Interpreting J-V Curves: Insights into Solar Cell Performance
If you work in any solar cell research, you know that achieving optimal device performance can be challenging. There are many factors which can hinder your device performance including:
- Device shunting or shorting
- Charge build-up at device interlayers
- Mismatch of energy levels leading to poor extraction
- Trap state formation
- Hysteresis (particularly for perovskite solar cells)
With so many variables in a PV device, it can be difficult to pinpoint the exact issue affecting your solar cell's performance. In these cases, J-V curves can be incredibly useful to help uncover the root of your issue. Here, we have some examples of common issues seen in solar cell I-V curves. These were tested on aged perovskite devices.
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Device Shorting
If you notice a linear current-voltage relationship instead of the characteristic I-V shape in your solar cell, it could indicate that your device is functioning as an ohmic resistor rather than a solar cell. In simpler terms, there is a direct pathway, or short-cut, between the contacts of your device. This means that a hole or defect exists within your device area, creating a direct connection between the electrodes. As a result, electrons bypass the absorbing material, traveling straight through your device. This phenomenon is known as device shorting.
Even a minor defect can have a significant impact on your J-V curve. Take a look at the following graph, which demonstrates this behaviour. Here, the forward voltage sweep follows the path of a normal solar cell, but occasional spikes indicate non-diode behaviour. Over the course of this measurement, we see this behaviour moves from diode-like to a more linear relationship. Although the pixel in question has not yet fully shorted, it is likely that continued operational bias will eventually lead to complete device shorting. An interesting observation is that the calculated power conversion efficiency for this device falls between 11% and 13%, as reported by processing software. Therefore, it is crucial to always examine the J-V sweeps of your devices, as calculated metrics alone may not provide the full picture.
If you encounter an unusual J-V sweep, it is advisable to perform additional stability measurements. These measurements will help confirm the accuracy of the metrics obtained and expose any underlying device faults.
Device shorting often arises from defects within your solar cell, and once a defect is present, it cannot be rectified. Therefore, you should try to minimise the amount of defect wherever possible. In thin films, this is often easier said than done. However, there are many things you can do to reduce defects and comets.
- Thoroughly clean your substrates and expose them to UV-Ozone treatment
- Filter precursor solutions before deposition
- Coat your devices as soon as possible after UV Ozone treatment
- Utilize thicker film layers in your device
- Use a compressed gas source (such as N2 gun) to remove any contamination from the substrate before coating
By implementing these measures, you can mitigate the occurrence of defects and enhance the overall performance of your solar cells.
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S-Shaped Curve
S-shaped curves normally indicate an issue with charge carrier transport between the layers of your device. This is most likely due mismatched energy levels at your absorber/contract interfaces or the formation of an energy barrier at these interfaces. This effect is often seen in combination with low fill factor - this makes sense as the squareness of the graph is obviously compromised by this different curve. This effect has been observed in C-Si solar cells, organic photovoltaics and perovskite solar cells. In most cases, this characteristic is linked with an energy barrier at absorber/contact interfaces.
If you see this shape in your I-V curve, you will need to take action to improve charge extraction at your device interfaces. Actions you can take include:
- Ensure the smoothness and uniformity of all device layers. Rough and inconsistent layers are less likely to create a good contact, reducing the chances of efficient charge transfer.
- Dope your transport layers or absorber materials to improve energy level alignment in your device.
- Dopants can improve conductivity through your charge transport layers, preventing charge build up.
- Introduce passivation layers, charge transport blocking layers or other relevant layers to control the movement of charge through your device.
- Optimize the thicknesses of your device layers. This has been shown to reduce the prominence of S-shapes in I-V curves.
Hysteresis
Hysteresis is an effect seen in perovskite solar cells, which manifests as differing device performance between forward and reverse I-V sweeps. The exact cause of this phenomenon is still somewhat debated in the perovskite community, but the most commonly accepted theory attributes it to ion migration through the perovskite device under bias. Hysteresis of your solar cell will depend on the scan speed you take your J-V sweep at. Be wary of the term "hysteresis-free" devices, as this usually just means that the perovskite doesn't exhibit hysteresis at the quoted scan speed.
To truly understand the impact of hysteresis, additional measurements such as stabilized current measurements become essential. These extended measurements can provide you with valuable insights into the authentic operational performance of your device.
Hysteresis manifests as variations in the curves between forward and backward sweeps. Therefore, to check for hysteresis in your device, it is important that you measure both the forwards and backwards curve for your perovskite solar cell. Furthermore, conducting stabilized measurements becomes instrumental in verifying that the observed sweep accurately represents the operational performance of your device.
Kinks in I-V Curve
Small kinks in your I-V curve are commonly observed in perovskite solar cells, are closely associated with ion movements. These kinks are often linked to hysteresis effects and can be attributed to ion migration occurring over the course of a measurement. Typically, small, sharp kinks manifest in the forward sweep of the I-V curves. As the voltage approaches the open-circuit voltage (VOC) from the negative voltage region, the driving force for charge extraction from the device (VBI=V-VOC) diminishes. Consequently, charge carriers at these interfaces undergo rearrangements between measurement points, leading to a slight shift in VOC. Similar shifts can also occur above the VOC as ions continually readjust throughout the measurement process.
There are a few things to consider if you are consistently seeing kinks in your I-V curves. This feature is often seen in fast scans, so decreasing your scan speeds might decrease the scale of this effect. Improving the crystallinity and quality of perovskite layer might decrease the density of migrating ions and, subsequently, alleviate the occurrence of kinks. Additionally, it has been observed that hysteresis effects tend to be more significant in planar devices compared to mesoporous devices. However, as with all hysteresis-related phenomena, the true impact on your device's operational performance can be assessed through stabilized measurements, such as stabilizing current measurements.
These kinks can be remarkably subtle, as demonstrated in the above graphs, but they can also exhibit more significant manifestations, as seen in the graphs below. Unveiling the underlying mechanisms behind these kinks holds the key to further understanding and optimizing the performance of perovskite solar cells.
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References
- R. saive (2019) S-Shaped Current–Voltage Characteristics in Solar Cells: A Review. IEEE Journal of Photovoltaics. 9 DOI: 10.1109/JPHOTOV.2019.2930409
- Y. Zheng et al. (2015) Charge selective contacts, mobile ions and anomalous hysteresis in organic–inorganic perovskite solar cells Material Horizons 2. DOI: 10.1039/C4MH00238E
- C. Li et al. (2016) Iodine Migration and its Effect on Hysteresis in Perovskite Solar Cells Advanced Materiasl 12. DOI: 10.1002/adma.201503832
- S R Kurtz,J. M. Olson, D. J. Friedman, J.F. Geisz, A.E. Kibbler, & K.A. Bertness (1999) Passivation of Interfaces in High-Efficiency Photovoltaic Devices. United States: N. p.. DOI:10.1557/PROC-573-95.