Template Effects in Cu(I)-Bi(III) Iodide Double Perovskites: A Study of Crystal Structure, Film Orientation, Band Gap and Photocurrent Response
The toxicity of lead has inspired recent interest for making lead-free photovoltaic materials, among which the bimetallic double perovskite (DP) strategy is very promising. Unlike the popular Ag(I)-Bi(III) based DPs with wider band gaps ~ 2.0 eV we found the Cu(I)-Bi(III) recipe may have narrower band gaps ~ 1.6 eV in our previous study. Here we expand this strategy to various amines, from which we can see the cyclic aliphatic amines (CAAs) are critical to govern the structural dimensions, thin film orientations and band gaps of the Cu(I)-Bi(III) iodide DPs. Among these compounds, CAAs with "Z-shape" and six-member-rings result in two-dimensional structures (compounds 1 - 5), while CAAs with "C-shape" and five-member-rings result in 1D bimetallic chains (compounds 6 and 7). Regardless of the structural dimensions, all seven compounds show narrow band gaps between 1.53 - 1.67 eV, which can be correlated to the distortions of the Cu-I-Bi angles. We found that the closer the Cu-I-Bi angle approach to 180°, or, the smaller the octahedral tilt, the narrower the band gap is. Moreover, DFT calculations reveal such band gaps are flat and direct in nature. Interestingly, the thin films formed by these compounds show preferential orientations to the ITO substrate with respect to the symmetries of the amines. For mirror-symmetric diamine the inorganic layers tend to be perpendicular to ITO; for asymmetric diamine the inorganic layers tend to be randomly orientated on ITO; for the asymmetric mono-amines the inorganic layers tend to be parallel to ITO. Such a phenomenon is as far as we know unknown to the field of making perovskite thin films. To further reveal the potential of these materials for photovoltaic application we took compound 3 as a representative to study the photoelectric properties. From 20 oC to 130 oC, the conductivity of compound 3 increases for five orders of magnitude, ramping from 3.97×10-10 S/cm to 2.78×10-5 S/cm, indicating a formal semiconducting behaviour. Thus, the photo-current response experiments show ca. 17.5 nA difference between Ilight and Idark for 3, indicating the potential application of this material for light harvesting.