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.     

Bulk and contact resistance in P3HT:PCBM heterojunction solar cells

In this paper, the series resistance of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction (BHJ) organic solar cells (OSC) has been studied. The series resistance of thermal annealed and un-annealed devices with different active layer thicknesses was measured. The series resistance of the organic solar cells consists of the bulk resistance of the active layer itself and the specific contact resistance between the active layer and the electrode. The bulk resistance and contact resistance were extracted from the measured series resistance using the vertical transmission line model (TLM) method. By fabricating solar cell devices with different active layer thicknesses, a relationship of the series resistance with thickness was established from which bulk and contact resistances were derived. We have also found that thermal annealing helps reduce both contact resistance and bulk resistance significantly; the contact resistance dropped by a factor of 2, while the bulk resistance decreased by a factor of 8. Results have shown that for an annealed P3HT:PCBM device that has an active layer thickness of 85 nm (optimum thickness for high efficiency), 17% of the total series resistance was due to the contact resistance, and bulk resistance contributed the rest 83%. The bulk resistance value for thermal annealed organic solar cell device with an active area of 0.1 cm² was found to be 150 Ω, and the measured specific contact resistance was 3.1 Ω cm². The measured bulk and contact resistance values are much higher as compared to the high efficiency silicon solar cells. Bulk resistance and contact resistance need to be further decreased in order to achieve higher organic solar cell efficiency.     

Gettering effects by aluminum upon the dark and illuminated I–V characteristics of N–P–P silicon solar cells

Impurity gettering is an essential process step in silicon solar cell technology. A widely used technique to enhance silicon solar cell performance is the deposition of an aluminum layer on the back surface of the cell, followed by a thermal annealing. The aluminum thermal treatment is typically done at temperatures around 600°C for short times (10–30 min). Seeking a new approach of aluminum annealing at the back of silicon solar cells, a systematic study about the effect the above process has on dark and illuminated I–V cell characteristics is reported in this paper. We report results on silicon solar cells where annealing of aluminum was done at two different temperatures (600°C and 800°C), and compare the results for cells with and without aluminum alloying. We have shown that annealing of the aluminum in forming gas at temperatures around 800°C causes improvement of the electrical cell characteristics. We have also made evident that for temperatures below 250 K, the predominant recombination process for our cells is trap-assisted carrier tunneling for both annealing temperatures, but it is less accentuated for cells with annealing of aluminum at 800°C. For temperatures above 250 K, the recombination proceeds through Shockley–Read–Hall trap levels, for cells annealed at both temperatures. Furthermore, it seems from DLTS measurements that there is gettering of iron impurities introduced during the fabrication processes. The transport of impurities from the bulk to the back surface (alloyed with aluminum) reduces the dark current and increases the effective diffusion length as determined from dark I–V characteristics and from spectral response measurements, respectively. All these effects cause a global efficiency improvement for cells where aluminum is annealed at 800°C as compared to conventional cells where the annealing was made at 600°C.     

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.     

Thickness dependence of microcrystalline silicon solar cell properties

This paper addresses the performance of pin and nip solar cells with microcrystalline silicon (μc-Si:H) absorber layers of different thickness. Despite the reverse deposition sequence, the behavior of both types of solar cells is found to be similar. Thicker absorber layers yield higher short-circuit currents, which can be fully attributed to an enhanced optical absorption. Open-circuit voltage V_OC and fill factor FF decrease with increasing thickness, showing limitations of the bulk material. As a result of these two contrary effects the efficiency η varies only weakly for absorber layers of 1 to 4 μm thickness, yielding maximum values up to 8.1 %. For a-Si:H/μc-Si:H stacked solar cells an initial efficiency of 12% has been obtained.     

Microstructures and photovoltaic properties of C60 based solar cells with copper oxides,CuInS2, phthalocyanines,porphyrin, PVK,nanodiamond, germanium and exciton diffusion blocking layers

Abstract

Organic solar cells have a potential for use in lightweight, flexible, inexpensive and large scale solar cells. However, significant improvements of photovoltaic efficiencies are mandatory for use in future solar power plants. One of the improvements is donor–acceptor proximity in the devices, which are called bulk heterojunction solar cells. Bulk heterojunction is an efficient method to generate free charge carriers, and the charge transfer is possible at the semiconductor interface. The purpose of the present work is to fabricate and characterise C₆₀ based solar cells with copper oxides, CuInS₂, phthalocyanines, porphyrin, poly-vinylcarbazole, nanodiamond, germanium and exciton diffusion blocking layers. In the present work, C₆₀ and fullerenol [C₆₀(OH)_10–12] were used for n-type semiconductors, and metal copper oxides, metal phthalocyanine derivative, porphyrin and poly-vinylcarbazole were used for p-type semiconductors. In addition, nanodiamond and germanium based molecules were added into the active layers of the solar cells. The novel aspect of the research is to investigate the relation between properties and microstructures of the solar cells using transmission electron microscopy, X-ray diffraction and electronic structure calculation. The impact of the research concerns the study of organic solar cells by means of microstructural analysis, property measurements and theoretical calculations.     

Longterm stability of efficient inverted P3HT:PCBM solar cells

We report on the longterm stability of 1.5-3% efficient inverted P3HT:PCBM solar cells sealed with glass plates, which have been glued onto the solar cells. We employed two different electron contact materials, titanium and chromium, and two different P3HT:PCBM layer thicknesses. By means of Auger electron spectroscopy we could show that the electron contact is partially oxidised during the processing of the inverted solar cells. It turns out that both, the electrode material and the thickness of the photoactive layer, have an influence on the longterm stability of such devices. After 1500 h of continuous illumination under a sulphur plasma lamp with a light intensity of ca. at the most stable devices maintained 90% of the initial efficiency and the most efficient cells still had more than 2.5% power conversion efficiency. These best results are obtained with chromium as electron contact material. The light dose corresponds to approximately 1.5 years exposure to sunlight and is therefore a promising result. To our knowledge, this is the first systematic study on the longterm stability of organic solar cells using the inverted layer sequence. Compared to state of the art devices with usual layer sequence our results suggest that inverted organic solar cells can be at least as stable.     

A compact multi-chamber setup for degradation and lifetime studies of organic solar cells

A controlled atmosphere setup designed for long-term degradation studies of organic solar cells under illumination is presented. The setup was designed with ease-of-use and compactness in mind and allows for multiple solar cells distributed on four glass substrates to be studied in four different chambers with temperature and atmosphere control. The four chambers are situated at close proximity in the setup thereby allowing the solar cells to be subjected to as uniform an illumination distribution as possible for the given solar simulator employed. The cell substrates serve as the front window and present a tight seal. Hence no illumination correction needs to be performed due to transmission and reflection losses as otherwise seen with test chambers employing a window as a seal. The solar cells in each chamber are continuously and individually electrically monitored under biased conditions by means of a computer controlled multiplexer and source meter. The dimensions of the setup allow it to pass through a mid-size load lock in most common glove box systems allowing for mounting of tested samples under inert conditions.As a demonstration of the applicability of the chamber design, a degradation study of standard P3HT:PCBM solar cells was performed under four different environmental conditions.     

Effects of paste storage on the properties of nanostructured thin films for the development of dye-sensitized solar cells

The effects of paste storage on the properties of nanostructured thin films were investigated in the present study. To this aim, dye-sensitized solar cells were fabricated using nanostructured TiO₂ thin films and an organic dye as a sensitizer. Aggregation of TiO₂ nanoparticles was observed when paste was stored until deposition affecting film's porosity and surface roughness factor and as a consequence, reducing the efficiency of the solar cells. On the other hand, fewer cracks were developed during the drying process of the film when stored paste was used for film deposition instead of freshly prepared paste. This is due to the presence of large pores on films with significant particle agglomeration, which enhance evaporation of acetyl acetone and water, thereby reducing cracks. The development of cracks on the film's surface results to a decrease in the efficiency of the cell, albeit a slight one. One of the main aims of the present study was also the investigation of different methods for developing counter electrodes, as the properties of the counter electrodes can affect the efficiency of the solar cells considerably. Counter electrodes were prepared by two different methods, namely by electrodeposition using an aqueous solution of H₂PtCl₆ (0.002 M) and by thermal decomposition of H₂PtCl₆ from isopropanol (5 mM). Electrodeposited counter electrodes were found to present several advantages over electrodes prepared by thermal decomposition, such as increased photocurrent, reduced sheet resistance, as well improved fill factor for the resulting solar cell.     

ITO-free low-cost organic solar cells with highly conductive poly(3,4 ethylenedioxythiophene): p-toluene sulfonate anodes

Indium tin oxide (ITO)-free organic solar cells were fabricated with highly conductive and transparent tosylate-doped poly(3,4-ethylenedioxythiophene: p-toluene sulfonate) (PEDOT:PTS) anodes of various thicknesses that were prepared by the vapor-phase oxidative polymerization of EDOT using Fe(PTS)₃ as an oxidant. Both solution-processable layers - PEDOT:PSS and photoactive P3HT:PCBM - were spin coated. The anodes transmittance and conductivity varied with thickness. Power conversion efficiency was maximized at 1.4%. The ITO-free organic solar cells photovoltaic characteristics are qualitatively compared with those of ITO-based organic solar cells to explore the possibility of replacing costly, vacuum-deposited ITO with highly conductive, patterned polymer films fabricated by inexpensive vapor-phase polymerization.     

Review on dye-sensitized solar cells (DSSCs): Fundamental concepts and novel materials

The prosperity of human society largely relies on safe energy supply, and fossil fuel has been serving as the most reliable energy source. However, as a non-renewable energy source, the exhaustion of fossil fuel is inevitable and imminent in this century. To address this problem, renewable energy especially solar energy has attracted much attention, because it directly converts solar energy into electrical power leaving no environment affect. In the past, various photovoltaic devices like organic, inorganic, and hybrid solar cells were fabricated in succession. In spite of high conversion rate of silicon based solar cells, the high module cost and complicated production process restricted their application solely to astronautic and aeronautic technology. For domestic and other commercial applications, research has been focused on organic solar cells for their inherent low module cost and easy fabrication. In addition, organic solar cells have their lightweight and flexibility advantage over conventional silicon-based crystalline solar cells. Among all the organic solar cells, dye-sensitized solar cells (DSSCs) are the most efficient and easily implemented technology. Here, this study examines the working principle, present development and future prospectus for this novel technology.     

On the use of Ga–In eutectic and halogen light source for testing P3HT–PCBM organic solar cells

In this paper, we report a simple and inexpensive method for testing the photovoltaic characteristics of organic devices. We report measurement data on P3HT–PCBM organic solar cells spin-coated from chloroform solutions, obtained using a Gallium–Indium eutectic top contact, and a halogen light source. We propose a method to correct the spectral mismatch of this light source with the solar spectrum. The effect of annealing and changing the P3HT:PCBM ratio are studied, and similar results as with evaporated aluminum are obtained. We conclude that Ga–In eutectic and halogen light source can conveniently be used instead of evaporated aluminum and a solar simulator for the study of polymer–fullerene bulk-heterojunction devices.     

Ultra-thin metal layer passivation of the transparent conductive anode in organic solar cells

Efficiency of organic solar cells shows a strong improvement when the transparent conductive anode (indium tin oxide—ITO, aluminium-doped zinc oxide—AZO, fluorine-doped tin oxide—FTO), is covered with an ultra-thin metallic film. It is shown that the best results are achieved with a gold film (0.5 nm). The efficiency of the solar cells using AZO or FTO is improved up to one order of magnitude, while in the case of ITO it is at least 50%. It is shown that if the matching between the work function of the anode and the highest occupied molecular orbital (HOMO) of the organic electron donor is the most important factor limiting the hole transfer efficiency, others factors such as transparent conductive oxide (TCO) surface roughness and adhesion of the organic layer are also key factors.     

Thermo-economic analysis of solar thermal power cycles assisted MED-VC (multi effect distillation-vapor compression) desalination processes

Solar power assisted different techniques of MED-VC (multi effect distillation-vapor compression) processes is thermo-economically analyzed and evaluated. In this work, two techniques of solar power cycles are considered to power on MED-PF-TVC, MVC (multi effect distillation thermal and mechanical vapor compressions). In the first technique, the developed solar thermal power is directly transmitted from the solar collector field via boiler heat exchanger unit toward the steam ejector of the MED-PF-TVC process. In the second technique, the electrical power generated from the SORC (Solar Organic Rankine Cycle) is used to power on the vapor compressor of the MED-PF-MVC process. The comparison is implemented according to the operation of PTC (parabolic trough collector) with Toluene organic oil and Water working fluids (2nd technique). Therminol-VP1 HTO (Heat Transfer Oil) is considered across the solar field and water is considered for boiler heat exchanger (1st technique). A case study is performed according to 4545 m³/day of distillate product. As a result, reducing the value of compression ratio with increasing the evaporator’s numbers would reduce the specific power consumption, solar field area, and thermo-economic costs. Also it is clear that the operation of steam ejector would increase the gain ratio instead of increasing the evaporator’s numbers.     

Performance enhancement of dye‐sensitized solar cells via cosensitization of ruthenizer Z907 and organic sensitizer SQ2

Cosensitization is a highly effective technique to enhance the photovoltaic performance of a dye‐sensitized solar cell. The main objective of this work is to improve the performance of dye‐sensitized solar cell using cosensitization approach and investigation of the effect of the organic cosensitizer concentration on the power conversion efficiency of the fabricated solar cell devices. In this work, Z907, a ruthenium dye, has been cosensitized with SQ2, an organic sensitizer, and an overall efficiency of 7.83% has been achieved. The fabricated solar cells were evaluated using UV‐Vis spectroscopy, current‐voltage (I‐V) characteristics, and electrochemical impedance spectroscopy analysis. Our results clearly indicate that the concentration of organic cosensitizer strongly affects the photovoltaic performance of fabricated solar cells. Upon optimization, the cell fabricated with 0.3 mM Z907 + 0.2 mM SQ2 dye solution demonstrated J_sc (mA/cm²) = 21.38, V_oc (mV) = 698.37, FF (%) = 52.46, and power conversion efficiency of η (%)  = 7.83 under standard AM1.5G 1 sun illumination (100 mW/cm²). It was observed that the efficiency of cosensitized solar cells is significantly superior than that of individual sensitized solar cells (Z907 [η  = 5.08%] and SQ2 [η  = 1.39%]). This enhancement in efficiency could be attributed to the lower electron‐hole recombination rate, decrease in competitive absorption of I^?/I^?₃, and less dye aggregation because of the synergistic effect in cosensitized solar cells.     

Thermal performance characteristics of STC system with Phase Change Storage

Storing solar energy heat using Phase Change Materials (PCM) is an effective method. The combination of solar collector and PCM in one unit is being currently studied. The performance characteristics of the proposed Solar Tube Collector (STC) are being analysed analytically and experimentally. Fundamental experiments were performed to simulate a direct contact solar storage system, using two vertical cylindrical concentric tubes with the annular space between them filled Stearic acid (C₁₈H₃₈O₂, melting temperature 70 °C). Experimental testing apparatus has been set up to simulated real system conditions, for an assumed initial and boundary conditions, to provide quantitative information concerning the heat transfer and the timewise evolution of the solid-liquid interface and to identify the role and pattern of natural convection and of the movement of the boundary layer in the liquid phase. For the heat charging mode, the experimental results for different types of fin structures have shown that the effect of melting process is strongly effected by the variation of the imposed conditions, in addition to the different trends of the melting profiles along the axial direction due to the effect of natural convection.     

The significance of fullerene electron acceptors in organic solar cell photo-oxidation

Stability of organic solar cells requires development before their commercialisation is possible. This review will give a brief overview of organic solar cells and their stability, before focussing on the photochemical stability of the active layer. The photo-oxidation of the donor polymers will be looked at first which has been studied quite extensively and then fullerene electron acceptors, such as widely used phenyl-C61-butyric acid methyl ester, which has been considerably less studied. It has been shown that oxidation of the fullerene cage on phenyl-C61-butyric acid methyl ester results in oxides with a deeper lowest unoccupied molecular orbital (LUMO) level than the fresh electron acceptor. These oxides act as electron traps, leading to deterioration of the blend photoconductivity. The significance of fullerene photo-oxidation on device stability has been indirectly shown via research on: photoconductivity; organic solar cells made with an oxidised fullerene derivative and organic field effect transistors. Techniques that could be developed to increase photochemical stability of fullerene electron acceptor resistance to photo-oxidation include: reducing its LUMO level; increasing its crystallinity or aggregation and changing its chemical structure. Improving the photochemical stability of organic solar cells would move us one step closer to a more accessible solar power.     

Comparison of various sol-gel derived metal oxide layers for inverted organic solar cells

Inverted bulk-heterojunction solar cells have recently captured high interest due to their environmental stability as well as compatibility to mass production. This has been enabled by the development of solution processable n-type semiconductors, mainly TiO₂ and ZnO. However, the device performance is strongly correlated to the electronic properties of the interfacial materials, and here specifically to their work function, surface states as well as conductivity and mobility. It is noteworthy to say that these properties are massively determined by the crystallinity and stoichiometry of the metal oxides. In this study, we investigated aluminum-doped zinc oxide (AZO) as charge selective extraction layer for inverted BHJ solar cells. Thin AZO films were characterized with respect to their structural, optical and electrical properties. The performance of organic solar cells with an AZO electron extraction layer (EEL) is compared to the performance of intrinsic ZnO or TiO_x EELs. We determined the transmittance, absorbance, conductivity and optical band gap of all these different metal oxides. Furthermore, we also built the correlations between doping level of AZO and device performance, and between annealing temperature of AZO and device performance.     

A comparative study of tartrazine degradation using UV and solar fixed bed reactors

The fixed bed reactor was combined with a solar photoreactor and UV lamp reactor. This hybrid photoreactor used a heterogeneous photocatalysis process (TiO₂/UV) as a tertiary treatment for the degradation of tartrazine dye in water. The solar reactor removed almost all organic compounds from the wastewater. Photocatalysis was optimized using a parametric study to improve the influence of different parameters on the degradation efficiency. Color removals when using solar, UV lamp and hybrid reactors present 99%, 30%, and 99%, respectively. Tartrazine removal kinetics followed a pseudo-first order model. The hybrid solar and UV lamp system combination was a feasibility choice for removing both dyes from wastewater.