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1. Low Voltage‐Loss Organic Solar Cells Light the Way for Efficient Semitransparent Photovoltaics

   https://doi.org/10.1002/solr.202200135  

   Luginbuhl, Benjamin R. ; Ko, Seo-Jin ; Ran, Niva A. ; Hu, Huawei ; Becwar, Shona M. ; Karki, Akchheta ; Seifrid, Martin ; Okubo, Takashi ; Wang, Ming ; Ade, Harald W. ; et al ( March 2022 , Solar RRL)

   Organic solar cells that are transparent to visible light are highly desirable for applications such as window treatments or solar greenhouse panels. A key challenge is to simultaneously transmit most photons between 400 and 700 nm while retaining a high short‐circuit current and power conversion efficiency (PCE). Here, organic bulk heterojunction (BHJ) solar cells consisting of a donor polymer (PM2) is reported and the non‐fullerene acceptor ITIC‐Th achieves a PCE of 9.3%, and the BHJ thin films exhibit an average visible transmittance over 40%. This value is achieved primarily due to a very high open‐circuit voltage (V_OC) of 0.93 V, which represents a voltage loss of only 0.50 V relative to the material optical bandgap,E_opt. In PM2:PC₆₁BM devices, this voltage loss increases to 0.62 V (V_OC = 0.82 V). It is found that this difference inV_OCis due to higher nonradiative recombination in the fullerene‐based solar cell, suggesting that non‐fullerene acceptors may lead to better performance in semi‐transparent devices. The optoelectronic properties associated with PM2:ITIC‐Th and PM2:PC₆₁BM blends are further corroborated by different morphological features and local structures at the donor‐acceptor interfaces characterized by atomic force microscopy, X‐ray scattering, and solid‐state NMR spectroscopy techniques.

    

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2. Elucidating the impact of molecular weight on morphology, charge transport, photophysics and performance of all-polymer solar cells

   https://doi.org/10.1039/D0TA08195G  

   Tran, Duyen K. ; Robitaille, Amélie ; Hai, I. Jo ; Ding, Xiaomei ; Kuzuhara, Daiki ; Koganezawa, Tomoyuki ; Chiu, Yu-Cheng ; Leclerc, Mario ; Jenekhe, Samson A. ( October 2020 , Journal of Materials Chemistry A)

   null (Ed.)

   Understanding the influence of polymer molecular weight on the morphology, photophysics, and photovoltaic properties of polymer solar cells is central to further advances in the design, processing, performance and optimization of the materials and devices for large scale applications. We have synthesized six number-average molecular weight ( M n ) values (21–127 kDa) of biselenophene–naphthalenediimide copolymer ( PNDIBS ) via direct heteroarylation polymerization and used them to investigate the effects of the acceptor polymer molecular weight on the charge transport, blend photophysics, blend morphology, and photovoltaic properties of all-polymer solar cells (all-PSCs) based on PNDIBS and the donor polymer PBDB-T . The short-circuit current and power conversion efficiency (PCE) of the PBDB-T : PNDIBS blend devices were found to increase with increasing M n until reaching peaks at an optimal molecular weight of 55 kDa and then decreased with further increases in M n . The maximum PCE of 10.2% observed at the optimal M n value of 55 kDa coincided with optimal blend charge transport properties, blend photophysics, and blend morphology at this critical molecular weight. Compared to the bi-continuous network of ∼5.5–6.5 nm crystalline domains with predominantly face-on molecular orientations observed at 55 kDa, a relatively disordered microstructure with larger scale phase separation was evident at higher M n while more finely packed crystalline domains were seen at 21 kDa. The sensitivity of the device efficiency to the active layer thickness was found to also depend on the PNDIBS M n value. These results highlight the importance of tuning the molecular weight of the polymer components to optimize the morphology, charge transport, photophysics and efficiency of all-polymer solar cells. The results also provide new insights on structure–property relationships for a promising n-type semiconducting copolymer. 

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3. Alkyl Chain Length Effects of Polymer Donors on the Morphology and Device Performance of Polymer Solar Cells with Different Acceptors

   https://doi.org/10.1002/aenm.201901740  

   Pang, Shuting ; Zhang, Ruiwen ; Duan, Chunhui ; Zhang, Song ; Gu, Xiaodan ; Liu, Xi ; Huang, Fei ; Cao, Yong ( July 2019 , Advanced Energy Materials)

   The development of nonfullerene acceptors has brought polymer solar cells into a new era. Maximizing the performance of nonfullerene solar cells needs appropriate polymer donors that match with the acceptors in both electrical and morphological properties. So far, the design rationales for polymer donors are mainly borrowed from fullerene‐based solar cells, which are not necessarily applicable to nonfullerene solar cells. In this work, the influence of side chain length of polymer donors based on a set of random terpolymers PTAZ‐TPD10‐Cnon the device performance of polymer solar cells is investigated with three different acceptor materials, i.e., a fullerene acceptor [70]PCBM, a polymer acceptor N2200, and a fused‐ring molecular acceptor ITIC. Shortening the side chains of polymer donors improves the device performance of [70]PCBM‐based devices, but deteriorates the N2200‐ and ITIC‐based devices. Morphology studies unveil that the miscibility between donor and acceptor in blend films depends on the side chain length of polymer donors. Upon shortening the side chains of the polymer donors, the miscibility between the donor and acceptor increases for the [70]PCBM‐based blends, but decreases for the N2200‐ and ITIC‐based blends. These findings provide new guidelines for the development of polymer donors to match with emerging nonfullerene acceptors.

    

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4. Fluorinating π‐Extended Molecular Acceptors Yields Highly Connected Crystal Structures and Low Reorganization Energies for Efficient Solar Cells

   https://doi.org/10.1002/aenm.202000635  

   Swick, Steven M. ; Alzola, Joaquin M. ; Sangwan, Vinod K. ; Amsterdam, Samuel H. ; Zhu, Weigang ; Jones, Leighton O. ; Powers‐Riggs, Natalia ; Facchetti, Antonio ; Kohlstedt, Kevin L. ; Schatz, George C. ; et al ( May 2020 , Advanced Energy Materials)

   The synthesis and characterization of new semiconducting materials is essential for developing high‐efficiency organic solar cells. Here, the synthesis, physiochemical properties, thin film morphology, and photovoltaic response of ITN‐F4 and ITzN‐F4, the first indacenodithienothiophene nonfullerene acceptors that combine π‐extension and fluorination, are reported. The neat acceptors and bulk‐heterojunction blend films with fluorinated donor polymer poly{[4,8‐bis[5‐(2‐ethylhexyl)‐4‐fluoro‐2‐thienyl]benzo[1,2‐b:4,5‐b′]‐dithiophene‐2,6‐diyl]‐alt‐[2,5‐thiophenediyl[5,7‐bis(2‐ethylhexyl)‐4,8‐dioxo‐4H,8H‐benzo[1,2‐c:4,5‐c′]dithiophene‐1,3‐diyl]]} (PBDB‐TF, also known as PM6) are investigated using a battery of techniques, including single crystal X‐ray diffraction, fs transient absorption spectroscopy (fsTA), photovoltaic response, space‐charge‐limited current transport, impedance spectroscopy, grazing incidence wide angle X‐ray scattering, and density functional theory level computation. ITN‐F4 and ITzN‐F4 are found to provide power conversion efficiencies greater and internal reorganization energies less than their non‐π‐extended and nonfluorinated counterparts when paired with PBDB‐TF. Additionally, ITN‐F4 and ITzN‐F4 exhibit favorable bulk‐heterojunction relevant single crystal packing architectures. fsTA reveals that both ITN‐F4 and ITzN‐F4 undergo ultrafast hole transfer (<300 fs) in films with PBDB‐TF, despite excimer state formation in both the neat and blend films. Taken together and in comparison to related structures, these results demonstrate that combined fluorination and π‐extension synergistically promote crystallographic π‐face‐to‐face packing, increase crystallinity, reduce internal reorganization energies, increase interplanar π–π electronic coupling, and increase power conversion efficiency.

    

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5. Photochemical Decomposition of Y‐Series Non‐Fullerene Acceptors Is Responsible for Degradation of High‐Efficiency Organic Solar Cells

   https://doi.org/10.1002/aenm.202300046  

   Liu, Tianran ; Burlingame, Quinn C. ; Ivancevic, Marko R. ; Liu, Xiao ; Hu, Junnan ; Rand, Barry P. ; Loo, Yueh‐Lin ( March 2023 , Advanced Energy Materials)

   Organic photovoltaic cells that employ Y‐series non‐fullerene acceptors (NFAs) have recently achieved impressive power‐conversion efficiencies (>18%). To fulfill their commercial promise, it is important to quantify their operational lifetimes and understand their degradation mechanisms. In this work, the spectral‐dependent photostability of films and solar cells comprising several Y‐series acceptors and the donor polymer PM6 is investigated systematically. By applying longpass filters during aging, it is shown that UV/near‐UV photons are responsible for the photochemical decomposition of Y‐series acceptors; this degradation is the primary driver of early solar cell performance losses. Using mass spectrometry, the vinylene linkage between the core and electron‐accepting moieties of Y‐series acceptors is identified as the weak point susceptible to cleavage under UV‐illumination. Employing a series of device characterization, along with numerical simulations, the efficiency losses in organic photovoltaic cells are attributed to the formation of traps, which reduces charge extraction efficiency and facilitates non‐radiative recombination as the Y‐series acceptors degrade. This study provides new insights for molecular degradation of organic photovoltaic absorber materials and highlights the importance of future molecular design and strategies for improved solar cell stability.

    

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