Electronic transport properties of vertical and lateral graphene-based heterostructures

Graphene has some drawbacks, like the absence of an electronic band gap which leads to only small gate switching ratios, and its fast carrier recombination that limits its use in optoelectronics. In order to overcome these issues, the aim of this thesis was to tune the properties of graphene by two different modification approaches. The first approach involved chemical functionalization to create lateral graphene heterostructures, whereas the second approach was directed toward vertical heterostructures that combine graphene with another 2D material. For the former case, two novel chemical functionalization methods were investigated. The first method relied upon hyperthermal molecular ion collisions with 4,4¿-azobis(pyridine). This enabled the covalent functionalization of graphene with a functionalization degree of up to 3%, as demonstrated by Raman in combination with XPS. AFM studies revealed that the functionalized sheets retain their topographic integrity. Thus obtained stripe-like patterns of covalently graphene within extended graphene sheets enabled enhanced on/off ratios upon gate switching. In the second functionalization, OsO4 was used to selectively introduce hydroxyl groups to graphene. Best results were obtained using UV light activation, which yielded graphene of high covalent functionalization degree, as concluded from its optical transparency and gate-induced on/off switching ratios of up to 500. Temperature dependent electrical measurements revealed 2D hopping as the dominant transport mechanism. The first type of vertical heterostructure, gr-TiOx-Ti diodes were fabricated and their electrical properties in the dark and under visible light illumination studied. In contrast to conventional MIM diodes, the performance of the graphene-based diodes was found to increase with decreasing thickness of the oxide insulator. Bias-induced modulation of the work function of graphene was identified as the key to the operation mechanism of the graphene diodes, enabling them to reach a very high asymmetry and nonlinearity (9000 and 8). The diodes compete well with state-of-the-art MIM diodes. Furthermore, the graphene-TiOx-Ti diodes could be operated as photovoltaic cell, with a maximum open-circuit voltage of 0.3 V and a short-circuit photocurrent of 14 nA under global illuminationt. This finding constitutes the first proof-of-principle of hot carrier extraction from graphene, based on the photovoltaic effect. The second type of vertical heterostructures was based on black phosphorus (BP). As a first step, thin BP sheets were combined with n-GaAs into novel pn devices. They displayed pronounced rectification behavior that, in the low bias regime, approaches that of ideal diodes. Moreover, when operated as photodiodes they reached external quantum efficiencies (EQE) above 30% under higher reverse bias. On this basis, the bP was then combined with gr into a p+/p heterojunction. Thus obtained devices achieved a maximum internal quantum efficiency (IQE) and responsivity of 13% and 10.7 mA/W, respectively. This IQE is the highest thus far reported for diodes comprising BP and another 2D material. Moreover, with the aid of a top gate it was possible to modulate both, the photo-conversion efficiency and photocurrent generation mechanism of the black phosphorous-graphene diodes. Together with the possibility to tune the photocurrent response by the thickness of the black phosphorous, these diodes emerge as promising photodetectors.

Electronic and structural modification of titanium dioxide/zinc oxide photoanode for dye-sensitized solar cells

Aravind Kumar Chandiran

Dye-sensitized solar cells (DSC) are a new class of molecular photovoltaics that mimics the natural photosynthesis, for the direct conversion of sunlight into electricity. A typical DSC is a sandwich of a dye sensitized nanoparticle TiO2 film and a catalyst coated counter electrode, with a redox electrolyte filling the space in between. The high internal surface area of the mesoporous TiO2 film ensures higher dye uptake for an efficient light harvesting. Following the light illumination, an electron from the highest occupied molecular orbitals (HOMO) of the dye is excited to its lowest unoccupied molecular orbitals (LUMO). Due to an effective electronic orbital coupling between dye and TiO2, the excited electrons are injected into TiO2. The oxidized dye is regenerated by the electron donor present in the electrolyte. The circuit is completed by the transfer of photogenerated electrons in TiO2 via an external load, to counter electrode regenerating the oxidized electrolyte species. In a conventional DSC, the transport rate of the photogenerated electrons in TiO2 competes with the parasitic electron recombination, as these two processes occur at a similar ms time domain. This thesis work is aimed at the fundamental understanding of these electron transfer processes at sensitized photoanodes that were subject to different bulk and surface electronic structural modifications. The implications were shown to be of profound interest for the fabrication of high efficiency and low cost dye-sensitized solar cells. In the first part of the thesis, the bulk electronic structure of the anatase TiO2 nanoparticles is modified by the incorporation of different concentrations of aliovalent cations (Nb5+, Ga3+, Y3+) in the crystal lattice. The cationic doping/substitution in the nanoparticles is achieved by hydrothermal process. The modified nanoparticles retained the parent anatase crystal structure of TiO2. The transparent dye-sensitized solar cells made with these modified films, using our standard heteroleptic metal complex sensitizers and iodide/triiodide redox electrolyte, displayed enhanced photovoltaic performances, at certain added concentrations. Charge extraction measurements in DSC, reveals a formation of deeper trap states below the conduction band of titanium dioxide when Nb ion is incorporated into the TiO2 lattice. On the other hand, Ga3+ and Y3+ did not significantly affect the trap states distribution compared to the reference device. The modification in the trap states and the oxygen stoichiometry due to these cation inclusions significantly influenced the electron recombination and transport rates. These properties are investigated in detail using transient decay techniques. Following the study on the electronic structure of the bulk TiO2, the influence of the surface modification of the TiO2 nanoparticles on the electron transfer dynamics is investigated. The kinetics of electron recombination from TiO2 to the single electron redox shuttles is fast, leading to a significant loss in the open-circuit potential (VOC) of devices. The emergence of cobalt complexes as a potential electrolyte motivated us to investigate the role of surface passivation of the titanium dioxide nanoparticles. Insulating sub-nanometer oxide tunneling layers deposited by atomic layer deposition (ALD) are known to block the electron recombination, thereby, leading to an increase in the VOC and the collection efficiency of the solar cell. A general perception in the DSC community is that any insulating oxide layer can block the recombination. However, in this work, it is unraveled that, just the insulating property of oxides is not sufficient. In addition, the properties such as the conduction band position and the oxidation state of the insulating oxide, the electronic structural modification induced to the underlying TiO2 mesoporous film, modification of surface charges (isoelectric point) and charge of the electrolyte species have to be considered. A complete photovoltaic study has been done by depositing different cycles (by ALD) of four insulating oxides (Ga2O3, ZrO2, Nb2O5 and Ta2O5) and their recombination characteristics, surface electronic properties, transport rate and injection dynamics are investigated with a standard organic dye and Co2+/Co3+ redox mediator. A comparison is made with the conventional iodide/triiodide electrolyte. The second part of the thesis is focused at the development of new type of photoanodes for effective electron transport to increase the charge collection efficiency. We present a way of utilizing thin and conformal overlayer of titanium dioxide or zinc oxide on an insulating mesoporous template as a photoanode for dye-sensitized solar cells. Different thicknesses of TiO2 ranging from 1 to 15 nm are deposited on the surface of the template by ALD. This systematic study helps unraveling the minimum critical thickness of the TiO2 overlayer required to transport the photogenerated electrons efficiently. A merely 6 nm thick TiO2 film on a 3 μm mesoporous insulating substrate is shown to transport 8 mA/cm2 of photocurrent density along with ≈900 mV of open-circuit potential, when using our standard donor-π-acceptor sensitizer and Co(bipyridine)3 redox mediator. The three dimensional conformal shell on the insulating core also exhibits one order of magnitude higher transport rate compared to the conventional TiO2 nanoparticles. Similar nanostructures were made by depositing ZnO on an insulating template and the electron transfer properties are compared with titania. We also developed ALD recipe for the deposition of crystalline TiO2 or ZnO at low temperatures (< 200 °C) on arbitrary mesoporous insulating templates, considering their prospects of making photoanodes on flexible plastic substrates. The experience gained about the deposition of conformal and pin-hole free TiO2 layer in the previous section is extended for the high efficiency solid-state mesoscopic solar cells based on CH3NH3PbI3 perovskite absorbers. We show that a 2 nm as-deposited ALD overlayer on a mesoporous TiO2 film, printed on a bare conducting glass, is sufficient to arrest the recombination process that lead to a power conversion efficiency of 11.5 %. A detailed investigation on the electron back reaction is presented using electrochemical impedance spectroscopy.

EPFL2013

Strategies to Optimizing Dye-Sensitized Solar Cells

Sophie Wenger

Dye-sensitized solar cells (DSCs) constitute a novel class of hybrid organic-inorganic solar cells. At the heart of the device is a mesoporous film of titanium dioxide (TiO2) nanoparticles, which are coated with a monolayer of dye sensitive to the visible region of the solar spectrum. The role of the dye is similar to the role of chlorophyll in plants; it harvests solar light and transfers the energy via electron transfer to a suitable material (here TiO2) to produce electricity — as opposed to chemical energy in plants. DSCs are fabricated of abundant and cheap materials using inexpensive processes (e.g. screen-printing) and are likely to be a significant contributor to the future commercial photovoltaic technology portfolio. The work conducted during this thesis aimed at optimizing the DSC using three different strategies: The use of versatile organic sensitizers for stable and efficient DSCs, the study of tandem device architectures in combination with other solar cells to harvest a larger fraction of the solar spectrum, and the development of a validated optoelectric model of the DSC. Organic donor-π-acceptor dyes are an interesting alternative to the standard metal-organic complexes used in DSCs. Efficient photovoltaic conversion and stable performance could be demonstrated with three classes of donor systems, namely diphenylamine, difluorenylaminophenyl, and π-extended tetrathiafulvalene. The highest conversion efficiencies were obtained with a difluorenylaminophenyl donor system (η = 8.3 % with a volatile electrolyte and η = 7.6 % with a solvent-free ionic liquid, which was a new record for organic dyes at the time of publication). Surprisingly, efficient regeneration of the oxidized dye by the I-/I3- redox mediator was found with the π-extended tetrathiafulvalene system, even though the thermodynamic driving force was as low as 150 mV. So far driving forces of 300-500 mV had been regarded as necessary for efficient regeneration of the dye cation. Also, important structure-property relationships pertaining to the recombination of electrons with the electrolyte and to the stability of the device could be identified (i.e. effect of linear vs. branched structure, linker length, and moieties used). The power conversion efficiency of solar cells can be extended beyond the limit for a single cell (∼ 30 %) by using multiple cells with different optical gaps in a tandem device. DSCs and chalcopyrite Cu(In,Ga)Se2 (CIGS) solar cells have complementary optical gaps and are thus suitable systems for integration in a tandem device. It was shown that a monolithic DSC/CIGS tandem device has the potential for increased efficiency over a mechanically stacked device due to increased light transmission to the bottom cell, and a monolithic DSC/CIGS device with an initial efficiency of η = 12.2 % was demonstrated. The degradation of the devices — induced by the corrosion of the CIGS cell in contact with the I-/I3- redox mediator — could be retarded with a protective thin conformal ZnO/TiO2 oxide layer coated on the CIGS cell by atomic layer deposition. Finally, an experimentally validated optical and electrical model of the DSC has been developed to assist the optimization process, which is predominantly conducted by empirical means in the DSC research community. The optical model allows to accurately calculate the internal quantum efficiency of devices, i.e. the ratio of extracted electrons to absorbed photons by the dye, a crucial and so far difficult to determine characteristic. Intrinsic parameters — like injection efficiency, electron diffusion length, or distribution of trap states in the TiO2 — can be extracted from experimental steady-state and time-dependent data with the electric model. The model allows to make a comprehensive and quantitative loss analysis of the different optical and electric loss channels in the DSC. The model has been implemented with a graphical user interface for straightforward usage. All three optimization strategies — organic dyes, tandem architecture, and device modeling — developed during this thesis make a valuable contribution to the development and commercialization of inexpensive and high efficiency DSCs. They enable a comprehensive view of the system and pave the way for a systematic analysis and reduction of losses, which has been the ultimate route to success for several established photovoltaic technologies.

EPFL2010

Nanoscale Organic / Inorganic Hybrid Photovoltaic Devices

Thomas Dufaux

There is an increasing trend of using various types of nanostructures as components of solar cells. This highlights the need to gain a deeper understanding of the nanoscale interface in such devices. The present thesis aims to study individual photoactive junctions as the elementary functional unit of bulk photovoltaic devices, and exploit the gained knowledge toward further improving the solar cells' efficiency. Along this direction, Schottky-contacts to cadmium sulphide nanowires were thoroughly studied using scanning photocurrent microscopy (SPCM). In conjunction with theoretical simulations of the measured photocurrent profiles, it was found that charge carriers can be very efficiently extracted out of a few micrometer long nanowire segments below the metal contact. Moreover, the carrier combination rate in this section could be determined as a function of an applied bias or backgate voltage. These findings provide valuable clues about how electric fields are distributed within semiconductor nanowire-based devices. Moreover, SPCM was employed to explore the local photoresponse along graphene/CdS heterojunctions, representing a first step toward implementing graphene as an active acceptor material into solar cells. The short circuit current of such devices could be enhanced by two orders of magnitudes through chemical tailoring of the graphene-CdS interface. In addition, evidence was obtained that surface plasmon excitation in the metal contacts can make significant contributions to the generated photocurrent. This effect was exploited in the fabrication of novel surface plasmon detectors, which support different plasmon modes depending on the polarization direction of the incoming light. Finally, it was attempted to realize photovoltaic devices comprising a pn-junction implemented into a π-conjugated organic polymer. While the electric-field assisted exciton dissociation in this system could be successfully modelled in the framework of the Braun-Onsager model, photoinduced charge separation at the interface between differently gated regions of the polymer could not be achieved due to insufficient electron conduction of the material.

EPFL2012