Advanced photon-harvesting concepts for low-energy gap organic solar cells

Organic semiconductors usually show high absorption coefficients only over a limited spectral range. This has to be taken into account when designing organic solar cells with low-energy gap absorbers, as it is likely that these cells will have reduced harvesting abilities for photons of higher energy. We will present two routes to tackle this problem via luminescence and energy transfer and outline three novel concepts of how to integrate these mechanisms. We use an efficient low-energy gap organic solar cell based on zinc-phthalocyanine and fullerene C₆₀ as a model system.     

Efficiency enhancement of solar cells by luminescent up-conversion of sunlight

Significant improvements in the efficiency of solar cells by combination with luminescent up- or down-converters have recently been predicted theoretically. Here, we extend the theoretical analysis of the limiting efficiency of the up-conversion (UC)-system to realistic Airmass spectra and analyse the spectral robustness of the UC-system. We also present initial experimental results from prototypes involving bifacial silicon solar cells with UC-phosphors attached to the rear surface, and discuss the possibility of realizing efficient UC with low-band-gap solar cells in combination with a light emitting diode.     

Improved soldering technique for concentrator solar cells

For reliable concentrator module assembly and to ensure consistent operation in the field, good adhesion between the cell metallization and the underlying cell material is essential. The adhesion requirement is several times more stringent than that specified for non-concentrating assemblies. Pull tests are generally used to measure this adhesion. A new technique for soldering cell interconnects is described which dramatically improves the results obtained during such pull testing.     

An improved solar cell circuit model for organic solar cells

The validity of conventional circuit model for interpreting results obtained using organic solar cells is examined. It is shown that the central assumption in the model that photo-generated current remains constant from short-circuit to open-circuit condition may not hold for organic cells. An improved model based on the photovoltaic response of organic solar cells is proposed and a method of extracting the parameters of the model is presented.     

Improved anisotropic etching process for industrial texturing of silicon solar cells

An experimental study on the surface texturisation of monocrystalline wafers with solutions containing sodium-hydroxide and isopropanol was carried out. Both the composition and the temperature of the etching solution were optimised on the basis of etch-rate measurements on silicon samples having different crystallographic directions. It was found that the density and the size of the pyramids are influenced by the etch-rate of silicon in the 1 0 0 direction and also by the anisotropy factor of the solutions being the quotient of the etch rate in the 1 0 0 to 1 1 1 directions. Design of experiments and response surface methods were used to extract the etch rate as a function of different input parameters, such as the sodium hydroxide and isopropanol concentrations and the temperatures of the solutions. Optimum texturing conditions were found at a temperature of 80°C and a composition which causes a relatively high etch rate in the 1 0 0 direction with an anisotropy factor of 10. At the starting point of the etching process, the inhomogeneity of pyramid nucleation can be avoided by mixing an additive to the texturing solution. With such a solution, the pyramid size can be tuned by varying the etching time in order to obtain a low reflectivity from the textured silicon wafers. Based on our results the texturing process could be stabilised with respect to the reproducibility on a large scale of wafers.     

Harnessing of low-energy IR photons via oxygen-vacancies in SrTiO3 nanocrystals for photocatalytic hydrogen production

The surface-oxygen vacancies were generated to enhance the near-IR light response of the SrTiO₃ photocatalyst by heating SrTiO₃ in a highly reducing environment. The UV–visible diffuse reflectance spectra of SrTiO₃-Ov revealed strong absorption between 400 and 800 nm, while IPCE response was extended beyond 800 nm, confirming the near-IR light response of SrTiO₃-Ov. The hydrogen production rates for SrTiO₃-Ov at pH 3.0 under full solar spectrum, visible and IR radiation were 100, 25 and 10 μlg⁻¹h⁻¹, respectively. The respective normalized hydrogen with respect to light intensity were found to be 100, 50 and 33 μlg⁻¹h⁻¹ while hydrogen production of pristine SrTiO₃ was less than 2 μlg⁻¹h⁻¹ under full solar spectrum. The mid-gap O_v and Ti³⁺ states in SrTiO₃-Ov are key factors in the origin of the near-IR responses of the catalyst and the feasibility of using low-photonic-energy IR light to produce hydrogen by water splitting was demonstrated.     

Development of a suitable model for characterizing photovoltaic arrays with shaded solar cells

The aim of this study is to investigate the effects of non-uniform solar irradiation distribution on energy output of different interconnected configurations in photovoltaic (PV) arrays. In order to find which configuration is less susceptible to mismatch effects, a PV module model is developed. This model can take into consideration the effects of bypass diodes and the variation of the equivalent circuit parameters with respect to operating conditions. The proposed model can provide sufficient degree of precision as well as solar cell-based analysis in analyzing large scale PV arrays without increasing the computational effort. In order to produce more reliable and robust simulations, improved and extended algorithms are presented. Some results are discussed in detail and some recommendations are extracted by testing several shading scenarios.     

Multi-region uniform-field model of amorphous-silicon solar cells

The field in the i-layer of an amorphous-silicon solar cell may be assumed as uniform. We formulate the respective cell model without further approximations in terms of a “regional method”. The i-layer is subdivided for that purpose in many regions, and the regional solutions are chained together. Some exemplary simulations are presented. The model is in particular suited to demonstrate snake-like low-forward-voltage photocharacteristics, to be induced by near-surface excess carrier generation, as with blue illumination. Observed and simulated “blue-snake” characteristics are compared with each other. A snake indicates the transition from a bulk-recombination to a surface-recombination limited current regime.     

A model to determine financial indicators for organic solar cells

Organic solar cells are an emerging photovoltaic technology that is inexpensive and easy to manufacture, despite low efficiency and stability. A model, named TEEOS (Technical and Economic Evaluator for Organic Solar), is presented that evaluates organic solar cells for various solar energy applications in different geographic locations, in terms of two financial indicators, payback period and net present value (NPV). TEEOS uses SMARTS2 software to estimate broadband (280-4000 nm) spectral irradiance data and with the use of a cloud modification factor, predicts hourly irradiation in the absence of actual broadband irradiance data, which is scarce for most urban locations. By using the avoided cost of electricity, annual savings are calculated which produce the financial indicators. It is hoped that these financial indicators can help guide certain technical decisions regarding the direction of research for organic solar cells, for example, increasing efficiency or increasing the absorptive wavelength range. A sample calculation using solar hats is shown to be uneconomical, but a good example of large-scale organic PV production.     

Fluctuation model for p–n heterojunction solar cells

Influence of the roughness (microrelief) of an active interface in p–n junction solar cells (SC) on the photovoltage (the open-circuit voltage V_oc) has been studied. Nonuniformity of contact potential difference between p- and n-regions leads to barrier height fluctuation that are exponentially enhanced when dealing with barrier current. This results in some decrease of the V_oc value. Three theoretical models of averaging open-circuit voltage were used. Experimental results on p⁺-Al_xGa_1−xAs/p⁺-n-GaAs heterostructure SC with various microrelief, obtained by the anisotropic chemical etching, are compared with theoretical calculations.     

Plated contacts for solar cells with superior adhesion strength to screen printed solar cells

The improvement of adhesion strength and durability of plated contacts is required for cell manufacturers to gain confidence for large-scale manufacturing. To overcome weak adhesion at the metal/Si interface, new approaches were developed. These involve the formation of laser-ablated anchor points, or grooves in the extreme case of overlapping anchor points, in the heavily doped silicon surface. When plated, these features greatly strengthen the mechanical adhesion strength of the metal. A stylus-based adhesion tester was developed specifically for evaluating the effectiveness of plated contacts to smooth silicon surfaces. The use of such a tester was also extended in this work to textured and roughened surfaces to allow evaluation of different metal contacting approaches. The adhesion strengths for various metal contacting schemes were evaluated, including screenprinted silver contacts, nickel/copper (Ni/Cu) lightinduced plated (LIP) contacts for laser-doped selective emitter (LDSE) cells, buried-contact solar cells (BCSCs), and Ni/Cu LIP contacts formed with laser-ablated anchoring points in selective emitter (LAASE) cells. The latter has superior adhesion strength. The standard “peel test” of the industry was compared to the stylus-based adhesion testing, with the latter shown value for testing metal contacts on smooth surfaces but with caution needed for use with textured or roughened surfaces.     

Improved differential control for solar heating systems

One possibility to exploit solar energy better is the efficiency enhancement of the control of solar thermal heating systems. In this paper an improved differential control and the generally used ordinary differential control operating with fixed switch-on and switch-off temperature differences are compared in different efficiency viewpoints. The comparison is based on measured data of a particular system at the Szent István University, Gödöll? and on a TRNSYS model developed for solar heating systems. According to the results the improved control provides a higher value of utilizability and brings forth fewer switch-ons and switch-offs for the pumps. These advantages nevertheless result in extended operation time and thus extended parasitic consumption of the pumps. This drawback can, however, be moderated or even extinguished by modern pumps with low energy consumption or if supplied by renewable energy source. Comparing the amount of utilized solar energy and consumed parasitic energy increments, the improved control can be generally recommended.     

Improved photovoltaic method for measurement of minority carrier diffusion length applied to silicon solar cells

The photovoltage spectrum measured on back illuminated silicon solar cells of the (passivated emitor solar cell) (PESC) type without original bottom ohmic electrode is evaluated with the aim to find the diffusion length of minority carriers in bulk of the absorber (L). Two junctions, namely pn⁺ junction of the cell and that spontaneously created on the free surface generally exist in such samples. They give rise to two signals of opposite signs with one point of exact compensation. Six parameters (including L) are needed to characterize the spectrum. Special simple arrangement removes influence of spontaneously created junction on the free surface, which, in this way, reduces the number of parameters needed for fitting to three and enhances reliability of the measurement.     

Resource allocation model for planning R&D on solar cells

R&D planning is important for promoting R&D effectively and efficiently. The authors formulate planning of R&D on solar cells as an optimal resource allocation problem by using mathematical programming. The optimal resource allocation model based on this formulation includes technological progress submodels of solar cells and production and market submodels of solar cells. This model is expected not only to make clear the validity of R&D strategy on solar cells in the past, but also to give us the optimal plan of R&D on solar cells in the future.     

Improved photovoltaic energy output for cloudy conditions with a solar tracking system

This work describes measurements of the solar irradiance made during cloudy periods in order to improve the amount of solar energy captured during such periods. It is well-known that 2-axis tracking, in which solar modules are pointed at the sun, improves the overall capture of solar energy by a given area of modules by 30-50% versus modules with a fixed tilt. On sunny days the direct sunshine accounts for up to 90% of the total solar energy, with the other 10% from diffuse (scattered) solar energy. However, during overcast conditions nearly all of the solar irradiance is diffuse radiation that is isotropically-distributed over the whole sky. An analysis of our data shows that during overcast conditions, tilting a solar module or sensor away from the zenith reduces the irradiance relative to a horizontal configuration, in which the sensor or module is pointed toward the zenith (horizontal module tilt), and thus receives the highest amount of this isotropically-distributed sky radiation. This observation led to an improved tracking algorithm in which a solar array would track the sun during cloud-free periods using 2-axis tracking, when the solar disk is visible, but go to a horizontal configuration when the sky becomes overcast. During cloudy periods we show that a horizontal module orientation increases the solar energy capture by nearly 50% compared to 2-axis solar tracking during the same period. Improving the harvesting of solar energy on cloudy days is important to using solar energy on a daily basis for fueling fuel-cell electric vehicles or charging extended-range electric vehicles because it improves the energy capture on the days with the lowest hydrogen generation, which in turn reduces the system size and cost.     

Improving the performance of textured silicon solar cells through the field-effect passivation of aluminum oxide layers and up-conversion via multiple coatings with Er/Yb-doped phosphors

This paper examines the influence of field-effect passivation (from a coating of aluminum oxide) in conjunction with up-conversion (from multiple coatings containing Er/Yb-doped phosphors) on the performance of silicon solar cells. Note that the phosphors were applied to the rear surface of the cells. The surface morphology of the coatings was characterized by scanning electron microscopy and the chemical composition of Er/Yb-doped phosphors coating was examined using energy-dispersive X-ray spectroscopy. The fluorescence emissions of the coatings were examined using photoluminescence and optical image measurements. We examined the influence of field-effect passivation on dark current-voltage as well as photo-current density and external quantum efficiency (EQE). Improvements in photovoltaic performance after applying coatings containing Er/Yb-doped phosphors were estimated in terms of EQE and conversion efficiency. The field-effect passivation of Al₂O₃ and up-conversion provided by Er/Yb-doped phosphors resulted in EQE enhancements over a wavelength range of 600 to 1050 nm. Field-effect passivation was shown to enhance the conversion efficiency by 1.77% (from 16.91% to 17.21%), up-conversion enhanced conversion efficiency by 2.9% (from 17.21% to 17.71%), and a combination of field-effect passivation and up-conversion enhanced conversion efficiency by 4.73% (from 16.91% to 17.71%).     

Improved performance of MEH-PPV/ZnO solar cells by addition of lithium salt

The paper reports the improved performance by addition of lithium bis(trifluoromethanesulfonyl)amide (LiTFSI) to poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) in the hybrid solar cells consisting of MEH-PPV as an electron donor and vertically aligned ZnO nanorod array as an electron acceptor. Results show that, with increasing the weight ratio R of LiTFSI/MEH-PPV, the charge transfer efficiency at MEH-PPV/ZnO interface, the device short circuit current (J_sc) and open circuit voltage (V_oc) get increased for R ? 2/10, but decreased when R > 2/10, resulting in a peak power conversion efficiency of η = 0.48% for R = 2/10 at AM 1.5 illumination (100 mW/cm²). It is revealed that the increased J_sc is due to the improved charge transfer between the MEH-PPV and ZnO as a result of the interaction between LiTFSI and MEH-PPV, while the increased V_oc and the decreased charge recombination are attributed to the increased hole mobility of MEH-PPV; moreover, the decreased J_sc and V_oc at high R values are attributed to the morphology degradation in the active layer due to the high LiTFSI content.     

Improved photoconversion from MoSe2 based PEC solar cells

MoSe₂, a group VI TMDC, has been found to be highly stable against photocorrosion due to d→d transitions and has a band gap of 1.4 eV.Thus, it possesses high potential towards photoelectronic applications. As grown, MoSe₂ based photoelectrochemical (PEC) solar cells are generally found to show low conversion efficiency. It has been shown here that cleaving and controlled chemical and thermal processing can lead to viable levels of photoconversion with high stability. The observed improvement in the behaviour of such solar cells has been attributed to improvements in various parameters like series resistance, minority carriers, diffusion lengths etc.     

Calibration of solar simulator for evaluation of dye-sensitized solar cells

The photo-to-electricity energy conversion efficiencies of ruthenium-dye-sensitized solar cells (DSC) are measured under a solar simulator. The error in conversion efficiencies was compared under a variety of spectral conditions. Measurements of the conversion efficiencies of DSC between a solar simulator and outdoor sunlight result in about 10% error. This error was seen when the spectral intensity of a xenon-lamp solar simulator (imitating an air mass (AM) 1.5 spectrum) was adjusted by the short-circuit photocurrent I_Si of a crystalline silicon (c-Si) standard cell. In order to adjust the energetic intensity of AM 1.5 for DSC that has a spectrum response only in the visible region light, the c-Si reference cell is modified with a glass UV filter (KG-5, Schott) and the solar simulator was adjusted by I_{IR-cut Si}. The energetic spectrum of the solar simulator has a good accuracy over the wavelength range 300–750 nm, giving the conversion efficiency of DSC an accuracy of about 2%. The dependency of the ratio of I_Si to I_{IR-cut Si} on natural sun power is discussed in view of scattering of the visible light under changing natural sun light.