Room-Temperature Hot-Polaron Photovoltaics in the Charge-Ordered State of a Layered Perovskite Oxide Heterojunction

Harvesting of solar energy by hot carriers from optically induced intraband transitions offers new perspectives for photovoltaic energy conversion. Clearly, mechanisms slowing down hot-carrier thermalization constitute a fundamental core of such pathways of third-generation photovoltaics. The intriguing concept of hot polarons stabilized by long-range phonon correlations in charge-ordered strongly correlated three-dimensional metal-oxide perovskite films has emerged and been demonstrated for ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{Mn}\mathrm{O}}_3$ at low temperature. In this work, a tailored approach to extending such processes to room temperature is presented. It consists of a specially designed epitaxial growth of two-dimensional Ruddlesden-Popper ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{1.5}{\mathrm{Mn}\mathrm{O}}_4$ films on $\mathrm{Nb}$:${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ with a charge-ordering transition at $T_{\mathrm{CO}}$ ∼ 320 K. This opens the route to a different phonon-bottleneck strategy of slowing down carrier relaxation by strong coupling of electrons to cooperative lattice modes.

Figure 1

XRD patterns of a 30-nm-thick MAD RP PCMO film on (011) STO and a blank substrate. Inset: pseudotetragonal representation of the RP $({\mathrm{AA}}^{\prime }{\mathrm{BO}}_3{)}_1({\mathrm{AA}}^{\prime }\mathrm{O}{)}_1$ structure.

Figure 2

Cross-section TEM imaging of RP PCMO on (011) STNO. The TEM lamella is prepared by focused ion-beam etching with the $[01\overline{1}]$ zone axis of the STNO substrate. (a) High-resolution ADF STEM image of the film. (b) Enlarged picture of an area in (a). (c) Profile of the integrated EELS counts of the $\mathrm{Ca}$, $\mathrm{Ti}$, and $\mathrm{Mn}$ L as well as $\mathrm{Pr}$ M edge after power-law background subtractions normalized by the maximum. The concentration profiles across the interface between RP PCMO and STNO shows $\mathrm{Pr}$ segregation close to the interface. The RP layer sequence is visualized by the colored column markers. (d) Selective area electron diffraction pattern in the STO $[01\overline{1}]$ zone axis shows reflections from the RP PCMO film and the STNO substrate due to the finite size of the SAED aperture.

Figure 3

High-resolution ADF STEM image of the interface of MAD grown RP PCMO on STO in the STO [100] zone axis. A misfit dislocation is marked by the red symbol.

Figure 4

Transport and optical properties of epitaxial [100]/[010] RP PCMO films on (011) STO. (a) Temperature-dependent deviation of resistance from the HT Arrhenius behavior and obtained temperature dependence of the activation energy (inset) for 30- and 200-nm-thick films. The change in slopes indicates the charge-ordering temperature of T_CO ≈ 320 K. (b) Spectrum of the absorption coefficient α, measured for a 100-nm-thick RP PCMO film at room temperature. The experimental data is fitted by three Gaussian peaks, with E_A = 0.76 eV (optical polaron hopping), E_B = 1.3 eV (Jahn-Teller transition), and E_C = 3.5 eV (charge-transfer transitions). The discontinuity in signal-to-noise ratio below 1.3 eV stems from different spectrometers used for the UV and NIR regimes.

Figure 5

Density of states of Ruddlesden-Popper ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{1.5}{\mathrm{Mn}\mathrm{O}}_4$ obtained from density-functional calculations (for details see Supplemental Material [23]). Majority and minority spin directions are shown with positive and negative sign. The colors correspond to the total density of states (black) and projections onto O p (red), as well as $\mathrm{Mn}$ t_2g (green) and $\mathrm{Mn}$ e_g (yellow) orbitals of the spin-up manganese atoms. The projected densities of states are shown stacked on top of each other; their values correspond to their colored areas. The JT splitting of the $\mathrm{Mn}$ e_g states leads to an occupied nonbonding and three unoccupied states of σ and δ character. Small polaron absorption is due to the dipole-allowed optical transition within the $\mathrm{Mn}$ d shell (peak B) from the nonbonding to antibonding σ states. The dipole-allowed charge-transfer transitions (peaks C and D) take place from nonbonding O 2p states to majority and minority spin $\mathrm{Mn}$ e_g states, respectively.

Figure 6

Electric and photovoltaic characteristics of RP PCMO and STNO heterojunctions (a) Current density versus voltage as a function of temperature in the dark. (b) The same under illumination with low-pass filter for hω ≤ 2.0 eV photon energy. (c) Open-circuit voltage U_OC versus temperature for limited spectral illumination ranges. For comparison, data for heterojunctions of the 3D perovskite ${\mathrm{Pr}}_{0.66}{\mathrm{Ca}}_{0.34}{\mathrm{Mn}\mathrm{O}}_3$ on STNO is also included.