Advanced Contacts For Crystalline Silicon Solar Cells
Date
2016
Authors
Bullock, James
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Abstract
Mainstream dopant-diffused crystalline silicon (c-Si) solar cells have reached a point in their development where losses at the directly-metalized, heavily-doped regions have a significant, and often limiting effect on device performance. The conventional wisdom on addressing this issue is to drastically reduce the percentage of the contacted surface area–to less than 1% in some cases–significantly increasing the complexity of fabrication. An alternative approach is to focus on addressing the losses at the metal / cSi interface by implementing novel ‘carrier-selective’ contacting structures. This approach to solar cell contacting has the potential to increase the output power whilst significantly simplifying cell architectures and fabrication procedures. This thesis is centered on the conceptual and experimental development of a number of advanced contacting structures for c-Si solar cells, collectively referred to here as ‘heterocontacts’. The ‘carrier-selectivity’ of the contact, that is, how well it collects just one of the two carriers (whilst preserving the other), is used as a universal concept for comparing different contacting strategies, including mainstream contacts based on direct metallization of heavily doped c-Si. To provide a foundation on this topic the initial section of the thesis discusses the concept and theory of carrier-selectivity. This is complemented with an in depth literature review of current state-of-the-art contacting practices for c-Si solar cells. This provides a reference frame with which to compare the three experimental chapters that follow. In the first experimental chapter it is shown that a suitable initial stepping stone towards advancing solar c-Si cell contacts is to combine the benefits of conventional dopant-diffused regions with those of heterocontacts. A number of such hybrid systems are demonstrated and optimized at the contact level through multiple dedicated studies focused on using thin silicon oxide (SiOx), aluminum oxide (AlOx) or hydrogenated amorphous silicon (a-Si:H) passivating interlayers. These interlayers are shown to reduce carrier recombination at the contact surface by up to two orders of magnitude. In a later study we develop and demonstrate a novel a-Si:H enhanced Al / SiOx / c-Si(n+) heterocontact concept. This structure is also explored at the solar cell level, yielding an efficiency of 21% in the initial stages of development – equivalent to that of an analogous cell made with the conventional directly metallized partial contact technique. In the succeeding chapter, the logical next stage in the development of such a concept is explored, that is, to completely remove the heavily doped surface regions, instead using the heterocontacts exclusively to separate electrons and holes. It is demonstrated that this can be achieved using materials with extreme work functions. For the collection of holes, sub-stoichiometric molybdenum oxide MoOx is utilized, favored for its transparency and large work function. Over multiple studies, it is demonstrated that MoOx heterocontact systems, both with and without passivating interlayers can be used to effectively collect holes on both n and p-type c-Si absorbers. This enables its application to a number of novel solar cells architectures, most prominently a novel MoOx partial rear contact cell attaining conversion efficiencies over 20% in the initial proof-ofconcept stage. In the final experimental chapter, a complementary electron heterocontact system is developed, based on a low work function LiFx / Al electrode. This is shown to provide ix excellent electron collection characteristics, both with and without a-Si:H passivating interlayers. The exceptional contact characteristics enabled by this heterocontact allow the demonstration of a first-of-its-kind n-type partial rear contact cell already with an efficiency above 20% in its first demonstration. To conclude the thesis and demonstrate its premise, a novel c-Si cell is developed without the use of dopants. This cell, referred to as the dopant free asymmetric heterocontact (DASH) cell, combines the previously mentioned MoOx based hole contacts and LiFx based electron heterocontacts, both with passivating a-Si:H interlayers. A conversion efficiency of 19.4% is attained for this proof-of-concept device— an improvement by more than 5 percent absolute from the previous DASH cell record and more importantly the first demonstration of such a concept to be competitive with conventional cell designs.
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Silicon Solar Cells
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