Recombination activity associated with thermal donor generation in monocrystalline silicon and effect on the conversion efficiency of heterojunction solar cells

Abstract

The recombination properties of the carrier lifetime-limiting center formed during the generation of oxygen-related thermal donors (so called “old” thermal donors) in n-type Czochralski silicon were determined over a wide range of thermal donors' concentrations. The procedure involved (1) determining the various energy levels associated with dopants with the help of temperature Hall effect measurements, (2) clarifying which energy level limits the carrier lifetime by temperature lifetime spectroscopy, and (3) determining the recombination parameters of the involved defect from room-temperature carrier lifetime curves. Our results support the fact that a deep energy level in the range of 0.2–0.3 eV below the conduction band limits the carrier lifetime. The second family of thermal donors, featuring bistable properties, was tentatively identified as the corresponding defect. From the obtained experimental data, the influence of the defect on the amorphous/crystalline silicon heterojunction solar cell conversion efficiency was simulated. It is observed that for extended donor generation, the carrier lifetime is reduced by orders-of-magnitude, leading to unacceptable losses in photovoltaic conversion efficiency. A key result is that even for samples with thermal donor concentrations of 1015 cm−3—often met in seed portions of commercial ingots—simulations reveal efficiency losses greater than 1% absolute for state-of-the-art cells, in agreement with recent experimental studies from our group. This result indicates to crystal growers the importance to mitigate the formation of thermal donors or to develop cost-effective processes to suppress them at the ingot/wafer scale. This is even more critical as ingot cool-down is likely to be slower for future larger ingots, thus promoting the formation of thermal donors.

The versatility of passivating carrier‐selective silicon thin films for diverse high‐efficiency screen‐printed heterojunction‐based solar cells

“…But even for non‐gettered wafers, lifetime values are still largely acceptable for cell processing (>4 ms). The wafer resistivity is also significantly affected by this high‐temperature process (Figure B), most likely due to thermal donor annihilation or de‐activation . After gettering, both lifetime and resistivity show relatively flat profiles over the complete ingot length.…”

“…To potentially reduce even further recombination due to bulk transition metallic impurities (typically Fe, Cr, Ni, and Ti), half of the p ‐type wafers taken from each section of the ingot received at the very beginning of the cell process a high‐temperature POCl 3 gettering step (temperature between 810°C and 900°C), followed by a phosphorus silicate glass (PSG) removal and c‐Si etching step to remove the entire diffusion/gettering zone including all impurities gettered there . Note that before gettering, wafers close to the seed end of the ingot showed an inverse doping type (ie, n instead of p ) due to the presence of a high density of oxygen‐related thermal donors in this ingot region . All wafers were then textured in a potassium hydroxide (KOH) solution and wet‐chemically cleaned.…”

Thermal History Index as a bulk quality indicator for Czochralski solar wafers

“…Low-lifetime areas have been successively observed as black cores [2], rings [1][2][3][4] or black edges [5] using various wafer and cell characterization techniques. Most of these features were related to oxygen (O)-containing defects such as thermal donors (small oxygen clusters featuring a donor character, referred to as TD) [6,7], oxygen precipitates (OP) [2], oxidation-induced stacking faults (OiSF) [8] or vacancy-O pairs (V-O) [9]. For these defects, the interstitial O concentration ([O i ]) can be seen in a simplified picture as the "driving force" for defect formation while thermal energy, characterized by a couple time (t) -temperature (T), governs the formation extent of the defects towards equilibrium.…”

Homo-heterojunction concept: From simulations to high efficiency solar cell demonstration

“…Such phenomenon occurs at temperatures above 600°C [43] and is known to suppress recombination-active defects [44]. Complementary tests with very high passivation (i)/(n) stack (not shown) demonstrate that TD were shown to limit τ LIL at 2 ms at the concentration of 5 × 10 14 cm −3 [45]. Therefore, assuming samples to have the same interface recombination velocities for SHJ ref and SHJ 950ºC , TD dissolution is likely to explain the effective lifetime increase upon annealing.…”