Category Next Generation Photovoltaics High efficiency through full spectrum utilization

The effective acceptance angle

According to the definition given in section 13.1.2 for the acceptance angle a, 100% of the rays within the acceptance angle cone are collected. This acceptance angle lacks practical interest for two reasons. First, because an optical efficiency better than that with which the geometric rays are collected must be considered. Therefore, the discussion must be done in terms of the directional optical efficiency, nopt(u) i. e. the optical efficiency of the designed concentrator as a function of the direction unit vector u of the incoming light. Second, because it is obvious that if a few percent of the rays are not collected at a certain incidence angle, the concentrator is still useful at that incidence angle.

Ъе1(%)

Подпись: Figure 13.2. Examples of relative directional optical efficiencies. The dotted curve corresponds to a concentrator which does not achieve the isotropic illumination, while the full and broken curves do.
We will discuss in this section how an effective acceptance angle within ...

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Optical efficiency

A first definition of the optical efficiency [6, 8], nopt, i, is the light power transmission efficiency through the concentrator up to the cell surface for light rays impinging at the entry aperture from a given nominal direction v (usually refer to as the normal incidence). This definition is wavelength dependent and cell independent.

A second definition [1] nopt,2 is obtained when the nominal parallel incident rays produce one-sun irradiance with the solar spectrum at the entry aperture and it is given by the ratio of the concentrator photocurrent to the product of the geometrical concentration and the cell photocurrent when illuminated at one sun with the solar spectrum...

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Geometrical concentration

The different possible definitions of the geometrical concentration come from the definitions of both the entry aperture area and cell area.

With respect to the concentrator entry aperture area, sometimes the fully occupied area AE, full (for instance, the one defined to tessellate concentrator units) is used, while other times, if a portion of the aperture is clearly inactive by design (for instance, if there is a gap), it is excluded from the aperture area, leading to AE, act. It should be noted that the concentrator topology may affect the decision of how to define the aperture area: inactive portions in linear systems (e. g. the central gap in parabolic trough technologies) have commonly been excluded [6], [7], while in rotational optics the full aperture area has been used [9]...

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Definitions of geometrical concentration and optical efficiency

Several definitions of both the geometrical concentration Cg and optical efficiency nopt of PV concentrators are commonly used in the literature. Of course, the definitions do not affect the device performance and cost but this situation
may be confusing for the interested reader when trying to compare different concentrators. We will briefly discuss about these definitions next and we will adopt one criterion for this chapter.

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Concentration and acceptance angle

Concentrators can be accurately analysed in the framework of geometrical optics, where the light can be modelled with the ray concept. This is deduced from statistical optics [5], in which geometrical optics appears as the asymptotic limit of the electromagnetic radiation phenomenon when the fields can be approximated as globally incoherent (also called quasi-homogeneous). In this approximation sources and receivers are non-punctual (usually called extended sources and receivers) and the diffraction effects are negligible.

Assume that a certain PV concentrator, which points in one nominal direction given by the unit vector, v, ensures that all the light rays impinging on the concentrator entry aperture forming angles smaller than a with v are transmitted onto the cell with incidence angles...

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Desired characteristics of PV concentrators

The next-generation concepts need concentrators for which the features are, in many cases, not yet fully defined. It seems that most of the present next- generation devices will need the same features as those required for the present PV concentration cells and, thus, we will refer to them here.

The PV concentrator constitutes a specific optical design problem, with features that make it very different from other optical systems. Even designs for other optical concentration applications (like solar thermal energy, wireless optical communications, high-sensitivity sensors, etc) may not be suitable for PV. Generally speaking, PV concentrators must be [3]

(3) capable of producing high concentration (> 1000 x) for a high cell cost,

(4) noticeably insensitive to manufacturing and mounting inacc...

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Concentrator optics for the next-generation photovoltaics

P Benitez and J C Minano

Institute de Energia Solar—Universidad Politecnica de Madrid ETSI TelecomunicaciOn Ciudad Universitaria s/n—28040 Madrid, Spain

13.1 Introduction

Next-generation photovoltaic (PV) converters aim to be ultra-high efficiency devices. In order to be so efficient, it is expected that the cost of these converters will also be very high per unit area. Although it has been claimed [1] that the next-generation PV approaches should aim at high efficiency but also low-cost per unit area using thin-film technologies, perhaps it will be more probable that we first find a highly efficient next-generation solution with high cost per unit area as there will be fewer restrictions.

As an example, let us imagine that next-generation devices achieving 70% efficiency were to be in...

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Germanium layer transfer for photovoltaic applications

Multi-junctions solar cells can be processed on germanium substrates. However, these substrates are very expensive and the possibility of replacing them with germanium layers bonded on silicon substrates has been studied [10]. The process used consists in implanting H+ ions into germanium, to bond the implanted germanium wafer to a virgin silicon wafer and splitting the germanium layer. In this experiment, direct bonding of germanium and silicon was used, in order to obtain an ohmic contact without altering the transmission by a metallic layer. A specific contact resistance of 400 ^ cm2 was obtained after annealing at 350 °C. This value of resistance is too high for solar cells applications but it could be improved by annealing at higher temperature...

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