Steric minimization towards high planarity and molecular weight for aggregation and photovoltaic studies

Abstract

Taking into account the effect of planarity, alkyl chain steric hindrance and molecular weight towards high performance polymer solar cells, a "planar" semiconducting polymer PTOBDTDTffBT is designed. Minimal intra-and inter-molecular steric hindrances are realized by removing the alkyl chains at the 4,4′-positions of DTffBT monomer and introducing shorter octyloxy chains on the BDT unit, respectively. Such steric minimization strategy endows PTOBDTDTffBT with high number average molecular weight of 343.37 kg mo 1 , a planar conjugated backbone and strong inter-molecular aggregation characteristic as well. The optical properties indicate the aggregation can only be partially broken even at 160 °C, theoretical calculations indicate that the polymer backbone has small distortion and the thiophene-phenyl bridge can move the octyloxy chains 6.77 Å range outward the polymer backbone, both of which ensure the strong inter-chain aggregation. In spite of the high planarity and strong aggregation of the polymer, PTOBDTDTffBT is relatively amorphous in its film indicated by grazing incidence X−ray diffraction analysis. PTOBDTDTffBT exhibits a narrow bandgap of 1.71 eV together with a high ionization potential of 5.46 eV. Because of the strong aggregation of the polymer, PTOBDTDTffBT/PC71BM active layer exhibits temperature dependent photovoltaic performance. When the active layer is spin coated at a relative low temperature below 100 °C, PTOBDTDTffBT/PC71BM exhibits stable power conversion efficiency above 6.6%. However, when the temperature is elevated from 120 °C to 160 °C, the power conversion efficiency decreased gradually to 3.58% because of decreased short−circuit current density and fill factor. The temperature dependent photovoltaic is explained that the PC71BM is easily to be spun out from the solution at higher temperature confirmed by the UV−vis absorption and X−ray photoelectron spectroscopy analysis. High temperature can also lead to coarse morphology and large phase separation in the active layer observed from the atom force microscopy and transmission electron microscopy images. Finally, maximum power conversion efficiency of 7.68% is obtained when the active layer is spin coated at 80 °C, which is a significant improvement of the power conversion efficiency for "planar" PBDTDTffBT series. 12-15When semiconducting polymers are designed towards high PCE, firstly, a planar conjugated backbone is desired, the planar conjugated backbone ensures a close − stacking distance, which is relevant for inter-molecular aggregation and charge carrier transport. [16][17][18] The planarity also warrants an effective conjugation along the polymer backbone which is beneficial for narrower E g . 19 Secondly, indispensably introducing soluble side chains ensure solubility. 20 However, the non-chromophoric side chains can influence charge carrier transport, aggregation and BHJ morphology when blending the Please do not adjust margins the "tortuous" PBDTDTffBTs. 24,37,38,42 The structural absorption can be roughly observed eve...

Elevated Photovoltaic Performance in Medium Bandgap Copolymers Composed of Indacenodi-thieno[3,2-b]thiophene and Benzothiadiazole Subunits by Modulating the π-Bridge

“…values (Δλ) and the decreased absorption intensity (ΔI) were 14 nm (from 570 to 556 nm) and 15.1% for PIDTT-DTBT, 13 nm (from 602 to 589 nm) and 12.1% for PIDTT-TBT and 16 nm (from 602 to 586 nm) and 14.4% for PIDTT-TFBT, respectively. The observed similar vibration values with respect to absorption peak and intensity implied that the strategies of reducing π-bridge and fluorination produced the little impact on aggregation in CB solution [79].…”

“…In detail, after temperature was risen from 25 to 105 • C, the blue-shifted values (∆λ) and the decreased absorption intensity (∆I) were 14 nm (from 570 to 556 nm) and 15.1% for PIDTT-DTBT, 13 nm (from 602 to 589 nm) and 12.1% for PIDTT-TBT and 16 nm (from 602 to 586 nm) and 14.4% for PIDTT-TFBT, respectively. The observed similar vibration values with respect to absorption peak and intensity implied that the strategies of reducing π-bridge and fluorination produced the little impact on aggregation in CB solution [79]. values (Δλ) and the decreased absorption intensity (ΔI) were 14 nm (from 570 to 556 nm) and 15.1% for PIDTT-DTBT, 13 nm (from 602 to 589 nm) and 12.1% for PIDTT-TBT and 16 nm (from 602 to 586 nm) and 14.4% for PIDTT-TFBT, respectively.…”

A fluorine-induced high-performance narrow bandgap polymer based on thiadiazolo[3,4-c]pyridine for photovoltaic applications

“…[1][2][3][4] During the last decade, remarkable progress has been made in enhancing the power conversion efficiency (PCE) of the PSCs, such as development of high-performance polymer donors, [4][5][6][7][8][9][10][11][12][13][14] incorporation of efficient interfacial materials, [15][16][17] and advancement of device architectures. [1][2][3][4] During the last decade, remarkable progress has been made in enhancing the power conversion efficiency (PCE) of the PSCs, such as development of high-performance polymer donors, [4][5][6][7][8][9][10][11][12][13][14] incorporation of efficient interfacial materials, [15][16][17] and advancement of device architectures.…”

Effects of alkyl side chain length of low bandgap naphtho[1,2‐c:5,6‐c′]bis[1,2,5]thiadiazole‐based copolymers on the optoelectronic properties of polymer solar cells

“…It was realized that with the temperature raising the molecule motions of CPs are accelerated, which results in the absorption shoulder peaks related to aggregation caused by intermolecular π−π* transition will be reduced. Simultaneously, the distortion of the conjugated backbone is strengthened at the raised temperature, which leads to a decrease of effective conjugation length of polymer backbone, and thus the absorption peaks originated from the localized π−π* transitions and ones produced by ICT transitions are both blue‐shifted . To investigate the difference of side chain length on interchain π–π stacking in solution, temperature‐dependent absorption (TD‐Abs) spectra of two copolymers in CB solution were measured, as shown in Figure .…”