Urbach Energy and Open-Circuit Voltage Deficit for Mixed Anion–Cation Perovskite Solar Cells

Abstract

The Urbach energy indicating the width of the exponentially decaying sub-bandgap absorption tail is commonly used as the indicator of electronic quality of thin-film materials used as absorbers in solar cells. Urbach energies of hybrid inorganic−organic metal halide perovskites with various anion− cation compositions are measured by photothermal deflection spectroscopy. The variation in anion−cation composition has a substantial effect on the measured Urbach energy and hence the electronic quality of the perovskite. Depending upon the compositions, the Urbach energy varies from 18 to 65 meV for perovskite films with similar bandgap energies. For most of the perovskite compositions studied here including methylammonium (MA) + formamidinium (FA)-based Pb iodides, mixed Sn + Pb narrow-bandgap perovskites with low or intermediate Sn contents, and wide-bandgap FA + Cs-and I + Br-based perovskites, the correlation between the Urbach energy of the perovskite thin film and open-circuit voltage (V OC ) deficit for corresponding solar cells shows a direct relationship with reduction of the Urbach energy occurring with a beneficial decrease in the V OC deficit. However, due to issues related to material quality, impurity phases and stability in laboratory ambient air, and unoptimized film processing techniques, the solar cells incorporating Cs-based inorganic and mixed Sn + Pb perovskites with a higher than optimum Sn content show a higher V OC deficit even though the corresponding films show a lower Urbach energy.

Altering heating area assisted space confined method for growth of large scale and high quality MAPbBr3 single crystal thin films

“…The band gap of the SCF is between those of BSCs and polycrystalline films. 35,43,45,48 Comparing with the BSC, the absorption onset of the MAPbBr 3 SCF is steeper, suggesting its smaller Urbach tail and energy, 49 which reflects the lower density of sub-bandgap states near the band edge for the MAPbBr 3 SCF. Fig.…”

Heterovalent Tin Alloying in Layered MA3Sb2I9 Thin Films: Assessing the Origin of Enhanced Absorption and Self-Stabilizing Charge States

“…The corresponding absorption measurements over the visible spectrum are depicted in Figures 4b−d, with the fitted absorbance spectrum of the undoped reference shown in Figure 4a. The absorbance spectrum of pristine 2D-MA 3 Sb 2 I 9 can be fitted by Elliott's method in proximity of the band edge 41,42 and Urbach's method in the bandgap, 43,44 pointing to an overall excitonic absorption behavior with a bandgap (E g ) of 2.19 eV, an Urbach energy of 34.0 meV in the range of other reported Sn perovskites, 45 and a small exciton binding energy of 18.1 meV, which is probably overshadowed by the large inhomogeneous broadening component Γ of 81.4 meV. This broadening component is significantly higher than in regular lead-based perovskites (∼20 meV).…”

Reducing Energy Disorder for Efficient and Stable Sn−Pb Alloyed Perovskite Solar Cells.

“…The improved film crystallinity and antioxidant ability usually result in higher quality perovskite films with lower defect density. Our speculation is firstly tested by fitting the curves close to the band edge of the experimental external quantum efficiency (EQE) spectra to conduct preliminary estimate of the Urbach energy ( E u ) of perovskite films with different concentrations of BBMS [7] . As shown in Figure S4, the device based on perovskite film with BBMS of 0.5 mg mL −1 shows the lowest E u .…”

“…Our speculation is firstly tested by fitting the curves close to the band edge of the experimental external quantum efficiency (EQE) spectra to conduct preliminary estimate of the Urbach energy (E u ) of perovskite films with different concentrations of BBMS. [7] As shown in Figure S4, the device based on perovskite film with BBMS of 0.5 mg mL À 1 shows the lowest E u . We further compared the E u of control and BBMS films by the photothermal deflection spectroscopy (PDS) measurement, which more precisely measures the defect-induced sub-band gap optical absorption and evaluates the electronic energy disorder of the perovskite films.…”