Infrared light management in high-efficiency silicon heterojunction and rear-passivated solar cells

Abstract

Silicon heterojunction solar cells have record-high open-circuit voltages but suffer from reduced short-circuit currents due in large part to parasitic absorption in the amorphous silicon, transparent conductive oxide (TCO), and metal layers. We previously identified and quantified visible and ultraviolet parasitic absorption in heterojunctions; here, we extend the analysis to infrared light in heterojunction solar cells with efficiencies exceeding 20%. An extensive experimental investigation of the TCO layers indicates that the rear layer serves as a crucial electrical contact between amorphous silicon and the metal reflector. If very transparent and at least 150 nm thick, the rear TCO layer also maximizes infrared response. An optical model that combines a ray-tracing algorithm and a thin-film simulator reveals why: parallel-polarized light arriving at the rear surface at oblique incidence excites surface plasmons in the metal reflector, and this parasitic absorption in the metal can exceed the absorption in the TCO layer itself. Thick TCO layers-or dielectric layers, in rear-passivated diffused-junction silicon solar cells-reduce the penetration of the evanescent waves to the metal, thereby increasing internal reflectance at the rear surface. With an optimized rear TCO layer, the front TCO dominates the infrared losses in heterojunction solar cells. As its thickness and carrier density are constrained by anti-reflection and lateral conduction requirements, the front TCO can be improved only by increasing its electron mobility. Cell results attest to the power of TCO optimization: With a high-mobility front TCO and a 150-nm-thick, highly transparent rear ITO layer, we recently reported a 4-cm 2 silicon heterojunction solar cell with an active-area short-circuit current density of nearly 39 mA/cm 2 and a certified efficiency of over 22%. V C 2013 American Institute of Physics. [http://dx

Improving metal reflectors by suppressing surface plasmon polaritons: a priori calculation of the internal reflectance of a solar cell

“…Consequently, if the dielectric layer is thin and has non-zero extinction, it will strongly absorb p-polarized light at high angles of incidence. We observed this in the rear TCO in silicon heterojunction solar cells; 11 it is also the principle behind metal overlayer-attenuated total reflection, an IR spectroscopic technique to enhance the absorption of ultrathin layers under investigation. 19,20 Most interesting for this study, F z (z) is near-zero at the metal surface; instead, F x (z), and to a lesser extent F y (z), is resonant with the metal.…”

“…4,5 In a recent investigation of the TCO layers in amorphous silicon/crystalline silicon heterojunction solar cells, we confirmed this finding with both simulations and experiments, and demonstrated that J sc may be increased by 0.5 mA cm 22 with rear TCO layers that minimize the penetration of evanescent waves to the metallic reflector. 11 In this paper, we expose the physics of parasitic absorption in the metal of rear-passivated solar cells by calculating the electric field intensities in semiconductor/dielectric/metal structures. We then develop a simple formalism for predicting the rear internal reflectance and total IR reflectance of a solar cell given specific dielectric and metal layers, thereby facilitating the optimization of these layers.…”

Transparent Electrodes for Efficient Optoelectronics

“…[140] and ref. [141], respectively. The popularity of both TCOs for application as window layers in solar cells is partly due to their chemical and environmental stability, as well as their demonstrated largescale production.…”

Analysis of lateral transport through the inversion layer in amorphous silicon/crystalline silicon heterojunction solar cells

“…To overcome this disadvantage and achieve high efficiencies, a transparent conductive oxide (TCO) layer is required on top of the emitter for lateral transport of photogenerated charge carriers to metal finger contacts; the TCO also serves as a single-layer antireflection coating. To reduce optical losses associated with free-carrier absorption in TCOs, 5 new high-mobility materials such as hydrogenated indium oxide are being introduced. [6][7][8] Another feature of the a-Si:H emitter is that it forms a heterojunction with c-Si due to their different bandgaps and electron affinities.…”