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The modification of fused rings in the central backbone, side chains, and end groups of the non‐fullerene acceptor (NFA) for organic solar cells (OSCs) can modulate properties and photovoltaic performance. In order to investigate the effect of fused rings and side chains on photovoltaic performance, PBDB‐T is selected as electron donor and IDTIC, IDIC, IDIC‐PhC6, IDIC‐C4Ph, 4TIC, ITIC‐OE, IDTTIC, ITC6‐IC, ITIC, and C8‐ITIC are selected as NFAs. Based on quantum chemistry calculations, the geometries, electronic structures, excitation properties, excited‐state lifetimes, and electrostatic potentials (ESPs) of the monomer and the PBDB‐T:NFA complexes are studied, and the rate constants of charge transfer (CT), exciton dissociation (ED) and charge recombination (CR) processes are analyzed. The results show that the increase of fused‐ring in NFA's backbone by substituting phenyl or thienyl with bithiophene can elevate the highest occupied molecular orbital energies, reduce the gap of frontier molecular orbital energies, induce red‐shift of absorption spectrum, increase CT energy, improve CT and ED rates. This work provides a detailed understanding of tuning optoelectronic properties and photovoltaic performance by modifying NFA's side chains and extending backbones. [ABSTRACT FROM AUTHOR]

The optoelectronic properties of formamidinium CH(NH2)2+ (FA) doped double perovskite Cs2AgBiBr6 with corresponding formula Cs1.875FA0.125AgBiBr6 were investigated based on electronic structures and supercell models. The dopant formation energies, tolerance and octahedral factors support the doped structures are stable. FA dopant increases band gap about 0.15 eV without introducing dopant states in gap, but also increases the reduced effective masses, exciton binding energies, and elastic constants. However, it decreases exciton energies and charge carrier mobilities. Though the energy barriers among the different FA orientations in Cs1.875FA0.125AgBiBr6 are quite small (<30 meV), FA orientations in doped systems can remarkably impact on electron and hole mobilities. [ABSTRACT FROM AUTHOR]

• Introducing Ag dopant into CH 3 NH 3 PbI 3 is endothermic. • High concentration of Ag dopant increases the band gap. • Ag dopant leads to the increase of hole and electron effective masses. • Ag dopant enhances the exciton binding energy. In order to improve power conversion efficiency and stability, doping is commonly adopted strategy to tune and modify the structures and properties of CH 3 NH 3 PbI 3 materials in organic-inorganic hybrid perovskite emerging solar cells. Here, to understand the photovoltaic performance of Ag-doped perovskite CH 3 NH 3 PbI 3 at high dopant concentration, based upon density functional theory calculations for crystal structures, dopant formation energies, energy bands, density of states, optical absorptions, effective masses and exciton binding energies, we studied the effects of Pb replacement by Ag on electronic structures, optical and charge transport properties of perovskite CH 3 NH 3 PbI 3 with cubic, tetragonal and orthorhombic phases at Ag concentration of 12.5%. It was found that, introducing Ag dopant induces more significant distortion of AgI 6 octahedral. Also, introducing Ag dopant is possible to change the order of phase stability, and to improve the optical absorption properties of tetragonal phase. Furthermore, the Ag dopant shifts Fermi level to deep energy and slightly increases band gap without introducing extra dopant states. Whereas, the increased electron/hole effective masses by introducing Ag-dopant at this dopant concentration lead to the reduced charge carrier mobilities and imbalanced charge transport properties, and also result into more difficult exciton dissociation. [ABSTRACT FROM AUTHOR]

• Fluorination of end groups, thiophene modification. • Increasing the number of central rings. • These modifications can decrease the H-L gap. • Fluorine substitution promotes charge transfer at the heterojunction interface. • Increasing the central ring number lowers CR and enhances ED efficiency. Modifying the skeleton, side chains and end groups of non-fullerene acceptors (NFAs) is important approach to improve the photovoltaic performance of organic solar cells. We selected PBDB-T as the donor and INIC, INIC1, INIC2, ITCC, ITCPTC and ITIC as the acceptors in order to study the difference in NFAs. The geometries, electronic structures, excited state properties, excited-state lifetimes and rate constants of charge transfer (CT), charge recombination (CR) and exciton dissociation (ED) processes of the monomers and PBDB-T:NFA complexes have been investigated by means of density functional theory and time-dependent density functional theory. The research reveals that fluorination of end groups, thienyl substitution and increase in the number of central rings result in the decrease in the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. Fluorine substitution and increase in central ring numbers cause redshift of the absorption spectrum for NFAs. NFAs with fluorine substitution have a larger maximum electrostatic potential, which facilitates CT at the donor:NFA heterojunction interfaces. Fluorine substitution and thienyl substitution decrease the NFA's CT distances, indicating that both fluorine and thienyl substitutions affect the CT rate. Fluorine substitution, thienyl modification of end groups and increase in central ring numbers also reduce the exciton binding energy, which is beneficial for decreasing CR and improving ED efficiency. [ABSTRACT FROM AUTHOR]

The optoelectronic properties of formamidinium CH(NH2)2+ (FA) doped double perovskite Cs2AgBiBr6 with corresponding formula Cs1.875FA0.125AgBiBr6 were investigated based on electronic structures and supercell models. The dopant formation energies, tolerance and octahedral factors support the doped structures are stable. FA dopant increases band gap about 0.15 eV without introducing dopant states in gap, but also increases the reduced effective masses, exciton binding energies, and elastic constants. However, it decreases exciton energies and charge carrier mobilities. Though the energy barriers among the different FA orientations in Cs1.875FA0.125AgBiBr6 are quite small (<30 meV), FA orientations in doped systems can remarkably impact on electron and hole mobilities. [ABSTRACT FROM AUTHOR]

Abstract Understanding electronic structures and excitation properties is a fundamental issue to develop novel electron donor (D) and acceptor (A) molecular dyad that can be applied in optoelectronic systems. Here, the geometry, electronic structures, and excitation properties of three meso-pentafluorophenyl substituted zinc porphyrin fullerene ((F 15 P)ZnP C 60) dyad were analyzed based on density functional theory (DFT) and time dependent DFT (TDDFT) calculations. The geometrical parameters provide the D-A distance is about 18.19 Å. The electronic structure analysis indicates that the highest occupied molecular orbital and the lowest unoccupied molecular orbital are localized in porphyrin core and C 60 , respectively. The transition configurations and the MOs suggest the excited states at the absorption maxima are local excited states, while the charge transfer (CT) states are transient intermediates. The population analysis supports the first singlet/triplet excited states are local excitations in porphyrin (LEP). Quasi-degeneracy of excited states between LEP and local excitation in C 60 (LEC) enable the partial delocalization of eigenstates and excitation-energies over the D and A in dyad, while the quasi-degeneracy between CT and LEP generate synergistic enhancement effects for CT. The results of this work could be helpful to understand optoelectronic properties of (F 15 P)ZnP C 60 dyad. Graphical abstract Image 1 Highlights • The electronic structure of (F 15 P)ZnP C 60 dyad is analyzed. • The transition configurations and the MOs of (F 15 P)ZnP C 60 dyad are calculated. • The population of (F 15 P)ZnP C 60 dyad are studied. • Quasi-degeneracy of excited states between LEP and local excitation in C 60 (LEC), and between CT and LEP are discussed. [ABSTRACT FROM AUTHOR]

As electronic acceptor materials in heterojunction, fullerene derivatives (FDs) can significantly affect the power conversion efficiency of organic solar cells (OSCs). Here, in order to investigate the number and size effects of fullerene-cage in FDs for OSCs, the geometries, electronic structures and related properties, as well as photovoltaic parameters of several FDs, including MP, PCBM, BP, TP and PC 71 BM, were analyzed based upon density functional theory (DFT) and time dependent DFT calculations. The results indicate that the fullerene-size is more important than the number of fullerene-unit to affect the local geometrical parameters, energy level alignments, hyper-polarizabilities, and optical absorptions in visible region. Furthermore, the lowest unoccupied molecular orbital (LUMO) energies are almost same for these FDs, and the orbital energies near frontier molecular orbitals for BP and TP exhibit two- and three-fold quasi-degenerate, respectively. The transition configurations and molecular orbitals reveal the absorption bands in visible region of MP, PCBM, PC 71 BM and TP are local excitations, and that of BP are charge transfer excitations. The similar open-circuit voltages and fill factors of OSCs based upon P3HT/MP, P3HT/PCBM, P3HT/BP, P3HT/TP, and P3HT/PC 71 BM blend films result from the similar LUMO energy levels of these FDs. [ABSTRACT FROM AUTHOR]

The design and development of novel dye sensitizers are effective method to improve the performance of dye-sensitized solar cells (DSSCs) because dye sensitizers have significant influence on photo-to-current conversion efficiency. In the procedure of dye sensitizer design, it is very important to understand how to tune their electronic structures and related properties through the substitution of electronic donors, acceptors, and conjugated bridges in dye sensitizers. Here, the electronic structures and excited-state properties of organic JK dye sensitizers are calculated by using density functional theory (DFT) and time dependent DFT methods. Based upon the calculated results, we investigated the role of different electronic donors, acceptors, and π-conjugated bridges in the modification of electronic structures, absorption properties, as well as the free energy variations for electron injection and dye regeneration. In terms of the analysis of transition configurations and molecular orbitals, the effective chromophores which are favorable for electron injection in DSSCs are addressed. Meanwhile, considering the absorption spectra and free energy variation, the promising electronic donors, π-conjugated bridges, and acceptors are presented to design dye sensitizers. [ABSTRACT FROM AUTHOR]

[Display omitted] • The light harvesting capability of EH-IDTBR is far better than that of PC 71 BM. • Most of excited states of EH-IDTBR exhibit intramolecular charge transfer characters. • The many merits of PffBT4T-2OD/EH-IDTBR can prevent " burn-in " degradation. • The faster rates of ED/CT of PffBT4T-2OD/EH-IDTBR can resist "burn-in" degradation. In order to clarify the different photovoltaic performance and "burn-in" degradation mechanism, the electron donor poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3′″-di(2-octyldodecyl)-2,2′;5′,2″;5″,2′″-quaterthiophen-5,5′″-diyl)] (PffBT4T-2OD), fullerene derivative acceptor [6,6]-phenyl C 71 butyric acid methyl ester (PC 71 BM) and the nonfullerene acceptor indacenodithiophene core flanked with benzothiadiazole and rhodanine groups with the branched (2-ethylhexyl) chains (coded as EH-IDTBR) were selected to construct PffBT4T-2OD/EH-IDTBR and PffBT4T-2OD/PC 71 BM complexes as heterojunction interface models. The geometries, electronic structures, excitation properties and the rates of electron processes were extensively explored based upon quantum chemistry calculations. The results indicate that the light harvesting capability of EH-IDTBR is far better than that of PC 71 BM. Most of excited states of PffBT4T-2OD and EH-IDTBR exhibit intramolecular charge transfer characters. Compared with PffBT4T-2OD/PC 71 BM, the better planarity, stronger non-bonding interactions, and the hybrid excited states, as well as the larger electrostatic potential difference between the donor and acceptor of PffBT4T-2OD/EH-IDTBR are responsible for its better photovoltaic performance and the lower "burn-in" degradation. Although the rates of charge recombination of PffBT4T-2OD/EH-IDTBR complex is larger, its faster rates of exciton dissociation and charge transfer processes can be helpful to resist "burn-in" degradation. The results are helpful to develop novel OPV materials that are resistant to "burn-in" degradation. [ABSTRACT FROM AUTHOR]

The commonly existed vacancy defects in semiconductors can affect optoelectronic properties. Here, to understand the vacancy defect influences on double perovskite Cs 2 AgBiBr 6 , based upon density functional theory calculations and supercell model, we systematically investigated Cs, Ag, Bi, Br, Cs–Br and Ag–Br atomic pair vacancy effects on crystal structure, electronic structures, optical absorption, charge carrier and exciton binding energies. It was found that, the vacancy defects in double perovskite Cs 2 AgBiBr 6 induce slight deformation of crystal lattice. The vacancy defects cannot introduce extra defect states in the gap of energy band. The Cs, Ag, Bi, Ag–Br and Cs–Br defects not only change the band gap into the direct from the indirect of pristine system and reduce the band gap, but also promote the optical absorption capability, and result in red-shift of absorption spectra in low energy region. The Br vacancy leads to a heavy dopant character due to significant elevation of the Fermi level. The Cs, Ag, Bi, Ag–Br and Cs–Br vacancy defects also generate imbalanced charge transport properties, and increase exciton binding energies. • Vacancy defects cause small deformations of the crystal lattice. • No defect state is found in the band gap after introducing vacancy defects. • Vacancy defects promote light absorption and generate a red-shift. • Vacancy defects can cause unbalanced charge transport properties. [ABSTRACT FROM AUTHOR]

Single component molecular dyad donor-acceptor junction is an important type of organic solar cells. Understanding the optoelectronic properties of molecular dyad plays the critical role to develop active layer materials for such kind of solar cells. Here, diathiafulvalene-functionalized diketopyrrolopyrrole–fullerene (DFDPP-Ful) was selected as the representative system, and the geometries, electronic structures and excitation properties of DFDPP-Ful monomer and dimer were systematically investigated based on extensive quantum chemistry calculations. The transition configurations and molecular orbitals show that the effective electron donor and acceptor are DFDPP and fullerene moieties, respectively. It also found the light harvesting is dominated by local excitation in DFDPP moiety. Meanwhile, the hybridization and quasi-degeneration between charge transfer (CT) and local excitation exist. The dimer data suggest that the increased excited states contribute to the expanding of absorption spectra, and the excitations exhibit both the intermolecular and intra-molecular CTs. Also, the remarkable CT energy differences among the different dimer models for the lowest CT excited states support the strong interface and energy disorder in such system. Therefore, the suggestions for developing molecular dyad of single component organic solar cells would be the combination of increasing light absorption, enhancing CT and local excitation hybridization, as well as suppressing energy and interface disorder by the aid of molecular design. Unlabelled Image • DFDPP-Ful was selected as representative molecular dyad for single component OPV. • The electronic structures of DFDPP-Ful monomer and dimer were calculated. • The excitation properties of DFDPP-Ful monomer and dimer were analyzed. • The strong interface and energy disorders exist in such system. [ABSTRACT FROM AUTHOR]