Center for Light Energy Activated Redox Processes

Emerging nonfullerene acceptors (NFAs) with crystalline domains enable high-performance bulk heterojunction (BHJ) solar cells. Thermal annealing is known to enhance the BHJ photoactive layer morphology and performance. However, the microscopic mechanism of annealing-induced performance enhancement is poorly understood in emerging NFAs, especially regarding competing factors. Here, optimized thermal annealing of model system PBDB-TF:Y6 (Y6 = 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3′’:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]-thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) decreases the open circuit voltage (VOC) but increases the short circuit current (JSC) and fill factor (FF) such that the resulting power conversion efficiency (PCE) increases from 14 to 15% in the ambient environment. Here we systematically investigate these thermal annealing effects through in-depth characterizations of carrier mobility, film morphology, charge photogeneration, and recombination using SCLC, GIXRD, AFM, XPS, NEXAFS, R-SoXS, TEM, STEM, fs/ns TA spectroscopy, 2DES, and impedance spectroscopy. Surprisingly, thermal annealing does not alter the film crystallinity, R-SoXS characteristic size scale, relative average phase purity, or TEM-imaged phase separation but rather facilitates Y6 migration to the BHJ film top surface, changes the PBDB-TF/Y6 vertical phase separation and intermixing, and reduces the bottom surface roughness. While these morphology changes increase bimolecular recombination (BR) and lower the free charge (FC) yield, they also increase the average electron and hole mobility by at least 2-fold. Importantly, the increased μh dominates and underlies the increased FF and PCE. Single-crystal X-ray diffraction reveals that Y6 molecules cofacially pack via their end groups/cores, with the shortest π–π distance as close as 3.34 Å, clarifying out-of-plane π-face-on molecular orientation in the nanocrystalline BHJ domains. DFT analysis of Y6 crystals reveals hole/electron reorganization energies of as low as 160/150 meV, large intermolecular electronic coupling integrals of 12.1–37.9 meV rationalizing the 3D electron transport, and relatively high μe of 10–4 cm2 V–1 s–1. Taken together, this work clarifies the richness of thermal annealing effects in high-efficiency NFA solar cells and tasks for future materials design.

The end-capping group (EG) is the essential electron-withdrawing component of nonfullerene acceptors (NFAs) in bulk heterojunction (BHJ) organic solar cells (OSCs). To systematically probe the impact of two frequent EG functionalization strategies, π-extension and halogenation, in A-DAD-A type NFAs, we synthesized and characterized four such NFAs: BT-BIC, LIC, L4F, and BO-L4F. To assess the relative importance of these strategies, we contrast these NFAs with the baseline acceptors, Y5 and Y6. Up to 16.6% power conversion efficiency (PCE) in binary inverted OSCs with BT-BO-L4F combining π-extension and halogenation was achieved. When these two factors are combined, the effect on optical absorption is cumulative. Single-crystal π–π stacking distances are similar for the EG strategies of π-extension. Increasing the alkyl substituent length from BT-L4F to BT-BO-L4F significantly alters the packing motif and eliminates the EG core interactions of BT-L4F. Electronic structure computations reveal some of the largest NFA π–π electronic couplings observed to date, 103.8 meV in BT-L4F and 47.5 meV in BT-BO-L4F. Computed electronic reorganization energies, 132 and 133 meV for BT-L4F and BT-BO-L4F, respectively, are also lower than Y6 (150 meV). BHJ blends show preferential π-face-on orientation, and both fluorination and π-extension increase NFA crystallinity. Femto/nanosecond transient absorption spectroscopy (fs/nsTA) and integrated photocurrent device analysis (IPDA) indicate that π-extension modifies the phase separation to enhance film ordering and carrier mobility, while fluorination suppresses unimolecular recombination. This systematic study highlights the synergistic effects of NFA π-extension and fluorination in affording efficient OSCs and provides insights into designing next-generation materials.

This Forum Article describes the photocatalytic oxidation of benzyl alcohol by visible-light-absorbing colloidal CdS quantum dots (QDs), with 99% selectivity for benzaldehyde or 91% selectivity for C–C coupled products (primarily hydrobenzoin). The selectivity is tuned through the number of low-valent Cd atoms photodeposited on the surfaces of the QDs in situ; this deposition is enhanced by addition of a Cd2+ salt and suppressed by addition of an electron scavenger, anthroquinone-2-sulfonate, to the reaction mixture. In both cases, the external quantum efficiency (number of oxidative equivalents extracted from the QDs per incident photon) of the system is ∼1%.

This Letter describes the use of CdSe quantum dots (QDs) as photocatalysts for photoinduced electron transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerization of a series of aqueous acrylamides and acrylates. The high colloidal solubility and photostability of these QDs allowed polymerization to occur with high efficiency (>90% conversion in 2.5 h), low dispersity (PDI < 1.1), and ultralow catalyst loading (<0.5 ppm). The use of protein concentrators enabled the removal of the photocatalyst from the polymer and monomer with tolerable metal contamination (8.41 ug/g). These isolated QDs could be recycled for four separate polymerizations without a significant decrease in efficiency. By changing the pore size of the protein concentrators, the QDs and polymer could be separated from the remaining monomer, allowing for the synthesis of block copolymers using a single batch of QDs with minimal purification steps and demonstrating the fidelity of chain ends.

Abstract 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‐4 H ,8 H ‐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 (&lt;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.

We report two new poly-small-molecule acceptors, PYN-BDT and PYN-BDTF, which serve, by virtue of their π-extended naphthalene rings, as broad optical cross-section macromolecular absorbers (extending to ∼900 nm; ΔEopticalgap = 1.38 eV) in all-polymer solar cells (APSCs). APSCs fabricated by blending PYN-BDT or PYN-BDTF with PM6 exhibit power conversion efficiencies (PCEs) of 7.24 and 9.08%, respectively, while blends with PBDB-T exhibits far higher PCEs of 12.06 and 13.22%, respectively; the latter cell achieves Jsc = 22.28 mA cm–2, among the highest known for an APSC. The results of blend morphology, GIWAXS, charge transport, exciton and carrier dynamics, PL quenching efficiency, and impedance-based analysis indicate that the PBDB-TT:PYN-BDTF blends and their APSCs outperform the corresponding PM6:PYN-BDTF devices due to significantly suppressed bimolecular recombination. These results demonstrate that π-conjugative extension of individual polymer acceptor blocks represents an efficient strategy to broaden APSC optical cross sections, decrease bimolecular recombination, and achieve high-performance cells with enhanced Jsc metrics.

Mixed-dimensional heterojunctions, such as zero-dimensional (0D) organic molecules deposited on two-dimensional (2D) transition metal dichalcogenides (TMDCs), often exhibit interfacial effects that enhance the properties of the individual constituent layers. Here we report a systematic study of interfacial charge transfer in metallophthalocyanine (MPc) – MoS2 heterojunctions using optical absorption and Raman spectroscopy to elucidate M core (M = first row transition metal), MoS2 layer number, and excitation wavelength effects. Observed phenomena include the emergence of heterojunction-specific optical absorption transitions and strong Raman enhancement that depends on the M identity. In addition, the Raman enhancement is tunable by excitation laser wavelength and MoS2 layer number, ultimately reaching a maximum enhancement factor of 30x relative to SiO2 substrates. These experimental results, combined with density functional theory (DFT) calculations, indicate strong coupling between nonfrontier MPc orbitals and the MoS2 band structure as well as charge transfer across the heterojunction interface that varies as a function of the MPc electronic structure.

Abstract Non‐fullerene acceptor (NFA) end group (EG) functionalization, especially by fluorination, affects not only the energetics but also the morphology of bulk‐heterojunction (BHJ) organic solar cell (OSC) active layers, thereby influencing the power conversion efficiency (PCE) and other metrics of NFA‐based OSCs. However, a quantitative understanding of how varying the degrees of NFA fluorination influence the blend morphological and photovoltaic properties remains elusive. Here a series of three A‐DAD‐A type NFAs (D = π‐donor group and A = π‐acceptor EG) which systematically increase the degree of EG fluorination and comprehensively investigate the resulting blends with the polymer donor PM6 in terms of optical properties, electronic structure, film crystallinity, charge carrier transport, and OSC performance is reported. The results indicate that the most highly fluorinated NFA, BT‐BO‐L4F, achieves an optimal BHJ hierarchical morphology where enhanced NFA molecule intermolecular π–π stacking and optimal vertical phase gradation are achieved in the BHJ blend. These factors also promote optimum NFA‐cathode contact, more balanced electron and hole mobility, and suppress both monomolecular and bimolecular recombination. As a result, both the short‐circuit current density and fill factor in this OSC series progressively increase with increasing EG fluorine density, and the resulting PCEs increase from 9 to 16.8%.

Abstract Developing efficient interfacial hole transporting materials (HTMs) is crucial for achieving high‐performance Pb‐free Sn‐based halide perovskite solar cells (PSCs). Here, a new series of benzodithiophene (BDT)‐based organic small molecules containing tetra‐ and di‐triphenyl amine donors prepared via a straightforward and scalable synthetic route is reported. The thermal, optical, and electrochemical properties of two BDT‐based molecules are shown to be structurally and energetically suitable to serve as HTMs for Sn‐based PSCs. It is reported here that ethylenediammonium/formamidinium tin iodide solar cells using BDT‐based HTMs deliver a champion power conversion efficiency up to 7.59%, outperforming analogous reference solar cells using traditional and expensive HTMs. Thus, these BDT‐based molecules are promising candidates as HTMs for the fabrication of high‐performance Sn‐based PSCs.

Here we report facile, high-yield synthetic access to the difluoro BTA building block, 4,7-bis(5-bromo-4-(2-hexyl-decyl)-thiophen-2-yl)-5,6-difluoro-2-(pentadecan-7-yl)-benzo[d]thiazole (BTAT-2f), for use in donor (D)–acceptor 1 (A1)–D–acceptor 2 (A2) polymers [D = bithiophene; A1 = BTA-2f; A2 = benzothiadiazole (BT) derivative] for organic solar cells (OSCs). Fine tuning of polymer optical and electronic properties is achieved by incrementally varying the A2 fluorination level. Bulk-heterojunction (BHJ) PBTATBT-4f:Y6 solar cells deliver a noteworthy power conversion (PCE) efficiency of 16.08% (Voc = 0.81 V; Jsc = 27.25 mAcm–2; FF = 72.70%) without processing additives. In contrast, PBTATBT-2f:Y6 exhibits an irregular morphology and low PCE, ascribable to cocrystal formation-induced recombination, which is unprecedented for nonfullerene (NFA) OSCs. This result should be of guiding significance for future NFA design.

Abstract Fluorination of the donor and/or acceptor blocks of photoactive semiconducting polymers is a leading strategy to enhance organic solar cell (OSC) performance. Here, the effects are investigated in OSCs using fluorine‐free ( TPD‐3 ) and fluorinated ( TPD‐3F ) donor polymers, paired with the nonfullerene acceptor Y6. Interestingly and unexpectedly, fluorination negatively affects performance, and fluorine‐free TPD‐3 :Y6 OSCs exhibit a far higher power conversion efficiency (PCE = 14.5%) than in the fluorine‐containing TPD‐3F :Y6 blends (PCE = 11.5%). Transmission electron microscopy (TEM) analysis indicates that the TPD‐3F :Y6 blends have larger phase domain sizes than TPD‐3 :Y6, which reduces exciton dissociation efficiency to 81% for TPD‐3F :Y6 versus 93% for TPD‐3 :Y6. Additionally, grazing incidence wide‐angle X‐ray scattering (GIWAXS) reveals that the TPD‐3F :Y6 blends are less textured than those of TPD‐3 :Y6, while space‐charge limited currents reveal lower and unbalanced hole/electron mobility in TPD‐3F :Y6 versus TPD‐3 :Y6 blends. Charge recombination dynamic, transient absorption, and donor–acceptor miscibility assays additionally support this picture. Furthermore, conventional architecture TPD‐3 :Y6 OSCs deliver a PCE of 15.2%, among the highest to date for halogen‐free polymer donor OSCs. Finally, a large‐area (20.4 cm 2 ) TPD‐3 :Y6 blend module exhibits an outstanding PCE of 9.31%, one of the highest to date for modules of area &gt;20 cm 2 .

Accurate single-crystal X-ray diffraction data offer a unique opportunity to compare and contrast the atomistic details of bulk heterojunction photovoltaic small-molecule acceptor structure and packing, as well as provide an essential starting point for computational electronic structure and charge transport analysis. Herein, we report diffraction-derived crystal structures and computational analyses on the n-type semiconductors which enable some of the highest efficiency organic solar cells produced to date, 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC) and seven derivatives (including three new crystal structures: 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-propylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC-C3), 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(3-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (m-ITIC-C6), and 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-butylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC-C4-4F). IDTT acceptors typically pack in a face-to-face fashion with π-π distances ranging from 3.28-3.95 Å. Additionally, edge-to-face packing is observed with S⋯π interactions as short as 3.21-3.24 Å. Moreover, ITIC end group identities and side chain substituents influence the nature and strength of noncovalent interactions (e. g. H-bonding, π-π) and thus correlate with the observed packing motif, electronic structure, and charge transport properties of the crystals. Density functional theory (DFT) calculations reveal relatively large nearest-neighbor intermolecular π-π electronic couplings (5.85-56.8 meV) and correlate the nature of the band structure with the dispersion interactions in the single crystals and core-end group polarization effects. Overall, this combined experimental and theoretical work reveals key insights into crystal engineering strategies for indacenodithienothiophene (IDTT) acceptors, as well as general design rules for high-efficiency post-fullerene small molecule acceptors.

Significance Blade coating is a promising methodology for the large-scale printing of polymer electronics, affording nonnegligible microstructure control and properties enhancement. Nevertheless, in two-component systems, the optical/electrical/physical properties are largely dominated by phase separation and domain purity phenomena that are challenging to control. Here, we report a mixed-flow microfluidic printing approach to phase purity control, enabled by a printing blade design based on fluid flow simulations. The result is 50% efficiency enhancement for printed all-polymer solar cells vs. conventional printing and similar enhancements for polymer transistors. Mixed flow is a versatile approach to control domain purity in two-component polymeric semiconductor systems and offers a methodology for printing high-performance soft-matter electronics.

Achieving efficient polymer solar cells (PSCs) requires a structurally optimal donor-acceptor heterojunction morphology. Here we report the combined experimental and theoretical characterization of a benzodithiophene-benzothiadiazole donor polymer series (PBTZF4-R; R = alkyl substituent) blended with the non-fullerene acceptor ITIC-Th and analyze the effects of substituent dimensions on blend morphology, charge transport, carrier dynamics, and PSC metrics. Varying substituent dimensions has a pronounced effect on the blend morphology with a direct link between domain purity, to some extent domain dimensions, and charge generation and collection. The polymer with the smallest alkyl substituent yields the highest PSC power conversion efficiency (PCE, 11%), reflecting relatively small, high-purity domains and possibly benefiting from "matched" donor polymer-small molecule acceptor orientations. The distinctive morphologies arising from the substituents are investigated using molecular dynamics (MD) simulations which reveal that substituent dimensions dictate a well-defined set of polymer conformations, in turn driving chain aggregation and, ultimately, the various film morphologies and mixing with acceptor small molecules. A straightforward energetic parameter explains the experimental polymer domain morphological trends, hence PCE, and suggests strategies for substituent selection to optimize PSC materials morphologies.

The scope of the environmentally benign direct C–H arylation polymerization (DARP) process is validated and significantly extended in the synthesis of a high-performance benzodithiophene-based copolymer series, PBDT(Ar)-FTTE, with previously untested and systematically varied heteroaryl (Ar) substituents. Bulk-heterojunction (BHJ) polymer solar cells (PSCs) containing the high-performance nonfullerene acceptor (NFA) ITIC-Th and DARP-derived donors are fabricated and evaluated, yielding PCEs as high as 8%. The relationships between Ar-sensitive copolymer structure, BHJ morphology, and PSC performance are elucidated through in-depth characterization of structural order, phase separation, and charge transport using SCLC, AFM, GIWAXS, R-SoXS, and NEXAFS measurements, which conclusively demonstrate the important effects of Ar-tunable, dimensionally smaller, and well-blended copolymer domains for maximum PSC performance. Smaller BHJ copolymer domains having greater ITIC-Th miscibility definitively correlate with enhanced JSC, FF, and PCE metrics. Surprisingly regarding cell performance durability, while unencapsulated PBDTT-FTTE:ITIC-Th PSCs deliver the highest initial PCE, the unencapsulated PBDTTF-FTTE:ITIC-Th devices exhibit the optimum combination of high initial photovoltaic metrics and stability, retaining nearly 90% of the initial PCE after 51 days in ambient conditions and 83% of initial PCE after 180 min under simulated solar illumination. Importantly, for this PBDT(Ar)-FTTE:ITIC-Th series, PSC photovoltaic stability correlates with the presence of large pure BHJ domains, and moreover rivals or exceeds the stability of the analogous fullerene-based PSCs. Together, these results argue that solar cells prepared with the environmentally benign DARP process and NFAs are promising for both greener and more stable solar energy generation.

The dynamics of an excess electron in size-selected methanol clusters is studied via pump-probe spectroscopy with resolution of approximately 120 fs. Following excitation, the excess electron undergoes internal conversion back to the ground state with lifetimes of 260-175 fs in (CH3OH)n- (n=145-535) and 280-230 fs in (CD3OD)n- (n=210-390), decreasing with increasing cluster size. The clusters then undergo vibrational relaxation on the ground state on a time scale of 760+/-250 fs. The excited state lifetimes for (CH3OH)n- clusters extrapolate to a value of 157+/-25 fs in the limit of infinite cluster size.

Recently, the power conversion efficiencies (PCEs) of all-polymer solar cells (all-PSCs) have surpassed 18%, principally reflecting the introduction of polymerized small-molecule acceptors (PSMAs). However, the effect of PSMA backbone conjugation on both device photovoltaic and mechanical properties has been sparsely investigated. Here we report the synthesis of two PSMAs having fully conjugated or partially nonconjugated backbones by copolymerizing a 2,2′-((2Z,2′Z)-((12,13-bis(2ethylhexyl)-3,9-diundecyl-12,13-dihydro[1,2,5] thiadiazolo[3,4-e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(6-fluoro-7-bromo-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))-dimalononitrile) (YF-Br) core with thienylene-vinylene-thienylene (TVT) and thienylene-ethyl-thienylene (TET) units, respectively. The resulting PYFTVT and PYFTET polymers are then blended with the conjugated donor polymer PM6 to fabricate all-PSCs. The results reveal that the PCE and device stability upon mechanical deformation are dominated by the PSMA backbone conjugation and molecular mass. Thus, for comparable molecular masses, the PYFTET-based blend exhibits higher crack onset strain and toughness than the PYFTVT-based blend. Furthermore, increasing the PSMA molecular mass enhances the mechanical ductility. Overall, these results convey important implications for developing future ductile and stretchable electronics.

Symmetric and new asymmetric electron-transporting naphthalene diimide (NDI) copolymers comprising five-member ring heterocyclic (Het) donor units [Het = furan (Fu), thiophene (Th), and selenophene (Se)] were synthesized via anion radical polymerization of dibrominated Het-NDI-Het monomers, which avoids the use of toxic reagents. Product polymers and monomeric building blocks were characterized by a battery of techniques including nuclear magnetic resonance, optical absorption, and Raman spectroscopy, as well as cyclic voltammetry and single-crystal X-ray diffraction. DFT computations were carried out for trimer models of the polymers. These comprehensive data indicate that furan unit incorporation results in more planar blocks/polymer backbones with elevated HOMO energies but limited Fu-NDI skeletal π-connectivity versus those of the Th and Se analogues. The resulting polymers were employed to fabricate organic field-effect transistors (OFETs) and all-polymer solar cells (APSCs) affording electron mobilities ranging from 0.012 cm2 V–1 s–1 to 0.24 cm2 V–1 s–1 and power conversion efficiencies from 1.71% to 6.41%. In all cases, the Fu-containing polymers exhibit the lowest performance, and PCEs within the series of polymers based on asymmetric NDI building blocks outperform those with symmetric building blocks. Films of the pristine polymers and their blends with the polymer donor PBDB-T were characterized by AFM, TEM, and GIWAXS and, for the blends, also by space-charge limited current measurements. Together these results provide important geometry–electronic structure–performance correlations, illuminating the reasons underlying the key device performance trends.

The development of readily accessible polymer acceptors is imperative to preserve the guiding principles of all-polymer solar cells as a low-cost and sustainable technology for alternative energy production. In this study, we report a computationally guided design and facile synthesis of new acceptor polymers comprising the alternating copolymer, PNIT, and the corresponding random copolymer, r-PNIT, which incorporate structurally simple naphthalene diimide (NDI) and isoindigo (IID) units. PNIT was prepared via direct arylation polymerization (DArP), which proceeds via C–H activation and avoids the use of toxic reagents and extended synthetic pathways for the monomer synthesis. PNIT has broad optical absorption in the 300–850 nm range and an enhanced absorption coefficient (47 × 103 cm–1) compared to r-PNIT (300–850 nm; 40 × 103 cm–1). When incorporated in all-polymer solar cells (APSCs) using PBDB-T as the donor polymer, PBDB-T:PNIT provides greater than two times the average (maximum) power conversion efficiency (PCE) of 5.18 ± 0.08 (5.32)% compared to that of PBDB-T:r-PNIT, 2.38 ± 0.16 (2.57)%, due to increased Jsc (10.36 vs 5.45 mA cm–2) and fill factor (FF) (0.58 vs 0.50) metrics. The PBDB-T:PNIT PCE is among the highest reported for an IID-based APSC and demonstrates the viability of DArP for the synthesis of new APSC acceptor polymers. Detailed morphological and microstructural investigations using atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS), respectively, reveal enhanced texturing for PNIT, which enhances charge transport properties as supported by space-charge-limited current (SCLC) mobility measurements.

We investigate backbone fluorination effects in bulk-heterojunction (BHJ) polymer solar cells (PSCs) with the fluorine-poor PBDTT-FTTE and fluorine-rich PBDTTF-FTTE donor polymers, paired with the perylenediimide (PDI) 3D “propeller acceptor” Ph(PDI)3. The PBDTTF-FTTE:Ph(PDI)3 devices exhibit a >50% power conversion efficiency (PCE, up to 9.1%) increase versus PBDTT-FTTE:Ph(PDI)3. This enhancement reflects structurally optimized phase separation due to templating effects, affording reduced energy loss, higher electron mobility, greater free charge lifetimes and yields, and lower bimolecular recombination, as quantified by UPS, AFM, TEM, GIWAXS, SCLC, light intensity dependence measurements, and fs/ns transient absorption (TA) spectroscopy. In PBDTTF-FTTE, the DFT-computed dipole orientations of the ground and excitonic states are nearly antiparallel, explaining the longer free charge lifetimes, minimized recombination, and lowered exciton binding energy. The PBDTTF-FTTE:Ph(PDI)3 performance enhancement vs that of the fluorine-poor PBDTT-FTTE:Ph(PDI)3 analogue as well as the overall PSC performance exceeds that of the corresponding PC71BM- and ITIC-Th-based cells.