The physical mechanism behind the drop in open-circuit voltage of fophotovoltaic diodes at high temperatures stems from the combined effects of semiconductor material properties and device structure as temperature rises. As a core component of photovoltaic systems, fophotovoltaic diodes convert light energy into electricity through the photovoltaic effect, and their performance is highly dependent on the electrical properties of the semiconductor material. As ambient temperature rises, carrier behavior, band structure, and PN junction characteristics within the semiconductor undergo significant changes, collectively leading to a decrease in open-circuit voltage.
The sharp increase in intrinsic carrier concentration in semiconductor materials with increasing temperature is one of the fundamental reasons for the drop in open-circuit voltage. At room temperature, the carrier concentration (electrons and holes) in semiconductors is low, primarily determined by the dopant. However, as temperature rises, electrons in the valence band gain sufficient energy to transition to the conduction band, forming intrinsic carriers. This process causes a simultaneous increase in both the conduction band electron concentration and the valence band hole concentration, disrupting the charge balance within the semiconductor. For fophotovoltaic diodes, an increase in intrinsic carrier concentration weakens the built-in electric field strength of the PN junction, which is a key factor in maintaining the open-circuit voltage. This weakening of the built-in electric field directly reduces the separation efficiency of photogenerated carriers, leading to a decrease in the open-circuit voltage.
Changes in the band structure further exacerbate the decrease in open-circuit voltage. The bandgap of semiconductor materials has a negative temperature coefficient, meaning that the bandgap narrows as temperature increases. This change makes it easier for photogenerated carriers to cross the bandgap, transitioning from the valence band to the conduction band. However, this narrowing of the bandgap also reduces the potential value of the photogenerated voltage. In the photovoltaic effect, the open-circuit voltage is closely related to the bandgap: the narrower the bandgap, the lower the theoretical maximum open-circuit voltage. Therefore, the reduction of the bandgap at high temperatures directly limits the achievable open-circuit voltage of fophotovoltaic diodes.
The weakening of the built-in electric field in the PN junction is the direct cause of the open-circuit voltage drop at high temperatures. The PN junction is the core structure of a photovoltaic diode. Its built-in electric field is determined by the balance between diffusion current and drift current. At high temperatures, an increase in intrinsic carrier concentration enhances the diffusion current, while the drift current changes relatively little. This imbalance reduces the built-in electric field strength, which in turn weakens the separation of photogenerated carriers. When photogenerated electrons and holes cannot be effectively separated, the probability of recombination increases, resulting in a decrease in photogenerated current and a consequent drop in open-circuit voltage.
High temperatures also indirectly reduce open-circuit voltage by affecting carrier mobility and lifetime. As temperature rises, thermal motion of carriers intensifies, causing mobility to decrease. Reduced mobility slows the transport of photogenerated carriers and increases the probability of recombination during transport. Furthermore, high temperatures exacerbate carrier scattering by defects and impurities in the semiconductor material, further shortening carrier lifetime. This shortened carrier lifetime means that more photogenerated carriers recombine before reaching the electrode, reducing the contribution to the photogenerated current and ultimately leading to a decrease in open-circuit voltage.
Degradation of the fophotovoltaic diode's packaging material at high temperatures can also indirectly affect the open-circuit voltage. Encapsulation materials typically protect fophotovoltaic diodes from environmental influences. However, at high temperatures, these materials may soften, expand, or chemically decompose, resulting in decreased transmittance or reduced adhesion to the semiconductor material. This reduced transmittance reduces the luminous flux reaching the fophotovoltaic diode, thereby lowering the photogenerated current and open-circuit voltage. Degradation at the interface between the encapsulation material and the semiconductor material can introduce additional recombination centers, accelerating the recombination of photogenerated carriers and further weakening the fophotovoltaic diode's performance.
