Contribution of Jahn-Teller and charge transfer excitations to the photovoltaic effect of manganite/titanite heterojunctions

The effect of correlation effects on photovoltaic energy conversion at manganite/titanite heterojunctions is investigated. As a model system we choose a heterostructure consisting of the small polaron absorber Pr0.66Ca0.34MnO3 (PCMO) epitaxially grown on single-crystalline Nb-doped SrTi0.998Nb0.002O3 (STNO) substrates. The high structural and chemical quality of the interfaces is proved by detailed characterization using high-resolution transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) studies. Spectrally resolved and temperature-dependent photovoltaic measurements show pronounced contributions of both the Jahn-Teller (JT) excitations and the charge transfer (CT) transitions to the photovoltaic effect at different photon energies. A linear temperature dependence of the open-circuit voltage for an excitation in the PCMO manganite is only observed below the charge-ordering temperature, indicating that the diffusion length of the photocarrier exceeds the size of the space charge region. The photovoltaic response is compared to that of a heterojunction of lightly doped Pr0.05Ca0.95MnO3 (CMO)/STNO, where the JT transition is absent. Here, significant contributions of the CT transition to the photovoltaic effect set in below the Neel temperature. We conclude that polaronic correlations and ordering effects are essentials for photovoltaic energy conversion in manganites.


Introduction
Manganite perovskites with pronounced interactions between the spin, charge, orbital, and lattice degrees of freedom have attracted considerable interest because these correlations lead to a rich phase diagram [1,2]. Dependent on temperature and carrier doping, various exotic phases emerge, including orbital [3], charge [4], and magnetically ordered ones [5]. The optical excitations of such phases include polaronic and charge transfer excitations [6][7][8][9], as well as cooperative excited states which may even lead to optically induced phase transitions at high photon fluences [10,11]. Rectifying electrical behavior and photovoltaic responses to photon illumination have been widely demonstrated in manganite heterojunctions based on manganite films on n-type SrTiO 3 [12][13][14][15][16]. Such junctions thus offer opportunities to study new strategies for light harvesting, i.e. using polaronic excitations with long lifetimes [17,18] or tuning the junction properties by external electric or magnetic fields [19,20]. However, the understanding of the photovoltaic response of such heterojunctions is still very limited, since neither the involved optical transitions nor the mechanism of charge separation are clearly understood.
We have investigated manganite/titanite junctions, where epitaxial manganite films of two different doping levels (Pr 0.66 Ca 0. 34  Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. a degenerated semiconductor with weak electron-phonon coupling giving rise to a 'metal-like' temperature dependence of resistivity. In contrast, the mobility in the manganite is governed by a thermally activated transport of small polarons [21,22].
Since the band gap of the STNO is E G,STNO = 3.2 eV, spectrally resolved measurements at photon energies below 3.2 eV allow for investigations of the manganite contribution to the photovoltaic effect of manganite/ titanite heterojunctions (see also the spectrally resolved absorptance measurements; figure S1 is available at stacks.iop.org/NJP/19/063046/mmedia in supplemental material section A). In PCMO, the optical transition in the visible range up to 2 eV is governed by a dipole-allowed transition of polaronic nature from occupied lower Mn3d e g states to antibonding σ type O2p-Mn3d e g states that are split due to the Jahn-Teller (JT) effect [6,23]. In the following, we name this transition a JT transition. For photon energies above 2.5 eV, charge transfer (CT) transitions from O2p to minority spin Mn3d t 2g and e g states set in (see figure 1).
PCMO exhibits a first-order charge-ordering (CO) transition at about T CO ≈ 240 K and antiferromagnetic order below the Neel temperature T N ≈ 160 K. The CO transition in PCMO evolves via a two-phase regime of ordered and unordered domains over a broad temperature range [24][25][26], which is influenced by strain, twinning, and preparation-induced point defect disorder. As a consequence, the long-range polaron ordering in the CO phase can be considerably improved by additional post-growth annealing steps [27]. Based on temperature-dependent and spectrally resolved investigations of the photovoltaic parameters, i.e. the opencircuit voltage V OC and the short-circuit current density J SC , we conclude that both transitions contribute to the photovoltaic effect in the PCMO/STNO heterojunction. The results are compared to those of CMO/STNO junctions, where the JT splitting and the related 3d type polaronic excitations are entirely absent. Here, the optical excitations have a pure charge transfer character, presumably slightly modified by weak electronphonon coupling. Furthermore, charge and orbital ordering is absent in CMO, but antiferromagnetic ordering below T N = 120 K is present [28]. In those junctions, the contribution of manganite photoexcitations to the photovoltaic effect is strongly reduced.

Experimental details
PCMO and CMO thin films were prepared by ion beam sputtering of the respective targets in a high-vacuum chamber (p 0 < 10 −6 mbar). During deposition, the partial pressures were fixed to p Ar = 3 · 10 −4 mbar (beam neutralization), p Xe = 1 · 10 −4 mbar (sputtering) and p O2 = 1.4 · 10 −4 mbar (film oxidation). For both types of samples, the deposition temperature was T dep = 750°C and the sputter power on the target was fixed to 4 W cm −2 .
Commercially available STNO substrates from Crystec served as a template for epitaxial growth and as the n-type part of the heterojunctions. Here we compare three different heterojunctions: A CMO/STNO junction and two PCMO/STNO junctions, as-prepared (a.p.) and after a post-growth annealing step in a furnace (p.a.) at 900°C for 20 h in air. Such annealing steps cause partial annihilation of preparation-induced disorder defects, which significantly affect long-range ordering in PCMO. Since the alkaline earth-rich manganites are thermodynamically less stable and tend to chemical phase decomposition, we have not performed additional post-growth annealing steps of the CMO-based junctions.
The crystal quality of the manganite layers was checked by x-ray diffraction (XRD) studies, which confirmed epitaxial growth without misorientations besides the commonly observed (001/220) twinning of PCMO films. Structural properties and the chemical composition of the near-interfacial regions were investigated by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Both CMO/STNO and PCMO/STNO interfaces are almost dislocation free and the degree of intermixing is small (<2.5 nm). They mainly differ in the appearance of anti-phase boundaries (APBs) in the CMO, which extend toward the CMO/ STNO interface and may represent recombination active defects. Further details of these investigations are shown in the supplemental material section B.
The sample geometry for electric characterization is shown in figure 2. The manganite layers with a thickness of about 75-100 nm were deposited at a temperature of T dep = 750°C. In order to avoid pronounced Schottky barriers, a Ti layer (200 nm) with a protective Au layer (100 nm) was previously deposited on the rear side of the STNO substrates. The deposition temperature was T dep = 200°C. Subsequently, Pt top layers (200 nm) were deposited at T dep = 200°C ensuring an ohmic electric contact to the manganite layers. The electric characterization was performed in a cryostat with a Suprasil entry window at temperatures between room temperature and 75 K. In order to measure the current-voltage dependence of the heterojunctions in the dark and under illumination, the top and rear side contacts were connected to a Keithley 2430, which served as voltage source and ammeter. The voltage drop between top and bottom electrodes was measured using a Keithley 2182A Nanovoltmeter with the convention of positive voltages corresponding to a positive top electrode potential.
Two different types of illumination were used: (1) Broad-band illumination (polychromatic) with a LOT (150 W) Xe-UV xenon lamp with Suprasil glass housing, and (2) spectrally resolved illumination with a 300 W Xe lamp equipped with a LOT Omni 300 monochromator The latter allows for an energy resolution of about 20 meV at a wavelength of about 500 nm. For the polychromatic illumination condition, a Laserpoint thermopile sensor was used to adjust the input power to 175 mW. In case of monochromatic illumination, the input power was adjusted to 200 μW using a calibrated Laserpoint Silicon diode. Note that the spectral fluxes of incident beams are comparable in order of magnitude, i.e. of the order of 29 mW eV −1 (polychromatic) and 10 mW eV −1 (monochromatic). In the following, all current densities are normalized to the small-sized top contact area (1 × 4 mm 2 ). The illuminated area beside the contact is about a factor of 2-3 larger than the top contact size.

Current-voltage (J-V) characteristics of manganite/titanite heterojunctions under polychromatic illumination
All junctions reveal rectifying behavior (see also figure S3 in supplemental material section C), i.e. an almost exponential increase of current in the forward direction and a small, only weakly voltage-dependent current in the reverse direction (negative voltages). Polychromatic illumination causes a clear photovoltaic effect (figure 3) appearing as pronounced open-circuit voltages V OC (J = 0) and short-circuit current densities J SC (V = 0). Figure 4(a) shows that the open-circuit voltage of the different junctions increases linearly with decreasing temperature with small deviations from the linear behavior occurring at rather low temperatures (T < 150 K). Interestingly, the slopes dV OC /dT are very similar for all junctions but they differ in the intercept at T = 0. Post- Figure 2. Sketch of the sample geometry for photovoltaic characterization. The (1 × 4 mm 2 ) Pt top contacts and the (4 × 9 mm 2 ) Ti/ Au backside contact were prepared by using shadow masks. The illuminated area next to the top contact has a size of about 3 × 4 mm 2 (Xe-illumination) and 2 × 4 mm 2 (monochromator). All current densities mentioned in the text refer to the top electrode area. Illumination powers were fixed to 175 mW (Xe-illumination) and 200 μW (monochromator), respectively. The voltage polarity was set with respect to the top electrode, i.e. a positive current corresponds to a current from top to bottom.  growth annealing of PCMO/STNO junctions is accompanied by a significant increase of the extrapolation V OC (T → 0) from about 541(4) meV to 694(11) meV. For the CMO/STNO junction, the intercept amounts to 1175(6) meV.
The PCMO/STNO and CMO/STNO junctions exhibit pronounced differences in the temperature dependence of the short-circuit current densities ( figure 4(b)). While CMO/STNO junctions reveal an almost temperature-independent J SC , PCMO/STNO junctions show significantly higher short-circuit currents at high temperature, which steeply decrease with temperatures below 220 K, i.e. approximately below the chargeordering temperature of PCMO films. Again, post-annealing is accompanied by improved photovoltaic properties, i.e. an increase of J SC over the whole temperature range. The slope and intercept of the linear V OC temperature dependence and short-circuit current density at room temperature are summarized in table 1.

Spectrally resolved photovoltaic characterization of manganite/titanite heterojunctions
In order to investigate the photovoltaic contributions of manganite and titanite transitions, spectrally resolved measurements of J-V characteristics were performed. We used different cut-off filters for studies with limited spectral ranges but with fixed illuminating power (figure 5). Reduction of the maximum photon excitation energy establishes the crossover from excitation in the manganite and the titanite to selective excitation of the manganite. This significantly affects the temperature dependence of the open-circuit voltage from linear to nonlinear ( figure 5(a)). However, the extrapolated zero-temperature values of V OC are very similar. V OC (T → 0) is thus rather independent of the spectral excitation range. Figure 5(b) shows the temperature dependence of the short-circuit current density J SC . The current densities are normalized to J SC under polychromatic illumination. Excluding excitations in STNO (E ph <= 2.8 eV) current density yields about 20% of the full-xenon illumination current density at temperatures above 170 K and about 40% at low temperatures (T = 70 K). Successive reduction of the maximum excitation energies to the near-infrared range (NIR) drastically reduces the J SC . For the PCMO/STNO junctions, the crossover temperature from exponential to linear behavior of V OC (T) is at about T CR = 200 K ( figure 6(a)). Post-annealing does not strongly influence E B in the exponential regime, but a broadening of the crossover region is visible in the calculated apparent activation energies ( figure 6(b)). Remarkably, the activation barrier E B and the intercept V OC (T → 0) are nearly the same and are only slightly below the values of V OC (T → 0) for polychromatic illumination.
CMO/STNO junctions under monochromatic illumination (E ph = 2.0 eV) reveal a similar behavior, i.e. a crossover from exponential to linear temperature dependence (figure 6(c)) and an activation barrier E B close to V OC (T → 0) (table 2). However, the crossover temperature T CR = 125 K is lower and close to the Neel temperature of CMO and V OC (T → 0) is much smaller than when under polychromatic illumination.
The spectrally resolved contributions of manganite excitations, i.e. excitations with photon energies below the STNO band gap (E G,STNO = 3.2 eV) are shown in figure 7. In a.p. PCMO/STNO junctions, the open-circuit voltage ( figure 7(a)) and short-circuit current density ( figure 7(b)) are almost constant in the spectral range from about 2.5 to 3 eV. Below 2.5 eV both quantities decrease, but a clear contribution remains from low-energy excitations down to 1.3 eV. The change in shape at about 1.8 eV indicates that two transitions are involved. Postgrowth annealing does not change the overall dependence but significantly enhances V OC , which is accompanied by a pronounced reduction of J SC , presumably due to improved charge-order and the resulting reduced carrier mobility. In the post-annealed junction, the vanishing of J SC below 1.8 eV limits the photovoltaic effect, i.e. V OC is below the resolution limit of our measurement setup. The spectrally resolved behavior of the CMO/STNO junction is similar, i.e. V OC and J SC are almost constant above 2.6 eV, decrease simultaneously below 2.6 eV, and vanish at about 1.8 to 1.9 eV. These results clearly show that excitations in the manganites contribute to the photovoltaic effect.

Discussion
From the viewpoint of Shockley's theory, the J-V characteristic of a single diode p-n homojunction under illumination is given by [30,31]: where n is the ideality factor, k is the Boltzmann constant, J S is the dark saturation current density of the junction, and J SC the short-circuit current density. The open-circuit voltage V OC is the potential required to balance the generation current by the thermally activated recombination current, and equation (1) implies Assuming that charge separation in the p-n heterojunctions is governed by a single dominant contribution to the thermally activated recombination current represented by an activation barrier E B and a sufficiently large short-circuit current density J SC ? J S , equation (2) can be rewritten as where J 0 is the temperature-independent prefactor of an Arrhenius-type temperature dependence of J S , which depends on intrinsic material properties. In the case of homojunctions, E B is determined by the band gap; meanwhile, in heterojunctions, E B can be limited by the band offsets [29,32]. V OC should linearly scale with temperature and the logarithm of the short-circuit current density, and the relevant activation barrier can be deduced from the intercept E B = V OC (T → 0). Equation (3) is valid only if the diffusion length of the excess carriers is larger than the extension of the space charge region, thus reflecting a charge separation mechanism, where the photocurrent of minority charge carriers, i.e. the excess carrier diffusion from the bulk of the thin film absorber, is not the limiting factor. Under polychromatic illumination, the linear temperature dependence of V OC in the entire temperature range and the similar values of the slopes in PCMO/STNO and CMO/STNO junctions ( figure 4(a)) point to a dominant contribution to the photovoltaic effect, which originates from excess carriers generated in the STNO. This is in accordance with recently published low-energy scanning transmission electron beam-induced current observations for PCMO/STNO junctions [33]. In addition, a strong increase of the short-circuit current due to STNO excitations was observed in [20]. The activation barrier E B of PCMO/STNO junctions is increased for post-annealed samples from E B ≈ 540 meV to E B ≈ 695 meV. Post-growth annealing may affect E B due to small cation intermixing at the interface [16] by charged defects like oxygen vacancies at the PCMO/STNO interface [19], or by a shift of the valence band in PCMO, e.g. change in Coulomb repulsion [23]. The latter may be induced by a vacancy-induced change of doping.
Under illumination conditions, where excitations in the STNO are excluded, the temperature dependence of V OC of PCMO/STNO junctions reveals three regimes (see figure 6(c)). These regimes are also visible in the relation between V OC and ln(J SC ) ( figure 8(a)), as deduced from the data in figure 5 (variation of the illumination spectrum at different temperatures): (i) The low-temperature regime, which reveals non-linear relations between V OC and ln(J SC ) as well as T < 140 K, resembling a saturation behavior. In PCMO, charge mobility is controlled by thermally activated hopping processes of small polarons, which give rise to a high series resistance contribution at low temperatures. This seems to be the main origin of the strong decrease of the short-circuit current of  4(b)). Low mobility of localized charge carriers is also expected to cause saturation or reduction of V OC at low temperatures, e.g. in organic solar cells [34].
(ii) The medium-temperature regime, where T CO > T > 140 K. The photovoltaic effect is governed by excess carrier diffusion to the interface. The relation between V OC and T as well as ln(J SC ) is linear, which points to In the high-temperature regime, the photocurrent from the PCMO to the STNO seems to be the limiting factor because the saturation current J S is not much smaller than the short-circuit current J SC . Assuming J S ? J SC , equation (2) leads to an exponential temperature dependence of V OC . For such low photocurrents, mainly photoexcitations at the interface might lead to the excess carrier generation, and thus the charge separation rate might be influenced by interfacial dipoles. We suggest that a V OC ∝ exp(E B /kT) characteristic emerges, because the charge separation is governed by the competition between the photocurrent due to the photo-induced interface dipole forward transition and the thermally activated recombination current. An optical transition of the interface dipole is a dipole-allowed transition due to the different parity of the involved manganite e g -O2p and titanite t 2g states. The activation energy in this scenario is still E B . The spectral dependence of the photovoltaic properties in figure 7 suggests that two photovoltaic relevant excitations in PCMO contribute to the V OC , i.e. the JT transition from Mn3d e g to O2p-e g states and the CT transition between O2p states and t 2g states (figure 1). In figure 8  similar to the slope of the CT transition in PCMO. However, the short-circuit current densities in CMO/STNO junctions are much lower. In contrast to PCMO based junctions, the short-circuit current seems not to be limited by serial resistance contributions at low temperatures, resulting in a temperature-independent shortcircuit current density ( figure 4(b)). The reduction is most probably due to enhanced electron-hole recombination at APBs (see supplement section B).
The minimum excitation energy of about 1.3 eV, which gives rise to a photovoltaic effect in PCMO (figure 7), is larger than the band gap of PCMO in the CO phase (E G,PCMO = 0.37 eV) [6]. Transitions between Mn3d e g states are commonly dipole forbidden. Taking into account the hybridiziation of O2p states in a molecular orbital picture, the minimum excitation energies can be attributed to the dipole-allowed transition from non-bonding Mn3d e g to antibonding σ type O2p-e g states that set in at the absorption maximum of the polaron transition ferromagnetically coupled JT polarons at 1 .6 eV [35] (see also figure S1 supplement section A). In CMO, the JT splitting is absent and we therefore expect that the only photovoltaic relevant excitation in CMO is the CT transition between O2p states and e g states, i.e. excitations above the CMO band gap (see supplement section A).
The most remarkable difference between transitions in the STNO and the manganites is the crossover to a Shockley-like bulk generation of photocarriers in the manganite in the medium-temperature regime. For PCMO/STNO junctions, the role of charge-ordering is evident in the temperature dependence of V OC and J SC (figures 5, 6(c)). Photoexcitations in PCMO give rise to an exponentially small open-circuit voltage at high temperatures and a linear increase at low temperatures. The key to explain this behavior is the recently observed long lifetime of polaronic excitations [34]. Combining optical pump-probe experiments and first principle calculations, the authors conclude that cooperative behavior of polarons in highly doped manganites, e.g. the appearance of short-or long-range charge-order, is an essential prerequisite for long-living excited polaron states in PCMO. This is consistent with our analysis that below T CO , excess carrier diffusion from outside the space charge region to the interface gives rise to the photovoltaic effect. Post-growth annealing facilitates the formation of the long-range charge-ordered phase, which in the framework of our model is reflected in an increase of V OC ( figure 7(a)). Indeed, in the charge-ordered state this seems to enable not only long lifetimes of photo-excited polarons in the JT transition (energies between 1.3 and 2 eV) but also for transitions into the minority spin t 2g states, which are populated above 2.4 eV as indicated by the increase of the photocurrent for E ph < 1.9 to E ph < 2.8 eV in figure 7(b). The charge transfer transition in CMO also gives rise to a crossover from exponential to linear temperature dependence of V OC (figure 6), which takes place near the Neel temperature T N = 120 K of anti-ferromagnetic ordering. This might indicate that lifetimes of charge transfer type excitations depend on the magnetic long-range order in CMO.

Summary
Spectrally resolved and temperature-dependent analysis of photovoltaic properties provide insights into the contribution of three optical transitions to photovoltaic energy conversion in manganite/titanite junctions. Under polychromatic illumination including optical excitations in the titanite, the photovoltaic effect is mainly governed by generation of excess carriers in the STNO. If the photocurrent is dominated by the generation of electron-hole pairs in STNO, the Shockley model is applicable in the entire temperature range. The similarity of PCMO/STNO and CMO/STNO junctions is mainly visible in the agreement of the slope of V OC (T), which reflects the density of states and lifetime of the STNO excitations.
In contrast, if the photocurrent is generated by photoexcitations in the manganites, a more sophisticated model of the underlying photovoltaic mechanisms is required. Here V OC (T → 0) does not depend strongly on the involved manganite transitions, in contrast to the short-circuit current density J SC . This indicates that the barrier for charge separation and photovoltage generation does not depend on the type of transition, and both the JT and CT transition contribute to the photovoltaic effect in PCMO/STNO junctions. Three different regimes are observed: (i) an interface-dominated transition above T CO , (ii) a Shockley-like behavior below T CO , and (iii) a freeze-in regime, where the small polaron mobility at T < 140 K limits charge separation. JT and CT transitions lead to distinguishable diode properties in a Shockley-type analysis of the V OC -ln(J SC ) relation under monochromatic illumination.
The onset of a linear V OC (T) dependence below T CO in PCMO and T N in CMO suggests that sufficiently long lifetimes of photocarriers for diffusion from outside the space charge region to the interface are only present for long-range ordering of charge and spins in the manganites. However, the effect of charge-ordering on the photovoltaic performance is counteracting the ability to generate large photocurrents. In CMO/STNO junctions, due to the absence of CO, the photocurrents should be much higher but are most probably limited by the recombination at extended defects.