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International audience; We present a new process to deposit thick front emitter contact on multijunction solar cells using a self‐powered light‐induced plating (SP‐LIP). This electrochemical deposition method is based on the use of the light‐induced polarization of the cell to drive the plating current instead of the use of an external power supply as for standard electroplating. The electrical characterization of the process is performed by current and potential measurements at various light intensities and enables the deposition of thick layers of various metals (Ag, Ni, and Au) on triple‐junction solar cells with a precise control of the metal thickness. The potential of SP‐LIP for the deposition of an ohmic contact layer directly on the solar cell front side without any seed layer is also demonstrated. A triple‐junction solar cell fabricated with both ohmic and thick metal front contact deposited only by an electrochemical method is reported.

International audience; Multijunction solar cells must be electrically isolated from one to another at the end of the fabrication process; a step known as mesa isolation. In this study, three different techniques are assessed to perform this step: saw-dicing, wet etching and plasma etching. Triple junction solar cells were fabricated with each process and the open-circuit voltages were measured in order to compare the impact of each technique on the device performance. An optional wet treatment is also proposed to clean the sidewalls after the mesa isolation process. The mesa sidewalls were characterized by scanning electron microscopy and atomic force microscopy to assess the profile length and roughness respectively. The cell performance was then correlated to the sidewall length and roughness for all three isolation techniques. This study indicates that a plasma etching process followed by a wet clean is the process that maximizes the solar cell performance, thanks to a short profile length

The use of III-V and group IV compounds in the same heterostructure is of great interest for high performances solar cells under concentration. In fact, the combination of these III-V and group IV compounds can lead to interesting strategic bandgap choices and engineering to better match the absorption of the solar spectrum, and therefore better solar cell performance. The series-connected quad-junctions (4J) solar cell strategies have the potential to improve solar cells performance and therefore enable low-cost concentrator photovoltaic (CPV) systems, allowing lower levelized cost of electricity (LCOE) from a CPV system. This work presents the investigation of the performance of dual junction (2J) GaInP/GaAs that might be implemented as upper cells with group IV (SiGeSn) cells as bottom cells. The aim of this study is to validate the epitaxial structure and the fabrication process for future 4J cells development. Pitch is varied from 125 μm to 400 μm for two different size of cells, in order to optimize solar cells performance under concentration (X) In the range of 100X to 1000X. Solar cells demonstrated high fill factor (FF) values and ideality factors (n) approaching unity per subcell have been obtained in the range of 100X to 500X. A FF of 85% and 88% are obtained at a concentration of 1000X for the bigger and smaller cells respectively, for the narrowest pitch. These results close to the state-of-the-art are encouraging for the implementation of this 2J with IV bottom subcell for the purpose of high performance 4J.

Series-connected four (quad) junctions (4J) resulting from the combination of III-V and group IV materials have the potential to improve solar cells efficiency under concentration (X). In this work, we investigate the performance of dual junction (2J) GaInP/GaAs that might be used with group IV (SiGeSn) cells as bottom cells, to validate the epitaxial structure and the fabrication process of future 4J cells. We vary gridline spacings for two different solar cells sizes, to optimize solar cells performance in the range of 100X to 1000X. The obtained results are close to the state-of-the-art and are promising for high performance 4J.

We demonstrate the feasibility approach to deploy two types of dual junction GaInP/Ge and GaAs/Ge solar cells in a hybrid photovoltaic-solar thermal concentrator system, where they can operate at 250°C and at medium light concentration. These devices were designed to transfer absorbed heat to tube with a fluid, using high thermal conductivity of Ge substrate. Both heterostructures showed no significant degradation after a high temperature thermal storage of over 7 months long. Regarding their performance, the GaAs/Ge dual junction showed ∼1% higher absolute efficiency than the InGaP/Ge, at 250°C and 170 suns.

The main parameters of InGaP and GaAs thin solar cells (SCs) at average light concentration ratios (X) of ~54 and ~93 suns were investigated in the temperature range 25-250 °C. The main parameters of the two devices showed quasi-linear behaviors with increasing the operating temperature. The conversion efficiencies were found to drop ~27 and ~40 % in InGaP and GaAs, respectively independently of the two concentration ratios. Furthermore, the two devices showed a decrease in output power averagely ranging from 25 to 27 %, which yields to a difference less than 2%. In term of thermal reliability, the two devices did not show significant degradation after approximately 4 months of heat dumping. Hence, these results imply that GaAs still deliver more output power at 250 °C.

A1. Carbon incorporation A3. Chemical beam epitaxy B1. AlGaAs B3. Solar cells B3. Tunnel junction abstract Trimethyl aluminium (TMA) was used as an intrinsic dopant source to grow highly p-doped AlGaAs by chemical beam epitaxy (CBE). Growth parameters were varied to control doping level, and three sets of growth parameters were identified to maximize the hole concentration in CBE-grown AlGaAs: low temperature growth; low V/III ratio combined with high growth rate; aluminium-rich composition. AlGaAs/GaAs tunnel junctions were fabricated using each of these set of growth parameters and tunneling peak currents as high as 6136 A/cm 2 were obtained. These tunnel junctions are suitable for use

The design of antireflection coating (ARC) for multijunction solar cells is challenging due to the broadband absorption and the need for current matching of each subcell. Silicon nitride, which is deposited by plasma-enhanced chemical vapor deposition (PECVD) using standard conditions, is widely used in the silicon wafer solar cell industry but typically suffers from absorption in the short-wavelength range. We propose the use of silicon nitride deposited by low-frequency PECVD (LFSiN) optimized for high refractive index and low optical absorption as a part of the ARC design for III–V/Ge triple-junction solar cells. This material can also act as a passivation/encapsulation coating. Simulations show that the SiO $_{{\rm 2}}$ /LFSiN double-layer ARC can be very effective in reducing the reflection losses over the wavelength range of the limiting subcell for top subcell-limited, as well as middle subcell-limited, triple-junction solar cells. We also demonstrate that the structure’s performance is stable over expected variations in the layer parameters (thickness and refractive index) in the vicinity of the optimal values.

Four 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

In order to create suitable nanoholes for quantum dot (QD) localization on InP and GaAs surfaces, we used atomic force microscopy in an intermittent contact mode coupled with a modulated voltage to realized local anodization at a nanometer scale. This method leads, after a few tens of milliseconds of oxidation, to an oxide height saturation and a low lateral growth rate for both surfaces. These specific results were used to control separately both the depth and the diameter of holes and to obtain compatible pattern for QD growth. We also demonstrated the thermal stability of these patterns at compatible temperatures with the InAs QD growth. First, results of QD growth on these patterns are presented.

Microfabrication cycles of III-V multijunction solar cells include several technological steps and end with a wafer dicing step to separate individual cells. This step introduces damage at lateral facets of the junctions that act as charge trapping centers, potentially causing performance and reliability issues, which become even more important with today’s trend of cell size reduction. In this paper we propose a process of wet etching of microtrenches that allows electrical isolation of individual solar cells with no damage to the sidewalls. Etching with bromine-methanol, the solution that is typically used for nonselective etching of III-V compounds, results in the formation of unwanted holes on the semiconductor surfaces. We investigate the origin of holes formation and discuss methods to overcome this effect. We present an implementation of the isolation step into a solar cell fabrication process flow. This improved fabrication process opens the way for improved die strength, yield, and reliability.

AlGaAs/GaAs tunnel junctions for use in high concentration multijunction solar cells were designed and grown by chemical beam epitaxy (CBE) using trimethyl aluminium (TMA) as the p-dopant source for the AlGaAs active layer. Controlled hole concentration up to 4⋅1020 cm−3 was achieved through variation in growth parameters. Fabricated tunnel junctions have a peak tunneling current up to 6140 A/cm2. These are suitable for high concentration use and outperform GaAs/GaAs tunnel junctions.

The current density-voltage characteristic of an AlGaAs/AlGaAs tunnel junction is determined by taking a time-averaged measurement across the device. A tunnelling peak of ∼950A/cm 2 is recorded by this method. Measurements of the tunnelling peak and valley currents by the time averaging method are obscured due to the unstable nature of the negative differential resistance region of the current density-voltage characteristic. This AlGaAs/AlGaAs tunnel junction is then biased inside the negative differential resistance region of the current density-voltage characteristic, causing the current and the voltage to oscillate between the peak and the valley. The current and voltage oscillations are measured over time and then currents and voltages corresponding to the same time stamps are plotted against each other to form a time-dependent curve from which a tunnelling peak of a value larger than 1100A/cm 2 is determined. The peak determined by this method is 11-20% larger than previously determined using the time averaged measurement. An AlGaAs/InGaP tunnel junction having no negative differential resistance region is also presented.

The following four TJ designs, AlGaAs/AlGaAs, GaAs/GaAs, AlGaAs/InGaP and AlGaAs/GaAs are studied to determine minimum doping concentration to achieve a resistance of 5×10−4 ωcm2.

High concentration solar applications require appropriate thermal management to prevent cell efficiency losses due to overheating. Cell carrier thermal characteristics play a role in the temperature of the solar cell. Thermal analysis of a conventional, ceramic‐based, cell carrier shows that it is suitable for high concentration applications but is not optimal for heat transfer. Calibrated 3D finite element simulations at 280 suns show that a metallic carrier produced an increase in cell temperature that is 24.4 K less than for the ceramic carrier. An enhanced metallic carrier was introduced and fabricated. Electric breakdown tests showed it can withstand a voltage of at least 1000 V. Results of temperature profiling on the metallic carrier’s surface showed a smaller ΔT, indicating an improved heat distribution.

AlGaAs tunnel junctions are shown to be well-suited to concentrated photovoltaics where temperatures and current densities can be dramatically higher than for 1-sun flat-panel systems. Detailed comparisons of AlGaAs/AlGaAs tunnel junction experimental measurements over a range of temperatures expected during device operation in concentrator systems are presented. Experimental and simulation results are compared in an effort to decouple the tunnel junction from the overall multi-junction solar cell. The tunnel junction resistance is experimentally studied as a function of the temperature to determine its contribution to overall efficiency of the solar cell. The current-voltage behavior of the isolated TJ shows that as the temperature is increased from 25°C to 85°C, the resistance decreases from ∼4.7×10−4 Ω·cm2 to ∼0.3×10−4 Ω·cm2 for the operational range of a multi-junction solar cell under concentration.

We present the method of site-controlled growth of InAs quantum dots on InP(001) by solid-source molecular beam epitaxy. InAs dots are positioned using nanopatterns realized by electron beam lithography and dry etching. We have obtained the localized InAs dots for InAs deposit thickness inferior to the critical thickness for the two-dimensional/three-dimensional growth mode transition measured on a flat InP surface, implying that the dots can be actively positioned at predefined nucleation sites. Photoluminescence results show the emission of localized InAs dots on patterns overgrown with a thin InP buffer layer, at a wavelength around 1.5 μm at room temperature.

The localization of InAs quantum dots on nanostructured InP(001) surfaces is achieved. The sites for QDs nucleation are nanoholes defined by e-beam lithography. Photoluminescence results of QD area are exposed. Nanosource fabrication is described.

This work is devoted lo the localization of InAs quantum dots (QDs) grown by solid-source molecular beam epitaxy on nanostructured InP(001) surfaces, which will find application in quantum or classical devices operating at the wavelength of 1.55 /spl mu/m. The dots are grown on the mesas or in the holes, which are designed to be the sites of privileged nucleation of the quantum dots.

This work is devoted to the localization of InAs quantum dots grown by SSMBE on nanostructured InP(001) surfaces: mesas or holes are the sites of privileged nucleation of the quantum dots (QDs). Both nanostructuration methods are compared. The principle of fabrication of a photonic crystal cavity based source with localized QDs is described.

Low-energy (18 keV) phosphorus ion implantation and rapid thermal annealing at 650 °C for 120 s were used to create point defects and promote intermixing in InAs∕InP quantum stick structures grown by molecular beam epitaxy. With these soft conditions for ion-implantation-induced intermixing, photoluminescence measurements at low temperature show a very large blueshift up to 350 nm and a narrow emission linewidth (down to 30 nm for ion dose equal to 5×1013cm−2). The band gap tuning limit in this system was evaluated using implantation of phosphorus ions at various doses (1×1011–5×1014cm−2), at a temperature of 200 °C followed by rapid thermal annealing.

The current–voltage characteristics of AlGaAs/AlGaAs tunnel junctions for use in multi-junction solar cells are studied experimentally, where tunneling current peaks of 1100 A/cm2 and specific contact resistivities of 0.3 × 10-4Ω⋅cm2 at 7 A/cm2 (typical concentrated photovoltaic operating current) are measured. This represents an ideal tunnel junction design, with a very low resistance and one of the highest tunneling peak currents reported for solar cells. Normally, solar cell current–voltage characteristics are measured using time-averaged methods, which, in this study, reveal a tunneling peak current density of ~950 A/cm2. Due to nonlinear oscillations within the measurement circuit, the precise locations and magnitudes of the tunneling peak and valley current densities are obscured when using time-average measurement methods. Here we present an alternative method to determine the tunneling peak current density, in which the nonlinear oscillations in the current and voltage are recorded over time and a current density–voltage curve is reconstructed. This time-dependent method results in a measured tunneling peak current density of ~ 1100 A/cm2. The nonlinear oscillations of the experimental circuit are reproduced by modeling an equivalent circuit, resulting in qualitative agreement with the observed oscillations. This model predicts the capacitance and inductance of the equivalent circuit to be approximately 3 nF and 3.5 μH, respectively. This numerical model can be used to determine the inductance and the capacitance of any circuit having a negative differential resistance region.

We report on the effects of the As/P intermixing induced by phosphorus ion implantation in InAs/InP quantum dashes (QDas) on their photoluminescence (PL) properties. For nonintermixed QDas, usual temperature-dependent PL properties characterized by a monotonic redshift in the emission band and a continual broadening of the PL linewidth as the temperature increases, are observed. For intermediate ion implantation doses, the inhomogeneous intermixing enhances the QDas size dispersion and the enlarged distribution of carrier confining potential depths strongly affects the temperature-dependent PL properties below 180 K. An important redshift in the PL emission band occurs between 10 and 180 K which is explained by a redistribution of carriers among the different intermixed QDas of the ensemble. For higher implantation doses, the homogeneous intermixing reduces the broadening of the localized QDas state distribution and the measured linewidth temperature behavior matches that of the nonintermixed QDas. An anomalous temperature-dependent emission energy behavior has been observed for extremely high implantation doses, which is interpreted by a possible QDas dissolution.