Back contacted emitter GaAs solar cells

A new device structure to improve the performance of concentrator GaAs solar cells is described and the first experimental results are reported. The reason for such an improvement relies on a drastic reduction of the shadowing and series resistance losses based on the possibility of back contacting the emitter region of the solar cell. The experimental results obtained with devices of these types, with a simplified structure, fabricated by liquid phase epitaxy, demonstrate the feasibility and correct operation of the proposed back contact of the emitter of the cells.

Research on intermediate band solar cells and development of experimental techniques for their characterization under concentrated illumination

Abstract This work is a contribution to the research and development of the intermediate band solar cell (IBSC), a high efficiency photovoltaic concept that features the advantages of both low and high bandgap solar cells. The resemblance with a low bandgap solar cell comes from the fact that the IBSC hosts an electronic energy band -the intermediate band (IB)- within the semiconductor bandgap. This IB allows the collection of sub-bandgap energy photons by means of two-step photon absorption processes, from the valence band (VB) to the IB and from there to the conduction band (CB). The exploitation of these low energy photons implies a more efficient use of the solar spectrum. The resemblance of the IBSC with a high bandgap solar cell is related to the preservation of the voltage: the open-circuit voltage (VOC) of an IBSC is not limited by any of the sub-bandgaps (involving the IB), but only by the fundamental bandgap (defined from the VB to the CB). Nevertheless, the presence of the IB allows new paths for electronic recombination and the performance of the IBSC is degraded at 1 sun operation conditions. A theoretical argument is presented regarding the need for the use of concentrated illumination in order to circumvent the degradation of the voltage derived from the increase in the recombi¬nation. This theory is supported by the experimental verification carried out with our novel characterization technique consisting of the acquisition of photogenerated current (IL)-VOC pairs under low temperature and concentrated light. Besides, at this stage of the IBSC research, several new IB materials are being engineered and our novel character¬ization tool can be very useful to provide feedback on their capability to perform as real IBSCs, verifying or disregarding the fulfillment of the “voltage preservation” principle. An analytical model has also been developed to assess the potential of quantum-dot (QD)-IBSCs. It is based on the calculation of band alignment of III-V alloyed heterojunc-tions, the estimation of the confined energy levels in a QD and the calculation of the de¬tailed balance efficiency. Several potentially useful QD materials have been identified, such as InAs/AlxGa1-xAs, InAs/GaxIn1-xP, InAs1-yNy/AlAsxSb1-x or InAs1-zNz/Alx[GayIn1-y]1-xP. Finally, a model for the analysis of the series resistance of a concentrator solar cell has also been developed to design and fabricate IBSCs adapted to 1,000 suns. Resumen Este trabajo contribuye a la investigación y al desarrollo de la célula solar de banda intermedia (IBSC), un concepto fotovoltaico de alta eficiencia que auna las ventajas de una célula solar de bajo y de alto gap. La IBSC se parece a una célula solar de bajo gap (o banda prohibida) en que la IBSC alberga una banda de energía -la banda intermedia (IB)-en el seno de la banda prohibida. Esta IB permite colectar fotones de energía inferior a la banda prohibida por medio de procesos de absorción de fotones en dos pasos, de la banda de valencia (VB) a la IB y de allí a la banda de conducción (CB). El aprovechamiento de estos fotones de baja energía conlleva un empleo más eficiente del espectro solar. La semejanza antre la IBSC y una célula solar de alto gap está relacionada con la preservación del voltaje: la tensión de circuito abierto (Vbc) de una IBSC no está limitada por ninguna de las fracciones en las que la IB divide a la banda prohibida, sino que está únicamente limitada por el ancho de banda fundamental del semiconductor (definido entre VB y CB). No obstante, la presencia de la IB posibilita nuevos caminos de recombinación electrónica, lo cual degrada el rendimiento de la IBSC a 1 sol. Este trabajo argumenta de forma teórica la necesidad de emplear luz concentrada para evitar compensar el aumento de la recom¬binación de la IBSC y evitar la degradación del voltage. Lo anterior se ha verificado experimentalmente por medio de nuestra novedosa técnica de caracterización consistente en la adquisicin de pares de corriente fotogenerada (IL)-VOG en concentración y a baja temperatura. En esta etapa de la investigación, se están desarrollando nuevos materiales de IB y nuestra herramienta de caracterizacin está siendo empleada para realimentar el proceso de fabricación, comprobando si los materiales tienen capacidad para operar como verdaderas IBSCs por medio de la verificación del principio de preservación del voltaje. También se ha desarrollado un modelo analítico para evaluar el potencial de IBSCs de puntos cuánticos. Dicho modelo está basado en el cálculo del alineamiento de bandas de energía en heterouniones de aleaciones de materiales III-V, en la estimación de la energía de los niveles confinados en un QD y en el cálculo de la eficiencia de balance detallado. Este modelo ha permitido identificar varios materiales de QDs potencialmente útiles como InAs/AlxGai_xAs, InAs/GaxIni_xP, InAsi_yNy/AlAsxSbi_x ó InAsi_zNz/Alx[GayIni_y]i_xP. Finalmente, también se ha desarrollado un modelado teórico para el análisis de la resistencia serie de una célula solar de concentración. Gracias a dicho modelo se han diseñado y fabricado IBSCs adaptadas a 1.000 soles.

Advances in the modeling, characterization and reliability of concentrator multijunction solar cells

Los sistemas de concentración fotovoltaica (CPV) parecen ser una de las vías más prometedoras para generar electricidad a gran escala a precios competitivos. La investigación actual se centra en aumentar la eficiencia y la concentración de los sistemas para abaratar costes. Al mismo tiempo se investiga sobre la fiabilidad de los diferentes componentes que integran un sistema de concentración, ya que para que los sistemas de concentración sean competitivos es necesario que tengan una fiabilidad al menos similar a los sistemas basados en células de silicio. En la presente tesis doctoral se ha llevado a cabo el estudio de aspectos avanzados de células solares multi-unión diseñadas para trabajar a concentraciones ultra-altas. Para ello, se ha desarrollado un modelo circuital tridimensional distribuido con el que simular el comportamiento de las células solares triple-unión bajo distintas condiciones de funcionamiento, así mismo se ha realizado una caracterización avanzada de este tipo de células para comprender mejor su modo de operación y así poder contribuir a mejorar su eficiencia. Finalmente, se han llevado a cabo ensayos de vida acelerados en células multiunión comerciales para conocer la fiabilidad de este tipo de células solares. Para la simulación de células solares triple-unión se ha desarrollado en la presente tesis doctoral un modelo circuital tridimensinal distribuido el cuál integra una descripción completa de la unión túnel. De este modo, con el modelo desarrollado, hemos podido simular perfiles de luz sobre la célula solar que hacen que la densidad de corriente fotogenerada sea mayor a la densidad de corriente pico de la unión túnel. El modelo desarrollado también contempla la distribución lateral de corriente en las capas semiconductoras que componen y rodean la unión túnel. Por tanto, se ha podido simular y analizar el efecto que tiene sobre el funcionamiento de la célula solar que los concentradores ópticos produzcan perfiles de luz desuniformes, tanto en nivel de irradiancia como en el contenido espectral de la luz (aberración cromática). Con el objetivo de determinar cuáles son los mecanismos de recombinación que están limitando el funcionamiento de cada subcélula que integra una triple-unión, y así intentar reducirlos, se ha llevado a cabo la caracterización eléctrica de células solares monouni ón idénticas a las subcelulas de una triple-unión. También se ha determinado la curva corriente-tensión en oscuridad de las subcélulas de GaInP y GaAs de una célula dobleunión mediante la utilización de un teorema de reciprocidad electro-óptico. Finalmente, se ha analizado el impacto de los diferentes mecanismos de recombinación en el funcionamiento de la célula solar triple-unión en concentración. Por último, para determinar la fiabilidad de este tipo de células, se ha llevado a cabo un ensayo de vida acelerada en temperatura en células solares triple-unión comerciales. En la presente tesis doctoral se describe el diseño del ensayo, el progreso del mismo y los datos obtenidos tras el análisis de los resultados preliminares. Abstract Concentrator photovoltaic systems (CPV) seem to be one of the most promising ways to generate electricity at competitive prices. Nowadays, the research is focused on increasing the efficiency and the concentration of the systems in order to reduce costs. At the same time, another important area of research is the study of the reliability of the different components which make up a CPV system. In fact, in order for a CPV to be cost-effective, it should have a warranty at least similar to that of the systems based on Si solar cells. In the present thesis, we will study in depth the behavior of multijunction solar cells under ultra-high concentration. With this purpose in mind, a three-dimensional circuital distributed model which is able to simulate the behavior of triple-junction solar cells under different working conditions has been developed. Also, an advanced characterization of these solar cells has been carried out in order to better understand their behavior and thus contribute to improving efficiency. Finally, accelerated life tests have been carried out on commercial lattice-matched triple-junction solar cells in order to determine their reliability. In order to simulate triple-junction solar cells, a 3D circuital distributed model which integrates a full description of the tunnel junction has been developed. We have analyzed the behavior of the multijunction solar cell under light profiles which cause the current density photo-generated in the solar cell to be higher than the tunnel junction’s peak current density. The advanced model developed also takes into account the lateral current spreading through the semiconductor layers which constitute and surround the tunnel junction. Therefore, the effects of non-uniform light profiles, in both irradiance and the spectral content produced by the concentrators on the solar cell, have been simulated and analyzed. In order to determine which recombination mechanisms are limiting the behavior of each subcell in a triple-junction stack, and to try to reduce them when possible, an electrical characterization of single-junction solar cells that resemble the subcells in a triplejunction stack has been carried out. Also, the dark I-V curves of the GaInP and GaAs subcells in a dual-junction solar cell have been determined by using an electro-optical reciprocity theorem. Finally, the impact of the different recombination mechanisms on the behavior of the triple-junction solar cell under concentration has been analyzed. In order to determine the reliability of these solar cells, a temperature accelerated life test has been carried out on commercial triple-junction solar cells. In the present thesis, the design and the evolution of the test, as well as the data obtained from the analysis of the preliminary results, are presented.

Voltage recovery in intermediate band solar cells

The intermediate band solar cell (IBSC) is based on a novel photovoltaic concept and has a limiting efficiency of 63.2%, which compares favorably with the 40.7% efficiency of a conventional, single junction solar cell. It is characterized by a material hosting a collection of energy levels within its bandgap, allowing the cell to exploit photons with sub-bandgap energies in a two-step absorption process, thus improving the utilization of the solar spectrum. However, these intermediate levels are often regarded as an inherent source of supplementary recombination, although this harmful effect can in theory be counteracted by the use of concentrated light. We present here a novel, low-temperature characterization technique using concentrated light that reveals how the initially enhanced recombination in the IBSC is reduced so that its open-circuit voltage is completely recovered and reaches that of a conventional solar cell.

Understanding intermediate-band solar cells

The intermediate-band solar cell is designed to provide a large photogenerated current while maintaining a high output voltage. To make this possible, these cells incorporate an energy band that is partially filled with electrons within the forbidden bandgap of a semiconductor. Photons with insufficient energy to pump electrons from the valence band to the conduction band can use this intermediate band as a stepping stone to generate an electron-hole pair. Nanostructured materials and certain alloys have been employed in the practical implementation of intermediate-band solar cells, although challenges still remain for realizing practical devices. Here we offer our present understanding of intermediate-band solar cells, as well as a review of the different approaches pursed for their practical implementation. We also discuss how best to resolve the remaining technical issues.

Symmetry considerations in the empirical k.p Hamiltonian for the study of intermediate band solar cells

With the purpose of assessing the absorption coefficients of quantum dot solar cells, symmetry considerations are introduced into a Hamiltonian whose eigenvalues are empirical. In this way, the proper transformation from the Hamiltonian's diagonalized form to the form that relates it with Γ-point exact solutions through k.p envelope functions is built accounting for symmetry. Forbidden transitions are thus determined reducing the calculation burden and permitting a thoughtful discussion of the possible options for this transformation. The agreement of this model with the measured external quantum efficiency of a prototype solar cell is found to be excellent.

Numerical simulation of Ge solar cells using D-AMPS-1D code

A solar cell is a solid state device that converts the energy of sunlight directly into electricity by the photovoltaic effect. When light with photon energies greater than the band gap is absorbed by a semiconductor material, free electrons and free holes are generated by optical excitation in the material. The main characteristic of a photovoltaic device is the presence of internal electric field able to separate the free electrons and holes so they can pass out of the material to the external circuit before they recombine. Numerical simulation of photovoltaic devices plays a crucial role in their design, performance prediction, and comprehension of the fundamental phenomena ruling their operation. The electrical transport and the optical behavior of the solar cells discussed in this work were studied with the simulation code D-AMPS-1D. This software is an updated version of the one-dimensional (1D) simulation program Analysis of Microelectronic and Photonic Devices (AMPS) that was initially developed at The Penn State University, USA. Structures such as homojunctions, heterojunctions, multijunctions, etc., resulting from stacking layers of different materials can be studied by appropriately selecting characteristic parameters. In this work, examples of cells simulation made with D-AMPS-1D are shown. Particularly, results of Ge photovoltaic devices are presented. The role of the InGaP buffer on the device was studied. Moreover, a comparison of the simulated electrical parameters with experimental results was performed.

Impact of MOVPE Environment on Silicon Substrates for III-V-on-Si Multijunction Solar Cells

With the final goal of integrating III-V materials to silicon for tandem solar cells, the influence of the metal-organic vapor phase epitaxy (MOVPE) environment on the minority carrier properties of silicon wafers has been evaluated. These properties will essentially determine the photovoltaic performance of the bottom cell in a III-V-on-Si tandem solar cell device. A comparison of the base minority carrier lifetimes obtained for different thermal processes carried out in a MOVPE reactor on Czochralski silicon wafers has been carried out. The effect of the formation of the emitter by phosphorus diffusion has also been evaluated.

Extended description of tunnel junctions for distributed modeling of concentrator multi-junction solar cells

One of the key components of highly efficient multi-junction concentrator solar cells is the tunnel junction interconnection. In this paper, an improved 3D distributed model is presented that considers real operation regimes in a tunnel junction. This advanced model is able to accurately simulate the operation of the solar cell at high concentraions at which the photogenerated current surpasses the peak current of the tunnel junctionl Simulations of dual-junction solar cells were carried out with the improved model to illustrate its capabilities and the results have been correlated with experimental data reported in the literature. These simulations show that under certain circumstances, the solar cells short circuit current may be slightly higher than the tunnel junction peak current without showing the characteristic dip in the J-V curve. This behavior is caused by the lateral current spreading toward dark regions, which occurs through the anode/p-barrier of the tunnel junction.

Understanding phosphorus diffusion into silicon in a MOVPE environment for III-V on silicon solar cells

Dual-junction solar cells formed by a GaAsP or GaInP top cell and a silicon bottom cell seem to be attractive candidates to materialize the long sought-for integration of III?V materials on silicon for photovoltaic applications. When manufacturing a multi-junction solar cell on silicon, one of the first processes to be addressed is the development of the bottom subcell and, in particular, the formation of its emitter. In this study, we analyze, both experimentally and by simulations, the formation of the emitter as a result of phosphorus diffusion that takes place during the first stages of the epitaxial growth of the solar cell. Different conditions for the Metal-Organic Vapor Phase Epitaxy (MOVPE) process have been evaluated to understand the impact of each parameter, namely, temperature, phosphine partial pressure, time exposure and memory effects in the final diffusion profiles obtained. A model based on SSupremIV process simulator has been developed and validated against experimental profiles measured by ECV and SIMS to calculate P diffusion profiles in silicon formed in a MOVPE environment taking in consideration all these factors.

On-site measurement of limiting subcell in multijunction solar devices

It is well known that the response of any photovoltaic solar cell is dependent on the spectral characteristics of the incident radiation. This dependency is crucial in the output characteristics of a multijunction (MJ) cell where the spectral composition of the radiation determines the overall photocurrent produced, as either the top or the middle subcell will be limiting its response. The current mismatching between top and middle subcell is translated into energy losses, affecting the yield of the system. For research and commercial purposes it is interesting to measure accurately the incident solar radiation on a MJ cell, in terms of its spectral composition. This measurement will allows us to determine the photocurrent generated in each band of the multijunction device. Nowadays, the only way of measuring the photocurrent generated by each subcell is done with isotype cells or with spectroradiometers but there is no device capable of directly measuring each subcell photocurrent. In this paper it is described a device based on a commercial multijunction solar cell that is capable of measuring the direct irradiance for the top and middle bands thus it offers information of the limiting subcell (top or middle) in outdoors conditions.

Nano-imprinted rear-side diffraction gratings for absorption enhancement in solar cells

As wafer-based solar cells become thinner, light-trapping textures for absorption enhancement will gain in importance. In this work, crystalline silicon wafers were textured with wavelength-scale diffraction grating surface textures by nanoimprint lithography using interference lithography as a mastering technology. This technique allows fine-tailored nanostructures to be realized on large areas with high throughput. Solar cell precursors were fabricated, with the surface textures on the rear side, for optical absorption measurements. Large absorption enhancements are observed in the wavelength range in which the silicon wafer absorbs weakly. It is shown experimentally that bi-periodic crossed gratings perform better than uni-periodic linear gratings. Optical simulations have been made of the fabricated structures, allowing the total absorption to be decomposed into useful absorption in the silicon and parasitic absorption in the rear reflector. Using the calculated silicon absorption, promising absorbed photocurrent density enhancements have been calculated for solar cells employing the nano-textures. Finally, first results are presented of a passivation layer deposition technique that planarizes the rear reflector for the purpose of reducing the parasitic absorption.

Effect of concentration on the performance of quantum dot intermediate-band solar cells

Implementation of a high-efficiency quantum dot intermediate-band solar cell (QD-IBSC) must accompany a sufficient photocurrent generation via IB states. The demonstration of a QD-IBSC is presently undergoing two stages. The first is to develop a technology to fabricate high-density QD stacks or a superlattice of low defect density placed within the active region of a p-i-n SC, and the second is to realize half-filled IB states to maximize the photocurrent generation by two-step absorption of sub-bandgap photons. For this, we have investigated the effect of light concentration on the characteristics of QDSCs comprised of multi-layer stacks of self-organized InAs/GaNAs QDs grown with and without impurity doping in molecular beam epitaxy.

The unstoppable rise of photovoltaic solar energy: PV vs. PV

Photovoltaic (PV) solar energy has been growing during the last decade an explosive rate. Last year (2011) the solar cell production amounted to more than 37 GW. It is the energy technology most installed nowadays. The power generated by the 37 GW is similar to the one generated by about 7 nuclear units of 1 GW each. The solar industry is already a huge industry dominated by Asian countries led by China. It is not anymore a promise. It is just a reality.

ARXPS analysis of a GaAs/GaInP heterointerface with application in III-V multijunction solar cells

In this contribution, angle-resolved X-ray photoelectron spectroscopy is used to explore the extension and nature of a GaAs/GaInP heterointerface. This bilayer structure constitutes a very common interface in a multilayered III-V solar cell. Our results show a wide indium penetration into the GaAs layer, while phosphorous diffusion is much less important. The physico-chemical nature of such interface and its depth could deleteriously impact the solar cell performance. Our results probe the formation of spurious phases which may profoundly affect the interface behavior.