1 Introduction
Silicon solar cells with passivation contact structures utilize
hydrogenated amorphous silicon (a-Si:H) or silicon oxide
(SiOx) as a highly effective passivation
layer[1,2]. This layer is inserted between the
crystalline silicon (c-Si) wafer and the carrier transport layer,
resulting in drastically reduced interface recombination losses and
higher efficiency when compared to PERC (Passivated Emitter and Rear
Cell) solar cell. An exemplary type of solar cells using passivation
contacts is the silicon heterojunction (SHJ) solar cell, which is known
for its high conversion efficiency, simple preparation process and low
process temperature. Recently, the power conversion efficiency (PCE) of
SHJ solar cell has reached 26.8%[3]. However, the
short-circuit current density (Jsc) of SHJ solar cell
tends to be lower than that of PERC cell, which is mostly attributable
to the parasitic absorptions of intrinsic a-Si:H (a-Si:H(i)),
highly-doped nano-crystalline silicon (nc-Si:H), and transparent
conductive oxides (TCO) layer[4,5]. Compared to
the passivation intrinsic a-Si:H layer, the parasitic absorption loss
induced by the highly-doped nc-Si:H layer is more pronounced, as a
certain thickness is required to form an effective built-in potential.
In addition, highly-doped nc-Si:H also faces problems such as Auger
recombination loss and complex doping techniques using toxic gases.
In order to replace highly-doped nc-Si:H, some dopant-free passivation
contact structures have been explored. Compound semiconductors that
feature a wide bandgap were used as the carrier transport layer (contact
layer) in creating a silicon/compound heterojunction (SCH) solar cell
with a c-Si wafer. Theoretically, it has the potential to mitigate
energy loss associated with heavy doping, such as Auger recombination,
bandgap narrowing, and free carrier
absorption[6,7]. Furthermore, the range of work
function in compound semiconductors is broader compared to that of doped
a-Si:H or nc-Si:H. This characteristic is advantageous in designing and
producing devices with higher built-in potentials in principle.
Many oxides and fluorides have been adopted as carrier transport layers.
High work function transition metal oxides (TMO), such as
MoOx[8,9],
WOx[10], and
V2Ox[11], are
utilized as hole transport layers (HTL) in SCH solar cells. The ones
with low work function, such as
SnO2[12],
TiO2[13,14] and
LiF[15,16], are used as electron transport layers
(ETL). These ETL and HTL compounds can be prepared using some simple
techniques, such as thermal evaporation, atomic layer deposition (ALD)[17], electron beam
evaporation[18], and
spin-coating[19], which have the potential for
low-cost compared to plasma enhance chemical vapor deposition (PECVD)
commonly used for a-Si:H and nc-Si:H.
Nowadays, SHJ solar cells are mostly based on n-type c-Si wafers and
commonly use a front-contact back-junction structure, where n-type
nc-Si:H (nc-Si:H(n+)) layer and p-type a-Si:H (or
nc-Si:H) emitter are respectively located on the front and rear side of
the cell. The main reason for adopting this structure is that p-type
doping of a-Si:H or nc-Si:H has lower efficiency than n-type doping,
thus requiring a larger thickness, which leads to more severe parasitic
absorption when located on front side of the cell. Therefore, finding
new emitter materials with higher transmittance and larger work function
to replace p-type a-Si:H or nc-Si:H is more urgent compared to replacing
n-type nc-Si:H. MoOx with a wide bandgap and high work
function has been considered the most probable alternative to p-type
a-Si:H or nc-Si:H and the MoOx/c-Si(n) SCH solar cells
have received more and more attention [8,9,20]. A
PCE of 23.85% has been achieved by Cao and Paul Procel et al.
recently[21].
However, the thermal stability of MoOx/c-Si(n) SCH solar
cells is an important issue and it was found that the device performance
decreases significantly when annealed above 130 oC[8, 22]. The device degradation could mostly be
attributed to the formation of a SiOx barrier layer at
the interface between MoOx and
a-Si:H[14,23,24]. The large area metallization
process (screen printing Ag grid) commonly used in SHJ solar cells is
not suitable for SCH solar cells, since a solidification temperature of
about 200 oC is required. The metal electrode
preparation techniques reported in the literature for the SCH solar
cells are mainly electroplating[8] or thermal
evaporation [14]. Up to now, limited by the
metallization process, all the reported MoOx/c-Si(n) SCH
solar cells, no matter front-junction or back-junction, are small-area
devices. Besides the metallization process, deposition of
MoOx film is also an important factor hindering the
preparation of large-area SCH solar cell. So far, the
MoOx HTL in most of the reported high-efficiency SCH
solar cells[20,21] was prepared using thermal
evaporation which is not conducive for continuous deposition of uniform
large-area films. Exploring large-area SCH solar cell processes is of
great significance for future mass production.
In this paper, studies on large-area MoOx/c-Si(n) SCH
solar cells featuring a front-contact back-junction architecture were
conducted. Large-area MoOx films were deposited using
hot-wire oxidation-sublimation deposition (HWOSD)
technique[25]. By optimizing the device
fabrication process flow including screen printing of the silver grids,
the MoOx layer can avoid suffering from high-temperature
process. By using ICO/Ag as the back reflector, a power conversion
efficiency (PCE) of 21.59% was achieved on the champion SCH solar cell
with the size of 274.15 cm2 (M6).