New steps were made by E. Guk, V. Shuman et al. (Ioffe Institute), by authors (VIESH, VEI), by Sater B. (USA), et al, in developing new technology of manufacturing SCVJ, including, for example, direct bonding.

A method of manufacturing a high intensity SCVMJ (thermal compression bonding with siluminum) [13] was developed in VIESH. The main goal of this technology is to provide a structure for a photovoltaic cell which gives in result improved characteristics: improving quality of interconnecting soldering, eliminating of compensative influence of aluminum to n+-layers, high temperature tolerance and mechanical firmness (the cross section of classical vertical structures without antireflection coat is showed at Fig.3). According to this invention, the siluminum layers prevent a penetration of compensative impurities (Al) into n+- region. The produced solar cell structure gives in result high quality mechanical and electrical contact, high fill factor for I-V-characteristic (more than

0. 8), and increased tolerance to high temperature under high intensity.

The next technology [14] offers the method of manufacturing a high intensity solar cell using n-type silicon with high diffusion length.

In other version of solar cell’s design of this invention it is additionally expedient to form alloyed inversing layer and TiOx antireflection coatings at the front sensitive surface. In this case hybrid cells representing a combination of planar and vertical p-n-junctions are manufactured.

Advantage of such design is combining of two processes: soldering and diffusion, which moreover are carrying out at the lower temperatures. It allows canceling a number of technological steps, to decrease a possibility of introducing different contaminations and defects and, in general, to save a high diffusion length which leads to increasing efficiency and to decreasing energy consumption for manufacture.

A number of research works were carried out at A. F. Ioffe Physicotechnical Institute [17-18, 21-23].

The Silicon Direct Bonding (SDB) or fusion bonding technique, in principle, a rather simple process of room temperature mating of two polished wafers and subsequent annealing at higher temperature, allows the fabricating of special devices for many applications. Direct bonding of silicon to silicon, as well as bonded interfacial layers of
different materials such as siluminum, poly-silicon, silicon oxide or nitride is feasible by SDB. Bonding of patterned wafers containing microstructures is an important technique for micromechanical device fabrication. Aligned bonding of preprocessed wafers offers new possibilities for 3D-integration. For semiconductor device manufacturers the Si-Si bonded wafer offers a cost effective alternative to thick epitaxial layers that have traditionally been used for applications such as power devices and p-i-n diodes. Last several years a number of papers, devoted to using SDB for solar cells are appeared. Solid-phase direct bonding silicon wafers having the diffusion p+ — or n+-layers with high surface doping concentration is described in [22].

The SDB for the fabrication of solar cell structures with vertical p-n-junctions is demonstrated in [23]. This technology based on ion implantation and direct wafer bonding of p+-p — n+-structures. The internal quantum yield of such structures was near 1 in the wavelength range of 350-900 nm.

The several methods using SDB are proposed in [19].

According to the first method of manufacture the wafers with p+-p- and n+-n-structures are oriented at the same crystallographic direction and bonding under a pressure in a vacuum furnace at the temperatures 800-1000 oC. The technological steps are similar to [13, 14]. In accordance with the second method the p+-p — n+-structures are exposed to SDB and in accordance with the third method symmetrical p+-n — p+- or n+-p — n+- structures are used for SDB. The cross-sections of interdigitated contacts on the backside of SCVMJ are shown at the Fig.9.

These methods allow to avoid the processes of metallization right up to SDB and to reach high efficiency values.

Using optimized bonding process it is possible to join two wafers of any doping and orientation with an interface that is free of silicon dioxide, silicon defects, precipitates and any unwanted doping. All this means that holes and electrons can move freely across the interface as if it was not even there!

Last achievements of Sater B. also show good prospects for vertical multi-junction technology. He developed high voltage silicon vertical multi-junction (VMJ) solar cells that provide efficient operation for up to 1000 suns intensity. The VMJ cell is an integrally bonded, series-connected array of miniature vertical junction p-n-n+ unit cells. The design gives high voltage, low current operation and other performance advantages at high intensities. Fabrication processes are being finalized and efficiencies exceeding 20% at up to 1000 suns intensity are expected. Preliminary tests at about 500 suns intensity show a 0.78-cm2 VMJ cell containing 40 unit cells with a maximum output power density of 11.385 W/cm2 at 24.5 volts with an estimated efficiency of 20.2%.

Applications

High voltage photovoltaic SC “Photovolt” are intended for power supply of high-voltage radio, dosimetric, other electronic and special equipment (rated at 100 to 1,000 V) as well as household appliances (rated at 220/110 V).

Solar power installations are currently sufficient to provide lighting, pump water, and power telecommunications facilities, and home appliances in remote areas and in vehicles.

The structures with vertical p-n junctions can be used as a high-sensitive sensor of position of the light beam (including transparent type sensors in IR region near the edge of absorption).

Let us also mention about project developing 3-D radiation imaging detectors here based on the ability of deep etching in a semiconductor by new methods to form deep macrospores or trenches in the material in a pixel matrix that can be doped to form vertical p-n-junctions [24].

Thermo-photovoltaic conversion, laser transfer energy systems, space concentrator systems, medical power supply systems, photometry (including high intensity), detection of nuclear particles are also the interesting areas for application of SCVJ due to low series resistance, high temperature resistance, high voltage output, unusual geometry and identical receiving surfaces. Taking into account that concentrator companies are making progress toward increasing their market, the SCVJ may become a good option of concentrator SC.

Small-scale version of space concentrator with SCVJ on the backside (along the top edge) of reflectors is depicted in Fig. 10.

SCVJ are very good for special applications such as, for example, space solar probe where SC with high thermal and radiation tolerance intended for conversion solar radiation with intensity, which is changing from usual to very high levels.

Conclusion

A number of practical SCVJ was developed and new approaches were proposed in Russia and in the USA.

Theoretical estimations indicate the limit of efficiency of SCVJ is similar to the limit of efficiency of traditional (planar) SC, but they are more suitable for conversion of concentrated light and have improved performance in term of radiation tolerance.

The combined spectral-probe and other methods have been developed for the investigation of vertical multi-junction solar cells and other types of the solar cells with complex structure.

According to obtained experimental data limit specific power is equal to 3.6 kW/cm2and specific voltage is equal to 100V/cm2. These values are probably impossible to have for SC of planar type.

At the modern stage of development semiconductor technology it is possible to use new design, concepts, and approaches and new advanced technology to achieve the efficiency close to the theoretical limit.