Fabrication and Characterization of Fullerene-Based Bulk Heterojunction Solar Cells with Porphyrin , CuInS 2 , Diamond and Exciton-Diffusion Blocking Layer

Fullerene-based bulk heterojunction solar cells were fabricated, and the electronic and optical properties were investigated. C60 were used as n-type semiconductors, and porphyrin, CuInS2 and diamond were used as p-type semiconductors. An effect of exciton-diffusion blocking layer of perylene derivative on the solar cells between active layer and metal layer was also investigated. Optimized structures with the exciton-diffusion blocking layer improved conversion efficiencies. Electronic structures of the molecules were investigated by molecular orbital calculation, and energy levels of the solar cells were discussed. Nanostructures of the solar cells were investigated by transmission electron microscopy, electron diffraction and X-ray diffraction, which indicated formation of mixed nanocrystals.


Introduction
Carbon-based nanostructures such as fullerenes, nanocapsules, onions, nanopolyhedra, cones, cubes, and nanotubes have been widely reported and investigated [1][2][3][4].These carbon (C) nanomaterials with hollow cage structures show different physical properties, and have the potential of studying materials of low dimensionality within an isolated environment.By controlling the size, layer numbers, helicity, compositions, and included clusters, the cluster-included C nanocage structures with band-gap energy of 0-1.7 eV and nonmagnetism are expected to show various electronic, optical, and magnetic properties such as Coulomb blockade, photoluminescence, and superparamagnetism [4,5].
Recently, C 60 -based polymer/fullerene solar cells have been investigated and reported [6][7][8][9].These organic solar cells have a potential for use in lightweight, flexible, inexpensive and large-scale solar cells [10][11][12].However, significant improvements of photovoltaic efficiencies are mandatory for use in future solar power plants.One of the improvements is donor-acceptor (DA) proximity in the devices by using blends of donor-like and acceptor-like molecules or polymers, which are called DA bulk-heterojunction solar cells [13][14][15].
The purpose of the present work is to fabricate and characterize C 60 -based bulk heterojunction solar cells.In the present work, 5,10,15,20-Tetraphenyl-21,23H-porphin zinc (ZnTPP), CuInS 2 (CIS) and diamond was used for p-type semiconductors [9,16], and C 60 with excellent electron affinity was used for n-type semiconductors.Porphyrin has high optical absorption in the visible spectrum and high hole mobility [17][18][19], and was expected to form cocrystallites with C 60 [20,21] that would be suitable for the bulk heterojunction structure [22,23].I-III-VI group compounds, called chalcopyrite, are expected as next generation solar cell materials, and CIS is one of the representative chalcopyrite compounds.Chalcopyrite compounds have advantages of high optical absorption and high resistivity to cosmic rays compared to conventional silicon solar cells [24,25].In addition, they have a band structure of direct transition, which shows high quantum efficiency.
The second purpose is to investigate an effect of exciton-diffusion blocking layer (EBL).3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) is a perylene derivative with a simple structure, which was reported to be used for solar cells [26].In the present work, PTCDA was used as the EBL for ZnTPP:C 60 bulk heterojunction solar cells.EBL prevents hole transfer between active layer and anode, and improvement of conversion efficiency was expected by an introduction of the EBL.Device structures were produced, and efficiencies and optical absorption were investigated.The present work will indicate a guideline for new-type organic-inorganic solar cells using C 60 .

Experimental Procedures
A schematic diagram of the present C 60 -based bulk heterojunction solar cells is shown in Figure 1.A thin layer of polyethylenedioxythiophen doped with polystyrene-sulfonic acid (PEDOT:PSS) (Sigma Aldrich) was spin-coated on pre-cleaned indium tin oxide (ITO) glass plates (Geomatec Co., Ltd., ~10 Ω/□).The PEDOT:PSS has a role as an electron blocking layer for hole transport.Then, semiconductor layers were prepared on a PEDOT layer by spin coating using a mixed solution of C 60 (Material Technologies Research Ltd., 99.98%), ZnTPP (Sigma Aldrich) in 1 mL o-dichlorobenzene (Nacalai Tesque, Inc., 99%).Total weight of ZnTPP:C 60 was 18 mg, and weight ratio of ZnTPP:C 60 was 3:7.
The thickness of the blended device was ~150 nm.To increase efficiencies, PTCDA with a thickness of ~20 nm was also added over the active layers as shown in Figure 1.After annealing at 100 °C for 30 min in N 2 atmosphere, PTCDA (Wako Pure Chemical Industries Ltd.) was evaporated between active layer and metal layer.Finally, aluminum (Al) metal contacts were evaporated as a top electrode, and annealed at 140 °C for 20 min in N 2 atmosphere.
Current density-voltage (J-V) characteristics (Hokuto Denko Corp., HSV-100) of the solar cells were measured both in the dark and under illumination at 100mW/cm 2 by using an AM 1.5 solar simulator (San-ei Electric, XES-301S) in N 2 atmosphere.The solar cells were illuminated through the side of the ITO substrates, and the illuminated area is 0.16 cm 2 .Optical absorption of the solar cells was investigated by means of UV-visible spectroscopy (Hitachi U-4100).Transmission electron microscope (TEM) observation was carried out by a 200 kV TEM (Hitachi H-8100).The microstructures were also investigated by X-ray diffraction (XRD, Philips X' Pert-MPD System).
The isolated molecular structures were optimized by ab-initio molecular orbital calculations using Gaussian 03.Conditions in the present calculation were as follows: calculation type (FOPT), calculation method (RHF) and basis set (6-31G).Electronic structures such as energy gaps between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and electron densities were investigated.

ZnTPP:C 60 bulk heterojunction solar cells
Measured J-V characteristic of ZnTPP:C 60 bulk heterojunction solar cells under illumination is shown in Figure 2. The bulk heterojunction indicates a one layered composite structures with p-and n-type semiconductors, which is denoted as ZnTPP:C 60 .Effects of PTCDA addition to the ZnTPP:C 60 bulk heterojunction solar cells were also investigated, which is denoted as ZnTPP:C 60 /PTCDA.Each structure shows a characteristic curve for open circuit voltage and short circuit current density, and measured parameters of the present solar cells are summarized in Table 1.Power conversion efficiency, fill factor, short-circuit current density and open-circuit voltage are denoted as η, FF, J SC , and V OC , respectively.As shown in Figure 2 and Table 1, current density of ZnTPP:C 60 increased by PTCDA addition, and the best efficiency was obtained for the ZnTPP:C 60 /PTCDA sample.Figure 3 shows the optical absorption of C 60 , ZnTPP, ZnTPP:C 60 and ZnTPP:C 60 /PTCDA bulk heterojunction solar cells.The ZnTPP:C 60 /PTCDA structure provided higher photo-absorption in the range of 300 to 800 nm (which correspond to 4.0 and 1.5 eV, respectively), compared to the ZnTPP:C 60 structure.Exciton migration of C 60 can be efficiently suppressed by use of PTCDA [21], and exciton would be generated for both ZnTPP/C 60 and C 60 /PTCDA interfaces, which results in the increase of conversion efficiency, as listed in Table 1.
X-ray diffraction patterns of ZnTPP and ZnTPP:C 60 bulk heterojunction layers are shown in Figure 4(a) and 4(b), respectively.In Figure 4(a), diffraction peaks corresponding to ZnTPP crystal are observed.After formation of ZnTPP:C 60 bulk heterojunction layer, the diffraction peaks corresponding to ZnTPP disappeared, and C 60 peaks are observed as shown in Figure 4(b).In addition, a new diffraction peak is observed as indicated by an arrow, which would be believed to be porphyrin/C 60 cocrystallites [20,21].Electronic structures of the molecules were calculated, and energy levels of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are shown in Figure 6.HOMO levels in Figure 6(a), electrons are localized around the pyrrole rings of the ZnTPP.Energy levels of LUMO of C 60 and PTCDA are also shown in Figure 6(b,c), respectively.The separated carriers would transfer from ZnTPP to C 60 , and from ZnTPP/C 60 to PTCDA.where e is the elementary charge [29].The value of 0.3 V is an empirical factor, and this is enough for efficient charge separation [30].The present experimental data of V OC indicated smaller compared to the calculated ones from the equation, which might be due to the voltage descent at the metal/semiconductor interface, and control of the energy levels is also important to increase the efficiency.

PTCDA
In the present work, efficiencies of the solar cells were increased by addition of PTCDA layers, which would work as the exciton-diffusion blocking layer for porphyrin:C 60 bulk heterojunction solar cells.The PTCDA layers prevents hole transfer between the porphyrin:C 60 active layer and aluminum, and the conversion efficiencies were improved.
Since the microstructure of ZnTPP and C 60 bulk heterojunction layer is strongly dependent on the weight ratio of these, it is necessary to control the microstructure to form cocrystallites of ZnTPP:C 60 .In the present work, higher efficiencies were obtained for the ZnTPP:C 60 sample with the weight ratio of 3:7, which would be suitable for the cocrystallite formation, as observed for weak reflections in X-ray and electron diffraction patterns.Recombination of electrons of C 60 and holes of ZnTPP would occur in the bulk heterojunction layer with intermittent cocrystallite structure.If continuous cocrystallite structures form perpendicular to the thin films, it is believed that the recombination of electrons and holes could be suppressed, which would lead to improvement of conversion efficiency.

CuInS 2 :C 60 bulk heterojunction solar cells
Measured parameters of a CuInS 2 :C 60 bulk heterojunction structure are summarized in Table 1.A solar cell with CIS:C 60 bulk heterojunction structure provided power convergent efficiency of 8.0 × 10 −4 %, fill factor of 0.28 and open-circuit voltage of 0.18 V.The p-n interfaces, which are photoelectron conversion areas were increased by using blend structures of p-type and n-type semiconductors.
Figure 8 shows a measured optical absorption of the solar cells based on CIS.These solar cells show a wide optical absorption range from 400 to 800 nm, and the heterojunction solar cell show as higher optical absorption range from 350 nm to 550 nm than that of the bulk heterojunction.Since the FTO substrate was set as an incident side, the optical absorption of the CIS layer was high for the heterojunction structure.On the other hand, optical absorption of the bulk heterojunction would be lower compared to that of the heterojunction structure because C 60 were mixed with the CIS layer.Optimization of the nanocomposite structure with CIS and C 60 would increase the efficiencies of the bulk heterojunction solar cell.From the present TEM observation, CIS and C 60 were not mixed in a molecular scale.If the mixture structure of CIS and C 60 is improved to a nanoscale, it is believed that the area of the p-n junction interfaces is increased, and the efficiency would be improved.In addition, it is important to search the most suitable mixture ratio of the p-type and n-type semiconductors for bulk heterojunction solar cells.
An energy level diagram of CIS/C 60 solar cells is summarized as shown in Figure 12.Previously reported values were used for the energy levels of the figures by adjusting them to the present work [15,26,31].When light is incident from the FTO side, excitation by the light absorption happens in the p-n interface, and electrons and holes are produced by charge separation.Carriers would transport from -4.5 eV to -4.3 eV by hopping conduction.Improvement of the present bulk heterojunction solar cells would be possible by the introduction of a buffer layer, change of annealing conditions, and the improvement of the microstructure is also necessary to obtain high efficiency.The evaporation method provided high quality thin films, but a high vacuum and high temperature process are Although CISCuT method is a productive process, it requires a high temperature process [32].On the other hand, the present spin coating method is simpler compared to the other formation methods.In addition, we can apply the spin coating method to plastic substrates without high vacuum and high temperature processes.

Diamond:C 60 bulk heterojunction solar cells
Measured J-V characteristic of diamond:C 60 bulk heterojunction solar cell under illumination showed characteristic curve for open circuit voltage and short circuit current.A solar cell with the diamond:C 60 structure provided power convergent efficiency of 4.3 × 10 −5 %, fill factor of 0.35, short circuit current of 5.3 μA/cm 2 and open circuit voltage of 0.023 V, as listed in Table 1.
Figure 13 shows optical absorption of the diamond:C 60 bulk heterojunction solar cell.The diamond:C 60 bulk heterojunction structure provided photo-absorption in the range of 350 to 500 nm, and shows high absorption at 339, 402 and 506 nm, which correspond to 3.7, 3.1 and 2.5 eV, respectively.Absorption peaks of the C 60 were confirmed within the range from 300 to 400 nm, and an absorption peak of 506 nm corresponds to the diamond.X-ray diffraction pattern of diamond powder showed diffraction peaks of diamond powder, which were confirmed as 111, 220 and 311 of the diamond structure.A grain size of diamond powder was determined to be 12 nm, which was calculated by Scherrer's equation.An increase of photo-absorption under the long wavelength would be due to the nanostructure of nanodiamond particles, which will be discussed later.
An energy level diagram of diamond:C 60 solar cell is summarized as shown in Figure 14.Previously reported values were also used for the energy levels [15].An energy gap of diamond estimated from Figure 13, which corresponds to absorbance of 506nm, is used for the model.From a theoretical calculation [33], nanodiamonds are composed of three layers; a diamond core (sp 3 ), a middle core (sp 2+x ) and a graphitized core (sp 2 ).Therefore, a band gap of the nanodiamond is decreased by the existence of the sp 2+x bonding [34,35].The carrier transport mechanism is considered as follows; when light is incident from the ITO substrate, light absorption excitation occurs at the p-n heterojunction interface, and electrons and holes appear by charge separation.Then, the electrons transport through C 60 toward the Al electrode, and the holes transport through PEDOT:PSS to the ITO substrate.Since it has been reported that Voc is nearly proportional to band gaps of semiconductors [36], control of energy levels is important to increase the efficiency.The low cell performance would be due to the insufficient dispersion of diamond and C 60 in the composite layer, and further control of the nanocrystals is needed.An advantage for the bulk heterojunction structure is increased p-n heterojunction interface.However, due to disarray of the diamond:C 60 microstructure, electrons and holes could not transport smoothly by carrier recombination at the C 60 /Al interface, and at the PEDOT:PSS/diamond interface, respectively.To solve these problems, introduction of a layer preventing carrier recombination and improvement of crystalline structure with few defects are needed.
In the present work, an organic-inorganic hybrid solar cell was fabricated characterized.For the carbon-based solar cells in previous works, thin films are fabricated by a CVD method.In the present work, solar cells with C 60 as an organic semiconductor and diamond as an inorganic semiconductor were fabricated by a spin coating method, which is a low cost method.Boron nitride acts as p-type semiconductor, and diamond has a similar crystal structure as boron nitride.Combination of the present solar cells and nanomaterials such as diamond or boron nitride with various direct band gaps might be effective for increase of efficiencies [37].The performance of the present solar cells would be dependent on the nanoscale structures of the organic-inorganic materials, and control of the structure should be investigated further.

Conclusions
C 60 -based bulk heterojunction solar cells were fabricated and characterized.A device based on ZnTPP:C 60 /PTCDA provided η of 7.8 × 10 −2 %, FF of 0.38, J SC of 0.62 mA/cm 2 and V OC of 0.33 V. Conversion efficiency was increased by introduction of PTCDA layer because the exciton migration of C 60 can be efficiently suppressed by use of PTCDA.A device of bulk heterojunction structure based on CuInS 2 :C 60 and diamond:C 60 were also fabricated and characterized.Photovoltaic behavior including charge transfer and mobility can be described on the basis of the energy diagram of the bulk heterojunction solar cells from the present J-V measurements, optical absorption and structure analysis.Optimization of blended structures with C 60 would increase the efficiencies of solar cells.

Figure 1 .
Figure 1.Structure of C 60 -based bulk heterojunction solar cells with PTCDA layer.

Figure 5
Figure 5 is an electron diffraction pattern of ZnTPP:C 60 bulk heterojunction layer, taken along the [-123] direction of C 60 .A twin structure with the (112) twin plane is observed in Figure 5, as indicated by a dotted line.Diffraction spots which would correspond to cocrystallites of ZnTPP:C 60 are also observed as indicated by arrows.Electronic structures of the molecules were calculated, and energy levels of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are shown in Figure6.HOMO levels in Figure6(a), electrons are localized around the pyrrole rings of the ZnTPP.Energy levels of LUMO of C 60 and PTCDA are also shown in Figure6(b,c), respectively.The separated carriers would transfer from ZnTPP to C 60 , and from ZnTPP/C 60 to PTCDA.

Figure 8 .
Figure 8. Optical absorption spectra of heterojunction and bulk heterojunction solar cells.

Figure 10
Figure 10(a) is a TEM image of CIS, and many CIS particles are observed.Figure 10(b) is an electron diffraction pattern of CIS.Debye-Scherrer rings are observed in the diffraction pattern, which shows crystallite structures of CIS particles.

Figure 11 (
b) is an inverse Fourier transform of (a), and arrows show the interface of CIS and C 60 .Lattice fringes of {101} of CIS and {111} of C 60 were observed.The enlarged image of a part of C 60 in (b) is shown in Fig 11(c).Arrangements of C 60 molecules are observed in the image.CIS and C 60 have size distribution, and the crystal sizes of them observed in the TEM image are larger compared to the averaged sizes.

Figure 11 .
Figure 11.(a) Filtered Fourier transform of HREM image of CIS:C 60 bulk heterojunction layer.(b) Inverse Fourier transform of (a).(c) Enlarged image of a part of C 60 in (b).

Figure 12 .
Figure 12.Energy level diagram of CIS:C 60 solar cell.

Figure 13 .
Figure 13.Optical absorption spectrum of diamond:C 60 bulk heterojunction solar cell.

Figure 14 .
Figure 14.Energy level diagram of diamond:C 60 bulk heterojunction solar cell.

Table 1 .
Measured parameters of the present solar cells.