Transport and recombination of electrons in nanostructured solids

18 04 2014

In the late 1990s, Daniel Vanmaekelbergh, Laurie Peter, and others observed that electron diffusion coefficient in dye-sensitized solar cells is density dependent. It was recognized that the electron lifetime is also density dependent, i.e., it depends on the specific position of the steady state Fermi level of electrons. These variations where shown to be a consequence of the multiple trapping model, insofar as a displacement of the Fermi level changes the kinetics of trapping and detrapping. Thus, the dependence of the diffusion coefficient and lifetime does not affect the microscopic parameters of either phenomenon, namely, the jump rate, and the probability of charge transfer of an electron in a trap (J. Bisquert and V. S. VikhrenkoJ. Phys. Chem. B 1082313 (2004)  http://dx.doi.org/10.1021/jp035395y). However, there as some mechanisms as the Langevin recombination, where the speed of an electron during diffusion does determine recombination rates, since an encounter with a recombination center makes the electron disappear from the structure. It has been for some time a puzzle, what is happening in the case of DSCs, and how both types of regimes could be generally distinguished. It is an involved problem as diffusion and charge transfer are occurring at the same time. Furthermore it is not enough to run some simulations and see the results. It is needed to make a clear cut physical criterion, since we want to have an interpretation of the macroscopic time constants as the lifetime, that are effectively measured.

A clear solution to this problem has been completed by my friends Mehdi Ansari-Rad and Juan Antonio Anta, building on a previous paper that laid general definitions of the measurable quantities from basic stochastic behaviour (M. Ansari-Rad, J. A. Anta and J. BisquertJ. Phys. Chem. C 11716275 (2013). http://dx.doi.org/10.1021/jp403232b). In the new paper they show the structure of the lifetime from basic kinetic parameters, in both a reaction limited and diffusion limited regime. In the first one the lifetime is not dependent on any transport properties, and it occurs when the electrons are rapidly thermalized. This result supports the definition of the electron lifetime that has been applied for many years, as Rrec Cchem, where Rrec is recombination resistance and Cchem is the chemical capacitance.

I think this paper brings a lot of insight to the fundamental electronic processes in nanostructures, and it will be helpful for the interpretation of experiments.

Conditions for diffusion-limited and reaction-limited recombination in nanostructured solar cells

Mehdi Ansari-Rad, Juan A. Anta and Ezatollah Arzi

J. Chem. Phys. 140, 134702 (2014); http://dx.doi.org/10.1063/1.4869748

Abstract

The performance of Dye-sensitized solar cells (DSC) and related devices made of nanostructured semiconductors relies on a good charge separation, which in turn is achieved by favoring charge transport against recombination. Although both processes occur at very different time scales, hence ensuring good charge separation, in certain cases the kinetics oftransport and recombination can be connected, either in a direct or an indirect way. In this work, the connection between electron transport and recombination in nanostructured solar cells is studied both theoretically and by Monte Carlo simulation. Calculations using the Multiple-Trapping model and a realistic trap distribution for nanostructured TiO show that for attempt-to-jump frequencies higher than 1011–1013 Hz, the system adopts a reaction limited (RL) regime, with a lifetime which is effectively independent from the speed of the electrons in thetransport level. For frequencies lower than those, and depending on the concentration of recombination centers in the material, the system enters a diffusion-limited regime (DL), where the lifetime increases if the speed of free electrons decreases. In general, the conditions for RL or DL recombination depend critically on the time scale difference between recombination kinetics and free-electron transport. Hence, if the former is too rapid with respect to the latter, the system is in the DL regime and total thermalization of carriers is not possible. In the opposite situation, a RL regime arises. Numerical data available in the literature, and the behavior of the lifetime with respect to (1) density of recombination centers and (2) probability of recombination at a given center, suggest that a typical DSC in operation stays in the RL regime with complete thermalization, although a transition to the DL regime may occur for electrolytesor hole conductors where recombination is especially rapid or where there is a larger dispersion of energies of electron acceptors.





ChemPhysChem special number on HOPV

11 04 2014

ChemPhysChem

Hybrid Organic–Inorganic Photovoltaics

DOI: 10.1002/cphc.201400098

Editorial
Shahzada Ahmad, Mohammad Khaja Nazeeruddin, and Juan Bisquert





Dye solar cell façade at SwissTech convention center at EPFL, by Solaronix

8 04 2014

HOPV14 Conference will be celebrated at the brand new SwissTech convention center at EPFL campus in Lausanne. Here is the first pictures of the semitransparent façade formed by large dye solar cells panels, made by Solaronix. It looks great, and it produces electricity!

fig1

EPFL and the architects at Richter Dahl Rocha contracted Solaronix with an ambitious goal: to combine a technology showcase with an ornamental façade. A year later the challenge has been met. The all-new SwissTech Convention Center hosts the world’s first multicolored Dye Solar Cell façade.
This novel photovoltaic installation has been realized on the vast west side of the building. The 300 m2 installation encompasses a length of 36 meters and a maximum height of 15 meters. No less than 355 panels were installed for 200 m2 of active photovoltaic area. In order to fit with the inclination of the roof, panels from 1 m to 2.5 m were produced by grouping together two to five 50 cm modules. The panels are arranged in 65 colored columns that perfectly complement the sublime architecture of the edifice, fulfilling both the aesthetic ambition and energy awareness of the designers.The transparency of red, green, and orange panels were tuned to meet the overall light transmission target of the architects. The solar façade completes both functions: passively preventing incoming sunlight from overheating the majestic entrance hall while actively producing renewable electricity from sunlight. Mixed and matched, the arrangement of colors ingeniously designed by artist Catherine Bolle gives a unique dynamic to the façade while providing a smooth color tone to the light transmitted into the hall. The annual production of the SwissTech Convention Center solar façade is estimated at 2,000 kWh, a respectable figure given the high transparency and orientation of the façade.

more information

more pictures

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TiO2 nanomaterials for solar cells

29 03 2014

Titanium Dioxide Nanomaterials for Photovoltaic Applications

Chemical Reviews, 2014
  • 1. Introduction: Properties of TiO2 Nanomaterials
  • 2. Application in Dye-Sensitized Solar Cells (DSCs)
    • 2.1. Fundamentals of DSCs
    • 2.2. Interaction of Sensitizers with TiO2
      • 2.2.1. Anchoring of Sensitizers on TiO2 Surface
      • 2.2.2. Influence of Sensitizer Adsorption Behavior
    • 2.3. Interaction of Coadsorbents with TiO2
    • 2.4. Interaction of Electrolytes with TiO2
    • 2.5. Electrons Transport and Recombination in TiO2 Electrodes
    • 2.6. Nanostructured TiO2 Electrodes for DSCs
  • 3. Application in Polymer-Inorganic Hybrid Solar Cells
    • 3.1. Fundamentals of Polymer-Inorganic Hybrid Solar Cells
    • 3.2. Devices Based on Nanoporous TiO2
    • 3.3. Devices Based on Polymer-TiO2 Blends
    • 3.4. Devices Based on Nanorods and Nanotubes
  • 4. Application in Quantum Dot-Sensitized Solar Cells (QDSCs)
    • 4.1. Fundamentals of QDSCs
    • 4.2. Surface Treatments in QDSCs
    • 4.3. Nanostructured TiO2 Electrodes for QDSCs
  • 5. Application in Inorganic Solid-State Solar Cells
  • 6. Application in Perovskite Solar Cells
  • 7. Concluding Remarks




Perovskite solar cells at HOPV Conference

20 03 2014

The perovskite solar cells have become a major revolution to the field of organic-inorganic photovoltaics in a very short time. This year HOPV14 Conference in Lausanne will bear witness to the fast progress and new topics of study. A very large number of groups have presented contributions in this field, from only a few in 2013 edition.

See the list of abstracts





Organometal Halide Perovskites for Transformative Photovoltaics

12 03 2014

Organometal Halide Perovskites for Transformative Photovoltaics

This is a series of papers published on JACS and JPCL on this topic. All articles will remain free to non-subscribers until the appearance of the next JACS Select.

Organometal halide perovskite based solar cells have recently emerged as one of the transformative photovoltaic technologies. Power conversion efficiency attained with CH3NH3PbI3 has now exceeded 15%, making them competitive with thin film PV technology. In this virtual issue we present a few recent articles that focus on the new methods and the physical insights into the operation of organometal halide perovskite solar cells.

 





Solar energy in Spain

5 03 2014

Antonio Urbina has presented a detailed analysis of solar energy in the context of the renewable energies in Spain in the last decade

Solar electricity in a changing environment: The case of Spain. Renewable Energy, Volume 68, Pages 264–269, 2014.

Antonio Urbina

Antonio Urbina

Here are some of the highlights

Evolution of the Spanish electricity mix is analysed for the past 23 years.

Within a total installed capacity which has evolved from 43GW in 1990 to 102GW in 2013, the share of renewable installed capacity has increased up to 48%, of which wind represents 22% (of the total) and solar electricity 6% (4% PV, 2% thermoelectric)

Solar electricity reached 15% of power generation in August 2013.

At peak time (13:30) solar electricity power injected to the grid was 15% of the total, which is remarkable for only a 6% of share in total installed capacity

Annual capacity factor of solar photovoltaic at national level is above 20%.

Which represents a performance ratio of more than 80% when compared to the theoretical expected energy output from the installed capacity.

Wind and solar are surpassing fossil-fuel technology (gas) in capacity factor.

This fact emphasizes the excess of installed capacity in Spain; since renewable electricity is produced at a very low marginal cost (once the facility has been built), fossil fuel technologies, and in particular gas fired plants (combined cycle) are working at very low capacity factors, around 10% since February 2013, which makes impossible to get an economic benefit from the huge investments made on this technology by the big electricity companies.

SolarElectricitySpain_Mix2013_08_08

Energy mix in Spain

SolarElectricitySpain_CapacityFactors

Solar electricity in Spain: Capacity Factors

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