Heterojunction Hybrid Solar Cells Research Articles

Determination of electron and hole energy levels in mesoporous nanocrystalline TiO 2 solid-state dye solar cell

A study of a hybrid heterojunction solar cell based on nanocrystalline mesoporous TiO 2 and the hole conductor spiro-OMeTAD (2,2′7,7′-tetrakis( N, N′-di- p-methoxyphenyl-amine)-9,9′-spiro-bifluorene) has been realized. Impedance and cyclic voltammetry techniques were used to measure the interfacial properties of the hybrid heterojunction and establish the energy levels of the solid-state electrolyte. It was observed that the energy levels of the organic hole transport material are changed when it forms a film deposited onto indium-doped tin oxide (ITO). Moreover, the HOMO level of the mono oxidized spiro-OMeTAD is well coupled with the HOMO level of the dye N719 (Ru(4,4′-dicarboxy-2,2′-bipyridyI) 2(SCN) 2) which implies that it is not convenient to increase the doping of the hole conductor much further than this first oxidized state. This doping level ( n ≈ 10 19 cm −3) also assures a high enough hole conductivity. The implications of our results to the solid-state dye solar cell performance are discussed.

Highly Efficient Hybrid Solar Cells Based on an Octithiophene-GaAs Heterojunction

We report a new type of hybrid heterojunction solar cell based on rod-like octithiophene (8T) as the organic p-type semiconductor and GaAs(111) as the inorganic n-type semiconductor. By using a semitransparent gold layer as the front contact deposited onto the 8T films, solar-energy conversion efficiencies of up to 4.2 % could be obtained. The reduction in the contact resistance at the Au/8T interface induced by iodine doping is found to be a very crucial factor for the high efficiency. Furthermore, we demonstrate that hybrid solar cells can be successfully used to investigate the photovoltaic properties of organic semiconductors in detail. By means of external quantum efficiency (EQE) measurements, the influence of film morphology on the photocurrent collection length in 8T films is studied. The results show that, in hybrid solar cells using highly ordered microcrystalline 8T films, an active contribution of the organic-layer semiconductor to the total photocurrent exists. A very large photocurrent collection length of up to 100 nm has been estimated from EQE measurements, indicating that exciton diffusion is very efficient in microcrystalline 8T. On the other hand, the use of nanocrystalline 8T leads to high photocurrent losses in the organic part of the hybrid solar cell. The strong influence of the film morphology on the photocurrent collection in 8T is attributed to a reduction in the exciton diffusion length due to a high trap density in nanocrystalline 8T films. Thus, our results reveal the importance of high crystalline order for obtaining efficient photocurrent collection in 8T films.

Growth of organic semiconductors for hybrid solar cell application

In this paper we present a study of the growth of quaterthiophene (4T), a linear conjugated oligomer of thiophene behaving as a p-type semiconductor, on n-doped GaAs and Si substrates to form hybrid heterojunction solar cells. We focus on the influence of the substrate and the different surface preparations on the morphology of the organic thin films. The 4T deposited by thermic evaporation on GaAs forms films consisting of grains of some hundred of nanometers and large lamella defects. However, in the case of Si as substrate we demonstrate almost defect-free high ordered films with grain sizes of several micrometers up to a film thickness of 250 nm. The photovoltaic characteristics of the hybrid solar cells made by the heterojunction organic–inorganic semiconductor systems are investigated.