9:30 AM - **H1.1
Advances in the Research of the Intermediate Band (IB) Solar Cell.
Antonio Luque 1 , Antonio Marti 1 Show Abstract
1 Instituto de Energia Solar, Universidad Politecnica de Madrid, Madrid Spain
An IB solar cell is formed of an IB material situated between two ordinary semiconductors —n- and p-type respectively— that play the role of selective contacts to conduction band (CB) and valence band (VB) electrons respectively. The IB material has a band of states inside the band gap between the CB and the VB. In this way photons with less energy than the one necessary to pump an electron form the VB to the CB can be absorbed by transitions that pump an electron form the VB to the IB and form the IB to the CB. Thus a full VB->CB electron transition (or electron-hole pair generation) can be completed by means of two photons of energy below the band gap. This mechanism should increase the solar cell current.However, any increase of cell current is usually accompanied by a reduction of the voltage. To avoid this, it is necessary that three separate quasi-Fermi levels appear in the IB material, two of them associated to the VB and to the CB, as in ordinary solar cells, and the third one associated to the IB. The voltage extracted from the cell is precisely the difference of the CB and VB quasi-Fermi levels at the n- and p-contacts respectively (changed of sign and divided by the charge of the electron). Nevertheless, photons of lower energy than this voltage can contribute to the current thanks to the IB.Limit efficiency of this concept for maximum concentration (the one providing isotropic illumination on the cell with the radiance of the sun’s photosphere) is 63.2% to compare with the Shockley-Queisser limit of 40.7% for an ordinary cell in the same conditions.IB GaAs solar cells have been fabricated based on this concept using InAs quantum dots (QD) to form the IB. A small increase of the short circuit current has been measured by using two sources of photons of different energy and evidence of the electron-hole formation through the described two-photon mechanism has been produced. In addition, a separation of the quasi-Fermi levels has been experimentally found for cells forward biased. However the efficiency is not higher than the one of the cells without quantum dots mainly because of the low current enhancement due to the weak absorption in the QD, due to their inherent low density and low IB material thickness. The solution proposed for this is to use strain-balanced growth permitting the growth of more quantum dot layers and the improvement of the light on the quantum dots by using diffraction methods.While QD IB cells have revealed to be very useful for proving the basic principles, practical cells seem more promising based on alloys, with higher IB atoms density. Deep levels situated in the mid of the gap are know to be introduced by certain impurities and to act as effective centres of SRH recombination. But when their density exceeds the Mott transition the electrons in the level become delocalised, the deep level becomes an impurity band and the SRH recombination is thought to disappear.
10:00 AM - H1.2
Growth and Characterization of Intermediate Band Cells Containing InGaAs Quantum Dots Surrounded by Thin GaAsP Energy Fence Barrier Layers.
Andrew Norman 1 , Mark Hanna 1 , Fude Liu 1 , Pat Dippo 1 , Scott Ward 1 , Mowafak Al-Jassim 1 Show Abstract
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Wei and Forrest have proposed using thin, high band gap, energy fence barrier layers to improve the efficiency of quantum dot intermediate band solar cells by reducing charge trapping and subsequent recombination in the quantum dots. Their calculations indicated GaAs-based quantum dot intermediate band cells, employing such energy fence barrier layers, could achieve power conversion efficiencies as high as 45%. To the authors’ knowledge, this proposal has not so far been experimentally tested. We report here results obtained on the epitaxial growth and characterization of InGaAs/GaAs quantum dot intermediate band cells containing thin GaAsP energy fence barrier layers. The quantum dot pin cell samples, containing 10 layers of undoped InGaAs quantum dots (6.1 ML, In concentration ~ 0.47) embedded between thin undoped GaAsP energy fence barrier layers and separated by ~ 40 nm undoped GaAs barrier layers, were grown by metal organic chemical vapor deposition on p+ (100) and (311)B GaAs substrates. The structural and optical properties of these quantum dot samples have been characterized by atomic force microscopy, transmission electron microscopy, high resolution scanning transmission electron microscopy, and photoluminescence. Processed cells were characterized by current-voltage and quantum efficiency measurements. The results obtained from these structures indicate that 4 nm thickness GaAsP energy fence barrier layers are adequate to completely bury the InGaAs quantum dots. Initial cell measurements, however, indicate slightly lower efficiencies for the quantum dot cells containing the thin GaAsP energy fence barrier layers in comparison to InGaAs/GaAs quantum dot cells of nearly identical structure except the GaAsP energy fence barrier layers were not included.
10:15 AM - H1.3
High Open Circuit 1.1 eV Dilute Nitride III-V Quantum Well Solar Cell.
Aristotelis Fotkatzikis 1 2 , Andenet Alemu 1 , Lekhnath Bhusal 1 2 , Alex Freundlich 1 2 3 Show Abstract
1 Photovoltaics and Nanostructures Laboratories, Center for Advanced Materials, University of Houston, Houston, Texas, United States, 2 Physics Department, University of Houston, Houston, Texas, United States, 3 Electrical and Computer Engineering Department , University of Houston, Houston, Texas, United States
III-V dilute nitrides have attracted much interest over the past few years due to their captivating properties, which include a substantial reduction of the band gap and an increase of the electron effective mass upon incorporation of small amounts of N. These properties make this family of materials very attractive for a wide range of optoelectronic applications, including high-efficiency solar cells. The use of ~1.0 to 1.25 eV GaInNAs subcells, lattice matched to GaAs and Ge, has been suggested to enhance the efficiency of existing triple and quadruple junction solar cells. However unfavorable effects resulting from the addition of N in III-V semiconductors, such as poor minority carrier properties have thus far limited the success of this approach.(1)The use of III-V dilute nitride multi quantum wells (MQWs)inserted in the i-region of conventional III-V solar cells is a promising method to increase their efficiency, while alleviating many of the problems associated with bulk III-V based solar cells. Indeed, the insertion of multi quantum wells within the intrinsic region of conventional GaAs p-i-n solar cells has been predicted to yield practical efficiencies exceeding 35% at AM0. 2 III-V-N MQWs should extend the photon absorption range while alleviating minority carrier issues encountered in thick bulk-like GaInNAs. An added advantage of the use of III-V dilute nitride quantum wells is the stronger absorption coefficient, a result of the increase of the electron effective mass. In this work we discuss the development of 1.1 eV GaAsN/GaAs MQW solar cells grown by Chemical Beam Epitaxy (CBE). Device structures were fabricated on n-type GaAs (001) substrates by radio frequency (rf) nitrogen plasma-assisted CBE. 10 to 20 period MQWs of not intentionally doped strained GaAs1-xNx/GaAs quantum wells were inserted in the i-region of conventional GaAs solar cell structure at relatively low temperatures (440C
10:30 AM - **H1.4
Photogeneration and Carrier Collection in Quantum Confined p-i-n Solar Cells.
Alex Freundlich 1 2 Show Abstract
1 Center for Advanced Materials, University of Houston, Houston, Texas, United States, 2 Physics and ECE, University of Houston, Houston, Texas, United States
With staggering efficiency projections, quantum confined solar cells have been suggested and investigated to overcome the fundamental efficiency limitation of single junction solar cells. Thus far most practical realizations have relied on the introduction of a periodic array of low dimensional narrow bandgap semiconductor (wells, wires, dots) within the intrinsic (i) region of a wider bandgap p-i-n diode. While the approach has been shown to successfully extend the spectral sensitivity of the device, one major practical shortcoming resides in the difficulty of extracting photo-generated carriers from the quantum-confined region, which leads to important recombination losses and marginal (if any) efficiency enhancement. After a brief review of past and recent developments in quantum confined p-i-n diodes, the presentation will focus on the present understanding of recombination and escape mechanisms of photo-generated carriers in these devices and will discuss the importance of quantum confined region design parameters (well depth, hole and electron band discontinuities, carrier effective masses and i- region width,…) in maintaining an efficient collection of the photogenerated carriers. Finally the presentation will evaluate few emerging strategies (single confined-carrier devices, intermediate state assisted carrier escape) toward significantly enhancing practical efficiencies in quantum confined p-i-n solar cells.
11:00 AM - H1: Inter Band
H2: Inorganic Quantum Dot and Nanowire Solar Cells
Monday PM, November 26, 2007
Republic B (Sheraton)
11:30 AM - **H2.1
Photoluminescent Spectra of Nanostructured InAs Quantum Dots Enveloped in A GaAsSb Matrix.
Michael Levy 1 , Stephan Bremner 1 , Christiana Honsberg 1 Show Abstract
1 Electrical and Computer Engineering, University of Delaware, Newark, Delaware, United States
Material properties of nanostructured InAs quantum dots enveloped in a GaAsSb matrix are presented here. This material system is of interest for it is identified as an absorbing medium for a multi-transition solar cell, in principle, capable of achieving greater than 50% efficient photovoltaic solar energy conversion. The structural properties of the samples are investigated with X-ray diffractometry and atomic force microscopy. The optical properties of the samples are investigated with photoluminescent spectroscopy and absorptance measurements. The matrices that were grown include GaAs0.88Sb0.12 matrices. Experimental studies reveal radiative transitions resulting from two distinct photo-induced transitions. These transitions result from interactions with photons with energies greater than and less than the bandgap of the barrier matrix.
12:00 PM - H2.2
Controlled Formation of Nanocrystal Superlattices in Conjugated Polymer Composites.
Alexandros Stavrinadis 1 , Richard Beal 1 , Jason Smith 1 , Hazel Assender 1 , Andrew Watt 1 Show Abstract
1 Department of Materials, University of Oxford, Oxford United Kingdom
We report the formation of nanocrystal superlattices during the post-synthesis processing of PbS nanocrystals/ MEH-PPV composites. Composites were synthesized by a single-step surfactant free route yielding well dispersed nanocrystals with broad size distribution (1-10nm) as characterized by transmission electron microscopy (TEM). When the composites were precipitated from solution by the rapid injection of methanol cubic and triangular colloidal particles, ~200nm in size were formed. Despite apparently monocrystalline electron diffraction patterns bright and dark field imaging shows the particles consist of individual nanocrystals which have self assembled with a high degree of crystallographic alignment. Absorption spectroscopy is used to show that nanocrystals retain their quantum confined properties when assembled. Using a higher alcohol to induce colloid precipitation such as 1-hexanol results in a modification of the superlattice structures observed and improved dispersion properties.
12:15 PM - H2.3
Atomistic Theoretical Modeling of Auger Recombination in Si and Ge Nanocrystals.
Cem Sevik 1 , Ceyhun Bulutay 1 Show Abstract
1 Physics and Institute of Materials Science and Nanotechnology, Bilkent University, Ankara Turkey
12:30 PM - H2.4
Photon Management for Solar Cells.
Andries Meijerink 1 , Peter Vergeer 1 , Linda Aarts 1 , Bryan van der Ende 1 Show Abstract
1 Chemistry, Debye Institute, Utrecht Netherlands
The largest energy loss in solar cells is related to the spectral mismatch between the absorption spectrum of the semiconductor material and the solar spectrum. For the most widely used c-Si solar cell, the main loss is related to the relaxation of high energy charge carriers created after absorption of relatively high energy photons (2-4 eV) to the 1 eV bandgap of c-Si. Alternatively, wider bandgap solar cells (like the Gratzel cell) experience a large energy loss due to the fact that the low energy part of the solar spectrum is not absorbed. In this contribution new strategies to reduce these losses will be presented based on photon management using the well-defined energy levels of lanthanides. The underlying mechanisms for the spectral conversion involve up- and downconversion processes where one higher energy photon is split into two lower energy photons (downconversion) or two lower energy photons are added up to one higher energy photon (upconversion). Recently we have shown that it is possible with downconversion to split high energy vacuum ultraviolet photons into two visible photons with a190% efficiency . Here it will be shown that both resonant energy transfer and cooperative sensitization are promising for downconversion processes resulting in a conversion efficiency from visible to infrared close to 200%. For the couple Tb-Yb efficient downconversion from green to infrared is observed in Y1-xYbxPO4:Tb1%. Upon excitation in the 5D4 level of Tb3+ energy transfer to two neighboring Yb3+ ions occurs. The energy transfer mechanism is shown to be cooperative energy transfer via dipole-dipole interaction. In the fully concentrated material (x=1) the theoretical limit is 185% conversion efficiency. The actual quantum efficiency of the Yb3+ emission is significantly reduced by concentration quenching. Downconversion by resonant energy transfer is studied for a number of couples of lanthanide ions: Er-Yb, Ho-Yb, Pr-Yb and Tm-Yb. In a number of oxide and fluoride matrices effici