Hannu S. Laine

Impact of Iron Precipitation on Phosphorus-Implanted Silicon Solar Cells

Ion implantation is a promising method to implement a high-performance emitter for crystalline silicon solar cells. However, an implanted emitter redistributes and mitigates harmful metal impurities to a different degree than a diffused one. This paper quantitatively assesses the effect of iron contamination level on the bulk diffusion length and open-circuit voltage of phosphorus-implanted solar cells manufactured with varying gettering parameters. By synchrotron-based micro-X-ray fluorescence measurements, we directly observe a process-dependent iron precipitate size distribution in the implanted emitters. We show that controlling the iron precipitate size distribution is important when optimizing final cell performance and discover a tradeoff between large shunting precipitates in the emitter and a high density of recombination active small precipitates in the wafer bulk. We present a heterogeneous iron precipitation model capable of reproducing the experimentally measured size distributions. We use the model to show that the dominant gettering mechanism in our samples is precipitation and that implanted emitters with surface phosphorus concentrations around 2×1019 cm-3 induce little-to-no segregation-based gettering. Based on this finding, we discuss optimal gettering strategies for industrial silicon solar cells with implanted emitters.

Black silicon significantly enhances phosphorus diffusion gettering

Black silicon (b-Si) is currently being adopted by several fields of technology, and its potential has already been demonstrated in various applications. We show here that the increased surface area of b-Si, which has generally been considered as a drawback e.g. in applications that require efficient surface passivation, can be used as an advantage: it enhances gettering of deleterious metal impurities. We demonstrate experimentally that interstitial iron concentration in intentionally contaminated silicon wafers reduces from 1.7 × 1013 cm−3 to less than 1010 cm−3 via b-Si gettering coupled with phosphorus diffusion from a POCl3 source. Simultaneously, the minority carrier lifetime increases from less than 2 μs of a contaminated wafer to more than 1.5 ms. A series of different low temperature anneals suggests segregation into the phosphorus-doped layer to be the main gettering mechanism, a notion which paves the way of adopting these results into predictive process simulators. This conclusion is supported by simulations which show that the b-Si needles are entirely heavily-doped with phosphorus after a typical POCl3 diffusion process, promoting iron segregation. Potential benefits of enhanced gettering by b-Si include the possibility to use lower quality silicon in high-efficiency photovoltaic devices.

Cu gettering by phosphorus-doped emitters in p-type silicon: Effect on light-induced degradation

The presence of copper (Cu) contamination is known to cause relevant light-induced degradation (Cu-LID) effects in p-type silicon. Due to its high diffusivity, Cu is generally regarded as a relatively benign impurity, which can be readily relocated during device fabrication from the wafer bulk, i.e. the region affected by Cu-LID, to the surface phosphorus-doped emitter. This contribution examines in detail the impact of gettering by industrially relevant phosphorus layers on the strength of Cu-LID effects. We find that phosphorus gettering does not always prevent the occurrence of Cu-LID. Specifically, air-cooling after an isothermal anneal at 800°C results in only weak impurity segregation to the phosphorus-doped layer, which turns out to be insufficient for effectively mitigating Cu-LID effects. Furthermore, we show that the gettering efficiency can be enhanced through the addition of a slow cooling ramp (-4°C/min) between 800°C and 600°C, resulting in the nearly complete disappearance of Cu-LID effects.

Accelerating synchrotron-based characterization of solar materials: Development of flyscan capability

Synchrotron-based μ-XRF is a powerful tool to measure elemental distributions non-destructively with high spatial resolution and excellent sensitivity. Recently, we implemented on-the-fly data collection (flyscan) at Beamline 2-ID-D at the Advanced Photon Source at Argonne National Laboratory, making data acquisition faster than 300 ms per pixel practical. We show that flyscan mode at Beamline 2-ID-D enables (a) traditional elemental maps to be completed twenty times more quickly while maintaining reasonably high sensitivity, and (b) practical studies of materials with an order of magnitude lower total impurity concentration and sparser spatial density of impurities. We highlight opportunities for flyscan to enable qualitatively new forms of microscopy, leveraging the accelerated data-acquisition rate for multi-dimensional mapping.

An extendible beyond 20% efficiency cost-efficient bifacial cell using boron & phosphorus implantation technology and its prospects for the future production

This paper presents an extendible beyond 20% efficiency cost-efficient roadmap for bifacial solar cell, which is suitable to both p-type and n-type substrate. The roadmap is based on boron & phosphorus implantation and screen print technology. We experimentally demonstrate here bifacial cells using industrial p-type c-Si substrate with front side efficiency of 20% and rear side efficiency of 16-16.5%. Due to the bifacial generation of electricity, these cells are expected to perform at high effective efficiency under various environments with different ground reflectance. Furthermore, very high quality boron implanted emitter with emitter saturation current as low as 5E-15 Acm-2 and open circuit voltage above 655 mV was obtained, which demonstrates the potential of implantation technology for the production of also other high efficiency cell architectures.

Analysis of different models of iron precipitation in multicrystalline silicon

Simulation of solar cell processing enables inexpensive and rapid process optimization. Over the last twenty years, several models describing the distribution and behavior of iron point defects and iron-silicide precipitates have been developed and incorporated into process simulations. The goal of this work is to elucidate what physics are needed to accurately describe industry-relevant as-grown impurity and defect distributions and processing conditions by simulating different material-processing combinations with each model. This rigorous comparison helps scientists and engineers choose the appropriate level of model complexity, and consequently simulation run time, based on material characteristics and processing conditions.

Iron precipitation upon gettering in phosphorus-implanted Czochralski silicon and its impact on solar cell performance

Phosphorus implantation can provide a direct route to a high-performing emitter, with no surface dead layer and improved blue response, and potentially higher open-circuit voltage. Here, iron precipitation during gettering is investigated in phosphorus-implanted, low-oxygen monocrystalline silicon and its impact on device performance evaluated. Previously, it has been shown that higher levels of initial iron contamination lead to lower final interstitial iron concentration after gettering with ion-implanted emitters, resulting in longer final bulk diffusion lengths in the more-highly contaminated materials. In this contribution, we show that despite longer bulk diffusion lengths, the open circuit-voltage of devices made from the highly iron-contaminated material can be strongly reduced. Using synchrotron-based Xray fluorescence we reveal the presence of micron-sized iron precipitates in the near surface region. While not measured over wafer-sized areas, the density of these precipitates correlates with the annealing profile. Slow-cooling from the activation anneal and proceeding directly to a 620-750°C gettering anneal results in large precipitates that are indicated as the underlying cause for the disastrous open-circuit voltage. On the other hand, quickly cooling to room temperature and then re-inserting the wafers for gettering results in very small precipitates that do not appear to have significant detrimental affects on open-circuit voltage. It is thus critical to consider the precipitation behavior of iron during gettering of ion-implanted emitters - even in monocrystalline silicon - and during low-temperature annealing in general.

Vertically integrated modeling of light-induced defects: Process modeling, degradation kinetics and device impact

As photovoltaic (PV) device architectures advance, they turn more sensitive to bulk minority charge carrier lifetime. The conflicting needs to develop ever advancing cell architectures on ever cheapening silicon substrates ensure that various impurity-related light-induced degradation (LID) mechanisms will remain an active research area in the silicon PV community. Here, we propose vertically integrated defect modeling as a framework to accelerate the identification and mitigation of different light induced defects. More specifically, we propose using modeled LID-kinetics to identify the dominant LID mechanism or mechanisms within complete PV devices. Coupling the LID-kinetics model into a process model allows development of process guidelines to mitigate the identified LID-mechanism within the same vertically integrated simulation tool. We use copper as an example of a well-characterized light-induced defect: we model the evolution of copper during solar cell processing and light soaking, and then map the deleterious lifetime effect of Cu-LID onto device performance. We validate our model using intentionally Cu-contaminated material processed on an industrial PERC-line and find that our model reproduces the LID-behavior of the manufactured solar cells. We further show via simulations that Cu-LID can be mitigated by reducing the contact co-firing peak temperature, or the cooling rate after the firing process.

Elucidation of Iron Gettering Mechanisms in Boron-Implanted Silicon Solar Cells

To facilitate cost-effective manufacturing of boron-implanted silicon solar cells as an alternative to BBr3 diffusion, we performed a quantitative test of the gettering induced by solar-typical boron-implants with the potential for low saturation current density emitters (<50 fA/cm2). We show that depending on the contamination level and the gettering anneal chosen, such boron-implanted emitters can induce more than a 99.9% reduction in bulk iron point defect concentration. The iron point defect results as well as synchrotron-based nano-X-ray-fluorescence investigations of iron precipitates formed in the implanted layer imply that, with the chosen experimental parameters, iron precipitation is the dominant gettering mechanism, with segregation-based gettering playing a smaller role. We reproduce the measured iron point defect and precipitate distributions via kinetics modeling. First, we simulate the structural defect distribution created by the implantation process, and then we model these structural defects as heterogeneous precipitation sites for iron. Unlike previous theoretical work on gettering via boron- or phosphorus-implantation, our model is free of adjustable simulation parameters. The close agreement between the model and experimental results indicates that the model successfully captures the necessary physics to describe the iron gettering mechanisms operating in boron-implanted silicon. This modeling capability allows high-performance, cost-effective implanted silicon solar cells to be designed.

Rapid thermal anneal activates light induced degradation due to copper redistribution

While it is well known that copper impurities can be relatively easily gettered from the silicon bulk to the phosphorus or boron–doped surface layers, it has remained unclear how thermally stable the gettering actually is. In this work, we show experimentally that a typical rapid thermal anneal (RTA, a few seconds at 800 °C) used commonly in the semiconductor and photovoltaic industries is sufficient to release a significant amount of Cu species from the phosphorus-doped layer to the wafer bulk. This is enough to activate the so-called copper-related light-induced degradation (Cu-LID) which results in significant minority carrier lifetime degradation. We also show that the occurrence of Cu-LID in the wafer bulk can be eliminated both by reducing the RTA peak temperature from 800 °C to 550 °C and by slowing the following cooling rate from 40–60 °C/s to 4 °C/min. The behavior is similar to what is reported for Light and Elevated Temperature degradation, indicating that the role of Cu cannot be ignored when studying other LID phenomena. Numeric simulations describing the phosphorus diffusion and the gettering process reproduce the experimental trends and elucidate the underlying physical mechanisms.While it is well known that copper impurities can be relatively easily gettered from the silicon bulk to the phosphorus or boron–doped surface layers, it has remained unclear how thermally stable the gettering actually is. In this work, we show experimentally that a typical rapid thermal anneal (RTA, a few seconds at 800 °C) used commonly in the semiconductor and photovoltaic industries is sufficient to release a significant amount of Cu species from the phosphorus-doped layer to the wafer bulk. This is enough to activate the so-called copper-related light-induced degradation (Cu-LID) which results in significant minority carrier lifetime degradation. We also show that the occurrence of Cu-LID in the wafer bulk can be eliminated both by reducing the RTA peak temperature from 800 °C to 550 °C and by slowing the following cooling rate from 40–60 °C/s to 4 °C/min. The behavior is similar to what is reported for Light and Elevated Temperature degradation, indicating that the role of Cu cannot be ignored when ...