《Science(IF>63.832)》The LinXole AB FENG WANG team - High-Efficiency and Stable Perovskite Solar Cells - Achieved over 25% high power conversion efficiency and significantly improved device stability under thermal and illumination conditions
Highlights
- The research team developed an innovative doping approach for the hole transport layer spiro-OMeTAD in perovskite solar cells, using stable organic radicals as dopants along with ionic salts to modulate the work function.
- This ion-modulated radical doping strategy achieved power conversion efficiencies over 25% and significantly improved device stability.
- The doping method allows decoupled tuning of conductivity and work function, providing inspiration for optimization of other optoelectronic devices.
Background
Perovskite solar cells have shown outstanding photovoltaic performance and rapidly become a hot research topic. The hole transporting material commonly used in perovskite solar cells today is 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD). Spiro-OMeTAD needs to be doped to attain sufficient conductivity and proper work function. However, conventional doping with lithium salts and 4-tert-butylpyridine requires prolonged oxidation and also leads to poor device stability. Therefore, researchers sought to develop new doping strategies to enhance efficiency and stability.
Results
The research team proposed an innovative doping strategy using stable organic radicals as dopants along with ionic salts to modulate the work function, avoiding the lengthy oxidation process. They chose the biradical precursor DBP-TAA which rapidly converts to the stable monoradical DBP-TAA ̇ in spiro-OMeTAD. The radical provides polaronic charge carriers that instantly increase conductivity and work function. Added ionic salts such as lithium salts can further tune the work function to the desired value.
Methods
- Synthesized the biradical precursor DBP-TAA.
- Co-doped DBP-TAA and ionic salts into spiro-OMeTAD. Ionic salts include Li-TFSI etc.
- Monitored doping process through in-situ conductivity measurements and transient absorption spectroscopy.
- Fabricated perovskite solar cells with doped spiro-OMeTAD as hole transport layer.
- Measured open circuit voltage, short circuit current, fill factor, efficiency etc. of the solar cells.
- Tested long-term stability of cells under accelerated aging conditions.
This ion-modulated radical doping strategy achieved perovskite solar cells with power conversion efficiencies over 25% along with significantly enhanced stability under thermal and illumination stress. Compared to conventional doping, this method can rapidly and effectively increase the conductivity of spiro-OMeTAD and tune its work function.
Conclusion
This work developed an innovative doping strategy for the hole transport layer spiro-OMeTAD in perovskite solar cells. Using stable organic radicals as dopants along with ionic salts for work function modulation avoids lengthy oxidation and improves device stability. This provides new insights into realizing more efficient and stable perovskite solar cells, and guides the development of other optoelectronic devices.
Fig. 1. Comparison of perovskite solar cells (PSCs) based on the conventional and ion-modulated (IM) radical doping strategies.
(A) Illustration of the complex in situ reaction processes in the conventional doping process (top) and the clean, instant IM radical doping strategy (bottom) of spiro-OMeTAD. The radical and ionic salt were dissolved in 1,1,2,2-tetrachloroethane. (B) Current density–voltage (J–V) curves of PSCs (SnO2 electron transport layer) based on conventional doping, radical doping, and IM radical doping of spiro-OMeTAD. (C) J–V curves of PSCs (mesoporous TiO2 electron transport layer) based on conventional and IM radical doping of spiro-OMeTAD. (D and E) PCE tracking of unencapsulated PSCs based on conventional and IM radical doping of spiro-OMeTAD under 70 ± 5% humidity (D) and 70 ± 3°C thermal aging (E). Error bars denote SD.
Fig. 2. Effects of radicals and ionic salts on conductivity and energetics.
(A and B) J–V curves of the hole-only devices (A) and conductivity of the spiro-OMeTAD films doped with different radical amounts (B). Inset of (B) shows the structure of the hole-only devices. (C) J–V curves of the hole-only devices with 14 mol % radicals and different amounts of TBMP+TFSI–. The inset shows conductivity. (D and E) Fermi-level and HOMO onsets of the spiro-OMeTAD films doped with different radical amounts (D) and different TBMP+TFSI– amounts (14 mol % radicals) (E). Error bars denote SD. (F) Illustration of the band alignment between the perovskite layer and the HTL with different WF values.
Fig. 3. Molecular-level doping mechanisms of the IM radical doping.
(A) Illustration of the charge transfer and doping mechanisms with spiro-OMeTAD•+TFSI– radicals. (B) 1H hr-NMR spectra of spiro-OMeTAD (black line), spiro-OMeTAD/spiro-OMeTAD•+TFSI– mixture without TBMP+TFSI– (red line; inset shows magnified view of d+e+f peak in the range of 9 to ~7.5 ppm), and spiro-OMeTAD/spiro-OMeTAD•+TFSI– mixture with TBMP+TFSI– (blue line) in the range 9 to 6.25 ppm. (Peaks b, c, and d+e+f refer to the aromatic protons signal.) (C) EPR signals of doped spiro-OMeTAD (14 mol % radicals) with and without TBMP+TFSI– at low temperature and room temperature. (D) Zoom-in spectra near the valence band of neutral spiro-OMeTAD, 14 mol % radical–doped spiro-OMeTAD film, and 14 mol % radical and 20 mol % TBMP+TFSI––doped spiro-OMeTAD films. (E) Illustration of ionic salts’ effect on WF modulation in the framework of the IM radical doping strategy. (F) Temperature-dependent conductivity evolution of doped spiro-OMeTAD (14 mol % radicals) with and without TBMP+TFSI–.
Fig. 4. Generality of the IM radical doping strategy.
(A) J–V curves of different PSCs based on spiro-OMeTAD HTL following the IM radical doping strategy. (B) Molecular structures of six additional ionic salts used in this work and their effects on the energetic levels of the doped spiro-OMeTAD films. EDMPA, ethyldimethylpropylammonium; PMPlm, 3-methyl-1-propylpyridinium; BMIM, 1-propyl-3-methyl-imidazolium. (C) J–V curves of PSCs based on the spiro-OMeTAD HTL doped with six additional ionic salts (14 mol % radicals).
References
Original Article:
