Diverging surface reactions at TiO2- or ZnO-based photoanodes in dye-sensitized solar cells

Ulf Anders Hagfeldt, Kazuteru Nonomura, Nikolaos Vlachopoulos, Gunther Wittstock
ROYAL SOC CHEMISTRY, 2019
Journal paper

Abstract

Fast recombination of electrons from semiconductors with the oxidized redox species in the electrolyte represents a major bottleneck in the improvement of ZnO-based dye-sensitized solar cells (DSSCs). While processes at the semiconductor-electrolyte interface are well studied on TiO2 electrodes, the interactions of the ZnO surface with the electrolyte solution in DSSCs are less explored. This work aims at clarifying the different impact of the two contrasting redox couples I-3(-)/I- or Co(bpy-pz)(2) (bpy-pz = bis(6-(1H-pyrazol-1-yl)-2,2 '-bipyridine)) in electrolytes containing either no additives or Li+ ions and/or 4-tert-butlypyridine (TBP) in DSSCs using screen-printed nanoparticulate TiO2 (NP-TiO2) or electrodeposited ZnO (ED-ZnO) photoanodes sensitized with the indoline dye DN216. A detailed photoelectrochemical study is performed to investigate light absorption, charge transfer and mass transport in these cells. We demonstrate that the chemical nature of the semiconductor directly influences the affinity of adsorbates. This drastically influences the energy levels and recombination kinetics at the semiconductor-electrolyte interface, electron and ion transport in the porous system as well as light absorption of dye molecules by the Stark effect. The present investigation reveals the origin of major performance losses in DSSCs based on ED-ZnO photoanodes as well as the relevance of ionic interactions with NP-TiO2 photoanodes that can both serve as the starting point for rationally guided improvement of their conversion efficiencies.

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https://infoscience.epfl.ch/record/267884?ln=en

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Ulf Anders Hagfeldt, Kazuteru Nonomura, Nikolaos Vlachopoulos, Gunther Wittstock
ROYAL SOC CHEMISTRY, 2019
Journal paper

Abstract

Official source

https://infoscience.epfl.ch/record/267884?ln=en

About this result

Related concepts (20)

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Renewable energy deployment and implementation of a circular economy will play a crucial role in decreasing CO2 emissions to meet the target from the Paris agreement. Dye-sensitized solar cells (DSSCs) can be designed in various colors and transparency for building-integrated photovoltaics (BIPV). This emerging technology has gained significant attention for its remarkably high power conversion efficiency (PCE) under indoor lighting up to 34%. To improve the performance of DSSCs, various approaches to study and reduce the interfacial electron recombination from the conduction band of the semiconductor to the electrolyte were implemented in this thesis. One of the approaches was the design of a tandem electrolyte containing two redox species, the radical AZADO (AZ+/0) and the [Co(bpy)3]2+/3+ complex. DSSCs with tandem electrolytes showed increased open-circuit voltage (VOC) up to 1009 mV and enhanced PCE of 9.15%. The VOC enhancement was determined to result from the higher redox potential of (AZ+/0) and the slower electron recombination rate. These findings demonstrate the advantages of employing tandem electrolytes to improve the VOC and overall performance of DSSCs. To decrease the solvent's volatility and the interfacial electron recombination in cobalt-based electrolytes, hyperbranched networks (HB) were designed through a click thiol-ene reaction between a thiol-siloxane and an acrylate monomer. The formed polymers were denoted HB1 (thiol-siloxane and PEGMA) and HB2 (thiol-siloxane and PEGMA). The addition of 20% of HB polymers to a cobalt-based electrolyte retained up to 98% of their initial weight until 150°C. The devices employing 10%(v/v) of HB1 attained the highest PCE under AM1.5 of 8.52%, comparable to 9.36% for the cobalt reference electrolyte. All the HB-electrolytes presented higher VOC values than the cobalt reference due to an upward shift in the CB of TiO2 and longer electron lifetimes. It was determined that the HB-polymers reduce the electron recombination and enhance the photovoltaic performance under low light intensity leading to a PCE of 23.19% under 1000 lux compared to 20.37% for the Co-reference. We then developed a new dye, R7, with a strong electron-donating PAH core and BPT2 as the central building block and a bulkier HF donor to decrease electron recombination at the interface TiO2/electrolyte. We found that in the presence of a copper electrolyte, the dye R7 outperformed devices with R6. EIS revealed that the bulky HF-donor introduction effectively suppressed the electron recombination between the TiO2 surface and the redox shuttle. Additionally, we employed a co-sensitized system of R7+Y123 to reach the outstanding parameters of JSC of 16.15 mAcm-2, VOC of 1035 mV, FF of 0.76 to achieve a top PCE of 12.7%. To the best of our knowledge, this is the highest for copper-based DSSCs using a blue photosensitizer. Lastly, the interfacial phenomena in DSSCs with carbon counter electrodes (CCEs) and copper-based electrolytes were studied to determine the feasibility of the production of monolithic DSSCs with copper-based hole-transporting materials. It was found that the electron transport in devices with CCEs is in the same order of magnitude as those with PEDOT. However, the devices with CCEs presented a decrease of 2 orders of magnitude in the recombination resistance, resulting in decreased VOC and JSC, leading to a PCE of 17% compared to 24.5% for PEDOT devices under 1000 lux illumination.

EPFL2021

Room-temperature molten salt electrolytes for photoelectrochemical solar cells

Ulf Anders Hagfeldt

In 1991, Gratzel and co-workers published promising results from dye-sensitized, nanocryst., photoelectrochem. solar cells (DNSCs), with huge internal surface areas. A Gratzel cell consists of a dye-sensitized TiO2-electrode, a platinized counter electrode and an electrolyte contg. a redox pair of iodide/triiodide ions. Room-temp. molten salts have qualities, such as high cond., ionic mobility, thermal stability and negligibly volatility, which make them suitable as electrolytes for solar cells. Two systems of potential electrolytes have been successfully synthesized and analyzed, based on the redox pair I-/I3- using different cations: 1. trialkylsulfonium iodides, (R2R'S)I 2. trialkylsulfonium iodides with addn. of metal iodides The capability to act as electrolytes in DNSCs was investigated by measuring the overall light-to-elec. energy conversion efficiency in 100-1000 W/m2 illumination (AM 1.5) using a solar simulator, and also the incident monochromatic photon-to-current conversion efficiency (IPCE). Results showed that liq., iodine-doped trialkylsulfonium iodide-salts work very well as electrolytes in DNSCs.

American Chemical Society2004

Rhodanine dyes for dye-sensitized solar cells: spectroscopy, energy levels and photovoltaic performance

Ulf Anders Hagfeldt

Three new sensitizers for photoelectrochem. solar cells were synthesized consisting of a triphenylamine donor, a rhodanine-3-acetic acid acceptor and a polyene connection. The conjugation length was systematically increased, which resulted in two effects. First, it led to a red shift of the optical absorption of the dyes, resulting in an improved spectral overlap with the solar spectrum. Secondly, the oxidn. potential decreased systematically. The excited state levels were, however, calcd. to be nearly stationary. The exptl. trends were in excellent agreement with d. functional theory computations. The photovoltaic performance of this set of dyes as sensitizers in mesoporous TiO2 solar cells was investigated using electrolytes contg. the iodide/triiodide redox couple. The dye with the best absorption characteristics showed the poorest solar cell efficiency, due to losses by recombination of electrons in TiO2 with triiodide. Addn. of 4-tert-butylpyridine to the electrolyte led to a strongly reduced photocurrent for all dyes due to a reduced electron injection efficiency, caused by a 0.15 V neg. shift of the TiO2 conduction band potential.

2009

EPFL2021