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1. 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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2. New D‐A‐A’‐Configured Small Molecule Donors Employing Conjugation to Red‐shift the Absorption for Photovoltaics

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

   Wang, Chun‐Kai; Che, Xiaozhou; Lo, Yuan‐Chih; Li, Ya‐Ze; Wang, Yung‐Hao; Forrest, Stephen R.; Liu, Shun‐Wei; Wong, Ken‐Tsung (June 2020, Chemistry – An Asian Journal)

   Four new donor-acceptor-acceptor' (D-A-A')-configured donors, CPNT, DCPNT, CPNBT, and DCPNBT equipped with naphtho[1,2-c:5,6-c']bis([1,2,5]-thiadiazole) (NT) or naphtho[2,3-c][1,2,5]thiadiazole (NBT) as the central acceptor (A) unit bridging triarylamine donor (D) and cyano or dicyanovinylene acceptor (A'), were synthesized and characterized. All molecules exhibit bathochromic absorption shifts as compared to those of the benzothiadiazole (BT)-based analogues owing to improved electron-withdrawing and quinoidal character of NT and NBT cores that lead to stronger intramolecular charge transfer. Favorable energy level alignments with C70 , together with the good thermal stability and the antiparallel dimeric packing render CPNT and DCPNT suitable donors for vacuum-processed organic photovoltaics (OPV)s. OPVs based on DCPNT : C70 active layers displayed the best power conversion efficiency (PCE)=8.3%, along with an open circuit voltage of 0.92 V, a short circuit current of 14.5 mA cm-2 and a fill factor of 62% under 1 sun intensity, simulated AM1.5G illumination. Importantly, continuous light-soaking with AM 1.5G illumination has verified the durability of the devices based on CPNT:C70 and DCPNT : C70 as the active blends. The devices were examined for their feasibility of indoor light harvesting under 500 lux illumination by a TLD-840 fluorescent lamp, giving PCE=12.8% and 12.6%, respectively. These results indicate that the NT-based D-A-A'-type donors CPNT and DCPNT are potential candidates for high-stability vacuum-processed OPVs suitable for indoor energy harvesting. 

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3. Quantum Simulations of Charge Separation at a Model Donor-Acceptor Interface: Role of Delocalization and Local Packing

   https://doi.org/10.1155/2018/6834908  

   Kelley, Allen; Patel, Kush; Bittner, Eric R. (January 2018, Advances in Condensed Matter Physics)

   Organic Polymer-based photovoltaic systems offer a viable alternative to more standard solid-state devices for light-harvesting applications. In this study, we investigate the electronic dynamics of a model organic photovoltaic (OPV) heterojunction consisting of polyphenylene vinylene (PPV) oligomers and a [ 6,6 ] -phenyl C61-butyric acid methyl ester (PCBM) blend. Our approach treats the classical molecular dynamics of the atoms within an Ehrenfest mean-field treatment of the π - π ⁎ singly excited states spanning a subset of donor and acceptor molecules near the phase boundary of the blend. Our results indicate that interfacial electronic states are modulated by C=C bond stretching motions and that such motions induce avoided crossings between nearby excited states thereby facilitating transitions from localized excitonic configurations to delocalized charge-separated configurations on an ultrafast time-scale. The lowest few excited states of the model interface rapidly mix allowing low frequency C-C out-of-plane torsions to modulate the potential energy surface such that the system can sample both intermolecular charge-transfer and charge-separated electronic configurations on sub-100 fs time scales. Our simulations support an emerging picture of carrier generation in OPV systems in which interfacial electronic states can rapidly decay into charge-separated and current producing states via coupling to vibronic degrees of freedom. 

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4. Organic single crystals of charge-transfer complexes: model systems for the study of donor/acceptor interactions

   https://doi.org/10.1039/D1MH01214B  

   Goetz, Katelyn P.; Iqbal, Hamna F.; Bittle, Emily G.; Hacker, Christina A.; Pookpanratana, Sujitra; Jurchescu, Oana D. (January 2022, Materials Horizons)

   The charge-transfer (CT) state arising as a hybrid electronic state at the interface between charge donor and charge acceptor molecular units is important to a wide variety of physical processes in organic semiconductor devices. The exact nature of this state depends heavily on the nature and co-facial overlap between the donor and acceptor; however, altering this overlap is usually accompanied by extensive confounding variations in properties due to extrinsic factors, such as microstructure. As a consequence, establishing reliable relationships between donor/acceptor molecular structures, their molecular overlap, degree of charge transfer and physical properties, is challenging. Herein, we examine the electronic structure of a polymorphic system based on the donor dibenzotetrathiafulvalene (DBTTF) and the acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) in the form of high-quality single crystals varying in the donor–acceptor overlap. Using angle-resolved photoemission spectroscopy, we resolve the highest occupied molecular orbital states of the CT crystals. Analysis based on field-effect transistors allows us to probe the sub-gap states impacting hole and electron transport. Our results expand the understanding on the impact of donor and acceptor interactions on electronic structure and charge transport. 

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5. 18.73% efficient and stable inverted organic photovoltaics featuring a hybrid hole-extraction layer

   https://doi.org/10.1039/d2mh01575g  

   Lin, Yuanbao; Zhang, Yadong; Magomedov, Artiom; Gkogkosi, Eleftheria; Zhang, Junxiang; Zheng, Xiaopeng; El-Labban, Abdulrahman; Barlow, Stephen; Getautis, Vytautas; Wang, Ergang; et al (April 2023, Materials Horizons)

   Developing efficient and stable organic photovoltaics (OPVs) is crucial for the technology's commercial success. However, combining these key attributes remains challenging. Herein, we incorporate the small molecule 2-((3,6-dibromo-9 H -carbazol-9-yl)ethyl)phosphonic acid (Br-2PACz) between the bulk-heterojunction (BHJ) and a 7 nm-thin layer of MoO 3 in inverted OPVs, and study its effects on the cell performance. We find that the Br-2PACz/MoO 3 hole-extraction layer (HEL) boosts the cell's power conversion efficiency (PCE) from 17.36% to 18.73% (uncertified), making them the most efficient inverted OPVs to date. The factors responsible for this improvement include enhanced charge transport, reduced carrier recombination, and favourable vertical phase separation of donor and acceptor components in the BHJ. The Br-2PACz/MoO 3 -based OPVs exhibit higher operational stability under continuous illumination and thermal annealing (80 °C). The T 80 lifetime of OPVs featuring Br-2PACz/MoO 3 – taken as the time over which the cell's PCE reduces to 80% of its initial value – increases compared to MoO 3 -only cells from 297 to 615 h upon illumination and from 731 to 1064 h upon continuous heating. Elemental analysis of the BHJs reveals the enhanced stability to originate from the partially suppressed diffusion of Mo ions into the BHJ and the favourable distribution of the donor and acceptor components induced by the Br-2PACz. 

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