Category Next Generation Photovoltaics High efficiency through full spectrum utilization

Experiments in view of developing a four-junction cells using metallic wafer bonding have been conducted [11]. The most difficult point is the interconnection between the cells. It must meet three requirements: it must be optically transparent to allow non-absorbed light to be transmitted from one cell to the one under; it must be electrically conductive; and mechanically rugged.

Bonding with metallic layers has been obtained. However, the properties of the contacts (optical transmission of 60% for the Sn layer used for bonding, and ohmic contact with contact resistivity of 2.5 ^ cm2) must be improved to apply this technique to multi-junction cell fabrication.

Transistors made on III-V films transferred onto silicon have been realized [20]. Lattice-matched InAlAs/InGaAs layers were grown in the reverse order compared to conventional HEMT structures on a 2 in InP substrate with etch stop layers between the substrate and the InAlAs/InGaAs. The InP wafer with the grown layers was bonded onto a 2 in FZ silicon wafer by means of SiO2-SiO2 bonding. After the InP substrate and etch stop layers were removed, the HEMT structure was realized (figure 12.8). These devices showed cut-off frequencies close to those of conventional HEMT transistors, indicating the good quality of the transferred layers...

The techniques of film transfer have already been used for transistors and solar cells applications. We give here some examples of structures and devices realized using bonding and transfer.

(b)

The transfer of thin films with patterned structures onto different substrates is very attractive for applications such as thin-film transistors on glass or quartz for TFT-

LCDs, intelligent sensors and actuators or smart power. For example, metal lines embedded in an oxide have been transferred onto a Si substrate [15].

In this example (see figure 12.5), the process was the following: 380 nm thick oxide growth on a silicon substrate, 340 nm thick aluminium-copper and 40 nm titanium nitride deposition and patterning, CVD deposition of 1.7 ^m thick oxide. This structure is then planarized, hydrogen is implanted through it and silicon wafer 1 is bonded to silicon wafer 2. Heat treatment at 450 °C has been chosen here to split this structure at the implanted layer.

The Smart-Cut®...

C Jaussaud1, E Jalaguier2 and D Mencaraglia3

1 CEA/DTEN

2 CEA /DTS

3 Supelec/LGEP

12.1 Introduction

Wafer-bonding and film transfer have been developed in the microelectronic industry and these techniques are presently used to make silicon-on-insulator (SOI) structures. They are also of interest for photovoltaic cells and many studies have been done to develop thin-film cells based on film transfer [1-9]. They have also been studied to make multi-junction cells [10,11]. In this chapter, we will describe recent developments in film transfer which could be of interest for multi-junction photovoltaic cells. Presently, multi-junction cells are made either by direct epitaxy onto a substrate or by mechanical stacking...

Lattice-matched In0.53Ga0.47As/InP heterostructures were developed initially for fabricating infrared photodetectors. Then, these diodes were used as laser power converters [64] and as bottom cells for monolithic InP(top)/InGaAs(bottom) [62] or mechanically stacked GaAs (top)/InGaAs(bottom) [31,32] tandems. InGaAs layers were grown on InP substrates by LPE [31,32,63] and by MOCVD [62] methods. A 6% efficiency (AM0, 100 suns) was measured [31] in the LPE cells illuminated through the InP substrate and the GaAs IR transparent filter. TPV cells fabricated by simultaneous Zn and P diffusion into the LPE-grown InGaAs layer have demonstrated VOC = 0.42-0.5 V and FF = 0.75-0.78 at JSC = 1­15 A cm-2 [63].

Lattice-matched (or mismatched) InGaAs/InP monolithic interconnected modules (MIMs) have als...

Epitaxial InAsSbP/InAs heterostructures for TPV cells have been grown [55,71­73] by the LPE method. Narrow-gap epitaxial InAsSbP (0.45-0.48 eV) cells were fabricated [73] from p-InAsSbP/n-InAsSbP/n-InAs heterostructures grown on (100) n-InAs substrates. Epitaxial growth of n-InAsSbP quaternary layers

lattice-matched to InAs was carried out by LPE from an Sb-rich melt. The lattice-mismatch ratio Aa/a = 0.15% was estimated from x-ray diffraction measurements. Liquid-solid phase equilibrium estimations were obtained using a simple or regular solution model...

The heterostructure of the monolithic tandem TPV cell (figure 11.15) consists of an n-GaSb (substrate); an n-p InxGai-xAsySbi-y (EG = 0.56 eV, 1-3 ^m thick n-layer and 0.2-0.5 ^m thick p-layer) bottom cell; a p++-n++GaSb (0.8 ^m total thickness) tunnel junction; an np GaSb (n-layer 3-5 ^m thick and a p-layer 0.2-0.5 ^m thick) top cell [41,50]. These structures were fabricated using two-

stage LPE growth and two Zn diffusions. The first step included LPE growth of an n-InGaAsSb layer and a GaSb cap layer from the Sb-rich melt on the GaSb substrate. Zn diffusion was carried out after the first LPE growth to form both the pn-junction in a quaternary solution and the first layer (p++ GaSb) of the tunnel junction in GaSb...