Over the past decade, perovskite and organic solar cells have achieved remarkable efficiency improvements. However, their efficiency testing methods show significant spectral mismatch differences compared to crystalline silicon cells’ spectral response. To avoid measurement errors, when conducting precise measurements of new solar cells like perovskite and OPV, we need to consider factors such as response time, spectral mismatch correction, and light source intensity calibration to reduce testing errors and uncertainty. 

Just as perovskite efficiency has shown spiral improvement trends, precise measurement techniques for perovskites have also grown through researchers’ continuous attempts, accumulated experience, and forward progress. Let’s review the development history and important milestones of precise measurement for perovskite and OPV solar cells over the past decade.

An Unexpected Discovery

In 2017, an article was published in GMCA comparing results from two ISO-certified laboratories (CSIRO, NREL) and eight non-ISO-certified laboratories (device laboratories), using NREL test results as the benchmark.

Through comparison, we found that traditional crystalline silicon cells showed differences of around 3.8%. However, for slower-responding perovskite cells, the differences between these laboratories could reach up to 35%. These incorrect results could lead perovskite development down the wrong path, affecting industry development. This laid the groundwork for subsequent industry advancement in precise measurement methods for perovskite and OPV.

Continuous Improvement Journey

Since the rise of OPV in 2006, academia has emphasized the importance of precise perovskite measurements. Perovskites, which gained attention in 2010 due to hysteresis effects, brought this issue into broader focus.

In 2014, Nature published three articles explaining the importance of this issue, mainly discussing how inaccurate measurements could mislead research directions and hinder solar cell development.

In 2015, all Nature series journals required authors to provide a “Solar Checklist” to avoid publishing incorrect results that could affect Nature’s credibility and field development. This document required authors to detail laboratory testing conditions, with the core purpose of ensuring data reproducibility, demonstrating the importance of precise measurement in leading journal publications.

Currently, submissions to Nature series journals for both perovskite and organic solar cells require the aforementioned “Solar Checklist.” For example, Professor Jingbi You’s perovskite solar cell research and Professor Jianhui Hou’s organic OPV research in 2019 both included their submitted “Solar Checklist.”

Careful Progress in Standard Development

Returning to the 35% measurement difference mentioned at the beginning, what causes such large timing differences in testing between perovskite/OPV cells and traditional crystalline silicon cells?

There are four significant differences:

1. First is spectral response range – silicon cells respond up to about 1100nm, while perovskite and OPV currently respond around 800nm-900nm.
2. Second is physical appearance – one is on silicon wafer, the other on glass substrate.
3. Third is external circuit connection method – one is substrate, the other superstrate.
4. Fourth is different internal charge transport mechanisms – OPV mainly uses excitons for charge transport.

So how do we precisely measure perovskite and OPV?

First, we can reference UCLA’s 2006 article – the first systematic paper discussing precise measurement in OPV. 2006 was the PCBM era, with efficiency just breaking 4.7%. Knowledge of precise OPV measurement was quite limited then, so UCLA collaborated with NREL on this paper detailing simulator calibration and spectral mismatch for precise OPV measurement.

For precise measurement of perovskite and OPV electrical characteristics, including conversion efficiency η, short-circuit current, short-circuit current density, open-circuit voltage, fill factor, measurements must be conducted under Standard Test Conditions (STC). The efficiency formula below shows factors affecting precise measurement. Conversion efficiency η is maximum power divided by input irradiance, which is 1000 W/m² under STC. Maximum power expanded is current times voltage times fill factor, divided by device area.

This formula shows that besides STC conditions, factors affecting precise measurement include qualitative testing, device area, and light intensity calibration.

STC Conditions and Their Significance:

• STC stands for Standard Test Conditions.
• IEC 60904-1 standard specifies that all solar cell device testing must be conducted under STC conditions: AM1.5G solar spectrum, 1000 W/m² irradiance, and sample temperature controlled at 25±1℃ for current-voltage and maximum power measurements.

Spectrum, irradiance intensity, and temperature significantly affect solar cell maximum power output. Therefore, to make measurements comparable across different locations and times, a universally accepted and followed measurement condition was needed, leading to STC standards.