Dopant-free passivating contacts
for photovoltaics have the potential to be deposited at low costs
while providing excellent surface passivation and low contact resistance.
However, one pressing issue of dopant-free carrier selective contacts
is their lower environmental stability compared to conventional silicon-based
contacts. In this contribution, we study the degradation in the ZnO/LiF
x
/Al electron selective nanocontact with experiments
and simulations and suggest design modifications for higher performance
and stability. Using a thicker metallization and optimal ZnO deposition
temperature (130 °C), we improved open-circuit voltage and fill
factor, together with improved stability with retention of over 93
and 88% of the initial open-circuit voltage and fill factor after
storage in air for 380 h. The champion device has reached an efficiency
of 21.3% with V
OC of 727 mV, J
SC of 37.6 mA/cm2, and FF of
78.0%. Furthermore, the enhanced stability in vacuum, scanning transmission
electron microscopy (STEM) images, and the current-exchange simulation
suggests that the degradation of the a-Si:H(i)/ZnO/LiF
x
/Al contact is caused by a drop of the LiF
x
/Al work function, due to interaction with air.
This work has developed a deep understanding of the degradation mechanism
and the methodology of stability analysis for dopant-free silicon
solar cells.
Herein, challenges in the fabrication of full dopant‐free bifacial silicon solar cells are discussed and efficient devices utilizing a MoO3/ indium tin oxide (ITO)/Ag hole‐selective contact and ZnO/LiFx/Al electron‐selective contacts with up to 79% short‐circuit current bifaciality are demonstrated. The ZnO/LiFx/Al rear electron contact features a full‐area ZnO antireflective coating and a LiFx/Al finger contact, allowing sunlight absorption from the back side, thus producing more overall power. The ZnO/LiFx/Al electron contacts with a thinner ZnO layer and a larger contact fraction display a better selectivity and a lower resistance loss. When considering rear‐side irradiance of 0.15 sun, the dopant‐free bifacial solar cell with 60 nm ZnO and 50% LiFx/Al metal contact fraction achieves a 3% estimated output power density improvement compared with its monofacial counterpart (21.0 mW cm−2 compared to 20.3 mW cm−2) using the full‐area back contact. Both the efficiency and bifaciality factor of this dopant‐free device are still significantly lower than those of state‐of‐the‐art devices relying on doped‐silicon‐based layers. The required improvement for this technology to become industry‐relevant is discussed.
The window-layer stack limits the efficiency of both-side-contacted silicon heterojunction solar cells. We discuss here the combination of several modifications to this stack to improve its optoelectronic performance. These include the introduction of a nanocrystalline silicon-oxide p-type layer in lieu of the amorphous silicon p-type layer, replacing indium tin oxide with a zirconium-doped indium oxide for the front transparent electrode, capping this layer with a silicon-oxide film, and applying a post-fabrication electrical biasing treatment. The influence of each of these alterations is discussed, as well as their interactions. Combining all of them finally enables the fabrication of a highly transparent and electrically well-performing windowlayer stack, leading to a screen-printed silicon heterojunction solar cell with 24.1% efficiency. Paths towards industrialization and for further improvements are finally discussed.
We recently showed that silicon heterojunction solar cell with MoOx-based hole-selective contact could reach 23.5% in efficiency with MoOx layers of 4 nm. Such thin MoOx layer enables a considerable current-density gain of over 1 mA/cm 2 compared to the use of p-type amorphous silicon, and outperforms thicker MoOx layers. In this study, we investigated the impact of the MoOx hole-selective layer for thickness between 0 and 4 nm. Based on opto-electrical characterization of the device at various processing stage, we discuss the optical and electrical effects of such variation on the solar-cell performances. We notably identify a loss of passivation and selectivity for MoOx films thinner than 4 nm, that we link to a reduced work-function for such thin MoOx films. We confirm experimentally that the optimal MoOx thickness is around 4 nm, yet evidence that close to 0.5 mA/cm 2 is still parasitically absorbed in such a thin layer.
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