T. Glatfelter

Band-gap profiling for improving the efficiency of amorphous silicon alloy solar cells

We have developed an amorphous silicon alloy based solar cell with a novel structure in which the optical gap of the intrinsic layer changes in a substantial portion of the bulk. Computer simulation studies show that for a given short circuit current, it is possible with this structure to obtain higher open circuit voltage and fill factor than in a conventional cell design. Experimental cell structures have been made and confirm the theoretical prediction. The new cell design shows a considerable improvement in efficiency. Incorporation of this structure in the bottom cell of a triple device has resulted in the achievement of 13.7% efficiency under global AM1.5 illumination.

Amorphous Silicon Alloy Photovoltaic Technology - from R&D to Production

The key requirements for photovoltaic modules to be accepted for large-scale terrestrial applications are (i) low material cost, (ii) high efficiency with good stability, (iii) low manufacturing cost with good yield and (iv) environmental safety. Thin films of amorphous silicon alloy are inexpensive; the products are also environmentally benign. The challenge has been to improve the stable efficiency of these modules and transfer the R&D results into production. Using a Multijunction, Multi-bandgap approach to capture the solar spectrum more efficiently, we have developed one-square-foot modules with initial efficiency of 11.8%. After 1000 h of one-sun light soaking, a stable efficiency of 10.2% was obtained. Both the efficiency values were confirmed by National Renewable Energy Laboratory. The technology has been transferred to production using an automated roll-to-roll process in which different layers of the cell structure are deposited in a continuous manner onto stainless steel rolls, 14” wide and half a mile long. The rolls are next processed into modules of different sizes. This inexpensive manufacturing process produces high efficiency modules with subcell yields greater than 99%. The key features of the technology transfer and future scope for improvement are discussed.

Experimental study of p layers in ‘‘tunnel’’ junctions for high efficiency amorphous silicon alloy multijunction solar cells and modules

The role of the p layer in the formation of good quality ‘‘tunnel’’ junctions and its dependence on the attainment of high efficiency hydrogenated amorphous silicon alloy (a‐Si:H) multijunction cells has been investigated. A new technique, namely, the evaluation of the current‐voltage characteristics of the NIPN structure consisting of a single‐junction n‐i‐p cell with an overlying doped n layer, has been developed for the determination of losses at the ‘‘tunnel’’ junction. Using an optimized p layer, an initial conversion efficiency of 11.4% has been obtained on a double junction a‐Si:H module of aperture area ∼900 cm2.

A comparison of the errors in determining the conversion efficiency of multijunction solar cells by various methods

Summary Methods of accurately measuring the conversion efficiency with respect to a given set of standard reporting conditions have been established for single-junction photovoltaic (PV) devices. Efficiency measurements on two- terminal tandem PV devices with more than one active junction are more difficult since the junction limiting the current and efficiency is determined by the source spectrum. The various methods of determining the illuminated current vs. voltage parameters of two-terminal and multiterminal multijunction solar cells with respect to standard reporting conditions (temperature, spectral irradiance and total irradiance) will be discussed. The methods discussed include a solar simulator and reference cell for each junction, one solar simulator with one reference cell, and one or more solar simulators with the absolute quantum efficiency for each junction of the multijunction cell. All three methods require the measurement of the quantum efficiency of each active junction of the cascade structure. A brief description and error analysis of the various methods will be discussed. Data will be presented comparing the I-V charac- teristics for several tandems using the various methods. 1. Introduction Multijunction solar cell structures are being developed by several research groups as a path towards higher photovoltalc (PV) efficiencies [1 - 5]. The efficiency in per cent for any PV structure is normally expressed as

High efficiency multi-junction solar cells using amorphous silicon and amorphous silicon-germanium alloys

Using a novel cell design the authors have achieved a 13.7% conversion efficiency with amorphous silicon and amorphous silicon-germanium alloys in a three-cell stacked-junction configuration. 13.0% conversion efficiency was achieved in the tandem configuration. The efficiency value was measured using a triple-source solar simulator adjusted for global AM1.5 test conditions. This device has a structure of stainless steel/textured silver/zinc oxide/ni/sub 1/p/ni/sub 2/p/ni/sub 3/p/ITO/grid. The authors used an amorphous silicon-germanium alloy with the new design in the i/sub 1/ layer, and amorphous silicon alloys in the i/sub 2/ and i/sub 3/ layers. The J-V characteristic shows J/sub sc/=7.66 mA/cm/sup 2/, V/sub oc/=2.55 V, and fill factor= 0.70 with an active area of 0.25 xcm/sup 2/. The quantum efficiency of this device is 60% collection at 400 nm, 93% at the peak, 55% at 800 nm, and 21% at 850 nm. The total photocurrent density obtained by integrating the quantum efficiency envelope with the global AM1.5 spectrum is 23.5 mA/cm/sup 2/.<<ETX>>

A novel design for amorphous silicon alloy solar cells

The authors have developed an amorphous silicon alloy-based solar cell with a novel structure. Computer simulation studies show that for a given short-circuit current, it is possible to obtain a higher open-circuit voltage and fill factor than in a conventional cell design. For a nominal 1.5 eV a-SiGe alloy, the fill factor under red illumination can be improved from 0.55 to 0.64 for the same short-circuit current. Experimental cell structures confirm the theoretical prediction. The novel cell design shows a considerable improvement in efficiency. Dynamic internal collection efficiency measurements show reduced recombination in these cells, which gives rise to the observed higher fill factors. Incorporation of this structure in the bottom cell of a triple device has resulted in the achievement of 13.7% efficiency under global AM1.5 illumination.<<ETX>>

Crucial parameters and device physics of amorphous silicon alloy tandem solar cells

Abstract We have previously shown that tandem solar cells exhibit higher efficiency and better long-term stability than single junction cells. We shall discuss crucial parameters for achieving high performance solar cells and certain aspects of device physics associated with the tandem structure. We will present data on what effect changes in intrinsic layer thicknesses, p + and n + layer thicknesses, back reflector quality, and solar spectral variation have on these multijunction devices.

An optical study of each layer in a-Si:H solar cells

Parasitic optical losses in amorphous silicon based solar cells were determined by evaluating the film layers as they actually exist in the device structure. The absorption coefficient, refractive index, and thickness of all the layers in the cells were calculated simultaneously using absolute spectral response and reflection measurements. A computer program fitted these measurements to the cells structure using semiconductor and damped harmonic oscillator models. The algorithm was based on minimization of error using a simplex method. The optical stack was treated coherently, taking into account interference effects that were evident in the spectral response and reflection spectra. Results were tested for consistency by the comparison of paired samples with slightly different structures. It was concluded that the thicknesses, absorption and dispersion of the individual layers in these solar cells could be resolved.<<ETX>>

Outdoor performance studies of a-Si alloy multi-junction solar cells using simulated solar illumination

A comparison of various a-Si alloy photovoltaic devices-singles, tandems, and triples-under incident solar irradiance with widely different spectral content was made in an outdoor performance study. The spectral irradiance was varied over a range of the three most crucial atmospheric parameters that affect the incident spectrum: air mass, water vapor and turbidity. A multisource solar simulator was used to simulate the spectrum throughout the day for each of five different days. I-V curves of the solar cells were then measured under these various spectral conditions, and their performances were compared. The results reveal that the power output of the multijunction devices, when normalized at solar noon and integrated over the day, shows a difference of only +or-4% with respect to the single-type cells, indicating a similarity in response among the different types of devices.<<ETX>>

A PV module that emulates an asphalt shingle

The authors have developed a shingle roofing module designed to emulate the conventional asphalt shingle in form, structural function and installation. The PV shingle module consists of a series of interconnected, coated stainless steel tabs laminated together in EVA/Tefzel polymers. The PV shingle design allows the mechanical and electrical installation to be performed independently, thereby minimizing coordination between the roofing and electrical tradesman. The installation procedure is so similar to conventional asphalt shingles that an experienced roofing contractor, with minimal training, can install the modules. The authors show results of testing that demonstrate that the PV shingle serves the dual function of electrical generator as well as a roofing material. Finally, they demonstrate the feasibility of the PV shingle by describing a 1.8 kW AC system installed on the Southface Energy Institutes Energy and Environmental Resource Center House in Atlanta, Georgia, USA.