Acid Methyl Ester Blend Films Research Articles

Effect of optical intensity distribution on device performances of PTB7-Th:PC71BM-based organic photovoltaic cells

Abstract In order to realize high performance of organic photovoltaic cells (OPVs), it is indispensable to optimize device structures according to composing materials. In particular, the optical density distribution in stacked layers is one of important factors to enhance the optical absorption within the photoactive layer, resulting in the improved photoconversion efficiency. In this study, we optimized the film thickness of the photoactive layer as a fundamental way to control the optical density distribution. The influence of the photoactive layer thickness on photovoltaic performances of OPVs was first investigated using, poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6diyl-alt-(4-(2-ethylhexyl)-3-uorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] and [6,6]-phenyl C 71 -butyric acid methyl ester blend films in normal and inverted OPV structures, respectively. The short circuit current density (J sc ) showed a peak at the thickness at around 100–120 nm and 80–100 nm in normal and inverted OPVs, respectively. In addition, J sc of inverted OPVs were higher than those of normal OPVs for all the thicknesses. The FF decreased continuously with increasing the thickness in both device structures. The fill factor of normal OPV decreased with increasing thickness more slowly than that of the inverted OPV at the thickness of over than 200 nm. Furthermore, we simulated the optical density distribution in the stacked layers using the algorithm utilized matrix formulas based on Abeles's formulas. The simulated optical intensity distribution as a function of the position and the wavelength is in a good agreement with the experimental spectra of external quantum efficiency. The result indicates a close correspondence between the optical density distribution and the photovoltaic performance. At the optimum photoactive layer thickness of 116 nm in the normal and 76 nm in the inverted OPVs, we obtained the highest PCEs of 9.25 and 10.4%, respectively.

Effect of temperature on carrier formation efficiency in organic photovoltaic cells

The internal quantum efficiency (ϕIQ) of an organic photovoltaic cell is governed by plural processes. Here, we propose that ϕIQ can be experimentally decomposed into carrier formation (ϕCF) and carrier transfer (ϕCT) efficiencies. By combining femtosecond time-resolved and electrochemical spectroscopy, we clarified the effect of temperature on ϕCF in a regioregular poly(3-hexylthiophene) (rr-P3HT)/[6,6]-phenyl C61-butyric acid methyl ester blend film. We found that ϕCF (=0.55) at 80 K is the same as that (=0.55) at 300 K. The temperature insensitivity of ϕCF indicates that the electron-hole pairs at the D/A interface are seldom subjected to coulombic binding energy.

Determination of free polaron lifetime in organic bulk heterojunction solar cells by transient time analysis

A transient response technique that is widely used to measure the minority carrier lifetime in inorganic semiconductors is proposed to measure the lifetime of free polarons in a polymer:fullerene bulk heterojunction (BHJ) solar cell. A numerical model that can be used to describe the transient behavior of BHJ devices has been developed. Using the proposed method, the lifetime of free polarons in poly (3-hexylthiophene) and [6, 6]-phenyl C61-butyric acid methyl ester blend film is estimated to be in the range of 300–400 ns.

Dependence of Charge Separation Efficiency on Film Microstructure in Poly(3-hexylthiophene-2,5-diyl):[6,6]-Phenyl-C61 Butyric Acid Methyl Ester Blend Films

Herein we address the factors controlling photocurrent generation in P3HT:PCBM blend films as a function of blend composition and annealing treatment. Absorption, photoluminescence, and transient absorption spectroscopy are used to distinguish the role of exciton dissociation, charge pair separation, and charge collection. Variations in blend film microstructure with composition and annealing treatment are studied using X-ray diffraction. While the trend in photocurrent generation with composition and annealing [Muller, et al., Adv. Mater. 2008, 20, 3510] does not follow the trend in exciton dissociation, it closely follows the trend in charge pair generation. Moreover, charge pair generation efficiency is positively correlated to the degree of polymer crystallization and the appearance of large domains of both polymer and fullerene phases. We argue that larger domains assist charge pair separation by increasing the probability of escape from the P3HT:PCBM interface, thus reducing geminate charge recombination.

Ultrafast spectroscopic studies in polyfluorene: [6,6]-phenylC61-butyric acid methyl ester blend films: monitoring the photoinduced charge transferprocess

We present a study of charge transfer in polyfluorene: [6,6]-phenylC61-butyric acid methyl ester (PCBM) blend films. After photoexciting the conjugatedpolymer, electrons are very quickly transferred onto the PCBM leaving positive charges inthe polymer. The presence of new photogenerated species in both polymer andPCBM can be demonstrated by photoinduced optical absorption studies. Usingultrafast spectroscopic experiments, not only are we able to demonstrate the chargetransfer process in such composites but also we can time resolve the dynamics ofthe process. Charge transfer is, in this way, estimated to take place ps after photoexcitation. After ps all charges have already been transferred and the process concludes. The resultingseparated charges have the potential to produce a photocurrent if they are successfullydriven through the bulk and collected by suitable electrodes.

Composition dependence of electron and hole transport in polyfluorene:[6,6]-phenyl C61-butyric acid methyl ester blend films

We present a study of charge transport in polyfluorene:[6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend films. The electron and hole mobilities, measured via time-of-flight photocurrent experiments, show a strong dependence on the PCBM concentration. As a result, ambipolar transport can be obtained in blends of two otherwise unipolar materials. Both the magnitude of the mobility and the shape of the current transient are strongly affected by the presence of PCBM molecules in the film. Highly dispersive electron and hole transport for the pure polymer becomes nondispersive when PCBM is added.