The Influence of ITO Dopant Density on J-V Characteristics of Silicon Heterojunction Solar Cells: Experiments and Simulations

The TCO/a-Si:H(p) contact is a critical part of the silicon heterojunction solar cell. At this point, holes from the emitter have to recombine loss free with electrons from the TCO. Since tunneling is believed to be the dominant transport mechanism, a high dopant density in both adjacent layers is critical. In contrast to this, it has been reported that high TCO dopant density can reduce field effect passivation induced by the a-Si:H(p) layer. Thus, in this publication, we systematically investigate the influence of a thin (∼10 nm) ITO contact layer with dopant densities ranging from Nd = 1019 - 1021 cm-3 placed between an ITO bulk layer of 70 nm with Nd= 2·1020 cm-3 and the a-Si:H(p) emitter on the J-V characteristics, with the aim to find an optimum Nd. We accompanied our experiments by AFORS-HET simulations, considering trap-assisted tunneling and field dependent mobilities in the a-Si:H(p) layer. As expected, two regimes are visible: For low Nd the devices are limited by inefficient tunneling, resulting in S-shaped J-V characteristics. For high Nd a reduction of the field effect passivation becomes visible in the low injection range. We can qualitatively reproduce these findings using device simulations

Crystalline silicon solar cells with thin poly-SiO_x carrier-selective passivating contacts for perovskite/c-Si tandem applications

Single junction crystalline silicon (c-Si) solar cells are reaching their practical efficiency limit whereas perovskite/c-Si tandem solar cells have achieved efficiencies above the theoretical limit of single junction c-Si solar cells. Next to low-thermal budget silicon heterojunction architecture, high-thermal budget carrier-selective passivating contacts (CSPCs) based on polycrystalline-SiOx (poly-SiOx) also constitute a promising architecture for high efficiency perovskite/c-Si tandem solar cells. In this work, we present the development of c-Si bottom cells based on high temperature poly-SiOx CSPCs and demonstrate novel high efficiency four-terminal (4T) and two-terminal (2T) perovskite/c-Si tandem solar cells. First, we tuned the ultra-thin, thermally grown SiOx. Then we optimized the passivation properties of p-type and n-type doped poly-SiOx CSPCs. Here, we have optimized the p-type doped poly-SiOx CSPC on textured interfaces via a two-step annealing process. Finally, we integrated such bottom solar cells in both 4T and 2T tandems, achieving 28.1% and 23.2% conversion efficiency, respectively.</p

Nanokristalline Silizium- und Siliziumoxid-basierte Kontaktschichten für Silizium-Heterokontakt Solarzellen

Crystalline-silicon solar cells are presently the main technology to supply the rapidly growing market for competitive photovoltaic electricity. To drive conversion efficiency of such cells further up, silicon heterojunction (SHJ) solar cells are promising. This thesis addresses strategies to improve the efficiency of such cells by the integration of nanocrystalline silicon (nc-Si:H) and nanocrystalline silicon oxide (nc-SiOx:H) contact layers grown by plasma enhanced chemical vapor deposition, replacing the commonly used amorphous silicon (a-Si:H) contact layers. The aim is to take advantage of both the improved charge carrier transport and contact resistance and a reduced parasitic absorption. The key challenge is to develop very thin, highly crystalline layers that require fast nucleation. As prerequisite, we first had to establish a reference process with p-doped a-Si:H with a high degree of reproducibility. With this we reached a conversion efficiency of 21.3%. We investigated the relation between deposition gas composition, optoelectronic material properties and structural features of (p)nc-Si:H films on cell stacks. The a-Si:H passivation layer growth regime (slight epitaxial or fully amorphous) is relevant for the nanocrystalline evolution and critically influences the fill factor (FF) and open circuit voltage of completed solar cells. Furthermore, nc-Si:H layers exhibited the ability to enhance surface passivation similarly to what was found with post-deposition hydrogen plasma treatment. A CO2 plasma treatment of the (i)a-Si:H surface prior to the emitter deposition was optimized as method to fast nucleate the thin nc-Si:H films without deteriorating the underlying passivation. The combination of optimized deposition parameters, plasma treatments and film thickness resulted in solar cells exhibiting an open circuit voltage of 727 mV, a short circuit current density of 38.9 mA/cm2, a FF of 74.6% and a conversion efficiency of 21.1%. The optical interaction between the incoming light and the multilayer stack placed on the illuminated side of the c-Si absorber was simulated varying the doped contact material (a-Si:H, nc-Si:H, nc-SiOx:H and doping type) in stack with (i)a-Si:H passivation and In2O3:Sn layers. We identified the optimal film thickness and refractive index as function of the substrate texture to maximize the generated current. The use of a tailored nc-SiOx:H layers allowed for the creation of a stack with refractive indexes that consecutively decreases from silicon to the ambient air resulting in a high short circuit current density of 40.4 mA/cm2 with a gain of 2 mA/cm2 as compared to using the (p)a-Si:H reference, but with limited FF of 72.9%. The causes of the FF limitations experimented were ascribed to both the poor contact to the front transparent conductive oxide and the initial stage of growth of the thin nc-Si:H emitter. Finally, an alternative procedure showed the potential to overcome both problems by passivating the silicon wafer surfaces with an ultra-thin amorphous silicon oxide film growth by PECVD. An exceptionally high crystalline volume fraction of 72% was measured in a thin layer on a cell stack. Preliminary cells exhibited a high FF and reduced current loss but a low open circuit voltage due to insufficient passivation. However, the approach opens up perspectives to further exploit the full potential of nc-Si:H contact layers.Kristalline Silizium-Solarzellen sind derzeit die wichtigste Technologie, um den schnell wachsenden Markt für wettbewerbsfähigen Photovoltaik-Strom zu bedienen. Um den Wirkungsgrad dieser Solarzellen weiter zu steigern, sind Solarzellen basierend auf einem Silizium Hetereoübergang (SHJ) vielversprechend. Diese Doktorarbeit befasst sich mit Strategien zur Effizienzsteigerung dieses Zelltyps. Dafür werden nanokristalline Silizium- (nc-Si:H) und nanokristalline Siliziumoxid- (nc-SiOx:H) Kontaktschichten, welche mittels plasmaunterstützter chemischer Gasphasenabscheidung (PECVD) abgeschieden werden, verwendet. Diese Schichten ersetzen die für gewöhnlich verwendeten Kontaktschichten aus amorphem Silizium (a-Si:H). Das Ziel ist, von dem verbesserten Ladungsträgertransport und Kontaktwiderstand als auch der reduzierten parasitären Absorption zu profitieren. Die Hauptherausforderung ist die Entwicklung sehr dünner, hoch kristalliner Schichten, welche eine schnelle Nukleation aufweisen. Zunächst war es notwendig einen Referenzprozess für SHJ Solarzellen mit p-dotierten a-Si:H Emittern mit einem hohen Grad an Reproduzierbarkeit zu etablieren. Damit erzielten wir einen Wirkungsgrad von 21,3%. Wir untersuchten daraufhin den Zusammenhang zwischen der Zusammensetzung des Depositionsgases, den optoelektronischen Materialeigenschaften und strukturellen Eigenschaften von (p)nc-Si:H Schichten auf Zellstapeln. Das Wachstumsverhalten der a-Si:H Passivierungsschicht (entweder epitaktisch oder vollständig amorph) ist ausschlaggebend für die nanokristalline Entwicklung und beeinflusst wesentlich den Füllfaktor und die Leerlaufspannung der Solarzelle. Ähnlich der Nachbehandlung mit einem Wasserstoffplasma führt die Abscheidung nanokristalliner Schichten zu einer verbesserten Oberflächenpassivierung. Eine CO2 Plasmabehandlung der (i)a-Si:H Oberfläche wurde vor der Emitter-Abscheidung durchgeführt. Diese Behandlung wurde im Hinblick auf die Emitter Nukleation optimiert, wobei die Qualität der Passivierung der darunterliegenden Schicht unverändert bleiben sollte. Die Kombination aus optimierten Depositionsparametern, Plasmabehandlungen und Filmdicken resultierte in Solarzellen, welche eine Leerlaufspannung von 727 mV, eine Kurzschlussstromdichte von 38,9 mA/ cm2, einen Füllfaktor von 74,6% und einen Wirkungsgrad von 21,1% aufwiesen. Der Schichtstapel auf der beleuchteten Seite des c-Si Absorbers wurde optisch simuliert. Dabei wurde das dotierte Kontaktmaterial (a-Si:H, nc-Si:H, nc-SiOx:H und Dotiertyp) im Stapel mit (i)a-Si:H Passivierung und der ITO Schichten variiert. Wir ermittelten die optimale Filmdicke und den optimalen Brechungsindex als Funktion der Substrattextur in Hinblick auf eine maximale Stromstärke. Die Verwendung geeigneter nc-SiOx:H Schichten erlaubte die gezielte Reduktion des Brechungsindex-Übergangs vom Silizium zur Luft. Damit konnte eine im Vergleich zur (p)a-Si:H Referenz um 2 mA/cm2 höhere Kurzschlussstromdichte von 40,4 mA/cm2 erreicht werden, wobei der Füllfaktor auf 72,9% begrenzt war. Diese Füllfaktorbegrenzung wurde einer schlechten Kontaktierung des Vorderseiten-TCOs und dem unzureichenden initialen Wachtumsverhaltens (Nukleation) des dünnen Emitters zugeschrieben. Die Passivierung der Silizium-Waferoberfläche mit einer ultradünnen, amorphen PECVD Siliziumoxid-schicht bietet potentiell die Möglichkeit beide o.g. Probleme zu umgehen. Ein sehr hoher kristalliner Volumenanteil von 72% konnte in dünnen Schichten auf Zellstapeln gemessen werden. Erste Solarzellen weisen einen hohen Füllfaktor und einen verringerten Stromverlust auf. Allerdings ist die Leerlaufspannung aufgrund einer geringen Passivierungsqualität niedrig. Dennoch eröffnet dieser Ansatz neue Perspektiven, um das Potential von nc-Si:H basierten Kontaktschichten vollständig auszuschöpfen.EC/FP7/ENERGY/OptiSolar Projec

Versatility of Nanocrystalline Silicon Films: from Thin-Film to Perovskite/c-Si Tandem Solar Cell Applications

Doped hydrogenated nanocrystalline (nc-Si:H) and silicon oxide (nc-SiOx:H) materials grown by plasma-enhanced chemical vapor deposition have favourable optoelectronic properties originated from their two-phase structure. This unique combination of qualities, initially, led to the development of thin-film Si solar cells allowing the fabrication of multijunction devices by tailoring the material bandgap. Furthermore, nanocrystalline silicon films can offer a better carrier transport and field-effect passivation than amorphous Si layers could do, and this can improve the carrier selectivity in silicon heterojunction (SHJ) solar cells. The reduced parasitic absorption, due to the lower absorption coefficient of nc-SiOx:H films in the relevant spectral range, leads to potential gain in short circuit current. In this work, we report on development and applications of hydrogenated nanocrystalline silicon oxide (nc-SiOx:H) from material to device level. We address the potential benefits and the challenges for a successful integration in SHJ solar cells. Finally, we prove that nc-SiOx:H demonstrated clear advantages for maximizing the infrared response of c-Si bottom cells in combination with perovskite top cells

Efektivní pasivace povrchu černého křemíku vrstvou hydrogenizovaného amorfního křemíku naneseného pomocí plazmou podpořené depozice z plynné fáze.

Solární články na bázi černého křemíku (b-Si) se ukázaly ve fotovoltaice (PV) jako nadějné a přesahující 22% účinnost. Pro dosažení vysoké účinnosti u povrchů b-Si je nejdůležitějším krokem efektivní pasivace povrchu. Dosud je nejúčinnější doba životnosti minoritních nosičů dosahována depozicí několik atomů tenké vrstvy Al2O3 nebo tepelného SiO2. Plazmou podpořená chemická depozice z par (PECVD) vrstvy hydrogenizovaného amorfního křemíku (a-Si: H) jako pasivace b-Si je jen zřídka hlášena kvůli problémům s konformitou. V této současné studii jsou b-Si povrchy superponované na standardní pyramidální textury, také známé jako modulované povrchové textury (MST), úspěšně pasivovány konformními vrstvami a-Si:H nanesenými PECVD. Je ukázáno, že za správných podmínek plazmou podpořené depozice mohou efektivní doby životnosti minoritních nosičů vzorků vybavených přední MST a zadní standardní pyramidální strukturou dosáhnout až 2,3 ms. Cesta ke konformnímu růstu je popsána a vyvinuta za pomoci transmisních elektronových mikroskopických (TEM) obrazů. Pasivované vzorky MST vykazují méně než 4% odraz v širokém spektrálním rozsahu od 430 do 1020 nm.Solar cells based on black silicon (b-Si) are proven to be promising in photovoltaics (PVs) by exceeding 22%efficiency. To reach high efficiencies with b-Si surfaces, the most crucial step is the effective surface passivation. Up to now, the highest effective minority carrier lifetimes are achieved with atomic layer-deposited Al2O3 or thermal SiO2. Plasmaenhanced chemical vapor deposition (PECVD)-grown hydrogenated amorphous silicon (a-Si:H) passivation of b-Si is seldom reported due to conformality problems. In this current study, b-Si surfaces superposed on standard pyramidal textures, also known as modulated surface textures (MSTs), are successfully passivated by PECVD-grown conformal layers of a-Si:H. It is shown that under proper plasma-processing conditions, the effective minority carrier lifetimes of samples endowed with front MST and rear standard pyramidal textures can reach up to 2.3 ms. A route to the conformal growth is described and developed by transmission electron microscopic (TEM) images. Passivated MST samples exhibit less than 4% reflection in a wide spectral range from 430 to 1020 nm

Thermodynamics of a nanowire solar cell: Towards the ultimate limit

A lossless solar cell operating at the Shockley-Queisser (S-Q) limit generates an open-circuit voltage (VOC) equal to the radiative limit. At VOC, the highly directional beam of photons from the sun is absorbed and subsequently externally re-emitted into a 4p solid angle, providing a large photon entropy loss. Moreover, due to many total internal reflections and low internal radiative efficiency, a lot of light is lost in nonradiative recombination events. In our research, we perform a nanophotonic optimization of a semiconductor nanowire geometry with a top microlens in order to decrease the photon entropy loss and to increase the photon escape probability for the nanowire, therefore increasing the output voltage. The optimization leads us to a maximum VOC of 1178 mV which is 141 mV higher than the radiative limit and 172 mV lower than the ultimate limit. The photon entropy loss is also studied fundamentally from the thermodynamics point of view to better understand where the entropy is generated during the absorption-emission processes

Efficient Continuous Light-Driven Electrochemical Water Splitting Enabled by Monolithic Perovskite-Silicon Tandem Photovoltaics

Solar-assisted water electrolysis is a promising technology for storing the energy of incident solar irradiation into hydrogen as a fuel. Here, an integrated continuous flow electrochemical reactor coupled to a monolithic perovskite-silicon tandem solar cell is demonstrated that provides light-driven electrochemical solar-to-hydrogen conversion with an energy conversion efficiency exceeding 21% at 1-Sun equivalent light intensity and stable operation during three simulated day-night cycles

High-Efficiency Silicon Heterojunction Solar Cells: Materials, Devices and Applications

Photovoltaic (PV) technology offers an economic and sustainable solution to the challenge of increasing energy demand in times of global warming. The world PV market is currently dominated by the homo-junction crystalline silicon (c-Si) PV technology based on high temperature diffused p-n junctions, featuring a low power conversion efficiency (PCE). Recent years have seen the successful development of Si heterojunction technologies, boosting the PCE of c-Si solar cells over 26%. This article reviews the development status of high-efficiency c-Si heterojunction solar cells, from the materials to devices, mainly including hydrogenated amorphous silicon (a-Si:H) based silicon heterojunction technology, polycrystalline silicon (poly-Si) based carrier selective passivating contact technology, metal compounds and organic materials based dopant-free passivating contact technology. The application of silicon heterojunction solar cells for ultra-high efficiency perovskite/c-Si and III-V/c-Si tandem devices is also reviewed. In the last, the perspective, challenge and potential solutions of silicon heterojunction solar cells, as well as the tandem solar cells are discussed