German scientists develop novel heterojunction solar cells

Technische Universität Dresden researchers have created a phase heterojunction (PHJ) solar cell – a new kind of heterojunction PV device that uses two polymorphs made of perovskite.

“The concept of a phase heterojunction for photovoltaics could be applicable to different classes of polymorphic materials, which means such devices may find applications in many different areas,” researcher Yana Vaynzof told pv magazine. “Phase heterojunctions can be deposited on solid substrates for long term PV installations, but can also be made on flexible substrates for portable electronics or Internet-of-Things applications.”

“The advantage of the phase heterojunction concept is that a single material is used on both sides of the junction, just in a different crystalline phase,” she continued. “This eliminates the need for doping and prevents the negative impact of ion migration. Such an approach demonstrates that crystalline engineering can lead to efficient heterojunction solar cells from a single material without the need for doping or other means to control their properties.”

Vaynzof noted that the novelty of the new approach lies in the realization of a junction based on the combination of two components with different optoelectronic properties. The scientists chose the β- and γ-phases of a perovskite material known as CsPbI₃ for the two polymorphs.

“Our choice of the β-CsPbI₃ and γ-CsPbI₃ phases for the demonstration of the PHJ concept was motivated by their relatively low-temperature processing, unlike the α-CsPbI₃ that requires annealing at less than 300 C,” they explained. “Considering that solution processing of all polymorphs of CsPbI₃ requires the use of the same type of polar solvent³⁸, we utilized a hybrid deposition approach that combines both solution processing and thermal evaporation.”

The team used scanning electron microscopy (SEM) to characterize the microstructure of the deposited layers and their surface. It found that the solution-processed β-CsPbI₃ shows uniform, large grain size with an average diameter of around 300 nm. They also found that the energy bandgap of the γ- and β-CsPbI₃ was 1.74 eV and 1.69 eV, respectively.

“Their two slightly different bandgaps result in the absorbance spectra of the PHJ samples displaying features originating from both phases,” they said, adding that their optimal thickness is between 500 nm and 370 nm.

The researchers said the cell configuration offers an increase in all photovoltaic parameters. It has a power conversion efficiency of 20.17% and a maximum fill factor of 84.17%.

“Advanced spectroscopic analysis revealed that this improvement in performance is associated with increased light absorption and the formation of an advantageous energetic alignment between the two phases,” said Vaynzof.

The academics presented the new cell technology in “Perovskite phase heterojunction solar cells,” which was recently published in Nature Energy.

“The approach demonstrated here offers new possibilities for the development of photovoltaic devices based on polymorphic materials,” they said.

*The article was updated on November 17 to add some statements from researcher Yana Vaynzof.