Sputtered Tungsten Oxide as Hole Contact for Silicon Heterojunction Solar Cells

Abstract

Reactively sputtered tungsten oxide (WOx) was investigated as hole contact on n-type crystalline silicon. Varying the oxygen gas flow during sputtering enables variation of the WOx conductivity from 0.01 to 1000 Ω/cm, while the band bending at the interface and the implied fill factor (FF) change by 70 meV and 1.5 %. Sputtered WOx shows higher resistivity and higher absorption in the visible range compared to ITO. Therefore stacks of WOx and ITO are used in solar cells. It was found that at least 20 nm thick WOx is needed to prevent detrimental effects of the ITO work function on the band bending at the junction, the implied FF and real FF of solar cells. WOx hole contacts of different thicknesses and conductivity were applied in solar cells and it was found that the highest FF is achieved using about 20 nm thick interlayers of WOx with the highest possible conductivity. It was found that sputtering enables a drastic improvement of WOx/silicon solar cells compared to thermal evaporation, due to the precise control of the WOx conductivity. Unfortunately the resistivity of sputtered WOx is still limiting the FF of these devices.

Passivating Contacts for Crystalline Silicon Solar Cells: From Concepts and Materials to Prospects

“…It should also be noted that, irrespective of the MoO x deposition method, an interlayer such as a-Si:H is still needed between MoO x and c-Si for good surface passivation. Furthermore, the WF value can be significantly lowered by the presence of oxygen vacancies, as has been demonstrated for tungsten oxide (WO x ) [101], [104], [111], which also owes its hole selectivity to a high WF value, similar to MoO x . However, the demonstrated high conversion efficiencies clearly underline the promising potential of MoO x in high-efficiency SHJ solar cells and hybrid combinations of solar cell structures, in which advantages in terms of transparency and simple manufacturing are combined, can be devised as well [12], [17].…”

“…For c-Si, MoO x can mainly provide selectivity toward holes through its high WF (>5.5 eV) in combination with its wide bandgap, which induces an accumulation of holes at the silicon surface. Based on these properties, MoO x can be considered as part of a family of materials together with WO x [97], [98], [101], [102], and vanadium oxide (VO x ) [97], [98], [103]. These metal oxides have been used on the front side of a classical SHJ solar cell to replace the p-type a-Si:H layer [94], [97], [99], [101], [102], [104] or to improve the contact between the p-type a-Si:H layer and the TCO [102], [103], [105].…”

Mitigating Plasmonic Absorption Losses at Rear Electrodes in High‐Efficiency Silicon Solar Cells Using Dopant‐Free Contact Stacks

“…Moreover, their fabrication methods are generally simple and only require a low processing temperature (<200 °C). As HTLs, organic semiconductor materials such as PEDOT:PSS, as well as metal oxides (particularly MoO x , NiO x , WO x , V 2 O x ,), are at present extensively explored. For example, our research group has successfully demonstrated efficient hole selectivity of MoO x , to replace p‐type hydrogenated amorphous silicon [a‐Si:H(p)] in silicon heterojunction solar cells .…”

Evolution of Optical, Electrical, and Structural Properties of Indium Tungsten Oxide upon High Temperature Annealing

“…[7] Novel contact stacks, in which thin layers of different transition metal oxides are applied as carrier selective contacts, [8] have become a broad field of interest. Especially, high work function materials, such as MoO x , [1,9,10] VO x , [11] and WO x , [1,12,13] have the potential to improve carrier selectivity and, therefore, both the device's open circuit voltage and the fill factor. [14] Tungsten(VI) oxide (WO 3 ) is a wellknown material, used as electrochromic layer in shading windows [15] and as an anode in water splitting devices.…”

“…The drawback of stoichiometric tungsten(VI) oxide as contact layer is the comparably low conductivity. [12,18] Other metal oxides, e.g., indium(III) oxide (In 2 O 3 ), are known for their high conductivity, which can be even increased by doping with various elements, such as hydrogen, [19] zinc, [19] molybdenum, [20] and tungsten. [21] Different to the well-known dopant tin, these dopants increase the conductivity by increasing not only the charge carrier concentration, but also the charge carrier mobility.…”