Bandgap and molecular energy level control are of great importance in improving photovoltaic properties of conjugated polymers. A common approach to tuning these parameters is to modify the structure of conjugated polymers by copolymerizing with different units. In this paper, research work focuses on the synthesis of benzo[1,2-b:4,5-b′]dithiophene (BDT) with different conjugated units and their photovoltaic performance. Eight new BDT-based polymers with commonly used conjugated units, including thiophene, benzo[c][1,2,5]thiadiazole (BT), thieno [3,4-b]pyrazine (TPZ), etc., were synthesized. The bandgaps of the polymers were tuned in the range of 1.0-2.0 eV, and their HOMO and LUMO energy levels could also be tuned effectively. The absorption spectra as well as electrochemical and photovoltaic properties of these polymers were investigated systematically. Some units exhibiting the same effect of bandgap lowering exhibited different effects on molecular energy levels of the polymers. For example, the TPZ unit can reduce the bandgap by lowering the LUMO energy level and elevating the HOMO level of the polymer, but the BT unit can lower the bandgap only by depressing the LUMO level. Since open-circuit voltage (V oc ) of the heterojunction polymer solar cell is believed to be inversely proportional to the HOMO level of electron donor material, V oc of the devices based on H9, the copolymer of BDT and TPZ, was ca. 0.5 V lower than that of the device based on H7, the copolymer of BDT and BT. The effects of seven commonly used units on bandgap, molecular energy level, and photovoltaic properties of the BDT based polymers are studied and discussed in this paper, which can provide a guideline not only for design of photovoltaic materials but also for materials of various other electronic devices. In addition, the PCE of the device based on PCBM and H6, one of the BDT-based polymers, reached 1.6%, and V oc , I sc , and FF of the device were 0.75 V, 3.8 mA/cm 2 , and 56%, respectively, which indicates that BDT is a promising common unit for photovoltaic conjugated polymers. Since we have developed the synthetic method of the 4,8-bisalkoxy-BDT monomer, the BDT unit will play an important role in future research on conjugated polymer design.

Polymer tandem solar cells with a PCE of 5.8% are demonstrated by employing a p–n junction as an interlayer between the two subcells. The role of the interlayer and several important issues of the tandem structure is addressed including optical optimization, interfacial engineering, and accurate efficiency characterization (see image). It is revealed that the interlayer acts as a metal/semiconductor contact as opposed to a traditional tunnel junction in inorganic tandem cells.

Synthesizing metal oxides through the sol–gel process provides a convenient way for forming a nanostructured layer in wide band gap semiconductors. In this paper, a unique method of introducing dopants into the metal oxide semiconductor is presented. The doped TiO2 is prepared by adding a Cs2CO3 solution to a nanocrystalline TiO2 solution that is synthesized via a non‐hydrolytic sol–gel process. The properties of the TiO2:Cs layer are investigated and the results show stable nanostructure morphology. In addition to providing morphological stability, Cs in TiO2 also gives rise to a more desirable work function for charge transport in organic electronics. Polymer solar cells based on the poly(3‐hexylthiophene) (P3HT): methanofullerene (PC70BM) system with the addition of a TiO2:Cs interfacial layer exhibit excellent characteristics with a power conversion efficiency of up to 4.2%. The improved device performance is attributed to an improved polymer/metal contact, more efficient electron extraction, and better hole blocking properties. The effectiveness of this unique functionality also extends to polymer light emitting devices, where a lower driving voltage, improved efficiency, and extended lifetime are demonstrated.

Tandem solar cells have the advantage of enhancing the absorption range of polymer solar cells. A three‐terminal tandem cell based on two polymer bulk heterojunctions that have complementary absorption profile is demonstrated. In this device configuration the two subcells are connected in parallel through a common semitransparent metal interlayer and an efficiency of 4.8% with short circuit current of 15.1 mA cm−2 is achieved.