World-record solar-to-electricity energy conversion efficiency has been previously achieved by photovoltaic devices that maximize the use of the solar spectrum, such as multi-junction tandem solar cells. These cells are made of III-V materials whose high cost is a strong limitation on their widespread commercial application. One solution to suppress the cost of these types of devices is to grow III-V solar cells on low-cost carrier materials such as silicon. We will discuss the material, structure and analysis of GaAsP/SiGe-on-silicon multi-junction tandem solar cells. A low threading dislocation density is realized by effective lattice-matching of the top and bottom cells which demonstrate a device that achieves high open-circuit voltage in the top solar cell. The GaAsP/SiGe solar cells have reached a measured efficiency of 20.6% under one sun concentration. Analysis of these results based on the product of the best parameters shows efficiency potential of 26% under one sun, 30.8% at 20× and 35.1% at 400×.

We studied the InGaP/GaAs//InGaAsP/InGaAs four-junction solar cells grown by molecular beam epitaxy (MBE), which were fabricated by the novel wafer bonding. In order to reach a higher conversion efficiency at highly concentrated illumination, heat generation should be minimized. We have improved the device structure to reduce the thermal and electrical resistances. Especially, the bond resistance was reduced to be the lowest value of 2.5 × 10-5 Ohm cm2 ever reported for a GaAs/InP wafer bond, which was obtained by the specific combination of p+-GaAs/n-InP bonding and by using room-temperature wafer bonding. Furthermore, in order to increase the short circuit current density (Jsc) of 4-junction solar cell, we have developed the quality of InGaAsP material by increasing the growth temperature from 490 °C to 510 °C, which leads to a current matching. In a result, an efficiency of 42 % at 230 suns of the four-junction solar cell fabricated by room-temperature wafer bonding was achieved.

A-D-A architectured quinoxaline and bezodithiophene based small molecule exhibiting low band gap was designed and synthesized for bulk heterojunction solar cell application. Two dimension conjugation to the molecule was imparted by using alkylated aromatic units. This effective extension of conjugation broadens the absorption spectra. Optical and electrochemical properties suggest that the synthesized molecule has a low band-gap and well matching frontier molecular orbital energy levels with that of PCBM. A photovoltaic device using this A-D-A molecule as the active layer was fabricated to evaluate the optoelectronics properties.

This paper reports on a monolithic 10 cm × 10 cm area PV module integrating an array of 72 a-Si:H n-i-p cells on a 100 μm thick polyethylene-naphtalate substrate. The n-i-p stack is deposited using a PECVD system at 150 °C substrate temperature. The design optimization and device performance analysis are performed using a two-dimensional distributed circuit model of the photovoltaic cell. The circuit simulator SPICE is used to calculate current and potential distributions in a network of sub-cell circuits, and also to map Joule losses in the front TCO electrode and the metal grid. Experimental results show that the shunt leakage is one of the factors reducing the device performance. Current-voltage characteristics of individual a-Si:H p-i-n cells were analyzed to estimate a variation of shunt resistances. Using the LBIC technique, the presence of multiple shunts in the n-i-p cell was detected. To understand the nature of electrical shunts, the change in the surface roughness of all device layers was analyzed throughout fabrication process. It is found that surface defects in plastic foils, which are thermally induced during the device fabrication, form microscopic pinholes filled with highly conductive top electrode material.

A simple model to describe the fundamental absorption of amorphous hydrogenated silicon carbide thin films based on band fluctuations is presented. It provides a general equation describing both the Urbach and Tauc regions in the absorption spectrum. In principle, our model is applicable to any amorphous material and it allows the determination of the bandgap. Here we focus on the bandgap engineering of amorphous hydrogenated silicon carbide layers. Emphasis is given on the role of hydrogen dilution during the deposition process and post deposition annealing treatments. Using the conventional Urbach and Tauc equations, it was found that an increase/decrease of the Urbach energy produces a shrink/enhancement of the Tauc-gap. On the contrary, the here proposed model provides a bandgap energy which behaves independently of the Urbach energy.

Hybrid solar receivers utilizing both photovoltaic cells and thermal collectors are capable of collecting the entire solar spectrum for use in energy systems. Such systems provide efficient solar energy conversion using PV in addition to dispatchability through thermal storage by incorporating a thermal collector in conjunction with the PV. Proposed hybrid systems typically invoke spectrum splitting so to redirect photons optimized for PV electric conversion to a cell while non-PV efficient photons are directed to a thermal absorber. This work discusses a hybrid system with a selective solar filter using a suspended nanoparticle fluid to directly absorb non-PV photons. Non-absorbed photons pass through the filter and impact the PV. Choice of nanoparticles in the fluid allow absorption and transmission of specific wavelengths. Nanoparticles were chosen based on optimization simulations for a bandpass filter to a cSi solar cell. The synthesized fluid has been experimentally characterized to show the effects of high temperature on nanoparticle stability and optical properties. Thermodynamic modeling of the system suggests solar to electric efficiency of the total system is 23.2% if all thermal energy is converted to electricity through an organic Rankine cycle (ORC). However, high temperature generation could be used for industrial process heat at a specific temperature by changing parameters such as absorbed energy and flow rates. Furthermore, a prototype is being developed with 14x concentration to demonstrate the technology on-sun with initial testing targeted for the 2nd quarter of 2016. Overall, the hybrid nanoparticle filter concentrating solar collector can be modified to fit a variety of applications through easily changeable parameters in the system.

Gas Technology Institute (GTI), together with its partners University of California at Merced (UC Merced) and MicroLink Devices Inc. (MicroLink) are developing a full spectrum solar energy collection system to deliver variable electricity and on-demand heat. The technology uses secondary optics in a solar receiver to achieve high efficiency at high temperature, collects heat in particles for low fire danger, stores heat in particles instead of molten salt for low cost, and uses double junction (2J) photovoltaic (PV) cells with backside infrared (IR) reflectors on the secondary optical element to raise exergy efficiency. The overall goal is to deliver enhancement to established trough technology while exceeding the heliostat power tower molten salt temperature limit. The use of inert particles for heat transfer may make parabolic troughs safer near population centers and may be valuable for industrial facilities.

Epitaxial Ge films are useful as a substrate for high-efficiency solar cell applications. It is possible to grow epitaxial Ge films on low cost, cube textured Ni(001) sheets using CaF2(001) as a buffer layer. Transmission electron microscopy (TEM) analysis indicates that the CaF2(001) lattice has a 45o in-plane rotation relative to the Ni(001) lattice. The in-plane epitaxy relationships are CaF2[110]//Ni[100] and CaF2[ $\bar 1$ 10]//Ni[010]. Energy dispersive spectroscopy (EDS) shows a sharp interface between Ge/CaF2 as well as between CaF2/Ni. Electron backscatter diffraction (EBSD) shows that the Ge(001) film has a large grain size (∼50 μm) with small angle grain boundaries (< 8o). The epitaxial Ge thin film has the potential to be used as a substrate to grow high quality III-V and II-VI semiconductors for optoelectronic applications.

The enhancement of photovoltaic efficiency by incorporation of down-shifting phosphor materials in optically active and inactive regions of solar modules is presented. Thin film photovoltaic modules suffer from various optical losses, including front glass reflectance, thermalization loss of absorbed high energy photons, window layer absorption, and the loss of photons to scribe regions. There have been various efforts to improve the performance of solar modules by application of down-shifting (DS), down-converting, and up-converting materials systems. Here we show results towards the development of a low-cost phosphor film system tuned to the solar spectrum and specifically designed for CdTe thin film modules.