Quantum Efficiency of Organic Solar Cells: Electro-Optical Cavity Considerations (original) (raw)
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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.
Solar Energy Materials and Solar Cells, 2012
We demonstrate experimentally that an appropriate combination of the layer thicknesses in an inverted P3HT:PCBM cell leads to an optical interference such that the EQE amounts to 91% of IQE. We observe that reflectivity between layers is minimized in a wavelength range of more than 100 nm. In that range the EQE closely matches the IQE. The role played by the optical interference in improving the performance of the fabricated solar cells is confirmed by EQE calculated numerically using a model based on the transfer matrix method. Additionally, we observed that a similar cell with an active material 1.7 times thicker exhibited a lower PCE. The poor photon harvesting in the later cell configuration is attributed to an EQE that amounts only to 72% of the IQE.
Journal of Applied Physics, 2007
The realization of highly efficient organic solar cells requires the understanding and the optimization of the light path in the photoactive layer. We present in this article our approach to measure and model the optical properties of our bulk-heterojunction devices, and to control them in order to enhance the photovoltaic performances. We report our recent observations on the dependence of the external quantum efficiency ͑EQE͒ on the incidence angle of the light, and our results on the determination of internal quantum efficiency based on EQE measurement and optical modeling cross-checked by reflection measurements. We investigate poly͑3-hexylthiophene͒: 1-͑3-methoxy-carbonyl͒ propyl-1-phenyl͓6,6͔C 61 based solar cells with two different thicknesses of the active layer ͑170 and 880 nm͒, and show that in the thin ones the absorption is enhanced for oblique incident radiation.
a b s t r a c t A 2-dodecyl benzotriazole and 9,9-dioctylfluorene containing alternating conjugated polymer, poly((9,9-dioctylfluorene)-2,7-diyl-(4,7-bis(thien-2-yl) 2-dodecyl-benzo[1,2,3]triazole)) (PFTBT), was blended with PCBM (1:1, w/w) and spin coated on ITO substrates using varying rotational speeds to obtain different active layer thicknesses. J–V characteristics of the constructed devices were investi-gated both in dark and under simulated sunlight (AM 1.5G, 100 mW/cm 2). For the determination of hole mobilities the space charge limited current (SCLC) method was used and found as 1.69 Â 10 À 6 cm 2 /Vs. In addition, the power conversion efficiency (PCE) of the devices was varied according to active layer thickness and the best power conversion efficiency was recorded as 1.06%. Moreover, incident-photon-to-current-efficiency (IPCE) measurements were carried out and the best efficiency was found to be 51%. Morphology of the active layers was probed using AFM and TEM techniques...
Optimizing the organic solar cell efficiency: Role of the active layer thickness
Solar Energy Materials and Solar Cells, 2013
A 2-dodecyl benzotriazole and 9,9-dioctylfluorene containing alternating conjugated polymer, poly((9,9-dioctylfluorene)-2,7-diyl-(4,7-bis(thien-2-yl) 2-dodecyl-benzo[1,2,3]triazole)) (PFTBT), was blended with PCBM (1:1, w/w) and spin coated on ITO substrates using varying rotational speeds to obtain different active layer thicknesses. J-V characteristics of the constructed devices were investigated both in dark and under simulated sunlight (AM 1.5G, 100 mW/cm 2 ). For the determination of hole mobilities the space charge limited current (SCLC) method was used and found as 1.69 Â 10 À 6 cm 2 /Vs. In addition, the power conversion efficiency (PCE) of the devices was varied according to active layer thickness and the best power conversion efficiency was recorded as 1.06%. Moreover, incident-photon-to-current-efficiency (IPCE) measurements were carried out and the best efficiency was found to be 51%. Morphology of the active layers was probed using AFM and TEM techniques.
Optical modeling and optimization of multilayer organic photovoltaic cells
Journal of Applied Spectroscopy
We show that the spectral position of the maxima in the exciton generation rate G in a photovoltaic cell, taking into account the spectral energy distribution in the AM1.5G solar spectrum, is determined by the absorption bands of its donor and acceptor materials. It varies slightly as the thicknesses of the layers in the cell change. Interference of light affects only the magnitude of these maxima. For a cell based on a CuPc (copper phthalocyanine)-C 60 (fullerene) heterojunction, the G maxima are located at 640 nm, 720 nm (absorption in CuPc) and close to 495 nm (absorption in C 60). The photovoltaic cell can be optimized using the ratio of the magnitudes of these maxima and their variations as layer thicknesses are varied and the exciton diffusion length is taken into account.