Monocrystalline silicon (mono-Si or c-Si) is silicon which consists of a continuous solid single crystal. The silicon grown for photovoltaic (PV) applications is grown in a cylindrical form with a typical diameter of 8 inches (~200 mm). The surface of the cylinder is then trimmed to make a pseudo-square shape. These ingots can be prepared as either intrinsic, p-type doped or n-type doped silicon. P-type doping is typically achieved using boron while n-type doping is achieved using phosphorus. Solar cells fabricated from mono-Si comprises an estimated 35 % (30 % p-type and 5 % n-type) of all silicon wafer-based solar cells [1]. The typical thickness of mono-Si used PV solar cell production is in the160‑190 μm range. In 2019, the largest mono-Si silicon wafer manufacturer was Xi’an Longi Silicon Materials Corporation.
The Cz method—named after Jan Czochralski—is the most common method of mono-Si production. This method has a relatively low thermal stress resistance, short processing time, and relatively low cost. The silicon grown via the Cz process is also characterised by a relatively high oxygen concentration that may assist internal gettering of impurities. The industry standard of the crystal diameter is from 75‑210 mm with a <100> crystallographic orientation. High purity polysilicon (solar grade silicon) material with additional dopants, most commonly boron (for p-type doping) or phosphorus (for n-type doping) is used as the feedstock for the process. A single crystal silicon seed is placed on the surface, rotated and gradually drawn upwards. This draws the molten silicon out of the melt so that it can solidify into a continuous single crystal from the seed. The temperature and the pulling speed are carefully adjusted to eliminate dislocation in the crystal, which can be generated by the seed/melt contact shock. Controlling the speed can also affect the diameter of the crystal. The typical oxygen and carbon concentrations are [O] ≈ 5‑10 × 1017 cm-3 and [C] ≈ 5‑10 × 1015 cm-3, respectively. Due to solubility variability of oxygen in silicon [2] (from 1018 cm-3 at the silicon melting point to several orders of magnitude lower at room temperature), oxygen can precipitate [3]. The oxygen that is not precipitated can become electrically active defects, and further, the thermal donors from the oxygen can affect the resistivity of the material. Alternatively, precipitated oxygen can facilitate an internal gettering of impurities. The interstitial form of oxygen [Oi] in boron-doped p-type silicon can severely affect the performance of the silicon. Under illumination or current injection, the interstitial oxygen forms a boron-oxygen defect with the background dopant, boron [4]. This is known to reduce the efficiency of a completed solar cell by up to 10 % relative.
Another disadvantage of the standard Cz process is the fact that the dopant distribution is not uniform along the ingot because the segregation coefficient of boron (0.8) and phosphorous (0.3) are not unity. This results in a relatively low dopant concentration, hence higher resistivity, at the start of the Cz pulling process and a higher dopant concentration, hence lower resistivity, towards the end of the pulling process. Due to the relatively low segregation process of phosphorous, this is mainly an issue for n-type mono-Si resulting in a wide resistivity range for n-type ingots.
The Cz process and subsequent ingot and wafer cutting process is shown in the animation below.
Another variant of the Cz process is the continuous Cz process. In the continuous Cz process, new material is added to the melt during the ingot-pulling. This allows for significantly shallower crucibles, reducing the interaction with the crucible walls, and also allows you to control the dopant concentration in the melt and consequently the dopant concentration in the ingot can be constant. This can thus lead to much more uniform ingots in terms of resistivity which are also longer as you are no longer limited to the starting melt volume. A disadvantage of the continuous Cz method is, however, that impurities with a low segregation coefficient can be built up in the melt resulting in high concentrations in the latter part of the pulling process. A short animation of the process is shown below.
[1] – International Technology Roadmap for Photovoltaics (ITRPV) 10th Edition, 2019