Emanuele Cornagliotti

On the blistering of atomic layer deposited Al 2 O 3 as Si surface passivation

This work proves that blistering is the partial delamination of a thick enough Al<inf>2</inf>O<inf>3</inf> layer caused by gaseous desorption in the Al<inf>2</inf>O<inf>3</inf> layer upon thermal treatments above a critical temperature: the Al<inf>2</inf>O<inf>3</inf> layer acts as a gas barrier and bubble formation occurs. First, using an atmospheric pressure rapid thermal processor with an atmospheric pressure ionization mass spectrometry, desorbing species upon heating of Si/Al<inf>2</inf>O<inf>3</inf> samples are identified: evident desorption peaks are observed around 400 °C for all spectra. The spectrum for m/e = 18, an indication of H<inf>2</inf>O, illustrates that gaseous desorption from Al<inf>2</inf>O<inf>3</inf> and from the Si substrate itself continues up to 600 °C and 700 °C, respectively. Also, it is shown that in the case of a 30 nm Al<inf>2</inf>O<inf>3</inf> layer, blistering starts at same annealing temperatures as gaseous desorption begins. In the case of a thin enough (< 10 nm) Al<inf>2</inf>O<inf>3</inf> film, blistering does not show. To complete the proof, elastic recoil detection measurements clearly show that after annealing a thick Al<inf>2</inf>O<inf>3</inf> film above 400 °C the H content is higher near the c-Si interface as compared to the near surface. Fortunately, effective lifetime and capacitance voltage measurements show that 5 to 10 nm Al<inf>2</inf>O<inf>3</inf> layers can still be adequate passivation layers after being annealed in N<inf>2</inf> environment at temperatures up to 500–700 °C: (i) interface trap densities (D<inf>it</inf>) can remain below 1×10¹¹ cm⁻² and (ii) fixed charge densities (Q<inf>f</inf>) stay negative and in the order of −3×10¹² cm⁻² Random local Al back surface field (BSF) solar cells, fabricated using a blistered film as rear surface passivation and no additional contact opening step, clearly show that random local BSFs are created upon firing of a blistered rear passivation layer covered by metal. Therefore, it is clear that blistering should be avoided, since it will reduce the overall rear surface passivation. The key to avoid blistering is using 5 to 10 nm Al<inf>2</inf>O<inf>3</inf> passivation layers and performing an annealing step prior to capping and co-firing. Al<inf>2</inf>O<inf>3</inf>/SiN<inf>x</inf> passivated local Al BSF p-type Si solar cells are made using an out-gassing step with temperatures up to 700 °C. For these cells, the reduction in blistering and hence improvement in rear surface passivation is clearly reflected in the gain in average Voc as a function of out-gassing temperature.

The impact of silicon solar cell architecture and cell interconnection on energy yield in hot & sunny climates

Extensive knowledge of the dependence of solar cell and module performance on temperature and irradiance is essential for their optimal application in the field. Here we study such dependencies in the most common high-efficiency silicon solar cell architectures, including so-called Aluminum back-surface-field (BSF), passivated emitter and rear cell (PERC), passivated emitter rear totally diffused (PERT), and silicon heterojunction (SHJ) solar cells. We compare measured temperature coefficients (TC) of the different electrical parameters with values collected from commercial module data sheets. While similar TC values of the open-circuit voltage and the short circuit current density are obtained for cells and modules of a given technology, we systematically find that the TC under maximum power-point (MPP) conditions is lower in the modules. We attribute this discrepancy to additional series resistance in the modules from solar cell interconnections. This detrimental effect can be reduced by using a cell design that exhibits a high characteristic load resistance (defined by its voltage-over-current ratio at MPP), such as the SHJ architecture. We calculate the energy yield for moderate and hot climate conditions for each cell architecture, taking into account ohmic cell-to-module losses caused by cell interconnections. Our calculations allow us to conclude that maximizing energy production in hot and sunny environments requires not only a high open-circuit voltage, but also a minimal series-to-load-resistance ratio.

Large-Area n-Type PERT Solar Cells Featuring Rear p + Emitter Passivated by ALD Al 2 O 3

We present large-area n-type PERT solar cells featuring a rear boron emitter passivated by a stack of ALD Al₂O₃ and PECVD SiO_x. After illustrating the technological and fundamental advantages of such a device architecture, we show that the Al₂O₃/SiO_x stack employed to passivate the boron emitter is unaffected by the rear metallization processes and can suppress the Shockley-Read-Hall surface recombination current to values below 2 fA/cm², provided that the Al₂O₃ thickness is larger than 7 nm. Efficiencies of 21.5% on 156-mm commercial-grade Cz-Si substrates are demonstrated in this study, when the rear Al₂O₃ /SiO_x passivation is applied in combination with a homogeneous front-surface field (FSF). The passivation stack developed herein can sustain cell efficiencies in excess of 22% and V_oc above 685 mV when a selective FSF is implemented, despite the absence of passivated contacts. Finally, we demonstrate that such cells do not suffer from light-induced degradation.

Hydrogen Passivation of Laser-Induced Defects for Laser-Doped Silicon Solar Cells

Hydrogen passivation of laser-induced defects (LasID) is shown to be essential for the fabrication of laser-doped solar cells. On first-generation laser-doped selective emitter solar cells where open-circuit voltages were predominately limited by the full-area back surface field, a 10-mV increase and 0.4% increase in the pseudo-fill factor were observed through hydrogen passivation of defects generated during the laser doping process, resulting in an efficiency gain of 0.35% absolute. The passivation of such defects becomes of increasing importance when developing higher voltage devices and can result in improvements in implied open-circuit voltage on test structures up to 50 mV. On n-type PERT solar cells, an efficiency gain of 0.7% absolute was demonstrated with increases in open-circuit voltage and pseudo-fill factor by applying a short low-temperature hydrogenation process using only hydrogen within the device. This process was also shown to improve the rear surface passivation, increasing the short-circuit current of approximately 0.2 mA/cm2 of wavelengths from 950 to 1200 nm compared with that achieved using an Alneal process. Subsequently, an average efficiency of 20.54% was achieved.

Impact of firing on surface passivation of p-Si by SiO2/Al and SiO2/SiNx/Al stacks

Firing impacts on surface passivation provided by a SiO2 and SiO2/SiNx stack with evaporated Al films are studied by capacitance-based techniques on MIS capacitors. For devices with insulator layers consisting solely of as-deposited SiO2, the densities of either interface states (Dit) or fixed charges (Qfc) are hardly influenced by firing. Capping the SiO2 layer with a SiNx layer results in a shift of the peak activation energy of Dit toward the valence band (Ev) of Si. Firing this SiO2/SiNx stack leads to an increase of Qfc, a reduction of Dit, and a moderate shift of peak activation energy of Dit toward Ev. Co-firing with the Al film on top significantly reduces the Qfc, Dit, and Dit peak activation energy, which is resulting from the atomic hydrogen passivation. These results are of particular interest for the development of solar cells with rear surface passivation and local contacts.

Electrical characterization of ALD Al 2 O 3 - HfO 2 and PECVD Al 2 O 3 passivation layers for p-type CZ-Silicon PERC solar cells

This work characterizes p-type Silicon surface passivation using a high-k material (Al2O3 or HfO2) combining capacitance voltage (CV) and lifetime measurements. For AI2O3 samples, the Silicon substrate bulk and surface quality is equivalent to CZ Silicon used in industrial solar cell processing. While AI2O3 has been proven to provide high quality surface passivation on p-type doped Silicon surfaces, the influence of the growth conditions and the post-deposition annealing is not yet completely understood. The dielectric thin film has been deposited by common techniques (ALD, PECVD) on H-/OHterminated Silicon surfaces (hydrophobic and hydrophilic, respectively). The impact of the roughness of the surface prior to the deposition has been also considered. Then, the passivation of each layer has been investigated as a function of different AI2O3 thicknesses (5 to 20 nm) and post-deposition annealing temperatures (300 to 800°C). CV measurements have been used to characterize chemical passivation (= interface trap density, Dit) and field effect passivation (= fixed charge density, Qf). Lifetime measurements have been used to assess the effective surface passivation. The results of both types of electrical characterization fit well together. (i) Prior post-deposition anneal, only either chemical passivation (ALO) or field effect passivation (PECVD) is adequate, resulting in lower effective lifetimes. (ii) At higher annealing temperatures, a negative net charge in the AI2O3 and a low Dit at the interface are measured, ideal for p-type CZ Silicon passivation and causing maximal effective lifetimes. (iii) At too high annealing temperatures, chemical passivation is destroyed resulting in decreasing effective lifetimes even though negative field effect remains in many cases. Another candidate as passivation layer on Silicon is HfO2. Being a new material in photovoItaics, it has been studied on FZ Silicon substrates and its electrical characterization has demonstrated interesting passivation properties at low anneal temperatures (also without thermal treatment).

Opportunities for Silicon Epitaxy in Bulk Crystalline Silicon Photovoltaics

This work presents an overview of the opportunities in bulk crystalline silicon photovoltaics that have been explored using silicon epitaxy as doping technology. Epitaxy demonstrates to be an elegant and versatile technology which brings a lot of new opportunities to further simplify and improve the design and performance of bulk solar cells. Advantages are the doping profile flexibility, the reduced thermal budget, the absence of additional steps to remove glassy layers or activate dopants, the simplified integration of local doping by means of selective epitaxy, and the possibility of single-side deposition. The results presented herein demonstrate the potential of epitaxy by applying the process in three cell structures to grow a boron-doped layer. First, epitaxy is used to grow blanket doped layers as emitters on the full rear side of n-type PERT cells. Second, selective epitaxy is applied to locally grow the interdigitated emitter in n-type IBC cells. Third, selective epitaxy is applied to form the local BSF in p-type PERL cells. For each of these cell concepts, silicon epitaxy helped to simplify the reference BBr3 diffusion-based process, while keeping high efficiencies: 20.5 % for n-type PERT (226 cm cell), 22.8 % for IBC (4 cm cell) and at least +0.5 mA/cm and +10 % escape reflectance for p-type PERL cells compared to the standard PERC.

Illumination level independence in relation to emitter profiles on industrial high efficiency local Al-BSF cells

If the worlds answer to alternative energy production is to be silicon photovoltaics, the fabricated devices need to be robust and highly efficient in varying operating conditions. Rear oxide passivated local Al-BSF cells have a prevalent issue that hinders their performance in particular operating conditions, this issue being bias light dependence (reduced response at low illumination levels). It has been demonstrated by many [1,2,3] that to obtain high quantum efficiency at longer wavelengths, the cells need to be illuminated with a sufficient high level of bias light. If quantum efficiency is shown to be bias light dependant, at low light conditions the cell will simply underperform. Although most cells suffer from this type of degradation, in practice some high efficiency cells reach maximum spectral responsivity already at 0.3 suns and are considered to be bias independent. Regardless if in practice, cells can be named bias independent, the mechanisms for bias dependency is a very relevant characteristic of high efficiency solar cells of the present and future. In this paper we present a phenomenon that has been observed and repeated in separate experiments. We compare differences in rear passivation, specifically between a fresh deposited silicon oxide versus one that has been used also as a diffusion mask. We observe a relationship between bias dependence and the process flows as well as a relationship with the densification recipe. As expected the 90 ohm/sq emitter outperforms the higher doped emitters in the UV wavelength range. What is not expected is that when measuring without a bias light, the higher sheet resistance emitters outperform the lower sheet resistance emitters in wavelengths above 700nm. Another observation is that the cells that have been passivated with fresh silicon oxide indicate a large bias dependence at ultra low bias levels below .01 sun, but saturate performance above .05 sun. The diffused silicon oxide cells increase their quantum efficiency as the bias light is increased. The cells studied in this paper are fabricated using 148.25 cm2, 160μm p-type Cz-Si wafers with screen printed Ag front contacts and are rear-side passivated with a deposited rear silicon oxide/silicon nitride stack. The highest efficiency of the cells studied is 19.2 % with a Voc of 637mV, Jsc of 38.2 mA/cm2 and a fill factor of 79.1%

High sensitivity photoconductivity based measurement setup for the determination of effective recombination lifetime in silicon wafers

We describe a high sensitivity measurement setup for the determination of recombination parameters in semiconductors at low levels of carrier injection. The setup is based on a lock-in amplifier and on a commercially available contactless conductivity detector. The information on recombination is extracted through the analysis, assuming quasi-steady-state conditions, of the low frequency, sinusoidally modulated photoconductivity signal induced by the illumination of a 950 nm light emitting diode array. Experimental results show a substantial increase in sensitivity with respect to traditional transient or quasi-steady-state techniques based on the same detection principle. The sensitivity bonus can be exploited for the extension of the carrier injection range for which effective recombination lifetime is measurable, both in the case of p-type and n-type wafers.

Kerfless Epitaxial Mono Crystalline Si Wafers With Built-In Junction and From Reused Substrates for High-Efficiency PERx Cells

This paper proposes a kerfless wafer structure with built-in p-n junctions in n-type silicon wafers grown using Crystal Solars high throughput epitaxy technology. Compared with a conventional p-type emitter by boron diffusion, ion implantation, or epitaxy, the built-in p-type emitter has a reduced and uniform doping concentration and increased thickness. The epitaxially grown wafers and conventional Czochralski (CZ) n-type wafers were processed into solar cells. A best efficiency of 22.5% with epitaxially grown wafers was achieved, with a 6 mV gain in open-circuit voltage, suggesting a high wafer quality and superiority of the deep epitaxial emitter over a standard boron-diffused emitter. Substrate reuse associated with the kerfless epitaxy technology is studied as well, with respect to its impact on solar cell efficiency. The data suggest no degradation in cell efficiency due to substrate reuse.