Thieno[2

3-b]thiophene-diketopyrrolopyrrole (DPPTTZ)

a novel cross-conjugated electron-deficient unit

has been synthesized.This unit was brominated and then copolymerized with 2

5-bis(trimethylstannyl)thiophene

2

5-bis(trimethylstannyl)selenophene and 2

5-bis(trimethyl-stannyl)-N-methylpyrrole respectively

via Stille polymerzation

to form three donor-acceptor (D-A) type copolymers

poly{2

5-bis(2-octyldodecyl)-3-[5-(thiophen-2

5-yl)thieno[2

3-b]thiophen-2-yl]- 6-(thieno[2

3-b]thiophen-2

5-yl)pyrrolo[3

4-c]pyrrole-1

4(2H

5H)-dione} (pDPPTTZ-T)

poly{2

5-bis(2-octyldodecyl)-3-[5-(selenophen-2

5-yl)thieno[2

3-b]thiophen-2-yl]-6-(thieno[2

3-b]thiophen-2

5-yl)pyrrolo[3

4-c]pyrrole-1

4(2H

5H)-dione} (pDPPTTZ-Se) and poly{2

5-bis(2-octyldodecyl)-3-[5-(N-methylpyrrole-2

5-yl)thieno[2

3-b]thiophen-2-yl]-6-(thieno[2

3-b]thiophen-2

5-yl)pyrrolo[3

4-c]pyrrole-1

4(2H

5H)-dione} (pDPPTTZ-Py).These polymers exhibit deep HOMO energy levels (-5.26~-5.37 eV)

leading to high open-circuit voltage (0.68~0.81 V) in OPV devices

and good stability of the devices towards atmospheric oxygen.The UV-Vis spectra show broad features with long-wavelength absorption maximum around 620 nm and optical band gap of 1.86

1.83 and 1.85 eV for pDPPTTZ-T

pDPPTTZ-Se and pDPPTTZ-Py respectively

which give polymers a good overlap of solar spectrum.Bulk heterojunction solar cells were fabricated by blending these polymers with [6

6]-phenyl-C71-butiric acid methyl ester (PC71MB)

and the power conversion efficiency (PCE) reaches 3.05% for the as cast pDPPTTZ-T:PCBM-based device without optimization

under simulated AM 1.5 G irradiation of 100 mW/cm2.