We report on the steady-state and optical modulation characteristics of a luminescence down-converting colloidal quantum dot/polyimide nanocomposite system suitable for integration with gallium nitride optoelectronics. The approach provides solution-processable and environmentally stable composite materials whose optical conversion and intrinsic modulation properties were evaluated at wavelengths from 535 to 624 nm. A nanocomposite for white-light generation upon excitation and mixing with 450-nm light was also obtained by blending colloidal quantum dots of different sizes in the same matrix. The forward external quantum efficiencies of the resulting nanocomposites were found to depend on the wavelength and can be as high as 33%. Optical modulation bandwidth above 25 MHz, which is an order of magnitude higher than for typical phosphor-based color-converters for GaN LEDs, and wavelength-converted data with an open-eye diagram at 25 Mb/s are demonstrated under external gallium nitride light-emitting diode excitation. These modulation characteristics are correlated with carrier lifetimes. This work provides guideline parameters and creates a possible path to integrated hybrid visible light sources for scientific and communications applications.
© 2012 OSA
Hybrid visible light-emitting diodes (LEDs) [1–4], where GaN-based optoelectronic sources are overcoated or otherwise integrated with a light-emitting nanocomposites or with neat chromophores are currently being explored for applications including solid-state lighting , (bio)-instrumentation [6,7] and photopumped organic lasers . In such hybrid components, charge carriers are electrically-injected into GaN-based heterostructures. Depending on the device configuration, this excitation is then transferred radiatively [9,10] and/or non-radiatively [1–4] to the active elements of the nanocomposites, which can subsequently emit down-converted light. A suitable range of nanocomposite materials could in this way enable efficient light-emission sources across the visible spectrum with the added functionality of electrical-control. This last feature is particularly relevant for applications requiring modulated visible light or pulses, such as for example time-resolved fluorimetry , optogenetics  and visible light communications (VLC) [11,12].
VLC uses visible light sources such as LEDs to transmit data and can be embodied in formats including fiber-based (e.g. polymer optical fiber or POF) single-color and free-space white light communications systems. The latter, which are proposed to combine data transmission with illumination, rely in their current state of development on AlInGaN LEDs color-converted with tailored solid-state phosphors dispersed in a polymer. This approach is relatively efficient for light generation but the use of phosphors with long upper state lifetime (~μs to ms) limits the modulation speed of the white sources to below 3 MHz with the intrinsic bandwidth of the phosphors being much lower . On the other hand, some formats of AlInGaN LEDs have been demonstrated to have bandwidth in excess of 200 MHz [13,14] in the visible making them promising for both POF and free-space communications. However, their emission efficiency is highest at 450 nm and drops significantly at longer wavelengths (the so-called ‘green gap’). For VLC and other applications requiring yellow-green and white-light LEDs, a color-conversion approach is therefore attractive. Consequently, there is a need for viable and efficient light-emitting materials with short upper state lifetime that can be simply integrated with GaN LEDs.
Colloidal quantum dots (CQDs) are an attractive class of chromophores for photonics applications. They benefit from a narrow, shape and size-tunable emission spectrum with high quantum yield and photostability and they can be processed from solution onto a wide range of materials. Core-shell CdSe/ZnS CQDs are particularly suited for luminescence transfer of blue and UV emitting GaN optoelectronics to longer wavelengths and even for the generation of white-light by color mixing [15,16]. At room temperature and above they have typical luminescence decay times of the order of a few nanoseconds to tens of nanoseconds and should therefore respond more quickly to modulated light than the traditional phosphors. The compatibility of these nanoparticles with polymers also means that tailored functional materials can be readily designed by incorporating CQDs (the ‘guests’) into a ‘host’ matrix. Such guest-host nanocomposites benefit from the combined properties of their constituents and enhance the processability and applicability of CQDs [17,18]. The choice of the host matrix can prove critical however, especially for color-conversion and direct integration with LEDs. An ideal matrix needs to be transparent in the visible part of the spectrum, protect the CQDs from the environment by encapsulation and be itself environmentally stable, i.e. its optical characteristics should not change with heat (generated for example by the down-converting process or by the optoelectronic device) or when exposed to air, humidity and light radiation. It should also be relatively simple to process. Polymers fulfil the later requirement of processing flexibility. Among them, polyimides would appear to be the most suited because they can operate at much higher temperatures than other polymeric materials, thanks to their higher glass transition temperature (>250°C). They are also quite strong and chemically resistant, making them suited for applications in harsh conditions. However, polyimides are not usually transparent in the visible and therefore their utilisation as matrices for color-converting composites has been hampered. Nevertheless, the very recent commercial development of novel polyimides with specifically improved transmission characteristics makes it timely to consider them as suitable candidates for the task.
In this paper, a guest-host nanocomposite system based on CdSe/ZnS CQDs for color-converting modulated optical signals generated by blue GaN LEDs is reported. Crucially, the approach uses a solution-processable polyimide matrix, which is transparent in the visible. The results presented in the paper show that the nanocomposite system could be applied to obtain modulated hybrid visible LEDs.
In Section 2 the nanocomposite fabrication is described. The steady-state and dynamic optical characterization results are reported and discussed in Section 3 and 4 respectively.
2. Material fabrication
CdSe/ZnS CQDs dispersed in toluene (Evident Technology) with mean diameters of 7.5 nm, 8 nm, 8.2 nm and 8.7 nm were used for the preparation of, respectively, 535-nm, 573-nm, 600-nm and 624-nm peak emission composite samples. A nanocomposite for white-light generation upon appropriate mixing with 450-nm light was also synthesised by incorporating CQDs of different sizes in the same matrix. The polyimide matrix used for the composites was Corin-XLS from Mantech. This is a fluorinated polyimide that offers high atomic oxygen and oxygen plasma durability and, importantly, is transparent in the visible. The matrix glass transition temperature is above 250°C and it is compatible with organic solvents which should enable composite deposition by spray-coating and ink-jet printing, for example. The refractive index at 589 nm is given as 1.54 by the manufacturer .
This polyimide matrix material was processed from a 30 mg/mL solution of C-XLS in Tetrahydrofuran (THF). The transmission spectrum of the solid matrix was verified by measuring the light from a fibre-coupled Tungsten halogen source passing through a ~20-µm thick sample. For this, a solid matrix sample was prepared by drop-coating the polyimide solution onto a glass slide and heating it up to 60C for 1 minute. Figure 1 confirms the optical clarity of the matrix with 94.5% transmission at 450 nm (the peak emission wavelength of typical blue InGaN LEDs) and a transmission of between 97.5% and 98.5% for the wavelengths corresponding to the emission of the CQDs used in this work.
The color-converting nanocomposites were fabricated as follow. The CQDs were incorporated, after a solvent exchange, into a polyimide-THF solution at a given ratio: 50 mg of CQDs per mL of THF/polyimide (30 mg of polyimide per mL of THF). The composite for white-light generation was obtained by incorporating 535, 573, 600 and 624-nm CQDs at respective volume percentages of 83.3%, 14%, 1.7% and 1%. The higher percentage of the smaller CQDs to obtain a desired spectrum is made necessary by energy transfer effects (both radiative and non-radiative) funnelling excitons to the bigger CQDs. We note that using this blend, we were able to obtain white light with (0.32, 0.34) coordinates when appropriately mixing the composite emission with blue light (see section 3.1). Samples for testing were prepared by drop-casting the nanocomposite solution onto 150-μm-thick glass substrates and heating to 60°C for 1 minute in order to accelerate the evaporation of THF. The resulting materials had a CQD-to-polyimide volume ratio of around 1% - the calculated ratio for each sample is given in Table 1 . Typical sample thicknesses were between 100 μm and 300 μm as measured by micrometer.
3. Steady-state optical characteristics
In this section, the steady-state optical characteristics of the nanocomposites are assessed. First, the photoluminescence (PL) spectra of the samples are studied and then their respective forward external photon conversion efficiency, which is an important parameter to estimate how well a hybrid LED using such nanocomposites would operate, are given.
PL spectra for all samples under 450-nm LED excitation are shown in Fig. 2(a) . The spectrum for each sample has a typical bell shape with a full width at half maximum close to ~30 nm, except for the white-light mix which is broader. In order to check that the CQD emission spectrum is not significantly affected when they are incorporated into the matrix, normalised PL spectra of the CQDs taken at different stages of the fabrication, i.e. when they were in toluene and in both the (THF) liquid- and the solid-state form of the composite, were taken and compared. Typical results are given in Fig. 2(b) in the case of the 624-nm sample. While the PL peak is blue shifted by 3 nm when the CQDs are in THF, the emission spectra in toluene and in the solid-state composite overlap closely.
The capability of the ‘mix’, or white-light composite, to effectively generate white light under proper color mixing with 450 nm is demonstrated in Fig. 3 . For this a 300-µm layer of nanocomposite on glass was placed directly on top of a 450-nm LED and spectra were recorded at different continuous-wave LED driving voltages (from 3V to 6.5V). Results are shown in Fig. 3. The spectral contribution around 450 nm is from non-absorbed LED light while the longer wavelength features are from the light down-converted by the nanocomposite. The corresponding chromaticity coordinates are (0.32, 0.34) and do not vary significantly over the LED driving condition used.
The forward external photon conversion efficiency, which we also call external quantum efficiency in the rest of the paper, was measured for all composite samples and taken as the ratio of the number of photons emitted from the front-face by the nanocomposite layer to the number of pump photons (at 450 nm) absorbed within the same layer. This ratio is obviously lower than the overall PL quantum efficiency, as both back-face-emitted and waveguided photons are not taken into account. For the measurement, the samples were put physically into contact with a GaN broad-area LED emitting at 450 nm. Typical IV and optical power characteristics of these LEDs have been reported elsewhere . The LED, acting here as the excitation source, was operated in continuous-wave (DC) at constant voltage (3.8 V – typical output power ~0.5mW) and the front-face PL emission from the samples was collected by an integrating sphere placed in contact. Because of the experimental configuration, the characteristics of the measured light were similar to what would be obtained from a hybrid LED with a colour-converter directly integrated on top of the semiconductor chip. Diffuse light from the sphere was tapped by a 600-μm-core optical fibre whose output was coupled to a grating-CCD spectrometer (1.5-nm resolution). The emission of the bare LED, i.e. with no colour-converter, was first measured through a 150-μm-thick glass slide, the integration of the resulting spectrum yielding a value denoted XLED. The emission spectrum with the sample in place was then recorded. From the area under the LED emission spectrum region, one then deduced XLED’ and A, the ratio of absorbed LED light: A = 1-XLED’/ XLED. The forward external quantum efficiency is given by:
In Eq. (1) Xnano is the integrated spectrum of the nanocomposite’s emission and λe and λp are, respectively, the composite emission wavelength and the LED wavelength. The external quantum efficiencies as defined above for the 535-nm, 573-nm, 600-nm and 624-nm samples were found to be, respectively, 8.5%, 14%, 33% and 18%, while it was 8% for the white-light composite (see Table 1). These values are consistent with previous efficiency report of other types of CdSe/ZnS CQD guest-host composites . The relatively high external quantum efficiency for the 573-nm sample, 33%, is attributed to an initial CQD solution of higher quality leading to a composite having lower non-radiative recombination compared to the other wavelengths. Similar behaviours were seen for films of corresponding neat CQDs. Apart from this particular value, there is a general trend of increase in efficiency with increasing emission wavelength. This effect could be explained by a higher contribution of non-radiative recombination losses for smaller CQDs although the higher quantum defect (i.e. the difference between the excitation and emission energy) for the longer-wavelength samples, arguably beneficial for improved efficiency in CdSe/ZnS CQDs as reported in , might also be a factor. The efficiency of the white-light sample indicates that it is dominated by the contribution from the 535-nm CQDs.
Overall, the data of this section shows that the proposed material system is a viable option for color-conversion in the visible.
4. Dynamic optical characteristics
The study of the dynamic characteristics of the down-converted light obtained by illuminating the nanocomposites with a modulated gallium nitride LED is presented next. The particular type of LED for this work operated at a wavelength of 450 nm, and consisted of an 8 x 8 array of emitting micro-pixels. A single 84-μm ‘micro-LED’ pixel was used for the measurements. Full details on the modulation characteristics of these devices have been reported elsewhere , but, in short, these pixels show −3dB bandwidths above 200 MHz and have been used bare in data transmission demonstrations at rates of up to 1 Gb/s. The 84-μm-diameter pixel was electrically contacted using a high-speed ground-signal-ground electrical probe (Cascade Microtech, ACP40-A-GSG-125). The center pin of the probe was placed on top of the pixel p-contact, and at least one of the outer ground pins was placed on the n-contact of the array. The LED pixel, otherwise DC-biased at 75 mA, was RF modulated with a small-signal chirped sine wave from an Agilent 8753ES network analyzer. Light from the LED pixel was imaged onto the nanocomposite samples and the down-converted emission was collected. In order to remove any remnant of the LED pump light and only record the frequency response of the nanocomposites, the converted light was separated by optical filters and focused onto a high-speed detector (Newport 818-BB-21A, 75kHz-1.2GHz bandwidth) whose amplified output was fed back to the network analyzer via a 50-dB external electrical amplifier (HP8447D - dual stage) having a 100-kHz to 3-GHz frequency range. The LED pixel response was separately recorded and subsequently subtracted from the overall measured signal in order to extract the intrinsic nanocomposite response.
The measured frequency responses of the nanocomposites are shown superposed in Fig. 4 . The y-axis of the figure represents the electrical response in decibels while the x-axis gives the frequency in logarithmic scale. It is evident from this figure that the two longer wavelength samples (600nm and 624nm) have a lower bandwidth than the other three.
The data were fitted using the non-least square method and assuming a mono-exponential PL decay, in order to recover the effective carrier lifetimes and the −3dB optical (−6dB electrical) frequency for each sample. While the PL decay is not truly mono-exponential in the CQD nanocomposites studied here, the effective, or mean, PL lifetime that results from all the recombination (radiative and non-radiative) processes is the relevant parameter for the modulation response of the converted light, justifying the approach. The following expressions were used for the fit:
In Eq. (2) M represents the normalized electrical response of the color-converter, f is the modulation frequency and τmean is the fit parameter representing the effective, or mean, exciton lifetime. In Eq. (3) fco is the cut-off frequency, i.e. the optical bandwidth. The fits of the data for each sample are given in Fig. 5 . For the 535-nm, 573-nm, 600-nm, 624-nm and white-light spectrum samples the cut-off frequency is found to be, respectively, 23 MHz, 27.5 MHz, 10.8 MHz, 11.1 MHz and 19.7 MHz. Such values are at least an order of magnitude higher than in the case of phosphorescent color-converters and indicate the advantage of using CQD nanocomposites over phosphors for applications requiring modulated light. The corresponding effective lifetimes are plotted in Fig. 6 . For comparison, the effective carrier lifetime of each sample was also measured with a time-gated PL system . Measurements from both techniques agree within the experimental errors (see Fig. 6). The longer effective lifetime that was found for the longer emission wavelengths (and the corresponding decrease in optical bandwidth) is attributed to the combination of lower non-radiative recombination and/or slower PL radiative decay rates in bigger CQDs. This is discussed in the following.
The PL radiative decay rates (or exciton radiative decay) cannot be properly deduced from the previous measurements because knowledge of the total PL quantum yield (PLQY) is required. However, it is possible to estimate a ratio of these PL radiative decays in order to compare the different samples emitting at different wavelengths. For this, it has to be assumed that the color-converter emission pattern and the amount of light extracted are the same in each of the samples, which is reasonable here. With this assumption, the forward quantum efficiency η has the same coefficient of proportionality with respect to the PLQY for all samples. While we do not know this factor of proportionality we can directly calculate the ratio of the PL decay rate for each sample. This ratio, taken arbitrarily here over the decay rate of the 535-nm sample, is obtained by:
In Eqs. (4) and (5) Rsample is a value in seconds which is proportional to the radiative decay rate for the given sample and R535nm represents this same value in the case of the 535nm sample. Parameters η and τmean are, respectively, the external quantum efficiency and the mean recombination lifetime of a given sample as defined previously. The ratio is plotted for all samples in Fig. 7 . Considering solely the 575nm, 600nm and 624nm samples, the ratio, hence the radiative decay lifetime, is seen to increase with the wavelength. This behavior can be attributed to the dependence of the spontaneous emission radiative rate on the emission frequency [23,24]. The radiative decay lifetime of an ideal two-level exciton as given by Fermi’s rule depends linearly on the wavelength , i.e. it is inversely proportional to the CQD size. In practise, the population distribution of excitons between bright and dark states of CdSe CQDs can modify this dependence from linear to supralinear . In any case, the result is that bigger CQDs, which emits at lower frequencies, are expected to have a longer radiative decay lifetime. Interestingly, the 535nm nanocomposite does not follow the trend and has a radiative lifetime almost as long as for the 624-nm sample. This behavior could be attributed to the fact that the 535-nm CQDs of the nanocomposites are directly excited into their first excitonic absorption line by the 450-nm LED light: such a feature could have repercussions for the steady-state exciton population distribution within the lowest energy manifolds and in the turn on the mean decay lifetime. This issue is still under investigation. The ratio for the white-light is also plotted in Fig. 7 for indication and shows a radiative decay lifetime comparable to the 535-nm and 624-nm nanocomposites.
Finally, in order to show that the CQD nanocomposites color-converters can indeed be used to transmit information, the LED was modulated with NRZ data and the data pattern from the composite down-converted emission was recorded. Optical filters were again used to remove the LED emission in order to measure the sole contribution of the composite. Figure 8 demonstrates the principle by showing an open eye diagram at 25 Mb/s which was obtained with the white-blend nanocomposites. The modulation bandwidth of these types of color-converters and the rate at which they can transmit information is at least an order of magnitude higher than typical rare-earth phosphors. They should readily enable transmission of several tens of Mb/s per color.
Assessment of the steady-state and dynamic optical properties of a wavelength-versatile, light-emitting guest-host nanocomposites system based on CQDs in a transparent polyimide matrix was reported. This material is intended to be integrated with GaN-heterostructures to act as a color-converter in hybrid LEDs and, as such, colour conversion and modulation measurements were undertaken with GaN LED excitation. Characterisation showed that the material can have forward external photon conversion efficiency (quantum efficiency) up to ~33% and modulation bandwidth above 20 MHz depending on the emission wavelength. Given such results, hybrid LEDs made from this type of nanocomposite look attractive for possibly visible light communications and for applications requiring modulated visible optical signals.
The authors would like to thank the Engineering and Physical Sciences Research Council for funding under the grant EP/F05999X/1, HYPIX, Hybrid organic semiconductor/gallium nitride/CMOS smart pixel arrays. They also acknowledge Dr. Garrett D. Poe from Mantech for providing the Corin-XLS polyimide material.
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