Effects of Hydroiodic Acid Concentration on the Properties of CsPbI3 Perovskite Solar Cells.

Inorganic cesium lead triiodide (CsPbI3) perovskite materials are becoming increasingly attractive for use in perovskite/silicon tandem solar cells, due to their almost ideal band gap energy (E g) of about 1.7 eV. To be useful as photovoltaic absorbers, the CsPbI3 must form the cubic or black phase (α-CsPbI3). To do so at relatively low temperatures, hydroiodic acid (HI) is required as a solution additive. This paper demonstrates CsPbI3 perovskite solar cells with an efficiency of 6.44%, formed using a HI concentration of 36 µL/mL. This value is higher than the previous most commonly used HI additive concentration. Herein, by undertaking a systematic study of the HI concentration, we demonstrate that the structural, morphological, optical, and electrical properties of CsPbI3 solar cells, processed with this HI additive concentration, are superior.

Adsorbed carbon nanomaterials for surface and interface-engineered stable rubidium multi-cation perovskite solar cells.

The current work reports the simultaneous enhancement in efficiency and stability of low-temperature, solution-processed triple cation based MA0.57FA0.38Rb0.05PbI3 (MA: methyl ammonium, FA: formamidinium, Rb: rubidium) perovskite solar cells (PSCs) by means of adsorbed carbon nanomaterials at the perovskite/electron transporting layer interface. The quantity and quality of the adsorbents are precisely controlled to electronically modify the ETL surface and lower the energy barrier across the interface. Carbon derivatives namely fullerene (C60) and PC71BM ([6,6]-phenyl C71 butyric acid methyl ester) are employed as adsorbents in conjunction with ZnO and together serve as a bilayer electron transporting layer (ETL). The adsorbed fullerene (C60-ZnO, abbreviated as C-ZnO) passivates the interstitial trap-sites of ZnO with interstitial intercalation of oxygen atoms in the ZnO lattice structure. C-ZnO ETL based PSCs demonstrate about a 19% higher average PCE compared to conventional ZnO ETL based devices and a nearly 9% higher average PCE than PC71BM adsorbed-ZnO (P-ZnO) ETL based PSCs. In addition, the interstitial trap-state passivation with a C-ZnO film upshifts the Fermi-level position of the C-ZnO ETL by 130 meV, with reference to the ZnO ETL, which contributes to an enhanced n-type conductivity. The photocurrent hysteresis phenomenon in C-ZnO PSCs is also substantially reduced due to mitigated charge trapping phenomena and concomitant reduction in an electrode polarization process. Another major highlight of this work is that, C-ZnO PSCs demonstrate a superior device stability retaining about 94% of its initial PCE in the course of a month-long, systematic degradation study conducted in our work. The enhanced device stability with C-ZnO PSCs is attributed to their high resistance to aging-induced recombination phenomena and a water-induced perovskite degradation process, due to a lower content of oxygen-related chemisorbed species on the C-ZnO ETL. The intricate mechanisms behind the efficiency and stability enhancement are investigated in detail and explained in the context of enhanced surface and interfacial electronic properties.

Solution-Processed Lithium-Doped ZnO Electron Transport Layer for Efficient Triple Cation (Rb, MA, FA) Perovskite Solar Cells.

The current work reports the lithium (Li) doping of a low-temperature processed zinc oxide (ZnO) electron transport layer (ETL) for highly efficient, triple-cation-based MA0.57FA0.38Rb0.05PbI3 (MA: methylammonium, FA: formamidinium, Rb: rubidium) perovskite solar cells (PSCs). Lithium intercalation in the host ZnO lattice structure is dominated by interstitial doping phenomena, which passivates the intrinsic defects in ZnO film. In addition, interstitial Li doping also downshifts the Fermi energy position of Li-doped ETL by 30 meV, which contributes to the reduction of the electron injection barrier from the photoactive perovskite layer. Compared to the pristine ZnO, the power conversion efficiency (PCE) of the PSCs incorporating lithium-doped ZnO (Li-doped) is raised from 14.07 to 16.14%. The superior performance is attributed to the reduced current leakage, enhanced charge extraction characteristics, and mitigated trap-assisted recombination phenomena in Li-doped devices, thoroughly investigated by means of electrochemical impedance spectroscopy (EIS) analysis. Li-doped PSCs also exhibit lower photocurrent hysteresis than ZnO devices, which is investigated with regard to the electrode polarization phenomena of the fabricated devices.

A high performance and low-cost hole transporting layer for efficient and stable perovskite solar cells.

Here we report a small molecule oxidant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ) doped, low cost 2',7'-bis(bis(4-methoxyphenyl)amino)spiro[cyclopenta[2,1-b:3,4-b']dithiophene-4,9'-fluorene] (FDT) hole transporting layer (HTL) for efficient mixed organic cation based MA0.6FA0.4PbI3 (MA = methyl ammonium, FA = formamidinium) perovskite solar cells (PSCs), fabricated via a highly reproducible controlled nucleation assisted restricted volume solvent annealing method, having full temperature compatibility with flexible substrates. The optimized (1 wt%) F4TCNQ doped FDT HTL based devices (F-FDT devices) demonstrate simultaneous enhancement of photovoltaic performance and device stability as well as significant reduction in photo-current hysteresis, as compared to conventional bis(trifluoromethylsulfonyl)amine lithium (Li-TFSI) additive based FDT HTL devices (L-FDT devices). Adding to the merits, F-FDT PSCs exhibit about 75% higher device stability compared to conventional L-FDT devices during the course of three weeks. Mott-Schottky analysis and in-depth charge transport characterization were carried out using electrochemical impedance spectroscopy (EIS) of the fabricated devices to understand the superior performance of the F-FDT devices. In addition, detailed polaronic intensity characterization of the doped HTL films was performed via ultraviolet-visible near-infrared (UV-vis-NIR) spectroscopy to investigate the underlying mechanism. Mitigated photocurrent hysteresis in the F-FDT devices has also been examined in terms of the inherent electrode polarization phenomenon. Furthermore, the superior device stability of the F-FDT PSCs has been probed in terms of variation in electronic properties, surface wettability, crystallinity, and microstrain dislocation density, and a detailed picture of the underlying mechanism behind stability enhancement is presented.