier Exciton in inorganic SCs. Here, we explore the Eb of different variants of PBnDT-FTAZ polymer. We focus on the two most common methods used to measure Eg: combination of Ultraviolet Photoelectron Spectroscopy and Inversed Photoelectron Spectroscopy (UPS-IPES) and Solid-State Cyclic Voltammetry (CV). We show that Eb measured by the abovementioned methods are not consistent or correlated with each other. The Eg measured by UPS-IPES technique is comparable with (or even smaller than) Eopt leading to small Eb. On the other hand, CV usually measures larger Eg compared to the Eopt resulting in larger values of Eb that are scattered between 200meV-1eV depending on the molecular structure of the materials. This discrepancy is the result of lack of both an exclusive theoretical and a functional definition of Eb that includes the relaxation effects, an important characteristic of Frenkel excitons. Moreover, due to the nature of each measurement method, they might measure different parameters than the actual properties of the bulk in a photovoltaic device. Our results elucidate the current conundrum on determination of Eb in OSCs and emphasize the importance of establishing standard theoretical and practical guidelines on how to properly estimate Eb.

Proceedings Article | 10 September 2019 Presentation

Sequential deposition of organic films with eco-compatible solvents enables high-efficiency nonfullerene solar cells (Conference Presentation)

Long Ye, Yuan Xiong, Zheng Chen, Qianqian Zhang, Reece Henry, Brendan O'Connor, Wei You, Harald Ade

Proceedings Volume 11094, 110940O (2019) https://doi.org/10.1117/12.2529802

KEYWORDS: Solar cells, Polymers, Polymer thin films, Thin film devices, Organic photovoltaics, Molecules, Crystals, Instrument modeling, Performance modeling, X-rays

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Casting of a donor:acceptor bulk-heterojunction structure from a single ink has been the predominant fabrication method of solution-processed organic photovoltaics (OPVs). Despite the success of such bulk-heterojunction, the task of controlling its microstructure in a single casting process has been challenging and arduous and alternative approaches are desired. To achieve and even improve OPVs with a desirable microstructure, a facile and eco-compatible sequential deposition approach is demonstrated for nonfullerene polymer/small molecule pairs. Using a known weakly crystalline polymer FTAZ as the model material, we show the profound influence of casting solvent on the molecular ordering of the film, and thus the device performance and mesoscale morphology of sequentially deposited OPVs can be tuned[1]. Static and in-situ X-ray scattering indicate that applying the green solvent limonene is able to greatly promote the molecular order of FTAZ and form the largest domain spacing exclusively, which correlate well with the best efficiency in sequentially deposited devices. The sequentially cast device generally outperforms its control device[2] based on traditional single-ink bulk-heterojunction structure. Investigations of distinct material systems suggest that our approach be applicable to many conjugated polymers and nonfullerene acceptors, which yield consistently higher fill factors than traditional bulk-heterojunction devices. Moreover, the relationships between polymer:solvent interactions, thin-film microstructure, and device performance are discussed for these sequentially deposited devices. It is noted that polymer:solvent interaction parameter χ positively correlates with domain spacing in the devices. Our findings shed light on innovative approaches to rationally create ink-stable, environmentally friendly, and highly efficient nonfullerene solar cells. Reference [1] Ye, L.; Xiong, Y.; Chen, Z.; Zhang, Q.; Fei, Z.; Henry, R.; Heeney, M.; O’Connor, B.; You, W.; Ade, H. Adv. Mater. 2019, under review. [2] Ye, L.; Xiong, Y.; Zhang, Q.; Li, S.; Wang, C.; Jiang, Z.; Hou, J.; You, W.; Ade, H. Adv. Mater. 2018, DOI: 10.1002/adma.201705485.

Proceedings Article | 10 September 2019 Presentation

Semi-transparent organic solar cells for greenhouse application (Conference Presentation)

Yuan Xiong, Eshwar Ravishankar, Jennifer Swift, Harald Ade, Ronald Booth, Melodi Charles, Reece Henry, Brendan O'Connor, Jeromy Rech, Carole Saravitz, Heike Sederoff, Long Ye, Wei You

Proceedings Volume 11094, 110940N (2019) https://doi.org/10.1117/12.2529997

KEYWORDS: Organic photovoltaics, Solar cells, Botany, Physics, Carbon, Electronics, Aerospace engineering, Biology, Chemistry, Photovoltaics

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Semi-transparent Organic Solar Cells for Greenhouse Application Yuan Xiong1*, Eshwar Ravishankar2, Jennifer Swift3, Harald Ade1*, Ronald Booth2, Melodi Charles4, Reece Henry1, Brendan O’Connor2, Jeromy James Rech5, Carole Saravitz3, Heike Sederoff4, Long Ye1, Wei You5 1. Department of Physics, Organic and Carbon Electronics Lab (ORaCEL), North Carolina State University, Raleigh, NC 27695, USA 2. Department of Mechanical and Aerospace Engineering and ORaCEL, North Carolina State University, Raleigh, NC 27695, USA 3. Department of Plant Biology, North Carolina State University, Raleigh, NC 27695, USA 4. Department of Plant and Microbial Biology, North Carolina State University, Raleigh, NC 27695, USA 5. Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA E-mail: yxiong8@ncsu.edu; hwade@ncsu.edu Semitransparent organic solar cells (ST-OSCs) show great potential in building-integrated photovoltaics due to the advantages in solution processability, flexibility, and transparency. Herein, we present a systematic study on the application of high-performance ST-OSC filters in a greenhouse by utilizing three representative systems with different spectral responses, namely, FTAZ:PC71BM[1], FTAZ:IT-M[2, 3], and PTB7-Th:IEICO-4F[4]. Specifically, the cultivation of red leaf lettuce is conducted in a controlled environment growth chamber, which is possible to duplicate any climate, and under different ST-OSC filters. In principle, the ST-OSCs absorb a portion of the solar spectrum for power generation and lettuce utilizes the penetrated light for photosynthesis. Furthermore, we quantitatively investigate the leaf area and number profiles, plant biomass, and photosynthetic rate under the as-prepared ST-OSC filters treatments. On the base of statistical analysis after the growth cycle, we can identify the best ST-OSC for plant growth. These results thus pave the way to integrate ST-OSCs with greenhouses. [1] S. C. Price, A. C. Stuart, L. Yang, H. Zhou, W. You, Journal of the American Chemical Society 2011, 133, 4625. [2] L. Ye, Y. Xiong, Q. Zhang, S. Li, C. Wang, Z. Jiang, J. Hou, W. You, H. Ade, Advanced Materials 2018, 30, 1705485. [3] Y. Xiong, L. Ye, A. Gadisa, Q. Zhang, J. J. Rech, W. You, H. Ade, Advanced Functional Materials 2019, 29, 1806262. [4] X. Song, N. Gasparini, L. Ye, H. Yao, J. Hou, H. Ade, D. Baran, ACS Energy Letters 2018, 3, 669.