D. A.
Vithanage
a,
A. L.
Kanibolotsky
bc,
S.
Rajbhandari
de,
P. P.
Manousiadis
a,
M. T.
Sajjad
a,
H.
Chun
d,
G. E.
Faulkner
d,
D. C.
O’Brien
d,
P. J.
Skabara
*b,
I. D. W.
Samuel
*a and
G. A.
Turnbull
*a
aOrganic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews, St. Andrews, KY16 9SS, UK. E-mail: gat@st-andrews.ac.uk; idws@st-andrews.ac.uk
bWestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, G1 1XL, UK
cInstitute of Physical-Organic Chemistry and Coal Chemistry, 02160 Kyiv, Ukraine
dDepartment of Engineering Science, University of Oxford, Oxford, OX1 3PJ, UK
eCentre for Mobility & Transport, School of Computing, Electronics and Mathematics, Coventry University, Coventry, CV1 2JH, UK
First published on 5th September 2017
For white light data communications, broadband light emitting materials are required, whose emission can be rapidly modulated in intensity. We report the synthesis, photophysics and application of a novel semiconducting polymer for use as a high bandwidth colour converter, to replace commercial phosphors in white LEDs. The high modulation bandwidth (470 MHz) is 140 times higher than that measured using a conventional LED phosphor.
Organic colour converters3–5 have the potential to overcome this limit, as they have a shorter emission lifetime (in the order of nanoseconds), and consequently higher communication bandwidths can be obtained.6–8 Semiconducting polymers are low cost, solution-processable materials that can be integrated on a wide range of substrates. They can have high photoluminescence quantum yields (PLQY) and broadband emission that can be tuned by changing the molecular structure, both of which, along with short lifetime, are required in colour converters for VLC. In one example, light from a blue LED was combined with photoluminescence from the commercial polymer Super Yellow (in a concentrated solution with modulation bandwidth of >90 MHz).6 The source achieved white light data transmission at 1.8 Gbps using advanced modulation techniques, but with a modest colour rendering index (CRI) of 53. BODIPY cored materials (in solution) have been used to extend VLC conversion wavelengths to the red, as optical transmitters with 39 MHz bandwidth and data rates (with simple on–off keying) of 98 Mbit per s.8 To produce a material with a short lifetime and improved colour rendering, a solid blend of two organic materials was made using the green emitter, poly[2,5-bis(2′,5′-bis(2′′-ethylhexyloxy)phenyl)–p-phenylenevinylene] (BBEHP–PPV) and the red emitter, poly[2-methoxy-5-(2′-ethyl-hexyloxy)–p-phenylene-vinylene] (MEH–PPV).7 The blend gave a broad emission spectrum with modulation bandwidth of 200 MHz, which had a high CRI value of 76 when mixed with blue LED light, but was limited in efficiency by the 17% PLQY of MEH–PPV. A key materials challenge for high performance VLC sources is therefore to develop new orange-red emitters that can combine fast modulation with high PLQY.
We present here the design of a novel, fast, orange-emitting polymer which combines efficient emission with an exceptionally high modulation bandwidth. The material is a poly(phenylenevinylene) (PPV) derivative, poly[(2,5-bis((2′,5′-bis((2′′-ethylhexyl)oxy)benzyl)oxy)–p-phenylene)vinylene] (BBEHBO–PPV) and is the first example of a conjugated polymer that has been custom designed for application in VLC. The extended conjugation of semiconducting polymers offers the fastest radiative rates, and we show here that the polymer structure can be designed to combine high solid-state PLQY and fast orange light emission. Bulky side groups separate the conjugated polymer backbones9,12 to achieve high PLQY in thin films, while their alkoxy bridges allow the emission wavelength to be tuned and give a high radiative rate.
The molecular design was inspired by the high bandwidth achieved using the blend of BBEHP–PPV and MEH–PPV.7 BBEHP–PPV9 and MEH–PPV are well known green- and red-emitting polymers, which have been researched for OLEDs,10 lasers and optical amplifiers,9,11–13 sensors for explosives9,14 and solar cells.15,16 BBEHBO–PPV (Scheme 1 and Fig. S1 in ESI†) is designed to have a poly(2,5-dialkoxy-p-phenylene vinylene) backbone with bulky substituents similar to those of BBEHP–PPV, but with an alkoxy bridge within the side groups, as applied in MEH–PPV. BBEHBO–PPV thereby combining advantages of the two materials: the high PLQY of BBEHP–PPV with the fast orange PL lifetime of MEH–PPV. This allows us to achieve a modulation bandwidth of 470 MHz for a fluorescent film of the material. This is 140 times higher than that in commercially available phosphor-coated light emitting diodes (LEDs) and currently the highest recorded bandwidth for an organic colour converter.
The target polymer BBEHBO–PPV was obtained as a red resin-like (due to low Tg, vide infra) solid in a fair yield of 68%. GPC analysis revealed the molecular weight Mn = 499 kDa with PDI = 2.67 (Fig. S8, ESI†).
BBEHBO–PPV turned out to be fairly thermally stable. Thermogravimetric analysis (Fig. S9, ESI† solid line) revealed a decomposition temperature of 290 °C (5% weight loss). Due to the bulky substituents and high internal volume of the polymer repeat unit the target compound BBEHBO–PPV exhibited a low glass transition temperature (Tg = −17 °C) in a differential scanning calorimetry experiment (Fig. S9, ESI† dashed line), which is responsible for the resin-like appearance of this polymer at ambient temperature.
The PL lifetime, absorption and PL spectra for BBEHBO–PPV, BBEHP–PPV, MEH–PPV are shown in Fig. 1. In solution and in film, BBEHBO–PPV has absorption peaks at 490 and 488 nm, respectively and covers the blue LED emission region. Transient absorption measurements show an excited state absorption band at 1100 nm (Fig. S2, ESI†). The emission peaks (0–0) are at 544 and 555 nm with a shoulder (0–1) at 585 and 586 nm in solution and film, respectively. PL lifetime measurements were conducted by exciting at 393 nm and detecting at 550 nm. The lifetimes were fitted using a multi exponential decay. The fit values are given in Table 1 and Table S1 (ESI†). In solution, the decay was single exponential with a lifetime of 0.7 ns. In the film, there was a double exponential decay with the dominant component at 0.37 ns. The PLQY was measured to be 67% in solution, which dropped to 45% in films. The CIE coordinates of BBEHBO–PPV lie between BBEHP–PPV and MEH–PPV, and are red-shifted from Super Yellow.7 The blue absorption band, short lifetime and high PLQY values show that the design of the material, using a PPV derivative, successfully meets the requirements in colour converters for VLC.
BBEHBO–PPV (solution) | BBEHBO–PPV (film) | BBEHP–PPV (film) | MEH–PPV (film) | |
---|---|---|---|---|
PLQY (%) | 67 | 45 | 85 | 17 |
τ 1 (ns) | 0.65 | 0.371 | 0.722 | 0.295 |
A 1 | 1 | 0.996 | 1 | 0.885 |
τ 2 (ns) | — | 1.63 | — | 0.658 |
A 2 | — | 0.004 | — | 0.116 |
τ 3 (ns) | — | — | — | 7.34 |
A 3 | — | — | — | 0.001 |
CIE coordinates | 0.46, 0.53 | 0.46, 0.53 | 0.25, 0.57 | 0.57, 0.43 |
In comparison, the absorption spectrum of BBEHBO–PPV closely resembles that of MEH–PPV at λ > 375 nm with similar emission peaks. The emission of BBEHBO–PPV is red-shifted compared to that of BBEHP–PPV which has a peak at 535 nm, overlapping the PL spectra of both BBEHP–PPV and MEH–PPV over a range of 300 nm. The PLQY of BBEHBO–PPV is lower than BBEHP–PPV which is highly luminescent at 75% (as a film) but an improvement from the <20% quantum yield in MEH–PPV. To compare with BBEHBO–PPV, PL lifetime measurements were conducted on BBEHP–PPV and MEH–PPV, exciting at 393 and 470 nm, respectively. The resulting emission was detected at 530 and 595 nm for BBEHP–PPV, and MEH–PPV, respectively. In solution, the lifetime of BBEHBO–PPV is very similar to that of BBEHP–PPV, with both materials showing a lifetime of ∼0.7 ns for the same concentration of 0.05 mg ml−1 (Table S1 in ESI†). This is longer than MEH–PPV which has a lifetime of 0.4 ns. In the film however, BBEHBO–PPV has a shorter lifetime which is similar to MEH–PPV films (see Fig. 1). The radiative rates were 2.6 × 109, 1.4 × 109 and 2.94 × 109 s−1 for BBEHBO–PPV, BBEHP–PPV and MEH–PPV, respectively. BBEHBO–PPV has a faster rate (due to the short lifetime) than BBEHP–PPV, much closer to that of MEH–PPV, indicating the effect of the oxymethylene bridges between the benzene ring and bulky substituents, which may enhance the optical transition from an excited state to the ground state.
The modulation bandwidth of the colour converter depends on the PL lifetime and BBEHBO–PPV has the shortest life time among a number of colour converters such as BBEHP–PPV7 and BODIPY cored oligo-fluorenes.8 A solution with concentration of 0.025 mg ml−1 and solid state film spin-coated from a solution of concentration 5 mg ml−1 were tested. Fig. 2a shows the measured electrical power attenuation against the frequency of BBEHBO–PPV in film and solution. A reference −3 dB power point (which refers to the electrical bandwidth of the system) is also shown. The BBEHBO–PPV film has a bandwidth of ∼470 MHz independent of the concentration of solutions used for spin-coating (see ESI†). This is significantly higher than that of the phosphor which is measured at 4.4 MHz7 and other organic semiconductors, such as BBEHP–PPV, Super Yellow and BODIPY Y3, which exhibited bandwidths of 130, 90 and 39 MHz, respectively.6–8 In solution, it has a lower bandwidth of 280 MHz, which is consistent with the longer PL lifetime.
In this paper, we used Pulse Amplitude Modulation (PAM) schemes with 2 and 4 levels (see ESI† for latter) to undertake data transmission. The study of higher multilevel modulation schemes is not presented as they offered inferior performance in comparison to 2 and 4-PAM. The measured bit error rate is shown as a function of transmitted data rate in Fig. 2b. Eye-diagrams at different data rates are shown in ESI.† A clear eye-opening is demonstrated at 500 Mbps (Fig. S6 and S7, ESI†) indicating error free performance at this rate. Considering a forward error correction (FEC) error floor of 3.8 × 10−320 (shown as solid line in Fig. 2b), the achievable data rate for PAM-2 and PAM-4 is >550 Mbps. The performance in the current set up is limited by the APD receiver which has bandwidth of less than 100 MHz. By adopting a complex modulation scheme, such as orthogonal frequency division multiplexing and/or equalisation, a significantly higher data rate could be achieved.21
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7tc03787b |
This journal is © The Royal Society of Chemistry 2017 |