Poly-Si(O)x passivating contacts for high-efficiency c-Si IBC solar cells

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Delft University of Technology

Poly-Si(O)x passivating contacts for high-efficiency c-Si IBC solar cells

Yang, Guangtao; Zhang, Yue; Procel, Paul; Weeber, Arthur; Isabella, Olindo; Zeman, Miro DOI

10.1016/j.egypro.2017.09.257 Publication date

2017

Document Version Final published version Published in

Energy Procedia

Citation (APA)

Yang, G., Zhang, Y., Procel, P., Weeber, A., Isabella, O., & Zeman, M. (2017). Poly-Si(O)x passivating contacts for high-efficiency c-Si IBC solar cells. Energy Procedia, 124, 392-399.

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Available online at www.sciencedirect.com

Energy Procedia 124 (2017) 392–399

Peer review by the scientific conference committee of SiliconPV 2017 under responsibility of PSE AG. 10.1016/j.egypro.2017.09.257

Available online atwww.sciencedirect.com

ScienceDirect

www.elsevier.com/locate/procedia

Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.

The 15th International Symposium on District Heating and Cooling

Assessing the feasibility of using the heat demand-outdoor

temperature function for a long-term district heat demand forecast

I. Andrić

a,b,c

_{*, A. Pina}

a

_{, P. Ferrão}

a

_{, J. Fournier}

b

_{., B. Lacarrière}

c

_{, O. Le Corre}

c a_{IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal}

b_{Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France}

c_{Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France}

Abstract

District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat sales. Due to the changed climate conditions and building renovation policies, heat demand in the future could decrease, prolonging the investment return period.

The main scope of this paper is to assess the feasibility of using the heat demand – outdoor temperature function for heat demand forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were compared with results from a dynamic heat demand model, previously developed and validated by the authors.

The results showed that when only weather change is considered, the margin of error could be acceptable for some applications (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations.

Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.

Keywords: Heat demand; Forecast; Climate change

10.1016/j.egypro.2017.09.257

Peer review by the scientific conference committee of SiliconPV 2017 under responsibility of PSE AG.

1876-6102

ScienceDirect

7th International Conference on Silicon Photovoltaics, SiliconPV 2017

Poly-Si(O)x passivating contacts for high-efficiency c-Si IBC solar

cells

Guangtao Yang

1

_{, Yue Zhang, Paul Procel, Arthur Weeber, Olindo Isabella, Miro Zeman}

Delft University of Technology, Photovoltaic Materials and Devices group, Mekelweg 4, 2628 CD Delft, the Netherlands

Abstract

Highest conversion efficiency in crystalline silicon (c-Si) solar cells can be enabled by quenching minority carriers’ recombination at c-Si/contact interface owing to carrier-selective passivating contacts. With the semi-insulating poly-crystalline silicon (SIPOS, poly-Si) a very good passivation of c-Si surfaces was obtained. We have explored these passivating structures on IBC solar cells and obtained over 22% efficiency with over 23% within reach on the short term. We present in detail the passivation quality of p-type and n-type ion-implanted LPCVD poly-crystalline silicon (poly-Si) and its relation to the doping profile. Optimized poly-Si layers in the role of emitter and BSF showed excellent passivation (J0,emitter = 11.5 fA/cm2 and

J0,BSF = 4.5 fA/cm2) and have been deployed in FSF-based IBC c-Si solar cells using a simple self-aligned patterning process.

Applying an optimized passivation of FSF by PECVD a-Si:H/SiNx layer (J0,FSF = 6.5 fA/cm2) leads to a cell with efficiency of

22.1% (VOC= 709 mV, JSC= 40.7 mA/cm2, FF = 76.6%). Since over 83% FF has been reached with adjusted metallization

technology on similar IBC structures, we believe 23% efficiency is within reach on the short term. Further improvement, especially at JSC level, is expected by deploying less absorbing carrier-selective passivating contacts based on poly-Si or wide

bandgap poly-SiOx layers (J0 ~ 10 fA/cm2).

Keywords: poly-crystalline silicon oxide alloys; passivating contact; IBC c-Si solar cell;

1. Corresponding author. Tel.: +31 (0)15 27 86288; fax: +31 (0)15 27 82968 E-mail address: G.Yang@tudelft.nl

ScienceDirect

7th International Conference on Silicon Photovoltaics, SiliconPV 2017

Poly-Si(O)x passivating contacts for high-efficiency c-Si IBC solar

cells

Guangtao Yang

1

_{, Yue Zhang, Paul Procel, Arthur Weeber, Olindo Isabella, Miro Zeman}

Abstract

22.1% (VOC = 709 mV, JSC = 40.7 mA/cm2, FF = 76.6%). Since over 83% FF has been reached with adjusted metallization

Guangtao Yang / Energy Procedia 00 (2017) 000–000 1. Introduction

Highest conversion efficiency in crystalline silicon (c-Si) solar cells is enabled by quenching minority carriers’ recombination velocity at c-Si/contact interface owing to carrier-selective passivating contacts [1]. The semi-insulating poly-crystalline silicon (SIPOS, poly-Si), with which very good passivation of c-Si surface was obtained [ 2 ], has recently attracted attention in several research groups as a high-temperature stable carrier selective contact [3,4,5]. Its structure comprises n-type or p-type doped poly-Si and tunnelling SiO2on c-Si wafer. With such

structure high efficiencies were obtained both on front / rear contacted cells and interdigitated back contacted (IBC) cells [3,4,6,7].

In this work, optimized low sheet resistance, ion-implanted LPCVD poly-Si passivating contacts are discussed alongside their implementation into n-type bulk, FSF-based IBC solar cells. The passivation quality of such passivating contacts is found to be closely related to the doping profile at the poly-Si/c-Si interface; while the optimal thickness of tunneling SiO2 is carried out directly at device level. A main drawback in using poly-Si layer is

the higher parasitic absorption, mainly due to free carrier absorption. To reduce it, in-situ doped thin PECVD Si or SiOx layers are optimized as passivating contacts to replace thick ion-implanted LPCVD poly-Si passivating

contacts, aiming to improve the IBC cell efficiency from both the electrical and optical points of view. 2. Experimental

The process to make and the techniques to characterize symmetrical structures based on poly-Si passivating contacts and related IBC solar cells are described in this section. Our typical poly-Si passivating contact consists of an ultra-thin tunneling SiO2 layer and a doped poly-Si layer, fabricated in a four-step process. First, the tunneling

SiO2 layer is formed on both sides of the wafer by a wet-chemical method; second, the intrinsic amorphous silicon

(a-Si) is also deposited on both sides of the wafer by means of low-pressure chemical vapor deposition (LPCVD); third, an ex-situ single-sided doping process is realized using ion-implantation technique; fourth, a high temperature annealing step is carried out to activate and drive-in the implanted dopants while also turning the a-Si into poly-Si. In this work, <100> oriented, 1~5 Ωcm, 285-μm thick, double-side polished FZ wafers were used. The parameters used for the poly-Si passivating contacts preparation are listed in Table 1. The detailed descriptions of each step are presented in the following sub-sections.

The tunneling SiO2layer is formed by the method of Nitric Acid Oxidation of Silicon (NAOS). Before the NAOS

process, in order to remove the native oxide, the Si wafer is dipped into HF, 0.55% for 4 min. The thickness of the obtained NAOS-based SiO2 layer is ~1.5 nm (see Section IV.A). The a-Si layer is then deposited on the

NAOS-based SiO2 layer on both sides of the wafer by a Tempress LPCVD tube furnace at a temperature of 580 ºC. After

the a-Si layer deposition, an annealing step at a temperature of 600 ºC for 1 hour is used to release the stress. In order to obtain a doped a-Si layer, a Varian Implanter E500HP is used to implant P or B atoms into the a-Si layer. With such an implanter, the implantation of B or P atoms can be done with a minimum energy of 5 keV and maximum dose of 1016 _cm-2_{. Therefore, the influence of the implantation parameters on the final passivation}

properties of poly-Si passivating contacts is studied. In this work, the P-implantation is done at fixed implantation energy of 20 keV and variable implantation dose; on the other hand, as the penetration depth of B is larger than P during the implantation, due to B atoms being smaller than P atoms, the B-implantation is done at lower fixed implantation energy of 5 keV and implantation dose of 5×1015_cm-2_{. After the ion-implantation, a high temperature}

process is used to activate and drive-in the dopants. A Tempress tube furnace is used to anneal the samples in N2

atmosphere. The annealing is here done at the temperature between 850 and 950 ºC. The ramping rate for heating or cooling is 10 C/min. The effect of annealing time and atmosphere on the final passivation properties of poly-Si passivating contacts is also studied.

The injection-dependent minority carrier lifetime (τ) and implied open-circuit voltage (iVOC) were measured by the

Photoconductance Lifetime Tester (Sinton, WCT-120) using Quasi-Steady State Photoconductance (QSSPC) mode and transient mode. The values reported in this paper are taken from the transient analysis mode with an optical constant of 0.7 and at the minority carrier density of 1 x 1015_cm-3_{. Four point probe measurement was used to obtain}

the sheet resistance (RSH) of the passivating contacts. In order to ensure an accurate measurement, the c-Si bulk used

for the RSHmeasurement exhibited opposite doping type than the one of the passivating contacts under test.

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Guangtao Yang et al. / Energy Procedia 124 (2017) 392–399 393

ScienceDirect

7th International Conference on Silicon Photovoltaics, SiliconPV 2017

Poly-Si(O)x passivating contacts for high-efficiency c-Si IBC solar

cells

Guangtao Yang

1

_{, Yue Zhang, Paul Procel, Arthur Weeber, Olindo Isabella, Miro Zeman}

Abstract

ScienceDirect

7th International Conference on Silicon Photovoltaics, SiliconPV 2017

Poly-Si(O)x passivating contacts for high-efficiency c-Si IBC solar

cells

Guangtao Yang

1

_{, Yue Zhang, Paul Procel, Arthur Weeber, Olindo Isabella, Miro Zeman}

Abstract

Guangtao Yang / Energy Procedia 00 (2017) 000–000 1. Introduction

Highest conversion efficiency in crystalline silicon (c-Si) solar cells is enabled by quenching minority carriers’ recombination velocity at c-Si/contact interface owing to carrier-selective passivating contacts [1]. The semi-insulating poly-crystalline silicon (SIPOS, poly-Si), with which very good passivation of c-Si surface was obtained [ 2 ], has recently attracted attention in several research groups as a high-temperature stable carrier selective contact [3,4,5]. Its structure comprises n-type or p-type doped poly-Si and tunnelling SiO2 on c-Si wafer. With such

structure high efficiencies were obtained both on front / rear contacted cells and interdigitated back contacted (IBC) cells [3,4,6,7].

In this work, optimized low sheet resistance, ion-implanted LPCVD poly-Si passivating contacts are discussed alongside their implementation into n-type bulk, FSF-based IBC solar cells. The passivation quality of such passivating contacts is found to be closely related to the doping profile at the poly-Si/c-Si interface; while the optimal thickness of tunneling SiO2 is carried out directly at device level. A main drawback in using poly-Si layer is

the higher parasitic absorption, mainly due to free carrier absorption. To reduce it, in-situ doped thin PECVD Si or SiOx layers are optimized as passivating contacts to replace thick ion-implanted LPCVD poly-Si passivating

contacts, aiming to improve the IBC cell efficiency from both the electrical and optical points of view. 2. Experimental

The process to make and the techniques to characterize symmetrical structures based on poly-Si passivating contacts and related IBC solar cells are described in this section. Our typical poly-Si passivating contact consists of an ultra-thin tunneling SiO2 layer and a doped poly-Si layer, fabricated in a four-step process. First, the tunneling

SiO2 layer is formed on both sides of the wafer by a wet-chemical method; second, the intrinsic amorphous silicon

(a-Si) is also deposited on both sides of the wafer by means of low-pressure chemical vapor deposition (LPCVD); third, an ex-situ single-sided doping process is realized using ion-implantation technique; fourth, a high temperature annealing step is carried out to activate and drive-in the implanted dopants while also turning the a-Si into poly-Si. In this work, <100> oriented, 1~5 Ωcm, 285-μm thick, double-side polished FZ wafers were used. The parameters used for the poly-Si passivating contacts preparation are listed in Table 1. The detailed descriptions of each step are presented in the following sub-sections.

The tunneling SiO2 layer is formed by the method of Nitric Acid Oxidation of Silicon (NAOS). Before the NAOS