Effective Surface Recombination Research Articles

The coupling effects of the different carrier generation rate distributions and recombination caused by nanostructures on the all-back-contact ultra-thin silicon solar cell performances

Ultra-thin silicon solar cell is a kind of good cost-effective energy solution. Moreover, nanostructured front surface and all-back contact design have been proposed as the promising methods to make ultra-thin silicon an effective solar irradiance absorber. Different nanostructures result in the different generation rate distributions and surface recombination, even though their corresponding integrated generation rates are the same. In addition, under the effect of carrier lifetime, the different generation rate distributions determine the number of carriers that reach the electrodes. Therefore, the grating nanostructures with three different section shapes are chosen for the investigation of the coupling effects of the different generation rate distributions and recombination caused by nanostructures. After consideration of the effect of the surface enhancement ratio of nanostructure, the deviation of electrical characteristics (fill factor), which is caused by the different generation rate distributions caused by nanostructures with the same integrated generation rate, is 0.20% at most as the effective surface recombination increases. Meanwhile, as the carrier lifetime decreases, the deviation of electrical characteristics (short circuit current) is only 1.80% at maximum and does not change with carrier lifetime. Therefore, it is found that, in all-back-contact ultra-thin silicon solar cell, the different generation rate distributions caused by nanostructures with the same integrated generation rate have little effect on the electrical characteristics, no matter how the effective surface recombination and carrier lifetime change. Based on the conclusion, a simplified method is developed for the electrical characteristics simulations. As the effective surface recombination and carrier lifetime change, the minimum error of electrical characteristics calculated by the simplified method is 0.06%, and the maximum is just 3.10%, which proves its accuracy.

Effective Surface Recombination of p+ Layers Doped Using Ion Implantation or Surface Deposited B Sources

Two key factors need to be fulfilled in order to enable high efficiency bifacial p-type PERT cell. High bulk lifetime values retention during production processes, and minimized effective surface recombination, Seff in the p+ layer. The influence of boron doped layer parameters on the effective surface recombination in the p+ layer of an n+-p-p+ bifacial PERT solar cell is evaluated. Back IQE data of n+-p-p+ cells with over-doped p+ layer were used for evaluation of Seff values in the p+ layer. Simulation was made for several doping profiles with the surface doping concentration, Bs, in the range 1018 - 1020 cm-3. According to simulation, the dominating parameter controlling the effect of the built-in charge in the passivation layer on the effective surface recombination is the Bs level. For Bs values above ∼1019 cm-3 this charge has no significant influence. p+ layer doping was made by B ion implantation as well as by thermal diffusion from deposited B source. Measurements were performed on symmetrically B doped n-Si wafers with different passivation. Good agreement between simulation and measurements was found.

Electronic Properties of Al p+ Surfaces Formed by Laser Doping from Aluminium Oxide Precursors: Implications for PERC Cell Design and Performance

Aluminium oxide (Al2O3)functions doubly as a high-quality surface passivation material for crystalline silicon and as an aluminium (Al) p-type precursor for laser doping. Thus, p+ doping based on laser ablation of Al2O3 thin-films deposited on a silicon substrate is an attractively simplified process for concurrent local contact definition and aligned surface doping. A number of studies have demonstrated this process, but a careful examination of the influence of laser parameters on the electronic properties of the Al laser doped p+ surface itself, including the influence of post-doping annealing, has yet to be presented. Such information is valuable for establishing process windows and for providing parameters by which the contact geometry can be optimized and the performance of locally Al2O3laser doped solar cells predicted. In this work, we present accurate characterization of the electronic properties of primary importance to solar cell performance: effective surface recombination and contact resistivity. Recombination at the Al2O3laserdoped p+ surface is found to exceed that of equivalently-doped p+ silicon from furnace diffused boron, while contact resistivity to vacuum evaporated Al is up to two orders of magnitude less than screen-printed, fired Al. Based on this characterization, computersimulations demonstrate that with optimized rear contact geometries, an industrially relevant PERC cell can approach 21 % efficiency, and the high-performance UNSW PERL structure can exceed 24 %.

High-Efficiency Large-Area Screen-Printed Solar Cell on Epitaxial Thin Active Layer With Porous Si Back Reflector Using Standard Industrial Process

Large area 17.3% high-efficiency screen-printed solar cells on a 90-μm-thick epitaxial silicon (epi-Si) active layer with a porous silicon (PSI) back reflector were fabricated using a 182-cm 2 epitaxial wafer equivalent (EpiWE) structure and a standard industrial process. The PSI layer was studied and optimized to serve as an efficient back reflector in the finished device. An effective back surface recombination velocity (BSRV) and back internal reflectance (Rb) of 90 cm/s and 88%, respectively, were extracted by PC1D modeling of the EpiWE cell. These values of BSRV and Rb are superior to a standard industrial full Al-BSF Si solar cell, where BSRV and Rb are usually ≥200 cm/s and ~65%, respectively. Model calculations showed very little drop in cell efficiency if the thickness of active epi-Si layer is reduced to ~30 μm because of the good-light trapping provided by the optimized PSI back reflector.

The Effect of Al-BSF on S<sub>eff</sub> and R<sub>b</sub> in Industrialized Mono-Silicon Solar Cells

In this work, the effect of aluminum back surface field formed by screen printed various amount of Al paste on the effective rear surface recombination velocity (Seff) and the internal rear reflectance coeffeicient (Rb) of commercial mono-silicon solar cells was investigated. We demonstrated the effect of Seffand Rbon the performance of Al-BSF solar cells by simulating them with PC1D. The simulated results showed that the lower Seffcould get higher open circuit voltage (Voc), at the same time, the larger Rbcould get higher short-circuit current (Isc). Experimentally, we investigated the Seffand Rbthrough depositing Al paste with various amount (3.7, 5, 6, and 8 mg/cm2) for fabricating Al-BSF mono-silicon solar cells. Four group cells were characterized by light I-V, spectral response, hemispherical reflectance and scanning electron microscope (SEM) measurements. It was found that, a minimum Seffof 350 cm/s was gotten from the cells with Al paste of 8 mg/cm2, which was extracted by matching quantum efficiency (QE) from 800 nm to 1200 nm with PC1D, and a maximum Rbof 53.5% was obtained from Al paste of 5 mg/cm2by calculating at 1105 nm with PC1D. When the amount of Al paste was higher than 5mg/cm2, there were less Seffand lower Rb. On the other hand, when Al amount was 3.7mg/cm2, it was too little to form a closed BSF. Based on the SEM graphs and simulations with PC1D, a simple explaination was proposed for the experimental results.

Effective optical generation rate and carrier surface recombination velocities in variband film photodetecting structures of MBE p-MCT

The profile is calculated for photogenerated nonequilibrium electron concentration in thickness of a three-layer film p-MCT structure in crossed electric and magnetic fields, in which the band gap of the central region is constant, but increases in adjacent variband regions. It is shown that a three-layer structure with variband regions can be replaced by a homogeneous film via introducing effective surface generation and surface recombination velocities at boundaries of central and variband regions.

Understanding and implementation of rapid thermal technologies for high-efficiency silicon solar cells

Rapid and potentially low-cost process techniques are analyzed and successfully applied toward the fabrication of high-efficiency monocrystalline Si solar cells. First, a methodology for achieving high-quality screen-printed (SP) contacts is developed to achieve fill factors (FF's) of 0.785-0.795 on monocrystalline Si. Second, rapid emitter formation is accomplished by diffusion under tungsten halogen lamps in both beltline and rapid thermal processing (RTP) systems (instead of in a conventional infrared furnace). Third, a combination of SP aluminum and RTP is used to form an excellent back surface field (BSF) in 2 min to achieve an effective back surface recombination velocity (S/sub eff/) of 200 cm/s on 2.3 /spl Omega/-cm Si. Next, a novel dielectric passivation scheme (formed by stacking a plasma silicon nitride film on top of a rapid thermal oxide layer) is developed that reduces the surface recombination velocity (S) to approximately 10 cm/s on the 1.3 /spl Omega/-cm p-Si surface. The essential feature of the stack passivation scheme is its ability to withstand short 700-850/spl deg/C anneal treatments (like the ones used to fire SP contacts) without degradation in S. The stack also lowers the emitter saturation current density (J/sub oe/) of 40 and 90 /spl Omega//sq emitters by a factor of three and ten, respectively, compared to no passivation. Finally, the above individual processes are integrated to achieve (1) >19% efficient solar cells with emitter and Al-BSF formed by RTP and contacts formed by vacuum evaporation and lift-off, (2) 17% efficient manufacturable cells with emitter and Al-BSF formed in a beltline furnace and contacts formed by SP, and (3) 17% efficient gridded-back contact (bifacial) cells with surface passivation accomplished by the stack and gridded front and back contacts formed by SP and cofiring.

18% efficient silicon photovoltaic devices by rapid thermal diffusion and oxidation

For the first time, cells formed by rapid thermal processing (RTP) have resulted in 18%-efficient 1 and 4 cm/sup 2/ 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/spl times/10/sup 5/ to about 2/spl times/10/sup 4//spl times/10/sup 4/ cm/s. This improvement resulted in an increase of about 1% (absolute) in energy conversion efficiency, up to 20 mV in V/sub ot/, and about 1 mA/cm/sup 2/ in J/sub 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 /spl ges/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/sub 0c/ by nearly a factor of ten), these RTP/RTO cells have become base dominated (J/sub 0b//spl Gt/J/sub 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.

Fabrication and characterization of 18.6% efficient multicrystalline silicon solar cells

Solar cell efficiencies as high as 18.6%(1 cm/sup 2/ area) have been achieved by a process which involves impurity gettering and effective back surface recombination velocity reduction of 0.65 /spl Omega/-cm multicrystalline silicon (mc-Si) grown by the heat exchanger method (HEM). Contactless photoconductance decay (PCD) analysis revealed that the bulk lifetime (/spl tau//sub b/) in HEM samples after phosphorus gettering can exceed 100 /spl mu/s. At these /spl tau//sub b/ levels, the back surface recombination velocity (S/sub b/) resulting from unoptimized back surface field (BSF) design becomes a major limitation to solar cell performance. By implementing an improved aluminum back surface field (Al-BSF), S/sub b/ values in this study were lowered from 8000-10000 cm/s range to 2000 cm/s for HEM mc-Si devices. This combination of high /spl tau//sub b/ and moderately low S/sub b/ resulted in the 18.6% device efficiency. Detailed model calculations indicate that lowering S/sub b/ further can raise the efficiency of similar HEM mc-Si devices above 19.0%, thus closing the efficiency gap between good quality, untextured single crystal and mc-Si solar cells. For less efficient devices formed on the same material, the presence of electrically active extended defects have been found to be the main cause for the performance degradation. A combination of light beam induced current (LBIC) scans as well as forward-biased current measurements have been used to analyze the effects of these extended defects on cell performance.

Measurement of Base Lifetime and Effective Back Surface Recombination Velocity in BSF Solar Cells

Rose and Weaver suggested that the open-circuit voltage decay and the short-circuit current decay experiments can be conveniently used to determine the surface recombination velocity and base lifetime of a BSF solar cell. In this paper, the analysis has been extended to include the effect of recombination in the emitter to determine the above parameters. It is shown that a prior knowledge of the emitter dark saturation current is necessary to determine their values accurately.

Photovoltage decay in p-n junction solar cells including the effects of recombinations in the emitter

The expressions for the open circuit photovoltage decay in a p-n junction solar cell are derived, including the effects of recombinations in the emitter. It is shown that for a cell with base thickness wB≫LB, the base diffusion length, the voltage decay rate for small values of time depends on the emitter dark saturation current JE0; the larger the value of JE0 , the faster is the initial rate of voltage decay. For large values of time, the rate of voltage decay is solely determined by the minority carrier lifetime in the base τB and is independent of JE0 . However, for a cell with wB≲LB, the voltage decay is linear from the very beginning and the decay rate is of the form (kT/e)[(1/τB) +(1/t1)]. The time constant t1 is independent of τB . It, however, depends on the base thickness, the effective back surface recombination velocity, and the emitter dark saturation current JE0 . The value of JE0 depends on emitter thickness, front surface recombination velocity, drift field in the emitter, band-gap narrowing, and Auger recombinations in the emitter.