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The spectral response of Poly(3-hexylthiophene) (P3HT): 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) heterojunction film is between 350 nm and 650 nm, meaning that a lot of the sunlight is lost at ultraviolet and infrared regions. We fabricated solar cells by the attachment of a fluorescence layer which absorbs UV light, and emit visible light which will be re-used by P3HT, and thus the absorption spectrum is expanded. Since N,N’-bis(3-methylphenyl)-N,N’-bis(phenyl)-benzidine (TPD) has high reflectance in the visible range, the usage of UV light will not manifest; when LiF is added as an antireflection layer, PCE was enhanced from 2.50% to 2.68%.
©2011 Optical Society of America
1. Introduction
Recently, organic photovoltaic devices (OPVs) have attracted great attention due to their peculiar merits, such as flexibility, easiness of large area fabrication and low cost etc. [1,2]. By now, the power conversion efficiency (PCE) of organic solar cells has risen to ~7% [3,4], which approaches to the forecast of 10% for industrialization of OPVs [5]. There are still many improvements to be realized for OPVs, one of which is to expand the absorption spectrum range. The photon flux reaching the surface of the earth from the sun have wide spectrum from ultraviolet to infrared light, while P3HT, the most widely used as organic semiconductor in OPVs, may only harvest 46% of the available solar photons in the wavelength range between 350 nm and 650 nm [6]. Several methods could be used to expand the polymer absorption spectrum. New materials [7,8], especially the low band gap materials, have been synthesized and applied for the OPVs. Another way to expand the absorption spectrum range is to design special device structures, such as tandem solar cells [9–12]; or to employ other optic-absorption materials which could be doped into the active layer [13]. For tandem solar cells, the balance of electrons and holes from each cell is required because the current extracted from the tandem cell will be determined by the current generated in either the front or back cell, whichever is smaller, and the unbalance current can generate the accumulation of space charges to destroy the performance of tandem solar cell; the balance of open-circuit voltage of each cell is required in parallel tandem. These requirements increase the difficulty to fabricate devices. When the optic-absorption material is mixed into the basic solar cells, the principle of excitons separating and electrons transporting shall be more complicated, and the result may be not as well as we expected. Another concept what is called “Fluorescence concentrator” has been reported in several papers previously [14–16], and the common method is to build one or more thick fluorescence layer on a mirror, and the fluorescence layer absorbs sunlight and then emits special fluorescence, and the light would be reflected by the mirror and transport to the solar cells attached to the side of fluorescence layer. Besides these methods, up-conversion or down-conversion is employed in solar devices [17,18]; while for these solar cells, the increase was not that much because of the low up or down conversion efficiency.
In this paper, a similar method was reported: a kind of solar cells was fabricated by attaching a fluorescent material on common solar cells; the material absorbs near UV light, and then emits visible light whose wavelength can be absorbed by the active layer of the OPVs. To raise the utilization rate of the fluorescence, we cover a LiF layer onto fluorescent film to trap the emission light in the devices. The advantage of using this structure is that it cuts loose the current-balance or voltage-balance requirement in the common tandem solar cells. The sticking point is that the fluorescent material should possess high fluorescence quantum yield. TPD is used as the candidate in our work. TPD is usually used as a blue-violet light emitting material or host material on the phosphorescence organic light emitting diodes for its wide energy band is about 3.2 eV with highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) −5.5 eV and −2.3 eV respectively [19–21]. The fluorescence quantum yield was measured to be 0.69 for TPD in 1,4-dioxane [22] and it was about 39% for solid TPD film [23]. The difference results from that the distance between atoms is much shorter in solid films than that in solution and self-quenching is much stronger. Here, TPD was attached to common devices to translate near ultraviolet light to visible light, which could be absorbed by P3HT, and then the PCE of OPVs increased from 2.50% to 2.68% after optimizing the TPD layer. The structure and principle scheme is shown in Fig. 1 .
2. Experimental
2.1. Device fabrication
OPVs fabrication: the ITO glass was cleaned by de-ionized water, acetone and ethanol successively in an ultrasonic sound cleaner. And then, it was translated into an O2 plasma generator for 5 minutes to remove the residual carbon on it. Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT-PSS) was spin-coated at 3000 rpm (revolutions per minute) for 30 seconds, and annealed at 200 °C for 5 minutes in air and 15 minutes in vacuum. After being stirred with a magnetic stirrer for 2 hours, the mixture of P3HT (15 mg/ml) and PCBM (12 mg/ml) dissolved in ortho-dichlorobenzene was spin-coated onto PEDOT-PSS layer at 700 rpm for 15 seconds. The samples with the active layer were annealed in vacuum for 30 minutes at 150 °C before being transferred to a vacuum deposition system at 6 × 10−4 Pa. 0.5 nm LiF and 120 nm Al were deposited for the cathode. Having fabricated the standard devices, the fluorescence layer (FL) was evaporated at the back of basic solar cells at 6 × 10−4 Pa and the evaporation temperature is about 100-120 °C. The structure of the tandem solar cells is: Fluorescence layer/glass/ITO/PEDOT:PSS/ P3HT:PCBM (15 mg/ml:12 mg/ml)/0.5 nm LiF/120 nm Al.
2.2. Characterization
The light transmission was studied with UV-visible spectrophotometer (Agilent 8453). The photovoltaic characteristics of OPVs were measured using AM 1.5G simulated sunlight source (Oriel 300), and Keithley 2611 System Source Meter. The external quantum efficiency (EQE) was taken at short circuit using monochromatic white light from a xenon lamp. Photoluminescence spectrum was studied with fluorescence spectrophotometer F-2500.
3. Results and discussion
According to the absorption spectrum of P3HT, as are shown in Fig. 2 , the absorption range is just between 350 nm and 650 nm; for PCBM, there is a large absorption coefficient below 400 nm, but since the exciton diffusion length in PCBM is too small, about 60% of the excitons generated in PCBM decay before reaching the donor-acceptor interface [24]. The absorption wavelength of TPD is mainly below 370 nm; the peak wavelength of the photoluminescence (PL) is 405 nm in dichloromethane; for a concentrated film, the PL peak is red shift to be 419 nm because of the intermolecular mutual effect. The PL light of TPD can be absorbed by P3HT. Therefore, the solar cells based on TPD can increase the utilization ratio in the near ultraviolet region.
Fig. 2 Normalized absorption spectrum of P3HT (red dash dot line), TPD (violet solid line), PCBM (green dot line) and photoluminescence spectrum of TPD (blue dash line); Inset: Structural formula of TPD.
In order to make the performances of common solar cells and with solar cells with fluorescence layers comparable, we wiped out the fluorescence film thoroughly as soon as the performances were measured, and then the performances of same device was measured again immediately. Theoretical calculation was made for our determining the optimized thickness of TPD. Since TPD has small absorption between 400 nm and 600 nm, absorption was not considered here. When the films are thinner than the coherence length of natural light, interference effect should be considered, and the reflection coefficient (r) is:
In Eq. (1), a structure with 3 interfaces is considered; r is the reflection coefficient; k is the wave vector; d is the thickness of the films; 0, 1, 2, 3 stands for air, LiF layer, TPD layer and the glass substrate. The refractive index of air, TPD, and glass is about 1, 1.7 [22] and 1.5. We calculated |r| dependent on the thickness of TPD and LiF. Normal incidence light at the wavelength of 500 nm was considered and the result was given in Fig. 3 .Fig. 3 Theoretical calculation of reflection coefficient dependent on the thickness of TPD and LiF. |r| increased as the color changed from dark blue to red.
The |r| shows periodic variation with the thickness of TPD and LiF. 90 nm LiF is almost suitable for all the TPD thickness, and it is set at that point. In the first cycle, 90 nm-150 nm TPD could be chosen to get a small |r|. Here, 150 nm TPD is determined to absorb more UV light. Thicker TPD is not suitable for its small absorbance in visible region and more scattering, evaporating time and more cost would occur when the film is too thick.
After the thickness of TPD and LiF is chosen, |r| dependent on wavelength and thickness of TPD is also given to show the final effect of 147 nm TPD and 90 nm LiF on the whole range of solar spectrum. The result is shown in Fig. 4 . Results indicate that 147 nm TPD/90 nm LiF could suppress the reflectance of the structure in most of the visible range, especially between 400 nm and 600 nm which is exactly among the P3HT absorption spectrum.
Fig. 4 |r| dependent on wavelength and thickness of TPD; LiF is set to be 90 nm. |r| increased as the color changed from dark blue to red. The blue region shows that 150 nm TPD/90 nm LiF keeps |r| small from 400 nm to 600 nm.
When we fabricated the devices, about 150 nm TPD was evaporated as the fluorescence layer firstly. In the room light, we could see the blue fluorescence of TPD film in the sunshine clearly. The J-V characteristics were measured under the simulated sunlight with the intensity of 100 mw/cm2, as was shown in Fig. 5 . For the device with TPD, the short circuit current density J SC = 6.43 mA/cm2, open circuit voltage V OC = 0.57 V, fill factor FF = 0.65, PCE = 2.38%; while for the common device, J SC = 6.81 mA/cm2, V OC = 0.57 V, FF = 0.64, PCE = 2.50%. For LiF/TPD devices, J SC increased from 6.81 mA/cm2 to 7.35 mA/cm2 nearly without changing V OC and FF, and PCE turned out to 2.68%, increasing about 7.20% compared with standard OPVs, as was shown in Fig. 5.
Fig. 5 J-V characteristics of standard OPVs (black solid line), OPVs with TPD backward (red dash line) and OPVs with TPD/LiF backward (blue dot line) when illuminated at 100 mW/cm2 air mass 1.5 global (AM1.5 G) simulated sun light; Inset: the schematic structure of the devices.
Real Reflectance was measured, as is shown in Fig. 6 . The reflectance is very small below 300 nm, and it is no more than 5%. So much of the light at this wavelength will be absorbed or transmit. In the visible range, the reflectance of the TPD device is enhanced by about 3%, and that is the reason why TPD shows negative result. For the LiF/TPD structure, reflectance sharply drops down in the visible range, especially between 400 nm and 600 nm. And that is consistent with the calculation result. Besides, since the refractive index of TPD is higher than LiF, part of the fluorescence light coming from TPD might be totally reflected at the TPD/LiF interface, and this part of light would be trapped and go to the active layer. Well, there came a question then: whether did the enhancing of PCE result from only LiF or both of TPD and LiF? We measured the transmittance of ITO/glass, and that with a layer of TPD, TPD/LiF, and only LiF at the back of glass with quartz as the blank. The data was shown in Fig. 7 : for the TPD situation, the transmittance reduced across the whole spectrum compared with nothing backward; when there being a LiF layer, the transmittance was raised about 3% across the whole spectrum; for TPD/LiF, the transmittance below 400 nm does not change, and it was raised to the same level as only LiF exist. That was to say, the PCE increase was partly from the LiF contribution in the visible region. Well what about in the near ultraviolet region? In order to answer this question, the EQE of the devices was measured, as shown in Fig. 8 .
Fig. 6 The reflectance of standard OPVs (black solid line), OPVs with TPD backward (red dash line) and OPVS with TPD/LiF backward (blue dot line).
Fig. 7 The transmittance of TPD (red dash line), TPD/LiF (blue dot line), LiF (green dash dot line), and noting at the back of ITO/glass (black solid line), the data was tested with quartz glass as blank.
Fig. 8 a. EQE of standard OPVs (black square), and OPVs with TPD backward (red circle); b. standard OPVs (black square) and OPVs with TPD/LiF backward (blue up-triangle) in short wavelength. Inset: EQE of OPVs in long wavelength which was measured with EQE measurement system.
When measuring the EQE, we mainly tested J SC /(λ·I λ), which was directly proportional to EQE, and I λ was the light intensity at wavelength λ. From the reflectance data and the EQE data, we know that the small reflectance by TPD has no effect on the final usage in the ultraviolet region (220 nm to 300 nm), and compared with the standard solar cells, the EQE increases much from 220 nm to 300 nm, and the peak is at 280 nm, with 80% increasing. The difference between the absorbance peaks and the EQE peak may result from the absorbance of P3HT. P3HT has strong absorbance from 400 nm to 600 nm, and it also has small absorption below 400 nm and a superposition with absorbance band of TPD exists. P3HT’s conversion yield from absorption light to excitons is nearly 100%, and the fluorescence yield of TPD film is 39%. The superposition of absorbance between TPD and P3HT will cause the blue shift of EQE. EQE of device with LiF/TPD increased at the same level as TPD structure compared with standard devices. From transmittance, reflectance and EQE data, we can deduce that about 40%-60% of the near violet light in average can be absorbed by TPD. Naturally, the increase of EQE in the near-UV band is attributed to the PL of TPD. In the visible region, EQE of devices with TPD is lower than that of the common ones because of the larger Fresnel reflection loss as shown in the inset of Fig. 8(a). This problem can be overcome by introduction of the anti-reflection LiF layer, and the inset of Fig. 8(b) shows that EQE of devices with LiF/TPD is slightly larger than that of the common ones. The two factors, PL of TPD in ultraviolet region and anti-reflection effect of LiF layer in visible region, bring on the larger photo-current and the higher power conversion efficiency in the devices with LiF/TPD.
4. Conclusion
In summary, we expanded the absorption spectrum of organic solar cells with a fluorescence layer, which could emit visible wavelength light when illuminated with near ultraviolet light. In this paper, we used TPD with a high fluorescence quantum yield as the fluorescence layer, and the standard solar cells were based on P3HT:PCBM hegerojunction. After optimizing the fluorescence layer, we made PCE of devices enhance about 7.20% compared with standard OPVs. In nowadays, researchers have been synthesizing narrow band organic materials in order to increase the usage of the yellow and red light. But the optical band of organic materials is usually limited, and thus the usage of blue and violet light would be reduced. Fluorescence layer structure is expected to be used in these kinds of OPVs more successfully.
Acknowledgments
The authors are thankful to Xinyuan Xia, Liming Zhang in the College of Chemistry and Molecular Engineering and Zheng Zhang for their kindly help. This work was supported by the National Natural Science Foundation of China under grant No. 10934001, 60878019 and 10821062, and the National Basic Research Program under grant No. 2007CB307000 and 2009CB930504.
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