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The concentration of boron-oxygen defects generated in compensated p-type Czochralski silicon has been measured via carrier lifetime measurements taken before and after activating the defect with illumination. The rate of formation of these defects was also measured. Both the concentration and the rate were found to depend on the net doping rather than the total boron concentration. These results imply that the additional compensated boron exists in a form that is not able to bond with the oxygen dimers, thus prohibiting the formation of the defect. This could be explained by the presence of boron-phosphorus complexes, as proposed in previous work. Evidence for reduced carrier mobilities in compensated silicon is also presented, which has implications for photoconductance-based carrier lifetime measurements and solar cell performance.

Degradation of minority carrier lifetime under illumination occurs in boron-containing Czochralski silicon of both p- and n-type. In n-Si, the recombination centre responsible for degradation is found to be identical to the fast-stage centre (FRC) known for p-Si, where it is produced at a rate proportional to the squared hole concentration, p2. Holes in n-Si are the excess minority carriers—of a relatively low concentration; hence, the time scale of FRC generation is increased by several orders of magnitude when compared to p-Si. The degradation kinetics, which is non-linear, due to dependence of p on the current concentration of FRC, is well reproduced by simulations. The injection level dependence of the lifetime shows that FRC exists in 3 charge states (− 1, 0, + 1) possessing 2 energy levels. Comparison of n-Si samples of various electron concentrations shows that FRC emerges by the reconstruction of a latent BsO2 complex of a substitutional boron and an oxygen dimer (while the major recombination centre in p-Si denoted SRC was previously found to emerge by reconstruction of BiO2 defect involving an interstitial boron atom). A model of the BsO2 reconfiguration into FRC through an intermediate state accounts for the rate constant dependence on p, which is reduced to a p2 proportionality, under certain conditions.

Solar cells whose breakdown current exceeds a certain limit cannot be used because such cells may thermally damage the module in case of unintentional reverse biasing by local shading (hot-spot problem [1]). In order to reduce the number of off-specification cells, the reason for the high reverse currents must be identified. The physical mechanisms leading to breakdown of reverse-biased p-n junctions are internal field emission (Zener effect) and impact ionization (avalanche effect). They exhibit a characteristic temperature dependence, which can be used for their identification: for internal field emission the current increases slightly with rising temperature due to band-gap lowering, but it decreases considerably for impact ionization due to increased phonon scattering. Moreover, multiplication of photo-generated carriers takes place only for avalanche breakdown [2]. Both mechanisms require a certain electric field strength, which normally is not reached in standard multicrystalline (mc) Si solar cells. According to that field strength, however, the breakdown voltage should be four times higher than observed in practice [3].In this letter, we present a systematic study of the breakdown mechanism in commercial, 156 × 156 mm 2 p-type base mc-Si solar cells. We employ special lock-in thermography (LIT) imaging techniques to identify the type of breakdown occurring at the hot spots, and various electron microscopy techniques to reveal the microscopic nature of the breakdown sites. The cells investigated were free from ohmic shunts. A typical reverse current-voltage characteristic is shown in Fig. 1, given for two different temperatures.At lower reverse voltages, only weak currents occur, which up to approximately -13 V increase only slightly (pre-breakdown). Beyond -13 V, however, a steep current increase is observed, which is typical for a hard breakdown. For the solar cells under investigation, the pre-breakdown current increases with temperature, whereas the hardbreakdown current decreases (for a given voltage). This indicates that in general, different breakdown mechanisms are involved, a fact which also other authors have observed, using electroluminescence (EL) at reverse bias [4].Lock-in thermography has been established as a standard technique for locating and characterizing leakage currents in solar cells [5]. For the investigation of breakdown currents we have recently proposed several LIT-based imaging techniques [6], performed either in the dark (DLIT) or under illumination (ILIT). In all these techniques, the -90° LIT signal, which can be interpreted quantitatively [7], is used. The temperature variation of the current at a given bias voltage is displayed by the Temperature-Coef-Multicrystalline silicon solar cells typically show hard breakdown beginning from about -13 V bias, which leads to the well-known hot-spot problem. Using special lock-in thermography techniques, hard breakdown has been found to occur in regions of avalanche multiplication. A systematic study of these regions by various electro...

We study ion implantation for patterned doping of back-junction back-contacted solar cells with polycrystallinemonocrystalline Si junctions. In particular, we investigate the concept of counterdoping, that is, a process of first implanting a blanket emitter and afterward locally overcompensating the emitter by applying masked ion implantation for the back surface field (BSF) species. On planar test structures with blanket implants, we measure saturation current densities J 0 ,p oly of down to 1.0 ± 1.1 fA/cm 2 for wafers passivated with phosphorusimplanted poly-Si layers and 4.4 ± 1.1 fA/cm 2 for wafers passivated with boron-implanted poly-Si layers. The corresponding implied pseudofill factors pF F im p l . are 87.3% and 84.6%, respectively. Test structures fabricated with the counterdoping process applied on a full area also exhibit excellent recombination behavior (J 0 ,p oly = 0.9 ± 1.1 fA/cm 2 , pF F im p l. = 84.7%). By contrast, the samples with patterned counterdoped regions exhibit a far worse recombination behavior dominated by a recombination mechanism with an ideality factor n > 1. A comparison with the blanket-implanted test structures points to recombination in the space charge region inside the highly defective poly-Si layer. Consequently, we suggest introducing an undoped region between emitter and BSF in order to avoid the formation of p + /n + junctions in poly-Si.