ABSTRACTWe present the integrated development of back‐contacted solar cells and an adequate module interconnection for a high‐efficiency photovoltaic module. We report on a large area (125 × 125) mm2 back‐junction back‐contact n‐type solar cell metallized with aluminum having a total area conversion efficiency of 20.7%. To transfer the high conversion efficiency to the module, we use the laser welding process named aluminum‐based mechanical and electrical laser interconnection for module integration of these back‐contacted solar cells. We further analyze the impact of the busbars of our back‐junction back‐contact cells on the cell performance and report on optimization of the cell/module interface. The new cells have an adapted rear‐side geometry by omitting the emitter busbar. A proof‐of‐concept module consisting of these cells is presented. Copyright © 2012 John Wiley & Sons, Ltd.

Increasing the area of interdigitated back-contact (IBC) solar cells featuring a busbar contact geometry requires the use of longer fingers. The finger resistance will, thus, be increased if the thickness of the metallization is kept constant. In order to maintain a thin metallization, it is beneficial to increase the number of busbars per contact. However, using more than one busbar for each polarity implies an asymmetric contact geometry. As a consequence, under operation, the busbars of the same polarity carry different currents. Due to voltage drops over unavoidable electrical resistances, this may lead to significant potential differences between these busbars. Since current–voltage characteristics are usually measured using separate sense contacts for the voltage measurement, the position and number of these contacts may considerably affect the shape of the resulting current–voltage characteristic and, thus, the fill factor. By means of simulations with the circuit simulator LTSpice, we show that a permanent contacting with soldered tabs allows for a correct determination of the fill factor. A chuck used for temporary contacting should feature at least one sense contact per busbar and pin contacting resistances below 30 mΩ in order to keep the fill factor error below 0.5% absolute.

In this paper, an interconnection method for the module integration of silicon solar cells by laser microwelding of the Al-metalized rear side of the solar cell to a metalized substrate is introduced. This laser microwelding process forms a direct mechanical and electrical connection between two Al-layers without the need for any soldering, conductive adhesives, or Ag-pastes. With a tensile tester, we measure tear-off stresses of up to 303 kPa for our laser weld spots. Furthermore, carrier lifetime measurements show that no defects are induced into the Si-crystal by the laser process over a wide range of laser pulse energies and number of laser pulses. In order to demonstrate the applicability of this laser-based interconnection method, we present a proof-of-concept module consisting of five n-type back-junction back-contact solar cells with a conversion efficiency of 20.0%.

We demonstrate the interconnection of silicon solar cells with evaporated aluminum back contacts using an aluminum foil which is attached to a silicone encapsulant. The aluminum-based mechanical and electrical laser interconnection (AMELI) process forms laser weld spots using single laser pulses. These laser welds resist high mechanical stresses and have a low electrical contact resistivity. No solder, conductive adhesives, or Ag-pastes are required for interconnection. We find the electrical contact resistivity to be below ρc = 0.01 mΩ·cm2. The contact resistance is constant under accelerated aging of 300 humidity-freeze cycles. With a tensile testing machine, we measure tear-off stresses in the perpendicular direction of up to 380 kPa for our laser weld spots. We present a proof-of-concept module which consists of five n-type back-junction back-contact solar cells with a conversion efficiency of 20.4%. The unchanged fill factor FF and open circuit voltage Voc verify a damage- and loss-free interconnection which is supported by electroluminescence measurements.

We investigate the artificial aging of PV modules interconnected with the aluminum-based mechanical and electrical laser interconnection (AMELI) process. AMELI forms a laser welded connection between the Al-metallization on the rear side of the solar cell and an Al-layer on an interconnector substrate. AMELI avoids soldering, conductive adhesives, and Ag-pastes. We investigate glass and encapsulant films as possible interconnector substrates that carry an Al-film. Modules processed with high temperature-deposited aluminum on the cells and an encapsulant as interconnector substrate do not degrade during 200 humidity-freeze cycles. The conversion efficiency η, the fill factor FF, the open circuit voltage Voc, and the short circuit current Isc are unaffected by the artificial aging within an accuracy of 1%.

The literature describes various techniques for fabricating thin monocrystalline Si films without the need of sawing. Layer transfer using epitaxy on porous Si (PSI) and subsequent layer separation from the growth substrate is one particular attractive option. A 40 μm thick epitaxial Si cell from this so-called PSI process is capable of saving about 80 % of the Si that is consumed by a 180 μm-thick Si wafer plus 100 μm Si kerf loss. In addition, the two crystallizations of growing the poly-feedstock and of growing the ingot are replaced by a single crystallization: the epitaxial growth of a thin Si layer. Applying the PSI process, we demonstrate an independently confirmed aperture efficiency of 19.1 % for a 4 cm2-sized layer transfer cell that has a thickness of only 43 μm. This is the highest efficiency ever reported for a thin-film (<;50 μm) crystalline Si layer transfer cell. We achieve this record with a passivated emitter and rear contact (PERC) structure. The passivation layer is Al2O3 from atomic layer deposition (ALD). The contacts on the rear side are formed by laser ablation using ultrashort ps pulses (LCO). The ISFH roadmap that our PSI process development is a part of is also discussed in this contribution. Our program aims at developing thin-film/wafer hybrid technologies that shall combine the high efficiency potential of thin monocrystalline Si films with the low cost per area of the Si-thin-film world. 19%-efficient and less then 50 μm-thick layer transfer cells mark a large step forward towards the realization of such thin-film/wafer hybrid technologies.

Highly aluminum-doped p-type silicon regions are formed by in-line high-rate evaporation of aluminum. We deposit aluminum layers of 28μm thickness at dynamic deposition rates of 20μm×m/min on p-type silicon substrates. Due to the high substrate temperature of up to 770°C during deposition an Al-doped p+ region is formed. Using the camera-based dynamic infrared lifetime mapping technique we measure emitter saturation current densities of 695±65fA/cm2 for the fully metalized Al-p+ regions, which corresponds to an implied solar cell open-circuit voltage of 635±2mV.

We introduce an n-type Si back-junction back-contact solar cell based on an Al-doped p+ rear emitter fabricated by means of screen-printing and firing instead of the commonly applied high-temperature boron diffusion. In order to demonstrate the applicability of this easy-to-fabricate p+ emitter to a back-junction back-contact solar cell we present experimental results showing 19.0% cell efficiency. The structuring of the cell is performed by laser processing omitting any photolithography. Using two-dimensional device simulation we determine a realistic efficiency limit of 21.6% for this cell type.

We present independently confirmed efficiencies of 21.4% for PERC cells with plasma-assisted atom-ic-layer-deposited (plasma ALD) Al2O3 rear passivation and 20.7% for cells with thermal ALD-Al2O3. Additionally, we evaluate three different industrially relevant techniques for the deposition of surface-passivating Al2O3 layers on 1-cm p-type silicon wafers, namely high-rate spatial ALD (spatial ALD), plasma-enhanced chemical vapour deposi-tion (PECVD) and reactive sputtering. Using spatial ALD and PECVD, surface recombination velocities (SRVs) be-low 10 cm/s are obtained. Sputtered Al2O3 layers still provide an SRV of 35 – 70 cm/s. Despite their lower passiva-tion quality compared to the Al2O3 films deposited by spatial ALD and by PECVD, we demonstrate that the sputtered Al2O3 layers are still suitable for the fabrication of 20.1% efficient PERC cells. After firing at ~800°C in a conveyor-belt furnace the SRV provided by the Al2O3 films deposited by spatial ALD is still below 20 cm/s, indicating an ex-cellent firing stability. Both PECVD and sputtered Al2O3 passivation layers degrade to SRVs larger than 100 cm/s af-ter firing. Hence, the firing stability of PECVD and – in particular – sputtered Al2O3 needs further optimisation

We show that the carrier lifetime in highly aluminum-doped silicon regions experiences a pronounced degradation during thermal treatment at temperatures above 850 °C. The defect formation is shown to be directly linked to the simultaneous presence of oxygen and aluminum and thus can be attributed to the formation of aluminum–oxygen complexes. Temperature-dependent measurements of the defect generation rate provide evidence that the formation of the Al–O complex is thermally activated with an activation energy of 0.25 ± 0.08 eV.

Applying an a-Si passivation layer to the rear-side of a screen-printed Al-p emitter on an nnp solar cell structure fabricated on 3 Ωcm n-type phosphorus-doped Cz-Si, we achieve an independently confirmed conversion efficiency of 20%. In a second approach, we apply an Al2O3/SiNx passivation stack to the screen-printed Al-p + emitter surface of our solar cells, where we demonstrate a conversion efficiency of 19.8% and a record-high open-circuit voltage of 649 mV. Furthermore, we introduce an industrial-type n-type Cz-Si solar cell featuring a screen-printed Al-p emitter at the rear and a selective front surface field at the front. This cell has an area of 100 cm and shows a stable conversion efficiency of 18.0%. This is the highest efficiency reported so far for an all-screen-printed n-type solar cell on Cz-Si without any boron diffusion.

The capacitance-voltage (CV) measurement is a precise and fast method to determine base-dopant concentrations of crystalline silicon solar cells. Since available measurement equipment is usually limited in its current output, the application of CV analysis has been limited to small-area solar cells in research laboratories. We present an experimental setup that is capable of measuring CV curves with a current output of up to 2 A. Using this setup, we demonstrate the applicability of CV measurements to large-area industrial solar cells for base-dopant concentrations ranging between 6.0 × 1014 cm-3 and 4.2 &times;1016 cm-3. An area enhancement factor f quantifying the relation between the macroscopic cell area and the active junction area is determined for alkaline textured mono- and isotextured multicrystalline silicon solar cells. Comparing the base dopant of the CV analysis with four-point probe measurements, we achieve an agreement with an uncertainty of 10%. For alkaline textured monocrystalline silicon solar cells, we demonstrate that the area enhancement factor can be extracted from the ratio of the pyramid base length and emitter thickness.

The next generation of industrial silicon solar cells aims at efficiencies of 20% and above. To achieve this goal using ever-thinner silicon wafers, a highly effective surface passivation of the cell front and rear is required. In the past, finding a suitable dielectric layer providing a high-quality rear passivation has been a major challenge. Aluminium oxide (Al2O3) grown by atomic layer deposition (ALD) has only recently turned out to be a nearly perfect candidate for such a dielectric. However, conventional ALD is limited to deposition rates well below 2nm/min, which is incompatible with industrial solar cell production. This paper assesses the passivation quality provided by three different industrially relevant techniques for the deposition of Al2O3 layers, namely high-rate spatial ALD, plasma-enhanced chemical vapour deposition (PECVD) and reactive sputtering.

The prospective efficiency improvements due to advanced metallisation technology of industrially fabricated Czochralski (Cz) silicon solar cells are assessed by means of highly predictive numerical modelling in two- and three dimensions. The analysis shows that an improved emitter design is only possible with advanced metallisation concepts, and that about 19% efficiency can be achieved with this. Then, the fully metallised back surface field (BSF) limits cell performance. If this BSF is replaced by point contacts without substantial BSF (PERC design), the efficiency is limited to about 20% because the wafer resistivity must be 0.5 Ωcm at maximum, so Cz material has a very limited carrier lifetime due to the boron-oxygen complex. If local BSFs are formed at the rear point contacts (PERL design), higher wafer resistivities can be used, so the base operates under high-injection conditions where the boron-oxygen complex is less detrimental. However, such improvements to the rear side increase cell efficiency only substantially if the lifetime in the Cz material, in particular its interstitial oxygen density, is reduced.

An important point of comparison between POCl3 emitter diffusion in a quartz tube furnace and in-line diffusion using sprayed phosphoric acid is the microscopic homogeneity of the diffusion, i.e. the homogeneity along the texture of a silicon surface. Two characterization methods were used. In each case, the cross-section of cleaved mc-Si and Cz-Si textured samples was observed in a scanning electron microscope (SEM). First, the thickness of the phosphosilicate glass (PSG) was measured. Second, the emitter was observed on SEM images which showed the n-type silicon as a darker region. The results show comparable homogeneity for in-line and POCl3 diffusion.

The carrier lifetimes in screen-printed Al-p+ regions are shown to be 3 orders of magnitude larger than the values expected from the extrapolation of the lifetime data measured on Al-doped Czochralski-grown silicon wafers. Device simulations show that the lifetime of 130 ns measured in Al-p+ regions enable open-circuit voltages of 670 mV and efficiencies of 21% on n-type silicon wafers. These results prove that the efficiency potential of screen-printed Al-p+ emitters for the application to rear-junction n-type cells is much higher than traditionally assumed.

We show that the recombination lifetimes measured in highly aluminum-doped p-type silicon (Al-p+) regions, fabricated by means of screen printing and firing of aluminum paste, are three orders of magnitude larger than the lifetimes expected from the extrapolation of the lifetime data measured on aluminum-doped Czochralski-grown silicon wafers. Solar cell simulations demonstrate that the lifetime of 130 ns measured in Al-p+ regions with an aluminum doping concentration of ∼2×1018 cm−3 enables solar cell open-circuit voltages of 670 mV and energy conversion efficiencies of 21% on n-type silicon wafers. These results prove that the efficiency potential of screen-printed Al-p+ emitters for the application to rear-junction n-type silicon solar cells is much higher than traditionally assumed.