Photonic crystal light trapping: Beyond 30% conversion efficiency for silicon photovoltaics

Sayak Bhattacharya,Sajeev John

doi:10.1063/1.5128664

Abstract

The power conversion efficiency of single-junction silicon solar cells has increased only by 1.5% despite extensive efforts over the past two decades. The current world-record efficiencies of silicon solar cells, within the 25%–26.7% range, fall well below the thermodynamic limit of 32.3%. We review the recent progress in photonic crystal light-trapping architectures poised to achieve 28%–31% conversion efficiency in flexible 3–20 μm-thick, single-junction crystalline-silicon solar cells. These photonic crystals utilize wave-interference based light-trapping, enabling solar absorption well beyond the Lambertian limit in the 300–1200 nm wavelength range. Using experimentally feasible doping profiles, carrier lifetimes, surface recombination velocities, and established Auger recombination losses, we review considerations leading to the prediction of 31% efficiency in a 15 μm-thick silicon photonic crystal cell with interdigitated back-contacts. This is beyond the conversion efficiency of any single-material photovoltaic device of any thickness.

Highlights

Sunlight striking the earth provides approximately 173 000 TW of continuous power, roughly 10 000 times more than all worldwide power consumption
This review focuses on combining the efficient light-trapping capability of photonic crystals (PhC)19,20 with the non-toxicity, abundance, and well-developed fabrication techniques of Si to develop a cell technology that encompasses the advantages of the existing thin-film photovoltaics (TFPV) and paves the way toward and beyond 30% power conversion efficiency, a far-reaching goal of the photovoltaic industry
This review highlights the importance of wave-interference based light-trapping to overcome previously reported barriers to solar absorption and cell efficiency

Summary

INTRODUCTION

Sunlight striking the earth provides approximately 173 000 TW of continuous power, roughly 10 000 times more than all worldwide power consumption. We compare the light-trapping performances of a wide variety of PhC architectures with unit cells consisting of nano-wires, conical-pores, inverted pyramids, and parabolic-pores.48,49 From this comparison, we discuss why the 10–15 μm-thick inverted pyramid PhC designs provide the best experimentally feasible route to solar energy absorption well beyond the Lambertian limit and silicon solar cells with conversion efficiency in the 28%–31% range. In contrast to the 110 μm optimum thickness of the hypothetical Lambertian cell, the optimum thickness of the silicon PhC cell with Auger recombination and experimentally achievable carrier lifetime falls between 10 and 15 μm This suggests that thin-film silicon photovoltaics has the potential to leap ahead of competing technologies and surpass the efficiency of any single-junction solar cell made of a single active material

LAMBERTIAN LIGHT-TRAPPING AND EFFICIENCY LIMITS OF CONVENTIONAL SOLAR CELLS

WAVE-INTERFERENCE BASED LIGHT-TRAPPING IN PHOTONIC CRYSTALS

MICRO-PYRAMID PHOTONIC CRYSTAL DESIGN OPTIMIZATION

ELECTRONIC PERFORMANCE OF THIN-FILM PHOTONIC CRYSTAL SOLAR CELLS

Findings

CONCLUSION