A molecular photosensitizer achieves a Voc of 1.24 V enabling highly efficient and stable dye-sensitized solar cells with copper(II/I)-based electrolyte

Abstract

To develop photosensitizers with high open-circuit photovoltage (V_oc) is a crucial strategy to enhance the power conversion efficiency (PCE) of co-sensitized solar cells. Here, we show a judiciously tailored organic photosensitizer, coded MS5, featuring the bulky donor N-(2',4'-bis(dodecyloxy)-[1,1'-biphenyl]-4-yl)-2',4'-bis(dodecyloxy)-N-phenyl-[1,1'-biphenyl]-4-amine and the electron acceptor 4-(benzo[c][1,2,5]thiadiazol-4-yl)benzoic acid. Employing MS5 with a copper (II/I) electrolyte enables a dye-sensitized solar cell (DSC) to achieve a strikingly high V_oc of 1.24 V, with the V_oc deficit as low as 130 mV and an ideality factor of merely 1.08. The co-sensitization of MS5 with the wider spectral-response dye XY1b produces a highly efficient and stable DSC with the PCE of 13.5% under standard AM1.5 G, 100 mW cm^-2 solar radiation. Remarkably, the co-sensitized solar cell (active area of 2.8 cm²) presents a record PCE of 34.5% under ambient light, rendering it very attractive as an ambient light harvesting energy source for low power electronics.

Fig. 1. Dye molecular structures and their optical and electronic properties.

a Molecular structures of dyes (NT35, MS4, MS5, and XY1b) and copper complex ([Cu^(I)(tmby)₂][TFSI] and [Cu^(II)(tmby)₂][TFSI]₂, tmby = 4,4′,6,6′-tetramethyl-2,2′-bipyridine; TFSI = bis(trifluoromethylsulfonyl)imide). b UV–Vis absorption spectra of NT35, MS4, MS5, and XY1b adsorbed on 2.2 μm thick transparent TiO₂ films. c Energy levels diagram of TiO₂, dyes, and [Cu^(I)tmby)₂][TFSI].

Fig. 2. Photovoltaic performance and interfacial charge recombination of co-sensitizers.

a Incident photon-to-electron conversion efficiency (IPCE) of the dye-sensitized solar cells (DSCs) based on the co-sensitizers of NT35, MS4, and MS5. The solid lines show the corresponding integrated photocurrent calculated from the IPCE. b Current density–voltage curves of the DSCs with NT35, MS4, and MS5 measured under AM1.5G, 100 mW cm⁻² condition. c The ideality factors of the DSCs based on the dyes NT35, MS4, and MS5. d Comparison of electron lifetimes measured with the small-pulse transient photovoltage decay method against voltage.

Fig. 3. Photovoltaic performance of solar cells based on XY1b and MS5+XY1b.

a Incident photon-to-electron conversion efficiency (IPCE) of the dye-sensitized solar cells (DSCs) based on XY1b and the co-sensitization of MS5+XY1b. The solid lines show the corresponding integrated photocurrent calculated from the IPCE. b Current density–voltage curves of the DSCs with XY1b and the co-sensitization of MS5 + XY1b measured under AM1.5G, 100 mW cm⁻² condition. c Histogram of power conversion efficiency (PCE) of the DSCs based on co-sensitization of MS5+XY1b (16 samples). d Evolution of PCE of the DSCs based on MS5+XY1b measured under AM1.5G sunlight (100 mW cm⁻²) during continuous light soaking at 45 °C for 1000 h.

Fig. 4. The loss mechanism of photovoltaic parameters of XY1b and MS5 + XY1b.

a Photovoltaic performance losses due to non-ideal absorption (red), non-radiative recombination (blue and dark gray) and charge transport (light gray). The subscripts stand for measured current density–voltage (J−V) curve (meas), quasi Shockley–Queisser limit (qSQ), and transport limit (TL). V_oc and FF stand for open-circuit photovoltage and fill factor, respectively. b Nano-second Flash photolysis measurements of dyes-sensitized TiO₂ films immersed in a [Cu^(II/I)(tmby)₂][TFSI]_2/1 redox electrolyte (EL) or an inert electrolyte (tmby = 4,4′,6,6′-tetramethyl-2,2′-bipyridine; TFSI = bis(trifluoromethylsulfonyl)imide). The solid lines are monoexponential fittings. Pump wavelength: 520 nm; probe wavelength, 815 nm. c The ideality factors of the DSCs based on the dyes XY1b and MS5 + XY1b. d The transport time as a function of applied voltage.

Fig. 5. The theoretical maximal PCE and the photovoltaic performance of MS5 + XY1b-based DSC under indoor lighting.

a Band gap dependent simulations of the theoretical power conversion efficiency (PCE) limitations (Shockley–Queisser limit) for different intensities of the model Osram 930 Warm White fluorescent tube light. b Current density–voltage curves and power output of the dye-sensitized solar cell (DSC) with a photoactive area of 2.80 cm² under different ambient light intensities. c Summary of the best indoor PCEs of various types of solar cells with the photoactive area of at least 1 cm², including amorphous silicon (a-Si), GaAs, organic polymers (OPV), metal halide perovskite (PSC), and DSC. d Summarized the ideality factors of representative publications on DSCs under ambient light.