This PDF file contains the front matter associated with SPIE Proceedings Volume 8620, including the Title Page, Copyright Information, Table of Contents, Introduction and Conference Committee listing.

The excitation of multiple surface-plasmon-polariton (SPP) waves guided by the interface of a metal and a onedimensional photonic crystal in the grating-coupled configuration was studied both experimentally and theoretically. Only p-polarized incident light was considered in the visible and near-infrared regimes. When the absorptance was plotted against the angle of incidence, the excitation of an SPP wave was indicated by an absorptance peak whose angular location did not change with the number of periods (beyond a threshold) of the photonic crystal. A decrease in the period of the metal grating resulted in shifting the excitation of the SPP waves to smaller wavelengths.

We present a design of implementing plasmonic nanoparticles made from silver onto the surface of amorphous silicon based solar cells. When adding these silver nanoparticles we expect to see enhancements to the solar cells due to the plasmonic effects induced by the metal nanoparticles. The nanoparticles are used as subwavelength scattering elements to couple and trap light within the cell. In addition, the excited surface plasmon-polaritons promote a strong localized field enhancement which increases the cells ability to absorb light. Our choice of geometry of the nanoparticle is cubic rather than the traditional spherical geometry. We expect to see the cell perform better with the cubic shape due to the larger surface area it spans. We investigate the effects of these particles on to the performance of the solar cells, as well as introduce an intrinsic layer between the active p and n region creating a p-i-n solar cell configuration. We report the use of an FDTD simulator to characterize the optical performance of the solar cell. Both cubical and spherical nanoparticles made from silver were studied. Our simulations predict an overall increase of 67% (from 7.5% to 12.5) based on the p-i-n configuration with inclusion of the plasmonic particles onto the surface of the cells. Experimentally we verified the results by first fabricating a crystalline silicon-based solar cell with a p-n configuration and then placing the silver nanocubes onto the surface of the cell. An overall increase of about 28% was experimentally demonstrated (from 3.97% to 5.081%). We anticipate further increases with the p-i-n configuration.

The reflectances of a thin-film solar cell were computed, using the rigorous coupled-wave approach, as functions of the angle of incidence and the free-space wavelength for illumination by linearly polarized plane waves. The metallic back-reflector was taken to be periodically corrugated and the solar cell was considered to be a tandem solar cell made of amorphous-silicon alloys. Low-reflectance bands in the reflectance spectrums were correlated with the solutions of the underlying canonical boundary-value problem to delineate the excitation of surface plasmon-polariton (SPP) waves. The total reflectance was lowered in the near-infrared spectral regime when multiple SPP waves of both linear polarization states were excited, thereby enhancing the absorption of light.

The energy transfer from plasmons in metallic nano-sphere deposited on the semiconductor surface to substrate electron band-system is investigated upon the scheme of Fermi golden rule. The analysis of PV efficiency dependence with respect to the nano-sphere size for plasmon mediated channel is presented for the case of photo-diode system with metallic nano-components deposited on the photo-active surface. The trade off between PV efficiency enhancement accompanying surface plasmon dipole magnitude increase with the nano-sphere size growth versus lowering tendency due to quenching of indirect inter-band transitions in substrate semiconductor, significant for smaller radii, is demonstrated in an analytical way. Plasmons in metallic nano-sphere are described within the semiclassical random phase approximation (RPA) framework, sufficiently accurate for large nanoparticles, with radius a ∼ 5 − 60 nm (for Au or Ag). Irradiation induced plasmon damping is analyzed via the Lorentz friction mechanism. The comparison with the experimental data is given supporting the formulated explanation of giant PV efficiency increase due to plasmon effect.

We have studied time-resolved intraband transition from the intermediate state to the continuum state of the conduction band in InAs/GaAs self-assembled quantum dots embedded in a one-dimensional photonic cavity structure using a two-color photoexcitation spectroscopy. The photonic gap was tuned to enhance the excitation from the intermediate state to the conduction band, whose energy is selected to be less than the interband transition energy between the intermediate state and the quantized hole state. The photoluminescence intensity was observed to be dramatically reduced by selectively pumping carriers in the intermediate state. This effect has been analyzed by modeling detailed carrier relaxation process.

CIGS is a material showing high efficiencies in photovoltaic devices although numerous questions remain about its physical mechanisms. Among them is the influence of the polycrystalline nature on global efficiencies. In order to study the spatial fluctuations of the optoelectronic parameters, two original setups are developed. Firstly a Hyperspectral Imager is used to investigate spectrally resolved luminescence images. It is also possible to calibrate it in absolute values, which means that the signal is measured in number of photons. From photoluminescence measurement we deduce maps of the quasi-Fermi level splitting with variation of 30 meV. These results are compared with a more common confocal microscope, which shows that the carrier transport has to be taken into account for the interpretation of these experiments. Using electroluminescence and reciprocity relations, we calculate map of the External Quantum Efficiency with the Hyperspectral Imager. For this investigation a second setup is developed, using Light Beam Induced Current with different excitation wavelengths. Therefore mapping of the diffusion lengths is possible, exhibiting a distribution around 1.09 μm with standard deviation of 0.10 μm.

Conventional light trapping techniques are inefficient at the sub-wavelength scale. This is the main limitation for the thickness reduction of thin-film solar cells below 500nm. We propose a novel architecture for broadband light absorption in ultra-thin active layers based on plasmonic nano-cavities and multi-resonant mechanism. Strong light enhancement will be shown numerically for photovoltaic materials such as CIGSe and GaAs. First experiments on ultrathin nano-patterned CIGSe solar cells will be presented.

Nanostructured active or absorbing layers of solar cells, including photonic crystals and wire arrays, have been increasingly explored as potential options to enhance performance of thin film solar cells because of their unique ability to control light. We show that 2D photonic crystals can improve light trapping by an enhanced density of optical states and improved incoupling, and demonstrate, using FDTD simulation, absorption enhancements in 200nm thick crystalline silicon solar cells of up to 205% from λ = 300nm to 1100nm compared to a planar cell with an optimized two-layer antireflection coating. We report here a method to further enhance absorption by introducing a lattice of coupled defect modes into the photonic crystal, which modify the available optical states in the absorber. Our results show that 2D photonic crystals are a viable and rich research option for light trapping in thin film photovoltaics.

In silicon-based solar cells, a substantial part of the energy losses is related to the charge carriers thermalization in the UV-blue range and the week carriers collection at these wavelenghts. To avoid this issue, we introduce a new concept which combines a rare-earths doped thin layer with a photonic crystal (PC) layer, allowing an efficient conversion from UV-blue photons to near-IR photons. We report on the feasibility of such a nanostructured down-converter module using an active rare-earth doped CaYAlO₄ thin layer and a silicon nitride PC on top. By means of optical numerical simulations, the promising potentialities of the concept are demonstrated.

Light trapping structures are a promising approach to increase light absorption in ultrathin absorbers of interest for photovoltaic applications. To be integrated within real systems, their fabrication approach need to be simple and scalable, insuring that the gain in efficiency is worth the extra manufacturing steps. Here, we optimize through simulation using a RCWA code, a variety of 1D and 2D structures that can be fabricated with such simple and scalable techniques. In particular, we investigate the effect 1D and 2D gratings fabricated by Nanoimprint Lithography, onto Bragg mirrors that could be made using simple liquid process approaches. The optimized structures exhibit a significant gain in absorption with almost twice as much absorption in the wavelength range 0.5 to 1 micron, compared to a bare absorbing film of GaAs chosen as our reference. We also identify the various phenomena behind the absorption peaks observed and study the angular dependency of absorption with these structures.

We numerically study the absorption enhancement of amorphous Si (α-Si) solar cells, in which a dual grating structure combining front dielectric grating and back metal grating is proposed to improve light absorption in the 300-900 nm wavelength range. The front dielectric grating scatters the incident light into active layer which can reduce reflection without much energy loss, especially at the short wavelengths. The back metal grating causes the absorption enhancement at long wavelengths due to the excitation of surface plasmon polaritons (SPPs) at the interface of metal/semiconductor and/or photonic modes in the active layer. When these two gratings are combined, a large, broadband absorption enhancement over the entire spectrum can be realized. For better comparison, the flat structure without any gratings is chosen as a reference. In our work, the absorption enhancement of the solar cells with dual gratings is superior to the structures with a front dielectric or back metal grating alone in almost over the entire wavelength range 300-900 nm. For wavelengths in the range 300-900 nm, 72.4% absorptivity is observed for 100-nmthickness flat α-Si solar cell, 76.9% and 75.1% for front and back grating cases, and up to 82.6% for dual grating case at the grating period of 360 nm.

The concept of "intermediate band solar cell" (IBSC) is, apparently, simple to grasp. However, since the idea was proposed, our understanding has improved and we feel now that we can explain better some concepts than we initially introduced. Clarifying these concepts is important, even if they are well-known for the advanced researcher, so that efforts can be driven in the right direction from start. The six pieces of this work are: Does a miniband need to be formed when the IBSC is implemented with quantum dots?; What are the problems of each of the main practical approaches that exist today? What are the simplest experimental techniques to demonstrate whether an IBSC is working as such or not? What is the issue with the absorption coefficient overlap? and Mott's transition? What the best system would be, if any?

We propose a new intermediate band GaAs solar cell comprising an Al_xGa_1-xAs absorber with built-in GaSb type-II quantum dots (QDs) [a gradual Al_xGa_1-xAs absorber with built-in Al_yGa_1-ySb QDs (0<x=y<0.40) as a variant] separated from the depletion region. We study the modification of the band alignment at type-II interface by two-photon absorption of concentrated sunlight. Our calculation shows that photogenerated carriers produce localized exciton-like electron-hole pairs spatially separated at QDs. Local field of such pairs may essentially modify potential barrier surrounding QDs, increase recombination lifetime of mobile carriers and additional photocurrent generated by two photon absorption. Concentration of about 300-sun pushes by 15% up the conversion efficiency as compared to the efficiency of the reference single junction GaAs solar cell without QDs.

The incorporation of nanostructures, such as quantum dots (QD), into the intrinsic region of III-V solar cells has been proposed as a potential route towards boosting conversion efficiencies with immediate applications in concentrator photovoltaic and space power systems. Necessary to the optimization process of this particular class of solar cells is the ability to correlate nanoscale properties with macroscopic device characteristics. To this purpose, the physics-based software Crosslight APSYS has been developed to investigate the design parameters of QD enhanced solar cells with particular focus on the InAs/GaAs system. This methodology is used to study how nanoscale variables, including size, shape and material compositions, influence photovoltaic performance. In addition, device-level engineering of the nanostructures is explored in optimizing the overall device response. Specifically, the effect of the position of the QDs within the intrinsic regions is investigated. Preliminary simulations suggest strategically placing the QDs off-center reduces non-radiative recombination and thereby the dark saturation current, contributing to a marked increase in opencircuit voltage and fill factor. The short-circuit current remains unchanged in the high field region resulting in an increase in overall conversion efficiency. To further explore this finding, a series of three samples with the QDs placed in the center and near the doped regions of a pin-GaAs solar cell have been grown using MOCVD, fabricated and fully characterized. Contrary to predictions, the emitter-shifted devices exhibit a marked decrease in open-circuit voltage and fill factor. This behavior is attributed to non-negligible n-type background doping in the intrinsic region which shifts the region of maximum recombination towards the p-type emitter.

Epitaxially formed indium arsenide quantum dot (QD) structures formed by the Stranski-Krastanov growth mode have been investigated with respect to how quantum confinement and lattice strain behavior affects the optoelectronic performance in p-i-n type InGaAs devices. The introduction of a correction layer and the proper selection of the QD capping layer’s alloy and thickness parameters allowed the control and management of the lattice misfit in two QD structures, which led to reduced defects and improved dark current behavior under forward bias conditions when compared to an InGaAs p-n homojunction (HOM) device without quantum-dots. Although the dark-current of the HOM devices behaved as expected under forward and reverse biases, the QD device structures displayed an apparent anomalous behavior in their dark-current densities under forward and reverse biases. Closer analysis reveals that this behavior is not anomalous; instead the information gained can be used to extract greater understanding about how to optimize the optoelectronic performance in quantum confined structures. In addition, the analysis suggests that lattice strain behavior continues to be a critical benchmark for defining and optimizing the performance of epitaxially formed= QD devices.

Triple-junction solar cells offer extremely high power conversion efficiency with minimal semiconductor material usage, and hence are promising for large-scale electricity generation. To fully exploit the broad absorption range, antireflective schemes based on biomimetic nanostructures become very appealing due to sub-wavelength scale features that can collectively function as a graded refractive index (GRIN) medium to photons. The structures are generally fabricated with a single-type dielectric material which guarantees both optical design robustness and mechanical durability under concentrated illumination. However, surface recombination and current matching issues arising from patterning still challenge the realization of biomimetic nanostructures on a few micrometer thick epitaxial layers for MJSCs. In this presentation, bio-inspired antireflective structures based on silicon nitride (SiN_x) and titanium dioxide (TiO₂) materials are demonstrated on monolithically grown Ga_0.5In_0.5P/In_0.01Ga_0.99As/Ge triple-junction solar cells. The nano-fabrication employs scalable polystyrene nanosphere lithography, followed by inductively-coupled-plasma reactive-ion-etching (ICP-RIE). We show that the fabricated devices exhibit omni-directional enhancement of photocurrent and power conversion efficiency, offering a viable solution to concentrated illumination with large angles of incidence. Moreover, a comprehensive design scheme is also presented to tailor the reflectance spectrum of sub-wavelength structures for maximum photocurrent output of tandem cells.

In high X III-V concentrator applications sunlight is focused onto the surface of cell with a wide angular distribution that limits the effectiveness of conventional thin-film AR coatings. Furthermore the transmission properties are generally degraded non-uniformly over the electromagnetic spectrum which in the case of multi-junction solar cells leads to additional sub-cell current matching related losses. Here, and in an attempt to identify a better alternative to the conventional dual layer ARCs, we have undertaken a systematic analysis of design parameters and angular dependent antireflective properties of dielectric grating formed through the implementation of sub-wavelength arrays of 2D pyramidal and hemispherical textures. The evaluation indicates that through a careful selection of the design and dielectric material these structures can significantly surpass the performance of planar double layer ARCs (i.e. MgF₂/ZnS), and the total number of reflected photons over the 380-2000 nm wavelength range can be reduced to less than 2%. Finally it is shown that the implementation of these structures for a typical 3 or 4 junction solar cells (i.e. inverted metamorphic) and for acceptance angles ranging from 0-60 degrees, reduces total losses of reflected photons for each subcell (and to some extent the resulting current degradation) to less than 4%. Anti-reflection and angular tolerant properties of 2D TiO₂ surface texturing made by nano imprinting technique were simulated and measured in this work. It has been proved that from both simulation and experimental work textured surface surpasses both antireflection and angular tolerant characters of planar ARC, which supplies a potential candidate AR structure for concentrated photovoltaic system.

Silicon quantum dot (Si QD) tandem solar cell is a promising cell structure for realising high efficiency at low cost. The tandem solar cell effectively harnesses energy from the solar spectrum by stacking two or more cells together in the order of descending band gaps. Due to quantum confinement, the band gap of silicon based nanostructures such as Si QDs can be tailored by varying the size of the QDs. Solar cells and light emitting diodes based on Si QDs have been realised in experiments. However, current crowding due to high lateral resistance remains to be a major problem for Si QD devices grown on quartz substrates. Annealed silicon rich carbide (SRC), owing to its electrical conductivity, thermal stability and energy band gap compatible with Si QD cell fabrication, has the potential to overcome this problem. Further, this quasi-transparent thin-film can be used as either substrate or superstrate of a Si QD solar cell and therefore provides flexibility in cell structure design. Here, we investigate the physical, optical and electrical properties of the new material as functions of silicon concentration and doping conditions via a number of characterisation techniques including X-ray diffraction, Raman spectroscopy, ultraviolet-visible-infrared spectroscopy and four-point probe measurement. Some discoveries, including the lower crystallisation temperature of SiC within SRC, are also discussed. The research may provide some insight into the optimisation of annealed SRC as the new conductive material for Si QD solar cell and may boost the final arrival of all-silicon tandem solar cell.

A novel device concept utilizing the approach of selectively extracting carriers at the respective contacts is outlined in the work. The dominant silicon solar cell technology is based on a diffused, top-contacted p-n junction on a relatively thick silicon wafer for both commercial and laboratory solar cells. The V_OC and hence the efficiency of a diffused p-n junction solar cell is limited by the emitter recombination current and a value of 720 mV is considered to be the upper limit. The value is more than 100 mV smaller than the thermodynamic limit of V_OC as applicable for silicon based solar cells. Also, in diffused junction the use of thin wafers (< 50 um) are problematic because of the requirement of high temperature processing steps. But a number of roadmaps have identified solar cells manufactured on thinner silicon wafers to achieve lower cost and higher efficiency. The carrier selective contact device provides a novel alternative to diffused p-n junction solar cells by eliminating the need for complementary doping to form the emitter and hence it allows the solar cells to achieve a VOC of greater than 720 mV. Also, the complete device structure can be fabricated with low temperature thin film deposition or organic coating on silicon substrates and thus epitaxially grown silicon or kerfless silicon, in addition to standard silicon wafers can be utilized.

Since the early beginnings of the space age in the 1950s, solar cells have been considered as the primary choice for long term electrical power generation of satellites and space systems. This is mainly due to their high power/mass ratio and the good scalability of solar modules according to the power requirements of a space mission. During the last decades, detailed solar cell material studies including the non-trivial interaction with high-energy space particles have led to continuous and significant improvements in device efficiency. This allowed the powering of advanced space systems like the International Space Station, rovers on the Martian surface as well as satellites which have helped to understand the universe and our planet. It is noteworthy that in addition to their success in space, these photovoltaic technologies have also broken ground for the application of photovoltaic systems in terrestrial systems. This paper discusses the development of space solar cells, gives insight into related experiments like the analysis of the interaction with space particles and provides an overview on challenges and requirements for future space missions.

Reduction of defects by use of thick sophisticated graded metamorphic buffers in inverted metamorphic solar cells has been a requirement to obtain high efficiency devices. With increase in number of metamorphic junctions to obtain higher efficiencies, these graded buffers constitute a significant part of growth time and cost for manufacturer of the solar cells. It's been shown that ultrathin 3 and 4 junction IMM devices perform better in presence of dislocations or/and radiation harsh environment compared to conventional thick IMM devices. Thickness optimization of the device would result in better defect and radiation tolerant behavior of 0.7ev and 1.0ev InGaAs sub-cells which would in turn require thinner buffers with higher efficiencies, hence reducing the total device thickness. It is also shown that for 3 and 4 junc. IMM, with an equivalent 10¹⁵ cm^-2 1 MeV electron fluence radiation, very high EOL efficiencies can be afforded with substantially higher dislocation densities (<2×10⁷ cm^-2) than those commonly perceived as acceptable for IMM devices with remaining power factor as high as 0.85. The irregular radiation degradation behavior in 4-junc IMM is also explained by back photon reflection from gold contacts and reduced by using thickness optimization of 0.7ev and 1.0ev InGaAs sub-cells.

Quantum dot triple junction solar cells (QD TJSCs) have potential for higher efficiency for space and terrestrial applications. Extended absorption in the QD layers can increase efficiency by increasing the short circuit current density of the device, as long as carrier extraction remains efficient and quality of the bulk material remains high. Experimental studies have been conducted to quantify the carrier extraction probability from quantum confined levels and bulk material. One studies present insight to the carrier extraction mechanisms from the quantum confined states through the use of temperature dependent measurements. A second study analyses the loss in carrier collection probability in the bulk material by investigating the change in minority carrier lifetimes and surface recombination velocity throughout the device. Recent studies for space applications have shown response from quantum structures to have increased radiation tolerance. The role strain and bonding strength within the quantum structures play in improving the radiation tolerance is investigated. The combination of sufficiently good bulk material and device enhancement from the quantum confinement leads to temperature dependent measurements that show TJSCs outperform baseline TJSCs near and above 60°C. Insight into the physical mechanisms behind this phenomenon is presented.

We present a predictive computational approach that may reduce the need for extensive inputs from Deep Level Transient Spectroscopy (DLTS) experiments. Three-dimensional NanoTCAD simulations are used for physics-based prediction of space radiation effects in a p⁺n GaAs solar cell with AlGaAs window, and validated with experimental data. The computed dark and illuminated I-V curves, as well as corresponding performance parameters, matched experimental data very well for 2 MeV proton irradiation at various fluence levels. We analyze the role of majority vs. minority and deep vs. shallow carrier traps in the solar cell performance degradation. The defects level parameters used in the simulations were taken from DLTS data obtained at NRL. It was determined from numerical simulations that the degradation of the photovoltaic parameters could be modeled and showed similar trends when a only a single deep level defect was considered compared to a spectrum of defect levels. This led to the development of an alternate method to simulate the degradation of a solar cell by using only a single deep level defect whose density is calculated by the Stopping and Range of Ions in Matter (SRIM) code. Using SRIM, we calculated the number of vacancies produced by 2 MeV proton irradiation for fluence levels ranging from 6x10¹⁰ cm^-2 to 5x10¹² cm^-2. Based on the SRIM results, we applied trap models in NanoTCAD and performed I-V simulations from which the degradation of the photovoltaic parameters (I_sc, V_oc, FF, P_max) was calculated. The simulations using SRIM-derived defect concentrations showed reasonable agreement with simulations using parameters determined from DLTS.

Doping superlattice devices have been pursued in part because of their inherent radiation hardness which results from long lifetimes and minimum diffusion length requirements in the range of nanometers. Diffusion length requirements are reduced because of the multiple closely spaced doped layers in the superlattice. Higher doping levels in conjunction with close superlattice spacing result in large electric fields in the range of 5x10⁵ V/cm that quickly collect carriers into the majority doped layers. The effect of the alternative solar cell structure will be studied by irradiating multiple device structures with 5.057 MeV alpha particles. Comparisons will be made between doping superlattice devices and single junction pin structures. Previous work developing a simulation routine to characterize the radiation response for these devices will be extended to confirm the predictive model developed. This work signifies a step forward in understanding the radiation effects of doping superlattice devices, and their potential for high radiation environments.

In order to develop photovoltaic devices with increased efficiency using less rare semiconductor materials, the concentrating approach was applied on Cu(In,Ga)Se₂ thin film devices. Microscale solar cells down to a few micrometers wide were fabricated. They show, at around x475, an efficiency of 21.3%, thanks to concentrated illumination (532 nm laser), compared to 16% efficiency under non-concentrated illumination. Due to the miniaturization, ultrahigh fluxes can be studied (< ×1000), without damaging the device. We analyse the high concentration regime of these micro-devices. Under ultrahigh light fluxes the collection efficiency decreases on certain devices. We attribute this effect to the screening of the electric field at the junction under high illumination. Numerical simulations of p-n junctions under intense fluxes corroborate this hypothesis. We built a homemade finite element method program, solving Poisson and continuity equations without resorting to the minority carrier approximation. We study the electric field at a p-n junction as a function of illumination intensity, and highlight the screening phenomena. Cu(In,Ga)Se2 thin films prove to be appropriate for a use under concentration, leading to significant gains in terms of efficiency and material usage. On these particular devices, ultrahigh illuminations can be used and the electric regime studied.

State of the art solar concentrators use free-space, non-imaging optics to concentrate sunlight. Mechanical actuators keep the focal spot on a small solar cell by tracking the sun’s position. Planar concentrators emerged recently that employ a waveguide slab to achieve high concentration by coupling the incident sunlight into the waveguide. We report on the development of an opto-fluidic waveguide coupling mechanism for planar solar concentration. The self-adaptive mechanism is light-responsive to efficiently maintain waveguide coupling and concentration independent of incoming light’s direction. By using an array of axicons and lenses, an array of vapor bubbles are generated inside a planar, liquid waveguide, one for each axicon-lens pair. The mechanism uses the infrared part of the solar spectrum on an infrared absorbing medium to provide the energy needed for bubble generation. Visible light focused onto the bubble is then reflected by total internal reflection (TIR) at the liquid-gas interface and coupled into the waveguide. Vapor bubbles inside the liquid are trapped by a thermal effect and are shown to self-track the location of the infrared focus. We show experimental results on the coupling efficiency of a single bubble and discuss the effect of angular coupling. Furthermore the effect of an array of bubbles inside the waveguide (as produced by a lensarray) onto the coupling efficiency and concentration factor is analyzed.

In this paper we discuss optical considerations and present design simulation results for a self-tracking (passive) solar concentrator. The self-tracking mechanism uses a reversible in-plane paraffin thermal actuator to couple shortwavelength light into a lightguide at the position of the solar focus. By splitting the solar spectrum using a longpass dichroic faceted reflector for actuation energy, this device adaptively self-tracks and concentrates solar light into a planar waveguide.

Research interests on sunlight applications are booming in recent years, due to the worldwide green-energy trends. Either using PV cells to store sunlight then convert to electricity, or to use sunlight for direct illumination source are among the many research projects which deserve investigation.

In this research, we focus a design combined the above two features together: direct sunlight illumination, and store the sunlight for later usage. Our design structure is as follows: 1. On the surface of outer layer, we use the liquid-prism structure to increase the angle tolerance range of solar concentrator; 2. Combine the micro structure of the solid-state prism and aspheric surfaces to produce a planar light guide structure, which compresses the plane light source into line light source, then guide the light into solar cells area; 3. Design a light switch using the liquid-prism of inside layer, and guides the sunlight into solar cells channel or indoor illumination channel.

We apply it in the NLIS^® developed at NTUST, not only retain the advantages of the static concentrator modules, but also eliminate the complex procedure of transmitting and emitting, reduce the loss and cost of energy transfer.

Cu₂ZnSnS₄ (CZTS) may exhibit both kesterite and stannite polymorphs and shows promise as an absorber layer in thin film photovoltaic solar cells to be produced at terawatt scales. This study examines the effects of CZTS polymorphism and inhomogeneous distributions of CZTS polymorphs on device characteristics under scenarios of single phase films, a sinusoidal variation between kesterite and stannite with depth, and single phase films with thin layers of the other polymorph at both interfaces. In general, stannite-only devices are predicted to have higher efficiency than kesterite-only devices and sinusoidal grading results in efficiency between those of the pure phases. However, the device performance is relatively insensitive to the wavelength of the sinusoidal grading and rather is very sensitive to the phase present at the CdS interface. Predicted AM1.5 current-voltage (J-V) curves and descriptive metrics as well as wavelength-resolved quantum efficiencies are reported for all models. Based on these results, we propose control of cation ordering in CZTSSe as a mechanism for device design using bandgap grading and interface engineering without variation of stoichiometry.

GaN microdomes are studied as a broadband omnidirectional anti-reflection structure for high efficiency multi-junction concentrated photovoltaics. Comprehensive studies of the effect of GaN microdome sizes and shapes on the light collection efficiency were studied. The three dimensional finite difference time domain (3-D FDTD) method was used to calculate the surface reflectance of GaN microdomes as compared to that of the flat surface. Studies indicate significant reduction of the surface reflectance is achievable by properly designing the microdome structures. Formation of the GaN microdomes with the flexibility to tune the size and shape has been demonstrated by using reactive ion etching (RIE) of both GaN and the self-assembled silica monolayer microspheres. Characterizations of the angle-dependence light surface reflectance for both micro-domes and flat surface show the similar trend as the simulation.

In the past years, reducing the thickness of the absorber layer in CIGS-based solar cells has become a key issue to reduce the global Indium consumption and thus increased its competitiveness. As the absorber thickness is reduced, less photons are absorbed and consequently the efficiency decreases. It is well known that scattering light in the absorbing layer increases the effective optical length, which results in enhanced absorption. In this study, we have deposited a transparent conductive oxide as a back contact to the cell with a white paint on the rear surface to diffuse the light back to the cell. A proof of concept device is realized and optically characterized. Modeling scattering by rough surfaces can be done by brute force numerical simulations but does not provide a physical insight in the absorption mechanisms. In our approach, we regard the collimated solar light and its specular reection/transmission as coherent. On an irregular surface, part of the collimated light is scattered in other directions. To model this diffuse light, we adopt the formalism of the radiative transfer equation, which is an energy transport equation. Thus, interference effects are accounted for only in the coherent part. A special attention is dedicated to preserving reciprocity and energy conservation on the interface. It is seen that most of the absorption near the energy bandgap of CIGS is due to the diffuse light and that this approach can yield very significant photocurrent gains below 500nm absorber thickness.

The intrinsic layer in an amorphous-silicon solar cell is usually several orders of magnitude thicker than the p- and n-layers to increase the electron-hole pair generation in the intrinsic layer and to decrease the recombination losses in the p- and n-layers. We hypothesized that a nohomogeneous intrinsic layer may trap the incident light better and increase the generation rate of charge carriers. The nonhomogeneity can be introduced by varying the composition of amorphous silicon alloys during chemical vapor deposition. The effect of intrinsic layer nonhomogeneity of various schemes was studied theoretically on the short-circuit current of a single-junction thin-film amorphous-silicon solar cell. The absorption of light was calculated using the rigorous coupled-wave approach for an AM1.5 solar irradiance spectrum for a wavelength range of 400-1100 nm. An antireection coating consisting of two layers of homogeneous dielectric materials was also used. The backing metallic layer of the solar cell was taken to be periodically corrugated. The short-circuit current of the solar cell with nonhomogeneous intrinsic layer was found to be higher than the solar cell with a homogeneous intrinsic layer.

Nano-enhanced solar cells incorporating III-V quantum wells or quantum dots have the potential to revolutionize the performance of photovoltaic devices. Extended spectral response characteristics have been widely demonstrated in both quantum well and quantum dot solar cells using a variety of different III-V materials. To fully leverage the increased spectral response of nano-enhanced solar cells, new device designs are discussed that can both maximize the current generating capability of the limited volume of narrow band-gap material and minimize the unwanted carrier recombination that degrades the voltage output.

III-V Dilute Nitride multi-quantum well structures are currently promising candidates to achieve 1 sun efficiencies of <40% with multi-junction design (InGaP/ GaAs/ GaAsN/ Ge). Previously under the assumption of complete carrier collection from wells, we have shown that III-V Dilute Nitride GaAsN multi-quantum well (MQW) structures included in the intrinsic region of the third cell in a 4 junction configuration could yield 1 sun efficiencies greater than 40%. However for a conventional deep well design the characteristic carrier escape times could exceed that of radiative recombination hence limiting the current output of the cell, as has been indicated by prior experiments. In order to increase the current extraction here we evaluate the performance of a cascaded quantum well design whereby a thermally assisted resonant tunneling process is used to accelerate the carrier escape process (<30ps lifetime) and hence improve the photo generated carrier collection efficiency. The quantum efficiency of a p-i-n subcell where a periodic sequence of quantum wells with well and barrier thicknesses adjusted for the sequential extraction operation is calculated using a 2D drift diffusion model and taking into account absorption properties of resulting MQWs. The calculation also accounts for the E-field induced modifications of absorption properties and quantization in quantum wells. The results are then accounted for to calculate efficiencies for the proposed 4 junction design, and indicate potential for reaching efficiencies in excess of this structure is above 42% (1 sun) and above 50% (500 sun) AM1.5.

The present paper proposes Carrier Collection Efficiency (CCE) as a useful evaluation measure to investigate the carrier transport in quantum well solar cells. CCE is defined as the ratio of the carriers extracted as photocurrent to the total number of the carriers that are photo-excited in the p-n junction area, and can be easily calculated by normalizing the collected current, i.e. the difference between the current under light irradiation and that in the dark, to its saturation value at reverse bias. By measuring CCE as a function of the irradiation wavelength and the applied bias, we can directly and quantitatively evaluate the efficiency of the carrier extraction under operation of the cell, and clarify the underlying problem of the carrier transport. The proposed derivation procedure of CCE is based on the assumption that the saturation of the collected current at reverse bias indicates 100% collection of the photo-excited carriers. We validated this hypothesis by studying the balance between the number of the photo-excited carriers that can be collected at a sufficiently large reverse bias and the number of the photons absorbed in the wells. As a result, the absorption fraction in the MQW region well agreed with the saturated external quantum efficiency as we predicted, indicating CCE defined in this study is an appropriate approximation for the collection efficiency of the carrier generated in the active region of a solar cell device.

In this work, we use an analytical drift-diffusion model, coupled with detailed carrier transport and minority carrier lifetime estimates, to make realistic predictions of the conversion efficiency of InP-based triple junction cells. We evaluate the possible strategies for overcoming the problematic top cell for the triple junction, and make comparisons of the more realistic charge transport model with incumbent technologies grown on Ge or GaAs substrates.

Light trapping techniques are one of the key research areas in thin film silicon photovoltaics. Since the 1980s randomly rough textured front transparent oxides (TCOs) have been the methods of choice as light trapping strategies for thin-film devices. Light-trapping efficiency can be optimized by means of optical simulations of nano-structured solar cells. We present a FEM based simulator for 3D rigorous optical modeling of amorphous silicon / microcrystalline silicon tandem thin-film solar cells with randomly textured layer interfaces. We focus strongly on an error analysis study for the presented simulator to demonstrate the numerical convergence of the method and investigate grid and finite element degree refinement strategies in order to obtain reliable simulation results.

We review our recent work concerning the development of dilute nitride solar cells by molecular beam epitaxy. This epitaxial technology enables a high level of control of the growth conditions and alleviates known issues related to epitaxy of dilute nitrides ultimately enabling to achieve high quality materials suitable for solar cell developments. In particular, we focus on discussing the mechanisms linking the epitaxial and annealing conditions to the operation of dilute nitride solar cells. We also report operation of a single junction dilute nitride solar cell with a short circuit current density as high as ~39 mA/cm² under 1 sun illumination.

Dilute nitride materials have been used in a variety of III-V photonic devices, but have not been significantly explored in photoelectrochemical applications. This work focuses on using dilute phosphide nitride materials of the form (Al,In)P_1-xN_x as photocathodes for the generation of hydrogen fuel from solar energy. Heteroepitaxial MOCVD growth of AlPN thin films on GaP yields high quality material with a direct bandgap energy of 2.218 eV. Aligned epitaxial growth of InP and GaP nanowires on InP and Si substrates, respectively, provides a template for designing nanostructured photocathodes over a large area. Electrochemical testing of a AlPN/GaP heterostructure electrode yields up to a sixfold increase in photocurrent enhancement under blue light illumination as compared to a GaP electrode. Additionally, the AlPN/GaP electrodes exhibit no degradation in performance after galvanostatic biasing over time. These results show that (Al,In)P_1-xN_x is a promising materials system for use in nanoscale photocathode structures.

Low resistance ohmic contacts have been successfully fabricated on n-GaSb layers grown by MBE on semi-insulating (SI) GaAs substrates using the Interfacial Misfit Dislocation (IMF) technique. Although intended for photovoltaic applications, the results are applicable to many antimonide-based devices. The IMF technique enables the growth of epitaxial GaSb layers on semi-insulating GaAs substrates resulting in vertical current confinement not possible on unintentionally doped ~ 1e17 cm^-3 p-doped bulk GaSb. Results for low resistance ohmic contacts using NiGeAu, PdGeAu, GeAuNi and GeAuPd metallizations for various temperatures are reported. Specific transfer resistances down to 0.12 Ω-mm and specific contact resistances of < 2e-6 Ω-cm² have been observed.

III-V multi-junction solar cells are based on a triple-junction design that consists of an InGaP top junction, a GaAs middle junction, and a bottom junction that employs either a 1eV material grown on the GaAs substrate or InGaAs grown on the Ge substrate. The most promising 1 eV material that is currently under extensive investigation is bulk dilute nitride such as InGaAsN(Sb) lattice matched to GaAs substrates. Both approaches utilizing dilute nitrides and lattice-mismatched InGaAs layers have a potential to achieve high performance triple-junction solar cells. In addition, it will be beneficial for both commercial and space applications if III-V triple-junction solar cells can significantly reduce weight and can be manufactured cost effectively while maintaining high efficiency. The most attractive approach to achieve these goals is to employ full-wafer epitaxial lift off (ELO) technology, which can eliminate the substrate weight and also enable multiple substrate re-usages. For the present study, we employed time-resolved photoluminescence (TR-PL) techniques to study carrier dynamics in MOVPE-grown bulk dilute nitride layers lattice matched to GaAs substrates, where carrier lifetime measurements are crucial in optimizing MOVPE materials growth. We studied carrier dynamics in InGaAsN(Sb) layers with different amounts of N incorporated. Carrier lifetimes were also measured from InGaAsN(Sb) layers at different stages of post-growth thermal annealing steps. Post-growth annealing yielded significant improvements in carrier lifetimes of InGaAsNSb double hetero-structure (DH) samples compared to InGaAsN DH samples possibly due to the surfactant effect of Sb. In addition, we studied carrier dynamics in MOVPE-grown GaAs-InAl(Ga)P layers grown on GaAs substrates. The structures were grown on top of a thin AlAs release layer, which allowed epitaxial layers grown on top of the AlAs layer to be removed from the substrate. The GaAs layers had various doping densities and thicknesses. We present our TR-PL results from both pre- and post-ELO processed GaAs-InAl(Ga)P samples.

In recent year, InGaN-based alloy was also considered for photovoltaic devices owing to the distinctive material properties which are benefit photovoltaic performance. However, the Indium tin oxide (ITO) layer on top, which plays a role of transparent conductive oxide (TCO), can absorb UV photons without generating photocurrent. Also, the thin absorber layer in the device, which is consequent result after compromising with limited crystal quality, has caused insufficient light absorption. In this report, we propose an approach for solving these problems. A hybrid design of InGaN/GaN multiple quantum wells (MQWs) solar cells combined with colloidal CdS quantum dots (QDs) and back side distributed Bragg reflectors (DBRs) has been demonstrated. CdS QDs provide down-conversion effect at UV regime to avoid absorption of ITO. Moreover, CdS QDs also exhibit anti-reflective feature. DBRs at the back side have effectively reflected the light back into the absorber layer. CdS QDs enhance the external quantum efficiency (EQE) for light with wavelength shorter than 400 nm, while DBRs provide a broad band enhancement in EQE, especially within the region of 400 nm ~ 430 nm in wavelength. CdS QDs effectively achieved a power conversion efficiency enhancement as high as 7.2% compared to the device without assistance of CdS QDs. With the participation of DBRs, the power conversion efficiency enhancement has been further boosted to 14%. We believe that the hybrid design of InGaN/GaN MQWs solar cells with QDs and DBRs can be a method for high efficiency InGaN/GaN MQWs solar cells.

We used AlGaSb/AlGaAs material system for a theoretical study of photovoltaic performance of the proposed GaAsbased solar cell in which the type-II quantum dot (QDs) absorber is spatially separated from the depletion region. Due to inelastic scattering of photoelectrons on QDs and proper doping of both QDs and their spacers, concentrated sunlight is predicted to quench recombination through QDs. Our calculation shows that 500-sun concentration can increase the Shockley-Queisser limit from 35% to 40% for GaAs single-junction solar cells.

III-V Dilute Nitride multi-quantum well structures are currently promising candidates to achieve 1 sun efficiencies of >40% with multi-junction design (InGaP/ GaAs/ GaAsN/ Ge). In other works, we have discussed the design having III-V Dilute Nitride GaAsN multi-quantum well (MQW) structures with resonant tunneling setup in the intrinsic region, in order to improve the response potentially yielding 1 sun efficiencies greater than 40%. Earlier efforts in this direction had yielded samples with considerable incorporation of N at the QW/barrier interface, leading to the formation of nitridation and reducing the overall quantum efficiency. In this work we discuss the results of the growth of MQW solar cells in MBE, with a modified run-vent system for the RF N-plasma setup aimed at increasing the sharpness of the well-barrier transition, and the change in quality of the quantum wells grown.

There are so many existed static sunlight concentrators and SunLego® is the one of them. SunLego has the advantages of high concentrate efficiency and easy to produce. However, the concentrate efficiency will decrease dramatically of different sunlight incident azimuth angle. Current azimuth tolerance is from negative 30 degree to positive 30 degree. This issue will cause unstable lighting situation. We proposed an innovative static compound parabolic concentrator (CPC) and it will increase the concentrate efficiency largely. It composes of two main optical components; one is the innovative CPC on the up side to increase incident azimuth angle. The down side component is the prism structure to increase output fiber coupling efficiency.

Sunlight is the most environmental-friendly energy. Through the Natural Light Illumination Systems (NLIS^® ), we can guide sunlight indoors to use. However, due to the construction cost of the NLIS^® , the most cost is spent on the optical fiber. Therefore we intend to design an element that can re-shape the beam from the SunLego^reg; to be narrower, where SunLego^® is the building unit of NLIS^® for light collection. The element consists of two functional optical structures: the first structure is to convert light from SunLego^® to parallel beam, technically, is to convert stray light to parallel. We design the first structure based on free form principle. The second is a fan-shaped structure, which is to reduce the light transmission area , this design is based on the E'tendue principle. The reduction of area means we can use smaller fiber for later light transmission, which is crucial to reduce material cost.

The best benefit of Natural Light Illumination System (NLIS^®) is to reduce energy consumption that compare to traditional lighting system. However, the propagation efficiency will decrease dramatically when there is the long distance propagation in NLIS^®. Therefore, this paper has proposed an innovative modulated guiding structure with high propagation efficiency. The base structure is consisting of two Fresnel lenses and the distance between two lenses is two times of focal length. Furthermore, the light will be focused by first Fresnel lens and diverge as original input again before the second lens due to two times of focal length design. The advantage of the innovative design is to avoid energy loss when propagation. Based on two times of focal length design method and connecting several base structures in the way of cascading, it could make the structure become modulated. The efficiency of a base module structure will reach above 80%. We have proposed an innovative modeled structure that is with high propagation efficiency. By the Fresnel lens, the structure has the benefit of low cost and easy to produce that compare to traditional natural light system.

We demonstrate the GaAs solar cells which utilize the high-transmittance textured polydimethylsiloxane (PDMS) film can outstanding increase the short circuit current density and power conversion efficiency of solar cells. The transmittance of PDMS film is exceeded 90%, which can pass through almost all the light of GaAs Solar cells can be absorbed. We used a special imprint technology to let the PDMS film possess a highly textured surface. Then we measured the characteristics of textured PDMS film and found out that it has a very excellent Haze performance. The effect of flexible textured PDMS film on the suppression of surface reflection in GaAs solar cells is also investigated. The presented technology provides an inexpensive surface anti-reflection process, which can potentially replace typically complex anti-reflection coating (ARC) layer. The GaAs solar cells with textured PDMS layer can effectively enhance the short-circuit current density from 22.91 to 26.54 mA/cm² and the power conversion efficiency from 18.28 to 21.43 %, corresponding to a 17 % enhancement compared to the one without textured PDMS. The open-circuit voltage (V_oc) and the fill-factor (FF) of GaAs solar cells exhibit negligible change, because the textured PDMS film was pasted up on the surface of GaAs solar cells and did not interfere with the diode operation. At the same time, we observed through the EQE measurement that the textured PDMS film not only proved wonderful light scattering effect but also generated more electron-hole pairs in all absorption spectrum range. Finally, through this simple PDMS process, we believe this technology shall be a great candidate for next generation of highly efficient and low-cost photovoltaic devices.

A vertical structure with a back contact layer is suggested for silicon quantum dots (Si QDs) solar cells to overcome the current crowding effect arising from the high lateral resistance in the emitter layer of the existing mesa-structured Si QDs solar cells on quartz substrates. Molybdenum (Mo) is widely used as the back contact layer in CIGS solar cells due to its high electrical conductivity, good optical reflectance and chemical stability. This paper will focus on the feasibility of Mo as a back contact layer deposited between a quartz substrate and a sputtered silicon rich oxide (SRO) bilayers structure to obtain a fully vertical Si QDs solar cell. In this structure, the desired previously mentioned electrical and optical properties of the Mo thin film have to be maintained during and after a high temperature annealing process. This high temperature process is unavoidable in this structure as it is required to form the Si QDs. This paper aims to study factors that have impacts on critical properties of the Mo thin films processed in contact with Si and SiO₂ at high temperatures. Characterizations including film thickness, microstructure, sheet resistance and optical reflectance measurements are also performed. Furthermore, interfacial properties between the Mo layer and the upper SRO bilayers are investigated.

Efficient light management is one of the key issues in modern energy conversion systems, might it be to collect optical power or to redistribute light generated by high power light emitting diodes. One problem remains: How can one realize small size elements with high quality of light management. We propose a novel scheme by using miniaturized angle transformers or concentrators that have size of several millimeters. none In this size range diffraction effects play rarely a role and the design can be based on classical ray tracing. Dimensions are chosen to allow effective solution for high power light emitting diodes as well as solar cells. In most solar cell designs, the photocurrent is extracted through a conducting window layer in combination with a silver grid at the front of the device. The trade-off between series resistance and shadowing requires either buried contacts or screen printing of narrow lines with high aspect ratio. We propose an alternate approach where an array of parabolic concentrators directs the incoming light into the cell. The front metallization can thus be extended over the area between the paraboloids without shadowing loss. High power light emitting diodes are source with certain far field distribution and composed often out of several chips. Applying the concentrator array technology not on the whole source but locally on each chip promises small and effective solutions. We demonstrate realization of linear and hexagonal arrays of micro-concentration systems, discuss details of application and results of simulation of their optical properties in applications.

In this study, the GaAs epilayer is quickly separated from GaAs substrate by epitaxial lift-off (ELO) process with mixture etchant solution. The HF solution mixes with surfactant as mixture etchant solution to etch AlAs sacrificial layer for the selective wet etching of AlAs sacrificial layer. Addiction surfactants etchant significantly enhance the etching rate in the hydrofluoric acid etching solution. It is because surfactant provides hydrophilicity to change the contact angle with enhances the fluid properties of the mixture etchant between GaAs epilayer and GaAs substrate. Arsine gas was released from the etchant solution because the critical reaction product in semiconductor etching is dissolved arsine gas. Arsine gas forms a bubble, which easily displaces the etchant solution, before the AlAs layer was undercut. The results showed that acetone and hydrofluoric acid ratio of about 1:1 for the fastest etching rate of 13.2 μm / min. The etching rate increases about 4 times compared with pure hydrofluoric acid, moreover can shorten the separation time about 70% of GaAs epilayer with GaAs substrate. The results indicate that etching ratio and stability are improved by mixture etchant solution. It is not only saving the epilayer and the etching solution exposure time, but also reducing the damage to the epilayer structure.

Knowledge of carrier transfer, in quantum dot sensitized solar cells, is the key to engineering the device structure and architecture optimization. In this work, Zinc oxide (ZnO) nanowire (NW) arrays were synthesized on glass wafers and on GaN thin films for application in photovoltaic and light-emitting devices. The nanowires grown on glass wafers were incorporated with CdSe/ZnS quantum dots (QD) and their steady state and lifetime photoluminescence (PL) were studied to investigate the feasibility of electron transfer from excited QDs to ZnO NWs. The results provide an indication that the injected electrons, from excited high quantum efficiency QDs, live longer and hence facilitate electron transport without undergoing non-radiative recombination at surface trap states. Morphology and optical properties of the ZnO nanowires on GaN film were also studied for application in light-emitting devices.

Hydrogenated nanocrystalline silicon (nc-Si:H) based alloys have strong potential in cost-effective and flexible photovoltaics. However, nc-Si:H undergoes light induced degradation (LID), which degrades the device efficiency by over 15%. The microstructural processes responsible for the LID are still under debate. Several recent studies suggest that the generation of metastable defects at grain/ grain-boundary (GB) interface enhances density of traps, which limits the charge collection efficiency. Conventional characterization techniques can measure transport properties such as electrical conductivity or carrier mobility averaged over large sample volumes. However, nanoscale characterization tools, such as Scanning Kelvin probe Force Microscopy (KFM), reveal local electronic properties of grains and GBs which may lead to better understanding of microscopic process of metastability. The optoelectronic properties of nc-Si:H films were measured in dark and under illumination to study the effect of LID at the nanoscale. The surface potential and charge distribution were measured in as-deposited and photo-degraded samples using a custom-designed scanning probe microscopy tool installed in an environment controlled glove-box. Photodegradation resulted in an upward bending of the conduction band edge, suggesting accumulation of photo-generated charges at GBs. This effect is attributed to the generation of acceptor like defects (traps) at GBs during illumination. Density of defects is estimated from grain/GB width and absolute value of band bending.

In traditional III-nitride solar cells, the polarization-induced charges and potential barrier in the hetero-interfaces are demonstrated to be harmful for carrier collection. To solve these challenges, the elimination or mitigation of the abrupt hetero-interfaces should be efficient. In this study, various kinds of solar cell structures are investigated numerically. The structures under various situations of indium composition and degree of polarization are systematically explored. Specifically, the photovoltaic performance, energy band diagrams, electrostatic fields, and recombination rates are analyzed. Then, according to the simulation results, the appropriate solar cell structure which possesses high conversion efficiency is proposed.

A cavity enhanced one-dimensional grating structure is proposed to improve the light absorption within the α-Si thin film solar cell. Typically, dielectric or metal structure including gratings is added for the light absorption enhancement. Not only does the structure form the guided modes, and increase the surface area/surface angle, but also the thin film itself forms a cavity allowing light trapping for better absorption. However, the structure is optimized in these two mechanisms separately. In this paper, finite element method (FEM) was used to optimize thicknesses of two cavities and then combine them into a one –dimensional grating structure. Comparing to the flat thin film solar cell, we have get absorption enhancement factors of 1.12 and 1.51 normalized for the AM 1.5 spectrum for 300 nm to 950 nm by the two proposed structures.

The Natural Light Guiding System(NLIS®) developed at NTUST is designed to guild the outdoor sunshine for indoors illumination. The system can be divided into three subsystems: the light collecting sub-system(LCS), the light transmitting sub-system(LTS), and the light emitting subsystem( LES). These three sub-systems are intimately connected with one another. However, a great deal of energy is wasted during the process of passing the sunlight from the LCS to the LCS. In this research, we design an optical coupler based on a combination of “The Power Index of Energy” and “Free-form” structure to improve the efficiency. Six time periods were selected when the sunlight was the strongest during a day. Six different angles were produced by these six rays of the sunlight. And the six angles of incidence could be derived from the “Equation of Internal Circle .” Using this method accompanying with the “Discrete surface” built by “The Power Index of Energy,” the “Reflect surface” was formed. Through the stimulations and analyses of this “Reflect surface” by the optical software, the results showed that the efficiency of the light transmission was improved more than three times comparing to the original system.