Full-area Aluminum Back Surface Research Articles

Recombination Parameters of the Diffusion Region and Depletion Region for Crystalline Silicon Solar Cells under Different Injection Levels

In order to maximize performance in all conditions of use, and to model exactly the performance of solar cells, it is very important to study the recombination parameters under different injection levels. In this paper, the recombination parameters and their effect on the output performance of solar cells are investigated under different injection levels for the full-area aluminum back surface field (Al-BSF) solar cell and passivated emitter and rear cell (PERC) solar cell for the first time. It is found that the recombination parameter J01 of the diffusion region and the recombination parameter J02 of the depletion region for the PERC solar cell are smaller than those of the Al-BSF solar cell under the same injection level. A new finding is that the recombination parameter J01 of Al-BSF solar cells increases quickly with the decreasing injection level compared with PERC solar cells. Finally, the J01/J02 of Al-BSF and PERC solar cells is investigated, and the effects of J01/J02 on the electrical parameters are also analyzed for Al-BSF and PERC solar cells under different injection levels. The obtained conclusions not only clarify the relationship between the recombination parameters and injection levels, but also help to improve cell processes and accurately model daily energy production.

Temperature Coefficients and Operating Temperature Verification for Passivated Emitter and Rear Cell Bifacial Silicon Solar Module

In this article, the temperature coefficients of the bifacial passivated emitter and rear cell (PERC) solar module and the full-area aluminum back surface field (Al-BSF) solar module were calculated based on the theoretical model under different module temperatures and irradiances. Meanwhile, the measured temperature coefficients were obtained by testing the performance of bifacial PERC solar module and conventional Al-BSF solar module under different module temperatures and irradiances. The results show that the laboratory measurements are close to the calculated values, and the normalized temperature coefficients of output power for the bifacial PERC solar module are lower than that of the conventional Al-BSF solar module under the same operating conditions. An interesting finding is that the normalized temperature coefficients of short-circuit current for the bifacial PERC solar module are larger than that of the conventional Al-BSF solar module under the same irradiance. This is attributed to lower back surface recombination velocity and higher carrier collection efficiency of PERC solar cells compared with conventional Al-BSF solar cells. Additionally, the operating temperatures of the bifacial PERC solar module and conventional Al-BSF solar module were surveyed outdoor. It is found that the operating temperature of the bifacial PERC solar module is higher than that of the conventional Al-BSF solar module in some cases. However, this tendency is reversed in other cases when these two solar modules work under the same environmental conditions. The conclusion was explained reasonably according to the actual operating conditions of solar modules. The obtained results in this work can provide useful insight into evaluating the field performance of the bifacial PERC solar module.

Low cost multicrystalline bifacial PERC solar cells – Fabrication and thermal improvement

It is important to find a more cost-effective way of putting multicrystalline silicon as the mainstream silicon materials for industrial silicon solar cell fabrications. This paper presents a low-cost, bifacial multicrystalline PERC (passivated-emitter and rear cell) cells approach. Instead of the traditional PERC cells with full-area aluminum back surface field, cells rear-side is treated with aluminum grid pattern and firing-through aluminum paste. Thus, the bifacial power generation is obtained, and the laser ablation step is avoid and the consumption of aluminum paste is reduced. Rear surface passivation and metal contact are optimized via enhancing the corrosiveness of aluminum paste, matching the passivation film and improving the co-firing process. LID (light induced degradation) phenomenon of this kind of cell is controlled within a reasonable range by both light injection annealing and electric injection annealing. In addition, an anneal at 385 °C can further increase the cell efficiency by 0.2% absolute. The average batch efficiency of the front-side can reach 19.63%. The champion cell efficiency is 19.86% on the front and 13.65% on the rear.

Active area cell efficiency (19%) monocrystalline silicon solar cell fabrication using low-cost processing with small footprint laboratory tools

A high efficiency (> $$18\%$$ ) industrial large area crystalline silicon wafer solar cell fabrication process generally requires industrial equipment with large footprint, high capital and running costs. Stricter processing window, continuous monitoring and automated functioning are the reasons for it. However, for any conventional laboratory (lab) it is always difficult to manage these requirements with limited available lab space or insufficient fund and other related resources. In this work, we report a novel way to fabricate high efficiency full area aluminium back surface field monocrystalline silicon wafer solar cells in our lab using low-cost processing with small-footprint fabrication tools for 6 inch pseudo-square industrial wafers. The novelty of our work includes optimization of every fabrication process step, e.g., texturization, emitter diffusion, emitter passivation and anti-reflection coating deposition, edge-isolation, screen printing and co-firing individually. These modifications include tuning of processing tools and processes, utility changes and inclusion of additional process steps. Beaker-based chemical processes, manual diffusion furnace, introduction of low temperature oxidation, low temperature silicon nitride deposition processes, plasma-edge isolation tool, single manual screen printer, single oven drying of metal pastes and co-firing using rapid thermal processing tools were used at our lab. For our cells, actual and active area efficiencies of 18.5 and 19% (measured under AM1.5G 1 Sun condition), respectively, were achieved.

Advanced passivation of laser-doped and grooved solar cells

In this work, we investigate the use of advanced hydrogenation and low-temperature diffusion processes (a 3 h 700 °C process after emitter diffusion) for the electrical neutralization of laser-induced defects for laser doped and grooved solar cells. Despite the laser doping and grooving (LDG) process being performed before silicon nitride passivation to avoid thermal expansion mismatch between the silicon and the silicon nitride layer, some crystallographic defects are still formed during the process. The application of a low-temperature diffusion process increases implied open circuit voltages by 14 mV, potentially due to phosphorus diffusion of dislocated regions induced during laser processing. Laser hydrogenation is shown to be capable of passivating the majority of the remaining laser-induced defects. Over 1% absolute improvement in efficiency is achieved on cells with a full area aluminum back surface field. Preliminary results with minimal optimization demonstrate efficiencies of over 19% with a full area Al back contact cell. The potential to achieve much higher voltages when used with a passivated rear is also demonstrated.

Crystallization of multicrystalline silicon from reusable silicon nitride crucibles: Material properties and solar cell efficiency

In this paper experimental results on High Performance multicrystalline silicon ingots (HPmc-Si) grown from reusable silicon nitride crucibles are presented. Five ingots are grown from the same crucible and the material is analyzed for carbon and oxygen, and metallic impurities, by means of Fourier transform infrared (FTIR) spectroscopy and Neutron Activation Analysis (NAA), respectively. Microwave Photoconductance Decay (μW-PCD) and Quasi-Steady-State Photoconductance (QSSPC) is applied on samples to reveal the resulting charge carrier lifetime distribution. Full-area aluminum back surface field (BSF) solar cells are processed from 156 mm × 156 mm wafers and the solar cell efficiency is analyzed. Material properties and solar cell efficiencies are comparable to reference material grown from high purity electronic grade (EG) quartz crucibles. Obtained results are discussed with respect to results published recently.

Impact of the Presence of Busbars During the Fast Firing Process on Contact Resistances

Uniform and low contact resistances between Ag grids and Si emitters are very important for reducing series resistance losses in screen-printed silicon solar cells. In this paper, we studied systematically the contact resistance of firing-through Ag pastes on samples that were processed in a fast firing furnace with and without the presence of busbars. We fabricated p-type full-area aluminum back surface field and n-type passivated emitter rear totally diffused solar cells with various doping concentrations and surface morphologies. For all tested conditions, lower contact resistances were obtained when firing without busbars. For instance, by omitting the busbars during firing, on a 138 $\Omega$ /sq phosphorus emitter, the median contact resistance dropped from 7.5 to 0.7 m $\Omega$ $\cdot$ cm $^{2}$ , and on an 85 $\Omega$ /sq boron emitter, the median contact resistance decreased from 80 to 1 m $\Omega$ $\cdot$ cm $^{2}$ . Furthermore, we found that the pure Ag paste outperformed the AgAl paste in contacting boron emitters. An explanation of these results might be the short-circuit effect, which is a fundamental mechanism that strongly influences contact formation. In addition, in spite of the significant impact of the presence of busbars during firing on contact resistances, its impact on the $J_{0,\text{met}}$ was found to be only marginal.

Low-cost and low-temperature chemical oxide passivation process for large area single crystalline silicon solar cells

Although thermally grown silicon dioxide is an excellent surface passivation layer for Si wafers, its industrial applicability for solar cell surface passivation is limited due to low throughput and high cost. This has prompted the exploration of low temperature and chemical silicon dioxide processes for silicon solar cells. In this work, a low-cost, low-temperature (40 °C), non-acidic and safe chemical oxide passivation process (named as NCPRE-oxide) with only sodium hypochlorite (NaOCl) solution is proposed and investigated on large area wafers (125 mm × 125 mm). This process uses a single-component chemical solution to grow ultra-thin silicon oxide (SiOx) layer of thickness of about 1.5 nm. The chemical SiOx layer capped with hydrogenated amorphous silicon nitride (SiNx:H) improves passivation on the n-type silicon surface. A considerable enhancement in the effective minority carrier lifetime (τeff) is observed for SiNx:H/SiOx stack on the phosphorous diffused pyramidal textured silicon surface. The versatility of the NCPRE-oxide is verified by transmission electron microscopy, ellipsometry, X-ray photoelectron spectroscopy, effective minority carrier lifetime and photoluminescence imaging measurements. The introduction of present SiNx:H/SiOx stack in our baseline cell fabrication process resulted in the improvement in cell efficiency by 0.3% (absolute) for screen-printed full area aluminum back surface field cells.

Record high efficiency of screen-printed silicon aluminum back surface field solar cell: 20.29%

We have achieved a record high cell efficiency of 20.29% for an industrial 6-in. p-type monocrystalline silicon solar cell with a full-area aluminum back surface field (Al-BSF) by simply modifying the cell structure and optimizing the process with the existing cell production line. The cell efficiency was independently confirmed by the Solar Energy Research Institute of Singapore (SERIS). To increase the cell efficiency, for example, in four busbars, double printing, a lightly doped emitter with a sheet resistance of 90 to 100 Ω/□, and front surface passivation by using silicon oxynitride (SiON) on top of a silicon nitride (SiNx) antireflection layer were adopted. To optimize front side processing, PC1D simulation was carried out prior to cell fabrication. The resulting efficiency gain is 0.64% compared with that in the reference cells with three busbars, a single antireflection coating layer, and a low-sheet-resistance emitter.

Selective emitter solar cell through simultaneous laser doping and grooving of silicon followed by self-aligned metal plating

Both buried contact solar cells (BCSC) and laser doped selective emitter (LDSE) solar cells have achieved considerable success in large-scale manufacturing. Both technologies are based on plated contacts. High metal aspect ratios achieved by BCSC allow low shading loss while the buried metal contacts in the grooves provide good contact adhesion strength. In comparison, although the LDSE cell achieves significantly higher efficiencies and is a much simpler approach for forming the selective emitter region and self-aligned metal plating, the metal adhesion strength falls well short of that achieved by the BCSC. Recent studies show that plated contacts based on the latter can be more durable than screen-printed contacts. This work introduces a new concept of laser doping with grooving to form narrow grooves with heavily doped walls in a simultaneous step, with the self-aligned metal contact subsequently formed by plating. This process capitalizes on the benefits of both BCSC and LDSE cells. The laser-doped grooves are only 3–5µm wide and 10–15µm deep; the very steep walls of these grooves remain exposed even after the subsequent deposition of the antireflection coating (ARC). This unique feature significantly reduces the formation of laser-induced defects since the stress due to the thermal expansion mismatch between the ARC and silicon is avoided. Furthermore, the exposed walls allow nucleation of the subsequent metal plating. This novel structure also benefits from greatly enhanced adhesion of the plated contact due to it being buried underneath the silicon surface in the same way as the BCSC. Cell efficiencies over 19% are achieved by using this technology on p-type Czochralski (Cz) wafers with a full area aluminum (Al) back surface field (BSF) rear contact. It is expected that much higher voltages and consequently higher efficiencies could be achieved if this technology is combined with a passivated rear approach.

Reduced Module Operating Temperature and Increased Yield of Modules With PERC Instead of Al-BSF Solar Cells

We demonstrate a reduced operating temperature of modules made from passivated emitter rear cells (PERCs) compared with modules made from cells featuring an unpassivated full-area screen-printed aluminum rear side metallization aluminum back surface field (Al-BSF). Measurements on specific test modules fabricated from p-type silicon PERC and Al-BSF solar cells reveal a 4 $^{\circ }$ C lower operating temperature for the PERC module under 1400 W/m $^2$ halogen illumination, if no temperature control is applied. For detailed analysis of the temperature effect, we perform a 3-D ray tracing analysis in the spectral range from 300 to 2500 nm to determine the radiative heat sources in a photovoltaic (PV) module. We combine these heat sources with a 1-D finite element method model solving the coupled system of semiconductor, thermal conduction, convection, and radiation equations for module temperature and power output. The simulations reveal that the origin of the reduced temperature of the PERC modules is a higher efficiency, as well as a higher reflectivity, of the cells rear side mirror. This reduces the parasitic absorptions in the rear metallization and increases the reflection for wavelengths above 1000 nm. This operating temperature difference is simulated to be linear in intensity. The slope depends on the spectral distribution of the incoming light. Under 1000 W/m $^2$ in AM1.5G, our simulations reveal that the operating temperature difference is about 1.7 $^{\circ}$ C. The operating temperature can be lowered another 3.2 $^{\circ}$ C, if all parasitic absorption for wavelengths longer than 1200 nm can be prevented. Standard testing conditions applying a temperature control to the module do not show this effect of enhanced performance of the PERC modules. Yield calculations for systems in the field will thus systematically underestimate their electrical power output unless the inherently lower operating temperature of PERC modules is taken into account.

Optical Simulation and Analysis of Iso-textured Silicon Solar Cells and Modules Including Light Trapping

Solar cells made from multicrystalline silicon (mc-Si) wafers play an important role in photovoltaics. Nevertheless, tools for the optical simulation of these devices are scarce. In the present work, the reflectance and charge carrier generation of mc-Si cells and modules are for the first time simulated successfully in the complete spectral range including light trapping and escape light, as the comparison with measured reflectance of the finished cells and mini-modules shows. The “spherical caps” geometry is used to model the front surface reflection of iso-textured silicon solar cells. The characteristic angles of the spherical caps are determined from the reflectance of iso-textured wafers for three different texture strengths. Based on this calibration, the reflectance and charge carrier generation rates of cells encapsulated with EVA and glass are simulated and analysed. Iso-textured cells with full-area aluminium back surface field (Al-BSF) and with passivated emitter and rear (PERC) are quantitatively compared regarding the photo-generated current density jPh. The simulations demonstrate that the direct cell-to-module loss of iso-textured mc-Si cells with Al-BSF (0.7 mA/cm2) is smaller than for PERC cells (1.2 mA/cm2).

0.4% absolute efficiency increase for inline-diffused screen-printed multicrystalline silicon wafer solar cells by non-acidic deep emitter etch-back

Emitter formation is one of the most critical and crucial process steps in the fabrication of standard silicon wafer solar cells. Typically the photovoltaic industry uses tube based phosphorus diffusion, using phosphorus oxychloride as the dopant source. Alternately, a low-cost inline diffusion using phosphoric acid as a dopant source can be used. However, proper process conditions must be used to meet solar cell energy conversion efficiencies obtained by tube diffusion. In this work, we present the application of a non-acidic homogeneous emitter etch-back process – the ‘SERIS etch’ – for inline-diffused emitters in order to raise the efficiency of multicrystalline silicon (multi-Si) wafer solar cells. We apply both light and heavy emitter etch-backs on inline-diffused emitters with sheet resistance (Rsq) values in the 40–60Ω/sq range to achieve emitters with a target Rsq of ~70Ω/sq. The emitter surface reflectance and doping uniformity are maintained even after an etch-back that results in a Rsq change of ~30Ω/sq. An average cell efficiency gain of 0.4% (absolute) is reported for cells with heavy etch-back when compared to the as-diffused non-etch-back screen-printed full-area aluminum back surface field solar cells and efficiencies up to 17.9% are achieved. Besides, best lot of the etch-back inline-diffused cells shows a 0.2% (absolute) efficiency gain over the standard tube-diffused cells. These results show that the ‘SERIS etch’ etch-back process can enable higher-efficiency industrial inline-diffused multi-Si wafer solar cells.

Photoluminescence imaging for determining the spatially resolved implied open circuit voltage of silicon solar cells

Photoluminescence imaging has widely been used as a characterisation tool for the development of silicon solar cells. However, photoluminescence images typically only give qualitative information due to the presence of an unknown calibration constant. In this work, quasi-steady-state photoconductance measurements on partially processed solar cells and I-V measurements on finished solar cells are used to determine the calibration constants to yield spatially resolved implied open circuit voltage images. This technique is then applied to determine the implied open circuit voltage of laser doped selective emitter solar cells at various stages of cell fabrication after the formation of the full area aluminium back surface field when other characterisation techniques such as photoconductance cannot be used.

Novel selective emitter process using non-acidic etch-back for inline-diffused silicon wafer solar cells

In this work, a novel etch-back approach for the fabrication of a selective emitter (SE) structure is reported for inline-diffused p-type monocrystalline silicon wafer solar cells. The complete SE process, named the ‘SERIS SE’ process, involves screen printing of an etch mask, use of a HF-free etch-back solution and screen-printed metallisation. Both the emitter etch-back and etch-mask dissolution are performed simultaneously in a single processing step, thereby reducing the number of processing steps as compared to other etch-back based SE technologies. For inline-diffused emitter (ILDE) solar cells, the ‘SERIS SE’ process actually requires only one additional processing step, as an emitter etch-back is typically applied to remove the detrimental top layer. An average cell efficiency gain of 0.4% (absolute) is reported for the SE cells fabricated using the single-step SERIS SE process, as compared to etch-back homogeneous-emitter (HE) solar cells. An average batch efficiency of 18.5% is achieved for screen-printed p-type 156 mm pseudo-square Cz mono-Si full-area aluminium back surface field (Al-BSF) SE solar cells. The minimal increase in the number of processing steps, reliance on the robust screen printing process, HF-free non-acidic chemical etch-back and applicability to both tube and inline-diffused emitters make the ‘SERIS SE’ process suitable for industrial application on both mono- and multicrystalline silicon wafers.

Optimization of Rear Local Contacts on High Efficiency PERC Solar Cells Structures

A local contact formation process and integration scheme have been developed for the fabrication of rear passivated point contact solar cells. Conversion efficiency of 19.6&#x25; was achieved using <svg style="vertical-align:-0.1638pt;width:66.612503px;" id="M1" height="11.225" version="1.1" viewBox="0 0 66.612503 11.225" width="66.612503" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(.017,-0,0,-.017,.062,10.963)"><path id="x31" d="M384 0h-275v27q67 5 81.5 18.5t14.5 68.5v385q0 38 -7.5 47.5t-40.5 10.5l-48 2v24q85 15 178 52v-521q0 -55 14.5 -68.5t82.5 -18.5v-27z" /></g><g transform="matrix(.017,-0,0,-.017,8.222,10.963)"><path id="x35" d="M153 550l-26 -186q79 31 111 31q90 0 141.5 -51t51.5 -119q0 -93 -89 -166q-85 -69 -173 -71q-32 0 -61.5 11.5t-41.5 23.5q-18 17 -17 34q2 16 22 33q14 9 26 -1q61 -50 124 -50q60 0 93 43.5t33 104.5q0 69 -41.5 110t-121.5 41q-53 0 -102 -20l38 305h286l6 -8&#xA;l-26 -65h-233z" /></g><g transform="matrix(.017,-0,0,-.017,16.381,10.963)"><path id="x36" d="M137 343l67 33q37 17 63 17q79 0 129.5 -53t50.5 -131q0 -92 -58 -156.5t-147 -64.5t-147 68t-58 182q0 63 17 119t43 95.5t61.5 72t69 52t67.5 31.5q62 22 128 33l6 -32q-56 -11 -108 -35q-149 -71 -184 -231zM227 337q-47 0 -95 -27q-6 -23 -6 -70q0 -93 36 -155.5&#xA;t96 -62.5q53 0 78 45.5t25 105.5q0 68 -35 116t-99 48z" /></g><g transform="matrix(.017,-0,0,-.017,28.314,10.963)"><path id="xD7" d="M528 54l-36 -38l-198 201l-198 -201l-36 38l197 200l-197 201l36 38l198 -202l198 202l36 -38l-197 -201z" /></g><g transform="matrix(.017,-0,0,-.017,42.066,10.963)"><use xlink:href="#x31"/></g><g transform="matrix(.017,-0,0,-.017,50.226,10.963)"><use xlink:href="#x35"/></g><g transform="matrix(.017,-0,0,-.017,58.385,10.963)"><use xlink:href="#x36"/></g> </svg>&#x2009;mm, pseudo square, p-type single crystalline silicon wafers. This is a significant improvement when compared to unpassivated, full area aluminum back surface field solar cells, which exhibit only 18.9&#x25; conversion efficiency on the same wafer type. The effect of rear contact formation on cell efficiency was studied as a function of contact area and contact pitch, hence the metallization fraction. Contact shape and the thickness of Al-BSF layer were found to be heavily dependent on the laser ablation pattern and contact area. Simulated cell parameters as a function of metallization showed that there is a tradeoff between open circuit voltage and fill factor gains as the metallization fraction varies. The rear surface was passivated with an Al<sub >2</sub>O<sub >3</sub> layer and a <svg style="vertical-align:-3.27605pt;width:35.3125px;" id="M2" height="15.5125" version="1.1" viewBox="0 0 35.3125 15.5125" width="35.3125" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(.017,-0,0,-.017,.062,11.363)"><path id="x53" d="M409 504l-29 -5q-16 60 -44.5 96t-86.5 36q-54 0 -82.5 -32t-28.5 -77q0 -51 30.5 -82.5t96.5 -65.5l35.5 -18t33 -19.5t33.5 -23.5l27 -25.5t24.5 -32t13.5 -36.5t6 -45q0 -80 -62 -134.5t-160 -54.5q-47 0 -98 15q-22 7 -50 21q-8 23 -27 155l30 7q7 -27 18 -52&#xA;t30 -51.5t49 -42.5t67 -16q56 0 88 32.5t32 87.5q0 51 -31.5 82t-98.5 67q-80 44 -110 73q-55 53 -55 124q0 75 56 126.5t150 51.5q53 0 126 -23z" /></g><g transform="matrix(.017,-0,0,-.017,8.018,11.363)"><path id="x69" d="M135 536q-20 0 -35 15.5t-15 35.5q0 22 15 37t36 15t35.5 -15t14.5 -37q0 -21 -15 -36t-36 -15zM252 0h-220v26q48 5 59 17t11 63v206q0 47 -9 58t-54 18v24q78 12 142 39v-345q0 -51 11.5 -63t59.5 -17v-26z" /></g><g transform="matrix(.017,-0,0,-.017,12.574,11.363)"><path id="x4E" d="M719 650v-28q-43 -2 -62 -15t-22 -44q-6 -47 -6 -169v-403h-31l-426 524h-2v-251q0 -111 6 -169q4 -37 24 -50.5t72 -16.5v-28h-237v28q45 2 64.5 16t23.5 49q6 62 6 171v220q0 54 -3 68.5t-17 32.5q-16 19 -34.5 26.5t-54.5 10.5v28h147l418 -502h3v246q0 117 -7 166&#xA;q-4 34 -24.5 47t-73.5 15v28h236z" /></g> <g transform="matrix(.012,-0,0,-.012,25.2,15.45)"><path id="x1D44B" d="M775 650l-6 -28q-60 -6 -81.5 -16t-61.5 -54l-175 -191l125 -243q30 -58 48.5 -71t82.5 -19l-5 -28h-275l7 28l35 4q31 4 37 12t-6 34l-108 216q-140 -165 -177 -219q-16 -22 -10.5 -30.5t41.5 -13.5l22 -3l-7 -28h-244l8 28q52 4 75 15.5t67 52.5q48 46 206 231&#xA;l-110 215q-26 51 -44 63t-72 17l6 28h250l-6 -28l-27 -4q-30 -5 -35 -10t3 -27q17 -43 95 -190q70 78 154 185q15 21 10 29.5t-33 12.5l-30 4l5 28h236z" /></g> </svg> capping layer. The rear surface contact pattern was created by laser ablation and the contact geometry was optimized to obtain voids free contact filling, resulting in a uniform back surface field. The efficiency gain in rear passivated cells over the reference cells is mainly due to improved short circuit current and open circuit voltage.

Record Efficiency on Large Area P-Type Czochralski Silicon Substrates

In this work we report a world record independently confirmed efficiency of 19.4% for a large area p-type Czochralski grown solar cell fabricated with a full area aluminium back surface field. This is achieved using the laser doped selective emitter solar cell technology on an industrial screen print production line with the addition of laser doping and light induced plating equipment. The use of a modified diffusion process is explored in which the emitter is diffused to a sheet resistance of 90 Ω/□ and subsequent etch back of the emitter to 120 Ω/□. This results in a lower surface concentration of phosphorus compared to that of emitters diffused directly to 120 Ω/□. This modified diffusion process subsequently reduces the conductivity of the surface in relation to that of the heavily diffused laser doped contacts and avoids parasitic plating, resulting an average absolute increase in efficiency of 0.4% compared to cells fabricated without an emitter etch back process.

20.1% Efficient Silicon Solar Cell With Aluminum Back Surface Field

We present a standard p+pn+ solar cell device exhibiting a full-area aluminum back surface field (BSF) and a conversion efficiency of 20.1%. The front side features a shallow emitter which has been exposed to a short oxidation step and reduces the emitter dark saturation current density j0e to 160 fA/cm2 on a textured surface. The front contact is formed by light-induced nickel and silver plating. Also, devices featuring screen-printed front contacts have been realized that reach a conversion efficiency of 19.8%. PC1D simulations are presented in order to extract the electronic parameters of the BSF. Therefore, external quantum efficiency and reflectance have been determined for modeling the internal quantum efficiency by adapting surface recombination and lifetime of the PC1D-simulated silicon device. As a result, a recombination velocity of SBSF = 283 cm/s and a dark saturation current density of jBSF = 274 fA/cm2 in the Al BSF are determined. This results in an effective diffusion length Leff = 1150 μm .

Hydrogen passivation of defects in EFG ribbon silicon

To enhance the bulk lifetime of multicrystalline silicon material, gettering of impurities and hydrogen passivation of defects are investigated. In edge-defined film-fed grown (EFG) ribbon silicon, an aluminium-enhanced hydrogenation of defects by silicon nitride has been reported. On thin wafers, the formation of a full area aluminium back surface field will lead to wafer bending due to different thermal expansion coefficients of aluminium and silicon. To circumvent this problem, remote plasma-enhanced chemical vapour deposited (PECVD) silicon nitride (SiN x ) as passivation scheme for the front and rear surface is proposed. In this work, the bulk passivation by hydrogenation is investigated using two different hydrogen passivation techniques: (i) passivation in a remote hydrogen plasma and (ii) passivation due to a post-deposition anneal of remote PECVD-SiN x in a lamp-heated conveyor belt furnace. Measurements of the bulk lifetime show that the lifetime improvement due to remote hydrogen plasma passivation degrades under illumination with white light. In contrast, the hydrogen passivation by a post-deposition SiN x anneal is only effective if a phosphorous-doped emitter is present below the SiN x layer during the hydrogenation. This lifetime improvement is stable under illumination.