1. Introduction

To meet the future energy needs, there is a need to develop low-cost alternative energy sources to complement the conventional energy sources (e.g., oil, gas, coal) as well as to address the pressing environmental issues associated with the latter. Hence, energy-related technology roadmaps are actively being released and revised toward the future energy needs. One good example is the International Technology Roadmap for Photovoltaic (ITRPV) [1]. In general, a successful deployment of any selected solar cell technology will be mainly dominated by (i) cost-effectiveness of the material and processes, (ii) scalability to high-volume manufacturing, (iii) device performance, and (iv) long-term stability of product. To progress toward item (iii), ITRPV predicts a continuous reduction of recombination losses in the wafer as well as at the front and rear surfaces of the solar cell. According to Ref. [2–4], given the considerable improvements in the wafer bulk, and surface passivation layers, the main source of recombination losses in high-efficiency solar cells is now dominated by the metal contacts. Thus, the ability to greatly reduce the recombination losses underneath the solar cell metal contacts (i.e., contact passivation) coupled with other technological advancements will be instrumental toward attaining the increasing solar cell efficiency targets.

One of the earliest examples of contact passivation can be found in the heterojunction silicon wafer solar cells, which utilizes a stack of intrinsic and doped amorphous silicon (a-Si:H) heterojunction layers [5–7] on both surfaces of the silicon wafer. The ultrathin (<5 nm) intrinsic a-Si:H layer not only serves to passivate the silicon surface but also to selectively enable hole or electron transport across this "tunnel layer," sandwiched between the overlying conductive a-Si:H layer and the crystalline silicon wafer. In this application, the contact-related recombination losses with the intrinsic/doped a-Si:H stack is significantly lower than utilizing the doped a-Si:H layers alone on the crystalline silicon wafers [5], hence establishing contact passivation for the former case. It can then be generalized that contact passivation can be established by deploying ultrathin passivating (and even in principal insulating, if thick) tunnel layers capped with a highly doped capping layer material with a suitable doping polarity or work function to form either hole-selective or electron-selective passivated contacts. Some examples of high/low work function capping layer materials such as transition metal oxides (WOx, VOx, etc.) and doped organic materials had been reported [4, 8].

Some prominent examples of single-junction silicon wafer-based high-efficiency (≥25%) solar cell concepts which adopt contact passivation include the amorphous silicon heterojunction interdigitated back contact (IBC) solar cell by Kaneka (26.6%) [9], the tunnel layer passivated interdigitated back contact (IBC) solar cell by SunPower (25.2%) [10], the polysilicon on oxide (POLO) passivated contact interdigitated back contact (IBC) solar cell by ISFH (26.1%) [11], and the conventionally front- and rear-contacted tunnel layer passivated contact solar cell (TOPCon) by the Fraunhofer ISE team (25.7%) [12]. The excellent performance of the TOPCon cell (despite being conventional front- and rear-contacted, instead of

## Double-Sided Passivated Contacts for Solar Cell Applications: An Industrially Viable Approach… DOI: http://dx.doi.org/10.5772/intechopen.85039

being contacted in an all-back-contact configuration) can be attributed to the highly effective and simplified full-area rear-side passivating contact scheme, which inserts an electron-selective tunnel layer passivated rear-side contact between the wafer and the full-area rear-side contact of the solar cell, comprising a wetchemically formed silicon oxide tunnel layer (wet-SiOx) and a highly n-doped polysilicon capping layer. This achieves both excellent interface passivation toward the silicon wafer and a highly selective collection of excess electron charge carriers. Although this work was established on a small-sized (4 cm2 ) float-zone n-type silicon wafer, adopting a conventional front-side selective emitter, photolithography processes, and evaporated contacts, it has set the stage for immense research interests such as those reported in Refs. [12–26]. Contact passivation presents a clear advantage over the popular passivated emitter rear contact (PERC) solar cell concept by UNSW [27], which is currently a large scale adopted by the industry (as of Jan. 2019), as an even higher solar cell efficiency can be reached (i.e., by directly passivating the metal solar cell contacts instead of "only" reducing the metal contact area fraction).

An ideal tunnel layer, suited for contact passivation, (i) exhibits a tunneling relevant thickness (i.e., <2 nm) [14], (ii) exhibits excellent interface passivation toward the crystalline silicon wafer [28, 29], and (iii) contributes only minimally to the total contact resistance of the solar cell (in the order of maximal 1 Ω cm<sup>2</sup> ) [30]. Furthermore, an ideal capping layer, suited for contact passivation, should be either (i) highly doped or (ii) exhibit a high/low work function [31] in order to ensure selective excess charge carrier extraction.

The already proven success on electron-selective passivated contacts is also generating huge interest and research activities on hole-selective passivated contacts now. Pertaining to the feasibility studies of different tunnel layer candidates for hole-extracting passivated contacts, most previous reports had focused on using silicon-based oxides formed via either wet-chemical approaches (wet-SiOx) or UV/ozone photo-oxidation (ozone-SiOx) approaches. In our published works [28, 29, 32–34], a comprehensive evaluation of passivation quality and interface properties of silicon-based oxides (SiOx) and atomic layer-deposited aluminum oxides (ALD-AlOx) had revealed a larger potential for ALD-AlOx to be integrated in hole-selective passivated contacts as compared to the commonly used wet-SiOx or ozone-SiOx. This stems from a significantly higher negative fixed interface charge density (1 order of magnitude higher at <sup>6</sup> 1012 cm<sup>2</sup> ) even at a tunneling relevant thickness (just a few ALD cycles) while maintaining a relatively low interface defect density (Dit) of <sup>2</sup> <sup>10</sup><sup>12</sup> cm<sup>2</sup> eV<sup>1</sup> , which is comparable to the Dit of SiOx-based tunnel layers. The high negative fixed interface charges of the ALD-AlOx tunnel layer will accumulate holes at the c-Si interface, which will simultaneously enhance hole extraction probability and reduce surface recombination rates due to an efficient field-effect passivation in addition to the chemical passivation at the interface, as evident from the higher measured effective carrier lifetime (two orders of magnitude higher) than the passivation by either wet-SiOx or ozone-SiOx alone on symmetrically tunnel layer passivated n-type Cz wafers in our previous work [28]. These findings were consistent with literature for much thicker AlOx layers [35–39]. For hole-extracting capping layer materials, various candidates had been suggested, which includes highly p-doped polysilicon, transition metal oxide films with high work function such as molybdenum oxide (MoOx) [40–45], tungsten oxide (WOx), vanadium oxide (V2O5), cuprous oxide (Cu2O) [46], or alternatively organic polymers, such as poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) [47–49], among others. It is worthy to highlight that the transition metal oxide films exhibit a tunable work function between 4.7 and 7 eV [50, 51] by an appropriate combination of materials, while organic

our efficiency potential prediction (using the measured j<sup>0</sup> and R<sup>c</sup> values of our developed contact passivation layers), bifacial double-sided passivated contact solar cells (efficiency potential of 23.2%, using our layers) can clearly outperform rear-

Keywords: passivated contacts, contact passivation, silicon solar cells, double-sided

To meet the future energy needs, there is a need to develop low-cost alternative energy sources to complement the conventional energy sources (e.g., oil, gas, coal) as well as to address the pressing environmental issues associated with the latter. Hence, energy-related technology roadmaps are actively being released and revised toward the future energy needs. One good example is the International Technology Roadmap for Photovoltaic (ITRPV) [1]. In general, a successful deployment of any selected solar cell technology will be mainly dominated by (i) cost-effectiveness of the material and processes, (ii) scalability to high-volume manufacturing, (iii) device performance, and (iv) long-term stability of product. To progress toward item (iii), ITRPV predicts a continuous reduction of recombination losses in the wafer as well as at the front and rear surfaces of the solar cell. According to Ref. [2–4], given the considerable improvements in the wafer bulk, and surface passivation layers, the main source of recombination losses in high-efficiency solar cells is now dominated by the metal contacts. Thus, the ability to greatly reduce the recombination losses underneath the solar cell metal contacts (i.e., contact passivation) coupled with other technological advancements will be instrumental

side-only passivated contact solar cells (efficiency potential of 22.5%).

toward attaining the increasing solar cell efficiency targets.

One of the earliest examples of contact passivation can be found in the heterojunction silicon wafer solar cells, which utilizes a stack of intrinsic and doped amorphous silicon (a-Si:H) heterojunction layers [5–7] on both surfaces of the silicon wafer. The ultrathin (<5 nm) intrinsic a-Si:H layer not only serves to passivate the silicon surface but also to selectively enable hole or electron transport across this "tunnel layer," sandwiched between the overlying conductive a-Si:H layer and the crystalline silicon wafer. In this application, the contact-related recombination losses with the intrinsic/doped a-Si:H stack is significantly lower than utilizing the doped a-Si:H layers alone on the crystalline silicon wafers [5], hence establishing contact passivation for the former case. It can then be generalized that contact passivation can be established by deploying ultrathin passivating (and even in principal insulating, if thick) tunnel layers capped with a highly doped capping layer material with a suitable doping polarity or work function to form either hole-selective or electron-selective passivated contacts. Some examples of high/low work function capping layer materials such as transition metal oxides (WOx, VOx, etc.) and doped organic materials had been reported [4, 8].

Some prominent examples of single-junction silicon wafer-based high-efficiency (≥25%) solar cell concepts which adopt contact passivation include the amorphous silicon heterojunction interdigitated back contact (IBC) solar cell by Kaneka (26.6%) [9], the tunnel layer passivated interdigitated back contact (IBC) solar cell by SunPower (25.2%) [10], the polysilicon on oxide (POLO) passivated contact interdigitated back contact (IBC) solar cell by ISFH (26.1%) [11], and the conven-

tionally front- and rear-contacted tunnel layer passivated contact solar cell

(TOPCon) by the Fraunhofer ISE team (25.7%) [12]. The excellent performance of the TOPCon cell (despite being conventional front- and rear-contacted, instead of

passivated contacts

Silicon Materials

1. Introduction

90

polymers can also exhibit a tunable work function from 3.0 to 5.8 eV by chemical modification [8]. As an example, Zielke et al. [52] has demonstrated a cell efficiency of 18.3 and 20.6% for both n-type silicon and p-type silicon solar cells, respectively, which deploys a rear-side tunnel layer passivated hole-extracting metal contact using their specifically adapted organic PEDOT:PSS blend as capping layer. Such findings could open up new opportunities for potentially low-cost novel material integration for high-efficiency solar cell concepts in the future.

polysilicon passivated contacts for effective contact passivation on both textured and planar silicon surfaces. A major highlight of this work is the demonstration of a practical solar cell efficiency potential approaching 24% on a large area (6-inch wafer), by deploying the in-house developed passivated contact layers on both sides of an otherwise conventionally processed silicon solar cells with industrial screen-

Double-Sided Passivated Contacts for Solar Cell Applications: An Industrially Viable Approach…

Firstly, the in-house development of device quality passivated contacts based on wet-SiOx/poly-Si(doped), ozone-SiOx/poly-Si(doped), or in situ thermal-SiOx/ poly-Si(doped) stack was established using simple planar symmetrical lifetime test structures as sketched in Figure 1. Such structures are convenient for assessing (i) the resulting tunnel layer/doped capping layer stack thickness; (ii) the passivation quality, attributing from the passivated contacts alone (i.e., determining minority carrier lifetime τeff, reverse saturation current density J0, and implied open-circuit voltage iVOC); and (iii) the tunneling resistance (i.e., determining the contact resistance Rc). Starting from bare diamond-wire cut Cz silicon wafers (NorSun, 190 μm thick, and

Schematic of the (a-e) symmetrical lifetime and (f) contact resistance test structures utilized for assessing/ optimizing the passivation quality and minimizing the contact resistance of both, electron-selective and holeselective passivated contacts, developed on either a wet-SiOx, ozone-SiOx or an in-situ thermal-SiOx tunnel layer. Using the sketched symmetrically passivated contact test structures, the developed SiOx/poly-Si passivated contacts were characterized in terms of their passivation quality, doping profiles, film uniformity and contact

printed contacts.

Figure 1.

resistance.

93

2. Experimental details

DOI: http://dx.doi.org/10.5772/intechopen.85039

Regarding contact passivation, however, it is to be noted that in most of these reports, the carrier-selective passivated contacts were mostly deployed at the rear side of the solar cell, while the front side composes of a conventionally diffused silicon surface followed by the standard anti-reflection coatings and screen-printed fire-through metal contacts. Since the rear-side deployed passivated contacts can achieve an excellently low contact recombination loss, instinctively the next focus will be to reduce the contact recombination loss at the front side as well in order to improve device performance. With varying degrees of success using either electronselective or hole-selective passivated contacts in a standalone configuration, the question arises on the feasibility to integrate both electron-selective and holeselective passivated contacts together in a typical silicon solar cell architecture. Regardless of the technological advances, the fundamental driving factors toward industry adoption will still be the same as outlined earlier (i.e., cost-effectiveness of the material and processes, manufacturing scalability, device performance, and product stability). Hence, it is of keen interest in this paper to evaluate the feasibility of combining our optimized electron-selective and hole-selective passivated contacts obtained via industrial relevant processes onto an otherwise conventional front and rear screen-printed silicon solar cells and comparing that to solar cells with only a rear-side passivated contact scheme.

In this work, we will investigate "conventional" SiOx/poly-Si passivated contacts to be deployed on both sides of the solar cell, instead of only being deployed rear side. Using different lifetime test structures and solar cell structures, the following topics are investigated: (i) the influence of the tunnel oxide choice on the passivation quality, comparing wet-chemically formed oxides (wet-SiOx), UV photooxidation-formed ozone oxides (ozone-SiOx), and in situ thermal oxidation-formed oxides (thermal-SiOx); (ii) the impact on the contact passivation quality after doped silicon capping layers were applied upon the tunnel oxide layers on the same lifetime test structures (formed via tube diffusion doping of low-pressure chemical vapor deposition (LPCVD) of intrinsic polysilicon layers, to serve as either electronselective or hole-selective capping layers); (iii) the influence of the surface conditions on the passivation quality by both types of electron- or hole-selective passivated contacts; (iv) the integration of the optimal passivated contacts onto a practical double-sided passivated contact solar cell structure and studies on the resulting passivation quality, both prior to and after subsequent anti-reflection passivation layers (i.e., SiNx layers) were applied; (v) the influence of the capping layer thickness on the absorbable cell current and various parasitic absorption losses via numerical analysis (SunSolve™) and our experimental approaches to realize ultrathin poly-Si capping layers; and (vi) the ability to apply screen-printed metal contacts on the developed electron-selective and hole-selective passivated contacts.

In addition, from the measured passivation quality results on lifetime test structures, a numerical calculation of the practical solar cell efficiency potential adopting both of our developed electron-selective and hole-selective passivated contacts was performed by utilizing the measured saturation current density J<sup>0</sup> and the measured contact resistance R<sup>c</sup> from our investigated tunnel layer passivated contact test structures. This work demonstrates the feasibility and attractiveness of using industrial relevant processes to develop device quality tunnel oxide/doped

Double-Sided Passivated Contacts for Solar Cell Applications: An Industrially Viable Approach… DOI: http://dx.doi.org/10.5772/intechopen.85039

polysilicon passivated contacts for effective contact passivation on both textured and planar silicon surfaces. A major highlight of this work is the demonstration of a practical solar cell efficiency potential approaching 24% on a large area (6-inch wafer), by deploying the in-house developed passivated contact layers on both sides of an otherwise conventionally processed silicon solar cells with industrial screenprinted contacts.
