3. Optimization and Morphology Analysis of Nonfullerene Acceptors with Ester Functionalized
3.2 Results and Discussion
3.2.1 Optical and electrochemical characterizations
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To investigate the effect of the termination of EstIC with the variation of terminal symmetricity on optical and electrochemical properties of NFAs, we analyzed the absorption spectra in both dilute chloroform (CF) solution and thin-film state by using UV-vis spectroscopy. The absorption spectra are plotted in Figure 3.1 b-c and the detailed optical and the parameters are summarized in Table 3.1. In CF solution (Figure 3.1 b), all three NFAs show nearly overlapped spectra with the main absorption range from 630 nm to 810 nm. In thin-film state (Figure 3.1 c), NFAs exhibit the distinctly red-shifted and widened absorption bands compared to solution, indicating the strong enhancement of intermolecular π-π stacking when going solution to the solid state. We considered these optical commonalities of three NFAs originated from same configuration of backbone. However, on closer inspection, asymmetric BTP-Est2F and BTP-Est2Cl film have larger absorption coefficient at main absorption range from 650 nm to 830 nm than BTP-2Est, suggesting asymmetrical termination of EstIC is advantageous to more efficient light-harvesting capability. To sum up the optical investigation, the termination of EstIC little influences on main absorption ranges of both symmetric and asymmetric NFA, but just in case of the asymmetrical usage of EstIC, it is beneficial for high JSC among the photovoltaic parameters.
We determined subsequently the frontier molecular orbital energy levels for BTP-2Est, BTP-Est2F, and BTP-Est2Cl using CV. The energy level diagrams of three NFAs and the corresponding CV curves are shown in Figure 3d. The calculated HOMO and LUMO values, and bandgaps of BTP-2Est, BTP-Est2F and BTP-Est2Cl are -5.62/-3.90 eV, 1.72 eV, -5.63/-3.88 eV, 1.75 eV and -5.64/-3.89 eV, 1.75 eV, respectively, in agreement with theoretically determined energy levels using DFT. All three NFAs exhibit similar HOMO and LUMO levels with an upward tendency, compared to commonly used NFAs (Y6, Y7), indicating the possibility of achieving high VOC from the EstIC terminated NFAs based OSCs devices. Yet, in comparison between symmetric NFA and asymmetric ones, there was no pronounced electrochemical difference. Thus, from these uneventful results, we can suppose that the combining EstIC with other counter terminal groups in NFAs is promising method to apply the terminal engineering for efficient OSCs without the fluctuation of energy levels.
27 3.2.2 Photovoltaic Characterizations
To investigate the photovoltaic performance of EstIC terminated NFAs based OSCs, the devices were fabricated with a conventional structure of Al/PDINO/Active layer/PEDOT:PSS/ITO. The PM6:NFA blends were spin-coated from the chloroform solution with 1-chloronaphthalene (CN) as the additive at room temperature and underwent no post-treatment. The J-V characteristics of the devices measured with the intensity of 100 mW cm-2 under AM 1.5G illumination, shown in Figure 3.2 a; the detailed
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photovoltaic parameters are summarized in Table 3.2. The device based on PM6:BTP-2Est blend shows PCEs of 14.2%, with VOC of 0.903 V, JSC of 23.0 mA cm-2, and FF of 68.4 %. the BTP-Est2F and BTP- Est2Cl blends exhibit increased PCEs of 16.4% and 15.3% with balanced VOC of 0.877 V and 0.872 V, JSC of 26.0 mA cm-2 and 25.2 mA cm-2, and FF of 71.9% and 69.7. Interestingly, despite no difference between energy levels of three blends, the BTP-2Est based device showed the highest VOC value than the others, indicating that the contents of EstIC in NFAs can significantly affect to reduce the energy loss. However, the PM6:BTP-2Est based devices show low JSC and FF, resulting in relatively poor PCE.
To complement the worsen photovoltaic performance, the asymmetrical termination of EstIC was an effective way to achieve the enhanced FF and JSC with retaining high VOC value.
The photocurrent density (Jph) versus effective voltage (Veff) curve was studied to examine the exciton dissociation and charge collection abilities, as shown in Figure 4c and the efficiencies are summarized in Table S1. The Jph was defined by Jph = JL – JD, where JL is the current density under illumination and JD is that in the dark. Veff = V0 – VA, where V0 is the voltage when Jph is equal to 0, and VA is the applied bias voltage. At Veff > 2 V, the Jph of all the devices approached saturation (Jsat), representing the complete exciton dissociation and charge collection. The detailed measurements are summarized in Table S2. The exciton dissociation efficiency (ηdiss = Jsc/Jsat) were calculated to be 96.7%, 98.6%, and 98.2% for BTP-2Est, BTP-Est2F, and BTP-Est2Cl based devices, respectively. The charge collection efficiency (ηcc = Jmax power/Jsat), where Jmax is current density under maximum power output conditions, were measured as 83.0%, 87.1%, and 84.4% for BTP-2Est, BTP-Est2F, and BTP-Est2Cl based devices, respectively. Overall, the highest values of ηdiss and ηcc are the most likely reason for the largest JSC. Furthermore, the dependence of VOC and JSC on Plight was measured to explore the charge recombination for three different devices. The power-law, which may be characterized as the pattern of connection between Plight and JSC, is JSC ∝ Plightα. 61, 62 The α close to unity reflects the suppressed bimolecular recombination in the device. As shown in Figure 3.2 d, the values of α for the BTP-2Est, BTP-Est2F, and BTP-Est2Cl were calculated to be 0.982, 0.987, and 0.983, respectively, indicating that the BTP- Est2F based device suffers from the least extent of bimolecular recombination compared to the others.
Additionally, VOC and Plight have a connection that can be explained as VOC ∝ nkT/qln(Plight). When a slope gets near kT/q, it is expected that the existence of trap-assisted recombination is recessive. As shown in Figure 3.2 e, the slopes of the BTP-2Est, BTP-Est2F, and BTP-Est2Cl based devices are 1.16, 1.18, and 1.32, respectively. This implies that the predominant recombination of all three devices is bimolecular recombination, rather than trap-assisted recombination.
To investigate the impact of the termination of EstIC on the Eloss, we analyzed FTPS-EQE and EL
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measurements. The Eloss, one of the important parameters in the performance of OSCs, is determined by three parts: Eloss = ΔErad + (Egopt – ECT) + ΔEnonrad = ΔE1 + ΔE2 + ΔE3, where ΔErad and ΔEnonrad mean radiative and nonradiative energy loss, and ECT represents charge transfer state energy.55-57 On the basis of the Shockley-Queisser theory, the three devices have almost same values of ΔE1 of 0.262, 0.263, and 0.263 eV for PM6:BTP-2Est, PM6:BTP-Est2F, and PM6:BTP-Est2Cl, respectively. These results demonstrate that the symmetricity of termination of EstIC in NFAs little influences on the ΔE1. The ΔE2
were measured using FTPS-EQE spectra and normalized EL. The ΔE2 of PM6:BTP-2Est device (0.055 eV) is less than that of asymmetric PM6:BTP-Est2F (0.060 eV) and PM6:Est2Cl (0.066 eV), which means a tidy driving force for exciton separation. For the ΔE3, the EQEEL measurement were performed and the values of PM6:BTP-2Est, -Est2F, and -Est2Cl are 4.80 × 10-4, 1.96 × 10-4, and 2.10 × 10-4, respectively. According to the equation, ΔEnonrad = -(kT/q)ln(EQEEL), the ΔEnonrad are calculated to be 0.197, 0.220, and 0.218 eV for PM6:BTP-2Est, -Est2F, and -Est2Cl, respectively.58-60 Considering all of these, BTP-2Est based device shows the lowest value in both ΔE2 and ΔE3, which agree well with the highest VOC of 0.902 V of PM6:BTP-2Est.
3.2.3 Microstructural and Charge Transport
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To morphologically verify the higher photovoltaic performance of asymmetric NFAs comprising EstIC terminal group, AFM and TEM analysis were performed for the blend films without thermal annealing.
As shown in Figure 3.3 a-c, all the blend films present the fibril textures with the low mean-square surface roughness (Rq) values of 0.773-0.943 nm and the flower-like D/A phase separation features.
Especially, the PM6:BTP-Est2F blend presents the lowest aggregation behavior and the most well- interpenetrating network, as supported by the highest miscibility between donor and acceptors, determined via Flory-Huggins interaction parameter (χ) values.64 Thus, we can deduce that the asymmetrical termination of EstIC with 2FIC is advantageous to form the favorable blend morphology for the efficient intermolecular charge transport.
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GIWAXS analysis were performed to analyze the crystalline feature and the molecular stacking properties of the neat films and the blend films with PM6. As presented in Figure 3.4, all neat films were obviously stacked with the face-on direction, as evidenced by the strong π-π stacking peak at ~1.71 Å-1 in OOP direction and two lamellar stacking peaks of (110) at ~0.27 Å-1 and (11-1) at ~0.40 Å-1 in IP direction. The line-cut profiles, and the corresponding parameters were shown in Figure Xc and Table 3.3. Interestingly, despite the nearly same π-π d-spacing values (3.5-3.6 Å) of all the neat films at ~3.5 Å, the OOP directional crystal coherence length (CCL) slightly increases in agreement with the strength of intermolecular binding energy provided above.27, 65 Likewise, all the blend films with PM6 commonly exhibit the distinct (010) peaks at coterminous q values at ~1.70 Å-1 with d-spacing of ~3.6 Å and (100) peaks at 0.29 Å-1, indicating the face-on direction.66 Moreover, the similar trend of the π–π d-spacing value are demonstrated with similar hole and electron mobility of the EstIC terminated NFAs based the blend films.
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In SCLC measurements, the hole (μh) and electron (μe) mobility of neat and blend films were evaluated to explore how the asymmetrical termination of EstIC influences the charge transport properties. The outcomes are exhibited in Figure 3.5, and the corresponding values are summarized in Table 3.4. The asymmetric BTP-Est2F neat films exhibited the highest electron mobility among three of EstIC terminated NFAs, demonstrating the enhancement of ICT by electronic unbalance between terminal groups. In addition, the best balanced ratio was also achieved from the PM6:BTP-Est2F blend (1.08), supporting the highest JSC and FF parameters of the device based on this system.
33 3.2.4 Thermal Stability
As inspired by the more favorable morphology in the asymmetric NFAs based active layers even without a thermal annealing treatment, we performed thermal stability test to reveal the correlation between the dipole moment of NFAs and the degradation of the device performance (morphological stability) for BTP-2Est, -Est2F, and -Est2Cl based OSC devices. The test conditions were established as the following conditions of thermal stress of 85oC for 455 h in the glove box without the deposition of ETL and the metal electrode on the above-described optimal active layers.67-69 The complete devices were fabricated after the corresponding time of thermal stress, followed by the estimation of photovoltaic parameters and the normalized photovoltaic parameters are illustrated in Figure 3.6.
During 455 h of thermal stress, the plots of normalized JSC and VOC for all devices shows an insignificant degradation, indicating an unchangeability of light absorptivity and the energy losses under the standard OSCs operating temperature for long time. Whereas, as implied by Figure Xc-d, the distinct difference in normalized FF and PCE is observed between symmetric NFA and asymmetric ones. After 455 h, the
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symmetric BTP-2Est based device shows the rapid degradation of FF and PCE to 69.2% and 62.5% of its initial value. In contrast, the asymmetric BTP-Est2F and -Est2Cl based devices effectively preserve their initial values of FF as 87.1% and 88.5%, respectively, and PCE as 83.7% and 84.3 %, respectively.
Based on data, thermal stability of the EstIC terminated NFAs can be arranged in order of BTP-Est2Cl
> -Est2F > -2Est. These results support that the large and biased dipole moment of NFA make it possible thermally stable OSC device through morphological locking with strengthened intermolecular interactions. Consequently, it is summarized that not only the termination of EstIC into NFA prevents the thermal-induced degradation of JSC and VOC. Moreover, if EstIC is asymmetrically introduced into NFA, FF and PCE are successfully preserved without a significant thermal-induced degradation, affording the ultimate thermally stable OSC device.70, 71