Báo cáo hóa học: Characterization of silicon heterojunctions for solar cells

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Báo cáo hóa học: " Characterization of silicon heterojunctions for solar cells"Kleider et al. Nanoscale Research Letters 2011, 6:152http://www.nanoscalereslett.com/content/6/1/152 NANO EXPRESS Open AccessCharacterization of silicon heterojunctions forsolar cellsJean-Paul Kleider1*, Jose Alvarez1, Alexander Vitalievitch Ankudinov2, Alexander Sergeevitch Gudovskikh3,Ekaterina Vladimirovna Gushchina2, Martin Labrune4,5, Olga Alexandrovna Maslova1,3, Wilfried Favre1,Marie-Estelle Gueunier-Farret1, Pere Roca i Cabarrocas4, Eugene Ivanovitch Terukov2 Abstract Conductive-probe atomic force microscopy (CP-AFM) measurements reveal the existence of a conductive channel at the interface between p-type hydrogenated amorphous silicon (a-Si:H) and n-type crystalline silicon (c-Si) as well as at the interface between n-type a-Si:H and p-type c-Si. This is in good agreement with planar conductance measurements that show a large interface conductance. It is demonstrated that these features are related to the existence of a strong inversion layer of holes at the c-Si surface of (p) a-Si:H/(n) c-Si structures, and to a strong inversion layer of electrons at the c-Si surface of (n) a-Si:H/(p) c-Si heterojunctions. These are intimately related to the band offsets, which allows us to determine these parameters with good precision. (so-called “ intrinsic ” layer, which leads to the “ HIT ” -Introduction heterojunction with intrinsic thin layer denomination [2]).In the field of silicon solar cells, recent progress has This limits interface recombination and leads to very highbeen achieved in two directions: silicon heterojunctions open circuit voltages [3]. Band offsets between a-Si:H andand silicon nanowires. These two topics are briefly c-Si also play a crucial role because they determine theaddressed here and we show some new characterization band bending, which influences the carrier collection. Weresults that use conductive-probe atomic force micro- here demonstrate the existence of a conduction channelscopy (CP-AFM) measurements. along both the (n) a-Si:H/(p) c-Si and the (p) a-Si:H/(n) Silicon heterojunctions are formed between crystalline c-Si interfaces from direct CP-AFM measurements per-silicon (c-Si) and hydrogenated amorphous silicon (a-Si: formed on cleaved sections of solar cells. We show fromH). Solar cell efficiencies of up to 23% have been demon-strated on high quality n-type c-Si wafers with layers of additional planar conductance measurements and simula-p-type a-Si:H deposited at the front (as the emitter) and tions that these are related to strong inversion regionsn-type a-Si:H deposited at the back (as the back surface at the interfaces. From the temperature dependence, we determine the values of band offsets.field), respectively [1]. Since transport properties are quitepoor in a-Si:H due to the large amount of defects andband gap states and low carrier mobilities, the doped a-Si: Experimental detailsH layers are used to form the junctions, but their thickness Solar cell structurehas to be kept very low. The front a-Si:H layer has to be A typical solar cell structure based on a-Si:H/c-Si het- erojunctions formed with n -type c -Si is presented invery thin in order to minimize absorption of incomingphotons and to privilege absorption in c-Si. One key fea- Figure 1. A similar structure stands for p -type c -Si, replacing the n-type a -Si:H by p-type a-Si:H and viceture of the Si heterojunctions is the very good passivationpr ...