Burhan Muhsin

Quality control of polymer solar modules by lock-in thermography

We have characterized lateral imperfections of photovoltaic modules based on solution processed polymer-fullerene semiconductor blends by means of lock-in thermography (LIT). The active layer of the solar cell modules is based on the heterogeneous organic semiconductor system poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester and the power conversion efficiency of the modules reached nearly 2% under irradiation of an AM 1.5 solar simulator. Applying highly sensitive LIT allowed us to detect several kinds of laterally distributed defects originating from imperfections in the respective functional layers as well as in the quality of encapsulation. We show that LIT is a powerful method for the quality control of large area polymer solar cells and modules, enabling fast feedback for optimization of production parameters.

Irradiation-induced degradation of PTB7 investigated by valence band and S 2p photoelectron spectroscopy

Monochromatic radiation with known absolute radiant power from an undulator at the electron storage ring Metrology Light Source (MLS) was used to irradiate PTB7 (a thieno[3, 4-b]thiophene-alt-benzodithiophene polymer) thin films at wavelengths (photon energies) of 185 nm (6.70 eV), 220 nm (5.64 eV), 300 nm (4.13 eV), 320 nm (3.88 eV), 356 nm (3.48 eV) and 675 nm (1.84 eV) under ultra-high vacuum conditions for the investigation of radiation-induced degradation effects. The characterization of the thin films is focused at ultraviolet photoelectron spectroscopy (UPS) of valence bands and is complemented by S 2p x-ray photoelectron spectroscopy (S 2p XPS) before and after the irradiation procedure. The radiant exposure was determined for each irradiation by means of photodiodes traceably calibrated to the international system of units SI. The valence band spectra show the strongest changes for the shortest wavelengths and no degradation effect at 356 nm and 675 nm even with the highest radiant exposure applied. In the spectral range where the Sun appears bright on the Earths surface, no degradation effects are observed.

Current density and heating patterns in organic solar cells: modelling and imaging experiments(Conference Presentation)

We developed finite element models of organic solar cells in order to investigate current pathways and dissipative losses under different geometries. The models are of purely resistive nature, as this is sufficient to describe the effects under consideration. The overall behaviour of the current mostly steers the resistive behaviour of the device and is a delicate consequence of the interplay between the individual layer properties, namely the resistivities and layer thicknesses in combination. The model calculations solely based on external material parameters, i.e. without fitting, yield the spatial distribution of the current densities, potentials and the according resistive losses. In particular, the current pathways are spread out from the entire length of the top contact towards the entire width of the ground contact, running along the electric potential gradient. On the other hand, current crowding appears at the foremost part of the top electrode, resulting in a respective concentration of the resistive loss in this vicinity. The resistive loss in turn is the origin of the heat pattern, which is visible in DLIT/ILIT experiments. The comparison between experiment and simulation shows remarkable agreement. Having established the description of defect free solar cells, defects were simulated. We utilized the micro-diode-model as another established simulation method to model shunt or blocking contact defects in combination with electro luminescence imaging methods. The respective heat patterns were calculated in FEM. Nice agreement is found between the various experimental and simulation methods. The respective heat patterns then allow identifying several classes of defects such as shunt defects or blocking contact defects in accordance with their patterns from various imaging measurements, bridging the gap between theory and experiment to further the detailed analysis of organic solar cells.

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

Optimal geometric design of monolithic thin-film solar modules: Architecture of polymer solar cells

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...