Amorphous Silicon/Crystalline Silicon Heterojunction Solar Cells

Christophe Ballif*,1; Stefaan De Wolf*; Antoine Descoeudres*; Zachary C. Holman†    * Photovoltaics and Thin-Film Electronics Laboratory, Institute of Microengineering (IMT), Ecole Polytechnique Fédérale de Lausanne (EPFL), Neuchâtel, Switzerland
† School of Electrical, Computer, and Energy Engineering, Arizona State University, Tempe, Arizona, USA
1 Corresponding author: email address: christophe.ballif@epfl.ch

1 Introduction

Silicon wafer-based solar cells have dominated the photovoltaics market for decades and may well continue to do so for years to come. Several key factors explain the success of this technology: Silicon is a well-studied semiconductor with known optoelectronic properties; it is abundant and nontoxic, and the price of multicrystalline silicon has witnessed an unprecedented drop in the last few years, partially because of a temporary production overcapacity, especially in Asia; and silicon solar cell technology has greatly benefited from the accumulated knowledge in semiconductor processing developed by the microelectronics community. An important strength of the current industrial silicon solar cell technology is its fabrication simplicity. Only a few steps suffice to fabricate a full device, where each step often fulfills several roles. Examples of this are the emitter diffusion process, which simultaneously getters impurities from the bulk of the wafer, and the metal contact firing through the silicon nitride anti-reflection coating, during which bulk hydrogenation of the wafer also occurs. A drawback of this simplicity is that further improvements in device performance must rely on the increasing sophistication of existing processes, while fundamental shortcomings of the technology are hard to overcome. One such critical limitation is carrier recombination at the electrical contacts.

Carrier recombination in silicon is a well-understood phenomenon and its minimization is a key factor in obtaining high-efficiency solar cells. We make a distinction between intrinsic recombination (Auger and band-to-band radiative recombination) and deep-defect-mediated recombination (Richter et al., 2013). Importantly, the latter type of recombination can in theory be completely eliminated by using perfect crystals, combined with an “ideal” solar cell architecture. This structure should feature perfectly passivated surfaces and contacts. Taking a 100-μm-thick wafer, such a solar cell would yield an open-circuit voltage (Voc) of about 770 mV (Richter et al., 2013; Tiedje et al., 1984). Note that, with perfect contacts, the Voc represents the energetic distance between the quasi-Fermi levels, which themselves express the density of excess charge carriers present in the material as a consequence of shining light on it. An important reason why the Voc can never equal the bandgap of the absorber—1.12 eV for crystalline silicon (c-Si) at room temperature—is not only the operating temperature of the device but also the aforementioned intrinsic recombination losses. Despite this, it is possible to come close to the 770 mV limit in real devices with excellent surface and contact passivation.

Defect recombination in the bulk of c-Si has been successfully combated in recent decades, as evidenced by the ever-increasing quality of silicon wafers on the market. Surface passivation has also improved: A number of dielectric passivation layers are available that can passivate p-type and n-type surfaces very well. These include materials like silicon oxides (Benick et al., 2011; Deal and Grove, 1965; Green, 2009; Schultz et al., 2004; Zhao et al., 1998), amorphous silicon nitrides (Lanford and Rand, 1978; Lauinger et al., 1996) and aluminum oxide (Agostinelli et al., 2006; Hezel and Jaeger, 1989; Hoex et al., 2006). Surface passivation can be accomplished in two fundamentally different ways: Either the surface defect states are removed, or the excess charge carriers are screened from the surface defects by an internal electrical field. The former is known as chemical surface passivation and can be obtained by, e.g., hydrogenation of these defects. The latter is known as field-effect passivation and is usually obtained by deposition of a fixed-charge dielectric on the surface under study, thereby repelling minority carriers inside the wafer from the defective surfaces. Positive-fixed-charge dielectrics repel the positively charged holes inside the semiconductor from the surfaces, and are ideally suited to passivate n-type surfaces. A prime example is silicon nitride, which has been used for the passivation of phosphorus-doped emitters in homojunction technology (Lanford and Rand, 1978; Lauinger et al., 1996). Negative-fixed-charge dielectrics repel negatively charged electrons from the surfaces and are used to passivate p-type surfaces. Here, the most studied dielectric is aluminum oxide, which is a material that received significant attention in the last few years as a potential passivating layer for the rear surface of homojunction solar cells (Agostinelli et al., 2006; Hezel and Jaeger, 1989; Hoex et al., 2006). Silicon nitride layers can be relatively easily integrated into existing c-Si solar cell processing, whereas the successful integration of aluminum oxide layers into industrial solar cells has proven to be more of a challenge.

In all cases, contacts are needed to extract carriers from the solar cell. In standard homojunction solar cell technology, where the junction is fabricated by thermal diffusion or ion implantation, these contacts are usually defined by locally opening the dielectric passivating layers and making a direct Ohmic contact between the metal and semiconductor. Whereas the contact resistances of such contacts can be made low, the minority-carrier recombination occurring at their surfaces is of significant concern. This issue is fundamentally resolved by silicon heterojunction technology, where a thin, wider-bandgap layer is inserted between the metal contact and the optically active absorber (i.e., the silicon wafer). Qualitatively, this type of contact can be considered as a semi-permeable membrane for carrier extraction. On the one hand, it should prevent generated carriers from being collected instantaneously, as this will lower the energetic splitting of the quasi-Fermi levels and thus reduce the voltage of the device. On the other hand, the contacts should be sufficiently electronically transparent to guarantee that carriers can be collected at the device terminals before they recombine in the wafer due to intrinsic recombination processes. In principle, such contacts can be fabricated in several ways. Irrespective of the materials used, passivated contacts should feature excellent (chemical) surface passivation while also giving charge carriers an incentive to be driven toward either the electron- or the hole-collecting layers. In this chapter, the focus will be on heterojunction solar cells with layers fabricated from thin films of amorphous silicon or related materials.

2 Passivating c-Si Surfaces with a-Si:H

For silicon wafer-based devices, thin films of hydrogenated amorphous silicon (a-Si:H) are particularly appealing candidates for passivated-contact formation. First, a-Si:H passivates c-Si surfaces very well, with electrical properties that are on par with the best dielectrics available. The passivation is mostly chemical, principally due to hydrogenation of surface states. Second, such layers can be doped relatively well, either n- or p-type, by adding the appropriate process gasses during deposition. This property enables the fabrication of contacts that are selective for either electron collection (when n-type a-Si:H is used), or hole collection (when p-type a-Si:H is used). This is of significant utility, as it allows us to not simply make passivating contacts but also to escape the need for a homojunction in the wafer. As the lateral conductivity of the a-Si:H layers is quite low, transparent electrodes that serve electrical and optical roles are usually applied. Another important reason for the success of such layers is the available knowledge regarding thin-film deposition technology. Whereas silicon homojunction solar cell technology has benefited greatly from developments taking place within...