C61-butyric Acid Methyl Ester Blend Research Articles

Functional solid additive modified PEDOT:PSS as an anode buffer layer for enhanced photovoltaic performance and stability in polymer solar cells

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is most commonly used as an anode buffer layer in bulk-heterojunction (BHJ) polymer solar cells (PSCs). However, its hygroscopic and acidic nature contributes to the insufficient electrical conductivity, air stability and restricted photovoltaic (PV) performance for the fabricated PSCs. In this study, a new multifunctional additive, 2,3-dihydroxypyridine (DOH), has been used in the PEDOT: PSS buffer layer to obtain modified properties for PEDOT: PSS@DOH and achieve high PV performances. The electrical conductivity of PEDOT:PSS@DOH films was markedly improved compared with that of PEDOT:PSS. The PEDOT:PSS@DOH film exhibited excellent optical characteristics, appropriate work function alignment, and good surface properties in BHJ-PSCs. When a poly(3-hexylthiohpene):[6,6]-phenyl C61-butyric acid methyl ester blend system was applied as the photoactive layer, the power conversion efficiency of the resulting PSCs with PEDOT:PSS@DOH(1.0%) reached 3.49%, outperforming pristine PEDOT:PSS, exhibiting a power conversion enhancement of 20%. The device fabricated using PEDOT:PSS@DOH (1.0 wt%) also exhibited improved thermal and air stability. Our results also confirm that DOH, a basic pyridine derivative, facilitates adequate hydrogen bonding interactions with the sulfonic acid groups of PSS, induces the conformational transformation of PEDOT chains and contributes to the phase separation between PEDOT and PSS chains.

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.

Performance of a Printed Photodetector on a Paper Substrate

A multilayer polymeric photodetector fabricated on a paper substrate by inkjet and aerosol jet printing has been demonstrated. It employs a poly(3-hexylthiophene) and C61-butyric acid methyl ester blend (P3HT:PCBM) as a photoactive layer sandwiched between a silver bottom electrode and a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) top electrode. A deoxyribonucleic acid biopolymer interlayer between P3HT:PCBM and PEDOT:PSS top electrode enables the printing of the PEDOT:PSS on P3HT:PCBM. The printed photodetector exhibits a photoresponse when photoexcited by four different light emitting diodes with center wavelengths of 405, 465, 525, and 635 nm. The highest responsivity was observed at 405 nm. The responsivity to pulsed light reveals a strong frequency dependence from 25 to 1000 Hz.

P3HT:PCBM Incorporated with Silicon Nanoparticles as Photoactive Layer in Efficient Organic Photovoltaic Devices

Silicon nanoparticles doped poly(3‐hexylthiophene) and [6,6]‐phenyl C61‐butyric acid methyl ester blends (P3HT:PCBM: Si NP) have been produced as the photoactive layer of organic photovoltaic devices (OPVs). The silicon nanoparticles’ size is between 80 and 100 nm checked by transmission electron microscope (TEM). The 0.35 wt% Si NP doping OPVs exhibit higher power conversion efficiency (PCE) than other OPVs. The PCE of the OPVs increases from 3.01% to 3.38% mainly due to increasing short‐circuit current density from 8.38 to 9.48 mA/cm2, while the open‐circuit voltage remains the same. The Si NP can provide extra exciton separation and electron pathways in hybrid solar cells.

Low Band Gap Star-Shaped Molecules Based on Benzothia(oxa)diazole for Organic Photovoltaics

A set of star-shaped molecules (SSMs) with benzothiadiazole and benzoxadiazole groups in the arms were designed and synthesized as a low band gap material for use in organic photovoltaic devices. Benzene and silicon atoms were exploited as core moieties to yield three-arm and four-arm SSMs, respectively. Absorption spectra of all SSMs show two major bands around 370 and 530 nm with the latter having significant charge transfer character. The band gaps of SSMs vary between 1.83 and 2.05 eV. Nevertheless, benzoxadiazole containing structures display the lowest band gaps and the deepest highest-occupied molecular orbital levels, suggesting benzoxadiazole is a stronger electron-withdrawing group than benzothiadiazole. Meta-substitution inhibits excitonic delocalization over the three-arm SSMs. Thus, both four-arm and three-arm structures display very similar photophysical properties as verified by experiment as well as theory. The films of SSMs are amorphous in nature; however, hole mobilities of four-arm structures are larger than those of three-arm analogs which has been attributed to isotropic transport of carriers in condensed phase. Atomic force microscopy images reveal similar SSM/[6,6]-phenyl C61-butyric acid methyl ester blend morphology for all SSMs studied in this work. Yet, benzothiadiazole-containing SSMs exhibit better photovoltaic performance than the SSMs with benzoxadiazole groups. One remarkable observation is that the hole reorganization energies determine the magnitude of hole mobilities in an Arrhenius-like law with essentially negligible energetic disorder. The results highlight the importance of minimization of internal reorganization energies for improving carrier mobilities in disordered materials.

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.

Impact of interfacial dipole on carrier transport in bulk heterojunction poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester blends

The electron transport properties in various poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) blend films, prepared by various process conditions, were investigated by admittance spectroscopy at different temperatures. It was found that the electron mobility and the dispersive transport behavior showed a strong dependence on the thermal treatment condition; the blend with the fastest growth rate had orders of magnitude reduction in the mobility and a much more dispersive transport. Using the Gaussian disorder model, it was found that the energetic disorder of the density-of-states between blends plays a significant role in the observed phenomena. It is proposed that the difference in the energetic disorder is due to the interfacial dipole effect at the P3HT/PC61BM heterojunctions in the various blend films.

Enhanced charge collection in polymer photovoltaic cells by using an ethanol-soluble conjugated polyfluorene as cathode buffer layer

We report enhanced polymer photovoltaic (PV) cells by utilizing ethanol-soluble conjugated poly(9, 9-bis (6′-diethoxylphosphorylhexyl) fluorene) (PF-EP) as a buffer layer between the active layer consisting of poly(3-hexylthiophene)/[6, 6]-phenyl C61-butyric acid methyl ester blend and the Al cathode. Compared to the control PV cell with Al cathode, the introduction of PF-EP effectively increases the shunt resistance and improves the photo-generated charge collection since the slightly thicker semi-conducting PF-EP layer may restrain the penetration of Al atoms into the active layer that may result in increased leakage current and quench photo-generated excitons. The power conversion efficiency is increased ca. 8% compared to the post-annealed cell with Al cathode.

Influence of composition and heat-treatment on the charge transport properties of poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester blends

Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blends have been demonstrated to form highly efficient polymer photovoltaic devices. In this letter, the time-of-flight technique is used to investigate the influence of composition and heat treatment on charge transport properties of P3HT and PCBM blends. The transport of electrons and holes both display a transition from dispersive to nondispersive and return to dispersive again as the percentage of PCBM increases. A balanced mobility of both electron and hole is obtained at a composition of 1:1 weight ratio, and it is nearly independent of the electrical field in the range of our test. The increase in carrier mobility is attributed to the formation of a more-ordered structure in the blend. This structural ordering is further enhanced by slowly evaporating the solvent during film formation which results in additional increase in carrier mobility. However, no such effect is observed in thick films (∼200nm), indicating the presence of such high-degree ordering due to heat treatment.

Influence of composition and heat-treatment on the charge transport properties of poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester blends

Influence of composition and heat-treatment on the charge transport properties of poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester blends

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.