Saptharishi Ramanathan

High-Efficiency Large-Area Rear Passivated Silicon Solar Cells With Local Al-BSF and Screen-Printed Contacts

This paper describes the cell design and technology on large-area (239 cm2) commercial grade Czochralski Si wafers using industrially feasible oxide/nitride rear passivation and screen-printed local back contacts. A combination of optimized front and back dielectrics, rear surface finish, oxide thickness, fixed oxide charge, and interface quality provided effective surface passivation without parasitic shunting. Increasing the rear oxide thickness from 40 to 90 Å in conjunction with reducing the surface roughness from 1.3 to 0.2 μm increased the Voc from 640 mV to 656 mV. Compared with 18.6% full aluminum back surface field (Al-BSF) reference cell, local back-surface field (LBSF) improved the back surface reflectance (BSR) from 65% to 93% and lowered the back surface recombination velocity (BSRV) from 310 to 130 cm/s. Two-dimensional computer simulations were performed to optimize the size, shape, and spacing of LBSF regions to obtain good fill factor (FF). Model calculations show that 20% efficiency cells can be achieved with further optimization of local Al-BSF cell structure and improved screen-printed contacts.

20% Efficient Screen-Printed Cells With Spin-On-Dielectric-Passivated Boron Back-Surface Field

This paper reports on the characteristics of a spin-on dielectric which has been used as the rear-surface passivation layer to achieve 20% efficient screen-printed (SP) boron back-surface field (B-BSF) solar cells. The dielectric provides, in a single thermal step, both stable passivation of a heavily doped p+ surface and strong gettering of iron which is a common contaminant in high-temperature boron diffusion processes. It was found that gettering of silicon substrates, contaminated during boron diffusion, is most effective when the dielectric is deposited on top of the boron-doped layer. The effect of dielectric charge density on passivation of p+ surfaces was also studied and a very high charge density of -1013 cm-2 was found to be necessary to significantly improve the passivation on surfaces with a boron concentration.

20% efficient screen printed LBSF cell fabricated using UV laser for rear dielectric removal

Low-cost high efficiency solar cells are the key to achieving grid parity with photovoltaic devices. Laser processing in silicon photovoltaics is being incorporated at various stages to achieve this target. This paper details the fabrication, characterization and analysis of 4 cm2 screen printed cells with efficiency over 20% achieved using a UV laser ablation for selective opening of rear dielectric. These cells are compared to cells fabricated using a screen printed etching paste for opening vias through the rear dielectric. Microscopy was used to examine the impact of laser pulses on the silicon surface and quality of the BSF and compare it with vias opened using etching paste. Characterization and analysis of these cells is performed using IQE measurements and supported by PC1D modeling. It was found that while laser ablation had an effect on the morphology of the silicon surface, the overall quality of the local back surface field and dielectric rear passivation were maintained, resulting in high cell efficiencies and VOC.

Fabrication of 20 % efficient cells using spin-on based simultaneous diffusion and dielectric anneal

Low-cost high efficiency solar cells are the key to achieving grid parity with photovoltaic devices. This paper details the fabrication, characterization and analysis of 4 cm2 screen printed cells with efficiency over 20% achieved using a local back surface field cell structure in conjunction with a 75 ohm/sq emitter and efficient back surface reflector. A streamlined process sequence involving a single high temperature step for simultaneous formation of emitter and rear passivation was used. The combination of LBSF and improved rear reflector resulted in a peak efficiency of 20.1% with JSC of 39.4 mA/cm2 and VOC of 652 mV. Detailed characterization and modeling involving IQE and escape reflectance measurements revealed that the increase in VOC and JSC was the result of increased BSR from 67% to 93% and reduced BSRV from 325 cm/s to 125 cm/s. Improved rear passivation was found to be an effective method for controlling charge-induced inversion or parasitic shunting at the rear surface. Future experiments and further optimization are expected to result in efficiencies of over 20% on thin wafers using a similar cell structure.

Understanding and Fabrication of 20% Efficient Cells Using Spin-on-Based Simultaneous Diffusion and Dielectric Passivation

Low-cost high-efficiency solar cells are the key to achieve grid parity with photovoltaic devices. High-quality rear passivation is essential for the achievement of this goal. Thick thermal oxide is known to provide the required back surface passivation, but it can lead to long process steps at a high temperature. A combination of 2-D simulations and experiments is used to identify a dielectric stack that provides passivation comparable with that of a thick thermal oxide. This dielectric stack, in conjunction with a local back surface field and 75-Ω/sq emitter, produced solar cell efficiency exceeding 20%. In addition, a streamlined process sequence, involving a single high temperature step for simultaneous formation of emitter and rear passivation, is used. The peak efficiency of 20.1% was achieved with JSC of 39.4 mA/cm2 and VOC of 652 mV on float zone wafers of 2.3 Ω·cm resistivity. Detailed characterization and modeling revealed that the increase in VOC and JSC is the result of increased back surface reflectance from 67% to 93% and reduced back surface recombination velocity from 325 to 125 cm/s. Improved rear passivation was found to be an effective method to control charge-induced inversion or parasitic shunting at the rear surface. According to model calculations, further optimization can result in efficiencies of over 20% on much thinner wafers (~100 μm) using an identical cell structure.

Enhanced Front and Rear dielectric passivation for commercially grown Czochralski silicon for high efficiency solar cells

Commercial silicon solar cell efficiencies have been improving consistently over the last few years with the implementation of novel techniques. Along with larger wafer area and thinner substrates, low-cost processing of high efficiency solar cells can help achieve grid parity using crystalline silicon. In this work, large area cells were fabricated using conventional diffusion, oxidation and screen printing technologies. Several rear passivation schemes were compared to achieve low rear surface recombination velocities and combined with good front passivation to obtain cell VOC of ∼650 mV. Cells fabricated using these schemes resulted in an efficiency of 19.7% on 4 cm2 cells and 18.5% on 62 cm2 cells. Analysis and characterization of these cells reveals the possibility of achieving cell efficiencies greater than 19% on large area substrates.

Elucidating and engineering recombination-active metal-rich precipitates in n-type multicrystalline silicon

Solar cells based on n-type upgraded metallurgical grade multicrystalline silicon (mc-Si) substrates may be a promising path for reducing the cost per watt of photovoltaics. The detrimental effect of metal point defects in both n- and p-type silicon is known, but the recombination activity of metal-silicide precipitates, especially in n-type mc-Si, is still not well established, impeding modeling and process optimization efforts. In this contribution, we provide a rationale for why metal-rich precipitates may limit minority-carrier lifetime in n-type mc-Si, in contrast to as-grown p-type mc-Si, which is dominated by metal point defects. Using μ-XRF, we identify metal-rich precipitates along a recombination active grain boundary in the low-lifetime “red zone” region of n-type wafers from a corner brick. To reduce the concentration of precipitated metals, we phosphorus-diffuse the wafers. Grain boundaries remain recombination active, which may be attributed to incomplete gettering of point defects and dissolution of recombination-active metal-rich precipitates.

20% efficient screen printed boron BSF cells using spin-on dielectric passivation

This paper reports on the characteristics of a negatively charged spin-on dielectric which has been used as the rear surface passivation layer to achieve 20% efficient screen-printed boron back surface field (B-BSF) solar cells. Solar cell schemes which utilize a heavily doped boron back surface field often face the challenges of avoiding wafer contamination during a long, high-temperature diffusion process and achieving good surface passivation of the heavily doped surface. The dielectric examined in this work addresses these challenges as it provides both impurity gettering and surface passivation in a single thermal step. Gettering studies have been performed to compare POCl3 and dielectric gettering and to examine the relationship between gettering strength and the proximity of the gettering site to the doped layer. Passivation and charge density studies have been used to identify the passivation mechanism and to examine the stability of the dielectric passivation.

Practical challenges of achieving high efficiency boron back surface field solar cells

This work examines two challenges facing the commercialization of boron diffused crystalline silicon solar cells - unintentional iron contamination and the passivation of a textured boron diffused surface. We find that when Fe is introduced into silicon wafers by boron diffusion, the Fe often remains trapped in the boron surface layer and undetectable using bulk lifetime measurements. However this trapped Fe is still a threat to bulk lifetime since subsequent thermal processes can inject this Fe into the bulk. Importantly, POCl3 appears to be ineffective at gettering the trapped Fe; POCl3 gettering works only when the trapped Fe is injected into the bulk prior to POCl3 deposition. An alternate strategy is to directly getter the trapped Fe by using a negatively charged dielectric that is placed on the boron diffused surface. Random pyramid texturing was found to be detrimental to the spin-on SiO2 induced passivation of a boron doped surface. This passivation loss can be minimized by removing the texture with a KOH etch and ∼ 650 mV VOC could be achieved on both FZ and Cz material.

Optimization of phosphoric acid based limited-source-diffusion to obtain high quality emitter for screen printed contacts

Limited source diffusion using commercial dopant sources have been used for obtaining high quality emitters for high efficiency cells. In this work, the effect of gas flow on the quality of limited source diffusion is investigated. An emitter is diffused using an in-house prepared limited phosphoric acid dopant source. The effect of gas flow in this process is studied by measuring the uniformity of sheet resistance and emitter saturation current density. In addition, to improve these two parameters, an alternate diffusion process was developed which involves the use of a short in-situ oxidation at a lower temperature (700 °C) before the actual diffusion that resulted in excellent Joe < 100 fA/cm2. The modified diffusion process also made this process much less sensitive to ambient humidity. 4 cm2 full Al BSF cells fabricated on 0.6 Ω·cm p-type FZ silicon wafers using this emitter with screen printed contacts resulted in a peak efficiency of 19.6% with a VOC of 648 mV and a JSC of 37.5 mA/cm2.