Quantifying Photon Recycling in Solar Cells and Light-Emitting Diodes: Absorption and Emission Are Always Key.

Photon recycling has received increased attention in recent years following its observation in halide perovskites. It has been shown to lower the effective bimolecular recombination rate and thus increase excitation densities within a material. Here we introduce a general framework to quantify photon recycling which can be applied to any material. We apply our model to idealized solar cells and light-emitting diodes based on halide perovskites. By varying controllable parameters which affect photon recycling, namely, thickness, charge trapping rate, nonideal transmission at interfaces, and absorptance, we quantify the effect of each on photon recycling. In both device types, we demonstrate that maximizing absorption and emission processes remains paramount for optimizing devices, even if this is at the expense of photon recycling. Our results provide new insight into quantifying photon recycling in optoelectronic devices and demonstrate that photon recycling cannot always be seen as a beneficial process.ARB acknowledges funding from a Winton Studentship, Oppenheimer Studentship and the Engineering and Physical Sciences Research Council (EPSRC) Doctoral Training Centre in Photovoltaics (CDT-PV). MA acknowledges funding from the Marie Skłodowska-Curie actions (grant agreement No. 841386) under the European Union’s Horizon 2020 research and innovation programme. SDS acknowledges the Royal Society and Tata Group (UF150033). We thank Luis Pazos-Outón for supplying data for MAPbI3 solar cells. This work was supported by EPSRC grant EP/S030638/1

Magnetic Field Modulation of Recombination Processes in Organic Photovoltaics

Polymer:fullerene photovoltaics have potential in small-scale power production but low open-circuit voltages limit their efficiency. Understanding the processes affecting the charge recombination rate is key to increasing device efficiency through optimizing open-circuit voltage. Most polymer-fullerene systems have an intramolecular triplet exciton state lower in energy than the interfacial charge-transfer state, and its formation can provide a terminal recombination pathway that may limit device performance. We used magnetic fields to modulate intersystem crossing in a prototypical system, and monitored the effect on the open-circuit voltage to infer changes in the steady-state carrier density and hence in the net recombination rate constant. We analyzed these effects using density-matrix modeling, and quantified the various recombination rate constants for a working device

Spin-dependent recombination probed through the dielectric polarizability.

Despite residing in an energetically and structurally disordered landscape, the spin degree of freedom remains a robust quantity in organic semiconductor materials due to the weak coupling of spin and orbital states. This enforces spin-selectivity in recombination processes which plays a crucial role in optoelectronic devices, for example, in the spin-dependent recombination of weakly bound electron-hole pairs, or charge-transfer states, which form in a photovoltaic blend. Here, we implement a detection scheme to probe the spin-selective recombination of these states through changes in their dielectric polarizability under magnetic resonance. Using this technique, we access a regime in which the usual mixing of spin-singlet and spin-triplet states due to hyperfine fields is suppressed by microwave driving. We present a quantitative model for this behaviour which allows us to estimate the spin-dependent recombination rate, and draw parallels with the Majorana-Brossel resonances observed in atomic physics experiments.This work was supported by the Engineering and Physical Sciences Research Council [Grants No. EP/G060738/1]. A. D. C. acknowledges support from the E. Oppenheimer Foundation and St Catharine's College, Cambridge. S. L. B. is grateful for support from the EPSRC Supergen SuperSolar Project, the Armourers and Brasiers Gauntlet Trust and Magdalene College, Cambridge.This is the final published version of the article. It was originally published in Nature Communications (Bayliss et. al, Nature Communications 2015, 6, 8534, doi:10.1038/ncomms9534). The final version is available at http://dx.doi.org/10.1038/ncomms953

Spin-dependent recombination probed through the dielectric polarizability.

Despite residing in an energetically and structurally disordered landscape, the spin degree of freedom remains a robust quantity in organic semiconductor materials due to the weak coupling of spin and orbital states. This enforces spin-selectivity in recombination processes which plays a crucial role in optoelectronic devices, for example, in the spin-dependent recombination of weakly bound electron-hole pairs, or charge-transfer states, which form in a photovoltaic blend. Here, we implement a detection scheme to probe the spin-selective recombination of these states through changes in their dielectric polarizability under magnetic resonance. Using this technique, we access a regime in which the usual mixing of spin-singlet and spin-triplet states due to hyperfine fields is suppressed by microwave driving. We present a quantitative model for this behaviour which allows us to estimate the spin-dependent recombination rate, and draw parallels with the Majorana-Brossel resonances observed in atomic physics experiments.This work was supported by the Engineering and Physical Sciences Research Council [Grants No. EP/G060738/1]. A. D. C. acknowledges support from the E. Oppenheimer Foundation and St Catharine's College, Cambridge. S. L. B. is grateful for support from the EPSRC Supergen SuperSolar Project, the Armourers and Brasiers Gauntlet Trust and Magdalene College, Cambridge.This is the final published version of the article. It was originally published in Nature Communications (Bayliss et. al, Nature Communications 2015, 6, 8534, doi:10.1038/ncomms9534). The final version is available at http://dx.doi.org/10.1038/ncomms953

In situ optical measurement of charge transport dynamics in organic photovoltaics.

We present a novel experimental approach which allows extraction of both spatial and temporal information on charge dynamics in organic solar cells. Using the wavelength dependence of the photonic structure in these devices, we monitor the change in spatial overlap between the photogenerated hole distribution and the optical probe profile as a function of time. In a model system we find evidence for a buildup of the photogenerated hole population close to the hole-extracting electrode on a nanosecond time scale and show that this can limit charge transport through space-charge effects under operating conditions.This work was supported by the EPSRC [Grant number EP/ G060738/1].This is the author accepted manuscript. The final published version is available at http://pubs.acs.org/doi/abs/10.1021/nl503687u

Charge Transfer in Single Chains of a Donor-Acceptor Conjugated Tri-Block Copolymer.

The photophysics of a conjugated triblock copolymer comprising poly(9,9-dioctylfluorene-co-bis-N,N'-(4-methylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine) (PFM) electron donor and poly(4-(9,9-dioctyl-9H-fluoren-2-yl)benzo[c][1,2,5]-thiadiazole) (F8BT) electron acceptor blocks has been studied in solution, in films, and as single chains. While an additional long-wavelength emission apparent in neat films of the copolymer is attributed to interchain exciplex formation, no such long-wavelength emission is apparent in solution or from single molecules. However, in these cases, time-resolved fluorescence measurements indicate the presence of a delayed fluorescence. The kinetics of the delayed emission can be interpreted in terms of an equilibrium between a locally excited and a charge-transfer state at the interface of the copolymer block components. Rate constants and thermodynamic quantities associated with these processes have been evaluated. The single-molecule results allow the assignment of an intramolecular charge-transfer state in an isolated conjugated block copolymer chain.We thank the Australian Research Council (ARC) for financial support of this research through grants DP0986166, LE100100131 and LE110100161. ENH acknowledges an Australian Postgraduate Award. We acknowledge Sam Ashworth for technical assistance with data collection. NCG is grateful to the School of Chemistry, The University of Melbourne for a Wilsmore Fellowship, and to Queen’s College, Melbourne for a Sugden Fellowship.This is the accepted manuscript. The final version is available from ACS at http://pubs.acs.org/doi/abs/10.1021/jp510769p

Charge Dynamics in Solution-Processed Nanocrystalline CuInS2 Solar Cells.

We investigate charge dynamics in solar cells constructed using solution-processed layers of CuInS2 (CIS) nanocrystals (NCs) as the electron donor and CdS as the electron acceptor. By using time-resolved spectroscopic techniques, we are able to observe photoinduced absorptions that we attribute to the mobile hole carriers in the NC film. In combination with transient photocurrent and photovoltage measurements, we monitor charge dynamics on time scales from 300 fs to 1 ms. Carrier dynamics are investigated for devices with CIS layers composed of either colloidally synthesized 1,3-benzenedithiol-capped nanocrystals or in situ sol-gel synthesized thin films as the active layer. We find that deep trapping of holes in the colloidal NC cells is responsible for decreases in the open-circuit voltage and fill factor as compared to those of the sol-gel synthesized CIS/CdS cell.We gratefully acknowledge support by EPSRC (Grant No. EP/G060738/1), Cambridge Display Technology and the Worshipful Company of Armourers & Brasiers for a Gauntlet Trust award, as well as the MacDiarmid Institute for use of their EM facilities.This is the final version of the article. It first appeared from ACS via http://dx.doi.org/10.1021/acsnano.5b0043

Efficient Singlet Fission and Triplet-Pair Emission in a Family of Zethrene Diradicaloids.

Singlet fission offers the potential to overcome thermodynamic limits in solar cells by converting the energy of a single absorbed photon into two distinct triplet excitons. However, progress is limited by the small family of suitable materials, and new chromophore design principles are needed. Here, we experimentally vindicate the design concept of diradical stabilization in a tunable family of functionalized zethrenes. All molecules in the series exhibit rapid formation of a bound, spin-entangled triplet-pair state TT. It can be dissociated by thermally activated triplet hopping and exhibits surprisingly strong emission for an optically "dark" state, further enhanced with increasing diradical character. We find that the TT excited-state absorption spectral shape correlates with the binding energy between constituent triplets, providing a new tool to understand this unusual state. Our results reveal a versatile new family of tunable materials with excellent optical and photochemical properties for exploitation in singlet fission devices

Multiple-exciton generation in lead selenide nanorod solar cells with external quantum efficiencies exceeding 120.

Multiple-exciton generation-a process in which multiple charge-carrier pairs are generated from a single optical excitation-is a promising way to improve the photocurrent in photovoltaic devices and offers the potential to break the Shockley-Queisser limit. One-dimensional nanostructures, for example nanorods, have been shown spectroscopically to display increased multiple exciton generation efficiencies compared with their zero-dimensional analogues. Here we present solar cells fabricated from PbSe nanorods of three different bandgaps. All three devices showed external quantum efficiencies exceeding 100% and we report a maximum external quantum efficiency of 122% for cells consisting of the smallest bandgap nanorods. We estimate internal quantum efficiencies to exceed 150% at relatively low energies compared with other multiple exciton generation systems, and this demonstrates the potential for substantial improvements in device performance due to multiple exciton generation.NJLKD thanks the Cambridge Commonwealth European and International Trust, Cambridge Australian Scholarships and Mr Charles K Allen for financial support. MLB thanks the German National Academic Foundation (“Studienstiftung”) for financial support. MT thanks the Gates Cambridge Trust, EPSRC and Winton Programme for Sustainability for financial support. F.W.R.R. gratefully thanks financial support from CNPq [Grant number 246050/2012-8]. C.D. acknowledges financial support from the EU [Grant number 312483 ESTEEM2]. This work was supported by the EPSRC [Grant numbers EP/M005143/1, EP/G060738/1, EP/G037221/1] and the ERC [Grant number 259619 PHOTO-EM].This is the final version of the article. It first appeared from Nature Publishing Group via http://dx.doi.org/10.1038/ncomms925

Triplet diffusion in singlet exciton fission sensitized pentacene solar cells

Singlet fission sensitized photovoltaics have the potential to surpass the Shockley-Queisser limit for a single-junction structure. We investigate the dynamics of triplet excitons resulting from singlet fission in pentacene and their ionization at a C60 heterojunction. We model the generation and diffusion of excitons to predict the spectral response. We find the triplet diffusion length in polycrystalline pentacene to be 40 nm. Poly(3-hexylthiophene) between the electrode and pentacene works both to confine triplet excitons and also to transfer photogenerated singlet excitons into pentacene with 30% efficiency. The lower bound for the singlet fission quantum efficiency in pentacene is 180 ± 15%