Role of Hot Singlet Excited States in Charge Generation at the Black

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Role of Hot Singlet Excited States in Charge Generation at the Black Dye/TiO2 Interface Ajay Ram Srimath Kandada,† Simona Fantacci,‡ Simone Guarnera,†,§ Dario Polli,† Guglielmo Lanzani,†,§ Filippo De Angelis,*,‡ and Annamaria Petrozza*,§ †

Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), Istituto CNR di Scienze e Tecnologie Molecolari, Via Elce di Sotto 8, I-06123, Perugia, Italy § Center for Nano Science and Technology @ PoliMi, Istituto Italiano di Tecnologia, via Pascoli 70/3 20133 Milano, Italy ‡

S Supporting Information *

ABSTRACT: Photoinduced electron transfer at low-band-gap ruthenium-based dye/TiO2 has been investigated by means of ultrafast transient absorption and DFT/TDDFT calculations. We demonstrate that although the charge generation mechanism is triplet mediated upon band gap excitation, as already proven in high band gap dyes such as the well-known N3 and N719, when excess energy is provided which allows to reach high energy singlet states still in the visible spectral range, ultrafast electron transfer takes place. No intersystem crossing process is observed and charge generation happens only from the singlet excited state. KEYWORDS: black dye, DSSC, charge generation, hot injection, DFT, transient absorption

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INTRODUCTION

Photoinduced electron injection in nanocrystalline TiO2 films has been deeply studied in mesoporous metal oxide samples sensitized by a variety of ruthenium complexes. It has been shown that the injection rate and hence the cell efficiency vary with the photon energy.21 For the well-known N3 and N719 dyes, it is generally recognized that the process happens through a two-step mechanism.8−11 The first step sees an ultrafast (