Mathias Nyman | Åbo Akademi (original) (raw)
Papers by Mathias Nyman
Advanced Energy Materials, 2014
The theory of 2D Langevin recombination is extended to the case with high trap density and demons... more The theory of 2D Langevin recombination is extended to the case with high trap density and demonstrated experimentally on the model system poly [2,5bis(3-tetradecylthiophen-2-yl)thieno[3,2-b] thiophene] (PBTTT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) using both transient and steady-state techniques. The recombination capture coeffi cients are derived for trap-assisted and bandto-band recombination, showing that anisotropic charge transport reduces the capture coeffi cients in both cases resulting in a reduced overall recombination.
Organic Electronics, 2014
We have extended the analytical framework of the charge extraction by linearly increasing voltage... more We have extended the analytical framework of the charge extraction by linearly increasing voltage (CELIV) theory to the case with doping, taking the effect of built-in voltage, diffusion and band-bending into account. Not taking the built-in voltage into account in dark CELIV will lead to an underestimation of the charge carrier density, while the mobility is overestimated. Furthermore, based on the analysis we propose to use CELIV in the doping-induced capacitive regime for direct determination of doping concentration and built-in voltage from extraction current transients in the time-domain of doped thin-film semiconductor devices. The analytical framework is confirmed numerically with a one-dimensional drift-diffusion model and experimentally on aged P3HT:PCBM bulk heterojunction solar cell devices. An excellent agreement between the experimental extraction current transients and the analytical prediction is found. The presented analytical treatment is not limited to sandwich-type thin-film devices, but is more general and the technique can also be extended to pn-junctions.
We demonstrate transient capacitance measurements using charge extraction by a linearly increasin... more We demonstrate transient capacitance measurements using charge extraction by a linearly increasing voltage (CELIV) on the small molecule naphthalene-tetra-carboxylic- di-imide (NTCDI) based organic transistors. The OFETs use Aluminum (Al) and Aluminum Oxide (AlOx) as bottom gate and dielectric, with gold (Au) source and drain electrodes. The Al/AlOx gate is modified using two different self assembled monolayers, triethoxy(octyl)silane and perfluorooctyltriethoxysilane, in order to tune the turn-on voltage. We have clarified the voltage dependent capacitance in diode structures and found that when the transistor is in the fully on state a charge reservoir is formed at the AlOx interface and a saturation of the steady-state capacitance is seen, equaling the capacitance of the AlOx layer. When the transistor is in the fully off state the steady state capacitance saturates to the capacitance of the semiconductor bulk. We interpret this as a build-up of a charge reservoir in the semi con...
Physical Review Applied, 2014
Interfaces play an important role in emerging organic electronic applications. In order to optimi... more Interfaces play an important role in emerging organic electronic applications. In order to optimize and control the performance in organic devices such as organic solar cells, a comprehensive understanding of the contacts is essential. However, despite the vast progress made, a fundamental theory of the physical processes taking place at the contacts is still lacking. In this work, a numerical device model is used to clarify the effect of imperfect contacts in organic bulk heterojunction solar cells. The effect of increased injection barriers, reduced surface recombination, interfacial minority carrier doping, and traps for majority carriers at the electrodes causing reduced efficiencies is simulated. Two distinctly different underlying mechanisms leading to different S-shaped features are found, both leading to an effective shift of the builtin voltage. In the case of an extraction barrier to majority carriers at the contact, such as reduced surface recombination, the S kink is due to an induced diffusion potential. In the case of interfacial doping or traps, the S kink results from band bending caused by the fixed or trapped space charge. We derive analytical expressions describing the effective reduction of the built-in voltage and the (effective) open-circuit voltage providing means to quantify and distinguish the mechanisms. We show how to experimentally differentiate between these effects and provide tools to extract the relevant physical parameters.
We have measured the effect of degradation on the I-V characteristics in APFO3:PCBM solar cells. ... more We have measured the effect of degradation on the I-V characteristics in APFO3:PCBM solar cells. The solar cell devices were subject to ambient air under simulated solar illumination. We found that the degradation resulted in a lowering of the fill factor and short circuit current while the open circuit voltage remained unchanged. In order to gain insight into what has caused the degraded I-V characteristics we have studied the active layer film using various techniques. We found clear spectral changes both in absorption and in photoinduced absorption spectroscopy correlated with increased carrier lifetimes and lowered mobility when comparing the degraded film with a pristine one. The results show a significant degradation of the active layer causing a lower fill-factor and short circuit current.
Synthetic Metals, 2015
ABSTRACT We present charge transport properties of intercalated and non-intercalated PBTTT:fuller... more ABSTRACT We present charge transport properties of intercalated and non-intercalated PBTTT:fullerene based bulk heterojunctions measured with the photo-CELIV technique. The mobilities are seen to vary greatly with fullerene loading in intercalated systems whereas the mobility in non-intercalated blends is fairly constant. The transport activation energies and widths of the Gaussian density of states (DOS) indicates lower energetic disorder in intercalated blends. The highest room temperature mobility of 2 × 10−3 cm2/Vs was obtained for a blend of 1:4 weight ratio PBTTT:PCBM, which is an order of magnitude higher than the other blends studied. We attribute the high mobility to lower energetic disorder and stronger electronic coupling between the transport sites due to shorter inter-site distances in the intercalated blend.
Organic Electronics, 2014
We have extended the analytical framework of the charge extraction by linearly increasing voltage... more We have extended the analytical framework of the charge extraction by linearly increasing voltage (CELIV) theory to the case with doping, taking the effect of built-in voltage, diffusion and band-bending into account. Not taking the built-in voltage into account in dark CELIV will lead to an underestimation of the charge carrier density, while the mobility is overestimated. Furthermore, based on the analysis we propose to use CELIV in the doping-induced capacitive regime for direct determination of doping concentration and built-in voltage from extraction current transients in the time-domain of doped thin-film semiconductor devices. The analytical framework is confirmed numerically with a one-dimensional drift-diffusion model and experimentally on aged P3HT:PCBM bulk heterojunction solar cell devices. An excellent agreement between the experimental extraction current transients and the analytical prediction is found. The presented analytical treatment is not limited to sandwich-type thin-film devices, but is more general and the technique can also be extended to pn-junctions.
Synthetic Metals, 2013
ABSTRACT Using charge extraction techniques we have clarified the charge transport properties of ... more ABSTRACT Using charge extraction techniques we have clarified the charge transport properties of novel polythiophene derivatives intended for use as sensitizers in polymer sensitized solar cells. The polymers are of low molecular weight in order to enable penetration into a mesoporous titanium dioxide film. Hydrophilic anchoring units are attached to the thiophene ring to facilitate attachment of the polymer to the titanium dioxide. The hole transporting properties were determined in order to clarify the possibility of using the polymers both as sensitizers and hole transporting material in a hybrid type device. We found that the mobilities are in the range of 1–4 × 10−8 cm2/(V s). The low mobility is compensated by very long charge carrier lifetimes in the order of 100 ms resulting in a very promising charge carrier drift distance of 1 μm.
![Research paper thumbnail of Origin of equilibrium charges in poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester solar cell devices](https://attachments.academia-assets.com/45152326/thumbnails/1.jpg)
](Chemical Physics, 2012
We have clarified the origin of equilibrium charges in poly(3-hexylthiophene):[6,6]-phenyl-C 61 -... more We have clarified the origin of equilibrium charges in poly(3-hexylthiophene):[6,6]-phenyl-C 61 -butyric acid methyl ester devices by using charge extraction by a linearly increasing voltage. By varying the work function of the contacts we find that the equilibrium charges are injected from the contacts due to energy level alignment caused by Fermi-level pinning.
Applied Physics Letters, 2013
ACS Applied Materials & Interfaces, 2013
Leakage currents through the gate dielectric of thin film transistors remain a roadblock to the f... more Leakage currents through the gate dielectric of thin film transistors remain a roadblock to the fabrication of organic field-effect transistors (OFETs) on ultrathin dielectrics. We report the first investigation of a self-assembled monolayer (SAM) dipole as an electrostatic barrier to reduce leakage currents in n-channel OFETs fabricated on a minimal, leaky ∼10 nm SiO2 dielectric on highly doped Si. The electric field associated with 1H,1H,2H,2H-perfluoro-octyltriethoxysilane (FOTS) and octyltriethoxysilane (OTS) dipolar chains affixed to the oxide surface of n-Si gave an order of magnitude decrease in gate leakage current and subthreshold leakage and a two order-of-magnitude increase in ON/OFF ratio for a naphthalenetetracarboxylic diimide (NTCDI) transistor. Identically fabricated devices on p-Si showed similarly reduced leakage and improved performance for oxides treated with the larger dipole FOTS monolayer, while OTS devices showed poorer transfer characteristics than those on bare oxide. Comparison of OFETs on both substrates revealed that relative device performance from OTS and FOTS treatments was dictated primarily by the organosilane chain and not the underlying siloxane-substrate bond. This conclusion is supported by the similar threshold voltages (VT) extrapolated for SAM-treated devices, which display positive relative VT shifts for FOTS on either substrate but opposite VT shifts for OTS treatment on n-Si and p-Si. Our results highlight the potential of dipolar SAMs as performance-enhancing layers for marginal quality dielectrics, broadening the material spectrum for low power, ultrathin organic electronics.
Advanced Energy Materials, 2011
In the pursuit of developing new materials for more effi cient bulk-heterojunction solar cells, t... more In the pursuit of developing new materials for more effi cient bulk-heterojunction solar cells, the blend poly (3-hexylthiophene):[6,6]-phenyl- C 61 -butyric acid methyl ester (P3HT:PCBM) serves as an important model system. The success of the P3HT:PCBM blend comes from effi cient charge generation and transport with low recombination. There is not, however, a good microscopic picture of what causes these,
Advanced Energy Materials, 2014
The theory of 2D Langevin recombination is extended to the case with high trap density and demons... more The theory of 2D Langevin recombination is extended to the case with high trap density and demonstrated experimentally on the model system poly [2,5bis(3-tetradecylthiophen-2-yl)thieno[3,2-b] thiophene] (PBTTT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) using both transient and steady-state techniques. The recombination capture coeffi cients are derived for trap-assisted and bandto-band recombination, showing that anisotropic charge transport reduces the capture coeffi cients in both cases resulting in a reduced overall recombination.
Organic Electronics, 2014
We have extended the analytical framework of the charge extraction by linearly increasing voltage... more We have extended the analytical framework of the charge extraction by linearly increasing voltage (CELIV) theory to the case with doping, taking the effect of built-in voltage, diffusion and band-bending into account. Not taking the built-in voltage into account in dark CELIV will lead to an underestimation of the charge carrier density, while the mobility is overestimated. Furthermore, based on the analysis we propose to use CELIV in the doping-induced capacitive regime for direct determination of doping concentration and built-in voltage from extraction current transients in the time-domain of doped thin-film semiconductor devices. The analytical framework is confirmed numerically with a one-dimensional drift-diffusion model and experimentally on aged P3HT:PCBM bulk heterojunction solar cell devices. An excellent agreement between the experimental extraction current transients and the analytical prediction is found. The presented analytical treatment is not limited to sandwich-type thin-film devices, but is more general and the technique can also be extended to pn-junctions.
We demonstrate transient capacitance measurements using charge extraction by a linearly increasin... more We demonstrate transient capacitance measurements using charge extraction by a linearly increasing voltage (CELIV) on the small molecule naphthalene-tetra-carboxylic- di-imide (NTCDI) based organic transistors. The OFETs use Aluminum (Al) and Aluminum Oxide (AlOx) as bottom gate and dielectric, with gold (Au) source and drain electrodes. The Al/AlOx gate is modified using two different self assembled monolayers, triethoxy(octyl)silane and perfluorooctyltriethoxysilane, in order to tune the turn-on voltage. We have clarified the voltage dependent capacitance in diode structures and found that when the transistor is in the fully on state a charge reservoir is formed at the AlOx interface and a saturation of the steady-state capacitance is seen, equaling the capacitance of the AlOx layer. When the transistor is in the fully off state the steady state capacitance saturates to the capacitance of the semiconductor bulk. We interpret this as a build-up of a charge reservoir in the semi con...
Physical Review Applied, 2014
Interfaces play an important role in emerging organic electronic applications. In order to optimi... more Interfaces play an important role in emerging organic electronic applications. In order to optimize and control the performance in organic devices such as organic solar cells, a comprehensive understanding of the contacts is essential. However, despite the vast progress made, a fundamental theory of the physical processes taking place at the contacts is still lacking. In this work, a numerical device model is used to clarify the effect of imperfect contacts in organic bulk heterojunction solar cells. The effect of increased injection barriers, reduced surface recombination, interfacial minority carrier doping, and traps for majority carriers at the electrodes causing reduced efficiencies is simulated. Two distinctly different underlying mechanisms leading to different S-shaped features are found, both leading to an effective shift of the builtin voltage. In the case of an extraction barrier to majority carriers at the contact, such as reduced surface recombination, the S kink is due to an induced diffusion potential. In the case of interfacial doping or traps, the S kink results from band bending caused by the fixed or trapped space charge. We derive analytical expressions describing the effective reduction of the built-in voltage and the (effective) open-circuit voltage providing means to quantify and distinguish the mechanisms. We show how to experimentally differentiate between these effects and provide tools to extract the relevant physical parameters.
We have measured the effect of degradation on the I-V characteristics in APFO3:PCBM solar cells. ... more We have measured the effect of degradation on the I-V characteristics in APFO3:PCBM solar cells. The solar cell devices were subject to ambient air under simulated solar illumination. We found that the degradation resulted in a lowering of the fill factor and short circuit current while the open circuit voltage remained unchanged. In order to gain insight into what has caused the degraded I-V characteristics we have studied the active layer film using various techniques. We found clear spectral changes both in absorption and in photoinduced absorption spectroscopy correlated with increased carrier lifetimes and lowered mobility when comparing the degraded film with a pristine one. The results show a significant degradation of the active layer causing a lower fill-factor and short circuit current.
Synthetic Metals, 2015
ABSTRACT We present charge transport properties of intercalated and non-intercalated PBTTT:fuller... more ABSTRACT We present charge transport properties of intercalated and non-intercalated PBTTT:fullerene based bulk heterojunctions measured with the photo-CELIV technique. The mobilities are seen to vary greatly with fullerene loading in intercalated systems whereas the mobility in non-intercalated blends is fairly constant. The transport activation energies and widths of the Gaussian density of states (DOS) indicates lower energetic disorder in intercalated blends. The highest room temperature mobility of 2 × 10−3 cm2/Vs was obtained for a blend of 1:4 weight ratio PBTTT:PCBM, which is an order of magnitude higher than the other blends studied. We attribute the high mobility to lower energetic disorder and stronger electronic coupling between the transport sites due to shorter inter-site distances in the intercalated blend.
Organic Electronics, 2014
We have extended the analytical framework of the charge extraction by linearly increasing voltage... more We have extended the analytical framework of the charge extraction by linearly increasing voltage (CELIV) theory to the case with doping, taking the effect of built-in voltage, diffusion and band-bending into account. Not taking the built-in voltage into account in dark CELIV will lead to an underestimation of the charge carrier density, while the mobility is overestimated. Furthermore, based on the analysis we propose to use CELIV in the doping-induced capacitive regime for direct determination of doping concentration and built-in voltage from extraction current transients in the time-domain of doped thin-film semiconductor devices. The analytical framework is confirmed numerically with a one-dimensional drift-diffusion model and experimentally on aged P3HT:PCBM bulk heterojunction solar cell devices. An excellent agreement between the experimental extraction current transients and the analytical prediction is found. The presented analytical treatment is not limited to sandwich-type thin-film devices, but is more general and the technique can also be extended to pn-junctions.
Synthetic Metals, 2013
ABSTRACT Using charge extraction techniques we have clarified the charge transport properties of ... more ABSTRACT Using charge extraction techniques we have clarified the charge transport properties of novel polythiophene derivatives intended for use as sensitizers in polymer sensitized solar cells. The polymers are of low molecular weight in order to enable penetration into a mesoporous titanium dioxide film. Hydrophilic anchoring units are attached to the thiophene ring to facilitate attachment of the polymer to the titanium dioxide. The hole transporting properties were determined in order to clarify the possibility of using the polymers both as sensitizers and hole transporting material in a hybrid type device. We found that the mobilities are in the range of 1–4 × 10−8 cm2/(V s). The low mobility is compensated by very long charge carrier lifetimes in the order of 100 ms resulting in a very promising charge carrier drift distance of 1 μm.
Chemical Physics, 2012
We have clarified the origin of equilibrium charges in poly(3-hexylthiophene):[6,6]-phenyl-C 61 -... more We have clarified the origin of equilibrium charges in poly(3-hexylthiophene):[6,6]-phenyl-C 61 -butyric acid methyl ester devices by using charge extraction by a linearly increasing voltage. By varying the work function of the contacts we find that the equilibrium charges are injected from the contacts due to energy level alignment caused by Fermi-level pinning.
Applied Physics Letters, 2013
ACS Applied Materials & Interfaces, 2013
Leakage currents through the gate dielectric of thin film transistors remain a roadblock to the f... more Leakage currents through the gate dielectric of thin film transistors remain a roadblock to the fabrication of organic field-effect transistors (OFETs) on ultrathin dielectrics. We report the first investigation of a self-assembled monolayer (SAM) dipole as an electrostatic barrier to reduce leakage currents in n-channel OFETs fabricated on a minimal, leaky ∼10 nm SiO2 dielectric on highly doped Si. The electric field associated with 1H,1H,2H,2H-perfluoro-octyltriethoxysilane (FOTS) and octyltriethoxysilane (OTS) dipolar chains affixed to the oxide surface of n-Si gave an order of magnitude decrease in gate leakage current and subthreshold leakage and a two order-of-magnitude increase in ON/OFF ratio for a naphthalenetetracarboxylic diimide (NTCDI) transistor. Identically fabricated devices on p-Si showed similarly reduced leakage and improved performance for oxides treated with the larger dipole FOTS monolayer, while OTS devices showed poorer transfer characteristics than those on bare oxide. Comparison of OFETs on both substrates revealed that relative device performance from OTS and FOTS treatments was dictated primarily by the organosilane chain and not the underlying siloxane-substrate bond. This conclusion is supported by the similar threshold voltages (VT) extrapolated for SAM-treated devices, which display positive relative VT shifts for FOTS on either substrate but opposite VT shifts for OTS treatment on n-Si and p-Si. Our results highlight the potential of dipolar SAMs as performance-enhancing layers for marginal quality dielectrics, broadening the material spectrum for low power, ultrathin organic electronics.
Advanced Energy Materials, 2011
In the pursuit of developing new materials for more effi cient bulk-heterojunction solar cells, t... more In the pursuit of developing new materials for more effi cient bulk-heterojunction solar cells, the blend poly (3-hexylthiophene):[6,6]-phenyl- C 61 -butyric acid methyl ester (P3HT:PCBM) serves as an important model system. The success of the P3HT:PCBM blend comes from effi cient charge generation and transport with low recombination. There is not, however, a good microscopic picture of what causes these,