Hole transporting materials for methylammonium lead halide (CH3NH3PbX3) perovskite solar cells
Date of Issue2017
Interdisciplinary Graduate School
Energy Research Institute @ NTU
Organic-inorganic lead halide perovskites have shown promise as cost effective high-performance material for solar cells. Hole transport materials (HTMs) are currently the bottleneck for the realization of efficient cost effective and stable devices. In this work, novel hole-transporting molecules based on triptycene (T101, T102 and T103), furan (F101) and silafluorene (S101) cores were synthesized using short routes with high yields are presented. In HTMs T101, T102 and T103 triptycene core is linked with diarylamine, triarylamine and thiophene-linked triarylamine sidechains, respectively. The bulky and twisted structure of triptycene provides high thermal stability, high glass transition temperature (Tg) and high solubility in common organic solvents. HTM F101 consists of an electron-rich furan core linked to triphenylamine moieties. The planar structure and smaller size of the O atom in the ring was expected to result in closer intermolecular π-π stacking, which could be beneficial for enhancing the hole transport and also increase the lifetime of charge carriers. In HTM S101 an electron rich silafluorene core is linked to triphenylamine moieties and hexyl chains attached to the silicon atom to produce high solubility inorganic solvents and so improve processing. Quantum calculations were performed to predict the opto-electronic properties and were found to be consistent with the experimental data. The power conversion efficiency (PCE) of the perovskite solar cells (PSCs) fabricated with T101, T102, and T103 as HTM was 8.42%, 12.24% and 12.38% respectively, as compared to 12.87% for spiro-OMeTAD. These results show that the performance of these three materials in perovskite solar cells was profoundly influenced by the sidechains. The devices based on F101 produced high PCE of 13.12%, which was higher than the efficiency obtained using spiro-OMeTAD (13%). The F101 HTM based device exhibited a JSC of 19.63 mA/cm2 and a remarkably high VOC of 1.1 V. The charge carrier dynamics of perovskite/ HTM interface which gives insight into the charge transfer process, were studied via steady state and time resolved PL. From PL steady state spectra, it was evident that both HTMs significantly quench the perovskite emission signal, with HTM F101 demonstrating slightly better PL quenching efficiency, ~ 97%, compared to spiro-OMeTAD (~ 93%). Both HTMs spiro-OMeTAD and F101 showed similar decay lifetimes of ~ 1 ns. These results suggest that furan is potentially an excellent unit for incorporation in HTMs and furans should be further investigated as replacements for thiophene units in HTMs. The devices fabricated with HTM S101 shows a PCE of ~ 11% which is comparable to that (η = 12.3%) obtained with spiro-OMeTAD. From the PL spectra it was evident that both HTMs significantly quench perovskite emission signal, with HTM spiro-OMeTAD having a slightly better PL quenching efficiency (ca. 93%) relative to S101 (ca. 89%). Time-resolved PL measurements of CH3NH3PbI3/spiro-OMeTAD samples showed a decay time of 0.8 ns whereas CH3NH3PbI3/S101 samples showed a longer decay life time of 1.1 ns. This indicates that both HTMs are quenching the holes but spiro-OMeTAD has a slightly better hole extraction ability as compared to S101. These results suggest silafluorene derivatives can produce at least comparable cost-per-unit power performance to spiro-OMeTAD. HTMs based on different design principles were synthesized and their properties and device performances studied in detail. The triptycene based HTMs are bulky, star shaped and non-planar with high Tg. By contrast the furan and silafluorene-based HTMs, i.e., F101 and S101 are linear and planar, respectively and possess lower Tg. This suggests high Tg is not necessary for high efficiency, though more detailed device stability tests need to be performed to see if it is important for extending lifetime. The performance of all these molecules having different dimensional symmetry (star shaped, linear and planar molecules) in PSCs is quite encouraging for the development of cost effective technologies.