Back Surface Field Solar Cells Research Articles

Importance of the emitter in thin back-surface field solar cells

The variation in the short-circuit current density J sc, the open-circuit voltage V oc and the efficiency η with the base thickness of an n +-p-p + back-surface field (BSF) solar cell with a resistivity of 10 Ω cm is reported for 1 sun air mass 0 illumination. It was found that when the cell thickness is small the cell characteristics are determined by the properties of the emitter. The effects of different emitter dark saturation current densities J E0 and different base diffusion lengths L B on the variation in V oc with the change in base thickness were calculated. If the base diffusion length is taken to be equal to or greater than 1000 μm for J E0 = 2 × 10 −12 A cm −2, the calculated variation in V oc with the base thickness is found to be in good agreement with the observed result that V oc is independent of the cell thickness in the base thickness range 100–300 μm. The calculations which were made for the case when a light reflector is present at the back of the BSF cell show that the maximum efficiency, which is obtained at a base thickness w B ≈ 200 μm, is appreciably increased. The effective surface recombination velocity S eff at the edge of the low-high junction is calculated as a function of the thickness w H of the high region for various values of the recombination velocity S r at the back contact. The calculations show that for S r > S v (where S v is the diffusion velocity in the high region and is equal to D H/ L H, and D H and L H are the diffusion coefficient and the diffusion length of the high region respectively) small values of S eff are obtained when the high thickness w H is greater than the high region diffusion length L H ( w H ≈ 3 L H). For S r = S v, S eff is independent of the thickness of the high region and has a value given by S v N B/ N H; when S r < S v, the high region must be thin but the thickness must be larger than the depletion width of the low-high junction. The temperature degradation coefficient of V oc for a BSF cell was calculated with the band gap narrowing in both the emitter and the high region taken into account. The band gap narrowing in both these regions reduces the temperature degradation coefficient of V oc.

Investigation of polycrystalline silicon back surface field solar cells

Solar cells fabricated from "bictystalline" silicon wafers with respective orientations of

Determination of the minority carrier lifetime in the base of a back-surface field solar cell by forward current-induced voltage decay and photovoltage decay methods

Theoretical expressions are derived for both the forward current-induced voltage decay (FCVD) and the photovoltage decay (PVD) in a base-dominated back-surface field (BSF) solar cell of arbitrary thickness. The expressions apply to the general case where the effective surface recombination velocity S at the edge of the low-high junction in the base can have any value. If the value of S is taken as infinity, the expressions become applicable to conventional cells with finite base thicknesses. The case of an effective BSF cell in which S = 0 and that of a conventional cell in which S = ∞ are considered in detail. Large errors were found to occur in the values of minority carrier lifetimes τ B when lifetimes in the bases of thin conventional cells were determined by the FCVD method using the formula obtained previously; this formula applies only to thick cells. The difference between the FCVD and the PVD is large for W = ∞ ( W = w/ L B where w is the width of the base and L B is the diffusion length), as shown in previous papers by other workers. As W decreases, the FCVD and the PVD plots become linear, approach each other and are almost identical for W ⩽ 0.5, both for S = 0 and for S = ∞. For S = 0, however, the decay becomes slower as W decreases and the reciprocal of the slope of the FCVD or the PVD plot is equal to the minority carrier lifetime in the base. In contrast, for S = ∞ the voltage decay rate increases rapidly as W decreases, i.e. the FCVD and the PVD plots become steeper; the reciprocal of the slope then becomes equal to τ B/(1 + π 2/4 W 2). For small W the advantage of using PVD rather than FCVD for the determination of τ B disappears. The PVD in both effective BSF and conventional cells after light pulses of very short duration, and of arbitrary duration, is also discussed. It is shown that in a thick base solar cell the PVD after short pulse excitation can conveniently be used to determine the minority carrier lifetime, even if the pulse is of arbitrary duration, provided that 1/α (where α is the absorption coefficient) is larger than L B. However, if the base is thin ( W ⩽ 0.5), the FCVD, the PVD and the PVD after short pulse excitation become almost identical. The method by which the lifetime and the effective surface recombination velocity S can be determined from the observed PVD or FCVD is also discussed for the general case when S is not zero.

Effect of thickness on silicon solar cell efficiency

Semiconductor material cost is one of the factors which determines the performance-cost ratio and economical feasibility of silicon solar cells for terrestrial power generation. Decreasing the cell thickness would lower the silicon material cost. The energy conversion efficiency of a back-surface field solar cell will have a peak as the silicon film thickness is reduced due to two opposing factors: 1) the open-circuit voltage increases and 2) the short-circuit current decreases with decreasing cell thickness. A computer-aided-design study on the dependence of this efficiency peak on the concentrations of the recombination and dopant impurities is presented in this paper. The illuminated current-voltage characteristics of over 100 cell designs were obtained using the transmission line circuit model to numerically solve the Shockley equations. Using an AM1 efficiency of 17 percent as a target value, which is the highest encapsulated silicon cell efficiency used in the Block IV modules of the Low-Cost Solar Array Project, it is shown that the efficiency versus thickness dependence has a broad maximum which varies less than 1 percent over more than a three-to-one range of cell thickness from 30 to 100 µm. Optical reflecting back surface will give only a slight improvement of AM1 efficiency, about 0.7 percent, in this thickness range. The sensitive dependence of efficiency on patchiness across the back-surface field low-high junction in thin cells is noted.

Temperature coefficients of the open-circuit voltage of p-n junction solar cells

An analytic expression for the tempoerature coefficient of the open-circuit voltage has been derived in general, which may be applied to different kinds of p-n junction solar cells. Based on the dominance of the saturation dark current density, the simplified expression have also been developed and justified by experimental measurements. It has been shown that the negative temperature coefficient of the open-circuit voltage is decreased with the increasing light level and the interaction between minority carriers and the high-low junction. Hence, the temperature coefficient of the open circuit voltage is good measure for providing a guide to test the effectivness of a high-low junction in a back-surface-field (BSF) solar cell. Moreover, based on the measured temperature coefficient of the open-circuit voltage from the BSF solar cells fabricated on thin epitaxial substrate, a new method for measuring the effective energy-gap narrowing in the highly doped emitter has been proposed and studied. It has shown that the effective energy-gap narrowing measured is in good agreement with the eempirical expression proposed by Slotboom.

Theory of back surface field silicon solar cells

Back surface field silicon solar cells with n + pp + (or sometimes p + nn +) structures are found to have better characteristics than the conventional solar cells. The existing theories have not been able to satisfactorily predict the experimentally observed parameters on these cells. A theory, based on the transport of both minority and majority carriers under the charge neutrality condition, has been developed in the present paper which explains the behavior of the back surface field solar cells. Good agreement is achieved between the results obtained by using this theory and the experimental observations of earlier workers.

Analysis of the photo voltage decay (PVD) method for measuring minority carrier lifetimes in P-N junction solar cells

When the diffusion length of minority carriers becomes comparable or even larger than the thickness of a P-N junction solar cell, the characteristic decay of the photogenerated voltage becomes a mixture of contributions with different time constants. The minority carrier recombination lifetime τ and the time constant l2/D, where l is essentially the thickness of the cell and D the minority carrier diffusion length, determine the signal as a function of time. It is shown that for ordinary solar cells (N+-P junctions), particularly when the diffusion length L of the minority carriers is larger than the cell thickness l, the excess carrier density decays according to exp(−t/τ−π2Dt/4l2), τ being the lifetime. Therefore, τ can be readily determined by the photo voltage decay (PVD) method once D and l are known. For ideal back-surface-field (BSF) cells (N+-P-P+ junctions) under the same circumstances the excess number density of carriers decays purely exponentially as exp(−t/τ). However most BSF solar cells are not ideal, possessing an effective surface recombination velocity seff of 100 to 1000 cm/sec at the high-low junction. Therefore, PVD measurements for BSF cells must be treated with caution and must be supplemented with other nonstationary methods recently developed. These facts are important for a determination of diffusion lengths since steady-state methods are notoriously unreliable when the cell thickness is smaller than the diffusion length. Finally a connection will be made with older work on the same subject.

High efficiency bifacial back surface field solar cells

The first high efficiency p +-n-n + bifacial solar cells are presented. Efficiencies of 15.7% and 13.6% were measured under front and back air mass one illumination respectively at 28°C. At 7 air mass one illumination and 28°C the front efficiency increases to 16.5%. A pilot production of 200 cells was made following a fabrication process as simple as that for conventional back surface field cells. Mean efficiencies of 13.4% and 10.7% were obtained under front and back illumination respectively. The production yield is higher than 80%.

An induced back surface field solar cell employing a negative barrier metal-insulator-semiconductor contact

A simple theory of current flow in negative barrier metal-insulator-semiconductor (MIS) junctions is developed. The properties of negative barrier MIS contacts are found to be closely related to those of positive barrier MIS junctions in which the semiconductor surface is strongly inverted. It is shown that by an appropriate choice of barrier metal work function and insulator thickness it should be possible to produce a negative barrier MIS contact which does not impede the flow of majority carriers, but which acts as a surface of low effective recombination velocity for minority carriers. It is demonstrated experimentally that such minority-carrier reflecting negative barrier contacts can provide an enhancement in solar cell open circuit voltage equal to that obtained with a diffused back surface field. This is the first report of an induced back surface field solar cell.

Physics underlying the performance of back-surface-field solar cells

A description of the physics of back-surface-field (BSF) solar cells is presented, in which several key approximations, valid for effective BSF cells, have been used to express the results of the analysis in ways that make them useful in understanding the performance of high-efficiency BSF cells. A silicon p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -n-n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> BSF solar cell, developed in conjunction with the theoretical treatment, provides experimental support for the theory. Measured current-voltage characteristics, together with the analysis, are used to describe the important physical mechanisms that control the performance of the BSF solar cell. For example, the analysis enables the determinations of the carrier lifetimes in the base region and of the effective surface recombination velocity at the low-high junction.

The open-circuit voltage of back-surface-field (BSF) p- n junction solar cells in concentrated sunlight

Simple analytical expressions for the open-circuit voltage of the n +− p− p + and p +− n− n + BSF solar cells, which are valid for both the low- and high-levels of optical illumination, are derived. Based on the principle of superposition the open-circuit voltage of both the n +− p− p + and p +− n− n + solar cells are expressed in terms of the short-circuit current and the known saturated dark current. Effects of the high-low junction doping, the energy-gap shrinkage, and the dimensions of the BSF solar cells on the open-circuit voltage are included. The numerical results of the derived expressions are found to be in good agreement with the exact numerical analysis of Fossum et al. The optimal design considerations based on the known characteristics of the open-circuit voltage are also discussed.

Optimisation of p+ doping level of n+-p-p+ bifacial b.s.f. solar cells by ion implantation

High-low junctions of n+-p-p+ bifacial back surface field solar cells have been fabricated by B11+ implantation. After an isothermal annealing step at 900°C for 10 min, an optimum surface concentration of 5 × 1019 cm−3 is observed. Its origin is found to lie in the partial electrical activation of the implanted impurities.

A simple theory of back surface field (BSF) solar cells

A theory of an n-p-p+ junction is developed, entirely based on Shockley’s depletion layer approximation. Under the further assumption of uniform doping the electrical characteristics of solar cells as a function of all relevant parameters (cell thickness, diffusion lengths, etc.) can quickly be ascertained with a minimum of computer time. Two effects contribute to the superior performance of a BSF cell (n-p-p+ junction) as compared to an ordinary solar cell (n-p junction). The sharing of the applied voltage among the two junctions (the n-p and the p-p+ junction) decreases the dark current and the reflection of minority carriers by the builtin electric field of the p-p+ junction increases the short-circuit current. The theory predicts an increase in the open-circuit voltage (VOC) with a decrease in cell thickness. Although the short-circuit current decreases at the same time, the efficiency of the cell is virtually unaltered in going from a thickness of 200 μm to a thickness of 50 μm. The importance of this fact for space missions where large power-to-weight ratios are required is obvious.

Analysis of Silicon Solar Cells with Multi-Layer Structure

The multi-layer silicon solar cells have been theoretically analyzed at AM 0 by a simplified one dimentional first order solar cell model, which neglects depletion region recombination, high injection and drift currents. It is shown that the six-layer solar cell is an optimum structure. The six-layer solar cell is compared with the conventional p+-n solar cell and the back-surface field (BSF) p+-n-n+ solar cell. It is proven that the multi-layer solar cells can show still higher conversion efficiencies than the BSF solar cell and the conversion efficiency can approach about 21% at AM 0.