Seungmin Kim

Effects of the incorporation of bithiophene instead of thiophene between the pyrrolo[3,4-c]pyrrole-1,3-dione units of a bis(pyrrolo[3,4-c]pyrrole-1,3-dione)-based polymer for polymer solar cells

A new wide band gap polymer, P(BDTT–BTBDPPD), consisting of electron rich 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDTT) and electron deficient bithiophene-incorporated bis(pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione) (BTBDPPD) derivative was prepared to improve the photovoltaic performances of a reported polymer, P(BDTT–TBDPPD), containing BDTT and thiophene-incorporated bis(pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione) (TBDPPD) derivative. Polymer P(BDTT–BTBDPPD) exhibited maximum absorption at 478 nm and the calculated optical band gap was 2.10 eV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of P(BDTT–BTBDPPD) were estimated to be −5.44 eV and −3.34 eV. The hole mobility of P(BDTT–BTBDPPD) was 3.22 × 10−4 cm2 V−1 s−1. The polymer solar cells (PSCs) prepared using P(BDTT–BTBDPPD) : PC70BM (1 : 2 wt%) + 3 vol% DIO blend offered a maximum power conversion efficiency (PCE) of 4.62% with an open-circuit voltage (Voc) of 0.90 V, a short-circuit current (Jsc) of 7.99 mA cm−2, and a fill factor (FF) of 64%. This study suggests that the replacement of the thiophene spacer unit located between the pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione units of bis(pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione) derivative with a bithiophene unit did not considerably alter the energy levels and charge transport properties of the resulting polymer. However, the overall photovoltaic performance was improved due mainly to the enhanced morphology of the photoactive layer.

Cyclic creep behaviour under tension–tension loading cycles with hold time of modified 9Cr–1Mo steel

Abstract Cyclic creep behaviour of modified 9Cr–1Mo steel was investigated by a series of cyclic creep (CC) tests at 600°C, which were performed under controlled tension–tension loading cycles with the magnitude of stress ranges in a constant stress ratio (R = 0·1). Hold time was applied for a 10 min hold at the maximum stress (σmax) and minimum stress (σmin). The CC properties were compared with the static creep (SC) using Norton’s power law, Larson–Miller plot, and Monkman–Grant relation, and the microstructure was examined. For the test conditions employed in the present investigation, retardation in the CC behaviour in terms of a lower creep rate and longer rupture time compared to those in the SC was obtained. The retardation was ascribed to the effects associated with anelastic recovery during the 10 min hold time at the minimum load of the cyclic loading. The creep rupture ductility decreased with a general decrease in stress, and there was no difference in the creep ductility between the CC and SC. The steel displayed a transgranular fracture characterised by the presence of dimples resulting from micro-void coalescence. Carbide precipitation was more coarsened with increasing in exposure time in the CC tests.