Absorption cross-sections of hole polarons in glassy and beta-phase polyfluorene

F. Montilla acknowledges financial support provided by Ministerio de Economía y Competitividad (Ramón y Cajal RYC1463/06 and José Castillejo JC2009-00 227 Programs). The authors also acknowledge the Engineering and Physical Sciences Research Council for financial support.Absorption induced by electrochemically injected holes is studied in poly-9,9-dioctylfluorene (PFO) films. Injected charges form positive polarons which are delocalised over four fluorene units in the glassy phase and about seven fluorene units in its beta-phase. Polaron absorption cross-sections at the 640 nm peak are similar to the published values of chemically reduced oligofluorenes in solution. The absorption cross-section of polaron in the beta-phase at 470 nm is about eight times smaller than the stimulated emission cross-section derived from published data. This indicates that beta-phase-rich PFO is an attractive candidate for a lightemitting layer in double-heterostructure organic laser diodes.Publisher PDFPeer reviewe

Light harvesting for organic photovoltaics

The authors are grateful to the Engineering and Physical Sciences Research Council of the UK (grants EP/J009016/1 and EP/L017008/1) and the European Research Council (grant number 321305) for financial support. IDWS also acknowledges a Royal Society Wolfson Research Merit Award.The field of organic photovoltaics has developed rapidly over the last 2 decades, and small solar cells with power conversion efficiencies of 13% have been demonstrated. Light absorbed in the organic layers forms tightly bound excitons that are split into free electrons and holes using heterojunctions of electron donor and acceptor materials, which are then extracted at electrodes to give useful electrical power. This review gives a concise description of the fundamental processes in photovoltaic devices, with the main emphasis on the characterization of energy transfer and its role in dictating device architecture, including multilayer planar heterojunctions, and on the factors that impact free carrier generation from dissociated excitons. We briefly discuss harvesting of triplet excitons, which now attracts substantial interest when used in conjunction with singlet fission. Finally, we introduce the techniques used by researchers for characterization and engineering of bulk heterojunctions to realize large photocurrents, and examine the formed morphology in three prototypical blends.Publisher PDFPeer reviewe

Distance dependence of excitation energy transfer between spacer-separated conjugated polymer films

We report a systematic study of the scaling with distance of electronic energy transfer between thin films of conjugated polymers separated by a silica spacer. The energy-transfer kinetics were obtained directly from time-resolved photoluminescence measurements and show a 1/ z3 distance dependence of the transfer rate between the excited donor and the acceptor film for z≥8 nm. This is consistent with Förster theory; but at shorter separations the energy transfer is slower than predicted and can be explained by the breakdown of the point-dipole approximation at z∼5 nm. The results are relevant for organic photovoltaics and light-emitting devices, where energy transfer can provide a means of increasing performance

Enhancing exciton diffusion length provides new opportunities for organic photovoltaics

Authors acknowledge support from the European Research Council (grant 321305) And are also grateful to EPSRC for support from grants (EP/L017008/1) and (EP/M025330/1).Organic semiconductors can potentially revolutionize solar cell technology by offering very thin, lightweight, and flexible modules for outdoor and indoor power generation. Light absorption in organic semiconductors generates a bound electron-hole pair (exciton), which needs to travel to the interface between electron donor and acceptor materials to dissociate into charge carriers. Because the exciton diffusion length in organic semiconductors is typically much shorter than the light absorption depth (∼100 nm), planar donor-acceptor heterojunctions are inefficient, and most effort has been dedicated to optimization of bulk heterojunctions with nanoscale phase separation. In this Perspective, we review recent findings and new approaches to increase the exciton diffusion length and discuss how these improvements can benefit environmentally friendly production of solar modules using organic nanoparticles or graded heterojunctions obtained by sequential deposition of electron donor and acceptor.PostprintPeer reviewe

Effect of a high boiling point additive on the morphology of solution-processed P3HT-fullerene blends

Funding: UK Engineering and Physical Sciences Research Council (EPSRC) EP/L505079/1, EP/G03673X/1, EP/J009016/1The use of high boiling point additives in solution processing has been widely employed to control the active layer morphology in bulk heterojunction organic solar cells. The morphology of the heterojunction is crucial in controlling charge separation and extraction by the electrodes, and therefore the power conversion efficiency (PCE) of the device. This paper presents a study of time-resolved fluorescence quenching in blends of P3HT containing varying concentrations of the fullerenes PC61BM or PC71BM. The relationship between the fluorescence quenching rate and fullerene concentration indicates that the fullerene molecules are dispersed within the P3HT film for up to 5% by mass of fullerene. For higher fullerene concentrations, the additional fullerene molecules aggregate and form fullerene domains. The high degree of phase segregation observed in these blends is beneficial for solar cell performance because the segregated fullerene phase provides electron percolation pathways through the blend. The addition of 1,8-diiodooctane (DIO) to the solutions for spin coating into films changes the scale of fullerene segregation when the ratio by mass of fullerene exceeds 20%. At high fullerene concentrations the rate of fluorescence quenching decreases in P3HT:PC61BM blends when prepared with DIO indicating a larger scale phase separation. The effect of DIO on the morphology of P3HT:PC71BM blends is the opposite in that it causes faster quenching in the blends. Overall the results show that DIO can be used to control the morphology of photovoltaic blends of P3HT with fullerenes.Publisher PDFPeer reviewe

Charge pair dissociation and recombination dynamics in a P3HT–PC60BM bulk heterojunction

The authors thank the EPSRC, SUPA, the Condensed Matter Doctoral Training Centre and the European Union Seventh Framework programme (grant agreement 321305) for financial support.The mechanism by which Coulombically bound charge pairs dissociate into free carriers in photovoltaic donor–acceptor blends is of great interest. Here, we use polarization-sensitive transient absorption (TA) to study the diffusion of photogenerated holes in a polythiophene (P3HT)–fullerene (PC60BM) blend. We observe an initial anisotropy value of 0.4 for the absorption of photogenerated holes, indicating that holes generated on a 100 fs time scale are localized on the same polymer chain as their precursor excitons. Depolarization dynamics indicate fast initial hole motion on a 0.3 ps time scale and slower migration up to 100 ps. Charge pair recombination is found to occur on a much longer time scale of 10–1000 ns via a purely bimolecular process independent of excess energy. Our results show that nearly all charge pairs get separated by at least 7 nm in the absence of an external field and indicate that high charge mobility is crucial for charge separation.PostprintPeer reviewe

Controlling exciton diffusion and fullerene distribution in photovoltaic blends by side chain modification

The influence of crystallinity on exciton diffusion and fullerene distribution was investigated by blending amorphous and semicrystalline copolymers. We measured exciton diffusion and fluorescence quenching in such blends by dispersing fullerene molecules into them. We find that the diffusion length is more than two times higher in the semicrystalline copolymer than in the amorphous copolymer. We also find that fullerene preferentially mixes into disordered regions of the polymer film. This shows that relatively small differences in molecular structure are important for exciton diffusion and fullerene distribution

Exciton-polaron interactions in polyfluorene films with β phase

The authors acknowledge financial support from the European Research Council (grant 321305), Spanish Ministry of Economy Explora Ciencia Project MAT2013-49534-EXP and the Engineering and Physical Sciences Research Council (grants EP/L017008/1 and EP/J009016/1). I.D.W.S. also acknowledges support from a Royal Society Wolfson Research Merit Award.Fluorescence quenching by electric charges is an important loss mechanism in high-brightness organic light emitting diodes (OLEDs) but its effect is difficult to quantify in working devices. Here we combine an electrochemical technique to control the charge density with time-resolved photoluminescence to distinguish between different quenching mechanisms. The material studied was the blue electroluminescent polymer poly(9,9-dioctylfluorenene) with β phase. Our results show that quenching occurs by Förster resonance energy transfer and is mediated by exciton diffusion. We determine the quenching parameters over a wide range of charge concentrations and estimate their impact on the OLED efficiency roll-off at high current density. We find that fluorescence quenching by charges and singlet-triplet exciton annihilation are the two main mechanisms leading to the efficiency roll-off. Our results suggest that hole polarons are not very effective quenchers of singlet excitons which is important for understanding current devices and encouraging for the development of high-brightness OLEDs and lasers.PostprintPeer reviewe

Interface limited hole extraction from methylammonium lead iodide films

Small solar cells based on metal halide perovskites have shown a tremendous increase in efficiency in recent years. These huge strides in device performance make it important to understand processes such as accumulation and extraction of charge carriers to better address the scalability and stability challenges which have not been solved yet. In most studies to date it is unclear whether the limiting factor of charge extraction is charge transport in the bulk of the perovskite or transfer across the interface with the charge extracting layer, owing largely to the inaccessibility of buried interfaces. Separating bulk and interfacial effects on charge extraction can help the search for new charge extracting materials, improve understanding of charge transport in active layer materials and help optimise device performance; not only in the laboratory setting but also for commercial production. Here we present a method to unambiguously distinguish between bulk and interface effects on charge extraction dynamics which is based on time-resolved photoluminescence with different excitation density profiles. We use this method to study charge extraction from solution-deposited CH3NH3PbI3 films to NiO and PEDOT:PSS layers. We find that NiO shows faster hole extraction than PEDOT:PSS from the 300 nm thick perovskite film on the time scale of 300 ps which is independent of charge carrier density in the region of 1016–1017 cm−3. The interface with NiO is found to only slightly limit charge extraction rate at charge densities exceeding 1016 cm−3 as the extraction rate is fast and does not decrease with time. This is in contrast to PEDOT:PSS where we find the charge extraction rate to be slower, decreasing with time and dependent on charge density in the region 1016–1017 cm−3 which we interpret as charge accumulation at the interface. Hence we find that charge extraction is severely limited by the interface with PEDOT:PSS. These findings are confirmed by transient absorption spectroscopy. A hole diffusion coefficient of D = (2.2 ± 0.5) cm2 s−1 was determined in the perovskite film that is independent of charge density. This indicates a band-like hole transport regime, not observed for solution processed films before. Our findings stress the importance of interface optimization in devices based on perovskite active layers as there is still room for improvement of the hole extraction rate even in the case of the superior NiO layer

Nanoscale heterogeneity in CsPbBr3 and CsPbBr3:KI perovskite films revealed by cathodoluminescence hyperspectral imaging

Funding: We are grateful for funding from the EPSRC under grant code EP/L017008/1. Dr. L. K. Jagadamma acknowledges support from a Marie Skłodowska-Curie Individual Fellowship (European Commission) (MCIF: No. 745776).The nanoscale morphology of solar cell materials strongly affects their performance. We report direct evidence for the existence of multiple length scales of heterogeneity in halide perovskites such as CsPbBr3 and CsPbBr3:KI. Contrary to the general notion of two distinct phases, our study suggests the presence of multiple phases in mixed halide perovskites. Highly spatially resolved (≈50 nm) cathodoluminescence maps reveal that the length scale of heterogeneity is composition dependent: smaller (≈ 200 nm) for CsPbBr3, and larger (≈500–1000 nm) for CsPbBr3:KI. Moreover, these nano-/micro-scale heterogeneities exist both laterally and vertically in mixed halides and correlate with high densities of carrier traps and fast trap-assisted recombination. The observed heterogeneities also lead to reduced power conversion efficiency of solar cells, higher hysteresis loss, and faster degradation. These insights argue for advanced nanoscale characterization of halide perovskites to guide reduction of heterogeneity and so improve device performance and stability.PostprintPeer reviewe