Ultrafast Spectroscopy for Printable Photovoltaics

<p>The development of efficient and low cost photovoltaic technologies is key to a more sustainable energy pathway for future generations. Research efforts aimed at improving the performance of organic photovoltaic (OPV) materials have resulted in a continuous growth in power conversion efficiency (PCE) over time, with a recent maximum PCE value of 18.22% in a single bulk heterojunction device. However, further improved efficiency, stability and cost reduction are required in order for OPVs to succeed in the market. To produce better performing OPV devices in a rational way, it is necessary to understand the relationships between material properties (e.g. energy levels, recombination rates, charge carrier mobilities) and the photovoltaic parameters. This requires combining different fundamental techniques, such as spectroscopic, electrical and structural studies of the materials. In this thesis work we contribute to the understanding of the mechanisms of charge photo-current generation in OPV layers by using transient absorption spectroscopy (TAS) to directly measure the fate of the photo-excited species created upon light absorption. In particular, we contribute to the understanding of the dynamical properties of tightly bound, interfacial charge-transfer (CT) states at the donor:acceptor heterojunction. We disentangle the contributions from individual transient species to the overall TAS signal via the soft-modelling algorithm known as Multivariate Curve Resolution by Alternating Least Squares (MCR-ALS), and we use simple kinetic models to retrieve associated kinetic rates. Our first study explores the photo-physics of a family of polymers derived from the low-band-gap alternating copolymer PTBT where the sulphur atom in the thiadiazole unit was substituted with oxygen or selenium. The literature shows that replacing a single atom in the donor or acceptor unit of a polymer donor can cause large changes in the photovoltaic parameters, which cannot be explained considering only the variations in the optical band-gap. Opposite results have been reported on systems where a sulfur atom is replaced by selenium, and spectroscopic studies were lacking. Our TAS results on PTBO and PTBSe systems explain the superior photovoltaic performance of the original sulfur-containing variant PTBT, highlighting the low tolerance of these materials to backbone substitutions. In both PTBO and PTBSe systems, we identify strong recombination of geminate CT pairs as the major limiting factor of the Jsc and FF photovoltaic parameters. This is attributed to unfavourable electronic and conformational properties at the donor:acceptor interface. In the particular case of PTBSe:PC61BM, the recombination pathway of CT states with triplet character into the triplet exciton manifold is facilitated by the heavy atom effect, in addition to a highly intermixed morphology. Our second study comprises the spectroscopic comparison between fullerene and nonfullerene (NFA) OPV layers. The PCE of OPV devices was reaching a plateau in past years, which was overcomed thanks to the development of high efficiency NFA acceptors. Here, we compare charge generation and recombination between three systems featuring the same polymer donor PPDT2FBT matched with three different acceptors, namely the fullerene acceptor PC70BM, the small molecule nonfullerene acceptor NIDCS-HO and the polymeric acceptor N2200. Our results provide insight on the processes that limit the performance of each device, showing that small molecule NFA are promising acceptors, since morphology and disorder, the factors that we have found to be limiting the device performance, could potentially be tuned for the development of more efficient materials. For the all-polymer device based on the N2200 acceptor, we find that both geminate and nongeminate recombination are limiting the photovoltaic performance. Lastly, we investigate charge carrier dynamics in a series of solar devices composed predominantly of C60 and small amounts of organic small molecule donors, where their CT state energies are systematically varied. The well-defined microstructure in low-donor-content OPV blends makes it easier to correlate macroscopic properties to molecular parameters. Our results, in combination with time-delayed collection field (TDCF), and external quantum efficiency measurements (EQE) measurements at different bias performed by our collaborators, allow us to identify geminate recombination as the major loss channel. We find that the dynamics of the CT decay are connected to the CT state energy via the energy-gap law. In this way, the energy of the CT state is identified as the main parameter determining the efficiency of photocurrent generation in these morphologically well-defined donor:acceptor blends. Overall, the contributions in this thesis work demonstrate how TAS measurements can provide valuable information to construct a comprehensive picture of the underpinning mechanisms of charge photo-current generation in OPV layers, in particular by isolating the dynamical properties of interfacial charge-transfer (CT) states at the donor:acceptor heterojunction via modelling.</p>

Ultrafast Spectroscopy for Printable Photovoltaics

<p>The development of efficient and low cost photovoltaic technologies is key to a more sustainable energy pathway for future generations. Research efforts aimed at improving the performance of organic photovoltaic (OPV) materials have resulted in a continuous growth in power conversion efficiency (PCE) over time, with a recent maximum PCE value of 18.22% in a single bulk heterojunction device. However, further improved efficiency, stability and cost reduction are required in order for OPVs to succeed in the market. To produce better performing OPV devices in a rational way, it is necessary to understand the relationships between material properties (e.g. energy levels, recombination rates, charge carrier mobilities) and the photovoltaic parameters. This requires combining different fundamental techniques, such as spectroscopic, electrical and structural studies of the materials. In this thesis work we contribute to the understanding of the mechanisms of charge photo-current generation in OPV layers by using transient absorption spectroscopy (TAS) to directly measure the fate of the photo-excited species created upon light absorption. In particular, we contribute to the understanding of the dynamical properties of tightly bound, interfacial charge-transfer (CT) states at the donor:acceptor heterojunction. We disentangle the contributions from individual transient species to the overall TAS signal via the soft-modelling algorithm known as Multivariate Curve Resolution by Alternating Least Squares (MCR-ALS), and we use simple kinetic models to retrieve associated kinetic rates. Our first study explores the photo-physics of a family of polymers derived from the low-band-gap alternating copolymer PTBT where the sulphur atom in the thiadiazole unit was substituted with oxygen or selenium. The literature shows that replacing a single atom in the donor or acceptor unit of a polymer donor can cause large changes in the photovoltaic parameters, which cannot be explained considering only the variations in the optical band-gap. Opposite results have been reported on systems where a sulfur atom is replaced by selenium, and spectroscopic studies were lacking. Our TAS results on PTBO and PTBSe systems explain the superior photovoltaic performance of the original sulfur-containing variant PTBT, highlighting the low tolerance of these materials to backbone substitutions. In both PTBO and PTBSe systems, we identify strong recombination of geminate CT pairs as the major limiting factor of the Jsc and FF photovoltaic parameters. This is attributed to unfavourable electronic and conformational properties at the donor:acceptor interface. In the particular case of PTBSe:PC61BM, the recombination pathway of CT states with triplet character into the triplet exciton manifold is facilitated by the heavy atom effect, in addition to a highly intermixed morphology. Our second study comprises the spectroscopic comparison between fullerene and nonfullerene (NFA) OPV layers. The PCE of OPV devices was reaching a plateau in past years, which was overcomed thanks to the development of high efficiency NFA acceptors. Here, we compare charge generation and recombination between three systems featuring the same polymer donor PPDT2FBT matched with three different acceptors, namely the fullerene acceptor PC70BM, the small molecule nonfullerene acceptor NIDCS-HO and the polymeric acceptor N2200. Our results provide insight on the processes that limit the performance of each device, showing that small molecule NFA are promising acceptors, since morphology and disorder, the factors that we have found to be limiting the device performance, could potentially be tuned for the development of more efficient materials. For the all-polymer device based on the N2200 acceptor, we find that both geminate and nongeminate recombination are limiting the photovoltaic performance. Lastly, we investigate charge carrier dynamics in a series of solar devices composed predominantly of C60 and small amounts of organic small molecule donors, where their CT state energies are systematically varied. The well-defined microstructure in low-donor-content OPV blends makes it easier to correlate macroscopic properties to molecular parameters. Our results, in combination with time-delayed collection field (TDCF), and external quantum efficiency measurements (EQE) measurements at different bias performed by our collaborators, allow us to identify geminate recombination as the major loss channel. We find that the dynamics of the CT decay are connected to the CT state energy via the energy-gap law. In this way, the energy of the CT state is identified as the main parameter determining the efficiency of photocurrent generation in these morphologically well-defined donor:acceptor blends. Overall, the contributions in this thesis work demonstrate how TAS measurements can provide valuable information to construct a comprehensive picture of the underpinning mechanisms of charge photo-current generation in OPV layers, in particular by isolating the dynamical properties of interfacial charge-transfer (CT) states at the donor:acceptor heterojunction via modelling.</p>

WO3 as Additive for Efficient Photocatalyst Binary System TiO2/WO3

Two different methods of synthesis of TiO2/WO3 heterostructures were carried out with the aim to increase photocatalytic activity. In this study, anodic TiO2 nanotube films were synthesized by electrochemical anodization of titanium foil. WO3 particles were applied to anodic Ti/TiO2 samples in two different ways – by electrophoretic deposition (EPD) and insertion during the anodization process. Structural and photocatalytic properties were compared between pristine TiO2 and TiO2 with incorporated WO3 particles. Raman mapping was used to character-ise the uniformity of EPD WO3 coating and to determine the structural composition. The study showed that deposition of WO3 onto TiO2 nanotube layer lowered the band gap of the binary system compared to pristine TiO2 and WO3 influence on photo-electrochemical properties of titania. The addition of WO3 increased charge carrier dynamics but did not increase the measured photo-current response. As the WO3 undergoes a phase transition from monoclinic to orthorhombic at approximately 320 ℃ proper sequence WO3 deposition could be beneficial. It was observed that secondary heat treatment of WO3 lowers the photocurrent.

Effect of Co2+ Doped Seed Layer on Morphology and Photoelectric Properties of ZnO Nanoarray

ZnO nanoarray were synthesized by hydrothermal method on Co2+-doped Zn1-xCoxO (x（mol%）=0.00, 0.01, 0.02, 0.03, 0.04, 0.05) seed layers pre-coated on ITO substrate. The effects of different Co2+ doping concentrations on morphology and photoelectric properties of ZnO nanoarray including transient photo-current and charge transfer resistance were investigated. The addition of Co2+ in the seed layer could perfect the oriented growth of ZnO nanoarray and apparently enhance its photo current. FESEM observation confirmed that the ZnO nanoarray were grown in the way perpendicular to ITO substrate along the direction of (002). Meanwhile, UV-vis tests shown that the band gap energy was decreased from 3.37 eV to 3.16 eV due to Co2+ doping and the ZnO nanoarray had a strong visible region in the range 400-650 nm. The transient photo-current was found to vary from 0.005 to 0.15 mA/cm2 under AM 1.5G simulated sunlight illumination. Photoelectric properties was correlated with the recombination of photo-generated charge carriers, which was inhibited with optimal Co2+ doping concentrations and was beneficial for application in perovskite solar cells.

Automatic solution for solar cell photo-current prediction using machine learning

In this paper, we discuss the prediction of future solar cell photo-current generated by the machine learning algorithm. For the selection of prediction methods, we compared and explored different prediction methods. Precision, MSE and MAE were used as models due to its adaptable and probabilistic methodology on model selection. This study uses machine learning algorithms as a research method that develops models for predicting solar cell photo-current. We create an electric current prediction model. In view of the models of machine learning algorithms for example, linear regression, Lasso regression, K Nearest Neighbors, decision tree and random forest, watch their order precision execution. In this point, we recommend a solar cell photocurrent prediction model for better information based on resistance assessment. These reviews show that the linear regression algorithm, given the precision, reliably outperforms alternative models in performing the solar cell photo-current prediction Iph

The Liquid Crystal Click Procedure for Oligothiophene-Tethered Phthalocyanines – Self-Assembly, Alignment and Photo Current

A series of star-shaped liquid crystals (LC) with a phthalocyanine donor core, oligothiophene antennae and fullerene acceptors have been successfully prepared. The hierarchical self-assembly results in a nanosegregated helical donor-acceptor-antennae...