New single-diode model tackles recombination losses in solar cells

The single-diode model (SDM) is an electrical equivalent circuit used to represent the behavior of solar cells and is widely used to simulate and predict the current–voltage (I–V) characteristics of solar panels under different conditions. It consists of a current source, one diode, and typically a series and shunt resistance to model internal losses. The diode represents the p–n junction behavior of the solar cell, while the resistances account for practical inefficiencies.

Despite its widespread adoption, the SDM is also known for assuming ideal diode behavior and not fully capturing the complex recombination mechanisms within a solar cell. Moreover, it may be less accurate under low irradiance conditions and near the open-circuit voltage.

In order to address these limitations, a research team from the University of New South Wales (UNSW) has developed an adjusted version of the SDM that can reportedly reduce the its root mean square error (RMSE) for I–V data measurements by a factor of three.

“As commercial silicon solar cells approach their intrinsic efficiency limits of over 26%, the SDM increasingly fails to accurately describe device behaviour, particularly near open-circuit voltage and the maximum power point,” lead author, Bram Hoex, told pv magazine. “As silicon PV approaches its physical efficiency ceiling, explicitly incorporating solid-state physics into yield models is no longer optional; it becomes necessary for accurate field-performance simulation.”

The research team said its intrinsic-adjusted extension of the single-diode model can explicitly account for radiative and Auger recombination in the silicon bulk.

Radiative recombination occurs when an electron recombines with a hole and emits a photon. It represents an intrinsic loss mechanism in solar cells, limiting the maximum achievable voltage and overall efficiency. Auger recombination, by contrast, is a non-radiative process in which the recombination energy is transferred to a third carrier rather than emitted as light. This energy is subsequently dissipated as heat, further reducing device performance.

The proposed SDM also requires parameters such as dopant concentration and silicon volume. It splits the series resistance into internal and external components, separating different physical loss mechanisms within the solar cell.

The adjusted method involves correcting voltage for external resistance, calculating radiative and Auger recombination currents, applying a modified single-diode fit, and combining all contributions into a final equation.

In order to assess its performance, the team compared two intrinsic-adjusted models based on different resistance treatments with a datasheet-based method and a standard single-diode fit. Curve fitting was performed using weighted RMSE minimization, with particular emphasis on short-circuit current and maximum power point (MPP) accuracy.

“For simulated high-efficiency device data, the intrinsic-adjusted model reduced RMSE by up to an order of magnitude compared to the standard SDM,” said Hoex. “For measured I–V data, the adjusted model reduced RMSE by roughly a factor of three, with notably improved accuracy near open-circuit voltage and MPP.”

The team added that separating internal and external series resistance further improved fit quality and avoided artefacts near open-circuit conditions. The model also captured key effects on temperature coefficients and low-light behaviour, as Shockley–Read–Hall (SRH) recombination, a non-radiative process that reduces voltage and efficiency, became “negligible.”

“Most yield simulation tools rely on the classical SDM, which implicitly assumes SRH-dominated recombination,” Hoex emphasized. “In ultra-high-efficiency devices, intrinsic recombination becomes non-negligible and alters the voltage and fill factor behaviour. Accurate modelling near the intrinsic limit is essential for reliable yield predictions, mismatch simulations, and bankability assessments of next-generation TOPCon and heterojunction (HJT) modules.”

The new version of the SDM was presented in “The intrinsic adjusted single-diode model: Solid State Physics meets accurate yield simulation,” published in Solar Energy Materials and Solar Cells. 

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