Dr Jonas Mattiasson Bjuggren

In this work, a new potential electron transport material, poly[(N,N'-bis(3-(N,N-dimethyl)-N-ethylammonium)propyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-2,5-thiophene)]dibromide P(NDI3N-T-Br), is studied in conjunction with active layers formed by poly[2,3-bis(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) and poly[[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)] (N2200). The energy levels of the active layer TQ1 and N2200 in contact with P(NDI3N-T-Br) have been determined by photoelectron spectroscopy. The dipole formed at the interface of the active layer and P(NDI3N-T-Br) is significantly different to the dipole formed at the respective interface with ITO. The dipole between the active layer and P(NDI3N-T-Br) blocks the transfer of holes from TQ1 to P(NDI3N-T-Br) which is desired, but the hole still transfers from the N2200 to P(NDI3N-T-Br) due to N2200 being an electron acceptor. This energy level alignment meets the expectation of using P(NDI3N-T-Br) as an interface layer in blocking the hole transfer to the interface layer when TQ1:N2200 is used in the active layer. The present work provides an understanding of P(NDI3N-T-Br) as charge extraction layer and indicates its potential to be used in organic photovoltaics.

Synthesizing copolymer acceptors based on a mix of three co-monomers is a facile and effective strategy to control the aggregation and crystallinity of semiconducting polymers which has been exploited to improve the photovoltaic performance of all-polymer solar cells (all-PSCs). Applying this strategy to the well-studied electron-transporting polymer acceptor PNDI2OD-T2, different amounts of 3-octylthiophene (OT) are used to partially replace the bithiophene (T2) unit, resulting in three copolymer acceptors PNDI-OTx where x = 5, 10, or 15%. Another polymer, namely PNDI2OD-C8T2, consisting of naphthalene diimide (NDI) polymerized with 3-octyl-2,2'-bithiophene (C8T2) is also synthesized for comparison. It is found that the solution aggregation and thin-film crystallinity of PNDI-OTx are systematically tuned by varying x, evidenced by temperature-dependent UV-vis and grazing incidence wide-angle X-ray scattering measurements. PNDI2OD-C8T2 is also found to have reduced solution aggregation and thin-film crystallinity relative to PNDI2OD-T2. However, the photovoltaic performance of all-PSCs based on J71:PNDI-OTx and J71:PNDI2OD-C8T2 blends are much lower than that of the reference J71:PNDI2OD-T2 system. Extensive morphological studies indicate that reduced aggregation and crystallinity do not guarantee a more favorable blend morphology, with coarser phase separation found in J71:PNDI-OTx and J71:PNDI2OD-C8T2 blends compared to J71:PNDI2OD-T2 blends. Furthermore, the OT-modified copolymers with reduced crystallinity are found to have reduced electron mobilities. The results here suggest that reduced aggregation and less crystallinity of random copolymer acceptors do not always produce favorable morphology in polymer/ polymer blends and do not guarantee for improvement in the photovoltaic performance.

The development of non-fullerene acceptors (NFAs) such as ITIC and Y6 has greatly improved the efficiency of polymer solar cells. Therefore, focus should now shift towards the design of donor polymers with better compatibility with these NFAs to attempt to push efficiencies to higher levels. The indacenodithienothiophene (IDT) unit has up till now been typically incorporated into donor polymers for fullerene-based solar cells; however, the application of IDT-based polymers in NFA solar cells is rare. In order to increase the number of donor polymer candidates, we have synthesized two new polymers PIDT-T8BT and PIDT-T12BT conjugating IDT and perfluorinated benzothiadiazole moieties through octyl and dodecyl thiophene bridging units, respectively. These polymers were studied and revealed to be completely amorphous without aggregates, and their glass transition temperatures were distinct owing to the different lengths of the alkyl groups attached to the thiophene units. In addition, the length of the side groups greatly affects the solar cell performance. The longer dodecyl side groups in the polymer PIDT-T12BT resulted in a lower glass transition temperature and favorable thermal annealing conditions of the active layer blend with Y6. A promising power conversion efficiency of over 12% was achieved for PIDT-T12BT when paired with the Y6 acceptor and thermally annealed at 170 °C, which is the highest reported value so far for IDT-based donor polymers.