P-type Passivated Emitter And Rear Cell Research Articles

Enhancement of efficiency in monocrystalline silicon solar cells

As the representative of the first generation of solar cells, crystalline silicon solar cells still dominate the photovoltaic market, including monocrystalline and polycrystalline silicon cells. With the development of silicon materials and cut-silicon wafer technologies, monocrystalline products have become more cost-effective, accelerating the replacement of polycrystalline products. In addition, the conversion efficiency of monocrystalline products increases gradually through high-efficiency cell technologies such as Passivated Emitter and Rear Cell (PERC). This paper will start with the solar cell efficiency and combine cost factor, the P-type PERC cell and additional four types of high-efficiency N-type cell technologies to improve the conversion efficiency for exploration, and will analyze and predict the future solar cell industrialization technologies. The study finally concludes that the N-type Tunnel Oxide Passivated Contact solar cell technology has a large industrial potential and could become a mainstream technology after the perc cell technology. At the same time, Interdigitated back contact cell structure and selective all-passive contact technologies are reliable paths to obtain solar cells with high conversion efficiency, integrated cost, and efficiency, depending on the advantages and compatibility of various cells. It can create conditions for the industrialization of low- cost and high-efficiency monocrystalline silicon solar cells.

LeTID mitigation via an adapted firing process in p-type PERC cells from gallium-doped Czochralski silicon

In this work, we analyse the light and elevated temperature induced degradation (LeTID) behaviour of gallium (Ga)-doped Czochralski (Cz) passivated emitter and rear cells (PERC). We show that these cells, based on industrial precursors, are prone to LeTID, degrading on average 8.3%rel in efficiency. The degradation and regeneration is slower than in boron (B)-doped PERC cells leading to a greater loss of energy over a 20 year lifespan of a solar module on a German rooftop, since, according to our experiments, regeneration does not start during that time frame. We show that the method of mitigating LeTID by slightly altering the temperature profile during fast firing, previously introduced on B-doped PERC cells, also works on Ga-doped PERC cells, reducing the maximum degradation to between 1%rel and 4%rel. This process does only slightly reduce the initial efficiency of the cell by up to 0.8%rel. Lastly, we show that increasing illumination and temperature from 0.15 suns eq. to 2 suns eq. and 75 °C to 140 °C, respectively, is an adequate way to accelerate LeTID testing on Ga-doped samples by more than two orders of magnitude while achieving qualitatively and quantitatively similar results to those obtained in a standard test.

Suppression of potential-induced degradation in monofacial PERC solar cells with gradient-designed capping layer

The potential-induced degradation (PID) of p-type crystalline silicon passivated emitter and rear cell (PERC) is a critical issue causing severe output power loss. In this paper, a gradient-designed capping layer consisting of silicon nitride/silicon oxynitride/silicon oxide (SiNx/SiNxOy/SiOx) is proposed as the surface coating for p-type PERC solar cells. The capping layer improves the short wavelength spectral response due to its better anti-reflection effect. An impressive efficiency of 23.52% has been achieved on mass production line. A long-term PID evolution up to 672 h is performed for glass/back sheet PERC cell modules under stringent conditions with the ambient temperature of 85 °C, relative humidity of 85%, and the applied voltage of 1500 V. The current–voltage (light I-V, dark I-V) parameters of the modules were measured, as well as the electroluminescence, to evaluate the effect of sodium ion migration in PERC solar cells. It is shown that monofacial PERC solar modules suffer from shunt of PN junction under PID stress, which can be obviously suppressed by gradient-designed capping layers. The optimized PERC solar modules show excellent PID resistance, showing great potential for lower levelized cost of energy (LCOE) of photovoltaic power generation.

Improved Bifacial Properties of P‐Type Passivated Emitter and Rear Cell Solar Cells toward High Mass Production Efficiency

The p‐type passivated emitter and rear cell (PERC) has achieved great success, and the bifacial PERC product is predicted as the mainstream of photovoltaic market. Herein, the higher diffusion sheet resistance on laser‐doped selective emitter combined with different rear‐side laser ablation designs and their impact on passivation and electrical performance are investigated. A high diffusion sheet resistance can restrain front surface recombination and improve short‐wavelength spectral response. The minority carrier lifetime evolution is evaluated on a semi‐PERC structure, which is significantly affected by the laser ablation design on the rear surface. The measured efficiency analysis reveals that there is a negative relation between enlarged laser ablation area and open‐circuit voltage. The conversion efficiency of bifacial PERC cells with dash ablation design increases by 0.29% (absolute efficiency gain) comparing with base line samples with straight‐line ablation. A high front‐side average efficiency of 22.34% and a high bifaciality of 76.87% are achieved and further optimized to 22.52% and 78%, respectively, for a p‐type PERC cell in a mass production line. After applying a multipassivation structure and careful optimization, an impressive mass production efficiency of 23.24% is achieved recently, showing great potential for low‐cost photovoltaic power generation.

STABILIZATION OF MINORITY CARRIER LIFETIME IN PERC STRUCTURED SILICON SOLAR CELL

This paper reports on the regeneration of the minority carrier lifetime in passivated emitter and rear cell (PERC) structured silicon solar cells. It is observed that minority carrier lifetime in the cells can degrade, recover and then stabilize with illumination level of ~1 sun (1000 W/m2) at 80oC. The exposure to ~1 sun illumination at 80oC enables the release of H from B-H bonds at ~1.3 eV energy to supplement the interstitial H in Si to passivate the B-O defects responsible for the minority carrier lifetime instability. Passivation of these B-O defects is therefore, dependent on temperature and time, hydrogenation and high carrier injection level. It was interesting to note that sequential process or single regeneration step led to same conclusion that minority carrier lifetime in a p-type PERC cell first degrades, due to B-O complexes, recovers and then stabilize with time. There is therefore, no need to degrade the cells in a separate step in order for regeneration to occur, because regeneration encompasses the three states: degradation, recovery and stabilization.

Bifacial PERC Solar Cell Designs: Bulk and Rear Properties and Illumination Condition

We report on the evaluation of cell performances of the bifacial passivated emitter and rear cell (PERC) structures for both p- and n-type Cz-Si. We compared four conditions: Front-side illumination, with and without the rear metal contact, rear-side illumination, and double-side illumination. Furthermore, the effects of the rear contact area and the rear passivation surface recombination velocity (SRV) were evaluated. The numerical simulations were based on the experimental results of the p-type bifacial PERC performance, and we attempted to determine optimized cell designs for each condition. The results showed that the rear passivation SRV determined the bulk condition: An n-type bulk resistivity of 1 $\Omega$ -cm and p-type bulk resistivity with BO-cluster deactivation of 0.5 $\Omega$ -cm were suitable for a smaller and larger SRV, respectively.

Enhanced open-circuit voltage in p-type passivated emitter and rear cell by doped polysilicon layer as passivation contact

The most promising and feasible approach to achieving high-efficiency silicon solar cells must be passivation contact compatible with current homojunction thermal processes. In this paper, an easy-to-implement carrier-selective passivation contact with doped poly-Si/SiOx for produing p-type passivated emitter and rear cell (PERC) was proposed. The passivation contact structure consisted of an ultra-thin SiOx layer capped with an intrinsic amorphous silicon (a-Si) layer prepared by low pressure chemical vapor deposition, which was deposited onto a p-type silicon substrate and then was doped and re-crystallized by a thermal phosphorus diffusion at 850 °C for 75 min. By optimizing the diffusion profile and cleaning time, a low dark recombination current density J0 ≈ 3 fA/cm2 and large minority carrier lifetime of 3000 μs were achieved on the 180-μm-thick n-type wafers with a resistivity of 3 Ω·cm, corresponding to an implied open-circuit voltage of 700 mV. P-type PERC cells with the highly doped poly-Si/SiOx passivated emitter demonstrated 683.97 mV of open-circuit voltage, showing more than 10 mV enhancement comparing with regular PERC cells. The results demonstrated an attractive potential of the carrier-selective passivation contacts with doped poly-Si/SiOx under front metallizatin area in photovoltaic application.

Impact of Postplating Annealing on Defect Activation in Boron-Doped PERC Solar Cells

In this article, the impact of postplating annealing on the regenerated state of boron-doped p-type passivated emitter and rear cell (PERC) solar cells with plated Ni/Cu/Ag front-side contacts is characterized. The assessment of different plating annealing profiles in the temperature range of 200–300 °C and their impact on light-induced degradation as well as on additional defects are realized by lifetime measurements of nonmetallized solar cell precursors before and after annealing. An observed lifetime degradation indicates that the current process sequence might facilitate bulk defect activation. An alternative process sequence is tested and promising results are presented.

Eliminating light- and elevated temperature-induced degradation in P-type PERC solar cells by a two-step thermal process

Light- and elevated temperature-induced degradation (LeTID) can have a severe impact on the carrier lifetime of silicon substrates used in solar cell production and thus remains a crucial challenge for manufacturers. In this work, we introduce a two-step annealing process to mitigate LeTID in multi-crystalline silicon (mc-Si) passivated emitter and rear cell (PERC) solar cells. We demonstrate that the first annealing step (450 °C) with a slow belt speed (0.5 m/min) plays a primary role in mitigating LeTID in the cells, but also results in an increase in contact resistance. The application of a second annealing step at a similar temperature (400–500 °C) with a faster belt speed (1.4 m/min) recovers the contact resistance whilst maintaining the stability of the cell. Applying this approach to the p-type mc-Si PERC solar cells resulted in a reduction of efficiency loss during light soaking from ~6%rel (control) to ~1%rel (treated sample). This finding is significant for p-type mc-Si solar cell manufacturers, as the process can be applied to finished cells using a standard belt firing furnace to stabilise cell efficiency for long term operation in the field.

Efficiency improvement of B/Ga co-doped p-type multicrystalline silicon passivated emitter and rear cells induced by B/Al co-doping

In the present work, electrical performance of B/Ga co-doped p-type multicrystalline silicon passivated emitter and rear cell (PERC) was improved by B/Al co-doping. By combining the Al-Si alloying process with the relative high solid solubility of B in Si, high doping concentration in B/Al BSF was realized with the maximum doping concentration of 1.86E19 cm−3, which was almost one magnitude higher than Al doping. Benefiting from the relative high doping concentration, the saturation current density expelled the influence of emitter decreased from 135.1 fA cm−2 to 121.1 fA cm−2. The suppression of saturation current together with the J01 both indicated the electrical improvement of PERC solar cell, with the open-circuit voltage (VOC) enlargement of 1.5 mV and efficiency improvement of 0.02%. After body defect passivation, the electrical parameters of all PERC solar cells increased slightly, while the superiority of the B/Al paste aggravated with the efficiency enlargement of 0.05%. All these efforts will be helpful to deepen the understanding of B/Ga co-doped p-type multicrystalline PERC solar cell as well as B/Al co-doping.

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%.

Analysis of nanosecond and femtosecond laser ablation of rear dielectrics of silicon wafer solar cells

Laser ablation of rear dielectrics of PERC (Passivated Emitter and Rear Cell) type silicon solar cells is by far the most cost-effective method to open a variety of patterns for local contact formation. In this paper, rear dielectric ablation of p-type PERC solar cells with an implanted phosphorus emitter is investigated using two laser sources with green wavelength and with pulse durations in the nanosecond (ns) and femtosecond (fs) range. The ns laser source emits 38 ns pulses and the ablation behaviour is linear for the entire range of available pulse fluences. The fs laser source emits 480 fs pulses and has two operating regimes, gentle and strong, based on the incoming pulse fluence. To evaluate the impact of laser induced damages on PERC solar cells, four different laser conditions are considered: fs gentle regime, fs strong regime, ns low fluence and ns high fluence. The solar cells’ one-sun I–V parameters show that the cells processed in the fs strong regime have a severe reduction in the open-circuit voltage by almost 40 mV, fill factor loss by ~7% and efficiency loss by 3–4%. Fill factor loss analysis shows that a loss of almost 7% absolute comes entirely from J02 recombination, revealing a possibility of severe damage to the front side emitter. The solar cells processed with the other three laser conditions have comparable I–V characteristics. SEM analysis of the front surface of samples with laser treated rear surface shows that the fs strong regime produces severe damage to the front-surface texture resulting in lower Jsc, whereas this effect is not observed for the other laser conditions.

Evaluating root cause: The distinct roles of hydrogen and firing in activating light- and elevated temperature-induced degradation

The root cause of light- and elevated temperature-induced degradation (LeTID) in multicrystalline silicon p-type passivated emitter and rear cell (PERC) devices is still unknown. Microwave-induced remote hydrogen plasma (MIRHP) is employed to vary the concentration of bulk hydrogen and to separate the effects of hydrogen and firing temperature in LeTID-affected wafers. We find that hydrogen is required for degradation to occur, and that samples fired prior to the introduction of hydrogen do not degrade. Importantly, samples with hydrogen that have not been fired do degrade, implying that the firing time-temperature profile does not cause LeTID. Together, these results suggest that the LeTID defect reaction consists of at least two reactants: hydrogen and one or more defects that can be separately modified by high-temperature firing. We assess the leading hypotheses for LeTID in the context of our new understanding of the necessary reactants.

Assisted passivation by a chemically grown SiO2 layer for p-type selective emitter-passivated emitter and rear cells

This paper presents the development of industrial-sized p-type passivated emitter and rear cells (PERCs) with a selective emitter (SE) structure. We focus on different assisted passivation schemes and use nitric acid oxidation of silicon (NAOS) to form an ultrathin SiO2 layer. The results show that all I–V parameters of the PERCs are significantly modified when SiO2 is present on the front and/or rear sides of the cell. For front-emitter passivation by NAOS-SiO2, we observe an increase in the open-circuit voltage (Voc) and short-circuit current density (Jsc) and strong enhancement of the internal quantum efficiency (IQE) for short-wavelength photons. This can be attributed to the high level of chemical passivation, as indicated by the reduced interface trap density (Dit), due to the reduced Shockley–Read–Hall recombination. By contrast, all I–V parameters associated with SiO2 rear passivation decrease due to the increased Dit, which results in enhanced surface recombination. The different types of surface passivation for n-type (front) and p-type (rear) surfaces are presumably due to different doping surface concentrations. These results suggest that NAOS surface pretreatment is a very promising technique to improve the level of surface passivation and thereby enhance the performance of industrial-sized PERCs.

Mass production of p-type Cz silicon solar cells approaching average stable conversion efficiencies of 22 %

Within this work, both the performance and reliability of industrial p-type monocrystalline solar cells with dielectrically passivated rear side and corresponding modules are investigated. Results of the mass production of Q.ANTUM solar cells at Hanwha Q CELLS on boron-doped p-type Czochralski-grown silicon (Cz-Si) substrates are presented, exceeding 21.5 % average conversion efficiency. Without power-enhancing measures such as the use of half cells, multi-wire approaches or light-capturing ribbons, essentially all currently (as of March 2017) produced Cz-Si Q.ANTUM solar modules exhibit output powers of > 300 Wp with 60 full 4-busbar cells. In terms of reliability, light-induced degradation (LID) is investigated in detail, with conditions relevant for the activation of, both, the boron-oxygen (BO) defect, and, so-called “Light and Elevated Temperature Induced Degradation” (LeTID). While the formation of the BO defect has been considered the most prominent LID mechanism in boron-doped p-type Cz-Si, LeTID has so far been discussed mainly as a potential issue for passivated emitter and rear cells (PERC) on multicrystalline silicon (mc-Si) substrates. This work shows that, if not adequately suppressed, LeTID can also occur in p-type Cz-Si PERC with a degradation in output power of up to > 6 %, which cannot be suppressed in a straightforward manner by conventional processing steps to permanently deactivate the BO defect. In contrast to conventional PERC, Hanwha Q CELLS’ Q.ANTUM technology is shown to reliably suppress, both, LID due to BO defect formation, and, LeTID in modules manufactured from, both, p-type mc-Si and Cz-Si substrates.

Analysis of contact recombination at rear local back surface field via boron laser doping and screen-printed aluminum metallization on p-type PERC solar cells

The passivated emitter and rear cell (PERC) has entered the solar cell market, and it is expected that the amount of PERC production will increase yearly in the future. Improvements in the cell efficiency have a great influence on reducing the cost of power generation. In this study, we focused on the loss of the back surface field (BSF) at the rear side of a PERC, fabricated PERCs having a local BSF including boron diffused by boron laser doping (B-LD), and evaluated this effect. The results from an analysis using scanning electron microscopy (SEM) indicated that the thickness of the BSF fabricated with B-LD was thicker than that of the BSF fabricated without B-LD. The average value of the saturation current density of the metallized area (including BSF) J0, contact was 554 fA/cm2 in the PERCs fabricated with B-LD and a dedicated Al paste, and the lowest value achieved was approximately 300 fA/cm2. The number of Kirkendall voids for the PERCs fabricated with B-LD decreased in comparison with the PERCs fabricated without B-LD. The PERCs fabricated with B-LD exhibited reduced rear-side recombination.

Lifetime Spectroscopy Investigation of Light-Induced Degradation in p-type Multicrystalline Silicon PERC

When untreated, light-induced degradation (LID) of p -type multicrystalline silicon (mc-Si)-based passivated emitter and rear cell (PERC) modules can reduce power output by up to 10% relative during sun-soaking under open-circuit conditions. Identifying the root cause of this form of LID has been the subject of several recent investigations. Lifetime spectroscopy analysis, including both injection and temperature dependencies (IDLS and TIDLS), may offer insight into the root-cause defect(s). In this paper, to illustrate the root-case defect identification method, we apply room-temperature IDLS to intentionally Cr-contaminated mc-Si. Then, we apply this technique to the p -type mc-Si that exhibits LID in PERC devices, and we provide further insights by analyzing qualitatively the injection-dependent lifetime as a function of temperature. We quantify the sensitivity of the capture cross-section ratio to variations in the measured lifetime curve and in the surface recombination. We find that the responsible defect most likely has an energy level between 0.3 and 0.7 eV above the valence band and a capture cross-section ratio between 26 and 36. Additionally, we calculate the concentrations of several candidate impurities that may cause the degradation.

Model Based Continuous Improvement of Industrial p-type PERC Technology Beyond 21% Efficiency

In this work, we present our progress in the industrial p-type PERC technology [1, 2]. Based on device simulations we continuously develop an efficiency roadmap for a steady improvement of our PERC (passivated emitter and rear cell) process. Following this simulation based approach, we effectively improve the front side metallization and the emitter characteristics. Currently, our best prototype process has reached a conversion efficiency well over 21% which enables the manufacturing of a 60-cell based module with a power of 310W. Our best cell so far has a conversion efficiency of 21.5% which has been confirmed by the calibration laboratory of Fraunhofer ISE. This is to our knowledge the highest efficiency reported for industrial-size silicon solar cells with screen-printed metal front and rear contacts.

Investigation on the passivated Si/Al2O3 interface fabricated by non-vacuum spatial atomic layer deposition system.

Currently, aluminum oxide stacked with silicon nitride (Al2O3/SiNx:H) is a promising rear passivation material for high-efficiency P-type passivated emitter and rear cell (PERC). It has been indicated that atomic layer deposition system (ALD) is much more suitable to prepare high-quality Al2O3 films than plasma-enhanced chemical vapor deposition system and other process techniques. In this study, an ultrafast, non-vacuum spatial ALD with the deposition rate of around 10 nm/min, developed by our group, is hired to deposit Al2O3 films. Upon post-annealing for the Al2O3 films, the unwanted delamination, regarded as blisters, was found by an optical microscope. This may lead to a worse contact within the Si/Al2O3 interface, deteriorating the passivation quality. Thin stoichiometric silicon dioxide films prepared on the Si surface prior to Al2O3 fabrication effectively reduce a considerable amount of blisters. The residual blisters can be further out-gassed when the Al2O3 films are thinned to 8 nm and annealed above 650°C. Eventually, the entire PERC with the improved triple-layer SiO2/Al2O3/SiNx:H stacked passivation film has an obvious gain in open-circuit voltage (Voc) and short-circuit current (Jsc) because of the increased minority carrier lifetime and internal rear-side reflectance, respectively. The electrical performance of the optimized PERC with the Voc of 0.647 V, Jsc of 38.2 mA/cm2, fill factor of 0.776, and the efficiency of 19.18% can be achieved.

Resistive Power Loss Analysis of PV Modules Made From Halved 15.6 × 15.6 cm2 Silicon PERC Solar Cells With Efficiencies up to 20.0%

In photovoltaic (PV) modules, the interconnection of solar cells is critical in terms of mechanical stability and resistive power losses. In this study, we analyze the interconnection of large-area 15.6 × 15.6 cm 2 industrial p-type passivated emitter and rear cell (PERC) solar cells in terms of resistive losses. For our analysis, we prepare a 3 × 3 minimodule from PERC solar cells with soldering pads and efficiencies up to 20.0%. We measure a significant cell-to-module (CTM) power loss of 8% at this module. For comparison, we prepare a 3 × 6 module consisting of halved 7.8 × 15.6 cm 2 PERC solar cells. Using a nanosecond laser to cut the finished solar cell in two pieces, no additional power loss is introduced by cutting. The CTM factor of 1.0 determined at the 3 × 6 module is explained using an analytical model describing the series resistance of the module interconnection. Using this model, we estimate for our current PERC cell generation and module process an output power of 275 W for 60 full-size cells and 285 W for 120 halved cells.