Advanced Functional Materials and Devices (AFMD) Group

Abstract:

Current matching of the subcells is crucial to optimize the performance of tandem solar cells. Due to the thin film optics of organic solar cells, the position of the two subcells relative to the reflecting electrode becomes a very important issue. This is demonstrated for an indium tin oxide (ITO)/pin/pii/Al structure with thin intrinsic absorbing layers consisting of zinc-phthalocyanine and fullerene C60 and a metal-free lossless recombination contact between the subcells. By keeping the thickness of the absorbing layers constant and changing only the thickness of the inner p-doped transparent layer in 16 steps from 0 to 186 nm, the distance of the ITO-sided subcell from the reflecting electrode (Al) is systematically varied. Thus, the p-doped layer works as an optical spacer between both subcells. The influence of its thickness on the thin film optics is shown in optical simulations and confirmed with current-voltage measurements. If both subcells are separated only by the recombination contact, they are positioned in the first interference maximum of the incident light and the currents of the individual subcells nearly matches. By increasing the spacer layer thickness, the ITO-sided subcell is moved to the first interference minimum, limiting the measured short circuit current density jsc of the tandem solar cell to about 1/2 of its initial value without spacer. At a spacer thickness of about 140 nm, jsc recovers in the second interference maximum to nearly its original value. Within this series, an almost constant high fill factor of about 59% as well as a constant open circuit voltage of 1.06 V is observed, showing that the Ohmic losses in the spacer are negligible. The power conversion efficiency of these devices reaches nearly 4% in the first and approximately 3.6% in the second interference maximum, respectively, in an outdoor test at 1 sun. Furthermore, it is shown that for thicker absorber layers, an optimized current density cannot be reached in the first, but only in the second optical interference maximum, making the presented optical spacer an essential component for efficient organic tandem devices. © 2010 American Institute of Physics.

Abstract:

We investigate the end-capped oligothiophene derivative α,ω -bis-(dicyanovinylene)-sexithiophene with ethyl side chains (DCV6T) as donor material in heterojunctions with C60. The effect of the substrate temperature on the morphology and related photophysical properties of single DCV6T and mixed DCV6T:C60 layers is investigated. Single layers of DCV6T show crystalline features in UV-visible absorption and x-ray diffraction when grown on a substrate heated to 90 °C. Investigations of DCV6T:C60 mixed layers by atomic force microscopy, UV-visible absorption, and photoluminescence measurements reveal that the elevated substrate temperature induces an increased phase separation between the two materials with larger domain size and higher surface roughness. Based on these observations, we present mixed heterojunction solar cells where the power conversion efficiency (ηPCE) is increased from 1.6% to 3.8% by increasing the substrate temperature from 30 to 90 °C, respectively. © 2010 American Institute of Physics.

Abstract:

This work addresses the long-term aging of organic solar cells based on a flat zinc phthalocyanine (ZnPc)/C60 heterojunction. We investigate both the typical degradation behavior of the short circuit current and of the saturated photocurrent, defined as I(-1V). The latter remains constant after a relatively small initial decay, which is directly related to a substantial reduction of the contribution of excitons generated in C60 to the external quantum efficiency. Mass spectroscopy analysis of the organic material after aging revealed oxidized C60 and sub-products thereof. The reduction of I(-1V) is thus attributed to the reaction of C60 molecules with oxygen impurities. The results strongly suggest that ZnPc/C60 photovoltaic cells are intrinsically very stable on a time scale of thousand of hours if such reactions are prevented. © 2009 Elsevier B.V.

Abstract:

Cu and Zn Phtalocyanines (CuPc and ZnPc), and C60 are materials frequently used for organic Solar cell engineering. Their energy levels form a donor-acceptor junction, and they have high absorption coefficients and a complementary absorption for the Sun spectrum. We have investigated ageing properties of ZnPc/C60 Solar cells as they are influenced by the charge carrier mobility and variation of the potential barrier height of the ZnPc/C60 interface. The structures ITO/ZnPc/C60/C60:AOB/Al with a reasonable energy conversion efficiency of ∼1.5% were investigated. The samples were aged for 1300 hours upon illumination with blue LED, with peak emission at 475nm, and incident light power density of 10mW/cm2. The aged devices showed a strong and fast degradation of the short circuit current and of the fill factor after several hours followed by an almost constant behaviour of these values. The reference samples kept in the dark at the room temperature did show only very small changes in their I-V curves. Carrier mobility dependencies on electric field strength at different temperatures were measured by the Charge Extraction by Linearly Increasing Voltage (CELIV) method. It was demonstrated that mobility values decrease during degradation as compared to the reference samples. Nevertheless only mobility changes cannot explain the observed drop of device current. The increase of the effective barrier height at the interface of ZnPc and C60 by about 0.1eV from ∼0.55eV up to ∼0.65eV was observed in the aged samples. Meanwhile thermal activation energy values of the electrical conductivity grew from about 0.28eV prior to degradation up to about 0.34eV after ageing. © Taylor & Francis Group, LLC.

Abstract:

We have investigated conductivity and ageing of ZnPc/C60 solar cells as they are influenced by the charge carrier mobility and variation of the potential barrier height. The test structures with a reasonable energy conversion efficiency of ∼1.5% were investigated. The samples were aged for 1300 h upon illumination by the blue LED with peak emission at 475 nm. Upon ageing the devices showed a strong and fast degradation of the efficiency, short circuit current and of the fill factor within several hours followed by a much slower decrease of them. The reference samples kept in the dark at the room temperature did show only very small changes in their I-V curves. Carrier mobility dependencies on electric field strength at different temperatures were measured by the Charge Extraction by Linearly Increasing Voltage (CELIV) method. It was demonstrated that mobility values decrease during degradation as compared to the reference samples. Nevertheless only mobility changes cannot explain the observed drop of device current. The increase of the effective barrier height by about 0.1 eV from ∼0.55 eV up to ∼0.65 eV was observed in the aged samples. Meanwhile thermal activation energy values of the electrical conductivity grew from about 0.28 eV prior to degradation up to about 0.34 eV after ageing. © 2010 Elsevier B.V. All rights reserved.