Low-Bandgap Poly(Thiophene-Phenylene-Thiophene) Derivatives with Broaden Absorption Spectra for Use in High-Performance Bulk-Heterojunction Polymer Solar Cells

Abstract

Two low-bandgap (LGB) conjugated polymers ( P1 and P2) based on thiophene-phenylene-thiophene (TPT) with adequate energy levels have been designed and synthesized for application in bulk-heterojunction polymer solar cells (PSCs). The absorption spectral, electrochemical, field effect hole mobility and photovoltaic properties of LGB TPT derivatives are investigated and compared with poly(3-hexylthiophene) (P3HT). Photophysical studies reveal bandgaps of 1.76 eV for P1 and 1.70 eV for P2, which could effectively harvest broader solar spectrum. In addition, the thin film absorption coefficients of P1 and P2 are 1.6 x 10 (5) cm (-1) (lambda approximately 520 nm) and 1.4 x 10 (5) cm (-1) (lambda approximately 590 nm), respectively. Electrochemical studies indicate desirable HOMO/LUMO levels that enable a high open circuit voltage while blending them with fullerene derivatives as electron acceptors. Furthermore, both materials show sufficient hole mobility (3.4 x 10 (-3) cm (2)/Vs for P2) allowing efficient charge extraction and a good fill-factor for PSC application. High-performance power conversion efficiency (PCE) of 4.4% is obtained under simulated solar light AM 1.5 G (100 mW/cm (2)) from PSC device with an active layer containing 25 wt% P2 and 75 wt% [6,6]-phenyl-C71-butyric acid methyl ester (PC 71BM), which is superior to that of the analogous P3HT cell (3.9%) under the same experimental condition.

Identifying a Threshold Impurity Level for Organic Solar Cells: Enhanced First‐Order Recombination Via Well‐Defined PC84BM Traps in Organic Bulk Heterojunction Solar Cells

“…Polymer-fullerene solar cells were fabricated using blends of the copolymer poly[N-9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole) (PCDTBT) and the fullerene derivatives, [6,6]phenyl C 61 butyric acid methyl ester (PC 60 BM) and [6,6]-phenyl C 84 butyric acid methyl ester (PC 84 BM). Devices of reproducible quality were fabricated on indium tin oxide (ITO) coated glass substrate with the within experimental error.…”

“…The form of the equation for V oc , assuming purely bimolecular recombination, has been extensively shown to be a universal equation for optimized polymer:fullerene solar cells. [ 52 , 60 ] For a bulk recombination process incorporating both bimolecular recombination and trap-assisted recombination, V oc can be expressed as a combination of Equation (5) and (6) . A simplifi ed case of SRH recombination predicts a slope of *V oc = 2kT/e in the plot of V oc versus the natural logarithm of the light intensity, rather than a slope of *V oc = kT /e as shown previously for bimolecular recombination.…”

“…The fullerene derivatives [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 60 BM) and [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) have been widely used as the electron-accepting material. Small amounts of these fullerene materials have been found to quench luminescence in the highly absorptive and fl uorescent semiconducting polymer materials.…”

Solution‐Processed Bulk Heterojunction Photovoltaic Cells from Gradient π‐Conjugated Thienylene Vinylene Dendrimers

“…[24] G0-4-2 has the highest HOMO level and the lowest LUMO level among our compounds. Comparison with the HOMO levels of À6.1 eV and the LUMO levels of À4.3 eV for PCBM [33] (PCBM = [6,6]-phenyl-C 61 butyric acid methyl ester) suggests possible photoinduced intermolecular charge transfer between our molecule and PCBM. [34] …”

The effect of intramolecular donor–acceptor moieties with donor–π-bridge–acceptor structure on the solar photovoltaic performance

“…One of the effective ways to lower the band gap of conjugated polymers is to make -(A) m -(B) n -random type copolymers by adjusting the ratio of the two monomer unit [1][2][3]. On the other hand, it is found that the introduction of electron-acceptor units onto the side chain of conjugated polymers could extend the absorption breadth of sunlight and lower the energy gap to some extent [4,5].…”