I am grateful for the financial support to this thesis from the Catalan Institute of Chemical Investigation (ICIQ) of Tarragona given by the pre-doctoral fellowship with reference 05/09 of Dr. Margherita Bolognesi, Marta Tessarolo, Tamara Posati, Morena Nocchetti, Valentina Benfenati, Mirko Seri, Giampiero Ruani and Michele Muccini, ''Efficiency Improvement of P3HT:PCBM Solar Cells Containing Scattering Zn-Al Hydrotalcite Nanoparticles in the PEDOT:PSS Layer ', accepted in Organic Photonics and Photovoltaics, 2013. Mirko Seri, Margherita Bolog. , Zhihua Chen, Shaofeng Lu, Wouter Koopman, Antonio Facchetti, and Michele Muccini, ''Fine Structural Tuning of New Cyanated Dithieno[3,2-b:2',3'-d]silole-oligothiophene Copolymers: Synthesis, Characterization and Photovoltaics response”, submitted for publication.
History of photovoltaics
In parallel, the discoveries that led to the birth of modern organic chemistry in the late 19th and early 20th centuries led to increased scientific and commercial interest in organic materials research. Advances in PV technology had a strong impulse after the oil crisis of the 1970s, which caused scientific and industrial research to reach about 15% of production. Therefore, scientific and industrial research in recent decades has been focused mainly on increasing device efficiency and reducing materials and production costs, and these efforts have led to currently emerging PV technologies such as organic photovoltaics.
Organic photovoltaic (OPV)
- Inorganic and organic semiconductors
- Single layer OPV devices
- Donor/acceptor bilayer heterojunction OPV devices
- Donor/acceptor bulk heterojunction (BHJ) OPV devices
- Polymer:fullerene BHJ OPV devices
The intrinsic electronic diversity between inorganic and organic materials causes, first of all, a significant difference in the relative importance of interfacial processes in inorganic PV and OPV devices, which is closely related to the charge generation mechanisms described. Throughout the history of OPV devices, research efforts have been directed at exploiting the described advantages of the properties of organic materials, while trying to A logical way to improve the D/A efficiency of OPV binary heterojunction devices is to maximize the D/A interfacial area while minimizing the exciton diffusion path to the D/A interface.
Working principles in BHJ OPV devices
- Light absorption and exciton formation
- Exciton diffusion to the donor/acceptor interface
- Charge separation at the donor/acceptor interface
- Charge transport in the D and A phases
- Charge collection at the electrodes
The third factor determining the light absorption efficiency of a solar cell's photoactive layer is the overlap between the absorption spectrum and the solar radiation spectrum. This internal field is mainly determined by the different work functions of the electrodes on both sides of the device. The charge transport mechanism in organic semiconductors depends on the degree of order of the molecular species in the BHJ film.
Fundamental photovoltaic parameters
Equivalent circuit of BHJ OPV devices
The load on the RL circuit to which the cell is connected develops a potential difference between the terminals of the device even when the cell is not illuminated. Even BHJ OPVs deviate from ideality, and by analyzing their deviations, we can upgrade the equivalent circuit of the cell to a more representative one and define other important parameters that better describe the behavior of the solar cell. The overall effect of increasing Rs or decreasing Rp on the I(V) characteristics of the cell is summarized in Figures 6b and 6c, respectively.
I-V curves: PCE, Isc, Voc, FF
As a result, as demonstrated by several studies,[44] the Voc in BHJ OPV devices strongly depends on the energy difference between the HOMOD-LUMOA offset at the D/A interface of the cell, rather than on the D band gap . The maximum electric power produced by the OPV cell under illumination is the maximum of the power curve (Pmax) and corresponds to the maximum power point (MPP) in the I(V) curve. The solar cell power conversion efficiency is the ratio of the electrical output of a solar cell to the incident light power to which the cell is exposed.
Quantum efficiency
Strategies to decrease contact resistances (thereby decreasing Rs) include proper selection and processing of buffer layers and electrodes, while strategies to decrease shunt currents bypassing the D/A interface (increasing Rp) include improving BHJ morphology such as and the control and tuning of the energy and kinetics involved in the charge separation process. PCE measured at standard temperature (25°C) and illumination conditions (incident irradiance PL = 100 mW/cm2 and spectral shape AM1.5) is the conventional efficiency value (η) given for each solar cell. The EQE is smaller than the internal quantum efficiency (IQE), which represents the conversion of absorbed photons into charges inside the cell, because the light absorption capacity of the active layer and possible losses due to reflection and scattering are also taken into account in the EQE. account.
Limiting factors in BHJ OPV devices efficiency
This decrease in the internal electric field results in a slowing down of the charge carrier extraction time at the electrodes, resulting in an increase in the steady-state charge carrier density in the photoactive layer toward the open circuit. This increase in charge carrier density can lead to a significant increase in non-geminate recombination losses as the cell voltage increases. It has been shown that these non-geminate losses can be quantified as a function of bias and light intensity and used to understand the light intensity dependence of FF and VOC in many BHJ OPV cells.[51] It should also be noted that the reduction of the internal electric field near open circuit conditions also affects the dissociation efficiency of photogenerated charge carrier pairs; therefore, the magnitude of geminate recombination may directly depend on this field and therefore on the cell voltage.
The aim of this work: strategies to improve the BHJ OPV cells performance. 24
Bruder, Organic solar cells: Correlation between molecular structure, morphology and device performance, PhD thesis, Max-Planck-Institut f¨ur. 30] ASTM E927 – 10, http://www.astm.org/Standards/E927.htm, Standard Specification for Solar Simulation for Terrestrial Photovoltaic Testing.
Photoactive polymers, the donor materials
Advantages of using polymeric over small molecule donors
Traditional donor polymers and recent achievements
In particular, many useful models of the energy and kinetics of OPV devices were proposed based on their foundations in these studies. In parallel, a wide number of strategies aimed at optimizing the morphology of the active layer in order to improve the device performance have been investigated: the use of solvent mixtures or additives, different photoactive layers. Most importantly, the gradual understanding of the operation of OPV devices and the results coming from the optimization of materials flowed into the design of promising new π-conjugated donor polymers, resulting in today's record efficiencies above 9% (certified). [7 ] In the following sections, some of the strategies that have been pursued to improve BHJ OPV device performance by precisely designing and processing novel donor polymers are illustrated.
Requisites for polymeric semiconductors to work as donor in BHJ OPV
From an energetic point of view, a prerequisite for the use of a polymer semiconductor as a donor in BHJ OPV devices is the appropriate position of its frontier orbital energies with the energies of the electron acceptor material constituting the BHJ active layer. On the other hand, a low PDI usually allows greater reproducibility of the chemical and physical properties of a polymer film with a given Mw. In the following sections, some strategies aimed at optimizing the performance of BHJ OPV devices through precise design and processing of donor polymers are presented.
Polymeric donor design and processing strategies to improve BHJ OPVs
- A new class of efficient Si-DT-based polymers
- Experimental
- Results and discussion
- Conclusions
- A new push-pull polymer with unsaturated spacer
- Introduction
- Experimental
- Results and Discussion
- Conclusions
Moreover, a lower lying HOMO level could also improve the chemical stability of the polymer towards moisture oxidizing species. The thickness of the different films was measured with a profilometer (KLA Tencor, P-6) on glass/ITO/PEDOT:PSS substrates. A first optimization of the P(1)-P(4) based BHJ solar cells was performed using PC61BM as acceptor material.
The introduction of MoO3 (5 nm) mainly increases the FF and JSC values. The EQE plots correspond to the broad optical absorption spectra of the active mixtures (Figure 15A and B). After analyzing the optical and electrical properties of the P(1)-P(4):PCXXBM active layers, morphological analyzes were performed (by tapping-mode AFM) to gain a better understanding of how the molecular structure of polymers P(1)-P(4) affects the output parameters of the solar cell.
To better understand the evolution of the OPV device performance of the most efficient P(3)-P(4) polymers, the surface morphology of the polymer:PC71BM film, produced with and without additive (DIO, 2% v /v), also investigated. It is worth noting that the introduction of the -CN group on the polymer backbone, going from P(1) to P(2), increases the fluorescence lifetimes by ~5 times. As expected, the presence of the -CN substituents in the backbone of P(2) lowers the band gap and LUMO energy of the polymer, improving its optical and photophysical properties compared to the unsubstituted polymer P(1).
In addition, the presence of the longer alkylthiophene groups in P(3) and P(4) significantly increases the polymer's solubility, electrical properties (light absorption and hole mobility), and improves the nanostructuring and optoelectrical properties of the corresponding blend films. , compared to P(1) and P(2). Thus, the recalculated HOMO/LUMO levels of the P(BDT-BTD) were in excellent agreement with those of P(BDT-V-BTD). The two thin films have similar morphologies but different domain size, which depends on the nature of the acceptor in the mixture.
Conclusions
Bibliography
Charge extracting (buffer) layers
TiO x and ZnO based CBL: effects on the charge recombination and efficiency of
- Experimental
- Results and discussion
- Metal oxide thin film morphology
- Devices photovoltaic characterization
- Charge recombination dynamics
In the case of the inverted devices, the metal oxide layers for i-OPVs were prepared with different procedures depending on the material. The Voc of the device before laser disruption under this light illumination intensity was measured with a simple voltmeter. On the other hand, when different contact architectures were used, the observed differences in the I-V curve characteristics could be attributed to the effects of the selective contacts used.
In terms of FF, the biggest difference is between the TiOx-based i-OPV device and the other two: only 52% FF is measured for the first device compared to 56–57% for the other two devices. The inhomogeneity of the TiOx surface compared to the ZnO or PEDOT:PSS surfaces shown by SEM analysis seems to support this hypothesis. The potential decay with time varies with the discharge process of the device, including charge recombination kinetics.
Since σ is representative of the perturbation of the DOS distribution centered at E0, as defined above, this result indicates a broadening of the DOS distribution for the TiOx-based inverted device compared to the other two. This broadening can be attributed to the presence of the TiOx layer that either: (a) provides, in addition to the energetic states determined by P3HT and PCBM, additional trap states that can be filled by free charge carriers during device performance, or (b) affects the morphology of the overlying organic layer and therefore its DOS distribution, or (c) both. The filling of the HOMO and LUMO levels with a wider DOS distribution then leads to a lower effective energy gap (indicated in the figure as q · Voc2), while the filling of the HOMO and LUMO levels with a narrower DOS distribution leading to a higher effective energy gap (q · Voc1).
Analysis of the charge density distribution in the device under operating conditions under different light biases revealed a.
Light managing techniques in OPVs
Scattering particles in the PEODT:PSS layer of P3HT:PCBM-based solar cells
- Materials and methods
- HTs/PEDOT:PSS thin-films preparation and characterization .
- Device fabrication and characterization
- HTs/PEDOT:PSS thin-film properties
- Scattering of HTs:PEDOT:PSS films
- Device performances
- Conclusions
Bibliography
In the first part of this work, special attention was paid to optimizing the performance of the BHJ OPV devices through a precise adjustment of the chemical structure of the polymeric donor materials. The presence of the cyano substituents in the polymer backbone has been shown to effectively lower its band gap and HOMO and LUMO energies, improving its optical, photophysical and photovoltaic properties compared to the unsubstituted polymer. As a result, the PCEs of the corresponding BHJ OPV devices were improved from 1% to about 5%.
In particular, the use of a vinylene spacer allowed increased π delocalization along the polymer backbone relative to the most common direct aryl–aryl linked copolymer. Electrochemical investigation highlighted the ambipolar character of the new polymer, where both p- and n-doping processes could be observed. On the other hand, the presence of the vinylene spacer resulted in a lower optical bandgap and improved thermal stability compared to the reference polymer with a single bond spacer, probably due to increased planarization of the backbone resulting in stronger intermolecular interactions.
In a second part of the work, the focus was shifted to optimizing the performance of BHJ OPV devices by a deeper study of the role of buffer layers. The analysis of the charge density distribution in the devices under operating conditions under different light biases revealed a difference in the density of states distribution of the TiOx-based device, compared to the ZnO-based and standard ones. The dispersing nanoparticles (Zn-Al layered double hydroxide, or hydrotalcite nanoparticles) were easily dispersed in the PEDOT:PSS layer, thereby avoiding any change of the standard array of the BHJ OPV device.
Finally, this thesis provides an overview of the different possible strategies that can be adopted for improving the performance of BHJ OPV devices.
