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35th European Photovoltaic Solar Energy Conference and Exhibition; 278-280, We have elaborated contactless method of measurement and evaluation of doping profiles in silicon polished wafers based on infrared reflectance under high angle of incidence. We have found higher angle of incidence increases sensitivity, however approaching Brewster angle increases also experimental error, therefore 65 angle has been chosen. Moreover, to increase reproducibility we divide the measured spectra by reference spectra taken on an undoped sample, and further we rescale the spectra to fixed value in the region of 4000 cm-1–7000 cm-1. To reduce number of evaluated parameter, the carrier profile in boron-doped samples was parametrized by 3 parameters and that in phosphorous-doped samples was parametrized by 4 parameters, using additional empirically determined assumption that the first part of the profile is a constant plateau and that the following two exponential tails are joined at a value of 3x1019 cm-3. As a physical model, the classical Drude equation is used and revised, leading to adjustment of effective mass and assuming concentration-dependent relaxation time and the contribution from band-to-band excitations. The relation of concentration-dependent relaxation time was represented in the range of interest by a power law that significantly deviates from typically used empirical data. The reason is that the empirical data had been obtained as a function of dopant density and not free carrier density. Because the doping efficiency drops at higher concentration this empirical data could not be used. This is one of the most important advancement of the free carrier absorption model. Finally, the model parameters were calibrated independently by infrared ellipsometry. Satisfactory agreement with the profiles measured by the electrochemical capacitance-voltage method was reached. The measurement using Fouriertransform infrared spectrophotometer takes few seconds and can be implemented as an in-line characterization technique in industry.

Diamond is a unique semiconductor with a wide bandgap which usually is easily doped with boron and is acknowledged as one of the best materials for electrochemical applications. Heavily boron doped, high quality single crystal synthetic diamond can reach electrical conductivity as high as 103 S.cm, whereas polycrystalline material usually reaches c.a. 102 S.cm. However, many potential applications are restricted by the deposition temperature and limited coating area of conventional MW PECVD systems. Deposition of boron doped nano-crystalline diamond (BNCD) layers using a microwave PECVD system with linear antenna delivery (MW-LA-PECVD), enabling large area coating, was first reported in 2014 [1]. However, layers showed lower electrical conductivity in comparison to BNCD layers deposited using conventional PECVD systems. In addition, deposition of BNCD by MW-LA-PECVD is complicated by the necessity for the addition of oxygen species, which are known to limit boron incorporation and the competitive growth of silicon carbide at low CO2 concentrations [2, 3]. In this work, we further study the effect of deposition conditions on the synthesis of BNCD using the MW-LA-PECVD technique. In order to produce highly conductive BNCD with a low sp2 fraction, we have investigated in greater detail the effect of deposition temperature, from 250 °C up to 750 °C, using temperature controlled substrate stages and the effect of precursor gas compositions.

28th European Photovoltaic Solar Energy Conference and Exhibition; 399-402, We report on hydrothermal growth of single (or multi)-crystalline ZnO nanocolumns arrays, perpendicularly oriented towards the TCO (transparent conductive oxide) substrate. In order to get well defined spacing and arrangement over the area, we used electron beam lithography. Vertically aligned ZnO nanocolumns were grown in an aqueous solution of zinc nitrate hexahydrate and hexamethylenetetramine at 75÷95 ºC, with the growth rate 0.5÷1μm/h. The morphology of the nanostruct ures was visualized by scanning electron microscopy (Raith e-LiNE). The electrical, optical and surface properties of ZnO nanocolumns, which are decisive for a high solar cell efficiency, were investigated by optical and photothermal spectroscopy. We will present our results on optimization of nanostructured TCO/glass substrates being designed for very thin amorphous Si cells and for tandem a-Si/a-Si and a-Si/microcrystalline Si solar cells and report on influence of hydrogen and oxygen plasma treatment.

26th European Photovoltaic Solar Energy Conference and Exhibition; 554-558, The exciton diffusion length is a crucial parameter characterizing polymer materials for solar cells. Method of surface photovoltage (SPV) used here for its evaluation is simple and quick. It utilizes an electric field at surface of the sample for dissociation of photogenerated excitons, which can then lead to generation of the photocurrent in an external circuit. In our case, such field is formed in the space charge region (SCR) at the ITO/polymer interface. The advantage of our SPV model is possibility to apply it to samples with arbitrary ratio of thickness of the SCR and of the bulk. The samples under study were MEH-PPV layers prepared by spin-coating technique using CHCl3 or THF solvent. Ellipsometric measurement yielded data on thickness of the samples, absorption coefficients needed for evaluation of SPV spectra were obtained from transmittance and reflectance measurements. Fitting of theory to experiment yielded the exciton diffusion lengths (13-15 nm). CdS nanoparticles were synthesized by thermal decomposition of cadmiumethylxanthate in triethanolamine or from cadmium chloride and ammonium thiosulfate solutions. Sigma-Aldrich CdS nanocrystals were also studied. TEM and SPV techniques were applied to characterize the size of the nanoparticles.

26th European Photovoltaic Solar Energy Conference and Exhibition; 2325-2328, Design and properties of the front transparent conductive oxide (TCO) are decisive for high efficiency of amorphous Si and tandem amorphous/microcrystalline Si (Micromorph) solar cells. In our previous work we described a new, 3 dimensional design of thin film silicon solar cells based on ZnO nanocolumns or nano- or micro-holes etched into ZnO layer (”Swiss cheese” design). Very thin amorphous and microcrystalline layers are then folded over ZnO nanocolumns or into the holes in ZnO. Here we have demonstrated results of hydrothermal growth of ordered ZnO nanocolumns over the area of 0.5 cm2. Electron beam lithography was used to define hexagonally ordered arrays of nanocolumns. For the “Swiss cheese” design of Micromorph solar cell, already successfully realized in our past work, we performed rigorous optical modeling with the help of FDTD (Finite Difference in the Time Domain) method to show the large increase of short circuit current with increased depth of the micro-holes in ZnO layer.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 2763-2766, At the 24th European PV SEC in Hamburg we presented a new, nanostructured 3-dimensional “folded cell” design and demonstrated the first experimental results of the amorphous silicon structure deposited over such ZnO columnar substrates. Now we present a “bottom up” approach for deposition of ZnO nanocolumns by direct low temperature hydrothermal synthesis, together with the results of complementary design with nano- or micro-holes etched into the ZnO (“Swiss cheese” design), which could be more easily up-scaled to much larger areas. Electron beam lithography was used for defining the openings in resist for selective ZnO nanocolumns growth in the first case, optical lithography and reactive ion etching have been used to fabricate the holes into ZnO substrate in the later case. Requirements for the optical properties of ZnO are discussed and Monte Carlo modeling of these new, 3D solar cells, using already materials developed on the industrial scale, is presented. Calculations show that stable efficiency of amorphous silicon p-i-n solar cells in over 12% range is possible and for the Micromorph cells efficiency can reach over 15%, with the layer thickness of amorphous Si below 200 nm and microcrystalline Si around 500 nm. The first “proof of concept” Micromorph cell with the “Swiss cheese” design is presented.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 2286-2289, Increased optical absorption and full stability of thin film silicon solar cells, together with industrial requirements for short deposition time and simple process are the main goals of all thin film silicon solar cell developments. Stable panel efficiency should reach levels above 10%. Here we present briefly 5 ways how to increase optical absorption in thin film Si and suggest how to diminish the limitations due to Staebler-Wronski effect in amorphous Si. Then we in details describe the new 3-dimmensional design of amorphous silicon, microcrystalline silicon and micromorph tandem cells which represent the concept of “electrically thin” and “optically thick” solar cells. Details of experimental realization of specially nanostructured TCO superstrates are shown and the results of optical modeling and the first results on amorphous Si deposition over such superstrate are presented.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 2309-2312, Thin film silicon technology is one of the most promising branches of the growing photovoltaic industry. For its lower cost, roll-to-roll deposition process on sacrificial Al superstrate is being tested for use in large industrial production at Helianthos bv. Fourier Transform Photocurrent Spectroscopy (FTPS) is a modern method widely used for thin film silicon solar cell diagnostics. At the same time the optical modeling of light management in thin film solar cells is a key tool for structure optimization. Here we refer to a comprehensive study of a single a-Si:H or tandem a-Si:H/μc-Si:H solar cell comprising material quality assessment by FTPS, optical constants evaluation by PDS (Photothermal Deflection Spectroscopy) and T/R spectroscopy, quantum efficiency measurements by FTPS and optical simulations by our numerical model Cell.. This is not an easy task because the aluminum foil, that is later in the process etched off, complicates the FTPS and optical measurement. The result of our study is the assessment of optical parameters of all layers forming the cell and the calculation of limiting efficiencies for several proposed cell structures. For absorbers we can evaluate the electronic quality by the measurement of sub-bandgap absorption in a single solar cell and newly also in a tandem solar cell. We also refer to the possibility and to our experimental experience with such measurement performed on completed modules.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 574-577, Titanyl phthalocyanine (TiOPc) thin films were prepared using evaporation and surface polymerization by ion-assisted deposition (SPIAD) which causes dimerization between adjacent TiOPc molecules. All films were characterized by the surface photovoltage method using absorption coefficients evaluated from measurement of the optical transmission and reflection. The surface photovoltage method is applied here to these organic films, allowing determination of exciton diffusion length, which characterizes signal from the bulk. Drift lengths of the charge carriers in the space charge region (SCR) showed recombination, influencing the voltage from this depletion region. Typically, the thickness of the SCR was higher than that of the bulk and the diffusion length of excitons was ~15 nm. No differences in the surface photovoltage spectra or the exciton diffusion lengths were observed between the SPIAD and evaporated TiOPc films more than one year after the samples preparation. An aging process was indicated at the evaporated films.

Remarkably big increase of the substrate temperature during the low-temperature MBE growth of GaMnAs layers is observed by means of band gap spectroscopy. It is explained and simulated in terms of changes in the absorption/emission characteristics of the growing layer. Options for the temperature variation damping are discussed., 5 pages, 5 figures Submitted to Journal of Applied Physics