7-я Международная научно-практическая конференция

Институтов сельскохозяйственной инженерии стран Центральной и Восточной Европы

(СЕЕ АgEng)

INVESTIGATION OF HIGH-EFFICIENCY SILICON SOLAR CELLS

Dmitry S. Strebkov and Vladimir I. Poljakov

State Scientific Institution The All-Russia Scientific-Research Institute for Electrification

of Agriculture (VIESH)1st Vyeshnyakovsky pr, 2 Moscow 109456 , Russia

tel. 7-499-171-19-20; e-mail: viesh@dol.ru

8-10 июня 2011 годаг. Минск, Беларусь

On a commercial scale planar solar cells with 20 % efficiency are produced by the Sun Power company (USA) but they have only limited applications because of their high price. All known silicon solar cells are not used under high concentrated sunlight due to drastic efficiency reduction at increased illumination. Now all factories in Russia and abroad are manufacturing silicon solar cells with 15-18 % efficiency. To raise SC efficiency above 25% new physical principles, new designs and technologies are needed.

In 1967 we developed and tested first generation matrix solar cells (MSC) [2]. Solar cells are made in the form of a silicon solid matrix consisting of series or parallel commutated microcells with vertical p-n junctions. Microcells density on the MSC working area was 25 cm-2 and their efficiency was 1-2% with illumination intensity of 7700 kW/m2.

In 1970 the technology of ion-implantation doping was used to develop second generation MSC with 10% efficiency under illumination of 2,5 kW/m2. Microcells density was increased 10 times – up to 250 cm-2, and a MSC with 4 cm2 area had voltage of 400 V [3]. Solar cell array consisting of MSC with 1 m2 area and 32 V voltage was developed and tested in 1972. Solar Module based on 6 MSC with dimensions of 4025 mm and operating voltage 28 V each was installed at the interplanetary station “Venus-70” and successfully tested. Under illumination of 10 kW/cm2 by a pulse mode laser 36% MSC efficiency and 3,6 kW/cm2 electrical capacity were achieved.

The most prominent are works by D.L. Sater [4, 5], who developed MSC experimental models with 20% efficiency under 2500 kW/m2 illumination.

The purpose of our research is to develop and study high-efficiency Third-generation (3G) matrix solar cells on the basis on single-crystal silicon.

Matrix nano solar cell on the basis of single-crystal silicon

MSC presented at Fig. 1, is composed of miniature solar cells 1, containing р+–n junctions 2, isotype junctions 3, 4 n–type working area and the doped isotype р+ layer 5, external metal contacts 6, internal metal contacts 7, passivating film 8, nanoclusters 9, antireflection dielectric 10 coating 10 at the working area 11 receiving solar radiation 12. р+–n junctions 2 are located perpendicularly to the working area 11. One or two linear dimensions of micro SC 1 are comparable with diffusion length of minority charge carriers in the base region 4. Passivating film 8 is located at the area 11 free of n-p junctions; the film 8 thickness is 10–30 nm. Nanoclusters 9 with 10-40 nm dimensions are located at the passivating film 8; the interval between them is 60–120 nm. Nanoclusters 9 are covered by a layer of passivating antireflection dielectric 10 coating [6].

In our work we consider a simple MSC model (Fig. 2), where nanoclusters 9 are located inside passivating film 8, and antireflection

coating 10 at passivating film 8 is absent.

Matrix nano solar cell with nanoclusters inside passivating film

MSC are operating in the following way: electromagnetic emission 12 reaching the working surface 11 through passivating antireflection coating 10 and passivating film 8 come to surfaces 11 of micro SC 1, free of n–р junctions 2. In this process photons are absorbed accompanied by the formation of electron-hole pairs and the emergence of excess minority charge carriers. Electron-hole pairs are divided by field which brings forth photogenerated current directed to the base area 4. Simultaneously emission reaches the nanoclusters 9. The frequency of plasma resonance of nanoclusters 9 corresponds to the frequency of incidence electromagnetic radiation, which makes it possible to reemit incident radiation through the insulating film 8, thus forming medium wherein electromagnetic wave is propagated. In the result the number of minority charge carriers is radically increased and the generation function is enhanced.

The patent on the given third generation MSC and their fabrication technique was listed among the 100 best inventions chosen from 42 000 Russian patents by the RF Federal Service for Intellectual Property, Patents and Trademarks [6].

At the XI International Forum “High Technologies of the XXI Century” (April 12-22, 2010) the State Scientific Institution “VIESH” became a Laureate of the competition “High Technologies – the Basis of the Modernization of Economy and Industrial Development” and was awarded a medal for the competitive project “Photovoltaic Energy-Efficient Silicon Cells (24% Efficiency) for Solar Plants with Concentrators”.

The 3G MSC with 20-29% efficiency were developed and tested in 2009 [7]. It was demonstrated that MSC with 6 cm2 area under 493 kW/cm2 has 60 W electrical capacity at 15 V voltage and 4 A current. A stationary solar module (SM) composed of a cascade of 36 125x125 mm SC under standard 1 000 W/m2 illumination, 25 °C temperature and 1,5 AM spectrum has the same 60 W electrical capacity, 15 V voltage and 4 A current. It should be mentioned that the areas of MSC and planar SM differ by 1000 times which implies 1 000-fold reduction of silicon consumption per 1 Wp in case of MSC operation with the use of solar concentrator [8].

3G MSC are fabricated at the pilot solar cells plant of the SSI VIESH under the direction of the head of this plant V.I. Polyakov. The technology of 3G MSC is adapted for mass production. It does not require such time-consuming operations as multistage diffusion, metal-screen printing, photolithography, vacuum metallization, etc. We succeeded in excluding silver for contacts manufacturing, as silver consumption amount at the world production of SC of 12 GW capacity exceeds 100 t per year, which causes serious problems for further development of solar energy.

The MSC current-voltage characteristic at total solar radiation: 1 - 700 W/m2 and 12оС temperature. The measurement date - 5.10.2010 , 15.00; 2 – 312 W/m2 and -8оС temperature. The measurement date – 22.12.2010, 11.40, in VIESH (Moscow). The MSC dimensions: 10100.5 mm, 25 micro cells

Solar radiation was measured with the use of a pyranometer and an actinometer. The measurements results were confirmed by solar radiation data submitted by the meteorological observatory at the Moscow State University named after M.V. Lomonosov.

Direct normal insolation was 0,63 kW/m2, diffused radiation on the horizontal surface – 0,07 kW/m2.

Short circuit current was 1,65 mA, or 41,25 mA on 1 cm2 of the working area of microcells. Optimal output power was 15,36 mW at 12 V voltage and 1,28 mA operating current, 21,9% efficiency, 14,44 V photo e.m.f. and 0,671 fill factor of volt-ampere characteristic. Relative error in efficiency definition was 4%, absolute error - 0,876%, minimal efficiency value considering possible measurement error is equal to 21,9%-0,876% = 21,14%. In recalculation in terms of 1000 W/m2 peak illumination, short circuit current density was 58,93 mA/cm2, electrical capacity – 21,9 mW.

In laboratory conditions under concentrated illumination the MSC characteristics were measured at a pulse simulator. The simulator uses a xenon flash lamp whose emission spectrum is close to solar spectrum. Flash duration is about 5 ms. Variable electronic load make it possible to measure current voltage characteristic for the period from 2 to 5 ms. Measurements were carried out with the use of a OTSZS-02-6 storage oscillograph and then processed with the use of special VAC software.

Current-voltage characteristic of MSC with 1 cm2 area measured under a pulse simulator. 1 – Illumination of MSC before measurement,

2 – measurement of MSC in darkness.

IV curve 1 corresponds SMC, exposed to a light intensity near AM 1,5 1 kW/m2 during 5 minutes, before measurement, IV curve 2 reflects the results of flash test of MSC in darkness. Passivation layer has charge centers, which induce an electrostatic field at the illuminated surface affecting the surface recombination velocity. These centers are activated by illumination, resulting spectral response and efficiency dependence on illumination time (Fig. 5).

Spectral characteristics of matrix solar cell, measured in darkness (1)

and after one minute of solar irradiation (2).

The MSC parameters for IV curve 1 are as follows: short circuit current - 77 mA, photo-e.m.f. – 17,79 V (0,71 V per one microcell), operational current – 72,8 mA, operating voltage – 16,54 V (0,66 V per one microcell), fill factor – 0,879, optimum output power - 1204,1 mW, efficiency – 36,8%. Due to increased illumination intensity, short circuit current was increased by 46,7 times in comparison with the parameters presented at Fig 1, IV curve 1: photo-e.m.f. – by 1,228 times, optimum load voltage – by 1,355 times, optimum load current – by 56,875 times, fill factor – by 1,31, efficiency – by 1,68 times. Considering possible 4% error of total insolation calculation, minimal possible illumination intensity under the pulse simulator will amount to 33,9976 kW/m2, and maximum possible efficiency – to 36,42%, while possible absolute error in efficiency calculation was 1,38%, and relative error – 3,75%.

The collection efficiency of the matrix silicon solar cell: 1 - theory. 2 - experiment.

The dependence of collection efficiency on the wave length of the solar cell:

where I/E - spectral sensitivity, q - electron charge, - quantum efficiency. с - light velocity, h - Plank constant, R - reflection coefficient, - wave length.

Rhc

qEI

Q

1

/

Assuming I/E [А/Вт]. [мкм], R in relative units, = 0,8,

we obtain:

For = 400 nm = 1,3; for = 450 mn = 1,14; for = 500 nm = 1,02; for 500 < < 1150 nm we assume = 1.

REI

Q

18,0

/

hc

q

Theoretical limit of photocurrent for silicon solar cells is defined in accordance with the following formula:

Iph=qNsi()[1-R()]Q(), (1)where q – electron charge; Nsi – photon beam density in solar spectrum with

energy Еph, exceeding the silicon energy gap Еgsi

=1,12 eV;() – quantum efficiency; R() – reflection ratio; Q() – collection efficiency.Plugging in q=1,610-19 C, Nsi = 3,51017 cm-2с-1,

()=1, R()=0,2, Q()= 1, we shall obtain:Iph = 44,8 mА/cm2.

It is experimentally confirmed that due to the absence of series resistance of the doped layer and photoconductivity modulation in the base region with increasing intensity, MSC is characterized by linear dependence of short circuit current on illumination. Hence for MSC measured under two different illumination intensities Е1 и Е2, Е2 > Е1, the following relation is true:

(2)

where Isc2 and Isc1 – short circuit currents under illuminations Е2 and Е1, k – concentration ratio.

kЕ

Е

I

I

sc

sc 1

2

1

2

Efficiency growth exceeds illumination increase due to increased fill factor, to longer effective lifetime and collection efficiency at the cost of electric fields contribution in base areas and in isotypes junctions areas, as well as to changed character of recombination processes in MSC base areas. Experiments confirm that the use of concentrated radiation allows to obtain maximum MSC efficiency.

MSC efficiency is defined by the following expression:

, (3)

where m2, m1 and V0 2, V0 1 – load factor of current voltage characteristic (CVC) and MSC photo- e.m.f. under illumination Е2 and Е1.

2

2.0222 Е

IVm sc1

10211 Е

IVm sc

Plugging in Е2 = кЕ1 и Isc2 = кIsc1 in relation to

, we shall obtain:

(4)

Plugging in to (4) m2 = 0,879, m1 = 0,671, V0 2 = =17,79 V, V0 1 = 14,44 V, we shall obtain

=1,614, that with 0,32% accuracy agrees with the measured value = 1,68.

1

2

01

02

1

2

1

2

V

V

m

m

1

2

1

2

Fill factors under illuminations Е2 and Е1 are equal to:

, , (5)

where Il 2, Il 1, Vl 2, Vl 1 – operating currents and voltage at the CVC maximum power point load under illuminations Е2 and Е1.

202

222

sc

ll

IV

VIm

101

111

sc

ll

IV

VIm

Plugging in Isc = kIsc in relation considering (5), we shall obtain:

. (6)

Relation (4) will be as follows:

. (7)

1

2

m

m

0211

0122

1

2

VVIk

VVI

m

m

ll

ll

11

22

1

2

ll

ll

VкI

VI

Due to increased CVC fill factor for MSC Il2 > kIl1, that confirms that it is reasonable to use MSC for operation under increased illumination intensity. In our case k = 46,7,

Plugging in to (7) Il2 = 72,8 мА, Il1 = 1,28 mA, k = 46,7, Vl2 = 16,54 V, Vl1 = 12 V, we shall obtain: 1,6786, that with 0,1% accuracy

agrees with the measured value 1,68.

875,561

2 l

l

I

I

1

2

1

2

MSC production cost is comparable with the manufacturing cost of planar silicon SC and in terms per one area unit, and is 100 times smaller than the production cost of cascade solar heterostructure SC on the basis of АIIIBV compounds, while their efficiency is similar. The development of the technology of 3G MSC on the basis of single-crystal silicon will make it possible to construct solar energy plants with lower costs per 1 kW of installed capacity and higher efficiency of electricity production in comparison with thermal power plants operating on coal.

1. Third-generation matrix silicon solar cells with 21% efficiency under solar illumination have been developed.

2. The study of MSC under high intensive illumination by a pulse simulator demonstrated that short circuit current is growing linearly with illumination increased up to 32,66 kW/m2, photo-e.m.f. is growing by 1,228 times, optimum load voltage – by 1,355 times, CVC fill factor – by 1,31 times, efficiency – by 1,68 times. Measured efficiency value amounted to 35,42-36,8%.

3. We have considered the ways of further raising MSC efficiency at the cost of using antireflection coating and reflection losses reduction from 20% to 4%, while MSC efficiency is expected to increase by 1,16 times and will exceed 41%.

4. The production cost of the 3G MSC is comparable to that of planar silicon SC. The development of the technology of the 3G MSC on the basis of single-crystal silicon will make it possible to create fuel-free solar energy plants using solar concentrators with lower unit costs per 1 kW of installed capacity and higher efficiency in comparison with coal-fired power plants.

The authors express their thanks to Cand. Sc. (Engineering) Vladimir S. Vershinin and Cand. Sc (Engineering) Irina S. Persitz for assistance in fabricating experimental models of 3G MSC, to Doctor of Physical and Mathematical Sciences V.I. Yevdokimov, Cand. Sc. (Physics and Mathematics) Yu. D. Arbuzov and Cand. Sc (Engineering) V.N. Mayorov for measuring MSC parameters at the impulse simulator and under solar illumination. The authors acknowledge the support provided by the Russian Foundation for Fundamental Research.

REFERENCES 1. Martin Green. “Perl cell”. The university of New South Wales.

www.nsinnovations.com.au. 2. Dmitry S. Strebkov. “Matrix Solar Cells”. Vol. 1. Second Edition, Moscow,

2010, Published by VIESH, 118 p.p. 3. Dmitry S. Strebkov. “Matrix Solar Cells”. Vol. 2. Second Edition, Moscow,

2010, Published by VIESH, 266 p.p. 4. Sater D.L. et all. “The multiple Junction Edge Illuminated Solar Cells”.

Conf. Res. Tenth IEEE Photovoltaic Specialists Conf. 1973, p. 188-193. 5. Sater D.L. “Method of making a high intensity Solar Cell”. US Pat. №

4516314, 1985. 6. Dmitry S. Strebkov, Olga V. Schepovalova, Vitaly V. Zadde.

“Semiconductor Photoelectric generator”. Russian Patent № 2336596, 2008, № 29.

7. Dmitry S. Strebkov. “Matrix Solar Cells”. Vol. 3. Second Edition, Moscow, 2010, Published by VIESH, 346 p.p.

8. Dmitry S. Strebkov, Vladimir I. Poljakov. “24% efficiency photoelectric silicon modules with for solar concentrator Power Plants”. Proceedings of International conference “Perspective directions of alternative energy system and energy saving technologies”. Chimkent, Republic of Kazakhstan. South Kazakhstan State university 2010, vol. 1, p. 137-145.