Roland Rösch

The ISOS-3 inter-laboratory collaboration focused on the stability of a variety of organic photovoltaic devices

Seven distinct sets (n ≥ 12) of state of the art organic photovoltaic devices were prepared by leading research laboratories in a collaboration planned at the Third International Summit on Organic Photovoltaic Stability (ISOS-3). All devices were shipped to RISO DTU and characterized simultaneously up to 1830 h in accordance with established ISOS-3 protocols under three distinct illumination conditions: accelerated full sun simulation; low level indoor fluorescent lighting; and dark storage with daily measurement under full sun simulation. Three nominally identical devices were used in each experiment both to provide an assessment of the homogeneity of the samples and to distribute samples for a variety of post soaking analytical measurements at six distinct laboratories enabling comparison at various stages in the degradation of the devices. Over 100 devices with more than 300 cells were used in the study. We present here design and fabrication details for the seven device sets, benefits and challenges associated with the unprecedented size of the collaboration, characterization protocols, and results both on individual device stability and uniformity of device sets, in the three illumination conditions.

Luminescence imaging of polymer solar cells: Visualization of progressing degradation

We apply luminescence imaging as tool for the nondestructive visualization of degradation processes within bulk heterojunction polymer solar cells. The imaging technique is based on luminescence detection with a highly sensitive silicon charge-coupled-device camera and is able to visualize with time advancing degradation patterns of polymer solar cells. The devices investigated have been aged under defined conditions and were characterized periodically with current-voltage (I-V) sweeps. This allows determining the time evolution of the photovoltaic parameters and—in combination with the luminescence images—understanding differences in the observed degradation behavior. The versatile usability of the method is demonstrated in a correlation between local reduction of lateral luminescence and a fast decrease of the short-circuit current (Isc) due to the loss of active area. Differences in the degradation of photovoltaic parameters under varied aging conditions are discussed.

Subbandgap absorption in polymer-fullerene solar cells

We present external quantum efficiency (EQE) studies of poly(3-hexylthiophene-2,5-diyl):[6,6]-phenylC61-butyric acid methyl ester (P3HT:PCBM) based bulk heterojunction polymer solar cells with improved intensity resolution in the subbandgap (SBG) region, i.e., the energy range below the optical bandgaps of the pristine materials. Varying the P3HT:PCBM blending ratio, we find that in addition to a Gaussian profile an exponential tail is needed for a quantitative description of the SBG EQE spectra. While the exponential contribution can be reliably assigned to disorder effects, the SBG EQE Gaussian profile can be due to charge-transfer absorption between P3HT and PCBM or due to absorption of PCBM at the interface or in the polymer-rich phase.

Photon recycling across a ultraviolet-blocking layer by luminescence in polymer solar cells

UV-blocking layers can increase the long term stability of organic solar cell devices; however, they limit the amount of light that can be utilized for energy conversion. We present photon recycling and down-conversion via a luminescent layer across a UV-blocking TiO2 layer. Our results show that the use of an additional UV-blocking layer does not necessarily reduce the overall efficiency of organic solar cells, since the loss in photocurrent due to the UV-absorption loss can be partially compensated using high energy photon down-conversion via luminescence layers.

Quantitative analysis of electroluminescence images from polymer solar cells

We introduce the micro-diode-model (MDM) based on a discrete network of interconnected diodes, which allows for quantitative description of lateral electroluminescence emission images obtained from organic bulk heterojunction solar cells. Besides the distributed solar cell description, the equivalent circuit, respectively, network model considers interface and bulk resistances as well as the sheet resistance of the semitransparent electrode. The application of this model allows direct calculation of the lateral current and voltage distribution within the solar cell and thus accounts well for effects known as current crowding. In addition, network parameters such as internal resistances and the sheet-resistance of the higher resistive electrode can be determined. Furthermore, upon introduction of current sources the micro-diode-model also is able to describe and predict current-voltage characteristics for solar cell devices under illumination. The local nature of this description yields important conclusio...

Combined characterization techniques to understand the stability of a variety of organic photovoltaic devices: the ISOS-3 inter-laboratory collaboration

This work is part of the inter-laboratory collaboration to study the stability of seven distinct sets of state-of-the-art organic photovoltaic (OPVs) devices prepared by leading research laboratories. All devices have been shipped to and degraded at the Danish Technical University (DTU, formerly RISO-DTU) up to 1830 hours in accordance with established ISOS-3 protocols under defined illumination conditions. In this work we present a summary of the degradation response observed for the NREL sample, an inverted OPV of the type ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag/Al, under full sun stability test. The results reported from the combination of the different characterization techniques results in a proposed degradation mechanism. The final conclusion is that the failure of the photovoltaic response of the device is mainly due to the degradation of the electrodes and not to the active materials of the solar cell.

Stability and degradation of organic photovoltaics fabricated, aged, and characterized by the ISOS 3 inter-laboratory collaboration

Seven distinct sets (n ≥ 12) of state of the art organic photovoltaic devices were prepared by leading research laboratories in a collaboration planned at the Third International Summit on Organic Photovoltaic Stability (ISOS-3). All devices were shipped to DTU and characterized simultaneously up to 1830 h in accordance with established ISOS-3 protocols under three distinct illumination conditions: accelerated full sun simulation; low level indoor fluorescent lighting; and dark storage with daily measurement under full sun simulation. Three nominally identical devices were used in each experiment both to provide an assessment of the homogeneity of the samples and to distribute samples for a variety of post soaking analytical measurements at six distinct laboratories enabling comparison at various stages in the degradation of the devices. Characterization includes current-voltage curves, light beam induced current (LBIC) imaging, dark lock-in thermography (DLIT), photoluminescence (PL), electroluminescence (EL), in situ incident photon-to-electron conversion efficiency (IPCE), time of flight secondary ion mass spectrometry (TOF-SIMS), cross sectional electron microscopy (SEM), UV visible spectroscopy, fluorescence microscopy, and atomic force microscopy (AFM). Over 100 devices with more than 300 cells were used in the study. We present here design of the device sets, results both on individual devices and uniformity of device sets from the wide range of characterization methods applied at different stages of aging under the three illumination conditions. We will discuss how these data can help elucidate the degradation mechanisms as well as the benefits and challenges associated with the unprecedented size of the collaboration.

Laser structuring of thin films for organic solar cells

Many R&D activities currently concentrate on low-cost production concepts for photovoltaics. Because of their low material price organic thin film cells possess the potential to provide module market prices below

Consensus stability testing protocols for organic photovoltaic materials and devices

1/Wp. In this publication we report on the development of a roll-to-roll production technology for the monolithic series connection of organic thin film solar cells based on laser structuring.Due to the different material properties of the layers, structuring experiments are performed with a broad range of laser sources. Various parameters like wavelength (e.g. 355 nm, 532 nm, and 1064 nm), pulse durations, pulse energy and spot-to-spot overlap are studied to obtain a profound understanding of the laser-material interaction mechanisms. The focus of the experiments is to obtain an optimal edge quality without damaging the layer structure.Results are presented, obtained with optimized parameters for the laser structuring of the several layers used in organic thin film modules. Good results are obt...