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Luminescent Sr2CeO4 nanocrystals for applications in organic solar cells with conjugated polymers
Journal of Luminescence ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescent Sr2CeO4 nanocrystals for applications in organic solar cells with conjugated polymers M. Dusza a,b,n, M. Stefanski b, M. Wozniak a, D. Hreniak b, Y. Gerasymchuk b, L. Marciniak b, F. Granek a, W. Strek b a b
Wroclaw Research Centre EIT þ , Stablowicka 147, 54-066 Wroclaw, Poland Institute of Low Temperature and Structure Research, Polish Academy of Science, Okolna 2, 50-422 Wroclaw, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 14 September 2014 Received in revised form 3 August 2015 Accepted 5 August 2015
High energy photons (UV) are not efficiently utilized in the organic solar cells mainly due to the negative impact on long term stability and the high absorption coefficient of flexible substrate and transparent electrodes. Powder of nanocrystalline strontium cerium oxides (Sr2CeO4) was used for fabrication of thin luminescent layers. The down-shifting properties of Sr2CeO4 could improve the stability of organic solar cell without significant losses of short-circuit current. Various design aspects associated with deposition of thin films and their luminescent properties are discussed. The measurement method allowed for determining the down-shifting properties (in the qualitatively way) of fabricated samples which were placed on the top of organic solar cells with the P3HT:PCBM bulk heterojunction. & 2015 Elsevier B.V. All rights reserved.
Keywords: Down-shifting Luminescent Thin Films Sr2CeO4 Organic solar cells Photovoltaics
1. Introduction Luminescent solar concentrators (LSCs) have been a subject of many publications since the late 1970s [1,2]. The LSC concept involves the application of large area glasses with luminescent dyes and small solar cells on the edges, as a solar energy converter. Luminescent dyes inside the glass emit light and due to total internal reflection a significant amount of light reaches the edges and thereby also the solar cells, which leads to the collection of light from a much larger area corresponding to the dimensions of the solar cells [3–5]. Nowadays photovoltaic technology is much cheaper than 40 years ago and the economic profit by reducing the area of photovoltaic cells is not as high as before. Luminescent properties of thin films based on Sr2CeO4 on the top of low-cost solar cells were investigated. Sr2CeO4 has an orthorhombic structure. Octahedra CeO6 are linked by strontium ions (edge-sharing) [6]. The luminescence properties of Sr2CeO4 are based on charge-transfer (CT) transitions [7]. A few years ago the high photoluminescent (PL) properties of oxide based materials were investigated for the display industry, especially as a component in field emission displays (FEDs) [8]. Due to the high efficient luminescence of Sr2CeO4 under UV, n Corresponding author at: Wroclaw Research Centre EIT þ , Stablowicka 147, 54-066 Wroclaw, Poland. Tel.: þ 48 71 734 71 56. E-mail address: [email protected] (M. Dusza).
cathode ray and X-ray excitation, this compound could be applied in X-ray detectors or lamps [9–12]. In this paper Sr2CeO4 was applied as a down-shifting thin film for potential application in organic solar cells with conjugated polymers. The organic photovoltaic (OPV) technology gained serious attention in the past few years. In the last fourteen years the highest efficiency obtained from organic solar cells has risen from 2.5% to over 11% [13–15]. It was the most rapid progress among all photovoltaic technologies. The state-of-the-art of OPV is represented by bulk heterojunction (BHJ) structures [14,16]. It is a blend of two materials with different electron affinities, so-called donors and acceptors. Conjugated polymers as donors and fullerene derivatives as acceptors are commonly applied. Photon absorption leads to the generation of a bound electron–hole pair (exciton) [17]. In organic materials, Coulomb attraction between the electron and the hole is strong and the dissociation of the exciton into free charge carriers is efficient only at the donor–acceptor interface. Moreover exciton diffusion length is in the range of 1–20 nm [18–21]. Due to this fact the nanoscale morphology of BHJ is required for efficient separation of excitons [14,22]. The most common and well known BHJ is P3HT:PCBM (Poly(3-hexylthiophene-2,5-diyl):Phenyl-C61-butyric acid methyl ester) blend and based on these materials the efficiency of solar cells close to 5% was achieved [23–25]. These structures have a great potential in reducing the energy consumption of manufacturing processes and the synthesis costs of active materials. Taking into account the
http://dx.doi.org/10.1016/j.jlumin.2015.08.006 0022-2313/& 2015 Elsevier B.V. All rights reserved.
Please cite this article as: M. Dusza, et al., J. Lumin. (2015), http://dx.doi.org/10.1016/j.jlumin.2015.08.006i
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Fig. 1. I–V curve of inverted organic solar cell directly after fabrication. The I–V curves measured after 18 and 26 days after fabrication are shown as well. As an inset a photo of the fabricated inverted organic solar cells is shown.
application possibilities of flexible and large area OPV modules, organic solar cells are an alternative for heavy and non-flexible 1st generation solar cells [26,27]. Roll-to-roll production of organic solar cells is high throughput what is important in the economic point of view for photovoltaic technology [28–30]. Unfortunately, the low-cost OPV has a still short operational time. The main processes of degradation are photo-oxidation and photolysis of organic materials [31,32]. Organic solar cells are very sensitive for UV illumination which induces strong degradation of conjugated polymers [33–35]. High-energy photons break the carbon bonds in the conducting polymer structure [36,37]. Down-shifting of UV light into VIS region could be a very useful technique to protect the organic solar cells from the high energy photons without significant loss of photocurrent [38–40]. Moreover, high-energy UV radiation is often not effectively utilized by the OPV module, because of the substrate absorption, which partially cuts off this spectral range, especially in case of flexible plastic substrates. The first papers about application of luminescent material in the OPV technology were published few years ago and were focused mainly on up-conversion or LSC as a large area glass [41–43]. A purpose of this paper was to report results of studies on fabrication of thin films deposited from nanopowders of Sr2CeO4 phosphor as a down-converter and their application in organic solar cell technology based on P3HT:PCBM bulk heterojunction.
2. Experimental 2.1. Fabrication of the inverted organic solar cells Organic solar cells based on P3HT:PCBM were fabricated in the inverted structure ITO/ZnO/P3HT:PCBM/MoO3/Ag. Glass substrates covered by pre-patterned indium tin oxide ITO (Ossila) were cleaned in acetone and isopropanol, for 30 min each, using an ultrasonic bath. Zinc oxide layers were spin-coated from ethanol ZnO nanoparticles dispersion (Sigma-Aldrich) and were annealed at 500 °C in air. Poly(3-hexylthiophene-2,5-diyl) (P3HT, SigmaAldrich) and phenyl-C61-butyric acid methyl ester (PCBM, SigmaAldrich) at mass ratio 1:0.8 were dissolved in the 1,2-dichlorobenzene (60 mg/ml) by 4 h stirring at 60 °C and 18 h aging of the solution at room temperature. The active layers were spin-coated at 2000 rpm under nitrogen atmosphere. Thin films of MoO3 ( 12 nm) and silver electrodes ( 4100 nm) were thermally evaporated under vacuum of 2 10 6 mbar. Post-annealing treatment
at 150 °C for 120 s was applied. Active area of inverted solar cell was defined by cross-bar of electrodes and was equal to 4.5 mm2. I–V measurements of organic solar cells were performed using sun simulator (model #SS80AAA, Photo Emission Tech. and I–V Tracer Auxiliary Unit, PV Test Solutions) and Keithley 2401 Low Voltage SourceMeter. Due to the instability of organic cells in the first few days after fabrication, the measurements with luminescent thin films were performed after 18 days. The solar cells were kept in air and in the dark during the initial aging. I–V curves, measured under STC (Standard Test Conditions-spectrum AM1.5, 1000 W/m2, 25 °C), of inverted organic solar cell after fabrication and after 18 and 26 days are shown in Fig. 1. Differences between measurement after 18 and 26 days were very small, which means that the cell properties were unchanged during the measurements with luminescent samples. 2.2. Synthesis and fabrication of the luminescent thin films The Sr2CeO4 phosphor was prepared with modified sol–gel technique. Sr(NO3)2 (Acros Organics, 99þ %) and Ce(NO3)3 6H2O (SERVA, pure) were used as starting materials. Stoichiometric amounts of raw materials were weighted and dissolved in deionized water. Subsequently, appropriate amount of citric acid (POCH, 99.5%) was added under 60 °C and intense stirring. After few hours, ethylene glycol (POCH, pure) was added dropwise into the mixture. Several hours later the solution was placed in laboratory dryer at 90 °C for few days. As a result a brown resin was obtained. Finally, the sample was annealed at 800 °C in the muffle furnace under air atmosphere. In consequence, Sr2CeO4 fine powder was synthesized. In order to carry out the Sr2CeO4 powder to the solution the sample was dissolved in appropriate amount of chloroform. The size of the agglomerates was reduced using ultrasonic dispersant from SPE UKRROSPRIBOR LTD model: UZDN-M900T with ultrasonic power equal to 900 W, range of operating frequencies: 20– 25 kHz and 6 mm width head. Obtained suspension was introduced to the solution of PMMA in chloroform. The particles size of Sr2CeO4 powder was determined using Rietveld refinement which was around 55 nm. XRD spectra were measured with X’Pert PRO powder diffractometer (PANalitycal) equipped with a linear PIXcel detector and using Cu Kα radiation (λ ¼ 1.54056 Å). The deagglomeration process was performed using ultrasonic dispersant. The spectroscopic properties of Sr2CeO4 were measured using the FLS980 Fluorescence Spectrometer from Edinburgh
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Fig. 2. Spectral characteristics of Sr2CeO4 nanocrystals (a) and the comparison of Sr2CeO4 emission spectrum and external quantum efficiency of fabricated organic solar cell based on P3HT:PCBM (b).
Fig. 3. Surface images of Sr2CeO4 layer deposited from chloroform.
Instruments. All the presented spectra were corrected with respect to the detector sensitivity and the lamp spectrum. The emission spectrum of Sr2CeO4 matches well with the external quantum efficiency of fabricated inverted organic solar cells based on P3HT: PCBM (Fig. 2). Photoluminescence efficiency of Sr2CeO4 nanocrystals excited at 273 nm was equal to 9.5%. More details on synthesis and properties of Sr2CeO4 can be found in the paper [44]. In this work the luminescent samples were deposited from dispersion of Sr2CeO4 nanocrystals in chloroform and in chloroform with the PMMA addition. Thin films of the phosphor were spin-coated with various rotational speeds (from 1000 up to 7000 rpm) or drop-casted on glass. After deposition the layers were stored under fume hood for 24 h for complete evaporation of
the solvent. The last sample was produced by form-casting and the bulk piece of PMMA with phosphor was obtained.
3. Measurement method, results and discussion Using the profilometer (Dektak XT), roughness and thickness of spin-coated layers were measured. Thickness of samples deposited from chloroform with PMMA addition was equal to 3.1 and 2.1 μm for 1000 and 2000 rpm, respectively. Samples deposited without PMMA addition were discontinuous and mean value of the separated grains height were in the range of 200–400 nm. The spincoated samples of Sr2CeO4 were transparent but high scattering of
Please cite this article as: M. Dusza, et al., J. Lumin. (2015), http://dx.doi.org/10.1016/j.jlumin.2015.08.006i
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Fig. 4. The mercury lamp emission spectrum and transmission of the optical filter.
Fig. 5. Schematic illustration of the measurement setup. On the left (a) solar cell with the optical filter on the top (cut-off the UV light). On the right (b) solar cell with optical filter and luminescent element on the top (down-shifting phenomenon of UV light).
light due to the high roughness (Ra ¼0.3–0.4 μm) of layers was observed. Using the PVE300 Photovoltaic Device Characterisation System (Bentham Instruments Ltd.) with integrated sphere and specular light trap, the diffusion reflection of samples with PMMA addition was estimated to 3575% of the total reflection. Dropcasted and form-casted samples were not transparent, due to large layer thickness in comparison to spin-coated films. Drop-casted film had a few hundreds of micrometers of thickness and it was inhomogeneous. Form-casted sample was 2 mm thick. Using the SEM (Scanning Electron Microscopy-FEI Helios NanoLab 450 HF) surface of the drop-casted sample on silicon substrate was imaged (Fig. 3). High agglomeration of grains was observed. Due to the scattering and reflection losses of transparent luminescent layers an increase in the efficiency was not observed under STC of solar cells. Accordingly to the large optical losses of luminescent layers it was decided to perform the qualitative measurements of the down-shifting properties under UV light. As a UV light source (illumination maximum at 254 nm) the mercury lamp was used. Optical filter was used to cut-off the wavelengths below 450 nm (Fig. 4). Thereby the organic solar cell generated very low current from the weak background light – Ibackground_light ¼ 13.5 nA. On the top of structure, shown in the Fig. 5a, phosphor samples were placed (Fig. 5b) and under UV lamp short circuit currents of organic solar cell (Isc_UV) were measured. This measurement method allows to determine the current gain only due to
Fig. 6. Short circuit currents of the inverted organic solar cells under UV-light with optical filter and luminescent samples on the top.
application of the luminescent samples based on Sr2CeO4 and their down-shifting properties. The drop-casted and bulk PMMA with phospors exhibited the highest increase of short circuit current of solar cell and thus the highest down-shifting properties. Unfortunately the thick layers were nontransparent. Application of the Sr2CeO4 layers deposited from chloroform at 1200 and 2400 rpm and slight increases of current were observed (Fig. 6). Much better down-shifting properties were obtained for layers deposited from chloroform with
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Advanced Materials” - NanoMat (POIG.01.01.02-02-002/08) financed by the European Regional Development Fund (Operational Programme Innovative Economy, 1.1.2).
References [1] [2] [3] [4] [5] [6]
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Fig. 7. Photograhs of the luminescent samples during the measurements under UV lamp. Drop-casted layer of Sr2CeO4 on glass (a). Form-casted bulk PMMA with phosphor (b). Spin-coated sample from Sr2CeO4 chloroform dispersion on glass at 1200 rpm (c). Spin-coated sample from Sr2CeO4 chloroform with PMMA addition at 2000 rpm (d) and 1000 rpm (e). The short circuit currents of organic solar cell measured under UV lamp (Isc_UV) is shown for all samples for comparison.
[21] [22] [23] [24] [25] [26] [27] [28]
PMMA addition. In Fig. 7 luminescent samples during the measurement are shown.
[29] [30]
4. Conclusions
[31]
The Sr2CeO4 nanocrystals were synthesized and their emission spectrum matches well to the external quantum efficiency of fabricated inverted organic solar cells based on P3HT:PCBM. The luminescent samples for application on the top of solar cell were fabricated from nanocrystals dispersed in chloroform and chloroform with PMMA addition by spin-coating and drop-casting deposition methods. Due to high scattering, reflection and total internal reflection losses improvement of efficiency under STC was not observed but using the UV-light source and luminescent layer on the top of inverted organic solar cells based on P3HT:PCBM down-shifting phenomenon was observed. The results of present work are preliminary for further studies of low-cost luminescent layers in organic and hybrid solar cells to increase the efficiency and protect the device from the high-energetic UV light. The future research will be focused on fabrication and non-scattering and transparent luminescent layers and integration of the phosphor material into the solar cell structure.
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Acknowledgments The work was supported by Wroclaw Research Centre EIT þ within the project "The Application of Nanotechnology in
Please cite this article as: M. Dusza, et al., J. Lumin. (2015), http://dx.doi.org/10.1016/j.jlumin.2015.08.006i
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescent Sr2CeO4 nanocrystals for applications in organic solar cells with conjugated polymers M. Dusza a,b,n, M. Stefanski b, M. Wozniak a, D. Hreniak b, Y. Gerasymchuk b, L. Marciniak b, F. Granek a, W. Strek b a b
Wroclaw Research Centre EIT þ , Stablowicka 147, 54-066 Wroclaw, Poland Institute of Low Temperature and Structure Research, Polish Academy of Science, Okolna 2, 50-422 Wroclaw, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 14 September 2014 Received in revised form 3 August 2015 Accepted 5 August 2015
High energy photons (UV) are not efficiently utilized in the organic solar cells mainly due to the negative impact on long term stability and the high absorption coefficient of flexible substrate and transparent electrodes. Powder of nanocrystalline strontium cerium oxides (Sr2CeO4) was used for fabrication of thin luminescent layers. The down-shifting properties of Sr2CeO4 could improve the stability of organic solar cell without significant losses of short-circuit current. Various design aspects associated with deposition of thin films and their luminescent properties are discussed. The measurement method allowed for determining the down-shifting properties (in the qualitatively way) of fabricated samples which were placed on the top of organic solar cells with the P3HT:PCBM bulk heterojunction. & 2015 Elsevier B.V. All rights reserved.
Keywords: Down-shifting Luminescent Thin Films Sr2CeO4 Organic solar cells Photovoltaics
1. Introduction Luminescent solar concentrators (LSCs) have been a subject of many publications since the late 1970s [1,2]. The LSC concept involves the application of large area glasses with luminescent dyes and small solar cells on the edges, as a solar energy converter. Luminescent dyes inside the glass emit light and due to total internal reflection a significant amount of light reaches the edges and thereby also the solar cells, which leads to the collection of light from a much larger area corresponding to the dimensions of the solar cells [3–5]. Nowadays photovoltaic technology is much cheaper than 40 years ago and the economic profit by reducing the area of photovoltaic cells is not as high as before. Luminescent properties of thin films based on Sr2CeO4 on the top of low-cost solar cells were investigated. Sr2CeO4 has an orthorhombic structure. Octahedra CeO6 are linked by strontium ions (edge-sharing) [6]. The luminescence properties of Sr2CeO4 are based on charge-transfer (CT) transitions [7]. A few years ago the high photoluminescent (PL) properties of oxide based materials were investigated for the display industry, especially as a component in field emission displays (FEDs) [8]. Due to the high efficient luminescence of Sr2CeO4 under UV, n Corresponding author at: Wroclaw Research Centre EIT þ , Stablowicka 147, 54-066 Wroclaw, Poland. Tel.: þ 48 71 734 71 56. E-mail address: [email protected] (M. Dusza).
cathode ray and X-ray excitation, this compound could be applied in X-ray detectors or lamps [9–12]. In this paper Sr2CeO4 was applied as a down-shifting thin film for potential application in organic solar cells with conjugated polymers. The organic photovoltaic (OPV) technology gained serious attention in the past few years. In the last fourteen years the highest efficiency obtained from organic solar cells has risen from 2.5% to over 11% [13–15]. It was the most rapid progress among all photovoltaic technologies. The state-of-the-art of OPV is represented by bulk heterojunction (BHJ) structures [14,16]. It is a blend of two materials with different electron affinities, so-called donors and acceptors. Conjugated polymers as donors and fullerene derivatives as acceptors are commonly applied. Photon absorption leads to the generation of a bound electron–hole pair (exciton) [17]. In organic materials, Coulomb attraction between the electron and the hole is strong and the dissociation of the exciton into free charge carriers is efficient only at the donor–acceptor interface. Moreover exciton diffusion length is in the range of 1–20 nm [18–21]. Due to this fact the nanoscale morphology of BHJ is required for efficient separation of excitons [14,22]. The most common and well known BHJ is P3HT:PCBM (Poly(3-hexylthiophene-2,5-diyl):Phenyl-C61-butyric acid methyl ester) blend and based on these materials the efficiency of solar cells close to 5% was achieved [23–25]. These structures have a great potential in reducing the energy consumption of manufacturing processes and the synthesis costs of active materials. Taking into account the
http://dx.doi.org/10.1016/j.jlumin.2015.08.006 0022-2313/& 2015 Elsevier B.V. All rights reserved.
Please cite this article as: M. Dusza, et al., J. Lumin. (2015), http://dx.doi.org/10.1016/j.jlumin.2015.08.006i
M. Dusza et al. / Journal of Luminescence ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
Fig. 1. I–V curve of inverted organic solar cell directly after fabrication. The I–V curves measured after 18 and 26 days after fabrication are shown as well. As an inset a photo of the fabricated inverted organic solar cells is shown.
application possibilities of flexible and large area OPV modules, organic solar cells are an alternative for heavy and non-flexible 1st generation solar cells [26,27]. Roll-to-roll production of organic solar cells is high throughput what is important in the economic point of view for photovoltaic technology [28–30]. Unfortunately, the low-cost OPV has a still short operational time. The main processes of degradation are photo-oxidation and photolysis of organic materials [31,32]. Organic solar cells are very sensitive for UV illumination which induces strong degradation of conjugated polymers [33–35]. High-energy photons break the carbon bonds in the conducting polymer structure [36,37]. Down-shifting of UV light into VIS region could be a very useful technique to protect the organic solar cells from the high energy photons without significant loss of photocurrent [38–40]. Moreover, high-energy UV radiation is often not effectively utilized by the OPV module, because of the substrate absorption, which partially cuts off this spectral range, especially in case of flexible plastic substrates. The first papers about application of luminescent material in the OPV technology were published few years ago and were focused mainly on up-conversion or LSC as a large area glass [41–43]. A purpose of this paper was to report results of studies on fabrication of thin films deposited from nanopowders of Sr2CeO4 phosphor as a down-converter and their application in organic solar cell technology based on P3HT:PCBM bulk heterojunction.
2. Experimental 2.1. Fabrication of the inverted organic solar cells Organic solar cells based on P3HT:PCBM were fabricated in the inverted structure ITO/ZnO/P3HT:PCBM/MoO3/Ag. Glass substrates covered by pre-patterned indium tin oxide ITO (Ossila) were cleaned in acetone and isopropanol, for 30 min each, using an ultrasonic bath. Zinc oxide layers were spin-coated from ethanol ZnO nanoparticles dispersion (Sigma-Aldrich) and were annealed at 500 °C in air. Poly(3-hexylthiophene-2,5-diyl) (P3HT, SigmaAldrich) and phenyl-C61-butyric acid methyl ester (PCBM, SigmaAldrich) at mass ratio 1:0.8 were dissolved in the 1,2-dichlorobenzene (60 mg/ml) by 4 h stirring at 60 °C and 18 h aging of the solution at room temperature. The active layers were spin-coated at 2000 rpm under nitrogen atmosphere. Thin films of MoO3 ( 12 nm) and silver electrodes ( 4100 nm) were thermally evaporated under vacuum of 2 10 6 mbar. Post-annealing treatment
at 150 °C for 120 s was applied. Active area of inverted solar cell was defined by cross-bar of electrodes and was equal to 4.5 mm2. I–V measurements of organic solar cells were performed using sun simulator (model #SS80AAA, Photo Emission Tech. and I–V Tracer Auxiliary Unit, PV Test Solutions) and Keithley 2401 Low Voltage SourceMeter. Due to the instability of organic cells in the first few days after fabrication, the measurements with luminescent thin films were performed after 18 days. The solar cells were kept in air and in the dark during the initial aging. I–V curves, measured under STC (Standard Test Conditions-spectrum AM1.5, 1000 W/m2, 25 °C), of inverted organic solar cell after fabrication and after 18 and 26 days are shown in Fig. 1. Differences between measurement after 18 and 26 days were very small, which means that the cell properties were unchanged during the measurements with luminescent samples. 2.2. Synthesis and fabrication of the luminescent thin films The Sr2CeO4 phosphor was prepared with modified sol–gel technique. Sr(NO3)2 (Acros Organics, 99þ %) and Ce(NO3)3 6H2O (SERVA, pure) were used as starting materials. Stoichiometric amounts of raw materials were weighted and dissolved in deionized water. Subsequently, appropriate amount of citric acid (POCH, 99.5%) was added under 60 °C and intense stirring. After few hours, ethylene glycol (POCH, pure) was added dropwise into the mixture. Several hours later the solution was placed in laboratory dryer at 90 °C for few days. As a result a brown resin was obtained. Finally, the sample was annealed at 800 °C in the muffle furnace under air atmosphere. In consequence, Sr2CeO4 fine powder was synthesized. In order to carry out the Sr2CeO4 powder to the solution the sample was dissolved in appropriate amount of chloroform. The size of the agglomerates was reduced using ultrasonic dispersant from SPE UKRROSPRIBOR LTD model: UZDN-M900T with ultrasonic power equal to 900 W, range of operating frequencies: 20– 25 kHz and 6 mm width head. Obtained suspension was introduced to the solution of PMMA in chloroform. The particles size of Sr2CeO4 powder was determined using Rietveld refinement which was around 55 nm. XRD spectra were measured with X’Pert PRO powder diffractometer (PANalitycal) equipped with a linear PIXcel detector and using Cu Kα radiation (λ ¼ 1.54056 Å). The deagglomeration process was performed using ultrasonic dispersant. The spectroscopic properties of Sr2CeO4 were measured using the FLS980 Fluorescence Spectrometer from Edinburgh
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Fig. 2. Spectral characteristics of Sr2CeO4 nanocrystals (a) and the comparison of Sr2CeO4 emission spectrum and external quantum efficiency of fabricated organic solar cell based on P3HT:PCBM (b).
Fig. 3. Surface images of Sr2CeO4 layer deposited from chloroform.
Instruments. All the presented spectra were corrected with respect to the detector sensitivity and the lamp spectrum. The emission spectrum of Sr2CeO4 matches well with the external quantum efficiency of fabricated inverted organic solar cells based on P3HT: PCBM (Fig. 2). Photoluminescence efficiency of Sr2CeO4 nanocrystals excited at 273 nm was equal to 9.5%. More details on synthesis and properties of Sr2CeO4 can be found in the paper [44]. In this work the luminescent samples were deposited from dispersion of Sr2CeO4 nanocrystals in chloroform and in chloroform with the PMMA addition. Thin films of the phosphor were spin-coated with various rotational speeds (from 1000 up to 7000 rpm) or drop-casted on glass. After deposition the layers were stored under fume hood for 24 h for complete evaporation of
the solvent. The last sample was produced by form-casting and the bulk piece of PMMA with phosphor was obtained.
3. Measurement method, results and discussion Using the profilometer (Dektak XT), roughness and thickness of spin-coated layers were measured. Thickness of samples deposited from chloroform with PMMA addition was equal to 3.1 and 2.1 μm for 1000 and 2000 rpm, respectively. Samples deposited without PMMA addition were discontinuous and mean value of the separated grains height were in the range of 200–400 nm. The spincoated samples of Sr2CeO4 were transparent but high scattering of
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Fig. 4. The mercury lamp emission spectrum and transmission of the optical filter.
Fig. 5. Schematic illustration of the measurement setup. On the left (a) solar cell with the optical filter on the top (cut-off the UV light). On the right (b) solar cell with optical filter and luminescent element on the top (down-shifting phenomenon of UV light).
light due to the high roughness (Ra ¼0.3–0.4 μm) of layers was observed. Using the PVE300 Photovoltaic Device Characterisation System (Bentham Instruments Ltd.) with integrated sphere and specular light trap, the diffusion reflection of samples with PMMA addition was estimated to 3575% of the total reflection. Dropcasted and form-casted samples were not transparent, due to large layer thickness in comparison to spin-coated films. Drop-casted film had a few hundreds of micrometers of thickness and it was inhomogeneous. Form-casted sample was 2 mm thick. Using the SEM (Scanning Electron Microscopy-FEI Helios NanoLab 450 HF) surface of the drop-casted sample on silicon substrate was imaged (Fig. 3). High agglomeration of grains was observed. Due to the scattering and reflection losses of transparent luminescent layers an increase in the efficiency was not observed under STC of solar cells. Accordingly to the large optical losses of luminescent layers it was decided to perform the qualitative measurements of the down-shifting properties under UV light. As a UV light source (illumination maximum at 254 nm) the mercury lamp was used. Optical filter was used to cut-off the wavelengths below 450 nm (Fig. 4). Thereby the organic solar cell generated very low current from the weak background light – Ibackground_light ¼ 13.5 nA. On the top of structure, shown in the Fig. 5a, phosphor samples were placed (Fig. 5b) and under UV lamp short circuit currents of organic solar cell (Isc_UV) were measured. This measurement method allows to determine the current gain only due to
Fig. 6. Short circuit currents of the inverted organic solar cells under UV-light with optical filter and luminescent samples on the top.
application of the luminescent samples based on Sr2CeO4 and their down-shifting properties. The drop-casted and bulk PMMA with phospors exhibited the highest increase of short circuit current of solar cell and thus the highest down-shifting properties. Unfortunately the thick layers were nontransparent. Application of the Sr2CeO4 layers deposited from chloroform at 1200 and 2400 rpm and slight increases of current were observed (Fig. 6). Much better down-shifting properties were obtained for layers deposited from chloroform with
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Advanced Materials” - NanoMat (POIG.01.01.02-02-002/08) financed by the European Regional Development Fund (Operational Programme Innovative Economy, 1.1.2).
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[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Fig. 7. Photograhs of the luminescent samples during the measurements under UV lamp. Drop-casted layer of Sr2CeO4 on glass (a). Form-casted bulk PMMA with phosphor (b). Spin-coated sample from Sr2CeO4 chloroform dispersion on glass at 1200 rpm (c). Spin-coated sample from Sr2CeO4 chloroform with PMMA addition at 2000 rpm (d) and 1000 rpm (e). The short circuit currents of organic solar cell measured under UV lamp (Isc_UV) is shown for all samples for comparison.
[21] [22] [23] [24] [25] [26] [27] [28]
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4. Conclusions
[31]
The Sr2CeO4 nanocrystals were synthesized and their emission spectrum matches well to the external quantum efficiency of fabricated inverted organic solar cells based on P3HT:PCBM. The luminescent samples for application on the top of solar cell were fabricated from nanocrystals dispersed in chloroform and chloroform with PMMA addition by spin-coating and drop-casting deposition methods. Due to high scattering, reflection and total internal reflection losses improvement of efficiency under STC was not observed but using the UV-light source and luminescent layer on the top of inverted organic solar cells based on P3HT:PCBM down-shifting phenomenon was observed. The results of present work are preliminary for further studies of low-cost luminescent layers in organic and hybrid solar cells to increase the efficiency and protect the device from the high-energetic UV light. The future research will be focused on fabrication and non-scattering and transparent luminescent layers and integration of the phosphor material into the solar cell structure.
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Acknowledgments The work was supported by Wroclaw Research Centre EIT þ within the project "The Application of Nanotechnology in
Please cite this article as: M. Dusza, et al., J. Lumin. (2015), http://dx.doi.org/10.1016/j.jlumin.2015.08.006i