Design of Cyanovinylene‐Containing Polymer Acceptors with Large Dipole Moment Change for Efficient Charge Generation in High‐Performance All‐Polymer Solar Cells

Abstract

over fullerene-based PSCs, including easily tunable polymer properties, simultaneous light absorption by both donors and acceptors, and enhanced stability against mechanical and thermal stresses. [1] However, few all-PSCs have been reported to exhibit power conversion efficiencies (PCEs) higher than 7%, as many systems have relatively low short-circuit current densities (J SC ) and fill factors (FF). [1d,e,2] The low performance of all-PSCs is mainly attributed to (i) low electron mobility of polymer acceptors within the photoactive layer and (ii) inefficient exciton dissociation at donor/acceptor (D/A) interfaces as a result of the anisotropic packing structure of donor and acceptor polymers. [3] Another hurdle for efficient exciton dissociation is lower dielectric constant of the polymers than that of fullerene derivatives, which increases the binding energy of the excitons in all-PSCs. [4] To develop polymer acceptors with high electron mobility, various n-type polymers have been designed and synthesized, among which naphthalenediimide (NDI)-based copolymers have attracted great attention due to strong π-π interactions between NDI units and facile functionalization through the N-position of NDI moiety. [1b,c,2a,c,5] However, lowest unoccupied molecular orbital (LUMO) of NDI-based polymers is often largely localized on the NDI units due to the high electron affinity of NDI, which hinders efficient intermolecular electron transport. [5n,6] Thus, insertion of strong electron-withdrawing groups into the electron donating moieties of the NDI-based copolymers would be a promising approach to enhance electron transport and intermolecular interactions by delocalizing the LUMO over the polymer backbone and generating stronger orbital overlaps between the adjacent polymer chains. [6a,b] Charge generation at the interfaces of the D/A polymer domains within the photoactive layer depends significantly on the interfacial dipole moment between the donor and acceptor and the internal dipole moment of the polymers. [3a,5l,7] Additionally, for conjugated polymers with a large dipole moment difference between the ground and excited states (Δµ ge ), the electron-hole separation distance within the polymer chain increases as the polarized exciton is formed, which reduces the Designing polymers that facilitate exciton dissociation and charge transport is critical for the production of highly efficient all-polymer solar cells (all-PSCs). Here, the development of a new class of high-performance naphthalenediimide (NDI)-based polymers with large dipole moment change (Δµ ge ) and delocalized lowest unoccupied molecular orbital (LUMO) as electron acceptors for all-PSCs is reported. A series of NDI-based copolymers incorporating electron-withdrawing cyanovinylene groups into the backbone (PNDITCVT-R) is designed and synthesized with 2-hexyldecyl (R = HD) and2-octyldodecyl (R = OD) side chains. Density functional theory calculations reveal an enhancement in Δµ ge and delocalization of the LUMO upon the incorporation of cyano...

11.2% All‐Polymer Tandem Solar Cells with Simultaneously Improved Efficiency and Stability

“…All‐polymer solar cells (all‐PSCs) that contain both p‐type and n‐type polymeric materials blended together as light‐absorption layers have attracted much attention, since the blend of a polymeric donor and acceptor should present superior photochemical, thermal, and mechanical stability to those of fullerene‐based PSCs. For example, by comparing the thermal and photostabilities of all‐PSCs and fullerene‐based PSCs based on the same polymer donor, all‐PSCs exhibited remarkable thermal and photostabilities, while the power conversion efficiencies (PCEs) of devices based on fullerene derivatives decreased rapidly upon thermal annealing or light illumination .…”

Recent Advances in the Synthesis of Electron Donor Conjugated Terpolymers for Solar Cell Applications

“…The high PCE of the device based on P44 is attributed to the improved charge transport and enhanced light absorption coefficient. P45 was chosen as an acceptor due to its miscibility with BDT based monomers (Cho et al, 2018).…”

Imide‐Functionalized Polymer Semiconductors

“…Additionally, introducing the cyano group rendered the LUMO more delocalized along the backbone, which improved the polymer μ e,SCLC . A highly encouraging PCE of 7.4 % with a large V oc of 0.94 V and a sizable FF of 65 % was achieved when 28 a was mixed with D11 (Figure ) as the donor polymer . Facchetti and co‐workers reported a series of NDI‐based polymers with tailored π‐conjugation length by introducing non‐conjugated units into the main chain.…”

“…Thel arger Dm ge should facilitatee xciton dissociation by reducing the coulombic attraction and exciton binding energy.A dditionally,i ntroducing the cyano group rendered the LUMO more delocalized along the backbone, which improved the polymer m e,SCLC .Ahighly encouraging PCE of 7.4 %w ith al arge V oc of 0.94 Vand as izable FF of 65 %w as achieved when 28 a wasm ixed with D11 ( Figure 2) as the donor polymer. [40] Facchetti andco-workers reportedaseries of NDI-based polymers with tailored p-conjugation length by introducing non-conjugated units into the main chain.P olymers 29 were synthesized from NDI with different ratios of the conjugated TVT (dithienylvinylene)and non-conjugated TET( dithienylethane) co-monomerss howeds ignificant differences in thermal, optoelectronic, morphological, and charge transport properties. Increasing the TET content resulted in al ower melting point, blueshifted absorption, and reduced crystallinity accompanied by the change of polymer chain orientation from face-on for PNDI-TVT100t oahighly crystallized structure as its unconnected analog like for PNDI-TET100.A st he LUMO is mainly located on the NDI and the HOMO is delocalized along the polymer backbone, electron transport was less affectedc ompared with the m h,OTFT ,a nd the m e,OTFT was reduced to 0.3 cm 2 V À1 s À1 ,h alf of that of parent polymer,w hen 60 %T ET was incorporated into the backbone.…”