The Hong Kong Polytechnic University has achieved a new breakthrough in the anode interface layer of organic solar cells

On May 16, 2022, the Journal Matter published the research results of Professor Gang Li’s team at The Hong Kong Polytechnic University entitled “Ambipolar-Transport Wide-Bandgap Perovskite Interlayer for Organic Photovoltaics with over 18% Efficiency”.

For the first time, the research group used the wide-band gap perovskite material MAPbBr3 as the anode interface layer to construct a highly efficient non-fullerene organic solar cell. Among them, MAPbBr3 can not only be used as a hole transport layer, but also as a downconversion layer, converting ultraviolet light and blue light into long-wavelength light, and transferring energy to the donor material PM6 through Foster energy transfer, which can also enhance the crystallinity of the active layer and show excellent material system universality and bipolar transmission characteristics.

The first author of the paper is Dr. Yan Cenqi, and Professor Li Gang is the corresponding author of the paper.

The anode interface layer of organic solar cells is critical to device efficiency and stability. Currently only PEDOT:PSS and inorganic p-type transition metal oxygen/sulfides are widely used. PEDOT: The acidic and hygroscopic properties of PSS can compromise the long-term stability of the device. The inorganic anode interface layer usually requires vacuum thermal evaporation and has strict requirements for thickness (usually < 10 nm), which poses a challenge to the large-scale processing of the device. The energy levels and band gaps of emerging perovskite materials can be controlled by adjusting the composition of halides, metal cations, and other cations. Wide bandgap perovskites exhibit excellent carrier transport characteristics with excellent electron/hole diffusion lengths and carrier lifetimes. Therefore, in this work, Professor Li Gang’s team for the first time developed a non-fullerene organic solar cell based on the hole transport layer of the broadband gap perovskite.

As shown in Figure 1, MAPbBr3 shows an absorption band edge of 539 nm and an optical band gap of 2.3 eV. MAPbBr3 exhibited high and balanced hole and electron mobility of 1.74 and 2.04 cm2V-1S-1, respectively. Ultraviolet photoelectron spectroscopy (UPS) shows that the work functions of PEDOT:PSS and MAPbBr3 are 5.2 and 5.0 eV, respectively. Kelvin Probe Microscopy (KPFM) showed a contact potential difference of -463 mV between the probe tip and the MAPbBr3 surface, while the contact potential difference between the probe tip and the PEDOT:PSS surface was -600 mV, consistent with the UPS results.

Figure 1: (a) UV-visible absorption spectrum of MAPbBr3; (b-c) dark current density-voltage curve; (d) UPS spectra; and (e) KPFM image.

As shown in Table 1, the unnealed device without a hole transport layer exhibits an efficiency of 9.09%, an open-circuit voltage of 0.68V, a short-circuit current density of 22.5 mA cm-2, and a pack factor of 59.2%. PEDOT:PSS-based unnealed devices show a PCE of 15.6%, an open-circuit voltage of 0.84V, a short-circuit current density of 25.2 mA cm-2, and a fill factor of 74.0%. MAPbBr3(14nm)-based devices exhibit 15.5% efficiency, an open-circuit voltage of 0.82V, a fill factor of 72.6%, and a short-circuit current density of 25.2 mA cm-2. The use of F4TCNQ to modify MAPbBr3 is conducive to increasing the function of the interface layer, and the hole mobility and electron mobility of MAPbBr3 are increased to 2.49 and 2.88 cm2V-1S-1, respectively. By using F4TCNQ doping, the device achieves an efficiency of 17.3%, an open-circuit voltage of 0.84V, a short-circuit current density of 26.5 mA cm-2, and a pack factor of 77.6%. Devices using MAPbBr3 have higher EQE peaks at wavelengths <400 nm (380 nm to 70%) than devices using PEDOT:PSS (380 nm to 62%). As shown in the fluorescence spectrum, the reason for the enhancement of the short wavelength EQE response is due to the Foster resonance energy transfer from MAPbBr3 to PM6. The relationship between current-light intensity and voltage-light intensity shows that F4TCNQ doping can effectively improve the efficiency of charge collection and inhibit trap-assisted recombination.

Table 1: Device PV performance

Figure 2: (a)J-V curve and (b) EQE spectra(c) ITO/PEDOT: PSS/PM6: BO-4Cl/PFNBr/Ag and ITO/MAPbBr3(14 nm)/F4TCNQ/PM6: EQE difference of BO-4Cl/PFNBr/Ag devices (d) PL spectrum (λex=450 nm) (e) J-V curve (f) Photocurrent density (Jph)-Effective voltage (Veff) curve (g) JSC’s dependence on Plight (h)VOC’s dependence on Plight.

Contact angle measurement and atomic force microscopy (AFM) can study the surface energy (γ) and surface topography of MAPbBr3 and PEDOT:PSS films. The surface tension of the MAPbBr3 film is 61.6 mN m-1, γd is 13.5 mN m-1, and γP is 48.1 mN m-1. PEDOT: The surface tension of the PSS interface is 71.9 mN m-1, γd is 13.9 mN m-1, and γP is 58.0 mN m-1. The AFM heightmap shows that PEDOT:PSS has a smooth surface with an Rq of 1.64 nm, while the MAPbBr3 has a rough surface and an Rq of 5.37 nm. The authors further employ grazing-incident wide-angle X-ray scattering (GIWAXS) to study crystalline structures at the molecular scale. PEDOT: The PSS sample showed only very weak scattering rings, while MAPbBr3 showed a clear orientation. The crystalline coherence length of the π-π peak of PM6: BO-4Cl (PEDOT: PSS substrate) is 21.0 Å, which is slightly smaller than the PM6:BO-4Cl (23.0 Å) on the MAPbBr3 substrate, indicating that the MAPbBr3 film is conducive to the aggregation of active layer materials.

Figure 3: (a) contact angle, (b) AFM altitude plot (1 μm×1 μm), GIWAXS(c) 2D plot and (d) inner and outer plane curves.

The bipolar transmission properties of MAPbBr3 may allow it to act as a cathode interface layer in organic solar cells, so the authors further constructed an inverted device. Devices based on the ITO/MAPbBr3/PM6: BO-4Cl/MoO3/Ag architecture have only a small short-circuit current density and efficiency, because MAPbBr3 and MoO3 have similar function functions and cannot be aligned with the LUMO layer of BO-4Cl. Inverted devices based on ITO/PM6:BO-4Cl/MoO3/Ag architecture exhibit an open-circuit voltage of 0.15V, a short-circuit current density of 17.4mA cm-2, a fill factor of 31.0%, and an efficiency of 0.799%. Devices based on ITO/MAPbBr3 (~50 nm)/Naphen-DPO/PM6:BO-4Cl/MoO3/Ag architecture demonstrated 8.75% efficiency, 0.59V open-circuit voltage, 25.8 mA cm-2 short-circuit current density, and 57.3% fill factor, demonstrating the bipolar transmission capability of MAPbBr3 in organic solar cells.

The authors further use the PM6:BTP-eC9:PC71BM and PM6:PY-IT architectures to verify the excellent ubiquity of the MAPbBr3 interface layer. For the PM6:BTP-eC9:PC71BM architecture, the unnealed device using the MAPbBr3(14 nm)/F4TCNQ interface layer achieves 18.3% efficiency, an open-circuit voltage of 0.86V, a fill factor of 79.5%, and a short-circuit current density of 26.7 mA cm-2. The PEDOT: PSS control device exhibits 17.4% efficiency, an open-circuit voltage of 0.86V, a fill factor of 76.6%, and a short-circuit current density of 26.5mA cm-2. For PM6:PY-IT full polymer systems, unneased devices containing MAPbBr3 (14 nm)/F4TCNQ exhibited a PCE of 15.1%, compared to PEDOT:PSS devices (14.2%).

Figure 4: (a) J-V curve, (b) EQE spectrum, (c) J-V curve, and (d) EQE spectrum.

The study demonstrates the potential of a solution-processed wide-bandgap perovskite material (MAPbBr3) as an efficient anode interface layer for organic solar cells, providing a unique perspective for designing interface layers for high-performance and cost-effective organic photovoltaics.

The research collaborators include Professor Hao Jianhua, Department of Applied Physics, The Hong Kong Polytechnic University, and Professor Lu Xinhui of the University of hong kong Chinese. (Source: Science Network)

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