John R. Wilcox

Multi-terminal dual junction InGaP 2 /GaAs solar cells for hybrid system

The performance of two and three-terminal solar cells under 13-sun AM1.5G illumination is reported. The solar cells are comprised of monolithically grown InGaP2 and GaAs single junction cells. The two-terminal device consists of single junctions in series. The three-terminal device structure comprises the single junctions independently interconnected by an InGaP2 layer serving as the middle contact. Device optimization was based on modeling of the InGaP2 top junction band gap for various spectral irradiance profiles. It was found that the optimal band gap combination for a 2.6eV filtered AM1.5G spectrum is achieved with 1.84eV and 1.43 eV for the top and bottom junctions, respectively. External quantum efficiency and illuminated current-voltage (I–V) measurements of the component two and three-terminal tandem cells are discussed.

Correlated Nonideal Effects of Dark and Light I–V Characteristics in a-Si/c-Si Heterojunction Solar Cells

a-Si/c-Si (amorphous Silcon/crystalline Silicon) heterojunction solar cells exhibit several distinctive dark and light I-V nonideal features. The dark I-V of these cells exhibits unusually high ideality factors at low forward-bias and the occurrence of a “knee” at medium forward-bias. Nonidealities under illumination, such as the failure of superposition and the occurrence of an “S-type” curve, are also reported in these cells. However, the origin of these nonidealities and how the dark I-V nonidealities manifest themselves under illumination, and vice versa, have not been clearly and consistently explained in the current literature. In this study, a numerical framework is used to interpret the origin of the dark I-V nonidealities, and a novel simulation technique is developed to separate the photo-current and the contact injection current components of the light I-V. Using this technique, the voltage dependence of photo-current is studied to explain the failure of the superposition principle and the origin of the S-type light I-V characteristics. The analysis provides a number of insights into the correlations between the dark I-V and the light I-V. Finally, using the experimental results from this study and from the current literature, it is shown that these nonideal effects indeed affect the dark I-V and the light I-V in a predictable manner.

Efficiency of multijunction photovoltaic systems

In this paper, an expression for the PV system efficiency is derived that can be used in conjunction with measured device performance and detailed numerical modeling to analyze PV system performance. Such an analysis will help identify design trade-offs and also help to identify which system and cell design changes will be of greatest benefit to the enhancement of PV system performance.

Design of a GaInP/GaAs tandem solar cell for maximum daily, monthly, and yearly energy output

Solar concentrator cells are typically designed for maximum efficiency under the AM1.5d standard spectrum. While this methodology does allow for a direct comparison of cells produced by various laboratories, it does not guarantee maximum daily, monthly, or yearly energy production, as the relative distribution of spectral energy changes throughout the day and year. It has been suggested that achieving this goal requires designing under a nonstandard spectrum. In this work, a GaInP/GaAs tandem solar cell is designed for maximum energy production by optimizing for a set of geographically-dependent solar spectra using detailed numerical models. The optimization procedure focuses on finding the best combination of GaInP bandgap and GaInP and GaAs sub-cell absorber layer thicknesses. It is shown that optimizing for the AM1.5d standard spectrum produces nearly maximum yearly energy. This result simplifies the design of a dual-junction device considerably, is independent of the optical concentration up to at least 500 suns, and holds for a wide range of geographic locations. The simulation results are compared to those obtained using a more traditional, ideal-diode model.

Peak efficiency of multijunction photovoltaic systems

The maximum efficiency of multijunction photovoltaic systems is determined for independently connected solar cells that are optically in series. The maximum efficiency is computed as a function of solar concentration using both the Shockley-Queisser detailed balance radiative limit for the reverse saturation current density and a simple empirical expression for the reverse saturation current density obtained from published “state-of-the-art” solar cells performance characteristics. It is shown that there are an optimal number of junctions for peak efficiency after which the system efficiency will decrease as the number of junctions increases.

Estimating Saturation Current Based on Junction Temperature and Bandgap

This paper describes a method for estimating a solar cells reverse saturation current density, JO(T,EG(300 K)), based upon the bandgap energy at 300 K and the junction operating temperature. In an easy to use functional form, the solar cell performance can be calculated without knowing material specific parameters. This method can be used to optimize the bandgaps for highest conversion efficiency, during the initial design phase of a multijunction system.

Numerical modeling of loss mechanisms resulting from the distributed emitter effect in concentrator solar cells

Power loss associated with lateral current flow in solar cell emitter layers is known to cause fill-factor degradation. In this work, a unique approach to modeling the lateral current flow in solar cell emitter layers is developed using MATLAB™ and current-generation models based upon one-dimensional detailed numerical cell models. An example is employed to illustrate the high degree of accuracy of this approach and that in addition to joule losses in the emitter layer and conducting electrodes, a third loss mechanism, which is the result of the potential gradient across the cell surface, also plays an important role. The relationship between these loss mechanisms is explored over a wide range of concentrations under both uniform and non-uniform illumination for the example cell.

A diagnostic tool for analyzing the current-voltage characteristics in a-Si/c-Si heterojunction solar cells

Understanding the effects of device parameters on carrier transport is essential for the design of high efficiency aSi/c-Si heterojunction solar cells. It is well known that the dark current-voltage (I-V) characteristics are correlated to fundamental PV parameters that dictate cell performance, and therefore an analysis of dark I-V may be a useful diagnostic tool to monitor changes in the device parameters (indirectly correlated to solar cell efficiency). In this paper, we first measure and then interpret the forward bias dark I-V characteristics of a-Si/c-Si solar cells by numerical and analytical models. Several well-known (but poorly understood) features of dark I-V characteristics are shown to be correlated to parameters of fundamental interest to solar cell design and can therefore be used as important markers of device parameter variation during the development process.

A case study of system power efficiency loss mechanisms in a multijunction, spectral splitting, concentrator solar cell system

In multijunction solar cell concentrator systems, it is important for system and cell designers to understand the relative magnitude of each loss mechanism. This investigation of potential system power efficiency improvements for a high-efficiency concentrator system focuses on quantifying the effects that various known loss mechanisms have on the overall system performance. Each of the loss mechanisms that were investigated play a part in the degradation of the devices performance and each of these losses can be reduced by appropriate engineering. This paper will address the extensive optimization of the system power efficiency of a multijunction concentrator solar cell system for the DARPA Very High Efficiency Solar Cell (VHESC) project. These results are useful to the system and cell designers and guide the efforts to reduce losses and to maximize the system power efficiency.

Combining solar cell and optical modeling in multijunction systems

High efficiency solar cell concentrator systems, particularly non-tracking systems, may utilize optical systems that involve lenses, secondary concentrators, dichroic mirrors and other optical components in conjunction with a variety of cells that may be optically and electrically connected in series or parallel configurations. This paper describes the value of combining realistic PV models into a complex optical model, making it possible to simulate the performance of the complete PV system to simplify the task of optimizing the system design. The model also allows for the assessment of many important optics-to-PV interface effects such as angle of incidence of the rays on cell performance. The embedded PV models used are based on readily measured cell performance, which can be characterized external to the complete PV system. Agreement between model predictions and measured performance is presented.