The photovoltaic performance of poly(3-hexylthiophene) (P3HT) has been improved greatly by using indene–C60 bisadduct (ICBA) as acceptor instead of phenyl-C61-butyric acid methyl ester (PCBM). However, the solvent of dichlorobenzene (DCB) used in fabricating polymer solar cells (PSCs) limited the application of the PSCs, because of the environmental problem caused by the harmful halogenated solvent. In this work, we fabricated the PSCs based on P3HT/ICBA processed with four low-harmful non-halogenated solvents of toluene, o-xylene, m-xylene, and p-xylene. The PSCs based on P3HT/ICBA (1:1, w/w) with toluene as the solvent exhibit the optimized power conversion efficiency (PCE) of 4.5% with open-circuit voltage (Voc) of 0.84 V, short circuit current density (Jsc) of 7.2 mA/cm2, and fill factor (FF) of 71%, under the illumination of AM 1.5G at 100 mW/cm2. Upon using 1% N-methyl pyrrolidone (NMP) as a solvent additive in the toluene solvent, the PCE of the PSCs was greatly improved to 6.6% with a higher Jsc of 10.3 mA/cm2 and a high FF of 75%, which is even higher than that of the devices fabricated with halogenated DCB solvent. The X-ray diffraction (XRD) measurement shows that the crystallinity of P3HT increased with the NMP additive. The investigations on morphology of the active layers by atomic force microscopy (AFM) and transmission electron microscopy (TEM) indicate that the NMP additive promotes effective phase separation and formation of nanoscaled interpenetrating network structure of the active layer, which is beneficial to the improvement of Jsc and PCE for the PSCs fabricated with toluene as the solvent.Keywords: indene−C60 bisadduct; N-methyl pyrrolidone additive; non-halogenated solvents; polymer solar cells; toluene solvent;

A novel small molecule with a acceptor–donor–acceptor (A–D–A) structure, IDT(BTTh2)2, containing an electron-rich indacenodithiophene (IDT) unit as core, flanked by electron-deficient 2,1,3-benzothiadiazole (BT) units and end-capped with hexyl-substituted bithiophene units, has been synthesized for photovoltaic application. IDT(BTTh2)2 shows a broad absorption in the visible range with an optical band gap of ca. 1.80 eV and possesses a relatively deep HOMO level at −5.21 eV. The solution-processed bulk heterojunction solar cells based on the blend of IDT(BTTh2)2/PC71BM (1:3, w/w) without using any treatment such as a solvent additive or thermal annealing, showed a power conversion efficiency (PCE) of 4.25% with a high open-circuit voltage (Voc) of 0.93 V, a short-circuit current (Jsc) of 9.42 mA cm−2 and a fill factor (FF) of 48.5%, under the illumination of AM 1.5G at 100 mW cm−2. These results indicate that indacenodithiophene-based small molecules are promising for bulk heterojunction solar cells.

A new D–A copolymer, poly(thieno[3,2-b]thiophene-alt-bithiazole) (PTTBTz), based on thieno[3,2-b]thiophene donor unit and bithiazole acceptor unit, was synthesized by the Pd-catalyzed Stille-coupling reaction for the application as donor material in polymer solar cells (PSCs). PTTBTz film possesses high thermal stability with 5% weight-loss temperature at 450 °C, a lower-lying HOMO energy level at −5.20 eV, a higher hole mobility of 6.45 × 10–3 cm2/(V s), and a crystalline structure. Photovoltaic performance of the polymer was investigated with [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) or indene-C60 bisadduct (ICBA) as acceptor and with 3% DIO additive. The power conversion efficiency (PCE) of the PSC based on PTTBTz:ICBA (1:1 w/w) reached 5.35% with a high Voc of 1.03 V, a Jsc of 8.55 mA/cm2, and an FF of 0.608, whereas the PCE of the PSC based on PTTBTz:PC70BM (1:1 w/w) was 4.57% with a Voc of 0.82 V, a Jsc of 9.89 mA/cm2, and an FF of 0.563, under the illumination of AM1.5, 100 mW/cm2. PTTBTz is one of the D–A copolymers that shows better photovoltaic performance with ICBA as acceptor than PC70BM. PTTBTz/ICBA could be a promising front active layer for high-efficiency tandem PSC because of its high Voc.

A novel small molecule with a acceptor–donor–acceptor (A–D–A) structure, IDT(BTTh2)2, containing an electron-rich indacenodithiophene (IDT) unit as core, flanked by electron-deficient 2,1,3-benzothiadiazole (BT) units and end-capped with hexyl-substituted bithiophene units, has been synthesized for photovoltaic application. IDT(BTTh2)2 shows a broad absorption in the visible range with an optical band gap of ca. 1.80 eV and possesses a relatively deep HOMO level at −5.21 eV. The solution-processed bulk heterojunction solar cells based on the blend of IDT(BTTh2)2/PC71BM (1:3, w/w) without using any treatment such as a solvent additive or thermal annealing, showed a power conversion efficiency (PCE) of 4.25% with a high open-circuit voltage (Voc) of 0.93 V, a short-circuit current (Jsc) of 9.42 mA cm−2 and a fill factor (FF) of 48.5%, under the illumination of AM 1.5G at 100 mW cm−2. These results indicate that indacenodithiophene-based small molecules are promising for bulk heterojunction solar cells.