Simple processing of back-contacted silicon heterojunction solar cells using selective-area crystalline growth

Abstract

For crystalline-silicon solar cells, voltages close to the theoretical limit are nowadays readily achievable when using passivating contacts. Conversely, maximal current generation requires the integration of the electron and hole contacts at the back of the solar cell to liberate its front from any shadowing loss. Recently, the world-record e ciency for crystalline-silicon singlejunction solar cells was achieved by merging these two approaches in a single device; however, the complexity of fabricating this class of devices raises concerns about their commercial potential. Here we show a contacting method that substantially simplifies the architecture and fabrication of back-contacted silicon solar cells. We exploit the surface-dependent growth of silicon thin films, deposited by plasma processes, to eliminate the patterning of one of the doped carrier-collecting layers. Then, using only one alignment step for electrode definition, we fabricate a proof-of-concept 9-cm 2 tunnel-interdigitated backcontact solar cell with a certified conversion e ciency >22.5%. I n recent decades, the market of photovoltaics has been consistently growing and the yearly installed photovoltaic capacity has increased from 328 MW peak in 2001 to 50 GW peak in 2015. This resulted in 2016 in a cumulative capacity of 235 GW peak (ref. 1), largely based on crystalline-silicon (c-Si) solar-cell technologies 2 , and contributing to about 1.3% of the global electricity production 3 . To further increase this number, the cost-competitiveness of photovoltaics must surpass that of classic, non-renewable energy sources, and one route towards this goal is to raise the conversion efficiency of industrial c-Si solar cells 4,5 .High power conversion efficiencies require maximizing the solar-cell electrical parameters: open-circuit voltage (V oc ), fillfactor (FF) and short-circuit current density (J sc ). For the V oc and FF, this is possible by using passivating contacts, employing silicon oxide or hydrogenated amorphous silicon (a-Si:H) thin films to minimize charge carrier recombination at the electrical contacts to the c-Si wafer, with demonstrated record efficiencies for two-sidecontacted solar cells of 25.1% (refs 6,7). Maximum J sc values can be achieved using a back-contacted architecture, eliminating front metal electrode shadowing and minimizing optical reflection and absorption losses at the front. Small-sized back-contacted solar cells, based on diffused silicon homo-junctions, were realized at several research institutes (refs 8-11), showing a best conversion efficiency up to 24.4% (ref. 12). Industrially, the back-contacted architecture was pioneered by Sunpower, recently reporting on large-area devices with very high J sc values and efficiencies surpassing 25% (ref. 13). Considering these achievements, integrating passivating contacts in a back-contacted architecture is the obvious c-Si single-junction solar-cell design towards highest conversion efficiencies. Such an approach has increasingly been researched in both academia and indust...

Improved Optics in Monolithic Perovskite/Silicon Tandem Solar Cells with a Nanocrystalline Silicon Recombination Junction

“…[36,37] Here, the Cs 0.19 MA 0.81 PbI 3 perovskite bandgap is 1.58 eV, well below that optimal value. As recently demonstrated for the case of interdigitated back-contacted SHJ cells, [33] diffraction measurements confirmed the presence of the perovskite phase ( Figure S2, Supporting Information). [10] Considering a gain of 2 mA cm −2 in the top cell due to avoiding parasitic absorption in the spiro-OMeTAD layer and using a perovskite absorber with a bandgap of 1.63 eV (Figure 6b), a tandem cell current density of 18.4 mA cm −2 could be reached.…”

“…Steady-state efficiencies of 22.0% and 21.2% during maximum power-point tracking for 1000 s, were measured (Figure 2a,b). [33] In comparison, a rear-side-textured single-junction SHJ solar cell reference for the bottom cell, comprising the nc-Si:H layer stack at the front, shows an efficiency of 16.45% with a V oc of 693 mV for an aperture area of 0.25 cm 2 ( Figure S3, Supporting Information). These tandem cells showed a high open-circuit voltage (V oc ) of >1750 mV, which demonstrates the capability of the nc-Si:H junction to efficiently recombine electrons from the top cell with holes from the bottom cell.…”

“…[26][27][28] A first demonstration of a silicon-based recombination junction for perovskite/silicon tandem cells was made by Mailoa et al, on a diffused-junction c-Si solar cell, although with a limited efficiency of 13.7% and requiring a hightemperature annealing step that is not compatible with SHJ cells. [32][33][34] Here we present a nc-Si:H recombination junction for monolithic perovskite/SHJ tandem cells, which reduces reflection and parasitic absorption losses in comparison to a commonly used TCO recombination layer. [27,30] By adding either trimethylboron or phosphine to the silane/hydrogen mixture in the reactor, highly doped nc-Si:H can be obtained, [31] which is required for recombination junctions exhibiting low resistance and narrow-potential energy barrier widths.…”

Laser-induced BSF: A new approach to simplify IBC-SHJ solar cell fabrication

“…Figure 5 shows the Raman measurements performed on compensated layer stacks to determine if there was significant crystallization of the layers in the "good compensation" regime as microcrystalline layers are necessary to form the tunnel contact, which may explain the compensation effect. 8 This would be visible in the peak around 520 cm -1 . The 15 mW and "no laser" represent the "under compensated" regime, the 17 and 24 mW laser powers represent the "good compensation" regime and the 25 and 33 mW laser powers represent the "over compensated" regime.…”

“…One method proposed to simplify IBC patterning has been proposed by Tomasi et al by introducing a tunnel junction concept for the BSF fingers of the solar cell. 8,9 This concept allows self-alignment between the a-Si:H deposition steps. Additionally, the use of laser ablation for SHJ-IBC patterning is being investigated by various research groups.…”