Development of GaInP/GaInAs/Ge TRIPLE-junction solar cells for CPV applications

La concentración fotovoltaica (CPV) es una de las estrategias más prometedoras para reducir el coste de la electricidad de origen fotovoltaico, y está basada en células multiunión de alta eficiencia. En este contexto, esta Tesis trata sobre el desarrollo de células monolíticas de triple unión (GaInP/Ga(In)As/Ge) para sistemas de CPV. Para ello, se ha transferido una estructura de doble unión de GaInP/GaAs —previamente desarrollada en el grupo de Semiconductores III-V del IESUPM— a un sustrato de Ge. Además, dicha estructura semiconductora se ha seguido desarrollando para poder explotar todo el potencial de la misma como dispositivo fotovoltaico de alta eficiencia. El proceso de desarrollo de la célula de triple unión se ha llevado a cabo de una manera integral, i.e., se han abarcado aspectos desde el punto de vista de la preparación de la superficie de Ge en un reactor MOVPE, hasta la optimización de la célula de triple unión, pasando por el desarrollo independiente de cada una de las subcélulas que conforman la célula de triple unión. En primer lugar, se aborda el crecimiento de semiconductores III-V en sustratos de Ge en un reactor MOVPE. Se lleva a cabo la caracterización de diversas superficies que resultan de interés para el crecimiento epitaxial por MOVPE, incluyendo la superficie de Ge limpia, la terminada con monohidrógeno, la terminada con As y por último la terminada con P. Además, se desarrolla una nueva heteronucleación de GaInP en Ge, que es necesaria para poder optimizar la subcélula de Ge. En ambos aspectos, se presta especial atención a la señal de RAS, que permite un control y caracterización in situ de las superficies de Ge y del crecimiento epitaxial de GaInP en Ge. A continuación, cada subcélula de la célula de triple unión se desarrolla y analiza de manera independiente. En primer lugar, se muestra cómo se crea la unión del Ge durante la heternonucleación de GaInP y se incluye una caracterización de las células de Ge resultantes. Adicionalmente, se aborda la medida de la eficiencia cuántica de la subcélula de Ge en una estructura completa de triple unión, prestando especial atención al efecto no trivial que tiene la baja tensión de ruptura de la célula de Ge. Respecto a la subcélula de Ga(In)As, para mejorar su fotocorriente —respecto la estructura doble previa—, se desarrolla una unión túnel de alto ancho de banda prohibida. Además, se incluye un 1% de In en la estructura para tener una subcélula perfectamente ajustada en red con el sustrato de Ge. Por otro lado, para mejorar el funcionamiento de la subcélula de GaInP, se estudia el crecimiento de GaInP con Sb. De esta manera, su energía de banda prohibida (i.e., su voltaje de circuito abierto) se incrementa, ya que se reduce el orden de tipo CuPt en la subred del grupo III del GaInP. Como dicho orden puede ser controlado con el uso de Sb, se incluye también un estudio del funcionamiento de la subcélula de GaInP en función de su grado de orden. Finalmente, se presentan los cuatro diseños más representativos de las células solares de triple unión desarrolladas en esta Tesis, en donde los desarrollos parciales previamente descritos son introducidos de manera gradual en la estructura semiconductora para poder analizar su impacto en el funcionamiento de la célula solar de triple unión. Las células solares se analizan mediante medidas de eficiencia cuántica, curvas I-V y medidas en concentración. Como resultado final, se ha obtenido una célula solar con una eficiencia calibrada del 39.2% a 400 soles, lo que sitúa la tecnología de célula solar del IES-UPM a niveles internacionales del estado del arte. ABSTRACT Concentrator photovoltaics (CPV) is one of the most promising technologies to reduce the cost of PV-electricity, and it is based on highly efficient multijunction solar cells. Within this framework, this Thesis deals with the development of monolithic GaInP/Ga(In)As/Ge triple-junction solar cells for CPV applications. For that purpose, a GaInP/GaAs dual-junction solar cell structure— previously developed by the III-V Semiconductor Group of the IES-UPM— is transferred to a Ge substrate. Moreover, the semiconductor structure is further developed to fully exploit its potential efficiency conversion. Such process has been undertaken in a comprehensive manner, i.e., it was addressed from the point of view of the Ge surface preparation in a MOVPE reactor to the optimization of the whole triple-junction solar cell, including the independent development of each subcell that constitute the monolithic stack. In the first place, the growth of III-V semiconductors on Ge substrates in a MOVPE reactor is addressed. A characterization of the different Ge surfaces that are of interest for the epitaxial growth by MOVPE is carried out, including the clean, the monohydride-, the As- and the Pterminated Ge surfaces. Besides, a new heteronucleation routine of GaInP on Ge is developed, which is needed to optimize the Ge subcell. In both aspects, special attention is paid to the RAS signal, which allows in situ control and characterization of the Ge surfaces and the GaInP on Ge growth. Then, each subcell of the triple-junction solar cell structure is developed and analyzed separately. Firstly, it is shown how the formation of the Ge bottom cell takes place during the GaInP heteronucleation process and a full characterization of the resulting Ge solar cells performance is also included. Besides, the quantum efficiency measurement of the Ge bottom cell in a complete triple-junction solar cell is addressed, paying special attention to the not-so-trivial effect of the low breakdown voltage of the Ge subcell. Regarding the middle cell, to further increase its photocurrent —regarding the previous dual-junction structure—, a high bandgap tunnel junction is developed. Besides, a 1% In content is included in the semiconductors structure to achieve a middle cell lattice-matched to the Ge substrate. Concerning the GaInP top cell performance, the growth of GaInP with Sb is studied. In this way, its energy bandgap (i.e., its open circuit voltage) is increased by reducing the CuPt-type ordering on the group III-sublattice of the GaInP. As the degree of order is tuned with the use of Sb, the effect of the degree of order in the GaInP top cell performance is also studied. Finally, the four most representative triple-junction solar cells developed within this Thesis are introduced, in which the aforementioned partial developments are gradually implemented in the semiconductor structure to analyze its impact on the triple-junction solar cell performance. The solar cells are analyzed in terms of quantum efficiency, I-V curves and concentration response measurements. As a result, a calibrated efficiency of 39.2% at 400X is attained, which brings IES-UPM triple-junction solar cell technology to state-of-the-art levels

Optical in situ calibration of Sb to grow disordered GaInP by MOVPE

Reflectance anisotropy spectroscopy (RAS) was employed to determine the optimal specific molar flow of Sb needed to grow GaInP with a given order parameter by MOVPE. The RAS signature of GaInP surfaces exposed to different Sb/P molar flow ratios were recorded, and the RAS peak at 3.02 eV provided a feature that was sensitive to the amount of Sb on the surface. The range of Sb/P ratios over which Sb acts as a surfactant was determined using the RA intensity of this peak, and different GaInP layers were grown using different Sb/P ratios. The order parameter of the resulting layers was measured by PL at 20 K. This procedure may be extensible to the calibration of surfactant-mediated growth of other materials exhibiting characteristic RAS signatures

Implications of low breakdown voltage of component subcells on external quantum efficiency measurements of multijunction solar cells

The electrical and optical coupling between subcells in a multijunction solar cell affects its external quantum efficiency (EQE) measurement. In this study, we show how a low breakdown voltage of a component subcell impacts the EQE determination of a multijunction solar cell and demands the use of a finely adjusted external voltage bias. The optimum voltage bias for the EQE measurement of a Ge subcell in two different GaInP/GaInAs/Ge triple-junction solar cells is determined both by sweeping the external voltage bias and by tracing the I–V curve under the same light bias conditions applied during the EQE measurement. It is shown that the I–V curve gives rapid and valuable information about the adequate light and voltage bias needed, and also helps to detect problems associated with non-ideal I–V curves that might affect the EQE measurement. The results also show that, if a non-optimum voltage bias is applied, a measurement artifact can result. Only when the problems associated with a non-ideal I–V curve and/or a low breakdown voltage have been discarded, the measurement artifacts, if any, can be attributed to other effects such as luminescent coupling between subcells

Why can't I measure the external quantum efficiency of the Ge subcell of my multijunction solar cell?

The measurement of the external quantum efficiency (EQE) of low bandgap subcells in a multijunction solar cell can be sometimes problematic. In particular, this paper describes a set of cases where the EQE of a Ge subcell in a conventional GaInP/GaInAs/Ge triple-junction solar cell cannot be fully measured. We describe the way to identify each case by tracing the I-V curve under the same light-bias conditions applied for the EQE measurement, together with the strategies that could be implemented to attain the best possible measurement of the EQE of the Ge subcell

Roadmap towards efficiencies over 40% at ultra-high concentrations (> 1000 suns)

High efficiency solar cells working under ultra-high concentrations (>;1000X) have been shown to be a promising solution to decrease the cost of PV electricity, increase the efficiency and circumvent the material availability restrictions for massive PV penetration. A detailed analysis of the limitations of our current triple junction solar cell (36.2% at 700X), in the quest to maximize efficiency at 1000X, shows that the main improvements to tackle are: a) implementation of a high band gap tunnel junction; b) increase the band gap of the top cell; c) fine current matching tune; d) enhancement of the front contact process. This constitutes our roadmap to reach an efficiency over 41

Capacitance Measurements for Subcell Characterization in Multijunction Solar Cells.

On this paper we present an alternative way to analyze de electronic properties of each subcell from the complete device. By illuminating the cell with light sources which energy is near one of the subcell bandgaps, it is possible to “erase” the presence of such subcell on the CV curve. The main advantages of this technique are that it is not destructive, it can be measured on the complete cell so can be easily implemented as a diagnostic technique for controlling electronic deviations

Status of Ultra-High Concentrator Multijunction Solar Cell Development at IES-UPM.

After the successful implementation of a record performing dual-junction solar cell at ultra high concentration, in this paper we present the optimization of key aspects in the transition to a triple-junction device, namely the hetero nucleation of III-V structures onto germanium substrates. This optimization is based on in-situ RAS measurements during the MOVPE growth of the triple-junction solar cell structure and subsequent AFM analysis. The correlation between RAS and AFM allows detecting which RAS features correlate with good morphology and low RMS roughness. TEM analysis confirms that the quality of the triple-junction structures grown is good, revealing no trace of antiphase disorder, and showing flat, sharp and clear interfaces. Triple-junction solar cells manufactured on these structures have shown a peak efficiency of 36.2% at 700X, maintaining an efficiency over 35% from 300 to 1200 suns

Optical in situ monitoring of hydrogen desorption from Ge(100) surfaces

Molecular hydrogen strongly interacts with vicinal Ge(100) surfaces during preparation in a metal organic vapor phase epitaxy reactor. According to X-ray photoemission spectroscopy and Fourier-transform infrared spectroscopy results, we identify two characteristic reflection anisotropy (RA) spectra for H-free and monohydride-terminated vicinal Ge(100) surfaces. RAS allows in situ monitoring of the surface termination and enables spectroscopic hydrogen kinetic desorption studies on the Ge(100) surface. Comparison of evaluated values for the activation energy and the pre-exponential factor of H desorption evaluated at different photon energies reflects that H unevenly affects the shape of the RA spectrum

Highly conductive p++-AlGaAs/n ++-GaInP tunnel junctions for ultra-high concentrator solar cells

Tunnel junctions are key for developing multijunction solar cells (MJSC) for ultra-high concentration applications. We have developed a highly conductive, high bandgap p  + + -AlGaAs/n  + + -GaInP tunnel junction with a peak tunneling current density for as-grown and thermal annealed devices of 996 A/cm 2 and 235 A/cm 2, respectively. The J–V characteristics of the tunnel junction after thermal annealing, together with its behavior at MJSCs typical operation temperatures, indicate that this tunnel junction is a suitable candidate for ultra-high concentrator MJSC designs. The benefits of the optical transparency are also assessed for a lattice-matched GaInP/GaInAs/Ge triple junction solar cell, yielding a current density increase in the middle cell of 0.506 mA/cm 2 with respect to previous designs

Effect of Sb on the quantum efficiency of GaInP solar cells

The energy bandgap of GaInP solar cells can be tuned by modifying the degree of order of the alloy. In this study, we employed Sb to increase the energy bandgap of the GaInP and analyzed its impact on the performance of GaInP solar cells. An effective change in the cutoff wavelength of the external quantum efficiency of GaInP solar cells and an effective increase of 50 mV in the open-circuit voltage of GaInP/Ga(In)As/Ge triple junction solar cells were obtained with the use of Sb. Copyright © 2016 John Wiley & Sons, Ltd