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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]

Understanding organic photovoltaic (OPV) work principles and the materials' optoelectronic properties is fundamental for developing novel heterojunction materials with the aim of improving power conversion efficiency (PCE) of organic solar cells. Here, in order to understand the PCE performance (>13%) of OPV device composed of the non-fullerene acceptor fusing naphtho[1,2-b:5,6-b′]dithiophene with two thieno[3,2-b]thiophene (IDCIC) and the polymer donor fluorobenzotriazole (FTAZ), with the aid of extensive quantum chemistry calculations, we investigated the geometries, molecular orbitals, excitations, electrostatic potentials, transferred charges and charge transfer distances of FTAZ, IDCIC and their complexes with face-on configurations, which was constructed as heterojunction interface model. The results indicate that, the prominent OPV performance of FTAZ:IDCIC heterojunction is caused by co-planarity between the donor and acceptor fragments in IDCIC, the the charge transfer (CT) and hybrid excitations of FTAZ and IDCIC, the complementary optical absorptions in visible region, and the large electrostatic potential difference between FTAZ and IDCIC. The electronic structures and excitations of FTAZ/IDCIC complexes suggest that exciton dissociation can fulfill through the decay of local excitation exciton in acceptor by means of hole transfer, which is quite different from the OPVs based on fullerenes acceptor. The rates of exciton dissociation, charge recombination and CT processes, which were evaluated by Marcus theory, support the efficient exciton dissociation that is also responsible for good photovoltaic performance. [ABSTRACT FROM AUTHOR]

Molecular stacking modes that determine morphology of bulk‐heterojunctions are important because of their effects on photovoltaic performance of organic solar cells (OSCs). Here, in order to investigate how molecular stacking pattern in OSCs heterojunction impact on electronic structures and properties, and then influence photovoltaic performance, this work selected D18:Y6 OSC as a representative system. On the basis of semi‐empirical quantum chemistry and density functional theory calculations, we studied the geometries, electronic structures and excitation properties, as well as electron and hole couplings of D18 and Y6 molecules, Y6 dimers, D18/Y6 and D18/Y6‐dimer complexes which are heterojunction interface models. The results indicate that the molecular stacking effects on open‐circuit voltage (VOC) can be attributed to the influences on charge transfer (CT) excitation energies because molecular stacking modes at donor/acceptor heterojunction interface can significantly change excitation energies. Furthermore, the molecular stacking modes can cause drastically different electron and hole couplings, and then affect charge transfer/transport rates. Hence, the molecular stacking effects on short‐circuit current density (JSC) can be understood from the effects on electron and hole couplings. The molecular stacking modes that are favorable to improving VOC and JSC of D18:Y6 OSC were also discussed. [ABSTRACT FROM AUTHOR]

Modifying the backbone, side chains and end groups of non-fullerene acceptors (NFAs) in organic photovoltaics (OPVs) can tune their properties and impact performance. Hence, understanding how different functional groups influence properties and performances is critical for developing novel NFAs for OPVs. Herein, to investigate the effects of introducing methyl groups and substituting dicyano with O, based upon extensive quantum chemistry calculations, we selected a PM6:IT-4F heterojunction as the reference system and studied the geometries, electronic structures, excitation properties, and electrostatic potentials (ESPs) of PM6, IT-4F, IM-4F, and IO-4F as well as PM6:IT-4F, PM6:IM-4F, and PM6:IO-4F complexes as interface models. The rate constants of the charge transfer (CT), exciton dissociation (ED), and charge recombination processes at the heterojunction interface were also calculated. The results indicated that the superior performance of PM6:IM-4F and PM6:IT-4F systems could be ascribed to the redshift of optical absorptions that assure more photon harvest, larger ESP difference between PM6 and NFAs, and larger excitation energy difference between NFA's local excitation and CT that is favorable for efficient ED. However, the larger open-circuit voltage of PM6:IO-4F OPV results from the substitution of dicyano with O, causing a significant increase in the lowest unoccupied molecular orbital and CT excitation energies of PM6:NFA complexes, whereas the smallest short-circuit current density is mainly related to less CT excitations, blue-shift optical absorption that is not complementary with that of PM6, inefficient ED, and the smaller ESP difference between PM6 and IO-4F. [ABSTRACT FROM AUTHOR]

Molecular engineering of electron donors and acceptors for organic bulk heterojunction solar cells is critical to develop novel materials with improved power conversion efficiency. Here, the non-fullerene acceptors (NFAs) BTPT-4F and BTPTT-4F combined with an electron donor, P2F-EHp, were taken as a representative system to theoretically study the effects of the fusion of thienyl into the backbone of NFAs. On the basis of extensive quantum chemistry calculations, it was found that fusing thienyl into the backbone elevated the highest occupied molecular orbital (HOMO) energy, reduced the gap between the HOMO and the lowest unoccupied molecular orbital, decreased the dipole moment, enhanced the light absorbance and generated red-shifts in the absorption bands. The data for the P2F-EHp/BTPT-4F and P2F-EHp/BTPTT-4F complexes indicate that fusing thienyl into the backbone of the electron acceptor modifies the excitation character of the donor/acceptor complexes, increases the charge transfer (CT) excitation energy and the number of local excitation and CT hybridized low-lying excited states, amplifies the CT rate, reduces the exciton dissociation rate, and dramatically suppresses the charge recombination (CR) rate, leading to better performance of BTPTT-4F. However, the poorer performance of BTPT- 4F can mainly be attributed to its larger dipole moment and faster CR rate. [ABSTRACT FROM AUTHOR]

• The introducing of electric field can modify the interface energy level alignments. • The CT energetic loss increases with the increasing of interfacial electric field. • Electric field effectively modulate the dynamics of interface electronic processes. • Electric field pointing from A to D with suitable strength dramatically suppresses CR. The applied electric field on organic heterojunction interface can significantly affect organic photovoltaic (OPV) performance. Here, to explore electric field effects on OPV heterojunction interface, the pentacene/C 60 complexes with face-on and edge-on configurations were constructed as model system, based upon quantum chemistry calculations that take into account of solid polarization effects and apply density functional with optimally tuned range separation parameters, we systematically studied the electric fields effects on the complexes' geometries, electronic structures, excitation and spectral properties, as well as the rate constants of charge transfer (CT), exciton dissociation (ED) and charge recombination (CR) processes. The electric field can effectively modify interface electronic structures and energy level alignments, and the increasing of electric field that is perpendicular to interface can increase CT energetic loss, reduce the lowest CT excitation energy and the open-circuit voltage. The introducing electric field can modulate the kinetics of electron processes at heterojunction interface. [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]