Optimization of efficient perovskite photovoltaics

The development of low-cost and stable metal electrodes is crucial for the mass production of perovskite solar cells (PSCs). Being an element common in the earth, Cu becomes an alternative candidate to replace noble metal electrodes such as Au and Ag due to its comparable physicochemical properties with good stability and low cost. However, undesirable band alignment associated with the device architecture hinders the study of efficient Cu-based PSCs. To solve this problem, researchers from China investigated the difference in energy levels at different interfaces and proposed a potential way to achieve more efficient copper-electrode clamped PSCs.

They published their work on July 8 in Advances in energetic materials.

“The development of cost-effective and high-performance PSCs is imperative,” said paper author Huangping Zhou, a professor at Peking University’s (PKU) School of Materials Science and Engineering. “Currently Cu electrode has attracted much attention due to its low cost and good stability, but it is limited in performance for PSCs with a clamp structure.”

Zhou explained that the copper electrode has several significant advantages as an alternative to Au or Ag, especially as the back electrode responsible for carrier transport in the device.

“Honey is an element that is abundant on Earth, and its cost is less than 1/80th that of Ag and 1/5500th that of Au,” Zhou said. “Cu is a promising candidate as a PSC electrode due to its comparable physical properties (such as conductivity) to Au and Ag and good stability.”

But Cu-based PSCs cannot exhibit high photovoltaic performance. According to Zhou, the main obstacle is that the Fermi level of the hole transport layer (HTL, e.g. Spiro-OMeTAD, –4.19 eV) is very different from the work function of Cu (–4.7 eV), resulting in a large bar Schottky’s era. at the HTL/Cu interface. This phenomenon does not exist in contact PSCs because the Fermi level of the commonly used C₆₀ (electron transport layer) is about –4.5 eV, which is similar to the output performance of Cu. This is why Cu-based pin PSCs can exhibit high optoelectronic performance, while Cu-based PSCs cannot.

To solve this problem, Zhou and her team systematically adjusted the Fermi level of the HTL according to the yield function of the copper electrode, so that the energy difference at the HTL/Cu interface could be reduced for better carrier transport. However, the difference between energy Perevskite (The Fermi level is –4.08 eV) and the copper electrode is unchanged, so a smaller energy difference between HTL and Cu means a larger energy difference between perovskite and HTL, which is detrimental to carrier extraction. How to balance the energy difference between the perovskite/HTL and HTL/Cu interfaces becomes important for PSC performance.

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

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

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

“The most efficient PSC with a copper electrode achieved a PCE of 20.10% with a Voc of 1.084 V and an FF of 78.77%,” Zhou said. “The devices also demonstrated good stability, able to remain at 92% of their original PCE after 1,000 hours of storage. This discovery not only expands the understanding of the band alignment of the adjacent functional layer of the semiconductor device architecture to improve the final performance, but also indicates the great potential of the copper electrode for applications in the PSC community.”

Courtesy of Beijing Institute of Technology Press Co., Ltd