Nallan Chakravarthi

A simple structured and efficient triazine-based molecule as an interfacial layer for high performance organic electronics

Achieving the state-of-the-art performance of solution processable and flexible organic electronics requires efficient, stable, and cost-effective interfacial layers (ILs). Here, we report an alcohol soluble phosphine oxide functionalized 1,3,5-triazine derivative (PO-TAZ) as an IL, which remarkably tailors the work function of conductors including metals, transparent metal oxides and organic materials, making it an ideal candidate for an interfacial material in organic electronics. Consequently, PO-TAZ thin films enable the fabrication of organic and organic–inorganic (perovskite) solar cells with power conversion efficiencies of 10.04% and 16.41%, respectively, and n-channel organic field-effect transistors with an electron mobility of 8 cm2 V−1 s−1. Owing to the low-cost processing associated with PO-TAZ and the tremendous improvement in device performances as compared to the devices without PO-TAZ along with ambient stability, PO-TAZ is a good choice for efficient organic electronics in large area printing processes.

Acridine-based novel hole transporting material for high efficiency perovskite solar cells

An acridine-based hole transporting material (ACR-TPA) without the spirobifluorene motif is synthesized via non complicated steps. The ACR-TPA film including Li-TFSI and 4-tert-butylpyridine (tBP) additives exhibits a hole mobility of 3.08 × 10−3 cm2 V−1 s−1, which is comparable to the mobility of the classical spiro-MeOTAD (2.63 × 10−3 cm2 V−1 s−1), and its HOMO level of −5.03 eV is slightly lower than that of spiro-MeOTAD (−4.97 eV). ACR-TPA layers with different thicknesses are applied to MAPbI3 perovskite solar cells, where power conversion efficiency (PCE) increases as the ACR-TPA layer thickness increases due to increased recombination resistance and fast charge separation. The best PCE of 16.42% is achieved from the ca. 250 nm-thick ACR-TPA, which is comparable to the PCE of 16.26% for a device with spiro-MeOTAD in the same device configuration. It is thus anticipated that ACR-TPA can be a promising alternative to spiro-MeOTAD because of its lower cost and comparable photovoltaic performance.

The effect of with/without resonance-mediated interactions on the organic solar cell performance of new 2D π-conjugated polymers

With the goal of discovering a new and plausible approach to utilizing the conjugated side chains (CSCs), other than for the previously reported purpose of two-dimensional (2D) π-conjugation extension in π-conjugated polymers, we report, for the first time, the impact of with/without interactions induced via resonance in CSCs on the molecular weight (Mw) and photovoltaic performance of polymers. For this, we designed two donor (D)–acceptor (A) polymers, represented as PBDTBPA(H)-DPP and PBDTBPA(F)-DPP, containing alkoxy-BPA(H) and alkoxy-BPA(F) [BPA = biphenylethynyl] on the benzodithiophene (BDT) unit as novel CSCs, respectively. The introduction of these CSCs generated bis-tolane as an integrated part of the BDT unit, which allowed us to address the difference in the polymerization, photophysical, and photovoltaic properties of PBDTBPA(H)-DPP and PBDTBPA(F)-DPP, as a function of the structural variation of CSCs, which has never been investigated in organic solar cells (OSCs). In contrast to the weak electron-donating nature of BPA(H), BPA(F) exhibited a strong electron-donating ability due to the interaction of the lone pair electrons of the fluorine atom with the triple bond via resonance, which decreased the rigidity of the triple bond, whereas in PBDTBPA(H)-DPP the rigidity of the triple bond was retained with no such interaction. The striking differences in the rigidity and different electron-withdrawing tendencies of the CSCs were well correlated with Mw and with the highest occupied molecular orbital (HOMO) energy levels of the polymers. As a result, the inverted OSCs based on PBDTBPA(H)-DPP achieved an open-circuit voltage (Voc) of 0.74 V, and power conversion efficiency (PCE) of 5.58%, which was 38% higher than that of PBDTBPA(F)-DPP-based inverted OSCs. More significantly, the inverted OSC devices were highly stable, retaining 80% of their original PCE after 60-day storage in air, even without encapsulation. To the best of our knowledge, this 5.58% is the highest PCE reported to date for the arylethynyl-substituted BDT donor-based OSCs. These results reveal that bis-tolane [BDTBPA(H)] as an integrated part of the new BDT unit is a promising donor building block for high Mw donor polymers in addition to 2D extended π-conjugation for high performance bulk heterojunction (BHJ) OSCs.

Synthesis, characterization and photovoltaic properties of low band gap donor-acceptor polymers containing benzodithiophene donor unit with fluorenylthiophene as 2D-conjugated side for organic solar cell application

ABSTRACT Two donor−acceptor low band gap polymers P1 (octyl as solubilizing group) and P2 (ethylhexyl as solubilizing group) containing fluorenylthiophene-substituted benzoditihiophene as an electron-rich unit and 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione as an electron-deficient unit are designed and synthesized for polymer solar cells application. Compared with P2 based on ethyl hexyl group, P1 with octyl group displays well resolved vibronic shoulder peak in absorption spectra, stronger intermolecular interactions, and higher hole mobility. Polymer solar cells based on P1 and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) exhibit a maximum power conversion efficiency of 1.78% under AM 1.5G illumination (100 mW/cm2).