Session U38: Novel Photophysics and Transport in NanoPV II

Hybrid passivated colloidal quantum dot solids for photovoltaics

Colloidal quantum dot (CQD) films are an attractive photovoltaic material due to their large-area-compatible solution processing and bandgap tuning through the quantum size effect. However, the large internal surface areas make CQD films prone to high trap state densities, leading to recombination of charge carriers. We quantify the density of midgap trap states in PbS CQD solids and show that the current photovoltaic performance is limited by these states. We develop a robust hybrid passivation scheme that involves introducing halide anions during the end stages of the synthesis process, which can passivate trap sites that are inaccessible to much larger standard organic ligands, and combine this with an organic crosslinking strategy to form the film. We use our hybrid passivated CQD solid to fabricate a solar cell with a certified efficiency of 7.0\%, which is a record for a CQD photovoltaic device.   

Elimination of deep surface traps in charged colloidal PbS and CdSe quantum dots

Colloidal quantum dots (CQDs) offer a promising path towards high efficiency, scalable, solution and room processed photovoltaics and electronics. Their promise is curtailed today by difficulty of doping, inefficient transport, nonradiative recombination, and blinking, all generally attributed to electronic trap formation. Using first-principles simulations on off-stoichiometric colloidal quantum dots, we show that preparing a CQD free of traps is possible. However, self-compensating defects can form deep electronic trap states in response to charging or doping even in the most idealized CQDs. Surface traps arise from atomic dimers whose energy levels reside within the bandgap. The same states can also form upon photoexcitation, providing an atomistic mechanism for blinking. We show that avoiding the trap formation upon doping is possible by incorporation of select cations on the surface which shift the dimer energy levels above the quantum-confined bandedge.   

High pressure core structures of Si nanoparticles for solar energy conversion

Multiple exciton generation (MEG) in semiconductor nanoparticles (NPs) is a promising path towards surpassing the Shockley-Queisser limit in solar energy conversion efficiency. Recent studies demonstrate MEG to be more efficient in NPs than in the bulk, including Si [1]. However, the increased efficiency is observed only on a relative energy scale in units of the gap: quantum confinement (QC) effects believed to be responsible for efficient MEG in NPs, also increase their optical gap, swiftly shifting the MEG threshold beyond the solar spectrum. Device applications require NPs with low gaps despite the QC enhanced Coulomb interaction. We propose that Si NPs with a core structure resembling that of high pressure Si phases, especially Si-III/BC8, exhibit significantly lower optical absorption thresholds than Si-I NPs, while retaining efficient MEG. The existence of such NPs was recently demonstrated [2]. Our predictions [3] are based on density functional and many body perturbation theory calculations of the electronic, optical and impact ionization properties of hydrogenated Si NPs with high pressure core structures. [1] M. Beard, JPCL 2, 1282 (2011); [2] M. Smith et al., JAP 110, 053524 (2011); [3] S. Wippermann et al. (submitted); [4] M. Voros et al. (submitted)   

Exploring the Influence of the Chemical Passivation on Electron Relaxation in Silicon Quantum Dots Using First-Principles Surface Hopping Methods

The generation of hot carriers in nano-materials is an exciting phenomenon that could potentially increase the efficiency of photovoltaic and photo-electrochemical cells significantly. The electron relaxation dynamics of a system is related to both the electronic and phononic contributions. Given that both of these contributions are ultimately derived from the electronic structure of a system, chemical substitutions may play a significant role in augmenting and controlling the electron relaxation dynamics in nano-materials. With greater insight into the phenomenon from the first-principles theory, engineering new nano-materials with novel opto-electronic properties via a chemical functionalization of its surface becomes a more realistic avenue. A first-principles surface hopping approach based on density functional theory calculations is used to elucidate the relaxation dynamics in silicon quantum-dots. We explore how varying the passivating species on silicon quantum dots influences the electron relaxation dynamics in the system. The two systems considered here are hydrogen-passivated and fluorine-passivated silicon quantum dots. We present a detailed analysis of the electron relaxation dynamics in these nano-materials.   

Germanium nanoparticles for solar energy conversion

We propose a strategy to enhance the efficiency of solar energy conversion by elemental germanium, by using Multiple Exciton Generation (MEG) in Ge nanoparticles with a ST12 core structure. The latter is the structure of a high pressure phase of solid Ge. MEG is more efficient in bulk Ge in the diamond phase than in several other semiconductors, e.g.\ Si. In principle it may be further improved at the nanoscale, due to an increased effective Coulomb interaction. However the electronic energy gap of semiconducting nanoparticles may be too large compared to the visible solar spectrum and their density of states (DOS) too low for efficient solar energy conversion. Using ab initio calculations we found that ST12 Ge nanoparticles of $\sim$1-2~nm exhibit high impact ionization rates and thus presumably efficient MEG, as well as a gap of $\sim$2~eV and a sizable DOS in the low energy part of the spectrum. Therefore these nanoparticles appear to be promising materials for solar energy conversion exploiting MEG.   

Temperature-Dependent Electron Transport in Si and Ge Nanoparticle Photovoltaics

We have studied both Si and Ge nanoparticle-based photovoltaic (PV) devices fabricated in a layered structure via spin-coating of the colloidal Si or Ge solution. With the low toxicity and high abundance of these group IV elements, combined with the relatively low costs of manufacturing via solution deposition, large-scale device processing offers high dollar-per-Watt opportunities as efficiencies continue to improve. To that end, we previously reported temperature effects of solution-processed PbS quantum dot (QD) PVs, wherein the capping ligand's thermal properties were shown to have strong effects on device performance. Here we show similar ligand effects in group IV QD devices. Current-voltage (I-V) measurements at temperatures from 100 to 360 K under dark conditions were fit to the ideal diode equation revealing the electron transport mechanism, with fit parameters matching transport models. The illuminated I-V data provide insight into each device's built-in potential, carrier mobility, and activation energy. In addition, modulating the illumination intensity gives the ideality factors of the solar cells. We show how these variations with temperature and light-intensity can be used to increase device performance for future studies.   

Carrier Multiplication Effects Between Interacting Nanocrystals for Solar Cell Applications

Carrier multiplication is a carrier relaxation process that results in the generation of multiple electron-hole pairs after absorption of a single photon. Such effect can potentially increase power conversion efficiency in solar cells by minimizing effects induced by thermalization loss processes.The possibility of increasing carrier multiplication efficiency by exploiting nanocrystals interplay have been recently demonstrated in both PbSe \footnote{A. Aerts, et al., Nano Lett. 11 4485 (2011)} and Silicon \footnote{D. Timmerman, et al., Nat. Photon. 2, 105 (2008)} \footnote{M.T. Trinh, et al., Nature Photon. 6, 316 (2012)} strongly coupled nanocrystals. In this talk we will analyze the role played by nanocrystal-nanocrystal interaction on carrier multiplication dynamics considering a system of interacting silicon nanoparticles. Using first-principles calculations, quantum cutting energy-transfer processes will be quantified and a new carrier multiplication effect, defined by us Coulomb driven Charge Transfer, will be introduced. Conditions that maximize effects induced by nanocrystals interplay on Carrier Multiplication dynamics will be pointed out and the role played by wavefunctions delocalization will be clarified \footnote{M. Govoni, et al., Nature Photon. 6, 672 (2012)}.   

Monte Carlo modeling of charge transport in nanocrystalline PbSe films

The electronic properties of three-dimensional nanocrystalline (NC) PbSe materials are of particular interest for next generation solar energy conversion technologies. With size-tunable optical and electronic properties, solution processing, and multiple exciton generation, these materials could represent an exciting new class of cost-effective and efficient solar cells. Two models, a multiple trapping random walk(MTRW) and a hopping model, were developed to simulate electron and hole transport in films of PbSe nanoparticles crosslinked with ethane dithiol ligands. This Monte Carlo code could easily be adapted to model solar cell current-voltage characteristics and variety of experimental conditions and device structures. In both simulations, films are represented by a regular cubic lattice, transport is carried out as a series of hopping events between neighboring nanocrystals, and electrons occupy energetic states determined by the particle size of the PbSe nanocrystals. We find that the hopping model represents a simpler parameter set and a better match to experimental measurements. This presentation will discuss the two transport mechanisms and the effects of particle size, energetic disorder, and coulomb blockade effects on electron and hole mobilities.   

3D engineering of potential profile by charged quantum dots for effective photovoltaic conversion

Charging of quantum dots (QDs) is an effective tool for managing of potential profile at micro- and nanoscales. Without radiation, QDs are charged as electrons from the dopants fill QDs. Filling of QDs under solar radiation is determined by the condition of equality of electron and hole capture rates. Because of strong difference in effective masses of electrons and holes, an electron level spacing in QDs substantially exceeds a level spacing for holes. Therefore, QDs play a role of deep traps for electrons, but they are just shallow traps for holes. The holes trapped in QDs may be excited by thermal phonons, while excitation of localized electrons requires IR radiation. Therefore, n-doping of QD structures is strongly preferable for photovoltaic applications. Optimized selective n-doping of QD medium provides micro- and nanoscale potential profiles favorable for effective photovoltaic conversion.   

Toward an Impurity Band PV: Dynamics of Carriers Generated via Sub-band gap Photons

Intermediate band solar cells are a pathway to cells that surpass the Shockley-Queisser limit by enabling the utilization of sub-band gap photons. A proposed method for fabricating an intermediate band material is to use impurities that introduce electronic levels within the band gap. At sufficiently high dopant concentrations, band formation may lead to a suppression of Shockley-Reed-Hall recombination, an idea known as ``lifetime recovery'' [1]. We investigate a proposed intermediate band material, silicon hyper-doped with sulfur. This material system exhibits strong sub-band gap optical absorption and metallic conductivity at sufficiently high sulfur concentrations [2], which makes it a strong candidate for an impurity-band material. We employ low-temperature photoconductivity using sub-band gap light to estimate the trapping rate of electrons in the conduction band. We vary the sulfur concentration near the critical value for the metal-insulator transition to test the idea of ``lifetime recovery'' in the S:Si system.\\[4pt] [1] A. Luque and A. Mart\'i, Adv. Mater. 22, 160 (2010).\\[0pt] [2] M. T. Winkler et.al. Phys. Rev. Lett. 106, 178701 (2011)   

Intermediate Band Performance of GaSb Type-II Quantum Dots Located in n-Doped Region of GaAs Solar Cells

The intermediate band (IB) electronic states assist sub-bandgap photons in generation of additional photocurrent in single-junction solar cells. Such non-linear effect of resonant two-photon absorption of concentrated sunlight attracts much attention because it promises up to 63{\%} conversion efficiency in IB solar cells. The main obstacle to achieving high performance is involvement of IB-states in electron-hole recombination that is drastically increasing the dark current and reducing the open circuit voltage of IB solar cells. The IB-states can be composed of quantum dots (QDs). Concentration of sunlight limits recombination through type-II QD IB-states located outside of the depletion region. In this work we model GaAs solar cell with strained GaSb type-II QDs separated from the depletion region. The focus is on type-II QDs located in n-doped region of p-n-junction. Our calculation shows that photovoltaic performance can be essentially improved by concentration of sunlight, and that this improvement is highly sensitive to the doping of materials and the shape of potential barriers surrounding type-II QDs. For instance, strained GaSb type-II QDs may increase the performance of GaAs solar cell by 20{\%} compared to the reference GaAs solar cell without QDs.   

Single Element n-p Co-doped Wide Band-gap Semiconductors as Candidate Materials for Intermediate-Band Solar Cells

Non-compensated n-p codoping by different element combinations has proven to be an effective approach to introduce intermediate bands in wide band-gap semiconductors. In this approach, the electrostatic attraction within an n-p dopant pair helps to enhance both the thermodynamic and kinetic solubilities of the dopants. Here we present a conceptually new and appealing approach to achieve non-compensated n-p codoping by substitutionally occupying the anionic and cationic sites in the host materials with a single element. The validity of this approach is demonstrated using first-principles calculations, showing that half filled energy bands are created within the forbidden gaps of the semiconductors because of the non-compensated nature of the codpants. Moreover, the electrostatic attraction between the neighboring dopant pairs enhances their thermodynamic and kinetic solubilities in the host semiconductors. Efforts on experimental confirmation of the single element n-p co-doping concept will also be discussed.   

Optical conductivity of GaP alloys studied by hybrid-density functional theory

Highly-mismatched semiconductor alloys are promising to produce multiple gaps utilizing wider frequency range of the solar spectrum. Quantitative first-principles calculations of the optical conductivity, which is of importance to access the performance of solar cells, are out of reach for the standard generalized gradient approximation (GGA) in density functional theory (DFT) due to well-known underestimation of the band gap. To overcome this problem, hybrid-DFT scheme is quite useful, which incorporates nonlocality of the exchange interaction reducing the self-interaction error in the GGA. In this work, highly-mismatched GaP alloys are studied as candidates for intermediate-band solar cells, where the optical conductivity is calculated on the basis of hybrid-DFT combined with time-dependent perturbation theory. Thanks to the practical computational costs of hybrid-DFT compared with the GW approximation, structures with realistic dopant concentrations are handled with 216-site supercells. Ideal composition of alloys in the sense of active optical transition energies and the formation energy are compared, where calculated results propose that the optimal doping condition is Mg-O co-doping [Y. Gohda and S. Tsuneyuki, Appl. Phys. Lett., in press.]   

Study of vertical correlation in type-II ZnCdTe/ZnCdSe submonolayer quantum dots for efficient intermediate band solar cells.

Intermediate band solar cells (IBSCs), having an intermediate band (IB) of states within the bandgap of the host semiconductor that enhances the light absorption without reducing the open circuit voltage, are substantially more efficient than single-gap devices. The IB, in principle, can be fabricated using quantum dots (QDs) embedded in the host semiconductor; however, there are many growth and material issues related to fabricating practical devices. We tackle some of these problems by growing the type-II ZnCdTe/ZnCdSe submonolayer QD system that lack the wetting layer. We present results of high resolution x-ray diffraction based reciprocal space map studies, complemented by photoluminescence, showing that this material system is an excellent candidate for IBSCs. Specifically, we found that the sample with larger Te fractions has larger QDs with increased vertical correlation. The vertical correlation is particularly important to have sufficient overlap of the hole wavefunctions, to facilitate the IB formation in this material system.   

Multiple Exciton Generation in Colloidal Si Nanocrystals at the Energy-Conservation-Limit

Silicon covers more than 90{\%} of photovoltaic cell production and is the 2$^{\mathrm{nd}}$ most Earth-abundant element. In a Bulk Silicon solar cell about half of the total absorbed energy is lost as heat, following the detailed balance Shockley-Queisser (SQ) analysis. Generating multiple excitons (MEG) in quantum confined Nanocrystals per absorbed high energy photon is a route to circumvent some of the heat losses and thereby enhance photoconversion efficiency. However, to utilize the absorbed excess energy for MEG and to break the SQ limit it is desirable to establish MEG threshold as close as possible to 2 x E$_{\mathrm{g}}$. Using femtosecond transient absorption spectroscopy, we demonstrate for the first time the generation of multiple excitons \textit{right at the energy-conservation-limit (2 x E}$_{g})$ in colloidal Si nanocrystals. The observed `near hard MEG-onset' is independent of the size of the nanocrystals studied (2.8 nm and 3.5 nm dots). Unlike Lead chalcogenides, the effect of photocharging on MEG yield is not observed in Si nanocrystals even at moderate pump-photon fluences ($\sim$ 10 nJ), much higher than the fluence typically used to measure MEG (\textless\ 1nJ). The efficient MEG and the observation of `near hard MEG-onset' at 2 x E$_{\mathrm{g}}$ in an indirect band gap semiconductor is extremely promising and has strong implications for third generation photovoltaics and is expected to enhance photoconversion efficiencies.