Query: Passivated rear  contacts for high-efficiency n-type Si solar cells providing high interface  passiv

Candidate papers:

abstract: Abstract In this work passivated rear contacts are used to replace point contact passivation schemes for high efficiency n type crystalline silicon solar cells. Our structure is based on an ultra thin tunnel oxide (SiO2) and a phosphorus doped silicon layer, which significantly reduce the surface recombination at the metal semiconductor interface. The passivation and transport mechanisms of this passivated contact will be addressed within this paper. Particular consideration will be given to the tunnel oxide's impact on interface passivation and on the I V characteristics of n type Si solar cells featuring a boron diffused emitter. It will be shown that the tunnel oxide is a vital part of this passivated contact and that it is required to achieve excellent interface passivation for both open circuit and maximum power point (MPP) conditions (implied open circuit voltage iVoc>710 mV and implied fill factor iFF>84% It will also become clear that the transport barrier arising from the tunnel oxide does not constrict the majority carrier flow. Thus, a low series resistance is obtained, which in conjunction with the high iFF enables FFs well above 82% Investigations on cell levels lead to an independently confirmed conversion efficiency of 23.0% for n type cells with a boron diffused emitter and the herein developed passivated rear contact, in which the efficiency is not limited by the electrical properties of our passivated contacts.
author_names: Frank Feldmann

Martin Bivour

Christian Reichel

Martin Hermle

Stefan W Glunz
title: Passivated rear contacts for high efficiency n type Si solar cells providing high interface passivation quality and excellent transport characteristics
year: 2014

abstract: Due to the improvements in material quality and surface passivation, high efficiency solar cells are often limited by the recombination at the metal semiconductor contacts. As a solution to this problem, Swanson proposed "to put a heterojunction with a band gap larger than silicon between the metal and silicon"[1] also known as passivated contact. In this work, a tunnel oxide passivated contact (TOPCon) structure allowing both an excellent surface passivation and an effective carrier transport is presented. High efficiency n type solar cells featuring this novel passivated rear contact instead of a point contact structure at the rear side yield a maximum efficiency of 23.7 a FF of 82.2 and a Voc of 703 mV.
author_names: Stefan W Glunz

Martin Hermle

Christian Reichel

Martin Bivour

Frank Feldmann
title: A Passivated Rear Contact for High Efficiency n Type Si Solar Cells Enabling High Voc's and FF>82
year: 2013

abstract: Abstract Bifacial crystalline silicon (c Si) solar cells have currently attracted much attention due to the front high efficiency and additional gain of power generation from the back side. Here, we have presented n type passivated emitter and rear totally diffused (n PERT) bifacial c Si solar cells featuring front selective emitter (SE) and polysilicon (poly Si) based passivating contacts. The SE formation was scanned with laser doping based on front boron diffusion p+ emitter. The poly Si based passivating contacts consisting of nano layer SiOx of ~1.5 nm thickness grown with cost effective nitric acid oxidation and phosphorus doped polysilicon exhibited excellent passivation for high open circuit voltage. We have successfully achieved the large area (156 x 156 mm2) n PERT bifacial solar cells yielding top efficiency of 21.15% together with a promising short circuit current density of 40.40 mA/cm2. Theoretical calculation has further demonstrated that the optimal thickness of SiOx nano layer will increase from 1.5 nm to 1.8 nm if the density of interface defect state decreases by one magnitude from 1 x 1010 cm 2/eV, and the cell efficiency can be improved up to 24.64% with open circuit voltage over 0.720 V by optimizing the parameters of functional materials and interface layers. The present work has indicated that the commercialization of low cost and high efficiency n PERT bifacial c Si cells is possible due to the processes compatible with existing production lines.
author_names: Dongyan Ding

Guilin Lu

Zhengping Li

Yue-heng Zhang

Wen-zhong Shen
title: High efficiency n type silicon PERT bifacial solar cells with selective emitters and poly Si based passivating contacts
venue: Solar Energy
year: 2019

abstract: Abstract In this work, we used the numerical simulation method to study the tunnel oxide passivated carrier selective contacts (TOPCon) structured solar cells, with the focus especially on the paths towards excellent surface passivation and low contact resistance. The presence of an ultra thin silicon oxide (SiO 2 with high quality (typically low interface states density, D it 1 x 10 10 cm 2 eV 1 and low pinhole density, D ph 4 suppresses the recombination of carriers at the rear surface. As a result, implied open circuit voltage (i V oc could be promoted by a value of more than 30 mV comparing with the solar cell without oxide layer, which is the primary benefit originated from TOPCon structure. Corresponding, the iV oc and recombination current density J oe could reach ~745 mV and ~9.5 fA/cm 2 (Dn 5 x 10 15 cm 3 for the 1 O cm and 200 mm n type wafer covered with high quality oxide and n Si layers. In addition to passivation, a well designed SiO 2 /n Si backside structure is also critical for carrier collection. The tunneling current is susceptible to oxide thickness, i.e. a 0.2 nm increase in SiO 2 thickness results in the decrease of the tunneling current by more than one magnitude under certain circumstance. Fortunately, raising the doping in n Si layer enhances the tunneling possibility of electron, which allows for a thicker oxide that is favorable to a stable mass production. The simulation suggests that to obtain a high fill factor FF >84% a minimum forward bias saturated tunneling current of about 0.01 A/cm 2 more favorable of 0.1 A/cm 2 is required for the Si/SiO 2 /n Si structure. Generally, our work offers an improved understanding of tunnel oxide, doping layer and their combined effects on TOPCon solar cells. Besides simulation, we also discuss the practical manufactures of how to control the above mentioned parameters, as well as the problems needed to be solved for further work.
author_names: Yuheng Zeng

Tong Hui

Cheng Quan

Cai Liang

Zhenhai Yang

Kangmin Chen

Zhi Zhong Yuan

Chung-Han Wu

Baojie Yan

Pingqi Gao

Jichun Ye
title: Theoretical exploration towards high efficiency tunnel oxide passivated carrier selective contacts (TOPCon) solar cells
year: 2017

abstract: High efficiency silicon heterojunction (SHJ) solar cells have already reached more than 26% (Yoshikawa et al. 2017) efficiency when tunnel oxide passivated contacts solar cells just broke the 26% barrier at the beginning of February 2018 (ISFH, 2018) For such devices, major losses occur at both the front and rear contacts where parasitic recombination can be very high. Carrier selective contacts use a special design in order to build a barrier that would block one specific charge carrier and let the other one go through. A Hybrid solar cell is a combination between a heterojunction solar cell and a TOPCon device, featuring then carrier selective contacts at both sides. In this thesis, a p type TOPCon structure is implemented at the rear when the front contact is made of n type amorphous silicon (a Si:H) Intrinsic amorphous silicon (a Si:H) used as front passivation layer, is deposited on top of crystalline silicon and requires an interface with as few defects as possible to minimize the parasitic recombination velocity. A new pretreatment method studied in this thesis involves the growth of a silicon oxide (SiO2) on a crystalline silicon substrate, that will allow to get rid of most of the superficial defects after etching and before a Si:H deposition. Lifetimes of up to 6 ms and saturation current density (J0) as low as 14 fA/cm2 can be reached with a 200 nm thick oxide. As a Si:H presents a very low lateral conductivity, a transparent conductive oxide (TCO) is needed to transport the charge carriers towards the front metal contacts. The resistivity of such a material should be as low as possible. While increasing the deposition temperature of Indium Tin Oxide (ITO) it has been possible to decrease the resistivity up to 2.5 *10 4 O.cm at a temperature of 130degC, without reducing the optical properties of such a layer. Finally, manufacturing defects are often introduced during the fabrication process, leading to some shunt losses (low shunt resistance) As we are fabricating several solar cells per wafer, it is most of the time necessary to cut them to get rid of the shunt before doing the measurements. This sensitive step could be avoided with a better isolation of each solar cell. Patterned ITO and metal have been developed in this thesis, allowing to reduce the shunt power losses in the range of 1 2% without the need to cut the cells.
author_names: Guillaume le Boucher d'Herouville
title: High Efficiency SHJ/Poly Si Hybrid Solar Cells
year: 2018

abstract: To improve the conversion efficiency of c Si solar cells, it is crucial to quench the recombination losses at c Si/metal interface. Such recombination losses are able to be reduced by using carrier selective passivating contacts by means of hydrogenated amorphous silicon (a Si:H) and poly crystalline silicon (poly Si)/tunneling silicon oxide (SiOx) In this thesis, a novel hybrid solar cell concept is presented. The novel structure combines the advantages of silicon heterojunction (SHJ) as front surface field (FSF) and poly Si passivating contact as rear emitter. This project aims to improve the passivation quality of FSF composed of intrinsic n type (i/n) a Si:H stacks and evaluate transparent conductive oxide (TCO) layer. Considering FSF optimization, effective cleaning pre treatment has been developed to provide ultra clean surface and prevent cross contamination. Lifetime measurements has shown that three cycles of NAOC improves effective lifetime significantly to 12.6 ms on symmetric structure. Plasma cleaning of chamber and plasma deposition on substrate holder decreases saturation current density (J0) by 10.9 fA/cm2. Comparing to post deposition annealing in air, H2 environment annealing shows more appealing passivation quality which improves effective lifetime to 4.7 ms and reduces J0 to 28.7 fA/cm2 on solar cell precursors. VOC demonstrates improvements for thicker i a Si:H films, but leading to more parasitic absorption losses. This effect results in a trade off between VOC and JSC. Furthermore, thicker i a Si:H layer degrades the carrier transport by means of FF. In terms of TCO optimization, novel hydrogen doped indium oxide (IO:H)/indium tin oxide (ITO) stacks is presented as an attractive alternative to replace conventional single ITO layer. Hall effect measurements have shown that post annealing in vacuum is an effective method to decrease carrier density and increase mobility of IO:H/ITO stacks. Accordingly, the opto electrical properties of IO:H/ITO stacks, single ITO layer and single IO:H layer are compared. 180 degC annealed sample shows optimal performance with high mobility of 117 cm2/(Vs) and low carrier density of 1.07 x 1020 cm 3. Either IO:H/ITO stacks or IO:H single layer shows nearly 4 times higher mobility and half carrier density compared to single ITO film after post deposition annealing. Spectra measurements reveal that the parasitic absorption of IO:H/ITO stacks is lower than that of ITO film over the complete spectral range. Before post deposition annealing, it was observed that TCO sputtering decreases passivation quality, but it is less pronounced for IO:H than ITO deposition. However, after post deposition annealing, passivation degradation is able to be nearly fully recovered for both sputtering: IO:H and ITO.
author_names: Haonan Ge
title: Development of High Efficiency SHJ/Poly Si Passivating Contact Hybrid Solar Cells
year: 2017

abstract: To improve the conversion efficiency of c Si solar cells, it is crucial to quench the recombination losses at c Si/metal interface. Such recombination losses are able to be reduced by using carrier selective passivating contacts by means of hydrogenated amorphous silicon (a Si:H) and poly crystalline silicon (poly Si)/tunneling silicon oxide (SiOx) In this thesis, a novel hybrid solar cell concept is presented. The novel structure combines the advantages of silicon heterojunction (SHJ) as front surface field (FSF) and poly Si passivating contact as rear emitter. This project aims to improve the passivation quality of FSF composed of intrinsic n type (i/n) a Si:H stacks and evaluate transparent conductive oxide (TCO) layer. Based on aforementioned optimization, the highest conversion efficiency of 20.5% was achieved (VOC 704 mV, JSC 39.5 mA/cm2 and FF 73.8%
author_names: Haonan Ge
title: Development of High Efficiency SHJ/Poly Si Passivating Contact Hybrid Solar Cells
year: 2017

abstract: This paper presents an analysis of physical mechanisms related to operation and optimization of interdigitated back contact (IBC) poly silicon based devices. Concepts of carrier selectivity and tunneling are used to identify the parameters that impact on the fill factor. Then, based on technology computer aided design (TCAD) numerical simulations, we describe the device performance in terms of transport and passivation. A validation of the model is performed by matching measured and simulated R, T, and external quantum efficiency spectra and electrical parameters. As result of such process, the opto electrical losses of the reference device are identified. Then, we execute a study of the impact of process parameters on the performance of the IBC device under analysis. Assuming a uniform SiO2 layer, simulation results reveal that both n type and p type poly Si contacts can be theoretically perfect (i.e. approx. lossless) if assuming no interface recombination but considering tunneling of both carrier types. In other words, there exists an optimum oxide thickness (1 nm) for which majority carriers tunneling works already very well, and minority tunneling is still low enough to not result in significant recombination. Moreover, SiO2 thickness up to maximum 1.6 nm is crucial to achieve high efficiency. Regarding rear geometry analysis, the efficiency curve as a function of emitter width peaks at 70% of pitch coverage. Further, it is shown that diffused dopants inside crystalline silicon make the device resilient to passivation quality. Finally, the calibrated model is used to perform an optimization study aiming at calculating the performance limit. The estimated performance limit is 27.3% for a 100 mm thick bulk, 20 nm thick poly silicon layers, silver as rear contact, and double ARC.
author_names: Paul Procel

Guangtao Yang

Olindo Isabella

Miro Zeman
title: Numerical Simulations of IBC Solar Cells Based on Poly Si Carrier Selective Passivating Contacts
venue: IEEE Journal of Photovoltaics
year: 2019

abstract: The goal of this thesis is the fabrication of high efficiency interdigitated back contact (IBC) c Si solar cell at low temperature and low cost manufacturing technology. This thesis proposes a new concept and at the same time a simple and elegant fabrication process that has been fully developed and culminated with the fabrication of a "cold" IBC solar cell as a proof of concept. To carry out this proposal, we focus our research on the study and application of low temperature processes such as Atomic Layer Deposition (ALD) and Plasma enhanced Chemical Vapor Deposition (PECVD) to deposit dielectric layers. A process based on laser techniques was also developed to be applied on these dielectric layers to form p+ and n+ regions into the c Silicon sample. The laser highly doped regions are formed in a point like structure avoiding the classical high temperature diffusion process. The dielectrics used, Al2O3 and a SiCx (n) stack play the role of aluminum and phosphorous dopant sources respectively. A detailed study has been accomplished to find the best laser parameters and obtain the optimal p+ and n+ junction. At the same time, these layers work as excellent surface passivating films and improves the front and the rear reflectance. To get the film which better fulfills these tasks, an extensive investigation has been performed to optimize the deposition and post deposition processes in terms of temperature, time and layer thickness. In order to fabricate the "cold" IBC cell, we firstly developed a IBC cell performed on p type FZ c Si with a conventional phosphorous diffusion. The SiCx(n) stack passivated the n region interface as well as provided phosphorous atoms to create n+ regions or selective emitter structures after laser processing. The aluminium atoms supplied by the Al2O3 layer formed a p+ region or Back surface field (BSF) after the laser processing and simultaneously passivated the p region interface. A promising efficiency of 18.7% (Jsc= 39.1 mA/cm2, Voc= 638 mV, FF= 75.3% was obtained as a result of this new concept. The first "cold" IBC cell was obtained after elimination of conventional phosphorous diffusion and rearrangement of the fabrication steps in order to reduce the thermal budget and the complex photolithographic steps. The laser employed is a pulsed Nd YAG lamp pumped working at 1064 nm in nanosecond regime. The efficiency achieved was 18.0% (Jsc 39.2 mA/cm2, Voc= 647mV, FF= 71.1% on 280 micras thick 2.5 Ohmcm n type FZ Si with a designated area of 9 cm2. The final efficciency achieved of the final "Cold" IBC cell was 20% (Jsc= 40.5 mA/cm2, Voc 650 mV and FF 76.4% using a pulsed Nd:YVO4 Laser operating at 355 nm (UV) The total fabrication process was carried out at low temperatures (below 400 oC) avoiding the high temperature difussion processes.
author_names: G Rodriguez
title: Interdigitated back contacted(IBC) c Si solar cells based on laser processed dielectric layers
year: 2016

abstract: Silicon heterojunction technology (HJT) uses silicon thin film deposition techniques to fabricate photovoltaic devices from mono crystalline silicon wafers (c Si) This enables energy conversion efficiencies above 21 also at industrial production level. In this presentation we review the present status of this technology and point out recent trends. We first discuss how the properties of thin hydrogenated amorphous silicon (a Si:H) films can be exploited to fabricate passivating contacts, which is the key to high efficiency HJT solar cells. Such contacts enable very high operating voltages, approaching the theoretical limits, and yield small temperature coefficients. With this approach, an increasing number of groups are reporting devices with conversion efficiencies well over 20 on both sides contacted n type cells, Panasonic leading the field with 24.7 Exciting results have also been obtained on p type wafers. Despite these high voltages, important efficiency gains can still be made in fill factor and optical design. This requires improved understanding of carrier transport across device interfaces and reduced parasitic absorption in HJT solar cells. For the latter, several strategies can be followed: Short wavelength losses can be reduced by replacing the front a Si:H films with wider bandgap window layers, such as silicon alloys or even metal oxides. Long wavelength losses aremore mitigated by introducing new high mobility TCO's such as hydrogenated indium oxide, and also by designing new rear reflectors. Optical shadow losses caused by the front metallization grid are significantly reduced by replacing printed silver electrodes with fine line plated copper contacts, leading also to possible cost advantages. The ultimate approach to minimize optical losses is the implementation of back contacted architectures, which are completely devoid of grid shadow losses and parasitic absorption in the front layers can be minimized irrespective of electrical transport requirements. The validity of this approach was convincingly demonstrated by Panasonic, Japan in 2014, reporting on an interdigitated back contacted HJT cell with an efficiency of 25.6% setting the new single junction c Si record. Finally, given the virtually perfect surface passivation and excellent red response of HJT solar cells, we anticipate these devices will also become the preferred bottom cell in ultra high efficiency c Si based tandem devices, exploiting better the solar spectrum. Such tandem cells have the potential to overcome the fundamental single junction limit of silicon solar cells (29.4% Combining HJT cells with perovskite solar cells as top cell appears to be particularly appealing. less
author_names: Stefaan De Wolf

Jonas Geissbuehler

Philipp Loper

S Martin de Nicholas

Johannes Peter Seif

Andrea Tomasi

Christophe Ballif
title: High efficiency silicon heterojunction solar cells: Status and perspectives
year: 2015