Barrierless slow dissociation of photogenerated charge pairs in high-performance polymer-fullerene solar cells

The work in St Andrews was supported by the Engineering and Physical Sciences Research Council (grants EP/L017008/1, EP/J009016/1 and EP/G03673X/1) and the European Research Council (grant 321305). The work in Vilnius was supported by the Research Council of Lithuania (project MIP-85/2015). I.D.W.S. acknowledges support from a Royal Society Wolfson Research Merit Award. D.A.V. is grateful to Supergen SuperSolar Hub for the travel grant. The research data supporting this publication can be accessed at http://dx.doi.org/10.17630/7ec84b4b-d2ab-493c-aaf6-5503a44c0eb5Broadband transient absorption spectroscopy is combined with ultrafast carrier drift measurements to study dissociation of photogenerated charge pairs in efficient photovoltaic blends of the electron donating polymer PTB7 with the acceptor PC71BM. A high ensemble-average mobility sum of electrons and holes is observed which is independent of applied electric field above 12 V/μm and indicates nearly barrier-less pair dissociation at room temperature on a picosecond time scale. High efficiency of pair dissociation in this material is achieved by a combination of high electron mobility in fullerene clusters and hole delocalization along the polymer chain which increases by 30% during dissociation. Our results suggest a predominantly diffusive charge pair dissociation mechanism which requires persistent mobility of both carriers and preferably some delocalization of at least one of them.PostprintPostprintPeer reviewe

Optical antennas for wavelength division multiplexing in visible light communications beyond the étendue limit

Funding: UK EPSRC via “Ultra-parallel visible light communications (UP-VLC)” (Programme Grant EP/K00042X/1) and “Super Receivers for Visible Light Communications” (related grants EP/R00689X/1 and EP/R005281/1).This paper presents a novel fluorescent receiver for visible light communications that combines a broad field of view (FoV) with a significant gain, and simultaneously enables wavelength division multiplexing (WDM) for data communications. These characteristics are achieved by creating a receiver that includes a stack of two fluorescent optical antennas, each designed to receive a different part of the spectrum and create two separable communication channels. The device outperforms the theoretical étendue limit in terms of the combination of FoV and gain it can achieve. Furthermore, the demonstrated de‐multiplexing of blue and green spectral components enables the parallel streaming of data by WDM. Since these devices are inexpensive, simple, and compact, they can easily be integrated into portable electronic devices such as phones, tablets, and laptops.PostprintPeer reviewe

Flexible Glass Hybridized Colloidal Quantum Dots for Gb/s Visible Light Communications

Color converting films of colloidal quantum dots (CQDs) encapsulated with flexible glass are integrated with microsize GaN LEDs (μLEDs) in order to form optical sources for high-speed visible light communications (VLC). VLC is an emerging technology that uses white and/or colored light from LEDs to combine illumination and display functions with the transmission of data. The flexible glass/CQD format addresses the issue of limited modulation speed of typical phosphor-converted LEDs while enhancing the photostability of the color converters and facilitating their integration with the μLEDs. These structures are less than 70 μm in total thickness and are directly placed in contact with the polished sapphire substrate of 450-nm-emitting μLEDs. Blue-to-green, blue-to-orange and blue-to-red conversion with respective forward optical power conversion efficiencies of 13%, 12% and 5.5% are reported. In turn, free-space optical communications up to 1.4 Gb/s VLC is demonstrated. Results show that CQD-converted LEDs pave the way for practical digital lighting/displays with multi-Gb/s capability

Ultra-wide coverage VLC system with alignment-free receiver

In this work we present an ultra-wide coverage visible light communication (VLC) system based on a mechanical beam steering system at the transmitter terminal and a fluorescent optical concentrator at the receiver. The transmitter and receiver full field of view (FFOV) are 100° and 120°, respectively. The coverage area of the transmitter system at 2m is 12.5m2 and a data link of 300Mbps has been achieved at this range.Postprin

Visualizing the non-equilibrium dynamics of photoinduced intramolecular electron transfer with femtosecond X-ray pulses

Ultrafast photoinduced electron transfer preceding energy equilibration still poses many experimental and conceptual challenges to the optimization of photoconversion since an atomic-scale description has so far been beyond reach. Here we combine femtosecond transient optical absorption spectroscopy with ultrafast X-ray emission spectroscopy and diffuse X-ray scattering at the SACLA facility to track the non-equilibrated electronic and structural dynamics within a bimetallic donor–acceptor complex that contains an optically dark centre. Exploiting the 100-fold increase in temporal resolution as compared with storage ring facilities, these measurements constitute the first X-ray-based visualization of a non-equilibrated intramolecular electron transfer process over large interatomic distances. Experimental and theoretical results establish that mediation through electronically excited molecular states is a key mechanistic feature. The present study demonstrates the extensive potential of femtosecond X-ray techniques as diagnostics of non-adiabatic electron transfer processes in synthetic and biological systems, and some directions for future studies, are outlined

Novel fast color-converter for visible light communication using a blend of conjugated polymers

Visible Light Communications (VLC) is a promising new technology which could offer higher data transmission rates than existing broadband RF/microwave wireless technologies. In this paper, we show that a blend of semiconducting polymers can be used to make a broadband, balanced color converter with a very high modulation bandwidth to replace commercial phosphors in hybrid LEDs for visible light communications. The resulting color converter exploits partial Förster energy transfer in a blend of the highly fluorescent green emitter BBEHP-PPV and orange-red emitting MEH-PPV. We quantify the efficiency of the photoinduced energy transfer from BBEHP-PPV to MEH-PPV, and demonstrate modulation bandwidths (electrical-electrical) of ∼200 MHz, which are 40 times higher than commercially available phosphor LEDs. Furthermore, the VLC data rate achieved with this blend using On–Off Keying (OOK) is many times (∼35) higher than that measured with a commercially available phosphor color converter

The influence of blend ratio and processing additive on free carrier yield and mobility in PTB7:PC71BM photovoltaic solar cells

This work was supported by the Research Council of Lithuania (project MIP-85/2015), the Engineering and Physical Sciences Research Council of the UK (grants EP/J009016/1 and EP/L012294/1) and the European Research Council of the European Union (grant 321305). I.D.W.S. also acknowledges support from a Royal Society Wolfson Research Merit Award. D.A.V. is grateful to Supergen SuperSolar Hub for the travel grant.Charge separation and extraction dynamics were investigated in high performance bulk heterojunction solar cells made from the polymer PTB7 and the soluble fullerene PC71BM on a broad time scale from sub-picosecond to microseconds using ultrafast optical probing of carrier drift and the integral-mode photocurrent measurements. We show that the short circuit current is determined by the separation of charge pairs into free carriers which is strongly influenced by blend composition. This separation is found to be efficient in fullerene-rich blends where a high electron mobility of > 0.1 cm2V-1s-1 is observed in the first 10 ps after excitation. Morphology optimization using the solvent additive 1,8-di-iodooctane (DIO) doubles the charge pair separation efficiency and the short circuit current. Carrier extraction at low internal electric field is slightly faster from the cells prepared with DIO which can reduce recombination losses and enhance a fill factor.Publisher PDFPeer reviewe

Novel fast color-converter for visible light communication using a blend of conjugated polymers

Visible Light Communications (VLC) is a promising new technology which could offer higher data transmission rates than existing broadband RF/microwave wireless technologies. In this paper, we show that a blend of semiconducting polymers can be used to make a broadband, balanced color converter with a very high modulation bandwidth to replace commercial phosphors in hybrid LEDs for visible light communications. The resulting color converter exploits partial Förster energy transfer in a blend of the highly fluorescent green emitter BBEHP-PPV and orange-red emitting MEH-PPV. We quantify the efficiency of the photoinduced energy transfer from BBEHP-PPV to MEH-PPV, and demonstrate modulation bandwidths (electrical-electrical) of ∼200 MHz, which are 40 times higher than commercially available phosphor LEDs. Furthermore, the VLC data rate achieved with this blend using On-Off Keying (OOK) is many times (∼35) higher than that measured with a commercially available phosphor color converter