Electrical analysis of c-Si/CGSe monolithic tandem solar cells by using a cell-selective light absorption scheme

Ah Reum Jeong,Sung Bin Choi,Jong-Keuk Park,Inho Kim,Jihye Choi,Jeung-Hyun Jeong,Won Mok Kim

doi:10.1038/s41598-017-15998-y

Ah Reum Jeong, Sung Bin Choi + Show 5 more

Open Access

https://doi.org/10.1038/s41598-017-15998-y

Copy DOI

Abstract

A monolithic tandem solar cell consisting of crystalline Si (c-Si)/indium tin oxide (ITO)/CuGaSe2 (CGSe) was demonstrated by stacking a CGSe solar cell on a c-Si/ITO solar cell to obtain a photovoltaic conversion efficiency of about 10%. Electrical analyses based on cell-selective light absorption were applied to individually characterize the photovoltaic performances of the top and bottom subcells. Illumination at a frequency that could be absorbed only by a targeted top or bottom subcell permitted measurement of the open-circuit voltage of the target subcell and the shunt resistance of the non-target subcell. The cell parameters measured from each subcell were very similar to those of the corresponding single cell, confirming the validity of the suggested method. In addition, separating the light absorption intensities at the top and bottom subcells made us measure the bias-dependent photocurrent for each subcell. The series resistance of a c-Si/ITO/CGSe cell subjected to bottom-cell limiting conditions was slightly large, implying that the tunnel junction was a little resistive or slightly beyond ohmic. This analysis demonstrated that aside from producing a slightly resistive tunnel junction, our fabrication processes were successful in monolithically integrating a CGSe cell onto a c-Si/ITO cell without degrading the performances of both cells.

Highlights

Multi-junction solar cells designed with the aim of achieving high photovoltaic conversion efficiencies (PCEs) offer a key technology for reducing the levelized costs associated with photovoltaic energy generation[1]
Because the top and bottom subcells were connected in series in the tandem cell, the voltage across the tandem cell (VTOT) was determined by the summation of each subcell’s voltage (VT + VB), and the current was determined by the lowest current in the subcells
Together with the previous external quantum efficiency (EQE) results, which indicated that the photocurrent of the top subcell was equal to that of the single CGSe cell, these results demonstrated that the CGSe cell performance was preserved in the crystalline Si (c-Si) bottom cell without loss

Summary

Introduction

Multi-junction solar cells (or tandem solar cells) designed with the aim of achieving high photovoltaic conversion efficiencies (PCEs) offer a key technology for reducing the levelized costs associated with photovoltaic energy generation[1]. The primary factor underlying the low PCE of the CGSe solar cell is a high VOC deficit (=Eg/q − VOC), attributed to the high recombination rate resulting from either a high density of bulk and/ or surface defects[19,20] or inappropriate alignment of the conduction band minima (CBO) of the CdS buffer and CGSe absorber[21] Another problem is that c-Si and CGSe solar cells do not have an optimal tunnel junction to enable easy transport of the carriers between the top and bottom subcells. The highest PCE achieved to date using a tandem structure was only 5.1%11

Methods

Results

Conclusion