Experimental and theoretical evaluation of boron diffused high-low junctions for BSF solar cells

An evaluation of boron diffused pp⁺ high-low junctions for BSF purposes, modelled in terms of an effective recombination velocity S_pp⁺, is presented. S_pp⁺ values are determined experimentally for three typical pp⁺ high-low junctions by means of performance analysis of grating MIS solar cells with a non-inverting antireflection coating; this type of cell is very sensitive to the minority carrier reflecting capability of the back surface field, due to the two-dimensional collection mechanism. The measured S_pp⁺ values are related to the physical structure of the high-low junctions using a general mathematical formulation. It is shown that, by including as much relevant data as are now known concerning the profile of electrically active impurities N_A, Auger and SRH recombination and dopant and field dependent diffusivities, and by employing the Slotboom and de Graaff bandgap-narrowing model (degeneracy included), a good agreement is obtained between the measured S_pp⁺ values and the theoretical values. It is also established that Auger recombination has a major impact on the minority carrier reflecting capability of the high-low junctions.     

Modified drift field model for high-low transition in solar cells

A theoretical discussion is presented for the understanding of the back surface pp⁺ transition used in solar cells. It is shown that quasi-neutrality of space charge is a good approximation if the back surface diffusion is fairly deep. The error involved in the drift field model developed by assuming a quasi-neutrality of the space charge is compared with that inherent in the abrupt high-low junction model. The analysis shows that the back surface boundary, when measured from the heavily doped p⁺ side, effectively exists at a distance much larger than the impurity diffusion depth and the recombination current in the base is always less than its value estimated from the abrupt junction model. The voltage in the pp⁺ transition is due to the change in electric field by the excess carriers injected by light and drops across those regions of the cell where the injected carrier density is appreciable.     

Study of injection effects on BSF silicon solar cells

The behavior of p⁺-n-n⁺ and n⁺-p-p⁺ silicon solar cells in terms of short-circuit current, open-circuit voltage, fill factor, and efficiency is studied as a function of base doping and illumination levels. A theoretical model that is valid for any injection level in the base region is used. Experimental results for cells of n-type base (in the range of 0.3 to 1000 Ω-cm) and a p-type base (0.4 to 300 Ω-cm) are presented. The theoretical model is able to explain phenomena such as the superlinearity of I_sc with concentration and the degradation of short-circuit current and efficiency at very high concentrations. These effects are seen as connected with the ohmic electric field in the base region. For the emitter saturation currents considered here, it can be concluded that, for p-type substrates, low base resistivities (≅1 Ω-cm) are necessary to achieve high efficiencies under concentrated light (≅100 suns), while for flat-array cells a particular resistivity is not required. For n-type substrates, it is found that any resistivity level can be used for both flat-array and concentrator cells     

Physics of BSF solar cells at high levels of illumination

The theory of BSF solar cells by Fossum et al. [1] is extended to high levels of injection. For such intensities, more generalJ_{SC}-V_{OC}relationships are derived which show deviation from the idealexp (qV/kT)relationship.     

Interpretation of C/V characteristics for heterojunctions and high-low junctions

Useful information about the interface can be obtained for heterojunctions or high-low junctions by simple inspection of the graphs of C?2 against V. The distance between the intercepts of the two linear portions of the graph on the voltage axis is simply related to the doping densities and the interface charge.     

Analysis of efficiency limitations in high-low emitter back surface field silicon solar cells

Analytical and numerical models have been used to analyse the electrical behaviour of HLE-BSF n+-n-p-p+ silicon solar cells. It is shown that this structure should have lower open-circuit voltage values than the simple HLE solar cell due to the recombination current in the p+ region. Back surface passivation is proposed as a means to improve the Voc, that can be as high as 650 mV.     

Investigation of Al back contacts and BSF formation by in situ TEM for silicon solar cells

Back contacts for Si solar cells made by Al evaporation and screen printing Al paste were studied by transmission electron microscopy. Si was found to diffuse into the Al during heating. Si diffusion formed vacancies in the Si wafer and Al could then penetrate the Si wafer in spiked formations. The Al spikes retracted during cooling, leaving a doped back surface field region. Copyright © 2012 John Wiley & Sons, Ltd.     

The spectral response of BSF silicon solar cells fabricated through masked ion implantation

The work presents the main peculiarities of the spectral response of back-surface-field (BSF) silicon solar cells fabricated through masked ion implantation of the n+-p junction. The emphasis is on the shift of maximum responsivity toward the visible spectrum and on the large bandwidth of these n+-p-p+ optical sensors. The dependence of these parameters on technological parameters are outlined in the communication.     

背电场硅太阳能电池离子辐照效应

空间太阳能电池受电子、质子，以及重离子辐照后，要受到损伤。文章对背电场太阳能电池低能质子、高能质子、碳、氧离子辐照效应进行了研究，对辐照前后太阳能电池光电参数进行测量，并用Ｍａｔｔｅ－Ｃａｒｌｏ方法对辐照粒子的硅中的能量损失过程进行了模拟计算。结果表明，各种离子辐照技术电池的损伤是不同的，低能质子对开路电压的损伤比对睡电压的损伤大得多，而高能质子 地开路电压的损伤较低能质子的反而要小，对短路电流的     

A high-low junction emitter structure for improving silicon solar cell efficiency

A new device structure which is designed to substantially increase the open-circuit voltage and the power-conversion efficiency of p-n junction silicon solar cells is described. The structure differs from the conventional cell structure in that it contains a high-low (H-L) junction in the emitter. Based on numerical solutions of the fundamental Shockley differential equations, efficiency improvements of about 15 percent at AM1 and about 40 percent at 50 suns can be epxected. The improvement at 50 suns results in an efficiency of about 20 percent at 27°C.     

Analysis of silicon solar cells with stripe geometry junctions

Silicon solar cells with stripe geometry junctions are analyzed. The base region collection efficiency is found to be insensitive to the transmissivity of the electrode and/or the diffused layer but quite sensitive to the width and separation of the stripe junctions. The additional losses of carriers are mainly due to the increased bulk recombination rather than the surface recombination. In one case analyzed the collection efficiency decreases by 22% when the junctions are separated by about one sixth of the diffusion length with the surface recombination velocity at 300 cm/sec. Possible uses of the stripe-junction design in p-n junction cells and Schottky barrier cells are re-examined in the light of the new calculation and found to be less attractive than previously suggested.     

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.     

High-low junctions for solar cell applications

A new theoretical model to calculate the effective surface recombination velocity (S_eff) of a high-low junction with an arbitrary impurity distribution is presented. The model is applied to erfc-diffused pp⁺ junctions using experimental data of bandgap narrowing, lifetime and mobility. Bandgap narrowing is shown to degrade the minority carrier reflecting properties of the high-low junction. Computer results are applied for the design of BSF solar cells and to study other solar cells structures based on high-low junctions.     

Characteristic features of silicon multijunction solar cells with vertical p-n junctions

A relatively simple technology (without photolithography) based on diffusion welding and ion-plasma deposition of an insulating coating has been developed for fabricating multijunction silicon solar cells with vertical p-n junctions. The effective collection factor for such structures is independent of the wavelength of the incident light in the wavelength range λ=340–1080 nm. Fiz. Tekh. Poluprovodn. 31, 855–857 (July 1997)     

Performance comparison of AlGaAs,GaAs and InGaP tunnel junctions for concentrated multijunction solar cells

Four tunnel junction (TJ) designs for multijunction (MJ) solar cells under high concentration are studied to determine the peak tunnelling current and resistance change as a function of the doping concentration. These four TJ designs are: AlGaAs/AlGaAs, GaAs/GaAs, AlGaAs/InGaP and AlGaAs/GaAs. Time‐dependent and time‐average methods are used to experimentally characterize the entire current–voltage profile of TJ mesa structures. Experimentally calibrated numerical models are used to determine the minimum doping concentration required for each TJ design to operate within a MJ solar cell up to 2000‐suns concentration. The AlGaAs/GaAs TJ design is found to require the least doping concentration to reach a resistance of <10⁻⁴ Ω cm² followed by the GaAs/GaAs TJ and finally the AlGaAs/AlGaAs TJ. The AlGaAs/InGaP TJ is only able to obtain resistances of ≥5 × 10⁻⁴ Ω cm² within the range of doping concentrations studied. Copyright © 2010 John Wiley & Sons, Ltd.     

Determination of minority carrier lifetime and effective back surface recombination velocity in BSF silicon solar cells from transient measurements

The method of determining the base lifetime ?B and the effective surface recombination velocity Seff in a BSF solar cell from the transient decay of open-circuit voltage and short-circuit current is extended to include emitter recombinations. If the emitter recombinations in modern Si solar cells are neglected in interpreting the experimental data, the experimental value of Seff is found to be in large error.     

Operating regimes for second generation luminescent solar concentrators

In this paper, we introduce the concept of first and second generation luminescent solar concentrators. Traditional, first generation devices are characterised by their randomly oriented molecules that absorb sunlight and emit luminescence isotropically. By applying detailed balance to the absorbed and emitted photon fluxes we derive the Shockley–Queisser limit for these devices. It is found that they have inherently low efficiency due to optical losses (the well known reabsorption problem) and also that device performance is strongly affected by the areal ratio between the top and edge surfaces. This latter property makes it very difficult to achieve significant cost reductions because as the edge area is reduced (to lessen the amount of expensive photovoltaic material required for conversion), the efficiency of the system diminishes. First generation concentrators have now approached the fundamental limits which we predict here, thus to achieve a stand‐alone luminescent concentrator that enables significant cost reductions, second generation approaches are now needed. New, second generation devices are characterised by either directional emitters or photonic filters which enhance the waveguiding mechanism, allowing high efficiency and large sizes to be achieved simultaneously. Here we define the fundamental operating regime in which second generation technology must reach to surpass the limit of first generation devices. Copyright © 2011 John Wiley & Sons, Ltd.     

Upper limits to absorption enhancement in thick solar cells using diffraction gratings

The application of diffraction gratings to solar cells is a promising approach to superseding the light trapping limits of conventional Lambertian structures. In this paper a mathematical formalism is derived for calculating the absorption that can be expected in a solar cell equipped with a diffraction grating, which can be applied to any lattice geometry and grating profile. Furthermore, the formalism is used to calculate the upper limit of total absorption that can theoretically be achieved using a diffraction grating. The derived formalism and limits are valid when the solar cell thickness is greater than the coherence length of the illuminating solar spectrum. Comparison is made to the upper limit achievable using an angularly selective Rugate filter, which is also calculated. Both limits are found to be considerably higher than the Lambertian limit within the range of sunlight concentration factors practically employed in photovoltaic systems (1–1000×). The upper limit of absorption using the diffraction grating is shown to be equal to the thermodynamic limit for all absorbances and concentration factors. The limit for the Rugate filter is generally lower, but tends to the thermodynamic limit for lower cell absorbances. Copyright © 2011 John Wiley & Sons, Ltd.     

Operating limits for RF power amplifiers at high junction temperatures

LDMOS RF––power amplifier components usually operate under severe conditions challenging long-term reliability. These components are subjected to high power dissipation and consequently high junction temperatures. Failure mechanisms are highly temperature dependent and driven by coupled electro-thermo-mechanical fields as a function of stress time. In this work we have investigated the reliability of such a component. Power cycling was used to assess its reliability by introduction of temperature gradients and transient at elevated junction temperatures. The experimental lifetime acceleration conditions provided transient thermal constraints to the thermo-mechanical strength of the silicon die. Power dissipation has been adjusted to cover a broad temperature range (T_{j max}: 200–300 °C) in the peak of a single power cycle. Different failure modes have been observed and related to the different temperature ranges. The experimental results have been combined with thermo-mechanical FE-simulations in ANSYS, leading to the validation of simulation models and implementation in a larger simulation network. The power cycling approach as applied in this paper provided a useful addition to the steady state reliability information. In this way, clear information about margins for safe operation under dynamical conditions has been obtained. This information is needed to fully exploit the functional capability of the component and avoid over-specification in the final application. Overall, the LDMOS RF–PA component showed excellent reliability which makes it suitable for application in telecom devices.