Recombination in passivating contacts: investigation into the impact of the contact work function on the obtained passivation

Improving the passivation of contacts in silicon solar cells is crucial for reaching high-efficiency devices. Herein, the impact of the contact work function on the obtained passivation is examined and quantified using a novel method—quasi-steady-state photoluminescence—which provides access to the surface saturation current density after metallization (J 0s,m). The obtained J 0s,m indicates an improvement of the surface passivation when contacts with high work function are applied onto Si wafers passivated with aluminum oxide, regardless of the wafer doping type. This improvement is mainly due to the amplification of the imbalance between the electron and hole concentrations near the Si interface. The passivation quality is reduced when using contacts with low work function in which the recombination rate increases via the charge-assisted carrier population control. Herein, the vital importance of selecting suitable metals to minimize contact recombination in high-efficiency solar cells is pointed

Recombination and trapping in multicrystalline silicon

Minority carrier recombination and trapping frequently coexist in multicrystalline silicon (mc-Si), with the latter effect obscuring both transient and steady-state measurements of the photoconductance. In this paper, the injection dependence of the measured lifetime is studied to gain insight into these physical mechanisms. A theoretical model for minority carrier trapping is shown to explain the anomalous dependence of the apparent lifetime with injection level and allow the evaluation of the density of trapping centers. The main causes for volume recombination in mc-Si, impurities and crystallographic defects, are separately investigated by means of cross-contamination and gettering experiments. Metallic impurities produce a dependence of the bulk minority carrier lifetime with injection level that follows the Shockley-Read-Hall recombination theory. Modelling of this dependence gives information on the fundamental electron and hole lifetimes, with the former typically being considerably smaller than the latter, in p-type silicon. Phosphorus gettering is used to remove most of the impurities and reveal the crystallographic limits on the lifetime, which can reach 600 ms for 1.5 Wcm mc-Si. Measurements of the lifetime at very high injection levels show evidence of the Auger recombination mechanism in mc-Si. Finally, the surface recombination velocity of the interface between mc-Si and thermally grown SiO2 is measured and found to be as low as 70 cm/s for 1.5 Wcm material after a forming gas anneal and 40 cm/s after an alneal. These high bulk lifetimes and excellent surface passivation prove that mc-Si can have an electronic quality similar to that of single crystalline silicon

Upgraded metallurgical-grade silicon solar cells with efficiency above 20%

We present solar cells fabricated with n-type Czochralski–silicon wafers grown with strongly compensated 100% upgraded metallurgical-grade feedstock, with efficiencies above 20%. The cells have a passivated boron-diffused front surface, and a rear locally phosphorus-diffused structure fabricated using an etch-back process. The local heavy phosphorus diffusion on the rear helps to maintain a high bulk lifetime in the substrates via phosphorus gettering, whilst also reducing recombination under the rear-side metal contacts. The independently measured results yield a peak efficiency of 20.9% for the best upgraded metallurgical-grade silicon cell and 21.9% for a control device made with electronic-grade float-zone silicon. The presence of boron-oxygen related defects in the cells is also investigated, and we confirm that these defects can be partially deactivated permanently by annealing under illumination.This work was supported by the Australian Renewable Energy Agency (ARENA) through the Australian Center for Advanced Photovoltaics (ACAP), Project RND009, and their Postdoctoral Fellowships program. D.M. acknowledges the support from the Australian Research Council through the Future Fellowships program

Simplified PERC solar cells passivated with PECVD silicon nitride

Stoichiometric plasma enhanced chemical vapor deposited silicon nitride films have been used to passivate the front and rear surface of simplified PERC silicon solar cells. These films have the distinctive properties that they can provide excellent surface passivation, are easily patterned using photolithography and wet chemical etching, and are compatible with aluminium layers for a back surface optical reflector. Cells with planar surfaces and random pyramid texturing have been fabricated. Open circuit voltages up to 667mV have been measured on float-zone substrates and 655mV on multicrystalline material, proving the outstanding surface passivation provided by the silicon nitride films. Conversion efficiencies of 18.5% and 16.1% have been obtained respectively

Evidence of impurity gettering by industrial phosphorus diffusion

The possible benefits of phosphorus gettering as applied to production multicrystalline silicon wafers have been evaluated. After optimization of an open tube POCl3 process, relatively low temperatures and short times have been found to significantly improve the minority carrier lifetime of most wafers. The possible gettering action stemming from the industrial process of phosphorus diffusion has also been investigated and found to be similarly effective. Average lifetimes of 45ìs (diffusion length of 360ìm) were obtained, with some wafers reaching maximum values up to 130ìs. Lifetime monitoring of a commercial cell fabrication line has also enabled characterization of the voltage limits imposed by the standard emitter and aluminum back-surface-field. The results indicate that the bulk, as improved by emitter gettering, is generally not the limiting factor on cell performance

Fluorine passivation of defects and interfaces in crystalline silicon

Defects and impurities in silicon limit carrier lifetimes and the performance of solar cells. This work explores the use of fluorine to passivate defects in silicon for solar cell applications. We present a simple method to incorporate fluorine atoms into the silicon bulk and interfaces by annealing samples coated with thin thermally evaporated fluoride overlayers. It is found that fluorine incorporation does not only improve interfaces but can also passivate bulk defects in silicon. The effect of fluorination is observed to be comparable to hydrogenation, in passivating grain boundaries in multicrystalline silicon, improving the surface passivation quality of phosphorus-doped poly-Si-based passivating contact structures, and recovering boron−oxygen-related light-induced degradation in borondoped Czochralski-grown silicon. Our results highlight the possibility to passivate defects in silicon without using hydrogen and to combine fluorination and hydrogenation to further improve the overall passivation effect, providing new opportunities to improve solar cell performance

Texturing industrial multicrystalline silicon solar cells

Three potential techniques for texturing commercial multicrystalline silicon solar cells are compared on the basis of reflectance measurements. Wet acidic texturing, which would be the least costly to implement, produces a modest improvement in reflection before antirflection coating and encapsulation, whereas maskless reactiveion etching texturing, and especially masked reactive-ion etched ‘pyramids’, generate a larger gain in absorption. After antireflection coating and encapsulation however, the differences between the methods are reduced. Short-circuit current measurements on wet acidic textured cells reveal that there is a significant additional current gain above that expected from the reduced reflection. This is attributed to both light-trapping and oblique coupling of incident light into the cell, resulting in generation closer to the junction

Effect of the metal work function on the recombination in passivating contacts using QSSPL

Understanding the impact of metal contacts on the recombination within a passivated crystalline silicon (c-Si) wafer is crucial for the optimization of various photovoltaic devices such as passivating-contact-based solar cells. In this type of device, the metal contacts are offset from the c-Si wafer surface by additional layers [1]. The latter (1) minimize recombination losses—either by chemically reducing the density of defects at the wafer surface or by increasing the imbalance between the majority and minority carrier density near the surface—and (2) are selective for one type of carrier [1, 2]. An asymmetric population of electrons and holes near the c-Si surface can be obtained by applying a contact layer with a different work function than that of c-Si [2]. Therefore, we expect that the presence of a metal contact forms an extremely thin accumulation or inversion layer close to the wafer surface. This imbalance in concentration of the two carrier types will change the recombination statistics at the passivated surface.Usually, the well-established quasi-steady-state (QSS) photoconductance (PC) technique is used to measure the injection-dependent minority-carrier lifetime (τeff) and to extract the surface recombination current density (J0s). However, the QSSPC technique is not easily applicable to metallized structures due to the dominating conductivity of metals in comparison with the semiconductor conductivity. Several other techniques exist that allow determining the saturation current density at the metallized surface (J0m) [3-5]. Yet, each of them has its own limitations, as discussed in Ref. [6]. Recently, Dumbrell et al. presented a robust and contactless method based on QSS photoluminescence (PL) from which τeff of any metallized structure can be obtained, giving access to J0m [7].In this study, we use J0m as figure of merit to investigate the impact of the metal work function on the recombination in passivating contacts. N-type Czochralski-grown silicon wafers are passivated by aluminum oxide (AlOx) films with a thickness of 5 or 20 nm deposited by atomic layer deposition. Five different metals were thermally evaporated at the rear side of Structures A and B, as shown in Fig. 1(a). We measured τeff of the samples using the QSSPL method, which enabled us to extract their total surface saturation current density (J0s,total). J0m is then calculated from J0s,total of the metallized samples and J0s of Structures C and D [Fig. 1(b)].We find that applying metals with a work function smaller than that of n-type c-Si (4.2 eV) on the 5-nm-thick AlOx passivation layer, a decrease in the electron density and an increase in the hole density near the c-Si surface is expected. The presence of negative fixed charges in the AlOx layers amplifies this asymmetric population of electrons and holes. Consequently, the recombination rate at the AlOx/c-Si interface is reduced. Therefore, we expected that J0m will be reduced in comparison with J0s of the non-metallized reference sample. The high J0m of the sample with the metal work function of 5.22 eV might be attributed to the abnormally poor surface passivation of this sample. The impact of the metal work function on J0m is strongly reduced in the case of thicker passivation layer (Structure B). We attribute this to the reduced charge transfer probability between the c-Si and the metal through the AlOx layer once it becomes too thick. Hence, the asymmetric carrier population induced by the difference in work function between them is not achieved.In summary, we find that J0m increases with the metal work function and that this effect is modulated with the passivation layer thickness. It is more pronounced for thinner passivation layers, which can be attributed to a significant change in the populations of electrons and holes near the silicon surface induced by the metal. Meanwhile thicker layers prevent the charge transfer between the silicon wafer and the metal leading to insignificant changes in J0m. Based on these findings, we suggest that suitable metals should exhibit work function values below that of n-type c-Si—or above for the case of p-type c-Si—to benefit from the asymmetric carrier population induced by the metal in passivating-contact-based solar cells

In situ recombination junction between p-Si and TiO2 enables high-efficiency monolithic perovskite/Si tandem cells

Increasing the power conversion efficiency of silicon (Si) photovoltaics is a key enabler for continued reductions in the cost of solar electricity. Here, we describe a two-terminal perovskite/Si tandem design that increases the Si cell's output in the simplest possible manner: by placing a perovskite cell directly on top of the Si bottom cell. The advantageous omission of a conventional interlayer eliminates both optical losses and processing steps and is enabled by the low contact resistivity attainable between n-type TiO2 and Si, established here using atomic layer deposition. We fabricated proof-of-concept perovskite/Si tandems on both homojunction and passivating contact heterojunction Si cells to demonstrate the broad applicability of the interlayer-free concept. Stabilized efficiencies of 22.9 and 24.1% were obtained for the homojunction and passivating contact heterojunction tandems, respectively, which could be readily improved by reducing optical losses elsewhere in the device. This work highlights the potential of emerging perovskite photovoltaics to enable low-cost, high-efficiency tandem devices through straightforward integration with commercially relevant Si solar cells