**3.6 Photovoltaic properties**

The advanced performance photovoltaic devices exhibit fundamentally significant power conversion efficiency (PCE). For high PCE, the photovoltaic devices should possess high fill factor (FF) and large open-voltage circuit (VOC). In fact, these conditions dispose a challenge for narrow band gap materials to cover as much of the solar spectrum as possible.

BHJ-OSCs contain principally an electron donor material blinded with an electron acceptor fullerene derivative named (6,6)-Phenyl-C71 Butyric Acid Methyl Ester ([70] PCBM). Accordingly, we have proposed a schematic energy diagram of BHJ-OSCs (P-CPDTBT3/SM-CPDTDPP: [70] PCBM), as shown in **Figure 8**. The experimental [70] PCBM energy level values of were recorded in ref. [67].

FF is one of the crucial factors that influence the PCE and can be estimated using the expression above [68]:

$$FF = \frac{\upsilon\_{\alpha^\star} - \ln\left(\upsilon\_{\alpha^\star} + \mathbf{0}.72\right)}{\upsilon\_{\alpha^\star} + \mathbf{1}}\tag{5}$$

Where, *oc v* is the dimensionless voltage:

$$\boldsymbol{\nu}\_{\rm oc} = \frac{e\boldsymbol{V}\_{\rm oc}}{k\_B T} \tag{6}$$

**75**

**Table 4.**

*Designing Well-Organized Donor-Bridge-Acceptor Conjugated Systems Based…*

Here, *e*, , *k T <sup>B</sup>* and VOC are the elementary charge, Boltzmann's constant,

*Schematic energy diagram of the proposed (P-CPDTBT3/SM-CPDTDPP: [67] PCBM) BHJ OSCs.*

Where, e, HD and LA are the elementary charge, HOMO of donor and LUMO of

The calculated photovoltaic parameters are listed in **Table 4**. As we can see from the table, there is a growth tendency of the photovoltaic parameters from SM-CPDTDPP to P-CPDTBT3. The P-CPDTBT3 copolymer exhibits larger Voc which is expected as this latter dispose a deeper HOMO energy level value.

Further, Scharber diagram was used to estimate the power conversion efficiency (PCE) of BHJ-OSCs [70]. From **Figure 9**, the predicted PCE of P-CPDTBT3 and SM-CPDTBT materials are found to be 9.5% and 8.2%, respectively. Thus, we can reveal from these results the fruitful molecular design of the investigated compounds to ensure a promising PCE for developing efficient materials for

**Compound** *Voc oc v FF* **P-CPDTBT3** 1.11 43.22 0.89 **SM-CPDTDPP** 0.86 33.48 0.86

( ) <sup>1</sup> 0.3 *D A V HL oc <sup>e</sup>* = −− (7)

temperature and open circuit voltage, respectively. The VOC can be approximated as [69]:

*Photovoltaic properties calculated at DFT/B3LYP/6-311 g(d,p) level.*

acceptor, respectively.

**Figure 8.**

BHJ-OSCs.

*DOI: http://dx.doi.org/10.5772/intechopen.94874*

*Designing Well-Organized Donor-Bridge-Acceptor Conjugated Systems Based… DOI: http://dx.doi.org/10.5772/intechopen.94874*

**Figure 8.** *Schematic energy diagram of the proposed (P-CPDTBT3/SM-CPDTDPP: [67] PCBM) BHJ OSCs.*

Here, *e*, , *k T <sup>B</sup>* and VOC are the elementary charge, Boltzmann's constant, temperature and open circuit voltage, respectively.

The VOC can be approximated as [69]:

$$V\_{oc} = \frac{1}{\varepsilon} (\left| H^D \right| - \left| L^A \right|) - \mathbf{0}.\,\mathbf{3} \tag{7}$$

Where, e, HD and LA are the elementary charge, HOMO of donor and LUMO of acceptor, respectively.

The calculated photovoltaic parameters are listed in **Table 4**. As we can see from the table, there is a growth tendency of the photovoltaic parameters from SM-CPDTDPP to P-CPDTBT3. The P-CPDTBT3 copolymer exhibits larger Voc which is expected as this latter dispose a deeper HOMO energy level value.

Further, Scharber diagram was used to estimate the power conversion efficiency (PCE) of BHJ-OSCs [70]. From **Figure 9**, the predicted PCE of P-CPDTBT3 and SM-CPDTBT materials are found to be 9.5% and 8.2%, respectively. Thus, we can reveal from these results the fruitful molecular design of the investigated compounds to ensure a promising PCE for developing efficient materials for BHJ-OSCs.


**Table 4.**

*Solar Cells - Theory, Materials and Recent Advances*

*TDM plots at the first excited state (S1) of the investigated materials.*

**3.6 Photovoltaic properties**

**Figure 7.**

of the solar spectrum as possible.

using the expression above [68]:

The weaker coupling of electron and holes makes easier the dissociation of exciton. The contour plots of TDM show also the exciton dissociation in the studied molecules may be easy regarding the weak electron–hole correlation that involves the charge transfer from main CPDT units to the dicyanomethylene bridge group [66]. The coefficients correlation of D-A within P-CPDTBT3 are slightly higher than those of SM-CPDTNDPP. Hence, the exciton dissociation is expected to be comparatively easier in the case of SM-CPDTNDPP than that in the case of P-CPDTBT3. The TDM analysis demonstrates the efficiency of charge separation within these molecules which leads to a considerable improvement of the Jsc.

The advanced performance photovoltaic devices exhibit fundamentally significant power conversion efficiency (PCE). For high PCE, the photovoltaic devices should possess high fill factor (FF) and large open-voltage circuit (VOC). In fact, these conditions dispose a challenge for narrow band gap materials to cover as much

BHJ-OSCs contain principally an electron donor material blinded with an electron acceptor fullerene derivative named (6,6)-Phenyl-C71 Butyric Acid Methyl Ester ([70] PCBM). Accordingly, we have proposed a schematic energy diagram of BHJ-OSCs (P-CPDTBT3/SM-CPDTDPP: [70] PCBM), as shown in **Figure 8**. The experimental [70] PCBM energy level values of were recorded in ref. [67].

FF is one of the crucial factors that influence the PCE and can be estimated

*oc oc oc*

*v* − + <sup>=</sup> <sup>+</sup>

*v v*

*oc*

*v*

*FF*

Where, *oc v* is the dimensionless voltage:

ln 0.72 ( ) 1

*oc*

*k T* <sup>=</sup> (6)

*B eV* (5)

**74**

*Photovoltaic properties calculated at DFT/B3LYP/6-311 g(d,p) level.*

**Figure 9.**

*Scharber diagram for estimating the power conversion (PCE) efficiency of the studied compounds.*
