Over 16% effectivity natural photovoltaic cells enabled by a chlorinated acceptor with elevated open-circuit voltages

Supplies design and theoretical calculations

In our earlier work, now we have demonstrated that chlorination has nice potential for setting up OPV supplies with superior performances in comparison with fluorination40. Very lately, Zou et al. reported a fluorinated non-fullerene small molecular acceptor Y6 and obtained an excellent photovoltaic efficiency21, which motivates us to discover the functions of its chlorinated spinoff in OPV cells. Determine 1a reveals the molecular constructions of the fluorine-containing and chlorine-containing non-fullerene acceptors and the used polymer donor PBDB-TF. As proven in Supplementary Fig. 1, the synthesis of BTP-4Cl is just like BTP-4F within the reported literature, the place the chlorine-containing terminal group was used as an alternative of the fluorine-containing unit21. BTP-4Cl could be dissolved in solvents like chloroform (CF) and chlorobenzene. The detailed synthesis process and characterization could be discovered within the experimental half within the Supplementary Strategies.

Fig. 1figure1

Molecular construction, optical, and electrochemical properties. a Chemical construction of the BTP-4X acceptors and the polymer donor PBDB-TF. b Molecular dipoles within the optimized molecular fashions for the BTP-4X acceptors. c Calculated UV–Vis absorption spectra of the BTP-4X. d Normalized UV–vis absorption spectra of the donor and acceptors as skinny movies. e Schematic vitality degree alignment of the supplies measured by the SWV technique. f 2D GIWAXS patterns of the neat BTP-4X movies. g Extracted 1D profiles alongside the IP and OOP instructions

To review the affect of the trade of halogen atoms on the geometries and electrical properties, we carried out molecular simulations utilizing density practical concept with the B3LYP (6–31G**) foundation set, the place the lengthy alkyl facet chains have been simplified to methyl or ethyl teams to assemble the molecular fashions. As displayed in Supplementary Fig. 2, the optimized molecular geometries and wavefunction distributions of the frontier orbitals together with the best occupied molecular orbits (HOMOs) and LUMOs present little distinction between the 2 acceptors. It needs to be famous that from the fluorinated BTP-4F to chlorinated BTP-4Cl, the molecular vitality ranges of the HOMO (−5.60 to −5.65 eV) and LUMO (−three.55 to −three.63 eV) are downshifted. Because the alternative of halogen atoms has a extra pronounced influence on the LUMO degree (80 meV, in contrast with 50 meV for the HOMO degree), the BTP-Cl shows a bandgap that’s narrowed by 30 meV in comparison with that of BTP-4F. These outcomes are predictable in response to the established molecular design theories, and decrease VOCs are anticipated when making use of the BTP-4Cl in OPV cells.

Not like the acceptors with centrosymmetric options (e.g. within the case of ITIC42), apparently, the BTP-4X (X represents F or Cl) molecules possess axisymmetric constructions. For ITIC and analogs, though they’ve robust ICT results, the general molecular dipole moments are extraordinarily small37. As introduced in Fig. 1b, compared, the molecular dipole moments are zero.8653 and zero.6882 Debye for BTP-4F and BTP-4Cl, respectively. For the reason that chlorine–carbon bond has a bigger dipole second than that of the fluorine–carbon bond, the dipole route in BTP-4Cl is turned to the other of that in BTP-4F. Though it’s laborious to narrate the dipole properties to the photovoltaic efficiency of OPV supplies, there are research that counsel giant dipoles moments are useful for cost separation in donor:acceptor blends43 and are useful for attaining fill components (FFs) within the units44.

We carried out calculations of the excited states of BTP-4X. Supplementary Determine three reveals the cost density distributions of the bottom excited states, from which the Coulomb engaging energies between the electrons and holes are calculated to be 2.24 and a couple of.21 eV for BTP-4F and BTP-4Cl, respectively. The diminished engaging vitality in BTP-4Cl could be ascribed to the extra delocalization of the cost density and could also be useful for cost switch within the donor:acceptor combos with low-energy offsets. Determine 1c reveals the calculated absorption spectra of BTP-4X, the place the principle peak of BTP-4Cl is redshifted by eight nm from that of PTP-4F. The molar absorption coefficients of BTP-4X are virtually the identical (1.05 × 105 and 1.03 × 105 M−1 cm−1 for BTP-4Cl and BTP-4F, respectively).

Optical, electrochemical, and crystalline properties

We measured the ultraviolet–seen (UV–Vis) absorption spectra of BTP-4X in diluted options and as skinny movies. As proven in Fig. 1d and Supplementary Fig. 4a, the principle absorption peak was positioned at 732 nm for BTP-4F, whereas that of BTP-4Cl redshifts by 14 nm (746 nm). From options to movies, vital redshifts of 84 and 93 nm are noticed for BTP-4F and BTP-4Cl, respectively, and the principle absorption bands find within the vary of 600–900 nm. The bigger redshift in BTP-4Cl could also be associated to the stronger intermolecular π–π packing brought on by the bigger atomic measurement of chlorine and bigger size of the chlorine–carbon bond. The absorption coefficients are 9.90 × 104 and 1.09 × 105 cm−1 (Supplementary Fig. 4b) for the BTP-4F and BTP-4Cl movies, respectively. The broader optical absorption vary and better absorption coefficient of BTP-4Cl are useful for the simpler utilization of the photo voltaic photon.

We measured the electrochemical vitality ranges of the BTP-4X movies by way of the square-wave voltammetry (SWV) technique, and the detailed present–voltage curves are plotted in Supplementary Fig. 5. BTP-4Cl reveals downshifted HOMO (by 30 meV) and LUMO (by 100 meV) ranges in comparison with these of BTP-4F, leading to an electrochemical bandgap that’s narrowed by 70 meV (Fig. 1e). We additionally carried out ultraviolet photoelectron spectroscopy (UPS) measurements to check with the SWV outcomes. As supplied in Supplementary Fig. 6, BTP-4Cl has the next ionization potential (IP) of 5.55 eV in comparison with BTP-4F (5.48 eV), which is per the theoretical calculations.

The molecular packing properties of the acceptors have been investigated by grazing-incidence wide-angle X-ray scattering (GIWAXS). From the two-dimensional (2D) patterns proven in Fig. 1f, clear π–π stacking (zero10) diffraction indicators are noticed within the out-of-plane (OOP) route for each movies, suggesting they’ve a preferential face-on orientations with respect to the substrate. In distinction, the height within the BTP-4Cl movie is extra pronounced than that within the BTP-4F for comparable movie thicknesses, which can point out a extra orderly molecular packing construction. From the one-dimensional (1D) profiles extracted alongside the OOP route from the 2D patterns (Fig. 1g), the π–π stacking distances are calculated to be round three.60 Å for the BTP-4X movies. These crystalline outcomes are per our earlier reviews40.

From fluorination to chlorination, the adjustments within the optical and electrochemical properties are extremely per the theoretical calculations and could be simply understood by the established molecular design theories for OPV supplies. When making use of the BTP-4Cl in photo voltaic cell units, it’s troublesome to foretell whether or not it would exhibit higher PCEs than BTP-4F. Nonetheless, larger VOCs usually are not anticipated due to the downshifted LUMO degree.

Photovoltaic efficiency and cost dynamics

To review the photovoltaic efficiency of BTP-4X, we fabricated OPV cells with an inverted configuration of indium tin oxide (ITO)/ZnO nanoparticles/PBDB-TF:BTP-4X/MoO3/Al, the place PBDB-TF was chosen because the electron donor materials. First, we measured the photoluminescence (PL) spectra of the neat and mix movies to analyze the photo-induced cost switch within the donor:acceptor mix movies. As displayed in Supplementary Fig. 7, the fluorescence of PBDB-TF or BTP-4X could be totally quenched by the presence of the opposite within the corresponding mix movies, suggesting that there’s environment friendly cost switch between the PBDB-TF and BTP-4X.

To acquire the most effective gadget efficiency, we fastidiously optimized the fabrication circumstances, together with the donor:acceptor ratio, additive, and thermal annealing temperature (Supplementary Desk 1). Determine 2a reveals the present density–voltage (J–V) curves, and the detailed parameters are collected in Desk 1. The PBDB-TF:BTP-4F-based gadget yields an excellent PCE of 15.6%, with a VOC of zero.834 V, a JSC of 24.9 mA cm−2, and a FF of zero.753, which is near worth within the reported literature21. For the gadget based mostly on BTP-4Cl because the acceptor, the VOC unexpectedly enhance to zero.867 V, which is 33 mV larger than that of the BTP-4F-based gadget. The JSC and FF are 25.four mA cm−2 and zero.750, respectively. Total, an excellent PCE of 16.5% is recorded for the PBDB-TF:BTP-4Cl-based gadget, which represents the highest consequence for single-junction OPV cells and is far larger than the outcomes obtained by non-halogenated45 and fluorinated acceptors21. Determine 2b shows the statistical diagram of the efficiencies of 100 units, and the common worth reaches 16.1%. To fastidiously consider the excessive PCE, we despatched the most effective gadget to the Nationwide Institute of Metrology, China (NIM) for certification and obtained a PCE of 15.83% (Fig. 2c and Supplementary Fig. eight). We fabricated the PBDB-TF:BTP-4Cl units with diverse energetic layer thicknesses from 70 to 300 nm. As proven in Supplementary Fig. 9 and Supplementary Desk 2, we will discover that the units can ship above 14% PCEs from 100 to 250 nm.

Fig. 2figure2

System efficiency. a J−V curves of the PBDB-TF:IT-4X-based units. b Statistical diagram of PCEs for 100 PBDB-T:BTP-4Cl-based cells. c J−V curves of the units measured by the NIM, China. d EQE curves of the PBDB-TF:BTP-4X mix cells. e Photograph-CELIV curves of the units for provider mobility calculations. f Service lifetimes below diverse mild intensities obtained from TPV measurements

Desk 1 Detailed photovoltaic parameters of the OPV cells

The photovoltaic efficiency mentioned above relies on cells with a small energetic space of 9 mm2. As large-area fabrication of the OPV cells is of nice significance for sensible functions, we additionally fabricated PBDB-TF:BTP-4Cl units with a 1 cm2 space. As introduced within the J–V curve (the inset in Fig. 2a reveals the true gadget) and summarized in Desk 1, the gadget yields a powerful PCE of 15.three%. The consequence was additionally licensed by NIM utilizing a zero.807 cm2 masks, and a PCE of 15.08% is recorded (Fig. 2c and Supplementary Fig. eight). We famous that the most effective outcomes for the units with comparable areas have been solely round 12–13% in earlier printed reviews12,22.

Determine second shows the exterior quantum effectivity (EQE) curves of the most effective cells. Each units exhibit EQEs of over 75% within the vary of 500–800 nm, and the utmost EQE values are near 85%. By comparability, the BTP-4Cl-containing gadget has a broader photo-response vary by roughly 15 nm than the gadget based mostly on BTP-4F because the acceptor, which needs to be ascribed to the redshift of the optical absorption of BTP-4Cl. The built-in JSCs from the EQE spectra are calculated to be 24.7 and 25.three mA cm−2, that are very near the values obtained from J–V measurements.

We measured the sunshine depth dependence of the J–V traits to grasp the recombination kinetics of the units. As proven in Supplementary Fig. 10, by becoming the curves, we will discover that each units possess weak bimolecular and trap-assisted recombination, which can be associated to the environment friendly cost transport within the units (Supplementary Desk three)46. We measured the mobilities of the sooner provider parts by way of photo-induced charger-carrier extraction in linearly rising voltage (photo-CELIV) measurements47. From the curves proven in Fig. 2e, the mobilities are calculated to be 7.45 × 10−5 and 1.82 × 10−four cm2 V−1 S−1 for the BTP-4F-containing and BTP-4Cl-containing cells, respectively. We subsequently carried out transient photovoltage (TPV) measurements to analyze the cost provider lifetimes (τ). As proven in Fig. 2f, the outcomes counsel that the PBDB-TF:BTP-4Cl-based gadget reveals a barely longer τ (2.four µs) than the PBDB-TF:BTP-4F-based gadget (1.5 µs), which can assist to acquire the excessive JSC and FF of the gadget at such a low-energy loss48,49.

Mix morphology characterization

We carried out morphology characterizations of the mix movies by way of atomic power microscopy (AFM), transmission electron microscopy (TEM), and GIWAXS. As proven within the peak photographs in Fig. 3a, each movies have easy surfaces. The mean-square floor roughness (Rq) of the PBDB-TF:BTP-4Cl movie is 1.68 nm, which is barely larger than that of the PBDB-TF:BTP-4F movie (1.33 nm). The AFM part photographs (Fig. 3b) and TEM patterns (Supplementary Fig. 11) counsel that each movies kind nanoscale phase-separated morphologies with applicable area sizes within the floor and bulk. Determine 3c reveals the second patterns from the GIWAXS measurements. The acceptors maintained their earlier orientations from the blended movies after mixing the polymer donor PBDB-TF. Calculated from the 1D profiles, the (zero10) coherence lengths are round nm for the mix movies (Fig. 3d and Supplementary Desk four).

Fig. threefigure3

Morphology characterizations of the PBDB-TF:BTP-4X mix movies. a AFM peak photographs. b AFM part photographs. c 2D GIWAXS patterns. d 1D plots extracted from the 2D patterns alongside the OOP and IP instructions

Non-radiative vitality loss

To analyze the explanations behind the bizarre enhance of the VOC, we studied the detailed vitality losses in each units (Desk 2). Based on the reported method6,50, the full vitality loss (ΔE) could be divided into three components: (1) ΔE1, radiative recombination loss above the bandgap; (2) ΔE2, radiative recombination loss beneath the bandgap; and (three) ΔE3, non-radiative vitality loss, additionally referred to as Eloss,nr. First, we estimated the optical bandgaps (Eg) by the intersections between the absorption and emission of the low bandgap BTP-4X50. Extracted from the plots proven in Supplementary Fig. 12, the Egs are calculated to be 1.407 and 1.400 eV.

Desk 2 Detailed VOC losses of the PBDB-TF:BTP-4X-based OPV cells

Based mostly on the Shockley–Queisser (SQ) concept51, each units exhibit comparable values of ΔE1 of about zero.263 eV. Subsequently, the SQ restrict output voltages (left( V_^ proper)) are estimated to be 1.143 and 1.137 V for the units based mostly on BTP-4F and BTP-4Cl, respectively. We then measured the extremely delicate EQE spectra of the units to guage the ΔE2. As proven in Fig. 4a, the mix movies of PBDB-TF:BTP-4X have very comparable sensitive-EQE spectra to the neat acceptors (Supplementary Fig. 13). As well as, the measured electroluminescence (EL) spectra of the mix movies are additionally fairly just like the corresponding neat acceptors with out extra emission peaks from the charge-transfer states. This phenomenon is usually noticed in extremely environment friendly donor:acceptor techniques with low-energy offsets and is useful for decreasing ΔE2. In comparison with the PBDB-TF:BTP-4F gadget, the band fringe of PBDB-TF:BTP-4Cl is extra abrupt, resulting in a barely diminished ΔE2 of zero.065 eV (zero.074 eV for BTP-4F-containing gadget).

Fig. fourfigure4

Vitality loss. a Extremely delicate EQE curves of each units. b EL quantum efficiencies of the photo voltaic cells at varied injected present densities. c Radiative and non-radiative vitality losses within the OPV cells

By measuring the EQEEL of the units, the third a part of vitality loss, ΔE3, could be evaluated utilizing the next equation: (E_3 = – kT,,(_))52. As proven in Fig. 4b, each units show excessive EQEEL values with magnitudes of 10−four, that are very excessive values among the many extremely environment friendly OPV techniques50,53. The PBDB-TF:BTP-4Cl-based gadget reveals the next EQEEL of three.47 × 10−four than the PBDB-TF:BTP-4F-based gadget (1.40 × 10−four) and the calculated ΔE3 is zero.206 eV for the gadget based mostly on PBDB-TF:BTP-4Cl, which is decrease than that within the PBDB-TF:BTP-4F-based cell by 24 meV (Fig. 4c). In contrast with PBDB-TF:BTP-4F mix, the decrease ΔE3 in PBDB-TF:BTP-4Cl mix could also be related to its decrease reorganization vitality as an alternative of the upper EQEEL of the BTP-4Cl (Supplementary Fig. 14)54,55. The diminished non-radiative vitality loss needs to be the foremost contribution to the VOC enhance within the PBDB-TF:BTP-4Cl-based gadget. The upper EQEEL of BTP-4Cl-containing combos can be noticed in different techniques when mixing completely different polymer donors (Supplementary Fig. 15 and Supplementary Desk 5).

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