Final Report Summary - HYBRIDSOLAR2010 (Development of inorganic / organic hybrid heterojunction solar cells)
The aim of this project is to harness the advantages of electrochemistry to assemble organic/inorganic hybrid materials with organized nanoscale architectures. Such hybrid materials are eminently attractive candidates for both energy harvesting and storage applications. Careful selection of the synthetic parameters allows achieving fine tuning of both composition and morphology of the composites. By optimizing all key processes of solar energy harvesting (light absorption, exciton generation, and charge transport) we aim to prepare hybrids possessing improved photoelectrochemical properties. As the first task we aimed to achieve the synthesis of well-ordered inorganic oxide nanostructures, through electrochemical anodization. This task has been fully achieved already; I got solid experience on the electrosynthesis and characterization of well-ordered oxide nanoarchitectures, including TiO2 and Nb2O5 nanotube arrays and nanoporous WO3. During the reintegration phase, these methods have been established in our research group. Several graduate and undergraduate students were involved in this dissemination, and they can utilize this knowledge in their own projects. I am also capable of fine tuning of the morphological features and geometrical parameters (diameter, wall thickness, length) of these materials, which is crucially important to obtain suitable materials for the designated applications. In addition to the above mentioned n-type oxide semiconductors, I contributed to the development of various methods to obtain p-type semiconductor nanostructures, most importantly vertically aligned Cu2O, CuO nanorod arrays and SiC quantum dots. Results related to the application of the Cu-containing oxides in value added environmental remediation (CO2-reduction) were published in two peer-reviewed articles in prestigious journals (Chemical Communications, ChemPhysChem). These publications were featured through various channels, including a note in the Chemical&Engineering News magazine, and a cover image of ChemPhysChem. As for SiC, we extended the concept of photoelectrochemical anodization to particulate systems from the well-known flat surfaces. These results were also published in a high level journal, namely Electrochemistry Communications.
From the hybrid perspective, the main objective of the outgoing phase was to achieve the homogenous infiltration of different conducting polymers into inorganic oxide matrices. Particular care was devoted to maintain the electroactivity of the polymer in the hybrid configuration. In some special cases simple electrodeposition was a feasible approach to obtain such composites (e.g. WO3/Polyaniline). These hybrid assemblies exhibited a quasi-reversible voltammetric behavior, which was the exact sum of their components. Photoelectrochemical data revealed a complex behavior of the composites. Specifically the hybrid exhibited the properties of both individual components: cathodic (p-type) photocurrents at negative potentials (related to the polymer) and anodic (n-type) photoactivity at positive potentials (arising from the oxide). Importantly, the cathodic photocurrents related to the CPs were ca. 3− 4 times larger in the hybrid configuration compared to values obtained for the neat polymeric film on a Au electrode. Such enhanced photocurrent may be utilized in applications, for example, the photo-electrocatalytic reduction of O2 and CO2 and such studies are in progress. These results were published in one of the flagship journals of the field (Journal of Physical Chemistry C).
As a result of our work a proof of concept was presented for photo-electrodeposition of conjugated polymers on nanostructured matrices of wide bandgap inorganic oxide semiconductors. Photocatalytic deposition preceding the electropolymerization step during potentiodynamic synthesis was shown to play a key role in the formation of homogeneous and organized assemblies of these hybrids. Employing illumination to deposit photo-electrochemically a very thin seed layer on the surface of an oxide semiconductor helps to overcome the main challenge, namely, its limited electroactivity. We have shown that by varying the applied potential window during the potentiodynamic polymerization, the contribution of photoelectrochemical and electrochemical polymerization to the overall process can be separately expressed. Morphological features (as seen by FE-SEM) corroborated the decisive role of illumination in achieving homogeneous deposition of PANI and PPy on nanoporous WO3 and PANI on nanotubular TiO2. These results were published in the Journal of Physical Chemistry C, and the article was featured on the cover page of the issue. Further mechanistic insights were gained on the photoelectrochemical oligomerization and published in Electrochimica Acta.
Building on the results obtained during the outgoing phase, we extended out studies to ternary hybrid assemblies to be exploited in solid state QD-sensitized solar cells. Photoelectrochemical synthesis of TiO2/CdX/PEDOT [X: S, Se; PEDOT = poly(3,4-ethylenedioxythiophene)] ternary hybrids was carried out by exploiting the semiconductor (SC) nature of both the oxide and the chalcogenide component. CdS and CdSe quantum dots were deposited on the nanotubes using successive ionic layer adsorption and reaction (SILAR). The conjugated polymer, PEDOT, was then grafted using photoelectrochemical excitation of the SC matrix and potentiodynamic deposition, to ultimately afford the ternary hybrid architecture. Photoelectrochemical deposition of the conducting polymer was carried out both through selective excitation of the chalcogenide sensitizer and the collective photoexcitation of the two SC components. Two precursor molecules, namely, EDOT or bis-EDOT were compared and contrasted in terms of their relative proclivity to oligomer/polymer formation. The use of UV or visible spectral wavelength components allowed for discrimination between the various polymer formation and SC photoexcitation pathways. This study is currently in the publication process.
During the outgoing phase of my international outgoing fellowship, my training included hands on experience of different characterization methodologies. I’ve learned the most in the field of photoelectrochemistry. Electrochemical phenomena on the semiconductor/electrolyte interface are rather complicated, especially under illumination. I believe that I can build on this knowledge, both in terms of synthesis and use of organic/inorganic hybrid assemblies. Furthermore, solar fuel generation will definitely be one of the main directions of my future research interest. Among others, the learned methods include photovoltammetry, IPCE (internal photon conversion efficiency) measurement and the analysis of the photocurrent transients. In terms of knowledge transfer, due to the regular group meeting I got acquainted with all the activities of the research group which broadened my knowledge significantly. I got inspiration related to various different fields of state of the art problems of energy related (photo)-electrochemistry, which definitely had an impact on my reintegration in Hungary. I was also working closely together with PhD students, which has helped me to improve my mentoring skills. Moreover, due to the regular consultations with Prof. Rajeshwar, my scientific writing skills and verbal skills developed significantly. During my reintegration period I started to build a state-of-the-art photoelectrochemistry laboratory, focusing on solar energy conversion. This last year enabled me to establish certain synthesis and characterization methods which I learned in the USA. Most importantly these include the electrochemical synthesis if semiconductor nanostructures and their photoelectrochemical characterization.
Overall, I firmly believe that the accomplished training and my progress is in full agreement with the objectives stated in the proposal. In addition, I feel that I was able to exploit most of the opportunities offered by my host institution. Consequently, I feel being at a more advanced level of scientific maturity than before my fellowship. This fact makes me ready to start my independent faculty career at the University of Szeged with the support of the Momentum Grant (700 k€) of the Hungarian Academy of Sciences. This financial support helps me to continue my work along the same principles and standards in the next five years.
From the hybrid perspective, the main objective of the outgoing phase was to achieve the homogenous infiltration of different conducting polymers into inorganic oxide matrices. Particular care was devoted to maintain the electroactivity of the polymer in the hybrid configuration. In some special cases simple electrodeposition was a feasible approach to obtain such composites (e.g. WO3/Polyaniline). These hybrid assemblies exhibited a quasi-reversible voltammetric behavior, which was the exact sum of their components. Photoelectrochemical data revealed a complex behavior of the composites. Specifically the hybrid exhibited the properties of both individual components: cathodic (p-type) photocurrents at negative potentials (related to the polymer) and anodic (n-type) photoactivity at positive potentials (arising from the oxide). Importantly, the cathodic photocurrents related to the CPs were ca. 3− 4 times larger in the hybrid configuration compared to values obtained for the neat polymeric film on a Au electrode. Such enhanced photocurrent may be utilized in applications, for example, the photo-electrocatalytic reduction of O2 and CO2 and such studies are in progress. These results were published in one of the flagship journals of the field (Journal of Physical Chemistry C).
As a result of our work a proof of concept was presented for photo-electrodeposition of conjugated polymers on nanostructured matrices of wide bandgap inorganic oxide semiconductors. Photocatalytic deposition preceding the electropolymerization step during potentiodynamic synthesis was shown to play a key role in the formation of homogeneous and organized assemblies of these hybrids. Employing illumination to deposit photo-electrochemically a very thin seed layer on the surface of an oxide semiconductor helps to overcome the main challenge, namely, its limited electroactivity. We have shown that by varying the applied potential window during the potentiodynamic polymerization, the contribution of photoelectrochemical and electrochemical polymerization to the overall process can be separately expressed. Morphological features (as seen by FE-SEM) corroborated the decisive role of illumination in achieving homogeneous deposition of PANI and PPy on nanoporous WO3 and PANI on nanotubular TiO2. These results were published in the Journal of Physical Chemistry C, and the article was featured on the cover page of the issue. Further mechanistic insights were gained on the photoelectrochemical oligomerization and published in Electrochimica Acta.
Building on the results obtained during the outgoing phase, we extended out studies to ternary hybrid assemblies to be exploited in solid state QD-sensitized solar cells. Photoelectrochemical synthesis of TiO2/CdX/PEDOT [X: S, Se; PEDOT = poly(3,4-ethylenedioxythiophene)] ternary hybrids was carried out by exploiting the semiconductor (SC) nature of both the oxide and the chalcogenide component. CdS and CdSe quantum dots were deposited on the nanotubes using successive ionic layer adsorption and reaction (SILAR). The conjugated polymer, PEDOT, was then grafted using photoelectrochemical excitation of the SC matrix and potentiodynamic deposition, to ultimately afford the ternary hybrid architecture. Photoelectrochemical deposition of the conducting polymer was carried out both through selective excitation of the chalcogenide sensitizer and the collective photoexcitation of the two SC components. Two precursor molecules, namely, EDOT or bis-EDOT were compared and contrasted in terms of their relative proclivity to oligomer/polymer formation. The use of UV or visible spectral wavelength components allowed for discrimination between the various polymer formation and SC photoexcitation pathways. This study is currently in the publication process.
During the outgoing phase of my international outgoing fellowship, my training included hands on experience of different characterization methodologies. I’ve learned the most in the field of photoelectrochemistry. Electrochemical phenomena on the semiconductor/electrolyte interface are rather complicated, especially under illumination. I believe that I can build on this knowledge, both in terms of synthesis and use of organic/inorganic hybrid assemblies. Furthermore, solar fuel generation will definitely be one of the main directions of my future research interest. Among others, the learned methods include photovoltammetry, IPCE (internal photon conversion efficiency) measurement and the analysis of the photocurrent transients. In terms of knowledge transfer, due to the regular group meeting I got acquainted with all the activities of the research group which broadened my knowledge significantly. I got inspiration related to various different fields of state of the art problems of energy related (photo)-electrochemistry, which definitely had an impact on my reintegration in Hungary. I was also working closely together with PhD students, which has helped me to improve my mentoring skills. Moreover, due to the regular consultations with Prof. Rajeshwar, my scientific writing skills and verbal skills developed significantly. During my reintegration period I started to build a state-of-the-art photoelectrochemistry laboratory, focusing on solar energy conversion. This last year enabled me to establish certain synthesis and characterization methods which I learned in the USA. Most importantly these include the electrochemical synthesis if semiconductor nanostructures and their photoelectrochemical characterization.
Overall, I firmly believe that the accomplished training and my progress is in full agreement with the objectives stated in the proposal. In addition, I feel that I was able to exploit most of the opportunities offered by my host institution. Consequently, I feel being at a more advanced level of scientific maturity than before my fellowship. This fact makes me ready to start my independent faculty career at the University of Szeged with the support of the Momentum Grant (700 k€) of the Hungarian Academy of Sciences. This financial support helps me to continue my work along the same principles and standards in the next five years.