Читать книгу Physics of Thin-Film Photovoltaics - Victor G. Karpov - Страница 14

We recall that PV effect in general is presented by the light generated voltage. The PV voltage when the system absorbed light creates electric asymmetry in the form of spatially separated opposite electric charges, electrons and holes, which can be extracted and utilized in the form of electric current. For the charge separation to occur, there must be a built-in electric field as illustrated in Fig. 1 presenting a basic design of PV devices made of two semiconductor layers. The built-in field originates from the junction formed by those layers. Historically, such junctions were between p- and n-types of semiconductor layers; hence, the name of p-n junctions traditionally associated with PV devices. However, p-n junctions are neither necessary nor sufficient elements of systems with built-in fields as discussed below. For example, semiconductor/metal junctions in Fig. 1 will generate their own built-in fields that can be either beneficial or detrimental to PV device functionality.

In general, the built-in electric fields always emerge with junctions of any two chemically different materials. The underlying physics is that one of the materials will be energetically more favorable for the electrons than the other. To minimize the system energy, the electrons will therefore move there leaving the one with unbalanced positive charge; hence, the built-in field between the spatially separated opposite charges of electrons and holes, qualitatively similar to that of electric capacitor. The number of electrons moving across the junction is determined by the balance between the above mentioned energy gain and the energy loss due to the necessity of overcoming the Coulomb attraction to the positive charges left behind. The built-in electric fields of that nature are omnipresent and are not limited to photovoltaics, or p-n junctions, or other artificial structures.

Fig. 1 Conceptual design of a solar cell. Front and back contacts are metallic, and the former one is transparent to light (shown in waving lines). The presented built-in electric field E is caused by the dark positive and negative charges shown as + and −. R is a load resistor. For specificity, the diagram presents a two semiconductor layer design, such as p- and n- materials with the field E in their junction proximity. However, sufficient electric fields can exist as well in the proximities of semiconductor-metal junctions.

Another reading of the latter statement is that photovoltaics do not necessarily have to be related to or understood in terms of p-n junctions. In reality, any (not only p-n) junctions of different materials produce built-in electric fields. Some of them, but not all, create photovoltaic effect.

For example, a junction of two metals produces the built-in field underlying the phenomenon of thermoelectricity, but not suitable for PV because the light does not penetrate in a metal deep enough and because that field is screened (by metal electrons) beyond a nanometer thin region, insufficient for light absorption. However the built-in fields of metal/semiconductor (rather than metal/metal) junctions can make good diodes and PV devices.

As another example, we point at a charge acquired by a solid particle immersed in a liquid. Curiously, that example explains how wines and many other liquid products consist of charged micro particles suspended in a somewhat ionized water. We will return to that example later in the book describing the so called “red wine effect” in PV.

Note that junction fields require electronic exchange between two materials while their direct physical contact is not necessary. As an illustration, chemically different metal electrodes of a capacitor will spontaneously acquire opposite charges proportional to the difference in their work functions. That process often called the ‘dielectric absorption’ takes time sufficient for the inter-plate electron transfer. Along the same lines, two different metals separated by a dielectric or semiconductor layer will exchange electrons forming the built-in electric field throughout that layer as described more in detail next (Sec. II).