Multijunction Concentrator Solar Cells Research Articles

Performance comparison of AlGaAs, GaAs and InGaP tunnel junctions for concentrated multijunction solar cells

AbstractFour tunnel junction (TJ) designs for multijunction (MJ) solar cells under high concentration are studied to determine the peak tunnelling current and resistance change as a function of the doping concentration. These four TJ designs are: AlGaAs/AlGaAs, GaAs/GaAs, AlGaAs/InGaP and AlGaAs/GaAs. Time‐dependent and time‐average methods are used to experimentally characterize the entire current–voltage profile of TJ mesa structures. Experimentally calibrated numerical models are used to determine the minimum doping concentration required for each TJ design to operate within a MJ solar cell up to 2000‐suns concentration. The AlGaAs/GaAs TJ design is found to require the least doping concentration to reach a resistance of <10−4 Ω cm2 followed by the GaAs/GaAs TJ and finally the AlGaAs/AlGaAs TJ. The AlGaAs/InGaP TJ is only able to obtain resistances of ≥5 × 10−4 Ω cm2 within the range of doping concentrations studied. Copyright © 2010 John Wiley & Sons, Ltd.

Is Auger recombination the ultimate performance limiter in concentrator solar cells?

In principle, Auger recombination should constitute the ultimate limiting mechanism for concentrator photovoltaic performance as optical concentration, and hence current densities, are raised to progressively higher values. In reality, however, the effect will be rate-limiting only in indirect-band-gap solar cells—a point well established for silicon but not yet explored for germanium which comprises the lowest-energy junction in today's ultraefficient commercial multijunction concentrator solar cells. Detailed calculations of when Auger recombination should play a substantial role are presented, and reinforced by existing data.

Narrow band gap GaInNAsSb material grown by metal organic vapor phase epitaxy (MOVPE) for solar cell applications

Bulk, lattice-matched GaInNAsSb material has been grown by metal organic vapor phase epitaxy (MOVPE) for applications in concentrated multi-junction solar cells. The present paper outlines the growth conditions for lattice-matched GaInNAsSb with band gap energies close to 1.0 eV. Metal organic antimony (Sb) precursors, triethyl antimony (TESb) and trimethyl antimony (TMSb), were utilized and incorporation studies were carried out over the growth temperature range of 515–550 °C. High-resolution X-ray diffraction (HRXRD) and photoluminescence (PL) emission spectra were used to optimize the growth conditions and compare the material properties between samples of differing nitrogen (N) content. The impact of a two-stage, post-growth annealing sequence on peak PL intensity was studied with respect to temperature, time (stabilized) and carrier gas overpressure (unstabilized). Optimization of the post-growth annealing conditions of this material system was found to improve the peak PL emission intensity by 7× compared to as-grown material.

Multijunction solar cell model for translating I–V characteristics as a function of irradiance, spectrum, and cell temperature

AbstractWe describe here a lumped diode model for concentrator multijunction solar cells, in which the temperature, irradiance and spectrum dependences are explicitly included. Moreover an experimental method based on it for the prediction of the I‐V curve under any irradiance‐spectrum‐temperature conditions from a single input measurement is proposed and applied to a set of commercial triple‐junction solar cells in order to demonstrate its validity. Component ‘isotype’ cells are used as reference cells for intensity and spectrum, sparing the measurement of light spectrum and cell spectral response. Finally, a mean RMS prediction error of 0.85% over a range of 100X‐25°C to 700X‐75°C is reported for the whole set when the model parameters inherent in the cell are assumed to be the same for every sample. If optimum parameters are extracted for every cell, the RMS error is reduced to 0.53%. Copyright © 2010 John Wiley & Sons, Ltd.

Localized irradiation effects on tunnel diode transitions in multi-junction concentrator solar cells

Multi-junction concentrator solar cells incorporate tunnel diodes that undergo a transition from high-conductance tunneling to low-conductance thermal diffusion behavior, typically at threshold current densities of the order of 10 2–10 3 mA/mm 2. We present experimental evidence of a prominent heretofore unrecognized dependence of threshold current density on the degree of localized irradiation, in different solar cell architectures. We also show that solar cells with non-uniform metallization can exhibit an additional spatial dependence to the tunnel diode threshold current density. These previously undiscovered phenomena – which should be observable in all non-uniformly irradiated photovoltaic tunnel diodes – are interpreted as being derived from the lateral spreading of excess majority carriers (analogous to current spreading in light-emitting diodes (LEDs)). The consequences for concentrator photovoltaics are addressed.

Spectral response and energy output of concentrator multijunction solar cells

AbstractThe spectral response of concentrator multijunction solar cells has been measured over a temperature range of 25–75°C. These data are combined with reference spectra representing the AM1·5 standard as well as annual spectral irradiance at representative geographical locations. The results suggest that higher performance in the field may be obtained if multijunction cells are designed for an effective air mass higher than AM1·5. Copyright © 2008 John Wiley & Sons, Ltd.

New Biaxial Flexure Test Device for Ultrathin Single Crystal Wafers

The demand for germanium (Ge) wafers is rapidly increasing due to the importance of multijunction concentrator solar cells. Because Ge is expensive, the Ge wafers tend to be thin so as to maximize the number of wafers from a given crystal boule. However, their consequent susceptibility to microcracking and fracture during processing reduces the yield and drives up the cost. Wafer manufacturers therefore desire a simple and improved wafer strength testing. In this research, an innovative and inexpensive biaxial flexure testing device was developed. The device provides for a uniform application of very small loads and continuous load‐deflection measurements on full wafers. Experimental data closely matched predicted values. Compared with a traditional three‐point bend test, this method yielded an average flexure strength value of 120 MPa (closer to the predicted values) vs 146–152 MPa from the three‐point method. This result, as well as the fine fracture pattern, suggests that a much larger wafer area is being subjected to uniform loading compared with three‐point bend tests.

Concentrator multijunction solar cell characteristics under variable intensity and temperature

AbstractThe performance of multijunction solar cells has been measured over a range of temperatures and illumination intensities. Temperature coefficients have been extracted for three‐junction cell designs that are in production and under development. A simple diode model is applied to the three‐junction performance as a means to predict performance under operating conditions outside the test range. These data may be useful in guiding the future optimization of concentrator solar cells and systems. Copyright © 2008 John Wiley & Sons, Ltd.

High-flux characterization of ultrasmall multijunction concentrator solar cells

A characterization study of ultrasmall ultraefficient multijunction concentrator solar cells is presented, with emphasis on performance sensitivity to light intensity and distribution. Information of this type is essential in the design and optimization of the latest generations of high-flux photovoltaic systems. Cell miniaturization allows increasing the concentration at which efficiency peaks, facilitates passive heat rejection, and permits the use of all-glass optics. However, few device measurements have been published on ultrasmall cells. Extensive measurements, up to ∼5000 suns, on the 1.0mm2 active region within the busbars are reported and analyzed.

A comparative study of BSF layers for GaAs-based single-junction or multijunction concentrator solar cells

An effective back surface field is a key structural element for a high-efficiency GaAs concentrator solar cell, either in a multijunction or in a single-junction device. In this paper, several BSF materials are analysed, namely: (1) p++GaAs(Zn), (2) p+Ga0.5In0.5P(Zn) and (3) p++Al0.2Ga0.8As(C). The results of the comparison demonstrate that the best option is C-doped Al0.2Ga0.8As, which exhibits a low series resistance and behaves as an excellent minority carrier mirror; p++GaAs(Zn) shows reduced minority carrier mirror properties resulting from Zn diffusion and p+Ga0.5In0.5P(Zn) is shown to produce important series resistance problems because of an unfavourable heterojunction with GaAs.