The base–emitter junction, which is the first facing the sun, is made of a high-bandgap semiconductor (designated as E H), and the collector is made of a low-bandgap semiconductor (designated as E L). Anticipating final results, the directions of the currents have been chosen in a way that they will all become positive under normal cell operation. Arrows indicating the direction that electron and hole current densities have through the base–emitter and base–collector junctions (when assumed positive) are also indicated. We will assume an npn structure but the same analysis trivially applies to a pnp structure by swapping the role of electrons and holes.
However, as we shall see, for the ideal performance of the solar cell structure we propose γ→0.įigure 1 shows the basic structure and simplified bandgap diagram of the three-terminal heterojunction bipolar transistor solar cell (HBTSC) that we propose. In order a BJT to approach its ideal performance, α T→1 and γ→1. The performance of a BJT is described in terms of its ‘transport factor’ ( α T) and ‘emitter injection efficiency’ ( γ). The structure we propose looks similar to the one of a bipolar junction transistor 11 (BJT). Three- or more- terminal solar cell structures have been proposed in the past, but consisted either in a mechanical stack of single-gap solar cells 8 or included a doped semiconductor layer to interconnect the solar cells in a (n/p)–p–(n/p) 9 or (n/p)–p–(p/n) 10 configuration. The proposed solar cell structure has three-terminals. In this work we propose the use of p/n/p (or n/p/n) structures instead, that exhibit the same limiting efficiency that a dual-junction solar cell, but without the need of using tunnel junctions or wafer bonding schemes for interconnecting the cells. More recently, a ‘wafer bonding’ scheme has been developed 7 to attach different solar cells each one consisting of a (p/n) junction. The problem could be solved thanks to tunnel diodes (a highly doped p ++–n ++ junction), invented by Esaki 3 and nowadays incorporated to multijunction solar cells 4, 5, 6. Wolf 2, in 1960, pointed out the difficulty in using tandem solar cells based on (p/n)–(p/n) semiconductor junctions to take to practice this approach because of the impossibility of having electrical current circulating across a p/n junction biased in reverse. The use of several semiconductors of different bandgaps to make a better use of the solar spectrum for photovoltaic energy conversion was first proposed by Jackson 1 in 1958.
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