Materials design and theoretical calculations

In our previous work, we have demonstrated that chlorination has great potential for constructing OPV materials with superior performances compared to fluorination40. Very recently, Zou et al. reported a fluorinated non-fullerene small molecular acceptor Y6 and obtained an outstanding photovoltaic performance21, which motivates us to explore the applications of its chlorinated derivative in OPV cells. Figure 1a shows the molecular structures of the fluorine-containing and chlorine-containing non-fullerene acceptors and the used polymer donor PBDB-TF. As shown in Supplementary Fig. 1, the synthesis of BTP-4Cl is similar to BTP-4F in the reported literature, where the chlorine-containing terminal group was used instead of the fluorine-containing unit21. BTP-4Cl can be dissolved in solvents like chloroform (CF) and chlorobenzene. The detailed synthesis procedure and characterization can be found in the experimental part in the Supplementary Methods.

Fig. 1 Molecular structure, optical, and electrochemical properties. a Chemical structure of the BTP-4X acceptors and the polymer donor PBDB-TF. b Molecular dipoles in the optimized molecular models 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 thin films. e Schematic energy level alignment of the materials measured by the SWV method. f 2D GIWAXS patterns of the neat BTP-4X films. g Extracted 1D profiles along the IP and OOP directions Full size image

To study the influence of the exchange of halogen atoms on the geometries and electrical properties, we performed molecular simulations using density functional theory with the B3LYP (6–31G**) basis set, where the long alkyl side chains were simplified to methyl or ethyl groups to construct the molecular models. As displayed in Supplementary Fig. 2, the optimized molecular geometries and wavefunction distributions of the frontier orbitals including the highest occupied molecular orbits (HOMOs) and LUMOs show little difference between the two acceptors. It should be noted that from the fluorinated BTP-4F to chlorinated BTP-4Cl, the molecular energy levels of the HOMO (−5.60 to −5.65 eV) and LUMO (−3.55 to −3.63 eV) are downshifted. As the replacement of halogen atoms has a more pronounced impact on the LUMO level (80 meV, compared with 50 meV for the HOMO level), the BTP-Cl displays a bandgap that is narrowed by 30 meV compared to that of BTP-4F. These results are predictable according to the established molecular design theories, and lower V OC s are expected when applying the BTP-4Cl in OPV cells.

Unlike the acceptors with centrosymmetric features (e.g. in the case of ITIC42), interestingly, the BTP-4X (X represents F or Cl) molecules possess axisymmetric structures. For ITIC and analogs, although they have strong ICT effects, the overall molecular dipole moments are extremely small37. As presented in Fig. 1b, in comparison, the molecular dipole moments are 0.8653 and 0.6882 Debye for BTP-4F and BTP-4Cl, respectively. Since the chlorine–carbon bond has a larger dipole moment than that of the fluorine–carbon bond, the dipole direction in BTP-4Cl is turned to the opposite of that in BTP-4F. Although it is hard to relate the dipole properties to the photovoltaic performance of OPV materials, there are studies that suggest large dipoles moments are beneficial for charge separation in donor:acceptor blends43 and are helpful for achieving fill factors (FFs) in the devices44.

We conducted calculations of the excited states of BTP-4X. Supplementary Figure 3 shows the charge density distributions of the lowest excited states, from which the Coulomb attractive energies between the electrons and holes are calculated to be 2.24 and 2.21 eV for BTP-4F and BTP-4Cl, respectively. The reduced attractive energy in BTP-4Cl can be ascribed to the more delocalization of the charge density and may be beneficial for charge transfer in the donor:acceptor combinations with low-energy offsets. Figure 1c shows the calculated absorption spectra of BTP-4X, where the main peak of BTP-4Cl is redshifted by 8 nm from that of PTP-4F. The molar absorption coefficients of BTP-4X are almost the same (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–visible (UV–Vis) absorption spectra of BTP-4X in diluted solutions and as thin films. As shown in Fig. 1d and Supplementary Fig. 4a, the main absorption peak was located at 732 nm for BTP-4F, while that of BTP-4Cl redshifts by 14 nm (746 nm). From solutions to films, significant redshifts of 84 and 93 nm are observed for BTP-4F and BTP-4Cl, respectively, and the main absorption bands locate in the range of 600–900 nm. The larger redshift in BTP-4Cl may be related to the stronger intermolecular π–π packing caused by the larger atomic size of chlorine and larger length 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 films, respectively. The broader optical absorption range and higher absorption coefficient of BTP-4Cl are beneficial for the more effective utilization of the solar photon.

We measured the electrochemical energy levels of the BTP-4X films via the square-wave voltammetry (SWV) method, and the detailed current–voltage curves are plotted in Supplementary Fig. 5. BTP-4Cl shows downshifted HOMO (by 30 meV) and LUMO (by 100 meV) levels compared to those of BTP-4F, resulting in an electrochemical bandgap that is narrowed by 70 meV (Fig. 1e). We also performed ultraviolet photoelectron spectroscopy (UPS) measurements to compare with the SWV results. As provided in Supplementary Fig. 6, BTP-4Cl has a higher ionization potential (IP) of 5.55 eV compared to BTP-4F (5.48 eV), which is consistent with the theoretical calculations.

The molecular packing properties of the acceptors were investigated by grazing-incidence wide-angle X-ray scattering (GIWAXS). From the two-dimensional (2D) patterns shown in Fig. 1f, clear π–π stacking (010) diffraction signals are observed in the out-of-plane (OOP) direction for both films, suggesting they have a preferential face-on orientations with respect to the substrate. In contrast, the peak in the BTP-4Cl film is more pronounced than that in the BTP-4F for similar film thicknesses, which may indicate a more orderly molecular packing structure. From the one-dimensional (1D) profiles extracted along the OOP direction from the 2D patterns (Fig. 1g), the π–π stacking distances are calculated to be around 3.60 Å for the BTP-4X films. These crystalline results are consistent with our previous reports40.

From fluorination to chlorination, the changes in the optical and electrochemical properties are highly consistent with the theoretical calculations and can be easily understood by the established molecular design theories for OPV materials. When applying the BTP-4Cl in solar cell devices, it is difficult to predict whether it will exhibit better PCEs than BTP-4F. However, higher V OC s are not expected because of the downshifted LUMO level.

Photovoltaic performance and charge dynamics

To study the photovoltaic performance of BTP-4X, we fabricated OPV cells with an inverted configuration of indium tin oxide (ITO)/ZnO nanoparticles/PBDB-TF:BTP-4X/MoO 3 /Al, where PBDB-TF was selected as the electron donor material. First, we measured the photoluminescence (PL) spectra of the neat and blend films to investigate the photo-induced charge transfer in the donor:acceptor blend films. As displayed in Supplementary Fig. 7, the fluorescence of PBDB-TF or BTP-4X can be thoroughly quenched by the presence of the other in the corresponding blend films, suggesting that there is efficient charge transfer between the PBDB-TF and BTP-4X.

To obtain the best device performance, we carefully optimized the fabrication conditions, including the donor:acceptor ratio, additive, and thermal annealing temperature (Supplementary Table 1). Figure 2a shows the current density–voltage (J–V) curves, and the detailed parameters are collected in Table 1. The PBDB-TF:BTP-4F-based device yields a good PCE of 15.6%, with a V OC of 0.834 V, a J SC of 24.9 mA cm−2, and a FF of 0.753, which is close to value in the reported literature21. For the device based on BTP-4Cl as the acceptor, the V OC unexpectedly increase to 0.867 V, which is 33 mV higher than that of the BTP-4F-based device. The J SC and FF are 25.4 mA cm−2 and 0.750, respectively. Overall, an outstanding PCE of 16.5% is recorded for the PBDB-TF:BTP-4Cl-based device, which represents the top result for single-junction OPV cells and is much higher than the results obtained by non-halogenated45 and fluorinated acceptors21. Figure 2b displays the statistical diagram of the efficiencies of 100 devices, and the average value reaches 16.1%. To carefully evaluate the high PCE, we sent the best device to the National Institute of Metrology, China (NIM) for certification and got a PCE of 15.83% (Fig. 2c and Supplementary Fig. 8). We fabricated the PBDB-TF:BTP-4Cl devices with varied active layer thicknesses from 70 to 300 nm. As shown in Supplementary Fig. 9 and Supplementary Table 2, we can find that the devices can deliver above 14% PCEs from 100 to 250 nm.

Fig. 2 Device performance. a J−V curves of the PBDB-TF:IT-4X-based devices. b Statistical diagram of PCEs for 100 PBDB-T:BTP-4Cl-based cells. c J−V curves of the devices measured by the NIM, China. d EQE curves of the PBDB-TF:BTP-4X blend cells. e Photo-CELIV curves of the devices for carrier mobility calculations. f Carrier lifetimes under varied light intensities obtained from TPV measurements Full size image

Table 1 Detailed photovoltaic parameters of the OPV cells Full size table

The photovoltaic performance discussed above is based on cells with a small active area of 9 mm2. As large-area fabrication of the OPV cells is of great significance for practical applications, we also fabricated PBDB-TF:BTP-4Cl devices with a 1 cm2 area. As presented in the J–V curve (the inset in Fig. 2a shows the real device) and summarized in Table 1, the device yields an impressive PCE of 15.3%. The result was also certified by NIM using a 0.807 cm2 mask, and a PCE of 15.08% is recorded (Fig. 2c and Supplementary Fig. 8). We noted that the best results for the devices with similar areas were only around 12–13% in earlier published reports12,22.

Figure 2d displays the external quantum efficiency (EQE) curves of the best cells. Both devices exhibit EQEs of over 75% in the range of 500–800 nm, and the maximum EQE values are close to 85%. By comparison, the BTP-4Cl-containing device has a broader photo-response range by approximately 15 nm than the device based on BTP-4F as the acceptor, which should be ascribed to the redshift of the optical absorption of BTP-4Cl. The integrated J SC s from the EQE spectra are calculated to be 24.7 and 25.3 mA cm−2, which are very close to the values obtained from J–V measurements.

We measured the light intensity dependence of the J–V characteristics to understand the recombination kinetics of the devices. As shown in Supplementary Fig. 10, by fitting the curves, we can find that both devices possess weak bimolecular and trap-assisted recombination, which may be related to the efficient charge transport in the devices (Supplementary Table 3)46. We measured the mobilities of the faster carrier components via photo-induced charger-carrier extraction in linearly increasing voltage (photo-CELIV) measurements47. From the curves shown in Fig. 2e, the mobilities are calculated to be 7.45 × 10−5 and 1.82 × 10−4 cm2 V−1 S−1 for the BTP-4F-containing and BTP-4Cl-containing cells, respectively. We subsequently conducted transient photovoltage (TPV) measurements to investigate the charge carrier lifetimes (τ). As shown in Fig. 2f, the results suggest that the PBDB-TF:BTP-4Cl-based device exhibits a slightly longer τ (2.4 µs) than the PBDB-TF:BTP-4F-based device (1.5 µs), which may help to obtain the high J SC and FF of the device at such a low-energy loss48,49.

Blend morphology characterization

We carried out morphology characterizations of the blend films via atomic force microscopy (AFM), transmission electron microscopy (TEM), and GIWAXS. As shown in the height images in Fig. 3a, both films have smooth surfaces. The mean-square surface roughness (R q ) of the PBDB-TF:BTP-4Cl film is 1.68 nm, which is slightly higher than that of the PBDB-TF:BTP-4F film (1.33 nm). The AFM phase images (Fig. 3b) and TEM patterns (Supplementary Fig. 11) suggest that both films form nanoscale phase-separated morphologies with appropriate domain sizes in the surface and bulk. Figure 3c shows the 2d patterns from the GIWAXS measurements. The acceptors maintained their previous orientations from the blended films after blending the polymer donor PBDB-TF. Calculated from the 1D profiles, the (010) coherence lengths are around 2.0 nm for the blend films (Fig. 3d and Supplementary Table 4).

Fig. 3 Morphology characterizations of the PBDB-TF:BTP-4X blend films. a AFM height images. b AFM phase images. c 2D GIWAXS patterns. d 1D plots extracted from the 2D patterns along the OOP and IP directions Full size image

Non-radiative energy loss

To investigate the reasons behind the unusual increase of the V OC , we studied the detailed energy losses in both devices (Table 2). According to the reported method6,50, the total energy loss (ΔE) can be divided into three parts: (1) ΔE 1 , radiative recombination loss above the bandgap; (2) ΔE 2 , radiative recombination loss below the bandgap; and (3) ΔE 3 , non-radiative energy loss, also called E loss,nr . First, we estimated the optical bandgaps (E g ) by the intersections between the absorption and emission of the low bandgap BTP-4X50. Extracted from the plots shown in Supplementary Fig. 12, the E g s are calculated to be 1.407 and 1.400 eV.

Table 2 Detailed V OC losses of the PBDB-TF:BTP-4X-based OPV cells Full size table

Based on the Shockley–Queisser (SQ) theory51, both devices exhibit similar values of ΔE 1 of about 0.263 eV. Therefore, the SQ limit output voltages \(\left( {V_{{\mathrm{OC}}}^{{\mathrm{SQ}}}{\mathrm{s}}} \right)\) are estimated to be 1.143 and 1.137 V for the devices based on BTP-4F and BTP-4Cl, respectively. We then measured the highly sensitive EQE spectra of the devices to evaluate the ΔE 2 . As shown in Fig. 4a, the blend films of PBDB-TF:BTP-4X have very similar sensitive-EQE spectra to the neat acceptors (Supplementary Fig. 13). In addition, the measured electroluminescence (EL) spectra of the blend films are also quite similar to the corresponding neat acceptors without additional emission peaks from the charge-transfer states. This phenomenon is commonly observed in highly efficient donor:acceptor systems with low-energy offsets and is beneficial for reducing ΔE 2 . When compared with the PBDB-TF:BTP-4F device, the band edge of PBDB-TF:BTP-4Cl is more abrupt, leading to a slightly reduced ΔE 2 of 0.065 eV (0.074 eV for BTP-4F-containing device).

Fig. 4 Energy loss. a Highly sensitive EQE curves of both devices. b EL quantum efficiencies of the solar cells at various injected current densities. c Radiative and non-radiative energy losses in the OPV cells Full size image