Ultrafast charge separation dynamics in opaque, operational dye-sensitized solar cells revealed by femtosecond diffuse reflectance spectroscopy

Abstract

Efficient dye-sensitized solar cells are based on highly diffusive mesoscopic layers that render these devices opaque and unsuitable for ultrafast transient absorption spectroscopy measurements in transmission mode. We developed a novel sub-200 femtosecond time-resolved diffuse reflectance spectroscopy scheme combined with potentiostatic control to study various solar cells in fully operational condition. We studied performance optimized devices based on liquid redox electrolytes and opaque TiO2 films, as well as other morphologies, such as TiO2 fibers and nanotubes. Charge injection from the Z907 dye in all TiO2 morphologies was observed to take place in the sub-200 fs time scale. The kinetics of electron-hole back recombination has features in the picosecond to nanosecond time scale. This observation is significantly different from what was reported in the literature where the electron-hole back recombination for transparent films of small particles is generally accepted to occur on a longer time scale of microseconds. The kinetics of the ultrafast electron injection remained unchanged for voltages between +500 mV and –690 mV, where the injection yield eventually drops steeply. The primary charge separation in Y123 organic dye based devices was clearly slower occurring in two picoseconds and no kinetic component on the shorter femtosecond time scale was recorded.

Complexity of Electron Injection Dynamics and Light Soaking Effects in Efficient Dyes for Modern DSSC

“…We believe that our results explain the previous discrepancies in the reported electron injection time constants for this dye: 2 ps in [21] and 12 ps or 25 ps in [20], which lie between the fastest and the slowest electron injection components revealed in this study. Most probably, the above time constants determined from transient absorption kinetics at a single wavelength (690 nm in [21] and 650 nm in two different environments in [20]) contained the specific partial contributions of the injection processes from the hot S 1 and relaxed S 1 states. As we have showed above, the contributions of the hot and relaxed states to the injection process are different at different probing wavelengths in the visible range.…”

“…At the same time, the above-mentioned advances have raised fundamental questions on the operation of systems comprising the state-of-the-art cell components. Although the number of studies focused on triphenylamine (TPA) dyes in combination with transition metals coordination compounds as redox mediators is relatively high [10,15,16,19], only a few authors have brought up a topic of ultrafast electron transfer dynamics of the benchmark TPA dye: 3-{6-{4-[bis(2 ,4 -dihexyloxybiphenyl-4-yl) amino-]phenyl}-4,4dihexyl-cyclopenta-[2,1-b:3,4-b'] dithiophene-2-yl}-2-cyanoacrylic acid coded as Y123 (or its popular counterpart coded as LEG4) [20,21], Supplementary Scheme S1A. In particular, no detailed information has been published on the examination of the excited state dynamics of Y123 interacting with mesoporous titania in broad visible light spectrum.…”

“…According to our knowledge, so far there have been only two reports about the electron injection dynamics for Y123. In the first report the process occurring within about 2 ps was detected using femtosecond diffuse reflectance spectroscopy [21]. In the second one, the contribution of the electron injection time constant of 12 ps was reported in the inert electrolyte, using the picosecond time-resolved transient absorption (TA) spectroscopy [20].…”

Charge separation and carrier dynamics in donor-acceptor heterojunction photovoltaic systems

“…Significantly slower charge injection into nanocrystalline titanium dioxide was witnessed for the plant-pigment betanine 39,40 and the D-π-A organic dye-sensitizer Y123, 41 where the interfacial charge transfer process was observed to take place with time constants of 6–8 ps and 2 ps, respectively. The conclusion provided by X-ray absorption experiments that electrons can be injected directly into trap states in the proximity of the dye-sensitizer molecules they originate from, rather than escaping quickly as hot carriers has important consequences.…”

Estudo teórico sobre a transferência de carga fotoinduzida de ftalocianinas de Ru (II) para um fragmento de anatase

“…Inúmeras são as possibilidades de aplicação desses materiais. Dentre essas se destacam fortemente a produção fotocatalítica de hidrogênio pela decomposição da água e células solares sensibilizadas por corantes, principalmente pela crescente demanda energética mundial e o apelo ambiental de geração de energia de maneira sustentável (DHOLAM; PATEL; MIOTELLO, 2011;HUANG et al, 2012;JOSHI;DINCER;REDDY, 2012;PATEL et al, 2012;HISATOMI;KUBOTA;DOMEN, 2014;GHADIRI et al, 2016). A associação de corantes fotossensibilizadores a semicondutores como o dióxido de titânio (TiO2) já se mostrou muito promissora para geração de energia por células solares (GRATZEL, 2005).…”