Oligomeric interface modifiers in hybrid polymer solar cell prototypes investigated by fluorescence voltage spectroscopy

Abstract

Carboxylated oligothiophenes were evaluated as interfacial modifiers between the organic poly(3-hexylthiophene) (P3HT) and inorganic TiO2 layers in bilayer hybrid polymer solar cells. Carboxylated oligothiophenes can be isolated using conventional purification techniques resulting in pure, monodisperse molecules with 100% carboxylation. Device prototypes using carboxylated oligothiophenes as interfacial modifiers showed improved performance in the open-circuit voltage and fill factor over devices using unmodified oligothiophenes as interfacial modifiers. X-ray photoelectron spectroscopy (XPS) studies supported the idea that interface layer adhesion was improved by functionalizing oligothiophenes with a carboxyl moiety. Wide-field fluorescence images revealed that devices made using carboxylated oligothiophenes had fewer aggregates in the P3HT layers atop the modified TiO2 surface. Hysteresis seen in the fluorescence intensity as a function of applied bias, obtained from In-Device Fluorescence Voltage Spectroscopy (ID-FVS), was found to be a diagnostic criterion of the quality of the hybrid interface modification. The best interfaces were found using oligothiophenes functionalized with carboxylates, which created smooth layers on TiO2, and showed no hysteresis, suggesting elimination of interfacial charge traps. However, this hysteresis could be re-introduced by increasing the scan rate of the applied bias, suggesting that smooth P3HT layers created by carboxylated oligothiophene interface modifiers were necessary but not sufficient for sustaining improved photovoltaic properties especially during long-term device operation.

A Quarterthiophene-Based Dye as an Efficient Interface Modifier for Hybrid Titanium Dioxide/Poly(3-hexylthiophene)(P3HT) Solar Cells

“…These dyes are metal ion-free dyes and have an electron-rich thiophene group. It was found that the dyes improve the V OC due to a dipole moment at the interface [16,28]. Oligothiophenes are discrete, monodisperse molecules, and are distinct from polythiophene, which inherently exists as a distribution of molecular weights.…”

“…Oligothiophenes are discrete, monodisperse molecules, and are distinct from polythiophene, which inherently exists as a distribution of molecular weights. A pure carboxylated oligothiophene can be isolated from any unfunctionalized oligomers via column chromatography and recrystallization [28]. Such organic semiconducting oligothiophenes have been intensively investigated and widely used in organic photovoltaic (OPVs) due to the presence of excellent charge transport properties and tunable optical/electrochemical properties [29].…”

Lithium doped poly(3-hexylthiophene) for efficient hole transporter and sensitizer in metal free quaterthiophene dye treated hybrid solar cells

“…Figure 3 a further shows that the insertion of 4T dye at the TiO 2 /P3HT and TiO 2 /doped P3HT interface resulted in a high open-circuit voltage around 0.87 V which is consistent with our previous work 31 . This is probably due to the increased number of electron-rich thiophene units present with 4T dye treated TiO 2 /P3HT and TiO 2 /doped P3HT nanocomposites, which leads to a dipole moment at the interface 30 , 33 , 47 . Furthermore, the current density of 4T dye treated devices with both doped Spiro-OMeTAD and doped P3HT were around 6.43 mA/cm 2 and 13.02 mA/cm 2 , respectively.…”

“…It was found that the insertion of quaterthiophene based 4T dye at Titanium dioxide/Poly (3-hexylthiophene) interface showed an enhanced efficiency 22 , 30 and, improved hole mobility of the polymer nanocomposite by up to two orders of magnitude compared to the corresponding control 31 . This electron-rich thiophene cyanoacrylic acid group containing metal-free dye led to devices with high via generation of dipole moments at the interface 30 , 32 , 33 . Molar extinction coefficient ( ε ) of 4T dye is also higher compared to commercial ruthenium based N719 and Z907 dyes 22 .…”

Roles of Interfacial Modifiers in Inorganic Titania/Organic Poly(3-hexylthiophene) Heterojunction Hybrid Solar Cells

“…Presence of strong electron withdrawing -CN moiety neighboring to the -COOH anchoring group facilitates a molecular dipole directing away from the TiO 2 surface, and enhances the electron affinity of the molecules, simultaneously enhances the J sc and V oc 2.87 [86] TiO 2 /carboxylated oligothiophene/P3HT Addition of carboxylated oligothiophenes enhanced surface coverage and improved interfacial interactions. 0.11 [87] TiO 2 /Z907/P3HT/PEDOT:PSS…”