Coevaporated Bisquaraine Inverted Solar Cells: Enhancement Due to Energy Transfer and Open Circuit Voltage Control

Abstract

There is currently enormous interest in the development of small molecule organic solar cells (SMSC), as in principle, these systems offer advantages over both conventional Si photovoltaics and organic polymer solar cells. Here, we report Forster Resonance Energy Transfer (FRET) enhanced inverted SMSC fabricated by coevaporating two different squaraine donors, a symmetrical squaraine (SQ, 2,4-bis-4-[(N,N-diisobutylamino)-2,6dihydroxyphenyl] squaraine), and an asymmetrical squaraine (ASSQ, 2,4-bis-[(N,N-diisobutylamino)-2,6-dihydroxyphenyl]-4-(4diphenyliminio) squaraine). ASSQ absorbs blue light (λ max 540 nm) and emits from 550 nm to the near-infrared region, which overlaps with SQ absorption (λ max 690 nm). Therefore, by utilizing a thin film containing the two squaraine donors as the active layer in a SMSC, we can both broaden the photovoltaic absorption spectrum, and reduce recombination loss as a result of FRET. This strategy has resulted in SMSC with power conversion efficiencies (PCE) which are up to 46% greater than those obtained by using a single squaraine donor. Ultrafast time-resolved photoluminescence and transient absorption spectroscopy provide clear evidence of FRET between the small molecules, with a rapid energy transfer time of ∼1 ps. At optimal blending, which correlates to the highest PCE measured, the efficiency of energy transfer is as high as 85%. Furthermore, in the devices containing two different squaraine molecules, the open circuit voltage (V OC ) is proportional to the fraction of the two donors in the blend, allowing us to predict the V OC as the ratio of the two donors is changed. SMSC with inverted structures also demonstrate long-term stability in ambient conditions compared to devices employing a conventional architecture.

Extending the Photovoltaic Response of Perovskite Solar Cells into the Near‐Infrared with a Narrow‐Bandgap Organic Semiconductor

“…These studies were performed by pumping at 374 nm to selectively excite CsFAMA, and detecting emission at 760 nm (corresponding to CsFAMA) and 855 nm (corresponding to IEICO‐4F). We observe that the emission decay of CsFAMA accelerates with increasing concentration of IEICO‐4F (Figure S5, Supporting Information), implying possible Förster resonance energy transfer (FRET) from CsFAMA to IEICO‐4F . The PL decay at 855 nm of the neat IEICO‐4F film is very close to the instrument response function (IRF), likely because pumping at 374 nm does not effectively excite neat IEICO‐4F, as IEICO‐4F has minimal absorption at this wavelength.…”

Status and prospects for ternary organic photovoltaics

“…70). Additionally, ultrafast (~1 picosecond) and efficient (85% efficiency) FRET between two co-evaporated SQ dyes was demonstrated 71 .…”

“…For example, by incorporating 0.058 wt% PFDTBT into a poly(N-9ʹ-heptadecanyl-2,7-carbazole-alt-5,5-(4ʹ,7ʹ-di-2-thienyl-2ʹ,1ʹ,3ʹ-benzothiadiazole)) (PCDTBT):PC 71 BM system, the PCE increased from 5.50% to 6.81% (ref. 65).…”

The Intriguing Case of the One‐Photon and Two‐Photon Absorption of a Prototypical Symmetric Squaraine: Comparison of TDDFT and Wave‐Function Methods

“…Squaraine dyes are obtained from the condensation of squaric acid with aromatic or heterocyclic components, [1,2] and are usually considered as donor-acceptor-donor (DÀ A-D) systems where the oxocyclobutenolate acts as the acceptor core. In the last decades, they have attracted large interest for different applications as solar cell dyes, [3][4][5][6][7] fluorescent probes, [8,9] photosensitizers for photodynamic therapy, [10][11][12] and nonlinear optical chromophores. [13][14][15][16][17][18][19][20] Figure 1 reports the structure of the simple squaraine dye that will be studied in this contribution, since now on referred to as SQ0.…”