FHJ solar cells

The first organic solar cell with two components was presented by C. W. Tang in 1986. The device comprised basically two active layers on top of each other.^[45] This geometry is called flat heterojunction, planar heterojunction or bilayer heterojunction. As shown in Figure 6, the FHJ configuration consists of a layer of the p-type donor material responsible for the hole transport and a layer of the n-type acceptor material that transports the electrons.^[23,88] This structure can be fabricated by two methods depending on the applied materials.^[108] The single layers are often

15 realized by subsequent vacuum deposition. Thus, usually small organic molecules like phthalocyanines as donor materials and fullerenes as acceptor materials are applied.^[4,109]

Besides vacuum deposition of small molecules that also allows the fabrication of multilayer structures with several functional layers, polymeric materials are solution processed via spin coating. The deposition of a second layer is difficult with polymer solutions as the underlying layer is dissolved upon spin coating of the upper material. Thus, multilayer devices are dependent on the insolubility of the underlying layer either achieved by orthogonal solvents or by crosslinking of the material.^[108]

Figure 6: Geometry of a FHJ solar cell.^[4,98,110]

In the first planar heterojunction solar cell by C. W. Tang, the two active layers comprising copper phthalocyanine as donor and 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole as acceptor are sandwiched between an ITO anode and a silver cathode. An efficiency of about 1% was achieved.^[45] In 1989, S. R. Forrest replaced the acceptor by 3,4,9,10-perylenetetracarboxylic dianhydride which he found out to be the better n-type material.^[111] N. S. Sariciftci was the first who applied C60 as acceptor material in 1993.^[50,51]

In FHJ solar cells, the interface between donor and acceptor is relatively small. Thus, the number of electrons that can contribute to the photocurrent is limited. Only excitons that are generated in a very thin layer close to the interface are able to reach the interface due to their small diffusion length. However, the optical absorption length is much higher than the diffusion length. In consequence, most of the generated excitons are lost by recombination processes.^[4]

Because of the low charge carrier mobilities in organic semiconductors, the free charge carriers can form space charges after exciton dissociation which influence the solar cell performance.^[60]

This can result in recombination of the charge carriers at the interface.^[112] The insertion of a transparent exciton blocking layer between the active layer and the metal electrode ensures that the excitons only migrate within the active layer and thus prevents the quenching of excitons at defect states of the interface between acceptor and cathode. Furthermore, also damage like trap levels due to the evaporation of the cathode is circumvented. In addition, the exciton blocking layer reduces the resistance between the organic material and the cathode and serves as an optical spacer that redistributes the optical density within the active layer enhancing the total absorption and the efficiency of the solar cell.[4,60,113,114] Applying the same organic semiconductors as C. W. Tang in his first efficient organic solar cell, the group of S. R. Forrest achieved an efficiency increase to 2.4% by integration of an exciton blocking layer

ITO Substrate

Donor Acceptor Metal electrode

16

made from bathocuproine (BCP).^[115] Moreover, by the exchange of the perylene derivative with C60 as a better acceptor, efficiencies of 3.6%^[116] and 4.2% can be reached, respectively.^[117]

Insertion of tris(4-(5-phenylthiophen-2-yl)phenyl)amine as an additional exciton blocking layer at the anode, a solar cell with an tetraphenyldibenzoperiflanthene donor, a C60 acceptor, and BCP as the cathode buffer layer achieved 5.3% efficiency.^[118] The highest efficiency for planar heterojunction solar cells with about 6% was reached by K. Cnops with the donor α-sexithiophene and the acceptor boron subnaphthalocyanine chloride in combination with a BCP exciton blocking layer towards the cathode.^[119] The chemical structures of the applied active materials are depicted in Figure 7. Further enhancement of the efficiency of FHJ structures is very difficult to achieve because the interface between the donor and the acceptor is small and the thickness of the two active layers is limited due to the short diffusion length of the excitons.^[110,120] In addition, the application of thick absorber layers would result in optical filter effects decreasing the photocurrent.^[98] However, efficiency increase is enabled by the combination of several active materials in multilayer devices. K. Cnops realized a three-layer planar heterojunction device that comprises the acceptor boron subphthalocyanine chloride in addition to the previously used subnaphthalocyanine chloride acceptor and the α-sexithiophene donor achieving a PCE of 8.4%.^[119] The additional acceptor material is also illustrated in Figure

7.

Figure 7: Chemical structures of α-sexithiophene, subphthalocyanine chloride, and subnaphthalocyanine chloride as used in the best FHJ solar cells.^[119]

Although the FHJ geometry is not suited for achieving very high efficiencies with organic semiconducting materials, this setup is frequently used for fundamental research concerning the processes at the donor-acceptor interface. The planar heterojunction is an ideal model system for such basic studies due to the inherent advantages of the structure. For the understanding of interfacial actions, the morphology of the interface is of vital importance and has to be controlled accurately. This is possible due to the planarity of the interface between donor and acceptor that restricts unpredictable and uncontrollable variations which arise from mixing of the materials. Thus, different material systems are comparable when using a planar structure. However, the conditions regarding solar cell fabrication have to be chosen carefully as the interfacial morphology can be significantly influenced by the mixing of donor and acceptor. This is possible if vacuum deposited low molecular weight materials diffuse into the underlying layer that consists either of evaporated small molecules as well or of solution processed polymers. Furthermore, the charge transport pathways are clearly separated in FHJ structures as the hole is transported within the donor and the electron migrates through the

17 acceptor. Because of the easier requirements, planar heterojunctions are often used for device simulations allowing the comparison of experiment and simulation.^[108]