Correlating photovoltaic properties of PTB7-Th:PC71BM blend to photophysics and microstructure as a function of thermal annealing

We acknowledge support from EPSRC (grant number EP/L012294/1) and the European Research Council (grant number 321305). I.D.W.S. also acknowledges a Royal Society Wolfson Research Merit Award. VS acknowledges support from the Office of Naval Research NDSEG fellowship. Research data supporting this paper is available at doi http://dx.doi.org/10.17630/eadf56f3-8c70-47da-ac6d-67f2d78b3f74Selective optimisation of light harvesting materials and interface properties has brought breakthroughs in power conversion efficiency (11-12 %) of organic photovoltaics (OPVs). However to translate this promising efficiency to economically viable applications, long term stability is a fundamental requirement. A number of degradation pathways, both extrinsic and intrinsic, reduce the long term stability of OPVs. Here, the photovoltaic properties of a highly efficient bulk heterojunction PTB7-Th:PC71BM blend were investigated as a function of thermal annealing. The changes in charge generation, separation, and transport due to thermal annealing were measured and related to changes in the microstructure and photovoltaic performance. A 30 % drop in power conversion efficiency of PTB7-Th:PC71BM blends upon thermal annealing at 150 oC was identified as mainly due to morphological instability induced by strong phase separation of donor and acceptor molecules of the blend films. Based on the insight gained from these investigations, enhanced thermal stability was demonstrated by replacing the PC71BM fullerene acceptor with the non-fullerene acceptor ITIC, for which power conversion efficiency dropped only by 9 % upon thermal annealing at 150 oC.PostprintPeer reviewe

Correlating photovoltaic properties of PTB7-Th:PC71BM blend to photophysics and microstructure as a function of thermal annealing

Selective optimisation of light harvesting materials and interface properties has brought breakthroughs in power conversion efficiency (11–12%) of organic photovoltaics (OPVs). However to translate this promising efficiency to economically viable applications, long term stability is a fundamental requirement. A number of degradation pathways, both extrinsic and intrinsic, reduce the long term stability of OPVs. Here, the photovoltaic properties of a highly efficient bulk heterojunction PTB7-Th:PC71BM blend were investigated as a function of ex situ thermal annealing. The changes in charge generation, separation, and transport due to thermal annealing were measured and related to changes in the microstructure and photovoltaic performance. A 30% drop in the power conversion efficiency of PTB7-Th:PC71BM blends upon thermal annealing at 150 °C was identified as mainly due to morphological instability induced by strong phase separation of donor and acceptor molecules of the blend films. Based on the insight gained from these investigations, enhanced thermal stability was demonstrated by replacing the PC71BM fullerene acceptor with a non-fullerene acceptor ITIC, for which power conversion efficiency dropped only by 9% upon thermal annealing at 150 °C

Impact of interfacial molecular orientation on radiative recombination and charge generation efficiency.

A long standing question in organic electronics concerns the effects of molecular orientation at donor/acceptor heterojunctions. Given a well-controlled donor/acceptor bilayer system, we uncover the genuine effects of molecular orientation on charge generation and recombination. These effects are studied through the point of view of photovoltaics-however, the results have important implications on the operation of all optoelectronic devices with donor/acceptor interfaces, such as light emitting diodes and photodetectors. Our findings can be summarized by two points. First, devices with donor molecules face-on to the acceptor interface have a higher charge transfer state energy and less non-radiative recombination, resulting in larger open-circuit voltages and higher radiative efficiencies. Second, devices with donor molecules edge-on to the acceptor interface are more efficient at charge generation, attributed to smaller electronic coupling between the charge transfer states and the ground state, and lower activation energy for charge generation.Molecular orientation profoundly affects the performance of donor-acceptor heterojunctions, whilst it has remained challenging to investigate the detail. Using a controllable interface, Ran et al. show that the edge-on geometries improve charge generation at the cost of non-radiative recombination loss

Tuning crystalline ordering by annealing and additives to study its effect on exciton diffusion in a polyalkylthiophene copolymer

M.C, M.T.S, A.R and I.D.W.S acknowledge support from the European Research Council (EXCITON grant 321305). I.D.W.S acknowledges Royal Society Wolfson Research Merit Award. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515The influence of various processing conditions on the singlet exciton diffusion is explored in films of a conjugated random copolymer poly-(3-hexylthiophene-co-3-dodecylthiophene) (P3HT-co-P3DDT) and correlated with the degree of crystallinity probed by grazing incidence X-ray scattering and with exciton bandwidth determined from absorption spectra. The exciton diffusion coefficient is deduced from exciton-exciton annihilation measurements and is found to increase by more than a factor of three when thin films are annealed using CS2 solvent vapour. A doubling of exciton diffusion coefficient is observed upon melt annealing at 200 °C and the corresponding films show about 50% enhancement in the degree of crystallinity. In contrast, films fabricated from polymer solutions containing a small amount of either solvent additive or nucleating agent show a decrease in exciton diffusion coefficient possibly due to formation of traps for excitons. Our results suggest that the enhancement of exciton diffusivity occurs because of increased crystallinity of alkyl-stacking and longer conjugation of aggregated chains which reduces the exciton bandwidth.Publisher PDFPeer reviewe

Triptycene as a supramolecular additive in PTB7:PCBM blends and its influence on photovoltaic properties

We acknowledge support from EPSRC (grant number EP/L012294/1) and the European Research Council (grant number 321305). I. D. W. S. also acknowledges a Royal Society Wolfson Research Merit Award.Additives play an important role in modifying the morphology and phase separation of donor and acceptor molecules in bulk heterojunction (BHJ) solar cells. Here, we report triptycene (TPC) as a small-molecule additive for supramolecular control of phase separation and concomitant improvement of the power conversion efficiency (PCE) of PTB7 donor and fullerene acceptor-based BHJ polymer solar cells. An overall 60% improvement in PCE is observed for both PTB7:PC61BM and PTB7:PC71BM blends. The improved photovoltaic (PV) performance can be attributed to three factors: (a) TPC-induced supramolecular interactions with donor:acceptor components in the blends to realize a nanoscale phase-separated morphology; (b) an increase in the charge transfer state energy that lowers the driving force for electron transfer from donor to acceptor molecules; and (c) an increase in the charge carrier mobility. An improvement in efficiency using TPC as a supramolecular additive has also been demonstrated for other BHJ blends such as PBDB-T:PC71BM and P3HT:PCBM, implying the wide applicability of this new additive molecule. A comparison of the photostability of TPC as an additive for PTB7:PCBM solar cells to that of the widely used 1,8-diiodooctane additive shows ∼30% higher retention of PV performance for the TPC-added solar cells after 34 h of AM 1.5G illumination. The results obtained suggest that the approach of using additives that can promote supramolecular interactions to modify the length scale of phase separation between donor and acceptor is very promising and can lead to the development of highly efficient and stable organic photovoltaics.PostprintPostprintPeer reviewe

Morphological, chemical, and electronic changes of the conjugated polymer PTB7 with thermal annealing

This work was supported by the Office of Naval Research NDSEG fellowship (V.S.) and the Department of Energy SCGSR Program (L.J.P.). Work was partially supported by the Department of the Navy, Office of Naval Research Award No. N00014-14-1-0580 (S.D.O., M.F.T.). L.K.J., I.R., and I.D.W.S. were supported by the Engineering and Physical Sciences Research Council (grants EP/L017008/1 and EP/L012294/1 ). I.D.W.S. also acknowledges support from a Royal Society Wolfson Research Merit Award. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.There is considerable interest in improving the performance of organic optoelectronic devices through processing techniques. Here, we study the effect of high-temperature annealing on the properties of the semiconducting polymer PTB7 and PTB7:fullerene blends, of interest as efficient organic photovoltaic (OPV) devices. Annealing to moderate temperature improves the PTB7 morphology and optoelectronic properties. High-temperature annealing also improves morphology but results in poorer optoelectronic properties. This is a result of side chain cleavage that creates by-products that act as trap states, increasing electronic disorder and decreasing mobility. We further observe changes to the PTB7 chemical structure after thermal cleavage that are similar to those following solar irradiation. This implies that side chain cleavage is an important mechanism in device photodegradation, which is a major ?burn-in? loss mechanism in OPV. These results lend insight into side chain cleavage as a method of improving optoelectronic properties and suggest strategies for improvement in device photostability.Publisher PDFPeer reviewe

Tuning crystalline ordering by annealing and additives to study its effect on exciton diffusion in a polyalkylthiophene copolymer

The influence of various processing conditions on the singlet exciton diffusion is explored in films of a conjugated random copolymer poly-(3-hexylthiophene-co-3-dodecylthiophene) (P3HT-co-P3DDT) and correlated with the degree of crystallinity probed by grazing incidence X-ray scattering and with exciton bandwidth determined from absorption spectra. The exciton diffusion coefficient is deduced from exciton–exciton annihilation measurements and is found to increase by more than a factor of three when thin films are annealed using CS2 solvent vapour. A doubling of exciton diffusion coefficient is observed upon melt annealing at 200 °C and the corresponding films show about 50% enhancement in the degree of crystallinity. In contrast, films fabricated from polymer solutions containing a small amount of either solvent additive or nucleating agent show a decrease in exciton diffusion coefficient possibly due to formation of traps for excitons. Our results suggest that the enhancement of exciton diffusivity occurs because of increased crystallinity of alkyl-stacking and longer conjugation of aggregated chains which reduces the exciton bandwidth

Triptycene as a Supramolecular Additive in PTB7: PCBM blends and its Influence on Photovoltaic Properties

Additives play an important role in modifying the morphology and phase separation of donor and acceptor molecules in bulk heterojunction (BHJ) solar cells. Here, we report triptycene (TPC) as a small-molecule additive for supramolecular control of phase separation and concomitant improvement of the power conversion efficiency (PCE) of PTB7 donor and fullerene acceptor-based BHJ polymer solar cells. An overall 60% improvement in PCE is observed for both PTB7:PC61BM and PTB7:PC71BM blends. The improved photovoltaic (PV) performance can be attributed to three factors: (a) TPC-induced supramolecular interactions with donor:acceptor components in the blends to realize a nanoscale phase-separated morphology; (b) an increase in the charge transfer state energy that lowers the driving force for electron transfer from donor to acceptor molecules; and (c) an increase in the charge carrier mobility. An improvement in efficiency using TPC as a supramolecular additive has also been demonstrated for other BHJ blends such as PBDB-T:PC71BM and P3HT:PCBM, implying the wide applicability of this new additive molecule. A comparison of the photostability of TPC as an additive for PTB7:PCBM solar cells to that of the widely used 1,8-diiodooctane additive shows ∼30% higher retention of PV performance for the TPC-added solar cells after 34 h of AM 1.5G illumination. The results obtained suggest that the approach of using additives that can promote supramolecular interactions to modify the length scale of phase separation between donor and acceptor is very promising and can lead to the development of highly efficient and stable organic photovoltaics

Impact of Polymer Side Chain Modification on OPV Morphology and Performance

Efficiencies of organic photovoltaic (OPV) devices have been steadily climbing, but there is still a prominent gap in understanding the relationship between fabrication and performance. Side chain substitution is one processing parameter that can change OPV device efficiency considerably, primarily because of variations in morphology. In this work, we explain the morphological link between side chain selection and device performance in one polymer to aid in the development of design rules more broadly. We study the morphology of an OPV active layer using a PBDTTPD-backbone polymer with four different side chain configurations, which are shown to change device efficiency by up to 4 times. The optimal device has the smallest domain sizes, the highest degree of crystallinity, and the most face-on character. This is achieved with two branched 2-ethylhexyl (2EH) side chains placed symmetrically on the BDT unit and a linear octyl (C8) side chain on the TPD unit. Substituting either side chain (C14 on BDT and/or 2EH on TPD) makes the orientation less face-on, while the TPD side chain primarily affects domain size. For all side chains, the addition of fullerene increases polymer crystallization compared to the neat film, but the degree of mixing between polymer and fullerene varies with side chain. Interestingly, the optimal device has a negligible amount of mixed phase. The domain sizes present in the optimal system are remarkably unchanged with a changing fullerene ratio between 10 and 90%, hinting that the polymer preferentially self-assembles into 10-20 nm crystallites regardless of concentration. The formation of this crystallite may be the key factor inhibiting mixed phase