Pulsed-laser hyperdoping and surface texturing for photovoltaics
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Introduction Most approaches to photon management in photovoltaics (PV) focus on increasing the total absorbed irradiance Iabs from the incident solar irradiance Isun through some modification of the photovoltaic material. The total absorbed irradiance of a material, obtained from Beer’s law, is:
I abs = ³ I sun (λ) [1 − R(λ)] [1 − exp[−α(λ) d ]] dλ ,
(1)
where R and α are the wavelength-dependent reflectance and absorption coefficients, respectively; d is the path length of a photon through the material; and the integration is over all optical wavelengths λ. Thus, neglecting wavelength conversion approaches, photon management requires manipulating one or more of three material parameters: α, R, or d. Pulsed-laser processing of semiconductors with nanosecond, picosecond, or femtosecond laser pulses offers two very different approaches to enhance photon absorption: pulsed-laser hyperdoping and surface texturing. For example, the “black silicon” process we have reviewed previously for the MRS Bulletin1 achieves nearunity, broadband absorption of visible and near infrared light in silicon with femtosecond laser processing. The effects of these techniques on light absorption are summarized in Figure 1. First, pulsed-laser irradiation can be used to introduce nonequilibrium concentrations of dopants into silicon, a process we refer to as hyperdoping. This process changes silicon’s
electronic structure and increases the absorption coefficient α.2–5 Second, pulsed-laser irradiation of a silicon wafer can produce micrometer- or nanometer-scale surface textures that are suitable for geometric light trapping.6–8 Hyperdoping significantly increases the absorption coefficient through the inclusion of a high concentration of new electronic states. The process depends on the fast resolidification that follows pulsed-laser melting, which traps dopant atoms at concentrations far above equilibrium solubility limits.9,10 Realizing such dopant concentrations with deep-level impurities is a possible route to fabricating an intermediate band photovoltaic cell, a high-efficiency PV concept.11 Surface texturing, on the other hand, increases the fraction of absorbed photons by increasing d and effectively decreasing R(λ) (Equation 1). Texturization relies on laser-induced ablation of silicon, which selectively removes material to create light trapping morphologies. These two pulsed-laser approaches to photon management are distinct and can be realized independently, although much research in this field has focused on a combination of these two effects.6,12–14 In this article, we review pulsed-laser hyperdoping and surface texturing, summarize the current state of the art for both techniques, and discuss outstanding questions and challenges for each application. We first describe the basic physics of laser-induced melting, how it leads to hyperdoping and changes in the absorption coefficient, and ongoing research directions in this field toward realizing efficient photovoltaic devices. Next, we describe laser
Meng-Ju Sher, Harvard University, Cambr
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