Naphthalenediimide-alt-Fused Thiophene D-A Copolymers for the Application as Acceptor in All-Polymer Solar Cells

Exploring a Fused 2-(Thiophen-2-yl)thieno[3,2-b]thiophene (T-TT) Building Block to Construct n-Type Polymer for High-Performance All-Polymer Solar Cells

In the field of all-polymer solar cells, exploring new electron-donating units (D) to match with electron-accepting units (A) is an important subject to promote the performance of D-A-type polymer acceptors. Herein, we developed a fused D unit 2-(thiophen-2-yl)thieno[3,2-b]thiophene (T-TT) derivated from the famous 2-(2-(thiophen-2-yl)vinyl)thiophene (TVT) unit. With classical naphthalene diimide (NDI) as A unit, the new D-A polymer PNDI-T-TT exhibits enhanced absorption coefficient, electron mobility, and miscibility with donor polymer in comparison with the analogous PNDI-TVT polymer. These advantages can be attributed to the enlarged conjugation and reduced rotamers due to the fused T-TT unit, leading to a stronger intermolecular interaction. When blending with the donor polymer PBDB-T, both NDI-based polymers can form better interpenetrating nanostructures than the corresponding blend films with the donor polymer J71. Finally, PBDB-T/PNDI-T-TT device achieves a power conversion efficiency of 6.1%, which is much higher than that of PBDB-T/PNDI-TVT device (4.24%). These results demonstrate that n-type polymer based on fused T-TT unit can ameliorate the absorption coefficient, molecular aggregation, and charge-carrier mobility and consequently achieve an improved photovoltaic performance in comparison with the classic TVT unit.

Aromatic-Diimide-Based n-Type Conjugated Polymers for All-Polymer Solar Cell Applications

All-polymer solar cells (all-PSCs) have attracted immense attention in recent years due to their advantages of tunable absorption spectra and electronic energy levels for both donor and acceptor polymers, as well as their superior thermal and mechanical stability. The exploration of the novel n-type conjugated polymers (CPs), especially based on aromatic diimide (ADI), plays a vital role in the further improvement of power conversion efficiency (PCE) of all-PSCs. Here, recent progress in structure modification of ADIs including naphthalene diimide (NDI), perylene diimide (PDI), and corresponding derivatives is reviewed, and the structure-property relationships of ADI-based CPs are revealed.

Recent Advances in n-Type Polymers for All-Polymer Solar Cells

All-polymer solar cells (all-PSCs) based on n- and p-type polymers have emerged as promising alternatives to fullerene-based solar cells due to their unique advantages such as good chemical and electronic adjustability, and better thermal and photochemical stabilities. Rapid advances have been made in the development of n-type polymers consisting of various electron acceptor units for all-PSCs. So far, more than 200 n-type polymer acceptors have been reported. In the last seven years, the power conversion efficiency (PCE) of all-PSCs rapidly increased and has now surpassed 10%, meaning they are approaching the performance of state-of-the-art solar cells using fullerene derivatives as acceptors. This review discusses the design criteria, synthesis, and structure-property relationships of n-type polymers that have been used in all-PSCs. Additionally, it highlights the recent progress toward photovoltaic performance enhancement of binary, ternary, and tandem all-PSCs. Finally, the challenges and prospects for further development of all-PSCs are briefly considered.

High-Performance All-Polymer Solar Cells Achieved by Fused Perylenediimide-Based Conjugated Polymer Acceptors

We report three n-type polymeric electron acceptors (PFPDI-TT, PFPDI-T, and PFPDI-Se) based on the fused perylene diimide (FPDI) and thieno[3,2- b]thiophene, thiophene, or selenophene units for all-polymer solar cells (all-PSCs). These FPDI-based polymer acceptors exhibit strong absorption between 350 and 650 nm with wide optical bandgap of 1.86-1.91 eV, showing good absorption compensation with the narrow bandgap polymer donor. The lowest unoccupied molecular orbital (LUMO) energy levels were located at around -4.11 eV, which are comparable with those of the fullerene derivatives and other small molecular electron acceptors. The conventional all-PSCs based on the three polymer acceptors and PTB7-Th as polymer donor gave remarkable power conversion efficiencies (PCEs) of >6%, and the PFPDI-Se-based all-PSC achieved the highest PCE of 6.58% with a short-circuit current density ( J_sc) of 13.96 mA/cm², an open-circuit voltage ( V_oc) of 0.76 V, and a fill factor (FF) of 62.0%. More interestingly, our results indicate that the photovoltaic performances of the FPDI-based polymer acceptors are mainly determined by the FPDI unit with a small effect from the comonomers, which is quite different from the others reported rylenediimide-based polymer acceptors. This intriguing phenomenon is speculated as the huge geometry configuration of the FPDI unit, which minimizes the effect of the comonomer. These results highlight a promising future for the application of the FPDI-based polymer acceptors in the highly efficient all-PSCs.

Effects of Inserting Thiophene as a π-Bridge on the Properties of Naphthalene Diimide-alt-Fused Thiophene Copolymers

On the basis of naphthalene diimide (NDI) units connected to thiophene (T), thienothiophene (TT), or dithienothiophene (DTT) units via a thiophene π-bridge, three new copolymers-PDTNDI-T, PDTNDI-TT, and PDTNDI-DTT, respectively-were synthesized and used in the fabrication of all-polymer solar cells (all-PSCs). The relationships between the structures of the polymers and their optoelectronic properties and photovoltaic performances as electron acceptors in all-PSCs were investigated in detail. As the number of copolymerized heteroaromatic rings in the DTNDI-based polymers increased, the power conversion efficiencies of the resulting all-PSCs were found to decrease. This decreasing trend in the photovoltaic performance is opposite to the results reported previously for NDI-based polymers lacking the thiophene π-bridge and naphthodithiophene diimide-based polymers. In addition, the three polymers were found to exhibit distinct molecular orientations: a face-on orientation for PDTNDI-T and edge-on orientations for PDTNDI-TT and PDTNDI-DTT. Our results indicate that large fused aromatic rings are not necessarily advantageous in the design of NDI-based polymers containing π-conjugated bridges.

A triptycene-cored perylenediimide derivative and its application in organic solar cells as a non-fullerene acceptor

A triptycene-cored PDI derivative with a 3D molecular structure was designed and synthesized as a promising acceptor in OSCs.