Sayan Saha

A novel low cost 25μm thin exfoliated monocrystalline Si solar cell technology

To achieve grid parity, photovoltaic (PV) technologies must reduce the production cost of PV modules to well below

Single heterojunction solar cells on exfoliated flexible ∼25 μm thick mono-crystalline silicon substrates

1/Wp. In crystalline Si (c-Si) solar cells the cost of raw Si wafers is over 40% of the module cost. There is an industry wide push to reduce the active Si content of the cell through a combination of thinner wafers and increased cell efficiency. However, cell manufacturers are struggling to reduce the wafer thickness below 150μm as there are no economically viable technologies for manufacturing very thin Si wafers and such thin silicon wafers impose stringent handling requirements as wafer breakage and yield loss impact final module cost. In this paper, we demonstrate for the first time, a novel exfoliation technology capable of producing large area (6-in diameter) 25μm thin flexible mono c-Si foils that will dramatically change the cost structure and form factor of high efficiency-Si solar cells without the yield losses and handling issues that are a major problem for traditional thin Si wafers. An un-optimized single side heterojunction cell has been formed with a 25μm exfoliated c-Si foil, which shows an efficiency of 12.5%. The cell characteristics of a 25μm thin c-Si cell with intrinsic a-Si passivation will be presented in the paper. Simulations show that with optimized texturing of the foil and passivation, higher efficiencies (20%) can be attained. Depending on the starting wafer or ingot thickness a final cell cost of between

Mechanical strength and reliability of a novel thin monocrystalline silicon solar cell

0.46/Wp to

Realization of dual-heterojunction solar cells on ultra-thin ∼25 μm, flexible silicon substrates

0.50/Wp can be achieved compared to

Light trapping in ultrathin 25 μm exfoliated Si solar cells

1.1/Wp for todays commercial thick crystalline Si cells.

A novel low-cost method for fabricating bifacial solar cells

Mono-crystalline silicon single heterojunction solar cells on flexible, ultra-thin (∼25 μm) substrates have been developed based on a kerf-less exfoliation method. Optical and electrical measurements demonstrate maintained structural integrity of these flexible substrates. Among several single heterojunction ∼25 μm thick solar cells fabricated with un-optimized processes, the highest open circuit voltage of 603 mV, short circuit current of 34.4 mA/cm2, and conversion efficiency of 14.9% are achieved separately on three different cells. Preliminary reliability test results that include thermal shock and highly accelerated stress tests are also shown to demonstrate compatibility of this technology for use in photovoltaic modules.

A low cost kerfless thin crystalline Si solar cell technology

Thin crystalline silicon solar cells, on the order of a few to tens of μm thick, are of interest due to significant material cost reduction and potentially high conversion efficiency. These thin silicon films impose stringent mechanical strength and handling requirements during wafer transfer, cell processing and module integration. Quantitative mechanical and fracture analyses to address reliability issues become necessary. Based on a bi-material foil composed of thin monocrystalline silicon and a supporting substrate fabricated from a novel SOM® (Semiconductor on Metal) kerf-less exfoliation process, closed-form mechanical analyses are introduced and developed to evaluate their strength and fracture behaviors. These analyses include the thermal stress field in the device silicon layer and supporting substrate, the fracture behavior and effects of pyramid structures from surface texturing and the energy release rate at the silicon-substrate interface. It is shown that the introduction of the intrinsic compressive residual strain in the SOM® substrate expands the processing temperature spectrum. The developed analysis and methodology can be readily extended to other thin film solar cell structures with various configurations of device layers and supporting substrates.

Exfoliated sub-10μm thin germanium for cost-effective germanium based photovoltaic applications

Silicon heterojunction (HJ) solar cells with different rear passivation and contact designs were fabricated on ∼25 μm semiconductor-on-metal (SOM) exfoliated substrates. It was found that the performance of these cells is limited by recombination at the rear-surface. Employing the dual-HJ architecture resulted in the improvement of open-circuit voltage (Voc) from 605 mV (single-HJ) to 645 mV with no front side intrinsic amorphous silicon (i-layer) passivation. Addition of un-optimized front side i-layer passivation resulted in further enhancement in Voc to 662 mV. Pathways to achieving further improvement in the performance of HJ solar cells on ultra-thin SOM substrates are discussed.

A novel non-photolithographic patterning method for fabricating solar cells

The optical absorption in 25-μm-thick, single-crystal Si foils fabricated using a novel exfoliation technique for solar cells is studied and improved in this work. Various light-trapping and optical absorption enhancement schemes implemented show that it is possible to substantially narrow the gap in optical absorption loss between the 25 μm Si foils and industry-standard 180-μm-thick Si wafer solar cells. An improvement of absorption by 58% in the near-infrared (740-1200 nm) range is observed for the 25 μm monocrystalline Si substrates with the use of antireflective coating and texturing. The back reflectance of the metal foil that provides mechanical support to the ultrathin Si semiconductor-on-metal foils is extracted to be ∼51.5%, based on the reflectance matching with the simulated escape reflectance in the sub-bandgap region. The back reflectance is enhanced to ∼58% by incorporating an intermediate silicon nitride layer on the back between the Si and the metal. The incorporation of Al as an improved metal reflector on top of the silicon nitride at the backside of the solar cell results in a 5.8 times enhancement in optical path length as a consequence of the improved effective back reflectance of ∼95%. A thin Si foil solar cell with an unoptimized amorphous Si/crystalline Si heterojunction with intrinsic-thin-layer design with implementation of such light-trapping schemes shows an efficiency of 13.28% with a short-circuit current density (JSC) of 35.97  mA/cm2, which approaches the JSC of industrial wafer-based Si solar cells.

Comparison of microstructure and surface passivation quality of intrinsic a-Si:H films deposited by remote plasma chemical vapor deposition using argon and helium plasma

In this work we propose and demonstrate a novel and cost-effective method to fabricate bifacial cells with conventional homojunction architecture. The method combines benefits of lithography-less, self-aligned patterning during deposition of antireflective coating (ARC) and simultaneous metallization of both surfaces aided by electroplating. We have fabricated a conventional diffused n+pp+ junction bifacial solar cell on a monocrystalline silicon (c-Si) substrate using this method. Electrochemically grown nickel is used to simultaneously form front and back electrodes. The bifacial solar cell fabricated with an un-optimized process has a front and rear efficiencies (under AM1.5G one sun illumination) of 12% and 8.66%, respectively. Part of the low performance of the cell is attributed to poor quality of the passivation layer and the post deposition annealing to reduce pinholes in deposited SiNx layer to prevent parasitic plating.