Large‐area p‐type HIP‐MWT silicon solar cells with screen printed contacts exceeding 20% efficiency

We present metal wrap through (MWT) silicon solar cells with passivated surfaces based on a simplified device structure. This so‐called HIP‐MWT structure (high‐performance metal wrap through) does not exhibit an emitter on the rear side and therefore simplifies processing. The confirmed peak efficiency of the fabricated solar cells with an edge length of 125 mm, screen printed contacts and solder pads is 20.2%. To our knowledge, this is the highest value reported for large‐area p‐type silicon solar cells to date.

     

21.63% industrial screen‐printed multicrystalline Si solar cell

In this paper, we demonstrate industrially feasible large‐area solar cells achieving energy conversion efficiency up to 21.63% on p‐type boron doped multicrystalline Si wafers. Advanced light trapping, passivation and hydrogenation technology are used to achieve excellent light absorption with very low surface recombination velocity. The bulk lifetime of the multi‐crystalline Si wafers used for the fabrication exceeds 500 μs after optimized gettering and hydrogenation processes. The high bulk lifetime and excellent surface passivation enable V_oc to exceed 670 mV. The metallization process is carried out by screen printing and firing in a conventional belt furnace. Detailed performance parameters and quantum efficiency of the cells will be illustrated in the paper. In addition, free energy loss analysis and cell simulation are also performed using the control parameters measured during cell fabrication processes.     

Investigation of fill factor losses on 20.2% efficient n‐type mono‐like silicon solar cells with laser contact opening

Large area (243.36 cm²) back‐junction passivated emitter, rear totally diffused (PERT) solar cells with laser contact opening (LCO) on n‐type mono‐like crystalline Si with efficiencies of 20.2% are presented. Boron emitters with high electrical quality (implied open circuit voltage iV_OC up to 700 mV) are formed during a co‐diffusion step using SiO_x:B layers. Increasing the rear metal contact coverage, we observed a decrease in fill factor (FF) instead of the expected increase due to the decrease of the back side series resistance. We show that it can be attributed to recombination centers (RCs) in the space charge region underneath the contact spots inducing an increasing second diode contribution. The presented empirical model for the RCs implemented in Synopsys Sentaurus TCAD allows for a successful reproduction of the FF, pseudo FF and V_OC behaviour with contact coverage. According to this model, the RCs induced by laser ablation and subsequently evaporation of Al have a shallow exponential distribution with a characteristic length of L_T = 0.2 µm and an effective surface density of N *_T0 = 25 cm^–1. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Dopant‐free back contact silicon heterojunction solar cells employing transition metal oxide emitters

The present study investigates the electrical properties of transition metal oxide (TMO) emitters in dopant‐free n‐Si back contact solar cells by comparing the properties of solar cells employing three TMOs (WO_x, MoO_x and V₂O_x) with varying electrical properties acting as p‐type contacts. The TMOs are found to induce large band bending in n‐Si, which reduces the injection level dependent interfacial recombination speed S_eff and contact resistivity ρ_c. Among the TMO/n‐Si contacts considered, the V₂O_x/n‐Si contact achieves the lowest S_eff of 138 cm/s and ρ_c of 0.034 Ω cm², providing the significant advantages over heavily doped a‐Si:H(p)/n‐Si contacts. The best device performance was achieved by the V₂O_x/n‐Si solar cell, demonstrating an efficiency of 16.59% and an open‐circuit voltage of 610 mV relative to solar cells based on MoO_x/n‐Si (15.09%, 594 mV) and WO_x/n‐Si (12.44%, 539 mV). Furthermore, the present work is the first to employ WO_x, V₂O_x and Cs₂CO₃ in back contact solar cells. The fabrication process employed offers great potential for the mass production of back contact solar cells owing to simple, metal mask patterning with high alignment quality and dopant‐free steps conducted at a lower temperature.     

21.0%‐efficient co‐diffused screen printed n‐type silicon solar cell with rear‐side boron emitter

Plasma enhanced chemical vapor deposition (PECVD) is applied to deposit boron silicate glasses (BSG) acting as boron diffusion source during the fabrication of n‐type silicon solar cells. We characterize the resulting boron‐diffused emitter after boron drive‐in from PECVD BSG by measuring the sheet resistances R_sheet,B and saturation current densities J_0,B. For process optimization, we vary the PECVD deposition parameters such as the gas flows of the precursor gases silane and diborane and the PECVD BSG layer thickness. We find an optimum gas flow ratio of SiH₄/B₂H₆= 8% and layer thickness of 40 nm. After boron drive in from these PECVD BSG diffusion sources, a low J_0,B values of 21 fA/cm² is reached for R_sheet,B = 70 Ω/□. The optimized PECVD BSG layers together with a co‐diffusion process are implemented into the fabrication process of passivated emitter and rear totally diffused (PERT) back junction (BJ) cells on n‐type silicon. An independently confirmed energy conversion efficiency of 21.0% is achieved on 15.6 × 15.6 cm² cell area with a simplified process flow. This is the highest efficiency reported for a co‐diffused n‐type PERT BJ cell using PECVD BSG as diffusion source. A loss analysis shows a small contribution of 0.13 mW/cm² of the boron diffusion to the recombination loss proving the high quality of this diffusion source. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

19.4%‐efficient large‐area fully screen‐printed silicon solar cells

We demonstrate industrially feasible large‐area solar cells with passivated homogeneous emitter and rear achieving energy conversion efficiencies of up to 19.4% on 125 × 125 mm² p‐type 2–3 Ω cm boron‐doped Czochralski silicon wafers. Front and rear metal contacts are fabricated by screen‐printing of silver and aluminum paste and firing in a conventional belt furnace. We implement two different dielectric rear surface passivation stacks: (i) a thermally grown silicon dioxide/silicon nitride stack and (ii) an atomic‐layer‐deposited aluminum oxide/silicon nitride stack. The dielectrics at the rear result in a decreased surface recombination velocity of S_rear = 70 cm/s and 80 cm/s, and an increased internal IR reflectance of up to 91% corresponding to an improved J_sc of up to 38.9 mA/cm² and V_oc of up to 664 mV. We observe an increase in cell efficiency of 0.8% absolute for the cells compared to 18.6% efficient reference solar cells featuring a full‐area aluminum back surface field. To our knowledge, the energy conversion efficiency of 19.4% is the best value reported so far for large area screen‐printed solar cells. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

n‐type silicon solar cells with surface‐passivated screen‐printed aluminium‐alloyed rear emitter

Aluminium‐doped p‐type (Al‐p⁺) silicon emitters fabricated by means of a simple screen‐printing process are effectively passivated by plasma‐enhanced chemical‐vapour deposited amorphous silicon (a‐Si). We measure an emitter saturation current density of only 246 fA/cm², which is the lowest value achieved so far for a simple screen‐printed Al‐p⁺ emitter on silicon. In order to demonstrate the applicability of this easy‐to‐fabricate p⁺ emitter to high‐efficiency silicon solar cells, we implement our passivated p⁺ emitter into an n⁺np⁺ solar cell structure. An independently confirmed conversion efficiency of 19.7% is achieved using n‐type phosphorus‐doped Czochralski‐grown silicon as bulk material, clearly demonstrating the high‐efficiency potential of the newly developed a‐Si passivated Al‐p⁺ emitter. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nano‐glass frit for inkjet printed front side metallization of silicon solar cells prepared by sol–gel process

Sol–gel derived nano‐sized glass frits were incorporated into the Ag conductive ink for silicon solar cell metallization. This mixture was specifically formulated for inkjet printing on textured Si wafers with 80 nm thick SiN_x anti reflection coating layers. The correlation between the contact resistance and interface microstructures were studied using scanning electron microscopy and transmission electron microscopy. In addition, the specific contact resistance between the front contact and emitter was measured at various firing conditions using the transfer length model. On an emitter with the sheet resistance of 60 Ω/sq, a specific contact resistance below 5 mΩ cm² could be achieved at a peak firing temperature around 800 °C. We found that the incorporated nano‐glass frit act as a very effective fire through agent, and an abundant amount of Ag crystallites was observed along the interface glass layer. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Surface passivation of phosphorus‐diffused n+‐type emitters by plasma‐assisted atomic‐layer deposited Al2O3

In recent years Al₂O₃ has received tremendous interest in the photovoltaic community for the application as surface passivation layer for crystalline silicon. Especially p‐type c‐Si surfaces are very effectively passivated by Al₂O₃, including p‐type emitters, due to the high fixed negative charge in the Al₂O₃ film. In this Letter we show that Al₂O₃ prepared by plasma‐assisted atomic layer deposition (ALD) can actually provide a good level of surface passivation for highly doped n‐type emitters in the range of 10–100 Ω/sq with implied‐V_oc values up to 680 mV. For n‐type emitters in the range of 100–200 Ω/sq the implied‐V_oc drops to a value of 600 mV for a 200 Ω/sq emitter, indicating a decreased level of surface passivation. For even lighter doped n‐type surfaces the passivation quality increases again to implied‐V_oc values well above 700 mV. Hence, the results presented here indicate that within a certain doping range, highly doped n‐ and p‐type surfaces can be passivated simultaneously by Al₂O₃. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Localization and characterization of annealing‐induced shunts in Ni‐plated monocrystalline silicon solar cells

The adhesion of Ni–Cu‐plated contacts requires annealing, which can lead to an electrical degradation of the solar cell. A combination of optical imaging methods and electron microscopical cross‐section analysis was used to investigate the annealing‐induced shunts on monocrystalline solar cells. The results show that Ni spikes cause the lowering of the pseudo fill factor and reveal that these nonlinear shunts show the same behavior as recombination active breakdown sites on multicrystalline silicon solar cells, including radiation of visible light while breakdown. Using reverse biased electroluminescence set‐up the shunts could be localized in top view with µm size lateral resolution. Subsequently, a cross‐section was prepared on a radiative breakdown spot. Advanced electron microscopic investigations reveal defect structures featuring nickel precipitates on the position of the light source. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Co‐diffusion from solid sources for bifacial n‐type solar cells

We present a simplified process sequence for the fabrication of large area n‐type silicon solar cells. The boron emitter and full area phosphorus back surface field are formed in one single high temperature step using doped glasses deposited by plasma enhanced chemical vapour deposition (PECVD) as diffusion sources. By optimizing the gas composition during the PECVD process, we not only prevent the formation of a boron rich layer (BRL), but also achieve doping profiles that exhibit a low dark saturation current density while allowing for contact formation by screen printing. The presented co‐diffusion process allows for major process simplification compared to the state of the art diffusion process relying on multiple high temperature processes, masking and wet chemistry steps.

     

Light‐induced degradation of PECVD aluminium oxide passivated silicon solar cells

Light‐induced degradation (LID) has been identified to be a critical issue for solar cells processed on boron‐doped silicon substrates. Typically, Czochralski‐grown silicon (Cz‐Si) has been reported to suffer from stronger LID than block‐cast multicrystalline silicon (mc‐Si) due to higher oxygen concentrations. This work investigates LID under conditions practically relevant under module operation on different cell types. It is shown that aluminium oxide (AlOx) passivated mc‐Si solar cells degrade more than a reference aluminium back surface field mc‐Si cell and, remarkably, an AlOx passivated Cz‐Si solar cell. The defect which is activated by illumination is shown to be doubtful a sole bulk effect while the AlOx passivation might play a certain role. This work may contribute to a re‐evaluation of the suitability of boron‐doped Cz‐ and mc‐Si for solar cells with very high efficiencies. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Enhanced rear‐side reflection and firing‐stable surface passivation of silicon solar cells with capping polymer films

Low refractive index polymer materials have been investigated with a view to form the back surface mirror of advanced silicon solar cells. SiO_x:H or AlO_y SiO_x:H polymer films were spun on top of an ultra‐thin (<10 nm) atomic‐layer‐deposited (ALD) Al₂O₃ layer, itself deposited on low‐resistivity (1 Ω cm) p‐type crystalline silicon wafers. These double‐layer stacks were compared to both ALD Al₂O₃ single layers and ALD Al₂O₃/plasma‐enhanced chemical vapour deposited (PECVD) SiN_x stacks, in terms of surface passivation, firing stability and rear‐side reflection. Very low surface recombination velocity (SRV) values approaching 3 cm/s were achieved with ALD Al₂O₃ layers in the 4–8 nm range. Whilst the surface passivation of the single ALD Al₂O₃ layer is maintained after a standard firing step typical of screen printing metallisation, a harsher firing regime revealed an enhanced thermal stability of the ALD Al₂O₃/SiO_x:H and ALD Al₂O₃/AlO_y SiO_x:H stacks. Using simple two‐dimensional optical modelling of rear‐side reflection it is shown that the low refractive index exhibited by SiO_x:H and AlO_y SiO_x:H results in superior optical performance as compared to PECVD SiN_x, with gains in photogenerated current of ～0.125 mA/cm² at a capping thickness of 100 nm. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Research on the boron contamination at the p/i interface of microcrystalline silicon solar cells deposited in a single PECVD chamber

This paper studies boron contamination at the interface between the p and i layers of μ c-Si:H solar cells deposited in a single-chamber PECVD system. The boron depth profile in the i layer was measured by Secondary Ion Mass Spectroscopy. It is found that the mixed-phase μ c-Si:H materials with 40% crystalline volume fraction is easy to be affected by the residual boron in the reactor. The experimental results showed that a 500-nm thick μ c-Si:H covering layer or a 30-seconds of hydrogen plasma treatment can effectively reduce the boron contamination at the p/i interface. However, from viewpoint of cost reduction, the hydrogen plasma treatment is desirable for solar cell manufacture because the substrate is not moved during the hydrogen plasma treatment.     

Light‐induced degradation and regeneration of multicrystalline silicon Al‐BSF and PERC solar cells

Light‐induced degradation (LID) is a well‐known problem faced by p‐type Czochralski (Cz) monocrystalline silicon (mono‐Si) wafer solar cells. In mono‐Si material, the physical mechanism has been traced to the formation of recombination active boron‐oxygen (B–O) complexes, which can be permanently deactivated through a regeneration process. In recent years, LID has also been identified to be a significant problem for multicrystalline silicon (multi‐Si) wafer solar cells, but the exact physical mechanism is still unknown. In this work, we study the effect of LID in two different solar cell structures, aluminium back‐surface‐field (Al‐BSF) and aluminium local back‐surface‐field (Al‐LBSF or PERC (passivated emitter and rear cell)) multi‐Si solar cells. The large‐area (156 mm × 156 mm) multi‐Si solar cells are light soaked under constant 1‐sun illumination at elevated temperatures of 90 °C. Our study shows that, in general, PERC multi‐Si solar cells degrade faster and to a greater extent than Al‐BSF multi‐Si solar cells. The total degradation and regeneration can occur within ～320 hours for PERC cells and within ～200 hours for Al‐BSF cells, which is much faster than the timescales previously reported for PERC cells. An important finding of this work is that Al‐BSF solar cells can also achieve almost complete regeneration, which has not been reported before. The maximum degradation in Al‐BSF cells is shown to reduce from 2% (relative) to an average of 1.5% (relative) with heavier phosphorus diffusion.     

Defect removal after low temperature annealing of boron implantations by emitter etch‐back for silicon solar cells

Ion implantation offers new possibilities for silicon solar cell production, e.g. single side doping that can be structured in‐situ with shadow masks. While phosphorus implantations can easily be annealed at low temperature, the annealing of boron implantations is challenging. In this study, we use low energy implantations of boron (1 keV and 5 keV) with a projected range of 5.6 nm and 21.2 nm that form defects causing charge carrier recombination after a low temperature anneal (950 °C, 30 min). An ozone‐based wet chemical etching step is applied to remove this near surface damage. With increasing chemical etch‐back the electrical quality (i.e. emitter saturation current density, J_0e) improves continuously. The calculated limit for J_0e was reached with an abrasion of 35 nm for 1 keV and 85 nm for 5 keV implantations, showing that the relevant defects causing charge carrier recombination are located very close to the surface, corresponding to the as‐implanted profile depth. This emitter etch‐back allows for the fabrication of defect free boron doping profiles with good sheet resistance uniformity (standard deviation <2%). With the resulting characteristics (sheet resistance <100 Ω/sq, surface doping concentration >5 × 10¹⁹ cm^–3, J_0e < 30 fA/cm²), these boron profiles are well suited for silicon solar cells. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Micro‐photoluminescence spectroscopy on heavily‐doped layers of silicon solar cells

We report and explain the photoluminescence spectra emitted from silicon solar cells with heavily‐doped layers at the surface. A micro‐photoluminescence spectroscopy system is employed to investigate the total spectrum emitted from both the heavily‐doped layer and the silicon substrate with micron‐scale spatial resolution. The two regions of the device give rise to separate photoluminescence peaks, due to band‐gap narrowing effects in the highly‐doped layer. Two key parameters, the absorption depth of the excitation wavelength, and the sample temperature, are shown to be critical to reveal the separate signatures from the two regions. Finally, this technique is applied to locally diffused and laser‐doped regions on silicon solar cell pre‐cursors, demonstrating the potential value of this micron‐scale technique in studying and optimizing locally doped regions. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Assessment of the illumination dependency of Al2O3 and SiO2 rear‐passivated p‐type silicon solar cells

This Letter discusses an important difference between positively charged SiO₂ and negatively charged Al₂O₃ rear‐passivated p‐type Si solar cells: their illumination level dependency. For positively charged SiO₂ rear‐passivated p‐type Si solar cells, a loss in short circuit current (J_SC) and open circuit voltage (V_OC) as a function of illumination level is mainly caused by parasitic shunting and a decrease in surface recombination, respectively. Hence, the relative loss in cell conversion efficiency, J_SC, and V_OC as a function of the illumination level for SiO₂ compared to Al₂O₃ rear‐passivated p‐type Si solar cells has been measured and discussed. Subsequently, an exponential decay fit of the loss in cell efficiency is applied in order to estimate the difference in the energy output for both cell types in three different territories: Belgium (EU), Seattle and Austin (US). The observed trends in the difference in energy output between both cells, as a function of time of the year and region, are as expected and discussed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Multi‐crystalline silicon solar cells with metal‐assisted nano‐texturing using HNO3 as hole injection agent

In this study, metal‐assisted etching (MAE) with nitric acid (HNO₃) as a hole injecting agent has been employed to texture multi‐crystalline silicon wafers. It was previously proven that addition of HNO₃ enabled control of surface texturing so as to form nano‐cone shaped structures rather than nanowires. The process parameters optimized for optically efficient texturing have been applied to multi‐crystalline wafers. Fabrication of p‐type Al:BSF cells have been carried out on textured samples with thermal SiO₂/PECVD‐SiN_x stack passivation and screen printed metallization. Firing process has been optimized in order to obtain the best contact formation. Finally, j_sc enhancement of 0.9 mA/cm² and 0.6% absolute increase in the efficiency have been achieved. This proves that the optimized MAE texture process can be successfully used in multi‐crystalline wafer texturing with standard passivation methods.

J –V curves and SEM images of the nano and iso‐textured samples. j_sc enhancement of 0.9 mA/cm² together with 0.6% absolute efficiency gain was observed on nano‐textured samples.