Optical modeling and optimization of multilayer organic photovoltaic cells (original) (raw)
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Global Journal of Pure and Applied Sciences, 2013
A theoretical study of a 3-D modelled bilayer heterojunction organic photovoltaic cell based on copper phthalocyanine (CuPc) as electron donor and fullerene (C 60 ) as electron acceptor is reported. The thin film multilayer stacking theory is applied to the bilayer heterojunction organic solar cell, with the optical matrix of the Abeles theory leading to new expression of generation rate and density of exciton photogenerated in the organic photoactive layer of CuPc/C 60 . The excitons density is investigated considering the excitons dissociation parameters at the different interfaces of the organic photoactive region, i.e. the exciton dissociation velocities S dp , S dg , S dj and S dn respectively associated to the transparent anode (ITO: Indium thin oxide)/donor (CuPc), the donor (CuPc)/acceptor (C 60 ) and the acceptor (C 60 )/metallic cathode (Al: aluminium) interfaces. Moreover, the influences of the monochromatic light wavelength λ and the thickness w of the organic photoactive layer (CuPc/C 60 ) on the exciton density are emphasized.
Optical modeling of organic solar cells based on CuPc and C_60
Applied Optics, 2008
We have investigated the influence of the poly(3,4-ethylenedioxythiophene)-blend-poly(styrenesulfonate) (PEDOT:PSS) layer on the short-circuit current density (J sc ) of single planar heterojunction organic solar cells based on a copper phthalocyanine (CuPc)-buckminsterfullerene (C 60 ) active layer. Complete optical and electrical modeling of the cell has been performed taking into account optical interferences and exciton diffusion. Comparison of experimental and simulated external quantum efficiency has allowed us to estimate the exciton diffusion length to be 37 nm for the CuPc and 19 nm for the C 60 . The dependence of short-circuit current densities versus the thickness of the PEDOT:PSS layer is analyzed and compared with experimental data. It is found that the variation in short-circuit current densities could be explained by optical interferences.
Quantum Efficiency of Organic Solar Cells: Electro-Optical Cavity Considerations
ACS Photonics, 2014
Organic solar cells (OSCs) are composed of one or more layers of order 100 nm thickness sandwiched between metallic and transparent electrodes. As such, they are low finesse, multilayer optical cavities where the optical field distribution is governed by the complex refractive indices and thicknesses of all layers in the "solar cell stack". Optical interference and parasitic absorbance in nonactive layers can have a dramatic effect on the shape of the measured external quantum efficiency (EQE), the parameter often used to optimize device structure and derive critical insight regarding charge generation and extraction. In this communication, we study a model high efficiency OSC system (PCDTBT/PC70BM) as a function of active layer thickness, blend composition and processing. The spectral shapes of the measured EQEs show strong thickness and blend ratio dependence. However, when correctly determined, the internal quantum efficiencies (IQEs) are spectrally flat. The differences in EQE spectral shape predominantly originate from optical interference and parasitic absorptions rather than charge generation or transport phenomena. We also demonstrate similar results for a second model system (PCPDTBT/PC60BM) in which an energy-dependent "IQE-like" response has recently been used to justify the existence of hot excitons. Once again, we show the origin of these spectral phenomena to be optical, not electronic. These cases highlight the fact that thin film organic solar cells (even single junction) must be properly considered as low finesse electrooptical cavities, a point that is not universally appreciated.
Spatial distribution of light absorption in organic photovoltaic devices
Solar Energy, 2005
In this paper we present methods for the optimization of light absorption of organic photoelectric bilayer devices like organic photodetectors and organic solar cells, which show the best performance if it is ensured that the spatial density of the absorbed light energy reaches its maximum in certain ''active'' areas. Mathematical simulations show interesting spatial distributions of light absorption depending on the thicknesses and the optical constants of the individual device layers. Our methods permit to dispose the maxima of absorption density to the area near the interface between the active layers of the bilayer device. We built photovoltaic devices according to the simulated configurations and gained improvements of the power conversion efficiencies of more than one magnitude. Parametric studies were carried out, which give us a suggestion for the potential active materials. Furthermore we analyzed the correlation between the photocurrent and the absorption density in given areas around the p-n junction, which will lead to better understanding of the diffusion range of dissociated charge carriers.
Materials Today: Proceedings, 2021
Organic photovoltaic is an emerging technology employing organic molecules as absorbers has gained tremendous attention in the past two decades due to its potential utilization as a renewable energy source. Contrary to conventional solar cells, excitons are formed under illumination which diffuse towards the heterojunction interface for dissociation into the photo charge-carriers. Heterojunction in the organic photovoltaic devices are formed either by stacking the layers of donor and acceptor molecules or by blending the both. Due to weak van-der Waals interaction between the molecules, intermixed states, in addition to the pure states of donor and acceptor molecules, are formed at the heterojunction interface which affects the electronic process and the power conversion efficiency. Significant efforts have been made to understand the photovoltaic process particularly in the bulk heterojunction solar cells however, least efforts have been devoted to investigate the governing mechanisms in the planner heterojunction solar cells. The motive of this study is to characterize the planar heterojunction solar cells based on the Copper Phthalocyanine (CuPc) and Fullerene (C 60) molecules under light illumination. The devices were fabricated under controlled condition in the normal device configuration, ITO (150 nm) |PEDOT:PSS (50 nm)|CuPc(20 nm)|C 60 (40 nm)|BCP(10 nm)| Ag(80 nm), and characterized performing the light intensity dependent measurement in the wide illumination range. The solar cell parameters such as open circuit voltage, short circuit current and the power conversion efficiency have been discussed as a function of illumination intensity in the wider illumination range.
Solar Energy Materials and Solar Cells, 2003
To determine the actual absorption in the photoactive layer of a plastic solar cell, e.g. consisting of blend of poly(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene vinylene) (MDMO-PPV) and a methanofullerene, w6,6x-Phenyl C -butyric acid methyl 61 ester (PCBM), a matrix formalism for the light propagation in this multi-layer system is applied. This calculation results in an upper limit for the incident photon to collected electron (IPCE) conversion efficiency for a given internal quantum efficiency. Comparisons with experimental results as well as conclusions for the optimal layer thickness are drawn. ᮊ
Journal of the American Chemical Society, 2014
The conventional picture of photocurrent generation in organic solar cells involves photoexcitation of the electron donor, followed by electron transfer to the acceptor via an interfacial charge-transfer state (Channel I). It has been shown that the mirror-image process of acceptor photoexcitation leading to hole transfer to the donor is also an efficient means to generate photocurrent (Channel II). The donor and acceptor components may have overlapping or distinct absorption characteristics. Hence, different excitation wavelengths may preferentially activate one channel or the other, or indeed both. As such, the internal quantum efficiency (IQE) of the solar cell may likewise depend on the excitation wavelength. We show that several model high-efficiency organic solar cell blends, notably PCDTBT:PC70BM and PCPDTBT:PC60/70BM, exhibit flat IQEs across the visible spectrum, suggesting that charge generation is occurring either via a dominant single channel or via both channels but with comparable efficiencies. In contrast, blends of the narrow optical gap copolymer DPP-DTT with PC70BM show two distinct spectrally flat regions in their IQEs, consistent with the two channels operating at different efficiencies. The observed energy dependence of the IQE can be successfully modeled as two parallel photodiodes, each with its own energetics and exciton dynamics but both having the same extraction efficiency. Hence, an excitation-energy dependence of the IQE in this case can be explained as the interplay between two photocurrent-generating channels, without recourse to hot excitons or other exotic processes.
Energy Level Tuning of Non-Fullerene Acceptors in Organic Solar Cells
Journal of the American Chemical Society, 2015
The use of non-fullerene acceptors in organic photovoltaic devices could lead to enhanced efficiencies due to increased open-circuit voltages (VOC) and improved absorption of solar light. Here we systematically investigate planar heterojunction devices comprising peripherally substituted subphthalocyanines as acceptor, and correlate device performance with heterojunction energetics. Due to a balance between VOC and photocurrent, tuning of the interface energy gap is necessary to optimize power conversion efficiency in these devices. In addition, we explore the role of the charge transport layers in the device architecture. It is found that non-fullerene acceptors require adjusted buffer layers with aligned electron transport levels to enable efficient charge extraction, while the insertion of an exciton blocking layer at the anode interface further boosts photocurrent generation. These adjustments result in a planar heterojunction OPV device with 6.9% efficiency and a VOC above 1 V.
This research problem bridges organic solar cells and flexible electronics based on polymer and fullerene active layer and their optimization based on SCAPS simulations. Their power conversion efficiency is around 3-5 percent and their thickness lies in sub-200nm range. The morphology of the bilayer and interface phenomena plays a great role in determining output parameters. The charge states which are created within the layer during photovoltaic generation involve excitons for example. The overall stability of cell depends on the collective effect of all layers in the multijunction. We have focused on thickness optimization based on SCAPS because this is a fundamental physical parameters that can be manipulated easily during fabrication. In case of inverted architecture of cell configuration, stability is enhanced. The output parameters are Voc, Jsc, eta and FF. We successfully compare these output parameters in different comparative configurations, different thickness ranges, different temperature ranges and even after addition of defects to compare simulations with real devices. Finally after addition of bulk and interface defects within acceptable range, we achieve the outputs of latest real devices. We found that the simulated defect density is around 10^7 cm^-2 and 10^11 cm^-2 respectively to achieve real device accuracy.
2008
Exciton harvesting is of fundamental importance for the efficient operation of organic photovoltaic devices. The quantum efficiencies of many organic and hybrid organicinorganic devices are still limited by low exciton harvesting efficiencies. This problem is most apparent in planar heterostructures that suffer from a direct tradeoff between light absorption and exciton harvesting. One approach to overcome small diffusion lengths is the use of triplet excitons. We have an ongoing project investigating pentacene/C60 solar cells to determine if triplets are the dominant exciton species following photoexcitation. Simulations of exciton harvesting suggest that excitons in pentacene are triplets. These triplets are likely formed by an exciton fission route which further has implications for beating the Shockley-Queisser limit and experiments are ongoing to verify this. In addition to using triplets to harvest excitons over long distances, we have developed a new scheme using long range r...