Bertrand Paviet-Salomon

Back-Contacted Silicon Heterojunction Solar Cells With Efficiency >21%

We report on the fabrication of back-contacted silicon heterojunction solar cells with conversion efficiencies above 21%. Our process technology relies solely on simple and size-scalable patterning methods, with no high-temperature steps. Using in situ shadow masks, doped hydrogenated amorphous silicon layers are patterned into two interdigitated combs. Transparent conductive oxide and metal layers, forming the back electrodes, are patterned by hot melt inkjet printing. With this process, we obtain high short-circuit current densities close to 40 mA/cm2 and open-circuit voltages exceeding 720 mV, leading to a conversion efficiency of 21.5%. However, moderate fill factor values limit our current device efficiencies. Unhindered carrier transport through both heterocontact layer stacks, as well as higher passivation quality over the minority carrier-injection range relevant for solar cell operation, are identified as key factors for improved fill factor values and device performance.

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.

Transparent Electrodes in Silicon Heterojunction Solar Cells: Influence on Contact Passivation

Charge carrier collection in silicon heterojunction solar cells occurs via intrinsic/doped hydrogenated amorphous silicon layer stacks deposited on the crystalline silicon wafer surfaces. Usually, both the electron and hole collecting stacks are externally capped by an n-type transparent conductive oxide, which is primarily needed for carrier extraction. Earlier, it has been demonstrated that the mere presence of such oxides can affect the carrier recombination in the crystalline silicon absorber. Here, we present a detailed investigation of the impact of this phenomenon on both the electron and hole collecting sides, including its consequences for the operating voltages of silicon heterojunction solar cells. Based on our findings, we define guiding principles for improved passivating contact design for high-efficiency silicon solar cells.

Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency

Tandem devices combining perovskite and silicon solar cells are promising candidates to achieve power conversion efficiencies above 30% at reasonable costs. State-of-the-art monolithic two-terminal perovskite/silicon tandem devices have so far featured silicon bottom cells that are polished on their front side to be compatible with the perovskite fabrication process. This concession leads to higher potential production costs, higher reflection losses and non-ideal light trapping. To tackle this issue, we developed a top cell deposition process that achieves the conformal growth of multiple compounds with controlled optoelectronic properties directly on the micrometre-sized pyramids of textured monocrystalline silicon. Tandem devices featuring a silicon heterojunction cell and a nanocrystalline silicon recombination junction demonstrate a certified steady-state efficiency of 25.2%. Our optical design yields a current density of 19.5 mA cm−2 thanks to the silicon pyramidal texture and suggests a path for the realization of 30% monolithic perovskite/silicon tandem devices.An optimized two-step deposition process allows the formation of uniform layers of metal halide perovskites on textured silicon layers, enabling tandem silicon/perovskite solar cells with improved optical design and efficiency.

Profilometry of thin films on rough substrates by Raman spectroscopy.

Thin, light-absorbing films attenuate the Raman signal of underlying substrates. In this article, we exploit this phenomenon to develop a contactless thickness profiling method for thin films deposited on rough substrates. We demonstrate this technique by probing profiles of thin amorphous silicon stripes deposited on rough crystalline silicon surfaces, which is a structure exploited in high-efficiency silicon heterojunction solar cells. Our spatially-resolved Raman measurements enable the thickness mapping of amorphous silicon over the whole active area of test solar cells with very high precision; the thickness detection limit is well below 1 nm and the spatial resolution is down to 500 nm, limited only by the optical resolution. We also discuss the wider applicability of this technique for the characterization of thin layers prepared on Raman/photoluminescence-active substrates, as well as its use for single-layer counting in multilayer 2D materials such as graphene, MoS2 and WS2.

Photolithography-free interdigitated back-contacted silicon heterojunction solar cells with efficiency >21%

We report on the development of interdigitated back-contacted silicon heterojunction solar cells with conversion efficiencies well above 21%. Doped hydrogenated amorphous silicon layers, needed for electron and hole collection, are patterned via in-situ shadow masking whereas transparent conductive oxide and metal layers, of the back electrodes, are defined via hot melt inkjet printing of an etch resist and subsequent wet etching. Our technology is therefore photolithography-free and avoids any high-temperature step. The best fabricated solar cell presents a high short-circuit current density of 39.9 mA/cm2, an open-circuit voltage of 724 mV and a fill factor of 74.5% resulting in a conversion efficiency of 21.5%, with a strong upside potential. We report also on a silver-free IBC-SHJ solar cell with conversion efficiency >20%.

Accurate Determination of Photovoltaic Cell and Module Peak Power From Their Current–Voltage Characteristics

We investigate the extraction of the peak power of photovoltaic (PV) cells and modules from their current-voltage (I-V) characteristics. Synthetic I-V curves are generated by numerically solving the two-diode equation in steady-state conditions with representative parameters for crystalline silicon-based solar cells. Parasitic effects that may affect the shape of the current- voltage curves are not considered yet. The cases of high- and low-voltage sampling frequencies are addressed. We propose and qualify a novel fit procedure, where the boundaries are defined as two independent power thresholds, and demonstrate a factor 3-4 improvement on the peak power estimation in comparison with other state-of-the-art approaches. We unveil the dependence of the fit accuracy on the devices parameters, especially their fill factor (FF). Interestingly, we show that an equally good fit accuracy is obtained when only five to ten points are placed neighboring the peak power, provided that these points are placed at the appropriate positions. We then broaden our approach to the extraction of the short-circuit current density and the open-circuit voltage from I-V curves. We validate our guidelines by extracting the maximum peak power from (I-V) curves measured on actual PV devices.

Boosting the efficiency of III-V/Si tandem solar cells

We have developed Si-based tandem solar cells with a certified 1-sun efficiency of 29.8% (AM1.5g). The four-terminal tandem devices consist of 1.8 eV rear-heterojunction GaInP top cells and silicon heterojunction bottom cells. The two subcells were fabricated independently in two different labs and merged using an optically transparent, electrically insulating epoxy. Work is ongoing to further improve the performance of each subcell and to push the tandem cell efficiency to > 30%.

22% efficient dopant-free interdigitated back contact silicon solar cells

In this study, we present dopant-free back contact heterojunction silicon solar cells employing MoOx and MgFx based stacks as hole-and electron-selective contacts deposited using a thermal evaporation process at low temperature. Only two masking steps and one alignment are required in this simple process flow. We investigate the effect of varying the MgFx film thickness as the electron contact layer on the rear side on IBC Si solar cells and define an optimal thickness 1.5 nm of MgFx for high VOC and FF. We compare different electron-selective contact materials including Mg-based and fluoride materials and discuss the suitable combinations. We fabricate dopant-free back contact solar cells by applying a stack of 1.5 nm MgF2/70 nm Al/800 nm Ag films on intrinsic a-Si:H, maintaining excellent passivation and show efficient carrier extraction. A 4.5-cm2 dopant-free back contact solar cells fabricated with these layers enables high VOC up to 709 mV and FF up to 75.6% still limited by series resistance due to too thin metal layers, a pseudo FF of 84.2% is yet measured. The cell exhibits very low front reflection and has outstanding collection efficiency, the IQE reach 98.2% - 99% ranging from 600 to 900-nm due to low recombination of MoOx and MgFx contacts results in a high JSC of 41.5 mA/cm2.In this study, we present dopant-free back contact heterojunction silicon solar cells employing MoOx and MgFx based stacks as hole-and electron-selective contacts deposited using a thermal evaporation process at low temperature. Only two masking steps and one alignment are required in this simple process flow. We investigate the effect of varying the MgFx film thickness as the electron contact layer on the rear side on IBC Si solar cells and define an optimal thickness 1.5 nm of MgFx for high VOC and FF. We compare different electron-selective contact materials including Mg-based and fluoride materials and discuss the suitable combinations. We fabricate dopant-free back contact solar cells by applying a stack of 1.5 nm MgF2/70 nm Al/800 nm Ag films on intrinsic a-Si:H, maintaining excellent passivation and show efficient carrier extraction. A 4.5-cm2 dopant-free back contact solar cells fabricated with these layers enables high VOC up to 709 mV and FF up to 75.6% still limited by series resistance due to ...

Transparent electrodes in silicon heterojunction solar cells: Influence on carrier recombination

Hole and electron collectors in silicon heterojunction solar cells consist of hydrogenated amorphous silicon layer stacks deposited on the crystalline silicon wafer surfaces. Charge carrier extraction from these layers is achieved by electrodes consisting of a transparent conductive oxide and a metal layer. Earlier, the mere presence of the transparent conductive oxide layer on top of the hole collecting stack was shown to alter minority carrier lifetimes, at low minority injection levels, of the crystalline silicon absorber. In this work, we present a detailed investigation of the magnitude and nature of these effects and discuss their impact on silicon heterojunction solar cell performance for the different device architectures.