Mapping the performance of PV modules, effects of module type and data averaging

A method is presented for estimating the energy yield of photovoltaic (PV) modules at arbitrary locations in a large geographical area. The method applies a mathematical model for the energy performance of PV modules as a function of in-plane irradiance and module temperature and combines this with solar irradiation estimates from satellite data and ambient temperature values from ground station measurements. The method is applied to three different PV technologies: crystalline silicon, CuInSe₂ and CdTe based thin-film technology in order to map their performance in fixed installations across most of Europe and to identify and quantify regional performance factors. It is found that there is a clear technology dependence of the geographical variation in PV performance. It is also shown that using long-term average values of irradiance and temperature leads to a systematic positive bias in the results of up to 3%. It is suggested to use joint probability density functions of temperature and irradiance to overcome this bias.     

Electro-optical characterization and modeling of thin film CdS–CdTe heterojunction solar cells

Optoelectronic characteristics of thin film CdTe–CdS solar cells fabricated at four different laboratories were measured and analyzed. Current versus voltage measurements revealed that, under one sun illumination, tunneling was the dominant current flow mechanism in all cells. Tunneling was also the dominant current flow mechanism in the dark for all types except P3 which exhibited a generation-recombination type current flow process in the dark. A theoretical model involving bulk traps in CdTe and a charged thin layer (T-layer) near the junction under forward bias and/or illumination was developed. The model is able to explain all significant features in the experimental results obtained from current versus voltage, and capacitance.     

Light and voltage dependence of the junction transport properties of CdTe/CdS photovoltaics

The J–V curve of CdTe/CdS photovoltaics does not consist of a simple superposition of a loss current and a light generated current with a considerable loss in conversion efficiency. This paper uses capacitance/voltage measurements and J–V measurements at a variety of temperatures and light levels to develop a model for this non-superposition. It was found that a light dependent tunneling mechanism dominates at low voltages. Moreover, the tunneling takes place from a trap level within the CdTe.     

Synthesis and characterization of CuInSe2 thin films from Cu, In and Se stacked layers using a closed graphite box

Various techniques have been used to produce CuInSe₂ but the problem of producing films with the desired properties for efficient device fabrication over large areas has always persisted. The Stacked Elemental Layer (SEL) technique has been demonstrated as a method for producing films over a large area, but the films normally annealed in vacuum or in Se ambient, mostly exhibited poor morphology with small grain sizes which result in poor devices. A method of synthesizing CuInSe₂ films by annealing or selenization of the Cu, In and Se elemental layers using a closed graphite box was developed. SEM, EDX, XRD, spectrophotometric and Hall measurements were used to characterize all annealed films. Results have shown single phase chalcopyrite films with improved crystal sizes of about 4 μm The film composition varied from Cu-rich to In-rich with electrical resistivities of 10⁻³ to 10⁴ Ωcm, cattier concentrations of 5 × 10¹⁵ to 10¹⁷ cm⁻³ and mobilities of 0.6 to 7.8 cm² V⁻¹ s⁻¹ An energy band gap of 0.99 eV and 1.02 eV was obtained for a Cu-rich and near stoichiometric In-rich films respectively. Heterojunction devices using the structure ZnO/CdS/CuInSe₂ were fabricated with electrical conversion efficiencies of 6.5%.     

Recent developments in high eficiency photovoltaic cells

Enormous progress has been made in recent years on a number of photovoltaic materials and devices in terms of conversion efficiencies. Efficiencies in the range of 18%–24% have been achieved in traditional silicon-based devices fabricated from both multicrystalline and single-crystal materials. Ultrahigh-efficiency (>30%) photovoltaic (PV) cells have been fabricated from gallium arsenide (GaAs) and its ternary alloys like gallium indium phosphide (GaInP₂). The high-efficiency GaAs-based solar cells are being produced on a commercial scale, particularly for space applications. Major advances in efficiency have also been made on various thin-film solar cells based on amorphous silicon (aSi:H), copper gallium indium diselenide (CIGS), and cadmium telluride materials. This paper gives a brief overview of the recent progress in PV cell efficiencies based on these materials and devices.     

Thin-film solar cells: An overview

Thin-film solar cells offer the most promising options for substantially reducing the cost of photovoltaic systems. A multiplicity of options, in terms of materials and devices, are currently being developed worldwide. Some of the leading contenders are: amorphous and polycrystalline silicon, compound semiconductor thin films such as CuInSe₂-based alloys, and CdTe thin-film solar cells. Enormous progress in device performance has been made in most of these technologies, and considerable effort is devoted to commercialization of these technologies. Exciting new developments are happening in some relatively new materials and devices.     

Formation of CuIn(S,Se)2 thin film by thermal diffusion of sulfur and selenium vapours into Cu–In alloy within a closed graphite container

The formation of CuIn(S,Se)₂ thin films by thermal diffusion of sulfur (S) and selenium (Se) vapours into co-sputtered Cu–In alloy within a closed-space graphite container is reported. All films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), Four-point-probe and hot-probe measurements. Cu–In alloy films with composition varying from Cu-rich to In-rich were deposited. The synthesized In-rich films yielded CuIn₅(S,Se)₈ spinel compound which gradually transformed into a single phase CuIn(S,Se)₂ as the film composition approached the Cu-rich region. The morphology of the CuIn₅(S,Se)₈ was found to differ from the stoichiometric and Cu-rich CuIn(S,Se)₂ as observed from SEM. EDX composition analysis of the films showed a Cu/In ratio varying from 0.36 to 1.54 and a (S+Se)/(Cu+In) varying from 0.97 to 1.32. The amount of S incorporated in the films was found to differ with changes in the composition. The resistivity of the films ranged between 10⁻¹ and 10⁷ Ω cm and it strongly followed the change in the alloy film composition.     

Thin film silicon photovoltaics: Architectural perspectives and technological issues

Thin film photovoltaics is a particularly attractive technology for building integration. In this paper, we present our analysis on architectural issues and technological developments of thin film silicon photovoltaics. In particular, we focus on our activities related to transparent and conductive oxide (TCO) and thin film amorphous and microcrystalline silicon solar cells. The research on TCO films is mainly dedicated to large-area deposition of zinc oxide (ZnO) by low pressure-metallorganic chemical vapor deposition. ZnO material, with a low sheet resistance (<8 Ω/sq) and with an excellent transmittance (>82%) in the whole wavelength range of photovoltaic interest, has been obtained. “Micromorph” tandem devices, consisting of an amorphous silicon top cell and a microcrystalline silicon bottom cell, are fabricated by using the very high frequency plasma enhanced chemical vapor deposition technique. An initial efficiency of 11.1% (>10% stabilized) has been obtained.     

pin double-heterojunction thin-film solar cell p-layer assessment

The simplest realization of a pin double-heterojunction thin-film solar cell would consist of a lightly doped, moderate-bandgap absorber i-layer; a heavily doped, wide-bandgap n-layer window (cathode); and a heavily doped, wide-bandgap p-layer window (anode) in which the anode and cathode are electrically contacted by at least one transparent conductor. The focus herein is on p-layer interfacial assessment, which is accomplished using modern Schottky barrier and heterojunction theory and is directed to the analysis of p-windows for copper indium gallium diselenide (CIGS) and cadmium telluride (CdTe) thin-film solar cells. A p-type window layer serves as an electron reflector and also aids in the formation of an ohmic anode contact. Ohmic anode contacts are particularly difficult to form in CIGS and CdTe thin-film solar cells since these materials have very large ionization potentials, i.e., IP_S=5.65 (CIGS) and 5.78 V (CdTe) and significant interfacial screening, characterized by extremely small Schottky barrier interface parameters, i.e., S=0.14 (CIGS) and 0.21 (CdTe). An ideal p-type window material would be heavily doped, p-type, and would have a wide bandgap, a large ionization potential, and a smaller charge neutrality level energy than that of the absorber layer.     

Calculation of CIS and CdTe module efficiencies

An accurate and fast method to calculate the efficiency of Cu(In,Ga)Se₂ (CIGS) and CdTe thin-film solar modules is presented here. This comprises a new method to calculate the fill factor as a function of discrete and distributed series resistance, and of shunt conductance: a three-dimensional, third-order polynomial approximation is presented, and the expansion of the coefficients as a power series of 1/V_oc is given. Analytical expressions are presented which fit experimental data of the optical absorption in ZnO as a function of its thickness or sheet resistance. Together with a calculation outline of the series and shunt effects of the module integration, this constitutes a practical module design tool. This is illustrated with results of dependence of module efficiency on cell length, window and absorber sheet resistance, interconnect contact resistance, “softness” of the cell I–V curve, and absorber material (CIGS or CdTe). Optimal or critical values for these parameters are given.     

Sprayed CuInS2 thin films for solar cells: The effect of solution composition and post-deposition treatments

CuInS₂ thin films were prepared by spray pyrolysis from solutions with different compositions. Etching in KCN solution and thermal treatments in vacuum and hydrogen were applied to as-deposited films. KCN etching removes conductive copper sulfide from the surface of Cu-rich films but has no effect on matrix composition. Vacuum annealing at 500°C and hydrogen treatment at 400–500°C purifies the films, prepared from the solutions with the Cu/In=1, from secondary phases, reduces chlorine content and improves crystallinity. Vacuum annealing results in n-type films due to the formation of In₂O₃ phase. Treatment in hydrogen reduces oxygen-containing residues and results in p-type CuInS₂ films with resistivity close to 10 Ω cm.     

Light soaking effect in a-Si:H based n–i–p and p–i–n solar cells

Hydrogenated amorphous silicon solar cells have been realised in both a p–i–n configuration on a Corning glass substrate as well as in a n–i–p configuration on stainless-steel substrate. The performance degradation of the two kinds of cell under solar illumination has been examined for a 140 h period. During degradation, the two devices were kept under load in the maximum power condition that is normally used in a solar plant. The performance of the Corning glass deposited device exhibited a higher rate of degradation with respect to the other cell. A discussion on the possible reasons for this behaviour is given.     

Thin film silicon solar cells for space applications: Study of proton irradiation and thermal annealing effects on the characteristics of solar cells and individual layers

The paper reports on the effects of a proton irradiation campaign on a series of thin-film silicon solar cells (single- and double-junction). The effect of subsequent thermal annealing on solar cells degraded by proton irradiation is investigated. A low-temperature annealing behaviour can be observed (at temperatures around 100 to 160°C) for microcrystalline silicon solar cells. To further explore this effect, a second proton irradiation campaign has been carried out, but this time on microcrystalline silicon layers. The effect of proton irradiation and subsequent thermal annealing on the optical and electronic properties of microcrystalline silicon is, thus, thoroughly investigated.     

Thin film nanocrystalline Ba0.5Sr0.5Co0.8Fe0.2O3: Synthesis, conductivity, and micro-solid oxide fuel cells

We report on single phase polycrystalline Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃ thin films grown on (1 0 0) (Y₂O₃)_0.08(ZrO₂)_0.92 substrates by radio frequency sputtering. Detailed studies on electrical conductivity as a function of temperature up to 500 °C are carried out for films in the 20-100 nm thickness range. Free-standing thin film micro-solid oxide fuel cells utilizing nanostructured Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃ cathodes are fabricated and tested for the first time. A maximum power density of 35 mW cm⁻² at 520 °C was obtained with Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃/(Y₂O₃)_0.08(ZrO₂)_0.92/Pt micro-solid oxide fuel cells. These results indicate the significance of microstructure on electrical properties of Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃ and present the first successful thin film micro-solid oxide fuel cells integrating Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃. We anticipate these results to be of relevance in advancing micro-solid oxide fuel cells for reduced temperature operation with dense oxide cathodes.