Jesse Appel

Understanding light-induced degradation of c-Si solar cells

We discuss results of our investigations toward understanding bulk and surface components of light-induced degradation (LID) in low-Fe c-Si solar cells. The bulk effects, arising from boron-oxygen defects, are determined by comparing degradation of cell parameters and their thermal recovery, with that of the minority-carrier lifetime (τ) in sister wafers. We found that the recovery of t in wafers takes a much longer annealing time compared to that of the cell. We also show that cells having SiN:H coating experience a surface degradation (ascribed to surface recombination). The surface LID is seen as an increase in the q/2kT component of the dark saturation current (J02). The surface LID does not recover fully upon annealing and is attributed to degradation of the SiN:H-Si interface. This behavior is also exhibited by mc-Si cells that have very low oxygen content and do not show any bulk degradation.

Some challenges in making accurate and reproducible measurements of minority carrier lifetime in high-quality Si wafers

Measurement of the minority carrier lifetime (τ) of high-quality wafers (having bulk minority carrier lifetime, τb > few milliseconds) requires surface passivation with very low surface recombination velocity, typically <; 1cm/s. Furthermore, for mapping large (e.g., 156 x156 mm) wafers, the passivation must also be stable and uniform over the entire wafer surfaces. These are very demanding requirements and it is a common experience that they are very difficult to achieve. Yet, they are necessary for performing defect analyses of the current N-type wafers. To understand the problems associated with these measurements, we have studied effect of wafer preparation (cleaning procedures, handling) and the passivation characteristics (stability, sensitivity to light, thickness of the passivation medium required for stable passivation) for many commonly used passivation media-iodine-ethanol (IE), quinhydrone-methanol (QHM), aluminum oxide (Al2O3), amorphous-silicon (a-Si), and silicon dioxide (SiO2). Here, we will discuss main factors that influence the accuracy and repeatability of lifetime measurements.

Light-induced degradation free and high efficiency p-type indium-doped PERC solar cells on Czochralski silicon

In this paper, novel and promising high efficiency light-induced degradation (LID) free indium-doped Cz Si cells are presented. Two different commercial grade and large area B-doped Cz materials were included for comparison. Ion-implanted large area (239 and 242.22 cm2) screen printed full Al-BSF cells as well as passivated emitter rear contact (PERC) cells with oxide passivation and local aluminum back surface field were fabricated. In-doped PERC cells achieved 20.3% efficiency while the B-doped cells gave the efficiencies of 20.7% and 20.5% from low (2 Ω-cm) and high resistivity (6.2 Ω-cm) substrates, respectively. Although the initial efficiency of In-doped PERC cells was slightly lower than B-doped cells, In-doped PERC cells surpassed the low and high resistivity B-doped PERC cells by 0.5% and 0.3%, respectively, in absolute efficiency after 0.8 sun 48-hour illuminations at 37°C.

Defect Generation and Propagation in Mc-Si Ingots: Influence on the Performance of Solar Cells

This paper describes results of our study aimed at understanding mechanism (s) of dislocation generation and propagation in multi-crystalline silicon (mc-Si) ingots, and evaluating their influence on the solar cell performance. This work was done in two parts: (i) Measurement of dislocation distributions along various bricks, selected from strategic locations within several ingots; and (ii) Theoretical modeling of the cell performance corresponding to the measured dislocation distributions. Solar cells were fabricated on wafers of known dislocation distribution, and the results were compared with the theory. These results show that cell performance can be accurately predicted from the dislocation distribution, and the changes in the dislocation distribution are the primary cause for variations in the cell-to-cell performance. The dislocation generation and propagation mechanisms, suggested by our results, are described in this paper.

P-Type Indium-Doped Passivated Emitter Rear Solar Cells (PERC) on Czochralski Silicon Without Light-Induced Degradation

Solar cells fabricated on boron (B)-doped Czochralski (Cz) Si wafers in the photovoltaic industry are known to suffer from light-induced degradation (LID) in efficiency. This paper reports on promising LID-free large-area indium (In)-doped Cz Si solar cells. Two different commercial-grade B-doped Cz materials were included for comparison. To study the impact of LID on the cell structure, ion-implanted large-area (239 and 242.22 cm2) screen-printed full aluminum (Al) back-surface field (BSF) baseline cells, as well as higher performance passivated emitter rear cells (PERC) with oxide passivation and local Al BSF, were fabricated. In-doped PERC cells achieved 20.3% efficiency, while the B-doped cells gave efficiencies of 20.7% and 20.5% from low- (2 Ω·cm) and high-resistivity (6.2 Ω·cm) substrates, respectively. It was found that initial efficiency of In-doped PERC cells was ~0.2% lower due to lower bulk lifetime and higher back-surface recombination velocity. However, In-doped PERC cells showed no LID and surpassed the B-doped PERC cell efficiency by 0.3-0.5% after 0.8-sun 48-h illumination at 37 °C.

Indium-doped mono-crystalline silicon substrates exhibiting negligible lifetime degradation following light soaking

Indium doping of Cz and CCz mono-crystalline silicon has been investigated as a means to reduce lifetime degradation after light soaking in wafers and cells. The lifetime degradation is due to the formation of a metastable boron-oxygen complex (B-O pairs) during incipient carrier injection and is proposed to be the mechanism responsible for light-induced degradation in p-type solar cells doped with boron. At higher resistivity (ρ > 2 ohm cm), it is possible to produce indium-doped wafers with reasonable lifetimes and which show negligible lifetime degradation after light soaking. However, at low resistivity (ρ ≤ 2 ohm cm), substrates contain a small fraction of un-ionized indium that does not contribute to base doping, but rather acts as a recombination center.

Effects of injection-level dependent bulk lifetime on cell properties

The injection-level dependent (ILD) lifetime of the silicon bulk material impacts the cell characteristics in multiple and underappreciated ways. Fill factor, FF, is commonly attributed to cell resistances but not to ILD lifetime. Most of the non-ideality behavior, m, and the J02, of a commercial p-type PERC cell with rear AlOx passivation is directly attributed to the positive sloped ILD lifetime and resistivity. This work demonstrates through theory and experiment various and counter-intuitive aspects of cell metrics originating from the ILD lifetime. Additional cell metrics are proposed to better optimize efficiency.