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In High-Performance mc-Si [1] random grain boundaries, although being recombination active, often enhance material quality by reducing dislocations. With this work, we take a step towards statistical large-scale investigations of crystal defects via a combined analysis of different inline-measurements on as-cut wafers: photoluminescence images for the extraction of recombination-active structures and reflection and infrared transmission images for the extraction of the grain structure. The combined extraction of recombination-active structures and grain structures allows isolating dislocations from grain boundaries for all material types. To discern dislocations from other recombination-active defect structures, an image-processing-based analysis technique has been developed. By applying this separation on wafers from various bricks of our material set, typical developments of grain structure and dislocations can be identified. As a particular application, we investigate the correlation between the development of dislocations in higher parts of the brick and grain size in the lower parts. The results support theory quantitatively: Dislocation ratio in the upper brick part shows a correlation with the square root of the weighted median of the grain size in the lower brick part (R≈0.85). However, the results also show that grain size distribution, in particular grain size homogeneity, has to be considered to account for a stronger distinction between materials.

The use of proper surface preparation and cleaning methods for Si wafers prior to the deposition of passivation layers is essential to minimize surface recombination and realize high efficiencies (> 20%) in crystalline Si photovoltaic cells. In this work, the influence of wafer cleaning on the quality of surface passivation achievable for boron-doped emitters was investigated, including the use of different combinations of HCl, HF, HNO3, and ozonated deionized water (DIO3). These different surface preparations and cleaning sequences were performed on undiffused and boron diffused n-type Cz Si wafers, followed by the deposition of either silicon nitride (SiNx) or an aluminum oxide film capped with SiNx (Al2O3/SiNx stack). Additionally, both planar and anisotropically textured wafers were used. Injection-level dependent photoconductance measurements and calibrated photoluminescence imaging were performed on symmetrical boron diffused samples based on the different cleaning processes and passivation materials described above. Additionally, non-contact corona-Kelvin measurements were used to extract the total charge and interface defect density at the Si surfaces. We found that cleaning variations strongly influence carrier lifetime for SiNx passivated Si, but the effect is less pronounced in the case of Al2O3/SiNx stacks. It was further observed that DIO3-last treatment resulted in higher lifetimes for the SiNx stacks. Overall, it emerged that the (DIO3 + HF + HCl → HF → DIO3) clean is a promising and potentially low cost cleaning sequence for the photovoltaics industry.

In this work, we present a novel technological approach to form highly boron-doped selective emitters. The selective emitters are formed by using a Laser Induced Forward Transfer Doping (DLIFT) process, which allows precise adjustment of the doping profile by tuning the laser parameters and choosing an appropriate doping source. Surface dopant concentrations of up to Ns≈1×1021 cm−3 were achieved by using DLIFT. Subsequent BBr3 tube furnace diffusion was performed to form a low surface concentration (Ns≈1×1019 cm−3) homogeneous emitter. In comparison to the conventionally applied BBr3 tube diffusion, an increase in open circuit voltage of up to 6 mV is predicted based on the photoluminescence measurements performed after passivation, screen-printing and firing processes. Simulations suggest that this voltage gain is most possibly due to a lower emitter saturation current density (j0e) of the highly doped DLIFT samples compared to the BBr3 diffusion samples. Moreover, it is observed that the laser-induced defects are reduced successfully after a subsequent boron tribromide (BBr3) tube diffusion, which is used to form the homogeneous emitter after the DLIFT process. The micro Raman spectroscopy results indicated that the compressive stress of −200 MPa in the silicon crystal lattice after DLIFT process is released by the subsequent BBr3 tube diffusion. These results demonstrate that the DLIFT process in combination with the conventionally applied BBr3 tube diffusion can be an effective approach to form selective boron emitters in high efficiency n-type crystalline silicon (c-Si) solar cells.

In this paper, we study the impact of change in emitter diffusion profiles on the electrical characteristics of nanotextured surfaces formed by an inline plasma-less dry-chemical etching process. Our experimental results and process simulations suggest that a deeper highly doped region and a significantly higher inactive P concentration in the emitter plays a determining role in defining recombination, as well as the resistive losses in nanotextured surfaces. Low emitter saturation current densities on phosphorous-diffused surfaces are achievable after passivation with either SiN x ( $j_{{\rm{0e,min}}}\,\approx \,{\text{81 fA/ cm}}^{2}$ ) or AlO x /SiN x ( $j_{{\rm{0e,min}}}\,{\rm{\approx}} \,\text{31 fA/ cm}^{2}$ ) if the emitter recombination channels are suppressed. Based upon macroscopic measurement of contact resistivity and microscopic analysis of the contact areas, we propose that the formation of numerous metal–semiconductor direct contact points on the peak and the plateaus of the nanostructures are mainly responsible for a low specific contact resistivity ( $\rho _{{\rm{c,min}}}\,{\rm{\approx}} \,\text{1.2}\; \text{m}\Omega \cdot \text{cm}^{2}$ ) achievable in these surfaces.

We present an approach for examining and understanding the impact of material and process variations on solar cell efficiencies using the example of an industrial feasible multicrystalline silicon (mc-Si) passivated emitter and rear cell (PERC) process. We fabricate and characterize more than 800 mc-Si PERC cells with a broad material variation and model the experimentally achieved solar cell efficiencies based on numerical 3-D device simulations, metamodeling, and Monte Carlo runs. We subject the simulated distribution of cell efficiencies to a variance-based sensitivity analysis, extracting and ranking the process- and material-related input parameters according to their share of the total variance of cell efficiencies and highlighting the parameters that need to be tuned and controlled most accurately. We are able to explain 90% of the measured total variance which divides into 68%abs. material- and 22%abs. process-related influences. Experimental indication of fill factor (FF) losses due to laterally inhomogeneous bulk lifetimes is found. The presented methodology and its findings provide a fundamental tool for a better understanding of the dependencies in a mc-Si PERC process and lays the groundwork for optimizing the quality and the yield of production lines. Furthermore, the approach is transferrable to other solar cell concepts and production lines.

Rotational flexographic printing technology is a highly promising approach to increase the productivity of the cost-intensive solar cell metallization process. The ability to realize narrow contact fingers with very low silver consumption makes this technology particularly attractive for the front side metallization of busbarless solar cells in combination with multi-wire interconnection like Meyer Burger’s SmartWire Connection Technology (SWCT). Within this work, we investigate the feasibility of this approach on solar cells with 156 mm edge length. Two types of silver inks are prepared and evaluated with focus on optical and electrical properties of the printed front side grid. Both inks achieve sufficient lateral finger resistances below 20 Ω/cm. A low specific contact resistance of ρc,95%=3.0±0.6 m Ω cm2 is obtained with ink A. Using flexographic printing, Aluminum back surface field Czrochalski-grown Silicon busbarless solar cells with a maximum conversion efficiency of η=19.4% (η∅=19.0%) are fabricated and interconnected to a mini-module. The mini-module obtains an aperture conversion efficiency of η=15.8%. The origin of the cell-to-module (CTM) losses are examined in detail. It is shown that a certain part of the CTM-losses originates from the characteristics of the used GridTOUCH I–V-measurement device. Further sources of possible CTM losses are investigated using electroluminescence measurement (EL) and discussed in detail.

This paper characterizes the adhesion quality of evaporated aluminum rear metallization on rear passivation layers of silicon solar cells. A peel-test is introduced for adhesion rating and the adhesion quality between various Al and passivation layers on test structures is compared and discussed. The results show that the adhesion strongly varies with passivation type and evaporation parameters. Strong adherence is attributed to the formation of a strongly bonded, extended interphase layer. We derive factors that influence adhesion quality on our sample structures and find that an intimate contact between the layers and elevated temperatures are very important parameters. Additionally, a sufficiently high adhesion quality for an evaporated rear metallization of passivated emitter and rear cells (PERC) is achieved for all investigated passivation layers.

In this paper, we study the effect of an enlarged surface area of nanotextured crystalline silicon wafers on the formation of n-type emitters using a tube diffusion process applying POCl 3 as P dopant source. A fast, single-step and industrially viable F 2 -based dry texturing process is used to perform nanotexturing of Si wafers. This process is presented as an alternative route of nanotexturing in comparison to the two-step nanotexturing approach of creating black silicon and then modifying it with an alkaline or acidic solution. Predictive simulations of phosphorous in-diffusion aided by microscopical characterization of the selected emitters are used to understand the formation of emitter in nanotextured surfaces. Based on these investigations, we show that the optimized emitter leads to a significant improvement in short circuit current density ( J sc ≥0.7 mA/cm 2 ) of nanotextured mc-Si solar cells in comparison to the industrial standard acidic textured solar cells.

In this paper, we study the influence of modifying the geometry of nanotexture on its electrical properties. Nanotexture is formed by an industrially feasible dry-chemical etching process performed entirely in atmospheric pressure conditions. A surface modification process is developed that allows low surface recombination velocities ( S eff,min ≤ 10 cm/s) on nanotextured surfaces. By simultaneously improving the surface passivation and the emitter diffusion processes, we achieve an equivalent passivation level ( V OC,impl ≥ 670 mV) for nanotextured surfaces to that of reference textured surfaces after applying either PECVD or ALD based deposition techniques.

In this paper we present a new update to PC1Dmod, which extends the original PC1D program by implementing Fermi-Dirac statistics and a range of state-of-the-art models in order to improve the accuracy of c-Si device simulation. In PC1Dmod 6.2 the list of models is further expanded to include a parameterization of incomplete ionization of dopants (Altermatt et al., J. Appl. Phys.100, 113715, 2006), with parameters for phosphorus, boron, arsenic, gallium and aluminum. The results have been verified against a previous implementation of the model. We show that the inclusion of incomplete ionization is of particular importance for moderately doped surface regions with doping densities in the range ∼5 × 10 17 to ∼5×10 19 cm -3 , resulting in a deviation of up to 15% in the simulated recombination current density and sheet resistance. The effect of incomplete ionization is also shown to be more pronounced in devices made from compensated Si material. Furthermore, the default solar spectrum has also been updated in PC1Dmod 6.2, taking advantage of the increased maximum file size to include a more realistic representation. The generation profiles calculated using PC1Dmod 6.2 together with input from OPAL 2 agree well with results from Sentaurus TCAD for both planar and pyramidally textured surfaces, thus facilitating a more direct comparison between PC1Dmod 6.2 and other simulation programs utilizing standard spectra. The simulation comparison is also extended to the electrical performance of the modeled devices, showing good agreement in the calculated short-circuit current density for a range of planar and textured solar cells with varying emitter doping. Finally, several new output options, including emitter saturation current and injection-dependent lifetime curves have been added, enabling a simpler and more direct way for users to access and plot important device properties.

Microcracks in silicon wafers reduce the strength of the wafers and can lead to critical failure within the solar-cell production. Both detection of the microcracks and their impact on fracture strength of the wafers are addressed within this study. To improve the accuracy of the crack detection in photoluminescence (PL) and infrared transmission (IR) images of as-cut wafers, we introduce a pattern recognition approach based on local descriptors and support-vector classification. The learning model requires a set of labeled data generated by an artificial insertion of cracks. Within this evaluation, the algorithm detects 81% of the cracks for PL-images and 98% for IR-images at precision rates above 98% in each case, which outperforms the quality of pure IR-intensity-based crack-detection systems with a hit-rate of 65% at a precision of 59%. The proposed algorithm may be combined with the images of the grain structure to avoid the confusion of cracks and grain boundaries. Moreover, the comprehensive set of wafers allows the impact of crack morphology on wafer strength to be investigated. Despite complex crack morphologies, the theoretically expected dependence between crack length and fracture strength is confirmed. Therefore, sorting criteria are derived to rate the cracks with respect to the expected fracture strength of the wafer based on the measured crack length only.

For the contacting of boron-doped emitters with screen-printed metallization in n-type silicon solar cells, usually a small amount of aluminum is added to the silver pastes. To date, low specific contact resistances ρ C in the range of a few mΩ cm 2 have only been achieved with these so-called silver–aluminum (Ag–Al) pastes. Within this work, ρ C is experimentally determined for two Ag–Al pastes for three different boron-doped emitters on alkaline textured and passivated silicon surfaces. The investigated boron doping profiles feature almost identical rising curve progressions up to a depth of ≈60 nm (which corresponds to the depletion zone), with a maximum dopant concentration N max ≈8·10 19 cm −3 . However, they have different junction depths between 570 nm and 980 nm. This work shows that the dopant concentrations with values well below N max following the depletion zone significantly affect the measured ρ C : the deeper the junction, the lower ρ C . This behavior is observed for both investigated Ag–Al pastes. For example, the mean ρ C for paste Ag–Al1 decreases from ρ C ≈4 mΩ cm 2 , determined on a 570 nm deep doping profile, to ρ C ≈2 mΩ cm 2 , determined on a 980 nm deep one. By using an analytical model to calculate ρ C with dependence on e.g. dopant concentration, metal crystallite coverage fraction, and crystallite penetration depth, the lower ρ C for deeper profiles can be explained by crystallites with penetration depths of several 100 nm. The calculations also reveal that the impact of the depletion zone is negligible with respect to ρ C for crystallites deeper than ≈100 nm.

We investigate the origin of the apparently reduced recombination parameter J 0 of highly doped regions obtained from e.g. QSSPC lifetime measurements at high injection densities as well as the influence of recombination at bulk defects on the recently updated J 0 -analysis of QSSPC measurements in the Sinton lifetime tester software. Using the example of crystalline silicon, we show that the frequently observed reduction and underestimation of J 0 , which is strongly pronounced at high injection levels, originate from neglecting the finite carrier diffusion coefficient. This work presents an analysis taking into account the finite carrier diffusion coefficient and thus enabling to evaluate charge carrier lifetime data at high injection levels leading to strongly increased consistency of the determined J 0 . Furthermore, we give a simple correction term for the recently published analysis leading to a total independence of a reasonably injection-independent Shockley–Read–Hall (SRH)-lifetime in the substrate. Combining the proposed methods, the determination of J 0 from numerically simulated lifetime-data is accurate within 3% of the input J 0 =100 fA/cm 2 over an injection range of 10 14 –3·10 16 cm −3 . The application of the proposed method to measured data of crystalline silicon devices is in excellent agreement with the simulations and shows a strongly improved linearity and thus constant extracted J 0 compared to the former and recently published methods, which underestimate J 0 at high injection levels, nearly independently from doping type and -level of the substrate. The new and simple method is implemented in a spreadsheet calculator enabling fast and robust data evaluation similar to the Sinton lifetime tester software.

The upgrade of state of the art p-type silicon solar cell production lines to passivated rear side technology (PERC) will be one of the major trends in the next years and new production processes for further cost reduction will continuously gain relevance. In 2007, we have introduced the laser based foil metallization technology “FolMet”: the rear electrode of p-type PERC devices as well as the local contact is fabricated by attaching conventional aluminum foil during the so-called laser fired contact process to the silicon wafer. This process features improved internal optical properties, a huge cost saving potential and a simplified cell production process. In this publication we focus on the acceleration of the laser process, which is together with module assembly issues a remaining challenge towards industrialization. We carried out comprehensive simulations, to better understand the correlation between different laser parameters on melting- and evaporation depth of the 8 μm thin aluminum foil. We determined lower limits for crucial laser pulse parameters to successfully attach the foil onto the substrate and validated these parameters experimentally. According to these results, we set up a system based on a pulsed high power laser featuring repetition rates Frep ≤ 2 MHz with an unique ultrafast polygon scanning system, allowing for scan-speeds vscan ≤ 1000 m/s. Thereby, we demonstrate processing times tpro < 0.8 s for industrial wafer, which corresponds to a reduction in laser process time by the factor of 20 compared to state of the art laser scanning technology.

The performance of PERC solar cells benefits from a smooth rear surface due to a reduced surface recombination and increased light trapping. State of the art surface roughness is defined solely by the polishing silicon removal. However the initial texture height can vary, therefore there is no universal definition of roughness. In this work roughness parameters from an earlier study will be used to determine the rear surface roughness of mono-crystalline industrial p-type solar cells [1]. The effects of rear side roughness on the passivation quality and thus minority carrier lifetime will be discussed. Additionally the influence of rear side roughness on reflection and therefore light trapping will be investigated. Furthermore solar cells with PERC concept are presented. Up to now used roughness parameters describe vertical structures and thus represent polishing results but they are not sensitive to horizontal structure expansion. Effective lifetime of polished samples first increases with decreasing roughness. For further polishing values stagnate or decrease. Implied VOC of saw damage etched surface is higher compared to polished surface although they show similar vertical roughness, hence there are more parameters influencing passivation quality than vertical roughness. According to roughness measurements nano-roughness, valleys and horizontal structure expansion seem to affect passivation quality in addition to vertical roughness. Decreasing roughness increases reflection at 1200 nm and thus increases light trapping. As the highest benefit in light trapping appears with decreasing structure height, vertical structure expansion seems to affect back reflection less than horizontal structures. Solar cell results cannot confirm the results of implied VOC and reflection measurements but prove the benefit of smooth rear sides. For polished polished rear sides the gain in efficiency is decreasing with increasing silicon removal.

In this work, a new approach for the front side metallization of silicon solar cells is presented. Molten solder (Sn96Ag3Cu) is directly printed in a non-contact manner on solar cell precursors via StarJet technology. The StarJet technology features a pneumatically driven, heatable printhead with a reservoir of molten metal and a star-shaped silicon nozzle chip. Using this printhead, a jet of molten metal with 55 µm ± 5 µm diameter is generated and used to apply busbars as well as contact fingers on prefabricated electroplated seed layers. After deposition via StarJet, printed fingers have a minimum width of 70 µm and a mean aspect ratio of 0.94. The printed metallization is evaluated optically and electrically. Aluminum back surface field silicon solar cells with front side electroplated NiAg seed layers and StarJet metallization (busbars and fingers) show efficiencies of up to 18.1% after degradation. Solder is about 30-40 times cheaper than silver and therefore may allow cost-efficient solar cell metallization. The StarJet metallization on electroplated NiAg seed layers is fully functional and requires no additional post-processing steps. Only 6 mg of Ag per cell are consumed for the seed layer. As a proof-of-principle, a module is demonstrated, which consists of four solar cells that are metallized via StarJet.

In this study, we use pump-probe microscopy to examine the melting and solidification dynamics of silicon during and after a UV laser pulse with a duration of 30ns. Below the ablation threshold, we observe lateral melt front contraction velocities of up to 600ms−1. The peak velocities spatially coincide with a ring of lower crystallinity within the formerly molten area, as we show with spatially resolved Raman spectroscopy.

Shingled interconnection of solar cells allows increased output power density pout by (i) increasing the active cell area within the module, (ii) decreasing shading losses, and (iii) reducing both series resistance per cell as well as interconnection losses. An additional increase in pout is possible by shingled interconnection using bifacial solar cells, allowing the light capture from the rear side as well. The ""p‐type silicon shingled passivated edge, emitter, and rear (pSPEER)"" solar cell concept introduced and examined in this work as an approach for fabrication of bifacial shingle solar cells is based on the passivated emitter and rear cell (PERC) concept. This article portrays bifacial pSPEER solar cells that are fabricated using industrial 6‐inch p‐type Czochralski‐grown silicon PERC precursors. After contact firing, the 6‐inch host wafers are separated by means of conventional laser scribing and manual mechanical cleaving into six pSPEER solar cells each with an area of 23 × 148 mm2. Current‐volt­age measure­ments of these bifacial pSPEER solar cells with front side illumination yield peak pout,f = 20.4 mW cm−2 on a black background (total cell area, busbar included). For an irradiance of 1000 W m−2, this pout,f is equivalent to an energy conversion efficiency of 20.4%. By additional rear side illumination with an irradiance of 100 W m−2, peak pout = 21.5 mW cm−2 is obtained for these pSPEER solar cells.

The concept of attaching a second deposition step at the end of boron tribromide (BBr3) diffusion is introduced, where second deposition describes an active ni­tro­gen flow through the BBr3 bubbler. This approach pro­vides a high­er boron dose in the borosilicate glass (BSG) which facilitates the formation of laser‐doped se­lec­tive emitters. It is found that the second deposition hardly impacts the as‐diffused charge carrier con­cen­tra­tion profile in comparison to BBr3 dif­fu­sion without second deposition. The emitter sheet re­sis­tance Rsh ≈ 110 O sq−1 and emitter dark saturation cur­rent den­si­ty j0e ≈ 25 fA cm−2 (alkaline textured, Al2O3/SiNX passi­va­tion) are similar for both processes. The BBr3 diffusion process forms a BSG/silicon dioxide (SiO2) stack layer on the silicon. The BBr3 diffusion with second deposition step results in an 8 nm thicker BSG/SiO2 stack layer (total thickness: 42 nm) with fac­tor two higher boron dose compared to the BBr3 diffusion without second deposition. After laser doping, the charge carrier concentration is higher for the BBr3 pro­cess with second deposition resulting in stronger local doping with about 10 O sq−1 lower Rsh. For laser‐doped and Al2O3/SiNX‐passivated areas, a promising process combination results in j0e = (250 ± 30) fA cm−2 at Rsh = (65 ± 1) O sq−1.

For bifacial applications, double-sided collecting solar cell structures can be beneficial. The recently introduced "BOth Sides COllecting and COntacted" (BOSCO) solar cell is such a structure and allows the use of standard module interconnection technology. The structure features emitter areas on both sides, which are interconnected by diffused vias. It favors the use of silicon substrates with low-to-medium diffusion length and low resistivity for the maximum benefit compared with other structures, such as Al-BSF and PERC. Within this study, we discuss the potential of the BOSCO cell structure and its applicability for certain silicon material types. Experimental results on different multicrystalline silicon (mc-Si) materials yield monofacial efficiencies (independently confirmed on a non-reflecting chuck) of 17.4% on large-area wafers from block-cast electronic-grade mc-Si and 16.9% for low-quality upgraded metallurgical-grade mc-Si. These values represent a gain of 0.6-0.7% (abs) compared with Al-BSF cells processed in parallel. The bifacial properties are investigated under outdoor testing conditions, yielding a gain in output power of 13% compared with monofacial operation.

For the extraction of spatially resolved solar cell parameters, a variety of methods has been introduced in the past years. Nearly all methods use the fact that the local luminescence intensity can be calibrated to local junction voltage. The different methods however use different approaches for the calibration. In this work we discuss the different approaches used throughout literature. We investigate the key assumption of two approaches which assumes that there are no lateral junction voltage gradients at sufficiently low excitation conditions. Our investigation is based on circuit simulations and experimental results on an example cell. We give an explanation for the remaining contrasts in luminescence images taken at these low excitation conditions. A third approach which does not rely on the key assumption is discussed with respect to measurement time in comparison to the other approaches. Finally the role of the short circuit current is discussed and a modification to the approach as known from literature is proposed.

The BOSCO (“ BO th S ides CO llecting and CO ntacted”) solar cell features a double-sided emitter and contact grids on both surfaces. The emitter region on the rear is connected to the front side by diffused vias. The structure allows the use of standard module interconnection technology and favours the use of silicon substrates with low to medium diffusion length and low resistivity for maximum benefit towards other structures, such as Al-BSF and PERC. Within this work, we summarize the latest results on multi-crystalline silicon (mc-Si). Monofacial efficiencies of 17.4% on large-area wafers from 9N (99.9999999% pure) block-cast mc-Si and 16.9% for low-quality 5N upgraded metallurgical-grade mc-Si have been achieved. These values represent a gain of 0.6 to 0.7% abs compared to Al-BSF cells processed in parallel. First tests of bifacial operation under outdoor conditions yield a gain in output power of 13% compared to monofacial operation. This effect makes the BOSCO solar cell concept a promising candidate to enable bifacial operation, even for low-cost wafers of low to medium diffusion length material.

Silicon Hetero Junction (SHJ) structures and ZnO:Al (AZO) were deposited completely by high throughput industrial inline CVD and PVD deposition systems, respectively. A design of experiment of the AZO deposition parameters was carried out, to find suitable AZO layers for the use in SHJ solar cells. A subsequential thermal treatment on the AZO layers was performed to investigate their performance in a solar cell production. Thin AZO layers with the lowest specific resistance of 0.69 10-3 Ωcm were achieved with low T annealing. The highest carrier mobility of 22 cm2 V-1 s-1 was reached with high temperature annealing.

The development of cost-effective and highly efficient solar cells is a primary goal of current research activities. In this work we investigate two different metallization concepts based on screen-printing technology that are aimed at achieving increased cell conversion efficiency of p-type silicon metal-wrap-through (MWT) solar cell structures. The first approach is the reduction of the pad area of the external rear n-type contacts. Furthermore a multilayer metallization concept is presented that allows for the decoupling of the rear external n-type contact and the p-type silicon base. In this context new pastes for via metallization are investigated-namely low-temperature silver-polymer pastes. The second approach is the optimization of the metallization layout of MWT solar cells to minimize cost of ownership. Therefore the number of contact rows, the finger width, and the number of n-type contact pads per contact row are varied methodically by using analytical simulations. The results are compared with experimental data. Both approaches show a potential for a further conversion efficiency increase of p-type silicon MWT solar cells.

In this paper, first generation back-contact back-junction (BC-BJ) silicon solar cells with cell efficiencies well above eta = 20% were fabricated. The process sequence is industrially feasible, requires only one high-temperature step (co-diffusion), and relies only on industrially available pattering technologies. The silicon-doping is performed from pre-patterned solid diffusion sources, which allow for the precise adjustment of phosphorus-and boron-doping levels. Based on the investigated process technologies, BC-BJ solar cells with gap and without gap between adjacent n+-and p+-doped areas were processed. On the one hand, a strong reduction of the process effort is possible by omitting the gap regions. On the other hand, parasitic tunneling currents through the narrow space charge region may occur. Hence, deep doped areas were realized to avoid tunneling currents in gap-free BC-BJ cells. This paper finishes with a detailed characterization of the manufactured cells including important cell measurements like I-V, SunsVOC, quantum efficiency, and an analysis of the cell specific fill factor losses.

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.

The loss analysis of state-of-the-art p-type Czochralski-grown silicon passivated emitter and rear solar cells (PERC) fabricated in a manner close to industrial production is presented in this paper. The 6-inch solar cells are featuring a homogeneous emitter on the front side, an Al2O3 passivation layer and local contacts on the rear side. The peak energy conversion efficiencies obtained are 21.1% for a standard antireflection coating (ARC) and 21.4% for a double-layer ARC. The loss analysis is based on an extended characterization of the solar cells and of special samples, which allow the separation of the contributions of each region of the solar cell including metallization. Their impact is determined experimentally on the open-circuit voltage, the short-circuit current density, and the fill factor. Based on the measurements, the devices are numerically simulated. The free energy losses are analyzed using the simulation model. Based on the results from the loss analysis, it is found that the integration of a selective emitter structure and the further improvement of the rear surface would allow for efficiencies above 22% in the short term.

This work points out that the application of phosphorus emitters with low surface concentration N surf of a few 10 19 cm -3 in combination with state-of-the-art screen-printed and fired silver contacts is no more limited by the specific contact resistance ρ C but by the dark saturation current densities underneath the metal contacts j 0,met . Eight emitter doping profiles have been designed with different N surf ranging between 3.3·10 19 cm -3 and 1.2·10 20 cm -3 which feature very similar junction depths of about 350 nm. The measured ρ C values for these emitters are determined to be between 2 mΩcm 2 and 5 mΩcm 2 for about 50 µm-wide contact fingers. Their emitter dark saturation current density (alkaline textured and passivated surface) range from 40 fA/cm² to 90 fA/cm². From these parameters and the current-voltage data of large-area Czochralski-grown silicon solar cells featuring an aluminum back surface field, we estimate j 0,met to be between 1200 fA/cm² and 3900 fA/cm². Our results show that for decreasing N surf , the recombination underneath the metal contacts increases significantly which mainly limits the cell’s open-circuit voltage. We thus conclude that the major challenge for emitters with low N surf consists in achieving reasonable low j 0,met values in order to benefit on the cell level from the lower recombination activity in the passivated, non-metallized surface area.

Although highly doped boron selective emitters (SEs) can be formed by using laser doping (LD) from the borosilicate glass remaining after boron tribromide tube diffusion, the dopant concentration is limited. This limitation can be overcome by using a laser-induced forward transfer (DLIFT) approach. In this work, SEs formed by DLIFT and LD are compared to a homogenous tube diffusion (Diff) emitter. We investigated the correlation of the emitter saturation current density ( j 0 e ) and the specific contact resistance $(\rho _{c})$ with the sheet resistance $(R_{{\rm{sheet}}})$ and the laser pulse energy density $(E_{p,d})$ . For passivated emitters, $j_{{0}e}$ of DLIFT and LD emitters was around ten times higher than $j_{{0}e}$ of Diff emitters. For metallized emitters, simulated $j_{{0}e}$ of DLIFT emitters was lower than $j_{{0}e}$ of LD and Diff emitters for $R_{{\rm{sheet}}}\,{\rm{ glt; \,40.0\,\Omega / sq}}$ . Moreover, we show how $j_{{0}e}$ can be further reduced by increasing the surface dopant concentration of the boron emitter and by reducing the laser-induced defects in the silicon crystal. Additionally, the metallization of DLIFT emitters with aluminum–silver paste by screen printing provided low contact resistances between metal and silicon ( ρc < 1.0 mΩ · cm2). Thus, p-type emitters can be optimized by forming an SE with the highly doped boron regions achieved by DLIFT under the screen-printed metallization.

We present different models for the internal resistance of passivated emitter and rear totally diffused (PERT) solar cells. First we apply the model of Gelmont and Shur for the spreading resistance of a passivated emitter and rear cell (PERC) structure with line-shaped contacts and adapt this model to account for the elliptic contact shape of Al-alloyed contacts. To account for the additional vertical current flow through the silicon wafer and the lateral current flow through the back surface field (BSF) of a PERT structure, we evaluate an alternative model using the analogy to the transfer length method (TLM) problem. For experimental verification, we have fabricated resistance test structures for various wafer resistivities, BSF doping levels and line distances (finger pitches) of the line-shaped contacts. Our experimental data shows good agreement with the presented models in distinct parameter ranges and thus offers a solid basis for analytical modelling of PERT-type solar cells.

The recombination induced by soldering pads, screen printed with a commercially available paste containing primarily silver and an aluminium amount of 1-5% wt , is investigated concerning the open-circuit voltage for aluminium back surface field (Al-BSF) solar cells. The saturation current density J o,pad due to soldering pads is found to be in the range of 26.7 10 3 to 85.4 10 3 fA/cm 2 for different conditions of the rear silicon surface. The recombination under soldering pads leads to a voltage drop of Δ V oc = 6.7 mV for Al-BSF solar cells with 6.2% of the rear side being covered with soldering pads compared to a cell with full-area Al-BSF. Within a microstructural analysis of the interface between the silicon surface and the soldering pads it is found that Al-alloying from the Al share in the soldering pad paste only occurs insularly. Furthermore, Al-BSF cells with and without residual rear side emitter and varying area fraction of soldering pads, using a soldering pad paste with and without an Al-share, are investigated. It is found that the non-overcompensated emitter regions under the soldering pads of Al-BSF solar cells affect cell performance dramatically in case of full area soldering pad, especially for an Al-free pad paste.

We investigate the formation of deep Phosphorus diffusion profiles with low surface concentrations in one single diffusion step. Such processes are suited for front surface field formation for n-type back-contact back-junction (BC- BJ) silicon solar cells or p-type silicon solar cells with alternative metallization techniques. The deposition temperature allows accurate control of the sheet resistance whereas the temperature of the in-situ oxidation strongly influences the profile-depth and surface concentration. The newly developed process leads to low dark saturation current densities well below 30 fA/cm 2 on alkaline-textured surfaces and low short circuit current loss at the front side. Due to their depth and adjustable surface concentration, the obtained profiles are promising for front-surface- fields (FSF) and novel n-type emitters. A first implementation in BC-BJ solar cells with aluminum-alloyed emitter shows a good blue response of the solar cells and an improved efficiency compared to the reference process by 0.6% abs .

Silicon-based films such as hydrogenated amorphous silicon (a-Si:H), nanocrystalline silicon (nc-Si:H), and hydrogenated amorphous silicon nitride (a-SiN x :H) can be deposited by hot-wire chemical vapor deposition (HW-CVD). The HW-CVD technology differs from conventional plasma-enhanced (PE)-CVD in a number of technological aspects, such as soft activation, high growth rates, and low system costs. To evaluate the HW-CVD technology for thin film deposition in solar industry an in-line hot-wire CVD system was used to deposit a-Si:H films for passivation of crystalline silicon solar cells as well as for the fabrication of thin film silicon solar cells. The HW-CVD system consists of seven vacuum chambers including three hot-wire systems with maximum deposition areas of 500 mm by 600 mm for each hot-wire activation source. The deposition processes were investigated by applying design of experiment to identify the effects and interactions of the process parameters on the deposition characteristics and film properties. The process parameters investigated were silane flow, deposition pressure, substrate temperature, film thickness, as well as temperature, diameter and number of wires, respectively. Growth rates up to 2.5 nm/s were achieved for a-Si:H films. Intrinsic a-Si:H films for passivation of different crystalline solar cell types yielded carrier lifetimes of more than 1000 μs for film thickness values below 20 nm. For n-doped a-Si:H films prepared with PH 3 as dopant gas, electrical resistivity is in the range of 10 2 Ω cm. P-doped a-Si:H films prepared with B 2 H 6 as dopant gas show electrical resistivity of about 10 5 Ω cm. Crystalline silicon heterojunction solar cells with intrinsic thin layer (HIT cells) exhibit energy conversion efficiencies of more than 17% when fabricated with intrinsic HW-CVD amorphous silicon films as passivation layers.

An approved measurement method for evaluating space charge region recombination (SCR-recombination) induced by the screen printed front side metallization is applied for industrial solar cells with aluminum back surface field (Al-BSF) on monocrystalline Czochralski silicon (Cz-Si). The H-patterned base grid of all processed solar cells is screen printed with a commercially available front side silver paste, ensuring sufficiently good contacting and therefore good electrical performance of the solar cells. Additionally, irregular contact fingers are realized between the regular contact fingers of the base grid by applying different screen printing layouts with commercially available front side pastes to increase the front side metallization fraction linearly as well as by applying various test pastes for the layout with the highest metallization fraction. Well-known components of glass frits as lead oxide, zinc oxide, bismuth oxide and boron oxide can affect SCR-recombination. Therefore we fabricated test pastes with these specific paste/glass frit components. By applying this test structure, a comparable qualitative evaluation of SCR-recombination with respect to the specific paste component is demonstrated. Quantitative evaluations regarding the electrical performance of the solar cells as well as related microstructural investigations are presented.

Photoluminescence (PL) imaging is a promising characterization technique for rating and sorting of multicrystalline silicon (mc-Si) as-cut wafers concerning to their material quality. It is inline applicable and yields high resolution images showing recombination active defects from crystallization which influence solar cell performance. In this contribution the basic concepts in ongoing work concerning relevant defects, rating results, statistics, algorithms and general approaches for the rating are summarized. Since 2009 wafers are sorted into five quality classes at Fraunhofer ISE. Details on the rating criteria are given and three examples for application are presented: (i) the impact of edge contaminations on the solar cell results, (ii) a comparison of wafer suppliers based on random samples from a statistical basis of 10,000 wafers and (iii) the results of an advanced rating and sorting of wafers for the manufacturing of highly efficient MWT-PERC solar cells. The results confirm that PL imaging can be used for a very precise rating of material quality.

The measurement of current-voltage (J-V) characteristics is one of the most straightforward methods for the characterization of solar cells. Consequently, an accurate knowledge of its meaning is of high relevance for the comprehension and technological feedback of these devices. The internal series resistance is one limiting parameter of the fill factor and the efficiency of these devices. A second limiting parameter is the p-n junction space charge region recombination. In this paper, we present a method to determine the lumped series resistance by combining the J-V characteristics in the dark and under 1-sun illumination. As a first approximation, the lumped series resistance under illuminated conditions is used for the dark J-V characteristic at small currents. Based on this, we present a method to quantify resistive losses and space charge region recombination only from the dark and illuminated J-V curves so that a simple separation of both losses becomes possible with all inline cell testers.

We present small-area metal wrap through solar cells with passivated rear side (MWT-PERC), screen-printed metallization, and a homogeneous industrial-like emitter showing an efficiency of 20.2%. A comparison of H-pattern reference cells and cells with aluminum back surface field confirms the advantages of the MWT-PERC approach and reveals significant synergistic effects of rear-surface passivation and wrap through metallization.

This paper is revisiting the problem of recombination on locally processed area (contacts, local doping …), based on the concept of point recombination rate (pLPA). The newly introduced effective point recombination rate (peff) can be easily determined from the effective surface recombination velocity. It also allows predictions and comparisons in most of the practical cases. Further analysis allows the separation of the influence of pLPA from the one of the transport of the carrier in a transparent way. Due to its simplicity, transparency and accuracy, the model proposed here is believed to be more suitable to recent local processing technology than the existing analytical models.

To enable a concentration control in the acidic texturing process we have closed the feedback loop from analytical data to the dosing mechanism of the used process tool. In order to analyze the process bath we used near-infrared spectroscopy in an online setup as well as ion chromatography as an inline method in a second approach. Using the developed dosing algorithm allows a concentration optimization of HF and HNO3 in dependence of the Si concentrations. This allows a further optimization of the acidic texturing process and leads to more homogeneous process results.

Cell concepts that feature via-holes, like the emitter wrap-through concept, suffer from higher series resistance losses than conventional solar cells. A part of these resistance losses is due to the fact that a small size element, in this case a via-hole, has to collect the current of a wide area on the front side, which leads to the so-called spreading resistance. An analytical equation already exists which describes the spreading resistance effects that are present in these cells. This equation is an accurate description for a quadratic or hexagonal pattern of via-holes. For cells that feature an interdigitated metallization grid on the rear side, like emitter wrap-through solar cells, the arrangement is often nonsquare because the distance between two via-holes is different in the x- and y-directions. In this paper, it is shown that the existing formula is in excellent correlation with the results from simulation program with integrated circuit emphasis (SPICE) simulation if a quadratic symmetry element is chosen. The SPICE simulation is then adjusted for nonsquare symmetry elements, showing that the existing formula is not valid for these arrangements and has to be modified. The derived analytical modification is adapted to yield a better description of the spreading resistance in the emitter of such a cell structure for variable via-hole distances in the x- and y-directions. The relative deviation between the simulation results and the analytic approach is less than 15%. This enables an accurate calculation of the resistance losses of relevant technological configurations.

Thermal silicon oxides are known to very effectively passivate silicon surfaces. Choosing a water vapor ambient instead of a dry oxygen atmosphere increases the oxidation rate by about one order of magnitude and considerably reduces process time and costs. State of the art pyrox systems produce steam by pyrolysis of hydrogen and oxygen gas. A new approach is the purification of vaporized deionized (DI) water. In this work we present a direct comparison of both steam generation systems, which are connected to the same quartz tube of an industrial high quality tube furnace. The higher steam saturation of the direct steam process enhances the growth rate by about 20% compared to a pyrolytic steam based process. On low-resistivity p-type substrates, excellent surface recombination velocities of around 25 cm/s are found for both systems after a forming gas anneal. Detailed characterization shows similar physical properties of the oxide layers grown by either steam from pyrol ysis or purified steam. Moreover, thermal oxide rear surface passivated silicon solar cells show similar high efficiencies exceeding 18.0% irrespective of the applied steam generation technology.

The firing of screen-printed aluminum pastes is well established for the formation of a back surface field (BSF) and back contacts since many years in silicon solar cell fabrication. In this paper we investigate the electrical and microstructural properties of Al-alloyed contacts and their layer system, consisting of (i) the Al-doped p+-layer (2–14 μm), the eutectic layer (1–15 μm) and the layer of paste residuals (20–100 μm). We show the influence of process parameters like the amount of printed paste, the alloying time and the peak temperature. Special emphasis is devoted to the properties of small alloyed screen-printed aluminum structures for the formation of local aluminum back contacts. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), electrochemical capacitance voltage (ECV) and conductivity measurements have been applied to characterize the samples. A simple model is qualitatively augmented to describe all effects occurring in a technical alloying process. For example, for increasing aluminum amounts, a saturation of the p+-layer thickness was found in the range of 10 mg/cm2. For small screen-printed structures, the p+-layer is formed very homogenously and with a greater thickness compared to samples with a full-area Al metallization, which have been processed with similar alloying conditions. For the eutectic layer a high electrical conductivity of about 16×106 S/m, only 2–3 times below that of pure aluminum has been determined. This is advantageous for the lateral conductivity especially for high paste amounts and small structures, which feature a relatively thick eutectic layer.

A series of hydrogenated microcrystalline silicon (μc-Si:H) p-layers for back surface field in crystalline silicon solar cells were deposited on glass substrates by the developed large area (45 cm×45 cm) plasma enhanced chemical vapour deposition processor operating at 13.56 MHz and various values of source gas trimethylboron (TMB) to H2 flowratio. The influence of deposition parameters on the large area p-layer performance was intensively studied, as well as the thin film uniformity, optical, electrical and structural performances by Raman, FTIR, Ellipsometry, etc. Arrhenius and Tauc plots were used to discuss the μc-Si:H thin film’s activation energy and the defects state distribution. When amorphous-microcrystalline transition state was obtained, the deposited p-doped μc-Si:H layers showed specific resistance of 38.3 Ω−1cm−1 at the flowratio of 0.66% and high crystallinity of 45%–50% with no further treatment. The effect of source gas flowratio, deposition rate, and source gas partial pressure on μc-Si:H thin film’s performance was also investigated.

Measuring the lifetime of excess charge carriers gives the opportunity to access the electric quality of the material. However, on as-cut wafers before production this quantity is strongly limited by the surface of the material. On a batch of solar cells we show that the open circuit voltage of the finished cells only scales with the lifetime, measured on as-cut wafers, if the material quality is very low. The difference between moderate and high material quality cannot be resolved. Using photoluminescence imaging the lifetime can be acquired with high spatial resolution. We show that by analyzing crystallization-related features in the images, certain defects can be identified: Such features are defects of crystal growth (e.g. dislocations) and areas of reduced lifetime that form at the edges of the crystallization crucible or near the top or bottom of a brick. Those features can be detected before production and we show their influence on cell parameters. By recognizing and rating these features, a more accurate quality control for wafers can be introduced.

A novel, industrially feasible metal wrap through (MWT) solar cell has been designed and realized utilizing a Si-oxide layer as diffusion barrier for separating the polarities on both sides and fine line screen-printing for the front contact grid. A maximum efficiency of 18.6% is measured under standard test conditions using the Am1.5 g spectrum (0.1 W/cm 2 ) according to the international standard (CEI/IEC 904-3). This is so far the highest conversion efficiency achieved by a silicon MWT solar cell. By adapting the grid for higher irradiation an efficiency of 19.5% at a concentration ratio of six and an efficiency exceeding 18% at 20 suns are achieved (each Am1.5 g).

During the manufacturing of crystalline silicon solar cells, several wet chemical etching and cleaning steps play an important role for the resulting solar cell efficiency. One important step especially for monocrystalline silicon is texturing by anisotropic alkaline etching which takes place in an aqueous solution of potassium hydroxide and isopropanol at a temperature between 70 and 80 °C. The evaporation of isopropanol and water, the consumption of potassium hydroxide and the accumulation of alkaline silicates change constantly the composition of the process bath. A replenishment of the converted chemicals is necessary to keep the texturisation results at a constant high level. To this day, dosing parameters are often based on operator's experience, but not on analytical data. In this paper, we investigated the suitability of near-infrared spectroscopy as a fast and cost-efficient method for the analysis of alkaline texturisation solutions. The effects of the chemical composition of the alkaline texturisation solution on the near-infrared spectra were investigated, as well as the influence of temperature variations and process induced gas bubbles in the circulation line. A partial least squares (PLS) model, which includes all the disturbing effects was created and validated in a semi-industrial scale texturing system. The development of the chemical concentrations was compared with reference measurements like titration and HPLC (high performance liquid chromatography) as well as those, calculated from the silicon weight loss of the etched silicon wafers.

We present an approach to predict the equilibrium temperature of hot spots in crystalline silicon solar cells based on the analysis of their temporal evolution right after turning on a reverse bias. For this end, we derive an analytical expression for the time-dependent heat diffusion of a breakdown channel that is assumed to be cylindrical. We validate this by means of thermography imaging of hot spots right after turning on a reverse bias. The expression allows to be used to extract hot spot powers and radii from short-term measurements, targeting application in inline solar cell characterization. The extracted hot spot powers are validated at the hands of long-term dark lock-in thermography imaging. Using a look-up table of expected equilibrium temperatures determined by numerical and analytical simulations, we utilize the determined hot spot properties to predict the equilibrium temperatures of about 100 industrial aluminum back-surface field solar cells and achieve a high correlation coefficient of 0.86 and a mean absolute error of only 3.3 K.We present an approach to predict the equilibrium temperature of hot spots in crystalline silicon solar cells based on the analysis of their temporal evolution right after turning on a reverse bias. For this end, we derive an analytical expression for the time-dependent heat diffusion of a breakdown channel that is assumed to be cylindrical. We validate this by means of thermography imaging of hot spots right after turning on a reverse bias. The expression allows to be used to extract hot spot powers and radii from short-term measurements, targeting application in inline solar cell characterization. The extracted hot spot powers are validated at the hands of long-term dark lock-in thermography imaging. Using a look-up table of expected equilibrium temperatures determined by numerical and analytical simulations, we utilize the determined hot spot properties to predict the equilibrium temperatures of about 100 industrial aluminum back-surface field solar cells and achieve a high correlation coefficient of 0...

We present 6-inch n-type Cz-Si metal wrap through (MWT) solar cells with screen-printed and fired contacts achieving energy conversion efficiencies of 20%. In order to decrease the occurrence of leakage currents under forward operation to a minimum after applying reverse bias load, the structuring of the rear side phosphorus doping in the area of the external p-type contacts is necessary. The fabrication of these so-called n-type high-performance MWT+ (n-HIP-MWT+) solar cells is considerably simplified by using laser processes to locally structure the rear side phosphorus doping. The n-HIP-MWT+ cells fabricated with laser structuring achieve the same peak energy conversion efficiency of 20% as conventionally fabricated ones, which are manufactured using an elaborate inkjet-based masking process prior to phosphorus diffusion. The loss of cell efficiency after reverse biasing is decreased to – 0.1% abs independent of the structuring method, and is three times smaller than the one observed for cells without structuring of the phosphorus doping.

Light-induced degradation (LID) of multicrystalline silicon (mc-Si) solar cell performance has been reported to be surprisingly strong for conditions relevant under field operation. In paticular, solar cells with dielectrically passivated rear sides such as passivated emitter and rear cells are affected by this LID effect that can cause a loss of more than 10%rel in cell efficiency. With the root cause for the observed degradation being unknown to date, the underlying defect, however, has also been reported to be permanently deactivated under the same conditions at even longer time scales. However, a severe power loss due to the mc-Si specific LID is observed before the cells recover due to the long time scales of the regeneration under these conditions. Hence, regeneration on short time scales similar to the fast regeneration processes being reported for the boron–oxygen defect within p-type Czochralski-grown silicon is highly desirable also for p-type mc-Si, especially since mc-Si currently dominates industrial solar cell production. Within this work, partial regeneration of the defect causing the mc-Si specific LID is shown to be possible within less than 30 s, reducing the impact of LID by up to 60%, which leads to a significantly increased performance of the regenerated mc-Si solar cells.