Mariano Campoy-Quiles - Academia.edu (original) (raw)

Papers by Mariano Campoy-Quiles

Synthetic Metals, 2005

We have used spectroscopic ellipsometry to characterise thin films of poly(9,9-dioctylfluorene) (... more We have used spectroscopic ellipsometry to characterise thin films of poly(9,9-dioctylfluorene) (PFO) spin coated on different substrates. Analysis of the ellipsometry data suggests that a model based on critical points of zero order (i.e. excitons) provides an appropriate framework to describe the data. It is found to be very accurate and to result in low standard deviations and a low correlation among the fitting parameters compared to other models, such as the Harmonic Oscillator Approximation or the Model Dielectric Function.

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2007

We investigate the potential for replacing inorganic semiconductors with polymeric semiconductors... more We investigate the potential for replacing inorganic semiconductors with polymeric semiconductors in medical X-ray imaging applications. Polymeric semiconductors are soft and can be fabricated using techniques such as spin coating and jet printing, leading to reductions in fabrication costs for large-area arrays, easy integration of heterostructures and composite materials and the possibility of using flexible substrates. By using a combined cascaded linear systems and Monte-Carlo model to simulate the imaging system, we establish a set of semiconductor requirements for a feasible flat-panel imager (FPI).

Journal of Physical Chemistry C, 2011

Synthetic Metals, 2012

ABSTRACT Post deposition annealing treatments are considered one of the most important and effect... more ABSTRACT Post deposition annealing treatments are considered one of the most important and effective ways to increase the performance of polymer-based thin films for organic photovoltaics (OPVs) and organic light emitting diodes (OLEDs).Hence, thermally induced morphological changes such as phase transitions are key phenomena which can have a determinant influence on the final properties and stability of the materials and devices based upon them. In this work, we have successfully proven that in-situ atomic force microscopy (AFM) and Raman spectroscopy can be used to measure the cold crystallization transition temperature of the widely studied blue-emitting polymer poly(9,9-dioctylfluorene) (PFO) and the workhorse system for photovoltaics based on mixtures of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Raman, as a bulk probe, evidences lower crystallization temperatures for PFO and P3HT films compared to those obtained at the surface with AFM which suggest the existence of morphological and/or molecular mobility differences between the bulk and the surface.

Organic Electronics, 2009

We present a detailed study of the effects of ternary mixing on blend morphology, charge carrier ... more We present a detailed study of the effects of ternary mixing on blend morphology, charge carrier mobility and organic solar cell performance. We investigate ternaries consisting of regio random poly(3-hexylthiophene) (P3HT), regio regular P3HT and soluble fullerene derivative, PCBM. By means of absorption, photoluminescence, atomic force microscopy and X-ray diffraction, we demonstrate that the structure of ternary films consists of crystallites of regular P3HT embedded into a random polymer matrix acting as a soft scaffolding where PCBM can only form nanoscale aggregates but cannot grow the detrimental micron size structures often observed in the conventional regular P3HT:PCBM case upon annealing. The ternary films exhibit higher degree of crystallinity than the conventional blends, but with smaller crystallite sizes. Moreover, we show that the addition of the random polymer chains does not prevent good charge 2 carrier transport for regio random P3HT concentrations up to 50% of the total polymer content. Finally, we prove that solar cells based on the ternary systems have a similar short circuit current than the conventional binary, but improved open circuit current (by ~100 mV), which leads to an overall enhancement of power conversion efficiency.

Journal of Materials Chemistry, 2012

ABSTRACT We demonstrate a solution-based method to create vertical nanoporous structures of semic... more ABSTRACT We demonstrate a solution-based method to create vertical nanoporous structures of semiconducting polymer poly(3-hexylthiophene) over large areas by taking advantage of the spontaneous phase segregation between poly(3-hexylthiophene) and polystyrene deposited from a single solution and, in a second step, removing polystyrene by selectively dissolving it. Nanoporous films with pore diameters which can be tuned down to 120 nm are produced by varying the composition. The potential of the method is further demonstrated by fabricating fully operational solar cells after deposition of [6,6]-phenyl-C61-butyric acid methyl ester from an adequate solvent. Under optimized conditions, the devices based on nanostructured thin films exhibit enhanced efficiencies with respect to graded bilayers and bulk heterojunction organic photovoltaic devices. We relate the increase in fill factor observed in the nanostructured devices to changes in the orientation of poly(3-hexylthiophene) chains induced by nanoconfinement and self-assembly with polystyrene resulting from this simple solution process without the use of any elaborate chemistry or soft lithography.

The Journal of Chemical Physics, 2005

We present a detailed structural and surface-enhanced Raman scattering ͑SERS͒ study of poly͑allyl... more We present a detailed structural and surface-enhanced Raman scattering ͑SERS͒ study of poly͑allylamine͒ modified with Os͑byp͒ 2 ClPyCHO ͑PAH-Os͒ and gold nanoparticles self-assembled multilayers ͓PAH-Os+ ͑Au-nanoparticles/ PAH-Os͒ n , n = 1 and 5͔. Atomic force microscopy and variable-angle spectroscopic ellipsometry measurements indicate that the first nanoparticle layer grows homogenously by partially covering the substrate without clustering. Analyzing the sample thickness and roughness we infer that the growth process advances thereafter by filling with nanoparticles the interstitial spaces between the previously adsorbed nanoparticles. After five immersion steps the multilayers reach a more compact structure. The interaction between plasmons of near-gold nanoparticles provides a new optical absorption around 650 nm which, in addition, allows a more effective SERS process in that spectral region than at the single-plasmon resonance ͑ϳ530 nm͒. We compare the electronic resonance Raman and SERS amplification mechanisms in these self-assembled multilayers analyzing Raman resonance scans and Raman intensity micromaps. As a function of nanoparticle coverage we observe large changes in the Raman intensity scans, with maxima that shift from the electronic transitions, to the plasmon resonance, and finally to the coupled-plasmon absorption. The Raman micromaps, on the other hand, evidence huge intensity inhomogeneities which we relate to "hot spots." Numerical discrete dipole approximation calculations including the interaction between gold nanoparticles are presented, providing a qualitative model for the coupled-plasmon absorption and redshifted Raman hot spots in these samples.

ACS Nano, 2013

Interfaces play a determining role in establishing the degree of carrier selectivity at outer con... more Interfaces play a determining role in establishing the degree of carrier selectivity at outer contacts in organic solar cells. Considering that the bulk heterojunction consists of a blend of electron donor and acceptor materials, the specific relative surface coverage at the electrode interfaces have an impact on the carrier selectivity. This work unravels how fullerene surface coverage at cathode contacts lies behind the carrier selectivity of the electrodes. A variety of techniques as variable-angle spectroscopic ellipsometry and capacitance-voltage measurements have been used to determine the degree of fullerene surface coverage in a set of PCPDTBT-based solar cells processed with different additives. A full screening from highly fullerene-rich to polymer-rich phases attaching the cathode interface has enabled the overall correlation between surface morphology (relative coverage) and device performance (operating parameters). The general validity of the measurements is further discussed in four donor/acceptor systems: PCPDTBT, P3HT, PCDTBT and PTB7 blended with fullerene derivatives. It is demonstrated that a fullerene rich interface at the cathode is a prerequisite to enhance contact selectivity, and consequently power conversion efficiency.

Advanced materials (Deerfield Beach, Fla.), Jan 8, 2016

UV-induced switching from p- to n-type character is demonstrated during deposition of carbon nano... more UV-induced switching from p- to n-type character is demonstrated during deposition of carbon nanotube-conjugated polymer composites. This opens the possibility to photopattern n-type regions within an otherwise p-type film, which has a potential for complementary circuitry or, as shown here, thermoelectric generators made from a single solution.

Nanoscale, Jan 19, 2015

The optical constants of methylammonium lead halide single crystals CH3NH3PbX3 (X = I, Br, Cl) ar... more The optical constants of methylammonium lead halide single crystals CH3NH3PbX3 (X = I, Br, Cl) are interpreted with high level ab initio calculations using the relativistic quasiparticle self-consistent GW approximation (QSGW). Good agreement between the optical constants derived from QSGW and those obtained from spectroscopic ellipsometry enables the assignment of the spectral features to their respective inter-band transitions. We show that the transition from the highest valence band (VB) to the lowest conduction band (CB) is responsible for almost all the optical response of MAPbI3 between 1.2 and 5.5 eV (with minor contributions from the second highest VB and the second lowest CB). The calculations indicate that the orientation of [CH3NH3](+) cations has a significant influence on the position of the bandgap suggesting that collective orientation of the organic moieties could result in significant local variations of the optical properties. The optical constants and energy band...

Chemistry of Materials, 2015

The Journal of Physical Chemistry Letters, 2015

We investigate the ferroelectric properties of photovoltaic methylammonium lead halide CH 3 NH 3 ... more We investigate the ferroelectric properties of photovoltaic methylammonium lead halide CH 3 NH 3 PbI 3 perovskite using piezoelectric force microscopy (PFM) and macroscopic polarization methods. The electric polarization is clearly observed by amplitude and phase hysteresis loops. However, the polarization loop decreases as the frequency is lowered, persisting for a short time only, in the one second regime, indicating that CH 3 NH 3 PbI 3 does not exhibit permanent polarization at room temperature. This result is confirmed by macroscopic polarization measurement based on a standard capacitive method. We have observed a strong increase of piezoelectric response under illumination, consistent with the previously reported giant photoinduced dielectric constant at low frequencies. We speculate that an intrinsic charge transfer photoinduced dipole in the perovskite cage may lie at the origin of this effect.

Energy Environ. Sci., 2015

ABSTRACT Organic photovoltaics is an emerging solar power technology which embodies properties su... more ABSTRACT Organic photovoltaics is an emerging solar power technology which embodies properties such as transparency, flexibility, and rapid, roll to roll manufacture, opening the potential for unique niche applications. We report a detailed techno-economic analysis of one such application, namely the photovoltaic greenhouse, and discuss whether the unique properties of the technology can provide advantages over conventional photovoltaics. The potential for spectral selectivity through the choice of OPV materials is evaluated for the case of a photovoltaic greenhouse. The action spectrum of typical greenhouse crops is used to determine the impact on crop growth of blocking different spectral ranges from the crops. Transfer matrix optical modelling is used to assess the efficiency and spectrally resolved transparency of a variety of commercially available semi-conducting polymer materials, in addition to a non-commercial low-band-gap material with absorption outside that required for crop growth. Economic analysis suggests there could be a huge potential for OPV greenhouses if aggressive cost targets can be met. Technical analysis shows that semi-transparent OPV devices may struggle to perform better than opaque crystalline silicon with partial coverage, however, OPV devices using the low-band-gap material PMDPP3T, as well as a high efficiency mid-band-gap polymer PCDTBT, can demonstrate improved performance in comparison to opaque, flexible thin-film modules such as CIGS. These results stress the importance of developing new, highly transparent electrode and interlayer materials, along with high efficiency active layers, if the full potential of this application is going to be realised.

J. Mater. Chem. A, 2015

ABSTRACT The distribution of electron donor and acceptor in the active layer is known to strongly... more ABSTRACT The distribution of electron donor and acceptor in the active layer is known to strongly influence the electrical performance of polymer solar cells for most of the high performance polymer:fullerene systems. The formulation of the solution from which the active layer is spincoated plays an important role in the quest for morphology control. We have studied how the choice of solvent and the use of small amounts of a low vapour pressure additive in the coating solution influence the film morphology and the solar cell performance for blends of poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM). We have investigated the lateral morphology using atomic force microscopy (AFM) and scanning transmission X-ray microscopy (STXM), the vertical morphology using dynamic secondary ion mass spectrometry (d-SIMS) and variable-angle spectroscopic ellipsometry (VASE), and the surface composition using near-edge X-ray absorption fine structure (NEXAFS). The lateral phase-separated domains observed in films spincoated from single solvents, increase in size with increasing solvent vapour pressure and decreasing PC70BM solubility, but are not observed when 1-chloronaphthalene (CN) is added. A strongly TQ1-enriched surface layer is formed in all TQ1:PC70BM blend films and rationalized by surface energy differences. The photocurrent and power conversion efficiency strongly increased upon the addition of CN, while the leakage current decreased by one to two orders of magnitude. The higher photocurrent correlates with the finer lateral structure and stronger TQ1-enrichment at the interface with the electron-collecting electrode. This indicates that the charge transport and collection are not hindered by this polymer-enriched surface layer. Neither the open-circuit voltage nor the series resistance of the devices are sensitive to the differences in morphology.

Synthetic Metals, 2005

We report significant improvements in the optical gain properties of highly oriented films of the... more We report significant improvements in the optical gain properties of highly oriented films of the fluorene-based conjugated polymer poly(9,9dioctylfluorene-co-benzothiadiazole) (F8BT). We demonstrate amplified spontaneous emission (ASE) in optically pumped aligned F8BT planar asymmetric waveguides. The influence of the polymer chain orientation and the excitation polarisation on the optical properties of the materials has been investigated. By selecting specific optimised excitation configurations, high net gain coefficients can be obtained, namely <47 cm −1 compared with <25 cm −1 for standard spin-coated films. The loss coefficients also reduce from α = 7.6 cm −1 (spin-coated film) to 0.9 cm −1 (aligned film). Ellipsometry allows determination of the optical constants and reveals that the refractive indices of the materials are also greatly affected by the orientation of the polymer chains.

Thin Solid Films

ABSTRACT We present a detailed investigation of the refractive index of polymer:fullerene blends ... more ABSTRACT We present a detailed investigation of the refractive index of polymer:fullerene blends for photovoltaic applications. The donor polymers poly[2,7-(9,9-dioctylfluorene)-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] (APFO3), poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1), and poly[2,7-(9,9-dioctylfluorene)-alt-5,5-(5,10-di-2-thienyl-2,3,7,8-tetraphenyl-pyrazino[2,3-g]quinoxaline)] (APFO Green9) were blended with either [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) . We measured variable angle spectroscopic ellipsometry for three systems, namely APFO3:PCBM, TQ1:PC71BM and APFO-Green9:PC71BM, as a function of composition and analyze the data employing a number of models. We found that Bruggeman effective medium approximations (EMA) are not precise for the description of the optical properties of these blends. This is due to a number of reasons. First, we find that there are energy shifts associated to changes in conjugation length that cannot be accounted for using EMA. Second, blending results in a strong reduction of anisotropy. And finally, our data suggest that there is some degree of vertical segregation between components. Therefore, our results support the idea that the optical properties of polymer:fullerene mixtures should be treated as alloys rather than non-interacting blends.

Organic Optoelectronics and Photonics III, 2008

We present single mode optically pumped lasing from a new resonator structure for polymer lasers ... more We present single mode optically pumped lasing from a new resonator structure for polymer lasers employing nonperiodic circular Bragg gratings. Our devices, using a polyfluorene derivative (BN-PFO) as gain medium, are the first blue-emitting circular grating semiconductor lasers (either organic or inorganic). They exhibit feature sizes as small as 47 nm and emit azimuthally polarized beams with a spectral linewidth ≈ 0.2 nm. We find a minimum lasing threshold energy density of 1.2 muJ/cm2 (10 Hz, 8 ns, 355 nm Nd:YAG laser excitation). The quality factor of the resonator modal fields is found to be at least 2200 for these devices.

Physical Review B, 2007

We present a detailed investigation on the effective dimensionality ͑associated with the degree o... more We present a detailed investigation on the effective dimensionality ͑associated with the degree of delocal-ization͒ of electronic excitations in thin organic films using the dielectric function as obtained from ellipsometry. To this end, we study first the best analytical representation of the optical dielectric function of these materials and compare different approaches found in the literature: ͑i͒ the harmonic oscillator approximation, ͑ii͒ the standard critical-point model ͑SCP͒, ͑iii͒ the model dielectric function ͑MDF͒, and ͑iv͒ the Forouhi-Bloomer model. We use these models to analyze variable angle spectroscopic ellipsometry raw data for a thin poly͑9,9-dioctylfluorene͒ ͑PFO͒ film deposited on quartz ͑taken as an archetypal sample͒. The superiority of the SCP model for PFO films and a wide range of other spin-coated conjugated polymers ͑and guest-molecules in polymers͒ is demonstrated. Moreover, we show how the SCP model can be used to gain physical information on the microscopic structure. As an example, we show that the delocalization of excitons decreases for nonconjugated polymers, such as polymethylmethacrylate and polyimide, while the conjugation length and exciton delocalization are, respectively, enhanced in cases where a planar conformation ͑e.g., ␤ phase of PFO͒ or a high degree of crystallinity ͓e.g., poly͑3-hexylthiophene͔͒ is achieved. As an additional example, we employ the SCP excitonic model to investigate the temperature dependence of the dielectric function of crystalline and glassy PFO films. We propose that the SCP excitonic model should be adopted as the standard choice to model the optical properties of polymer thin films from ellipsometry data.

Synthetic Metals, 2005

We have used spectroscopic ellipsometry to characterise thin films of poly(9,9-dioctylfluorene) (... more We have used spectroscopic ellipsometry to characterise thin films of poly(9,9-dioctylfluorene) (PFO) spin coated on different substrates. Analysis of the ellipsometry data suggests that a model based on critical points of zero order (i.e. excitons) provides an appropriate framework to describe the data. It is found to be very accurate and to result in low standard deviations and a low correlation among the fitting parameters compared to other models, such as the Harmonic Oscillator Approximation or the Model Dielectric Function.

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2007

We investigate the potential for replacing inorganic semiconductors with polymeric semiconductors... more We investigate the potential for replacing inorganic semiconductors with polymeric semiconductors in medical X-ray imaging applications. Polymeric semiconductors are soft and can be fabricated using techniques such as spin coating and jet printing, leading to reductions in fabrication costs for large-area arrays, easy integration of heterostructures and composite materials and the possibility of using flexible substrates. By using a combined cascaded linear systems and Monte-Carlo model to simulate the imaging system, we establish a set of semiconductor requirements for a feasible flat-panel imager (FPI).

Journal of Physical Chemistry C, 2011

Synthetic Metals, 2012

ABSTRACT Post deposition annealing treatments are considered one of the most important and effect... more ABSTRACT Post deposition annealing treatments are considered one of the most important and effective ways to increase the performance of polymer-based thin films for organic photovoltaics (OPVs) and organic light emitting diodes (OLEDs).Hence, thermally induced morphological changes such as phase transitions are key phenomena which can have a determinant influence on the final properties and stability of the materials and devices based upon them. In this work, we have successfully proven that in-situ atomic force microscopy (AFM) and Raman spectroscopy can be used to measure the cold crystallization transition temperature of the widely studied blue-emitting polymer poly(9,9-dioctylfluorene) (PFO) and the workhorse system for photovoltaics based on mixtures of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Raman, as a bulk probe, evidences lower crystallization temperatures for PFO and P3HT films compared to those obtained at the surface with AFM which suggest the existence of morphological and/or molecular mobility differences between the bulk and the surface.

Organic Electronics, 2009

We present a detailed study of the effects of ternary mixing on blend morphology, charge carrier ... more We present a detailed study of the effects of ternary mixing on blend morphology, charge carrier mobility and organic solar cell performance. We investigate ternaries consisting of regio random poly(3-hexylthiophene) (P3HT), regio regular P3HT and soluble fullerene derivative, PCBM. By means of absorption, photoluminescence, atomic force microscopy and X-ray diffraction, we demonstrate that the structure of ternary films consists of crystallites of regular P3HT embedded into a random polymer matrix acting as a soft scaffolding where PCBM can only form nanoscale aggregates but cannot grow the detrimental micron size structures often observed in the conventional regular P3HT:PCBM case upon annealing. The ternary films exhibit higher degree of crystallinity than the conventional blends, but with smaller crystallite sizes. Moreover, we show that the addition of the random polymer chains does not prevent good charge 2 carrier transport for regio random P3HT concentrations up to 50% of the total polymer content. Finally, we prove that solar cells based on the ternary systems have a similar short circuit current than the conventional binary, but improved open circuit current (by ~100 mV), which leads to an overall enhancement of power conversion efficiency.

Journal of Materials Chemistry, 2012

ABSTRACT We demonstrate a solution-based method to create vertical nanoporous structures of semic... more ABSTRACT We demonstrate a solution-based method to create vertical nanoporous structures of semiconducting polymer poly(3-hexylthiophene) over large areas by taking advantage of the spontaneous phase segregation between poly(3-hexylthiophene) and polystyrene deposited from a single solution and, in a second step, removing polystyrene by selectively dissolving it. Nanoporous films with pore diameters which can be tuned down to 120 nm are produced by varying the composition. The potential of the method is further demonstrated by fabricating fully operational solar cells after deposition of [6,6]-phenyl-C61-butyric acid methyl ester from an adequate solvent. Under optimized conditions, the devices based on nanostructured thin films exhibit enhanced efficiencies with respect to graded bilayers and bulk heterojunction organic photovoltaic devices. We relate the increase in fill factor observed in the nanostructured devices to changes in the orientation of poly(3-hexylthiophene) chains induced by nanoconfinement and self-assembly with polystyrene resulting from this simple solution process without the use of any elaborate chemistry or soft lithography.

The Journal of Chemical Physics, 2005

We present a detailed structural and surface-enhanced Raman scattering ͑SERS͒ study of poly͑allyl... more We present a detailed structural and surface-enhanced Raman scattering ͑SERS͒ study of poly͑allylamine͒ modified with Os͑byp͒ 2 ClPyCHO ͑PAH-Os͒ and gold nanoparticles self-assembled multilayers ͓PAH-Os+ ͑Au-nanoparticles/ PAH-Os͒ n , n = 1 and 5͔. Atomic force microscopy and variable-angle spectroscopic ellipsometry measurements indicate that the first nanoparticle layer grows homogenously by partially covering the substrate without clustering. Analyzing the sample thickness and roughness we infer that the growth process advances thereafter by filling with nanoparticles the interstitial spaces between the previously adsorbed nanoparticles. After five immersion steps the multilayers reach a more compact structure. The interaction between plasmons of near-gold nanoparticles provides a new optical absorption around 650 nm which, in addition, allows a more effective SERS process in that spectral region than at the single-plasmon resonance ͑ϳ530 nm͒. We compare the electronic resonance Raman and SERS amplification mechanisms in these self-assembled multilayers analyzing Raman resonance scans and Raman intensity micromaps. As a function of nanoparticle coverage we observe large changes in the Raman intensity scans, with maxima that shift from the electronic transitions, to the plasmon resonance, and finally to the coupled-plasmon absorption. The Raman micromaps, on the other hand, evidence huge intensity inhomogeneities which we relate to "hot spots." Numerical discrete dipole approximation calculations including the interaction between gold nanoparticles are presented, providing a qualitative model for the coupled-plasmon absorption and redshifted Raman hot spots in these samples.

ACS Nano, 2013

Interfaces play a determining role in establishing the degree of carrier selectivity at outer con... more Interfaces play a determining role in establishing the degree of carrier selectivity at outer contacts in organic solar cells. Considering that the bulk heterojunction consists of a blend of electron donor and acceptor materials, the specific relative surface coverage at the electrode interfaces have an impact on the carrier selectivity. This work unravels how fullerene surface coverage at cathode contacts lies behind the carrier selectivity of the electrodes. A variety of techniques as variable-angle spectroscopic ellipsometry and capacitance-voltage measurements have been used to determine the degree of fullerene surface coverage in a set of PCPDTBT-based solar cells processed with different additives. A full screening from highly fullerene-rich to polymer-rich phases attaching the cathode interface has enabled the overall correlation between surface morphology (relative coverage) and device performance (operating parameters). The general validity of the measurements is further discussed in four donor/acceptor systems: PCPDTBT, P3HT, PCDTBT and PTB7 blended with fullerene derivatives. It is demonstrated that a fullerene rich interface at the cathode is a prerequisite to enhance contact selectivity, and consequently power conversion efficiency.

Advanced materials (Deerfield Beach, Fla.), Jan 8, 2016

UV-induced switching from p- to n-type character is demonstrated during deposition of carbon nano... more UV-induced switching from p- to n-type character is demonstrated during deposition of carbon nanotube-conjugated polymer composites. This opens the possibility to photopattern n-type regions within an otherwise p-type film, which has a potential for complementary circuitry or, as shown here, thermoelectric generators made from a single solution.

Nanoscale, Jan 19, 2015

The optical constants of methylammonium lead halide single crystals CH3NH3PbX3 (X = I, Br, Cl) ar... more The optical constants of methylammonium lead halide single crystals CH3NH3PbX3 (X = I, Br, Cl) are interpreted with high level ab initio calculations using the relativistic quasiparticle self-consistent GW approximation (QSGW). Good agreement between the optical constants derived from QSGW and those obtained from spectroscopic ellipsometry enables the assignment of the spectral features to their respective inter-band transitions. We show that the transition from the highest valence band (VB) to the lowest conduction band (CB) is responsible for almost all the optical response of MAPbI3 between 1.2 and 5.5 eV (with minor contributions from the second highest VB and the second lowest CB). The calculations indicate that the orientation of [CH3NH3](+) cations has a significant influence on the position of the bandgap suggesting that collective orientation of the organic moieties could result in significant local variations of the optical properties. The optical constants and energy band...

Chemistry of Materials, 2015

The Journal of Physical Chemistry Letters, 2015

We investigate the ferroelectric properties of photovoltaic methylammonium lead halide CH 3 NH 3 ... more We investigate the ferroelectric properties of photovoltaic methylammonium lead halide CH 3 NH 3 PbI 3 perovskite using piezoelectric force microscopy (PFM) and macroscopic polarization methods. The electric polarization is clearly observed by amplitude and phase hysteresis loops. However, the polarization loop decreases as the frequency is lowered, persisting for a short time only, in the one second regime, indicating that CH 3 NH 3 PbI 3 does not exhibit permanent polarization at room temperature. This result is confirmed by macroscopic polarization measurement based on a standard capacitive method. We have observed a strong increase of piezoelectric response under illumination, consistent with the previously reported giant photoinduced dielectric constant at low frequencies. We speculate that an intrinsic charge transfer photoinduced dipole in the perovskite cage may lie at the origin of this effect.

Energy Environ. Sci., 2015

ABSTRACT Organic photovoltaics is an emerging solar power technology which embodies properties su... more ABSTRACT Organic photovoltaics is an emerging solar power technology which embodies properties such as transparency, flexibility, and rapid, roll to roll manufacture, opening the potential for unique niche applications. We report a detailed techno-economic analysis of one such application, namely the photovoltaic greenhouse, and discuss whether the unique properties of the technology can provide advantages over conventional photovoltaics. The potential for spectral selectivity through the choice of OPV materials is evaluated for the case of a photovoltaic greenhouse. The action spectrum of typical greenhouse crops is used to determine the impact on crop growth of blocking different spectral ranges from the crops. Transfer matrix optical modelling is used to assess the efficiency and spectrally resolved transparency of a variety of commercially available semi-conducting polymer materials, in addition to a non-commercial low-band-gap material with absorption outside that required for crop growth. Economic analysis suggests there could be a huge potential for OPV greenhouses if aggressive cost targets can be met. Technical analysis shows that semi-transparent OPV devices may struggle to perform better than opaque crystalline silicon with partial coverage, however, OPV devices using the low-band-gap material PMDPP3T, as well as a high efficiency mid-band-gap polymer PCDTBT, can demonstrate improved performance in comparison to opaque, flexible thin-film modules such as CIGS. These results stress the importance of developing new, highly transparent electrode and interlayer materials, along with high efficiency active layers, if the full potential of this application is going to be realised.

J. Mater. Chem. A, 2015

ABSTRACT The distribution of electron donor and acceptor in the active layer is known to strongly... more ABSTRACT The distribution of electron donor and acceptor in the active layer is known to strongly influence the electrical performance of polymer solar cells for most of the high performance polymer:fullerene systems. The formulation of the solution from which the active layer is spincoated plays an important role in the quest for morphology control. We have studied how the choice of solvent and the use of small amounts of a low vapour pressure additive in the coating solution influence the film morphology and the solar cell performance for blends of poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM). We have investigated the lateral morphology using atomic force microscopy (AFM) and scanning transmission X-ray microscopy (STXM), the vertical morphology using dynamic secondary ion mass spectrometry (d-SIMS) and variable-angle spectroscopic ellipsometry (VASE), and the surface composition using near-edge X-ray absorption fine structure (NEXAFS). The lateral phase-separated domains observed in films spincoated from single solvents, increase in size with increasing solvent vapour pressure and decreasing PC70BM solubility, but are not observed when 1-chloronaphthalene (CN) is added. A strongly TQ1-enriched surface layer is formed in all TQ1:PC70BM blend films and rationalized by surface energy differences. The photocurrent and power conversion efficiency strongly increased upon the addition of CN, while the leakage current decreased by one to two orders of magnitude. The higher photocurrent correlates with the finer lateral structure and stronger TQ1-enrichment at the interface with the electron-collecting electrode. This indicates that the charge transport and collection are not hindered by this polymer-enriched surface layer. Neither the open-circuit voltage nor the series resistance of the devices are sensitive to the differences in morphology.

Synthetic Metals, 2005

We report significant improvements in the optical gain properties of highly oriented films of the... more We report significant improvements in the optical gain properties of highly oriented films of the fluorene-based conjugated polymer poly(9,9dioctylfluorene-co-benzothiadiazole) (F8BT). We demonstrate amplified spontaneous emission (ASE) in optically pumped aligned F8BT planar asymmetric waveguides. The influence of the polymer chain orientation and the excitation polarisation on the optical properties of the materials has been investigated. By selecting specific optimised excitation configurations, high net gain coefficients can be obtained, namely <47 cm −1 compared with <25 cm −1 for standard spin-coated films. The loss coefficients also reduce from α = 7.6 cm −1 (spin-coated film) to 0.9 cm −1 (aligned film). Ellipsometry allows determination of the optical constants and reveals that the refractive indices of the materials are also greatly affected by the orientation of the polymer chains.

Thin Solid Films

ABSTRACT We present a detailed investigation of the refractive index of polymer:fullerene blends ... more ABSTRACT We present a detailed investigation of the refractive index of polymer:fullerene blends for photovoltaic applications. The donor polymers poly[2,7-(9,9-dioctylfluorene)-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] (APFO3), poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1), and poly[2,7-(9,9-dioctylfluorene)-alt-5,5-(5,10-di-2-thienyl-2,3,7,8-tetraphenyl-pyrazino[2,3-g]quinoxaline)] (APFO Green9) were blended with either [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) . We measured variable angle spectroscopic ellipsometry for three systems, namely APFO3:PCBM, TQ1:PC71BM and APFO-Green9:PC71BM, as a function of composition and analyze the data employing a number of models. We found that Bruggeman effective medium approximations (EMA) are not precise for the description of the optical properties of these blends. This is due to a number of reasons. First, we find that there are energy shifts associated to changes in conjugation length that cannot be accounted for using EMA. Second, blending results in a strong reduction of anisotropy. And finally, our data suggest that there is some degree of vertical segregation between components. Therefore, our results support the idea that the optical properties of polymer:fullerene mixtures should be treated as alloys rather than non-interacting blends.

Organic Optoelectronics and Photonics III, 2008

We present single mode optically pumped lasing from a new resonator structure for polymer lasers ... more We present single mode optically pumped lasing from a new resonator structure for polymer lasers employing nonperiodic circular Bragg gratings. Our devices, using a polyfluorene derivative (BN-PFO) as gain medium, are the first blue-emitting circular grating semiconductor lasers (either organic or inorganic). They exhibit feature sizes as small as 47 nm and emit azimuthally polarized beams with a spectral linewidth ≈ 0.2 nm. We find a minimum lasing threshold energy density of 1.2 muJ/cm2 (10 Hz, 8 ns, 355 nm Nd:YAG laser excitation). The quality factor of the resonator modal fields is found to be at least 2200 for these devices.

Physical Review B, 2007

We present a detailed investigation on the effective dimensionality ͑associated with the degree o... more We present a detailed investigation on the effective dimensionality ͑associated with the degree of delocal-ization͒ of electronic excitations in thin organic films using the dielectric function as obtained from ellipsometry. To this end, we study first the best analytical representation of the optical dielectric function of these materials and compare different approaches found in the literature: ͑i͒ the harmonic oscillator approximation, ͑ii͒ the standard critical-point model ͑SCP͒, ͑iii͒ the model dielectric function ͑MDF͒, and ͑iv͒ the Forouhi-Bloomer model. We use these models to analyze variable angle spectroscopic ellipsometry raw data for a thin poly͑9,9-dioctylfluorene͒ ͑PFO͒ film deposited on quartz ͑taken as an archetypal sample͒. The superiority of the SCP model for PFO films and a wide range of other spin-coated conjugated polymers ͑and guest-molecules in polymers͒ is demonstrated. Moreover, we show how the SCP model can be used to gain physical information on the microscopic structure. As an example, we show that the delocalization of excitons decreases for nonconjugated polymers, such as polymethylmethacrylate and polyimide, while the conjugation length and exciton delocalization are, respectively, enhanced in cases where a planar conformation ͑e.g., ␤ phase of PFO͒ or a high degree of crystallinity ͓e.g., poly͑3-hexylthiophene͔͒ is achieved. As an additional example, we employ the SCP excitonic model to investigate the temperature dependence of the dielectric function of crystalline and glassy PFO films. We propose that the SCP excitonic model should be adopted as the standard choice to model the optical properties of polymer thin films from ellipsometry data.