Chemistry

Easy non-fused electron acceptors for environment friendly and steady natural photo voltaic cells

Synthesis of NFRAs and molecular monformation

The totally non-fused acceptors, PTICH, PTIC, and PTICO, are designed and achieved with a two-step artificial route (Fig. 1), that are usually extra handy than the synthesis of FREAs (Supplementary Figs. 1 and a pair of). The chemical constructions of the studied NFRAs and polymer donors, together with a fused-ID4F acceptor are proven in Supplementary Figs. Three and4. PT-CHOs had been first constructed by Palladium (II)-catalyzed C–H activation coupling reactions between 1,Four-dibromo-2,5-bis((2-hexyldecyl)oxy) benzene and thiophene-2-carbaldehydes, featured with atomic economic system. Goal merchandise had been lastly accessed from Knoevenagel condensation of PT-CHOs and DFIC. It’s worthy to notice that NFRAs had been purified by recrystallization with out the necessity of silica gel column, featured with the easy and scalable work-up processes. The ultimate merchandise had been characterised by 1H-NMR, 13C-NMR, and mass spectra. The artificial advanced (SC) index of those NFRAs was additionally calculated based on reported methodologies24,34, whereby the SC index of PTIC is round 55%, a lot smaller than that of ID4F (97%) and different in contrast FREAs (usually past 80%) (Supplementary Desk 1). Primarily, the smaller SCs for NFRAs had been attributed from intrinsically quick artificial routes and ease of purification.

Fig. 1Fig. 1

Non-fused-ring acceptors (NFRAs) and their artificial route. a The chemical construction of FREA and NFRA. b The schematic presentation of NFRA conformation rearrangement from answer to stable state. c Two-step artificial route for NFRAs. Right here R1 is 2-hexyldecyl. Reacted situation: (1) Three-substituent-2-thenaldehyde, palladium-catalyzed C–H activation coupling and (2) DFIC, Knoevenagel condensation

To be able to perceive the molecular conformation of the above NFRAs, we first performed density useful concept (DFT) calculations (Fig. 2a and Supplementary Fig. 5). Inside NFRAs, two probably rotatable single C–C bonds (middle single bond between alkyloxybenzene and thiophene, famous as PT rotamer; terminal single bond between thiophene and DFIC, famous as TIC rotamer) had been current. The relaxed potential floor vitality scans of doable rotamers had been carried out (Fig. 2a). The vitality–torsion angle curves (E–θ curves) reveal that PT rotamer has a rotation vitality barrier of Four.Four kJ mol−1, from the conformation with the bottom complete vitality (10o PT rotamer featured with O–H interplay, O–H geometry), via 90° PT rotamer, to the conformation with the second lowest complete vitality (170° PT rotamer featured with O–S interplay between alkyloxybenzene and thiophene, O–S geometry). Word that O–H geometry has A barely decrease complete vitality than that of O–S geometry (Fig. 2a and Supplementary Fig. 5). It means that O–H geometry of NFRAs is the energetically preferential conformation, which, nonetheless, can thermodynamically entry O–S geometry if rotation barrier of PT rotamer is being overcome. As well as, the E–θ curves of the terminal C–C bond (TICH rotamer for PTICH, TIC rotamer for PTIC, and TICO rotamer for PTICO) had been in comparison with point out that substituted thiophene helps stopping the technology of a 180o rotamer (one of many doable isomers for NFRAs). It’s as a result of the steric hindrance between the thiophene substituent and DFIC considerably will increase vitality limitations over 45 kJ mol−1 (as a comparability, TICH rotamer with unsubstituted thiophene reveals solely a barrier of 6.25 kJ mol−1 at torsion angle of 180°). Theoretical investigations recommend that NFRAs adapt O–H geometry as their preferential conformation, whereby the middle single bond has a comparatively small rotation barrier, whereas, the terminal single bond reveals much less rotation tendency (Fig. 2a).

Fig. 2Fig. 2

Molecular conformation and optical properties. a Potential rotamers and their vitality–torsion angle (E–θ) curves: black curve for PT rotamer (left construction for O–H geometry and O–S geometry on the proper); TICH rotamer (pink), TIC rotamer (blue), and TICO rotamer (pink). b Temperature-dependent NMR of PTIC in d4-C6D4Cl2. c The heteronuclear multiple-bond correlation (HMBC) of PTIC in CDCl3. d The preferential conformation of PTIC. e The answer and f movie UV–vis absorption of ID4F and NFRAs. g The UV–vis absorption of I747 nm/I690 nm ratio for PTIC movie annealed at 100 oC for 320 min, with an inset of optical photographs

To probe the noncovalent O–H intramolecular interplay, we additional performed temperature-dependent 1H-NMR for NFRAs (Fig. 2b). Apparently, the chemical shift of thiophene proton (Ha) in PTIC (in d4-dichlorobenzene) steadily strikes from 7.83 to 7.70 ppm, upon rising temperature from 30 to 120 °C. It’s the attribute for breaking hydrogen bonds between Ha and oxygen atoms on alkyloxybenzene of the PT core on the elevated temperature. The hydrogen bond can be supported from the calculated Ha–O distance that’s shorter than the sum of Van der Waals radius for H and O atoms35. The proton (Hb) on the benzene core additionally barely shifts from 7.57 to 7.50 ppm, owing to the rotation of Hb away from DFIC. However, aliphatic protons (Hc and He) haven’t any apparent adjustments. Related outcomes are additionally noticed from the temperature-dependent 1H-NMR of PTICH and PTICO (Supplementary Figs. 6–eight).

From the 13C−1H correlation spectra of PTIC in CDCl3, the heteronuclear multiple-bond correlation (HMBC) displays a sign at (7.71, 154) of Ha and C3 correlation (Fig. 2c, Supplementary Fig. 9), suggesting the existence of a preferential hydrogen-bond-based conformation. Nonetheless, the coexistence of S–O geometry in answer shouldn’t exclude because of the low rotation vitality barrier of PT rotamer. To sum up, PTIC analogously adapts O–H geometry as a preferential conformation, which can be nonetheless thermodynamically accessible to O–S geometry in answer.

SSNMR of PTIC was additional performed to probe the molecular stacking behaviors in solids (Supplementary Figs. 10–14 and Supplementary Desk 2). The outcomes recommend “end-to-end” packing of PTIC in stable (Fig. 1b), which is analogous, however in nearer stacking distance to these reported FREAs (Supplementary Fig. 15)36,37. This statement is in settlement with GIWAXS measurements (which will probably be mentioned later). From these theoretical and experimental outcomes, we reveal that the NFRAs which have O–H geometry as a preferential conformation, which can be rotatable in answer (because of the low-energy barrier of middle single C–C bond), might be restrained into a comparatively inflexible and planar construction with dense “end-to-end” stacking in stable. From the fabric processing perspective, this conformational rearrangement from answer to stable would endow NFRA acceptors not solely good answer processability, but additionally wonderful movie optoelectronic properties.

Optoelectronic properties

The spine conformation drastically influences optoelectronic properties of a molecule. NFRAs (PTICH, PTIC, and PTICO) have options of rotatable conformation in answer, whereas being restrained right into a planar conformation upon stacking collectively within the movie. The ultraviolet–seen (UV–vis) absorption spectra of three NFRAs and fused ID4F in answer and skinny movies are proven in Fig. 2e. In answer, PTICH, PTIC, and PTICO exhibit broad absorption peaks with out obvious aggregation shoulders, with most absorption (λabssol) situated at 637, 648, and 647 nm on the molar extinction coefficients (εabssol) of zero.92 × 105 M−1 cm−1 (PTICH), 1.12 × 105 M−1 cm−1 (PTIC), and 1.18 × 105 M−1 cm−1 (PTICO), respectively. These extinction coefficients are apparently decrease than these of ID4F (εabssol of 1.75 × 105 M−1 cm−1 at 671 nm) in answer (Desk 1 and Fig. 2e). It’s as a result of that fused ID4F has a inflexible and planar construction in answer, guaranteeing stronger intramolecular cost switch (ICT) impact and aggregation than these of non-planar NFRAs in answer.

Desk 1 Optical properties of ID4F and NFRAs

Apparently, the utmost absorption peaks of movies (λabsfilm) are noticed at 709, 747, and 746 nm with sturdy aggregation shoulders for PTICH, PTIC, and PTICO, respectively (Fig. 2f). The λabsfilm of NFRAs shows a big redshift over 70 nm from answer to the movie, whereas ID4F has solely a 36-nm redshift. The extinction coefficients of movies (εabsfilm) of two NRFAs (PTIC, zero.99 × 105 cm−1 and PTICO, 1.01 × 105 cm−1) are nearly in the identical stage of ID4F (1.06 × 105 cm–1). These point out that NFRAs in movies not solely kind dense J-aggregation, but additionally adapt planar conformation with enhanced ICTs, leading to extra bathochromic and hyperchromic shifts than ID4F. Moreover, the PTIC reveals wonderful conformational and morphological stabilities within the movie, as indicated from temperature-dependent absorption measurements (Fig. 2g). When annealed from 25 to 200 °C, or at 100 oC as much as 360 min, each the depth ratios (I747 nm/I690 nm) and absorption edge stay the identical with no obvious adjustments (Fig. 2g, Supplementary Figs. 16 and 17). These outcomes coincide with the differential scanning calorimetry (DSC) of acceptors with out obvious thermal transitions under 200 °C (Supplementary Fig. 18). Quite the opposite, in chlorobenzene (CB) answer, the λabssol of PTIC is blue-shifted from 650 to 639 nm upon rising temperature from room temperature to 70 °C, which can be attributed to versatile and rotational conformation within the answer (Supplementary Fig. 19).

The photoluminescence (PL) spectra of ID4F and NFRAs had been additionally measured in each answer and movie (Supplementary Fig. 20 and Desk 1). The emission of PTIC reveals the dependence of solvent polarity, attributing the ICT impact of NFRAs upon excitation (Supplementary Fig. 21). The Stokes shift (SS) might be calculated from the distinction between most absorption and emission, which hyperlinks to the reorganization vitality of a molecule between floor and excited-state transition. In answer, the SS values of PTICH, PTIC, and PTICO are 85, 75, and 71 nm, that are all apparently bigger than 54 nm of ID4F, indicating that the nonradiative transition of NFRAs consumes extra energies than that of FREA ID4F. Apparently, within the movie, the SS values of NFRAs are largely diminished (48 nm for PTIC and 49 nm of PTICO), suggesting that NFRAs within the movie have much less (at the least related) nonradiative vitality losses than these of ID4F (51 nm), that are in step with the calculated reorganization vitality values (Supplementary Fig. 22). That is in settlement with the quantum efficiencies of non-fused acceptors within the stable state, that are apparently increased than these in answer; attributing to molecular packing in stable helps rigidifying the molecular construction and suppressing nonradiative decay, over these of versatile constructions in answer.

The vitality ranges of those acceptors had been estimated from cyclic voltammetry (CV) and ultraviolet photoelectron spectrometer (UPS) (Fig. 3b and Supplementary Desk Three). NFRAs possess comparatively upshifted vitality ranges over these of ID4F. They energetically match with the PBDB-TF polymer donor, apart from PTICO, exhibiting a small highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital offsets (zero.01 and zero.06 V, respectively) with PBDB-TF (Supplementary Fig. 23).

Fig. ThreeFig. 3

Vitality ranges and photovoltaic efficiency. a Inverted OSC system structure. b Vitality ranges for various lively parts. c The J–V traits and d EQE spectra of OSCs beneath AM 1.5 G illumination (100 mW cm–2). e JSC versus gentle depth of the OSCs. f Stabilities of encapsulated units beneath steady illumination of a metallic halide lamp with out UV filtration. The error bars characterize the usual deviation from 4 units

Photovoltaic properties

To analyze the affect of NFRA structural elements to their photovoltaic efficiency, we now have fabricated inverted OSCs with an structure of indium tin oxide (ITO)/ZnO/lively layer/MoO3/Ag (Fig. 3a), whereby the lively layer employs the PBDB-TF polymer donor and every acceptor (Fig. 3b), respectively. Word that every one units are fabricated from single-solvent processing with out an additive, which can be possible and straightforward for scale-up fabrication. All NFRA-based blends are solid with CB, and the ID4F mix is fabricated with CHCl3 resulting from its solubility choice. Their present density–voltage (J–V) traits are proven in Fig. 3c, and system performances are summarized in Desk 2. The ID4F-based system exhibited the PCE of 6.88%, with the JSC of 13.42 mA cm−2, VOC of zero.84 V, and FF of zero.61. NFRA-based units usually exhibit increased VOC values (zero.92 V for PTICH, zero.93 V for PTIC, and 1.01 V for PTICO) with mitigated voltage losses (zero.68 V for PTICH, zero.58 V for PTIC, and zero.55 V for PTICO), evaluating with these of fused ID4F (voltage losses of zero.80 V).

Desk 2 The system parameters of OSCs with totally different acceptors beneath AM 1.5 G illumination (100 mW cm–2). The resistances had been extracted from the J–V curves beneath gentle illumination

Encouragingly, PTIC-based system reveals the first rate efficiency with PCE of 10.27%, FF of zero.66, and JSC of 16.73 mA cm−2, and optimum collection resistance (Rs), in addition to shunt resistance (Rsh), regardless of that PTICH-based and PTICO-based units possess comparatively low JSC and FF values. When a non-halogenated solvent, resembling o-xylene is employed, the PBDB-TF: PTIC-based system can nonetheless attain 9.53% PCE (Supplementary Fig. 24). The exterior quantum effectivity (EQE) spectra of the PBDB-TF: PTIC mix show a broadened photoresponse from 310 to 820 nm (Fig. 3d). The calculated JSC values from the mixing of EQE curves are 15.93 mA cm−2 (PBDB-TF: PTIC), 12.80 mA cm−2 (PBDB-TF: ID4F), eight.19 mA cm−2 (PBDB-TF: PTICH), and 12.18 mA cm−2 (PBDB-TF: PTICO), respectively, in step with these from J–V measurements.

It’s fascinating to notice that the general EQE and photocurrent of the PTIC-based system are superior to different units in these research. To grasp the doable causes, electron mobilities of blends are measured by the area charge-limited present (SCLC) technique, revealing related ranges of three.Four × 10–5 cm2 V−1 S−1 (ID4F), 1.1 × 10–Four cm2 V−1 S−1 (PTICH), Four.Three × 10–5 cm2 V−1 S−1 (PTIC), and 1.7 × 10−5 cm2 V−1 S−1 (PTICO), respectively. The corresponding gap mobilities are 2.5 × 10−Four cm2 V−1 S−1 (ID4F), 2.Three × 10−Four cm2 V−1 S−1 (PTICH), 1.2 × 10−Four cm2 V−1 S−1 (PTIC), and 1.7 × 10−Four cm2 V−1 S−1 (PTICO). The ratios of cost provider mobilities (electron mobility/gap mobility) had been zero.13, zero.47, zero.37, and zero.10 (Supplementary Fig. 25). The cost recombination of units was additional investigated by way of monitoring the sunshine depth (Plight)-dependent J–V traits26. As proven in Fig. 3e, the slopes for JSC–Plight had been zero.97, zero.96, zero.98, and zero.89 for ID4F-based, PTICH-based, PTIC-based, and PTICO-based units, suggesting that every one units have good dissociation possibilities at short-circuit situation with slopes near 1, apart from PBDB-TF: PTICO-based system (with a smaller slope of zero.89). It may be ascribed to the small energetic offsets between PTICO and polymer, maybe hindering the exciton disassociation; thus there’s a monomolecular recombination of the associated OSCs, ensuing within the reasonable JSC (12.60 mA cm−2) and PCE (6.62%). The dependence of VOC on the Plight reveals that the slopes of units, are near 1 kT q−1 (Supplementary Fig. 26), indicating that open-circuit conduction units have primarily a bimolecular recombination with out trap-assisted Shockley–Learn–Corridor (SRH) recombination38.

In gentle of the first rate efficiency obtained from easy PTIC NFRA, we studied the stabilities of units (PBDB-TF: PTIC and PBDB-TF: ID4F) beneath steady 1-sun-equivalent illumination. Word metallic halide lamp with out UV filtration is employed as a lightweight supply and its gentle density is monitored with a silicon cell. Surprisingly, PBDB-TF: PTIC-based system permits sustaining about 70% of its preliminary PCE worth for 50-h illumination, regardless of the tough gentle supply (containing a portion of high-energy UV photons) employed in research. That is a lot better than that of PBDB-TF: ID4F-based units (the remaining 25% of its preliminary worth beneath the identical situation). 4 units are used for traditional deviation (Fig. 3f). To grasp the intrinsic photostability of acceptors themselves, we now have additional in contrast the neat PTIC and ID4F movies beneath fixed 1-sun-equivalent illumination, revealing that PTIC is extra steady than that of ID4F. As proven in Supplementary Fig. 27, neat PTIC movie remained as inexperienced colour and there was a comparatively regular absorption at 747 nm, for 32-h fixed illumination. Nonetheless, the fused ID4F and the unique inexperienced movie had been pale to almost clear after 16-h illumination, indicating the breaking of a conjugated system. The thermal stress stability of PTIC-based and ID4F-based OSCs was investigated, and stays insensitive to thermal remedies at 100 °C for 32 h (Supplementary Fig. 28). The soundness distinction might stem from the structural elements of NFRA with out extension-fused rings and tetrahedron sp3 bridge carbon of FREA, in addition to the aptitude of dense stacking in movies, which helps gaining extra photostabilities of NFRA-based OSCs.

Mix morphology

The highest-surface morphologies of NFRA mix movies had been measured by atomic power microscopy (AFM) (Supplementary Fig. 29). It reveals that PBDB-TF: PTICH movie reveals excessive floor roughness (Rq) of 21.6 nm, indicating the sturdy aggregation of such mix and non-ideal morphology. For PTIC-based and PTICO-based blends, the movie surfaces are comparatively easy with an Rq of 5.83 nm for PTIC and seven.73 nm for PTICO.

GIWAXS measurements had been carried out to review the molecular packing and crystallinity of the pure and mix movies25,37. The depth profiles within the out-of-plane and in-plane path and 2D scattering patterns are proven in Fig. Four and Supplementary Fig. 30, respectively. The height positions and d-spacing are summarized in Supplementary Desk Four. Pure ID4F movie displays face-on orientation indicated by the π–π peak situated at qz = 1.66 Å−1 (d = Three.79 Å), much like most FREAs. The extra peak at qz = zero.825 Å−1 corresponds to a layer spacing twice of the π–π stacking distance, which is more likely to originate from the spacing between the fused-ring core teams. Apparently, all three NFRAs in neat movies exhibit sturdy molecular stacking and excessive crystallinity, signified by sharp π–π peaks alongside the qr axis and as much as three orders of lamellar peaks alongside the qz axis. The π–π stacking distances of PTICH, PTIC, and PTICO might be extracted as Three.63 Å for PTICH, Three.59 Å for PTIC, and three.45 Å for PTICO. Not like a fused ladder construction, ID4F possesses two upward and downward pointing chains onto a tetrahedron sp3 bridge carbon, NFRAs with alkoxy chains guarantee shorter π−π stacking distances. This characteristic can be useful to cut back the Stoke shifts of NFRAs in movies.

Fig. FourFig. 4

Morphologies of neat and mix movies. The GIWAXS depth profiles alongside the out-of-plane (a) and in-plane (b) instructions

The scattering profiles of mix movies reveal that NFRA-based blends exhibit stronger lamellar peaks than the ID4F mix does, indicating increased crystallinity and extra ordered stacking for NFRA blends. Upon mixing, PBDB-TF:ID4F has a barely elevated acceptor lamellar distance (15.zero Å). The height at q = zero.305 Å−1 corresponds to the lamellar stacking of the donor. NFRAs in blends nonetheless exhibit related lamellar and π–π stacking distances as within the neat movies. It’s evident that by removing of the covalent chemical bonding in fused-ring acceptors, NFRAs are given with the next diploma of freedom in answer, in the end favoring the formation of upper crystallinity and tighter π−π stacking, in contrast with the fused-ring counterpart.

Because the PBDB-TF: PTIC BHJ confirmed environment friendly efficiency and a comparatively excessive VOC in single-junction OSCs, we additional employed them into monolithic tandem OSCs with the structure of ITO/ZnO/PBDB-TF: PTIC/MoO3/Ag/NP–ZnO/PFN-Br/PTB7-Th: IEICO-4F/MoO3/Ag (Fig. 5 and Supplementary Fig. 31). The interconnecting layer (ICL) is modified based on our earlier method27, whereby zero.5 nm of additional skinny silver layer is employed for good cost recombination and light-weight transmission. The tandem cells exhibited the JSC of 12.76 mA cm−2, VOC of 1.61 V, and FF of zero.68, leading to one of the best PCE of 13.97% (PCE of 13.49% measured with aperture). It’s extremely encouraging that an exceptionally easy acceptor constituted of two-step synthesis permits resulting in environment friendly and steady OSCs, which ought to be largely useful for his or her sensible functions.

Fig. 5Fig. 5

Environment friendly tandem photo voltaic cell. a Monolithic tandem structure and b J–V curves of the tandem system beneath the illumination of AM 1.5 G (100 mW cm−2). The inset is the cross-sectional SEM picture for tandem OSCs


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