Introduction

According to numerous publications in top-tier journals, open-circuit voltage loss (Voc-loss) analysis has emerged as the most effective method currently employed by researchers to continuously push the efficiency limits of Organic Photovoltaic (OPV) cells. 

Figure 1 Statistical analysis of published SCI papers on Voc loss analysis in organic solar cell efficiency improvement research in recent years

Figure 1 shows the statistics of SCI paper publications related to Voc loss analysis in the organic solar cell field. The trend indicates that since 2012, attention to open-circuit voltage Voc loss has gradually increased, showing linear growth. Although there was a stagnation period in 2019-2020 due to the Covid-19 pandemic, there was a doubling in the number of publications in 2021. As of May 2022, the number of SCI publications has already reached the level of the entire year 2021. It is projected that the number of publications in 2022 will show another doubling growth compared to 2021.

Next, let’s briefly review significant open-circuit voltage Voc loss research over the years to help readers quickly understand the role of Voc loss analysis in practical organic solar cell research.

In 2018, Nam-Gyu Park and Hiroshi Segawa published “Research Direction toward Theoretical Efficiency in Perovskite Solar Cells” in ACS Photonics, outlining development pathways for perovskite solar cells. The paper noted that in 2018, perovskite solar cells’ short-circuit current density (JSC) had reached 97% of its theoretical limit JSQ. In contrast, the open-circuit voltage (Voc) only achieved about 77% of its theoretical limit VSQ. Compared to high-efficiency GaAs solar cells, which achieved 95% of their theoretical Voc limit, the paper argued that research strategies focusing on improving Voc (reducing losses) were far more important than improving JSC to reach the theoretical efficiency limits of new solar cells (such as organic). Therefore, having a defect theory model to describe Voc losses and a measurement toolset for scientists to analyze Voc loss mechanisms became crucial work in organic solar cell research.

Figure 2 Trend analysis of improving new solar cell (e.g., organic solar cell) energy conversion efficiency through two approaches: increasing Jsc or reducing Voc loss

(Source: ACS Photonics; July 2, 2018; https://pubs.acs.org/doi/10.1021/acsphotonics.8b00124)

Since 2019, as the proportion of “open-circuit voltage” data in organic solar cell-related papers has increased annually, “reducing Voc loss” has become a crucial strategy for improving organic solar cell efficiency. For instance, a Nature Communications publication in 2019 describing a breakthrough 16% efficient organic solar cell mentioned that their strategy focused on reducing open-circuit voltage Voc loss. The study utilized Enlitech ELCT3011 (now Enlitech REPS Perovskite Photovoltaic Voc Loss Analysis System) to study and quantify Voc’s non-radiative recombination losses. Through spectral data combined with thermodynamic theory, they confirmed that using a low-bandgap material system improved Voc, enhancing the overall conversion efficiency of organic solar cells.

Figure 3 Successful demonstration of improving organic solar cell energy conversion efficiency through reducing open-circuit voltage Voc loss strategy (2019)

(Source: Nature Communication; June 7, 2019; https://www.nature.com/articles/s41467-019-10351-5)

Figure 4 Successful demonstration of improving organic solar cell energy conversion efficiency through reducing open-circuit voltage Voc loss strategy (2021)

(Source: Advanced Materials; November 13, 2021; https://onlinelibrary.wiley.com/doi/abs/10.1002/adma.202106316)

In 2021, Harald Ade from North Carolina State University and Wei Zhixiang from the Center for Nanoscience achieved a 16.2% efficiency in all-small-molecule organic solar cells (ASM-OSC) through high miscibility of ordered molecular stacking. This research breakthrough was published in Advanced Materials (IF 30.849) in November 2021. Their efficiency breakthrough strategy focused on open-circuit voltage Voc, using Enlitech ELCT3010 (now Enlitech REPS Perovskite and Organic Photovoltaic Voc Loss Analysis System) to quantitatively test and analyze the Voc loss characteristics of organic solar cells.

In February 2022, Energy & Environmental Science (IF 38.532) published the latest research results from Professor Jianhui Hou’s team at the Institute of Chemistry, Chinese Academy of Sciences. The research team used an asymmetric wide-bandgap non-fullerene acceptor called AITC, which helps form stable mixed phases in the blend, thereby improving the photoelectric conductivity of ternary OSCs, suppressing charge recombination, and reducing non-radiative voltage losses, achieving a high efficiency of 19.4% in tandem organic solar cells, currently the highest efficiency in the organic field.

The researchers used Enlitech ELCT-3010 (now Enlitech REPS Perovskite Photovoltaic Voc Loss Analysis System) and other instruments for experiments, showing that devices with AITC addition achieved significant improvements in both open-circuit voltage (Voc) and fill factor. The laboratory-tested efficiency reached a record 19.4% for tandem organic solar cells.

Figure 5 Professor Jianhui Hou’s OSC photovoltaic performance research results.

• Experimental Data (a): The J-V curves of devices with AITC addition show better performance compared to devices without AITC.
• Experimental Data (b): Comparison of EQE spectra between devices with and without AITC addition.
• Experimental Data (c): Temperature influence on short-circuit current for three types of OSCs.
• Experimental Data (d): Effect of charge density on non-geminate recombination rate constants for three types of OSCs.
• Experimental Data (e): EL spectra of OSCs with AITC addition.
• Experimental Data (f): EL EQE of OSCs with AITC addition.
• Experimental Data (g): Dark current curves at different thicknesses.
• Experimental Data (h): Efficiency variation with different thicknesses.

The Enlitech REPS Organic Photovoltaic VOC Loss Analysis System can measure electroluminescence quantum efficiency (EL EQE). It can detect extremely low EL-EQE signals (down to 10^-5%, spanning 7 orders of magnitude) and, through the SQ-VLA analysis software, match the calculated Voc-loss with the actual Voc-loss from device I-V curves, facilitating research progress and journal publication.

What is Voc-loss Analysis?

From our previous discussion, we understand that studying open-circuit voltage loss is the most active area in solar cell efficiency research. Therefore, it’s essential to first understand what we’re examining in open-circuit voltage loss analysis.

To comprehend open-circuit voltage loss, let’s start with the fundamental principles of solar cells. Solar cells consist of n-type and p-type semiconductors. When photon energy exceeds the semiconductor bandgap, photons are absorbed, generating electrons and holes. The entire mechanism operates through four stages: (1) Absorption, (2) Photocarrier Generation, (3) Transport, and (4) Collection, enabling solar cells to provide electrical power.

Figure 6 Solar cell concept and energy band diagram

(Source: NTU Electrical Engineering Department – Introduction to Solar Cell Principles and Applications, https://ee.ntu.edu.tw/upload/hischool/doc/2014.04.pdf)

The External Quantum Efficiency (EQE) of solar cells calculates the ratio of electrons transmitted to the external circuit under short-circuit conditions to the number of incident photons from monochromatic light, considering processes including photon absorption, photocarrier generation, charge transport, and charge collection. These four processes describe how incident photons are absorbed by the solar cell, converted to photocarriers, and transported to the electrodes. This entire process, represented by the EQE, indicates the ability/percentage of incident photons converted to electrons. Therefore, the EQE spectrum reflects information about all these processes.

In an ideal solar cell model, energy should only be converted through radiative recombination pathways, achieving 100% EQE. However, in reality, multiple non-radiative recombination pathways often affect cell performance, leading to additional voltage losses, known as Voc losses.

What are the Challenges in Breaking Through Organic Solar Cell Conversion Efficiency?

According to Wikipedia (https://en.wikipedia.org/wiki/Organic_solar_cell#Charge_carrier_mobility_and_transport), the challenges in organic solar cells include: Despite having good internal quantum efficiency, organic photovoltaic cells have lower external quantum efficiency (maximum 70%) compared to inorganic photovoltaic devices due to insufficient absorption in their ~100nm active layers. Additionally, organic solar cells’ instability to oxidation and reduction, recrystallization, and temperature changes leads to device degradation and performance decline over time. The extent of these issues varies for organic photovoltaic devices with different compositions, an area actively researched by many scientists. Other important factors include exciton diffusion length affected by impurities, charge separation, and charge collection.

Many scientists are working to improve organic solar cell performance through the following approaches:

• Charge Carrier Mobility and Transport: Optimizing carrier transport pathways
• Thin Film Morphology Impact: Studying film structure effects on performance
• Controllable Heterojunction Growth: Improving device interface characteristics
• Growth Technology Advancement: Developing new processing methods
• Vacuum Thermal Evaporation: Enhancing thin film quality
• Organic Vapor Deposition: Controlling film growth
• Organic Solar Inks: Developing new materials
• Light Harvesting: Improving light absorption efficiency
• Tandem Photovoltaic Implementation: Increasing overall efficiency
• Mechanical Behavior: Studying device mechanical properties

Based on these requirements, two aspects have naturally become the most crucial items in organic solar cell research: (1) How to achieve precise measurements? (2) How to quickly calculate and obtain analysis results of thermodynamic loss (ΔE1), radiative recombination loss (ΔE2), and non-radiative recombination loss (ΔE3) using inspection data? These are common pain points in this research field.

The Enlitech REPS Perovskite and Organic Photovoltaic Voc Loss Analysis System is a comprehensive system that helps scientists measure, calculate, and analyze Voc-loss in solar cells under study, providing insights for subsequent process improvements. REPS can not only precisely detect extremely low EL-EQE signals (down to 10^-5%, spanning 7 orders of magnitude) but also calculate thermodynamic Voc loss, radiative recombination Voc loss, and non-radiative recombination Voc loss (through its SQ-VLA software). Furthermore, it can analyze ΔV1, ΔV2, and ΔV3 losses between different types of devices in a bar chart. Quickly providing researchers with effective test data and analysis results not only saves significant time but also prevents errors from manual calculations.

Figure 7 Research results using the Enlitech REPS Perovskite and Organic Photovoltaic Voc Loss Analysis System for perovskite solar cell loss analysis improvement can be seamlessly integrated for rapid journal publication

Can We Understand Voc Loss More Systematically?

The Shockley-Queisser (SQ) limit theory provides the answer. In the SQ limit, Voc has three major losses: ΔV₁ thermodynamic loss, ΔV₂ radiative loss, and ΔV₃ non-radiative loss. Using SQ limit theory, we can explain the Voc loss process in detail.

Figure 8 Energy level diagram illustrating thermodynamic loss, radiative recombination loss, and non-radiative recombination loss in SQ limit theory.

Figure 9 For a detailed understanding of radiative and non-radiative recombination loss mechanisms, readers can refer to Adv. Energy Mater. 2017, 1602358. The paper provides more detailed Voc loss mechanisms caused by various defects.

Can We Obtain Open-Circuit Voltage Loss Values in Our Organic Solar Cell Research? How Can We Achieve This?

The answer is yes. According to the Shockley-Queisser limit, the open-circuit voltage Voc loss in solar cells is determined by three major losses, which can be obtained using the following relationship:

Where q is the elementary charge, ΔV is the total open-circuit voltage loss, ΔV1 is the thermodynamic loss, ΔV2 is the loss caused by radiative recombination, and ΔV3 is the open-circuit voltage loss caused by non-radiative recombination.

From the open-circuit voltage loss relationship, we can clearly see that by measuring E_g, V^SQ_OC, and V^rad_OC, we can obtain these three loss values. E_g, V^SQ_OC, and V^rad_OC respectively represent the solar cell’s bandgap E_g, open-circuit voltage under the Shockley-Queisser limit V^SQ_OC, and open-circuit voltage under full radiative recombination V^rad_OC.

How can we measure these three physical quantities? First, let’s write down their definitions:

Additionally, through the diode model, Voc can theoretically be defined as:

Next, we use Enlitech ELCT-3010 (now Enlitech REPS Perovskite Photovoltaic Voc Loss Analysis System) to first measure the solar cell’s electroluminescence external quantum efficiency EQE_EL.

Open the organic solar cell open-circuit voltage loss analysis software (SQ-VLA) and set a name for this analysis session.

Figure 10 Initial screen of the perovskite open-circuit voltage loss analysis software (SQ-VLA)

After importing the test data from QE-R, IVS-KA6000, and REPS, the analysis calculation results are immediately presented, including bandgap, open-circuit voltage under the Shockley-Queisser limit, open-circuit voltage under full radiative recombination, and each open-circuit voltage loss: thermodynamic loss ΔV1, radiative recombination loss ΔV2, and non-radiative recombination loss ΔV3.

Figure 11 Data import screen for QE-R, IVS-KA6000, and REPS test data

Figure 12 SQ-VLA software displaying calculation results for thermodynamic loss ΔV1, radiative recombination loss ΔV2, and non-radiative recombination loss ΔV3

By comparing the open-circuit voltage losses between control and experimental groups, we can clearly see whether the experimental group’s manipulated variables effectively reduce open-circuit voltage loss, thereby improving the organic solar cell’s open-circuit voltage.

Figure 13 SQ-VLA software overlays comparison results of thermodynamic loss ΔV1, radiative recombination loss ΔV2, and non-radiative recombination loss ΔV3 between control and experimental groups for intuitive comparison

What Other Practical Cases of Voc-loss Analysis Systems Are There?

Energy & Environmental Science published research by Yanming Sun and colleagues from Beihang University.

Organic solar cells (OSCs) are considered a promising solar energy conversion technology due to their unique advantages of low cost, light weight, and ease of manufacture. Using ternary OSCs containing either two donors/one acceptor or one donor/two acceptors is an effective method to improve device power conversion efficiency (PCE).

However, little attention has been paid to how to select suitable third components. This paper presents a strategy for selecting the third component in non-fullerene acceptor-based ternary organic solar cells.

Advanced Materials (IF 30.849) published a research breakthrough in November 2021. The research team achieved high-performance non-fused wide bandgap acceptors for multifunctional photovoltaic applications without using fused ring structures.

In organic photovoltaic (OPV) cells, wide bandgap (WBG) non-fullerene acceptors (NFAs) with non-fused conjugated structures play a crucial role. In light of this, the research team synthesized NFAs named GS-OEH, GS-OC6, and GS-ISO without using fused ring structures, with optical bandgaps greater than 1.70 eV. Among these three NFAs, GS-ISO demonstrated stronger crystallinity compared to GS-OEH and GS-OC6, resulting in smaller energetic disorder and larger exciton diffusion coefficients. Additionally, GS-ISO exhibited a high electroluminescence external quantum efficiency of 1.0 × 10^-2.

Under solar simulator illumination, calibrated with Enlitech standard cells, PBDB-TF:GS-ISO-based OPV cells achieved 11.62% power conversion efficiency (PCE). Furthermore, under 500 lux illumination with 2700 K color temperature, PBDB-TF:GS-ISO-based cells reached 28.37% PCE.

Through experiments using Enlitech QE-R Quantum Efficiency Measurement System, FTPS Fourier Transform Photocurrent Spectroscopy System, and REPS Photovoltaic Voc-loss Analyzer, results showed that tandem OPV cells using PBDB-TF:GS-ISO as the front sub-cell demonstrated an outstanding PCE of 19.10%. Importantly, GS-ISO-based OPV cells showed good stability under continuous simulated solar illumination. Enlitech’s quantum efficiency measurement system not only analyzes EQE (External Quantum Efficiency) spectra of solar cells but also provides Jsc (short-circuit current density) comparison under solar simulator illumination to verify experimental accuracy.

This research demonstrates that the molecular design strategy used has significant advantages in developing non-fused NFAs, and GS-ISO is a promising WBG acceptor for multifunctional photovoltaic applications.

Other top-tier journals like Advanced Materials and Energy & Environmental Science have pointed out that to improve organic solar cell efficiency, it’s essential to first understand where the main open-circuit voltage losses occur, then measure electroluminescence efficiency, and analyze and improve non-radiative recombination losses.

This article provides the following key insights:

• Solar Cell Efficiency Research: Open-circuit voltage loss analysis is currently one of the most effective research methods
• Voc-loss Analysis: Can effectively help researchers understand device loss mechanisms
• Measurement System Importance: Precise measurement systems are crucial for research development
• Future Development Direction: Reducing non-radiative recombination loss is key to improving efficiency

Through these research cases, we can see that Enlitech’s measurement systems play a crucial role in solar cell research, not only providing precise measurement results but also helping researchers conduct quick and effective analysis, accelerating research progress. In future solar cell research, these precise measurement and analysis systems will continue to play a key role in advancing solar technology.