Manoj Arora

Characterization of doped PEDOT: PSS and its influence on the performance and degradation of organic solar cells

The present work is a detailed study of the optical, morphological and electrical properties of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS, films doped with ethylene glycol (EG) and multi-walled carbon nanotubes (MWCNT). The conductivity of PEDOT:PSS films doped with EG and MWCNT is higher than pristine PEDOT:PSS film. The optical transparency of PEDOT:PSS film decreases insignificantly after addition of MWCNT and EG. The films were further studied using atomic force microscopy, ?x-ray diffraction, Raman spectroscopy and Kelvin probe work function measurement, after which films of PEDOT:PSS with EG and MWCNT were optimized for the fabrication of solar cells. The optimized film was used as a hole extracting layer in a typical ITO/PEDOT:PSS/P3HT:PCBM/Al solar cell. The suitable concentration for an optimized film was found to be 4% MWCNT and 1:4 ratio of EG to PEDOT:PSS. The performance of the device with doped PEDOT:PSS was found to improve in terms of short circuit current density (JSC) and efficiency (?). The solar cell with a doped PEDOT:PSS layer showed higher JSC?and ? due to the increase in the interchains among PEDOT chains along with the introduction of MWCNT channels in PEDOT:PSS matrix. The degradation behavior of the cells was studied and it was found that both pristine and doped PEDOT:PSS cells showed similar trends of degradation. The performance degradation with time was also studied under variable environmental conditions, which showed different aging rates for the two devices.

Role of donor–acceptor domain formation and interface states in initial degradation of P3HT:PCBM-based solar cells

This work is devoted to identifying the degradation mechanism in various structures of a poly(3-hexylthiophene) (P3HT):6,6-phenyl C61-butyric acid methyl ester (PCBM)-based solar cell. We have tried to identify the dominant initial degradation mechanism on the basis of experimental studies carried out on different structures of an organic solar cell. It is known that many of these problems can be solved by means of the following: using proper electrodes and a suitable annealing temperature and duration, improving the morphology of the active film and maintaining a donor–acceptor phase-segregated ordered network as far as possible. The present studies have been carried out both in the dark and under illumination, and it is suggested that initial degradation plays a key role in device performance. The dominant degradation mechanism is the growth of the donor–acceptor complex with time, which not only reduces the effective surface area but also hampers the charge separation. The little change in VOC and the significant change in JSC suggest that once the LiF/Al electrode is improved, one must modify the structure either by introducing a thin layer of high-molecular-weight P3HT between PEDOT:PSS (poly(3,4-ethylenedioxythiopene) poly(styrenesulfonate)) and photoactive P3HT:PCBM layers or by introducing an optimized content of P3HT nanofibrils/nanoparticles into the P3HT:PCBM blend. The best structure was found to be ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al and it can be improved by the above two methods.

An anomalous behavior in degraded bulk heterojunction organic solar cells

An anomalous behavior—a change in polarity with the passage of time in the bulk heterojunction poly(3-hexylthiophene) (P3HT):6,6-phenylC61 butyric acid methyl ester (PCBM) organic solar cells—is reported here. This work is a continuation of our previous work where the initial degradation of the organic solar cells, freshly prepared up to 4 h, was mainly due to domain formation in the active layer. With the passage of time, the activity at the interfaces starts becoming significant. A decrease of VOC and JSC, leading to a change in polarity, has been reported and explained up to 300 h after fabrication.

Effects of aging on the mobility and lifetime of carriers in organic bulk heterojunction solar cells

Mobility and lifetime of the carriers are two very crucial parameters, which can account for the aging of a photovoltaic device, because changes in morphology, electrode/polymer interfaces, etc., will finally result in decrease of mobility and lifetime of carriers. In the present work, we have tried to explain our experimental data with the help of theoretical analysis based on our earlier model in which we have incorporated the activity at electrode interfaces in terms of density of surface states, thickness of interface layer, and tunneling probability. Calculations show that decrease in mobility for a cell under illumination is much faster than in a dark cell. Photoinduced oxidation plays a dominant role in fast degradation of the mobility. The lifetime is also seen to decrease because of modification of parameters such as interface thickness, density of interface states, and roughness.

Thermoelectric power of the bulk amorphous GexSe1-x system

DC conductivity and thermoelectric power of bulk amorphous samples of GexSe1-x (0.1<x<0.4) have been studied as functions of temperature (77-350 K) and composition. The thermoelectric power was found to be positive. The thermo-electric power exhibits a less steep temperature dependence compared with that of the conductivity data. This discrepancy has been explained on the basis of the small polaron model, the chaotic band model and the two-channel model. It was observed that the discrepancy cannot be reconciled with the small polaron model and chaotic band model predictions but the data are best fitted to a parallel current path model, in extended and localised states at the band edges as proposed by Nagels et al. (1985). The present analysis also provides evidence for the existence of a small tail of localised states at the valence band edge.

Electrical transport properties of as evaporated and annealed amorphous GexSe1−x thin films

Abstract D.C. conductivity measurements were made on evaporated and annealed amorphous Ge x Se 1−x thin films as a function of temperature (80–330K) and composition ( x = 1.0, 0.9, 0.7, 0.5, 0.3, 0.1). It was observed that for Ge-rich films ( x = 1.0, 0.9, 0.7) the conduction in the high temperature region takes place due to thermally assisted tunneling of charge carriers in localized states at the band edges and at low temperatures conduction is due to variable range hopping in localized states near the Fermi level. For Se-rich films ( x =0.5, 0.3, 0.1) the conduction is intrinsic in the entire temperature range of measurements. It was observed that annealing reduces the density of states at the Fermi level. At the highest annealing temperature films become polycrystalline.

Degradation Analysis Of Organic Solar Cells Under Variable Conditions - See more at: http://www.vbripress.com/amp/articles/details/24/#sthash.jPCBhYhN.dpuf

In the present work, the degradation mechanism of ITO/PEDOT:PSS/P3HT:PCBM/Al solar cells has been studied under variable environmental conditions, i.e., in air and under vacuum. It was observed that the absorption for P3HT:PCBM film kept under normal atmospheric conditions decreased slightly after 350 hours of fabrication. When these films were kept under vacuum, no change in the absorption intensity was observed. However, when the P3HT:PCBM films with PEDOT:PSS layer were studied, an increase in absorption spectra was observed both under air and vacuum. This strongly suggests that the presence of hygroscopic PEDOT:PSS adversely affects the optical properties of thin films and hence the solar cells. The AFM images of the films after degradation showed presence of microscopic holes and mico-sized particles. The decrease in mobilities with time was also less when the devices were kept in vacuum. This suggests that the decrease in mobility is dependent on the decreasing crystallinity of P3HT:PCBM films as observed by XRD data and due to diffusion of impurities. The fall in efficiency of fabricated devices is higher for device exposed to the environment as compared to the fall for device kept under vacuum. These analyses give insight into the possible degradation pathways and help in eradicating the factors responsible for short shelf-life of organic solar cells, thus enabling better device performance in future. Copyright

Improving P3HT:PCBM based polymer solar cell: Role of doped PEDOT:PSS hole extracting layer towards degradation

Films of PEDOT:PSS and doped PEDOT:PSS, used as hole extracting layer in organic solar cells, have been studied using scanning electron microscopy, Kelvin probe work function measurement, Col-Cole plots, and conductivity measurements and current voltage (I-V) characteristics with time. A change in morphology is observed in PEDOT:PSS films doped with ethylene glycol and multi-walled carbon nanotubes. The doped films have rougher morphology as compared to the pristine films. The work function increases for doped sample thus improving the charge transfer. Further, the analyses of Cole-Cole plots show that the resistance of doped film is lower than pristine film. The change in conductivity with time has been measured, and the I-V characteristics of fresh and degraded samples indicates that fall in conductivity of HEL is one of the reasons for the decrease in device performance.

Dielectric Functions of Niobium Doped Bi3.25La0.75Ti3O12 Thin Films Using Spectroscopic Ellipsometry

Ferroelectric thin films of Nb doped Bi3.25La0.75Ti3O12 (BLT) were prepared by chemical solution deposition method onto Pt/Ti/SiO2/Si(100) substrates using spin coating technique. From the X-ray diffraction analysis it was found that films were polycrystalline with single phase. The optical properties of the Nb doped BLT thin films were investigated at room temperature in the 0.72–6.2 eV energy range using spectroscopic ellipsometry (SE). Double Tauc–Lorentz (DTL) dispersion relation was successfully fitted in the uv/visible region. Value of the full width at half maxima (Г) in dielectric function plots increased with Nb doping. This increase in FWHM may be attributed to the increase in the trap density in forbidden band which consequently decreases the value of Eg. The optical band gap energy (E g d) was found to decrease with increase in Nb doping concentration. This decrease in E g d with doping may be due to the changes in the defect concentration present in the structure.

Electrical Conduction Behaviour of CaCu3Ti4O12 Ceramics with Different Sintering Times

Effect of sintering time on ac conduction has been investigated at different frequencies over a wide range of temperatures. Results have been discussed in the light of different conduction mechanisms. Variation of frequency exponent, ‘n’ (σ = A f n) with temperature has been studied. Owing to its variation with temperature, conduction behavior may be attributed to correlated barrier hoping (CBH) mechanism. Frequency exponent n, which is degree of correlation among charge carriers, found to increase with sintering time. Variation of activation energy with sintering time and frequency has also been studied in detail. Activation energy is observed to decrease with increase in both sintering time and frequency.