Optimization and characterization of amorphous/crystalline silicon heterojunction solar cells

Abstract

Amorphous hydrogenated silicon/crystalline silicon (a‐Si:H/c‐Si) heterojunction solar cells are investigated and optimized with regard to efficiency and simplicity of processing. Starting with a survey of a‐Si:H/c‐Si heterojunction solar cell results from the literature, we describe the fabrication steps of our a‐Si:H/c‐Si technology and analyze the electronic device properties by quantum efficiency, current–voltage, admittance, and capacitance–voltage measurements. The open‐circuit voltage and the fill factor of the a‐Si:H/c‐Si heterojunction solar cells under investigation are limited by recombination in the neutral zone of the crystalline Si absorber. Recombination at the a‐Si:H/c‐Si‐interface is subsidiary in respect of the limitation of the open‐circuit voltage. Our best n‐type a‐Si:H/p‐type c‐Si solar cell prepared without high‐efficiency features such as back‐surface field or surface texturing has an independently confirmed efficiency of 14.1% and an open‐circuit voltage of 655 mV. Copyright © 2001 John Wiley & Sons, Ltd.

Plasma-initiated rehydrogenation of amorphous silicon to increase the temperature processing window of silicon heterojunction solar cells

“…24 Prolonged hydrogen plasma treatment also reduces lifetime, especially for thin a-Si:H layers. Consistent with many other reports, [20][21][22][23] we find that at least 5 nm of a-Si:H is necessary for millisecond lifetimes; otherwise, the a-Si:H layers after hydrogen etching are too thin to prevent tunneling to the defective surface 25 and the underlying c-Si substrate can be damaged by the hydrogen plasma. 3,19 For device applications, a 30-s hydrogen plasma treatment on an 8-nm-thick a-Si:H layer appears to be a good recipe because the implied V oc and implied voltage at maximum power point (V mpp ) are enhanced by 35 mV and 50 mV, respectively, compared to the annealed state.…”

“…18 However, hydrogen plasma treatment also etches the film, 12,19 with a 2 nm/min rate in this case ( Figure 1). As intrinsic a-Si:H layers that are thinner than approximately 5 nm poorly passivate c-Si surfaces, [20][21][22][23] prolonged hydrogen plasma treatment on a-Si:H layers should be avoided during SHJ cell processing. Figure 2 investigates the minority-carrier effective lifetime in textured wafers passivated with symmetric intrinsic a-Si:H layers as they undergo the dehydrogenation and rehydrogenation observed in Figure 1.…”

Current Losses at the Front of Silicon Heterojunction Solar Cells

“…A minimum tolerable thickness is set, however, by the V oc , which drops rapidly for i-layers thinner than 4 nm, triggering a small drop in FF as well. Others have observed this trend previously [10]- [12]. Consistent with the defect pool model [23], De Wolf and Kondo found that the defect formation energy in an a-Si:H i-layer is lowered when a p-layer is deposited on it, since the Fermi energy E F in the i-layer is pulled away from mid gap [24].…”

High-efficiency Silicon Heterojunction Solar Cells: A Review

“…The data point with no buffer layer and p-type emitter represents the J sc loss associated solely with parasitic absorption in the TCO film (compared to a SiN x film). Overall cell efficiency is maximized for layers that are thick enough to passivate and collect carriers, but no thicker [36,101,102].…”