Session H29: Physics and Materials for Inorganic Photovoltaics: III

Investigation of GaTlP for Use in Multijunction Photovoltaics

To achieve the highest possible conversion efficiencies in multijunction photovoltaics, the individual layers of the device must both be lattice-matched and have optimal band-gap spacing. Lattice-matched or strain-compensated epitaxy is required for the growth of junctions thick enough to elicit high quantum efficiency. Ideally, for spectral matching, one would have an infinite number of junctions that are current-matched; however, fabrication of a large number of junctions is neither easy nor desirable because of problems that arise from series resistance. In the end, it becomes a balancing act where the optimal number of junctions for a high efficiency concentrator cell is 3-6 junctions, with the conversion efficiency directly linked to how well spacing of the band gaps of the cell are optimized for absorption of the solar spectrum. Unfortunately, many of the optimal lower band-gaps for these multijunction cells do not occur in the dominant materials system (i.e. Ge and mixtures of In, Ga, Al, As, and P). As such, of late there has been a strong push to characterize new materials in hopes of providing more design options for photovoltaic cells. GaTlP is one such material, theorized to be useful as one of the lower junctions of 3+-junction cells while still being lattice-matched to GaAs and Ge. In this research, the change in lattice constant and band gap of GaTlP with varying compositions are investigated first by computational simulation and then with physical devices. New efficiency records should be achievable by incorporating these new optimal junction materials into the design for multijunction cells. This development will help solar concentrator cells achieve grid parity, thereby becoming a viable renewable energy choice.   

Materials Identification for Quantum Dot Intermediate Band Solar Cells Including the Effect of Strain

Heterostructures that include self-assembled quantum dots (SAQDs) have been suggested as model systems for the realization of novel high efficiency solar cells such as those based on intermediate bands (IBs). The lattice mismatch in the epitaxial growth of these structures, necessary for the formation of SAQDs, introduces strain throughout the structure, making the selection of materials systems with appropriate physical parameters problematic. The model solid theory is used to calculate the energy band edge alignment at $\Gamma$ point of such quantum dot (QD) heterostructures including the effects of strain. With the modified band gaps due to strain, a materials search was performed for high efficiency QD solar cells among III-V binaries and ternaries with negligible valence band offsets. This requirement of the valence band offset along with the limited band gap ranges for optimum efficiency results in only a few feasible materials systems being identified. The optimum barrier/dot material system found was Al$_{0.50}$In$_{0.50}$As/ InAs$_{0.41}$P$_{0.59}$ for fully strained system.   

Singlet fission in pentacene through multiple exciton quantum states

Multi-exciton generation (MEG) has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission is the molecular analogue of MEG and has been observed in various systems, including tetracene and pentacene, however, no fundamental mechanism for singlet fission has yet been described, although it may govern MEG processes in a variety of materials. Because photoexcited states have single-exciton character, singlet fission to produce a pair of triplet excitons must involve an intermediate state that: (1) exhibits multi-exciton (ME) character, (2) is accessible from S1 and satisfies the fission energy requirement, and (3) efficiently dissociates into multiple electron-hole pairs. Here, we use sophisticated \textit{ab initio} calculations to show that singlet fission in pentacene proceeds through a dark state (D) of ME character that lies just below S1, satisfies the fission energy requirement (E$_{D}>$2E$_{T0})$, and splits into two triplets (2$\times $T0). In tetracene, D lies just above S1, consistent with the observation that singlet fission is thermally activated in tetracene. Rational design of photovoltaic systems that exploit singlet fission will require \textit{ab initio} analysis of ME states such as D.   

Intermediate band solar cells

Intermediate band (IB)solar cells aim to exploit the energy of below bandgap energy photons in solar cells. They are based in a material that, in addition to the conventional conduction and valence bands, have an electronic band (named intermediate band) located inside the bandgap and separated from the conduction and valence band by a null density of states. The theoretical limiting efficiency of these cells is equivalent to that of a triple junction solar cell (63.2{\%} at maximum concentration) but requiring a single material instead. Several approaches are being followed worldwide to take to practice this concept. They can be classified into ``bulk'' or ``quantum dot'' approaches. In the ``bulk'' approach, the IB emerges from the insertion of impurities that can be incorporated inside the semiconductor at high densities (beyond the Mott's transition) without forming clusters and that typically would produce deep centers at low densities. Examples experimentally pursued worldwide under this approach are the following material systems: Si:Ti, InGaN:Mn, ZnTe:O and Cu(InGa)S2:Ti. In the ``quantum dot'' approach, the IB arises from the quantum confinement of the electrons usually in the conduction band. This system has allowed in the past to demonstrate some of the principles of operation of the IB solar cells on the basis mainly of the InAs/GaAs system. This system, however, is not optimum for IB solar cell operation and the challenge is now to find a feasible combination of materials that allow both to introduce the confined energy level at the appropriate position as well as to do it at the time the number of additional energy levels introduced between the intermediate band and the conduction band is minimized.   

Structural Characterization of Optically Active Defects in Selenium-Doped Silicon

We demonstrate that enhanced sub-bandgap absorption in ultra-doped Si is directly related to the chemical structure of the dopant atoms. Femtosecond-laser irradiation of a crystalline-Si wafer coated with a thin Se film results in doping concentrations of 1 at. {\%} Se in a layer extending 200 nm from the surface. This layer absorbs over 90{\%} of incident photons at wavelengths between 400 and 2500 nm, demonstrating the potential to increase the efficiency of Si-based solar cells. Se K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy on infrared-absorbing samples reveals clusters of two or more dopant Se atoms. Thermal annealing results in a decrease in infrared absorption and an evolution of Se atom chemical state to isolated interstitial point defects. These results indicate that a Se complex is responsible for enhanced optical absorption and suggest a method to alter the absorption coefficient of silicon.   

Intermediate Band Formation in Selenium Doped Silicon

The intermediate band solar cell is a promising approach to achieving high efficiency photovoltaic conversion, with theoretical limiting efficiencies in principle well exceeding those of conventional single gap semiconductors. The presence of an intermediate defect band within the normally forbidden band gap of the photovoltaic material allows the absorption of normally unused low energy solar photons. Using total energy electronic structure methods based on Density Functional Theory, we explore in detail the formation of a defect band as the concentration of Selenium defects in Silicon is increased from below to above the intermediate band transition limit. Two isolated defect states in Selenium, a double donor in Silicon, are observed. As the concentration of defects is increased, the localized defect states, with correspondingly flat bands, begin to overlap, resulting in electron delocalization and the formation of a continuous, dispersive defect band.   

Electronic transport in silicon doped with sulfur to non-equilibrium concentrations

Doping silicon with chalcogens (S, Se, Te) to highly non-equilibrium concentrations ($>$10$^{20}_{ }$cm$^{-3})$ yields intriguing optical properties, such as near-unity optical absorptance extending to photon energies lower than 0.5 eV --- significantly below the band gap of silicon. We have previously hypothesized that this absorption arises due to an impurity band formed from the high concentration of sulfur-dopant states, and could represent one of the first bulk impurity band absorbers. In this talk, we report temperature-dependent Hall effect and resistivity measurements of silicon doped with high sulfur concentrations; doping techniques include both fs-laser doping as well as ion implantation followed by pulsed laser melting and rapid resolidification. We report a sulfur-donor driven transition to metallic conduction, and identify the critical sulfur concentration for this effect. To our knowledge, this is the first report of a metal-insulator transition driven by such a deep state in silicon. We also will discuss the relevance of these findings to our hypothesis that the anomalous sub-band gap absorption represents an impurity-band effect.   

Time-domain ab initio studies of photoinduced electron dynamics in nanoscale semiconductors

Design of novel materials for energy harvesting and storage requires an understanding of the dynamical response on the nanometer scale. We have developed state-of-the-art non-adiabatic molecular dynamics techniques and implemented them within time-dependent density functional theory in order to model the ultrafast processes in these materials at the atomistic level and in real time. Quantum dots (QD) are quasi-zero dimensional structures with a unique combination of molecular and bulk properties. As a result, QDs exhibit new physical phenomena such as the electron-phonon relaxation bottleneck and carrier multiplication, which have the potential to greatly increase solar cell efficiencies. Photoinduced charge separation across molecular/bulk interfaces drives the dye-sensitized semiconductor solar cell. A subject of active research, it creates many challenges due to the stark differences between the quantum states of molecular and periodic systems, as well as the different sets of theories and experimental tools used by physicists and chemists. Our time-domain atomistic simulations create a detailed picture of these materials. By comparing and contrasting their properties, we provide a unifying description of quantum dynamics on the nanometer scale, resolve several highly debated issues, and generate theoretical guidelines for development of novel systems for energy harvesting and storage. \\[4pt] [1] O. V. Prezhdo ``Photoinduced dynamics in semiconductor quantum-dots: insights from time-domain ab initio studies'', \textit{Acc. Chem. Res.}, available online.\\[0pt] [2] O. V. Prezhdo, W. R. Duncan, V. V. Prezhdo, ``Photoinduced electron dynamics at semiconductor interfaces: a time-domain ab initio prospective'', \textit{Prog. Surf. Science}, \textbf{84}, 39 (2009).\\[0pt] [3] O. V. Prezhdo, et al., ``Dynamics of the photoexcited electron at the chromophore-semiconductor interface'', \textit{Acc. Chem. Res.}, \textbf{41}, 339 (2008).\\[0pt] [4] W. R. Duncan, O. V. Prezhdo, ``Theoretical studies of photoinduced electron transfer in dye-sensitized TiO$_{2}$'', Review, \textit{Ann. Rev. Phys. Chem.}, \textbf{58}, 143 (2007).\\[0pt] [5] C. F. Craig, W. R. Duncan, O. V. Prezhdo ``Trajectory surface hopping in the time-dependent Kohn-Sham theory for electron-nuclear dynamics'', \textit{Phys. Rev. Lett.}, \textbf{95} 163001 (2005).   

Advances in all-sputtered CdTe solar cells on flexible substrates

The University of Toledo II-VI semiconductor group has developed magnetron sputtering (MS) for the deposition of thin films of CdS, CdTe, and related materials for photovoltaic applications. On glass superstrates, we have reached air mass 1.5 efficiencies of 14{\%}.[1] Recently we have studied the use of MS for the fabrication of thin-film CdS/CdTe cells on flexible polyimide superstrates. This takes advantage of the high film quality that can be achieved at substrate temperatures below 300 C when RF MS is used. Our recent CdS/CdTe solar cells have reached 10.5{\%} on flexible polyimide substrates. [2] This all-sputtered cell (except for back contact) has a structure of polyimide/ZnO:Al/ZnO/CdS/CdTe/Cu/Au. The physics of this device will be discussed through the use of spectral quantum efficiency and current-voltage measurements as a function of CdTe layer thickness. Pathways toward further increases in device efficiencies will also be discussed. [1] Appl. Phys. Lett. 85, 684 (2004) [2] Phys. Stat. Sol. (B) 241, No. 3, 779--782 (2004)   

High-Efficiency Photovoltaic Energy Conversion using Surface Acoustic Waves in Piezoelectric Semiconductors

We propose a radically new design for photovoltaic energy conversion using surface acoustic waves (SAWs) in piezoelectric semiconductors. The periodically modulated electric field from SAW spatially separates photogenerated electrons and holes to the maxima and minima of SAW, thus preventing their recombination. The segregated electrons and holes are transported by the moving SAW to the collecting electrodes of two types, which produce dc electric output. Recent experiments [1] using SAWs in GaAs have demonstrated the photon to current conversion efficiency of 85\%. These experiments were designed for photon counting, but we propose to adapt these techniques for highly efficient photovoltaic energy conversion. The advantages are that the electron-hole segregation takes place in the whole volume where SAW is present, and the electrons and holes are transported in the organized, collective manner at high speed, as opposed to random diffusion in conventional devices.\\[4pt] [1] S. J. Jiao, P. D. Batista, K. Biermann, R. Hey, and P. V. Santos, \textit{J. Appl. Phys.} {\bf 106}, 053708 (2009).   

Studying nanoscale CuInSe$_{2}$ for high efficient tandem cell applications

We report Cu(In,Ga)Se$_{2}$ nanoparticle-based cells using transparent conductive oxide back contacts. The structure is very important for next-generation tandem quantum-dot solar cells. The nanoparticles were synthesized by \textit{in-situ} pulsed laser ablation and subsequently by a selenization process. X-ray diffraction, transmission electron microscopy, and atomic force microscopy of the nanoparticles were measured to study phase, crystalline information, chemical composition, and morphology. Optical characterization yielded transmission, bandgap shift, and absorption coefficient. Local current-voltage behaviors were also investigated with a contact-mode conducting scanning probe in dark and bright illuminating conditions. CdS buffer layers were found to be critical to obtain high external quantum efficiency (EQE) and energy conversion efficiency ($\eta )$. More than 20 {\%} of EQE and 1 {\%} of $\eta $ were obtained and are now being improved.