Kakaraparthi Kranthiraja

Side-Chain Fluorination: An Effective Approach to Achieving High-Performance All-Polymer Solar Cells with Efficiency Exceeding 7.

Side-chain fluorination of polymers is demonstrated as a highly effective strategy to improve the efficiency of all-polymer solar cells from 2.93% (nonfluorinated P1) to 7.13% (fluorinated P2). This significant enhancement is achieved by synergistic improvements in open-circuit voltage, charge generation, and charge transport, as fluorination of the donor polymer optimizes the band alignment and the film morphology.

High-Performance Long-Term-Stable Dopant-Free Perovskite Solar Cells and Additive-Free Organic Solar Cells by Employing Newly Designed Multirole π-Conjugated Polymers

Perovskite solar cells (PSCs) and organic solar cells (OSCs) are promising renewable light-harvesting technologies with high performance, but the utilization of hazardous dopants and high boiling additives is harmful to all forms of life and the environment. Herein, new multirole π-conjugated polymers (P1-P3) are developed via a rational design approach through theoretical hindsight, further successfully subjecting them into dopant-free PSCs as hole-transporting materials and additive-free OSCs as photoactive donors, respectively. Especially, P3-based PSCs and OSCs not only show high power conversion efficiencies of 17.28% and 8.26%, but also display an excellent ambient stability up to 30 d (for PSCs only), owing to their inherent superior optoelectronic properties in their pristine form. Overall, the rational approach promises to support the development of environmentally and economically sustainable PSCs and OSCs.

Influential effects of π-spacers, alkyl side chains, and various processing conditions on the photovoltaic properties of alkylselenyl substituted benzodithiophene based polymers

π-spacers and alkyl side chains play a key role in the optical, electrochemical, and photovoltaic properties of π-conjugated polymers. To investigate the collective effects of π-spacers, alkyl side chains, and various processing conditions on the photovoltaic properties, an array of four new low bandgap (LBG) π-conjugated polymers (P1–P4) was designed and synthesized for their application as donor materials in bulk heterojunction polymer solar cells (BHJ PSCs). These π-conjugated polymers contain a benzodithiophene (BDT) donor unit substituted with 2-ethylhexylselenyl or 2-hexyldecylselenyl as π-conjugated side chains and a dialkoxybenzothiadiazole (dialkoxyBT) electron deficient unit connected with thiophene or selenophene as π-spacers. Among the four polymers, the absorption spectra of P3 with the thiophene π-spacer showed a well enhanced vibronic shoulder peak between 620 and 650 nm, indicating that P3 possesses a strong interchain aggregation tendency and attains a planar backbone structure due to the non-covalent interactions arising between the sulfur atom in thiophene and the oxygen atom in dialkoxyBT. Under suitable device processing conditions optimized pristine PSCs of P3 showed a maximum power conversion efficiency (PCE) of 4.09%. After employing 1,8-diiodooctane as an additive, one of the PSC devices based on P2 displayed a PCE of 5.34%. The active layers of P1–P4 showed a positive response towards methanol treatment, especially the P3-based devices delivered an improved PCE of 5.63%, which was further assessed by electrical impedance spectroscopy. These findings in the current article provide a good specimen for efficiently fine tuning the optical and photovoltaic properties of π-conjugated polymers via varying the size of alkyl chains, π-spacer groups and device processing conditions for the imminent growth of LBG π-conjugated polymers.

Carbazole linked phenylquinoline-based fullerene derivatives as acceptors for bulk heterojunction polymer solar cells: effect of interfacial contacts on device performance

To understand the effect of interfacial contact between the hole transporting layer (HTL) and fullerene derivatives in the active layer of bulk heterojunction polymer solar cells (BHJ PSCs), carbazole (Cz) linked phenylquinoline (PhQ)-based fullerene derivatives, PhQHCz-C61BM and PhQEOCz-C61BM, have been successfully synthesized. They are used as acceptors with a poly(3-hexylthiophene) (P3HT) donor in the active layer, and PEDOT:PSS and MoO3 were used as the HTL. Both the derivatives are highly soluble in common organic solvents and possess high thermal stability. BHJ PSCs are fabricated with configurations of ITO/PEDOT:PSS/P3HT:PhQHCz-C61BM/LiF/Al, ITO/PEDOT:PSS/P3HT:PhQEOCz-C61BM/LiF/Al, and ITO/MoO3/P3HT:PhQHCz-C61BM/LiF/Al, ITO/MoO3/P3HT:PhQEOCz-C61BM/LiF/Al, and the device characteristics were measured under AM1.5G (100 mW cm−2). Both derivatives exhibited much lower power conversion efficiencies (PCE) of ∼0.1% when PEDOT:PSS was employed as the HTL. In contrast, the PCE increases to ∼2.2% upon replacing PEDOT:PSS with MoO3 as the HTL. This is due to the fact that protonation of the pyridyl nitrogen of the acceptor in the active layer by the –SO3H group of PEDOT:PSS in the HTL, establishes a charge injection barrier at the interfacial contact and leads to restricted charge collection at the electrodes. This was indirectly confirmed by protonation of pyridyl nitrogen in PhQHCz-C61BM by the –SO3H group in p-toluenesulphonic acid.

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.

Accomplishment of Multifunctional π-Conjugated Polymers by Regulating the Degree of Side-Chain Fluorination for Efficient Dopant-Free Ambient-Stable Perovskite Solar Cells and Organic Solar Cells

We present an efficient approach to develop a series of multifunctional π-conjugated polymers (P1-P3) by controlling the degree of fluorination (0F, 2F, and 4F) on the side chain linked to the benzodithiophene unit of the π-conjugated polymer. The most promising changes were noticed in optical, electrochemical, and morphological properties upon varying the degree of fluorine atoms on the side chain. The properly aligned energy levels with respect to the perovskite and PCBM prompted us to use them in perovskite solar cells (PSCs) as hole-transporting materials (HTMs) and in bulk heterojunction organic solar cells (BHJ OSCs) as photoactive donors. Interestingly, P2 (2F) and P3 (4F) showed an enhanced power conversion efficiency (PCE) of 14.94%, 10.35% compared to P1 (0F) (9.80%) in dopant-free PSCs. Similarly, P2 (2F) and P3 (4F) also showed improved PCE of 7.93% and 7.43%, respectively, compared to P1 (0F) (PCE of 4.35%) in BHJ OSCs. The high photvoltaic performance of the P2 and P3 based photovotaic devices over P1 are well correlated with their energy level alignment, charge transporting, morphological and packing properties, and hole transfer yields. In addition, the P1-P3 based dopant-free PSCs and BHJ OSCs showed an excellent ambient stability up to 30 days without a significant drop in their initial 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 and optoelectronic properties of new acceptor-donor-acceptor type solution processed conjugated small molecules for optoelectronics

ABSTRACT We designed and synthesized two new acceptor-donor-acceptor (A-D-A) small molecules SM-1 and SM-2 by connecting benzodithiophene (BDT) and dithienopyrrole (DTP) are electron rich donor units, dialkoxyphenylenedithiophene as π-spacer and 2,4 thiazolidinedione as terminal acceptor unit respectively. The SM-1 and SM-2 showed broad absorption windows with optical band gaps of 1.7 and 1.8 eV respectively, and also possess high thermal stabilities and suitable energy levels for optoelectronic applications. Especially, SM-2 shows a maximum power conversion efficiency of 1.26%, and hole mobility of 4.6 × 10−3 cm2/V.s in conventional organic solar cells and organic field effect transistor devices respectively.

Acceptor-donor-acceptor type planar hole-transporting materials for efficient dopant-free perovskite solar cells

ABSTRACT Acceptor-donor-acceptor (A-D-A) type organic small molecule (SM-1) has been explored as an efficient dopant-free hole transporting material (HTM) to replace spiro-OMeTAD in the perovskite solar cells. High quality smooth, uniform, and hydrophobic SM-1 HTM can be readily deposited through solution casting without any external dopants. The highly compatible highest occupied molecular orbital level of −5.15 eV of SM-1 with respect to the perovskite valence band, along with intrinsically rich hole conductive nature effectively extracts holes from perovskite and thereby showed a reasonably high power conversion efficiency of 10.54%. Henceforth, the SM-1 based PSCs results successfully demonstrate the advancement of dopant-free PSCs through design engineering of planar A-D-A type conjugated small molecules.

Impact of Topology of Alkoxy Side Chain in Alkoxyphenylthiophene Subsituted Benzodithiophene Based 2D Conjugated Low Bandgap Polymers on Photophysical and Photovoltaic Properties

We report a new series of low band gap (LBG) polymers (P1-P4), in which para or meta- alkoxyphenylthiophene (APTh) substituted benzodithiophene and 2,5-ethylhexyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione or 2-ethylhexyl-4,6-dibromo-3-fluorothieno[3,4-b]thiophene-2-carboxylate are key repeating units. All the polymers showed broad absorption profiles over 900 nm with reduced optical band gaps (Egopt). Interestingly, the straightforward modification (exchanging the topology of alkoxy side chain on phenyl group of APTh in donor unit) brought considerable changes in photophysical and photovoltaic properties of new polymers. In particular, meta-substituted polymers (P2, P4) showed reduced Egopt (1.26, 1.41 eV), deep highest occupied molecular orbitals (HOMOs) (-5.23, -5.28 eV) than para-substituted polymers P1, P3 (Egopt=1.33, 1.44 eV; HOMOs=-5.19, -5.20 eV). Furthermore, the optimized P2 and P4 based devices delivered an enhanced power conversion efficiency (PCE) of 4.39 and 4.33%, with open-circuit voltage (Voc) of 0.71 and 0.79 V, respectively, which are higher than P1 (PCE of 2.95 with Voc of 0.65) and P3 (PCE of 2.33% with Voc of 0.69 V) based devices.