Synthesis and characterization of dithienobenzothiadiazole-based donor–acceptor conjugated polymers for organic solar cell applications

New donor–acceptor conjugated polymers (P1 and P2) containing a fused-ring dithienobenzothiadiazole (DT-BTD building block) were synthesized by using the Stille copolymerization method. The synthesized polymers were characterized by ¹H NMR, GPC, and elemental analysis. The optical band gaps of the polymers were found to be 1.86 and 1.9 eV, respectively, as calculated from their film onset absorption edge. Upon annealing both produced a distinct shoulder peak in their film absorption spectra. The electrochemical studies of P1 and P2 revealed that the HOMO and LUMO energy levels of the polymer were −5.3, −5.1 eV, and −3.4, −3.2 eV, respectively. The polymers are thermally stable up to 250–350 °C.     

Pyrene-benzo[1,2,5]thiadiazole based conjugated polymers for application in BHJ solar cells

Ethylhexyloxy-functionalised pyrene (P_EH) was prepared and copolymerised with both dithienyl-benzo[c]-[1,2,5]thiadiazole and dibithiophenyl-benzo[c]-[1,2,5]thiadiazole via a Stille coupling polymerisation method to yield PP_EH-DTBT-8 and PP_EH-DT2BT-8, respectively. A comparative study was conducted to assess the impact of substituting thiophene for bithiophene repeat units upon the resulting properties of the conjugated polymers. PP_EH-DT2BT-8 which has bithiophene spacers between pyrene and benzothiadiazole repeat units, exhibited a narrower optical and electrochemical band gap relative to PP_EH-DTBT-8; a consequence of the incorporating bithiophene spacer units which promote intramolecular charge transfer between the electron donating and electron accepting moieties. Both PP_EH-DTBT-8 and PP_EH-DT2BT-8 showed deep HOMO levels of −5.54 and −5.50 eV, respectively. The polymers possess good thermal stabilities with degradation temperatures in excess of 310 °C. The photovoltaic performance of the two polymers was studied by fabricating bulk heterojunction (BHJ) photovoltaic devices using PC₇₀BM as the acceptor. PP_EH-DTBT-8 and PP_EH-DT2BT-8 demonstrated efficiencies of 0.33 and 1.83%, respectively. The higher efficiency of PP_EH-DT2BT-8 can be attributed to vastly improved FF and J_sc values.     

Impact of environmentally friendly processing solvents on the properties of blade‐coated polymer solar cells

High‐performance polymer solar cells (PSCs) are typically fabricated by spin coating in inert atmosphere from toxic halogenated solvents such as 1,2‐dichlorobenzene (o‐DCB) and chlorobenzene. This fabrication process is potentially hazardous for both the humans and the environment and dramatically impacts the possibility for the organic photovoltaic technology to be adopted at large scale. In this work, efficient PSCs blade coated in air using nonhalogenated 1,2,4‐trimethylbenzene (TMB) as processing solvent are demonstrated. The active layer, based on a previously synthesized benchmark polymer PFQ2T‐benzodithiophene blended with [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC₆₁BM), showed an enhanced solid‐state aggregation induced by the use of TMB. Compared to o‐DCB‐processed devices, the solar cells fabricated from TMB resulted 10% more efficient with a power conversion efficiency of 4.20%. Interestingly, the improved photovoltaic performance resulted from the combination of synergic effects promoted by a more favorable film morphology, such as high exciton dissociation efficiency and lower bimolecular recombinations resulting in higher charge collection efficiency at the electrodes. The positive effect of TMB, compared to that of commonly employed halogenated solvents, confirms the great potential of this approach for the development of efficient PSCs for practical applications with reduced environmental impact. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 487–494     

Pyrene–benzothiadiazole‐based copolymers for application in photovoltaic devices

The preparation and characterization of four narrow band gap pyrene–benzothiadiazole‐based alternating copolymers are presented. An investigation of the impact of attaching different solubilizing groups to the pyrene repeat units on the optical, electrochemical, and thermal properties of the resulting materials was undertaken along with studies on the aggregation of polymer chains in the solid state. Unsurprisingly, polymers which had the smaller 2‐ethylhexyl chains attached to the pyrene units (PP_EH‐DTBT and PP_EH‐DTffBT) displayed lower molecular weights relative to polymers with larger 2‐hexyldecyl substituents (PP_HD‐DTBT and PP_HD‐DTffBT). Despite this, the 2‐ethylhexyl substituted polymers displayed narrower optical band gaps relative to their analogous 2‐hexyldecyl substituted polymers. Of all polymers synthesized, PP_EH‐DTBT displayed the lowest optical band gap (1.76 eV) in the series. All polymers display degradation temperatures in excess of 300°C. Polymers with smaller alkyl chains on the pyrene units display shallower highest occupied molecular orbital levels, which could be due to increased intramolecular charge transfer between the donor and acceptor units. Preliminary investigations on bulk heterojunction solar cells with a device structure indium tin oxide/poly(3,4‐ethylenedioxythiophene) : polystyrene sulfonate /Polymer : PC₇₀BM/Ca/Al were undertaken. Polymer/PC₇₀BM blend ratios of one third were used in these studies and have indicated that PP_EH‐DTBT displayed the highest efficiency with a power conversion efficiency of 1.86%. Copyright © 2016 John Wiley & Sons, Ltd.     

A new quinoxaline and isoindigo based polymer as donor material for solar cells: Role of ecofriendly processing solvents on the device efficiency and stability

A new semiconducting polymer based on two different electron deficient (quinoxaline and isoindigo) and electron rich (benzodithiophene) moieties is synthesized, characterized and used as donor material for photovoltaic devices. Blade‐coated bulk heterojunction solar cells are fabricated in air by using chlorinated (o‐dichlorobenzene) and nonchlorinated (o‐xylene) solvents for the deposition of the active layer. The use of o‐xylene allows a ～10% improvement of the device efficiency in comparison to the analogous system processed from o‐dichlorobenzene. In addition, the evolution of the photovoltaic parameters of the resulting devices during thermal stress is monitored and compared, demonstrating a nearly identical resistance against temperature. The reported results not only highlight the promising properties of the new polymer in terms of environmental stability and compatibility with nonhalogenated solvents, but also show an easy and ecofriendly way to further improve the device performance without altering the corresponding thermal stability. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 234–242     

New small organic molecules based on thieno[2,3-b]indole for efficient bulk heterojunction organic solar cells: a computational study

ABSTRACT

This study aims to evaluate the characteristics of novel organic D-π-A-π-D class small-molecules by using carefully the density functional theory, and time-dependent density functional theory calculations. Thedesigned sequence of (D-A) BHJ-1a to BHJ-4a in organic Bulk Heterojunction (BHJ) solar cells has been comprehensively analysed. Thiéno[2,3-b]indole (TI) has been used as donor, and Diketopyrrolopyrrole (DPP) as acceptor for all compounds. In order to improve the electronic, photovoltaic, and opticalproperties, we have substituted thiophene unit with furan, thieno[2,3-b]thiophene, thiazole and thiazolothiazole as π-bridge moieties. Thus, the result shows that the wise choice of the π-bridge units plays a significant role in improving E_gap, producing a high bathochromic shift, and increasing V_OC as well as a theoretical power conversion efficiency (PCE) over 7%. Interestingly, BHJ-4a with suitable π-bridge presents the optimal electronic properties with low band gap (1.870?eV) and high V_OC (1.534?eV). Furthermore, we have modelled a Bulk heterojunction organic photovoltaic cells based on donor-PCBM complex in order to achieve the optimum E_gap and V_OC. Consequently, the obtained results provide a new way to design BHJ small molecule donors with higher power conversion efficiency.     

Dark currents in bulk heterojunction devices for imaging applications: The effect of a cathode interfacial layer

While sol–gel-processed metal oxides are widely used as an electron transport layer to enhance photovoltaic performances, their effect on photodetector application was not studied. We found sol–gel-processed titanium oxide deteriorated dark current characteristics in reverse biases by almost two orders of magnitude, whereas bare Al cathodes exhibited ideal dark current characteristics. Increased dark current came from space charge limited currents in microscopic p-i-p metal-semiconductor-metal configurations. The spatial variation of workfunction values was believed to form local leakage paths by partial filling of traps on the surface of sol–gel titanium oxide.