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In this paper, we extend the concept of back surface recombination through a study of a silicon mono facial solar cell in static regime and under polychromatic illumination. Back surface recombination velocities noted
*Sbe*,
*Sbj* and
*Sbr* are determined for which respectively we derived, the power, the fill factor and the conversion efficiency, that become constant whatever the thickness of the solar cell. We have then obtained the expression of the minority carrier’s density in the base from the continuity equation. We then have determined the photocurrent density, the photo voltage, the power, the fill factor and finally the conversion efficiency.

Today one of the main objectives in the domain of the photovoltaic energy is to improve the solar cell efficiency and to reduce the manufacturing cost. Until now, the conversion efficiency in photovoltaic solar is in the order of 20% in the laboratories but only in the order of 17% to the commercial level. This objective has to optimize the solar cell thickness (H) and the recombination parameters in the bulk and surfaces.

These recombination [

We consider an n^{+}p-p^{+} polycrystalline monofacial silicon solar cell. In this work, we overlook the contribution of the emitter to the photocurrent density compared to the base. When the solar cell is illuminated by the emitter front side.

Considering generation, recombination and diffusion phenomena of excess minority carriers in the base, the well-known continuity equation is written as:

∂ 2 n ( x ) ∂ x 2 − n ( x ) L n 2 = − G n ( x ) D n (1)

With:

n(x) is the excess minority carrier density at the position x in the base;

D n and L n are respectively the diffusion coefficient and the minority carriers diffusion length.

L n and D n are linked by the well-known Einstein expression:

L n = D n τ n (2)

τ n is the excess minority carrier lifetime

G n is the excess minority carriers generation rate.

The G n is in the form of a whole series [

G n ( x ) = ∑ λ 0 λ g F 0 λ ( 1 − r λ ) α λ e − α λ x (3)

where λ 0 (=0.3 μm) and λ g (=1.2 μm) are respectively the minimum and the maximum wavelength in the polychromatic source.

F 0 λ is the incident photon flux; α 0 λ is the monochromatic absorption coefficient of the material; r 0 λ is the monochromatic reflection coefficient at the surface of the material;

The solution of the continuity equation with the conventional method can be written as:

n ( x ) = A ⋅ cosh ( x L ) + B ⋅ sinh ( x L ) + ∑ λ = 1 λ = 45 K ⋅ e − α x (4)

A and B are determined by the boundary conditions.

The junction at x = 0

D n ∂ n ( x ) ∂ x | x = 0 = S f ⋅ n ( 0 ) (5)

where Sf is the minority carrier recombination velocity at the junction [

S f is expressed as the sum of two terms: S f 0 and S f m S f 0 is related to the shunt resistance. It represent the intrinsic junction recombination velocity.

S f m characterizes the photocurrent collected through the external load R m connected to the solar cell.

At the rear face:

D n ∂ n ( x ) ∂ x | x = H = − S b ⋅ n ( H ) (6)

H is the solar cell base thickness and Sb is the excess minority carrier recombination velocity at the back surface [

The solar cell photocurrent density is determined from the minority carrier density by the following relationship:

J n ( S f , S b ) = q ⋅ D n ⋅ ∂ n ( x , S f , S b ) ∂ x | x = 0 (7)

The Boltzmann relation gives us the following photovoltage expression as:

V p h ( S f , S b ) = V T ⋅ ln ( 1 + N B n i 2 n ( x , S f , S b ) | x = 0 ) (8)

With:

V T = K T q (9)

n_{i} is the intrinsic carrier density,

N_{B} is the base doping density,

V_{T} is the thermal voltage at a given temperature T.

The output electric power is an essential parameter for a solar cell. It indicates the capacity of the solar cell to provide an electricity to the external load. The electrical power produced by the solar cell under polychromatic illumination constant and for a given operating point is determined by:

P = J ⋅ V p h (9-a)

With: J = J p h − J d , and

J d = q ⋅ n 1 ⋅ S f 0 ( exp ( V p h V T ) − 1 ) (10)

J is the photocurrent density in the external load,

Jd represents the dark current density [

and n 1 = n 0 N b , n 0 : carrier density at equilibrium.

We plot on the curves 4 and 5, the profile of the photocurrent density versus junction recombination velocity ( S f ) respectively for a given thickness H and for different S b values

On

i) for the interval S f ≤ 10 cm / s , the photocurrent is zero. The minority carriers are blocked and stored at the junction. Any carrier crosses the junction: this is the open circuit condition.

ii) in the interval 10 cm / s ≤ S f ≤ 4 × 10 4 cm / s , j increases quickly.

iii) in the interval S f ≥ 4 × 10 4 cm / s , j becomes constant and retains its maximum value corresponding to the short-circuit photocurrent. All the photogenerated carrier crosses the junction: this is the short circuit. The effect of the back surface recombination velocity on the short-circuit photocurrent density is reflected by an increase of the photocurrent density when S b decreases (BSF effect).

On

In this interval, the short-circuit photocurrent density increases with the thickness (

For H ≥ 0.02 cm , the photocurrent density becomes nearly constant.

Therefore for thin photovoltaic solar cell (low thickness), the short-circuit

photocurrent density is higher for the low values of the back surface recombination velocity whereas when the recombination velocity is high, it is more important for thick solar cells.

We have plotted on

On

i) in the interval S f ≤ 2 × 10 2 cm ⋅ s − 1 , the photovoltage is constant and corresponds to the open circuit photovoltage V o c . Here the carriers photo generated are blocked and cannot cross the junction, their accumulations explain the maximum photovoltage value.

ii) in the interval S f ≥ 2 × 10 2 cm ⋅ s − 1 , the photovoltage decreases rapidly up to cancel itself. This is because all excess minority carriers photo generated gradually cross the junction until there are charges no more: this is the short circuit.

The curves show that for low S b values (

The open-circuit photo voltage is all the more slight that the solar cell is thin for the low back surface recombination velocity. Whereas for the thick solar cells the open-circuit photo voltage is low for high back surface recombination velocity values.

Recognizing the expression of the power, we represent respectively on

and

On

On the curve 9, It is noticed that for a given curve, the lower values of S f i.e.

S f ≤ 10 2 cm ⋅ s − 1 ), power tends to the nil value. Because of the proximity with the open circuit, the current is low (or even nil) this therefore brings about a low power or even nil.

In the interval 10 2 cm / s ≤ S f ≤ 4 × 10 4 cm / s , the junction recombination velocity increases, the photocurrent density increases also resulting in an increase of the power to reach a maximum value corresponding to the maximum power. This maximum power therefore corresponds to a junction recombination velocity optimum that can be also obtained by the following equation

∂ P ∂ S f = 0 (10-a)

Then for S f ≥ 4 × 10 4 cm / s , it tends towards the short circuit condition, then the photovoltage tends to nil value [

On the contrary,

In

Then, it decreases in the interval 2 × 10 2 cm / s ≤ S b ≤ 4 × 10 4 cm / s . Finally it reaches its minimum value for S b ≥ 4 × 10 4 m / s . As S b reflects the lost factor of recombination, when it is weak, the power reaches its most important value, then the power decreases when S b increases to finally take its smallest value for the large S b value. The curves show that for S b ≤ S b e the power decreases with the increase of the thickness. Whereas for S b ≥ S b e , it increases with the thickness

We can conclude that, for solar cells of different thicknesses, the most important power is obtained with the weakest solar cell in the case of lower back surface recombination velocity. But for high back surface recombination velocity, the most important power is obtained with the thinnest solar cell.

It is recognized that there is a back surface recombination velocity ( S b e ) which allows to obtain the same power for all thicknesses.

It represents the fraction of the lost power in the semiconductor material either by resistance effect or by recombination phenomena of the carrier’s photogenerated. It is given by the relationship:

F F = P max V o c ⋅ J s c (11)

P max is the maximum power value which can be extracted, V o c represents the open circuit photovoltage and J s c represents the short circuit photocurrent density. In

Then in the interval 2 × 10 2 ≤ S b ≤ 4 × 10 4 , the fill factor decreases gradually and finally becomes constant for S b ≥ 4 × 10 4 cm ⋅ s − 1 . For the low values of the back surface recombination velocity that is to say in the interval S b ≤ 2 × 10 2 cm ⋅ s − 1 , the fill factor reaches its maximum value. Because for small S b values, the loss factor of recombination is low therefore the lost power is low (

fill factor regardless of the solar cell thickness.

The conversion efficiency is the fraction between electrical power generated by the solar cell and the power of the incident flux ( P i n c ) received by this solar cell. The maximum efficiency is, the fraction between the maximum power charged in and the forward power ( P i n c = 1 sun = 100 mW / cm 2 ). The maximum conversion efficiency is given by the following relationship:

η = P m P i n c , with P i n c = 100 mW / cm 2 (12)

We present in

The conversion efficiency curves versus back surface recombination velocity have the same profile.

For a given thickness H, the conversion efficiency is constant for S b ≤ 2 × 10 2 cm ⋅ s − 1 , then in the interval 2 × 10 2 cm ⋅ s − 1 ≤ S b ≤ 4 × 10 4 cm ⋅ s − 1 , it decreases gradually and finally it becomes constant in the interval S b ≥ 4 × 10 4 cm ⋅ s − 1 . The curves show that for S b ≤ S b r , the conversion efficiency decreases with the increase of the thickness, while for S b ≥ S b r , it increases with the thickness. This allows to conclude that the thin solar cells are

more advantageous than the thick one with low S b values. On the other hand, in the range of high Sb values, thick solar cell are more advantageous. S b r is a back surface recombination velocity which allows to get the same conversion efficiency with the different thicknesses of the solar cell.

In this work, we have studied first the profile of photocurrent density, the profile of the photo voltage and of the power. The knowledge of these parameters has allowed then to study the fill factor and the conversion efficiency.

For the photocurrent density, solar cells generate a larger photocurrent when the junction recombination velocity is high ( S b ≥ 4 × 10 4 cm ⋅ s − 1 ). The thick solar cells (H = 0.025 cm) are more beneficial because they generate a higher photocurrent density. For the photo voltage, the open circuit photo voltage decreases with S b . It also decreases with the thickness for the small Sb values but increases with the thickness for the high back surface recombination velocity. For the power, fill factor and conversion efficiency, we have found S b e , S b j and S b r respectively, the back surface recombination velocities for which they are invariable whatever the solar cell thickness is.

Diasse, O., Diao, A., Wade, M., Diouf, M.S., Diatta, I., Mane, R., Traore, Y. and Sissoko, G. (2018) Back Surface Recombination Velocity Modeling in White Biased Silicon Solar Cell under Steady State. Journal of Modern Physics, 9, 189-201. https://doi.org/10.4236/jmp.2018.92012