S

S

X1

*

OR2O

S

S

OR1

OR1

X2

X2

*n

*solution prepared from dichlorobenzene #solution prepared from chloroform/diiodooctance (97/3, v/v)

The structures of polymers

The summary of solar cell devices

Polymers

Polymer/PC71BM(w/w ratio)

Jsc (mA/cm2) Voc (V) FF (%) PCE (%)

PTBF0 1:1* 14.1 0.58 62.4 5.3

PTBF1 1:1.5* 14.0 0.74 60.3 6.2

PTBF1 1:1.5# 14.1 0.74 68.9 7.2

PTB4 1:1# 14.8 0.70 64.6 7.0

PTBF2 1:1.5* 11.0 0.68 43.4 3.3b

PTBF2 1:1.5# 11.1 0.68 42.2 3.3

PTBF3 1:1.5* 9.1 0.75 39.4 2.8

PTBF3 1:1.5# 8.8 0.68 39.0 2.4

PTBF0 : X1 = H, X2 = HPTBF1 : X1 = F, X2 = HPTBF2 : X1 = H, X2 = FPTBF3 : X1 = F, X2 = F

TEM images of polymer/PC71BM blend films prepared from DCB solvent,

PTBF0 (a), PTBF1 (b), PTBF2 (c), PTBF3 (d). Scale bar = 200μm.

< Decrease in the optical density (max) of polymer films under UV light and ambient condition >

Our group has succeeded in developing a low bandgap polymer system, polythieno[3,4-b]thiophene-co-benzo[1,2-b:4,5-b]dithiophene (PTBx series), which showed a rather synergistic combination of the desired properties, e.g., low bandgap ( 1.6 eV), relatively high mobility, proper ∼orientation of the π-system, and controllable morphology. Introduction of a fluorine atom into the thieno[3,4-b]thiophene ring substantially improved the Voc, and the device achieved the highest PCE over 7.8%. To better understand the reasons behind the high performances of these polymers, we investigated the effects of fluorination on the properties of PTB polymers and have synthesized a series of fluorinated polymers (PTBFs). Detailed studies revealed That PTBF1 gave the best solar cell performance, mainly attributed to the lowering of the polymer HOMO energy level. Perfluorination of the polymer backbone led to poor compatibility with PC71BM molecules, thus poor solar energy conversion efficiency. Furthermore, it was found that perfluorination on the polymer backbone resulted in poor photochemical stability, which was explained based on charge density change after fluorination.

Thienothiophene based polymers for renewable organic solar cell with highest efficiencyLuping Yu, University of Chicago, DMR 1004195

J. Am. Chem. Soc., (2011), 133(6), 1885-1894

Education: Under this grant, one student, Hae Jung Son, has completed her Ph. D. research and graduated in this July. Four graduate students Bridget Carsten, Tao Xu, Luyao Lu and Tianyue Zheng are working in this project. Two post-doctoral scientists has joined this project: Dr. Feng He and Dr. Joseph Bullock. An undergraduate student, Michael Miller is also involved in this project from this summer.

Outreach: The PI has established close collaboration with a start-up company-Solarmer Energy for solar cell devices based on materials developed in PI’s lab (Dr. Gang Li and Dr. Yue Wu). The company has offered financial support for this project as well as technical assistance. The PI also enjoys close collaborations with several research groups and national and industrial labs: Prof Tobin J. Marks at Northwest University on device fabrication and characterization; Dr. Lin Chen at Argonne National Lab (ANL) on electronic dynamics of low band gap polymers; Dr. Hieke Riel at IBM labs at Zurich, Prof. Ivan Oleynik at U. of South Florida on theoretical studies, Prof. N. J. Tao at Arizona State University on molecular electronics; Dr. D.J. Liu at ANL on hydrogen storage polymers. For the publication, two recent papers from the group that have reached fifth and eighth most cited current publications in chemistry.

Luping Yu talks with ScienceWatch.com and answers a few questions about this month's Fast Breaking Paper in the field of Materials

Science.

Thienothiophene based polymers for renewable organic solar cell with highest efficiencyLuping Yu, University of Chicago, DMR 1004195

http://sciencewatch.com/dr/fbp/2011/11febfbp/11febfbpYu/