P/n-heterojunction thin-layer photovoltaic devices

Multi-layer vapour deposited solar cells

In 1986 the breakthrough in the field of p/n type organic solar cells occurred with the preparation of a two-layer solar cell based on phthalocyanine and perylenebisimide by Tang and co-workers (Tang cell)¹⁴. A glass plate coated with indium-tin oxide (ITO) served as transparent substrate on which subsequently thin films of hole transporting and electron transporting material were applied in high vacuum vapour deposition processes.

A 30 nm layer of Cu-phthalocyanine (CuPc) served as p-type material and hence as hole conductor whereas a perylene bisimidazole derivative (50 nm) acted as electron transporting component. Ag-electrodes were vapour deposited on top completing the device.

12 G. Horowitz Adv. Mater. 1990, 2, 287.

13 D. Wöhrle, D. Meissner Adv. Mater. 1991, 3, 129.

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1 Introduction

On illumination both photo-active layers exhibit excitation of electrons and thus the generation of electron-hole pairs (excitons) which can diffuse within the bulk of the films. But only at the interface between CuPc and perylene the charge separation takes place. Holes are preferrentially transported in the CuPc layer whereas electrons diffuse into the perylene bisimidazole phase. The efficiency of this exciton dissociation process can be attributed to the internal field and is dependend on the field strength. The Tang cell is characterized by power conversion efficiency of 0.95 %. Improvement of the original Tang cell with CuPc as hole transport material was achieved by doping phthalocyanine with fullerene C60. This was realized via depositing a mixture of ZnPc and C60 between the actual layers of ZnPc and perylene bisimide. All layers had been prepared by vapour deposition processes controlling the desired geometry by using appropriate masks. With the described setup it was possible to increase power conversion efficiency of this type of solar cell to 1.05 %¹⁵.

Polymer / fullerene solar cells

Another approach of thin-layer photovoltaic devices was applying C60 in combination with hole transport polymers and this was investigated intensively using a sandwich structure with a blend of p-type and n-type materials embedded between electrodes consisting of ITO and aluminium. Successful realization of this concept was carried out using a layer structure with the poly(p-phenylenevinylene) derivative poly[2-methoxy-5-(2´-ethylhexoxy)-p-phenylene]vinylene (MEH-PPV) as electron donor and C60 as electron acceptor – materials which provide satisfying results in solar cells due to compatible HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) levels:

Holes can be transferred easily from HOMO of MEH-PPV to the ITO electrode and electrons from LUMO of C60 to the aluminium electrode¹⁶.

15 J. Rostalski, D. Meissner Sol. En. Mat. & Solar Cells 2000, 61, 87.

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1 Introduction

The interface for the generation and separation of charges is limited to the surface area of the layers offering the possibility of device improvement via increasing the internal interface. This was achieved by applying a phase separated blend-system as photo-active unit consisting of substituted PPV and the soluble C60 derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)¹⁷. With this concept in the whole volume of the photoactive material charge separation is possible and recently power conversion efficiencies of 2.5 – 3 % were reported for mixtures of substituted PPVs and PCBM solution-processed from chlorobenzene^{18, 19}. A further improvement upto 4 % was achieved using better light absorbing hole conductors such as poly(3-hexylthiophene) instead of PPVs²⁰.

Polymer / polymer heterojunction solar cells

Another strategy for converting sun light into electricity is the use of phase separated polymer blends of hole transporting (donor) and electron transporting (acceptor) polymers. A large internal interface can be generated by controlling the morphology of the phase separation with the structure of the polymers. Moreover the light sensitivity of the system can be regulated by chosing polymers with adequate π → π* energy gaps which also enables the application for broad illumination wavelengths. First results in efficient charge generation and transfer were obtained using a phase separated blend system of MEH-PPV (p-type, donor) and a dimethoxy-cyano PPV-derivative poly[dimethoxy-cyano(phenylene)vinylene] (CN-PPV, n-type, acceptor)²¹. The structures of the two compounds are shown in Figure 1-1.

17 G. Yu, A. J. Heeger J. Appl. Phys. 1995, 78, 4510.

18 S. E. Shasheen, C. J. Brabec, N. S. Sariciftci, F. Padinger, T. Fromherz, J. C. Hummelen Appl. Phys. Lett.

2001, 78, 841.

19 C. J. Brabec, N. S. Sariciftci, J. C. Hummelen Adv. Funct. Mat. 2001, 11, 15.

20 C. J. Brabec Sol. En. Mat. & Solar Cells 2004, 83, 273.

21 J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H. Friend, S. C. Moratti, A. B. Homes

1 Introduction

O

O ⁿ MeO

OMe

MeO

OMe

NC

CN

n

MEH-PPV CN-PPV

Figure 1-1: Chemical structures of poly[2-methoxy-5-(2´-ethyl)-hexyloxy-p-phenylene vinylene] (MEH-PPV) and poly[dimethoxy-cyano(p-phenylene)vinylene]

(CN-PPV).

The solar cells were prepared via solution-casting of polymer thin films (150 nm – 200 nm) from xylene onto a glass substrate partly covered with ITO. Calcium or aluminium was vapour deposited as counter electrode. An increase in overall efficiency seemed to be possible by better control of the morphology and thus suppressing recombination processes.

In the group of Friend, a two-layer concept was developed consisting of conjugated, polymeric hole and electron conductors which were coated by a special lamination technique²². As photo-active materials a CN-PPV derivative (electron conductor) and poly[3-(4-octylphenyl)thiophene] (POPT, see Figure 1-2) were spin-coated onto two ITO covered glass subtrates from different solution compositions. One substrate was coated with a solution consisting of CN-PPV and 5 wt% POPT, the second substrate with a solution of POPT and 5 wt% CN-PPV. The two substrates were arranged one on top of the other and set under pressure which resulted in a p/n heterojunction device.

Aluminium or calcium served as electrode materials which were deposited onto the substrates before coating the photo-active layers.

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1 Introduction

With this setup power conversion efficiency of 1.9 % was achieved at irradiation with solar spectrum A.M. 1.5 (see chapter 1.2.4)²².

S ⁿ

POPT

Figure 1-2: Chemical structure of poly[3-(4-octylphenyl)thiophene] (POPT).

The main problems in this concept still are charge separation and charge transport to the electrodes. The light has to be absorbed directly at the donor-acceptor interface and at the same time the contact of the materials with the matching electrodes has to be guaranteed which turned out to be the limiting factor of this device structure. The reproducibility of the lamination technique also was not guaranteed for large area devices.

Polymer / semiconductor solar cells

Another concept in the development of solar cells are hybrid systems involving both conjugated polymers and inorganic semiconductor nanoparticles.

The first system of this kind was reported by Alivisatos et al. and employed MEH-PPV as conjugated polymer and CdSe and CdS as inorganic nanoparticles²³. With this approach it is possible to combine the easy processability of polymeric materials with the favourable absorption profile of inorganic materials and the charge transport properties of the inorganic particles can be optimized by tunig their size.

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1 Introduction

For optimized hybrid devices applying poly(3-hexylthiophene) as conjugated low band gap polymer and CdSe nanoparticles a power conversion efficiency of 1.7 % could be obtained²⁴.