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Impedance spectroscopy on organic bulk‐heterojunction solar cells
Abstract
We measured the electrical impedance spectra of organic bulk-heterojunction solar cells based on an absorber blend of poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester. Comparing the spectra of the non-treated device and after two consecutive treatments with applied forward bias voltage at 110 °C, we observed a region in the semiconductor with a low conductivity, which was expanding after the treatments. We concluded that this region is a depletion region at the aluminium contact. This was confirmed by the bias dependence of the impedance spectra. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
... Based on the postulated equivalent circuit, impedance and capacitance spectroscopy should allow the electrical parameters of solar cells, such as charge mobility, concentration, drift velocity, etc., to be described [19][20][21][22][23][24]. The photovoltaic devices most frequently studied with impedance spectroscopy are third-generation solar cells and are based on donor polymers and acceptor materials [25][26][27][28][29][30][31][32][33][34][35][36]. For this group, it is possible to identify devices that are an interesting alternative in terms of the production of ecologically renewable energy sources, i.e., standard [25][26][27][28][29]35,36] and inverted [30][31][32][33][34] organic solar cells. ...
... The photovoltaic devices most frequently studied with impedance spectroscopy are third-generation solar cells and are based on donor polymers and acceptor materials [25][26][27][28][29][30][31][32][33][34][35][36]. For this group, it is possible to identify devices that are an interesting alternative in terms of the production of ecologically renewable energy sources, i.e., standard [25][26][27][28][29]35,36] and inverted [30][31][32][33][34] organic solar cells. ...
... One of the methods that allowed us to investigate the described charge transfer phenomena in solar cells with an active layer based on a mixture of donor polymers and acceptor materials was the impedance spectroscopy method [25][26][27][28][29]35,36,[51][52][53][54][55][56]. Photoactive impedance spectroscopy (p-IS) experiments on the reference sample (ITO/PE-DOT:PSS/P3HT:PCBM/Al) and photovoltaic devices with an active layer modified by Figure 5. Absorption coefficient spectra of P3HT layers doped with 0%, 1%, 5% and 10% iodine from reference [37]. ...
In this work, macro- and nanodiagnostic procedures for working, third-generation photovoltaic devices based on a modified polymer:fullerene (P3HT:PCBM) absorber were conducted using atomic force microscopy (AFM) and impedance spectroscopy (IS) equipment. All experiments were performed both in the dark and under irradiation with a specific light wavelength. Photoactive Kelvin probe force microscopy (p-KPFM) and impedance spectroscopy (p-IS) experiments were conducted on half- and whole-solar cell devices. Based on the p-KPFM measurements, the surface potential (SP) and surface photovoltage (SPV) on top of the active layer at the micro/nanoscale were estimated for various light wavelengths (red, green, blue, and white). For light in the red spectrum range, which was associated with an optical absorption edge and acceptor states that occurred in the band gap of the P3HT material after doping the donor polymer with iodine, the SPV was measured at levels of 183 mV, 199 mV, and 187 mV for the samples with 0%, 5% and 10% iodine doping, respectively. In addition, a macroscale investigation enabling the determination of the electrical parameters of the studied organic solar cells (OSCs) was carried out using p-IS. Based on the data obtained during p-IS experiments, it was possible to propose a series electrical equivalent circuit to define and describe the charge transfer phenomenon in the OSCs. Estimations of data obtained from the fitting of the experimental results of p-IS under white light allowed us to evaluate the average diffusion time of electric charges at 8.15 µs, 16.66 µs, and 24.15 µs as a function of organic layer thickness for the device without doping and with 5% and 10% iodine doping. In this study, we demonstrated that correlating information obtained at the macro- and nanoscale enabled a better understanding of the electrical charge distribution of OSCs for indoor applications.
... A review on different methods of investigating the cause of the degradation/oxidation of the cells is given elsewhere [3]. Among the methods, the electrochemical impedance spectroscopy (EIS) was used [16][17][18][19][20] to provide physical/chemical information on the reason of the decrease of the photoelectric current of the cells. It was found that the conductivity of the heterojunction varies between the bulk of the heterojunction and the contact region between the metallic electrodes of organic materials [18,19] in the present of ambient environment. ...
... Among the methods, the electrochemical impedance spectroscopy (EIS) was used [16][17][18][19][20] to provide physical/chemical information on the reason of the decrease of the photoelectric current of the cells. It was found that the conductivity of the heterojunction varies between the bulk of the heterojunction and the contact region between the metallic electrodes of organic materials [18,19] in the present of ambient environment. The conductivity of the contact region between the metallic electrodes and the organic materials was found much lower than that in the bulk of the heterojunction. ...
... The conductivity of the contact region between the metallic electrodes and the organic materials was found much lower than that in the bulk of the heterojunction. This observation was interpreted due to the corrosion of the Al electrode to Al 2 O 3 , in the present of O 2 or a relative humidity, in which that the dielectric constant of the region increases by depletion of the Al materials and consequently, decreases the photoelectronic current [18,19]. In a similar manner, the corrosion of the Ag electrode to Ag 2 O, in the present of deionized water and polluted water, in which that the dielectric constant of the region increases by depletion of the Ag materials and consequently, decreases the photoelectronic current [16,17]. ...
... Äîäàòêîâó ³íôîðìàö³þ ïðî ðåëàêñàö³éí³ ïðîöåñè, îá'ºìí³ òà ³íòåðôåéñí³ õàðàêòåðèñòèêè ñòðóêòóð íà îñíîâ³ îðãàí³÷íèõ ìàòåð³àë³â ìîaeíà îòðèìàòè, âèêîðèñòîâóþ÷è åëåêòðè÷íó ³ìïåäàíñíó ñïåêòðîñêîï³þ [23]. ...
... ßêùî ÷àñòîòà çðîñòàº, ºìíîñò³ C J òà C B ïî÷èíàþòü âíîñèòè âêëàä ó ïðîöåñè ñòðóìîïðîõîäaeåííÿ. ßê â³äîìî [23][24][25], ó âèïàäêó ìîäåëþâàííÿ ³ìïåäàíñíèõ çàëåaeíîñòåé ä³îäíèõ ñòðóêòóð ç âèêîðèñòàííÿì åêâ³âàëåíòíî¿ ñõåìè (âñòàâêà ðèñ.5) ïîâèíåí ñïîñòåð³ãàòèñÿ íèçüêî÷àñòîòíèé ìàêñèìóì, ùî õàðàêòåðèçóº ïîòåíö³àëüíèé áàð'ºð ì³ae êîìïîçèòîì òà ìåòàëîì. Âàðòî çàóâàaeèòè, ùî â³äñóòí³ñòü ÿâíî âèðàaeåíîãî íèçüêî÷àñòîòíîãî ìàêñèìóìó íà çà-ëåaeíîñò³ Re(Z)-Im(Z) äîñë³aeóâàíî¿ ñòðóêòóðè, éìîâ³ðíî, çóìîâëåíà íèçüêîþ ñóìàðíîþ ïðîâ³-äí³ñòþ êîìïîçèòíîãî ìàòåð³àëó. ...
Сформована органічна фоточутлива композитна структура ПАН:пентацен з широким спектром фоточутливості (1,7-2,7 еВ) методом одночасного вакуумного напилення пентацену та поліаніліну на гнучку підкладку (PET) з шаром ITO. Випрямляючий контакт забезпечувався напиленням алюмінієвої плівки на поверхню композиту. Досліджено об’ємні та бар’єрні властивості композитної структури за допомогою вольт-амперних та імпедансних вимірювань.
... L'impédance des cellules organiques à base de P3HT : PCBM a été abondamment examinée dans la littérature scientifique [107,19,108,109,20,110,111,112,113,114]. La majorité de ces articles rapportent que les cellules à base de P3HT : PCBM se comportent comme des jonctions de Schottky. ...
... Les contraintes sur les propriétés de perméation des matériaux employés sont imposées par la durée de vie désirée pour les cellules [206]. 9 est l'oxydation de l'aluminium à son interface avec le TiO x [111,196]. ...
In order to reach commercialization, organic photovoltaic solar cells need to reach efficiencies above 10 % and achieve lifetimes of several thousands of hours. Tandem solar cells are a way of improving the efficiencies. The objectives of this work are therefore to study the fabrication process, the operation and the ageing of organic tandem solar cells. First, single solar cells based on the active material PCDTBT are used as model to investigate the factors governing the cells efficiencies. Using characterization techniques such as impedance spectroscopy, the roles played by each layer of the cells are identified. Based on these results, a protocol to make series tandem cells is developed. Each of its steps is dedicated to treating a key aspect of tandem cells : choice of complementary absorbing polymers, design of the intermediate layer (IML) and thickness optimization. The functioning of the IML is subjected to a particular attention. To optimize the thicknesses, optical phenomena are numerically simulated. The prediction thus made are then compared to experimental results. Finally, the ageing of single and tandem cells is investigated on time spans ranging from several dozens to several thousands of hours. It is shown that the device degradation can be linked to poor ageing of the interface layers, while the active layer stays stable. Organic tandem cells are promising candidates to reach both high efficiencies and long lifetime.
... In the literature, there are many reports showing practical applications of impedance spectroscopy to the analysis of organic devices [66][67][68][69][70]. ...
... In device analysis, the observation of a series resistance is generally attributed to the contact resistance that arises from the charge-injection problem at the metal/semiconductor interface [72,73]. where the conductivity of one region can be significantly lower than that of the other region [67]. ...
Despite impressive progresses in organic electronic devices, a full theoretical understanding of the device operation is still lacking. The thesis is dedicated to establishing applicable theoretical descriptions of organic electronic devices, and in particular physics-based compact models. Prototypical organic diodes and transistors with various architectures are investigated, with a consistent effort to view and present the effect of charge carrier transport and injection on the observable device phenomena. A particular effort is given to integrate these models in circuit simulators, thus connecting the device-level and system-level outlooks. The approaches used comprise equivalent circuit modeling by impedance spectroscopy, analytical development of physics equations, numerical finite-element-based 2-D simulations, and experimental validations. The results provide significant understanding on the effect of traps and injection barriers on the current-voltage characteristics. Original, fully analytical compact models for rectification diodes and organic field-effect transistors are proposed with reliable numerical and experimental proofs.
... The electron mobilities of doped devices are relatively higher than control device, which demonstrates that the doping NPs improved electron transfer properties of TiO 2 films. Impedance spectroscopy has been successfully used both in inorganic and OSC to obtain valuable information about kinetics and energetic processes governing the device performance323334. In the literature different equivalent circuit models have been proposed to simulate the impedance of OSCs, which results in perfect semicircular35363738 . ...
... The R 3 corresponds to the electrodes including the resistance of ITO, TiO 2 , MoO 3 and Ag. According to the reference323334, impedance spectroscopy was used to determine the electronic parameters, such as built-in voltage, the doping concentration, the effective recombination lifetime and diffusion time. The diffusion coefficient and the carrier mobility can be obtained from the diffusion time. ...
Phosphor materials can be applied to enhance the energy conversion efficiency of a solar cell. Such materials convert low-energy transmitted photons to higher-energy photons that can be absorbed by the cell, substantially decreasing the spectral mismatch between the cell and the solar spectrum. In this paper, the efficiency of organic solar cells (OSCs) was improved by incorporating phosphor nanoparticles as dual functionality into TiO2 cathode buffer layer. The dependence of devices performance on doping concentration of NaYF4:Yb3+,Er3+ nanoparticles was investigated. A high power conversion efficiency of 6.83% was achieved, which mainly attributes to the increase of short-circuit density. The absorption spectrum indicates that light-harvesting of doped films is higher than undoped film, which originates from scattering effect and NIR spectrum sensitization of phosphor nanoparticles. The measurement of electron-only devices shows that electron transport property of doped devices was apparently improved. Impedance spectroscopy reveals that the diffusion coefficient and carrier mobility were greatly enhanced. This study demonstrates that phosphor nanoparticles doping is useful for fabricating high performance OSCs.
... Now, in the case of the CPE included here (in parallel to R rec ), it could be associated to the depletion region in the device or to the chemical capacitance. However, as the semicircle at low frequencies associated to CPE does not expand its diameter at increasing reverse bias, it cannot be assigned to the depletion region [38]. Because of CPE describes distribution of relaxation process due to, for example, geometric (like fractal interfaces) or dynamical origins (as in the case of multiple trapping processes) [39], CPE can be related also to the particular properties of the p -n interface in our case. ...
We report on strategies that improve Se-derivative based solar cells performance. With this aim, a compact thin film based on ZnO nanoparticles is deposited onto fluorine doped tin oxide (FTO) as an electron-transport layer, in thermally evaporated Ga x Se 10-x based solar cells. ZnO nanoparticles films are synthesized by sol-gel process whereas Ga x Se 10-x material is obtained by mechanical alloying. Using current-voltage measurements, impedance spectroscopy, and capacitance-voltage profiling, device characteristics and performance limiting factors are revealed and discussed. Particularly, the use of ZnO nanoparticles results in improved device performance as well as long-term stability. In comparison to Se-only devices with the structure FTO/Se/Au (power conversion efficiency of 0.98%), under 100 mW/cm 2 AM 1.5 G illumination the devices achieved a power conversion efficiency of 2.7% with the structure FTO/ZnO/GaSe 9 /Au (open circuit voltage of 0.71 V, short-circuit current of 7.9 mA/cm 2). Hence, an increase of around 175% in the power conversion efficiency is obtained in comparison to Se-only devices. In addition, the effect of others parameters, like thickness of the active layer as well as the gallium contents in the alloy, are discussed.
... Glatthaar presented in several papers that Al 2 O 3 would lead the IV curve changed from a standard exponential diode curve to a curve with an inflection point, which might deteriorate the FF to some degree, or total fail the solar cell creating an S-shape IV curve. [34,86]. Silver is less reactive comparing to Ca and Al. ...
Currently, one of the biggest challenges of OPV being commercially competitive is to perform consistently throughout its lifetime. Generally, aging stresses, such as light source, temperature, and atmosphere, can induce degradation in all layers including electrodes, interfaces, and active layers. Heat and white light induced intrinsic degradation is deemed to be investigated and addressed with the highest priority. The investigation of the PhD program concentrates on microstructural evolution of bulk-heterojunction (BHJ) active layer under thermal- and/or photo- stress. The first attempt is to employ polymeric fullerenes. A polystyrene based side-chain polymeric fullerene is successfully synthesized and applied in solar cell. However, side-chain polymeric fullerenes suffer from poor solubility which limits its role of acting as the main acceptor in the photovoltaic application. Two main-chain polymeric fullerenes, PPC4 and PPCBMB, are employed as additives in PCE11:PCBM based solar cells. With up to 8 wt% addition, the efficiency of the ternary solar cells stay as high as the binary solar cells; however, with 20 wt% addition, the photovoltaic performance slightly decreases; moreover, the addition of the main-chain polymeric fullerenes fails to address the photo instability issue of polymer:fullerene solar cells. As increasing insights concerning the BHJ morphology are gained, here comes the strategy of combining materials with good miscibility to overcome the thermal instability of polymer:fullerene solar cells. We demonstrate that the low miscibility between PCBM and pDPPT5-2 or PTB7-Th is one of the fundamental origins of the low thermal stability. On the contrary, two novel fullerenes, PyF5 and FAP1, with a significantly higher chemical compatibility are introduced to overcome these limitations. Further, it is observed that the miscibility between the donor and acceptor dominates the optimized acceptor:donor ratios in polymer-fullerene BHJ systems. Owing to the extremely poor miscibility, the highly efficient PCE11:PCBM solar cells suffer from strong burn-in losses even under room temperature. We demonstrate that crystalline properties of PCE11 are highly influenced by molecular weight and polydispersity; furthermore, polymer-fullerene BHJ film morphology is largely affected by the crystalline nature of polymers and evolves differently under external stresses, like heat or light, which eventually results in solar cells with different burn-in loss and lifetime. A detailed analysis of the energy and intensity dependence of light-induced burn-in degradation suggests that photo-excited carriers do affect amorphous polymer segments in a similar way as thermal stress does.
... Because each circuit element For example, impedance spectroscopy and equivalent circuit modelling are used for an organic rectifying diode in this thesis (Chapter 4). In the literature, a number of reports have shown practical applications of impedance spectroscopy for the analysis of organic devices [135][136][137][138][139]. Nonetheless, the determination of the equivalent circuit is based on a knowledge-based guess and subsequent verification by comparing the modelled impedance spectrum to the measured one. ...
In spite of a remarkable improvement in the performance of organic electronic devices, there is still a lack of rigorous theoretical understanding on the device operation. This thesis is dedicated to establishing practical models of organic electronic devices with a full physical basis, namely a physically-based compact model. A physically-based compact model of a circuit element is a mathematical equation that describes the device operation, and is generally assessed by three criteria: whether it is sufficiently simple to be incorporated in circuit simulators, accurate to make the outcome of the simulators useful to circuit designers, and rigorous to capture physical phenomena occuring in the device. In this context, distinctive features of charge carrier injection and transport in organic semiconductors are incorporated in the models with a particular effort to maintain mathematical simplicity. The concomitant effect on the current-voltage characteristics of prototypical organic diodes and transistors are studied. Parameter extraction methods consistent to the models are presented which enable unambiguity determination of device parameters used for modeling device operation and assessing device performance and properties of organic thin-films and interfaces. The approaches encompass analytical developement of physical equations, two-dimensional numerical simulation based on finite-element method and experimental validation. The original and fully analytical compact models and parameter extraction methods provide fundamental understanding on how energetic disorder in an organic semiconductor thin-film, described by the Gaussian density of states, affects the observable current-voltage characteristics of the devices.Keywords : Organic electronics, device physics, analytical modeling, diodes, field-effect transistors, Gaussian density-of-states
... Furthermore, studies have also been done for the characterization and diagnosis of proton exchange membrane fuel cells (Yuan et al., 2007), solid oxide fuel cells (Huang et al., 2007), biofuel cells (Kashyap et al., 2014), solar cells (Fabregat-Santiago et al., 2005;Glatthaar et al., 2005;Rock et al., 2014) and microbial fuel cells (He and Mansfeld, 2009). ...
Electrical impedance spectroscopy (EIS), in which a sinusoidal test voltage or current is applied to the sample under test to measure its impedance over a suitable frequency range, is a powerful technique to investigate the electrical properties of a large variety of materials. In practice, the measured impedance spectra, usually fitted with an equivalent electrical model, represent an electrical fingerprint of the sample providing an insight into its properties and behavior. EIS is used in a broad range of applications as a quick and easily automated technique to characterize solid, liquid, semiliquid, organic as well as inorganic materials. This paper presents an updated review of EIS main implementations and applications.
... irradiation with total power of 100 mW cm À2 ) can be reached (Bisquert and Garcia-Belmonte, 2011). Impedance spectrometry is used for a couple of years as a technique providing valuable information about the kinetics and energetic processes governing the organic solar cell device's performance (Garcia-Belmonte et al., 2010, 2008Glatthaar et al., 2005Glatthaar et al., , 2007Kuwabara et al., 2009aKuwabara et al., , 2009bSekine et al., 2009). It is also a valuable tool to observe bulk and interfacial electrical properties that cannot be observed in direct current regime. ...
... A more advanced analysis is therefore needed to have a better understanding of the complex behavior of such parameters. The accurate extrapolation and analysis of photocurrent [9]- [11] and the study of the impedance spectroscopy [12], [13]-for example-represent further investigation tools in organic heterojunction solar cells that can reveal the behavior of important physical parameters such as generation rate, polaron charge transfer state separation and recombination rate, built-in potential, and carrier lifetime. Manuscript During the operational life of a photovoltaic module, several factors may concurrently contribute to the loss of cell performances. ...
We performed a constant current stress at forward bias on organic heterojunction solar cells. We measured current voltage curves in both dark and light at each stress step to calculate the photocurrent. An existing model applied to photocurrent experimental data allows the estimation of several parameters such as generation, recombination, dissociation rate, and nearly zero field voltage within the active layer as a function of the stress time. The analysis of extrapolated parameters shows that the stress mainly affects the recombination rate of the polaron charge transfer states.
... In addition, the structure was prepared in air and left in ambient conditions. Therefore, the series resistance value can be affected by many factors, such as reduced mobility and the changes in the contact barrier located at the space charge regions [33,34]. The ideality factor is an important parameter in the specification of the electrical behavior of the diodes which is assigned to the voltage drop in the interfacial layer [35]. ...
We present a comparative study between multiwalled carbon nanotubes (MWCNTs) thin films deposited on polyethylene terephthalate (PET) substrates using (i) spin-coating technique and (ii) deposition through a membrane. We deduce from transparence, electrical properties, and AFM image that deposition through membrane presents better properties than spin-coating method. The concentration comparison shows that the optimum result was achieved at a concentration of 1.2 mg·mL
−1
corresponding to a resistance (
R
s
) of 180 Ω·cm
−2
and an optical transparence of about 81% using a wavelength 550 nm. We will also demonstrate the use of the elaborated electrodes to fabricate the following flexible structure: PET-MWCNTs/MEH-PPV/Al. The series resistance
R
s
and the ideality factor
n
were calculated.
... This metal oxide layer is electrically insulating and creates a transport barrier eventually degrading the performance of the device as seen in the diode characteristics [20]. Glatthaar et al. showed in a series of letters that this layer of Al 2 O 3 gives rise to a capacitance that can be estimated from the layer thickness and that changes the standard exponential diode curve to one with an inflection point [65,66]. This may erode the fill factor to a point where the device no longer produces power. ...
We compared the degradation process in organic bulk heterojunction solar cells, upon exposure to a temperate (Belgium) and a sub-equatorial (Benin) climate. Differences in degradation of these devices have been attributed to humidity differences between the temperate and Beninese environments. Our analyses revealed, the decrease rate on 240 h of JSC is about 40% in Belgium and 45% in Benin, when the devices are exposed to light and to ambient air. The diffusion of H2O and O2 species through the layers is more rapid when the temperature is high.
... The field distribution in PSCs is often described in terms of a metal-insulator-metal model, where relatively uniform electric fields are assumed in the device. However, nonuniform electric fields will develop when there is electronic or chemical doping 19,38 or unbalanced charge carrier mobilities in the device. 21 In the case of balanced mobilities (the value of electron mobility is close to the hole mobility), the distribution of the electric field is closely related to the doping density of the active layer. ...
The change in doping density in P3HT:PCBM based polymer solar cells (PSCs) with different processing solvents and with/without post-fabrication thermal treatment is investigated with capacitance-voltage measurement and optical microscopic imaging. The results suggest that both slow drying and thermal treatment facilitate the phase-separation and crystallinity of P3HT and PCBM, leading to low defect density and thus low p-type doping. Direct links between the doping density and the performance of the PSCs, specifically the short-circuit current (Jsc) and open-circuit voltage (Voc), are observed. The results show that doping density is one of the decisive factors affecting the photocurrent of the PSCs. Lower doping density leads to a wider depletion region, which is beneficial for carrier collection. The agreement between the calculation and the experiment suggests that the Voc increases monotonically with increasing doping densities in the PSCs. These rules consistently explain our results on the change of Jsc and Voc after thermal annealing in the PSCs with different processing solvents. (C) 2014 AdIP Publishing LLC.
... The simulations show that improper thicknesses and low mobilities could easily lead to very poor EQE performance. High levels of P-type doping [111,[184][185][186][187] or reduced generation zones [188] may also be important factors affecting EQE and I sc . ...
This review summarizes the optical and electrical models of bulk heterojunction (BHJ) polymer solar cells (PSCs) and numerically simulates and analyzes the performance of the PSCs. A complete simulation of a conventional BHJ device based on the polymer P3HT is presented and results are compared with the experimental data. Key factors affecting the device performance, including the photo absorption, quantum efficiency, short-circuit current, fill factor, and open-circuit voltage of the device, are analyzed and summarized. Simulations on inverted, semitransparent, and large-area PSCs are performed and findings are compared with experimental results. Simulations reveal the effects of optical spacer layers, different thicknesses, carrier mobilities, light intensities, contact barriers, effective bandgaps, recombination coefficients, and energy-level bending on the quantum efficiency, short-circuit current, fill factor, and open-circuit voltage of the PSCs. Differences between conventional and inverted geometry, opacity and semitransparency, and small and large-area PSCs are discussed based on the simulations. A power conversion efficiency of 11.0% is predicted for the PSC based on P3HT. Results suggest the need to further reduce the series resistance in large-area PSCs.
... Finally, PEDOT:PSS has actually been shown to have relatively poor electron-blocking properties. 40,41 This combination of: (a) interdiffusion at the electrodes, (b) incomplete PCBM layer formation at the cathode, and (c) poor PEDOT:PSS electron-blocking capability at the anode would eliminate any blocking effect of these interface layers; consistent with the significant interface recombination and poor electron/hole blocking properties that are predicted by the Monte Carlo simulations and explaining why the best fit is obtained when assuming no blocking layers at all. In order to test this hypothesis, a fifth set of simulations were run using the electrode coverage indicated by the XPS measurements, i.e., 58% of the anode is in contact with P3HT and 60% of the cathode is in contact with PCBM. ...
Monte Carlo (MC) simulations have been used to fully model organic solar cells. The quantum efficiency and short-circuit current of these virtual devices are in excellent agreement with experimental measurements. Simulations show that, contrary to expectation, indium tin oxide/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)/poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methylester (PCBM)/aluminium devices lack effective charge blocking layers at the electrode interfaces. X-ray photoelectron spectroscopy depth profiling shows that despite a PCBM-rich region near the cathode, interface intermixing at the electrodes combined with incomplete PCBM coverage leads to significant interface recombination. This work highlights the effectiveness of MC simulations as a predictive tool and emphasizes the need to control electrode interface processes.
... In conventional semiconductors, the Fermi's level moves up or down by increasing or decreasing the number of electrons. The same rule is true for organic semiconductors, except that instead of adding a new material as the dopant to change the intrinsic balance of carriers, this change can be obtained by change in the weight ratio [53] or in the synthesis temperature [54] or the annealing time [55,56]. It was assumed that the electrons and holes have separate continuity equations with the same generation (G) and recombination rate (R). ...
The energy crises, along with the recent global warming trends, demand an immediate cut in the use of fossil fuel. Therefore, renewable sources of energy and especially solar power, have gained tremendous attention from the consumer countries as a possible candidate to replace the carbon-based energy supplies. Among all different types of solar cells, flexibility, cost-effective fabrication processes, combined with low-price materials and most importantly a great potential for improvement make organic solar cells an interesting topic for research. On the other hand, modeling provides a valuable opportunity to study device properties that experimentally are out of reach, expensive or need a long time to measure. These mentioned reasons motivated us to choose modeling of organic solar cells as the subject of our research.In this research, we tried to provide a complete study of the power generating procedure in organic solar cells by modeling all of the following processes: in-coupling of the photons, absorption of the photons, formation of the excitons, diffusion of the excitons, dissociation of the excitons, transportation of the charges and collection of the charges at the electrodes.To get a better understanding and also because of basic physical differences, the modeling isdivided into two parts: the optical section and the electrical section. Each section is also dividedinto two separate segments, analytical and numerical analysis.Using the optical models with different designs to improve the performance of the solar cells, the effect of the layer thickness and two- and three-dimensional light focusing apertures on the intensity of light at the junction of n-type and p-type materials (for bilayer heterojunction organic solar cells) are studied. Results show that for a certain design of the light focusing aperture, a 98% increase in the light absorption in a bilayer heterojunction solar cell can be obtained.The electrical performance of organic solar cell is also studied by using analytical modeling of exciton diffusion for bilayer heterojunction solar cells and numerical models based on driftdiffusion procedures by using COMSOL multiphysics software for bulk heterojunction solar cells. Based on the mismatch between the calculated results and measurement (counterdiode effect), a tunneling current correction is introduced. Finally, using the tunneling current model, the energy diagram of the organic active layer at the metallic contact is characterised.In summary, five different models are described in five separate sections, and at the end of each section, results are reported and compared with the literature that prove that the presented models can be used for a new design of organic solar cell characteristics to improve the performance of the device. Also, by introducing the tunneling current to model the counterdiode effect, we have contributed to the literature.
... It is believed that Cs 2 CO 3 layer device offer the possibility of fabricating high performance polymer solar cells using all solution processes. Polymer solar cells can be modeled as a combination of contact, bulk, and junction resistance (R C , R B , R J ), and bulk and junction capacitance (C B , C J ) [16]. The complete device can be modeled as the two different regions for carrier relaxation. ...
In this paper, we studied the effect of the electron injection layer, Cesium carbonate (Cs2CO3), thickness on the performance of organic solar cell (OSC) based on blends of poly (3-hexylthiophene) (P3HT) and [66.
Hoppe , H. ,
Sariciftci , N. S. , &
Meissner , D. ( 2002 ). Mol. Cryst. Liq. Cryst. , 385 , 113 . [Taylor & Francis Online]View all references,66.
Hoppe , H. ,
Sariciftci , N. S. , &
Meissner , D. ( 2002 ). Mol. Cryst. Liq. Cryst. , 385 , 113 . [Taylor & Francis Online]View all references]-phenyl-C61 butyric acid methyl ester fullerene derivative (PCBM). The polymer solar cell consists of molybdenum-oxide (MoO3) as a hole injection layer, P3HT and PCBM bulk hetero junction as an active layer, and Cesium carbonate (Cs2CO3) as an electron injection layer. We measured each device by current-voltage measurement and impedance spectroscopy which is widely used for equivalent circuit analysis of solid state structures. The device with the Cs2CO3 layer showed about 8–10% higher JSC and about 6–8% higher power conversion efficiency compared with the devices without the Cs2CO3 layer.
... After 3 days in ambient conditions, the increase in Rs2 for blend 2 cells was around five times greater than for blend 1 cells, which is in agreement with having a more interpenetrated morphology. The increase in series resistance observed when the devices are left in ambient conditions has been attributed to several factors as reduction of mobility, metal corrosion at the contact or changes in the contact barrier and charge space regions [14,15]. ...
In this work, we investigate the performance of Bulk Heterojunction (BHJ) Organic Solar Cells fabricated with P3HT films blended with varying weight fractions of PCBM[70]. To determine the optimal composition, P3HT:PCBM[70] blends at three different ratios (1:0.84, 1:1 and 1:1.21, wt%) were fabricated. The electrical parameters were extracted from the current - voltage characteristics (I-V) under dark and illuminated conditions. The degradation mechanisms involved in the devices were studied.
... Sufficiently high concentrations of dopants in P3HT allow to describe the P3HT/Al contact in terms of a Schottky barrier, involving the formation and modulation of a depletion region by an external bias (Dennler et al., 2005). Also for solar cells based on blends of P3HT and PCBM, a comparable behaviour has been suggested (Garcia-Belmonte et al., 2008; Glatthaar et al., 2005). In a recent paper (Seemann et al., 2009) we have shown that bulk heterojunction solar cells with inverted structure and gas permeable top electrodes suffer from a partially reversible loss in short circuit current (j sc ) when they are exposed to oxygen, especially under illumination. ...
The effect of oxygen on the degradation of inverted bulk heterojunction solar cells based on poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blends has been investigated by monitoring current–voltage (jV)-curves, impedance spectra and charge extraction by linearly increasing voltage (CELIV) traces during the degradation process. The use of gas permeable top electrodes allows monitoring the kinetics of degradation without being limited by the diffusion through a compact metal electrode. A computational model is used to provide a phenomenological explanation of the experimental results. The charge distribution inside the device is modeled by solving the fully coupled set of nonlinear differential equations for the quasi-one-dimensional transport of electrons and holes.Degradation of the cells in the presence of oxygen results mainly in the reduction of short circuit current (jsc), while the concomitant loss in light absorption is negligible. The rate of degradation is enhanced significantly by illumination. A significant part of the loss in jsc is reversible upon annealing under nitrogen or in vacuum.The irreversible part of the degradation is assigned to the photochemical formation of carbonyl and carboxylic groups, which act as traps for electrons. The reversible component of degradation is due to p-doping of the photoactive layer by oxygen, which results in the formation of mobile holes and immobile superoxide anions. This leads to the formation of a space charge region in front of the electron extracting electrode whose width depends on the doping level as well as on the applied bias. The space charge region shields the electric field inside the photoactive layer and hence hampers charge carrier extraction, which leads to the observed loss in short circuit current.
The article presents the results of studying how impedance analysis can be used for determination of monosodium glutamate in order to identify food fraud. We have suggested that the parameters of complex conductivity (admittance) of a two-terminal circuit could allow detecting monosodium glutamate (E 621), an additive used in the food industry to enrich the taste. The method involves passing current of different frequencies through solid foodstuffs and a cell with liquid foodstuffs, measuring the electrical conductivity, and determining and analysing the frequency dependence of admittance. The active G component and the reactive B component of the admittance have been measured at different frequencies, from 100 Hz to 100 kHz. For the experiment, food samples were prepared in accordance with the Codex Alimentarius recommendations for the dosage of the food additive E 621: orange juice with monosodium glutamate added in the amount of 0.3%, and mashed potatoes with glutamate added in the amount of 1%, of the total weight of the products. The temperature of the tested products was 22 ± 0.2°С. The results of the studies have shown the dependences of the admittance components on the frequency for the control samples of juice and mashed potatoes and for the samples with monosodium glutamate added. The dependence of the active component and the reactive component of the foodstuff admittance have been established, with monosodium glutamate (added in the above-specified proportion) and without it. The difference is in how the dependences change in their nature. The monosodium glutamate curves both in juice and in mashed potatoes are similar. The samples containing monosodium glutamate have far higher values of the active and reactive admittance component than the control samples do, with a distinct peak of the reactive component characteristic. Therefore, impedance analysis is a possible method to detect quickly the flavour enhancer monosodium glutamate in foods of different consistency and thus identify food fraud.
We report on strategies that improve Se-derivative based solar cells performance. With this aim, a compact thin film based on ZnO nanoparticles is deposited onto fluorine doped tin oxide (FTO) as an electron-transport layer, in thermally evaporated GaxSe10-x based solar cells. ZnO nanoparticles films are synthesized by sol-gel process whereas GaxSe10-x material is obtained by mechanical alloying. Using current-voltage measurements, impedance spectroscopy, and capacitance-voltage profiling, device characteristics and performance limiting factors are revealed and discussed. Particularly, the use of ZnO nanoparticles results in improved device performance as well as long-term stability. In comparison to Se-only devices with the structure FTO/Se/Au (power conversion efficiency of 0.98%), under 100 mW/cm² AM 1.5 G illumination the devices achieved a power conversion efficiency of 2.7% with the structure FTO/ZnO/GaSe9/Au (open circuit voltage of 0.71 V, short-circuit current of 7.9 mA/cm²). Hence, an increase of around 175% in the power conversion efficiency is obtained in comparison to Se-only devices. In addition, the effect of others parameters, like thickness of the active layer as well as the gallium contents in the alloy, are discussed.
A growing number of recent studies have demonstrated the substantial impact of the alkyl side chains on the device performance of organic semiconductors. However, detailed investigation of the effect of side‐chain engineering on the blend morphology and performance of ternary organic solar cells (OSCs) has not yet been undertaken. In this study, the performance of ternary OSCs is investigated in a given poly(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b;4,5‐b′]dithiophene‐2,6‐diyl‐alt‐(4‐(2‐ethylhexyl)‐3‐fluorothieno[3,4‐b]thiophene‐)‐2‐carboxylate‐2‐6‐diyl)):[6,6]‐phenyl‐C71‐butyric acid methyl ester (PTB7‐Th:PC71BM) host set by introducing various small molecule donors (SMDs) with different terminal side‐chain lengths. As expected, the performance of binary OSCs with SMDs depends greatly on the side‐chain length. In contrast, it is observed that all SMD‐based ternary OSCs exhibit almost identical and high power‐conversion efficiencies of 12.0–12.2%. This minor performance variation is attributed to good molecular compatibility between the two donor components, as evidenced by in‐depth electrical and morphological investigations. These results highlight that the alloy‐like structure formed due to the high compatibility of the donor molecules has a more significant effect on the overall performance than the side‐chain length, offering a new guideline for pairing donor components for achieving high‐performance ternary OSCs.
Developing air-stability, low cost, non-toxicity, and high transparency charge buffer layer is a critical strategy to achieve the high photoelectric conversion efficiency of polymer photovoltaic cells. This paper reports the remarkable improvement of device performance by employing a combination of Copper bromide (CuBr2) and molybdenum trioxide (MoO3) (CuBr2/MoO3) as the hole transport layer (HTL) of inverted-type polymer solar cells (PSCs). The bulk transport processes and resistive capacitance elements in the operating PTB7:PC71BM bulk hetero-junction PSCs were characterized using the impedance spectroscopy. The impedance response was modeled using two equivalent circuital models, which are the general transmission line circuit (GTLC) model and electrochemical polarization model. The effective carrier lifetime, conductivity, and mobility for both devices were extracted from the models. The improved hole injection and transport at the anode and the efficient electron transport blocking decreased interface recombination and contact resistances, resulting in an improved power conversion efficiency (PCE) from 7.30 % to 9.56 %. These results suggest the quantitative interpretation and modeling of the impedance spectroscopy provide an effective way to unravel the operating mechanism of photovoltaic devices.
Solar cells based on evaporated Cs2O/GaSe9 and Cs2O/Se (as comparison) active layers have been investigated. We correlate their performance with devices previously reported without Cs2O. GaSe9 appears as a very promising solar absorber because of its attractive optical and electrical properties. We improved device performance by around 30% by inserting a Cs2O thin interlayer. Current density-voltage, capacitance-voltage and impedance spectroscopy measurements were used to characterize the effect of Cs2O layer on device performance. The charge carrier concentration and the built-in potential were derived using Schottky-Mott relation. The enhancement promoted by Cs2O is attributed to an improvement in electron extraction efficiency by FTO, and the avoidance of Se sublimation during the annealing process. The insertion of the metallic oxide also increases the built-in potential at FTO/GaSe9 interface. Dynamic processes were clearly identified by impedance spectroscopy under complex modulus analysis, confirming better capabilities of charge collection by the Cs2O containing electrodes.
The effect of low-temperature dynamic vacuum annealing (DVA) of sol–gel ZnO films on inverted polymer solar cells (IPSCs), which are composed of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PC61BM), was systematically investigated. The results show that IPSCs with low-temperature DVA-based ZnO films exhibit 15.9% enhancement within a power-conversion efficiency (PCE) of 4.01%, compared with IPSCs with conventional annealing-process-based ZnO films. The surface morphology measurement of ZnO films, light intensity dependent solar cell behaviors, and impedance spectroscopy of devices were investigated to study the performance of IPSCs. The dramatic enhancement of PCE was attributed to the improved surface morphology, optimized carrier transport characteristics, prolonged charge carrier lifetime, and reduced recombination rate. These results indicate that the low-temperature DVA process could be a promising method for producing high-quality ZnO films and high-performance IPSCs.
We report on the characterization of crystalline GaSe9 thin films produced by thermal evaporation onto fluorine doped tin oxide (FTO) transparent electrode substrate, and its further application in photovoltaic devices. The physical properties are studied in order to understand the implications of gallium addition to the Se alloy. Hexagonal structure of the crystalline films after thermal annealing is confirmed by X-ray diffraction, whereas scanning electron microscopy images indicate improved morphology in GaSe9 films, in comparison to Se films. Photoelectrical properties of the devices are studied by current density–voltage, capacitance–voltage and impedance spectroscopy measurements. GaSe9 based devices show average power conversion efficiency of 1.37 %, under 100 mW/cm2 AM 1.5 irradiance, in a simple FTO/GaSe9/Au structure. It means an increase of 40 % with respect to Se devices under the same structure. Additional improvements are related with the built-in potential, charge carrier concentration and depletion width, which are derived by the Schottky–Mott relationship. Finally, from impedance spectroscopy measurements, an increase in the recombination time and a faster charge extraction in GaSe9-based devices are observed.
The photo-degradation mechanism of P3HT:PCBM bulk heterojunction solar cells is investigated by using the AMPS-1D (analysis of microelectronic and photonic structures) computer program. An effective medium model with exponential tail states and Gaussian mid-gap states is used to simulate the J-V characteristics of the photovoltaic devices. By using the proposed model, the J-V characteristics of both fresh and degraded devices could be nicely reproduced. The simulation results suggest that the main factors governing the mechanism of photo-degradation are the increase in the density of the mid-gap states, the increase of the p-type doping concentration, the slight increase in back contact barrier height and the reduction of charge carriers mobility. The deterioration of the photovoltaic performance is attributed to the increased recombination rate and insufficient charge collection.
IntroductionPhotoelectric Characterization of OPV DevicesInterlaboratory Studies of OPVsLifetime Testing and Reporting: Standardized ApproachConclusions
List of AbbreviationsReferences
A criterion of the degradation/oxidation susceptibility of organic photovoltaic (OPV) cells in aqueous solutions was proposed for the first time. The criterion was derived based on calculating the limit of the ratio value of the polarization resistance of an OPV cell in aqueous solution (Rps) to the polarization resistance of the OPV cell in air (Rpair). In other words, the criterion lim(Rps/Rpair) = 1 was applied to determine the degradation/oxidation of the OPV cell in the aqueous solution when Rpair became equal (increased) to Rps as a function of time of the exposure of the OPV cell to the aqueous solution. This criterion was not only used to determine the degradation/oxidation of different OPV cells in a simulated operational environment but also it was used to determine the electrochemical behavior of OPV cells in deionized water and a polluted water with fine particles of sand. The values of Rps were determined by the electrochemical impedance spectroscopy at low frequency. In addition, the criterion can be applied under diverse test conditions with a predetermined period of OPV operations.
In this chapter, the interface degradation mechanisms in organic photovoltaics (OPVs) will be discussed. The interface instability is mainly ascribed to the diffusion of oxygen and water into the electrode materials as well as the interface modification layer between electrode and organic layer. Furthermore, the commonly used characterization techniques are presented, which are quite helpful to understand the origin of the interface instability and their level of impact. In particular, these techniques reveal optical and electrical changes at the interface. Therefore, the analysis on the interface degradation and development of characterization techniques would contribute to the understanding of interface stability and further enhance the entire device lifetime.
In this study, molecular doping with polymer dots was designed to unravel its effect on the photoconductivity in organic solar cells. The photocurrent in organic solar cells exhibited a considerable increase under optimal doping concentration, leading to an ultimate enhancement of power conversion efficiency from 2.30% to 3.64%. This can be attributed primarily to the improvement of the initial boost in charge carriers due to the background carriers induced by the polymer dots and increased tail absorption by the active layer. Based on single carrier device and impedance measurements, polymer dopant can efficiently decrease charge recombination and improve charge carriers mobilities. The obtained achievements pave an approach of molecular doping in affecting the operation of organic solar cells.
In any microelectronic device, fundamental physical parameters must be well understood for successful electronic optimization. One such prominent parameter is energetic trap states, which are well-known to plague amorphous or otherwise impure semiconducting materials. Organic semiconductors are no strangers to such states and their electronic properties are evidently tied to these defects. Herein, this article discusses the identification, characterization and mitigation of bandgap residing trap levels in organic photovoltaic devices. A compilation of select studies to date is given and a general outlook is proposed. Organic photovoltaic materials are depicted as multiple trap-level systems with a seemingly continuous distribution of electronic states throughout the bandgap. Some elucidations as to the origins of these electronic states as well as recent works centered on defect removal are also presented.
http://chemgroups.ucdavis.edu/%7Eosterloh/pubs/ref_69.pdf Surface photovoltage (SPV) spectra are reported for separate films of (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) and for regioregular and regiorandom poly(3-hexylthiophene) (P3HT):PCBM bulk heterojunctions, as a function of wavelength, film thickness, thermal annealing, and substrate. In PCBM films, two photovoltage features are observed at 1.1–1.4 eV (F1) and 1.4–2.3 eV (F2), which are assigned to excitation of charge transfer states at the interface (F1) and in the bulk (F2) of the film. In BHJ films, five different photovoltage features are observed at 0.75–0.9 eV (F1), 0.9–1.3 eV (F2), 1.3–1.8 eV (F3), 1.8–2.0 eV (F4), and 2.0–2.4 eV (F5). This data can be analyzed on the basis of optical absorbance and fluorescence spectra of the films, and using SPV spectra for PCBM and P3HT only films, and for a BHJ film containing P3HT nanofibers for comparison. SPV features are assigned to states at the polymer–substrate interface (F1 and F2), the P3HT:PCBM charge transfer state (F3), the self-ionized (CT) state of P3HT (F4), and the band gap transition of P3HT (F5). This interpretation is also consistent with molecular orbital energy diagrams and electron microscopy-derived topological maps of the films. Photovoltage sign and substrate dependence can be understood with the depleted semiconductor model. Features F1–4 are caused by polarization of electrostatically bound charge pairs by the built-in electric field at the substrate–BHJ interface, whereas F5 is due to transport of free charge carriers through the film and through the substrate film interface. This work will promote the understanding of photochemical charge generation and transport in organic photovoltaic films.
In this study, we systematically investigated the stoichiometric dependence of titanium oxide (TiOx, x=1.56–1.93) as a cathode modifier on the device performance of polymer solar cells. Electronic structures of the synthesized TiOx modifier layers were controlled by tuning the compositions of various O/Ti ratios. The effective cathode work-functions and the corresponding device performances of polymer solar cells are systematically changed as a result of inserting the TiOx modification layers. Interfacial modification of the Al cathode with a low O/Ti ratio of TiOx layer yields the best performing photovoltaic device as a result of a largest built-in potential. The correlation of power conversion efficiencies and carrier dynamics of these devices by inserting various TiOx modification layer is further examined by using the Mott-Schottky analysis and the impedance spectroscopy technique. The consistent result shows an enhanced carrier collection efficiency and a reduced charge recombination rate of the device via adequate band alignment between the photoactive layer and the cathode using the TiOx modification layer with an optimized O/Ti ratio.
This work reports on indium tin oxide (ITO)-free organic solar cells with roll to roll (R2R) processed organic functional layers. The device stack comprises a chromium-aluminum-chromium (Cr-Al-Cr) electron contact layer on a polyethylene terephthalate (PET) film, a photoactive layer of poly(3-hexylthiophene) (P3HT): (6,6)-phenyl C61 butyric acid methyl ester (PCBM), a hole transport layer of Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) and a silver (Ag) grid for current collection. For the photoactive layer the non-halogenated solvent o-Xylene was used in order to reduce the impact on health and environment for R2R coating on ambient atmospheric conditions. The Cr-Al-Cr layers were sputtered onto the PET rolls in a batch process while the photoactive layer as well as the hole transport layer were applied in a continuous R2R process by slot die coating. The Ag grid was either thermally evaporated through a shadow mask as reference process or deposited by aerosol printing as a more production compatible process. Device efficiencies up to 2.9% on an active area of 1.1 cm(2) were obtained with no difference for the method of grid processing. These experimental results demonstrate that R2R coated organic functional layers in ITO-free devices obtain the same device performance as compared to spin coated laboratory cells. (c) 2014 Elsevier B.V. All rights reserved.
Defect levels play a significant role in altering organic photovoltaic (OPV) performance, affecting device aspects such as recombination, carrier transport, and Fermi-level pinning. In the ongoing effort to optimize the promising OPV technology, the identification, characterization, and potential mitigation or enhancement of such defect states remain important regions of interest. Herein, low frequency admittance spectroscopy is coupled with a high frequency, point-by-point capacitance versus voltage measurement to reveal a previously unknown deep-defect distribution in poly(3-hexylthiophene) based OPVs. The capacitance models of Cohen and Lang, Walter et al. and Kimmerling are employed alongside a trap-free dark current model to give good characterization and substantiation to the discovered band. Repetitions of the measurements on devices with and without a fullerene acceptor show the measured distribution to contain acceptor-like traps spatially located in the polymer bulk. The findings presented here are important for the understanding and optimization of organic solar cells and we expect the presented methods to be generally applicable to other OPV material sets.
A study of the optical and impedance behavior of optimized standard and inverted photovoltaic solar cells based on P3HT:PCBM active nano composites is presented. The standard cells sequence is ITO/HTL1/P3HT:PCBM/Ca/Al and the inverted cells one is ITO/ZnO/P3HT:PCBM/HTL2/Ag where HTL1 and HTL2 are Hole Transport Layers. Absorption and action spectra, together with I–V characteristics, are shown to be quite similar and lead to 4.05% and 3.90% energy conversions, for standard and inverted cells, respectively. Built-in potentials of 0.82–0.89 V and acceptor impurities concentrations of 1.6–2.4 1015 cm−3 are found through capacitance measurements. Impedance spectrometry shows the classical two-circle complex plan curves, one being related to the effective lifetime of charge carriers before recombination at low frequency, and the other one to the diffusion time of these carriers at high frequency. The shape of the curves is identical, showing the ohmic role of the ZnO layer. It is shown that overall resistances in the dark are higher for inverted cells as compared to standard ones, and that this feature is inverted under illumination, with a thousand-time decrease. Global mobilities are in the range 3.8–4.6 10−3 cm2 V−1 s−1, which is slightly higher as compared to the literature. Four different equivalent circuit models are tested on experimental results, and it is concluded that the classical RCPE model (or its Garcia–Belmonte variant) is suitable for this kind of cells.
We review the methods used to simulate the optoelectronic response of organic solar cells and focus on the application of one-dimensional drift-diffusion simulations. We discuss how the important physical processes are treated and review some of the experiments necessary to determine the input parameters for device simulations. To illustrate the usefulness of drift-diffusion simulations, we discuss several case studies, addressing the influence of charged defects on transport in bipolar and unipolar devices, the influence of defects on recombination, device performance and ideality factors. To illustrate frequency domain simulations, we show how to determine the validity range of Mott-Schottky plots for thin devices. Finally, we discuss an example where optical simulations are used to calculate the parasitic absorption in contact layers.
One of the important factors influencing on organic solar cells efficiency is doping of the polymer by atmospheric oxygen. It is known already that the doping may be partially reversed by annealing of the device, but the phenomenon is not well-studied yet. In this paper we studied processes of doping and dedoping of conjugated polymers, using well-known poly(3 hexylthiophene) (P3HT) and derivative of novel low-bandgap polymer poly(3,4-ethylenedioxyselenophene) (PEDOS-C12), by impedance spectroscopy.Possibility of dedoping under annealing was examined for both of the polymers. Influence of annealing atmosphere and sample preparation conditions on dedoping process were studied along with dynamics of self-doping and dedoping processes and influence of light on self-doping dynamics. Dedoping dynamics in inert atmosphere turn out to be biexponential.
Traditional inorganic solar cell models, originating with the work of Shockley, are widely used in understanding bulk heterojunction (BHJ) organic solar cell response (organic solar cells are also referred to as organic photovoltaics, or OPVs). While these models can be useful, there are several key points of departure from traditional solar cell behavior. In this Perspective, we discuss three important areas: (1) geminate pair and bimolecular recombination, (2) effects of interfacial layers inserted between the electrodes and active layer, and (3) resistance effects. Since organic solar cell materials typically have large Coulombic exciton binding energies (e.g., 0.3–0.5 eV), limited dissociation of photogenerated charge carriers can be a significant limitation in these cells that is not observed in traditional silicon solar cells. Additionally, the active layer morphology of BHJ organic solar cells allows free charge carriers to recombine before extraction from the cell, creating another photocurrent loss mechanism. Interfacial layers serve a unique role in BHJ organic solar cells; in addition to conventional functions such as photon transmission and charge injection, interfacial layers often act as “blocking” layers, ensuring that charge carriers are collected at their respective electrodes (i.e., electrons at the cathode and holes at the anode). Additionally, resistance effects in organic solar cells differ from traditional models in both field and cell area dependencies. Organic semiconductor mobilities and charge densities exhibit significant sensitivity to field strength, with mobility varying by 10x over typical cell voltage test ranges (1 V). This creates the need for alternative models to describe cell internal resistance. Finally, resistance losses are also sensitive to cell area, due to the limited conductivities of the transparent electrode materials used. Therefore, accommodation of the above deviations from traditional models is imperative for the design and synthesis of new generation high efficiency organic solar cell materials.
Semi-transparent P3HT:PCBM (poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester) solar cells with high conductivity PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)) top electrodes are used to study degradation. Due to the gas permeability of this type of electrode, the simultaneous exposure to oxygen and light leads to a strong decrease of the short circuit current on the timescale of minutes. The losses are not due to a change in the conductivity of the PEDOT:PSS layer, and the short circuit current can be partially recovered by a heating step, indicating that the observed degradation is taking place in the photo-active layer of the cell.
A study is done to understand the dependence of injection on the degree of doping of an organic semiconductor. A model organic semiconductor, tetra-methyl triphenyl diamine doped polycarbonate (PC:TMTPD) was used for these experiments. By substituting TMTPD molecules with a TMTPD+SbF6- salt, the degree of doping in the organic semiconductor was systematically varied. Changes in the electrical characteristics of devices with various electrodes were analyzed to yield the dependence of injection on the degree of doping. Along with the doping concentration, the temperature and distance between electrodes was also varied. This provided a better understanding of how doping, temperature and electrode spacing affect device performance.
This contribution analyses the performance potential of state of the art organic bulk-heterojunction photovoltaic devices and gives a guideline towards higher device efficiencies. The concept relies on the identification and determination of the relevant material parameters (lifetime, mobility, bandgap, trap density….) for the pristine components and for the blended photovoltaic composites. These material parameters are used as the input for a simulation model, which is demonstrated to correctly describe bulk heterojunction solar cells. Results from simulation are compared to experimental device performance. Comparison between experiment and simulation allows to analyse deficiencies of the pristine compounds, the composite as well as the device architecture. Based on these findings, material and device parameters for a highly efficient bulk heterojunction device are presented.
Efficiencies of organic solar cells based on an interpenetrating network of a conjugated polymer and a fullerene as donor and acceptor materials still need to be improved for commercial use. We have developed a postproduction treatment that improves the performance of solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) by means of a tempering cycle at elevated temperatures in which an external voltage is simultaneously applied, resulting in a significant increase of the short-circuit current. Using this postproduction treatment, an enhancement of the short-circuit current density, Isc, to 8.5 mA cm–2 under illumination with white light at an illumination intensity of 800 W m–2 and an increase in external quantum efficiency (IPCE, incident photon to collected electron efficiency) to 70 % are demonstrated.
To improve the efficiency of polymer solar cells, it is vital to understand which mechanisms control the current–voltage characteristics of a given device. Temperature and light intensity dependence of the main solar cell parameters are very informative for analyzing losses. We report on the current–voltage characteristics and the external photogeneration quantum yield of ITO/PEDOT:PSS/OC1C10-PPV:PCBM/Al as well as of ITO/PEDOT:PSS/P3HT:PCBM/Al devices investigated in the broad temperature range 120–325K under variable illumination, between 0.02 and 100mW/cm2. We discuss the recombination on traps and the low mobility of charge carriers caused by poor morphology of active layers as possible mechanisms limiting the efficiency of these devices.
Here we report enhanced efficiency bulk heterojunction organic solar cells using blend films of regioregular poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6) C <sub>61</sub> (PCBM) that are subjected to a thermal annealing process. Blend films ( P 3 HT : PCBM =1:1 by weight) were prepared using chlorobenzene and 1,2-dichlorobenzene in order to investigate the role of the solvent. Irrespective of the chosen solvent, the optimal device annealing temperature was found to be 140 °C. The highest power conversion efficiency, 3% under air mass 1.5 simulated solar illumination (100 mW / cm <sup>2</sup>) , was achieved by device annealing at 140 °C for 15 min using blend films prepared from chlorobenzene (2.3% for 1,2-dichlorobenzene).
Commercial poly-3-hexylthiophene was used to create thin films, which were analyzed to determine density, the dependence of the optical absorption on film thickness and the effects of impurities on the optical properties of the polymer. Rutherford backscattering spectroscopy (RBS) demonstrated Zn and Br impurities in the films, which could be removed with methanol extraction. These impurities affect the optical properties of the polymer. Film thicknesses were measured using atomic force microscopy, and in combination with the RBS data, the density of the polymer film was ascertained to be 1.33±0.07 g/cm3. The dependence of the polymer absorbance on the film thickness is well modeled by the linear function , where T is the film thickness and A(λmax) is the absorbance maximum.
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