The optical properties of high haze fluorine-doped tin oxide (FTO) glass substrates limit the short-circuit current density (Jsc) and photoelectric conversion efficiency (PCE) of chalcogenide solar cells (PSCs). In order to accurately quantify the optical parameters of the substrates, in this study, the transmittance, reflectance, haze, and surface roughness of the FTO substrates were characterized with high precision using a Millennial chalcogenide online transmittance tester.

By comparing the performance of different FTO substrates, it is found that FTO substrates with high haze and roughness can significantly reduce the light reflection at the NiOₓ/calcite interface and enhance the light absorption of the calcite layer through multiple reflections, thus improving Jsc and PCE.

PSC preparation for chalcogenide solar cells

The PSC structures used in this study are glass/FTO /NiOx (20 nm) /MAPbI3 (400 nm) /PCBM (40 nm) /BCP (10 nm) /Ag (80 nm) , comparing three different FTO-coated glass substrates: the FTO-N, the FTO-A, and the high haze FTO-HA.

Optical and Morphological Properties of FTO Glasses

 (a) Transmittance (T) spectra of different FTO glasses; (b) Haze spectra of different FTO glasses

Comparison of transmittance and haze

FTO-A has the highest transmittance in the full band, and FTO-HA has lower transmittance due to higher absorption.

The haze of FTO-HA is significantly higher than that of FTO-N and FTO-A, indicating its stronger light scattering ability.

Surface topography and roughness

SEM images of FTO-N, FTO-A, FTO-HA

AFM images of FTO-N, FTO-A, FTO-HA

SEM and AFM results show that FTO-HA has the largest surface roughness (RMS = 59 nm) and FTO-N has the smallest (30 nm).Surface roughness is positively correlated with haze, and high roughness enhances light scattering and reduces reflection loss through sub-wavelength structure.

Performance limitations under high temperature process

Performance statistics of PSCs on different FTO substrates under high-temperature process:(a) Jsc, (b) Voc, (c) FF, (d) PCE

(a) IPCE spectra of high-temperature process devices (b) J-V characteristics of high-temperature process devices

The FTO-A and FTO-HA substrates enhance the Jsc of the PSC to 20.83 mA/cm² and 20.53 mA/cm², respectively, which is a significant improvement over the FTO-N's 18.90 mA/cm².

Reflectance (R) spectra of FTO substrate (a) before deposition of chalcogenide layer (b) after deposition of chalcogenide layer

Schematic of multiple reflections of incident light at the NiOₓ-deposited high haze FTO/calcite interface

Incident photon-current conversion efficiency (IPCE) tests show that the high haze substrate improves light absorption efficiency in the 300-700 nm wavelength range by reducing interfacial reflections, and the IPCE of the FTO-HA is close to that of the FTO-A despite having the lowest transmittance.

However, high-temperature annealing leads to a sharp increase in the square resistance of FTO-A and FTO-HA from about 9 Ω/□ to more than 160 Ω/□, which in turn leads to an increase in the series resistance Rs, resulting in a decrease in the fill factor (FF), and ultimately restricts the increase in PCE.

Breakthrough in low-temperature process

J-V characterization of PSCs with low-temperature deposited NiOₓ

To the limitations of the high-temperature process, the square resistances of the FTO-A and FTO-HA substrates remain stable after the preparation of NiOx layers by low-temperature annealing at 320°C, and the device FFs are improved to 0.723 and 0.786, with PCEs reaching 16.0% and 17.1%, respectively. This improvement is attributed to the low-temperature process that avoids carrier scattering at the FTO grain boundaries and reduces the non-uniformity of the NiOx layer thickness. It is shown that FTO-HA exhibits optimal overall performance in the low-temperature process due to greater roughness, which results in stronger multiple reflection of light at the NiOx chalcogenide interface.

The high haze fluorine-doped tin oxide FTO substrate forms an efficient anti-reflective structure at the NiOx chalcogenide interface through surface roughness modulation, which significantly enhances the Jsc of the PSCs.Although the PCE is limited by the resistance increase in the high-temperature process, the low-temperature preparation technique successfully solves the problem, enabling the FTO-A and FTO-HA substrates to achieve stable performance at a PCE level of 16%-17%.

Millennial In-Line Transmittance Tester for Chalcogenide Solar Cells

Millennial In-Line Transmittance Tester for Chalcogenide Solar Cells is a real-time system for monitoring the optical transmittance of Chalcogenide thin films, transparent oxide glass or modules to optimize the process, ensure uniformity and improve cell efficiency.

High accuracy: Measurement accuracy reaches 0.01%, providing accurate transmittance ratio data

Good stability: Measurement stability <0.1%, providing stable data in 10 repetitions of the test, ensuring the reliability of the test results

High efficiency and automation: Large-area scanning (e.g., 0.6m x 1.2m substrate) can be completed in seconds

The core optical data support provided by Millennial chalcogenide online transmittance tester realizes high-throughput screening and real-time process monitoring of FTO substrate transmittance and haze, providing a key quality control means for the industrialization of anti-reflective design.

Original reference:Antireflection effect of high haze FTO for improving short circuit current density of perovskite solar cells