The first practical solar cells with vertical p-n-junctions were developed in Russia at the end of the sixties (there are British, German, and French Patents of Russian scientists) after considerable theoretical and experimental work in 1960th. The fist Russian invention was applied in 1967 (Strebkov D., Kozurev V.) Actually it is possible to tell: new class of silicon vertical multi-junction and high voltage cell technology were developed in 1967 (Landsman A., Strebkov D., Unishkov V. et al) [3-5].

In the early 1970s and later, in Russia a number of the new design ideas for high voltage SC for concentrated sunlight was put forward and realized [6].

The term multi-junctional edge-illuminated SC has been used in papers published in English to reflect untraditional geometry of vertical SCs and because such design involves many p-n junctions to form monolithic solar cell.

Some variations of vertical multijunction SC are shown at Fig. 2. One should remark that term multijunction is also used for tandem SC. So it is better to use adjective vertical to differentiate the dissimilar approaches.

Many of the design parameters of conventional SC are the result of a series of compromises between competitive phenomena. Many concentrator SC designs have been proposed to avoid disadvantages of conventional SC for high concentrated light and to decrease ohmic losses for concentration light. The practical Russian realization of SC with cross section (simple classical version), which is represented at Fig. 3, has also a name high-voltage SC or “Photovolt”.

They have a double-sided sensitive area and are transparent for the infrared non-active spectral band. The generator featured a high sensitivity to radiation with a wavelength of 1 pm. The maximum EMF density of generator is higher than 50 V / cm2, maximum current density — 10 A/ cm2. Ordinary silicon solar modules cannot achieve such ratings. More complicated design was realized and received name matrix SC (Fig. 4 and Fig. 5) Examples of realization of small high voltage solar modules 500 V and 1000 V are shown at Fig. 6.

There were made several investigations by Chadda T., Wolf M., Sater B. et al, and some patents were published in the USA in the middle of 1970th [7-10]. Some investigations have proceeded later [11-13].

In the early of 1990s the small industrial series of SCVJ were produced in All-Russian Electrical Engineering Institute (VEI) using technological processes developed to manufacture high voltage semiconductor devices.

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In spite of the interesting properties of vertical design of SC not many books give information on SCVJ. So authors believe that detailed (but unfortunately not full) list of references will be useful to readers. Now photovoltaic systems with concentrator are becoming commercial and one of the future practical options for concentrator systems is SCVJ.

It is possible to match the area of the photoactive face with the power distribution in the incident radiation flux, to reduce power losses in case of non-uniform incident radiation

fluxes and to increase the efficiency of the generator in case it receives highly concentrated radiation. A method of fabricating such solar cell generator is also described in this patent [33]. A cross-section and photograph of a sample of stair-like SC are shown at Fig. 7, 8.

Main characteristics

After the first stages of development SC technology when planar SC showed efficiency more than 6-8%, the SCVJ have not been considered as a real competitor of planar SC. It partly reflects the fact that main advantages of SCVJ: low series resistance and increased tolerance to the damaging radiation were not very important for usual mass terrestrial applications. But SCVJ having interesting characteristics and features can find definite niches.

The SCVJ developed in Russia showed output electric power about 3.6 kW/cm2 under the illumination of 10 kW/ cm2 Nd-laser beam with the wavelength 1.06 pm. It was obtained 32 kV from the SCVJ small module. There were achieved the voltage density about 106 V/cm2 and power density about 20 W/cm2 from SCVJ in concentration light. Temperature decreasing of voltage is about — 1.1 mV/K under illumination about 7500 “suns “.

Open circuit voltage was measured for photovolt under the impulse illumination by xenon lamp at the temperature of liquid nitrogen. For one p-n junction EMF was equal to 1.06 V (close to the value corresponding to energy gap). Temperature coefficient of voltage was increasing from 0.4 mV/oC to 2 mV/oC under increasing concentration ratio till 10W/cm2.

The spectral-probe method was developed to combine measuring of the spectral response with a scanning technique (for instance variable-wavelength laser beam probe used) [27].

The theoretical estimations [14] indicate the limit of efficiency of the SCVJ is similar to the limit of efficiency of traditional (planar) SC, and this limit can be achieved for concentrated light and with advanced technology.

We do not consider here (in short paper) review on vertical p-n-junctions using other materials but one should said that vertical multijunction solar cell technologies using GaAlAs-GaAs [29], germanium [30], silicone carbide [31], were developed in Russia at the end of 1970th.

Cascade SCVJ were made from silicon (top) and germanium (bottom); detailed investigation were published in [30].

SCVJ silicon carbide showed ability to work at very high temperatures till 200oC.

We also have to make reference to paper of researcher who does not belong to Russian or American groups [26].