Elisa García-Tabarés

Influence of PH3 exposure on silicon substrate morphology in the MOVPE growth of III–V on silicon multijunction 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. One of the first issues to be considered in the development of this structure will be the strategy to create the silicon emitter of the bottom subcell. In this study, we explore the possibility of forming the silicon emitter by phosphorus diffusion (i.e. exposing the wafer to PH3 in a MOVPE reactor) and still obtain good surface morphologies to achieve a successful III–V heteroepitaxy as occurs in conventional III–V on germanium solar cell technology. Consequently, we explore the parameter space (PH3 partial pressure, time and temperature) that is needed to create optimized emitter designs and assess the impact of such treatments on surface morphology using atomic force microscopy. Although a strong degradation of surface morphology caused by prolonged exposure of silicon to PH3 is corroborated, it is also shown that subsequent anneals under H2 can recover silicon surface morphology and minimize its RMS roughness and the presence of pits and spikes.

Optimization of the silicon subcell for III-V on silicon multijunction solar cells: Key differences with conventional silicon technology

Dual-junction solar cells formed by a GaAsP or GaInP top cell and a silicon (Si) bottom cell seem to be attractive candidates to materialize the long sought-for integration of III-V materials on Si for photovoltaic (PV) applications. Such integration would offer a cost breakthrough for PV technology, unifying the low cost of Si and the efficiency potential of III-V multijunction solar cells. The optimization of the Si solar cells properties in flat-plate PV technology is well-known; nevertheless, it has been proven that the behavior of Si substrates is different when processed in an MOVPE reactor In this study, we analyze several factors influencing the bottom subcell performance, namely, 1) the emitter formation as a result of phosphorus diffusion; 2) the passivation quality provided by the GaP nucleation layer; and 3) the process impact on the bottom subcell PV properties.

Numerical simulation and experimental facts about bottom-cell optimization for III-V on Silicon multijunction solar cells

This work reviews both theoretically - using numerical simulations with Silvaco TCAD- and experimentally several key features for the design and optimization of the Si bottom subcell for its integration on III-V on Si multijunction solar cells. We have focused on 1) the optimization of the p-n junction, i.e. the emitter and base configurations; 2) the impact of the BSF layer and other alternatives - point-contact rear metallization - for back surface passivation; and 3) the role of the GaP/Si interface in the device performance.

Optimizing bottom subcells for III-V-on-Si multijunction 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. Such integration would offer a cost breakthrough for photovoltaic technology, unifying the low cost of silicon and the efficiency potential of III-V multijunction solar cells. In this study, we analyze several factors influencing the performance of the bottom subcell of this dual-junction, namely, 1) the formation of the emitter as a result of the phosphorus diffusion that takes place during the prenucleation temperature ramp and during the growth of the III-V layers; 2) the degradation in surface morphology during diffusion; and 3) the quality needed for the passivation provided by the GaP layer on the emitter.

Impact of a Metal–Organic Vapor Phase Epitaxy 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.

Assessment of Rear-Surface Processing Strategies for III–V on Si Multijunction Solar Cells Based on Numerical Simulations

The manufacturing of high-efficiency III-V on Si multijunction solar cells needs the development of hybrid, i.e., adapted to both families of materials, solar cell processing techniques, able to extract the full photovoltaic potential of both the subcells. This fact especially impacts the processing of the silicon rear surface of the tandem, which cannot receive treatments commonly used in the single-junction Si solar cell industry [Al-back surface field (BSF), thermal SiO2, and so on], since these would result in an excessive thermal load that would deteriorate the III-V upper layers (top cell, tunnel junction, and buffer layer). However, the Si bottom cell requires an advanced design with good rear passivation, a good ohmic contact, and good carrier selectivity, so that its contribution to the efficiency of the tandem is maximized. Accordingly, in this paper, several low-temperature compatible rear-surface passivation techniques for the Si bottom subcell in a monolithic III-V/Si tandem solar cell are explored. In particular, aluminum BSFs, passivated emitter and rear cell (PERC)-like architecture, passivated emitter and rear locally diffused (PERL)-like architecture formed with low thermal loads, and heterojunction with intrinsic thin layer (HIT)-like processes are assessed using numerical simulations, and a comparison of the Si bottom cell performance for the mentioned alternatives in a GaAsP/Si dual-junction solar cell is presented.

Evolution of the silicon bottom cell photovoltaic behavior during III–V on Si multi-junction solar cells production

The evolution of the Si bulk minority carrier lifetime during the heteroepitaxial growth of III-V on Si multi-junction solar cell structures via metal-organic vapor phase epitaxy has been analyzed. Initially, the emitter formation produces important lifetime degradation. Nevertheless, a progressive recovery was observed during the growth of the metamorphic GaAsP/Si structure. A step-wise mechanism has been proposed to explain the lifetime evolution observed during this process. The initial lifetime degradation is believed to be related to the formation of thermally-induced defects within the Si bulk. These defects are subsequently passivated by fast-diffusing atomic hydrogen -coming from precursor (i.e. PH3 and AsH3) pyrolysis- during the subsequent III-V growth. These results indicate that the MOVPE environment used to create the III-V/Si solar cell structures has a dynamic impact on the minority carrier lifetime. Consequently, designing processes that promote the recovery of the lifetime is a must to support the production of high-quality III-V/Si solar cells.

Triple-junction solar cells for ultra-high concentrator applications

After the successful implementation of a record performing dual-junction solar cell at ultra high concentration, in this paper we present the transition to a triple-junction device. The semiconductor structure of the solar cells is presented and the main changes in respect to a dual-junction design are briefly discussed. Cross-sectional TEM analysis of samples confirms that the quality of the triple-junction structures grown by MOVPE is good, revealing no trace of antiphase disorder, and showing flat, sharp and clear interfaces between the layers. Triple-junction solar cells manufactured on these structures have shown a peak efficiency of 36.2% at 700X, maintaining the efficiency over 35% from 300 to 1200 suns. With some changes in the structure and a fine tuning of its processing, efficiencies close to 40% at 1000 suns are envisaged.

Optimizing diffusion, morphology and minority carrier lifetime in Silicon for GaAsP/Si dual-junction solar cells

Dual-junction solar cells formed by a GaAsP cell on a silicon bottom cell seem to be attractive candidates to materialize the long sought-for integration of III-V materials on Si for photovoltaic applications. In this study, we analyze several factors for the optimization of the bottom cell, namely, 1) the emitter formation as a result of phosphorus diffusion; 2) the growth of a high quality GaP nucleation layer; and 3) the process impact on the bottom subcell PV properties.

Alternatives for rear-surface passivation in III–V on Si multi-junction solar cells

In the recent developments in the field of III-V on Si dual-junction solar cells, low attention has been paid to optimizing the device configuration to maximize its photovoltaic performance. The few practical implementations reported heretofore have been based on III-V solar cell processing techniques. Although the fabrication of conventional Si structures is a well-known technology, certain steps have been found to be incompatible with III-V semiconductors, primarily due to their high thermal load. Accordingly, in this work we discuss the applicability of different alternatives for the Si rear-surface passivation (Al-BSF, PERC- and HIT-like schemes) in III-V/Si dual-junction solar cell structures. Using numerical simulations, a comparison of the Si bottom cell performance in a GaAsP/Si dual-junction solar cell structure is presented for the mentioned alternatives.