Back Surface Field Solar Cells 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.

Evolution of PERC from Al-BSF: optimization based on root cause analysis

Passivated emitter rear contact (PERC) solar cell involves upgraded adaptation of technology over the existing silicon wafer-based aluminium back surface field (Al-BSF) solar cell. The lower efficiency of Al-BSF is caused by a few drawbacks e.g. higher rear surface recombination and weaker electric field at the rear surface. These technical shortcomings can be mitigated by incorporating field-effect passivation (FEP) layer viz. aluminium oxide (Al2O3) at the rear surface with a capping layer of SiNX on it. Conventionally two new processing steps utilizing two expensive equipments, green laser (532 nm) for laser ablation of the FEP and SiNX capping layer stack at the rear surface and single side etcher for rear side polishing of wafer are used for manufacturing of PERC cells. In this paper, we have shown that instead of these two new expensive processing equipments we can use the IR laser (1024 nm) and RIE etcher, which are commonly available for Al-BSF manufacturing line, for the fabrication of PERC solar cells. Using p-type wafers of rather modest quality (minority carrier lifetime of ≥10 µs) we have demonstrated improvement of the efficiency from 18.71 to 19.62% and enhancements in VOC and JSC from 628 to 646 mV and 38.59 mA/cm2 to 39.82 mA/cm2, respectively. A thorough root-cause analysis has been carried out to optimize the two new approaches within the existing Al-BSF line for the fabrication of PERC solar cell.

Luminescence Image Analysis Using Finite-Element Models: Finished Solar Cell Analysis

A general computational routine which seeks cell parameters that best explain a set of luminescence imaging data is presented in this article. The routine combines Griddler, a finite-element simulator for solar cells, and tailored multivariate regression techniques. It is applied to a dataset of luminescence images from about 80 monocrystalline silicon Al back surface field solar cells with varying front metal grid contact resistance and recombination. The fitted cell parameters including contact resistance spatial distribution, saturation current densities of the wafer's passivated regions, and under the metal contacts lead to simulated I–V characteristics that are in agreement with experimental data. The extracted contact resistance also correlates well with transmission line method measurements. In a separate study of luminescence imaging data from a multicrystalline passivated emitter rear local contacted cell (PERC) cell, this analysis routine is also compared with an alternative high spatial resolution method, yielding comparable results.

Photovoltaic Performance of Crystalline Silicon Recovered from Solar Cell Using Various Chemical Concentrations in a Multi-Stage Process

In this study, using a wet chemical process, we evaluate the effectiveness of different solution concentrations in removing layers from a solar cell, which is necessary for recovery of high-purity silicon . A 4-step wet etching process is applied to a 6-inch back surface field(BSF) solar cell. The metal electrode is removed in the first and second steps of the process, and the anti-reflection coating(ARC) is removed in the third step. In the fourth step, high purity silicon is recovered by simultaneously removing the emitter and the BSF layer from the solar cell. It is confirmed by inductively coupled plasma mass spectroscopy(ICP-MS) and secondary ion mass spectroscopy(SIMS) analyses that the effectiveness of layer removal increases with increasing chemical concentrations. The purity of silicon recovered through the process, using the optimal concentration for each process, is analyzed using inductively coupled plasma atomic emission spectroscopy(ICP-AES). In addition, the silicon wafer is recovered through optimum etching conditions for silicon recovery, and the solar cell is remanufactured using this recovered silicon wafer. The efficiency of the remanufactured solar cell is very similar to that of a commercial wafer-based solar cell, and sufficient for use in the PV industry.

Investigation on Industrial Screen-Printed Aluminum Point Contact and Its Application in n-PERT Rear Junction Solar Cells

As compared with the traditional back surface field (BSF) solar cells, the n -type passivated emitter rear and totally diffused rear-junction ( n -PERT-RJ) solar cells have much lower carrier recombination losses on their rear sides. In order to obtain more benefits (higher open-circuit voltages) from the currently passivated rear sides, the implementation of the point contact concept is an intuitive solution. In this article, we present industrial screen-printed point contacts that are metallized with aluminum (Al) pastes containing silicon additives (Si-add). To understand the impacts of the Si-add on the performance our n -PERT-RJ solar cells, the detailed loss analysis on the open-circuit voltage ( $V_{OC}$ ), short-circuit current density ( $J_{SC}$ ), and fill factor are performed separately. The effect of Si-add and pitch of laser contact openings (LCO) on the point contact characteristics are also investigated. In the last part, we introduce an innovative “point-line” contact concept on the rear side, which is composed of dot-shaped LCO and Al metal finger design, resulting in an n -PERT-RJ solar cell to reach $V_{OC}$ of 692 mV and peak efficiency of 22%. The 60% bifaciality of the device opens the way for an additional power output on module level, which further reduces the levelized cost of electricity. These features make the rear-side point contact design more flexible to be applied in industrial mass production.

Optimization of ALD $$\hbox {Al}_{2}\hbox {O}_{3}$$ process parameters for passivation of c-silicon and its implementation on industrial monocrystalline silicon solar cell

Effect of process parameters on \(\hbox {Al}_{2}\hbox {O}_{3}\) deposited using Atomic Layer Deposition (ALD) for the surface passivation of c-silicon surface has been investigated. Surface passivation properties of \(\hbox {Al}_{2}\hbox {O}_{3}\) have been measured by evaluating the minority carrier lifetime and interface charges at the \(\hbox {Si}/\hbox {Al}_{2}\hbox {O}_{3}\) interface. It has been observed that surface passivation properties of \(\hbox {Al}_{2}\hbox {O}_{3}\) are strongly dependent on process parameters such as substrate temperature, annealing temperature, and thickness of the deposited \(\hbox {Al}_{2}\hbox {O}_{3}\) film. Minority carrier lifetime, effective charge density (\(Q_{{\mathrm{eff}}}\)), and interface defect density (\(D _{{\mathrm{it}}}\)) were observed to vary from 180 to \(355\,\upmu \hbox {s}\), \(-\,2.3 \times 10^{12}\) to \(-1.46 \times 10^{13}\,\hbox {cm}^{-2}\) and \(1.2 \times 10^{9}\) to \(1.9\times 10^{10}\,\hbox {eV}^{-1}\,\hbox {cm}^{-2}\), respectively, for various process parameters. \(\hbox {Al}_{2}\hbox {O}_{3}\) film based on the optimized process parameters were then used as a passivation layer in fabricating industrial PERC solar cells. Effect of \(\hbox {Al}_{2}\hbox {O}_{3}\) passivation in PERC solar cells has been demonstrated by comparing the characteristics of the PERC solar cells with that of the standard Al BSF solar cells. An efficiency improvement of \(\sim 0.8\%\) has been observed in passivated emitter rear cells (PERC) solar cells as compared to the standard aluminum back surface field (Al BSF) solar cells.

Application of SiO2 passivation technique in mass production of silicon solar cells

Effective surface passivation is one of the primary prerequisites for high-efficiency silicon solar cells. In this paper, high-quality silicon dioxide (SiO2) films with excellent surface passivation abilities have been realized by thermal oxidation and plasma-enhanced chemical vapor deposition, respectively. By employing SiO2 capped with hydrogenated silicon nitride (SiNx:H) (SiO2/SiNx:H stacks) as the front passivation layers, the emitter saturation current density has been reduced from 90.5 fA/cm2 to 62.3 fA/cm2. We have successfully mass-produced thermal-oxidized p-type Czochralski-Si (CZ-Si) solar cells with high conversion efficiency (η) of 20.1%, which is 0.2% absolutely higher than that of the conventional Al back surface field (Al-BSF) solar cells. With a rational design of process integration, we have further presented a cost-effective way to fabricate high-efficiency SiO2 passivated emitter and rear cells (PERCs) at an existing production line. The introduction of SiO2/SiNx:H stacks on the rear surface can effectively increase the long wavelength response and the rear surface recombination is also suppressed with a rather low surface recombination velocity of 26 cm/s achieved after a post-annealing process. The industrial SiO2 passivated p-type CZ-Si PERCs possess outstanding performances with the average η of 21.3% and the highest η of over 21.9%, absolute 1.3% increment in average η compared with the conventional Al-BSF solar cells. Moreover, we have demonstrated that the relative low illumination response (below 50 W/m2) loss of our SiO2 PERCs over a day is extremely limited to be less than 0.2%.

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.

Analytical modeling of the temporal evolution of hot spot temperatures in silicon solar cells

We present an approach to predict the equilibrium temperature of hot spots in crystalline silicon solar cells based on the analysis of their temporal evolution right after turning on a reverse bias. For this end, we derive an analytical expression for the time-dependent heat diffusion of a breakdown channel that is assumed to be cylindrical. We validate this by means of thermography imaging of hot spots right after turning on a reverse bias. The expression allows to be used to extract hot spot powers and radii from short-term measurements, targeting application in inline solar cell characterization. The extracted hot spot powers are validated at the hands of long-term dark lock-in thermography imaging. Using a look-up table of expected equilibrium temperatures determined by numerical and analytical simulations, we utilize the determined hot spot properties to predict the equilibrium temperatures of about 100 industrial aluminum back-surface field solar cells and achieve a high correlation coefficient of 0.86 and a mean absolute error of only 3.3 K.

The Effect of Bifacial AlOx Deposition on PERC Solar Cell Performance

Single-sided atomic layer deposition (ALD) is challenging as often some degree of wrap-around deposition onto the nontarget side occurs. This is particularly challenging for aluminum oxide (AlO x ) deposition on silicon solar cells, as is required on the rear of passivated emitter rear cell (PERC) solar cells. The effects of unintended ALD-deposited AlO x on the front side of PERC solar cells are explored in this work, with a specific focus on the impact on contact resistance. The contact resistance is determined from transmission line measurement structures on samples with up to 10 nm of AlO x on top of silicon nitride (SiN x ). These values are used as input parameters for computer simulations to simulate the effect of varying degrees of wrap-around on p -PERC cells. Finally, p -type aluminum back surface field solar cells with 3, 5, 7, and 10 nm AlO x layers on top of the SiN x are fabricated using various firing recipes and screen printing pastes to assess the effect on fabricated devices. It was found that a 3 nm AlO x layer can improve the cell performance relative to the uncapped case, whereas 5, 7, and 10 nm resulted in a lower solar cell efficiency due to higher series resistance.

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.

Modeling of Edge Losses in Al-BSF Silicon Solar Cells

Specific photovoltaic (PV) applications can require the use of limited-area solar cells. In that case, the edge of the device can have a considerable influence on the conversion efficiency. In this study, limited-area solar cells were fabricated by laser scribing and subsequent mechanical cleavage of large aluminum back surface field solar cells. First, laser parameters were optimized in order to limit the loss in conversion efficiency. It was shown that laser scribing must be performed on the rear side of the device in order to avoid the formation of shunts. The variation in the laser conditions could not improve the cell efficiency, as the discontinuity of the crystal lattice has a prominent impact compared with defects induced by laser scribing and cleaving. The fabrication of various cell geometries confirmed that the reduction of edge recombination was possible by simply limiting the cell periphery over its area. Specific characterization of the fabricated devices was carried out in order to understand the influence of the cell area on the performance. Using the two-diode model, two methods were used to extract the saturation current densities induced by metallization-related recombination ( $J_{{\rm{0m}}}$ ) and edge recombination ( $J_{{\rm{0e-dr}}}$ ). From this model, it is thus possible to predict the open-circuit voltage of any cell format. This study is particularly relevant for specific PV applications requiring the use of small-area solar cells and can be applied to other silicon cell technologies.

Influence of aluminum workfunction and capping dielectric thickness on the performance of local back surface field solar cell using numerical simulation

High efficiency partial rear contact solar cells forms a Metal-Insulator-Semiconductor (MIS) stack in the rear side. Aluminum is used as a local back surface field (localized heavily doped pregion) and a hole contact. The workfunction difference between the aluminum contact and the c-Si(p) causes inversion in the silicon surface of the MIS structure. The surface inversion affects the quality of passivation and hence, the performance of the solar cell which requires further understanding. In this work, we have analysed the influence of aluminum on surface passivation in two passivation stacks namely (i)AlOx/SiNx and (ii) SiOx/SiNx using Sentaurus TCAD simulation.Our simulation result shows that in case of positive dielectric stack (SiOx/SiNx), aluminum workfunction enhances the passivation (effective surface recombination velocity <15cm/s) for the injection condition of 2×1013cm-3 which is common at the rear side of AlLBSF solar cell. But, in negative dielectric stack (AlOx/SiNx), aluminum workfunction influence is negligible since it is surpassed by high negative fixed charge density. In addition, we have varied the capping layer (SiNx) thickness in the dielectric stack to alter the aluminum influence on the crystalline silicon surface. It shows up to 1.0% absolute improvement in the efficiency for reduced SiNx thickness in the SiOx/SiNx stack for the fixed charge density of 2×1011cm-2, interface trap density of 5×1011cm-2, assumed electron and hole capture cross section of 10-14cm2 and 4×10-16cm2 respectively. This is solely attributed to the aluminum workfunction influence on the rear passivation.

An industrial solution to light-induced degradation of crystalline silicon solar cells

Boron-oxygen defects can cause serious light-induced degradation (LID) of commercial solar cells based on the boron-doped crystalline silicon (c-Si), which are formed under the injection of excess carriers induced either by illumination or applying forward bias. In this contribution, we have demonstrated that the passivation process of boron-oxygen defects can be induced by applying forward bias for a large quantity of solar cells, which is much more economic than light illumination. We have used this strategy to trigger the passivation process of batches of aluminum back surface field (Al-BSF) solar cells and passivated emitter and rear contact (PERC) solar cells. Both kinds of the treated solar cells show high stability in efficiency and suffer from very little LID under further illumination at room temperature. This technology is of significance for the suppression of LID of c-Si solar cells for the industrial manufacture.

Performance improvement for epitaxially grown SiGe on Si solar cell by optimizing the back surface field

This letter reports on the performance improvement of an epitaxially grown SiGe on Si solar cell by optimizing the back surface field (BSF). First, a Si0.18Ge0.82 on silicon (Si) solar cell was fabricated with a 0.25 μm BSF layer. A 25 mV open‐circuit voltage (VOC) improvement was observed on this BSF solar cell compared with the reference solar cell without BSF layer. Then, a Si0.18Ge0.82 on Si solar cell with double BSF layers was designed and fabricated. The measured efficiency of this solar cell is 3.4% when filtered by a GaAs0.79P0.21 top cell. To the best of the authors' knowledge, the 3.4% efficiency reported here is the highest efficiency for SiGe on Si solar cells when filtered by a GaAs0.79P0.21 top cell. The previous best reported efficiency for high Ge composition SiGe on Si solar cell was only 1.7% when filtered by a GaAs0.79P0.21 top cell.

Surface Recombination Velocity of Local Al-contacts of PERC Solar Cells Determined from LBIC Measurements and 2D Simulation

Industrial production volumes of passivated emitter and rear contact (PERC) solar cells increase due to significantly higher cell efficiencies compared to full area back surface field (BSF) solar cells at similar costs. The main features of PERC cells are dielectric surface passivation of the rear and local contact formation with Al leading to a p+p junction beneath the Al/eutectic. For non-optimized process conditions, the eutectic in the local Al contact area does not form and so-called voids result. Since it is known that there are voids causing high or low recombination activity, a determination of the surface recombination velocity (SRV) is necessary for identification of the potential for process optimization. The passivation quality of the BSF, locally formed in the rear side contacts, is studied in detail via local internal quantum efficiency (IQE) measured by high resolution light beam induced current (LBIC). The significant spreading of the IQE values is attributed to a variation in local BSF layer thickness at different areas. The SRV of the local contact is determined by fitting the LBIC measurements of voids by 2D simulations. These simulations are based on a detailed modeling of SRV in local contact areas involving a non-uniform SRV in the void's vicinity. The non-uniform SRV in voids is traced back to laser induced damage nearby the local contact opening in the dielectric layer. Additionally the existence of laser damaged areas close to filled contacts is demonstrated in this work.

Intra-grain versus grain boundary degradation due to illumination and annealing behavior of multi-crystalline solar cells

Light induced degradation can cause a severe loss of efficiency on multi-crystalline PERC solar cells (mc-LID) of more than 10%rel. In this work, the kinetics of the mc-LID annealing process and its temperature dependence is analyzed. It is shown that the initial efficiency can be partly restored by annealing the cell in the dark. However, the degradation process is not completely reversible and the degradation rates of the first and subsequent degradation cycles are different. Furthermore, lateral variations of the degradation are investigated. Four regions showing a quantitatively different degradation behavior are identified. Mc-LID of the rear contacts shows similar degradation as for standard back surface field solar cells. The degradation of grain boundaries is weaker than intra-grain degradation and thus of particular interest for root cause analysis. Decorated grain boundaries are dominated by other recombination mechanisms suppressing the appearance of mc-LID. Physical explanations for these results of a laterally different degradation behavior and an increased degradation rate after annealing are discussed.

Improved radiation resistant properties of electron irradiated c-Si solar cells

Abstract This work investigates the radiation tolerance of c-Si solar cells under electron energy of 9 MeV with fluence of 5.09×1016 cm−2. The solar cells were fabricated and characterized before and after electron irradiation through current-voltage (I-V), capacitance-voltage (C-V), and frequency dependent conductance (Gp) measurements. The results revealed that all the output parameters such as short circuit current (Isc), open circuit voltage (Voc), series resistance (Rs), and efficiency (η) were degraded after electron irradiation. Capacitance-Voltage measurements show that there is a slight decrease in the base carrier concentration (ND), while a small increase in depletion layer width (WD) was due to an increase in the base carrier concentration. Enhancements in the density of interface states (Nss), and trap time constant (τ) have been observed after electron irradiation. The results has revealed that back surface field (BSF) solar cell with front surface passivation (FSP) presented lowest efficiency degradation ratio of 11.3% as compared to 15.3% of the solar cell without FSP. The subsequent annealing of irradiated Si solar cell devices revealed that the Si solar cell with FSP demonstrated high efficiency recovery ratio of 94% as compared to non-FSP solar cell.

Innovative Front Grid Design, Four-Streets and Five-Busbars (4S-5BB), for High Efficiency Industrial Al-BSF Silicon Solar Cell

This letter reports on an innovative front grid design that is capable of: 1) decreasing the metal shading; 2) increasing the short-circuit current ( $J_{\mathrm {SC}}$ ); 3) increasing the open-circuit voltage ( $V_{\mathrm {OC}}$ ); 4) maintaining the fill factor; and 5) increasing the efficiency of an industrial Al back surface field (Al-BSF) solar cell. Because the design incorporates four-streets and five-busbars (4S-5BB), it uses less front silver metal ( $\sim 73$ mg) than the conventional 3BB ( $\sim 129$ mg), hence the cost reduction from 6.07 to 3.426 ¢ per cell in metallization with no additional processing step. The 4S-5BB design has shown great potential for achieving >20% efficiency for the industrial size Al-BSF silicon solar cell. In this letter, the preliminary result for the 4S-5BB industrial Al-BSF cell having average efficiency of 19.66% compared with 19.3% 3BB counterpart is presented.

Characterization of local Al‐contacts by light beam induced current measurements and their verification by 2D simulation using flexPDE

In recent years the relevance of passivated emitter and rear contact solar cells for industrial application increased due to significantly higher cell efficiencies compared to full area back surface field solar cells. However, the formation of local Al contacts is particularly sensitive to the manufacturing process parameters. Under non‐optimized conditions, the eutectic in local Al contacts is missing and so‐called voids are formed. So far, their impact onto the electrical parameters of the solar cells is not fully understood, mainly because of their difficult spatially resolved detection. Within this work the application of scanning acoustic microscopy circumvents this difficulty and allows a classification of local contacts in terms of “voids” or “filled contacts” on large cell area. The passivation quality of the BSF in the local contacts is investigated in detail through the local internal quantum efficiency (IQE) measured by light beam induced current (LBIC). It is found that there is a large spreading for the IQE values, attributed to a variation in BSF layer thickness. In addition, LBIC measurements of voids are fitted by 2D simulations, according to a detailed modelling of the surface recombination velocities (SRV) in local contacts. The final model is based on a non‐uniform SRV in the void's vicinity, attributed to laser damage close to the interface. It is demonstrated that the electrical parameters of the solar cell are affected by voids only when their surface is not sufficiently passivated by a local BSF.