High open circuit voltage organic solar cells based upon fullerene free bulk heterojunction active layers

Abstract

We have recently reported on a small organic molecule containing a bithiophene core with end-capping phthalimide units (PthTh 2 Pth) that exhibited a H-aggregation tendency in the solid state and high electron mobility in organic field effect transistors. In this contribution, we have studied both the physical and electrical properties of poly(3-hexylthiophene) (P3HT) and PthTh 2 Pth thin films by measuring the optical absorption, Frontier molecular orbital energy levels, photoluminescence quenching, thermal properties, and photovoltaic response. Our results have provided a useful insight into the use of PthTh 2 Pth as an electron acceptor material for organic photovoltaic applications. In comparison with high-performance, fullerene-based, solution-processed bulk heterojunction solar cells reported in the literature, a relatively high open circuit voltage (ϳ0.94 V) was obtained for various donor-acceptor blend ratios. These results highlight the potential for PthTh 2 Pth to act as an alternative to fullerenes as acceptors in organic solar cell devices.Résumé : Nous avons récemment mis en évidence l'existence d'une petite molécule organique contenant du bithiophène en son coeur et des coiffes terminales phtalimide (PthTh 2 Pth). Cette molécule montrait une tendance à produire des agrégats d'hydrogène à l'état solide et présentait une mobilité électronique élevée dans des transistors organiques à effet de champ. Dans la présente étude, nous nous sommes intéressés aux propriétés physiques et électriques du poly(3-hexylthiophène) (P3HT) et de fines pellicules de PthTh 2 Pth en effectuant des mesures d'absorption optique, de niveaux d'énergie des orbitales moléculaires frontières, de désactivation de photoluminescence, des propriétés thermiques et de réponse photovoltaïque. Nos résultats ont fourni un aperçu utile de l'utilisation de PthTh 2 Pth comme matériau accepteur d'électrons dans les applications photovoltaïques organiques. En comparaison avec les cellules solaires à hétérojonction volumique traitées dans une solution à haute performance de fullerène décrites dans la littérature, un voltage en circuit ouvert relativement élevé (ϳ0,94 V) a été obtenu avec divers mélanges de proportions en donneurs et accepteurs différentes. Ces résultats mettent en évidence la capacité de PthTh 2 Pth à remplacer les fullerènes en agissant comme accepteurs dans les dispositifs équipés de cellules solaires. [Traduit par la Rédaction] Mots-clés : photovoltaïque organique, cellules solaires en plastique, hétérojonction volumique, traitement en solution, accepteurs non fullerèniques, petites molécules donneur-accepteur -conjuguées, agrégats d'hydrogène, voltage en circuit ouvert.

Key components to the recent performance increases of solution processed non-fullerene small molecule acceptors

“…HPI-BT can be synthesized in a two-step protocol and functionalized with low-cost phthalimide end cap units, another building block that has been incorporated in many electron-accepting materials. 79,[109][110][111][112][113] The synthesis of this material exemplies the straightforward and versatile synthesis of many non-PDI acceptors, wherein a single synthetic protocol can be adapted to access several materials. HPI-BT has one of the widest band-gaps of non-fullerene acceptors at 2.3 eV, with a relatively high lying LUMO level that sits at À3.5 eV.…”

Phthalimide-based π-conjugated small molecules with tailored electronic energy levels for use as acceptors in organic solar cells

“…The importance of ideal donor acceptor pairings is clearly illustrated in this work, a good example of this can be seen by comparing the devices made using M4 with each donor, for DTS(FBTTh 2 ) 2 : M4 devices a modest PCE of 0.43% is achieved; however, for the P3HT : M4 devices a PCE that is essentially negligible was observed (Table 2). Overall the PCEs are significantly lower than related devices fabricated with fullerene acceptors 37,39 but based on film morphology studies (see Fig. S31 and S32, ESI †) we believe the PCEs obtained for M1-M7 devices are currently limited by excessive phase separation.…”

“…36 This phthalimidethiophene based small molecule has also been used in our group to produce OSCs with high V oc , B1.0 V, when paired as an acceptor with P3HT. 37 Moving forward, we incorporated the diketopyrrolopyrrole (DPP) chromophore as a central building bock in this design and demonstrated that DPP and phthalimide can be used together to make low band gap electron transporting materials. 38 In another study we have investigated the use of the isoindigo (IS) chromophore as a central building block and demonstrated that it can also be used to produce low band gap materials, that when used as the electron-accepting component in OSCs, gave devices with a V oc of B1.0 V. 39 We have also performed a theoretical investigation on a series of electron deficient building blocks with acceptor-donor-acceptordonor-acceptor (A 1 DA 2 DA 1 ) architecture (where A 1 and A 2 have different electron affinities).…”

Synthesis and structure–property relationships of phthalimide and naphthalimide based organic π-conjugated small molecules

“…16 In this work we investigate the side chain engineering of a well-studied D-A p-conjugated molecular framework synthesized in our group, 5,5 0 (2,2 0 -bithiophene-5,5 0 -diyl)bis(2-alkylphthalimide). [17][18][19][20][21][22] In this D-A framework, the bithiophene electron-rich core (D) is flanked with electron-deficient imide terminal units (A). Imide-flanked materials have shown considerable success in organic p-conjugated materials, [23][24][25][26] and are attractive due to their well-studied self-assembly properties, and the ability to functionalize the imido-nitrogen with a wide range of alkyl side chains.…”