Boron-oxygen Complex Research Articles

Accelerated formation of the boron–oxygen complex in p‐type Czochralski silicon

This Letter reports on the acceleration of the rate of formation of the boron–oxygen defect in p‐type Czochralski silicon with illumination intensities in excess of 2.1 × 1017 photons/cm2/s. It is observed that increased light intensities greatly enhance the rate of defect formation, without increasing the saturation concentration of the defect. These results suggest a dependence of the defect formation rate upon the total majority carrier concentration. Finally, a method using temperatures up to 475 K and an illumination intensity of 1.68 × 1019 photons/cm2/s is shown to result in near‐complete defect formation within seconds. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

A unified approach to modelling the charge state of monatomic hydrogen and other defects in crystalline silicon

There are a number of existing models for estimating the charge states of defects in silicon. In order of increasing complexity, these are (a) the Fermi-Dirac distribution, (b) the Shockley-Last model, (c) the Shockley-Read-Hall model, and (d) the Sah-Shockley model. In this work, we demonstrate their consistency with the general occupancy ratio α, and show that this parameter can be universally applied to predict the charge states of both monovalent and multivalent deep levels, under either thermal equilibrium or steady-state conditions with carrier injection. The capture cross section ratio is shown to play an important role in determining the charge state under non-equilibrium conditions. The application of the general occupancy ratio is compared with the quasi-Fermi levels, which are sometimes used to predict the charge states in the literature, and the conditions where the latter can be a good approximation are identified. The general approach is then applied to the prediction of the temperature- and injection level-dependent charge states for the technologically important case of multivalent monatomic hydrogen, and several other key monovalent deep levels including Fe, Cr, and the boron-oxygen complex in silicon solar cells. For the case of hydrogen, we adapt the model of Herring et al., which describes the charge states of hydrogen in thermal equilibrium, and generalize it for non-equilibrium conditions via the inclusion of the general occupancy ratio, while retaining the pre-factors which make the model more complete. Based on these results, the impact of temperature and injection on the hydrogenation of the key monovalent defects, and other pairing reactions, are discussed, demonstrating that the presented model provides a rigorous methodology for understanding the impact of charge states.

Formation kinetics of copper-related light-induced degradation in crystalline silicon

Light-induced degradation (LID) is a deleterious effect in crystalline silicon, which is considered to originate from recombination-active boron-oxygen complexes and/or copper-related defects. Although LID in both cases appears as a fast initial decay followed by a second slower degradation, we show that the time constant of copper-related degradation increases with increasing boron concentration in contrast to boron-oxygen LID. Temperature-dependent analysis reveals that the defect formation is limited by copper diffusion. Finally, interface defect density measurements confirm that copper-related LID is dominated by recombination in the wafer bulk.

Invited; Light Induced Degradation in Compensated B-Doped Czochralski Silicon

Light induced degradation (LID) in B-doped Czochralski Si solar cells is a universal phenomenon, which reduces the conversion efficiency by 2-5% relatively. It is believed that Boron-Oxygen complexes are formed during light illumination, which are the effective recombination centers. Studies in the exclusively B-doped Si, B/Ga co-doped Si and p-type B/P compensated Si show that the saturated concentration (Nt*) of B-O defect is proportional to the B concentration (NB ) (or the net hole concentration p0 ) and the generation rate constant (Rgen ) proportional to p0 2 . Interestingly, the LID has also been found in n-type B/P compensated Si, which was supposed to be unlikely to happen. Hence, it brings out a fundamental question of whether the p- and n-type B-doped Si have the same formation mechanism of LID. In this presentation, we will report our recent work about LID in B-doped compensated Cz silicon. It is believed that the BO defect density in compensated silicon depends on the B concentration and the net doping concentration. Moreover, High B concentration in the compensated silicon results in very serious cell efficiency loss. Furthermore, The electrical parameters of the reference and compensated cells can get fully recovered by illumination at high temperatures. Moreover, the LID in n-type B-doped Cz silicon with thermal donors (TDs) compensation is also reported. The advantage of TDs compensation is able to obtain a variation of carrier density but with a constant NB . We find that the formation of B-O complex also occurs under the light illumination and can be annealed out quickly in the dark at 200 oC. The Nt* exhibits a nonlinear dependence on the majority carrier concentrations n0 , and the Rgen is independent on the n0 . The formation activation energy is determined to be 0.4 eV, identical to the value in p-type Si. Based on the results, we believe that the nature of LID in p- and n-type silicon should be the same.

Forward current enhanced elimination of the radiation induced boron-oxygen complex in silicon n+ -p diodes

Using forward current injection with densities in the range 15–30 A/cm2 we can effectively eliminate the radiation-induced boron–oxygen complex, which is the main compensating center in irradiated Si solar cells. It was found that for a given forward current density the elimination rate is decreasing with increasing irradiation dose. Additionally, some evidences have been obtained on the negative-U properties of the radiation-induced boron–oxygen complex.

Tunnel oxide passivated contacts as an alternative to partial rear contacts

Recently, n-type Si solar cells featuring a passivated rear contact, called TOPCon (Tunnel Oxide Passivated Contact) were reported. The high conversion efficiency of 24.4% and very high FF>82% demonstrates that the efficiency potential of this full-area passivated rear contact is as good as or even better than that of partial rear contact (PRC) schemes like PERL (passivated emitter and rear locally diffused) and in addition avoids complex structuring steps and features a 1D carrier transport. Likewise, a boron-doped passivated rear contact for p-type solar cells (p-TOPCon) is proposed as an alternative to p-PRC cells. The optimum device design of PRC cells has to account for two opposing effects: a low-loss 3D carrier transport requires a high base doping but Shockley–Read–Hall (SRH) recombination within the base due to the formation of boron–oxygen complexes in standard Cz silicon calls for a low base doping level. This conflict might be overcome by p-TOPCon because its performance is less sensitive to base doping. This will be discussed on the base of experimental results. It is shown that its high implied fill factor (iFF) of 84% combined with the 1D carrier transport in the base translates into a higher FF potential. First investigations on planar solar cells prove the good performance of the p-TOPCon with respect to passivation and carrier transport. A Voc of 694mV and a FF of 81% underline the efficiency potential of this rear contact.

Preventing light-induced degradation in multicrystalline silicon

Multicrystalline silicon (mc-Si) is currently dominating the silicon solar cell market due to low ingot costs, but its efficiency is limited by transition metals, extended defects, and light-induced degradation (LID). LID is traditionally associated with a boron-oxygen complex, but the origin of the degradation in the top of the commercial mc-Si brick is revealed to be interstitial copper. We demonstrate that both a large negative corona charge and an aluminum oxide thin film with a built-in negative charge decrease the interstitial copper concentration in the bulk, preventing LID in mc-Si.

Generation kinetics of boron-oxygen complexes in p-type compensated c-Si

Kinetics characteristics of boron-oxygen complexes responsible for light-induced degradation in p-type compensated c-Si have been investigated. The generation of B-O complexes is well fitted by a fast-forming process and a slow-forming one. Activation energies of complexes generation during the fast-forming process are determined to be 0.29 and 0.24 eV in compensated and non-compensated c-Si, respectively, and those during the slow-forming process are the same, about 0.44 eV. Moreover, it is found that the pre-exponential factors of complexes generation in compensated c-Si is proportional to the square of the net doping concentration, which suggests that the latent centers should exist.

A numerical simulation study of gallium-phosphide/silicon heterojunction passivated emitter and rear solar cells

The performance of passivated emitter and rear (PERC) solar cells made of p-type Si wafers is often limited by recombination in the phosphorus-doped emitter. To overcome this limitation, a realistic PERC solar cell is simulated, whereby the conventional phosphorus-doped emitter is replaced by a thin, crystalline gallium phosphide (GaP) layer. The resulting GaP/Si PERC cell is compared to Si PERC cells, which have (i) a standard POCl3 diffused emitter, (ii) a solid-state diffused emitter, or (iii) a high efficiency ion-implanted emitter. The maximum efficiencies for these realistic PERC cells are between 20.5% and 21.2% for the phosphorus-doped emitters (i)–(iii), and up to 21.6% for the GaP emitter. The major advantage of this GaP hetero-emitter is a significantly reduced recombination loss, resulting in a higher Voc. This is so because the high valence band offset between GaP and Si acts as a nearly ideal minority carrier blocker. This effect is comparable to amorphous Si. However, the GaP layer can be contacted with metal fingers like crystalline Si, so no conductive oxide is necessary. Compared to the conventional PERC structure, the GaP/Si PERC cell requires a lower Si base doping density, which reduces the impact of the boron-oxygen complexes. Despite the lower base doping, fewer rear local contacts are necessary. This is so because the GaP emitter shows reduced recombination, leading to a higher minority electron density in the base and, in turn, to a higher base conductivity.

Use of QSSPC and QSSPL to Monitor Recombination Processes in P-type Silicon Solar Cells

This paper reports the use of injection-dependent local ideality factors, obtained from quasi-steady state photoconductance and photoluminescence measurements, to investigate the effects of various cell processing steps on recombination in solar cells, fabricated using boron-doped Czochralski silicon wafers, with an ∼ 100Ω/□ phosphorus-doped emitter and silicon nitride passivation of both surfaces. It is shown that activation of boron-oxygen complexes in the cells by light soaking can impact the pseudo fill factor and is manifest in increased local ideality factors in the injection range between maximum power point and open circuit voltage. The introduced recombination was modeled as a single Shockley Reed Hall (SRH) recombination centre at Ec - Et = 0.41eV and an electron:hole capture cross section ratio of ∼14. The effects of boron and phosphorus laser doping on the injection-dependent local ideality factor were also investigated. Boron laser doping was shown to introduce additional recombination in cells, as indicated by the increased local ideality factor in the injection range between maximum power point and open circuit voltage. However, in this case, the additional recombination was not well-modelled by a single SRH recombination centre, especially in the mid-injection range. Finally, it is shown that high temperature belt furnace anneals can place cells into a specific recombination state, and cells can be returned to this state by subsequent anneals even after additional recombination is introduced into cell by processes like laser doping.

Towards 20% Solar Cell Efficiency Using Silicon from Metallurgical Process Route

In this work we present high efficient silicon solar cells based on low-cost UMG-Si purified by the metallurgical process route of Elkem Solar with efficiencies close to 20%. Solar cells were fabricated by two different PERC solar cell process sequences using only processes steps which are already well-known in industry and transferable to mass production. Simultaneously solar cells were fabricated by the state-of-the-art full Al-BSF process as reference. The investigated wafers were cut from a Cz grown crystal based on UMG-Si blended with 40% poly silicon and were compared to 100% poly-Si wafers. It is shown that applying PERC processes to the silicon wafers can enhance the cell efficiency by 0.8% absolute to reach average cell efficiencies of 19.8% and thus demonstrates the excellent quality of the fabricated c-Si solar cells. Furthermore it is proven that UMG-Si does not show feedstock-related limitations to high efficiency concepts such as the PERC process. To our knowledge these results present the highest obtained cell efficiencies for silicon solar cells based on UMG-Si so far.Studies of light induced degradation of PERC and full Al-BSF solar cells from both kinds of feedstock were conducted to determine the influence of recombination active boron-oxygen complexes on the respective solar cell performance. The obtained results reveal higher losses for PERC solar cells, which can be explained by the higher sensitivity of PERC solar cells to changes of the bulk lifetime. This is also confirmed by PC1D simulations.

Stability of the regeneration of the boron–oxygen complex in silicon solar cells during module certification

Light-induced degradation in boron-doped p-type Czochralski silicon is caused by a boron–oxygen complex, which may be permanently deactivated by simultaneous illumination and heating leading to a permanent regeneration of carrier lifetime and solar cell performance. To date, the long-term stability of the regenerated state under thermal stress has not been fully investigated. In this work, we investigate whether the regeneration gain achieved on solar cell level is stable upon accelerated aging tests such as thermal cycling and damp-heat testing as applied during module certification according to IEC and UL standards. It is shown that regeneration is mostly conserved upon thermal cycling and cells in the degraded state may even be regenerated if current is injected during the treatment. Damp-heat testing at 85°C in the dark for 1500h decreases the regeneration-induced gain significantly but does not annihilate it completely.

Stability of the regeneration of the boron–oxygen complex in silicon solar cells during module integration

Light-induced degradation (LID) in boron-doped p-type Czochralski (Cz) silicon is caused by a boron–oxygen (BO) complex, which may be permanently deactivated by simultaneous illumination and heating leading to a permanent regeneration of carrier lifetime and solar cell performance. Up to now, regeneration has only been investigated on wafer and solar cell level. In this work, we investigate whether the regeneration gain on solar cell level can be transferred to module level. For this purpose, we fabricated solar cells with passivated emitter and rear on boron-doped p-type Cz and float zone wafers. The cells are extensively characterised regarding losses in cell performance due to LID by preparing different defect states with certain temperature and illumination treatments. The effect of the injection-dependent carrier lifetime caused by the BO complex in its active state on fill factor is investigated in detail theoretically and experimentally, developing a descriptive explanation. Finally, it is shown by integrating solar cells in the degraded and regenerated state into solar modules, that the regeneration effect can be transferred to module level and is stable upon subsequent illumination of the module.

Experimental evidence of staggered oxygen dimers as a component of boron-oxygen complexes in silicon

We have investigated the relationship of the boron-oxygen (B-O) complexes to the staggered oxygen dimers (O2ist) in silicon. It is found that the saturated concentration of B-O complexes (Nt*) is proportional to the O2ist concentration ([O2ist]) in Czochralski silicon samples, whose [O2ist] are varied by annealing at different temperatures. This proportionality is further confirmed in a special silicon ingot with variable oxygen and carbon concentrations, in which the Nt* and [O2ist] show the similar dependences on the interstitial oxygen concentration. Therefore, our experimental data support that the O2ist should be an ingredient of the light-induced defects in silicon.

Light-induced degradation in n-type Czochralski silicon by boron-doping and thermal donor compensation

In this paper, the behaviors of boron-oxygen complexes related to the light-induced degradation (LID) have been studied in n-type boron-doped Czochralski silicon (Cz-Si) with different levels of thermal donors (TDs) compensation. The degradation is found to have two stages, a fast- and a slow-process, similar to that in p-type boron-doped Cz-Si. In order to compare with the properties of LID in p-type Si, we systematically investigate the dependence of the saturated defect concentration (Nt*) and the defect generation rate constant (Rgen) in our TD-compensated n-type Cz-Si on the carrier concentrations, i.e., the net electron concentration (n0) and the excess hole concentration (Δp). Moreover, we determine the activation energy for the slow-process to be 0.4 eV. Based on our results, we suggest that the LID effect in boron-doped Si, either in p-type case or in n-type case by compensation, should be attributed to the same defects.

Dislocation-induced variation of generation kinetics of boron–oxygen complexes in silicon

The behaviors of light-induced degradation (LID) using quasi-single-crystalline (QSC) silicon have been demonstrated. It is found that the dislocations, the main defect in QSC silicon, can significantly influence the generation kinetics of B–O complexes that are responsible for the LID effect. Compared to dislocation-poor samples, higher activation energy of 0.57±0.02eV for the B–O complex generation in dislocation-rich silicon has been obtained, and meanwhile, the pre-exponential factor is two orders of magnitude higher. It is believed that the dislocation-related electronic states charged with holes can cause an energy potential barrier for the capture of single-positive holes that is required for the transformation of B–O complexes from latent centers to immediate transient centers.

The role of excess minority carriers in light induced degradation examined by photoluminescence imaging

A new approach to investigate light induced degradation (LID) effects in boron-doped silicon has been developed. By studying spatial variations in LID resulting from localized carrier excitation (spot-LID), it is verified that the generation of the boron-oxygen complexes responsible for the degradation is directly related to the presence of excess minority carriers. Through the examination of the diffused minority carrier density distribution (during light exposure), from an exposed into an unexposed wafer area compared to the observed defect generation, we are able to monitor the generation of excess carrier induced defects over a range of carrier concentrations. The results show that very low concentrations of minority excess carrier densities are sufficient to generate the defects. For the investigated material carrier concentrations down to 1.7 ± 0.2 × 109 cm−3 are observed to cause lifetime degradation.

Behaviour of Light Induced Defect Generation and Carrier Lifetime Degradation in Solar Grade Silicon

Light-induced defect generation seriously reduces the minority-carrier lifetime of crystalline silicon (c-Si) wafers which causes a decrease in solar cell efficiency. In this paper we investigate the impact of boron-oxygen complexes and iron impurities on the light induced minority-carrier lifetime degradation in c-Si, comparing electronic grade and upgraded metallurgical grade materials. For the later, the characteristic of the decay process is shown to be composed of a fast initial decay and a subsequent slow asymptotic decay. We conclude that the dissociation of iron-boron pairs must be taken into account to explain the light-induced lifetime reduction.

Phosphorus gettering of iron by screen‐printed emitters in monocrystalline Czochralski silicon wafers

ABSTRACTIn this paper, we demonstrate single‐sided screen‐printed emitters in thin monocrystalline Czochralski silicon (Cz‐Si) wafers with an improved gettering of iron compared with conventional double‐sided POCl3 emitters. The phosphorus dopant pastes used have to be chosen carefully to provide a sufficiently low emitter sheet resistance and to avoid iron contamination. The iron concentration is determined in a non‐destructive way from the minority carrier lifetime obtained by quasi‐steady‐state photoconductance measurements, down to levels not yet demonstrated for screen‐printed emitters. In addition, the well‐known metastable boron–oxygen complexes in Cz‐Si have been transferred into a stable state by light‐induced degradation prior to these measurements. Copyright © 2012 John Wiley & Sons, Ltd.

Investigation of Self Interstitial Influences in Light and Dark Induced Degradation in p-type Compensated Silicon

In this study, based on p-type strongly compensated electronic grade monocrystalline Cz silicon, one tried to find if self interstitials have a direct or an indirect role in light and dark induced degradation (LID and DID respectively). Many studies have been carried out on LID phenomenon due to the formation of boron-oxygen complexes under light exposure. Some of them compare as-cut wafers and wafers which have been treated by phosphorus diffusion. They show that the LID phenomenon is reduced when samples have been phosphorus diffused. This effect has been explained by self interstitial injection in the bulk during the phosphorus diffusion, but the role of these self-interstitials on LID phenomenon is not yet well defined [1] [2] [3]. In this paper, investigations on the degradation kinetic are made in as-cut wafers, in phosphorus diffused wafers and in wafers which were annealed in the same conditions used for the phosphorus diffusion. Comparisons of the results allow us to pinpoint the role of self-interstitials on LID phenomenon. It seems that the presence of a high self interstitial concentration reduce the formation of BsO2i complexes responsible for the first faster degradation.