“Buckets effect” in efficient perovskite photovoltaics

Beijing Institute of Technology Press Co., Ltd

They published their work on July. 8th in Energy Material Advances.

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

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

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

But Cu-based n-i-p PSCs cannot exhibit high photovoltaic performance. According to Zhou, the major obstacle is that the Fermi level of hole transport layer (HTL, such as Spiro-OMeTAD, –4.19 eV) is quite different with the work function of Cu (–4.7 eV), which leads to large Schottky barrier at HTL/Cu interface. This phenomenon is not existed in p-i-n PSCs, because the Fermi level of commonly used C60 (electron transport layer) is about –4.5 eV, which is similar with the work function of Cu. This is why Cu-based p-i-n PSCs can exhibit high optoelectronic performance while Cu-based n-i-p PSCs cannot.

Regarding to this problem, Zhou and her team systematically adjusted the Fermi level of HTLs to match well with the work function of Cu electrode, so that the energy difference at HTL/Cu interface can be reduced for better carrier transportation. However, the energy difference between perovskite (Fermi level is –4.08 eV) and Cu electrode is constant, so the smaller energy difference between HTL and Cu means larger energy difference between perovskite and HTL, which is deleterious for carrier extraction. How to balance the energy difference between perovskite/HTL and HTL/Cu interfaces is becoming important for PSC performance.

“Just like the buckets effect, we hope 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, through adding different amount of PTAA into Spiro-OMeTAD.”

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

The researchers tested the optoelectronic performance of n-i-p PSCs based on Cu electrode and different HTLs. Through the photovoltaic parameters, Zhou said, smaller energy difference between HTL and Cu could lead to higher short-circuit current density (Jsc), while smaller energy difference between perovskite and HTL could lead to 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 higher fill factor (FF), eventually contributed to the improved power conversion efficiency (PCE).

“The best performing n-i-p PSC with Cu-electrode achieved a PCE of 20.10% with the Voc of 1.084 V and FF of 78.77%,” Zhou said. “The devices also exhibited good stability, which could remain 92% of their initial PCE after 1000 h storage. This finding not only extends the understanding on the band alignment of neighboring semiconductor functional layer in the device architecture to improve the resulting performance, but also suggests great potential of Cu electrode for application in PSCs community.”

Zhou is also affiliated with the Beijing Key Laboratory for Theory and Technology of Advanced Battery Materials, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, BIC-ESAT. Other contributors include Ziqi Xu, Nengxu Li and Huifen Liu, School of Materials Science and Engineering, Peking University; Xiuxiu Niu and Qi Chen, Experimental Centre for Advanced Materials, School of Materials Science and Engineering, Beijing Institute of Technology; and Guilin Liu, School of Science, Jiangnan University.

Other contributor Ziqi Xu is also affiliated with the China Automotive Technology and Research Center Co., Ltd.

National Natural Science Foundation of China (Grant No. 51972004), the National Key Research and Development Program of China (Grant No. 2020YFB1506400, 2017YFA0206701), and the Tencent Foundation through the XPLORER PRIZE. supported this work.

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