Jef Poortmans

Thin film solar cells : fabrication, characterization and applications

Series Preface. Preface. 1. Epitaxial thin-film crystalline Si solar cells on low-cost Si carriers (Jef Poortmans). 2.Crystalline Silicon Thin-Film Solar Cells on Foreign Substrates by High-Temperature Deposition and Recrystallization (Stefan Reber and Thomas Kieliba). 3. Thin-film polycrystalline Si solar cells (Guy Beaucarne and Abdellilah Slaoui). 4. Advances in microcrystalline silicon solar cell technologies (Evelyne Vallat-Sauvain, Arvind Shah and Julien Bailat). 5. Advanced Amorphous Silicon Solar Cell Technologies (Miro Zeman). 6. Chalcopyrite Based Solar Cells (Martha Ch. Lux-Steiner). 7. CdTe Thin Film Solar Cells: Characterization, Fabrication and Modelling (Marc Burgelman). 8.Charge carrier photogeneration in doped and blended organicSemiconductors (V. I. Arkhipov and H. Bassler). 9. Nanocrystalline Injection Solar Cells (Michael Gratzel). 10. Charge Transport and Recombination in Donor-Acceptor Bulk Heterojunction Solar Cells (A. J. Mozer and N. S. Sariciftci). 11. The Terawatt Challenge for Thin Film PV (Ken Zweibel).

Pinhole-free perovskite films for efficient solar modules

We report on a perovskite solar module with an aperture area of 4 cm2 and geometrical fill factor of 91%. The module exhibits an aperture area power conversion efficiency (PCE) of 13.6% from a current–voltage scan and 12.6% after 5 min of maximum power point tracking. High PCE originates in pinhole-free perovskite films made with a precursor combination of Pb(CH3CO2)2·3H2O, PbCl2, and CH3NH3I.

Trap density in conducting organic semiconductors determined from temperature dependence of J−V characteristics

Space charge limited currents in organic semiconductors are frequently observed to obey the power law J−Vm and are attributed to an exponential distribution of traps having two parameters, namely the characteristic distribution energy Et and the trap concentration Ht. We determine these parameters from the J(V) characteristics at two or more temperatures reported in literature.

Advanced manufacturing concepts for crystalline silicon solar cells

An overview is given concerning current industrial technologies, near future improvements and medium term developments in the field of industrially implementable crystalline silicon solar cell fabrication. The paper proves that considerable improvements are still possible, both in efficiency and in production cost. The paper also proves that a lot of effort is being put worldwide on thinner substrates and on thin-film crystalline silicon cells deposited on cheap carriers, in order to save in substrate cost and in order to gain more independence from availability problems of silicon feedback.

Extraction of bulk and contact components of the series resistance in organic bulk donor-acceptor-heterojunctions

Abstract Basic solar cell characteristics are examined in bulk donor-acceptor-heterojunction devices. Therefore, spin-coated organic blends of poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene-vinylene) (MDMO-PPV) and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) are used as active material sandwiched between a transparent IndiumTinOxide (ITO)-electrode and an Al backside contact. A comparison is made between cells with or without an extra interfacial layer of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) on top of the ITO-electrode. Furthermore, the effect of the thickness of the active layer on the photovoltaic performance of the devices is studied. It is seen that applying this extra PEDOT/PSS layer results in an important increase of the contact component of the series resistance of the cells. At the same time, open circuit voltage improved for devices with an interfacial layer while the fill factor was higher for cells with no PEDOT/PSS film. For thinner active layers, in both cases the bulk component of the series resistance decreased. Nevertheless, a decrease of short circuit current is seen at reduced illumination. Shunt resistance shows a slight increase resulting in an improvement of open circuit voltage. Also, the fill factor increases for thinner active layers. For the thinnest standard devices a power conversion efficiency of over 3% could be measured under AM1.5 conditions.

Self-Standing Porous Silicon Films by One-Step Anodizing

A novel technique for the formation and lift-off of thin porous silicon films from starting substrates in a single step by electrochemical etching in hydrofluoric acid-based solutions is described. Lift-off or separation of porous silicon (PS) film occurs under specific sets of current density and HF concentration, which also determines the PS film thickness that can vary from a few micrometers to a few tens of micrometers. A model based on a diffusion-limited mass transfer of HF molecules from the bulk of the solution to the point of reaction at the pore tip is proposed to explain the lift-off phenomena. Based on this an expression for expected PS film thickness and separation time as a function of current density and hydrofluoric acid concentration has been derived. The experimental results are in agreement with the proposed medel.

Porous silicon in crystalline silicon solar cells: A review and the effect on the internal quantum efficiency

Crystalline silicon (c-Si) is the dominant semiconductor material in use for terrestrial photovoltaic cells and a clear tendency towards thinner, active cell structures and simplified processing schemes is observable within contemporary c-Si photovoltaic research. The potential applications of porous silicon and related benefits are reviewed. Specific attention is given to the different porous silicon formation processes, the use of this porous material as anti-reflection coating in simplified processing schemes and for simple selective emitter processes and its light trapping and surface passivating capabilities, which are required for advantageous use in thin active cell structures. Our analysis of internal quantum efficiency data obtained on both conventional and thin-film c-Si solar cells has been performed with the aim of describing the light diffusing behaviour of porous Si as well as investigating the surface passivating capabilities. An effective entrance angle of 60° is derived, which corresponds to totally diffuse isotropic light, and the importance of a correction for absorption losses in the porous layer is illustrated. Furthermore, photoconductivity decay measurements of freshly etched porous Si on float-zone p-type Si indicate a strong bias-light dependency and a fast degradation of the surface recombination velocity.

Overview of solar cell technologies and results on high efficiency multicrystalline silicon substrates

Fabrication technologies for multicrystalline silicon (mc-Si) solar cells have advanced in recent years with efficiencies of mc-Si cells exceeding 18%. Intense efforts have been made at laboratory level to improve process technology, growth methods, and material improvement techniques to deliver better devices at lower cost. Deeper understanding of the physics and optics of the device led to improved device design. This provided a fruitful feedback to the industrial sector. Both screenprinting and buried-contact technologies yield cells of high performance. An increasingly large amount of research activity is also focussed on the fabrication of thin solar cells on cheap substrates such as glass, ceramic, or low quality silicon. Success of these efforts is expected to lead to high efficiency devices at much lower costs. Efforts are also being put on low thermal budget processing of solar cells based on rapid thermal annealing.

Injection- and space charge limited-currents in doped conducting organic materials

Most conducting organic materials have a background p-type doping varying in the range 1015–1017 cm−3. We report results of a theoretical and experimental study of carrier transport in p-doped organic Schottky diodes. The theory given in this article shows that in a doped organic material with ohmic contacts the current is ohmic at low voltages. If the ohmic contact at the cathode is replaced by an Al Schottky contact the current varies exponentially with the applied voltage V. The current changes to space charge limited current (SCLC) at high voltages. The voltage at which the change takes place depends on the doping concentrations. In the SCLC regime the current varies according to the well-known V2 law if there are no traps and the mobility is independent of the electric field. If either trapping or effect of field on mobility is important, the current varies as Vm, where m>2. We have investigated experimentally the I–V characteristics of Schottky diodes fabricated using the PPV-based oligomer 2,5-di-n...

Trap filled limit of conducting organic materials

According to the approximate theory of transport in a conducting organic material containing exponential traps, ln J versus ln V plots are straight lines with slope l=TC/T, where TC is the characteristic temperature of the trap distribution. It is assumed in this theory that the concentration pt of trapped holes is much larger than the concentration p of free holes. Our experiments and recent literature results show that at high applied voltages the observed ln J versus ln V plots deviate from the straight lines and bend down. The numerical solution presented in this article shows that at high voltages the contribution of p to the space charge does not remain negligible. Calculated ln J versus ln V plots do bend down consistent with our experimental results. The current approaches the trap-filled limit asymptotically as the applied voltage approaches infinity.