Increasing p-type dye sensitised solar cell photovoltages using polyoxometalates

Abstract

Dye-sensitized photocathodes (p-DSSCs) were first proposed as a way to increase the efficiency of dye-sensitized solar cells over a decade ago.1 Such photocathodes would replace the platinised counter electrode of the Grätzel cell, and by harvesting lower energy photons than the photoanode increase spectral coverage as well as increasing photovoltage. In this way, the p-DSSC could help drive advances from the current DSSC record of 13%, 2 towards (or beyond) the 25% attained by crystalline Si: 3 the resulting tandem DSSCs would have a maximum theoretical efficiency of 43%, vs. 33% for a single junction device. But to date, no tandem DSSC has exceeded the power conversion efficiency of a state-of-the-art, single-junction TiO 2 n-DSSC, because they are limited by the poor performance of the series connected p-DSSC. The only p-DSSC power conversion efficiencies 41% have been achieved using redox mediators whose negative redox potentials would severely reduce the photovoltage produced by the TiO 2 side of any tandem cell. 4 With the more typical n-DSSC I 3 À /I À redox couple, the p-DSSC record efficiency is just 0.61%. 5The poor performance of p-DSSCs results from the small energy difference between the valence band (VB) of the p-type semiconductor (usually NiO) and redox mediator, rapid back transfer of photogenerated holes from NiO to the electrolyte, and slow hole diffusion through NiO.6 Consequently, open circuit voltages (V OC ) are low -the maximum obtained with I 3 À /I À is 350 mV using special high crystallinity NiO and a dye with a highly extended conjugated system. 7 More typically, V OC is around 100 mV, and rarely exceeds 150 mV. At the same time, fill factors are low (ca. 30%) and photocurrents ( J SC ) are moderate (up to 8.2 mA cm À2 ). 8 While the improvements achieved through sensitizer design have been impressive, 4,6,8 the necessary step change in p-DSSC performance will require exploration of new strategies, and materials. One simple modification employed in n-DSSCs, but little investigated in p-DSSCs, 9a is the use of coadsorbents such as cholic acid derivatives and alkyl phosphonic acids. 9b-e These can prevent dye aggregation, and passivate the surface helping to suppress recombination, reduce dark current and increase stability and efficiency. We expect that in p-DSSCs co-adsorbents can play a similar role, while potentially also providing a remote electron acceptor, isolated from the NiO VB that can relay electrons to the mediator and impede recombination. This strategy may have advantages over incorporating the acceptor into the dye -it facilitates a rapid, combinatorial approach to new cells and should reduce communication between electron and hole. Owing to their fast electrochemistry, tunable potentials and stability to redox cycling, 10 polyoxometalates (POMs) appear a good but as yet untested choice of electron acceptor coadsorbent for p-DSSCs. Ultrafast electron transfer (ET) has been observed between POMs and dyes on TiO 2 , 11 and in n-DSSCs, POMs can enhance performance by incre...

Polyoxometalates: Tailoring metal oxides in molecular dimension toward energy applications

“…351,352 Cobaltbased polyoxometalates have been applied to energy systems such as the redox couple in dye-sensitized solar cells. 43,349,[353][354][355][356][357][358][359] A number of other transition metalbased polyoxometalates have been investigated, such as silver-containing polyoxometalates, 226,[360][361][362][363][364][365] zinccontaining polyoxometalates. 366 polyoxotantalate, [367][368][369][370][371][372][373][374][375][376][377][378][379][380][381][382][383][384] ruthenium-containing polyoxometalates, [385][386][387] coppercontaining polyoxometalates, [388][389][390] chromium-containing polyoxometalates, 391 and polyoxoaurates [392][393]…”

“…“Polyoxoiron” is rarely obtained, but the iron element is frequently introduced into polyoxometalates for various energy applications; for example, it has been employed to enhance the ZnO‐based solar cell performance via the construction of a metal‐organic framework owing to the excellent electron‐separation properties . Cobalt‐based polyoxometalates have been applied to energy systems such as the redox couple in dye‐sensitized solar cells . A number of other transition metal‐based polyoxometalates have been investigated, such as silver‐containing polyoxometalates, zinc‐containing polyoxometalates .…”

Covalent Grafting of Polyoxometalate Hybrids onto Flat Silicon/Silicon Oxide: Insights from POMs Layers on Oxides

“…photovoltaic and artificial photosynthesis (proton or CO2 reduction, water oxidation...), nanostructured transparent metal oxide semiconductors like ITO, TiO2, NiO or ZnO, and more recently the promising Fe2O3 or BiVO4 semiconductors, often modified by an organic dye, are usually used to build photo-electrodes. [12][13][14][15][16] Exploiting the redox activity of POMs in such applications thus requires mastering their deposition on oxide surfaces. Furthermore, the control of their organization on the surface is an essential and still challenging issue.…”

“…Whereas various strategies have been proposed to covalently anchor POMs on oxide nanoparticles surface [21][22][23] or mesoporous silica, 24 very few examples describe the grafting of POMs on flat oxide substrates. 13,25 In these last examples, few attention has been devoted to the control of the density and the characterization of the interface. In the present work, to ensure a strong bond between the POMs and the oxide, the POM hybrids with a pendant carboxylic acid group TBA4.…”

Lindqvist polyoxometalates as electrolytes in p-type dye sensitized solar cells

“…[19][20][21][22] POMs have also been used as co-adsorbents on NiO surfaces in p-type DSSCs, which showed a decreased recombination rate and increased V OC . 23 To our knowledge, POMs have not been employed as redox mediators in p-DSSCs.…”