Embrace the defects of perovskite, which leads the innovative transformation in solar cells

The future of perovskites breathtaking. Recently material scientist from North China Electric Power University developed the new planer p-n homojunction perovskite solar cells with highly efficiency of 21.3%. The research led by Prof. Meicheng Li was published in Nature Energy, and the same time, Ji-Sang Park, Shooter International Fellow in Royal Society and Aron Walsh, Department of Materials, Imperial College London, have commented that the finding open the way for perovskite solar cells.

Perovskite solar cells (PSCs) have emerged as an attractive photovoltaic technology thanks to their outstanding power conversion efficiency (PCE). Further improvement in the device efficiency is limited by the recombination of the charge carriers in the perovskite layer even when employing heterojunction-based architectures. The team of Prof. Meicheng Li proposed and demonstrated a p-type perovskite/n-type perovskite homojunction whose built-in electric field promotes oriented transport of the photo-induced carriers, thus reducing carrier recombination losses. By controlling the stoichiometry of the perovskite precursors, they are able to induce n-type or p-type doping. they integrated the homojunction structure in a planar PSC combining a thermally evaporated p-type perovskite layer on a solution-processed n-type perovskite layer. The PSC with a MAPbI3 homojunction achieves a PCE of 20.80% (20.5% certified PCE), whereas the PSC based on a FA0.15MA0.85PbI3 homojunction delivers a PCE of 21.38%. they demonstrated that the homojunction structure is an effective approach, beyond existing planar heterojunction PSCs, to achieve highly efficient PSCs with reduced carrier recombination losses.

Fig. Structure of planar PSCs with and without the homojunction and fabrication schematic of the homojunction PSC fabricated by the combined deposition method.

p-n-homojunction-perovskite_2.jpg
Fig. Carrier generation and recombination of the planar PSCs with and without the homojunction.

Ji-Sang Park and Aron Walsh indicate that this is important progress, as the existing p–i–n architecture limits the choice of materials and approaches to further enhance performance. A homojunction p–n architecture, instead, can benefit from internal electric fields within the perovskite absorber layer, thereby enhancing electron–hole separation. This initial demonstration of a halide perovskite homojunction may require further developments for long-term durability. It has been established that halide perovskites are mixed ionic-electronic conductors with ions and electrons being mobile at room temperature and even more mobile under illumination. Therefore, any gradient in doping profile (stoichiometry) could be removed by mass transport in response to current–voltage cycles of a solar cell under illumination or even in the dark over longer timescales. Therefore, even though the report no changes in device performance over the course of one month, an ultrathin interfacial barrier layer between the p-type and n-type regions may be needed to impede ion redistribution across the p-n junction over time.

These findings open the way for perovskite solar cells to adapt device configurations from mature technologies, such as silicon, that rely on homojunctions. Some of these will require higher n and p concentrations, which could be achieved by extrinsic doping; a task that has not yet proved successful.

In addition, the control of charge carrier concentrations and distributions in halide perovskite semiconductors illustrated by the researchers has wide ranging impacts beyond solar cells, including emerging applications in light emission (high current regime) and neuromorphic computing (low current regime). The time has come for lead halide perovskites to embrace their defects.

References:
Ji-Sang Park and Aron Walsh. (2019). Embrace your defects. Nature Energy. volume 4, pages95–96 (2019).

Planar p-n homojunction perovskite solar cells with efficiency exceeding 21.3%.
Peng Cui, Dong Wei, Jun Ji, Hao Huang, Endong Jia, Shangyi Dou, Tianyue Wang, Wenjing Wang & Meicheng Li.