Smallest Band Gap Bulk Ferroelectric Material: Realising an Elusive Goal

Any ferroelectric material is potentially an efficient solar energy converter. The common phenomenon in any solar cell, following the photoexcitation by sunlight of an electron-hole pair, is the separation of these electrons and holes to give rise to the current in an external circuit. The intrinsic dipolar field of a ferroelectric material is known to be very effective in separating the photoexcited electron-hole pair. Unfortunately, all known ferroelectric materials have large band gaps with little overlap with the solar spectrum and therefore, poor ability to absorb and convert sunlight to electricity. All past attempts to harvest a larger part of the solar spectrum by reducing the bandgap of bulk ferroelectric materials by different routes proved unsuccessful, mostly due to the rapid loss of ferroelectric properties.

Figure. Dielectric constant as a function of temperature for a) BaTi_1-x(Mn_1/2Nb_1/2)_xO₃, (BTMNO) x=0.075 and b) BaTi_1-x(Fe_1/2Nb_1/2)_xO₃, (BTFNO) x=0.075. c) Polarization vs electric field hysteresis loops of compositions BaTi_1-x(TM_1/2Nb_1/2)_xO₃ with TM = Mn, x = 0.075 (in black) and with TM = Fe, x = 0.075 (in red) at applied electric field frequency (f) = 100 Hz and at temperature (T) = 77 K. d) Kubelka-Munk function plotted as a function of incident energy of BaTiO₃, BTMNO, x = 0.05, 0.075 and BTFNO, x = 0.05 along with solar spectrum. Band gap of a few standard materials which were used in PV device has also been shown.

We recently discovered an efficient way to reduce the band gap of a classical ferroelectric material, BaTiO₃, without compromising its ferroelectric polarization significantly with the help of a charge-neutral dipole doping of Mn³⁺-Nb⁵⁺ pair replacing two Ti⁴⁺ ions. The Jahn-Teller distortion of Mn³⁺ ion plays an important role to stabilize the ferroelectric phase as we found replacing Mn³⁺ with non-Jahn-Teller Fe³⁺ deteriorates the ferroelectric property of BaTiO₃ rapidly. The substitution with Mn-Nb pair is superior to Fe-Nb pair in terms of ferroelectric transition temperature T_C, (Figure (a) and (b)), saturation polarization P_sat, (Figure (c)) and reduction of band gap E_g, (Figure (d)). This way, we could achieve a bulk ferroelectric oxide with the lowest bandgap (1.66 eV) with a sizable room temperature polarization of nearly 70% of BaTiO₃ by substituting 7.5% Ti⁴⁺ ions with Mn³⁺-Nb⁵⁺ pairs.

Designing a Lower Band Gap Bulk Ferroelectric Material with a Sizable Polarization at Room Temperature; Shyamashis Das, Somnath Ghara, Priya Mahadevan, A. Sundaresan, J. Gopalakrishnan, and D. D. Sarma; ACS Energy Lett. 2018, 3, pp 1176–1182; (DOI: 10.1021/acsenergylett.8b00492)

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