Belete Gonfa - Academia.edu (original) (raw)

Papers by Belete Gonfa

physica status solidi (b), 2010

In this work the use of different ZnO nanostructures has been studied to assess the effect of mor... more In this work the use of different ZnO nanostructures has been studied to assess the effect of morphology and surface modification on the performance of photovoltaic devices. ZnO nanostructures (nanoparticles, nanowires and nanofibres) have been produced by different techniques, and surface modified with pyrene-1-carboxylic acid (PCA). The materials prepared were characterized by XRD, UV-Visible spectroscopy, TEM and SEM. The photovoltaic devices have been prepared in two different configurations: glass/ITO/ PEDOT:PSS/photoactive layer/Al and glass/ITO/ZnO/photoactive layer/PEDOT:PSS/Ag paste using spin coating and were characterized by current-voltage characteristics measurement under simulated standard illumination conditions. Whilst ZnO nanoparticles yielded the best results, surface modification with PCA resulted in solar cells with higher short circuit current densities but lower open circuit voltage pointing to a better carrier collection but also higher recombination. Figure 1 (a) TEM image of ZnO nanoparticles, (b) SEM image of ZnO nanowires, (c) SEM image of ZnO fibres.

Advanced Functional Materials

ABSTRACT

Nanoscale, 2015

Near infrared (NIR) PbS quantum dots (QDs) have attracted significant research interest in solar ... more Near infrared (NIR) PbS quantum dots (QDs) have attracted significant research interest in solar cell applications as they offer several advantages, such as tunable band gaps, capability of absorbing NIR photons, low cost solution processability and high potential for multiple exciton generation. Nonetheless, reports on solar cells based on NIR PbS/CdS core-shell QDs, which are in general more stable and better passivated than PbS QDs and thus more promising for solar cell applications, remain very rare. Herein we report high efficiency bulk heterojunction QD solar cells involving hydrothermally grown TiO2 nanorod arrays and PbS/CdS core-shell QDs processed in air (except for a device thermal annealing step) with a photoresponse extended to wavelengths >1200 nm and with a power conversion efficiency (PCE) as high as 4.43%. This efficiency was achieved by introducing a thin, sputter-deposited, uniform TiO2 seed layer to improve the interface between the TiO2 nanorod arrays and the front electrode, by optimizing TiO2 nanorod length and by conducting QD annealing treatment to enhance charge carrier transport. It was found that the effect of the seed layer became more obvious when the TiO2 nanorods were longer. Although photocurrent did not change much, both open circuit voltage and fill factor clearly changed with TiO2 nanorod length. This was mainly attributed to the variation of charge transport and recombination processes, as evidenced by series and shunt resistance studies. The optimal PCE was obtained at the nanorod length of ∼450 nm. Annealing is shown to further increase the PCE by ∼18%, because of the improvement of charge carrier transport in the devices as evidenced by considerably increased photocurrent. Our results clearly demonstrate the potential of the PbS/CdS core-shell QDs for the achievement of high PCE, solution processable and NIR responsive QD solar cells.

Reviews in Nanoscience and Nanotechnology, 2012

Solar Energy Materials and Solar Cells, 2014

All solution processed depleted bulk heterojunction (DBH) solar cell devices based on near infrar... more All solution processed depleted bulk heterojunction (DBH) solar cell devices based on near infrared (NIR) PbS/CdS core-shell quantum dots (QDs) and films of rutile TiO 2 nanorod arrays have been investigated. The device fabrication was achieved through the layer-by-layer spin coating of PbS/CdS QDs, in ambient atmosphere, onto hydrothermally grown TiO 2 nanorod arrays film leading to the general device architecture consisting of fluorine doped tin oxide (FTO)/TiO 2 /QDs/interfacial layer/Au. The performance of these devices fabricated under different processing conditions was tested and compared with that of similar devices where the PbS/CdS QDs were replaced by a spin-coated layer of colloidal PbS QDs (processed under inert atmosphere). It was found that the maximum power conversion efficiency of the former devices is about 40% higher when MoO 3 was used as an interfacial layer (2.02% 70.15 vs 1.40% 70.11). The stability and ease of processing in air together with the higher performance of the PbS/CdS core-shell QDs, as compared to the PbS QDs, strongly suggest their high potential in solar cell applications. This work represents the first demonstration of the use of NIR PbS/CdS core-shell QDs in solar cells.

Nanoscale, 2014

We present for the first time detailed investigation of the charge transfer behavior of PbS@CdS c... more We present for the first time detailed investigation of the charge transfer behavior of PbS@CdS core@shell quantum dots (QDs) showing either a single emission peak from the core or intriguing double emission peaks from the core and shell, respectively. A highly non-concentric core@shell structure model was proposed to explain the origin of double emissions from monodisperse QDs. Their charge transfer behavior was investigated by monitoring photoluminescence (PL) intensity variation with the introduction of electron or hole scavengers. It was found that the PL quenching of the PbS core is more efficient than that of the CdS shell, suggesting more efficient charge transfer from the core to scavengers, although the opposite was expected. Further measurements of the PL lifetime followed by wave function calculations disclosed that the time scale is the critical factor explaining the more efficient charge transfer from the core than from the shell. The charge transfer behavior was also examined on a series of single-emission core@shell QDs with either different core sizes or different shell thicknesses and dominant factors were identified. Towards photovoltaic applications, these PbS@CdS QDs were attached onto multi-walled carbon nanotubes (MWCNTs) and their charge transfer behavior was compared with that in the PbS-QD/MWCNT system. Results demonstrate that although the CdS shell serves as an electron transfer barrier, the electrons excited in the PbS cores can still be transferred into the MWCNTs efficiently when the shell thickness is ∼0.7 nm. Considering their higher stability, these core@shell QDs are very promising for the development of highly efficient QD-based photovoltaic devices.

Advanced Functional Materials, 2011

A solution-processed nanoarchitecture based on PbS quantum dots (QDs) and multi-walled carbon nan... more A solution-processed nanoarchitecture based on PbS quantum dots (QDs) and multi-walled carbon nanotubes (MWCNTs) is synthesized by simply mixing the pre-synthesized high-quality PbS QDs and oleylamine (OLA) pre-functionalized MWCNTs. Pre-functionalization of MWCNTs with OLA is crucial for the attachment of PbS QDs and the coverage of QDs on the surface of MWCNTs can be tuned by varying the ratio of PbS QDs to MWCNTs. The apparent photoluminescence (steady-state emission and fl uorescence lifetime) "quenching" effect indicates effi cient charge transfer from photo-excited PbS QDs to MWCNTs. The as-synthesized PbS-QD/MWCNT nanoarchitecture is further incorporated into a hole-conducting polymer poly(3-hexylthiophene)-(P3HT), forming the P3HT:PbS-QD/MWCNT nanohybrid, in which the PbS QDs act as a light harvester for absorbing irradiation over a wide wavelength range of the solar spectrum up to near infrared (NIR, ≈ 1430 nm) range; whereas, the one-dimensional MWCNTs and P3HT are used to collect and transport photoexcited electrons and holes to the cathode and anode, respectively. Even without performing the often required "ligand exchange" to remove the long-chained OLA ligands, the built nanohybrid photovoltaic (PV) device exhibits a largely enhanced power conversion effi ciency (PCE) of 3.03% as compared to 2.57% for the standard bulk hetero-junction PV cell made with P3HT and [6,6]-Phenyl-C 61 -Butyric Acid Methyl Ester (PCBM) mixtures. The improved performance of P3HT:PbS-QD/MWCNT nanohybrid PV device is attributed to the signifi cantly extended absorption up to NIR by PbS QDs as well as the effectively enhanced charge separation and transportation due to the integrated MWCNTs and P3HT. Our research results suggest that properly integrating QDs, MWCNTs, and polymers into nanohybrid structures is a promising approach for the development of highly effi cient PV devices.

Nanoscale, 2014

N-type metal oxide solar cells sensitized by infrared absorbing PbS quantum dots (QDs) represent ... more N-type metal oxide solar cells sensitized by infrared absorbing PbS quantum dots (QDs) represent a promising alternative to traditional photovoltaic devices. However, colloidal PbS QDs capped with pure organic ligand shells suffer from surface oxidation that affects the long term stability of the cells.

physica status solidi (b), 2010

In this work the use of different ZnO nanostructures has been studied to assess the effect of mor... more In this work the use of different ZnO nanostructures has been studied to assess the effect of morphology and surface modification on the performance of photovoltaic devices. ZnO nanostructures (nanoparticles, nanowires and nanofibres) have been produced by different techniques, and surface modified with pyrene-1-carboxylic acid (PCA). The materials prepared were characterized by XRD, UV-Visible spectroscopy, TEM and SEM. The photovoltaic devices have been prepared in two different configurations: glass/ITO/ PEDOT:PSS/photoactive layer/Al and glass/ITO/ZnO/photoactive layer/PEDOT:PSS/Ag paste using spin coating and were characterized by current-voltage characteristics measurement under simulated standard illumination conditions. Whilst ZnO nanoparticles yielded the best results, surface modification with PCA resulted in solar cells with higher short circuit current densities but lower open circuit voltage pointing to a better carrier collection but also higher recombination. Figure 1 (a) TEM image of ZnO nanoparticles, (b) SEM image of ZnO nanowires, (c) SEM image of ZnO fibres.

Advanced Functional Materials

ABSTRACT

Nanoscale, 2015

Near infrared (NIR) PbS quantum dots (QDs) have attracted significant research interest in solar ... more Near infrared (NIR) PbS quantum dots (QDs) have attracted significant research interest in solar cell applications as they offer several advantages, such as tunable band gaps, capability of absorbing NIR photons, low cost solution processability and high potential for multiple exciton generation. Nonetheless, reports on solar cells based on NIR PbS/CdS core-shell QDs, which are in general more stable and better passivated than PbS QDs and thus more promising for solar cell applications, remain very rare. Herein we report high efficiency bulk heterojunction QD solar cells involving hydrothermally grown TiO2 nanorod arrays and PbS/CdS core-shell QDs processed in air (except for a device thermal annealing step) with a photoresponse extended to wavelengths >1200 nm and with a power conversion efficiency (PCE) as high as 4.43%. This efficiency was achieved by introducing a thin, sputter-deposited, uniform TiO2 seed layer to improve the interface between the TiO2 nanorod arrays and the front electrode, by optimizing TiO2 nanorod length and by conducting QD annealing treatment to enhance charge carrier transport. It was found that the effect of the seed layer became more obvious when the TiO2 nanorods were longer. Although photocurrent did not change much, both open circuit voltage and fill factor clearly changed with TiO2 nanorod length. This was mainly attributed to the variation of charge transport and recombination processes, as evidenced by series and shunt resistance studies. The optimal PCE was obtained at the nanorod length of ∼450 nm. Annealing is shown to further increase the PCE by ∼18%, because of the improvement of charge carrier transport in the devices as evidenced by considerably increased photocurrent. Our results clearly demonstrate the potential of the PbS/CdS core-shell QDs for the achievement of high PCE, solution processable and NIR responsive QD solar cells.

Reviews in Nanoscience and Nanotechnology, 2012

Solar Energy Materials and Solar Cells, 2014

All solution processed depleted bulk heterojunction (DBH) solar cell devices based on near infrar... more All solution processed depleted bulk heterojunction (DBH) solar cell devices based on near infrared (NIR) PbS/CdS core-shell quantum dots (QDs) and films of rutile TiO 2 nanorod arrays have been investigated. The device fabrication was achieved through the layer-by-layer spin coating of PbS/CdS QDs, in ambient atmosphere, onto hydrothermally grown TiO 2 nanorod arrays film leading to the general device architecture consisting of fluorine doped tin oxide (FTO)/TiO 2 /QDs/interfacial layer/Au. The performance of these devices fabricated under different processing conditions was tested and compared with that of similar devices where the PbS/CdS QDs were replaced by a spin-coated layer of colloidal PbS QDs (processed under inert atmosphere). It was found that the maximum power conversion efficiency of the former devices is about 40% higher when MoO 3 was used as an interfacial layer (2.02% 70.15 vs 1.40% 70.11). The stability and ease of processing in air together with the higher performance of the PbS/CdS core-shell QDs, as compared to the PbS QDs, strongly suggest their high potential in solar cell applications. This work represents the first demonstration of the use of NIR PbS/CdS core-shell QDs in solar cells.

Nanoscale, 2014

We present for the first time detailed investigation of the charge transfer behavior of PbS@CdS c... more We present for the first time detailed investigation of the charge transfer behavior of PbS@CdS core@shell quantum dots (QDs) showing either a single emission peak from the core or intriguing double emission peaks from the core and shell, respectively. A highly non-concentric core@shell structure model was proposed to explain the origin of double emissions from monodisperse QDs. Their charge transfer behavior was investigated by monitoring photoluminescence (PL) intensity variation with the introduction of electron or hole scavengers. It was found that the PL quenching of the PbS core is more efficient than that of the CdS shell, suggesting more efficient charge transfer from the core to scavengers, although the opposite was expected. Further measurements of the PL lifetime followed by wave function calculations disclosed that the time scale is the critical factor explaining the more efficient charge transfer from the core than from the shell. The charge transfer behavior was also examined on a series of single-emission core@shell QDs with either different core sizes or different shell thicknesses and dominant factors were identified. Towards photovoltaic applications, these PbS@CdS QDs were attached onto multi-walled carbon nanotubes (MWCNTs) and their charge transfer behavior was compared with that in the PbS-QD/MWCNT system. Results demonstrate that although the CdS shell serves as an electron transfer barrier, the electrons excited in the PbS cores can still be transferred into the MWCNTs efficiently when the shell thickness is ∼0.7 nm. Considering their higher stability, these core@shell QDs are very promising for the development of highly efficient QD-based photovoltaic devices.

Advanced Functional Materials, 2011

A solution-processed nanoarchitecture based on PbS quantum dots (QDs) and multi-walled carbon nan... more A solution-processed nanoarchitecture based on PbS quantum dots (QDs) and multi-walled carbon nanotubes (MWCNTs) is synthesized by simply mixing the pre-synthesized high-quality PbS QDs and oleylamine (OLA) pre-functionalized MWCNTs. Pre-functionalization of MWCNTs with OLA is crucial for the attachment of PbS QDs and the coverage of QDs on the surface of MWCNTs can be tuned by varying the ratio of PbS QDs to MWCNTs. The apparent photoluminescence (steady-state emission and fl uorescence lifetime) "quenching" effect indicates effi cient charge transfer from photo-excited PbS QDs to MWCNTs. The as-synthesized PbS-QD/MWCNT nanoarchitecture is further incorporated into a hole-conducting polymer poly(3-hexylthiophene)-(P3HT), forming the P3HT:PbS-QD/MWCNT nanohybrid, in which the PbS QDs act as a light harvester for absorbing irradiation over a wide wavelength range of the solar spectrum up to near infrared (NIR, ≈ 1430 nm) range; whereas, the one-dimensional MWCNTs and P3HT are used to collect and transport photoexcited electrons and holes to the cathode and anode, respectively. Even without performing the often required "ligand exchange" to remove the long-chained OLA ligands, the built nanohybrid photovoltaic (PV) device exhibits a largely enhanced power conversion effi ciency (PCE) of 3.03% as compared to 2.57% for the standard bulk hetero-junction PV cell made with P3HT and [6,6]-Phenyl-C 61 -Butyric Acid Methyl Ester (PCBM) mixtures. The improved performance of P3HT:PbS-QD/MWCNT nanohybrid PV device is attributed to the signifi cantly extended absorption up to NIR by PbS QDs as well as the effectively enhanced charge separation and transportation due to the integrated MWCNTs and P3HT. Our research results suggest that properly integrating QDs, MWCNTs, and polymers into nanohybrid structures is a promising approach for the development of highly effi cient PV devices.

Nanoscale, 2014

N-type metal oxide solar cells sensitized by infrared absorbing PbS quantum dots (QDs) represent ... more N-type metal oxide solar cells sensitized by infrared absorbing PbS quantum dots (QDs) represent a promising alternative to traditional photovoltaic devices. However, colloidal PbS QDs capped with pure organic ligand shells suffer from surface oxidation that affects the long term stability of the cells.