Photovoltaic response promoted via intramolecular charge transfer in pyrazoline-based small molecular acceptors: Efficient organic solar cells

Muhammad Khalid; Ayesha Mustafa; Sarfraz Ahmed; Muhammad Adnan Asghar; Tansir Ahamad; Ataualpa A.C. Braga; Suvash Chandra Ojha

doi:10.1016/j.arabjc.2023.105271

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Original article

11 2023

:16;

105271

doi:

10.1016/j.arabjc.2023.105271

Photovoltaic response promoted via intramolecular charge transfer in pyrazoline-based small molecular acceptors: Efficient organic solar cells

Muhammad Khalid^{^a,^b,⁎}, Ayesha Mustafa^{^a,^b}, Sarfraz Ahmed^{^c}, Muhammad Adnan Asghar^{^d}, Tansir Ahamad^{^e}, Ataualpa A.C. Braga^{^f}, Suvash Chandra Ojha^{^g,⁎}

a Institute of Chemistry, Khwaja Fareed University of Engineering & Information Technology, Rahim Yar Khan 64200, Pakistan

b Centre for Theoretical and Computational Research, Khwaja Fareed University of Engineering & Information Technology, Rahim Yar Khan 64200, Pakistan

c Wellman Center for Photomedicine, Harvard Medical School, Massachusetts General Hospital, Boston, MA 02114, United States

d Department of Chemistry, Division of Science and Technology, University of Education Lahore, Pakistan

e Department of Chemistry, College of Science, King Saud University, Saudi Arabia

f Departamento de Qúımica Fundamental, Instituto de Qúımica, Universidade de Saõ Paulo, Av. Prof. Lineu Prestes, 748, Sao Paulo 05508-000, Brazil

g Department of Infectious Diseases, The Affiliated Hospital of Southwest Medical University, Luzhou 646000, China

⁎Corresponding authors. khalid@iq.usp.br (Muhammad Khalid), suvash_ojha@swmu.edu.cn (Suvash Chandra Ojha)

Received: 2023-3-21, Accepted: 2023-9-14,

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Herein, a series of pyrazoline based non-fullerene compounds (THP1-THP8) having ladder-like backbone was designed by structural modulation with various electron accepting moieties. The density functional theory (DFT) and time-dependent functional theory (TD-DFT) study was executed at M06/6-311G(d,p) level for structural optimization and to determine the electronic and optical characteristics of the pyrazoline based chromophores. The optimized structures were employed to execute frontier molecular orbital (FMO), transition density matrix (TDM), density of state (DOS), open circuit voltage (V_oc) and reorganization energy analyses at the aforementioned level of DFT to comprehend the photovoltaic (PV) response of THP1-THP8. The red-shifted absorption spectrum (512.861–584.555 nm) with reduced band gap (2.507–2.881 eV) allow considerable charge transferal from HOMO to LUMO in all the studied compounds. Global reactivity parameters (GRPs) demonstrated high softness with considerable reactivity in THP1-THP8. Moreover, remarkable V_oc values (2.083–2.973 V) were noted for all the derivatives (THP1-THP8). However, THP2 with lowest energy gap (2.364 eV), highest λ_max (617.482 nm) and softness (0.423 eV) values is considered good candidate among afore-said chromophores. Hence, the studied chromophores with efficient properties are appropriate for experimentalists in terms of manufacturing of efficient OSCs.

Keywords

Pyrazoline-based molecules

DFT

Optical properties

TDM

Open circuit voltage

1 Introduction

Development in science and technology offers advanced methods to produce clean energy at an adaptable level (Wu et al., 2005). The scientific community of the contemporary world is directing its approach of energy generation from greenhouse gas emission technologies to eco-friendly power generation projects and photovoltaic energy is a pertinent alternate for this aim (Shafiq et al., 2023b; ul Ain et al., 2021). The most auspicious approach of transforming solar energy into electrical power is by using photovoltaic cells employing photoelectric effect (Khalid et al., 2022a). Photovoltaic technology has been industrialized in the existing decades however, it still needs to be upgraded to attain the utmost output (Yaqoob et al., 2021). Usually, 173,000 terawatts of solar energy is received by the earth’s surface which is 10,000 times greater than the overall energy consumption (Aboulouard et al., 2021). The energy generated by PV cells epitomizes around 1% of the total energy requirement. It turns out that the required electrical power can be domestically produced by decreasing electrical deficits as well as storage capacities (Traverse et al., 2017).

Based on the material and techniques utilized to engineer solar cells, the PV cells are categorized into three generations. Initially, silicon based first-generation solar cells were developed and these cells were used for a long duration due to their stability, heat constancy and high efficiency. Later on, second and third-generation thin-film PV cells were introduced to overcome the deficiencies of crystalline silicon photovoltaics i.e., high cost of production and problematic installation on large scale (Conibeer et al., 2008; Green, 2004; Heidarzadeh and Tavousi, 2019 ). Over the last 15 years, apparent improvements in ‘second generation’ thin-film cells have been made. With the use of thin film, the cost of the material has been reduced by eradicating silicon. Furthermore, power conversion efficiency (PCE) has been significantly improved. With the passage of time as technology advanced, the manufacturing cost has dominated over the cost of material (Green, 2002). Thus, further progress of thin-film photovoltaics (PVs) confronted bottlenecks and the research on third-generation PVs has been progressed. These cells do not contain p-n junction as in conventional solar cells (Arjunan and Senthil, 2013) and are suitable for large-scale installation of PVs (Conibeer, 2007). Third-generation PV cells include polymer, organic PVs, (Yan and Saunders, 2014) hot carrier solar cells, (Tavousi, 2019) multi-junction cells (Conibeer, 2007) etc.

A keen examination reveals that OSCs have the potential to be engineered at less cost, effective using simple synthetic route with lightweight and high flexibility (Chen et al., 2011; Mahmood et al., 2018; Zhan and Marder, 2019 ). Besides, the OSCs exhibit irregular absorption that can be supervised to transmit visible, infrared and ultraviolet light simultaneously (Aboulouard et al., 2021). The active layer in OSCs generally comprises a blend of electron acceptor and donor with a bulk heterojunction (BHJ) structure that facilitates charge separation and transfer of hole and electron to their respective electrodes (Zhan and Marder, 2019). During the last two decades, fullerene derivatives remained the most extensively used acceptors in OSCs as they embrace fascinating assets comprising high PCE and exclusive electron and hole mobility (Yu et al., 1995). However, their morphological instability and confined variability of optoelectronic properties limit the viable progress of this field, particularly restraining the upper attainable device efficacies (Zhan and Marder, 2019). Organic fullerene-free acceptors offer the probability to overcome these limitations. Less toxicity, facile synthesis, lower level of highest occupied molecular orbital (HOMO) and fine absorption spectrum made non-fullerene acceptors (NFAs) progress a mainstream of today’s research (Chan et al., 2017; Sun et al., 2018). Moreover, NFAs having acceptor–donor-acceptor (A-D-A) backbone presented around 17% PCE (Lin et al., 2019). They substantially enrich the carrier mobility with high intramolecular charge transfer rate within a molecule (Firdaus et al., 2020; Xu et al., 2019).

End-group modulation is viewed as an effective approach to modulate the optoelectronic behavior of materials utilized in PV devices (Khan et al., 2021a, 2021b, 2020 ). Herein, keeping in view the crystallinity and remarkable PCE of indacenodithiophene (IDT) (Arshad et al., 2022), we utilized it as π-spacer and enlarged π-conjugation with 1-phenyl-4,5-dihydro-1H-pyrazole. The THP1 derivative is developed by attaching 3-ethylidenepentanedinitrile as end-capped acceptor moiety with π-spacer. The remaining chromophores (THP2-THP8) are designed by amending terminal acceptor entities (Szukalski et al., 2017). A comprehensive DFT study of designed derivatives (THP1-THP8) is performed with the characterization of essential parameters like the frontier molecular orbital (FMO), density of state (DOS), global reactivity parameters (GRPs), transition density matrix (TDM), reorganization energy and open circuit voltage (V_oc) to explore the PV response of afore-said compounds. It is estimated that these innovative NFAs would play an indispensable role in the engineering of highly proficient OSCs.

2 Computational study

In the current investigation, non-fullerene electron acceptors having ladder-like backbone were designed by structural modulation of several end-capped acceptors. Gaussian 09 program package (Frisch et al., 2009) was utilized to compute the entire theoretical calculations of the studied chromophores (THP1-THP8). The optimization of THP1-THP8 was accomplished by using Minnesota 06 (M06) (Bryantsev et al., 2009) exchange-correlational functional along with 6-311G(d,p) (Andersson and Uvdal, 2005) basis set. Furthermore, to visualize the photovoltaic and optoelectronic properties, FMO, GRPs, DOS, V_oc, UV–Vis absorption and TDM analyses were accomplished at the TD-DFT/M06/6-311G(d,p) level. A number of software i.e., Avogadro, (Hanwell et al., 2012) Gaussum, (O’boyle et al., 2008) Chemcraft, (Zhurko and Zhurko, 2009) PyMOlyze 2.0, (UrRehman et al., 2022) Multiwfn 3.7 (Lu and Chen, 2012) and Gauss view 6.0 (Dennington et al., 2016) were utilized to elucidate the results from output files.

Usually, reorganization energy is utilized to study the rate of charge transference and is segregated into two main types i.e., internal reorganization energy (λ_int) and external reorganization energies (λ_ext). λ_int is concerned with the quick variations in internal composition, while λ_ext is directly linked with the external environmental relaxation (Janjua, 2012; Khalid et al., 2020b). In this work, λ_ext effect was neglected because of the little contribution of external environmental factors, so λ_int was considered. Hence, the reorganization energy of hole (λ_h) and electron (λ_e) can be computed with the aid of Eqs. (1) and (2) (Mehboob et al., 2021b).

(1)

λ_{e} = [E_{0}^{-} - E_{-}^{}] + [E_{-}^{0} - E_{0}^{}]

(2)

λ_{h} = [E_{0}^{+} - E_{+}^{}] + [E_{+}^{0} - E_{0}^{}]

where,

E_{0}^{+}

and

E_{0}^{-}

are the cationic and anionic energies acquired through optimized neutral molecular structure, whereas

E_{+}^{0}

and

E_{-}^{0}

are energies of neutral molecule evaluated at cationic and anionic state, respectively.

E_{-}^{}

and

E_{+}^{}

specify the energies of optimized anionic and cationic structures, correspondingly.

E_{0}^{}

epitomizes single energy at the ground state (Tang and Zhang, 2012).

3 Results and discussion

The present quantum chemical research envisages some effective NFAs having A-π-A framework with pyrazoline ring as a crucial component of each structure, are signified in Figure S1 and the optimized structures are illustrated in Fig. 2. In this report, eight novel molecules (Figure S1) are designed utilizing the end-capped modification approach to further discover the potential of IDT related compounds. The IUPAC names of compounds are given in Table S9 and structures of all the acceptors that are utilized in structural modulation are shown in Figure S2. Furthermore, their Cartesian coordinates are presented in Tables S1-S8. Literature survey discloses that acceptor groups are essential in tuning the absorption wavelength as well as energy gap of a compound (Khan et al., 2018). Thus, eight derivatives (THP1-THP8) are being designed by changing acceptor moieties to discover and enhance the electronic properties of OSCs. Moreover, to analyze how various acceptors affect the PV and spectral response, several parameters such as UV–Vis absorption wavelength, V_oc and GRP have been computed by TD-DFT and DFT calculations. The schematic demonstration of THP1-THP8 are presented in Fig. 1.

Schematic illustration of the designed compounds.

Optimized structures of all the theoretically computed molecules (THP1-THP8).

3.1

3.1 Frontier molecular orbital (FMO) analysis

FMO analysis is an effective approach for exploring the optoelectronic characteristics and chemical stability of the molecule (Javed et al., 2014). The mechanical modeling and absorption spectrum are greatly influenced by FMOs i.e., HOMO and LUMO considered as valence and conduction bands, respectively (Mandado et al., 2006). This analysis unveils the electronic distribution pattern and also provide information about the properties of PVs having the capability to accelerate the transmission of electric current (Khan et al., 2019c; Peng and Yu, 1994). The energy gap among FMOs ( $Δ E$ = E_LUMO-E_HOMO) is believed as a hallmark of aptitude to transfer charge (Janjua et al., 2012b, 2012a; Khan et al., 2019b ). The conjugation within the molecule enhances the carrier mobility of the designed molecular system owing to the electronic transitions (Mustafa et al., 2023). Energies of HOMO/LUMO and their band gaps are entirely coupled with the PCE of OSCs (Shafiq et al., 2023a). It also depicts the chemical hardness, softness, molecular interactions and reactivity within the compounds (Amiri et al., 2016). FMO analysis was executed at TD-DFT/M06/6-311G(d,p) level and the calculated energies as well as their E_gap are demonstrated in Table 1. Furthermore, the pictographic illustration of key orbitals is presented in Fig. 3 while the other molecular orbitals (HOMO-1/LUMO + 1 and HOMO-2/LUMO + 2) are displayed in Figure S3 and their relevant E_gap is tabulated Table S10.

Table 1 E_HOMO, E_LUMO, and energy gap (E_gap = E_LUMO-E_HOMO) of the THP1-THP8.

Compounds	HOMO	LUMO	*E_gap*
THP1	−5.525	−3.018	2.507
THP2	−5.381	−3.017	2.364
THP3	−5.111	−2.691	2.420
THP4	−5.080	−2.368	2.712
THP5	−4.921	−2.190	2.731
THP6	−5.349	−2.846	2.503
THP7	−5.180	−2.577	2.603
THP8	−5.009	−2.128	2.881

Units in eV.

From literature, it has been noticed that experimentally determined HOMO/LUMO values (-5.63/-3.86 eV) of IDIC-C8 compound (Zhang et al., 2020) shows harmonization with the DFT computed results of our designed chromophores. The theoretically calculated E_HOMO values of THP1-THP8 are −5.525, −5.381, −5.111, −5.080, −4.921, −5.349–5.180 and −5.009 eV, correspondingly while their E_LUMO values are −3.018, −3.017, −2.691, −2.368, −2.190, −2.846, −2.577 and −2.128 eV, respectively. The E_gap of THP1-THP8 is observed as 2.507, 2.364, 2.420, 2.712, 2.731, 2.503, 2.603 and 2.881 eV, respectively. The maximum value of E_gap is noticed in THP8 (having three fluoro groups at terminal acceptor) i.e., 2.881 eV which is the uppermost value among all the derivatives (THP1-THP8). The fluorine substituents have lowest electron withdrawing capability than nitro and cyano groups utilized in other chromophores which resulted in superior E_gap of THP8. The E_gap value is declined to 2.712 and 2.731 eV in THP4 and THP5, respectively owing to the cyano group at para and ortho positions of peripheral acceptor unit (2-vinylbenzonitrile). In THP7 and THP6, the E_gap is further dropped to 2.603 and 2.503 eV due to the 2-(1-cyanovinyl)benzonitrile and 4-(1-cyanovinyl) benzonitrile end-capped groups. Moreover, the E_gap of 2.507 eV is noticed in THP1 that might be due to the existence of two cyano groups at the terminal acceptors. The greater electronegativity of cyano groups increases the acceptors capability to pull the electrons towards itself, results in reduced E_gap among orbitals. Remarkably, the HOMO and LUMO energy difference is lowered to 2.420 eV in THP3 due to the presence of sole nitro group which has greater electron withdrawing capability. Moreover, the smallest value (2.364 eV) of E_gap is viewed in THP2 owing to the nitro group which has strong electron withdrawing (-I) nature that attracts the electron density towards itself. Further, the presence of cyano group in the vicinity of benzene ring also abridged the E_gap. Moreover, the overall decreasing trend of E_gap is as follows: THP8 > THP5 > THP4 > THP7 > THP1 > THP6 > THP3 > THP2. The effective charge mobility and lowest E_gap between orbitals is inspected in THP2 than all the examined chromophore that emerged as a competent PV-material.

The scheme of electronic charge distribution over the surface areas of THP1-THP8 is illustrated in Fig. 3. The electronic cloud is majorly occupied on π-spacer in HOMO, while in LUMO it is mainly concentrated on over terminal acceptors and minutely over π-spacer. This designates that substantial assistance of charge transmission from π-spacer towards acceptors is monitored in all the designed compounds (THP1-THP8).

3.2

3.2 Density of state (DOS)

The DOS investigation was performed at M06 functional with 6-311G basis set to interpret the findings of THP1-THP7. To envisage the path of charge transmission, we separated our investigated chromophores into two fragments i.e., acceptor and π-spacer that are portrayed by green and red lines, correspondingly as shown in Fig. 4. The DOS study has been executed to ascertain the valuable contribution of every fragment over the molecular system which has particular number of electronic states (Adeel et al., 2021). It has unveiled the charge dissemination from HOMO which has large capacity to donate electrons to LUMO that has tendency to accept electrons (Goszczycki et al., 2017). The pattern of charge dissemination is changed by varying acceptor units that is supported by HOMO/LUMO percentage of DOS (Table S11). Herein, acceptor demonstrates the contribution of electronic charges as: 4.5, 5.5, 5.0, 5.1, 5.0, 5.6, 5.1 and 5.0% to HOMO, while 52.2, 70.4, 81.7, 68.2, 69.2, 62.4, 60.2 and 61.2% to LUMO for THP1-THP8, correspondingly. Similarly, π-spacer reveals the distribution of charges as: 95.5, 94.5, 95.0, 94.9, 95.0, 94.4, 94.9 and 95.0% to HOMO, whereas 47.8, 29.6, 18.3, 31.8, 30.7, 37.6, 39.8 and 38.8% to LUMO for THP1-THP8, respectively. In DOS graphs, the negative values denote the HOMO, whereas the positive values show LUMO along x-axis and the E_gap expresses the distance among HOMOs and LUMOs (Khalid et al., 2022c). From DOS spectrum, it is anticipated that maximum charge density of HOMO is positioned over π-spacer at approximately −13 eV whereas LUMO is mainly resided over acceptor with highest peak at 3.5 eV in the aforesaid compounds. Overall, DOS analysis exhibits extensive charge transfer from π-bridge towards peripheral acceptor moieties in all the studied chromophores.

Graphical illustration of DOS of the titled compounds (THP1-THP8).

3.3

3.3 Optical response

TD-DFT computations are utilized to investigate the absorption spectrum of the titled chromophores (THP1-THP8) in gas phase at M06 with 6-311G(d,p) basis set. UV–Vis analysis gives substantial information about the nature of electronic transitions, possibility of charge transference as well as contributing configuration of the molecule. The oscillator strength (f_os), maximum absorption wavelengths (λ_max) and excitation energies (E) of lowest six singlet–singlet electronic transitions were examined and the outcomes are tabulated in Tables S13-S20, whereas some main results are depicted in Table S12. The optical absorption spectra of THP1-THP8 are exhibited in Fig. 5 in gaseous phase. The electronic excitation spectrum of the examined compounds is envisioned to acknowledge the influence of several acceptors on the spectral properties.

Absorption spectra of entitled compounds THP1-THP8.

The results mentioned in Table S12 has disclosed that all the studied molecules display absorption in the visible region. The simulated λ_max of THP1-THP8 are found in the range of 512.861––584.555 nm with oscillation strength of 1.807–2.951 and excitation energy of 2.008–2.417 eV in gaseous phase. The compound (THP2) depicted highest λ_max value of 617.482 nm with transition energy value of 2.008 eV and 1.948 oscillation strength presenting 93% molecular orbital (MO) contribution from HOMO to LUMO. The utmost value of λ_max in THP2 is ascribed to the extensively electron accepting nitro and cyano groups. The λ_max in THP3 is abridged to 589.166 nm owing to 1-nitro-4-vinylbenzene group. In addition, in THP6, the λ_max value is observed as 588.021 nm that is larger than THP1 and THP7 (584.555 and 567.096 nm, respectively). Furthermore, this value is squeezed to 539.202 and 537.123 nm in THP4 and THP5 due to the incorporation of cyano groups at the end-capped acceptors. The computed maximum wavelength for THP8 is observed at 512.861 nm with transition energy 2.417 eV and oscillation strength as 2.951 which is the lowermost value among all the studied compounds. The values of λ_max are noticed in the subsequent declining trend as: THP2 > THP3 > THP6 > THP1 > THP7 > THP4 > THP5 > THP8. Lower excitation energy and increase in wavelength disclose that THP1-THP8 molecules exhibit greater charge transfer aptitude, consequently easy transition might occur amongst HOMO and LUMO. In a nutshell, THP2 possess highest λ_max, least energy gap and minimum transition energy among all the derivatives which might be anticipated as a suitable material for OSCs.

3.4

3.4 Global reactivity parameters (GRPs)

The energy gap of HOMO and LUMO is utilized to compute the GRPs (Khalid et al., 2020a) of the entitled compounds (THP1-THP8) which in turn determine their reactivity as well as stability. Therefore, E_gap is a dynamic feature to evaluate the GRPs such as electron affinity (EA), electronegativity (X), ionization potential (IP), softness (σ), electrophilicity index (ω), hardness (η) and chemical potential (μ) (Sheela et al., 2014). The ionization potential (IP = -E_HOMO) and electron affinity (EA = -E_LUMO) are calculated utilizing the given formulas (Pearson, 1986). Koopman’s theorem (Koopmans, 1934) is used to calculate the electronegativity [X = (IP + EA)/2], chemical hardness [η = (IP – EA)/2] and chemical potential [μ = E_HOMO + E_LUMO)/2]. Parr et al. (Parr et al., 1978) described the global electrophilicity index that is computed via using the given equation (ω = μ²/2η) (Chattaraj and Roy, 2007). Moreover, the global softness is determined by the mentioned equation (σ = 1/2η) (Koopmans, 1934). The values of η helps in determining the chemical reactivity and stability, (Pearson, 2005) while μ describes the electronic movement within the molecule (Toro-Labbé, 1999). Further, ω aids in determining the stability of a molecule when an additional electronic charge is acquired by it from an external source (Parr et al., 1999).

Table 2 elucidates the outcomes of GRPs for THP1-THP8 which are discussed here. The IP and EA designates the donating and accepting ability of a specie and are identical to the potential needed to transmit an electron from HOMO to LUMO. The polarization of a molecule is in direct relation with the EA and IP, which discloses the reactivity of a molecule. THP1-THP8 exhibit the high IP values (5.009–5.525 eV) and low EA values (2.128–3.018 eV) as mentioned in Table 2. All the studied molecules manifest comparable EA values which endorse their accepting nature. This might be owing to the existence of vigorous acceptors at the terminal portions of molecules. The values of X and ω also upholds the abovementioned statement. The chemical potential, global softness and hardness are correlated with the E_gap and also give information about the reactivity. Thus, a compound with less E_gap is considered as highly reactivity, soft and less stable. However, the compound with high E_gap is assessed to be least reactive, hard and more kinetically stable.(Tahir et al., 2017) The increasing order of softness is: THP8 < THP5 < THP4 < THP7 < THP1 < THP6 < THP3 < THP2 with values as: 0.347 < 0.3662 < 0.3687 < 0.384 < 0.398 < 0.400 < 0.413 < 0.423 eV. Interestingly, THP2 exhibit the highest softness value that made it more polarized as well as reactive chromophore which might holds probable PV aptitude.

Table 2 GRP values of the studied chromophores (THP1-THP8) in eV.

Compounds	IP	EA	X	η	μ	ω	σ
THP1	5.525	3.018	4.271	1.253	−4.271	7.277	0.398
THP2	5.381	3.017	4.199	1.182	−4.199	7.458	0.423
THP3	5.111	2.691	3.901	1.210	−3.901	6.288	0.413
THP4	5.080	2.368	3.724	1.356	−3.724	5.113	0.368
THP5	4.921	2.190	3.555	1.365	−3.555	4.629	0.366
THP6	5.349	2.846	4.097	1.251	−4.097	6.708	0.400
THP7	5.180	2.577	3.878	1.301	−3.878	5.779	0.384
THP8	5.009	2.128	3.568	1.440	−3.568	4.420	0.347

3.5

3.5 Reorganization energy

Reorganization energy (λ) analysis is another essential tool utilized to disclose the performance and working of OSCs. Generally, λ is inversely correlated with the charge mobilities (hole and electron) (Köse et al., 2007). The lesser the reorganization energy, the greater will be the carrier mobility of a molecule. Reorganization energy is fluctuated by altering particular conditions, but these variations majorly influenced by the cationic and anionic geometries. Anionic geometry epitomizes electron transferal from the donor substance, whereas cationic geometry denotes hole transmission from acceptor material (Afzal et al., 2020). Reorganization energy is characterized into λ_int and λ_ext, but we preferably choose λ_int by neglecting λ_ext. The values of λ_h and λ_e are computed using Eqs. (1) and (2) and the results are collected in Table 3.

Table 3 Computed reorganization energies of THP1-THP8 chromophores.

Compounds	*λ_e*	*λ_h*
THP1	−0.0002	0.0006
THP2	−0.0005	−0.000007
THP3	−0.0004	−0.0002
THP4	−0.0004	−0.0004
THP5	−0.0008	−0.0003
THP6	−0.0007	−0.0005
THP7	−0.0006	−0.0002
THP8	−0.00004	−0.0004

Units in eV.

From literature survey, it has been observed that Bary et al., reported A1-A5 compounds which showed λ_e values in the range of 0.0077–0.0007 eV while λ_h values was found in the range of 0.0403–0.0003 eV (Bary et al., 2021) that are somewhat comparable to our designed compounds. Likewise, Adnan et al., reported that λ_e and λ_h values of R and CS1-CS5 are found in the range of 0.0184–0.0104 eV and 0.0206–0.0163 eV, respectively (Adnan et al., 2021). Kayani et al., observed that compounds R and Rm1-Rm4 depicted λ_e of 0.2290–0.0038 eV whereas λ_h values are noticed in the range of 0.0108–0.0072 eV (Kayani et al., 2021). Furthermore, Mehboob et al., studied R and H1-H5 chromophores which showed λ_e and λ_h values in the range of 0.0088–0.0104 eV and 0.0090–0.0060 eV, correspondingly (Mehboob et al., 2021c). Siddique et al., reported that R and D1-D4 compounds depicted λ_e values in the range of 0.1915–0.1344 eV whereas λ_h values are noticed in the range of 0.0834–0.0629 eV (Siddique et al., 2020) which are overestimated than out entitled compounds. The computed values of electron motilities for THP1-THP8 are −0.0002, −0.0005, −0.0004, −0.0004, −0.0008, −0.0007, −0.0006, −0.00004 eV, respectively. Among all studied chromophores, THP5 depicted the lowest value of λ_e which shows greater electron transference rate between HOMO and LUMO. Likewise, THP6 and THP2 have significantly better charge mobilities due to their lesser λ_e values. The escalating order of λ_e for the all the entitled compounds is: THP5 < THP6 < THP2 < THP4 = THP3 < THP1 < THP8. Similarly, the theoretically computed λ_h values for THP1-THP8 are 0.0006, −0.000007, −0.0002, −0.0004, −0.0003, −0.0005, −0.0002, −0.0004 eV, correspondingly. The ascending order of λ_h for all the entitled compounds are: THP6 < THP4 = THP8 < THP5 < THP3 = THP7 < THP2 < THP1. This analysis disclosed that λ_e values of all the examined chromophores are observed lower than λ_h values except THP8. Overall, the reduction in terms of λ_e designates that all the chromophores are promising applicants for transferal of electrons and can be used as competent PV materials.

3.6

3.6 Open circuit voltage (V_oc)

Open Circuit Voltage (V_oc) is indispensable to elucidate the performance of OSCs and shows the maximum magnitude of voltage that can be taken out from any optically active material at zero current (Tang and Zhang, 2012). It plays an indispensable role to get insights into the working mechanics of organic semiconductor materials (Irfan et al., 2017). Numerous features influence the V_oc i.e., light source, light intensity, temperature of OSCs, external environmental proficiency, charge carrier recombination and several other environmental factors (Saleem et al., 2021). Primarily, V_oc is influenced by light generation and saturation voltage that helps in the recombination of PV devices. The E_gap of donor and acceptor i.e., HOMO_PBDB-T-LUMO_acceptor is directly associated with the V_oc. In the present quantum chemical calculations, PBDB-T donor molecule (Zheng et al., 2020) (which has E_HOMO of −5.401 eV) is used to compute the V_oc and the findings are summarized in Fig. 6. The calculated V_oc outcomes of THP1-THP8 by using Eq. (3) developed by Scharber (Scharber et al., 2006) are grouped in Table 4.

(3)

Voc = (|E_{HOMO}^{D}| - |E_{LUMO}^{A}|) - 0.3

Diagrammatic representation of Voc of THP1-THP8 with PBDB-T.

Table 4 V_oc of the entitled compounds (THP1-THP8).

Δ E = E_{LUMO}^{A} - E_{HOMO}^{D}

Compounds	*V_OC (V)*	$Δ$ E (eV)
THP1	2.083	2.383
THP2	2.084	2.384
THP3	2.410	2.71
THP4	2.733	3.033
THP5	2.911	3.211
THP6	2.255	2.555
THP7	2.524	2.824
THP8	2.973	3.273

The computed values of V_oc of THP1-THP8 with regards to the energy gap of HOMO_donor –LUMO_acceptor are examined to be 2.083, 2.084, 2.410, 2.733, 2.911, 2.255, 2.524 and 2.973 V, correspondingly. The V_oc of the examined compounds are perceived to be in the subsequent declining order: THP8 > THP5 > THP4 > THP7 > THP3 > THP6 > THP2 > THP1. The highest value of V_oc is noticed in THP8 at 2.973 V and this larger V_oc value demonstrates the contribution to their superior LUMO values. Moreover, the V_oc values majorly depends on the energy difference of a molecule that is an innate property of semiconductors. Having less energy gap means relaxed excitation and maximum photons would have greater energy than required for excitation producing more electricity with greater PV response and higher PCE. In this report, we made BHJ device by blending PBDB-T donor polymer with our designed acceptor molecules. When the complex developed, we observed that low level of LUMO of our acceptors enhance the transfer of charge carriers from HOMO of PBDB-T and thus escalate the transitions which results in higher efficiencies.

3.7

3.7 Transition density matrix (TDM)

TDM analysis assists in interpreting the transition process, (Ans et al., 2019) electronic excitation state and hole-electron overlapping (Mehboob et al., 2021a). In addition, it is very valuable in evaluating the behavior of electronic transitions in the excited (S₁) state, interaction among acceptor and donor entities accompanied by electron and hole localization as well as the extent of intramolecular charge transference (Ans et al., 2018). The determination of these parameters aids in evaluating the function of OSCs. M06 method in conjunction with 6-311G(d,p) basis set was used to ascertain the absorption and emission of all the designed chromophores (THP1-THP8). This analysis offers a 3D heat map with appropriate color discrepancy i.e., exhibited by blue region. The impact of hydrogen atoms has been overlooked due to their minute contribution in transition. The pictorial display of TDM analysis is given in Fig. 7.

TDM graphs of the titled molecules (THP1-THP8).

In the current examination, we split our molecules (THP1-THP8) into two different portions i.e., acceptor and π-spacer. From FMO investigation, it is revealed that charge transmission is substantially executed over the molecule. The TDM pictographs disclosed that, there is an effective diagonal transmission of electron density from π-spacer towards terminal acceptors in all the chromophores, permitting good charge transference without any confinement. Moreover, electron hole pair generation and excitation of charge coherence also appear to proliferate non-diagonally over the TDM plots. Besides, calculation of TDM maps of THP1-THP8 indicate schematic, simpler and progressive exciton dissociation in the S₁ state which proves valuable for future applications.

3.8

3.8 Exciton binding energy (E_b)

Binding energy (E_b) is another distinctive feature correlated to TDMs that is utilized to evaluate the PV response of OSCs. It is a significant parameter for the evaluation of exciton dissociation capability and columbic force of interaction among electron and hole. The E_b is in inverse relation to the exciton dissociation and directly associated with columbic interaction (Farhat et al., 2020; Khalid et al., 2022b). The E_b of THP1-THP8 is computed from the difference in E_gap of LUMO-HOMO and optimization energy (E_opt) as shown by Eq. (4) (Köse, 2012).

(4)

E_{b} = E_{L - H} - E_{opt}

In Eq. (4), E_L-H indicates the energy difference among HOMO/LUMO and E_opt represents the lowest amount of energy necessary for excitation form S₀ to S₁ state, which generates an electron-hole pair (Janjua et al., 2012b; Khan et al., 2019a). The calculated outcomes of E_b are grouped in Table 5.

Table 5 Computed E_L-H and E_opt of THP1-THP8, units are in eV.

Compounds	E_H-L	*E_opt*	*E_b*
THP1	2.507	2.121	0.386
THP2	2.364	2.008	0.356
THP3	2.420	2.104	0.316
THP4	2.712	2.299	0.413
THP5	2.731	2.308	0.423
THP6	2.503	2.109	0.394
THP7	2.603	2.186	0.417
THP8	2.881	2.417	0.464

The values of E_b for THP1-THP8 are calculated as 0.386, 0.356, 0.316, 0.413, 0.423, 0.394, 0.417 and 0.464 eV, correspondingly. THP3 gives the lowermost value (0.316 eV) of E_b exhibiting higher efficiency of charge dissociation in S₁ state as compared to the other chromophores. Generally, the compounds having 1.9 eV or lesser E_b value are regarded as effective PV candidates with inspiring V_oc. Remarkably, our titled molecules (THP1-THP8) has expressed lower values of E_b than 1.9 eV. The descending trend of E_b is: THP8 > THP5 > THP7 > THP4 > THP6 > THP1 > THP2 > THP3. So, THP3 molecule possessed lowest E_b elucidating greatest potential of charge separation with exceptional optical and electronic properties.

4 Conclusion

Conclusively, state-of-the-art quantum chemical procedures have been utilized to inspect the electronic and photophysical capabilities of eight pyrazoline-based NFAs (THP1-THP8), designed by altering terminal acceptor moieties. The structural tailoring has demonstrated as a significant strategy to acquire inspiring PV compounds with remarkable optoelectronic possessions for OSCs. FMO study depicted that least band gap of 2.364 eV is perceived in THP2, while for all the derivatives, E_gap values fall in the following range of 2.507–2.881 eV with effective charge transference rate that is further endorsed by the DOS and TDM analyses. Furthermore, GRPs studies disclosed that extensive conjugation gives exceptional stability to the studied chromophores. Likewise, THP1-THP8 displayed red shifted absorption spectra at 512.861–584.555 nm range in gas phase. The V_oc of THP1-THP8 with regards to HOMO_PBDB-T-LUMO_acceptor are observed with satisfactory values. The binding energy of the entitled chromophores (THP1-THP8) are found comparable with each other resulted in higher exciton dissociation. Furthermore, least reorganization energy in the above-mentioned chromophores for electron with hole is also examined. Amongst all the designed chromophores, THP2 owing to the strong electron accepting capability is recognized as a significant candidate for OSCs applications with outstanding PV properties comprising highest λ_max (617.482 nm), lowest energy gap (2.364 eV) and highest softness (0.423 eV). Our outcomes indicated that incorporating electron-accepting groups is an effective strategy for designing promising NF based OSCs. The designed molecules specifically THP2 is suggested for production of high-performance OSCs devices.

Acknowledgment

Dr. Muhammad Khalid gratefully acknowledges the financial support of HEC Pakistan (project no. 20-14703/NRPU/R&D/HEC/2021). A. A.C.B. acknowledges the financial support of the São Paulo Research Foundation (FAPESP) (Grants 2014/25770-6 and 2015/01491-3), the Conselho Nacional de Desenvolvimento Cientı́fico e Tecnológico (CNPq) of Brazil for academic sup- port (Grant 309715/2017-2), and Coordenac¸ ão de Aperfeic¸ oa- mento de Pessoal de Nı́vel Superior – Brasil (CAPES) that partially supported this work (Finance Code 001). The authors thank the Researchers Supporting Project number (RSP2023R6), King Saud University, Riyadh, Saudi Arabia. S.C.O. acknowledges the support from the doctoral research fund of the Affiliated Hospital of Southwest Medical University.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Appendix A

Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2023.105271.

Appendix A

Supplementary material

The following are the Supplementary data to this article:

Supplementary data 1

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Introduction

Computational study

Results and discussion

Frontier molecular orbital (FMO) analysis
Density of state (DOS)
Optical response
Global reactivity parameters (GRPs)
Reorganization energy
Open circuit voltage (Voc)
Transition density matrix (TDM)
Exciton binding energy (Eb)

Conclusion

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Photovoltaic response promoted via intramolecular charge transfer in pyrazoline-based small molecular acceptors: Efficient organic solar cells

Abstract

Keywords

Pyrazoline-based molecules

DFT

Optical properties

TDM

Open circuit voltage

1 Introduction

2 Computational study

3 Results and discussion

3.1 Frontier molecular orbital (FMO) analysis

3.2 Density of state (DOS)

3.3 Optical response

3.4 Global reactivity parameters (GRPs)

3.5 Reorganization energy

3.6 Open circuit voltage (Voc)

3.7 Transition density matrix (TDM)

3.8 Exciton binding energy (Eb)

4 Conclusion

Acknowledgment

Declaration of Competing Interest

References

Appendix A

Supplementary material

Appendix A

Supplementary material

Supplementary data 1

Suggested read for related articles:

3.6 Open circuit voltage (V_oc)

3.8 Exciton binding energy (E_b)