A passivating contact concept compatible with a short thermal treatment

In this study we present a boron-doped silicon carbide layers as a hole-selective contact which is compatible with short annealing time (typically < 1 minute) as the one used for firing of metal pastes. The application of such layers on symmetrically processed test structures lead to implied open circuit voltages up to 715 mV and contact resistances below 75 m Omega.cm(2). Proof-of-concept p-type solar cells employing such passivating contact stack over the full-rear side and a POCl3 diffused emitter metallized with firing-through of Ag-paste were processed, leading to a first conversion efficiency of 21.4%

Phosphorous-Doped Silicon Carbide as Front-Side Full-Area Passivating Contact for Double-Side Contacted c-Si Solar Cells

We present an electron selective passivating contact based on a tunneling SiOx capped with a phosphorous doped siliconcarbideandpreparedwithahigh-temperaturethermalanneal. We investigate in detail the effects of the preparation conditions of theSiCx(n)(i.e.,gasflowprecursorandannealingtemperature)on the interface recombination rate, dopant in-diffusion, and optical properties using test structures and solar cells. On test structures, our investigation reveals that the samples annealed at temperatures of 800–850 °C exhibit an increased surface passivation toward higher gas flow ratio (r = CH4/(SiH4 + CH4)). On textured and planar samples, we obtained best implied open-circuit voltages (i-VOC) of 737 and 746 mV, respectively, with corresponding dark saturation current densities (J0) of∼8 and∼4 fA/cm2. The SiCx(n)layerswithdifferentrvalueswereappliedonthetextured front side of p-type c-Si solar cells in combination with a borondoped SiCx(p) as rear hole selective passivating contact. Our cell results show a tradeoff between VOC and short-circuit current density (JSC) dictated by the C-content in the front-side SiCx(n). On p-type wafers, best VOC = 706 mV, FF = 80.2%, and JSC = 38.0 mA/cm2 with a final conversion efficiency of 21.5% are demonstrated for 2 × 2 cm 2 screen-printed cells, with a simple and patterning-free process based on plasma depositions and one annealing step 800 °C < T < 850 °C for the formation of both passivating contacts

Interplay of annealing temperature and doping in hole selective rear contacts based on silicon-rich silicon-carbide thin films

We present a detailed optimization of a hole selective rear contact for p-type crystalline silicon solar cells which relies on full-area processes and provides full-area passivation. The passivating hole-contact is based on a layer stack comprising a chemically grown thin silicon oxide, an intrinsic silicon interlayer, and an in-situ boron doped non-stoichiometric silicon-rich silicon-carbide layer on top. After deposition, the structure is annealed at 775-900 degrees C to diffuse dopant impurities to the c-Si wafer and a hydrogenation step is carried out. It is shown that hydrogenation is essential to obtain high quality surface passivation. In particular, we compare the effect of annealing in forming gas and annealing with a silicon-nitride overlayer as hydrogen source. We present a systematic optimization of the hole-selective contact, for which we varied the doping concentration, annealing parameters and report the implied open circuit voltage (iV(oc)) and combined specific contact resistivity (p(c)). It is observed that for highly doped layers the optimum annealing temperature for high quality surface passivation is 800 degrees C while for lowly doped layers the optimum annealing condition shifts to 850 degrees C. Excellent surface passivation and efficient current transport is evidenced by an iV(oc) value of 718 mV which corresponds to a saturation current density (J(0)) of 11.5 fA/cm(2) and a p(c) of 17 mg Omega cm(2) on p-type wafers. Moreover, the evolution of the boron diffusion profiles with different annealing conditions is investigated. Finally, we demonstrate proof-of concept p-type hybrid solar cells employing the full-area hole-selective rear contact presented here and standard heterojunction front electron contact. The excellent efficiency potential of our passivating rear contact is highlighted by conversion efficiencies up to of 21.9%, enabling V-oc of 708 mV, FF of 79.9% and J(sc) of 38.7 mA/cm(2)

A Mixed-Phase SiOx Hole Selective Junction Compatible With High Temperatures Used in Industrial Solar Cell Manufacturing

We present a p-type passivating rear contact that complies with integration into standard solar cell manufacturing with phosphorus-diffused front side. Our contact structure consists of a thin SiOx tunneling layer grown by wet chemistry and a stack of layers deposited in one single run by plasma-enhanced chemical vapor deposition. The layers of the stack were tailored to protect the interfacial oxide layer, to act as a source for boron diffusion into the wafer and to connect to the external metallisation with low contact resistivity. We found that this stack tolerated annealing at 900 degrees C over a wide range of dwell times: for 15 min anneals we obtained dark saturation current densities (J(o)) as low as 10 fA center dot cm(-2) (after hydrogenation) and after 12-fold increase of the annealing time to 180 min, J(0) was only increased to 12 fA center dot cm(-2). These values corresponded to implied open circuit voltages (iV(oc)) of 718 and 715 mV, respectively. To test passivating rear contacts under realistic operation conditions, we combined them with an n-type heterojunction into hybrid solar cells. With conversion efficiencies abovementioned 22% and V-oc > 705 mV, these devices demonstrated high level of rear surface passivation. Finally, we demonstrated the integration of the hole selective rear contact with a POCl3 diffusion process. To this end, we added a phosphorus diffusion barrier to our layer stack by depositing one additional layer of amorphous SiOx on top of the stack. For symmetric samples with this layer structure on both sides, we observed iV(oc) values of 714 and 712 mV on n- and p-type silicon wafers after hydrogenation, respectively. Co-diffused cells with POCl3 front diffused emitter and rear passivating contact resulted so far in efficiencies of 20.4% and 20.1% for n- and p-type wafers, respectively

A passivating contact for silicon solar cells formed during a single firing thermal annealing

Passivating contacts are indispensable for achieving high conversion efficiency in crystalline-silicon solar cells. Their realization and integration into a convenient process flow have become crucial research objectives. Here, we report an alternative passivating contact that is formed in a single post-deposition annealing step called 'firing', an essential step for current solar cell manufacturing. As firing is a fast (750 degrees C) anneal, the required microstructural and electrical properties of the passivating contact are stringent. We demonstrate that tuning the carbon content of boron-doped silicon-based thin films inhibits firing-induced layer delamination without preventing a partial crystallization. The latter promotes charge-carrier selectivity, even in the absence of a diffused doped region beyond the oxide, by inducing hole accumulation near the wafer surface. We fabricated proof-of-concept solar cells employing the developed technology, demonstrating an open circuit voltage of 698 mV and an efficiency of 21.9%, and show how it could be a drop-in replacement for today's rear contacts based on locally opened dielectric passivation stacks

Raising the one-sun conversion efficiency of III-V/Si solar cells to 32.8% for two junctions and 35.9% for three junctions

Today's dominant photovoltaic technologies rely on single-junction devices, which are approaching their practical efficiency limit of 25-27%. Therefore, researchers are increasingly turning to multi-junction devices, which consist of two or more stacked subcells, each absorbing a different part of the solar spectrum. Here, we show that dual-junction III-V//Si devices with mechanically stacked, independently operated III-V and Si cells reach cumulative one-sun efficiencies up to 32.8%. Efficiencies up to 35.9% were achieved when combining a GaInP/GaAs dual-junction cell with a Si single-junction cell. These efficiencies exceed both the theoretical 29.4% efficiency limit of conventional Si technology and the efficiency of the record III-V dual-junction device (32.6%), highlighting the potential of Si-based multi-junction solar cells. However, techno-economic analysis reveals an order-of-magnitude disparity between the costs for III-V//Si tandem cells and conventional Si solar cells, which can be reduced if research advances in low-cost III-V growth techniques and new substrate materials are successful

Realization of GaInP/Si Dual-Junction Solar Cells With 29.8% 1-Sun Efficiency

Combining a Si solar cell with a high-bandgap top cell reduces the thermalization losses in the short wavelength and enables theoretical 1-sun efficiencies far over 30%. We have investigated the fabrication and optimization of Si-based tandem solar cells with 1.8-eV rear-heterojunction GaInP top cells. The III-V and Si heterojunction subcells were fabricated separately and joined by mechanical stacking using electrically insulating optically transparent interlayers. Our GaInP/Si dual-junction solar cells have achieved a certified cumulative 1-sun efficiency of 29.8% +/- 0.6% (AM1.5g) in four-terminal operation conditions, which exceeds the record 1-sun efficiencies achieved with both III-V and Si single-junction solar cells. The effect of luminescent coupling between the subcells has been investigated, and optical losses in the solar cell structure have been addressed

Hybrid sequential deposition process for fully textured perovskite/silicon tandem solar cells

Tandem solar cells that feature a high-bandgap perovskite cell on top of a lower bandgap silicon cell have the potential to reach efficiencies > 30%. Here, we present a versatile hybrid deposition method that yields conformal perovskite cells directly on textured silicon bottom cells, a prerequisite to achieve highest photocurrents and hence efficiencies. Furthermore, this low-temperature evaporation/spin-coating 2-step method produces high-quality perovskite materials with different bandgaps, here varied in the range of 1.5 eV to 1.8 eV. This flexibility enables the fabrication of monolithic 2-terminal perovskite/textured Si tandems that feature high photocurrents of about 19.5 mA/cm(2)