Aaltodoc Repository :: Browsing by Author "Pasanen, Toni"

Browsing by Author "Pasanen, Toni"

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Achieving surface recombination velocity below 10 cm/s in n-type Germanium using ALD Al2O3
(American Institute of Physics, 2021-11-01) Isometsä, Joonas; Fung, John; Pasanen, Toni; Liu, Hanchen; Yli-Koski, Marko; Vähänissi, Ville; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin Group
Desirable intrinsic properties, namely, narrow bandgap and high carrier mobility, make germanium (Ge) an excellent candidate for various applications, such as radiation detectors, multi-junction solar cells, and field effect transistors. Nevertheless, efficient surface passivation of Ge has been an everlasting challenge. In this work, we tackle this problem by applying thermal atomic layer deposited (ALD) aluminum oxide (Al2O3), with special focus on the process steps carried out prior to and after dielectric film deposition. Our results show that instead of conventional hydrofluoric acid (HF) dip, hydrochloric acid (HCI) pre-treatment is an essential process step needed to reach surface recombination velocities (SRVs) below 10 cm/s. The main reason for efficient surface passivation is found to be a high dielectric charge that promotes the so-called field-effect passivation. Furthermore, the results demonstrate that the post-deposition anneal temperature, time, and ambient play a role in passivating Ge-dangling bonds, but surprisingly, good surface passivation (SRV below 26 cm/s) is obtained even without any post-deposition annealing. The results pave the way for high-performance n-type Ge optoelectronic devices that could use induced junctions via negatively charged Al2O3 layers.
Alpha particle silicon detectors based on induced junction
(2021-01-25) Setälä, Olli; Pasanen, Toni; Sähkötekniikan korkeakoulu; Savin, Hele
Detection of alpha particles is an important safety measure in applications producing alpha radiation. Silicon-based alpha detectors have taken their place in the industry due to their desirable properties, such as small size and inexpensive manufacturing. The operation principle of the silicon detectors has traditionally based on a p-n junction created by external doping. However, doping results in an area of high recombination near the surface of the detector often called a dead layer. The energy resolution and efficiency of the detector suffer from the dead layer and thus it is rather surprising that this issue has not been solved yet in state-of-the-art detectors. In this thesis, a novel idea to overcome the above-mentioned dead layer issue is proposed. Instead of using external doping, the junction could be realized as an induced junction using a highly charged dielectric layer. The main goal is to study whether this idea is realistic and to develop a possible fabrication process for such induced junction alpha detectors. First, the principles of alpha particle detection are reviewed to recognize the most important aspects and a baseline alpha detector fabrication process is built. Leakage current is identified as an especially important property, and hence additional test measurements are performed to optimize it. The findings are then applied to create a more advanced process flow. Finally, both process flows are used to fabricate the actual detectors, which are then characterized with IV/CV and alpha spectroscopy measurements. The results show that functional alpha detectors can be fabricated without a need for external doping using the proposed idea for the induced junction technology. In fact, multiple induced junction detectors reach leakage currents close to 50 pA/mm2, which compares well with detectors produced by the market leaders. Test measurements show that the induced junction has a hundred times lower dark surface saturation current than implanted junction. Preliminary alpha spectroscopy demonstrates that the detectors can clearly distinguish particles emitted by different alpha emitters although the used setup was insufficient for accurate determination of energy resolution. The detectors are currently being measured in an industrial setting, which will provide accurate device performance data. Nevertheless, the leakage currents already indicate that utilizing the induced junction could be a promising option for achieving improved energy resolution with alpha detectors.
Black Silicon
(2020-04-24) Pasanen, Toni; Hele Savin Group; Department of Electronics and Nanoengineering
Black silicon back contact module with wide light acceptance angle
(Wiley, 2020-03-01) Ortega, Pablo; Garin, Moises; von Gastrow, Guillaume; Savisalo, Tuukka; Tolvanen, Antti; Vahlman, Henri; Vähänissi, Ville; Pasanen, Toni; Carrio, David; Savin, Hele; Alcubilla, Ramon; Department of Electronics and Nanoengineering; Hele Savin Group; Polytechnic University of Catalonia; Valoe Oyj; Endeas Oy
In this work a photovoltaic mini-module combining interdigitated back contacted solar cells with black silicon in the front was implemented as a proof of concept. The module consists of nine solar cells connected in series with an active area of 86.5 cm2. Both the assembly and panel encapsulation were made using industrial back contact module technology. Noticeable photovoltaic efficiencies of 18.1% and 19.9% of the whole module and the best individual cell of the module respectively, demonstrate that fragile nanostructures can withstand standard module fabrication stages. Open circuit voltage and fill factor values of 5.76 V and 81.6% respectively reveal that series interconnection between cells works as expected, confirming a good ohmic contact between cell busbars and the conductive back sheet. Additionally, the excellent quasi-omnidirectional anti-reflection properties of black silicon surfaces prevails at module-level, as it is corroborated by light incidence angle dependence measurements of the short circuit current parameter.
Black ultra-thin crystalline silicon wafers reach the 4n2 absorption limit – application to IBC solar cells
(Wiley-VCH Verlag, 2023-09-27) Garin, Moises; Pasanen, Toni; López, G.; Vähänissi, Ville; Chen, Kexun; Martín, I.; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin Group; Polytechnic University of Catalonia
Cutting costs by progressively decreasing substrate thickness has been a common theme in the crystalline silicon PV industry for the last decades, since drastically thinner wafers would significantly reduce the substrate-related costs. In addition to the technological challenges concerning wafering and handling of razor-thin flexible wafers, a major bottleneck is to maintain high absorption in those thin wafers. For the latter, advanced light-trapping techniques become of paramount importance. Here we demonstrate that by applying state-of-the-art black-Si nanotexture produced by DRIE on thin uncommitted wafers, the maximum theoretical absorption (Yablonovitch’s 4n2 absorption limit), i.e. ideal light trapping, is reached with wafer thicknesses as low as 40 µm, 20 µm and 10 µm when paired with a back reflector. Due to the achieved promising optical properties the results were implemented into an actual thin interdigitated back contacted solar cell. The proof-of-concept cell, encapsulated in glass, achieved a 16.4% efficiency with an JSC = 35 mA/cm², representing a 43% improvement in output power with respect to the reference polished cell. These results demonstrate the vast potential of black silicon nanotexture in future extremely-thin silicon photovoltaics.
Black-silicon ultraviolet photodiodes achieve external quantum efficiency above 130%
(American Physical Society, 2020-09-08) Garin, Moises; Heinonen, Juha; Werner, Lutz; Pasanen, Toni; Vähänissi, Ville; Haarahiltunen, Antti; Juntunen, M.; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin Group; Physikalisch-Technische Bundesanstalt; ElFys Inc.
At present ultraviolet sensors are utilized in numerous fields ranging from various spectroscopy applications via biotechnical innovations to industrial process control. Despite of this, the performance of current UV sensors is surprisingly poor. Here, we break the theoretical one photon - one electron barrier and demonstrate a device with a certified external quantum efficiency (EQE) above 130 external amplification. The record high performance is obtained using a nanostructured silicon photodiode with self-induced junction. We show that the high efficiency is based on effective utilization of multiple carrier generation by impact ionization taking place in the nanostructures. While the results can readily have a significant impact on the UV-sensor industry, the underlying technological concept can be applied to other semiconductor materials, thereby extending above unity response to longer wavelengths and offering new perspectives for improving efficiencies beyond the Shockley-Queisser limit.
Boron-implanted black silicon photodiode with close-to-ideal responsivity from 200 to 1000 nm
(American Chemical Society, 2023-06-21) Setälä, Olli; Chen, Kexun; Pasanen, Toni; Liu, Xiaolong; Radfar, Behrad; Vähänissi, Ville; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin Group
Detection of UV light has traditionally been a major challenge for Si photodiodes due to reflectance losses and junction recombination. Here we overcome these problems by combining a nanostructured surface with an optimized implanted junction and compare the obtained performance to state-of-the-art commercial counterparts. We achieve a significant improvement in responsivity, reaching near ideal values at wavelengths all the way from 200 to 1000 nm. Dark current, detectivity, and rise time are in turn shown to be on a similar level. The presented detector design allows a highly sensitive operation over a wide wavelength range without making major compromises regarding the simplicity of the fabrication or other figures of merit relevant to photodiodes.
Comparison of SiNx-based Surface Passivation Between Germanium and Silicon
(WILEY-VCH VERLAG, 2023-01) Liu, Hanchen; Pasanen, Toni; Fung, John; Isometsä, Joonas; Leiviskä, Oskari; Vähänissi, Ville; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin Group
Germanium (Ge) has attracted much attention as a promising channel material in nanoscale metal-oxide-semiconductor devices and near-infrared sensing because of its high carrier mobilities and narrow bandgap, respectively. However, efficient passivation of Ge surfaces has remained challenging. Herein, silicon nitride (SiNx)-based passivation schemes on Ge surfaces are studied and the observations are compared to Si counterparts. These results show that instead of a high positive charge density (Q(tot)) that is found in SiNx-passivated Si samples, similar Ge samples contain a high amount of negative Q(tot) (in the range of 10(12 )cm(-2)). The maximum surface recombination velocity of the samples is shown to reduce by a factor of three in both Si and Ge samples by a post-deposition anneal at 400 degrees C. The SiNx-coated samples are capped with an atomic-layer-deposited aluminum oxide (Al2O3) layer, which reduces the midgap interface defect density (D-it) after annealing to 7 x 10(10) and 4 x 10(11) cm(-2) eV(-1) in Si and Ge, respectively. Interestingly, while the Al2O3 capping seems to have no impact on Q(tot) of the Si samples, it turns the stack virtually neutral (similar to-1.6 x 10(11) cm(-2)) on Ge. The presented SiNx-based passivation schemes are promising for optoelectronic devices, where a low D-it and/or a low charge are favored.
Defect engineering in black silicon
(Aalto University, 2019) Pasanen, Toni; Elektroniikan ja nanotekniikan laitos; Department of Electronics and Nanoengineering; Electron Physics Group; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Savin, Hele, Prof., Aalto University, Department of Electronics and Nanoengineering, Finland
Black silicon (b-Si), i.e. a nanostructured silicon surface, is currently a subject of great interest within the photovoltaics (PV) community due to its excellent optics. While PV-related b-Si research has mainly focused on the reduction of reflectance and surface recombination, other possible effects of nanostructured surfaces have received less attention. This thesis investigates b-Si from a wider perspective and concentrates on engineering of surface and bulk defects in b-Si solar cells. The thesis first focuses on b-Si surfaces. It is demonstrated that no trade-offs are required between the optical and electrical properties of wet-chemically fabricated b-Si by the application of atomic-layer-deposited (ALD) aluminum oxide (Al2O3) surface passivation. The current mainstream solar cell architectures, however, have a phosphorus-doped emitter on the front, and thus, the negatively-charged Al2O3 is non-optimal. This work addresses the issue by using positively-charged ALD SiO2/Al2O3 stacks, which result in reduced recombination at diffused b-Si phosphorus emitter surfaces. In addition to affecting surface passivation, heavy phosphorus doping is shown to accelerate the consumption of silicon in standard cleaning solution, which strongly impacts both electrical and optical properties of b-Si emitters. All these results need to be considered in the design of high-efficiency b-Si solar cells. The second main theme of the work is engineering of bulk-related phenomena by b-Si. The nanostructures are shown to enhance gettering of detrimental metal impurities. Indeed, intentionally iron-contaminated b-Si wafers have more than three times higher minority carrier lifetime than polished samples after gettering (720 µs vs. 200 µs). In addition to impurity gettering, the impact of b-Si on another important bulk phenomenon, i.e., light-induced degradation (LID), is demonstrated. Black multicrystalline (mc-Si) passivated emitter and rear cells (PERC) show no or only slight degradation under illumination at elevated temperature, while standard acidic-textured equivalents suffer from severe LID. The increased gettering efficiency and reduced LID clearly demonstrate that benefits provided by b-Si are not limited only to the excellent optics. Finally, dry-etched b-Si is applied to industrial mc-Si PERC solar cells and modules. The fragile nanostructure is demonstrated to remain intact through cell and module fabrication at industrial production lines. Indeed, the prototype modules appear uniformly black after processing without anti-reflection coatings. Furthermore, the b-Si modules are shown to retain their performance until an incidence angle of 60°, whereas the modules with standard acidic-textured cells start to lose their performance already after a 30° tilt. Hence, the results demonstrate that the optical and electrical properties of b-Si can be maintained also at module level.
Effect of MACE Parameters on Electrical and Optical Properties of ALD Passivated Black Silicon
(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2019-07-01) Chen, Kexun; Pasanen, Toni; Vähänissi, Ville; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin Group
Metal-assisted chemical etching (MACE) enables efficient texturing of diamond-wire sawn multicrystalline silicon (mc-Si) wafers. However, the excellent optics are often sacrificed by polishing the surface to achieve better surface passivation with chemical-vapor-deposited (CVD) silicon nitride (SiNx). In this work, we show that a polishing step is not required when CVD SiNx is replaced with atomic-layer-deposited (ALD) aluminum oxide (Al2O3). Indeed, while polishing increases reflectance, it has in general only very modest effect on surface recombination velocity of ALD-passivated b-Si. Furthermore, since ALD Al2O3 is compatible with various surface morphologies due to its excellent conformality, the MACE parameters can be more freely adjusted. First, the concentration of silver nitrate (AgNO3) in AgNO3/H2O solution that is used to deposit Ag nanoparticles is shown to affect the final b-Si morphology. Instead of needle-shaped b-Si produced by 5 mmol/L AgNO3 concentration, two orders of magnitude lower AgNO3 concentration produces porous structures, which are more challenging to passivate. Additionally, we demonstrate that a separate Ag nanoparticle removal step in nitric acid (HNO3) is not a prerequisite for high carrier lifetime. Instead, Ag nanoparticles present during polishing in a HF/HNO3/H2O solution affect the final b-Si morphology by accelerating the etching of Si. The results demonstrate that no trade-offs are necessary between optical and electrical properties of MACE b-Si when using ALD.
Efficient surface passivation of black silicon using spatial atomic layer deposition
(Elsevier, 2017-09-21) Heikkinen, Ismo; Repo, Päivikki; Vähänissi, Ville; Pasanen, Toni; Malinen, Ville; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin Group; Beneq Oy
Nanostructured silicon surface (black silicon, b-Si) has a great potential in photovoltaic applications, but the large surface area requires efficient passivation. It is well known that b-Si can be efficiently passivated using conformal Atomic Layer Deposited (ALD) Al2O3, but ALD suffers from a low deposition rate. Spatial ALD (SALD) could be a solution as it provides a high deposition rate combined with conformal coating. Here we compare the passivation of b-Si realized with prototype SALD tool Beneq SCS 1000 and temporal ALD. Additionally, we study the effect of post-annealing conditions on the passivation of SALD coated samples. The experiments show that SALD passivates b-Si surfaces well as charge carrier lifetimes up to 1.25 ms are obtained, which corresponds to a surface recombination velocity Seff,max of 10 cm/s. These were comparable with the results obtained with temporal ALD on the same wafers (0.94 ms, Seff,max 14 cm/s). This study thus demonstrates high-quality passivation of b-Si with industrially viable deposition rates.
Efficient surface passivation of germanium nanostructures with 1% reflectance
(Institute of Physics Publishing, 2023-08-27) Fung, John; Isometsä, Joonas; Lehtiö, Juha Pekka; Pasanen, Toni; Liu, Hanchen; Leiviskä, Oskari; Laukkanen, Pekka; Savin, Hele; Vähänissi, Ville; Department of Electronics and Nanoengineering; Hele Savin Group
Germanium (Ge) is a vital element for applications that operate in near-infrared wavelengths. Recent progress in developing nanostructured Ge surfaces has resulted in > 99 % absorption in a wide wavelength range (300 nm- 1700 nm), promising unprecedented performance for optoelectronic devices. However, excellent optics alone is not enough for most of the devices (e.g. PIN photodiodes and solar cells) but efficient surface passivation is also essential. In this work, we tackle this challenge by applying extensive surface and interface characterization including transmission electron microscopy and X-ray photoelectron spectroscopy, which reveals the limiting factors for surface recombination velocity of the nanostructures. With the help of the obtained results, we develop a surface passivation scheme consisting of atomic-layer-deposited aluminum oxide and sequential chemical treatment. We achieve surface recombination velocity as low as 30 cm/s combined with ~1 % reflectance all the way from ultraviolet to NIR. Finally, we discuss the impact of the achieved results on the performance of Ge-based optoelectronic applications, such as photodetectors and thermophotovoltaic cells.
Excellent Responsivity and Low Dark Current Obtained with Metal-Assisted Chemical Etched Si Photodiode
(IEEE, 2023-04-01) Chen, Kexun; Setälä, Olli; Liu, Xiaolong; Radfar, Behrad; Pasanen, Toni; Serue, Michael; Heinonen, Juha; Savin, Hele; Vähänissi, Ville; Hele Savin Group; Department of Electronics and Nanoengineering; ElFys Inc.; Department of Electronics and Nanoengineering
Metal-assisted chemical etched (MACE; also known as MacEtch or MCCE) nanostructures are utilized widely in the solar cell industry due to their excellent optical properties combined with a simple and cost-efficient fabrication process. The photodetection community, on the other hand, has not shown much interest toward MACE due to its drawbacks, including insufficient surface passivation, increased junction recombination, and possible metal contamination, which are especially detrimental to p-n photodiodes. Here, we aim to change this by demonstrating how to fabricate high-performance MACE p-n photodiodes with above 90% external quantum efficiency (EQE) without external bias voltage at 200-1000 nm and dark current less than 3 nA/cm2 at -5 V using industrially applicable methods. The key is to utilize an induced junction created by an atomic layer deposited (ALD) highly charged Al2O3 thin film that simultaneously provides efficient field-effect passivation and full conformality over the MACE nanostructures. Achieving close to ideal performance demonstrates the vast potential of MACE nanostructures in the fabrication of high-performance low-cost p-n photodiodes.
Grass-like alumina coated window harnesses the full omnidirectional potential of black silicon photodiodes
(OPTICAL SOC AMER, 2021-11-20) Kauppinen, Christoffer; Pasanen, Toni; Isakov, Kirill; Serue, Michael; Heinonen, Juha; Vähänissi, Ville; Lipsanen, Harri; Savin, Hele; Department of Electronics and Nanoengineering; Harri Lipsanen Group; Hele Savin Group; ElFys Inc.
Packaged photodiodes suffer from Fresnel reflection from the package window glass, especially at high angles of incidence. This has a notable impact particularly on black silicon (b-Si) photodiodes, which have extreme sensitivity. In this work, we show that by adding a simple grass-like alumina antireflection (AR) coating on the window glass, excellent omnidirectional sensitivity and high external quantum efficiency (EQE) of b-Si photodiodes can be retained. We demonstrate that EQE increases at all angles, and up to 15% absolute increases in EQE at a 70° angle of incidence compared to conventional uncoated glass. Furthermore, even at the incidence angle of 50°, the double-sided coating provides higher EQE than bare glass at normal incidence. Our results demonstrate that grass-like alumina coatings are efficient and omnidirectional AR coatings for photodiode package windows in a wide wavelength range across the visible spectrumto near-infrared radiation.
High-sensitivity NIR photodiodes using black silicon
(SPIE - The International Society for Optical Engineering, 2020) Heinonen, Juha; Haarahiltunen, Antti; Serue, Michael; Vähänissi, Ville; Pasanen, Toni; Savin, Hele; Werner, Lutz; Juntunen, Mikko; ElFys Inc.; Department of Electronics and Nanoengineering; Hele Savin Group; Physikalisch-Technische Bundesanstalt
There is an increasing demand for highly sensitive near infrared (NIR) detectors due to many rapidly growing application areas, such as LiDAR and optical communications. Despite the limited NIR absorption, silicon is a common substrate material in NIR detectors due to low cost and maturity of the technology. To boost the NIR performance of silicon devices, one option is texturizing the front and/or back surface to reduce reflectance and extend the optical path by scattering. Here we demonstrate silicon photodiodes with nanostructured front surface (i.e. black silicon fabricated with reactive ion etching (RIE) that exhibit significantly enhanced external quantum efficiency (EQE) at NIR wavelengths compared to typical state-of-the-art silicon photodiodes. The detectors exhibit over 90% EQE with wavelengths up to 1040 nm and above 60% at 1100 nm. Identical detectors with a planar surface are also investigated revealing that the enhancement from black silicon effectively corresponds to increasing the substrate thickness up to 43% at 1100 nm depending on the thickness of the active layer and back surface structure. This confirms that in addition to reduced reflectance, scattering of transmitted light induced by black silicon plays a key role in the EQE enhancement which benefits especially devices such as backside illuminated photodetectors where very thin substrates are required. We also demonstrate that the high EQE of the black silicon detectors is maintained at incidence angles up to 60 degrees allowing excellent performance in applications where the light is not always perpendicular
Impact of Black Silicon on Light- and Elevated Temperature-Induced Degradation in Industrial Passivated Emitter and Rear Cells
(2019-10-21) Pasanen, Toni; Modanese, Chiara; Vähänissi, Ville; Laine, Hannu; Wolny, Franziska; Oehlke, Alexander; Kusterer, Christian; Heikkinen, Ismo T. S.; Wagner, Matthias; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin Group; SolarWorld Industries GmbH; SolarWorld Innovations GmbH
Light and elevated-temperature induced degradation (LeTID) is currently a severe issue in passivated emitter and rear cells (PERC). In this work, we study the impact of surface texture, especially a black silicon (b-Si) nanostructure, on LeTID in industrial p-type mc-Si PERC. Our results show that during standard LeTID conditions the b-Si cells with atomic-layer-deposited aluminum oxide (AlOx) front surface passivation show no degradation despite the presence of a hydrogen-rich AlOx/SiNx passivation stack on the rear. Furthermore, b-Si solar cells passivated with silicon nitride (SiNx) on the front lose only 1.5 %rel of their initial power conversion efficiency, while the acidic-textured equivalents degrade by nearly 4 %rel under the same conditions. Correspondingly, clear degradation is visible in the IQE of the acidic-textured cells, especially in the ~850–1100 nm wavelength range confirming that the degradation occurs in the bulk, while the IQE remains nearly unaffected in the b-Si cells. The observations are supported by spatially-resolved photoluminescence (PL) maps, which show a clear contrast in the degradation behavior of b-Si and acidic-textured cells, especially in the case of SiNx front surface passivation. The PL maps also suggest that the magnitude of LeTID scales with surface area of the texture, rather than wafer thickness that was recently reported, although the b-Si cells are slightly thinner (140 vs. 165 µm). The results indicate that b-Si has a positive impact on LeTID, and hence, benefits provided by b-Si are not limited only to the excellent optical properties, as commonly understood.
Improved emitter performance of RIE black silicon through the application of in-situ oxidation during POCl3 diffusion
(Elsevier Science B.V., 2020-06-15) Fung, Tsun Hang; Pasanen, Toni; Zhang, Yu; Soeriyadi, Anastasia; Vähänissi, Ville; Scardera, Giuseppe; Payne, David; Savin, Hele; Abbott, Malcolm; Department of Electronics and Nanoengineering; Hele Savin Group; University of New South Wales; Macquarie University
Nano-texture has the potential to reduce the optical losses of crystalline silicon solar cells. RIE fabricated black silicon enables near zero reflectance across a broad range of wavelengths and the angular dependence has been shown to be superior to existing technologies. However, in front-contact cells which are the current industrial mainstream architecture, the emitter is located on the front textured side and is typically realized by POCl3 diffusion. The interaction of this process with the nano-texture is complex, which makes it challenging to optimise the electrical performance of the phosphorus emitter. This paper studies the impact of in-situ oxidation during emitter formation to the electrical performance of a POCl3 diffused RIE nanotextured emitter surface. Additional corona charge was applied on the ALD SiO2/Al2O3 stack to avoid the limitation on the emitter performance due to non-ideal surface passivation conditions. After saturation with surface charge, the results demonstrate in-situ oxidation to be an effective technique to improve the electrical performance. An emitter recombination factor of 147 fA/cm2 was achieved for a 127 Ω/□ emitter formed on reactive-ion etched sample with surface area enhancement factor and effective slope index of 4.19 and 1.63, respectively. Further paths for improvement are identified, particularly relating to the collection of carriers generated by short wavelength light and how that relates to the shape of the texture used.
Industrial Applicability of Antireflection-Coating-Free Black Silicon on PERC Solar Cells and Modules
(2018) Pasanen, Toni; Vähänissi, Ville; Wolny, Franziska; Oehlke, Alexander; Wagner, Matthias; Juntunen, Mikko A.; Heikkinen, Ismo T. S.; Salmi, Emma; Sneck, Sami; Vahlman, Henri; Tolvanen, Antti; Hyvärinen, Jaakko; Savin, Hele; Hele Savin Group; Aalto Nanofab; SolarWorld Industries GmbH; SolarWorld Innovations GmbH; Naps Solar Systems Inc.; Beneq Oy; Department of Electronics and Nanoengineering; Endeas Oy
Black silicon (b-Si) is of particular interest within the photovoltaic community due to its ability to texture diamond wire-sawn multicrystalline silicon wafers. In this work, we apply a deep dry-etched black silicon nanotexture, which possesses less than 1 % solar-weighted reflectance with no antireflection coating (ARC), to industrial Passivated Emitter and Rear Cells (PERC). Additionally, the cells are processed further into small solar modules, which are characterized by their electrical and optical performance. The fragile nanostructures remain intact during industrial cell and module fabrication, maintaining the excellent optical properties until the final product. We show that b-Si modules with a typical cover glass retain their performance until incident angles larger than 60°, whereas the heavily increased reflectance of acidic-textured modules decreases their efficiency already after a 30° tilt. Furthermore, at an incidence angle of 70°, the efficiency of b-Si modules has reduced only 7 %, while that of the acidic-textured equivalents has decreased more than 25 %. Hence, the excellent optical properties of ARC-free b-Si are maintained also at module level. The results demonstrate that deep dry-etched b-Si nanostructures are fully applicable to current industrial PERC production facilities.
Millisecond-Level Minority Carrier Lifetime in Femtosecond Laser-Textured Black Silicon
(IEEE, 2022-08-15) Liu, Xiaolong; Radfar, Behrad; Chen, Kexun; Pälikkö, Elmeri; Pasanen, Toni; Vähänissi, Ville; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin Group
Femtosecond laser-textured black silicon (fs-bSi) is known to suffer from heavy minority carrier recombination resulted from laser irradiation. In this paper, we demonstrate that the thermal annealing step, generally used to recover the crystal damage, could improve the minority carrier lifetime of the fs-bSi wafers only from 8 μs to 12 μs, even when using as high temperature as 800 °C. However, with an optimized wet chemical etching process, we obtain a high minority carrier lifetime of 2 ms without sacrificing the optical properties of the samples, i.e., the absorptance remains above 90% in the studied wavelength range (250–1100 nm). Increasing the etching time further leads to a total recovery of the lifetime up to 10.5 ms, which proves that the damage originating from the fs-laser texturing extends only to the near-surface layer (a few μm) of silicon.
Modeling Field-effect in Black Silicon and its Impact on Device Performance
(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2020-04-01) Heinonen, Juha; Pasanen, Toni; Vähänissi, Ville; Juntunen, Mikko; Savin, Hele; Hele Savin Group; Department of Electronics and Nanoengineering; ElFys Inc.
Black silicon (b-Si) has improved the performance of solar cells and photodetectors due to the excellent optics and surface passivation achieved with atomic layer deposition (ALD) dielectric films. One major reason for the success is the strong field effect caused by the high density of fixed charges present in the dielectric. Depending on the device, the field effect can be utilized also in a more active role than for mere surface passivation, including the formation of floating and/or induced junctions in silicon devices. However, in order to utilize the field effect efficiently, a deeper understanding of the thin-film charge-induced electric field and its effects on charge carriers in b-Si is required. Here, we investigate the field effect in b-Si using the Silvaco Atlas semiconductor device simulator. By studying the electric field and charge-carrier profiles, we develop a model where the electrical properties of b-Si can be approximated with a planar surface, which significantly simplifies the device-level simulations. We validate the model by simulating the spectral response of a b-Si -induced junction photodiode achieving less than 1% difference compared with experimental device performance in a wide range of wavelengths. Finally, we apply the model to study how variation in surface recombination velocity affects the short-wavelength sensitivity and dynamic range in a b-Si photodiode.

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