James M. Gee

Emitter wrap-through solar cell

The authors present a new solar cell concept (emitter wrap-through or EWT) for a back-contact cell. The cell has laser-drilled vias to wrap the emitter on the front surface to contacts on the back surface and uses a potentially low-cost process sequence. Modeling calculations show that efficiencies of 18 and 21% are possible with large-area solar-grade multi- and monocrystalline silicon EWT cells, respectively.<<ETX>>

Characterization of random reactive ion etched-textured silicon solar cells

Hemispherical reflectance and internal quantum efficiency (IQE) measurements have been employed to evaluate the response of Si nanostructured surfaces formed by using reactive ion etching (RIE) random texturing techniques. Random RIE-textured surfaces typically exhibit broadband anti-reflection behavior with solar-weighted-reflectance (SWR) of /spl ap/3% over 300-1200-nm spectral range. RIE-texturing has been demonstrated over large areas (/spl sim/180 cm/sup 2/) of both single and multicrystalline Si substrates. Due to the surface contamination and plasma-induced damage, as formed RIE-textured solar cells do not provide enhanced short-circuit current. However, improved surface cleaning combined with controlled wet-chemical damage removal etches provide a significant improvement in the short-circuit current. For such textures, the internal quantum efficiencies are comparable to the random, wet-chemically-textured solar cells. In both the UV and near-IR wavelength regions, the RIE-textured subwavelength surfaces exhibit superior performance in comparison with the wet-chemically-textured surfaces. Due to their large area, low-reflection capability, random, RIE-texturing techniques are expected to find widespread commercial applicability in low-cost, large-area multicrystalline Si solar cells.

A 31%-efficient GaAs/silicon mechanically stacked, multijunction concentrator solar cell

The development and demonstration of the first solar cell to achieve an efficiency in excess of 30% are reported. The improved performance compared to previous GaAs/silicon mechanically stacked, multijunction (MSMJ) concentrator cells is due to improvements in the component cell technologies and to better optimization of the GaAs cell transmissivity. Preliminary analysis suggests that an efficiency approaching 35% is possible with GaAs-based MSMJ cells.<<ETX>>

The effect of parasitic absorption losses on light trapping in thin silicon solar cells

The effect of parasitic absorption (PA) losses on light trapping in thin silicon cells was investigated. Parasitic absorption refers to an optical absorption process which does not generate an electron/hole pair; it competes with band-to-band absorption to decrease the photocurrent. A simple model for light trapping that includes PA is described for interpretation of experimental data. The sub-bandgap reflectance is shown to be a sensitive and quantitative indicator of PA losses. Minimization of the PA losses is shown to be as important as optimization of the light-trapping geometry for increased photogeneration in light-trapping thin silicon solar cells.<<ETX>>

A comparison of different module configurations for multi-band-gap solar cells☆

Abstract The use of different module configurations with multijunction (MJ) solar cells was investigated. Module configuration refers to the electrical circuit in which the subcells of the MJ cell are connected. The simplest configuration has the subcells connected in series. The subcells may also be connected in parallel or operated independently. These circuits require MJ cells with more than two terminals. The performance of two-junction tandem cells with different module configurations was analyzed by means of computer modeling. Roughly the same performance was found for an optimized MJ cell with the different module configurations. However, several advantages for MJ cells with the subcells wired in parallel as opposed to series were identified, including wider selection of band gaps for optimal performance, less sensitivity to spectral variations, and possibly less sensitivity to radiation damage.

Diffraction grating structures in solar cells

Sub-wavelength periodic texturing (gratings) of crystalline-silicon (c-Si) surfaces for solar cell applications can be designed for maximizing optical absorption in thin c-Si films. The authors have investigated c-Si grating structures using rigorous modeling, hemispherical reflectance and internal quantum efficiency measurements. Model calculations predict almost /spl sim/100% energy coupling into obliquely propagating diffraction orders. By fabrication and optical characterization of a wide range of 1D and 2D c-Si grating structures, they have achieved broadband, low (/spl sim/5%) reflectance without an antireflection film. By integrating grating structures into conventional solar cell designs, they have demonstrated short-circuit current density enhancements of 3.4 and 4.1 mA/cm/sup 2/ for rectangular and triangular 1D grating structures compared to planar controls. The effective path length enhancements due to these gratings were 2.2 and 1.7, respectively. Optimized 2D gratings are expected to have even better performance.

Simplified high-efficiency silicon cell processing

Abstract We developed an emitter diffusion process that yields a near-ideal doping profile with a passivating oxide in a single furnace step. Because this process subjects the material to only one high-temperature thermal excursion, bulk lifetime is better preserved. This is especially true for lower-cost silicon materials containing a high concentration of oxygen or carbon. Using this process, we routinely obtain one-sun cell efficiencies over 19% on float-zone material and over 18% on Czochralski material. Using solar-grade Czochralski material, we have demonstrated record efficiencies of 18.3% at one sun and 20.0% under concentration. Simple processes that yield high-performance diffusion profiles are expected to become increasingly important as manufacturers adopt improved techniques for ohmic contacts.

Selective emitters using photonic crystals for thermophotovoltaic energy conversion

Photonic crystals use a periodic modulation of the refractive index to alter the photonic density of states. The photonic density of states is an important parameter in many phenomena involving radiation-matter interactions - including thermal emission of radiation. Hence, a photonic crystal can be used to engineer the emissivity of an emitter for thermophotovoltaic generators to match the spectral response of the TPV cell. The use of photonic crystals in TPV is described. A three-dimensional photonic crystal in tungsten is realized that exhibits an exceptionally large photonic bandgap and attenuation factor. The photonic crystal is shown to have promise for radiant energy conversion applications like TPV energy conversion.

Circuit modeling of the emitter-wrap-through solar cell

Back-contact solar cells have the potential to reduce module assembly costs and give a higher conversion efficiency. Such a device must be simple to fabricate on an industrial scale and be tolerant of low minority-carrier diffusion lengths. The emitter-wrap-through (EWT) cell is a device design that can meet these goals. In this device, the diffused junction is present on both sides and is connected by laser-drilled holes through the silicon. EWT cells were frequently found to have poor fill factors (FFs) due to shunt-like behavior. The holes were found to possess no defects that adversely affect device performance. However, detailed equivalent circuit modeling of the EWT cell was able to explain the shunt-like behavior. Experiments were performed to confirm the physical mechanisms described by the equivalent circuit model. Device optimization guided by the equivalent circuit model has led to the demonstration of a large area EWT cell with a FF of 77.64% and efficiency of 18.2%.

Application of a boron source diffusion barrier for the fabrication of back contact silicon solar cells

Interdigitated back contact and emitter wrap-through solar cells were fabricated using a diffusion barrier to achieve selective phosphorus diffusion for the patterning of the base and the emitter regions on the cell rear. The addition of boron to the diffusion barrier for emitter formation in the underlying n-type base and for the formation of a back surface field in the case of a p-type base was further examined. Boron was successfully incorporated into the n-type Si for the creation of rear p/sup +/ emitters in an interdigitated back contact cell. An order of magnitude improvement in the surface recombination velocity to 10/sup 3/ cm/s could be achieved with a p/sup +/ surface field applied to the base of p-type wafers. Incorporating this technology, best multi-crystalline emitter wrap-through cell performance could be gained with a 1k-2k /spl Omega///spl square/ surface field; however, the characteristics were rapidly dominated by increased saturation current as the surface field layer concentration was increased.