Fabrication of p-n junction GaAs solar cells by a novel method

p⁺-n diffused junction GaAs solar cells have been successfully fabricated using a novel method in which the diffusion source, anti-reflective coating and protective cover layer are all formed in a single chemical vapor deposition run. The advantage of this process lies in its simple, low cost fabrication procedure and in the fact that the GaAs surface is protected throughout the fabrication procedure.     

Influence of the dislocation density on the performance ofheteroepitaxial indium phosphide solar cells

The authors point out that heteroepitaxial indium phosphide solar cells developed to date have low efficiency due to misfit dislocations. Dislocations act as recombination centers and strongly influence the solar cell performance. Calculations have been made to study the dependence of heteroepitaxial InP solar cell efficiency on dislocation density. The effects of surface recombination velocity and cell emitter thickness are also considered. Calculated results are compared with the available experimental results on representative InP solar cells. It is shown that heteroepitaxial InP cells with over 20% AM0 efficiency could be fabricated if dislocation density can be reduced to <10⁵ cm^-2 and the surface recombination velocity reduced to <10⁵ cm/s     

Process and device characterization of high voltage gallium arsenide P-i-N layers grown by an improved liquid phase epitaxy method

GaAs P-i-N layers with an i-region net doping of less than 10¹² cm⁻³ were grown on P⁺ and N⁺ substrates by a modified liquid phase epitaxy (LPE) method. Doping profiles and structural data obtained by varius characterization techniques are presented and discussed. A P⁺-P-i-N-N⁺ diode with a 25 μm-wide i-region exhibits a breakdown voltage of 1000 V, a t_rr of 50 ns, and reverse current densities (at V_R = 800 V) of − 3 × 10⁻⁶ A/cm² at 25°C and 10⁻² A/cm² at 260° C.     

Simulation and analysis of amorphous silicon image sensor having a p−i−n structure

The current-voltage characteristics of a p−i−n a-Si : H contact image sensor under dark and illuminated conditions have been simulated by solving the Poisson's equation and the continuity equations, and the results are correlated with the experiments. The dependence of the dark and photo-currents on the parameters such as the density of states in the gap, intrinsic layer width, dopant concentrations of p⁺ layer and n⁺ layer are discussed.     

The role of an amorphous SiC:H `buffer' in the high-performanceμc-SiC:H/a-SiC:H/poly-Si heterojunction solar cells

High-efficiency solar cells have been developed using relatively simple processing at low temperatures up to 300°C. The cells studied were p⁺ μc-SiC:H/p a-SiC:H (buffer)/n poly-Si and n⁺ μc-SiC:H/n a-SiC:H (buffer)/p poly-Si heterojunctions fabricated by the electron cyclotron resonance (ECR) plasma chemical vapor deposition (CVD) method. The thin amorphous buffer layer played an important role in improving the photovoltaic performance. The optimization of the buffer layer thickness resulted in a conversion efficiency of η=15.4% under AM1 solar simulated radiation of 100 mW/cm²     

Rise time of silicon p-n-p photodiodes

Exact analytical expressions are derived for the short circuit photodiode currents excited by light pulses, under the assumption that the drift carrier velocity linearly depends on the electric field in the depletion layer. Reflection from the back surface of the photodiode is taken into account. Using the obtained expressions it is possible to establish a connection between the rise time t_rise and the product W_eff of the absorption coefficient (λ) and effective depletion layer width W_eff(W) at various ratios of the diode thickness and the effective depletion layer width. The influence of the RC-constant (where C is the photodiode effective capacity and R is the sum of the diode series and loading resistances) on the rise time is also analyzed.

One of the most important conclusions is that generally the rise time is larger for p-n-n⁺ photodiode configurations than for n-p-p⁺ configurations at the same substrate resistivity.     

GaAs Shallow-homojunction solar cells on Ge-coated Si substrates

Solar cells with conversion efficiencies of 12% (AM1) have been fabricated from single-crystal GaAs epilayers grown by CVD on Ge-coated Si substrates. The cells utilize an n⁺/p/p⁺shallow-homo junction GaAs structure on a thin (<0.2 µm) epitaxial Ge layer. These solar cells are the first reported GaAs devices fabricated on Si substrates.     

An optimized rapid aluminum back surface field technique forsilicon solar cells

Screen-printing and rapid thermal annealing have been combined to achieve an aluminum-alloyed back surface field (Al-BSF) that lowers the effective back surface recombination velocity (S_eff) to approximately 200 cm/s for solar cells formed on 2.3 Ω-cm Si. Analysis and characterization of the BSF structures show that this formation process satisfies the two main requirements for achieving low S_eff: (1) deep p⁺ regions and (2) uniform junctions. Screen-printing is ideally suited for fast deposition of thick Al films which, upon alloying, result in deep BSF regions. Use of a rapid alloying treatment is shown to significantly improve the BSF junction uniformity and reduce S_eff. The Al-BSFs formed by screen-printing and rapid alloying have been integrated into both laboratory and industrial-type fabrication sequences to achieve solar cell efficiencies in excess of 19.0 and 17.0%, respectively, on planar 2.3 Ω-cm float zone Si. For both process sequences, these cell efficiencies are 1-2% (absolute) higher than analogous cells made with unoptimized Al-BSFs or highly recombinative rear surfaces     

Cz bifacial solar cells

High efficiencies have been achieved with bifacial solar cells made on FZ silicon. In the adaptation of the process to the more commonly used Cz material, attention has been paid to the influence of thermal processing on Cz, trying to avoid internal gettering effects related to oxygen precipitation. Lifetime measurements at different steps of the process have been carried out to quantify this influence. Reduction of “thermal load” by growth of a thin passivating oxide and deposition of a double antireflecting coating gives 17.7% when illuminated from the n⁺ side and 15.2% when illuminated from the p⁺ one     

Effect of antireflection coatings and coverglasses on silicon solar cell performance

The high reflectivity of the polished silicon surface of the newer N⁺/P silicon solar cells has emphasized the need for properly designed antireflection coatings to obtain improved solar cell performance. The problem is complicated by the facts that solar cells are generally tested in air, but are for their final application covered with a glass or quartz slide which is adhesive-bonded to the cell surface, and further, that solar cells operating in a nuclear particle radiation environment change their spectral response and are frequently optimized for performance at the end of design-life. Experiments have been performed to explore the antireflection characteristics of thin films of silicon monoxide which have been evaporated on the solar cell surface. The effect of the antireflection coating thickness on cell response as a function of wavelength has been determined and the improvement in cell short circuit current for Air Mass Zero space sunlight evaluated. Included in this study was the evaluation of the antireflection characteristics after the application of a coverglass with adhesive over the antireflection coating. For comparison, coverglasses were also applied to bare cells with no antireflection coating present. In all cases the various coating comparisons were based on the cell short-circuit current performance in Air Mass Zero sunlight.     

Numerical modeling of an amorphous-silicon-based p-i-n solar cell

A simulation program for amorphous-silicon-based p-i-n solar cells which allows for accurate calculation of single-junction or multijunction cell response under monochromatic or global AM1.5 illumination is discussed. The device model is based on a complete set of Poisson and current continuity equations describing the amorphous intrinsic and microcrystalline or amorphous n⁺ and p⁺ contacts. It predicts solar cell behavior with uniform and nonuniform optical (mobility) bandgaps, spatially dependent doping densities, and various layer thicknesses, as demonstrated by the very good agreement between the experimental and simulated current-voltage characteristics of single cells, with the bandgaps in the range of 1.75 to 1.47 eV. The material parameters used in the simulation have been obtained from experimental results reported in the literature. The possibility of obtaining higher efficiencies using novel cell designs has also been investigated. Calculations have been carried out on cell structures in which the bandgap of the intrinsic layer is profiled to help hole transport. The most efficient structure, also confirmed by recent experimental data, incorporates normal profiling throughout the bulk of the intrinsic layer with a thin graded buffer at the p⁺ -intrinsic junction     

A new method of determination of minority carrier diffusion lengthin the base region of silicon solar cells

A new method of determination of the minority carrier diffusion length (L) in the base region of an n⁺-p-p⁺ silicon solar cell using the spectral response of the cell in a middle wavelength (λ) range, e.g., 0.75<λ<0.90 μm is presented. In this method Q_int or if required Q_int/f where Q_int is the internal quantum efficiency of the cell and f=exp(-(x_bα_λ ))(L²α_λ²/L² α_λ²-1), x_b being the distance of base region from the front surface, is plotted against the reciprocal absorption coefficient (α_λ^-1 of silicon. The Q_int versus α_λ ^-1 or else Q_int/f versus α_λ^-1, plot gives an intercept L_MW on the α_λ^-1-axis and a unit intercept on the other axis. The intercept length L_MW is related to L through d/L and S_B, where d is the thickness of the base region and S_B is the back surface recombination velocity of minority carriers. For d/L>2.5, L=L_MW and is independent of S_B. However, for d/L<2.5, the true value of L which may be somewhat different from L_MW can be determined if S_B is known. While most existing long wavelength spectral response (LWSR) methods require d/L to be large (d/L>2.5) is such that tanh(d/L)≈1, this method has no such restriction on d/L. It is highly suitable for cells for which L is large but x_b is small. We have applied the MWSR and LWSR methods to a few n⁺-p-p⁺ silicon solar cells and have found that the former is much superior to the latter if d/L<2.5     

Modeling multi-band effects of hot electron transport in silicon by self-consistent solution of the Boltzmann transport and Poisson equations

This paper presents a self-consistent numerical technique for the solution of the multi-band Boltzmann transport equation (BTE) and the Poisson equation in silicon. The effects of high energy bands ( 3 eV) are modeled in the formulation. The numerical technique utilizes a new curvilinear boundary-fitted coordinate grid which is tailored for self-consistent calculations. A new Scharfetter-Gummel like discretization of the BTE is presented. The numerical algorithm is tested on a n⁺ − n − n⁺ device structure.     

Analysis of the enhancement of the performance of a polysilicon solar cell due to simultaneous passivation of grain boundaries and back contact

The enhancements in the short circuit current, open circuit voltage and efficiency of an n⁺/p polysilicon solar cell due to simultaneous passivation of the grain boundaries and the back contact are studied analytically. The results show that these enhancements are greater than the sum of the enhancements obtained by passivating the grain boundaries only and by passivating the back contact only. That is the enhancements due to simultaneous passivation cannot be deduced from studies on passivation of each boundary at a time.     

High conversion efficiency p-n+InP homojunction solar cells

p-n⁺InP homojunction solar cells have been fabricated and investigated. The best experimental cell without an antireflection coating exhibits a conversion efficiency of 13.5 percent (active area) under AM1 illumination; the corresponding open-circuit voltage, short-circuit current density, and fill factor (F.F.) are 0.817 V, 21.0 mA/cm², and 0.787, respectively.     

Thermal noise of a silicon double-injection plasma diode

Measurements of the thermal noise of a silicon p⁺−π−n⁺ diode operating in the Lampert-insulating regime agree within 6 per cent with the prediction S_i = 4kT Re(Y). The noise measurements were performed in the cube-law regime with d.c.-currents from 100 μA to 4mA at room temperature.     

18% efficient silicon photovoltaic devices by rapid thermaldiffusion and oxidation

For the first time, cells formed by rapid thermal processing (RTP) have resulted in 18%-efficient 1 and 4 cm² single-crystal silicon solar cells. Front surface passivation by rapid thermal oxidation (RTO) significantly enhanced the short wavelength response and decreased the effective front surface recombination velocity (including contact effects) from 7.5×10⁵ to about 2×10⁴ ×10⁴ cm/s. This improvement resulted in an increase of about 1% (absolute) in energy conversion efficiency, up to 20 mV in V_ot, and about 1 mA/cm² in J_sc. These RTO-induced enhancements are shown to be consistent with model calculations. Since only 3 to 4 min are required to simultaneously form the phosphorus emitter and aluminum back-surface-field (BSF) and 5 to 6 min are required for growing the RTO, this RTP/RTO process represents the fastest technology for diffusing and oxidizing ⩾18%-efficient solar cells. Both cycles incorporate an in situ anneal lasting about 1.5 min to preserve the minority carrier lifetime of lower quality materials such as dendritic-web and multicrystalline silicon. These high-efficiency cells confirmed that RTP results in equivalent performance to cells fabricated by conventional furnace processing (CFP). Detailed characterization and modeling reveals that because of RTO passivation of the front surface (which reduced J_0c by nearly a factor of ten), these RTP/RTO cells have become base dominated (J_0b≫J_0c), and further improvement in cell efficiency is possible by a reduction in back surface recombination velocity (BSRV). Based upon model calculations, decreasing the BSRV to 200 cm/s is expected to give 20%-efficient RTP/RTO cells     

New experimental evidence for minority-carrier reflection at negative-barrier MIS contacts

Recently, measurements of the open-circuit voltage of solar cells with negative-barrier metal-insulator-semiconductor (MIS) back contacts have been used to demonstrate that such contacts can function as the electrical analogues of metallurgical high-low junctions. In this brief, further experimental evidence for the minority-carrier reflecting properties of the negative-barrier MIS junction is presented. First, it is shown that a negative-barrier Mg-SiOx-nSi back contact can be used to enhance the long-wavelength photoresponse of p⁺-n solar cells in the same manner as a diffused n⁺back-surface field. Secondly, measurements of the effective surface-recombination velocity for an Mg-SiOx-nSi contact and for a diffused n-n⁺high-low junction formed on an identical substrate are reported. Both junctions gave very low values of recombination velocity, on the order of 50 cm/s.     

Breakdown voltage of diffused epitaxial junctions

The quantitative aspects of the breakdown-voltage calculations in reach-through-limited p⁺−n−n⁺ junctions are revisited, using numerical simulation. It is shown that the conventional abrupt-junction approximation may underestimate the breakdown voltage of diffused epitaxial junctions by as much as 60%, depending on junction depth. Oppositely, but less erroneously, a combined abrupt/linearly-graded approximation overestimates the breakdown voltage by at most 15%. A set of numerically calculated plots are provided for the design of low-voltage power devices.