Black silicon solar cells with interdigitated back-contacts achieve 22.1% efficiency

The nanostructuring of silicon surfaces—known as black silicon—is a promising approach to eliminate front-surface reflection in photovoltaic devices without the need for a conventional antireflection coating. This might lead to both an increase in efficiency and a reduction in the manufacturing costs of solar cells. However, all previous attempts to integrate black silicon into solar cells have resulted in cell efficiencies well below 20% due to the increased charge carrier recombination at the nanostructured surface. Here, we show that a conformal alumina film can solve the issue of surface recombination in black silicon solar cells by providing excellent chemical and electrical passivation. We demonstrate that efficiencies above 22% can be reached, even in thick interdigitated back-contacted cells, where carrier transport is very sensitive to front surface passivation. This means that the surface recombination issue has truly been solved and black silicon solar cells have real potential for industrial production. Furthermore, we show that the use of black silicon can result in a 3% increase in daily energy production when compared with a reference cell with the same efficiency, due to its better angular acceptance.Peer reviewe

Recombination processes in passivated boron-implanted black silicon emitters

In this paper, we study the recombination mechanisms in ion-implanted black silicon (bSi) emitters and discuss their advantages over diffused emitters. In the case of diffusion, the large bSi surface area increases emitter doping and consequently Auger recombination compared to a planar surface. The total doping dose is on the contrary independent of the surface area in implanted emitters, and as a result, we show that ion implantation allows control of emitter doping without compromise in the surface aspect ratio. The possibility to control surface doping via implantation anneal becomes highly advantageous in bSi emitters, where surface passivation becomes critical due to the increased surface area. We extract fundamental surface recombination velocities Sn through numerical simulations and obtain the lowest values at the highest anneal temperatures. With these conditions, an excellent emitter saturation current (J0e) is obtained in implanted bSi emitters, reaching 20 fA/cm2 ± 5 fA/cm2 at a sheet resistance of 170 O/sq. Finally, we identify the different regimes of recombination in planar and bSi emitters as a function of implantation anneal temperature. Based on experimental data and numerical simulations, we show that surface recombination can be reduced to a negligible contribution in implanted bSi emitters, which explains the low J0e obtained.Postprint (published version

Passivation of black silicon boron emitters with atomic layer deposited aluminum oxide

The nanostructured surface – also called black silicon (b-Si) – is a promising texture for solar cells because of its extremely low reflectance combined with low surface recombination obtained with atomic layer deposited (ALD) thin films. However, the challenges in keeping the excellent optical properties and passivation in further processing have not been addressed before. Here we study especially the applicability of the ALD passivation on highly boron doped emitters that is present in crystalline silicon solar cells. The results show that the nanostructured boron emitters can be passivated efficiently using ALD Al2O3 reaching emitter saturation current densities as low as 51 fA/cm2. Furthermore, reflectance values less than 0.5% after processing show that the different process steps are not detrimental for the low reflectance of b-Si.Peer reviewe

N-type Black Silicon Solar Cells

Black silicon is an interesting surface texture for solar cells because of its extremely low reflectance on a wide wavelength range and acceptance angle. In this paper we present how black silicon (b-Si) texturization can be applied on the boron doped front surface of an n-type solar cell resulting in an efficiency of 18.7%. We show that the highly boron doped emitter can be formed on black silicon without losing its good optical properties and that atomic layer deposited aluminum oxide provides good surface passivation on these boron doped b-Si emitters.Peer reviewe

2D/3D Simulations of black-silicon interdigitated back-contacted c-si(n) solar cells

Black silicon (b-Si) reduces drastically light reflectance in the front side of c-Si solar cells to values near zero for the whole absorbed solar spectrum. In this work, we apply 2D and 3D simulations to explore the efficiency limits of interdigitated back-contacted c-Si(n) solar cells with line or point contacts respectively, using ALD Al2O3 passivated b-Si in the front surface. Realistic physical and technological parameters involved in a conventional oven-based fabrication process are considered in the simulations, especially those related to surface recombination on the b-Si as well as high doped p+/n+ strip regions. One important issue is the temporal stability of surface passivation on b-Si surfaces. In this work experimental long-term b-Si surface passivation data after two years and its impact on cell performance are studied. Simulations demonstrate initial and final photovoltaic efficiencies over 24.6% and 23.2% respectively for an emitter coverage of 80% independently of the cell contact strategy. A photocurrent loss about 1.3 mA/cm2 occurs when surface recombination velocity at the b-Si surfaces degrades from 6 cm/s to a final value of 28 cm/s.Postprint (author's final draft

Atomikerroskasvatetulla alumiinioksidilla passivoitu musta pii: sähköiset ominaisuudet ja soveltaminen tehokkaihin aurinkokennoihin

The term ’black silicon’ (bSi) refers to a particular silicon texture with surface features smaller than the wavelength of light in the ultraviolet to near-infrared range, which results in a reflectance close to zero. This material is consequently highly considered for solar cells applications. In those structures, carrier recombination can be critical due to the large surface area, and eventually affects the solar cell performance. This thesis proposes ways to improve control and understanding of recombination mechanisms in bSi. This work first introduces atomic layer deposition (ALD) processes for Al2O3 growth that rely on ozone to improve the standard water-based process. Significantly higher passivation quality is obtained with ozone-based processes and is shown to depend on ozone concentration. This can be explained by definite improvements in terms of electrical interface characteristics, which are studied in detail and discussed in relation with the chemical composition of the silicon/dielectric interface. While the quality of the dielectric film is essential for bSi passivation, bSi geometry also plays a decisive role. A relationship between bSi surface recombination velocity and surface charge is determined empirically; it indicates that the electric field in bSi is significantly stronger than in planar substrates. As a result, one type of charge carriers is efficiently repelled from the bSi surface, and surface recombination velocities below 7 cm/s are obtained in lowly-doped n-type bSi passivated by Al2O3. Such performance is typically relevant for interdigitated back-contact (IBC) solar cells, which rely on a lowly-doped and well-passivated front surface. IBC cells of nand p polarities are fabricated, and an external quantum efficiency close to 95 % is obtained in most of the sunlight spectrum due to low reflectance and effective surface passivation. This results in an excellent bSi solar cell efficiency of 22.1 %. Finally, recombination mechanisms are studied in highly-doped bSi for applications in front contact solar cells. This work shows that the excessive recombination often encountered in diffused emitters can be avoided by ion implantation, which enables strict control of dopant dose while preserving the low bSi reflectance, and by Al2O3 passivation. High-efficiency solar cells with front side emitter can thus also be envisaged.Musta pii kostuu piinanorakenteista ja vähentää huomattavasti valon heijastusta pinnalta. Musta pii onkin tullut viimeisinä vuosikymmeninä suosituksi tutkimuskohteeksi aurinkokennosovelluksissa. Mustassa piissä varauksenkuljettajien rekombinaatio kuitenkin lisääntyy suuremman pinta-alan takia, mikä heikentää aurinkokennon hyötysuhdetta. Tässä väitöskirjassa tutkitaan mustan piin rekombinaatioilmiöitä ja esitetään menetelmiä niiden vähentämiseksi. Työssä tutkitaan ensin atomikerroskasvatettuja aluminaohutkalvoja (Al2O3), joissa on käytetty otsonia lähtöaineena. Otsonipohjaisella prosessilla saavutetaan merkittävästi parempi passivointikyky kuin vesipohjaisella prosessilla. Tehokkaamman passivointikyvyn todettiin johtuvan piipinnan sähköisten ominaisuuksien paranemisesta, ja sen havaittiin riippuvan otsonipitoisuudesta. Väitöstutkimuksen perusteella Si/Al2O3-rajapinnan kemialliset ominaisuudet riippuvat hapetin-tyypistä ja vaikuttavat ohutkalvon passivoinnin tehokkuuteen. Lisäksi työssä kehitetään empiirinen kaava pinnan nanorakenteiden dimensioiden vaikutuksesta passivointitehokkuuteen, joka yhdistää pintarekombinaationopeuden ja pintavarauksen suuruuden. Al2O3-ohutkalvolla päällystetyn mustan piin pintavaraus aiheuttaa erittäin voimakkaan sähkökentän, mikä johtaa alle 7 cm/s pintarekombinaationopeuksiin heikosti seostetussa n-tyypin piissä. Tuloksia hyödynnetään takakontaktiaurinkokennoissa, jotka vaativat heikon etupinnan seostuksen ja tehokkaan pintapassivoinnin. Mustasta piistä valmistetuissa n- ja p-tyypin takakontaktikennoissa saavutetaan lähes 95 % ulkoinen kvanttihyötysuhde suurimmalla osalla auringonvalon spektrialuetta. Tämä vastaa 22.1 % kokonaishyötysuhdetta. Lopuksi tutkitaan boorilla seostetun mustan piin rekombinaatioilmiöitä. Diffuusiolla tehty seostus, joka on yleinen mikrovalmistustekniikka, aiheuttaa mustaan piihin voimakasta rekombinaatiota. Tässä työssä osoitetaan, että ioni-istutustekniikka mahdollistaa seostusannoksen tarkan kontrollin ja aiheuttaa vähemmän rekombinaatiota. Alumina-ohutkalvo edistää merkittävästi myös seostetun pinnan passivointia. Työssä esitelty menetelmä mahdollistaa siten korkean hyötysuhteen valmistamisen mustasta piistä myös etukontaktiaurinkokennoissa

Long-term stability of Al2O3 passivated black silicon

In this work we report on the long-term stability of black silicon surfaces passivated with atomic layer deposited (ALD) 20 nm thick Al2O3 films on p- and n-type FZ c-Si substrates. The results are directly compared with random pyramid textured counterparts. The effective surface recombination velocity Seff has been measured within a time frame of one year after activation of surface passivation. The results demonstrate that after an initial slight degradation during the first month Seff values stabilize around 45 and 25 cm/s on p- and n-type black silicon samples, respectively. These values are enough to guarantee stable high efficiency in interdigitated back-contacted (IBC) c-Si(n) solar cells (> 24.5%) using black silicon nanostructures on the front side. Similar, although weaker, losses are also observed in surface passivation on textured samples covered by Al2O3 with equal thickness, indicating that the origin of the instability might be independent of surface morphology.Peer reviewe

Effect of substrate pretreatments on the atomic layer deposited Al2O3 passivation quality

VK: T40310Peer reviewe

Effect of ozone concentration on silicon surface passivation by atomic layer deposited Al2O3

Abstract We study the impact of ozone-based Al2O3 Atomic Layer Deposition (ALD) on the surface passivation quality of crystalline silicon. We show that the passivation quality strongly depends on the ozone concentration: the higher ozone concentration results in lower interface defect density and thereby improved passivation. In contrast to previous studies, our results reveal that too high interface hydrogen content can be detrimental to the passivation. The interface hydrogen concentration can be optimized by the ozone-based process; however, the use of pure ozone increases the harmful carbon concentration in the film. Here we demonstrate that low carbon and optimal hydrogen concentration can be achieved by a single process combining the water- and ozone-based reactions. This process results in an interface defect density of 2 × 1011 eV−1 cm−2, and maximum surface recombination velocities of 7.1 cm/s and 10 cm/s, after annealing and after an additional firing at 800 °C, respectively. In addition, our results suggest that the effective oxide charge density can be optimized in a simple way by varying the ozone concentration and by injecting water to the ozone process.Peer reviewe