Étude De Dispositifs Électro-Optiques À Base De Matériaux Dérivés De L'Unité 2,7-CARBAZOLE
... Il m'est important de remercier également Eric Eastwick, Hiroshi Fukutani, Jeff Fras... more ... Il m'est important de remercier également Eric Eastwick, Hiroshi Fukutani, Jeff Fraser, Jean Frenette et Rodica Plesu pour leur aide technique et pour certaines analyses qu'ils ont effectuées pour ... électroluminescent dans les diodes (ITO/PEDOT:PSS/MEL/Ca) 149 XXV Page 26. ...
research is focussing on further improvements to this effi ciency, equally important fundamental ... more research is focussing on further improvements to this effi ciency, equally important fundamental research on the photophysics, photochemistry, and structure/function relationships of organic photovoltaics devices and their constituent materials remains an integral facet of this research area. Such research is vital for investigating all the steps comprising the operation of organic solar cells, from exciton generation, diffusion and dissociation to charge separation, transport and extraction. Optical and opto-electrical methodologies have been successfully applied in order to understand this organic solar cell behavior. Most common are charge carrier mobility studies, [ 4 ] using techniques such as time-of-fl ight, [ 5-8 ] photo-CELIV (photoinduced charge extraction by linearly increasing voltage), [ 5,9,10 ] space-charge limited current [ 11 ] and dark injection. [ 12 ] Other techniques such as electrochemical impedance spectroscopy (EIS), charge
The effect of altering the acceptor and exchanging a key atom in the polymer structure on the ext... more The effect of altering the acceptor and exchanging a key atom in the polymer structure on the extent of non-Langevin (suppressed) recombination has been examined using the polymer/fullerene photovoltaic blend PDTSiTTz:PC60BM. Time-of-flight data show that changing the acceptor from PC60BM to PC70BM maintains the non-Langevin recombination. In contrast, altering the donor polymer by exchanging the silicon bridging atom for a carbon considerably reduces the non-Langevin behavior. Importantly, the addition of a processing additive, diiodooctane (DIO), allows a partial recovery of this non-Langevin recombination. The addition of DIO also decreases the ionization potential of the polymer, which not only explains the drop in open circuit voltage but may also contribute to the partial recovery of non-Langevin behavior observed. It is proposed that localized, more crystalline areas of lower ionization potential (or higher electron affinity) within a mixed/amorphous phase may act as energy sinks for the holes (electrons), thus potentially inhibiting bimolecular recombination. Such a phenomenon could contribute to non-Langevin behavior in organic photovoltaic blends.
Photodegradation in encapsulated silole-based polymer: Pcbm solar cells Photodegradation in encap... more Photodegradation in encapsulated silole-based polymer: Pcbm solar cells Photodegradation in encapsulated silole-based polymer: Pcbm solar cells investigated using transient absorption spectroscopy and charge extraction investigated using transient absorption spectroscopy and charge extraction measurements measurements
Benzothiadiazole-cored regioregular oligothiophenes as building blocks for novel crystalline low band-gap conjugated polymers with solution processibility
2008 IEEE International Conference on Sustainable Energy Technologies, 2008
ABSTRACT Bulk heterojunction organic solar cells based on conjugated polymers and soluble fullere... more ABSTRACT Bulk heterojunction organic solar cells based on conjugated polymers and soluble fullerene derivatives have been intensively studied due to their simple device structure, easy thin-film casting process, low fabrication cost, and high power conversion efficiency. To further enhance the efficiency, it is of paramount importance to develop new p-type polymeric semiconductors with high charge mobility and narrow energy band gaps. In this paper, we report on an innovative way to synthesize low band-gap conjugated polymers with high crystallinity and reasonable solubility for wet process by using electron-acceptor cored regioregular oligothiophenes as building blocks. These novel building blocks can copolymerize with coplanar fused aromatic compounds, such as indolo[3,2-b]carbazole, to form crystalline polymers with enhanced interchain interaction. Two alternating copolymers P(InCzThnBTD) (n = 2-3) synthesized in this work show much narrower optical band gaps than widely used regioregular P3HT. In addition, the first ionization energy of both synthesized polymers remains high at ~5.17 eV, which is important to achieve large open-circuit voltage (Voc) and improve air stability. Remarkably, the bulk heterojunction photovoltaic (PV) cells fabricated from P(InCzTh2BTD) and PC61BM exhibited a promising PV performance with a PCE = 3.6 %, Voc = 0.69 V, Jsc = 9.17 mA/cm2, and FF = 0.57.
The transparent top contact layer of a solar cell is a distributed resistance that cannot be easi... more The transparent top contact layer of a solar cell is a distributed resistance that cannot be easily represented mathematically. We have used a finite element model to calculate the current distribution in this transparent layer. The calculation gives the voltage drop along the layer as well as the resulting reduction in the current generated in the active layer of the solar cell. IV curves generated by the model are used to investigate various methods of experimentally determining series resistance. It is found that the suns-V oc method is the most accurate. A fit of the finite element model to experimental IV data from an organic solar cell is used as an example of obtaining solar cell model parameters.
One polymer:PCBM blend has an emissive charge transfer state that is quenched by a processing add... more One polymer:PCBM blend has an emissive charge transfer state that is quenched by a processing additive, while the other's charge transfer state dissociates very efficiently.
A detailed analysis of the optical and photophysical properties of 2,7-bis(phenylene)-9,9-dioctyl... more A detailed analysis of the optical and photophysical properties of 2,7-bis(phenylene)-9,9-dioctylfluorene (PFP), 2,7-bis(biphenylene)-9,9-dioctylfluorene (BPFBP), 2,7-bis(2-thienyl)-9,9-dioctylfluorene (TFT), and 2,7-bis-(2,2 \-bithien-5-yl)-9,9-dioctylfluorene (BTFBT) in various environments are reported. The optical properties of the free molecules isolated in an alkane matrix are obtained and discussed in terms of the conformation adopted by each derivative in the electronic ground and first excited states. Also, conformational changes are responsible for the optical changes observed at high concentrations in an isopentane glass at 77 K. High quantum yields of all the oligofluorenes at 77 K indicate the absence of quenching effects such as excitonic or aggregation effects. The similar spectral and photophysical properties in matrix and glass environments are explained by the disorder introduced in oligofluorenes by long octyl chains at the C-9 position of the fluorene moiety. To study the effect of intermolecular interactions in the solid state, we recorded the spectra of thin films of these derivatives. The much red-shifted emission band in the solid state cannot be explained by conformational changes and has its origin in the π-stacking of conjugated oligomers in their relaxed S 1 state. As an evidence to show the importance of the role played by octyl chains at the C-9 position of the fluorene moiety, we synthesized two new model compounds: one, without octyl chains at the C-9 position of the fluorene moiety, 2,7-bis(2-thienyl)fluorene (TFTWC) and another with more octyl chains, 1,4-bis(9,9dioctylfluoren-2-yl)phenyl (FPF). The spectral properties of these derivatives have been studied at room temperature and at 77 K. These systems serve as excellent examples to show the effect of intermolecular interactions on optical properties of oligofluorenes.
Spectral and Photophysical Properties of Fluorene-Based Polyesters in Solution and in the Solid State
Macromolecules, 2002
... Soujanya Tirapattur, † ‡ Michel Belletête, † Nicolas Drolet, ‡ § Mario Leclerc, ‡ and Gilles ... more ... Soujanya Tirapattur, † ‡ Michel Belletête, † Nicolas Drolet, ‡ § Mario Leclerc, ‡ and Gilles Durocher* †. Laboratoire de photophysique moléculaire, Département de Chimie, Université de Montréal, CP 6128, Succ. Centre-Ville ...
Red green blue light-emitting diodes containing fluorene-based copolymers
Journal of Optics A: Pure and Applied Optics, 2002
... Nicolas Drolet1,2, Serge Beaupré1, Jean-François Morin1, Ye Tao2 and Mario Leclerc1,3 ... 20 ... more ... Nicolas Drolet1,2, Serge Beaupré1, Jean-François Morin1, Ye Tao2 and Mario Leclerc1,3 ... 20 L1942 [12] Ranger M, Rondeau D and Leclerc M 1997 Macromolecules 30 7686 [13] Donat-Bouillud A, Lévesque I, Tao Y, D'Iorio M, Beaupré S, Blondin P, Ranger M, Bouchard J ...
Supplementary Information Characterization. Number-average (M n) and weight-average (M w) molecul... more Supplementary Information Characterization. Number-average (M n) and weight-average (M w) molecular weights were determined by size exclusion chromatography (SEC) with a HPLC pump using a Waters UV-visible detector. The calibration curve was made with a series of monodispersed polystyrene standards in THF (HPLC grade, Aldrich). UV-visible absorption spectra were recorded on a Hewlett-Packard diode-array spectrophotometer (model 8452A) using 1-cm path length quartz cells. Optical band gaps were calculated from the onset of the visible absorption band. For solid-state measurements, polymer solutions in chloroform were cast on quartz plates. The fluorescence spectra were measured using a Varian Eclipse spectrofluorimeter. For fluorescence analysis in solution, the polymer concentration was about 10-6 M. The fluorescence quantum yield (φ F) for PCVP and PCVPA were determined in argon-saturated solutions at 298 K in chloroform against PQC10 (φ F = 0.11) in chloroform 1 as the standard. For PCCVP and PCVT, fluorescence quantum yield were determined against 1,3,5,7,8-pentamethyl-2,6diethylpyrromethane•BF 2 (φ F = 0.83) in ethanol. 2 For solid-state fluorescence analysis, the polymer was cast from a solution on a triangular quartz cell and placed at 45° with respect to the incident beam. Electrochemical analysis of polymers was carried out in a two-compartment cell with a Solartron potentiostat (model 1287) using platinum electrodes at a scan rate of 50 mV/s against Ag/Ag + (0.100 M of AgNO 3 in acetonitrile) reference electrode in an anhydrous and argon-saturated solution of 0.1 M of tetrabutylammonium tetrafluoroborate (Bu 4 NBF 4) in acetonitrile. Acetonitrile was dried over CaH 2 under inert atmosphere (argon) during 24 h. Bu 4 NBF 4 was dried at 100°C under reduced pressure. In these conditions, the oxidation potential (E ox) of ferrocene was
Bulk heterojunction solar cells with thick active layers and high fill factors enabled by a bithiophene-co-thiazolothiazole push-pull copolymer
Applied Physics Letters, 2011
A push-pull copolymer is presented which can be used in bulk heterojunction (BHJ) solar cells wit... more A push-pull copolymer is presented which can be used in bulk heterojunction (BHJ) solar cells with active layers greater than 200 nm and fill factors above 60%. The efficiencies of most BHJ solar cells are limited by the fact that they have active layers which are between 60 and 110 nm. While this thickness regime enables peak quantum efficiencies (EQE)
This work was supported by grants from the Natural Sciences and Engineering Research Council of C... more This work was supported by grants from the Natural Sciences and Engineering Research Council of Canada (NSERC) and Canada Research Chair Program. We thank Rodica Plesu, Jean Frenette, and Michel Beaulieu for assistance with some aspects of the experimental work. We have fabricated organic field-effect transistors based on thin films of 2,7-carbazole oligomeric semiconductors 1,4-bis(vinylene-(N-hexyl-2-carbazole))phenylene (CPC), 1,4-bis(vinylene-(N¢-methyl-7¢-hexyl-2¢-carbazole))benzene (RCPCR), N-hexyl-2,7bis(vinylene-(N-hexyl-2-carbazole))carbazole (CCC), and N-methyl-2,7-bis(vinylene-(7-hexyl-N-methyl-2-carbazole))carbazole (RCCCR). The organic semiconductors are deposited by thermal evaporation on bare and chemically modified silicon dioxide surfaces (SiO 2 /Si) held at different temperatures varying from 25 to 200 C during deposition. The resulting thin films have been characterized using UV-vis and Fourier-transform infrared spectroscopies, scanning electron microscopy, and X-ray diffraction, and the observed top-contact transistor performances have been correlated with thin-film properties. We found that these new p-conjugated oligomers can form highly ordered structures and reach high hole mobilities. Devices using CPC as the active semiconductor have exhibited mobilities as high as 0.3 cm 2 V ±1 s ±1 with on/off current ratios of up to 10 7. These features make CPC and 2,7-carbazolenevinylene-based oligomers attractive candidates for device applications.
Photodegradation in Encapsulated Silole-Based Polymer: PCBM Solar Cells Investigated using Transient Absorption Spectroscopy and Charge Extraction Measurements
ABSTRACT Light induced degradation has been observed in the performance of organic solar cells in... more ABSTRACT Light induced degradation has been observed in the performance of organic solar cells in the absence of oxygen and a detailed analysis of the effect of this photodegradation on optical and electrical features has been accomplished. This photodegradation study has been performed on encapsulated photovoltaic blend devices comprised of the silole-based donor-acceptor polymer KP115 blended with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Photodegradation induces an almost 20% decrease in power conversion efficiency, primarily as a result of a reduction in short circuit current, J(SC). The initial burn-in phase of the photodegradation has been examined using a combination of transient absorption spectroscopy and charge extraction measurements, including photo-CELIV (charge extraction by linearly increasing voltage) and time-resolved charge extraction using a nanosecond switch. These measurements reveal a bimodal KP115 polaron population, comprised of both delocalised and localised/trapped charge carriers. The photodegradation results are consistent with an alteration of this bimodal KP115 polaron population, with the polarons becoming trapped in a broader, deeper density of localised states. Under laser illumination and at open circuit conditions, this enhanced trapping after light soaking inhibits charges from undergoing bimolecular recombination, leading to higher extracted charge densities at long times. At the lower charge densities operating at short circuit conditions and under continuous white light illumination, where bimolecular recombination is much less significant, the J(SC) decreases after light soaking due to a reduction in the efficiency of trapped charge carrier extraction.
Quantifying the Relationship between the Maximum Achievable Voltage and Current Levels in Low-Bandgap Polymer Photovoltaics
The Journal of Physical Chemistry C, 2013
ABSTRACT A critical problem in the design of materials for organic photovoltaics is quantifying t... more ABSTRACT A critical problem in the design of materials for organic photovoltaics is quantifying the driving force needed for efficient charge separation without losses associated with a large overpotential. Here, we directly measured the effect of the molecular driving force on the charge transfer rate in films of low-bandgap push–pull type polymers mixed with a series of fullerene-based molecular acceptors using broadband near-infrared transient absorption spectroscopy. By systematically tuning the absolute energy levels of the donor and acceptor, as well as the relative offset between them, we determine the minimum voltage loss required to achieve a high short circuit current. A molecular donor–acceptor framework provides a quantitative description of the charge transfer rate constants in our system and describes the scaling of the photogenerated current with S1–LUMO energy offset. These results point to potential efficiency gains for high performing polymer devices through recovery of additional voltage without sacrificing current output.
Étude De Dispositifs Électro-Optiques À Base De Matériaux Dérivés De L'Unité 2,7-CARBAZOLE
... Il m'est important de remercier également Eric Eastwick, Hiroshi Fukutani, Jeff Fras... more ... Il m'est important de remercier également Eric Eastwick, Hiroshi Fukutani, Jeff Fraser, Jean Frenette et Rodica Plesu pour leur aide technique et pour certaines analyses qu'ils ont effectuées pour ... électroluminescent dans les diodes (ITO/PEDOT:PSS/MEL/Ca) 149 XXV Page 26. ...
research is focussing on further improvements to this effi ciency, equally important fundamental ... more research is focussing on further improvements to this effi ciency, equally important fundamental research on the photophysics, photochemistry, and structure/function relationships of organic photovoltaics devices and their constituent materials remains an integral facet of this research area. Such research is vital for investigating all the steps comprising the operation of organic solar cells, from exciton generation, diffusion and dissociation to charge separation, transport and extraction. Optical and opto-electrical methodologies have been successfully applied in order to understand this organic solar cell behavior. Most common are charge carrier mobility studies, [ 4 ] using techniques such as time-of-fl ight, [ 5-8 ] photo-CELIV (photoinduced charge extraction by linearly increasing voltage), [ 5,9,10 ] space-charge limited current [ 11 ] and dark injection. [ 12 ] Other techniques such as electrochemical impedance spectroscopy (EIS), charge
The effect of altering the acceptor and exchanging a key atom in the polymer structure on the ext... more The effect of altering the acceptor and exchanging a key atom in the polymer structure on the extent of non-Langevin (suppressed) recombination has been examined using the polymer/fullerene photovoltaic blend PDTSiTTz:PC60BM. Time-of-flight data show that changing the acceptor from PC60BM to PC70BM maintains the non-Langevin recombination. In contrast, altering the donor polymer by exchanging the silicon bridging atom for a carbon considerably reduces the non-Langevin behavior. Importantly, the addition of a processing additive, diiodooctane (DIO), allows a partial recovery of this non-Langevin recombination. The addition of DIO also decreases the ionization potential of the polymer, which not only explains the drop in open circuit voltage but may also contribute to the partial recovery of non-Langevin behavior observed. It is proposed that localized, more crystalline areas of lower ionization potential (or higher electron affinity) within a mixed/amorphous phase may act as energy sinks for the holes (electrons), thus potentially inhibiting bimolecular recombination. Such a phenomenon could contribute to non-Langevin behavior in organic photovoltaic blends.
Photodegradation in encapsulated silole-based polymer: Pcbm solar cells Photodegradation in encap... more Photodegradation in encapsulated silole-based polymer: Pcbm solar cells Photodegradation in encapsulated silole-based polymer: Pcbm solar cells investigated using transient absorption spectroscopy and charge extraction investigated using transient absorption spectroscopy and charge extraction measurements measurements
Benzothiadiazole-cored regioregular oligothiophenes as building blocks for novel crystalline low band-gap conjugated polymers with solution processibility
2008 IEEE International Conference on Sustainable Energy Technologies, 2008
ABSTRACT Bulk heterojunction organic solar cells based on conjugated polymers and soluble fullere... more ABSTRACT Bulk heterojunction organic solar cells based on conjugated polymers and soluble fullerene derivatives have been intensively studied due to their simple device structure, easy thin-film casting process, low fabrication cost, and high power conversion efficiency. To further enhance the efficiency, it is of paramount importance to develop new p-type polymeric semiconductors with high charge mobility and narrow energy band gaps. In this paper, we report on an innovative way to synthesize low band-gap conjugated polymers with high crystallinity and reasonable solubility for wet process by using electron-acceptor cored regioregular oligothiophenes as building blocks. These novel building blocks can copolymerize with coplanar fused aromatic compounds, such as indolo[3,2-b]carbazole, to form crystalline polymers with enhanced interchain interaction. Two alternating copolymers P(InCzThnBTD) (n = 2-3) synthesized in this work show much narrower optical band gaps than widely used regioregular P3HT. In addition, the first ionization energy of both synthesized polymers remains high at ~5.17 eV, which is important to achieve large open-circuit voltage (Voc) and improve air stability. Remarkably, the bulk heterojunction photovoltaic (PV) cells fabricated from P(InCzTh2BTD) and PC61BM exhibited a promising PV performance with a PCE = 3.6 %, Voc = 0.69 V, Jsc = 9.17 mA/cm2, and FF = 0.57.
The transparent top contact layer of a solar cell is a distributed resistance that cannot be easi... more The transparent top contact layer of a solar cell is a distributed resistance that cannot be easily represented mathematically. We have used a finite element model to calculate the current distribution in this transparent layer. The calculation gives the voltage drop along the layer as well as the resulting reduction in the current generated in the active layer of the solar cell. IV curves generated by the model are used to investigate various methods of experimentally determining series resistance. It is found that the suns-V oc method is the most accurate. A fit of the finite element model to experimental IV data from an organic solar cell is used as an example of obtaining solar cell model parameters.
One polymer:PCBM blend has an emissive charge transfer state that is quenched by a processing add... more One polymer:PCBM blend has an emissive charge transfer state that is quenched by a processing additive, while the other's charge transfer state dissociates very efficiently.
A detailed analysis of the optical and photophysical properties of 2,7-bis(phenylene)-9,9-dioctyl... more A detailed analysis of the optical and photophysical properties of 2,7-bis(phenylene)-9,9-dioctylfluorene (PFP), 2,7-bis(biphenylene)-9,9-dioctylfluorene (BPFBP), 2,7-bis(2-thienyl)-9,9-dioctylfluorene (TFT), and 2,7-bis-(2,2 \-bithien-5-yl)-9,9-dioctylfluorene (BTFBT) in various environments are reported. The optical properties of the free molecules isolated in an alkane matrix are obtained and discussed in terms of the conformation adopted by each derivative in the electronic ground and first excited states. Also, conformational changes are responsible for the optical changes observed at high concentrations in an isopentane glass at 77 K. High quantum yields of all the oligofluorenes at 77 K indicate the absence of quenching effects such as excitonic or aggregation effects. The similar spectral and photophysical properties in matrix and glass environments are explained by the disorder introduced in oligofluorenes by long octyl chains at the C-9 position of the fluorene moiety. To study the effect of intermolecular interactions in the solid state, we recorded the spectra of thin films of these derivatives. The much red-shifted emission band in the solid state cannot be explained by conformational changes and has its origin in the π-stacking of conjugated oligomers in their relaxed S 1 state. As an evidence to show the importance of the role played by octyl chains at the C-9 position of the fluorene moiety, we synthesized two new model compounds: one, without octyl chains at the C-9 position of the fluorene moiety, 2,7-bis(2-thienyl)fluorene (TFTWC) and another with more octyl chains, 1,4-bis(9,9dioctylfluoren-2-yl)phenyl (FPF). The spectral properties of these derivatives have been studied at room temperature and at 77 K. These systems serve as excellent examples to show the effect of intermolecular interactions on optical properties of oligofluorenes.
Spectral and Photophysical Properties of Fluorene-Based Polyesters in Solution and in the Solid State
Macromolecules, 2002
... Soujanya Tirapattur, † ‡ Michel Belletête, † Nicolas Drolet, ‡ § Mario Leclerc, ‡ and Gilles ... more ... Soujanya Tirapattur, † ‡ Michel Belletête, † Nicolas Drolet, ‡ § Mario Leclerc, ‡ and Gilles Durocher* †. Laboratoire de photophysique moléculaire, Département de Chimie, Université de Montréal, CP 6128, Succ. Centre-Ville ...
Red green blue light-emitting diodes containing fluorene-based copolymers
Journal of Optics A: Pure and Applied Optics, 2002
... Nicolas Drolet1,2, Serge Beaupré1, Jean-François Morin1, Ye Tao2 and Mario Leclerc1,3 ... 20 ... more ... Nicolas Drolet1,2, Serge Beaupré1, Jean-François Morin1, Ye Tao2 and Mario Leclerc1,3 ... 20 L1942 [12] Ranger M, Rondeau D and Leclerc M 1997 Macromolecules 30 7686 [13] Donat-Bouillud A, Lévesque I, Tao Y, D'Iorio M, Beaupré S, Blondin P, Ranger M, Bouchard J ...
Supplementary Information Characterization. Number-average (M n) and weight-average (M w) molecul... more Supplementary Information Characterization. Number-average (M n) and weight-average (M w) molecular weights were determined by size exclusion chromatography (SEC) with a HPLC pump using a Waters UV-visible detector. The calibration curve was made with a series of monodispersed polystyrene standards in THF (HPLC grade, Aldrich). UV-visible absorption spectra were recorded on a Hewlett-Packard diode-array spectrophotometer (model 8452A) using 1-cm path length quartz cells. Optical band gaps were calculated from the onset of the visible absorption band. For solid-state measurements, polymer solutions in chloroform were cast on quartz plates. The fluorescence spectra were measured using a Varian Eclipse spectrofluorimeter. For fluorescence analysis in solution, the polymer concentration was about 10-6 M. The fluorescence quantum yield (φ F) for PCVP and PCVPA were determined in argon-saturated solutions at 298 K in chloroform against PQC10 (φ F = 0.11) in chloroform 1 as the standard. For PCCVP and PCVT, fluorescence quantum yield were determined against 1,3,5,7,8-pentamethyl-2,6diethylpyrromethane•BF 2 (φ F = 0.83) in ethanol. 2 For solid-state fluorescence analysis, the polymer was cast from a solution on a triangular quartz cell and placed at 45° with respect to the incident beam. Electrochemical analysis of polymers was carried out in a two-compartment cell with a Solartron potentiostat (model 1287) using platinum electrodes at a scan rate of 50 mV/s against Ag/Ag + (0.100 M of AgNO 3 in acetonitrile) reference electrode in an anhydrous and argon-saturated solution of 0.1 M of tetrabutylammonium tetrafluoroborate (Bu 4 NBF 4) in acetonitrile. Acetonitrile was dried over CaH 2 under inert atmosphere (argon) during 24 h. Bu 4 NBF 4 was dried at 100°C under reduced pressure. In these conditions, the oxidation potential (E ox) of ferrocene was
Bulk heterojunction solar cells with thick active layers and high fill factors enabled by a bithiophene-co-thiazolothiazole push-pull copolymer
Applied Physics Letters, 2011
A push-pull copolymer is presented which can be used in bulk heterojunction (BHJ) solar cells wit... more A push-pull copolymer is presented which can be used in bulk heterojunction (BHJ) solar cells with active layers greater than 200 nm and fill factors above 60%. The efficiencies of most BHJ solar cells are limited by the fact that they have active layers which are between 60 and 110 nm. While this thickness regime enables peak quantum efficiencies (EQE)
This work was supported by grants from the Natural Sciences and Engineering Research Council of C... more This work was supported by grants from the Natural Sciences and Engineering Research Council of Canada (NSERC) and Canada Research Chair Program. We thank Rodica Plesu, Jean Frenette, and Michel Beaulieu for assistance with some aspects of the experimental work. We have fabricated organic field-effect transistors based on thin films of 2,7-carbazole oligomeric semiconductors 1,4-bis(vinylene-(N-hexyl-2-carbazole))phenylene (CPC), 1,4-bis(vinylene-(N¢-methyl-7¢-hexyl-2¢-carbazole))benzene (RCPCR), N-hexyl-2,7bis(vinylene-(N-hexyl-2-carbazole))carbazole (CCC), and N-methyl-2,7-bis(vinylene-(7-hexyl-N-methyl-2-carbazole))carbazole (RCCCR). The organic semiconductors are deposited by thermal evaporation on bare and chemically modified silicon dioxide surfaces (SiO 2 /Si) held at different temperatures varying from 25 to 200 C during deposition. The resulting thin films have been characterized using UV-vis and Fourier-transform infrared spectroscopies, scanning electron microscopy, and X-ray diffraction, and the observed top-contact transistor performances have been correlated with thin-film properties. We found that these new p-conjugated oligomers can form highly ordered structures and reach high hole mobilities. Devices using CPC as the active semiconductor have exhibited mobilities as high as 0.3 cm 2 V ±1 s ±1 with on/off current ratios of up to 10 7. These features make CPC and 2,7-carbazolenevinylene-based oligomers attractive candidates for device applications.
Photodegradation in Encapsulated Silole-Based Polymer: PCBM Solar Cells Investigated using Transient Absorption Spectroscopy and Charge Extraction Measurements
ABSTRACT Light induced degradation has been observed in the performance of organic solar cells in... more ABSTRACT Light induced degradation has been observed in the performance of organic solar cells in the absence of oxygen and a detailed analysis of the effect of this photodegradation on optical and electrical features has been accomplished. This photodegradation study has been performed on encapsulated photovoltaic blend devices comprised of the silole-based donor-acceptor polymer KP115 blended with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Photodegradation induces an almost 20% decrease in power conversion efficiency, primarily as a result of a reduction in short circuit current, J(SC). The initial burn-in phase of the photodegradation has been examined using a combination of transient absorption spectroscopy and charge extraction measurements, including photo-CELIV (charge extraction by linearly increasing voltage) and time-resolved charge extraction using a nanosecond switch. These measurements reveal a bimodal KP115 polaron population, comprised of both delocalised and localised/trapped charge carriers. The photodegradation results are consistent with an alteration of this bimodal KP115 polaron population, with the polarons becoming trapped in a broader, deeper density of localised states. Under laser illumination and at open circuit conditions, this enhanced trapping after light soaking inhibits charges from undergoing bimolecular recombination, leading to higher extracted charge densities at long times. At the lower charge densities operating at short circuit conditions and under continuous white light illumination, where bimolecular recombination is much less significant, the J(SC) decreases after light soaking due to a reduction in the efficiency of trapped charge carrier extraction.
Quantifying the Relationship between the Maximum Achievable Voltage and Current Levels in Low-Bandgap Polymer Photovoltaics
The Journal of Physical Chemistry C, 2013
ABSTRACT A critical problem in the design of materials for organic photovoltaics is quantifying t... more ABSTRACT A critical problem in the design of materials for organic photovoltaics is quantifying the driving force needed for efficient charge separation without losses associated with a large overpotential. Here, we directly measured the effect of the molecular driving force on the charge transfer rate in films of low-bandgap push–pull type polymers mixed with a series of fullerene-based molecular acceptors using broadband near-infrared transient absorption spectroscopy. By systematically tuning the absolute energy levels of the donor and acceptor, as well as the relative offset between them, we determine the minimum voltage loss required to achieve a high short circuit current. A molecular donor–acceptor framework provides a quantitative description of the charge transfer rate constants in our system and describes the scaling of the photogenerated current with S1–LUMO energy offset. These results point to potential efficiency gains for high performing polymer devices through recovery of additional voltage without sacrificing current output.
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