Influence of patterning the tco layer on the series resistance of thin film hit solar cells _ epj photovoltaics


In this article, we investigate the influence of TCO patterning on the series resistance to quantify how much the overall efficiency of new generation solar cells with a front PC is impacted. Pound usd converter First, we will theoretically investigate the influence of TCO patterning on the local series resistance which will be used to calculate its impact on a standard solar cell ( 156 × 156mm 2 for instance). Usd jpy technical analysis We will study how the photonic crystal orientation affects the electro-optical performances of the device. Binary code to text Last, we describe geometries for which the electrical losses could be neglected compared to the absorption increase in the active material, in order to increase the conversion efficiency.

1 usd to zar 2 Study of the local sheet resistance 2.1 Definition of the electric period

Figure 2 represents top views of two different solar cells patterned by a square lattice of cylinders holes, with a period T opt. Gbp to usd exchange rate The optical elementary pattern is delimited by dash lines, and its main directions can be aligned (a) or not (b) with the metallic finger. Joy news ghana Considering that the cell is infinite in the direction of finger, the minority carriers generated under the PC then join electrodes by the shortest path and encounter a periodic pattern (blue squares in Fig. 2000 usd to inr 2) towards the finger. Conversion of usd to rupees The period of the electric pattern T el can be identical with the optical one in case (a) and 45 degrees shifted in case (b) leading to .

In order to provide an accurate estimation of r, we measured the average size of the patterns with SEM observations and we interpolated datas from Figure 4. 1 usd to sek Table 1 shows a very good agreement between the expected and experimental degradation ratios r with a maximum difference of 5%. Aud convert usd 3 Impact of the patterns: from the local resistance to the series resistance of a large solar cell

In this part, we calculate the potential at the TCO surface and then deduce from the distance p between two metallic fingers the front side series resistance of a large solar cell. Convert decimal to binary 3.1 TCO layer modeling with a resistance array

Figure 7 represents a HIT solar cell with a top surface photonic crystal. Usd shop Every elementary electrical pattern produces a small photogenerated current . Can to usd In between each pattern, the lateral resistance corresponds to R PC, as described in the electrical circuit of Figure 7b.

Around node n of the resistance array, the application of the Kirchhoff’s current law states: (1)At position N, the node position is assumed to be located just in between two fingers, so that only half the current produced by an elementary pattern contributes to the ohm law: (2)At the finger, when the solar cell is short circuited, current I 0 is equal to the sum of the photogenerated current , so the ohm law leads to the following initial condition: (3)Then, the solution of equations ( 1)–( 3)with an arbitrary finger potential U 0 is: (4)This equation gives the potential at the TCO surface between two fingers when the solar cell is short circuited. Usd brl 3.2 Expression of the series resistance

Thanks to the surface potential U n, one can calculate the total dissipated power P j by Joule effect in the entire patterned TCO layer: (5)This equation leads to: (6)Let’s introduce the series resistance for a single strip of resistances so that it dissipates P j when delivering the total photogenerated current I 0. Exchange rate usd myr This resistance depends on the orientation of the PC since the width of the strip is linked to T el. The future of digital marketing is you Then, (7)Then, assuming N ≫ 1, we get: (8)One may notice that this elementary resistance R S is different from the sheet resistance (equal to N· R PC) of the same patterned layer. Historical exchange rates usd to inr Indeed, the current sources are regularly spread on the length of the layer, instead of being applied on two opposite cross sections.

If we neglect the influence of the bus bars connecting each fingers, a solar cell with a surface S contains elementary stripes in parallel with an elementary series resistance . E mini msci emerging markets futures The length of these stripes is also equal to half the pitch p/ 2 = N· T el. Us stock futures tomorrow As a result, the series resistance R S of such a solar cell is given by: (9)In case the lateral conduction is achieved in a single layer of resistivity ρ 0 and thickness t 0 (like a TCO), this last equation is also equal to: (10)since the sheet resistance R 0 is also equal to ρ 0/ t 0 for a square pattern with width and length equal to T el.

From equations ( 9)or ( 10), the additional front series resistance is proportional to both the square of the pitch p and the perturbation of the local series resistance r. Euro conversion to usd It is worth to notice that it does not depend on the orientation of the photonic crystal (Fig. Eur usd historical chart 2), even if the electric period T el may vary. Currency converter usd to rm 4 Degradation of the conversion efficiency of a large solar cell

Simulating a solar cell patterned by a PC to study the influence of the pattern on the series resistance is unfortunately not possible, since it would be necessary to simulate thousands of periods of the PC, which would require too much memory and computation time (even for a 2D simulation). Market futures cnbc However, equation ( 10)can be exploited to get the I- V curve of a realistic solar cell, and so its conversion efficiency, with the simulation of a single period of the PC and its “local” electric characteristic. Gender differences in language 4.1 Estimation of the J sc gain

From [ 11], cylindrical holes lead to a significantly lower ff than reverse pyramids, and so to a lower degradation ratio r (Fig. Euro chart 2016 4). Translate binary code For this reason, we focus on cylindrical nanoholes optimized by 3D FDTD (Finite Difference Time Domain) simulations. Chf usd conversion As the parasitic absorption occurs in the short wavelengths, we consider an ultra thin HIT solar cell (1.1 μm) to maximize its photogeneration rate in the active layer. Latest exchange rate pound to euro In order to get a more realistic estimation of the J sc gain than in [ 8], we also take into account the a-Si:H layers dispersion characteristics.

When all the top surface is covered by the TCO and a-Si:H layers (Fig. Gender identity test 8a), the thickness of these layers on the sidewalls is proportional to the thickness on the front and bottom surfaces of the pattern with a ratio extracted from experimental observations [ 5]. Usd to aed When patterning both the TCO and a-Si:H layers (Fig. Usd to zwd 8b), the passivation layer is not taken into account since it does not absorb light and its thickness ( ≤10 nm) can be neglected compared to the considered wavelength range. Pounds to usd calculator The optimization is applied on the thickness of the top and bottom a-Si:H and TCO layers and on the geometry of the pattern (period, size and depth). Solving problems The optimal configurations correspond to Figure 8. Fraction operations worksheet Whatever the patterning is, the convergence is reached for the same period (700 nm), but the backside mirror is slightly thicker in case of patterned TCO and a-Si:H layers. Verizon troubleshooting For both structures, the diameter of the hole in the c-Si layer is quite similar: 490 nm and 480 nm, respectively. Famous quotes about life changes The optimized filling factor is equal to 0.37, leading to r ≈ 2.15 from Figure 4.

As a consequence, for cylindrical holes, even taking into account the presence of the a-Si:H layers that are more absorbing than the c-Si layer, we can still expect a current density gain higher than 1 mA cm -2 thanks to a lower parasitic absorption in the blue range of the solar spectrum. Usd chf chart 4.2 Estimation of the conversion efficiency losses in a large solar cell

In order not to depend on the geometry of the PC, we will consider an ideal solar cell ruled by its open circuit voltage V oc, its short circuit current I sc and its ideality factor n. Usd to aed rate As the series resistance R S does not influence both V oc and I sc for any solar cell, the following parts will focus on the conversion efficiency. Futures and commodity market news Then, at first approximation, the current I and the voltage V of such a solar cell at temperature T are linked by; (11)derived from: (12)where the dark current I dark has been expressed as a function of V oc since it corresponds to I = 0.

Equation ( 11)can be used to express the voltage V as a function of the current I, and so, the available electric power P: (13)Then, the maximum conversion efficiency η of this ideal solar cell with a serial resistance R S can be found from this last expression by calculating the maximum of P( I).

In order to maximize the influence of R S, we will consider an efficient solar cell with a V oc, j sc close to the actual record for a HIT structure [ 16], with the same conversion efficiency η 0 = 24.7%. Stock futures meaning We assume that a 75 nm thick ITO layer is responsible for the front series resistance increase. Asian stock market cnn money Finally, all the parameters are summed up in Table 3.

Figure 9 represents the degradation of the conversion efficiency η of this ideal solar cell as a function of the pitch p between the fingers and the degradation ratio r with the same vertical axis as for Figure 4. Rmb usd chart The cases where r = 1 describe the evolution of the conversion efficiency for a TCO covering all the pattern (Fig. Euro vs usd forecast 1a). Stock plan connect morgan stanley When r> 1, the solar cell looks like Figure 1b with partial TCO and a-Si:H layers. Aud usd history White contour lines obtained thanks to equations ( 9)show that for a given degradation ratio r, the conversion efficiency of the solar cell with a patterned TCO is the same as for the un-patterned one if both the pitch p and the width of the fingers (same optical shading) are divided by . Jpy to usd exchange rate history Consequently, patterning the TCO layer can be compensated by reducing the distance between two fingers to prevent the increase of the series resistance. Convert us dollars to euros Reducing the fingers width with the same ratio would avoid extra shadowing, so parasitic absorption in the top metal layer would stay unchanged. Gbp usd live chart Furthermore, by also scaling their width, Joule losses along the fingers are unchanged, with no additional electrical losses. Aud to usd conversion Then, the solution presented in Figure 1b can be used to reduce the parasitic absorption in the TCO, and, under the condition that a smaller pitch with thinner fingers can be achieved, its conversion efficiency can be higher than for a solar cell like Figure 1a.

Relative degradation of the conversion efficiency for an ideal efficient solar cell (see Tab. Aed conversion to usd 3). Usd to ngn Only the front series resistance is taken into account to calculate the conversion efficiency degradation. Exchange rate eur usd White lines are contour levels.

For instance, if we consider that a PC patterned thin film solar cell can reach the same performance as the Panasonic reference (pitch estimated at 2.3 mm) [ 16], then a filling factor of 0.55 (pessimistic value since the previous part shows that it could be as low as 0.37) leads to a degradation ratio r ≈ 3.6 (Fig. Gold price in us 4). Usd to vnd Consequently, the minimum pitch is 1.2 mm for an ITO layer with a resistivity of 540 μΩcm. Usd cad forecast 2016 The width of the fingers should also be divided by a factor ≈1.9 to keep the same shadowing and series resistance contribution of the electrical fingers. Convert usd into gbp According to the International Technology Roadmap for Photovoltaic (ITRPV), it is expected that this 50% decrease of the fingers’ width is reached in 10 years, passing from 45 to 22.5 μm [ 17].

On the other hand, if technological considerations lead to constraints in terms of fingers width and periodicity, one can define a threshold value of the pitch p th for which the increase of the photogeneration is balanced by the increase of the series resistance in the TCO. Usd to inr exchange rate comparison This particular point depends on the solar cell itself (more particularly its J sc). Hkd to usd The main consequence is that our structure can be more efficient if p< p th. Singapore to usd exchange rate For instance, using the extreme example used for Figure 9 with a pitch of 1.5 mm (instead of 1.2 mm) for a PC with a ff equal to 0.55 induces a relative 2.4% conversion efficiency loss. The boxer lyrics In that case, the benefit of a 1 mA cm -2 current density increase would be compensated by the increase of the electrical losses, so the pitch should be chosen between 1.2 and 1.5 mm to increase the solar cell conversion efficiency. Future stock market prices However, the previous part suggests that the optimal ff of the PC could be lower than 0.55, requiring a scale factor : a 32% reduction of the pitch and finger’s width is enough to ensure that no impact on the series resistance is introduced. Binary to bcd converter Consequently, it should be possible to pattern the TCO so that the optical gain corresponds to a minimum +1 mA cm -2 increase of J sc. Cad usd history Then, our solution is still more interesting even if the pitch is not as low as possible.

Last, one may notice that for thin film HIT solar cells with a thinner c-Si absorber, the short circuit current is actually limited to lower values that reduce the impact of the series resistance, leading to less constraint on the pitch and the width of the fingers. Yuan to usd 4.3 Influence of the pattern’ shape

Reference [ 11] demonstrates that for reverse pyramids, the filling factor ff must be around 0.9 to maximize the photon current density in the active material. Decimal calculator that shows work From Figure 4 and Table A.1, this condition typically corresponds to r ≈ 19. Market futures On the contrary, with a pattern of cylinder holes, the optimal ff is in the range 0.3–0.6, leading to smaller degradation ratio (typ. Usd to cad exchange rate r< 4.3). Nzd usd investing Using Figure 9, we can deduce that for cylinder holes, the ideal pitch and width of the fingers is closer to the industrial standard processes.

The contour lines of Figure 9 and the relation between filling factor and the degradation ratio (Fig. Bloomberg pre market futures 4) can be used as an abacus to directly obtain the pitch of the patterned TCO solar cell with the same series resistance. Usa to india exchange rate In that case, using equations ( 9)or ( 10)and a the simulation of a single period of the photonic crystal, the additional series resistance to be taken into account to convert a local I- V curve into that one of a large device can be calculated. Binary translator to text Then, the conversion efficiency of the large solar cell can be calculated. Convert usd to pounds 5 Conclusion

We report on the increase of the series resistance in thin HIT solar cells when patterning the front TCO layer. Gender quotes We demonstrate that this degradation of the electric properties only depends on the shape and filling factor of the pattern, and not its orientation towards the position of the electric fingers.

We also explain that a single local simulation can be used to calculate the series resistance of a large solar cell, and its conversion efficiency, even taking into account the patterning of the TCO layer.

Last, we demonstrate that the increase of the series resistance can be compensated by lowering the distance between fingers and their width. Equity meaning in economics Then, for a HIT solar cell with a periodic photonic crystal, the increase of the absorption in the active material thanks to the reduction of the parasitic absorption can be completely used to increase the conversion efficiency. Stock market futures fair value The only limitation is the capacity to reduce the finger’s width to prevent the increase of optical shadowing. Learning articles Consequently, the lower parasitic absorption in the TCO layer of a thin HIT solar cell with a patterned TCO layer can increase the conversion efficiency at the cost of a different front contact design. Cnbc stock market futures A gain on the short circuit current density up to 1 mA cm -2 is expected. Gbp to usd rate Appendix

In order to ease the estimation of the overall performance of a large solar cell from a local simulation, we propose to interpolate r -1 with a polynomial expression of the filling factor ff: (A.1)Then, using the coefficients in Table A.1, one may easily assess the degradation of diamond, round and square patterns in the TCO layer (Fig. Pound exchange rate 4).