217 research outputs found
Effects of optical interference and energy transfer on exciton diffusion length measurements in organic semiconductors
Exciton diffusion is of great importance to the future design of high efficiency organic photovoltaics. Exciton diffusion studies require accurate experimental techniques. This paper addresses two important complications that can arise in exciton diffusion length measurements made by analyzing luminescence from thin films on quenching substrates: namely, the effects of optical interference and of energy transfer to the quencher. When there is modest contrast in the refractive indices of the quencher and organic material, as is the case for titania or C 60 and most organic materials, interference effects can overwhelm the measurement, thereby making it impossible to accurately determine the diffusion length of excitons in the organic material. We show that this problem can be fully eliminated by using thin ͑Ͻ5 nm͒ quencher films. The second complication that can occur is energy transfer to the quenching layer. We model the effect this has when fullerenes are used as quenchers. If energy transfer was ignored, one would falsely measure exciton diffusion lengths that are much greater than, and in some cases more than double, the actual diffusion length. Using titania as a quencher we eliminate the possibility of energy transfer, and by using thin titania films we eliminate the effects of interference and accurately measure a diffusion length of 6 ± 1 nm for the commonly used polymer poly͓2-methoxy,5-͑3,7-dimethyloctyloxy͔͒-1,4-phenylenevinylene
A 2-terminal perovskite/silicon multijunction solar cell enabled by a silicon tunnel junction
With the advent of efficient high-bandgap metal-halide perovskite photovoltaics, an opportunity exists to make perovskite/silicon tandem solar cells. We fabricate a monolithic tandem by developing a silicon-based interband tunnel junction that facilitates majority-carrier charge recombination between the perovskite and silicon sub-cells. We demonstrate a 1 cm[superscript 2] 2-terminal monolithic perovskite/silicon multijunction solar cell with a V [subscript OC] as high as 1.65 V. We achieve a stable 13.7% power conversion efficiency with the perovskite as the current-limiting sub-cell, and identify key challenges for this device architecture to reach efficiencies over 25%.Bay Area Photovoltaic Consortium (Contract DE-EE0004946)United States. Dept. of Energy (Contract DE-EE0006707
Highly soluble energy relay dyes for dye-sensitized solar cells
High solubility is a requirement for energy relay dyes (ERDs) to absorb a large portion of incident light and significantly improve the efficiency of dye-sensitized solar cells (DSSCs). Two benzonitrile-soluble ERDs, BL302 and BL315, were synthesized, characterized, and resulted in a 65% increase in the efficiency of TT1-sensitized DSSCs. The high solubility (180 mM) of these ERDs allows for absorption of over 95% of incident light at their peak wavelength. The overall power conversion efficiency of DSSCs with BL302 and BL315 was found to be limited by their energy transfer efficiency of approximately 70%. Losses due to large pore size, dynamic collisional quenching of the ERD, energy transfer to desorbed sensitizing dyes and static quenching by complex formation were investigated and it was found that a majority of the losses are caused by the formation of statically quenched ERDs in solution
Tuning the Properties of Polymer Bulk Heterojunction Solar Cells by Adjusting Fullerene Size to Control Intercalation
Pour la France contemporaine, René Lévy notait en 1996, l’hétérogénéité des recherches d’histoire du crime produites depuis le milieu des années 1980. Il en soulignait les raisons d’ordres divers : approche peu critique des sources, absence de dialogue entre deux pans de la corporation historienne : historiens de lettres et historiens du droit, faible spécialisation du domaine, difficultés de publication. Près d’une décennie plus tard, l’évolution est nette. Les deux derniers siècles ont fait..
Perovskite-perovskite tandem photovoltaics with optimized bandgaps
We demonstrate four and two-terminal perovskite-perovskite tandem solar cells
with ideally matched bandgaps. We develop an infrared absorbing 1.2eV bandgap
perovskite, , that can deliver 14.8 %
efficiency. By combining this material with a wider bandgap
material, we reach monolithic two
terminal tandem efficiencies of 17.0 % with over 1.65 volts open-circuit
voltage. We also make mechanically stacked four terminal tandem cells and
obtain 20.3 % efficiency. Crucially, we find that our infrared absorbing
perovskite cells exhibit excellent thermal and atmospheric stability,
unprecedented for Sn based perovskites. This device architecture and materials
set will enable 'all perovskite' thin film solar cells to reach the highest
efficiencies in the long term at the lowest costs
Architecture Optimization Dramatically Improves Reverse Bias Stability in Perovskite Solar Cells: A Role of Polymer Hole Transport Layers
We report that device architecture engineering has a substantial impact on
the reverse bias instability that has been reported as a critical issue in
commercializing perovskite solar cells. We demonstrate breakdown voltages
exceeding -15 V in typical pin structured perovskite solar cells via two steps:
i) using polymer hole transporting materials; ii) using a more
electrochemically stable gold electrode. While device degradation can be
exacerbated by higher reverse bias and prolonged exposure, our as-fabricated
perovskite solar cells completely recover their performance even after
stressing at -7 V for 9 hours both in the dark and under partial illumination.
Following these observations, we systematically discuss and compare the reverse
bias driven degradation pathways in perovskite solar cells with different
device architectures. Our model highlights the role of electrochemical reaction
rates and species in dictating the reverse bias stability of perovskite solar
cells
- …