New High PCE Non-Fullerene Acceptors Y6 and Y7
Posted on Wed, Nov 13, 2019 by Hunan Yi
The discovery of ITIC and the subsequent boom in the use of non-fullerene electron acceptors in organic photovoltaic solar cells (OPVs) has resulted in rapid improvements in device power conversion efficiencies (PCEs).
Despite this renewed interest, however, more research is needed before non-fullerene organic solar cells (NFOSCs) can be created with efficiencies that rival those of inorganic devices.
The development of new molecules has resulted in considerable jumps in performance. Most recently, devices have been created with PCEs of 15.7%  and 16.5%  using two promising new non-fullerene electron acceptors known as Y6 and Y7.
Y6 (BTP-4F) and Y7 (BTP-4Cl)
Y6, also known as BTP-4F, is a fluorinated fused-ring acceptor with an electron-deficient core. Specifically, it consists of a fused thienothienopyrrolo-thienothienoindole (TTP-TTI) electron rich core and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (2FIC) electron deficient end groups.
With an absorption that peaks around 810 nm and extends to about 1100 nm, Y6 and its polymer blends have the potential to absorb light across the entire visible and NIR spectrum.
The benzothiadiazole core allows for the creation of solar cells using the polymer PBDB-T-2F (PM6) as an electron donor. Impressive power conversion efficiencies of up to 15.7% have been demonstrated in optimised single-junction solar cells with both conventional and inverted architectures using Y6 as an electron acceptor and PM6 as an electron donor .
The success of the electron-deficient-core-based fused ring design of Y6 has led to the development of other non-fullerene acceptors based on the same principle. One such molecule is the chlorinated form of BTP-4F, known as Y7 or BTP-4Cl.
Using the same polymer donor, PM6, devices created using BTP-4Cl have achieved a record PCE of 16.5% .
The full chemical names of Y6 and Y7 are as follows:
- Y6 - 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2",3’':4’,5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (TTPTTI-4F, BTPTT-4F, BTP-4F, Y6F)
- Y7 - 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2",3’':4’,5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-dichloro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (BTP-4Cl, Y6Cl)
- Single-Junction Organic Solar Cell with over 15% Efficiency Using Fused-Ring Acceptor with Electron-Deficient Core, J. Yuan et al., Joule (2019); doi: 10.1016/j.joule.2019.01.004.
- Over 16% efficiency organic photovoltaic cells enabled by a chlorinated acceptor with increased open-circuit voltages, Y. Cui et al., Nat. Commun., 10, 2515 (2019); 515; doi: 10.1038/s41467-019-10351-5.
Author: Hunan Yi
Hunan is Ossila’s organic chemist, specialising in polymers and functional materials synthesis, polymeric semiconductors, and purification of conjugated polymers. Having achieved an MSc from Shanghai China, Hunan moved to St Andrews University to study towards his PhD in Dye-Sensitised Solar Cells where he then remained as Research Fellow until 2004. He then became a Postdoctoral Research Associate at the University of Sheffield, before working with both the University and Ossila as a KTA associate in 2015.