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Novel Photovoltaic Phenomenon in Manganite/ZnO Heterostructure

In this paper, we report a novel photovoltaic phenomenon in a low cost manganite/ZnO p-n heterojunction grown on ITO glass substrate by pulsed laser depositon (PLD) under relative low growth temperature. The heterostructure ITO/La0.62Ca0.29K0.09MnO3(LCKMO)/ZnO/Al exhibits reproducible rectifying characteristics and consists of four parts: aluminum top electrode, ITO bottom electrode, manganite and ZnO semiconductor films. Moreover, the light current can generate under continuous laser ({\lambda}=325nm) irradiation. In this article, we investigate the influence of manganite and ZnO film thickness on the electrical and photoelectric characteristics of the heterostructure at room temperature. The maximum power conversion efficiency (PCE) is achieved not only when the LCKMO and ZnO layer is thin enough, but also when the full space charge layer is sufficient. We obtain the maximum value(0.0145%) of PCE when the thickness of LCKMO and ZnO layer is 25nm and 150nm, respectively. Under this condition, the open circuit voltage is 0.04V, which can be attributed to internal photoemission.


Introduction
In the past decade, perovskite manganites, especially hole doped manganites, have attracted intense attention [1,2] of the academic community by displaying fascinating physical behaviors, including intrinsic colossal magnetoresistance (CMR) and metal-insulator (M-I) transition.
Many excellent rectifying behaviors have been observed in manganites and n-type semiconductors based heterostructures [3][4][5] . In comparison with ordinary semiconductor heterojunction, the manganite based heterojunctions have shown numerous fascinating characteristics such as picosecond photoelectric effect [6] , high magnetic sensitivity [7] and positive magnetoresistance [8] .
Sheng et al observed change in photovoltage due to external magnetic field in manganite-based heterojunction [9] . Influence of film thickness on the physical properties of manganite-based heterojunctions was also studied [10] . Nevertheless, the earliest works were based on single-crystal substrate, such as Nb: SrTiO 3 . It was also observed that the high growth temperature condition was always imperative for the epitaxial growth of La 0.7 Ce 0.3 MnO 3 /Nb:SrTiO 3 [11] , La 0.7 Ca 0.3 MnO 3 /Nb:SrTiO 3 [12] and La 0.7 Sr 0.3 MnO 3 /Nb:SrTiO 3 [13] .
ZnO is a traditional n-type semiconductor with potential application value in the next generation UV light emitting diode and laser diode. Although a large structural disorder exhibits between hexagonal ZnO and orthorhombic manganite layers, the excellent rectifying behaviors [14] and photovoltaic characteristics [15] of manganite/ZnO heterostructure still persist. There is a thin depletion layer at the junction interface due to high carrier concentrations of manganites and ZnO layers. The fabrication of ZnO nanowire/La 0.65 Sr 0.35 MnO 3 heterojunction and characterization of its rectifying behavior were reported [16] . Feng et al reported the effects of ZnO film thickness on electrical and magnetoresistance characteristics of La 0.8 Sr 0.2 MnO 3 /ZnO heterostructures [17] .
Herein, in the La 0.62 Ca 0.29 K 0.09 MnO 3 (LCKMO)/ZnO heterostructure on ITO glass substrate, we observe excellent rectifying I-V curve and photoelectric behavior when it is irradiated by continuous laser. We also investigate the influence of film thickness on the electrical and photoelectric characteristics of the junctions.

Experimental
The photovoltaic device has a structure of ITO/LCKMO/ZnO/Al. The ITO-conducting glass substrate (a sheet resistance of 15 Ω/□) was cleaned separately using acetone, ethanol, and de-ionized water for 15 min before deposition.
The LCKMO target was obtained by conventional ceramic sintering process. The single crystals of La 0.62 Ca 0.29 K 0.09 MnO3(LCKMO) were first synthesized by hydrothermal method and then were grounded, pressed, and sintered at 1200℃ in air for 4h to formulate a stoichiometric target. The pure ZnO target was commercially purchased from KJMT.
The LCKMO/ZnO heterostructure was deposited onto the ITO glass substrates by pulsed laser deposition (PLD) with a KrF excimer laser beam (Coherent, λ=248nm) having a fluence of 260mJ. Prior to the deposition, the chamber was vacuumed to a base pressure of 3×10 -6 Torr.
During deposition, the substrate temperature was kept at 300℃ and the distance from substrate to target was ~75mm for both LCKMO and ZnO layer deposition. The optimized oxygen partial pressure of 200 and 150 mTorr was used respectively and the film thickness of LCKMO and ZnO layers was precisely controlled by the number of laser pulses. After deposition, the substrates with films were annealed at the temperature of 300℃ for 0.5h and then naturally cooled to room temperature in oxygen pressure of 400 Torr. Subsequently, the thin films were covered with a homemade mask, then the aluminum (Al) pads with an area of 1mm 2 which got Ohmic contact were thermal evaporated on ZnO films as top electrodes.
The polycrystallinity of LCKMO and ZnO films was investigated by an X-ray diffractometer    [18,19] .  circuit voltage(0.04V) which can be attributed to internal photoemission [12] . An schematic energy diagram is shown in the inset of Figure 5.

Results and discussion
We also investigate the influence of film thickness of the junctions which have structures of ITO/LCKMO/ZnO/Al. As well known, the film thickness can affect the electric properties by influencing depletion width and interfacial barrier. For the photoelectric process, it may affect the diffusion and number of the nonequilibrium charge carriers. Eventually it will have an effect on the power conversion efficiency of the heterostructures [10] . The thickness of ZnO(n type) films is firstly fixed as 550nm and we change the thickness of LCKMO layers. Figure 4 Figure   4(a). When d=100nm, V oc is 0.04V and PCE is 0.0014%. The thickness of the LCKMO layer is sufficient to obtain full space charge layer. When d=50nm, V oc is 0.04V and PCE is 0.0047%. PCE is larger than that of 120nm for smaller resistance caused by thinner films. When d=25nm, V oc is 0.04V and PCE is 0.0097% as maximum value. When d=15nm, V oc is 0.02V and PCE is 0.0069%, the reduction of V oc indicates LCKMO layer is fully depleted and the thickness of space charge layer decreases. When d=10nm, V oc is 0.01V and PCE is 0.0025%. The thickness of space charge layer further decreases. When d=7.5nm, V oc and PCE are both 0. Because surface roughness of commercial ITO glass substrate is approximately 6nm(measured by atomic force microscope, not shown here)，the layer is so thin that we cannot prevent the contact of ZnO and bottom electrode.
Another possible reason may be tunneling transport of the charge carriers. The maximum of PCE is achieved when not only LCKMO layer is thin enough but also when the full space charge layer is confirmed.
Then we change the film thickness of ZnO. The thickness of LCKMO films is fixed as 25nm.
When d is 550nm, V oc is 0.04V and PCE is 0.0097%. When d decreases to 250nm, V oc is 0.04V and PCE increases up to 0.0099%. When d is 150nm, V oc is 0.04V and PCE is 0.0145% as maximum value. When d is 50nm, Voc and PCE are both 0. It can be observed that the influence of ZnO films thickness has the same feature as that of LCKMO. As well, we obtain the maximum value (0.0145%) of PCE when the thickness of LCKMO and ZnO is 25nm and 150nm respectively.
The current study of heterojunctions can be attributed to one of the Schottky (metal-semiconductor) barrier type, having the form ,where I 0 is the saturation current and ŋ is the ideality factor. Fitting results show I 0 of 7.2×10 -5 A and ŋ of 18 [ Figure 5]. As per results of the previous discussion, we surmise that the suitable transport mechanism is the thermionic emission model which is valid for the Schottky barriers, when the Further studies on the enhancement of power conversion efficiency and the modification of rectifying behavior are underway.

Conclusion
In