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In this work, we design and compare two intermediate layers (IMLs) for solution-processed tandem organic solar cells. The IMLs are made of PEDOT as the hole transport layer (HTL) coupled with either TiO x or ZnO nanoparticles as the electron transport layer (ETL). The IMLs are characterized with regard to the requirements usually found in the literature: process robustness, work function compatibilities, good optical transparency, good electrical conductivity, and finally, the effective presence of an ohmic contact between the ETL and the HTL. Specifically designed single and tandem cells are then made to compare the performances of the two IMLs. We show that the ohmic contact is the most important criterion, which has to be carefully probed in order to design a functional IML. We offer recommendations on how to properly investigate this property. We also show that the nature of the ETL/HTL contact cannot be predicted from the results of the materials characterization, particularly not from the work functions and neither from the conductivities. Finally, XPS and work function experiments suggest that the nature of the contact depends on phenomena arising at the interface between the HTL and the ETL. Specifically, a chemical interaction is observed between ZnO and PEDOT within the IML. We conclude that a ZnO-based IML is a better choice than a TiO x -based IML.

A study of the optical and impedance behavior of optimized standard and inverted photovoltaic solar cells based on P3HT:PCBM active nano composites is presented. The standard cells sequence is ITO/HTL1/P3HT:PCBM/Ca/Al and the inverted cells one is ITO/ZnO/P3HT:PCBM/HTL2/Ag where HTL1 and HTL2 are Hole Transport Layers. Absorption and action spectra, together with I–V characteristics, are shown to be quite similar and lead to 4.05% and 3.90% energy conversions, for standard and inverted cells, respectively. Built-in potentials of 0.82–0.89 V and acceptor impurities concentrations of 1.6–2.4 1015 cm−3 are found through capacitance measurements. Impedance spectrometry shows the classical two-circle complex plan curves, one being related to the effective lifetime of charge carriers before recombination at low frequency, and the other one to the diffusion time of these carriers at high frequency. The shape of the curves is identical, showing the ohmic role of the ZnO layer. It is shown that overall resistances in the dark are higher for inverted cells as compared to standard ones, and that this feature is inverted under illumination, with a thousand-time decrease. Global mobilities are in the range 3.8–4.6 10−3 cm2 V−1 s−1, which is slightly higher as compared to the literature. Four different equivalent circuit models are tested on experimental results, and it is concluded that the classical RCPE model (or its Garcia–Belmonte variant) is suitable for this kind of cells.

A novel series of regioregular bithiophene disubstituted with a carbonitrile group as electron acceptor and a hexyl or hexyloxy chain as electron donor, have been synthesized. The position and the regioregularity of substituents on bithiophene have been controlled via a head-to-head (HH) or head-to-tail (HT) coupling reaction to give four monomers: the HH and the HT carbonitrilehexylbithiophene monomers and the HH and the HT carbonitrilehexyloxybithiophene monomers. Each of them lead, by chemical polymerization, to four polymers constituted by a polythiophene backbone containing alternating electron-donating and electron-withdrawing units directly connected to the polythiophene. Optical and electrochemical characterizations reveal that all these polymers have a low band gap and absorb at longer wavelengths in comparison with a poly(3-hexylthiophene) (P3HT) film. The planarity of the polythiophene backbone was increased by introduction of sulphur–oxygen (S–O) interactions between hexyloxy chains and thiophene units. In addition, the introduction of carbonitrile groups allows to stabilize the LUMO and the HOMO energy levels of polymers at around −3.10 eV for the LUMO and −5.0/−5.3 eV for the HOMO, which were lower than the ones of P3HT (−2.19 eV for the LUMO and ∼−4.9 eV for the HOMO). Bulk heterojunction photovoltaic devices were fabricated using blends of these polymers with PCBM ([6,6]-phenyl-C61-butyric acid methyl ester). Power conversion efficiency of 1.07% was obtained under solar illumination AM 1.5 (100 mW cm−2) from a device containing the polymer synthesized from HT 3-carbonitrile-4′-hexyloxyl-2,2′-bithiophene monomers and PCBM as the active layer.