This PDF file contains the front matter associated with SPIE Proceedings Volume 10527, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Organic-inorganic hybrid perovskites are attracting much attention as photoabsorbers for high-efficiency thin film solar cells. For further improvement of the device performance, the photophysics in the actual devices have to be clarified. Here, we studied carrier recombination and extraction dynamics in CH₃NH₃PbI₃ perovskite solar cells using time-resolved photoluminescence (PL) and photocurrent (PC) measurements. It was found that the PL lifetime of the perovskite layer in the solar cell devices becomes longer with increasing excitation intensity, which is the exact opposite of the trend observed for the bare perovskite thin films. This slowdown of the PL decay reflects the bottleneck effect in the carrier extraction process at the interface, which also reduces the external quantum efficiency for PC. We clarified that the dynamical change of the carrier extraction efficiency plays an important role in the carrier recombination and transport in the perovskite solar cells. We successfully reproduced the experimental excitation intensity dependence by numerical simulations based on a simple model including carrier recombination and extraction processes. Furthermore, we performed microscopic imaging of PL and PC from CH₃NH₃PbI₃ solar cells. The observed negative correlation between the PL and PC intensities reflects the competition between radiative recombination and carrier extraction processes. Our simultaneous measurement of PL and PC provides essential information to establish high-performance devices. This allows us to analyze the photocarrier dynamics in the lead-free CH₃NH₃SnI₃ solar cells.

Thin film tandem solar cells, comprising of a perovskite top device and a radiation hard CIGS bottom solar cell would be attractive for space applications since they can be lightweight, flexible, and efficient. In this work, the ability to withstand the harsh radiation environment in space, consisting mainly of high-energy protons is demonstrated. In-situ measurements of the J-V characteristics during proton irradiation with a proton energy of 68 MeV show that organic-inorganic perovskites are radiation hard. The investigated CH3NH3PbI3 based perovskite solar cells possess a negligible degradation for proton doses of up to 1012 p cm-2. [1,2]This exceeds the proton dose at which c-Si degrades by almost 3 orders of magnitude. Even more surprising is the observation that the solar cell performance improves with time after the proton irradiation is terminated. Hence, the localized defects caused by proton irradiation are metastable and vanish with time. In addition, proton irradiation partially compensated the photo-degradation effect of the perovskite solar cells. Compared to untreated devices the open circuit voltage and the fill factor improved. This is mainly attributed to smaller recombination losses due to a decrease of the density of localized defects. Simultaneously, proton irradiation caused the formation of shallow defects, which act as doping centers. The observed radiation hardness and self-healing capabilities render inorganic-organic perovskite absorbers highly attractive for space applications. [1] F. Lang, N. H. Nickel, J. Bundesmann, S. Seidel, A. Denker, S. Albrecht, V. V. Brus, J. Rappich, B. Rech, G. Landi, H. C. Neitzert, Adv. Mater. 2016, 28, 8726. [2] V. V Brus, F. Lang, J. Bundesmann, S. Seidel, A. Denker, B. Rech, G. Landi, H. C. Neitzert, J. Rappich, N. H. Nickel, Adv. Electron. Mater. 2017, 3, 1600438.

This talk aims at introducing time-resolved fluorescence imaging (TR-FLIM) as an optical characterization method for optoelectronic devices. It allows to obtain time-resolved photoluminescence maps with a temporal resolution of 500 ps and a micrometric spatial resolution. A first experiment under wide-field illumination on a GaAs-based solar cell is presented as a proof of concept. Thanks to a model including diffusion and recombination of minority charge carriers, we could fit the experimental photoluminescence (PL) transients and decorrelate key optoelectronic properties for the considered device. For various fluence levels, we could determine a constant bulk lifetime τ_n = 75 ns, a constant effective diffusion length L_eff = 190 μm, and an injection-dependent contact recombination velocity S_n, which is explained by the saturation of interface states. The wide-field illumination notably avoids lateral diffusion artefacts leading to a significant underestimation of τ_n. TR-FLIM also has a noteworthy interest for optoelectronic materials showing heterogeneous properties, as organic-inorganic halide perovskite. With the same set-up, we could investigate various chemical compositions for this semi-conductor, and highlight the need for another self-consistent model linking TRPL transients with physical properties, as no clear definition of a lifetime appears. However, the crucial role of the perovskite/TiO₂ can be underlined, in particular for the sample containing Cesium, as well as the probable role of charge carrier in-depth diffusion following a pulsed excitation. As a conclusion, TR-FLIM appears as a versatile characterization method and we open the gate to further studies of other optoelectronic devices with this set-up.

Recent efforts to include a quantum dot array within the intrinsic region of a pin GaAs solar cell have focused on minimising the open circuit voltage (Voc) loss relative a control device without quantum dots [1]. Strategies include the addition of strain balancing (e.g. GaP, GaAsP) [2] or high energy (e.g. AlGaAs) barrier layers [3]. In this work we demonstrate a significant improvement of up to 260 mV in Voc by controlling only the size of the quantum dots at the nanometre scale using precise molecular beam epitaxial wafer growth. High resolution scanning transmission electron microscope (STEM) imaging is used to determine the dimensions of individual quantum dots, providing valuable input to a theoretical model. The modelling suggests that the performance improvement is a direct consequence of opening a clear energy gap between the conduction band and the quantum dot ensemble ground state energy e0. With appropriate quantum mechanical design this energy gap can be up to ~90 meV, giving rise to intermediate band behaviour rather than quantum dot solar cell behaviour at room temperature. Current-voltage measurements under air mass 1.5 conditions indicate an efficiency (active area) of 18.4% (19.7%) at 5-suns concentrations. Higher concentration measurements confirm the quality of the material with diode ideality factors as low as 1.16 and Voc ≈ 1 V at 500 suns. References: [1] Y. Okada et al., Appl. Phys. Rev. 2, 021302 (2015), [2] C. G. Bailey et al., Appl. Phys. Lett. 98, 163105 (2011), [3] A. Varghese et al., Nanoscale 8, 7248 (2016).

Hot carrier solar cells (HCSCs) have been proposed as potential systems to increase the conversion efficiency of single gap solar cells beyond the Shockley-Queisser limit. Despite a great deal of recent progress in HCSCs, designing an effective and efficient hot carrier absorber remains challenging. To evaluate the efficiency of any proposed absorber accurate determination of the carrier temperature is required. This can be non-trivial, particularly in the quantum wells (QW) where state-filling effects can complicate the simple extraction of carrier temperature from photoluminescence (PL) spectra. Specifically, the PL may be distorted if there are other linewidth broadening mechanisms prevalent in addition to non-equilibrium carriers. These may include; in addition to state filling, the effects of phonons and system inhomogeneities, all of which serve to perturb the PL linewidth. In this study, an InGaAsP QW with a type-II band alignment is investigated using continuous wave power and temperature dependent PL to evaluate the effects of broadening on the extraction of the true carrier temperature in the system [1]. Since there is a relatively small energy separation between the ground and first excited state transitions in the QW studied, state filling effects can be controlled and their contribution to the linewidth evaluated, using various conditions and combinations of excitation power and lattice temperature. The role of phonons is also presented, and supported within the framework of a theoretical model that includes the various phononic processes and their temperature dependent contribution to the PL. This work is supported by the National Science Foundation Grant ECCS #1610062 [1] Esmaielpour, Hamidreza, Vincent R. Whiteside, Louise C. Hirst, Joseph G. Tischler, Chase T. Ellis, Matthew P. Lumb, David V. Forbes, Robert J. Walters, and Ian R. Sellers. Progress in Photovoltaics: Research and Applications (2017).

Light concentration opens up the path to enhanced material efficiency of solar cells via increased conversion efficiency and decreased material requirement. For true material saving, a fabrication method allowing local growth of high quality absorber material is essential. We present two scalable fs-laser based approaches for bottom-up growth of Cu(In,Ga)Se₂ micro islands utilizing either site-controlled assembly of In(,Ga) droplets on laser-patterned substrates during physical vapor deposition, or laser-induced forward transfer of (Cu,In,Ga) layers for local precursor arrangement. The Cu(In,Ga)Se₂ absorbers formed after selenization can deliver working solar devices showing efficiency enhancement under light concentration.

The record in photovoltaic conversion efficiency is detained by multi-junction solar cells based on III-V semiconductors. However, the wide adoption of these devices is hindered by their high production cost, to a large extent due to the expensive III-V substrates. As an alternative, a hybrid geometry has been proposed [LaPierre JAP 2011], which combines a 2D Si bottom cell with a III-V nanowire top cell in a tandem device. This approach, which may reach theoretical efficiencies of approx. 34%, requires smaller amounts of expensive III-V materials compared to conventional III-V tandem cells and benefits from the nanowire light trapping effects. In this work, we report the fabrication and nanoscale characterization of two types of nanostructures for solar cells: radial GaAlAs and axial GaAsP p-n junction nanowires. Nanowires are grown by gallium-assisted molecular beam epitaxy using Be and Si as doping sources. The composition (probed by EDX and cathodoluminescence) was adjusted to tune the bandgap toward the optimal value for a III-V-on-Si tandem cell (approx. 1.7 eV). Local I-V characteristics and electron beam induced current (EBIC) microscopy under different biases are used to probe the electrical properties and the generation pattern of individual nanowires. For radial junction nanowires, EBIC mappings revealed a homogeneous collection of carriers on the entire nanowire length. For axial junction nanowires, the doping concentrations and the minority carrier diffusion lengths were extracted from the EBIC generation profiles. The effect of an epitaxial GaP passivating shell on the optical and generation properties was assessed.

Bandgap engineering, by means of alloying or inserting nanostructures, is the bedrock of high efficiency photovoltaics. III-V quaternary alloys in particular enable bandgap tailoring of a multi-junction subcell while conserving a single lattice parameter. Among the possible candidates, AlInAsSb could in theory reach the widest range of bandgap energies while being lattice-matched to InP or GaSb. Although these material systems are still emerging photovoltaic segments, they do offer advantages for multi-junction design. GaSbbased structures in particular can make use of highly efficient GaSb/InAs tunnel junctions to connect the subcells. There has been only little information concerning GaSb-lattice matched AlInAsSb in the literature. The alloy’s miscibility gap can be circumvented by the use of non-equilibrium techniques. Nevertheless, appropriate growth conditions remain to be found in order to produce a stable alloy. Furthermore, the abnormally low bandgap energies reported for the material need to be confirmed and interpreted with a multi-junction perspective. In this work, we propose a tandem structure made of an AlInAsSb top cell and a GaSb bottom cell. An epitaxy study of the AlInAsSb alloy lattice-matched to GaSb was first performed. The subcells were then grown and processed. The GaSb subcell yielded an efficiency of 5.9% under 1 sun and the tandem cell is under optimization. Preliminary results are presented in this document.

Here we study photovoltaic properties of GaInPNAs based quantum well and superlattice solar cells, that are either strainbalanced or lattice matched to silicon and evaluate their potential towards the development of high-efficiency tandem operating in conjunction with a silicon bottom cell. Quaternary dilute nitride compound semiconductors like GaAs_yP_1-x-yN_x and Ga_1-zIn_zP_1-xN_x are lattice matched with silicon at y=4.7*x-0.1, z=2.2*x-0.044 and have direct bandgap (with N<0.6%); thus, allowing monolithic integration of III-V optoelectronics with silicon technology and IIIV/Si tandem solar cells. Applying eight band k.p strained Hamiltonian, for both tensile and compressive strain, and Band Anti-crossing (BAC) model to the conduction band, in order to account for small amounts of nitrogen impurities, the electronic band structure and dispersion relation of these alloys can be determined. With small amount of nitrogen (<5%), bandgaps of these alloys lattice matching with silicon fulfil the optimum bandgap (1.7-1.9) eV requirement for III-V/Si tandem solar cell. Confinement energies in quantum structures can be computed using the transfer matrix approach. Optical absorption is evaluated by taking into account the inter-band transitions in the quantum well and barrier region, including excitonic transitions, using the fermi golden rule for both TE and TM polarization of incident light. Using these properties, we can determine whether the possible integration of p-i-n MQWs (or SL) solar cell on silicon is favorable by evaluating the predicted quantum efficiency and photo-current. The designs for a tandem III-V-N/Si represent an opportunity for achieving practical efficiencies of more than 30% under AM 1.5G spectrum.

Dilute nitride GaAsSbN is an ideal candidate to form the 1-1.15 eV lattice-matched sub-cell that would significantly enhance the performance of 3- and 4-junction solar cells. However, growth problems inherent to this quaternary alloy lead typically to a poor crystal quality that limits its applicability. Better compositional control and crystal quality have been recently reported by growing the material as a GaAsSb/GaAsN superlattice, because of the spatial separation of Sb and N that avoid miscibility problems. Moreover, these structures provide bandgap tunability trough period thickness. Here we study the performance of lattice-matched 1.15 eV GaAsSb/GaAsN type-II superlattice p-i-n junction solar cells with different period thickness and compare them with the bulk and GaAsSbN/GaAs type-I superlattice counterparts. We demonstrate carrier lifetime tunability through the period thickness in the type-II structures. However, the long carrier lifetimes achievable with periods thicker than 12 nm are incompatible with a high carrier extraction efficiency under short-circuit conditions. Only superlattices with thinner periods and short carrier lifetimes show good solar cell performance. Quantum kinetic calculations based on the non-equilibrium Green’s function (NEGF) formalism predict a change in transport regime from direct tunneling extraction to sequential tunneling with sizable thermionic emission components when passing from 6 nm to 12 nm period length, which for low carrier lifetime results in a decrease of extraction efficiency by more than 30%.

In_0.227GaAs/GaAsN_0.011 was introduced as a 1.2-eV multiple quantum well (MQW) with a flat conduction band (FCB) in which a conduction band edge of GaAsN was adjusted to be equal to that of InGaAs. This MQW was established as a candidate material for a middle absorption layer of three-junction solar cell since electron confinement was eliminated and a short electron lifetime in GaNAs was compensated by InGaAs layer. The band alignment of MQW was characterized by the power-dependent photoluminescence (PL) measurement under low temperature. According to the band-anti crossing model, the FCB is possibly constructed since a small amount of incorporated N can drastically reduce the energy of the conduction band edge of GaAsN. The PL results demonstrated that In_0.227GaAs/GaAsN MQW was a type-I structure when N content was below 1.1%, and became a type-II structure when N content was above 1.1%. The type-II MQW was characterized by the observation of blueshift of PL peak when increasing excitation power. This blueshift is a result of band-bending effect due to the accumulation of excited carriers at the interface between two materials, which is unique for the type-II MQW. In addition, it was observed that the activation energy estimated from the Arrhenius plot provided a minimum value in the structure with 1.1%N; the lowest activation energy indicated the weakest confinement energy of carriers in the structure. These results approved that a transition from type-I to type-II occurred when N content surpassed 1.1%, and our designed In_0.227GaAs/GaAsN_0.011 MQW was potentially the FCB structure.

We have proposed a two-step photon up-conversion solar cell (TPU-SC), which is a single junction solar cell comprising a wide gap semiconductor (WGS) and a narrow gap semiconductor (NGS) to break through the Shockley–Queisser limit for the single-junction solar cells. In the TPU-SC, below-gap photons of WGS excite the NGS and accumulate electrons at the WGS / NGS hetero-interface. The accumulated electrons at the hetero-interface are easily excited towards the WGS barrier by the low-energy photons, resulting in the efficient two-step up-conversion (TPU). We have experimentally demonstrated highly efficient current generation by the TPU. In this paper, we present the concept of the TPU-SC, theoretical prediction of the conversion efficiency of the TPU-SC, and experimental result of efficient photocarrier collection attributable to the TPU phenomenon.

Catalysts, chemical stability, and light harvesting are three major challenges in developing high performance photoelectrochemical water-splitting devices. Here we report the significant improvement of using bifacially designed schemes for achieving an ultrahigh solar to hydrogen efficiency of 18.22% with ultrafast hydrogen production rate and excellent chemical stability up to 370 hr. By deposition with appropriate catalyst (Pt) thickness, we have successfully balanced the trade-off among catalytic reaction, chemical protection, and light harvesting properties. In addition, the well-distributed catalytic and light harvesting sites provided by Si bifacial photoelectrochemical cells exhibit significantly higher omnidirectional hydrogen production capability as compared to conventional single-sided devices. Light-intensity dependent characterization is further demonstrated for realizing the bifacial photoelectrochemical cell in practical applications. The well-controlled, high efficiency, and chemically stable photoelectrochemical cell demonstrated herein can provide an important advance towards the development of next-generation renewable energy devices.

Flat silicon solar cells are the standard for solar technology implementations, due to the simplicity of its form and the low cost of its material. However, the bare material of such a technology is known to suffer from high reflectivity and low solar conversion efficiency. Engineered surfaces (e.g., textured) and bulk structures (e.g., quantum dots) are often used to diminish the reflection and enhance the efficiency, but such processes come at the expense of complexity and cost. The proposed work responds to these challenges by introducing a new architecture for traditional silicon solar technology. The architecture takes the form of a Smart Solar Sensing (S-Cubed) Array. It consists of a macroscopic close-packed array of corner-cube- (CC-) shaped solar cells. Each CC-cell has three silicon solar cells lining its interior corners. The three silicon solar cells establish multiple internal reflections for enhanced overall absorption. At the same time, the impedances of the three silicon solar cells in each CC-cell of the S-Cubed Array can be sensed and independently matched to a common load. This allows for maximized electric power transfer to the load over a broad range of illumination conditions. It is shown in this work that the collected energy density of the CC-cell array, over the course of a day, can be increased by 33.02% when compared to an array of conventional (flat) silicon solar cells. Such findings can lay the groundwork for future implementations of high-efficiency solar technology.

Recently, a systematic effort has been undertaken in developing efficient energy harvesting devices on thin Si films. Two main mechanisms have been identified for the efficient light harvesting. One is related to minimizing reflection losses, while the other is related to coupling to quasi-guided modes supported by the silicon film. However, the effects associated with the homogeneity and isotropy of the structures have not attracted much attention. Here, we employ hyperuniform disordered structures(HUDS) to achieve very efficient light harvesting in the wavelength range from 400 to 1000 nm. We show that the surface patterning has a dramatic impact on the number modes that are involved in the absorption process and that the structure needs to be optimized such that the scattering promotes minimization of the energy directed in radiative channels, i.e. inside the light cone of the surrounding air. To provide a through comparison, we also fully optimize a periodic structure taking into account the patterning of the AR layer refractive index. We then examine various HUDS architectures and demonstrate not only the importance of the scattering components, but also the dramatic impact of the structure homogeneity and isotropy on the devices performance. Using this design strategy, we report a broadband solar energy absorption of 84% in a one micron-thick Si membrane, which is, to the best of our knowledge, the best value achieved in such ultra-thin Si membranes.

Subwavelength metallic nanoparticles have been proposed for optimizing efficiency of current solar cells. However, their inherent ohmic losses limit their performance. High Refractive Index dielectric particles have been suggested as an alternative to metallic ones due to their low-losses in the visible and near infrared spectral regions and also to their magnetic response. The directionality properties that arise from the coherence effects between electric and magnetic resonances, make them very attractive for redirecting the incident radiation. In this work, we analyze the Scattering Directionality Conditions of a symmetric dimer made of High Refractive Index dielectric particles as a function of the gap. We demonstrate that, by using a dimer, it is possible to find, in the dipolar regime, two spectral regions where the incident radiation is redirected in the forward direction. They correspond to the Zero-Backward condition (also observed for isolated particles) and to a “near Zero-Backward” condition. The last would correspond to a “rotation” of the near Zero-Forward condition as a consequence of the interaction effects between the dimer components. The proposed scattering unit could constitute a new block for building more complex systems for applications in optical communications, light guiding and solar energy harvesting devices.

High reflection losses combined with low absorption capabilities and high velocity surface recombination are the main problems that deteriorate the efficiency of thin silicon solar cells. Therefore, Low cost and easy scalable fabrication of wide band, angle and self-cleaning antireflection coatings are of great importance for different optical applications especially solar cells. Random textured silicon nanocones are fabricated through electroless metal assisted chemical etching (EMACE) combined with ambient oxidation. Theoretical studies using Finite difference Time Domain (FDTD) simulation guided the experimental procedures in terms of dimensions and tolerance to reach the optimum dimensions and superior optical properties. The Optical numerical and experimental studies are revealed wide antireflection properties and strong trapping effects up to 60° through the entire visible wavelength. The textured structure modified the hydrophobicity of the solar cell into hydrophobic surface with self-cleaning properties.

Simulation and analysis of photovoltaic cells require both optical modelling and semiconductor device modelling. Often, both are very involved and therefore, the simulations or analyses of the photovoltaic cells make compromises in either modelling. Here, we present a simple coupled model, where the optical modelling is done using finite difference beam propagation method and the electronic modelling is done using the drift-diffusion model. Starting from the semi-classical Boltzman transport equation, we make valid approximations to arrive at the drift-diffusion equation which is further discretized by the Scharfetter-Gummel Scheme. For optical modelling, we start with Maxwell`s equations, apply successive valid approximations and arrive at the finite- difference beam propagation method. We then solve both the equations simultaneously, the input flux at every step used for calculation of carrier transport in the drift diffusion equation is supplied by the solution of the optical beam propagation. This coupled model would give us a fast and simple method to simulate and analyze novel optical structures for photovoltaic cells. Initial results of both the independent models are very encouraging. Equilibrium and non-equilibrium transport properties have been successfully described by the electronic modelling, similarly for low contrast photonic structures, FD-BPM is a long-proven method. We would present an integrated approach to arrive at a very fast, simple yet efficient model for simulation of optical structures for photovoltaic applications.

The purpose of this research is to analyze mathematically cylindrical shapes of flexible solar panels and compare their efficiency to the flat panels. The efficiency is defined to be the flux density, which is the ratio of the mathematical flux and the surface area. In addition we describe the trajectory of the Sun at specific locations: the North Pole, The Equator and a geostationary satellite above the Equator. The calculations were performed with software: Maple, Mathematica, and MATLAB.

A challenge is emerging for utilities having a significant reliance on solar power generation. Such a reliance leads to substantial solar power generation in the midday hours and insufficient solar power generation during the late-day hours. This yields high demand and an upswing in electrical power prices at the end of the day when the solar generating capacity is low. The upswing is titled the "Duck Curve" and it is a growing concern—particularly for California. This work tackles this challenge by introducing an engineered solar cell array that traps light over a broadened range of incident angles, which leads to increased late-day solar power generation. The proposed U-Groove Array consists of a periodic array of U-shaped grooves that are parallel and macroscopic in size. This work shows that a U-Groove Array, when engineered with the appropriate orientation and aspect ratio, promotes multiple reflection and absorption processes for low-angle light. This broadens the duration over which the array collects solar power, extending the generation into the high-value evening hours. The accumulated value of the solar power generated by the U-Groove Array is simulated and compared to that of two contemporary structures: a standard array of flat solar cells and an analogous V-Groove Array (which has been the subject of numerous recent investigations). It is shown that future implementations of the UGroove Array can outperform these contemporary structures and yield improved solar power generation.

Highly mismatched ZnTe_1-xO_x (ZnTeO) alloy is one of the potential candidates for an absorber material in a bulk intermediate band solar cell (IBSC) because a narrow, O-derived intermediate band IB (E-) is formed well below the conduction band CB (E+) edge of the ZnTe. We have previously demonstrated the generation of photocurrent induced by two-step photon absorption (TSPA) in ZnTeO IBSCs using n-ZnO window layer. However, because of the large conduction band offset (CBO) between ZnTe and ZnO, only a small open circuit voltage (V_oc) was observed in this structure. Here, we report our recent progress on the development of ZnTeO IBSCs with n-ZnS window layer. ZnS has a large direct band gap of 3.7 eV with an electron affinity of 3.9 eV that can realize a smaller CBO with ZnTe. We have grown n-type ZnS thin films on ZnTe substrates by molecular beam epitaxy (MBE), and demonstrated ZnTe solar cells and ZnTeO IBSCs using n-ZnS window layer with an improved V_OC. Especially, a n-ZnS/i-ZnTe/p-ZnTe solar cell showed an improved V_oc of 0.77 V, a large short-circuit current density of 6.7 mA/cm² with a fill factor of 0.60, yielding the power conversion efficiency of 3.1 %, under 1 sun illumination.

The concept of intermediate band solar cells (IBSC) promise a drastic increase in single junction efficiency (theoretically up to over 60% under concentration), exceeding theoretical predictions made by the Shockley-Queisser limit. In this concept, an intermediate band (IB) is formed within the forbidden gap of the host material, acting as an additional energy level where photons with energy less than the host bandgap can be absorbed, significantly reducing the transmission loss of a single junction solar cell. Unfortunately, thermal escape from the quantum dots limit the two-step photon absorption (TSPA) photocurrent collection to low temperatures. In this talk, we report on growth, fabrication, and characterization of type-II InP QD/InGaP IBSC via MOCVD and show high temperature TSPA photocurrent collection compared to conventional InAs and GaSb QD based IBSCs. Time resolved photoluminescence (TRPL), atomic force microscopy (AFM), and excitation-power dependent PL measurements reveal homogeneous formation of type-II InP QDs in an InGaP host matrix with long carrier lifetimes of up to 250 ns. TSPA response using two light sources (monochromatic light and an IR laser) show photocurrent collection at temperatures up to 250 K for the InP QD cells compared to less than 120 K for InAs and GaSb QD cells. These results clearly establish InP QD/InGaP as a viable material system for IBSC applications.

The Intermediate Band Solar Cell is an advanced concept, which has been predicted to overcome the Shockley-Queisser limit, despite efficiencies remaining below the best single junctions so far. Practical realizations with nanostructures suffer from two intrinsic deficiencies: narrow absorption widths and low radiative efficiencies. We evaluate in this paper the theoretical efficiency expectations with respect to those two properties, and consider in addition the possibility of including an electronic ratchet. We observe that an intermediate band solar cell using a ratchet becomes highly tolerant to non-ideal nanostructures, so that any combination of low absorption and low radiative efficiency becomes compatible with optimized performances above the Shockley-Queisser limit. We conclude that future practical realization may take advantage of quantum wells, which have been less considered so far than quantum dots, due to relatively higher nonradiative recombination rates. Such realizations would take advantage of the higher absorption properties of quantum wells.

We have quantitatively investigated the two-step photon absorption in quantum dot solar cells (QDSCs) by using absolute intensity calibrated photoluminescence (PL) spectroscopy. Multi-stacked InAs/AlGaAs QDs were fabricated in the i-region of p-i-n single-junction solar cells by molecular beam epitaxy. Hyperspectral imaging, which combines both the spatial and spectral dimensions of the luminescence, was used to investigate QD ground state PL at room temperature. Two lasers simultaneously excited the QDSCs to characterize two-step photon absorption. An excitation laser caused interband transition to generate photo-carriers in QDs, and the other infrared (IR) laser excited intraband transition from the QD states. As the result of two-step photon absorption, reduction in PL intensity was clearly observed under IR bias excitation. We compared absolute PL intensity with and without IR illumination, and obtain quasi-Fermi level splitting and two-step photon absorption efficiency in QDSCs under study. Compared with the photocurrent measurements, PL spectroscopy performed under open-circuit conditions, so that higher carrier filling ratio can be realized in QDs. Furthermore, PL can characterize fundamental transition on two-step photon absorption because photocurrent production needs carrier extraction to the external circuit. Quantitative analysis of two-step photon absorption by PL spectroscopy could clarify physical insights, and it would be beneficial to realize high efficiency intermediate band solar cells.

In this study, we report an easy and cheap fabrication technique of wide band omnidirectional antireflective black silicon surface based on porous and non-porous silicon nanowires (SINWs). This technique depends on one step silver electroless catalytic etching method (EMACE) in an aqueous solution of AgNO₃ and HF. We found a commensurate relationship between the dimensions and the etching time. The fabrication technique was examined for large scale production potential. Wide band and angle near zero reflection is reported in the visible region due to the strong trapping and antireflection properties. Quantum size effect and phonon scattering is confirmed for the fabricated structure through Raman measurement. Black silicon based on porous and non-porous SINWs shows promising potential for photovoltaic, optoelectronic and energy storage applications.

Gold nanarods (GNRs) with an aspect ratio equal to 3:1 and 4:1 and a major axis equal to 50 nm, silanized and capped with titania (c-GNRs), were embedded within the 6μ thick semiconductor layer of DSSCs based on an organic D-π-A sensitizer dye (L0 or L1) and a squaraine co-sensitizer (SQ2). These dye cocktails and a transparent titania paste were selected to maximize the transmittance of DSSCs in a region of maximum visual acuity. The thickness of the SiO₂ layer was consisted of about 4.0 nm and that of the TiO₂ layer of about 2.0 nm in order to maintain high the localized surface plasmon resonance (SPR) effects produced by GNRs and to prevent Au corrosion. The slight thickness of both types of capping was not sufficient to preserve the dimensions of the gold nanorods during sintering at 450 °C. Absorbance, transmittance and reflectance spectra of the films were measured after they were dyed. The plasmonic effects were distinguishable both in absorbance and absorptance spectra for all the types of dyes cocktails and GNRs aspect ratios used. Transmittance values depended on the cocktail used and on the adsorption of the dyes on the semiconductor film blended with c-GNRs. When the mix of L1 and SQ2 was used together with GNRs of a 3:1 aspect ratio, we achieved an increase of 23% in DSSC efficiency (from 3.50% to 4.32%) but transmittance values higher than 50% only between 560 nm and 630 nm.

We engineered an epsilon-near-zero (ENZ) material from suitably disordered metallic nanostructures. We create a new class of dispersionless composite materials that efficiently harnesses white light. By means of Atomic Force Microscopy (AFM) and Photoluminescence (PLE) measurements we experimentally demonstrate that this nanomaterial increases up to a record value the absorption of ultra-thin light harvesting films at visible and infrared wavelengths. Moreover, we obtained a 170% broadband increase of the external quantum efficiency (EQE) when these ENZ materials are inserted in an energy-harvesting module. We developed an inexpensive electrochemical deposition process that enables large-scale production of this material for energy-harvesting applications.

Organic solar cells (OSC) offer a promising alternative to achieve highly efficient solar cells with low cost and easy fabrication. However, light absorption efficiency in OSC is limited due to the low carrier diffusion length and low exciton mobility. Plasmonic nanostructures have the ability to localize light in the organic active layer and thus increasing the light path length without increasing its physical thickness. Here, we exploited a new type of plasmonic nanostructures to achieve high and broadband absorption enhancement in organic solar cells. Zirconium Nitride (ZrN), as an example of refractory plasmonics, has one of the highest localized surface plasmon resonance quality factor. In this work, several new ZrN nanostructures such as spherical nanoshells and nanodisks are incorporated in organic solar cells. A theoretical analysis using finite difference time domain (FDTD) simulations is implemented to thoroughly analyze and compare these structures. Mie scattering and absorption efficiencies are calculated to analyze these spherical nanoshells in a polymer environment. A high and broadband absorption enhancement is achieved after their incorporation in the organic solar cell.

III-V multijunction devices that incorporate a dilute nitride 1-1.2 eV bottom cell have already demonstrated conversion efficiencies of about 44% under high sunlight concentration (942 Suns). However, the poor minority carrier properties of dilute nitride have, thus far, prevented the full realization of the practical potential for tandem configuration (>40% 1 sun, and >50% at 500X and above). To overcome this shortcoming, our group, over the past years, have focused on dilute nitride-based devices where the degraded minority carrier diffusion length has a minimal impact on the device performance. We have shown that the incorporation of resonantly coupled GaAsN/GaP multi-quantum wells in the intrinsic region of p-i-n GaAs cells allows both a significant sub-GaAs-bandgap photon harvesting while maintaining a high open circuit voltage. Here, in order to gain a better understanding of photo-generated carrier escape and recombination mechanisms in these devices and further optimize the performance, we examine optical and electrical properties of such devices using various characterization techniques such as: photoluminescence (PL), modulated photo-reflectance (PR), photo-current (PC) as well as current-voltage (IV) measurements under dark or illuminated conditions. The temperature dependent analysis enables us to modulate and freezes carrier thermalization phenomena, while simultaneous measurement of photogenerated carrier extraction (SR) and recombination’s (PL) as a function of the applied load (bias) enables a close correlation between the evolution of I-V characteristics and the physics at play. Next, typical temperature and bias dependent activation energies reveal interesting details about carrier escape, intra-cells coupling and recombination sequences.

Thin film solar cells (TFSCs) where first introduced as a low cost alternative to conventional thick ones. TFSCs show low conversion efficiencies due to the used poor quality materials having weak absorption capabilities and to thin absorption layers. In order to increase light absorption within the active layer, specially near its absorption edge, photon management techniques were proposed. These techniques could be implemented on the top of the active layer to enhance the absorption capabilities and/or to act as anti-reflecting coating structures. When used at the back side, their purpose is to prevent the unabsorbed photons from escaping through the back of the cell.

In this paper, we coupled the finite difference time-domain (FDTD) algorithm for simulating light interaction within the cell with the commercial simulator Comsol Multiphysics 4.3b for describing carrier transports. In order to model the dispersive and absorption properties of various used materials, their complex refractive indices were estimated using the Lorentzian-Drude (LD) coefficients. We have calculated the absorption profile in the different layers of the cell, the external quantum efficiency and the power conversion efficiency achieved by adding dielectric nanospheres on the top of the active layer. Besides that, the enhancement observed after the addition of dielectric nanospheres at the back side of the active layer was computed. The obtained results are finally compared with the effects of using textured surface and nanowires on the top in plus of cascaded 1D and 2D photonic crystals on the back.

Enhancing the light absorption in ultrathin-film silicon solar cells is important for improving efficiency and reducing cost. In this paper, we report a highly effecient cosine periodic nanostructure as light trapping texture. The design and fabrication as well as measurement of cosine nanotextures were presented. The optimized structure yields an average reflectance of 7.07% at an equivalent silicon thickness of 10μm, much better than planar and random pyramid structures. The measurements demonstrate that the absorptions in ultrathin film solar cells are very close to the Yablonovitch limit for the entire solar spectrum and insensitive to the angle of the light. This approach is applicable to various thicknesses and promising in future glass-based thin film solar cells.