Konstantinos Fostiropoulos

Fluorinated Copolymer PCPDTBT with enhanced open-circuit voltage and reduced recombination for highly efficient polymer solar cells

A novel fluorinated copolymer (F-PCPDTBT) is introduced and shown to exhibit significantly higher power conversion efficiency in bulk heterojunction solar cells with PC(70)BM compared to the well-known low-band-gap polymer PCPDTBT. Fluorination lowers the polymer HOMO level, resulting in high open-circuit voltages well exceeding 0.7 V. Optical spectroscopy and morphological studies with energy-resolved transmission electron microscopy reveal that the fluorinated polymer aggregates more strongly in pristine and blended layers, with a smaller amount of additives needed to achieve optimum device performance. Time-delayed collection field and charge extraction by linearly increasing voltage are used to gain insight into the effect of fluorination on the field dependence of free charge-carrier generation and recombination. F-PCPDTBT is shown to exhibit a significantly weaker field dependence of free charge-carrier generation combined with an overall larger amount of free charges, meaning that geminate recombination is greatly reduced. Additionally, a 3-fold reduction in non-geminate recombination is measured compared to optimized PCPDTBT blends. As a consequence of reduced non-geminate recombination, the performance of optimized blends of fluorinated PCPDTBT with PC(70)BM is largely determined by the field dependence of free-carrier generation, and this field dependence is considerably weaker compared to that of blends comprising the non-fluorinated polymer. For these optimized blends, a short-circuit current of 14 mA/cm(2), an open-circuit voltage of 0.74 V, and a fill factor of 58% are achieved, giving a highest energy conversion efficiency of 6.16%. The superior device performance and the low band-gap render this new polymer highly promising for the construction of efficient polymer-based tandem solar cells.

On the function of a bathocuproine buffer layer in organic photovoltaic cells

The role of bathocuproine (BCP) buffer layer inserted between active layer and Al contact in photovoltaic cells based on phthalocyanine (Pc) and C60 was investigated. Photoluminescence (PL) experiments show exciton quenching at the C60–Al interface to be strongly reduced by inserting BCP. Current-voltage characteristics of photovoltaic cells with planar geometry front electrode/Pc∕C60∕BCP∕Al show that BCP dramatically improves electron transport out of the C60 film into the Al electrode. The tenfold increase in power conversion efficiency (η) with BCP can mostly be attributed to the latter effect. BCP does not improve η in photovoltaic cells with blend film geometry front electrode/Pc:C60∕Al.

On the Field Dependence of Free Charge Carrier Generation and Recombination in Blends of PCPDTBT/PC70BM: Influence of Solvent Additives

We have applied time-delayed collection field (TDCF) and charge extraction by linearly increasing voltage (CELIV) to investigate the photogeneration, transport, and recombination of charge carriers in blends composed of PCPDTBT/PC70BM processed with and without the solvent additive diiodooctane. The results suggest that the solvent additive has severe impacts on the elementary processes involved in the photon to collected electron conversion in these blends. First, a pronounced field dependence of the free carrier generation is found for both blends, where the field dependence is stronger without the additive. Second, the fate of charge carriers in both blends can be described with a rather high bimolecular recombination coefficients, which increase with decreasing internal field. Third, the mobility is three to four times higher with the additive. Both blends show a negative field dependence of mobility, which we suggest to cause bias-dependent recombination coefficients.

Effects of oxygen and illumination on the photovoltaic properties of organic solar cells based on phtalocyanine:fullerene bulk heterojunction

The authors report factors that limit the photovoltaic parameters of organic solar cells based on phtalocyanine:fullerene bulk heterojunction. They show that the device photovoltaic properties and their stability are strongly influenced by O2 impurities. Oxygen affects mostly the photoelectrical properties of the fullerene and the state of the interface between phtalocyanine and fullerene domains. The illumination of the devices, especially in the wavelength range <570nm, induces photodiffusion of the oxygen into fullerene material and promotes formation of O2-fullerene dipoles and additional recombination levels due to C–O reactions that accelerate the device degradation.

Formation of charge-selective Mg–Ag electrodes to CuPc:C60 blend layers

The formation of buffer-free charge carrier selective Mg–Ag cathodes for donor-copper phthalocyanine (CuPc):acceptor-C60 bulk heterojunction organic solar cells is investigated. An optimum charge carrier selectivity is achieved with Mg:Ag/Ag cathode structures, where the Mg:Ag alloy layer has a composition close to that of Ag3Mg. In addition, Mg diffusion into CuPc:C60 layer is observed. As a result, an interaction between Mg and Cu2+ with a concurrent change in oxidation state of both metals takes place. However, no formation of magnesium phthalocyanine is observed.

Morphology effects on charge generation and recombination dynamics at ZnPc:C60 bulk hetero-junctions using time-resolved terahertz spectroscopy

The influence of growth temperature induced phase segregation and crystallinity in ZnPc:C60 blend films on the charge generation and recombination dynamics is investigated with optical-pump terahertz-probe spectroscopy. While an ultrafast photo-induced charge generation process is observed for all morphologies, a subsequent sub-nanosecond photoconductivity rise depends on crystallinity and phase segregation. For higher intensities, the signal is dominated by a morphology-dependent bimolecular recombination process. High local mobilities of minimal μ ∼ 0.3 cm2/Vs are found. The increase of photoconductivity with film growth temperature correlates with formerly observed device photocurrent improvements.

Preparation and investigation of phthalocyanine/C60 solar cells

Phthalocyanine/C60 heterostructure organic solar cells were fabricated by evaporating highly purified materials under high vacuum conditions. 1%-2.5% external power conversion efficiencies were obtained with ITO / PEDOT:PSS / 300 angstrom ZnPc / 300 angstrom C60 / 170 angstrom BCP/Al layering. The role of the buffer layers at the interfaces to the electrodes and their influence on the I-V characteristics was studied. Degradation of the organic cell caused by air and light exposure was investigated. Atmospheric oxygen diffusing into C60 films was identified as external source of degradation. A second source was spotted in the interior of the cell.

Performance and Transport Properties of Phthalocyanine:Fullerene Organic Solar Cells

Copper phthalocyanine (CuPc)-fullerene (C60) photovoltaic cells are produced by organic vapour phase deposition reaching efficiencies of 3%. The electronic transport properties of the devices are investigated as a function of the CuPc:C60 absorber blend layer composition and its preparation temperature. The analysis of the transport properties of the devices employs the one-diode model. It is shown that the dominant recombination process takes place at the donor-acceptor interfaces of the CuPc and C60 absorber domains. The activation energy of recombination is related to the effective band gap of the blend layer.

Work Function Homogeneity of Monolayer Passivated ITO Surfaces for Organic Solar Cells

The ITO transparent electrode surface is passivated either by organic quinoline molecules or by ions of periodic and phosphoric acids for organic solar cells. Passivated surfaces are investigated by means of Kelvin probe force microscopy.