The band bending effect of LiI/NaI treated TiO2 photoanodes on the performance of dye-sensitized solar cells.

A new method has been developed for surface treatment of TiO2 photoanodes with LiI/NaI to enhance the photocurrent and, subsequently, the performance efficiency of the fabricated dye-sensitized solar cells (DSSCs). Three different concentrations (0.1, 0.25, and 0.5 mmol%) of LiI and NaI solutions were used to investigate the effect of this surface treatment on the device performance of DSSCs. A positive shift in the energy level of TiO2 has been experienced by surface treated devices, which is predominantly supported by the decrease in VOC. Furthermore, the introduction of LiI/NaI onto the TiO2 surface resulted in a reduction in the crystallite size, indicating an increase in the surface area which helps in more dye adsorption leading to higher JSC values of the devices. Besides, modification of the conduction band energy level, it also allows a fast electron injection process by shifting the density of states. Thus, this approach offers a simple but efficient route to enhance the photocurrent and efficiency of DSSCs.

Proton Radiation Hardness of Perovskite Tandem Photovoltaics.

Monolithic [Cs0.05(MA0. 17FA0. 83)0.95]Pb(I0.83Br0.17)3/Cu(In,Ga)Se2 (perovskite/CIGS) tandem solar cells promise high performance and can be processed on flexible substrates, enabling cost-efficient and ultra-lightweight space photovoltaics with power-to-weight and power-to-cost ratios surpassing those of state-of-the-art III-V semiconductor-based multijunctions. However, to become a viable space technology, the full tandem stack must withstand the harsh radiation environments in space. Here, we design tailored operando and ex situ measurements to show that perovskite/CIGS cells retain over 85% of their initial efficiency even after 68 MeV proton irradiation at a dose of 2 × 1012 p+/cm2. We use photoluminescence microscopy to show that the local quasi-Fermi-level splitting of the perovskite top cell is unaffected. We identify that the efficiency losses arise primarily from increased recombination in the CIGS bottom cell and the nickel-oxide-based recombination contact. These results are corroborated by measurements of monolithic perovskite/silicon-heterojunction cells, which severely degrade to 1% of their initial efficiency due to radiation-induced recombination centers in silicon.

Correlation between Electronic Defect States Distribution and Device Performance of Perovskite Solar Cells.

In the present study, random current fluctuations measured at different temperatures and for different illumination levels are used to understand the charge carrier kinetics in methylammonium lead iodide CH3NH3PbI3-based perovskite solar cells. A model, combining trapping/detrapping, recombination mechanisms, and electron-phonon scattering, is formulated evidencing how the presence of shallow and deeper band tail states influences the solar cell recombination losses. At low temperatures, the observed cascade capture process indicates that the trapping of the charge carriers by shallow defects is phonon assisted directly followed by their recombination. By increasing the temperature, a phase modification of the CH3NH3PbI3 absorber layer occurs and for temperatures above the phase transition at about 160 K the capture of the charge carrier takes place in two steps. The electron is first captured by a shallow defect and then it can be either emitted or thermalize down to a deeper band tail state and recombines subsequently. This result reveals that in perovskite solar cells the recombination kinetics is strongly influenced by the electron-phonon interactions. A clear correlation between the morphological structure of the perovskite grains, the energy disorder of the defect states, and the device performance is demonstrated.