Chellappan Vijila - Academia.edu (original) (raw)
Papers by Chellappan Vijila
Electronic Materials Letters, 2018
In this paper, temperature dependence of the excitonic bands in a mechanically exfoliated tungste... more In this paper, temperature dependence of the excitonic bands in a mechanically exfoliated tungsten diselenide (WSe 2) monolayer is studied using photoluminescence and circular dichroic photoluminescence (PL) in the temperature range between 8 and 300 K. The peak energies associated with the neutral exciton (A), charged exciton (trion) and localized excitons are extracted from the PL spectra revealing a trion binding energy of around 30 meV. The circular dichroic PL measured at 8 K shows about 45% valley polarisation that sharply reduces with increasing temperature to 5% at 300 K with photoexcitation energy of 1.96 eV. A detailed analysis of the emission line-width suggests that the rapid decrease of valley polarisation with the increase of temperature is caused by the strong exciton-phonon interactions which efficiently scatter the excitons into different excitonic states that are easily accessible due to the supply of excess photoexcitation energy. The emission linewidth broadening with the increase of temperature indicate residual exciton dephasing lifetime < 100 fs, that correlates with the observed rapid valley depolarisation.
IEEE Journal of Photovoltaics, 2016
Application of our newly synthesized low-band-gap polymer, i.e., PDTBT-TT, in organic solar cells... more Application of our newly synthesized low-band-gap polymer, i.e., PDTBT-TT, in organic solar cells (OSCs) has been studied. We have investigated the effects of molecular weight of the PDTBT-TT on the PDTBT-TT:PC 71 BM-based bulk heterojunction OSCs' performance. It was found that the OSCs fabricated with higher molecular weight (M n) PDTBT-TT polymer have the higher short-circuit current and power conversion efficiency. The better performance of the PDTBT-TT-based OSC with the higher M n PDTBT-TT polymer is attributed to the improved charge transfer resistance and the suppressed bimolecular recombination inside the PDTBT-TT:PC 71 BM blend active layer of the OSC. Moreover, the increased doping density observed in the higher M n PDTBT-TT:PC 71 BM-based OSCs increases the photocurrent and leads to the cell's higher power conversion efficiency.
Phys. Chem. Chem. Phys., 2014
Incorporation of fluorene and diketopyrrolopyrrole functionalities to generate a non-fullerene el... more Incorporation of fluorene and diketopyrrolopyrrole functionalities to generate a non-fullerene electron acceptor for solution-processable organic solar cells is shown to lead to devices with impressive open circuit voltage.
Materials Letters, 2016
Abstract Bismuth ferrite (BFO) is one of the most popular multiferroic oxides and has recently at... more Abstract Bismuth ferrite (BFO) is one of the most popular multiferroic oxides and has recently attracted interest of scientific community as a suitable material for light harvesting applications, due to its efficient polarization-governed mechanism of charge separation and ability to generate above-bandgap voltages. Identified as a promising photoferroic material, BFO has been a subject of various photoconversion studies in conjunction with other oxides and polymers commonly found in organic photovoltaics. In this report, we examined fundamental device parameters such as carrier lifetime, charge transport and diffusion coefficient within BFO/polymer bilayer, utilizing transient photovoltage and photocurrent spectroscopy techniques. Tested devices were fabricated via simple and inexpensive solution processing route, using nitrate precursors for BFO synthesis. Obtained results suggest formation of space-charge region within the device causing charge carrier lifetimes extension up to 350 μs in the absence of external voltage bias, as well as fast charge extraction (
Advanced Functional Materials
The Journal of Physical Chemistry C
- Charge generation and recombination in diketopyrrolopyrrole polymer: Fullerene bulk heteroj... more 2016) Charge generation and recombination in diketopyrrolopyrrole polymer: Fullerene bulk heterojunctions studied by transient absorption and timeresolved microwave conductivity.
ACS applied materials & interfaces, Jan 25, 2017
PbI2-enriched mixed perovskite film [FA0.81MA0.15Pb(I0.836Br0.15)3] has been widely studied due t... more PbI2-enriched mixed perovskite film [FA0.81MA0.15Pb(I0.836Br0.15)3] has been widely studied due to its great potential in perovskite solar cell (PSC) applications. Herein, a FA0.81MA0.15Pb(I0.836Br0.15)3 film has been fabricated with the temperature-dependent optical absorption spectra utilized to determine its exciton binding energy. A ∼13 meV exciton binding energy is estimated, and a near-unity fraction of free carriers out of the total photoexcitons has been obtained in the solar cell operating regime at equilibrium state. PSCs are fabricated with this mixed perovskite film, but a significant electron transport barrier at the TiO2-perovskite interface limited their performance. Cs2CO3 and CsI are then utilized as functional enhancers with which to substantially balance the electron and hole transport and increase the carriers (both electrons and holes) mobilities in PSCs, resulting in much-improved solar-cell performance. The modified PSCs exhibit reproducible power conversion e...
Nanoscale, Jan 7, 2017
Titanium dioxide (TiO2) nanoporous hemispheres (NHSs) with a radius of ∼200 nm are fabricated by ... more Titanium dioxide (TiO2) nanoporous hemispheres (NHSs) with a radius of ∼200 nm are fabricated by electrospraying a hydrothermally synthesized TiO2 nanoparticle (NP) suspension solution. The resulting TiO2 NHSs are highly porous, which are beneficial to the infiltration of perovskites and provide a larger contact area, as building blocks to construct a mesoporous TiO2 layer for FA0.81MA0.15Pb(I0.836Br0.15)3 based perovskite solar cells (PSCs). By varying the TiO2 NHS collecting period (15 s, 30 s, 60 s and 90 s) during the electrospraying process, the performance of PSCs changes with different TiO2 NHS distribution densities. The optimized PSC employing TiO2 NHSs (60 s) exhibits a photovoltaic conversion efficiency (PCE) as high as 19.3% with a Jsc of 23.8 mA cm(-2), a Voc of 1.14 V and a FF of 0.71. Furthermore, the PSC possesses a reproducible PCE value with little hysteresis in its current density-voltage (J-V) curves. The small perturbation transient photovoltage (TPV) measuremen...
J Photochem Photobiol a Chem, 2008
... Adv. Mater., 14 (2002), pp. 1834–1837. [9] M. Wohlgenannt, XM Jiang, ZV Vardeny and RAJ Janss... more ... Adv. Mater., 14 (2002), pp. 1834–1837. [9] M. Wohlgenannt, XM Jiang, ZV Vardeny and RAJ Janssen, Conjugation-length dependence of spin-dependent exciton formation rates in Π-conjugated oligomers and polymers. Phys. Rev. Lett., 88 (2002), pp. 197401–197404. ...
Org Electron, 2007
The charge transport properties in a novel electroluminescent poly{[2-(4 0 ,5 0 -bis(3 00 -methyl... more The charge transport properties in a novel electroluminescent poly{[2-(4 0 ,5 0 -bis(3 00 -methylbutoxy)-2 0 -p-methoxy-phenyl)phenyl-1,4-phenylene vinylene]-co-(9,9-dioctyl-2,7-fluorenylene vinylene)} (BPPPV-PF) have been studied using a time-of-flight (TOF) photoconductivity technique. The TOF transients for holes were recorded over a range of temperatures (207-300 K) and electric fields (1.5 · 10 5 -6.1 · 10 5 V/cm). The hole transport in this polymer was weakly dispersive in nature with a mobility at 300 K of 5 · 10 À5 cm 2 /V s at 2.5 · 10 5 V/cm. This increased to 8.4 · 10 À5 cm 2 /V s at 6.1 · 10 5 V/ cm. The temperature and field dependence of charge mobility has been analyzed using the disorder formalisms (Bässler's Gaussian disorder model (GDM) and correlated disorder model (CDM)). The fit with Gaussian disorder (GDM) model yielded the mobility pre-factor l 1 = 1.2 · 10 À3 cm 2 /V s, energetic disorder parameter r = 82 meV and positional disorder parameter R = 1.73. The average inter-site separation (a = 7 Å ) and the charge localization length (L = 3.6 Å ) was estimated by assuming the CDM type charge transport. The microscopic charge transport parameters derived for this polymer are almost identical to the reported values for fully conjugated polymers with high chemical purity. The results presented indicate that the charge transport parameters can be controlled and optimized for organic optoelectronic applications.
Asian Journal of Organic Chemistry, 2015
An electron-accepting chromophore based on fluorene and naphthalenediimide building blocks for so... more An electron-accepting chromophore based on fluorene and naphthalenediimide building blocks for solution-processable bulk heterojunction devices.
ACS Applied Materials & Interfaces, 2015
The effect of inserting an SU-8 dielectric interlayer into inverted bulk heterojunction (BHJ) org... more The effect of inserting an SU-8 dielectric interlayer into inverted bulk heterojunction (BHJ) organic solar cells (OSCs) was studied. Insertion of an ultrathin layer of SU-8 between the zinc oxide (ZnO) electron transport layer and the photoactive layer resulted in a smoother interface and a 14% enhancement in power conversion efficiency. The properties of devices with and without an SU-8 interlayer were investigated using transient photovoltage (TPV) and double injection (DoI) techniques, and it was found that devices with SU-8 show longer carrier lifetimes and greater mobility-lifetime (μ-τ) products than those without. Devices with SU-8 were also found to have improved stability. The results indicate that the insertion of an SU-8 interlayer reduces the recombination rate for photogenerated carriers without affecting the charge transport properties, improving overall performance and stability.
Organic Electronics, 2007
The charge transport properties in a novel electroluminescent poly{[2-(4 0 ,5 0 -bis(3 00 -methyl... more The charge transport properties in a novel electroluminescent poly{[2-(4 0 ,5 0 -bis(3 00 -methylbutoxy)-2 0 -p-methoxy-phenyl)phenyl-1,4-phenylene vinylene]-co-(9,9-dioctyl-2,7-fluorenylene vinylene)} (BPPPV-PF) have been studied using a time-of-flight (TOF) photoconductivity technique. The TOF transients for holes were recorded over a range of temperatures (207-300 K) and electric fields (1.5 · 10 5 -6.1 · 10 5 V/cm). The hole transport in this polymer was weakly dispersive in nature with a mobility at 300 K of 5 · 10 À5 cm 2 /V s at 2.5 · 10 5 V/cm. This increased to 8.4 · 10 À5 cm 2 /V s at 6.1 · 10 5 V/ cm. The temperature and field dependence of charge mobility has been analyzed using the disorder formalisms (Bässler's Gaussian disorder model (GDM) and correlated disorder model (CDM)). The fit with Gaussian disorder (GDM) model yielded the mobility pre-factor l 1 = 1.2 · 10 À3 cm 2 /V s, energetic disorder parameter r = 82 meV and positional disorder parameter R = 1.73. The average inter-site separation (a = 7 Å ) and the charge localization length (L = 3.6 Å ) was estimated by assuming the CDM type charge transport. The microscopic charge transport parameters derived for this polymer are almost identical to the reported values for fully conjugated polymers with high chemical purity. The results presented indicate that the charge transport parameters can be controlled and optimized for organic optoelectronic applications.
Organic Electronics, 2015
ABSTRACT The technique of photo-CELIV (charge extraction by linearly increasing voltage) is one o... more ABSTRACT The technique of photo-CELIV (charge extraction by linearly increasing voltage) is one of the more straightforward and popular approaches to measure the faster carrier mobility in measurement geometries that are relevant for operational solar cells and other optoelectronic devices. It has been used to demonstrate a time-dependent photocarrier mobility in pristine polymers, attributed to energetic relaxation within the density of states. Conversely, in solar cell blends, the presence or absence of such energetic relaxation on transport timescales remains under debate. We developed a complete numerical model and performed photo-CELIV experiments on the model high efficiency organic solar cell blend poly[3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene] (PDPP-TNT):[6,6]-phenyl-C71-butyric-acid-methyl-ester (PC70BM). In the studied solar cells a constant, time-independent mobility on the scale relevant to charge extraction was observed, where thermalisation of photocarriers occurs on time scales much shorter than the transit time. Therefore, photocarrier relaxation effects are insignificant for charge transport in these efficient photovoltaic devices.
International Journal of Nanotechnology, 2014
ABSTRACT Inverted organic solar cells with device structure ITO/ZnO/poly (3-hexylthiophene) (P3HT... more ABSTRACT Inverted organic solar cells with device structure ITO/ZnO/poly (3-hexylthiophene) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PCBM)/MoO&lt;sub align=&quot;right&quot;&gt; 3 /Ag were fabricated employing low temperature solution processed ZnO as electron selective layer. Devices with varying film thickness of ZnO interlayer were investigated. The optimum film thickness was determined from photovoltaic parameters obtained from current-voltage measurements. Furthermore, the distribution of localised energy states or trap depth and the ohmicity of the contacts in the optimised device were evaluated, using the temperature and illumination intensity dependent study. The results demonstrate the effect of trap depth distribution on the charge transport, device performance, and stability of the contacts.
RSC Advances, 2013
ABSTRACT We investigate the blend morphology and performance of bulk heterojunction organic photo... more ABSTRACT We investigate the blend morphology and performance of bulk heterojunction organic photovoltaic devices comprising the donor polymer, pDPP-TNT (poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene}) and the fullerene acceptor, [70]PCBM ([6,6]-phenyl C71-butyric acid methyl ester). The blend morphology is heavily dependent upon the solvent system used in the fabrication of thin films. Thin films spin-coated from chloroform possess a cobblestone-like morphology, consisting of thick, round-shaped [70]PCBM-rich mounds separated by thin polymer-rich valleys. The size of the [70]PCBM domains is found to depend on the overall film thickness. Thin films spin-coated from a chloroform:dichlorobenzene mixed solvent system are smooth and consist of a network of pDPP-TNT nanofibers embedded in a [70]PCBM-rich matrix. Rinsing the films in hexane selectively removes [70]PCBM and allows for analysis of domain size and purity. It also provides a means for investigating exciton dissociation efficiency through relative photoluminescence yield measurements. Devices fabricated from chloroform solutions show much poorer performance than the devices fabricated from the mixed solvent system; this disparity in performance is seen to be more pronounced with increasing film thickness. The primary cause for the improved performance of devices fabricated from mixed solvents is attributed to the greater donor-acceptor interfacial area and resulting greater capacity for charge carrier generation.
ECS Solid State Letters, 2012
ABSTRACT We report thermally stimulated current (TSC) experiments on graphene oxide ( GO) to stud... more ABSTRACT We report thermally stimulated current (TSC) experiments on graphene oxide ( GO) to study the effects of various defect levels near the GO Fermi level. The TSC peaks are ascribed to detrapping from defect levels to the GO hopping transport energy level, and are found to be in agreement with the GO density of states reported in the literature. This work will be useful in evaluating the use of GO in memory/dielectric/barrier applications.
![Research paper thumbnail of Charge transport studies in fluorene – Dithieno[3,2-b:2′,3′-d]pyrrole oligomer using time-of-flight photoconductivity method](https://attachments.academia-assets.com/65785892/thumbnails/1.jpg)
](Organic Electronics, 2009
Charge mobility characteristics of a newly synthesised 2,6-bis [2-(9,9-dihexyl-9H-fluorene)]-N-(4... more Charge mobility characteristics of a newly synthesised 2,6-bis [2-(9,9-dihexyl-9H-fluorene)]-N-(4-hexylphenyl)-dithieno[3,2-b:2 0 ,3 0 -d]pyrrole oligomer (DTP-FLU) was studied as a function of electric field and temperature using time-of-flight photoconductivity measurement. It is found that the DTP-FLU oligomer is a hole transporting material with a hole mobility of 7.7 Â 10 À6 cm 2 /Vs at an applied electric field of 2.9 Â 10 5 V/cm at 298 K. The dependence of hole mobility with applied electric field and temperature is studied in detail by analyzing the experimental results using the Bassler's Gaussian disorder model and Correlated disorder model. The energetic disorder parameter (r) = 100 meV, mobility pre-factor (l 1 ) = 6.1 Â 10 À4 cm 2 /Vs and positional disorder parameter (R) = 2.4 were extracted using Gaussian disorder model. The film morphology and photophysical properties of this new oligomer are also studied in detail.
ECS Solid State Letters, 2014
Electronic Materials Letters, 2018
In this paper, temperature dependence of the excitonic bands in a mechanically exfoliated tungste... more In this paper, temperature dependence of the excitonic bands in a mechanically exfoliated tungsten diselenide (WSe 2) monolayer is studied using photoluminescence and circular dichroic photoluminescence (PL) in the temperature range between 8 and 300 K. The peak energies associated with the neutral exciton (A), charged exciton (trion) and localized excitons are extracted from the PL spectra revealing a trion binding energy of around 30 meV. The circular dichroic PL measured at 8 K shows about 45% valley polarisation that sharply reduces with increasing temperature to 5% at 300 K with photoexcitation energy of 1.96 eV. A detailed analysis of the emission line-width suggests that the rapid decrease of valley polarisation with the increase of temperature is caused by the strong exciton-phonon interactions which efficiently scatter the excitons into different excitonic states that are easily accessible due to the supply of excess photoexcitation energy. The emission linewidth broadening with the increase of temperature indicate residual exciton dephasing lifetime < 100 fs, that correlates with the observed rapid valley depolarisation.
IEEE Journal of Photovoltaics, 2016
Application of our newly synthesized low-band-gap polymer, i.e., PDTBT-TT, in organic solar cells... more Application of our newly synthesized low-band-gap polymer, i.e., PDTBT-TT, in organic solar cells (OSCs) has been studied. We have investigated the effects of molecular weight of the PDTBT-TT on the PDTBT-TT:PC 71 BM-based bulk heterojunction OSCs' performance. It was found that the OSCs fabricated with higher molecular weight (M n) PDTBT-TT polymer have the higher short-circuit current and power conversion efficiency. The better performance of the PDTBT-TT-based OSC with the higher M n PDTBT-TT polymer is attributed to the improved charge transfer resistance and the suppressed bimolecular recombination inside the PDTBT-TT:PC 71 BM blend active layer of the OSC. Moreover, the increased doping density observed in the higher M n PDTBT-TT:PC 71 BM-based OSCs increases the photocurrent and leads to the cell's higher power conversion efficiency.
Phys. Chem. Chem. Phys., 2014
Incorporation of fluorene and diketopyrrolopyrrole functionalities to generate a non-fullerene el... more Incorporation of fluorene and diketopyrrolopyrrole functionalities to generate a non-fullerene electron acceptor for solution-processable organic solar cells is shown to lead to devices with impressive open circuit voltage.
Materials Letters, 2016
Abstract Bismuth ferrite (BFO) is one of the most popular multiferroic oxides and has recently at... more Abstract Bismuth ferrite (BFO) is one of the most popular multiferroic oxides and has recently attracted interest of scientific community as a suitable material for light harvesting applications, due to its efficient polarization-governed mechanism of charge separation and ability to generate above-bandgap voltages. Identified as a promising photoferroic material, BFO has been a subject of various photoconversion studies in conjunction with other oxides and polymers commonly found in organic photovoltaics. In this report, we examined fundamental device parameters such as carrier lifetime, charge transport and diffusion coefficient within BFO/polymer bilayer, utilizing transient photovoltage and photocurrent spectroscopy techniques. Tested devices were fabricated via simple and inexpensive solution processing route, using nitrate precursors for BFO synthesis. Obtained results suggest formation of space-charge region within the device causing charge carrier lifetimes extension up to 350 μs in the absence of external voltage bias, as well as fast charge extraction (
Advanced Functional Materials
The Journal of Physical Chemistry C
- Charge generation and recombination in diketopyrrolopyrrole polymer: Fullerene bulk heteroj... more 2016) Charge generation and recombination in diketopyrrolopyrrole polymer: Fullerene bulk heterojunctions studied by transient absorption and timeresolved microwave conductivity.
ACS applied materials & interfaces, Jan 25, 2017
PbI2-enriched mixed perovskite film [FA0.81MA0.15Pb(I0.836Br0.15)3] has been widely studied due t... more PbI2-enriched mixed perovskite film [FA0.81MA0.15Pb(I0.836Br0.15)3] has been widely studied due to its great potential in perovskite solar cell (PSC) applications. Herein, a FA0.81MA0.15Pb(I0.836Br0.15)3 film has been fabricated with the temperature-dependent optical absorption spectra utilized to determine its exciton binding energy. A ∼13 meV exciton binding energy is estimated, and a near-unity fraction of free carriers out of the total photoexcitons has been obtained in the solar cell operating regime at equilibrium state. PSCs are fabricated with this mixed perovskite film, but a significant electron transport barrier at the TiO2-perovskite interface limited their performance. Cs2CO3 and CsI are then utilized as functional enhancers with which to substantially balance the electron and hole transport and increase the carriers (both electrons and holes) mobilities in PSCs, resulting in much-improved solar-cell performance. The modified PSCs exhibit reproducible power conversion e...
Nanoscale, Jan 7, 2017
Titanium dioxide (TiO2) nanoporous hemispheres (NHSs) with a radius of ∼200 nm are fabricated by ... more Titanium dioxide (TiO2) nanoporous hemispheres (NHSs) with a radius of ∼200 nm are fabricated by electrospraying a hydrothermally synthesized TiO2 nanoparticle (NP) suspension solution. The resulting TiO2 NHSs are highly porous, which are beneficial to the infiltration of perovskites and provide a larger contact area, as building blocks to construct a mesoporous TiO2 layer for FA0.81MA0.15Pb(I0.836Br0.15)3 based perovskite solar cells (PSCs). By varying the TiO2 NHS collecting period (15 s, 30 s, 60 s and 90 s) during the electrospraying process, the performance of PSCs changes with different TiO2 NHS distribution densities. The optimized PSC employing TiO2 NHSs (60 s) exhibits a photovoltaic conversion efficiency (PCE) as high as 19.3% with a Jsc of 23.8 mA cm(-2), a Voc of 1.14 V and a FF of 0.71. Furthermore, the PSC possesses a reproducible PCE value with little hysteresis in its current density-voltage (J-V) curves. The small perturbation transient photovoltage (TPV) measuremen...
J Photochem Photobiol a Chem, 2008
... Adv. Mater., 14 (2002), pp. 1834–1837. [9] M. Wohlgenannt, XM Jiang, ZV Vardeny and RAJ Janss... more ... Adv. Mater., 14 (2002), pp. 1834–1837. [9] M. Wohlgenannt, XM Jiang, ZV Vardeny and RAJ Janssen, Conjugation-length dependence of spin-dependent exciton formation rates in Π-conjugated oligomers and polymers. Phys. Rev. Lett., 88 (2002), pp. 197401–197404. ...
Org Electron, 2007
The charge transport properties in a novel electroluminescent poly{[2-(4 0 ,5 0 -bis(3 00 -methyl... more The charge transport properties in a novel electroluminescent poly{[2-(4 0 ,5 0 -bis(3 00 -methylbutoxy)-2 0 -p-methoxy-phenyl)phenyl-1,4-phenylene vinylene]-co-(9,9-dioctyl-2,7-fluorenylene vinylene)} (BPPPV-PF) have been studied using a time-of-flight (TOF) photoconductivity technique. The TOF transients for holes were recorded over a range of temperatures (207-300 K) and electric fields (1.5 · 10 5 -6.1 · 10 5 V/cm). The hole transport in this polymer was weakly dispersive in nature with a mobility at 300 K of 5 · 10 À5 cm 2 /V s at 2.5 · 10 5 V/cm. This increased to 8.4 · 10 À5 cm 2 /V s at 6.1 · 10 5 V/ cm. The temperature and field dependence of charge mobility has been analyzed using the disorder formalisms (Bässler's Gaussian disorder model (GDM) and correlated disorder model (CDM)). The fit with Gaussian disorder (GDM) model yielded the mobility pre-factor l 1 = 1.2 · 10 À3 cm 2 /V s, energetic disorder parameter r = 82 meV and positional disorder parameter R = 1.73. The average inter-site separation (a = 7 Å ) and the charge localization length (L = 3.6 Å ) was estimated by assuming the CDM type charge transport. The microscopic charge transport parameters derived for this polymer are almost identical to the reported values for fully conjugated polymers with high chemical purity. The results presented indicate that the charge transport parameters can be controlled and optimized for organic optoelectronic applications.
Asian Journal of Organic Chemistry, 2015
An electron-accepting chromophore based on fluorene and naphthalenediimide building blocks for so... more An electron-accepting chromophore based on fluorene and naphthalenediimide building blocks for solution-processable bulk heterojunction devices.
ACS Applied Materials & Interfaces, 2015
The effect of inserting an SU-8 dielectric interlayer into inverted bulk heterojunction (BHJ) org... more The effect of inserting an SU-8 dielectric interlayer into inverted bulk heterojunction (BHJ) organic solar cells (OSCs) was studied. Insertion of an ultrathin layer of SU-8 between the zinc oxide (ZnO) electron transport layer and the photoactive layer resulted in a smoother interface and a 14% enhancement in power conversion efficiency. The properties of devices with and without an SU-8 interlayer were investigated using transient photovoltage (TPV) and double injection (DoI) techniques, and it was found that devices with SU-8 show longer carrier lifetimes and greater mobility-lifetime (μ-τ) products than those without. Devices with SU-8 were also found to have improved stability. The results indicate that the insertion of an SU-8 interlayer reduces the recombination rate for photogenerated carriers without affecting the charge transport properties, improving overall performance and stability.
Organic Electronics, 2007
The charge transport properties in a novel electroluminescent poly{[2-(4 0 ,5 0 -bis(3 00 -methyl... more The charge transport properties in a novel electroluminescent poly{[2-(4 0 ,5 0 -bis(3 00 -methylbutoxy)-2 0 -p-methoxy-phenyl)phenyl-1,4-phenylene vinylene]-co-(9,9-dioctyl-2,7-fluorenylene vinylene)} (BPPPV-PF) have been studied using a time-of-flight (TOF) photoconductivity technique. The TOF transients for holes were recorded over a range of temperatures (207-300 K) and electric fields (1.5 · 10 5 -6.1 · 10 5 V/cm). The hole transport in this polymer was weakly dispersive in nature with a mobility at 300 K of 5 · 10 À5 cm 2 /V s at 2.5 · 10 5 V/cm. This increased to 8.4 · 10 À5 cm 2 /V s at 6.1 · 10 5 V/ cm. The temperature and field dependence of charge mobility has been analyzed using the disorder formalisms (Bässler's Gaussian disorder model (GDM) and correlated disorder model (CDM)). The fit with Gaussian disorder (GDM) model yielded the mobility pre-factor l 1 = 1.2 · 10 À3 cm 2 /V s, energetic disorder parameter r = 82 meV and positional disorder parameter R = 1.73. The average inter-site separation (a = 7 Å ) and the charge localization length (L = 3.6 Å ) was estimated by assuming the CDM type charge transport. The microscopic charge transport parameters derived for this polymer are almost identical to the reported values for fully conjugated polymers with high chemical purity. The results presented indicate that the charge transport parameters can be controlled and optimized for organic optoelectronic applications.
Organic Electronics, 2015
ABSTRACT The technique of photo-CELIV (charge extraction by linearly increasing voltage) is one o... more ABSTRACT The technique of photo-CELIV (charge extraction by linearly increasing voltage) is one of the more straightforward and popular approaches to measure the faster carrier mobility in measurement geometries that are relevant for operational solar cells and other optoelectronic devices. It has been used to demonstrate a time-dependent photocarrier mobility in pristine polymers, attributed to energetic relaxation within the density of states. Conversely, in solar cell blends, the presence or absence of such energetic relaxation on transport timescales remains under debate. We developed a complete numerical model and performed photo-CELIV experiments on the model high efficiency organic solar cell blend poly[3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene] (PDPP-TNT):[6,6]-phenyl-C71-butyric-acid-methyl-ester (PC70BM). In the studied solar cells a constant, time-independent mobility on the scale relevant to charge extraction was observed, where thermalisation of photocarriers occurs on time scales much shorter than the transit time. Therefore, photocarrier relaxation effects are insignificant for charge transport in these efficient photovoltaic devices.
International Journal of Nanotechnology, 2014
ABSTRACT Inverted organic solar cells with device structure ITO/ZnO/poly (3-hexylthiophene) (P3HT... more ABSTRACT Inverted organic solar cells with device structure ITO/ZnO/poly (3-hexylthiophene) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PCBM)/MoO&lt;sub align=&quot;right&quot;&gt; 3 /Ag were fabricated employing low temperature solution processed ZnO as electron selective layer. Devices with varying film thickness of ZnO interlayer were investigated. The optimum film thickness was determined from photovoltaic parameters obtained from current-voltage measurements. Furthermore, the distribution of localised energy states or trap depth and the ohmicity of the contacts in the optimised device were evaluated, using the temperature and illumination intensity dependent study. The results demonstrate the effect of trap depth distribution on the charge transport, device performance, and stability of the contacts.
RSC Advances, 2013
ABSTRACT We investigate the blend morphology and performance of bulk heterojunction organic photo... more ABSTRACT We investigate the blend morphology and performance of bulk heterojunction organic photovoltaic devices comprising the donor polymer, pDPP-TNT (poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene}) and the fullerene acceptor, [70]PCBM ([6,6]-phenyl C71-butyric acid methyl ester). The blend morphology is heavily dependent upon the solvent system used in the fabrication of thin films. Thin films spin-coated from chloroform possess a cobblestone-like morphology, consisting of thick, round-shaped [70]PCBM-rich mounds separated by thin polymer-rich valleys. The size of the [70]PCBM domains is found to depend on the overall film thickness. Thin films spin-coated from a chloroform:dichlorobenzene mixed solvent system are smooth and consist of a network of pDPP-TNT nanofibers embedded in a [70]PCBM-rich matrix. Rinsing the films in hexane selectively removes [70]PCBM and allows for analysis of domain size and purity. It also provides a means for investigating exciton dissociation efficiency through relative photoluminescence yield measurements. Devices fabricated from chloroform solutions show much poorer performance than the devices fabricated from the mixed solvent system; this disparity in performance is seen to be more pronounced with increasing film thickness. The primary cause for the improved performance of devices fabricated from mixed solvents is attributed to the greater donor-acceptor interfacial area and resulting greater capacity for charge carrier generation.
ECS Solid State Letters, 2012
ABSTRACT We report thermally stimulated current (TSC) experiments on graphene oxide ( GO) to stud... more ABSTRACT We report thermally stimulated current (TSC) experiments on graphene oxide ( GO) to study the effects of various defect levels near the GO Fermi level. The TSC peaks are ascribed to detrapping from defect levels to the GO hopping transport energy level, and are found to be in agreement with the GO density of states reported in the literature. This work will be useful in evaluating the use of GO in memory/dielectric/barrier applications.
Organic Electronics, 2009
Charge mobility characteristics of a newly synthesised 2,6-bis [2-(9,9-dihexyl-9H-fluorene)]-N-(4... more Charge mobility characteristics of a newly synthesised 2,6-bis [2-(9,9-dihexyl-9H-fluorene)]-N-(4-hexylphenyl)-dithieno[3,2-b:2 0 ,3 0 -d]pyrrole oligomer (DTP-FLU) was studied as a function of electric field and temperature using time-of-flight photoconductivity measurement. It is found that the DTP-FLU oligomer is a hole transporting material with a hole mobility of 7.7 Â 10 À6 cm 2 /Vs at an applied electric field of 2.9 Â 10 5 V/cm at 298 K. The dependence of hole mobility with applied electric field and temperature is studied in detail by analyzing the experimental results using the Bassler's Gaussian disorder model and Correlated disorder model. The energetic disorder parameter (r) = 100 meV, mobility pre-factor (l 1 ) = 6.1 Â 10 À4 cm 2 /Vs and positional disorder parameter (R) = 2.4 were extracted using Gaussian disorder model. The film morphology and photophysical properties of this new oligomer are also studied in detail.
ECS Solid State Letters, 2014