Green Inks for the Fabrication of Organic Solar Cells: A Case Study on PBDTTPD:PC61_{61}BM Bulk Heterojunctions

Nonhalogenated ecofriendly solvents are an important asset to avoid costly safety precautions during the fabrication of organic solar cells by printing. Yet, in the past, the quest for suitable nontoxic solvents has widely used empirical approaches. Herein, a comprehensive solubility study is rolled out embracing Hansen solubility parameters (HSPs), tailoring of binary solvents and rational choices of solvent additives, identifying ecofriendly solvents or solvent combinations for the deposition of poly-benzodithophene-thienopyrroledione (PBDTTPD)/fullerene thin-film blends. A particular challenge is the low polymer solubility even in common halogenated solvents. Following the HSPs, initially, a list of suitable solvent candidates is identified which are tested toward their applicability in solar cell fabrication. Among the shortlisted solvents, significant differences between p-xylene and o-xylene are observed, which can be compensated using solvent additives. The ecofriendly green solvent eucalyptol in combination with benzaldehyde and p-anisaldehyde in a ternary solvent mixture gives rise to decent solar cell performances. Solar cells are produced with power conversion efficiencies matching those conventionally fabricated from state-of-the-art halogenated solvents comprising chlorobenzene and chloronaphthalene. Notably, the Hansen solubility approach provides an initial choice of solvents, but comes to its limits in predicting the best micromorphology formation, or if solvents react with the organic semiconductors

How the structural deviations on the backbone of conjugated polymers influence their optoelectronic properties and photovoltaic performance

The design of novel conjugated polymers with appropriate frontier orbital energy levels, low band gap (LBG) and suitable carrier transport properties are needed to improve the power conversion efficiency (PCE) of organic photovoltaic devices. In this review, a detailed structure–property relationship study is presented, by identifying those chemical entities in the backbone of conjugated polymers that are responsible for the modification of optoelectronic properties towards high photovoltaic performanc

Thermally stable blue emitting terfluorene block copolymers

Spectroscopic and morphological studies on a series of rod-coil block copolymers containing terfluorene segments as the rigid blocks and polystyrene as the flexible parts demonstrate an increase in the photoluminescence intensity and the exciton lifetime as well as formation of isolated spheres as thin films upon thermal annealing in air (200 degrees C for 30 min). Moreover, no appearance of the low energy emission band centered at 520 nm was found after the same thermal treatment which permits these copolymers to emit pure blue light

3,6-Dialkylthieno[3,2-b]thiophene moiety as a soluble and electron donating unit preserving the coplanarity of photovoltaic low band gap copolymers

International audienc

A [3,2-b]thienothiophene-alt-benzothiadiazole copolymer for photovoltaic applications: design, synthesis, material characterization and device performances

The synthesis of a new alternating copolymer (PTBzT(2)-C12) based on a thiophene-benzothiadiazole-thiophene (TBzT) segment and a thieno[3,2-b]thiophene unit and its utilization as electron donor in photovoltaic bulk heterojunctions are reported. The copolymer has been obtained via a Stille cross-coupling reaction. The material's optical and electrochemical properties, in solution or in thin films, have been investigated using UV-visible absorption as well as photoluminescence spectroscopy and cyclic voltammetry. A significant red-shift of the absorption edge is observed during film formation, leading to an optical bandgap of 1.88 eV. The polymer ionisation potential is as high as 5.3 eV and makes the material particularly interesting for photovoltaic applications since it is a prerequisite for high open circuit voltages and better chemical stability. Bulk heterojunction solar cells using blends of the copolymer, as the electron donor, and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), as the electron transporting material, have been elaborated. A power conversion efficiency of 1% is achieved with a 1:4 PTBzT(2)-C12:PCBM weight ratio and a 30 nm semi-transparent active layer. The open-circuit voltage is equal to 0.74 V and agrees with the expected value. It is anticipated from the copolymer optoelectronic properties, that this novel material should allow power conversion efficiencies above 5% after further optimization of the processing steps

Optimization of the power conversion efficiency in high bandgap pyridopyridinedithiophene-based conjugated polymers for organic photovoltaics by the random terpolymer approach

We report that the organic photovoltaic (OPV) performance of wide band gap pyridopyridinedithiophene-based conjugated polymers can be significantly improved by employing the random terpolymer approach for the development of new pyridopyridinedithiophene-based conjugated polymers. This is demonstrated by the synthesis of the alternating copolymer (P1) consisting of 3,3′-difluoro-2,2′-bithiophene and pyridopyridinedithiophene and the random terpolymer (P2) containing pyridopyridinedithiophene 3,3′-difluoro-2,2′-bithiophene and thiophene. OPV devices fabricated by P1 and P2 in combination with PC61BM and PC71BM in an inverted device configuration exhibited power conversion efficiencies (PCEs) of 1.5% and 4.0%, respectively. We identified that the main reason for the enhanced performance of the OPV devices based on the P2 random copolymer was the improved morphology (miscibility) between P2 and PCBM as compared to P1. More specifically, atomic force microscopy (AFM) and scanning electron microscopy (SEM) studies revealed that the P1 based films showed rougher surface with clear crystallization/precipitation of the polymer chains even after the addition of chloronaphthalene (CN) to the chloroform processing solvent which significantly limited the short circuit current density (JSC), fill factor (FF) and overall performance of the prepared photovoltaic devices. On the other hand, P2 based films showed better miscibility with the acceptor particularly when processed using 5% CN containing chloroform solvent giving a respectable improvement in the PCE of the photovoltaic devices