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RESEARCH ARTICLE | Updated:2022-07-21
    • Subtle Alignment of Organic Semiconductors at the Donor/Acceptor Heterojunction Facilitates the Photoelectric Conversion Process

    • Liu Yu-Xuan

      ,  

      Wang Liang

      ,  

      Zhou Ke

      ,  

      Wu Hong-Bo

      ,  

      Zhou Xiao-Bo

      ,  

      Ma Zai-Fei

      ,  

      Guo Sheng-Wei

      ,  

      Ma Wei

      ,  
    • Chinese Journal of Polymer Science   Vol. 40, Issue 8, Pages: 951-959(2022)
    • DOI:10.1007/s10118-022-2759-4    

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  • Yu-Xuan Liu, Liang Wang, Ke Zhou, et al. Subtle Alignment of Organic Semiconductors at the Donor/Acceptor Heterojunction Facilitates the Photoelectric Conversion Process. [J]. Chinese Journal of Polymer Science 40(8):951-959(2022) DOI: 10.1007/s10118-022-2759-4.

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    Abstract

    The aligned molecular packing structure is vital to the anisotropic charge transport in conjugated polymer and small molecule thin films. However, how this molecular packing motif influences the photoelectric conversion process at the donor/acceptor heterojunction is still mysterious. Herein, we employed a PM6/Y6 bilayer model to investigate the long-range alignment of molecular packing induced photoelectric conversion process. Both PM6 and Y6 layers were properly controlled to exhibit the uniaxially oriented molecular packing compared to their as-cast counterparts, as revealed by the polarized absorption spectra and transmission electron microscopy. After analyzing the photovoltaic performance of bilayer devices, the smaller energy loss, lower energetic disorder, and longer charge carrier lifetime can be observed in the bilayer devices with aligned Y6 molecules, which contribute to a higher power conversion efficiency (PCE) than the as-cast devices. While the molecular packing structure of PM6 layer exhibited negligible influence on the device performance, probably resulting from the intrinsic semicrystalline nature of PM6 molecules. Our results indicate that the alignment of small molecular acceptor at the donor/acceptor interfaces should be a powerful strategy to facilitate the photoelectric conversion process, which will definitely pave the way to highly efficient bulk heterojunction photovoltaic device.

    Keywords

    Molecular packing; Photoelectric conversion process; Planar heterojunction; Organic solar cells

    INTRODUCTION

    Organic solar cells (OSCs), which used conjugated polymers or small molecules as the electron donor and acceptor, have drawn intensive attention during last several decades due to their various advantages, such as light weight, semitransparent and flexibility.[

    1−6] Recently, with the continuous progress in the design and synthesis of donor/acceptor material and the optimization of device manufacture, the power conversion efficiency (PCE) of OSCs is approaching 20%,[7−9] indicating a bright future in the practical application. The crucial photoelectric processes, including photon absorption, exciton dissociation, and charge transport, substantially proceed in the bulk-heterojunction (BHJ) active layer of devices.[10] This invokes a fundamental issue on the phase-separated morphology of active layer, in which the molecular packing of donor and acceptor phases and their interfacial structures should be carefully controlled so as to meet the efficient charge generation and collection processes.[11−14]

    The solution-processed thin films consisting of conjugated polymers and small molecules typically feature hierarchical morphology, in which the nanostructures with different length scales can be identified with the evaporation of solvent.[

    15,16] Researchers have used various processing methods to optimize the morphology of active layer, not only aiming to promote the PCE of devices, but also trying to extend their device stability.[17−20] Indeed, the BHJ morphology is rather complex resulting from its binary or ternary components and their coupling process between the phase separation and molecular crystallization. This probably leads to a trial-and-error and time-consuming work when attempting to optimize the BHJ morphology. The bilayer structure, which is usually prepared by the PDMS-template-transfer[21] or floating-film-transfer method,[22] can provide the relatively sharp donor/acceptor interface and eliminate the effect of domain structure, therefore becomes the proper model system to investigate the relationship between the morphology and photoelectric process in OSCs.[23] For example, Nguyen et al.[24] found that donor molecules face-on to the acceptor interface had the higher charge-transfer state energy and less efficient charge generation in the p-SIDT(FBTTh2)2/C60 bilayer device. While Cho et al.[25] observed a different trend in the P3HT/PCBM bilayer device, in which photocurrent generation was more efficient at the P3HT face-on donor/acceptor interface than at the edge-on interface. Recently, we found the pure molecular orientation of conjugated polymer could effectively reduce the interfacial energetic disorder in the all-polymer bilayer devices.[21] Those results indicate that the molecular orientation largely dominates the charge generation process at the donor/acceptor interface. Although it was obtained in the bilayer devices, previous results suggest that those conclusions can also be generalized to the BHJ devices.[26,27] However, to the best of our knowledge, the present morphology study in the bilayer model systems is merely focused on the short-range molecular packing structure, such as molecular orientation.[28] The aligned molecular packing in the long-range scale has previously proven to be favorable to the charge transport in the conjugated polymer and small molecule thin films.[29] Considering that this kind of long-range packing motif probably provides more ordered donor/acceptor interface, their potential influence on the photoelectric conversion process should also be deserved to investigate thoroughly.

    In this work, the long-range alignments of PM6 and Y6 molecules were properly controlled and the corresponding bilayer devices were prepared to study their photoelectric conversion process. As revealed by the polarized absorption spectra and transmission electron microscopy (TEM), PM6 and Y6 films can be easily modulated to perform the subtly uniaxial alignment. After analyzing the photovoltaic performance of bilayer devices, we found that the bilayer device with aligned Y6 molecules exhibits the smaller energy loss, lower energetic disorder, as well as longer charge carrier lifetime. This should be the main reasons for its higher PCE than the as-cast device. While the molecular packing structure of PM6 layer exhibited negligible influence on the device performance, we inferred that the intrinsic semicrystalline nature of PM6 molecules typically makes it perform the relatively disordered packing structure in the films so that their uniaxial alignment can hardly change the electronic state of PM6 film and thus the related photoelectric conversion process in the bilayer devices. The conclusion obtained in this work suggests that the alignment of small molecular acceptor at the donor/acceptor interfaces should be a powerful strategy to facilitate the photoelectric conversion process, which can be one guideline to manufacture the highly efficient bulk heterojunction photovoltaic device.

    EXPERIMENTAL

    Materials

    PM6 and Y6 were purchased from Solarmer Materials Inc. o-Xylene (o-Xy), chloroform (CF), chlorobenzene (CB) were obtained from Sigma-Aldrich.

    Device Fabrication

    The PHJ OSCs were fabricated with an inverted configuration of ITO/ZnO/Y6/PM6/MoO3/Al. The patterned ITO substrate was cleaned by sequential ultrasonication in detergent water, deionized water, acetone, and isopropanol for 20 min in each step. After being dried with nitrogen flow, the ITO substrate was treated with UV ozone for 20 min to reform the surface. The ZnO precursor solution prepared by dissolving zinc acetate in 2-methoxyethanol with ethanolamine was spin-coated on the ITO substrate at 4500 r/min for 30 s, followed by thermal annealing at 200 °C for 30 min to form the electron-transporting layer of ZnO. And the PEDOT:PSS was deposited on the glass substrate by spin-coating at 2000 r/min for 30 s, followed by thermal annealing at 140 °C for 3 min. The donor PM6 was dissolved into CF, o-Xy, and CB, respectively, at the same concentration of 10 mg·mL−1. The acceptor Y6 was dissolved into CF at the concentration of 10 mg·mL−1. The active layer was prepared by a floating-film-transfer method. The PM6 and Y6 solutions were deposited on the PEDOT:PSS and ZnO layers, respectively. The PM6 dissolved in CF was spin-coated to form the i-PM6 film. The PM6 dissolved in o-Xy was spin-coated and blade-coated to form the i-PM6_o-Xy and ani-PM6 film, respectively. The PM6 dissolved in CB was spin-coated and blade-coated to form the i-PM6_CB and ani-PM6_CB film, respectively. The Y6 solution was spin-coated to form the i-Y6 film. The ani-Y6 film was obtained by rubbing the i-Y6 film dozens of times using wiper. The thickness of all films was controlled in ~30 nm. The PM6 film was floated on the water, and then it was scooped up by a Y6-coated cathode substrate. After placing the sample in a vacuum environment overnight, 10 nm of MoO3 and 100 nm of Al were sequentially deposited by thermal evaporation as the hole-transporting layer and anode at a vacuum level under 1×10−4 Pa. The active area of the measured device is 4 mm2.

    Film and Device Characterizations

    The thickness of a film was measured by a Bruker Dektak XT. The UV-Vis spectrum was measured on a Shimadzu UV-3600 Plus. The polarized UV-Vis absorption spectrum was obtained in transmission geometry through a polarizer crystal. The incident beam was polarized parallel and perpendicular to the shearing or rubbing directions. TEM measurement was performed using a FEI Talos F200c transmission electron microscope instrument with 200 kV accelerating voltage. PL spectra were performed on Edinburgh FLS980 Spectrophotometer. CV measurement was carried out on a CH-Instruments 650A Electrochemical Workstation. J-V characteristic was measured in the N2 glovebox under AM 1.5G (100 mW·cm−2) using an AAA solar simulator (SS-F5-3A, Enli Technology Co., Ltd.) calibrated with a standard photovoltaic cell equipped with a KG5 filter and a Keithley 2400 source meter. The temperature-dependent J-V characteristic was performed under the illumination of an AM 1.5 G solar simulator (100 mW·cm−2). The temperature of the device was controlled from −193 °C to 25 °C, using the TC 202 temperature controller. The EQE curve was obtained using a solar cell spectral response measurement system (QE-R3018, Enli Technology Co., Ltd.) with the calibrated light intensity using a standard single-crystal Si photovoltaic cell.

    GIWAXS Characterization

    GIWAXS measurements were carried out at SAXS/WAXS beamline, Australian Synchrotron ANSTO. The sample was prepared on the Si substrate. The 15.2 keV X-ray beam was incident at a grazing angle range of 0.08°–0.12°, selected to maximize the scattering intensity from the samples. The scattered X-rays were detected using a Dectris Pilatus 2M photon counting detector.

    Mobility Measurements

    The hole mobility (µh) of neat PM6 films were measured based on the space-charge-limited current (SCLC) method using hole-only device with the structure of ITO/PEDOT:PSS/PM6/MoO3/Al. The electron mobility (µe) of neat Y6 films were measured using electron-only device with the structure of ITO/ZnO/Y6/PFN-Br/Al. The mobilities were obtained by fitting the current density-voltage curves and calculated by the equation:

    J=9ϵ0ϵrμ(VapplVbiVs)2/8L3
    1

    where J is the current density, ε0 is permittivity of vacuum, εr is the relative permittivity of the material (assumed to be 3), μ is the carrier mobility, Vappl is the applied voltage, Vbi is the built-in voltage (0 V), Vs is the voltage drop from the substrate's series resistance (Vs = IR) and L is the film thickness.

    Energy Loss Measurements

    Electroluminescence (EL) measurement applied the direct-current meter (PWS2326, Tectronix) to provide a bias voltage to the device, then the luminous signals were collected by a fluorescence spectrometer (KYMERA-328I-B2, Andor technology LTD). For the sensitive-EQE measurement, monochromatic light was focused on the device to generate electrical signals which were amplified and subsequently collected by the phase-locked amplifier (Newport). The EQE spectrum was obtained by using the corrected Si standard detector (S1337-1010Br). The EQEEL measurement system consists of Keithley 2400 digital source meter, Keithley 6482 picoammeters and a standard Si detector (S1337-1010Br).

    TPV Measurement

    A LED light source was employed to generate the background illumination, and pulsed light was provided by an arbitrary wave generator (AFG322C, Tektronix). The TPV signals of the device were collected by an oscilloscope (MDO4104C, Tektronix).

    RESULTS AND DISCUSSION

    We used PM6 and Y6 as the donor and acceptor, respectively, and their chemical structures are shown in Fig. 1(a). Firstly, to control of the alignment of polymer PM6 chains, we fabricated the neat PM6 films by using spin-coating and blade-coating technologies, respectively. By using polarized UV-Vis absorption spectrum, the optical dichroism of an organic semiconductor film is obtained to probe the molecular alignment in the corresponding films.[

    30] The dichroic ratio R is defined to be the intensity ratio of 0–0 absorption peaks obtained by the parallel and perpendicular polarized light (R = A||/A
    ). The polarized UV-Vis absorption spectra of spin-coated and blade-coated PM6 films are shown in Figs. 1(c) and 1(d), respectively. For the spin-coated PM6 film from chloroform (CF) solution, two overlapping absorption curves for incident light in parallel and perpendicular polarization directions indicate that there is no optical dichroism from the film, implying that the alignment of PM6 chains is completely random along the long-range direction. Therefore, the spin-coated PM6 film is isotropic and donated as i-PM6 film. On the other hand, due to the low boiling point of CF, another solvent o-Xylene (o-Xy) was chosen to prepare the blade-coated PM6 film. It can be observed that the absorption in the light polarization direction parallel to the shearing direction is higher than that in the direction perpendicular to the shearing direction. The calculated dichroic ratio R=1.15 exhibits the subtly long-range orderly alignment of PM6 chains along the shearing direction, as the transition dipole moment is typically arranged along the polymer backbones. Therefore, the blade-coated PM6 film is anisotropic to some extent compared to its spin-coated counterpart and donated as ani-PM6 film, which suggests that the shearing strain to the solution induced by the blade enhances the orderly alignment along the shearing direction.[30] Besides, to construct the long-range orderly alignment in the Y6 film, we firstly processed the isotropic Y6 film by spin-coating (donated as i-Y6 film), then applied the uniaxial mechanical rubbing to form an anisotropic Y6 film (donated as ani-Y6 film).[31] The polarized UV-Vis absorption spectra of i-Y6 and ani-Y6 films are shown in Figs. 1(e) and 1(f), respectively. Similar to the i-PM6 film, the i-Y6 film displays the equal absorption for both polarization directions, meaning the dichroic ratio R=1. While the ani-Y6 film achieves a dichroic ratio of 1.11, indicating that the long-range alignment of Y6 molecules is improved along the rubbing direction. TEM measurements were performed to observe the morphology of these films. The TEM images of i-PM6 (Fig. 1g) and i-Y6 films (Fig. 1e) show the random alignment, demonstrating the isotropy of the two films. However, TEM images of ani-PM6 (Fig. 1h) and ani-Y6 films (Fig. 1j) reveal the obvious long-range orderly alignment along a certain direction, as the yellow arrows point out, corresponding to the polarized UV-Vis absorption spectra. To further study the surface morphology of these films, the atomic force microscopy (AFM) images are measured and shown in Fig. S1 (in the electronic supplementary information, ESI). The ani-PM6 film exhibits the higher root-mean-square (RMS) roughness value (3.32 nm) than the i-PM6 film (1.12 nm). After rubbing treatment, the RMS roughness value of Y6 film increases from 1.31 (i-Y6) to 3.33 nm (ani-Y6). The AFM results reveal that with the formation of the long-range orderly alignment, the surface roughness of the film also increases, which are consistent with the polarized UV-Vis absorption spectra and TEM measurement.

    fig

    Fig 1  (a) Chemical structures of PM6 and Y6; (b) Schematic molecular packing of i-PM6, ani-PM6, i-Y6, and ani-Y6 films; Polarized UV-Vis absorption spectra of (c) i-PM6, (d) ani-PM6, (e) i-Y6, and (f) ani-Y6 films; TEM images of (g) i-PM6, (h) ani-PM6, (i) i-Y6, and (j) ani-Y6 films.

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    Grazing incident wide angle X-ray scattering (GIWAXS) measurements were used to characterize the crystallinity of these films. The 2D patterns and corresponding line profiles are shown in Fig. S2 (in ESI). Both the i-PM6 and ani-PM6 films exhibit the coexistence of face-on and edge-on orientations, as evidenced by the (010) peaks of π-π stacking at q≈1.71 Å−1 and (100) peaks of lamellar stacking at q≈0.30 Å−1 in both the in-plane (IP) and out-of-plane (OOP) directions. Both i-Y6 and ani-Y6 films show strong π-π stacking peaks at q≈1.75 Å−1 in the OOP direction, indicating their preferential face-on orientation. The fitted structure parameters of these films based on the line profiles are summarized in Table S1 (in ESI). In the OOP direction, the crystallization coherence length (CCL) of π-π stacking slightly decreases from 36.7 Å (i-PM6) to 35.6 Å (ani-PM6), while that of lamellar stacking also reduces from 157 Å to 120 Å. This is probably due to the different property of processing solvent. Besides, the ani-Y6 film exhibits smaller CCL value of (010) π-π stacking peak in the OOP direction than the i-Y6 film, suggesting that the rubbing method induces weaker crystallinity. Based on the polarized absorption spectra, TEM images and GIWAXS data, the molecular alignment of the four films is illustrated in Fig. 1(b) in order to better understand the transformation from isotropy to anisotropy induced by shearing and rubbing methods.

    The PM6/Y6 planar heterojunction (PHJ) devices with different molecular alignments of films were fabricated, and their photovoltaic properties were measured. Fig. 2(a) shows the current density-voltage (J-V) characteristics under 1 sun illumination of these devices, and Table 1 lists the detailed photovoltaic parameters. The i-PM6/i-Y6 device exhibits an average PCE of 1.26% with the VOC of 0.845 V, the JSC of 2.42 mA·cm−2, and the FF of 61.28%. After rubbing the i-Y6 film, the prepared i-PM6/ani-Y6 device shows better photovoltaic performance than the i-PM6/i-Y6 device, suggesting that the transformation from random to long-range orderly alignment of Y6 films is beneficial to the photovoltaic performance. The VOC improves from 0.845 V to 0.858 V obviously, theJSC increases from 2.42 mA·cm−2 to 2.52 mA·cm−2, the FF enhances from 61.28% to 63.37%, thus resulting in the higher PCE of 1.38% in the i-PM6/ani-Y6 device. On the other hand, for the devices prepared with ani-PM6 films, the photovoltaic performance, especially the PCE, of the ani-PM6/ani-Y6 device is very close to that of the ani-PM6/i-Y6 device. The main difference is the VOC value, which increases from 0.836 V (ani-PM6/i-Y6 device) to 0.851 V (ani-PM6/ani-Y6 device). The corresponding external quantum efficiency (EQE) curves of these devices are showed in Fig. 2(b), where the variation of JSC value is identical with the results obtained in the measurements of J-V curves. The lower PCEs of ani-PM6-based devices than those of the corresponding i-PM6-based devices probably results from the decreased crystallinity of ani-PM6 film in the OOP direction, which inhibits the charge generation at the heterojunction interface and the charge transport toward to the electrodes. To investigate the effect of different arrangements on charge transport, the hole mobility (µh) of PM6 films and the electron mobility (µe) of Y6 films were measured based on the space-charge-limited current (SCLC) method using hole-only device of ITO/PEDOT:PSS/PM6/MoO3/Al and electron-only device of ITO/ZnO/Y6/PFN-Br/Al, respectively.[

    32] As shown in Fig. S3 and Table S2 (in ESI), the i-PM6 film exhibits the higher hole mobility (1.79×10−5 cm2·V−1·s−1) than ani-PM6 film (1.39×10−5 cm2·V−1·s−1), which corresponds to the decreased crystallinity of ani-PM6 film in the OOP direction. The electron mobility of ani-Y6 film (4.11×10−5 cm2·V−1·s−1) is higher than that of i-Y6 film (3.38×10−5 cm2·V−1·s−1), which demonstrates the long-range alignment in the IP direction can also improve the charge transport in the OOP direction to some extent.

    Table 1  Photovoltaic parameters of PM6/Y6 PHJ devices with different molecular alignments in films.
    DeviceVOC (V) JSC (mA·cm−2) FF (%)PCE a (%)
    i-PM6/i-Y6 0.845±0.004 2.42±0.03 61.28±1.07 1.26±0.03
    i-PM6/ani-Y6 0.858±0.004 2.52±0.10 63.87±1.26 1.38±0.06
    ani-PM6/i-Y6 0.836±0.005 2.15±0.09 62.64±1.68 1.13±0.07
    ani-PM6/ani-Y6 0.851±0.004 2.12±0.06 63.46±1.74 1.15±0.06

    a Average values are obtained from 15 devices.

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    fig

    Fig 2  (a) J-V and (b) EQE curves of PM6/Y6 PHJ devices with different molecular alignments in films. Dependence of the (c) VOC and (d) JSC on light intensity of PM6/Y6 PHJ devices.

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    In order to further verify the impact of molecular packing structure on the bilayer device performance, we prepared other isotropic and anisotropic PM6 films using chlorobenzene (CB) and o-Xy, and donated as i-PM6_CB, ani-PM6_CB, and i-PM6_o-Xy, respectively. Then, we fabricated and measured the corresponding PM6/Y6 PHJ devices. The J-V characteristics and the detailed photovoltaic parameters are shown in Fig. S4 and Table S3 (in ESI), respectively. The photovoltaic performance of the i-PM6_CB/ani-Y6 device is better than that of i-PM6_CB/i-Y6 (Fig. S4a, in ESI). Fig. S4b (in ESI) exhibits that the same phenomenon arises between the i-PM6_o-Xy/i-Y6 and i-PM6_o-Xy/ani-Y6 devices. Meanwhile, the JSC value of ani-PM6_CB/i-Y6 is comparable with the ani-PM6_CB/ani-Y6 device, and the enhanced the VOC and FF values can be observed. These results confirm that only the long-range alignment in the ani-Y6 film is beneficial to the enhancement of device performance, while the molecular packing structure of PM6 film in term of isotropic and anisotropic has the limited impact on the device performance. This probably results from the semicrystalline feature of PM6 film that the subtle alignment of PM6 molecules can hardly change the electronic states of the films and thus the photoelectric conversion process in the devices. Therefore, it should be further investigated to clarify the potential reasons for the improved device performance induced by the Y6 molecular alignment.

    The light intensity dependence of the VOC and JSC for different PM6/Y6 PHJ devices was measured to investigate the charge recombination kinetics,[

    33] as shown in Figs. 2(c) and 2(d). The dependence of VOC on the natural logarithm of light intensity exhibits a linear relationship with the slope ofkT/q, which reveals the Shockley-Read-Hall (SRH) recombination.[34] According to Fig. 2(c), the fitted linear slope of the i-PM6/ani-Y6 device is 1.38 kT/q which is close to that of the i-PM6/i-Y6 device (1.37 kT/q), while the ani-PM6/ani-Y6 device also shows a similar slope (1.47kT/q) compared to the ani-PM6/i-Y6 device (1.51kT/q), suggesting there is no difference in the SRH recombination. Meanwhile, JSC has a power-law dependence on light intensity (JSC
    Pα). The value of α closed to unity means that the bimolecular recombination is negligible.[35,36] As shown in Fig. 2(d), the four kinds of bilayer device show the comparable α values (0.90−0.93), meaning the similar bimolecular recombination. Those results suggest that the SRH and bimolecular recombination should not contribute to the PCE difference in the bilayer devices with different molecular packing structure.

    Due to the obvious VOC difference between the i-PM6/i-Y6 and i-PM6/ani-Y6 devices, their energy loss process was subsequently measured. Typically, the ultimate VOC is determined by analyzing three parts of energy losses shown in Eq. (2):[

    32,37,38]

    qΔVloss=EgapqVOC=(EgapqVSQOC)+(qVSQOCqVradOC)+(qVradOCqVOC)=(EgapqVSQOC)+qΔVrad,belowgapOC+qΔVnonradOC=ΔE1+ΔE2+ΔE3
    2

    where Egap

    is the optical bandgap, q is the elementary charge, ∆V is the total voltage loss, VSQOC
    is the maximum voltage by the Shockley-Queisser limit, VradOC
    is the open-circuit voltage when there is only radiative recombination, EgapqVSQOC
    (denoted as ∆E1) is the radiative recombination loss originating from the absorption above the bandgap which is unavoidable, qΔVrad,belowgapOC
    (denoted as ΔE2) is the radiative recombination loss from the absorption below the bandgap, qΔVnon-radOC
    (denoted as ΔE3) is the non-radiative recombination loss. The optical bandgap ( Egap
    ) of PM6/Y6 bilayer film is determined by the lower bandgap component, which can be obtained by the intersection point of the normalized absorption spectra and photoluminescence spectra of the PM6 and Y6 films (shown in Fig. S5 in ESI). To calculate the energy loss, the sensitive-EQE and electroluminescence (EL) spectra of i-PM6/i-Y6 and i-PM6/ani-Y6 devices are measured and displayed in Figs. 3(a) and 3(b), respectively. The detailed parameters are listed in Table 2. Compared to the i-PM6/i-Y6 device, the i-PM6/ani-Y6 device shows slightly reduced ΔE1 and obviously increased ΔE2, indicating that the improved radiative recombination loss due to the changing of molecular alignment in Y6 films. The electroluminescence external quantum efficiency (EQEEL) curves are shown in Fig. 3(c). The EQEEL is defined as photons emitted per electrons injected into the device, which means that the higher EQEEL values, the lower non-radiative recombination loss. The calculated ΔE3 value of the i-PM6/ani-Y6 device is 0.211 eV, which is lower than that of the i-PM6/i-Y6 device (0.224 eV). Therefore, it can be concluded that the decreased non-radiative recombination loss leads to the higher VOC in the i-PM6/ani-Y6 device.

    Table 2  Detailed voltage loss parameters of PHJ devices.
    DeviceΔEloss (eV) Egap (eV) qVOC (eV) VSQOC
    (V)
    VradOC
    (V)
    Vnon-radOC
    (V)
    ΔE1 (eV) ΔE2 (eV) ΔE3 (eV)
    i-PM6/i-Y6 0.555 1.386 0.831 1.139 1.055 0.224 0.247 0.084 0.224
    i-PM6/ani-Y6 0.549 1.395 0.846 1.155 1.057 0.211 0.240 0.098 0.211
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    fig

    Fig 3  Sensitive-EQE and normalized EL of the (a) i-PM6/i-Y6 and (b) i-PM6/ani-Y6 PHJ devices. (c) EQEEL measurements and (d) temperature dependence of VOC in i-PM6/i-Y6 and i-PM6/ani-Y6 PHJ devices.

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    In order to study the impacts of molecular alignment of Y6 films on the interfacial energetic disorder, the temperature-dependent VOC of these devices was measured. Generally, the VOC of OSCs can be expressed as the Eq. (3):[

    39,40]

    VOC=(EALUMOEDHOMOΔ)/qkBTqln(N2cnenh)
    3

    where EALUMO

    is the energy level of the lowest unoccupied molecular orbital (LUMO) of acceptor, EDHOMO
    is the energy level of the highest occupied molecular orbital (HOMO) of donor. Δ represents the Gaussian width of the density of states (DOS) at the D/A interface. The energetic disorder parameter σ is obtained by the expression Δ = σ2/kBT. Nc is the total DOS at the band edge of the acceptor and donor, nenh is the product of electron and hole densities at VOC in OSCs. As shown in Fig. 3(d), extrapolating the VOC in linear region to T=0 K, the effective bandgap ( Eeffg
    ) of bilayer device is obtained from the intercept based on Eq. (3). In addition, cyclic voltammetry (CV) measurement is performed to measure the HOMO and LUMO energy levels for PM6 and Y6 films with different molecular alignments (Fig. S6 and Table S4 in ESI).[41] Subsequently, the Δ and σ values are calculated and listed in Table 3. The σ value in the i-PM6/i-Y6 device is 90.03 meV and it decreases to 78.04 meV when changing the isotropy Y6 film into anisotropy Y6 film. The i-PM6/ani-Y6 device shows the lower interfacial energetic disorder, which is one reason to obtain the lower energy loss. Besides, the lower energetic disorder can also facilitate the charge generation process, leading to the improved JSC and FF in the i-PM6/ani-Y6 device.

    Table 3  The calculated interfacial energetic disorder for different PHJ devices.
    Device
    EALUMOEDHOMO
    (eV)
    Eeffg
    (eV)
    Δ (eV)σ (meV)
    i-PM6/i-Y6 1.371 1.029 0.342 90.03
    i-PM6/ani-Y6 1.364 1.107 0.257 78.04
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    On the other hand, we also evaluated the activation energy (Ea) of charge separation in the i-PM6/i-Y6 and i-PM6/ani-Y6 devices. Typically, the Arrhenius equation is used to extract the activation energy:[

    42,43]

    JSC=J0exp(EakT)
    4

    where J0 is the pre-exponential factor. Fig. 4(a) shows the temperature-dependent JSC and the corresponding fitted lines. We found out that the Ea of the i-PM6/ani-Y6 device is 11.88 meV which is lower than that of the i-PM6/i-Y6 device (13.74 meV), suggesting easier charge separation and thus higher JSC and FF. Besides, the transient photovoltage (TPV) measurements were applied to obtain the charge carrier lifetimes.[

    44] As shown in Fig. 4(b), the i-PM6/ani-Y6 device exhibits the longer charge carrier lifetimes than the i-PM6/i-Y6 device, which could promote the charge transport. Therefore, when changing the random alignment to long-range orderly alignment, the easier charge separation and longer charge carrier lifetime contribute the higher JSC for the i-PM6/ani-Y6 device.

    fig

    Fig 4  (a) Temperature dependence of JSC in i-PM6/i-Y6 and i-PM6/ani-Y6 PHJ devices; (b) Carrier lifetime under varied bias obtained from TPV measurement.

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    CONCLUSIONS

    The effect of long-range alignment of molecular packing structure on the photoelectric conversion process has been clarified in the PM6/Y6 bilayer model system. The PM6 and Y6 layers can be properly adjusted to exhibit the uniaxial oriented packing structure. The analysis of device performance in the different bilayer devices revealed that the smaller energy loss, lower energetic disorder, and longer charge carrier lifetime can be observed in the bilayer devices with aligned Y6 molecules, which contribute to a higher PCE than the as-cast devices. However, due to the intrinsic semicrystalline nature of PM6 molecules, their molecular packing structure exhibited negligible influence on the device performance. This indicates that the alignment of small molecular acceptor at the donor/acceptor interfaces should be a powerful strategy to facilitate the photoelectric conversion process, and control of the interfacial molecular packing structure in the BHJ devices should also be considered so as to further promote their device performance.

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