Showing papers by "Ajeet Rohatgi published in 2020"

26.7% Efficient 4-Terminal Perovskite–Silicon Tandem Solar Cell Composed of a High-Performance Semitransparent Perovskite Cell and a Doped Poly-Si/SiO x Passivating Contact Silicon Cell

Abstract: The rapid rise in single-junction perovskite solar cell (PSC) efficiencies, tunable bandgap, and low-cost solution processability make PSCs an attractive candidate for tandems with Si bottom cells. However, the challenge is to fabricate a high-performance semitransparent perovskite top cell in combination with an appropriate silicon bottom cell with high response to long wavelength photons that are filtered through the perovskite top cell. Currently, semitransparent perovskite cells show much lower performance compared with their opaque counterparts, while high-performance silicon bottom cells, such as heterojunction with intrinsic thin layer and interdigitated back contact, may be too expensive to meet the cost and efficiency targets for commercial viability. Here, we demonstrate a 26.7% perovskite–Si four terminal (4T) tandem cell comprising a highly efficient 17.8% CsFAMAPbIBr semitransparent, 1.63-eV bandgap perovskite top cell, and a ≥22% efficiency n-type Si bottom cell fabricated with a conventional boron diffused emitter on the front and carrier selective n+ poly-Si/SiOx passivated contact on the rear. This is among the highest efficiency perovskite/Si 4T tandems published to date and represents the first report of the use of the high temperature-resistant single side n-tunnel oxide passivated contact Si cell in a 4T configuration.

Quantitative Understanding and Implementation of Screen Printed p+ Poly-Si/Oxide Passivated Contact to Enhance the Efficiency of p-PERC Cells

Abstract: This paper reports on the modeling, optimization, and implementation of p-TOPCon (tunnel oxide passivating contacts) on the rear side of a PERC to enhance its cell efficiency. Local Al-BSF of a traditional PERC was replaced by p+ poly-Si/oxide passivated contact composed of ~15A thick chemically grown tunnel oxide, capped with 120-250nm thick p+ poly-Si layer grown by LPCVD. Process optimization resulted in full-area un-metallized recombination current density (J 0b , pass ) of 0b ,) of 65fA/cm2. Detailed analysis and device simulation showed that by replacing this LBSF with 250nm TOPCon developed in the study should produce a V oc enhancement of 9.2mV, consistent with the observed cell V oc increase of 10mV.

Boron Selective Emitter Formation With Un-metallized J 0e of 13fA/cm 2 For Silicon Solar Cell Applications

Abstract: This paper describes the development of a novel selective boron emitter formed a screen-printed negative resist to form vias for heavily doping regions. Ion-implantation and atmospheric pressure chemical vapor deposition (APCVD) are used to from the field (p+) and heavily doped regions (p++) of the emitters, respectively. This approach gives better control over the surface concentrations/junction depth for each region of the emitter resulting in very low full area emitter dark saturation current density (J0e) of 6fA/cm2 for the lightly doped regions and ∼93fA/cm2 for the heavily doped regions. Very low un-metallized J0e of 13fA/cm2 and 18fA/cm2 were achieved for selective emitters with $30\ \Omega/\square\ \mathrm{p}^{++}$ region in combination with 200 and $150\ \Omega/\square$ ion-implanted p+ regions respectively. A metallized J0e of 28fA/cm2 was achieved with 3% screen-printed metal grid contact coverage to p++ regions of the selective emitter. This selective emitter gave 0.4% absolute increase in efficiency over standard NPERT with a $150\ \Omega/\square$ homogeneous emitter.

Sulfurization as a promising surface passivation approach for both n- and p-type Si

Abstract: Sulfur is demonstrated to be an effective and promising surface passivation element for both n- and p-type Si wafers after reacting in dilute hydrogen sulfide gas (2 - 6% in argon) at 550°C. Effective minority carrier lifetimes of > 2 ms and > 0.25 ms are achieved for n- and p-type Si wafers, respectively, comparable to the industry standard thermal oxide and atomic layer deposited aluminum oxide passivation level. Surface characterization by x-ray photoelectron and emission spectroscopy reveals that sulfur is primarily bonded in a sulfide environment.

Laser Crystallization and Dopant Activation of a-Si:H Carrier-Selective Layer in TOPCon Si Solar Cells

Abstract: Herein, we present a pulsed-laser processing method for crystallization and dopant activation of a highly n-doped amorphous silicon (a-Si:H) carrier-selective layer in a tunnel oxide passivated contact (TOPCon) Si solar cell structure. The laser method provides enhanced conductivity and implied open circuit voltage while reducing emitter saturation current density and surface heating, as opposed to conventional high-temperature furnace annealing of the bulk Si wafer with a TOPCon structure. We identify an appropriate laser wavelength, fluence, and layer thickness using modeling and simulations. Raman and Hall effect measurements demonstrate increased crystallinity and dopant activation, whereas photoconductive decay shows enhanced surface and interface passivation quality. Additionally, we examine the role of subsequent SiNx deposition on further improving the passivation of laser-processed TOPCon layers to achieve a 5.9 kΩ/sq film sheet resistance, 2 ms effective carrier lifetime, 718 mV implied open-circuit voltage (iVOC), and 8.6 fA/cm2 one-side recombination current density (J0) with the potential for further passivation improvement via laser process and a-Si layer thickness optimization.

Design, Optimization, and In-Depth Understanding of Front and Rear Junction Screen-Printed Double-Side Passivated Contacts Solar Cells

Abstract: In this work, detailed numerical modeling is performed for front junction (FJ) and rear junction (RJ) n-type Si solar cells with screen-printed double-side poly-Si based tunnel oxide passivated contacts (TOPCon). A roadmap for efficiency projections of commercial-type RJ and FJ topologies reaching ~24% and >22.5% efficiencies, respectively, has been developed to quantify and explain the impact of various technological innovations on the performance of each design. By investigating several key parameters such as front poly sheet resistance and thickness, bulk material properties, and current transport in our simulation model, we determine and explain why RJ cells outperform FJ cells. Our findings reveal that FJ suffers from present technological limitations of p-poly based passivated contacts, namely, i) large recombination observed in textured p-TOPCon layers and ii) low boron solid solubility and hole mobility in p-poly Si which results in very high sheet resistance of the front p-poly emitter that contributes to FF degradation, especially when using thin poly layer to reduce absorption loss. RJ, on the other hand, desensitizes the cell efficiency to front sheet resistance allowing application of ultra-thin front n-poly layers and is therefore ideally suited for double-side TOPCon cells.