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14.3% certified large area OSC (organic solar cells); improved by adding inorganic electron transfer complex.

First Author: Qian Kang

Corresponding Author: Jianhui Hou、Bowei Xu

DOI: 10.1039/d0ta00999g

Highlights
  1. In this paper, a series of polynuclear metal oxide clusters (PMC) with gradually changing chemical composition and photoelectric properties have been synthesized, and a highly efficient and stable hole extraction layer has been developed to improve the efficiency of OSC.
  2. Compared with PEDOT:PSS device, the PCE of the OSC modified by PMC-4 increased from 15.7% to 16.3%.
  3. In addition, PMC-4 can be manufactured by solution processing without any post-processing, and the corresponding devices show higher long-term stability.
  4. The strong oxidizing properties of PMC can induce the formation of inorganic-organic electron transfer complexes with barrier-free interfaces, thereby effectively extracting holes.
  5. Experimental data and theoretical calculation results show that the molecular polarization of the mixed additional PMC can enhance the capacitance of the AIL/active layer interface. Therefore, the mixed addition of PMC can be processed by doctor-blade coating to produce 1 cm2 of large area OSC and obtain 14.3% certified PCE.
Introduction

  In February 2020, the Journal of Materials Chemistry A published the latest research results of organic solar cell materials by researcher Jianhui Hou from the Institute of Chemistry of the Chinese Academy of Sciences and Bowei Xu from the Institute of Chemistry of the Chinese Academy of Sciences as associate researchers. The interface engineering of electrode modification has proven to be an effective method to improve the power conversion efficiency (PCE) of organic solar cells (OSC).

  However, compared with the progress of the active layer, the research on interface modification is seriously lagging behind, and the contribution of electrode modification to the enhancement of PCE is marginalized. Polynuclear metal oxygen clusters (PMC) with gradually changing chemical composition and photoelectric properties have developed a highly efficient and stable hole extraction layer to improve the efficiency of OSC.

Background

  Benefiting from the rapid development of photosensitive layer materials, the power conversion efficiency (PCE) of OSCs has exceeded over 16%, reaching the threshold for practical applications. In addition to the active layer, the electrode intermediate layer is also the electrode interlayer helps to form a favorable built-in electric field and barrier-free contact at the interface between the electrode and the active layer, so the carriers can be driven and eventually collected. However, compared with the active layer, the development of the electrode interlayer is largely lagging behind. In particular, the electrode interlayer materials used for anode modification are extremely scarce, which severely limits the further improvement of OSC efficiency.

Key Results
14.3% certified large area OSC (organic solar cells); improved by adding inorganic electron transfer complex.

Figure 1. The chemical structure and band diagram of PMCs.

(a) Keggin structure and molecular formula of PMC in this work.
(b) XPS spectra of Mo 3d and W 4f core energy levels in PMC.
(c) Normalized absorption of PMC film.
(d) PMC, PEDOT: schematic diagram of the energy levels of PSS and PBDB-T-2F.
(e) The longitudinal (triangular, red) and transverse (square, blue) conductivity of PMC and PEDOT:PSS.
(f) AFM height and (g) phase image of PMC-4 surface.

14.3% certified large area OSC (organic solar cells); improved by adding inorganic electron transfer complex. J01 Figure 2. Structure of the OSC used in this journal.

Figure 2. 

(a) The device structure of the OSC used in this work.

(b) J-V curve and (c) EQE of PBDB-T-2F:Y6 device and PMC AIL. The inset is a PCE histogram of 50 independent cells using PMC-4 as AIL.

(d) Photovoltaic parameters of Voc, (e) Jsc, (f) FF and (g) PCE for devices with PMC thickness of 5, 20 and 40 nm.

  The solar simulator IV curve characterization and quantum efficiency graphs shown in Figure 2b and c and Table 1 show that the device modified by PMC-4 shows the best performance with a PCE of 16.3%, which is one of the highest PCE values of OSC. It is worth noting that the best PCE and average PCE of PMC-4 devices are higher than that of PEDOT:PSS devices, indicating the advantages of PMC-4 in achieving high efficiency.

  Moreover, although the HOMO energy level of PBDB-T-2F reaches 5.31 eV, the Voc value of PMC-4 devices can still reach the maximum Voc of PEDOT:PSS devices. No problem of Voc loss has been observed in PMC-4 devices, which means that PMC-4 AIL does not cause additional charge extraction barriers compared to PEDOT:PSS.

Photovoltaic parameters of devices

Table 1.

  As we can see from Table 1, the deviation between Jcal (EQE integration with AM1.5G, or called Jsc(EQE)) and Jsc measured from IV curves is within 1%. In general, not more than 5% difference between Jcal from EQE and Jsc from IV is acceptable for journal publication. In this article the authors use QE-R quantum efficiency measurement system and Enlitech’s solar simulator to take Jsc(EQE) (or Jcal) and Jsc(IV), respectively, which are highly coincident with each other. It also illustrates the reliable PCE results from IV testing data.

Application of PMC in large-area fabrication.

Application of PMC in large-area manufacturing.

(a) Schematic diagram of doctor blade coating process and optical photo of 1 cm2 device.

(b) PCE statistics of 30 independent 1 cm2 batteries.

(c) Photovoltaic results of a 1.0 cm2 device with a blade coating PMC-4 AIL certified by the National Institute of Metrology (NIM) of China.

Conclusion

  A series of PMCs with gradually changing chemical structures and AIL based on PMC-4 was developed as an excellent substitute for PEDOT: PSS for manufacturing OSC. Compared with PEDOT:PSS, PMC-4 has the advantages of adjustable performance, low cost and easy processing, without any post-processing.

  Thanks to optimized photoelectric performance, the synthesized PMC-4 is superior to PEDOT:PSS and commercial PMC in terms of improving the long-term stability of PCE and OSC; the device with PMC-4 shows an excellent PCE of 16.3%; this is one of the highest PCE values of OSC, with excellent reproducibility.

  By using ESR and UPS measurements, it is shown that PMC can form inorganic-organic electron transfer complexes with polymer donors, resulting in a band bending area of about 25 nm thick, which is essential for extraction of unobstructed holes.

  EIS analysis and DFT calculation results show that the molecular polarization of mixing additional PMC can enhance the capacitance at the AIL/active layer interface, which has been proven to be a new factor that improves the insensitivity of the interlayer thickness. PMC-4 is processed into a large area of 1 cm2 OSC through a doctor blade coating process, and has obtained a 14.3% certified PCE. These results show that PMC not only has advantages in manufacturing high-performance OSC devices, but also has great promise in the future industrial production of OSC.

Reference

An inorganic molecule-induced electron transfer complex for highly efficient organic solar cells†

Qian Kang, Yunfei Zu, Qing Liao, Zhong Zheng, Huifeng Yao, Shaoqing Zhang, Chang He, Bowei Xu and  Jianhui Hou  

https://pubs.rsc.org/en/content/articlelanding/2020/ta/d0ta00999g#!divAbstract

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