Optimizing efficient perovskite photovoltaics

Advances in energy equipment (2022). DOI: 10.34133/2022/9781073″ width=”716″ height=”530″/>

(a) Energy level of valance tape for perovskite (PVSK), HTL1-4 and Cu. (b) Valance band energy level difference for different HTLs between PVSK/HTL and HTL/Cu. (cf) The diagram of the energy level difference between PVSK/HTL/Cu and the corresponding hole transport behavior. Credit: Advances in energy equipment (2022). DOI: 10.34133/2022/9781073

Developing cheap and stable metal electrodes is crucial for the mass production of perovskite solar cells (PSCs). As a terrestrial element, Cu becomes an alternative candidate to replace noble metal electrodes such as Au and Ag, due to its similar physiochemical properties with good stability and low cost at the same time. However, the unwanted band alignment associated with the device architecture hinders the exploration of efficient Cu-based nip PSCs. To solve this problem, researchers in China examined the difference in energy level at different interfaces and offered a possible way forward to achieve more efficient nip PSCs with a Cu electrode.

They published their work on July 8 in Advances in energy equipment.

“The development of cost-effective and high-performance PSCs is imperative,” said author Huanping Zhou, professor at the School of Materials Science and Engineering, Peking University (PKU). “Currently, the Cu electrode has attracted a lot of attention because of its low cost and good stability, but it is limited in performance for nip structure PSCs.”

Zhou explained that the Cu electrode has several important advantages as an alternative to Au or Ag, especially as the back electrode, which is responsible for the transport of the carrier in the device.

“Cu is the earth element and it costs less than 1/80th of Ag and 1/5500th that of Au,” Zhou said. “Cu is the promising candidate to be PSC electrode because of its similar physical properties (i.e., conductivity) to Au and Ag, and good stability.”

But Cu-based nip PSCs cannot exhibit high photovoltaic performance. According to Zhou, the biggest obstacle is that the Fermi level of the hole transport layer (HTL, such as Spiro-OMeTAD, -4.19 eV) is very different with the work function of Cu (-4.7 eV), leading to a large Schottky barrier at HTL/Cu interface. This phenomenon does not exist in pin PSCs, because the Fermi level of commonly used C60 (electron transport layer) is about –4.5 eV, which is comparable to the work function of Cu. This is why Cu-based pin PSCs can exhibit high optoelectronic performance while Cu-based nip PSCs cannot.

To address this issue, Zhou and her team systematically matched the Fermi level of HTLs to the work function of the Cu electrode, so that the energy difference at the HTL/Cu interface can be reduced for better carrier transport. However, the energy difference between perovskite (Fermi level is -4.08 eV) and the Cu electrode is constant, so the smaller energy difference between HTL and Cu means a larger energy difference between perovskite and HTL, which is harmful for the extraction of carriers. How to balance the energy difference between perovskite/HTL and HTL/Cu interfaces becomes important for PSC performance.

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“Like the bucket effect, we hope that both perovskite/HTL and HTL/Cu interfaces are not the shortest buckets during device operation,” Zhou said. “In this paper, we carefully adjusted the Fermi level of HTLs to balance the energy difference at perovskite/HTL and HTL/Cu interfaces, by adding different amounts of PTAA to Spiro-OMeTAD.”

“We concluded that the balanced energy difference between perovskite/HTL and HTL/Cu interfaces could significantly improve charge collection and transport properties in the resulting nip PSC devices,” Zhou said.

The researchers tested the optoelectronic performance of nip PSCs based on the Cu electrode and various HTLs. Due to the photovoltaic parameters, Zhou said, a smaller energy difference between HTL and Cu could lead to a higher short-circuit current density (Jsc), while a smaller energy difference between perovskite and HTL could lead to a higher open circuit voltage (Voc). Finally, the balanced energy difference between perovskite/HTL and HTL/Cu interfaces could lead to moderate Jsc and Voc, especially a higher fill factor (FF), which ultimately contributed to the improved energy conversion efficiency (PCE).

“The best performing nip PSC with the Cu electrode achieved a PCE of 20.10% with the Voc of 1.084 V and FF of 78.77%,” said Zhou. “The devices also showed good stability, which could remain at 92% of their original PCE after 1000 hours of storage. Not only does this finding increase the understanding of the band alignment of the adjacent functional semiconductor layer in the device architecture to improve the resulting performance, but also suggests great potential of the Cu electrode for application in the PSC community.”


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More information:
Ziqi Xu et al, Balancing Energy Level Difference for Efficient Nip Perovskite Solar Cells with Cu electrode, Advances in energy equipment (2022). DOI: 10.34133/2022/9781073

Provided by Beijing Institute of Technology Press Co., Ltd

Quote: Optimization of Efficient Photovoltaic Perovskite (2022, October 3), retrieved October 3, 2022 from https://phys.org/news/2022-10-optimizing-efficient-perovskite-photovoltaics.html

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