Solution-processed semiconductor quantum dot solar cells offer a path towards both reduced fabrication cost and higher efficiency enabled by novel processes such as hot-electron extraction and carrier multiplication. Here we use a new class of low-cost, low-toxicity CuInSexS2−x quantum dots to demonstrate sensitized solar cells with certified efficiencies exceeding 5%. Among other material and device design improvements studied, use of a methanol-based polysulfide electrolyte results in a particularly dramatic enhancement in photocurrent and reduced series resistance. Despite the high vapour pressure of methanol, the solar cells are stable for months under ambient conditions, which is much longer than any previously reported quantum dot sensitized solar cell. This study demonstrates the large potential of CuInSexS2−x quantum dots as active materials for the realization of low-cost, robust and efficient photovoltaics as well as a platform for investigating various advanced concepts derived from the unique physics of the nanoscale size regime.

With the aim of increasing conversion efficiency, the series-internal resistance of dye-sensitized solar cells (DSCs) was investigated with electrochemical impedance spectroscopy measurement based on an equivalent circuit of DSCs. It was found that series-internal resistance correlates positively with the sheet resistance of the transparent conducting oxide and the thickness of the electrolyte layer and negatively with the roughness factor of the platinum counter electrode. A cell sensitized with a black dye with series-internal resistance of 1.8Ωcm2 was fabricated and showed conversion efficiency of 10.2% when measured with a metal mask under an air mass of 1.5 sunlight.

Colloidal CuInSexS2-x quantum dots (QDs) are an attractive less-toxic alternative to PbX and CdX (X = S, Se, and Te) QDs for solution-processed semiconductor devices. This relatively new class of QD materials is particularly suited to serving as an absorber in photovoltaics, owing to its high absorption coefficient and near-optimal and finely tunable band gap. Here, we engineer CuInSexS2-x QD sensitizers for enhanced performance of QD-sensitized TiO2 solar cells (QDSSCs). Our QD synthesis employs 1-dodecanethiol (DDT) as a low-cost solvent, which also serves as a ligand, and a sulfur precursor; addition of triakylphosphine selenide leads to incorporation of controlled amounts of selenium, reducing the band gap compared to that of pure CuInS2 QDs. This enables significantly higher photocurrent in the near-infrared (IR) region of the solar spectrum without sacrificing photovoltage. In order to passivate QD surface recombination centers, we perform a surface-cation exchange with Cd prior to sensitization, which enhances chemical stability and leads to a further increase in photocurrent. We use the synthesized QDs to demonstrate proof-of-concept QDSSCs with up to 3.5% power conversion efficiency.

Using transient absorption spectroscopy, we investigated the effects of 4-tert-butylpyridine (tBP) and Li ions on photoinduced electron injection efficiency in nanocrystalline TiO 2 films sensitized by black dye. The efficiency increased with the addition of Li ions but decreased with the addition of tBP molecules. These results indicate that a conduction band shift was induced by the solvation effect of the additives. Using recombination kinetic measurements, we examined that such a solvation effect was induced by the intercalation of Li ions into TiO 2 particles and by the adsorption of tBP on the TiO 2 surface. When both additives were used simultaneously, the efficiency was suppressed more than when only tBP was added. This difference in efficiency suggests that the local concentration of tBP near the TiO 2 surface increased with the addition of Li ions. This cooperative effect may have been caused by the formation of a complex between tBP and Li in solution. IntroductionSince highly efficient dye-sensitized solar cells (DSSCs) were first reported, 1 much research has been carried out to improve their performance. Solar cells consisting of N719 dye, in which two protons of N3 dye [cis-di(thiocyanato)-bis(2,2′-bipyridiyl-4,4′-dicarboxylate)ruthenium(II); Ru(dcbpy) 2 (NCS) 2 ] are replaced by tetrabutylammonium (TBA) cations, adsorbed on nanocrystalline TiO 2 films (N719/TiO 2 ) show high solar-energyto-electricity-conversion efficiency (η > 11%). 2 Black dye (BD; trithiocyanato(4,4′,4′′-tricarboxy-2,2′:6′,2′′-terpyridine)ruthenium(II); Ru(tcterpy)(NCS) 3 ) is also a promising sensitizer dye for DSSCs because its lowest unoccupied molecular orbital (LUMO) is located just above the conduction band of TiO 2 , and because its absorption edge is in the near-infrared range. [3][4][5] With further optimizations, such as the reduction of internal resistance and the preparation of a high-haze electrode, 6-8 η values greater than 11% can be achieved with BD-sensitized nanocrystalline TiO 2 (BD/TiO 2 ) DSSCs. 9 However, to further improve these DSSCs, more detailed knowledge of the mechanisms of DSSC processes, especially with regard to the electron injection mechanism, is required.To understand the primary processes that occur in DSSCs, the photophysical processes of dye-sensitized TiO 2 electrodes have been studied extensively by means of various experimental techniques. Electron injection, which is the most important primary process, has been studied mainly by means of transient absorption (TA) spectroscopy. 10-12 Electron injection dynamics can be studied by means of femtosecond TA measurements. For dye-sensitized films based on N3 and N719 complex dyes, nonexponential ultrafast electron injection has been observed in the 100-fs to 100-ps time range. 13-18 Furthermore, microsecond TA measurements are used to study the recombination

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