High‐Performance All‐Small‐Molecule Solar Cells Based on a New Type of Small Molecule Acceptors with Chlorinated End Groups

Abstract

On the other hand, the electron donor materials could generally be divided into two categories: polymers and small mole cules. Both types of donors have showed high performance with PCE over 11% in combination with fullerene acceptors. [6,8] Nevertheless, for NFAbased OSCs with high efficiencies, most donor materials are widebandgap polymers [24][25][26][27] and only a few small molecule donors [28][29][30][31] work well with several very limited NFAs. In consideration of the unique advantages of small molecule materials, [32,33] such as welldefined structure and thus less batchtobatch variation, versatile chemical structures and thus easier energy level control, much efforts have been made to develop allsmallmolecule (ASM) OSCs, which comprise small molecule donor and nonfullerene small molecule acceptor. Presently, promising PCEs with value over 10% have been achieved by Li and Hou's groups. [30,34,35] But so far, the electron acceptors with decent performance used in the reported ASMOSCs have only been found in the case of (2,2′[(4,4,9,9tetrahexyl4,9dihydrosindaceno[1,2b:5,6b′] dithiophene2,7 diyl)bis[methylidyne(3oxo1Hindene2,1(3H) diylidene)]]bispropanedinitrile) (IDIC). [36] Considering the versatile chemical structures and thus easier energy level control of small molecules for OSCs, it is highly possible and even better high performing acceptors could be designed and developed for ASMOSCs.Recently, our group has reported an A-D-A type small mole cule acceptor, 2,9bis(2methylene (3(1,1dicyanomethylene) indanone))7,12dihydro4,4,7,7,12,12hexaoctyl4H cyclopenta[2″,1″:5,6;3″,4″:5′,6′]diindeno[1,2b:1′,2′b′] dithiophene (FDICTF, F0Cl in Figure 1) and PCE over 10% has been achieved using widebandgap polymer PBDBT as the donor. [37] To further tune its band structure and morphology in active layer, another acceptor F1Cl through introduction of one chlorine atom on the end groups of F0Cl was developed and a PCE over 11% was obtained for the devices based on PBDB T:F1Cl, which was attributed to the redshifted absorption, enhanced molecular packing, and electron mobility of F1Cl. [38] Encouraged by the success of polymer donor devices, acceptor F1Cl might be a good choice for ASM OSCs. Just recently, it has been reported that acceptors with high crystallinity are favorable in ASMOSCs. [39][40][41] Whether the crystallinity of F1ClWhile a wide variety of nonfullerene acceptors are developed and perform well in combination with polymer donors, only a few nonfullerene acceptors can work well with small molecule donors. Here, all-small-molecule solar cells with high performance enabled by a new type of small molecule acceptors (F-0Cl, F-1Cl, and F-2Cl), which contain linear alkyl side chains and end groups substituted with various number of chlorine atoms, are reported. End group chlorination leads to redshifted absorption, enhanced crystallinity, and high electron mobility. These properties make them competitive as electron acceptors for all-small-molecule solar devices. When combined with two popular small molecule don...

Deciphering the Role of Fluorination: Morphological Manipulation Prompts Charge Separation and Reduces Carrier Recombination in All‐Small‐Molecule Photovoltaics

“…These advances noticeably increased the PCE by improving the short‐circuit current density ( J SC ) from 12.35 to 16.07 mA cm −2 and enhancing the fill factor (FF) from 60.4% to 70.9%. To the best of our knowledge, the PCE of 9.25% we achieved here is among the best values of oligothiophene‐donor‐based NF ASM OSCs, which indicates the effectiveness of acceptor fluorination in ASM OSCs. Our results provide insights into the complex effects of FREA fluorination in high‐performance ASM OSCs.…”

Over 12% Efficiency Nonfullerene All‐Small‐Molecule Organic Solar Cells with Sequentially Evolved Multilength Scale Morphologies

“…For example, Hou and co‐workers have recently demonstrated high J sc over 18 mA cm −2 in NFSM‐OSCs by combining a slightly lower bandgap acceptor, IT‐4F ( E g = 1.52 eV) with a mid‐bandgap donor . However, the photoresponse for all of the high performing NFSM‐OSCs reported so far are below 820 nm, which limits the potential to further increase J sc . Considering that near‐infrared (NIR) nonfullerene acceptors ( E g < 1.4 eV) have been widely used in NFP‐OSCs to achieve high J sc of >23 mA cm −2 , it is rational to apply similar NIR absorbing acceptor/donor to NFSM‐OSCs.…”

Difluorinated Oligothiophenes for High‐Efficiency All‐Small‐Molecule Organic Solar Cells: Positional Isomeric Effect of Fluorine Substitution on Performance Variations

“…Judging from previous reports, three key parameters are mandatory for achieving enhanced performance in oligothiophene‐based ASM OSCs: 1) reduced vibronic coupling between the ground states and the charge‐transfer (CT) states of the oligothiophene to lower the nonradiative voltage losses; 2) utilization of a more absorption‐complementary SM donor/NFA couple instead of an absorption‐overlapping one to improve short‐circuit current ( J SC ); and 3) enhanced crystallinity of the NFA to improve electron mobility and the PCE of devices . In practice, the parameters aforementioned may result from element substitutions.…”