Effect of the Push–Pull Electronic Structure of Nonfullerene Acceptor Molecules on the Interfacial Charge Dynamics in Heterojunction Organic Solar Cells

Abstract

Benefitting from the development of nonfullerene acceptors (NFAs), the power conversion efficiency (PCE) approaching 19% has been achieved in single-junction organic solar cells (OSCs). [1,2] Compared with fullerene acceptors (FAs), NFAs present different aspects of advantages, such as the tunable energy levels, [3,4] strong absorption in the visible-to-near-infrared wavelength range, [5,6] and various molecular aggregation structures. [7,8] Despite molecular diversity, most of the highperformance NFA molecules (e.g., ITIC and Y6) have push-pull electronic structure, where the central group has stronger electron push ability, the terminal groups have stronger electron pull ability, and sometimes π-bridges connecting the central group and the terminal groups. [9,10] Up to now, different strategies have been explored to strengthen the electron push ability of the central group, such as extending the conjugation length, [11,12] introducing strong electron push group (e.g., alkoxy or alkyl amino side groups), [13,14] and inserting quinone resonance structure. [15] To strengthen the electron pull ability of the terminal groups, one usually introduces halogen atoms (e.g., fluorine and chlorine). [16][17][18][19] The push-pull electronic structure of NFA molecules can result in their intramolecular charge transfer character, reducing the binding energy of their excited states. [20][21][22][23] Recent works have successively reported that the excited states in NFA molecules might behave in intra-and intermolecular charge transfer (CT) states, [24,25] which can dissociate into free charges even at room temperature without the assistance of a donor/acceptor (D/A) energy offset. [24][25][26] In FA-based OSCs, we know that polymer donors usually act as the light-absorbing components due to the poor absorption of FA molecules, [27] such that charges mainly originate from the donor excitation via experiencing the interfacial electron transfer into FA molecules. [28] In NFA-based OSCs, polymer donors and NFA molecules usually have complementary absorption; thus, charge generation presents diverse channels. When the donor molecule is photoexcited, the excited electron transfers into NFA molecule. [20,22,23] When NFA molecule is photoexcited, the excited hole transfers into donor molecule. [10,21,25] Recently, a third charge generation channel was revealed in

Revealing the Role of Donor/Acceptor Interfaces in Nonfullerene-Acceptor Based Organic Solar Cells: Charge Separation versus Recombination

“…For typical NFA based heterosystems, photoexcitation in NFA molecules plays a more important role in the total absorption, since NFA molecules present stronger absorption in the visible to near-infrared wavelength range than polymer–donor molecules . More interestingly, experimental − and theoretical , observations have both confirmed that donor excited states can also transfer into NFA molecules via the energy transfer process before charge transfer and turn into NFA molecular excited states. Therefore, in typical NFA based heterosystems, NFA molecular excited states usually dominate the charge processes, such that we only consider the photoexcitation in NFA molecules.…”