Salima Alem

Morphology control in polycarbazole based bulk heterojunction solar cells and its impact on device performance

Incremental increase in dimethyl sulfoxide (or dimethyl formamide) in ortho-dichlorobenzene solution of poly[N-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) gradually reduces the polymer-solvent interaction, the attraction forces between polymer chains become more dominant, and the polymer chains adopt a tight and contracted conformation with more interchain interactions, resulting in a progressive aggregation in both solutions and films. This was used to fine tune the morphology of PCDTBT/PC71BM based solar cells, leading to improved domain structure and hole mobility in the active layer, and significantly improved photovoltaic performance. The power conversion efficiency increased from 6.0% to 7.1% on devices with an active area of 1.0 cm2.

Highly efficient polycarbazole-based organic photovoltaic devices

We combined experimental and computational approaches to tune the thickness of the films in poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT)-based organic solar cells to maximize the solar absorption by the active layer. High power-conversion efficiencies of 5.2% and 5.7% were obtained on PCDTBT-based solar cells when using [6,6]-phenyl C61-butyric acid methyl ester (PC60BM) and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) as the electron acceptor, respectively. The cells are designed to have an active area of 1.0 cm2, which is among the largest organic solar cells in the literature, while maintaining a low series resistance of 5 Ω cm2.

New low band gap thieno[3,4-b]thiophene-based polymers with deep HOMO levels for organic solar cells

Two new soluble alternating alkyl-substituted benzo[1,2-b:4,5-b′]dithiophene and ketone-substituted thieno[3,4-b]thiophene copolymers were synthesized and characterized. We found that grafting 3-butyloctyl side chains to the benzo[1,2-b:4,5-b′]dithiophene unit at C4 and C8 afforded the resulting polymer (P1) a high hole mobility (∼10−2 cm2Vs−1) and a low-lying HOMO energy level (5.22 eV). Preliminary experiments in bulk heterojunction solar cells using P1 as the electron donor demonstrated a high power conversion efficiency of 4.8% even with PC61BM as the electron acceptor. The introduction of an electron-withdrawing fluorine atom into the thieno[3,4-b]thiophene unit at the C3 position (P2) lowers the HOMO energy level and consequently improves the open circuit voltage from 0.78 to 0.86 V. These values are about 0.1 V higher than those reported for their analogues based on alkoxy-substituted benzo[1,2-b:4,5-b′]dithiophene. This work demonstrates that the replacement of the alkoxy chains on the benzo[1,2-b:4,5-b′]dithiophene unit with less electron-donating alkyl chains is able to lower the HOMO energy levels of this class of polymers without increasing their band gap energy.

Synthesis of oligofluorene modified C60 derivatives for organic solar cell applications

New oligofluorene modified C60 derivatives were designed and synthesized viarhodium complex catalyzed direct coupling between C60 and oligofluorene boronic acids in H2O/1,2-dichlorobenzene (1 : 4). The product was first purified by silica gel column chromatography and then further purified by semipreparative HPLC equipped with a Buckyprep column using toluene as an eluent. The strong deep-blue fluorescence of the oligofluorene bridge was completely quenched after end-capped with C60, indicating an efficient energy and/or charge transfer between the π-conjugated bridge and the C60 cages. Cyclic voltammetric measurements show that the HOMO and LUMO energy levels of the synthesized C60 derivatives are similar to those of widely used PC61BM. This means that the energy levels of the C60 derivatives mainly depend on the π electron system of the C60 cage. However, the substituents have huge impacts on the other physical properties of the resulting C60 derivatives, such as solubility, crystallinity, and electron mobility.

Flexo printed sol-gel derived vanadium oxide films as an interfacial hole-transporting layer for organic solar cells

In this paper we report on the synthesis and development of vanadium oxide precursor flexographic ink for the printing of hole-transporting layers in organic solar cells. For the synthesis of vanadium oxide inks, a sol-gel methodology was utilized. By modifying the vanadium alkoxide precursor with a right type of coordinating ligands a stable and flexoprintable ink has been successfully developed. Flexo-printing afforded smooth and uniform vanadium oxide sol-gel films on top of PCDTBT:PC70BM films. The conversion of the synthesized sol-gel film into a corresponding vanadium oxide layer was followed by DSC/TGA and XPS analyses. The inks were used for the fabrication of inverted organic solar cells by flexo-printing. Power conversion efficiencies ranging between 3.5 % and 4.5 % were achieved, which are slightly lower than the reference cells using vacuum-deposited MoO3 as the hole-transporting layers.