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8th World Conference on Photovoltaic Energy Conversion; 1099-1104, In this work we give an overview of the module monitoring results achieved within the H2020 funded project called HighLite. A dedicated online monitoring platform has been developed for the project to group and treat the data from all partners with homogeneous procedures. Monitoring of advanced prototype modules developed in the project and based on various cells and interconnection approaches, like silicon-heterojunction (SHJ)-shingle, TopCon-shingle, cut interdigital back-contact cells (cut-IBC) are reported. We describe key learnings from monitoring modules for various applications including building-applied PV (BAPV) and building-integrated PV (BIPV). Annual energy yield up to 1900 kWh/kWp is expected in Cadarache (France) based on HighLite PV technology. Temperature corrected performance ratios are used for a ranking of the new technologies against the reference PERC modules.

36th European Photovoltaic Solar Energy Conference and Exhibition; 157-160, This work reports the latest results at imec of rear emitter bifacial Passivated Emitter, Rear Totally diffused (PERT) cells, processed on large area (243.3 cm2), 170 µm thick n-type Cz substrates. The cells feature a selective front side surface field (FSF) formed by laser doping using phosphorous silicate glass (PSG) as dopant source from POCl3 diffusion and a homogeneous rear Boron emitter with local p+ back surface field (BSF) formed during contact firing of Aluminium paste fingers. The FSF and rear emitter are passivated with thermal oxide (SiO2) + PECVD Silicon Nitride (SiNx), and with atomic layer deposited (ALD) Al2O3 + a stack of PECVD SiOx/SiNx respectively. The cells are fully screen-printed with 12 busbars and solderable Ag pads on the rear. We achieve an average cell efficiency of 23.10.1% with the best cell efficiency of 23.2% (both independently confirmed) with a reverse current (Irev) below 0.5A at -12V.

35th European Photovoltaic Solar Energy Conference and Exhibition; 354-358, In this paper, we present a simple processing sequence to fabricate large area (244.3 cm2) n-type Czochralski-grown (Cz) monofacial screen-printed rear-emitter Passivated Emitter, Rear Totally diffused (nPERT) cells that can be implemented in production as an upgrade to conventional p-type Cz Passivated Emitter Rear Cell (p- Cz PERC) manufacturing. We use conventional POCl3 and BBr3 tube diffusions, selective n++ laser doping from phosphorous silicate glass (PSG), atomic layer deposited (ALD) Al2O3 emitter passivation, and front and rear screenprinting metallization using commercially available pastes and conventional calendered mesh screens. In a first experiment optimizing rear emitter contact pitch, we achieve average cell energy conversion efficiencies of 22.6±0.1% (internal measurement). In a second experiment implementing fine line double printing (FLDP) and double antireflective coating (DARC), we improve average conversion efficiencies to 22.8±0.1% (internal measurement) with the champion cell measured at 23.03% (externally confirmed). Finally, we identify main loss mechanisms using Free Energy Loss Analysis (FELA) and perform Quokka2D simulations to define a feasible roadmap towards >24% efficiencies.

35th European Photovoltaic Solar Energy Conference and Exhibition; 580-583, We report detailed studies on laser doping from phosphorus silicate glass (PSG) to form the selective front surface field (FSF) for screen printed n-type passivated emitter, totally diffused (nPERT) solar cells. All relevant parameters for the laser doping process have been characterized at different laser conditions. These include measurements of sheet resistance, specific contact resistivity, recombination losses at the passivated laser-treated and metallized surface (measured by J0n++pass and J0n++metal respectively), optical and scanning electron microscopy SEM investigations. State of the art and very low dark saturation current densities have been achieved on passivated n+, passivated laser doped (n++) and metalized surfaces while maintaining low specific contact resistivity. The developed process for selective front surface field (FSF) with chosen doping parameters has been implemented in the monofacial nPERT devices with screen printed contacts resulting in independently confirmed efficiencies up to 23%.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 560-564, This work reports on the fabrication of advanced nPERT (passivated Emitter, Rear Totally diffused) cells by imec on epitaxial wafers from Crystal Solar obtained by direct gas to wafer technology. Crystal Solar has developed a process to grow full-epitaxial monocrystalline wafers of ~180-200μm thick with a limited stacking fault defect density of

32nd European Photovoltaic Solar Energy Conference and Exhibition; 439-442, Large area n-type rear-junction Passivated Emitter Rear Totally-diffused (PERT) solar cells with efficiency of 22.5% were recently reported by imec using rear sputtered Al. To develop a more industrially relevant process, screenprinting of Al is also currently investigated as the rear metallization. In this case, an Al p+ emitter is locally formed by Al- Si alloying during a firing step. The quality of this p+ emitter is crucial to the performance of the final devices. However, this is a challenging and less controlled process in terms of thickness of the doping layer and possible formation of voids. In this work, the contact geometry and the firing are optimized and their impact on the void and Al p+ layer formation is studied. Using the optimized processing parameters, large area n-PERT solar cells with screen-printed Al with efficiency up to 21.7% are fabricated. These cells demonstrate high values for the open circuit voltage (Voc) up to 690mV. This proves the feasibility of using screen-printed Al for high performance n-PERT cells.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 420-425, In this paper we introduce bifacial n-type PERT cells (Bi-PERT) with ultra-thin (< 3 m) and narrow (< 20 m) Ni/Ag plated fingers on both the n+ and p+ side of the cell. This cell architecture is designed for multi-wire interconnection and has the potential to deliver low Cost of Ownership at module level (< 0.40 $/Wp), as it can profit from bifacial gain, a low cost metallization sequence and the high efficiency potential of multi-wire interconnection. We describe the Bi-PERT cell features and show that the metallization sequence here proposed is well suited for integration with the multi-wire technology as it features finger section well below 60 m2 and no highly conductive Cu inter-layer. We also demonstrate a self-aligned fully plated metallization sequence able to form Ni/Ag electrodes on both sides of the cell with a specific contact resistance (c) below 1 m∙cm2 for both the n+ and p+ side. We benchmark this novel technique to a seed-and-plate sequence where the plating seed is obtained by physical vapor deposition of Ni (PVD-and-plate). Finally, we present light I-V results for rear-junction and front-junction Bi-PERT cells and report efficiency up to 21.0% for cells featuring the fully plated Ni/Ag metallization sequence and 22.6% for cells fabricated with the PVD-and-plate sequence.

31st European Photovoltaic Solar Energy Conference and Exhibition; 365-368, We describe the process for fabricating interdigitated back-contacted cells compatible with thin (40μm) monocrystalline silicon foils. Throughout the process, the foils are supported by either the silicon parent substrate, or by a host (glass) superstrate using adhesive bonding. For a number of process steps, we describe possible issues, boundary conditions and performance. Using the integrated process which is fully compatible with bonded foils, cells were fabricated on a freestanding, non-textured wafer. An efficiency of 18.0% was achieved, the main limitations being the absence of surface texturing and a non-optimized amorphous silicon emitter.

29th European Photovoltaic Solar Energy Conference and Exhibition; 417-420, This work deals with quantifying the internal optical losses in a Cu plated n-Si rear junction cell. The focus is on the following 2 losses : (i) Free Carrier Absorption (FCA) in the highly doped regions of the device and (ii) non-ideal internal reflectance at the rear of the device. For the FCA, optical measurements have been used to quantify the loss of photons in both the n+ diffused Front Surface Field (FSF) and p+ emitter regions of the device. It is shown that this loss is negligible for the P- and B-doped layers used in imec’s n-Si rear junction cells and amounts to a combined loss in short-circuit current of only 0.1 mA/cm2. External reflectance measurements have also been employed to quantify the internal rear reflectance in the device as function of the nature and thickness of the rear dielectric layers. Predictive calculations indicate an improved internal rear reflectance by switching from the original dielectric stack (thermal oxide + PECVD nitride) to a single dielectric layer (> 100nm thermal oxide), which has been confirmed experimentally. Such implementation into imec’s n-Si rear junction cells has helped to improve the efficiency to levels above 21%.

29th European Photovoltaic Solar Energy Conference and Exhibition; 528-531, We study different paths to reduce the front recombination losses of n-Si rear-junction cells featuring a laser patterned front and Ni/Cu contacts. At first, the laser induced recombination for different n+/n/n+ structures is quantified, showing that its dimensions depend strongly on the “high-low” junction depth. Selective laser doping is evaluated as an alternative solution for the patterning of the metal contacts. Different thermal sequences are tested to anneal the laser-induced defects for both laser ablated and laser doped cells. An absolute increase of jsc of 0.5 mA/cm2 for the laser doped cells is demonstrated, with conversion efficiencies over 21%.

28th European Photovoltaic Solar Energy Conference and Exhibition; 757-760, This work reports on the feasibility to implement a cost effective dry oxidation step in a Passivated Emitter and Rear Cell (PERC) structure on p-type Cz Silicon substrates. PERC solar cells have been extensively investigated in the recent years for industrial implementation. One of the main challenges is to combine the integration of cost effective process steps while maintaining high cell efficiency. In our previous work, we introduced at imec the use of a thermal oxide below the front ARC and below the rear dielectric PECVD SiOx/SiNx stack for enhanced surface passivation. In this study, we demonstrate that a low thermal budget of 800oC coupled with a drastic reduction of oxidation time down to 15 min is sufficient for the growth of a high quality thermal oxide without adversely affecting the emitter properties. We show that the level of Dit at the silicon interface after oxidation is not determinant for the level of surface passivation but rather the hydrogenation provided by the SiNx after firing. We also show that the resulting SIMS emitter Phosphorous profiles after the low temperature oxidation maintain a surface concentration above 1×1020/cm3, which is fully compatible with screen printing technology. Assisted by the latest developments on commercial silver paste, this enables to contact homogeneous emitters up to 90 Ω/ with measured FF above 79% on the cell level after firing optimisation. All devices were fabricated on substrates area of 156 mm × 156 mm with a base resistivity of 2 Ohm.cm and a thickness of 170 μm. The PERC cells have front Ag screen printed contacts with a 3- busbar pattern, a thin thermal oxide on both sides with a rear PECVD SiOx/SiNx stack. Average conversion efficiency above 19.7% are achieved on such large area substrates. Several optimisation cycles have led to best cell efficiency of 20.1%.

28th European Photovoltaic Solar Energy Conference and Exhibition; 1008-1012, Despite cost and technological advantages, the commercialization of nickel/copper (Ni/Cu) plated contacts has so far been limited. Reasons for this include increased process complexity, availability of suitable lowcost production tools, and doubts over long-term reliability. We previously reported on a simple Cu based metallization sequence consisting of i) defining the front contact pattern by ps-UV laser ablation ii) self-aligned plating of the contacts using Ni/Cu/Ag and finally iii) sintering in N2 for nickel silicidation. We also reported that 10 cells strings modules featuring these Ni/Cu/Ag contacts met extended (1.5x) IEC61215 criteria for thermal cycling and damp heat reliability. In this work, this process sequence is transferred to 156x156 mmm2 substrates using pilotline processing equipment available at imec that include inline plating and sintering tools. The process sequence is further simplified to yield an industrial 0.5μm deep emitter without compromising efficiency results nor ribbon pull force values (average >3N/mm). This 0.5μm emitter also proves to be more robust to changes in sheet resistance induced during processing than the previously used high-efficiency 1μm deep homogeneous emitter. Applying the 0.5μm deep emitter and optimizing both front grid design and plating thickness we demonstrate efficiencies above 20% on 109 cells (average of 20.5%), with the best cell achieving 20.7% (externally confirmed by FhG-ISE CalLab).

27th European Photovoltaic Solar Energy Conference and Exhibition; 1630-1633, This paper describes the progress in transferring a PERC type cell structure based on Al2O3/SiNx or SiO2/Al2O3/SiNx passivation into an industrial environment. Al2O3-layers deposited in an industrial type spatial ALD system are characterized using C-V, ellipsometry and lifetime measurements. A negative charge density above 1012 cm-2 was measured for a 6 nm thick layer. The Al2O3-layers are successfully incorporated in a PERC process sequence for monocrystalline Cz-Si solar cells. This process is based on a production type homogeneous POCl3- diffused emitter, rear surface polishing and screenprinted contacts. Average efficiencies above 19% have been obtained with a thin dielectric stack of Al2O3 and SiNx relying on accumulation layer passivation on the rear surface. A thin thermally grown oxide further improves the Voc due to a lower emitter dark saturation current density. On the rear surface this thin oxide, with Al2O3 and SiNx on top, results in inversion layer passivation. Care is taken to avoid inversion layer shunting by improving the Back Surface Field around the local contacts using the Extended Laser Ablation (ELA) process. This resulted in an average efficiency of 19.46% for a batch of 140 Cz-Si cells with a top efficiency of 19.7%.

27th European Photovoltaic Solar Energy Conference and Exhibition; 680-685, An easily implementable approach to increase cell efficiency without incurring too much cost is to introduce thermal oxidation [1,2,3,4]. Thermal oxide is well known to the microelectronics industry as one of the best dielectrics to passivate silicon surfaces. This paper will focus on the front side (diffused side) of passivated emitter rear contacted (PERC) silicon solar cells and on the enhancement brought upon thin thermal oxidation performed at 800°C in a POCl3 furnace. The resulting emitters are characterized by secondary ion mass spectroscopy (SIMS) as well as scanning spreading resistance profiling (SRP) measurements and emitter saturation current densities. We report that increasing oxidation growth (or time) significantly decreases phosphorus surface concentration (Ns) while increasing junction depth. The resulting profiles after oxidation have surface concentrations in the range of 1- 2x1020/cm3 which are fully activated and can be contacted with industrially available silver screen printing pastes. We also report that using a thin thermal oxide a lower contact resistance can be obtained especially when considering emitter sheet resistances in the 85+ Ω/ range. The low contact resistance from the front metallization is attributed to the presence of phosphorous in the thermal oxide. The final cell results show that increasing oxidation growth (or time) increases open circuit voltage as well as fill factor, but if the oxide is left on, the short circuit current is reduced partly due to optical loss. The cells studied in this paper are 148.8cm2, 155μm 1.5 ohm∙cm p-type Cz-Si wafers that have screen printed Ag front contacts and a thin thermal oxide on both sides with a rear deposited oxide/nitride stack. The highest efficiency of the cells studied is 19.9% with a Voc of 654mV, a Jsc of 38.4 mA/cm2 and a fill factor of 79.3% confirmed by the Fraunhofer ISE.

27th European Photovoltaic Solar Energy Conference and Exhibition; 1602-1606, An industrially applicable process flow for the formation of rear side passivated solar cells has been suggested by IMEC in 2005. Since then considerable improvements in solar cell efficiencies have been obtained with that type of process. In parallel simplifications and cost reductions were needed to bring it to production. IMEC’s Industrial Affiliate Programme (IIAP) offers industrial partners the possibility to actively join development efforts with IMEC and other partners sending residents to work at IMEC as part of a research team. Selected development results of this IIAP are presented here. On large area homogeneous emitter CZ-Si solar cells FhG-SE Cal-Lab confirmed solar cell efficiencies of 20.0 and 20.3% for screen printed Ag and reliable plated Cu front contacts, respectively. Fill factors of 80% have been achieved for fully screen printed (front and rear) solar cells with low cost rear side dielectric stack layers. Cu plated contacts show reliable module data in IEC test conditions. Latest results with average solar cell efficiencies of 20.4% over the length of a CZ-Si ingot with top efficiencies of 20.6% are just an intermediate step. IMEC sees a road to >21% efficient i-PERL solar cells in production.

27th European Photovoltaic Solar Energy Conference and Exhibition; 538-543, This work reports on using copper (Cu) as the main conductor as an alternative to screen printed silver (Ag) front contacts for homogeneous emitter silicon solar cells. The work is not only motivated by the limitations that Ag screen printed contacts have regarding solar cell efficiency (high contact shading, limited line conductivity, and poor contact resistance to moderately doped emitters) but also the PV industries desire to reduce Ag usage for cost reasons. A relatively simple process sequence is investigated consisting of i) defining the front contact pattern by ps-UV laser ablation ii) self-aligned plating of the contacts using Ni/Cu/Ag and finally iii) sintering in N2 for nickel silicidation. The process sequence is applied to 12.5x12.5 cm2 p-type CZ-Si PERC type solar cells with 1μm deep 120Ω/ homogeneous emitters. Solar cell efficiencies of up to =20.3% and average pull tab adhesion results >2N/mm are achieved. Thermal cycling and damp heat reliability data for single cell laminates and small modules are reported meeting extended (1.5x) IEC61215 criteria.

Atomic layer deposition (ALD) of thin Al2O3 (=10¿nm) films is used to improve the rear surface passivation of large-area screen-printed p-type Si passivated emitter and rear cells (PERC). A blister-free stack of Al2O3/SiOx/SiNx is developed, leading to an improved back reflection and a rear recombination current (J0,rear) of 92¿±¿6¿fA/cm2. The Al2O3/SiOx/SiNx stack is blister-free if a 700°C anneal in N2 is performed after the Al2O3 deposition and prior to the SiOx/SiNx capping. A clear relationship between blistering density and lower open-circuit voltage (VOC) due to increased rear contacting area is shown. In case of the blister-free Al2O3/SiOx/SiNx rear surface passivation stack, an average cell efficiency of 19.0% is reached and independently confirmed by FhG-ISE CalLab. Compared with SiOx/SiNx-passivated PERC, there is an obvious gain in VOC and short-circuit current (JSC) of 5¿mV and 0.2¿mA/cm2, respectively, thanks to improved rear surface passivation and rear internal reflection.

26th European Photovoltaic Solar Energy Conference and Exhibition; 1129-1131, Blister-free Al2O3-based surface passivation stacks for p-type Si passivated emitter and rear cells (PERCs) are developed. Measuring the blistering area and effective surface recombination velocity, it is shown that using (i) an Al2O3 layer ≤ 10 nm and (ii) out-gassing this Al2O3 film above 600 °C prior to capping is necessary. These blister-free Al2O3-based stacks are implemented as rear surface passivation for p-type Si PERC: 148.25 cm2 screen-printed cells with an average efficiency of 19.0 % have been made. There is an obvious gain in open circuit voltage of 5 mV compared to SiOx passivated PERC, thanks to enhanced rear surface passivation.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 1162-1166, The integrated project (IP) Crystal Clear has been finalized in June 2009 and thus a chapter of more than 5 years successful collaboration between European solar cell research institutes and industry is closed. This paper reviews the achievements within the 4th subproject (SP4) of Crystal Clear dealing with the development of advanced solar cell concepts. Within SP4, 12 of the 16 partners have participated and have formed a productive consortium. The goal of this subproject was the further reduction of the cell production costs to enable the psychological important Si PV module price below 1 Euro/Watt. This should be reached by novel cell designs and manufacturing processes in order to achieve a process cost reduction of 40% (cost per Wp). One major effort was put in the development of novel cell concepts and processes suited for thin wafers in industrial fabrication. Three cell concepts were distinguished and developed by the participating institutes: i-PERC, MWT with full Al, and solar cells with laser fired contacts (PERC-LFC). In addition to the effort to implement these concepts into an industrial type of process with large area wafers, excellent results have been achieved on EFG and RGS ribbons. World record results with laboratory-type processes could be reported, with efficiencies of 18.2 % on EFG ribbons and 14.4 % on RGS ribbons. The defect mechanism of these materials has been studied in detail and efficiency limits have been indicated. The paper will refer to the separate contributions submitted to the conference by the different partners on specific topics.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 1609-1612, The continuous decrease of the substrate thickness for industrial silicon solar cells results in an increased importance of surface passivation. The most natural way to achieve surface passivation is to grow a thermal oxide. However, the cost of ownership for thermal oxidation is much too high for the photovoltaic industry. In the present paper, the passivating properties of a deposited oxide are studied. It is found that this oxide offers poor passivation after deposition, but that the combination of a high temperature anneal and a forming gas anneal results in passivation of the surface. The role of the high temperature anneal is shown to be a reduction of the densities of the interface state, in particular of Pb centers, and of the oxide charges, while the forming gas anneal leads to de-activation of the remaining defects.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 1843-1846, To improve the front passivation phosphorus emitters for high efficiency Si solar cells require a low surface concentration and high sheet resistance. The fabrication of such emitters is studied with a process based on plasma doping or POCl3 diffusion. The improved passivation of POCl3 based emitters results in an increase in Voc. Plasma doping is shown to be a valuable candidate to simplify and better control the advanced emitter formation.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 1820-1822, Most emitters today are still homogeneous and consequently must fulfill a compromise between good contacting and as low as possible recombination. The selective emitter concept offers a way out by decoupling doping under contacting areas from the rest of the volume to be phosphorus-doped. In order to reach this, we propose to diffuse the emitter through a patterned diffusion barrier with a given diffusion transparency. The integration aspect arises by taking advantage of this same barrier layer as a passivating element for the solar cell and as a key element for the definition of front side metallization pattern. A first step towards an integrated approach for selective emitter formation is proposed here with the establishment of a reference process for emitter tuning by a thermal oxide barrier. The impact of such a barrier on diffusion profiles and related sheet resistance, emitter saturation current and IV characteristics has been investigated.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 1483-1486, Driven by further decreasing the price per watt peak to reach grid parity, the crystalline silicon solar cells roadmap is moving towards thinner wafers while further increasing the efficiency. While 200 ;m is the standard thickness used in industrial solar cell production today, solar cells on silicon wafers down to 80 μm are being developed. The influence of rear-side surface recombination on the losses of the solar cell is increasing by going to thinner silicon wafer. In order to decrease this rear-side recombination, new solar cell processes with back-side passivation by a dielectric layer and locally Al-alloyed contacts are being developed. One promising approach involves the local opening of the passivation layer by laser ablation, blanket deposition of Al, and a short thermal anneal. In this paper, we studied the local Al-alloyed contacts formed in this way. We showed that Si dissolution is an important issue and we gave a qualitative explanation for the observed contact structures. Applying dielectric passivation and local Al back surface field (Al-BSF), superior results on very thin multicrystalline (mc) and mono Si solar cells have been reported recently, like 16.6% efficiency on 80 ;m thick, 78 cm2 CZ-Si (4.3 cm) [3].

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 1171-1174, This work evaluates the application of an advanced solar cell process that features a dielectric rear surface passivation as well as a local BSF on BP Solar’s Mono2 TM material. The Mono2 TM material is a novel type of Si material for solar cell application and is obtained using a seeded directional solidification technique [1], producing wafers that are free of grain boundaries, or in some cases have small areas with crystal grains. It is a promising alternative to traditional high cost Czochralski and float-zone material. The applied solar cell process features a rear passivation stack of deposited oxide/PECVD SiN as well as local Al-alloyed contacts [2]. The target is high efficiency at low cost which is foreseeable with the combination of the presented advanced Si material and the proposed advanced solar cell process. Cell results obtained in this work are up to 18% efficient for the best Mono2 TM and an average of 17.2% for a batch of 41 cells all of which have a final thickness of 130 μm and area of 156 cm2.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 958-962, CrystalClear is a large integrated project funded by the European Commission that aims to drastically reduce the cost of crystalline Si PV modules, down to 1 Euro/Wp. Among the different subprojects, the one dealing with the development of advanced solar cells is relatively large (with 11 partners out of the 15 Crystal Clear partners taking part) and has a crucial role. The goal of the subproject is to develop cell design concepts and manufacturing processes that would enable a reduction in the order of 40% of the cell processing costs per Wp. In this paper, we give an overview of all the development work that has taken place in the CrystalClear solar cells subproject so far. World class results have been achieved, particularly on high efficiency cells on Si ribbons, and on industrial-type solar cells on very thin (120 μm thick) substrates.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 1600-1603, At IMEC the i-PERC concept of thermally fired local back-surface field (LBSF) cells, was developed to improve the rear surface passivation over full Al-BSF cells in order to be able to realize thinner and thinner bulk silicon solar cells without sacrificing efficiency. For the first time, the i-PERC concept has been applied using high quality float zone silicon. This industrial rear side process is combined with a lab-scale front side process, consisting of a lithography process and metal evaporation to define the front finger grid. Small area (4 cm2) i-PERC cells are compared with solar cells having a standard aluminum back surface field. Both silicon nitride and thermal silicon oxide are investigated to passivate the front surface. Comparing a standard Al BSF with the i-PERC process, shows a clear increase in open-circuit voltage of around 8 to 10 mV and an increase in short-circuit current density of approximately 1 mA/cm2, leading to a gain in energy conversion efficiency of 0.8 percent absolute.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 1193-1196, The fabrication of solar cells on thin (below 150 μm) and ultrathin (below 100 μm) Si wafers has received much attention in recent years due to the shortage and the high cost of Si feedstock. However, it is widely recognized that some modifications of the standard industrial solar cell structure and of the process sequence have to be made in order to keep the efficiency and yield high. Industrial-type passivated emitter and rear cell, or i-PERC in short, seems to be a very attractive candidate thanks to the improved rear surface passivation, the reduced cell bowing, and its compatibility with the present industrial solar cell production. In the i-PERC cell, the rear side consists of a dielectric/SiNx passivation stack which is locally opened by laser ablation. A localized back surface field (BSF) is formed in the openings after metallization and co-firing, instead of the formation of a full Al BSF as in the standard industrial Si solar cell. In this paper, the latest results of i-PERC solar cells processed on 80 μm thick 78 cm2 monocrystalline Si wafers will be reported. Furthermore, the formation of the local BSF, the impacts of the dielectric layer thickness and the contact pitch on the open circuit voltage and short circuit current will also be investigated and discussed.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 1200-1203, The continuous decrease of the substrate thickness for industrial silicon solar cells results in an increased importance of surface passivation. One approach which received much attention in a recent past is the use of a passivation stack formed by Si/SiOx/SiNx:H. In this kind of stack, the oxide layer ensures a low concentration of interface states, while the nitride layer acts as a source of hydrogen. While this hydrogen is known to contribute to bulk passivation, its effect on the Si/SiOx interface is poorly understood. In the present study, the hydrogenation process of a Si/SiOx interface is investigated, where SiOx is a deposited oxide. It is demonstrated that hydrogenation of this interface can lead to effective surface recombination velocities below 150 cm/s, and that this hydrogenation is effective both when atomic or molecular hydrogen is applied.