Douglas M. Powell

Crystalline silicon photovoltaics: a cost analysis framework for determining technology pathways to reach baseload electricity costs

Crystalline silicon (c-Si) photovoltaics are robust, manufacturable, and Earth-abundant. However, barriers exist for c-Si modules to reach US

Modeling the Cost and Minimum Sustainable Price of Crystalline Silicon Photovoltaic Manufacturing in the United States

0.50–0.75/Wp fabrication costs necessary for subsidy-free utility-scale adoption. We evaluate the potential of c-Si photovoltaics to reach this goal by developing a bottom-up cost model for c-Si wafer, cell, and module manufacturing; performing a sensitivity analysis to determine research domains that provide the greatest impact on cost; and evaluating the cost-reduction potential of line-of-sight manufacturing innovation and scale, as well as advanced technology innovation. We identify research domains with large cost reduction potential, including improving efficiencies, improving silicon utilization, and streamlining manufacturing processes and equipment, and briefly review ongoing research and development activities that impact these research domains. We conclude that multiple technology pathways exist to enable US

Assessing the drivers of regional trends in solar photovoltaic manufacturing

0.50/Wp module manufacturing in the United States with silicon absorbers. More broadly, this work presents a user-targeted research and development framework that prioritizes research needs based on market impact.

Shape and quality of Si single bulk crystals grown inside Si melts using the noncontact crucible method

We extend our cost model to assess minimum sustainable prices of crystalline silicon wafer, cell, and module manufacturing in the United States. We investigate the cost and price structures of current multicrystalline silicon technology and consider the introduction of line-of-sight innovations currently on the industry roadmap, as well as advanced technologies currently at an earlier stage of development. We benchmark the capability of these concepts to reach the U.S. Department of Energy SunShot module price target and perform a sensitivity analysis to determine high-impact research domains that have the greatest impact on price. This exercise highlights advanced c-Si manufacturing concepts with significant cost reduction potential and provides insight into strategies that could greatly reduce module prices in a financially sustainable manner.

Sorting Metrics for Customized Phosphorus Diffusion Gettering

The photovoltaic (PV) industry has grown rapidly as a source of energy and economic activity. Since 2008, the average manufacturer-sale price of PV modules has declined by over a factor of two, coinciding with a significant increase in the scale of manufacturing in China. Using a bottom-up model for wafer-based silicon PV, we examine both historical and future factory-location decisions from the perspective of a multinational corporation. Our model calculates the cost of PV manufacturing with process step resolution, while considering the impact of corporate financing and operations with a calculation of the minimum selling price that provides an adequate rate of return. We quantify the conditions of Chinas historical PV price advantage, examine if these conditions can be reproduced elsewhere, and evaluate the role of innovative technology in altering regional competitive advantage. We find that the historical price advantage of a China-based factory relative to a U.S.-based factory is not driven by country-specific advantages, but instead by scale and supply-chain development. Looking forward, we calculate that technology innovations may result in effectively equivalent minimum sustainable manufacturing prices for the two locations. In this long-run scenario, the relative share of module shipping costs, as well as other factors, may promote regionalization of module-manufacturing operations to cost-effectively address local market demand. Our findings highlight the role of innovation, importance of manufacturing scale, and opportunity for global collaboration to increase the installed capacity of PV worldwide.

Dislocation Density Reduction During Impurity Gettering in Multicrystalline Silicon

The noncontact crucible method enables production of Si bulk single crystals without crucible contact by intentionally establishing a distinct low-temperature region in the Si melt. In this contribution, we correlate crystal growth conditions to crystal material properties. The shape of the growing interface was generally convex in the growth direction. The quality of the Si ingots was determined by the spatial distributions of dislocations, resistivity, oxygen concentration, and minority-carrier lifetime. In an ingot with a convex bottom, swirl patterns with higher resistivity are present in the top, middle, and bottom of the ingot. The dislocation density decreased from the top (first to solidify) to the bottom of the ingot because dislocations in the ingot moved to the periphery from the center of the ingot during crystal growth owing to the convex growing interface. The oxygen concentration was concentrically distributed on the seed axis owing to the convex growing interface. The lifetime was as high as 1.8 ms after phosphorus diffusion gettering (PDG) and 205 µs before PDG at an injection level of 1 × 1015 cm−3. The lifetime was not strongly affected by the dislocation density, which was as low as 102–103 cm−2.

Exceptional gettering response of epitaxially grown kerfless silicon

Customized solar cell processing based on input material quality has the potential to increase the performance of contaminated regions of multicrystalline silicon ingots. This provides an opportunity to improve material yield and device efficiency without substantially reducing the overall throughput. Simulations and experiments show that in wafers from the top and border regions of an ingot containing as-grown iron concentrations ≳1014 cm-3, a high concentration of interstitial iron point defects, i.e., Fei, remains after standard phosphorus diffusion gettering (PDG), severely limiting electron lifetime and simulated efficiencies of PERC-type solar cells. It is shown that an extended PDG leads to a stronger reduction of Fei point defects, enabling high-efficiency devices, even on wafers from the red zone of the ingot. However, a satisfactory performance improvement after standard PDG is already achieved on wafers that contain as-grown total iron concentrations <;1014 cm-3, making the low-throughput extended PDG process unnecessary for a large fraction of the ingot. We propose using the total iron concentration and the corresponding photoluminescence contrast between grain boundaries and intragranular regions in the as-grown wafer as a simple sorting metric to determine when extended phosphorus diffusion is warranted.

Simulated co-optimization of crystalline silicon solar cell throughput and efficiency using continuously ramping phosphorus diffusion profiles

Isothermal annealing above 1250 °C has been reported to reduce the dislocation density in multicrystalline silicon (mc-Si), presumably by pairwise dislocation annihilation. However, this high-temperature process may also cause significant impurity contamination, canceling out the positive effect of dislocation density reduction on cell performance. Here, efforts are made to annihilate dislocations in mc-Si in temperatures as low as 820 °C, with the assistance of an additional driving force to stimulate dislocation motion. A reduction of more than 60% in dislocation density is observed for mc-Si containing intermediate concentrations of certain metallic species after P gettering at 820 °C. While the precise mechanism remains in discussion, available evidence suggests that the net unidirectional flux of impurities in the presence of a gettering layer may cause dislocation motion, leading to dislocation density reduction. Analysis of minority carrier lifetime as a function of dislocation density suggests that lifetime improvements after P diffusion in these samples can be attributed to the combined effects of dislocation density reduction and impurity concentration reduction. These findings suggest there may be mechanisms to reduce dislocation densities at standard solar cell processing temperatures.

Proof-of-concept framework to separate recombination processes in thin silicon wafers using transient free-carrier absorption spectroscopy

The bulk minority-carrier lifetime in p- and n-type kerfless epitaxial (epi) crystalline silicon wafers is shown to increase >500× during phosphorus gettering. We employ kinetic defect simulations and microstructural characterization techniques to elucidate the root cause of this exceptional gettering response. Simulations and deep-level transient spectroscopy (DLTS) indicate that a high concentration of point defects (likely Pt) is “locked in” during fast (60 °C/min) cooling during epi wafer growth. The fine dispersion of moderately fast-diffusing recombination-active point defects limits as-grown lifetime but can also be removed during gettering, confirmed by DLTS measurements. Synchrotron-based X-ray fluorescence microscopy indicates metal agglomerates at structural defects, yet the structural defect density is sufficiently low to enable high lifetimes. Consequently, after phosphorus diffusion gettering, epi silicon exhibits a higher lifetime than materials with similar bulk impurity contents but highe...

>1.8 millisecond effective lifetime in n-type silicon grown by the noncontact crucible method

Defect engineering is essential for the production of high-performance silicon photovoltaic (PV) devices with cost-effective solar-grade Si input materials. Phosphorus diffusion gettering (PDG) can mitigate the detrimental effect of metal impurities on PV device performance. Using the Impurity-to-Efficiency (I2E) simulator, we investigate the effect of gettering temperature on minority carrier lifetime while maintaining an approximately constant sheet resistance. We simulate a typical constant temperature plateau profile and an alternative “volcano” profile that consists of a ramp up to a peak temperature above the typical plateau temperature followed by a ramp down with no hold time. Our simulations show that for a given PDG process time, the “volcano” produces an increase in minority carrier lifetime compared to the standard plateau profile for as-grown iron distributions that are typical for multicrystalline silicon. For an initial total iron concentration of 5×1013 cm-3, we simulate a 30% increase in minority carrier lifetime for a fixed PDG process time and a 43% reduction in PDG process cost for a given effective minority carrier lifetime while achieving a constant sheet resistance of 100 Ω/□.