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Two small molecules BYG-1 and BYG-2 with fluorene donor and benzothiadiazole acceptor units connected to the terminal naphthamide group via ethyne linker were designed and synthesized. In this work we have discussed the effect of fluorine atoms connected with electron withdrawing benzothiadiazole unit to the fluorene core (BYG-1). In this study, we have fabricated solar cells with small-molecular donor and acceptor materials in the device architecture of bulk-heterojunction, using highly conjugated BYG-1 and BYG-2 as electron acceptors along with an appropriate small molecule donor (SMD). After improving the device architecture of the active layer using a suitable donor-to-acceptor weight ratio with solvent vapour annealing, we achieved power conversion efficiencies of 8.67% and 7.12% for BYG-1 and BYG-2, respectively. The superior photovoltaic performance of the fluorine-substituted BYG-1 can be attributed to its higher crystallinity, more balanced charge transport mobilities and efficient exciton dissociation.

The design and fabrication of solar cells have recently witnessed the exploration of non-fullerene-based acceptor molecules for higher efficiency. In this study, the optical and electronic properties of four new three-dimensional (3D) star-shaped acceptor molecules (M1, M2, M3, and M4) are evaluated for use as acceptor molecules in organic solar cells. These molecules contain a triphenylamine donor core with diketopyrrolopyrrole acceptor arms linked via a thiophene bridge unit. Molecules M1–M4 are characterized by different end-capped acceptor moieties, including 2-(5-methylene-6-oxo-5,6-dihydrocyclopenta-b-thiophen-4-ylidene)malononitrile (M1), 2-(2-methylene-3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (M2), 2-(5-methyl-2-methylene-3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (M3), and 3-methyl-5-methylnene-thioxothiazolidin-4-one (M4). The properties of the newly designed molecules were compared with a well-known reference compound R, which was recently reported as an excellent acceptor molecule for organic solar cells. Molecules M1–M4 exhibit suitable frontier molecular orbital patterns for charge mobility. M2 shows maximum absorption (λ_max) at 846.8 nm in dichloromethane solvent, which is ideal for the design of transparent solar cells. A strong electron withdrawing end-capped acceptor causes a red shift in absorption spectra. All molecules are excellent for hole mobility due to a lower value of λ_h compared to the reference R.

Due to limitations of fullerene based acceptor molecules for solar cell applications, research is recently diverted to explore non-fullerene acceptor molecules. In this regard, four new A-D-A type fused ring electron acceptor molecules (M1, M2, M3 and M4) are evaluated for their opto-electronic properties for transparent organic solar cells. These molecules contain strong electron donor undecacyclic linked with four different acceptor moieties, 2-(3-ethly-5-methylene-4-oxothiazolidin-2-yluidene)malononitrile (M1), 2-(5,6-dicyano-2-methylene-3-oxo -2,3-dihydroindene-1-ylidene)malononitrile (M2), 2-(5-methylene-6-oxo-tetrahydro-1H-cyclopenta-thiophene-4(5H)-ylidene)malononitrile (M3), and 3-ethyl-5-methylene-2-thioxothiazolidin-4-one (M4). The electronic and optical properties of these molecules are compared with the reference molecule R, which is recently reported as excellent non-fullerene based acceptor molecule. Among all molecules, M2 exhibits the maximum red shift where absorption appears 893.5 nm with B3LYP/6-31 + + G(d,p) level of theory due to highly extended conjugation between electron withdrawing end-capped acceptor moieties. The calculated Open circuit voltage (V-oc) of reference molecule R is 1.78 eV with donor polymer PTB7-Th while molecule M2 exhibits the V-oc value of 1.86 eV.

Three small molecules denoted SM1, SM2 and SM3, with the phenothiazine donor moiety connected to benzothiadiazole and 3-ethylrhodanine acceptor units through thiophene π-linkers have been synthesized for use in organic solar cells with PC71BM as electron acceptor. SM1 has a 2-ethylhexyl group at the phenothiazine nitrogen, while SM2 and SM3have a 2-(2-methoxyethoxy)ethyl group at that N. Opto-electronic and dielectric properties, charge carrier mobilities, morphology of active layers, and photovoltaic properties were investigated in detail. The three molecules have wide absorption bands with high molar absorption coefficients and relatively low HOMO levels (−5.21 to −5.27 eV). Compared with the N-alkylated SM1, both SM2 and SM3 exhibit red-shifts of the long-wavelength absorption band in thin films, and show enhanced crystallinity in thin films with smaller stacking distances, higher hole mobility, and higher dielectric constant. After solvent vapourannealing, the power conversion efficiencies (PCEs) were significantly improved for the solar cell devices, from 1.69 to 3.95% for SM1, 2.78–6.62% for SM2 and 3.22–7.16% for SM3. This increase in PCEs was due to the enhancement in Jsc and FF attributed to the formation of nanoscale domains of donor and acceptor resulting in efficient charge separation, balanced charge transport and suppressed charge recombination. These results demonstrate that use of an oxygen-containing side chain, in conjunction with alkyl side chains for solubility during solar cell processing, is an alternative and effective strategy for achieving high-performance small molecule donor materials.

Four new three-dimensional (3D) acceptor-acceptor-donor (A-A-D) type of small molecule acceptors (Ml, M2, M3 and M4) were designed for better optoelectronic properties in organic solar cells. These molecules contain spirobifluorene as a 3D core unit, flanked with 2,1,3- benzothiadiazole (BT) units linked with the end-capped acceptor groups 2-(4-oxo-4,5-dihydrocyclopenta-b-thiophene-6-ylidene)malononitrile (M1), 2-(3-oxo-2,3-dihydro-1H-indene-1-indene-1-ylidene)malononitrile (M2), 2-(5,6-difluoro-3-oxo-2,3-dihydroindene-1-ylidene) malononitrile (M3) and 2-(5,6-dimethyl-3-oxo-2,3-dihydroindene-1-ylidene)malononitrile (M4). The optoelectronic properties of M1 -M4 were compared with the well-known reference molecule R, which has the same central BT-spirobifluorene-BT structure as Ml-M4 but is end-capped with the 2-(2-dicyanomethylene)-3-ethyl-4-oxo-thiazolidin-5-ylidenemethyl group. Among these molecules, M3 has the most appropriate frontier molecular orbital diagram for optoelectronic properties as deduced from MPW1PW91 calculations and also shows the maximum absorption peak at longest wavelength (569 nm) by TD-MPW1PW91 calculations with a polarizable continuum model for chloroform solution. These properties are due to the strong electron-withdrawing end-capped acceptor group which causes a red shift in the absorption spectrum. Computed reorganization energies indicate that the electron mobilities for M1-M4 are higher compared to that of reference R.

Theoretical analysis of physical properties of organic solar cells (OSCs) are important in order to reveal the correlation between power conversion efficiencies (PCE), structure and properties. Five new A-D-A type small molecules M-1, M-2, M-3, M-4, and M-5 were designed by using dithienopyrrole (DTP) as electron rich donor unit with different types of pi-spacers and end capped acceptor units. Functional MPW1PW91/6-31G(d,p) level of theory was used to optimize the geometry of all molecules. For excited state calculation TD-MPW1PW91/6-31G(d,p) level of theory was used. The geometries, electronic structures, dipole moment, open circuit voltage, reorganization energies and charge transport properties of designed molecules M-5) have been scrutinized comparing with the reported compound R. The results revealed that the HOMO energy levels of molecules M-1, M-2, M-3, and M-5 were lower while M-4 was of high energy level thus facilitate the donation of electron as compared to references molecule R. While LUMO energy level of all the molecules were slightly high energy due electron withdrawing effects of spacer and acceptor moiety. Highest energy gap of HOMO-LUMO was observed in M-1 which was 2.48 eV and M-3 showed low energy gap (2.11 eV) as compared to other designed molecules. All molecules showed low values for lambda(e), so they have high rate of electron transfer as compared to R. All designed molecules exhibited higher value of dipole moment as compared to reference molecule R except M-1. Higher value of dipole moment of donor molecules contrary to reference means good solubility towards organic solvents which is beneficial for further solar cell device fabrication. All designed molecules show higher V-oc values except M-4 which has comparable V-oc with respect to reference molecule R. In short, choice of appropriate electron withdrawing and donating groups is very important for improving power conversion efficiencies of OSCs.

Three compounds consisting of the electron-donating triphenylamine–phenothiazine conjugate backbone and each of the electron-withdrawing groups 3-ethylrhodanine, malononitrile and 1,3-indandione have been synthesized and used as donors in blends with [6,6]-phenyl-C70-butyric acid methyl ester (PC₇₁BM) for organic solar cell devices. After improvements of the active layer structure by a selected donor-to-acceptor weight ratio and a two-step solvent and thermal annealing, the organic solar cells showed power conversion efficiency (PCE) values in the range of 4.79–7.25%. The highest PCE was obtained for the bulk heterojunction device with the indandione compound, which can be attributed to its better absorption profile, higher crystallinity, more balanced electron and hole transport, higher charge collection efficiency and reduced recombination, in comparison with the photovoltaic cells from the other two compounds. DFT-calculated characteristics, absorption spectra and cyclic voltammetry of the compounds, along with X-ray diffraction patterns of the blend films, are used to validate the photovoltaic results.

Five new Acceptor-Donor-Acceptor (A-D-A) type small donor molecules (M₁-M₅) namely; Dimethyl cyanoacetate terthiophene di(methylthiophene) dibenzosilole (DMCAO3TBS) (M1), Dimelononitrile terthiophene di(methylthiophene) dibenzosilole (DMCNTBS) (M2), Dimethyl rhodanine terthiophene di(methylthiophene) dibenzosilole (DMRTBS) (M₃), Dimelanonitrile terthiophene di(methylthiophene) methyl carbazole (DMCNTCz) (M₄) and Dimethyl rhodanine terthiophene di(methylthiophene) methyl carbazole (DMRTCz) (M₅) were designed and theoretically explored their electronic, photophysical and geometrical properties via DFT best functional MPW1PW91/6-311G (d,p) with respect to reference molecules Dioctyl cyanoacetate terthiophene di(octylthiophene) dioctylfluorene (DCAO3TF) (R_a) and Dioctyl cyanoacetate terthiophene di(octylthiophene) octylcarbazole (DCAO3TCz) (R_b). Among the designed donor molecules (M₁-M₅), M₂ and M₄ represented lowest band gap value (2.480 eV and 2.47 eV) with distinctive broad absorption peak at 598 nm and 601 nm in chloroform. Theoretically estimated reorganization energies of these molecules recommended excellent property of charge mobility. The designed donor molecules (M₁-M₅), demonstrated lower λ_e value with reference to their λ_h, showing that these molecules could be ideal candidates for the transfer of electron while M₂ and M₄ were found to be best molecules having lowest λ_e (0.006 eV and 0.005 eV respectively). Additionally the Voc of M₂ and M₄ are 2.01 eV and 1.85 eV respectively with respect to PCBM.

Three dimensional (3D) acceptor‐donor‐acceptor (A−D‐A) type small molecules (M1, M2, M3 and M4) are theoretically investigated for optoelectronic properties. The designed molecules contain spirobifluorene as core unit linked with end capped acceptors through four four thieno‐[3,2‐b]Thiophene (TT) units. The end capped acceptors are (3‐methyl‐2‐thioxothiazolidin‐4‐one) (M1), 2‐(2‐ethylidene‐5,6‐difluoro‐3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile (M2), 2‐(3‐ethyl‐4‐oxothiazolidin‐2‐ylidine)malononitrile (M3) and 2‐(2‐ethylidene‐5,6‐dicyano‐3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile (M4). The photovoltaic parameters of the designed molecules are compared with the recently reported reference compound R. Among all designed molecules, M4 is a low energy gap material (2.28 eV), broad absorption which is attributed to excellent communication between strong electron withdrawing end capped acceptors through extended conjugation. All newly designed molecules have lower binding energy as compared to reference molecule R which results in higher exciton dissociation in excited state. The reorganization energy calculations indicate good charge transfer ability of the designed molecules. M4 shows the lowest λ_e (0.0022) value with respect to the reference molecule R (0.034) which signifies its enhanced electronic transport behavior. The calculated open circuit voltages (V_oc) ranges from 1.97 to 2.36 eV, 2.11 to 2.49 eV and 1.9 eV to 2.28 eV with respect to three different well known donor materials PTB7‐Th, PBDB−T and P3HT, respectively.

Two new acceptor–π(donor)–donor–π(donor)-acceptor small molecules SM1 and SM2 with benzodithiophene (BDT) donor core unit linked via phenothiazine (PTZ) donor units with 1,3–indanedione and malononitrile end capping units, respectively, have been designed and synthesized and used as electron donor along with PC₇₁BM as acceptor for solution-processed bulk heterojunction organic solar cells. The influence of these end-capping groups on their thermal, optical, electrochemical properties and photovoltaic performance was investigated. After the optimization of the donor to acceptor weight ratio and solvent vapor annealing, the organic solar cells based on SM1:PC₇₁BM and SM2:PC₇₁BM active layers showed the power conversion efficiency (PCE) of 6.20% and 7.45%, respectively. Hence, the end-capping acceptor units for the two small molecules revealed a significant influence on the PCE of the organic solar cells, which resulted from differences in film absorption, molecular packing and charge transport properties.