Over the past few years, tremendous research effort has been made on all-solution processed bulk-heterojunction polymer solar cells (PSCs) in order to realize low-cost, lightweight, largearea and flexible photovoltaic devices. 1 To achieve high efficiency of PSCs, the most critical challenge at the molecular level is to develop the p-type conjugated polymers that possess (1) sufficient solubility to guarantee solution processability and miscibility with an n-type material, (2) low band gap (LBG) for strong and broad absorption spectrum to capture more solar photons and (3) high hole mobility for efficient charge transport. The general approach to produce a LBG polymer is to incorporate electron-rich donor and electrondeficient acceptor segments along the conjugated polymer backbone. Based on these polymers, researchers have made a breakthrough in fabricating PSC devices with PCEs over 5%. 2 Planarization of polyaromatic system facilitates π-electron delocalization and elongates effective conjugation length, providing another effective way to reduce the band gap. 3 Moreover, coplanar geometries and rigid structures can suppress the rotational disorder around interannular single bonds and lower the reorganization energy, which in turn enhances the intrinsic charge mobility. 4 Tricyclic 2, 7-carbazole 5 unit is an ideal electron-rich building block to construct donorÀacceptor polymers because its derivatives exhibit deeplying HOMO energy levels and good hole-transporting properties which are crucial prerequisites to achieve high open-circuit voltages (V oc ) and short circuit currents (J sc ), respectively. Poly-(2,7-carbazole-alt-dithienylbenzothiadiazole) (PCDTBT) has been shown to act as a superior p-type photoactive material for the application in PSCs (Scheme 1). 5 Inspired by the skeletons of the PCDTBT polymers, for the first time, we have successfully utilized a facile Friedel-Craft cyclization to develop a novel carbazole-based coplanar π-conjugated system, carbazole-dicyclopentathiophene (CDCT), where the 3-positon of two outer thiophenes are covalently connected with the 3,6-position of central carbazole cores by a sp 3 -hybridized carbon bridge (Scheme 1). 6 By copolymerizing this heptacyclic structure with electron-deficient benzothiadizole unit, an alternating poly(carbazole-dicyclopentathiophene-alt-benzothiadiazole) (PCDCTBT) was synthesized. 6 The two cyclopentadiene rings embedded in the CDCT structure allows for introducing four 4-(2-ethylhexoxy)phenyl groups to guarantee solubility and

Two new C60-based n-type materials, EGMC-OH and EGMC-COOH, functionalized with hydrophilic triethylene glycol groups (TEGs), have been synthesized and employed in conventional polymer solar cells. With the assistance of the TEG-based surfactant, EGMC-OH and EGMC-COOH can be dissolved in highly polar solvents to implement the polar/nonpolar orthogonal solvent strategy, forming an electron modification layer (EML) without eroding the underlying active layer. Multilayer conventional solar cells on the basis of ITO/PEDOT:PSS/P3HT:PC61BM/EML/Ca/Al configuration with the insertion of the EGMC-OH and EGMC-COOH EML between the active layer and the electrode have thus been successfully realized by cost-effective solution processing techniques. Moreover, the electron conductivity of the EML can be improved by incorporating alkali carbonates into the EGMC-COOH EML. Compared to the pristine device with a PCE of 3.61%, the devices modified by the Li2CO3-doped EGMC-COOH EML achieved a highest PCE of 4.29%. Furthermore, we demonstrated that the formation of the EGMC-COOH EML can be utilized as a general approach in the fabrication of highly efficient multilayer conventional devices. With the incorporation of the EGMC-COOH doped with 40 wt % Li2CO3, the PCDCTBT-C8:PC71BM-based device exhibited a superior PCE of 4.51%, which outperformed the corresponding nonmodified device with a PCE of 3.63%.