Terence John Stephen Dennis

High performance perovskite solar cell (PSC) technologies use electron transport layers (ETLs) to boost device performance. Here we show that single enantiomer chiral fullerene ETLs significantly enhanced thermal and operational stability of PSCs.

Environmental stability remains a major challenge for the commercialisation of organic solar cells and degradation pathways remain poorly understood. Designing materials for improved device stability requires an understanding of the relationship between the properties of the donor or acceptor molecule and different degradation mechanisms. Here we study the correlations between various molecular parameters of the fullerene derivative bis-PCBM and the degradation rate of polymer:bis-PCBM organic solar cells, based on the same carbazole-alt-benzothiadiazole polymer, in aerobic and anaerobic conditions. We compare eight high purity bis-PCBM isomers with different electronic, chemical and packing properties along with PCBM and the mixture of bis isomers. In the case of aerobic photodegradation, we find that device degradation rate is positively correlated to the LUMO energy of the bis-PCBM isomer and to the degree of crystallinity of the isomer, while the correlation of degradation with driving force for epoxide formation is unclear. These results support the idea that in these samples, aerobic photodegradation proceeds via superoxide formation by the photogenerated polaron on the fullerene, followed by further chemical reaction. In the absence of air, photodegradation rate is correlated with molecular structure, supporting the mechanism of microstructural degradation via fullerene dimerization. The approach and findings presented here show how control of specific molecular parameters through chemical design can serve as a strategy to enhance stability of organic solar cells.

With the aim of determining the best input geometry for DFT calculations of [60]PCBM, the geometry of 24 chemically possible [60]PCBM conformers were optimised and their electronic energies and average bond strains were determined. A DFT analysis of the relevant dihedral angles provided insights into the dynamical behaviour of the ester group through sterically restricted bond rotations. In addition, the ¹³C NMR spectra of the six better performing conformers were simulated and compared with an experiment. There is a close correlation between average bond strain, total electronic energy and mean absolute error of the simulated ¹³C NMR spectra of the ester carbons. The best overall candidate conformer for the input geometry had the C61-C4, C4-C3 and C3-C2 single bonds of the alkyl chain in syn, anti and anti arrangements, respectively, and had the C2-C1 and C1-O single bonds of the ester in syn and anti arrangements, respectively. This contrasts strikingly with most representations of PCBM in the literature, which depict all relevant bonds in anti arrangements.

The ¹H NMR spectrum of phenyl C₆₁ butyric acid methyl ester ([60]PCBM) was recorded at high resolution (600 MHz). All of the ¹H resonances expected of the C_s-symmetric molecule were observed. The spin–spin couplings in the ¹H NMR spectrum were not as expected at first order. Instead, the effects of AA′BB′-type second-order couplings were clearly observed for the protons attached to both ester carbons C3 and C4, which were analyzed in terms of seven coupling constants. This indicates that there is no free rotation of the σ bonds of the alkyl chain in the ester group, although rotation becomes free at a larger distance from the fullerene bridge carbon (C61). The ¹H NMR results further indicated that there is a 1:6:1 population ratio of the three staggered conformers (gauche:anti:gauche′) about the ester group C3–C4 bond. These results may aid in the understanding of the morphological interactions between [60]PCBM and its surroundings in condensed-phase organic electronic devices such as organic and perovskite photovoltaics.

Solubilized fullerene derivatives have revolutionised the development of organic photovoltaic devices, acting as excellent electron acceptors. The addition of solubilizing addends to the fullerene cage results in a large number of isomers, which are generally employed as isomeric mixtures. Moreover, a significant number of these isomers are chiral, which further adds to the isomeric complexity. The opportunities presented by single isomer, and particularly single enantiomer, fullerenes in organic electronic materials and devices are poorly understood. Here we separate 10 pairs of enantiomers from the 19 structural isomers of bis[60]PCBM, using them to elucidate important chiroptical structure-property relationships and demonstrating their application to a single enantiomer circularly polarized (CP) light detecting device. We find that larger chiroptical responses occur through inherent chirality of the fullerene cage and particularly through transitions with low CT character. When used in a single enantiomer organic field-effect transistor device, we demonstrate the potential to discriminate CP light with a fast light response time and with a very high photocurrent dissymmetry factor (gph = ±1.35). Our study thus provides key strategies to design fullerenes with large chiroptical responses for use as single enantiomer components of organic electronic devices. We anticipate that our data will position chiral fullerenes as an exciting material class for the growing field of chiral electronic technologies.

A number of recent studies have shown that the non-radiative voltage losses in organic solar cells can be suppressed in systems with low energetic offsets between donor and acceptor molecular states, but the physical reasons underpinning this remain unclear. Here, we present a systematic study of 18 different donor:acceptor blends to determine the effect that energetic offset has on both radiative and non-radiative recombination of the charge transfer (CT) state. We find that for certain blends, low offsets result in hybridization between charge-transfer and lowest donor or acceptor exciton states, which leads to a strong suppression in the non-radiative voltage loss to values as low as 0.23V associated with an increase in the luminescence of the CT state. Further, we extend a two-state CT-state recombination model to include the interaction between CT and first excited states, which allows us to explain the low non-radiative voltage losses as an increase in the effective CT to ground state oscillator strength due to the intensity borrowing mechanism. We show that low non‐radiative voltage losses can be achieved in material combinations with a strong electronic coupling between CT and first excited states, and where the lower band gap material has a high oscillator strength for transitions from the excited state to the ground state. Finally, from our model we propose that achieving very low non-radiative voltage losses may come at a cost of higher overall recombination rates, which may help to explain the generally lower FF and EQE of highly hybridized systems.

In recent times, as a result of its exceptional resistance to moisture and heat, cesium lead bromide (CsPbBr₃) has been established as a potential high-performance perovskite material for optoelectronics, which is inclusive of photodetectors and photovoltaics.

Organic semiconducting single crystals are ideal building blocks for organic field-effect transistors (OFETs) and organic photodetectors (OPDs) because they can potentially exhibit the best charge transport and photoelectric properties in organic materials. Nevertheless, it is usual for single-crystal OFETs to be built from one kind of organic material in which the dominant transport is either electron or hole; such OFETs show unipolar charge transport. Furthermore, single-crystal OPDs present high performance only in restricted regions because of the limited absorption of one-component single crystals. In an ideal situation, devices which comprise both electron- and hole-transporting single crystals with complementary absorptions, such as single-crystalline p–n heterojunctions (SCHJs), can permit broadband photoresponse and ambipolar charge transport. In this paper, a solution-processing crystallization strategy to prepare an SCHJ composed of C₆₀ and 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-PEN) was shown. These SCHJs demonstrated ambipolar charge-transport characteristics in OFETs with a balanced performance of 2.9 cm² V^–1 s^–1 for electron mobility and 2.7 cm² V^–1 s^–1 for hole mobility. This demonstration is the first of single-crystal OFETs in which both electron and hole mobilities were over 2.5 cm² V^–1 s^–1. OPDs fabricated upon as-prepared SCHJs exhibited highly sensitive photoconductive properties ranging from ultraviolet to visible and further to near-infrared regions as a result of complementary absorption between C₆₀ and TIPS-PEN, thereby attaining photoresponsivities that are among the highest reported values within the OPDs. This work would provide valuable references for developing novel SCHJ systems to achieve significant progress in high-performance ambipolar OFETs and broadband OPDs.

Solution‐processed organic single‐crystalline donor–acceptor heterojunctions (SCHJs) composed of N,N,N′,N′‐tetraphenylbenzidine (TPB) and phenyl‐C₆₁‐butyric acid methyl ester ([60]PCBM) are successfully obtained, and fundamental studies on its charge transport properties are demonstrated, revealing the advantages of applying SCHJs in photovoltaic devices. The SCHJs exhibit a balanced high‐mobility ambipolar charge transport with both hole and electron mobility being more than one‐order magnitude higher than its thin‐film heterojunction (TFHJ) counterparts. The difference between single‐crystalline and TFHJs in charge transport mechanisms was revealed, and it is shown that SCHJs present a more favorable band‐like charge transport properties at room temperature. Organic photovoltaics fabricated on SCHJs present much higher current density and a 32‐times higher power conversion efficiencies than TFHJ devices. The present work, which outlines comprehensive advantages of SCHJs in charge transport properties, should accelerate the application of organic single crystals for high‐performance photovoltaics.