**3. Results and Discussion**

## *3.1. QSPR Modeling*

The training set offered 7 features (descriptors) and 6 latent variables (LVs) based PLS equation (Equation (1)). To judge the goodness-of-quality of the presented equation along with predictivity of the test set molecules, we have checked a series of stringent statistical metrics and all of them passed the stipulated threshold values (Table 2).

$$\begin{array}{l} \text{PCE (\%)} = 2.50 + 1.84 \text{S}\_A (\text{chg})/A\_D\_D\_D/1\_2, s\_1.3s\_3.4a/6 \\ \quad - 0.78\text{Fr5}(\text{chg})/B\_C\_C\_C\_D/1\_4s\_2.3s\_2.4s\_3.4s\_4/4s\_4 \\ \quad - 0.06\text{Fr5}(\text{type})/C.3\_C\_C.3\_C.3\_C.3\_H/1\_2, s\_2.3s\_3.4s\_4.5s/ \\ \quad - 0.937\text{Fr5}(\text{at})/C\_C\_E\_E\_E/1\_3s\_2.4s\_3.5s\_4.5s/ \\ \quad - 0.11^\text{Fr5}(\text{type})/C.3\_C.3\_C.AR\_C.AR\_C.AR\_C/1\_4s\_2.3s\_2.5s\_4.5s\_4.5s/ \\ \quad + 0.61^\text{S}\_A (\text{type})/C.3\_C.3\_C.3\_C.3\_C.3\_C.3s\_3.3\_4s/5-0.008^\text{A}\_A P \end{array} \tag{1}$$

**Table 2.** Obtained statistical data from the developed quantitative structure-property relationship (QSPR) model.


Considering the complexity and diversity of FDs structures, the internal and external prediction variances are 0.74 and 0.73, respectively which are highly acceptable in QSPR modeling. The same value for Q2 F1 and Q2 F2 suggested stability between training and test sets followed by identical distribution of FDs. The model also passed the strict rm<sup>2</sup> metrics and Golbraikh and Tropsha's criteria. To check the randomness of the model which one support that the model is not developed by chance, we have performed process validation by generation of 100 random models. We found that average R<sup>2</sup> and Q2 values for 100 random models are 0.17 and −0.38, respectively which failed the stipulated value of 0.5 for both metrics. It supports the conclusion that the PLS model is not a result of correlation-by-chance. To check the applicability domain (AD), we have prepared ED-based AD study and found that all test compounds are falling within the AD zone created by the training set data which supports the reliability of prediction for test compounds.

## *3.2. Mechanistic Interpretation of Model*

Out of seven features, two features namely S\_A(chg)/A\_D\_D\_D/1\_2s,1\_3s,3\_4a/6 and S\_A(type)/C.3\_C.3\_C.3\_C.AR/1\_3s,2\_3s,3\_4s/5 contributed positively to PCE value. This signifies that the higher value of these features may increase the PCE value. On the contrary, the remaining five features affect the equation negatively suggesting lowering or no effect on the PCE value (absent of these features or fragments in the FDs).

S\_A(chg)/A\_D\_D\_D/1\_2s,1\_3s,3\_4a/6 defines a four-atomic fragment labeled by partial charges which are induced by -ortho directing groups in the benzene rings. FDs having the mentioned fragments (see Figure 1) have higher value for this descriptor and in a consequence promote higher PCE value. While, S\_A(type)/C.3\_C.3\_C.3\_C.AR/1\_3s,2\_3s,3\_4s/5 represents types of fullerene substituent connections demonstrated in Figure 2 are also good for increment of PCE value. This specific fragment portrayed that aromatic rings like phenyl, thiophene, pyrrole (electron acceptors) attached to the fullerene by a linker help the electron withdrawing capability of the fullerene. Fr5(chg)/B\_C\_C\_C\_D/1\_4s,2\_3s,2\_4s,3\_4s/ also defines partial charges portrayed by the molecular fragment in Figure 3 offers detrimental effects to PCE. This suggests that it is better to avoid such specific substituents to FDs. Fr5(type)/C.3\_C.3\_C.3\_C.3\_H/1\_2s,2\_3s,3\_4s,4\_5s/ describes the existence of saturated carbon chains like [C(sp3)- C(sp3)- C(sp3)- C(sp3)-H]) and offers inductive effects and reduces the mesomeric effects of aromatic rings in a FD, which has a negative effect on PCE. Fr5(att)/C\_C\_E\_E\_E/1\_3s,2\_4s,3\_5a,4\_5a/ is associated to the van der Waals attraction between 3 or higher-ortho substituents in benzene rings with negative contribution to the PCE. Fr5(type)/C.3\_C.3\_C.AR\_C.AR\_C.AR/1\_4s,2\_3s,2\_5s,4\_5a/ portrayed substituents to a pentagon of the fullerene core which is electrochemically more steady than general two-points substituted FDs. Higher number of attachments in the parent fullerene affects the unsaturation and aromaticity negatively and results in reduction of acceptor property of FDs. ASA\_P defines the solvent accessible surface area of polar atoms which is significant for the calculation of free energy changes as a result of shifting the molecule from a polar to a non-polar solvent during the formation of PSCs with BHJ layers.

**Figure 1.** Fragments like -ortho directing groups substituted in the benzene ring help in power conversion efficiency (PCE) increment.

**Figure 2.** Important fullerene substituents for higher PCE.

**Figure 3.** Fragment has detrimental effect on PCE value.

#### *3.3. Designing of Novel FDs as Acceptor*

Mechanistic interpretation of important fragments lead to higher PCE are considered here for the designing of lead FDs as acceptor for Fullerene-based PSCs. The major fragments illustrated in Figures 1 and 2 are included in the acceptor and fragments (Figure 3, C(sp3)- C(sp3)- C(sp3)- C(sp3)-H, 3 or higher-ortho substituents in benzene and substituents to a pentagon of the fullerene core) which are detrimental to PCE are avoided when possible. Ten FDs structures (7 C60 and 3 C70) are designed (Figure 4) and modeled descriptors are calculated following similar protocols mentioned in Section 2.1.2. Followed by QSPR model (Equation (1)) is implemented to predict the PCE of the designed FDs and AD study has also been performed to check their prediction reliability. All 10 FDs passed the Euclidean distance-based AD study portraying the PCE values to be reliable and can be considered for the further introspection to prove them as future efficient acceptor for fullerene-based PSCs. The predicted PCE of FDs range from 7.96 to 23.01 considering both C60 and C70 FDs. Here, FD7, FD8 and FD9 are C70 FDs whose PCE values varies from 7.96 to 12.11, while remaining FDs are C60 FDs whose PCE values varies from 12.03 to 23.01. All values are no doubt encouraging and higher than any existing FD acceptor for PSCs. To check the electrochemical properties of these FDs, we have selected the top three C60 and top C70 FDs for further analysis. Computed modeled descriptors value along with mean normalized distance for AD study provided in Table S2 in the Supplementary material section.
