Fully crystalline perovskite-perylene hybrid photovoltaic cell capable of 1 . 2 V output with a minimized voltage lossa

A fully crystalline heterojunction of organo-metal-halide perovskite, CH3NH3PbI3−xClx (X < 0.24), and perylene constitutes a planar photovoltaic cell that yields a photovoltage exceeding 1.2 V with a single junction cell absorbing up to 800 nm. Here, perylene not only works as a hole conductor but also contributes to photovoltage as a photoconductor. The crystalline plane orientation of perovskite prepared on TiO2 was controlled by thermal annealing such that the lead halide (110) plane that participates in carrier conduction is highly oriented to enhance the photovoltaic performance. The crystal orientation improves the heterojunction structure with perylene. For the best cell with high crystalline orientation, a total voltage loss is significantly minimized to 0.32 V with respect to the absorption band gap of 1.55 eV. The planar crystal cells generate high open-circuit voltages of 1.15–1.22 V, which is close to a theoretical maximal voltage (1.25–1.3 V) described by the Shockley-Queisser efficiency limit. The cell yielded energy conversion efficiency up to 4.96%. © 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4895039]

is particularly of our interest because of its small voltage loss, 0.45-0.55V.It is considered that a work function gradient of around 0.2 eV is normally required for rectified carrier transfers across a single heterojunction.Therefore, two heterojunction interfaces that sandwich perovskite layer may involve a thermal loss of energy up to 0.4 eV.In this respect, small voltage loss in the perovskite cell is a rare advantage ever found in inorganic and organic photovoltaic cells.Here, the V oc is strongly affected by hole transport material (HTM) in contact with perovskite.For example, the most widely used organic HTM spiroMeOTAD 16 (2,2 ,7,7 -tetrakis (N,N-dimethoxyphenylamine)-9,9 -spirobifluorene) (HOMO, 5.22 eV 17 ) shows a voltage loss of around 0.26 eV at the junction with perovskite, CH 3 NH 3 PbI 3 (valence band, 5.48 eV).
The metal oxide underlayer, such as TiO 2 and Al 2 O 3 , either as compact planar or mesoporous layer plays a significant role as a crystal type scaffold to assist the chemical synthesis and growth of oriented perovskite crystals.Relatively high V oc (1.1 V) has been obtained by formation of Cl-doped CH 3 NH 3 PbI 3 on an Al 2 O 3 scaffold. 3This indicates that quality and characteristics of the junction of perovskite with the scaffold and HTM is critical for the overall performance.The choice of HTM may also depend on junction characteristics.For instance, a crystalline/amorphous junction of perovskite and amorphous organic HTMs, such as spiroMeOTAD and conductive polymers, [18][19][20][21] requires doping with charged molecules or ions in order to achieve sufficient hole conductivity.However, replacement of amorphous HTMs with well-oriented crystals of organic or inorganic conductors can lead to formation of an ordered crystalline heterojunction interface.We expect that this fundamental change to minimize the loss in excitation energy results in higher photovoltage.
In the present study, a fully crystalline hybrid heterojunction between a planar film of organolead-halide perovskite and oriented crystalline film of perylene 22,23 was prepared.We employed Cl-doped perovskite, CH 3 NH 3 PbI 3−x Cl x , as the light absorber, which has been studied for its unique carrier conduction properties. 24,25 ong diffusion lengths of photo-electron and hole of CH 3 NH 3 PbI 3−x Cl x 10 are considered to contribute to high voltage performance.A crystalline film of CH 3 NH 3 PbI 3−x Cl x was prepared by spin-coating a 40 wt.% precursor solution of CH 3 NH 3 I and PbCl 2 (at a 3:1 molar ratio) in N,N-dimethylformamide 3 on the surface of a transparent conducting oxide (TCO) substrate that had been passivated with a thin TiO 2 compact layer (TiO 2 -CL).The TCO used here was composed of an indium and antimony tin oxide (ITO-ATO) composite on a glass substrate, which has relatively a flat surface.The TiO 2 compact layer was made by dip coating treatment of the ITO-ATO substrates with 50 mM aqueous TiCl 4 solution (for materials and experimental details, see the supplementary material). 32X-ray fluorescence analysis of the perovskite film showed that Cl occupies only 8% of the total amount of halogens (Cl:I = 6:70, see Table S1 in the supplementary material) 32 which are a slightly excess of stoichiometric ratio (3.0) with respect to lead.XRD reveals existence of a negligible amount of CH 3 NH 3 PbCl 3 contaminant perovskite (see Figure S1 in the supplementary material). 32The bulk composition of the perovskite layer is given as CH 3 NH 3 PbI 2.81 Cl 0.24 .In other way, we can say that fraction of Cl occupying the perovskite crystal cannot be more than 0.24.For hole conduction, a thin crystalline film of perylene was coated on the CH 3 NH 3 PbI 3-x Cl x perovskite surface by an atmosphere-controlled vapor deposition method, as reported previously. 22Au counter electrode (back contact) was deposited on top of the perylene film by thermal vapour deposition method.Figure 1 shows a schematic diagram of the perovskite-perylene photovoltaic cell and a scanning electron microscope (SEM) image of cross-sectional view of the cell.The thicknesses of the perovskite layer and the perylene layer are approximately 400 and 80 nm, respectively.
We controlled the crystallization process of perovskite film by changing annealing conditions after initial deposition of the crystal.To determine crystallinity and orientation of the crystalline places in the film, the two-dimensional X-ray diffraction (2D-XRD) was measured.The method allows fast collection of the X-ray scattering data and direct imaging of the crystal orientation 26,27 (see XRD measurements in the supplementary material). 32Figure 2(a) shows 2D X-ray diffraction patterns of perovskite film formed at different annealing conditions.The two main diffraction peaks at 14.1 • and 28.4 • were assigned to (110) and (220), respectively, which are consistent with Pb-I plane (see Figures S1 and S2 in the supplementary material). 32In the film that was formed on the TiO 2 compact layer by heating up to 100 • C from 25 • C gradually for 45 min (film A), the (110) and (220) reflections exhibited a noticeable spread along the Debye-Scherrer ring (Figure 2(a), left).The  and Pb in the plane of CH 3 NH 3 -Pb-X (see Figure S2 in the supplementary material). 32These results supported that the (200) crystal planes are vertically aligned in the film B with tilted alignment of the (110) plane.So, highly orientated crystalline perovskite film can be obtained by controlling the annealing process.
An 80 nm thick perylene was deposited on both of films A and B by vapour deposition method.Here, the substrate temperature and deposition rate were changed in the ranges from 50 • C to 100 • C and 0.1 Å/s to 0.5 Å/s, respectively, in an atmosphere of 2.0 × 10 −5 Pa.2D XRD patterns of perylene formed on the perovskite films A and B are shown in Figures 3(a) and 3(b), respectively.As reported previously, 23 the XRD pattern of perylene crystal typically shows a monoclinic structure with lattice parameters of a = 11.267(4),b = 10.835(3),c = 10.19 (3) and α = 90.00,β = 100.81(7), γ = 90.00.On the highly oriented perovskite film (film B), the diffraction patterns of perylene film were more enhanced than that on the weakly oriented film (film A).This indicates that highly crystalline perylene was formed on the highly orientated perovskite film, as exhibited in XRD patterns of Figure 3. Additionally, the 2D XRD diffraction pattern of the perylene film 23 corresponded to the single crystal β-phase, 28 in which 2D intermolecular π -π interactions are formed along the (001) direction that corresponds to the carrier conductive direction. 22Here, the orientated (110) plane of perovskite film seems to be assisting nucleation and crystal growth of perylene, leading to improved hole conductivity. 29hotocurrent density-voltage (J-V) characteristics for the TCO/TiO 2 -CL/CH 3 NH 3 PbI 3−x Cl x / perylene/Au photovoltaic cells were measured under AM 1.5 simulated sunlight.The active area of the cell receiving incident light was 2 mm × 2 mm by setting a black square mask.Voltage was scanned at a rate of 100 mV/s in terms of retention time of stepwise scanning (for detailed conditions, see photovoltaic measurements in the supplementary material). 32The effect of light intensity was studied by varying incident intensity between high (1.5 sun), standard (1 sun = 100 mW/cm 2 ), and low (0.25 sun) conditions.In order to examine the effect of light absorption by perylene on photovoltage, irradiation was done with a cut-off filter that removed the short wavelength light (<450 nm) where perylene absorbs.The results of J-V curves for perovskite films with high and low orientation were compared with that of a standard cell made with a mesoporous structure of TiO 2 (Figure 4(a)).Here, mesoporous TiO 2 film was annealed at 500 • C before coating the perovskite precursor solution and a perovskite film was formed by heating up to 100 • C from 25 • C gradually for 45 min.This heating process gave the best cell performance for the mesoporous TiO 2 sample.The J-V characteristics of these cells are summarized in Table I   A thin layer of highly crystalline perylene has a relatively deep HOMO level (−5.40 eV, see Figure S3 in the supplementary material), 32 compared to non-crystalline conductors such as spiroMeOTAD (−5.22 eV).This HOMO level is situated near the valence band of the perovskite (−5.48 eV, Figure S3 in the supplementary material). 32An energy gap of around 1.47 eV between the perylene HOMO level and conduction band (CB) of CH 3 NH 3 PbI 3−x Cl x (−3.93 eV) becomes a theoretical limit for photovoltage generation at the perovskite/perylene heterojunction, although it is also considered that the absolute levels in vacuum of HOMO and CB are not precisely applicable for the interfaces of actual cell.In our experiments, the photovoltaic cell with oriented crystalline junction of perovskite and perylene could generate a V oc of 1.15 V in the 1 sun incident condition.Further, with irradiance of 1.5 sun, the cell showed V oc as high as 1.22 V. Here, the resultant voltage loss of 0.25-0.32V from the perovskite/perylene gap of 1.47 eV is assumed to include a potential gap, 0.17 eV, which belongs to the interface between perovskite (CB, −3.93 eV) and TiO 2 (CB, −4.10 eV).In conclusion, the practical voltage loss due to the resistance of carrier conduction (IR loss) at the heterojunction interfaces as well as inside of the perovskite layer can be slightly more or less than 0.1 eV, which is remarkably a very small value for Gibbs energy loss.Due to thermodynamic constraints, the maximum V oc that a heterojunction-type solar cell can generate is 250-300 mV lower than the band gap of the absorber material, as described by the Shockley-Queisser treatment. 30For the perovskite material used here, the band gap is around 1.55 eV, meaning that the theoretical maximum voltage is between 1.25 and 1.3 V. V oc reaching 1.22 V, to our knowledge, is the highest voltage ever realized for the photovoltaic cell absorbing the entire visible light up to 800 nm.Total voltage loss with respect to the band gap energy is only 0.33 V, which is close to the Shockley-Queisser limit.Here, we also consider the contribution of the good conductivity (long diffusion lengths and lifetimes of carriers) of Cl-doped perovskite 10,24,25 to realize high voltage in combination with perylene.The above result is compared with our recent work of hybrid cells using metallocene complex dye absorbers which also could generate V oc up to 1.2 V. 23 However, the metallocene dye-based cell has a broad band of absorption (400-750 nm) peaking at around 480 nm.Strong absorption only occurs at wavelengths less than 700 nm (1.77 eV), which would have resulted in a voltage loss of more than 0.5 V with respect to absorption energy.Therefore, minimal voltage loss realized by the perovskite/perylene cells could be attributed to the oriented crystalline structure through the heterojunction of absorber and HTM.
Moreover, such orientation of crystal structures not only improved photovoltage but also increased the photocurrent.The cell with the oriented junction of perovskite and perylene crystal showed much higher photocurrent density than those of less oriented samples, as shown in Figure 4(a).The photocurrent enhancement is assumed to be due to reduction in internal resistance, as is reflected by an increase in fill factor (FF), which is possibly caused by highly oriented junction structure.A reference non-planar cell with a mesoporous TiO 2 underlayer yields a photocurrent density comparable with the highly orientated planar cell but apparently produces lower V oc , indicating that the non-planar structure causes a large loss in voltage.The corresponding incident photon-to-current conversion efficiency (IPCE) data are given in Figure 4(b).The action spectra cover a wavelength range extended to 800 nm and more, following the optical absorption of the CH 3 NH 3 PbI 3−x Cl x (see Figure S4 in the supplementary material). 32There is an additional peak at 400 nm, which can be assigned to the photoresponse intrinsic to the perylene crystal.It is therefore assumed that the perylene also contributes, in the short-wavelength region, to photovoltage generation as a p-type photoconductor. 31This was confirmed by the J-V measurement taken under illumination with wavelengths cut off below 450 nm, where perylene absorbs.In absence of perylene photo-excitation, V oc decreased from 1.15 V to 0.85 V, accompanied by a significant drop in FF and photocurrent (see Table I and Figure S5 in the supplementary material). 32With this evidence, we know that the high V oc and minimal energy loss realized by crystalline junction of perovskite and perylene is assisted by photoconductivity of perylene which participates in photovoltage generation.It is assumed that the mechanism and photovoltaic energetics in these perovskite-perylene full-crystalline cells can essentially differ from those of the most common cells with perovskite and amorphous organic HTMs.
Additionally, photoconductivity of perylene was observed to be affected by the underlying structures of perovskite.The V oc decreased to 1.0 V when a TiO 2 mesoporous film was used in the structure (Figure 4(a)).In Figure 4(b), the IPCE peak around 400 nm due to perylene is enhanced for the planar heterojunction cell than the non-planar cell with TiO 2 mesoporous layer.The enhanced perylene photoconductance is thought to contribute to maximization of the photovoltage.This effect was reproducibly proved by the planar structure cells.In contrast, use of mesoporous structures caused difficulty in constructing a flat surface of perovskite, which is required to control the orientation of perylene.And hence, mesoporous structure generated lesser V oc (1.0 V).
The planar and fully crystalline heterojunction cell (film B) yields 3.96% and 3.73% power conversion efficiency at incident power of 25 mW/cm 2 and 100 mW/cm 2 , respectively.The V oc ranged from 1.10 to 1.15 V (see Table I and Figure S6 in the supplementary material). 32The same cell achieved higher V oc of 1.22 V and 4.15% efficiency under incident power of 150 mW/cm 2 .Shortcircuit photocurrent density (J sc ) was linearly dependent on incident light intensity for the range of 0.25-1.5 sun.The V oc improves with increase in light intensity.The latter may reflect a relatively high internal resistance of the cell, as is indicated by low J sc s and FFs of the J-V curves.In our cells, the perylene layer affects the internal resistance and may be too thick (80 nm or more) to generate high photocurrent density.At 100 mW/cm 2 , the best cell fabricated with optimized thickness and uniformity of perylene yielded 4.96% efficiency with J sc 8.23 mA/cm 2 , V oc 1.11V, and FF 0.543 (see Figure S7 in the supplementary material for J-V curve). 32To minimize the thickness and resistance of crystalline perylene films, it is necessary to ensure the surface flatness of the perovskite layer to the level of 5-10 nm precision, which has been difficult by the present spin coating method.
In conclusions, we have demonstrated a fully crystalline hybrid photovoltaic cell based on organometal halide perovskites as light absorbers and crystalline perylene as hole conductors that is capable of yielding high open circuit voltage.Orientation of the perovskite crystal planes was controlled by varying post annealing treatments so as to orient the carrier conducting (110) plane along the film thickness.Oriented perovskite film improves the carrier transport properties of the heterojunction interface between perovskite and perylene and enhances hole conductivity in perylene, resulting in high voltage and photocurrent.The best cell with highly orientated junctions was found to be capable of generating V oc up to 1.22 V with conversion efficiency of 4.15%.A higher efficiency was obtained at 4.96% with V oc of 1.11 V. Here, total voltage loss that we could minimize with respect to the band gap (1.55 eV) is 0.33 V. Based on the Shockley-Queisser limit of V oc with respect to band gap, 30 the perovskite cell designed here works with V oc close to the theoretical maximum (1.25-1.30V) and a minimal loss of Gibbs energy.The minimized voltage loss indicates that the internal IR drop (thermodynamic loss) at the TiO 2 /perovskite/perylene junctions was significantly suppressed.As for potential performance of the cell, improvement of FF to 0.75 and photocurrent density to more than 20 mA/cm 2 can lead to conversion efficiency as high as 20% for a single cell capable of V oc 1.2 V and visible light harvesting up to 800 nm.This research is supported by the Japan Society for the Promotion of Science (JSPS) through the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST) Program and Japan Science and Technology Agency (JST) Advanced Low Carbon Technology R&D program (ALCA).We acknowledge Dr. Hitoshi Morioka and Mr. Manabu Mizuhira (Bruker AXS) for help with the X-ray fluorescence analysis and XRD measurements.
A.I. designed the experiments, fabricated and characterized the devices.T.M. planned the study and supervised the work.A.K.J participated in the discussion of the comparison of I-V performance in Table I.All authors discussed the results and contributed to the final version of the paper.
Table S1.The molar ratio obtained from X-ray fluorescence analysis of the mixed halide perovskite CH3NH3PbI3-xClx.

FIG. 1 .
FIG. 1.A schematic diagram of a perovskite-perylene photovoltaic cell with molecular components and the scanning electron microscope (SEM) cross-sectional image of the cell.Configuration: TCO/TiO 2 -CL/CH 3 NH 3 PbI 3−x Cl x /perylene/Au.Scale bar represents 500 nm.

χ
FIG. 3. 2D X-ray diffraction patterns of the perylene films.(a) Perylene on the perovskite film A and (b) perylene on the perovskite film B.
FIG. 4. J-V and IPCE spectra.(a) Photocurrent density-voltage characteristics for the TiO 2 -CL/CH 3 NH 3 PbI 3−x Cl x /perylene photovoltaic cell with lower (blue line) and higher (red line) film orientation, under AM 1.5 sunlight (100 mW/cm 2 ).A dotted black line shows the photovoltaic cell with mesoporous structure of TiO 2 .(b) The corresponding incident photon-to-electron conversion spectra.

Figure S1 .
Figure S1.Out-of-plane XRD patterns of the film A (week orientation, in blue line) and film B (high orientation, in red line), together with assignments of the Bragg reflections.

TABLE I .
J-V characteristics of perovskite-perylene photovoltaic cells with different film crystallinity, together with a meso-structured cell as a reference.