Everyone is using - Voc-loss analysis helps you break through the conversion efficiency of organic solar cells!
Research reports published in many top journals show that Voc-loss analysis is currently the most effective method used by many researchers to continuously break through the efficiency limit of organic solar cells (OSC).
Figure 1 Thesis Amount of adopting Voc-loss Analysis method for improving the PCE of Organic Solar Cells
Figure 1 shows the statistics of published SCI papers related to Voc loss analysis in the field of organic solar cells. It can be seen from the trend that the open circuit voltage Voc loss has gradually attracted attention since 2015, and it has increased linearly. Although in 2019~2020, the number of papers has a stagnation period due to the Covid-19 epidemic, in 2021 there is a doubled increase in the number of papers. According to statistics, the number of SCI papers published by May 2022 has reached the level of the whole year of 2021. It is estimated that the number of papers in 2022 will also double the number of papers compared to 2021.
Next, we briefly review the important open-circuit voltage Voc loss applications over the past years to help readers quickly understand the role of Voc open-circuit voltage loss 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. This paper presents a pathway for the development of perovskite solar cells. Mentioned in this article, the short-circuit current density JSC of perovskite solar cells has reached 97% of the theoretical limit of short-circuit current density JSQ in 2018. In contrast, the open circuit voltage Voc at the same time only reaches about 77% of the theoretical limit of the open circuit voltage VSQ. Compared with ultra-high conversion efficiency GaAs solar cells, Voc has reached 95% of the theoretical limit. Therefore, it is considered in this paper that to reach the theoretical limit of the conversion efficiency of perovskite solar cells, the research strategy of improving Voc (reducing loss) is far more important than improving JSC. Thus, if there is a defect theoretical model to describe the loss of Voc, and there is a set of measurement tools for scientists to measure and analyze the loss mechanism of Voc, this will be very helpful for the research of perovskite solar cells.
Since 2019, as the proportion of “open circuit voltage” in related papers on “organic solar cells” has increased year by year, “reducing Voc loss” has become an important strategy to improve the efficiency of organic solar cells. For example, the 16% organic solar cell that broke the world efficiency record at that time published in Nature Communications in 2019 mentioned that the strategy adopted was to reduce the loss of the open circuit voltage Voc. This research uses Enlitech ELCT3011 and Enlitech REPS perovskite and organic photovoltaic Voc loss analysis system to quantify non-radiative recombination loss of Voc. Through spectral data combined with thermodynamic theory confirmed the passivation effect of PEAI on the surface of organic thin films, increased open circuit voltage (Voc), and improved the overall conversion efficiency of organic solar cells.
Figure 3 A successful example of improving the energy conversion efficiency of organic solar cells by adopting the strategy of reducing the open circuit voltage Voc loss (2019).
(Quoted from: Nature Communication; June 7, 2019; https://www.nature.com/articles/s41467-019-10351-5)
Figure 4 A successful example of improving the energy conversion efficiency of organic solar cells by adopting the strategy of reducing the open circuit voltage Voc loss (2021).
(Quoted from: Advanced Materials; November 13, 2021; https://onlinelibrary.wiley.com/doi/abs/10.1002/adma.202106316)
In 2021, Harald Ade of North Carolina State University and Zhixiang Wei of the Center for Nanoscience and others have achieved an efficiency of 16.2% in all-small-molecule organic solar cells (ASM-OSCs) through the high miscibility of ordered molecular packing. The research results was published in Advanced Materials (IF 30.849) in November 2021. Among them, the efficiency breakthrough strategy focuses on the open circuit voltage Voc. Enlitech ELCT3010 (now Enlitech REPS perovskite & organic photovoltaic Voc loss analysis system) is used to quantitatively test and analyze the Voc loss characteristics of organic solar cells.
In February 2022, the journal Energy & Environmental Science (IF 38.532) published the latest research results of Professor Hou Jianhui from the Institute of Chemistry, Chinese Academy of Sciences. The research team used an asymmetric wide-bandgap non-fullerene acceptor called AITC, which facilitates the formation of a stable mixed phase in the mixture and improves the photoconductivity of ternary OSCs, suppresses charge recombination and reduces non-fullerenes. The radiation voltage loss enables the tandem organic solar cell to achieve a high efficiency of 19.4%, which is also the highest efficiency in the organic field.
The researchers used Enlitech ELCT-3010 (now Enlitech REPS perovskite and organic photovoltaic Voc loss analysis system) and other instruments to conduct experiments. The results showed that the open circuit voltage (Voc) and fill factor (Fill Factor) of the device after adding AITC were obtained a large improvement. Laboratory tests achieved a record efficiency of 19.4% for tandem organic solar cells.
Figure 5 Prof. Hou Jianhui’s research results of OSC photovoltaic performance.
(a) The J-V curve of the device after the addition of AITC is better than that of the device without AITC.
(b) Comparison of EQE spectra of devices with and without AITC added.
(c) The effect of temperature on the short-circuit current of the three OCSs
(d) The effect of the charge densities of the three OSCs on the non-twin recombination rate constant
(e) EL spectra of the OSCs after AITC addition.
(f) EL EQE of OSCs after AITC addition.
(g) Dark current curves of OSCs with different thicknesses.
(h) The effect of different thicknesses on the efficiency.
Enlitech’s perovskite and organic photovoltaic Voc loss analysis system REPS not only can detect the extremely low EL-EQE signal (as low as 10-5 %, which is 7 orders of magnitude), but can also calculate thermodynamic Voc, radiative recombination Voc, and non-radiative recombination Voc (through its software SQ-VLA). In addition, it can also analyze ΔE1, ΔE2, and ΔE3 losses between different type of devices in one histogram chart. Most importantly, the analyzing software can help users match the calculated Voc-loss with the real Voc-loss of the device IV curve, thereby promoting the research progress and journal publication.
What is Voc-loss analysis?
In the previous article, we roughly learned that in the field of solar cell efficiency research, the study of the loss of open circuit voltage is the most popular part. So, do we have to first understand what the open circuit voltage loss is looking at?
To understand the loss of open circuit voltage, let’s start with the basics of solar cells. Solar cells are composed of n-type and p-type semiconductors. When the energy of photons is greater than the energy gap of the semiconductors, the photons are absorbed by the semiconductors and generate electrons and holes. The entire operation mechanism goes through four stages: (1) Absorption, (2) Photocarrier Generation, (3) Transport, and (4) Collection, so that solar cells can provide us with electricity.
Figure 6. Concept of solar cell and energy band diagram of solar cell (Quoted from: National Taiwan University Department of Electrical Engineering, The principle and application of solar cells, https://ee.ntu.edu.tw/upload/hischool/doc/2014.04.pdf).
When light irradiates the solar cell, energy conversion occurs through the processes of photon absorption, photogenerated carriers, charge transport, and charge collection. The External Quantum Efficiency (EQE) of a solar cell is the ratio of the number of monochromatic light with a known number of photons irradiating the solar cell and the number of electrons transferred to the external circuit. The above four processes describe the solar cell being irradiated and absorbed by known incident photon, becoming photocarriers and transport to electrodes. The whole process is the external quantum efficiency EQE process and it shows the photon-electron transferring capability of solar cell. In other words, EQE spectrum represents the whole process of above mentioned 4 steps.
The ideal solar cell model should convert energy only through the radiative recombination pathway, i.e. obtain 100% external quantum efficiency EQE. However, in fact, there are often multiple non-radiative recombination pathways that affect the performance of the solar cell. It results in additional voltage loss, called Voc loss.
The pain points of breaking through the conversion efficiency of organic solar cells?
According to Wikipedia (https://en.wikipedia.org/wiki/Organic_solar_cell#Charge_carrier_mobility_and_transport), difficulties associated with organic photovoltaic cells include their low external quantum efficiency (up to 70%) compared to inorganic photovoltaic devices, despite having good internal quantum efficiency; this is due to insufficient absorption with active layers on the order of 100 nanometers. Instabilities against oxidation and reduction, recrystallization and temperature variations can also lead to device degradation and decreased performance over time. This occurs to different extents for devices with different compositions, and is an area into which active research is taking place. Other important factors include the exciton diffusion length, charge separation and charge collection which are affected by the presence of impurities.
Many scientists work on improving the performance of OPVs through the ways as follows: 1. Charge carrier mobility and transport, 2. Effect of film morphology, 3. Controlled growth heterojunction, 4. Progress in growth techniques, 5. Vacuum thermal evaporation, 6. Organic vapor phase deposition, 7. Organic solar ink, 8. Light trapping, 9. Use in tandem photovoltaics, 10. Mechanical behavior.
In order to quantify and analyze above mentioned improvements, two things have naturally become the most concerned factors in the field of organic research: (1) How to achieve accurate measurement? (2) How to quickly calculate and obtain thermodynamic loss (thermodynamic Voc Loss, ΔE1), radiative recombination Voc Loss (ΔE2) & non-radiative recombination Voc Loss (ΔE3) using measurement data Analysis results? This is also a common pain point in this research field.
Enlitech’s Perovskite and Organic Photovoltaic Voc Loss Analysis System (Enlitech REPS) is a complete system that can help scientists measure, calculate and analyze the Voc-loss of solar cells, and provide ideas for the next step of process improvement. Enlitech’s REPS not only can detect the extremely low EL-EQE signal (as low as 10-5 %, which is 7 orders of magnitude), but can also calculate thermodynamic Voc, radiative recombination Voc, and non-radiative recombination Voc (through its software SQ-VLA). In addition, it can also analyze ΔE1, ΔE2, and ΔE3 losses between different type of devices in one histogram chart. Quickly providing researchers with effective test data and analysis results will not only saves researchers’ time, but also avoids errors caused by human calculations.
Figure 7. Enlitech’s organic and organic photovoltaic Voc loss analysis system (REPS) is used to analyze and improve the loss of organic solar cells, and the research results produced by the system can be seamlessly integrated and quickly published in journals.
How to proceed Voc-loss analysis more systemically?
Physicist Shockley-Queisser’s SQ equilibrium limit theory provides the answer. In the SQ equilibrium limit, the three major losses of Voc include: ΔV₁ thermodynamic loss, ΔV₂ radiation loss, ΔV₃ non-radiative loss. Using the SQ equilibrium limit theory, the process of Voc loss can be explained in detail.
Figure 8. Schematic diagram of energy levels of thermodynamic loss, radiative recombination loss, and non-radiative recombination loss of SQ equilibrium limit theory.
Figure 9. For the explanation and understanding of the related mechanisms of radiative recombination loss and non-radiative recombination loss, readers can refer to Adv. Energy Mater. 2017, 1602358. In this paper, there are more detail information of Voc loss mechanisms caused by various defects.
For the research of organic solar cells, can we obtain the open circuit voltage loss value by ourselves? How to do it?
The answer is yes. According to the Shockley-Quiser limit, the open circuit voltage Voc loss of a solar cell is determined by the three losses, which can be obtained by the following relationship:
where q is the basic 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 relationship of the open circuit voltage loss, we can clearly see that as long as the energy is measured Eg、V SQOC、VradOC., these three loss values can be obtained. Eg、V SQOC、VradOC. respectively represent the band gap of the solar cell, the open circuit voltage under the Shockley-Quiser limit , the open circuit voltage under the total radiation recombination VradOC.
So what should we do to measure these three physical values including band gap Eg, Shockley-Quiser limit V SQOC and the open circuit voltage under the total radiation recombination VradOC ? Firstly, let’s write down the definitions of the three physical quantities:
In addition, with the diode model, theoretically Voc can be defined by the following formula:
Next, we use Enlitech ELCT-3010 (now Enlitech REPS perovskite photovoltaic Voc loss analysis system) to measure the external quantum efficiency of solar cell electroluminescence EQEEL.
To use the perovskite open-circuit voltage loss analysis software (SQ-VLA), you need to set a name for this measurement and analysis.
Figure 10. Start-up snapshoot of perovskite open-circuit voltage loss analysis software (SQ-VLA).
After implementing the measuring data of QE-R, IVS-KA6000 and REPS, the results of the analysis and calculation are presented immediately, including the band gap Eg, Shockley-Quiser limit VOCSQ, the open circuit voltage under the total radiation recombination VOCrad, thermodynamic loss ΔV1, radiative recombination loss ΔV2 and non-radiative recombination loss ΔV3.
Figure 11. Snapshot of the data implement from QE-R, IVS-KA6000 and REPS.
Figure 12. SQ-VLA software shows the thermodynamic loss ΔV1, radiative recombination loss ΔV2 and non-radiative recombination loss ΔV3
Comparing the open-circuit voltage loss between the control group and the experimental group, we can clearly see whether the manipulation variable of the experimental group can effectively reduce the open-circuit voltage loss, thereby increasing the open-circuit voltage of organic solar cells.
Figure 13. SQ-VLA shows the comparison result of the thermodynamic loss ΔV1, radiative recombination loss ΔV2 and non-radiative recombination loss ΔV3 of control group and experimental group.
More practical applications of the Voc-loss analysis systems
Energy & Environmental Science publishes the research of Sun Yanming et al. from Beijing University of Aeronautics and Astronautics.
Organic solar cells (OSCs) are considered to be a promising solar energy conversion technology due to their unique advantages of low cost, light weight, and easy fabrication. The use of ternary OSCs containing two donors/one acceptor or one donor/two acceptors is an effective method to improve the power conversion efficiency (PCE) of devices.
However, little attention has been paid to how to select the appropriate third component. This paper presents a strategy for selecting the third component in ternary organic solar cells based on non-fullerene acceptors.
Advanced Materials (IF 30.849) published a study in November 2021. The research team realized a high-performance non-fused wide-bandgap acceptor for multifunctional photovoltaic applications without using a fused ring structure.
Wide-bandgap (WBG) non-fullerene acceptors (NFAs) with non-fused conjugated structures play a key role in organic photovoltaic (OPV) cells. In view of this, the research team synthesized NFAs named GS-OEH, GS-OC6 and GS-ISO with optical band gaps larger than 1.70 eV without using fused ring structures. Among the three NFAs, GS-ISO exhibits stronger crystallinity than GS-OEH and GS-OC6, resulting in smaller energetic disorder and larger exciton diffusion coefficient. In addition, GS-ISO also exhibits a high electroluminescent external quantum efficiency of 1.0 × 10-².
Under the illumination of the solar simulator, the OPV cell based on PBDB-TF:GS-ISO achieves a power conversion efficiency (PCE) of 11.62% with light intensity correction with the standard cell of Enlitech. In addition, the PBDB-TF:GS-ISO based battery achieves 28.37% PCE under the illumination of 500 lux and 2700 K color temperature.
Experiments were carried out through the QE-R quantum efficiency measurement system of Enlitech, FTPS Fourier transform photocurrent spectroscopy system, REPS photovoltaic Voc-loss analyzer and other instruments. The OPV cell exhibited an excellent PCE of 19.10%. Importantly, the GS-ISO-based OPV cells exhibited good stability under continuous irradiation of simulated sunlight. In addition to the EQE (External Quantum Efficiency) spectral analysis of solar cells, the Quantum Efficiency Measurement System of Guangyan Technology also provides a Jsc (short-circuit current density) comparison for the short-circuit current of solar cells under a solar simulator to prove the accuracy of the experiment.
The results of this study demonstrate that the molecular design strategy used in the study has great advantages in developing non-fused NFAs and that GS-ISO is a promising WBG acceptor for multifunctional photovoltaic applications.
Other top journals such as Advanced Materials, Energy & Environmental Science have pointed out that in order to improve the efficiency of organic solar cells, we must first understand where the loss of open circuit voltage is mainly, and then measure the electroluminescence efficiency, analyze and analyze Improve the loss of non-radiative recombination.