Advances in Science and Technology Vol. 74

Abstract: The peculiarities of photo-electric processes in thin film CdS/CdTe solar cells (SC) with different back electrodes (Cu/Au, ITO, Cu/ITO) have been studied. As it was established by capacitance – voltage (C – V) characteristics, the potential barrier heights for CdTe/Cu/Au and CdTe/ITO were 0.3 eV and 2.2 eV, respectively. The concentrations of charge carriers near back contact consisted 91020 m–3 and 21021 m–3, respectively. A high carrier concentration and high potential barrier of the ITO back contact caused the tunnel – recombination mechanism of the charge transport. The investigations of CdS/CdTe/ITO SC spectral photosensitivity testify a negative impact of the developed grain-boundary surface of the base layer on the processes of diffusion and separation of non-equilibrium current carriers generated by short-wave radiation. It is shown that the deposition of Cu nanolayer before the deposition of ITO films give stable efficiency 10 % for bifacial CdS/CdTe solar cells.

Abstract: An integrated cell and module technology based on metal-wrap-through (MWT) cells has been developed and demonstrated. 243 cm2 large and 160 µm thin multicrystalline silicon MWT cells were made with a best cell efficiency of 17.9%. From 36 cells with an average efficiency of 17.8% a full-size module was made with an efficiency of 17.0% (aperture area). The module was made using a conductive rear-side foil with conductive adhesive for the interconnection. The module was constructed using a dedicated module manufacturing line that is designed to be able to work with extremely thin cells and provide a high through-put of one 60 cell module per minute.

Abstract: A theoretical design analysis using numerical two dimensional computer aided design tool (i.e., TCAD) is presented for a-Si/c-Si based heterojunction (HJ) solar cells. A set of optical beam propagation models, complex refractive index models and defect models for a-Si material implemented (in-built) in the simulation software are first evaluated for single (SHJ) and double heterojunction (DHJ) devices. Assessment is further carried out by varying physical parameters of the layer structures such as doping, thickness of the c-Si and a-Si layers, defect density in the a-Si layer and bandgap discontinuity parameter. With varying bandgap discontinuity and using standard transport model in numerical device simulation, HJ solar cell performance is undervalued (η = 19.5%). This is the result of poor photogenerated carrier collection due to the presence of heterojunction at the respective n and p-contacts of the device. Implementing thermionic field emission tunneling model at the heterojunction, we obtained improved performance (η = 24 %) over large range of bandgap discontinuities. Keeping improved efficiency of HJ cell, implementing a step graded a-Si layer, further helps to widen the range of bandgap discontinuity parameter.

Abstract: Quantum size effect (QSE) comprises a novel phenomenon where structural, mechanical, thermal, electronic and optical properties of solids are affected by the reduction in particle size. Size-dependent properties in semiconductors and dielectrics come into play especially by making one (thin films) or more (quantum wires and dots) dimensions of a sample very small, particularly going from micro to nano-dimensions. One or more dimensional QSE is accompanied with an increase of the light absorption and the blue-shift of the optical band-gap due to a creation or reduction of the crystallite sizes having different dielectric constants (relative permitivity). Understanding the nature of the size-induced properties is of fundamental importance for advanced technological applications. The size-dependent band-gap makes a material attractive for optical absorption-based applications. In this work, a-Si:H films different in thickness were prepared by PECVD technology using different hydrogen-diluted silane. One-dimensional QSE (when film thickness decreased) and three-dimensional QSE (when due to a high hydrogen-silane dilution the nano-crystalline structure of the films appeared) were observed.

Abstract: Intermediate band (IB) solar cells aim to exploit in solar cells the energy of below bandgap energy photons. They are based in a material that, in addition to the conventional conduction and valence bands, has an electronic band (named intermediate band) located inside the bandgap and separated from the conduction and valence band by a null density of states. The theoretical limiting efficiency of these cells (63.2 % at maximum concentration) is equivalent to a triple junction solar cell but requiring a single material instead. Several approaches are being followed worldwide to take to practice this concept that can be divided into two categories: quantum dots and bulk materials. This paper reviews the main experimental results obtained under both approaches.

Abstract: First-principles calculations carried out for compounds based on Si implanted with different species, as Ti or chalcogens (S, Se, Te), show them as solid candidates to intermediate band (IB) photovoltaic materials. This DFT study predicts electronic structures, formation energies, relaxed atomic structures, optoelectronic properties, diffusion paths, for supercells containing up to several hundreds of atoms. The knowledge of Si-based devices is a relevant factor to facilitate the creation of an IB solar cell. Crystalline samples with a concentration of Ti several orders of magnitude above the solubility limit have been already grown. Formation energy calculations agree with the experiment in showing mainly interstitial implantation. Calculated electronic structure presents an IB, which is in agreement with electrical measurements and models, and is expected to cause an increase of the absorption coefficient across the solar spectrum. Chalcogen-implanted Si is an efficient IR absorber when implantation is carried out at ultra-high concentrations. Substitutional implantation produces a filled band inside Si band-gap and our calculations predict that plausible co-doping with IIIA atoms (as Al, B) would allow to obtain an IB fulfilling all the needed requirements.

Abstract: A tandem dye-sensitized solar cell consisting of two electrodes in one cell is reported. The tandem cell (Cell TAN GF) has a floating electrode (bottom cell) and a TiO2 electrode prepared on a F doped SnO2 glass (top cell). The floating electrode is a flexible and self-standing composite film consisting of a porous titania/dye layer supported by a glass mesh. The Incident Photon to Current Conversion Efficiency (IPCE) curve for the Cell TAN GF had two peaks corresponding to visible absorptions of the two dyes. The open circuit voltage (Voc) of the Cell TAN GF (0.82 V) was higher than that of the corresponding single cell (0.6-0.64 V). These results demonstrated that the Cell TAN GF has a potential for tandem cells. The Voc of Cell TAN GF was almost the same as that of Cell TAN (0.88 V) in which the glass mesh was replaced by a conductive stainless steel mesh having a protective layer, leading to the conclusion that a conductive layer is not necessarily needed.

Abstract: The short circuit current density (Jsc) of polymer solar cells is strictly related to the absorption of the blend film. Recently it has been shown that the use of [70]PCBM as electron acceptor can improve the current output of such devices because C70 derivatives have a stronger and broader absorption compared to C60 ones. The aim of this work is to study the influence of the fullerene on the optical behaviour of the photoactive blend film of a polymer solar cell. We have determined the optical constants of a P3HT:[70]PCBM blend film and studied their variation as a function of the annealing temperature. Afterward, we simulated the optical absorption of the active layer inside the device structure and calculated the maximum achievable Jsc with the aim to correlate the variation of the optical constants to the device output current. We compared this value with that one obtained using a P3HT:[60]PCBM blend.

Abstract: Most commercially available photovoltaic solar cells are crystalline silicon cells. However, in indoor environments, the efficiency of Si-cells is poor. Typically, the light intensity under artificial lighting conditions is less than 10 W/m² as compared to 100-1000 W/m² under outdoor conditions. Moreover, the spectrum is different from the outdoor solar spectrum and there is more diffuse than direct light. Taken into account the predicted cheaper costs for the production of organic solar cells, a possible niche market for organic PV can be indoor applications. In this article, we study the properties and suitability of several bulk heterojunction organic solar cells (with distinct different absorption spectra) for different indoor conditions. We simulate different light environments and use a silicon solar cell as reference. Depending on the required power for the indoor device, we determine minimum requirements for the environment (light intensity and indoor spectrum) and for the organic solar cell (absorption spectrum and surface area). In this way we determine the appropriateness and conditions for a competitive indoor use of organic solar cells.

Abstract: Quantum dots are proposed as luminescent species in luminescent solar concentrators in combination with thin film silicon solar cells. As both tuning absorption and emission properties of quantum dots is possible by adapting process conditions, as well as tuning the band gap of thin film silicon solar cells, an optimum combination is expected to exist for which the conversion efficiency of the whole device is maximum. As a first step we have employed ray-tracing modeling to determine the efficiency of a luminescent concentrator using several quantum dots and heteronanocrystals with varying Stokes’ shift and absorption cross sections. A maximum efficiency of 5.9% is found for so-called Type II heteronanocrystals.