Photochemical upconversion: present status and prospects for its application to solar energy conversion

Abstract

All photovoltaic solar cells transmit photons with energies below the absorption threshold (bandgap) of the absorber material, which are therefore usually lost for the purpose of solar energy conversion. Upconversion (UC) devices can harvest this unused sub-threshold light behind the solar cell, and create one higher energy photon out of (at least) two transmitted photons. This higher energy photon is radiated back towards the solar cell, thus expanding the utilization of the solar spectrum. Key requirements for UC units are a broad absorption and high UC quantum yield under low-intensity incoherent illumination, as relevant to solar energy conversion devices, as well as long term photostability. Upconversion by triplet-triplet annihilation (TTA) in organic chromophores has proven to fulfil the first two basic requirements, and first proof-of-concept applications in photovoltaic conversion as well as photo(electro)chemical energy storage have been demonstrated. Here we review the basic concept of TTA-UC and its application in the field of solar energy harvesting, and assess the challenges and prospects for its large-scale application, including the long term photostablity of TTA upconversion materials.

Efficient Triplet Exciton Fusion in Molecularly Doped Polymer Light‐Emitting Diodes

“…This reaction obeys spin conservation and is referred to as triplet fusion or TTA-UC in the upconversion literature. [9][10][11][12][13][14] In Stage III, the triplet-fusion-generated singlet recombines and produces delayed emission.…”

“…[6][7][8] At the same time, photon upconversion through TTA has been widely considered for next-generation photovoltaic applications. [9][10][11][12][13][14] Effective upconverters have been demonstrated using combinations of a triplet sensitizer such as platinum octaethylporphyrin (PtOEP) [3] and a triplet acceptor/annihilator such as 9,10-diphenylanthracene (DPA), [15] perylene, [16] or rubrene.…”

“…[6][7][8] At the same time, photon upconversion through TTA has been widely considered for next-generation photovoltaic applications. [9][10][11][12][13][14] Effective upconverters have been demonstrated using combinations of a triplet sensitizer such as platinum octaethylporphyrin (PtOEP) [3] and a triplet acceptor/annihilator such as 9,10-diphenylanthracene (DPA), [15] perylene, [16] or rubrene.[17] TTA-upconversion (TTA-UC) quantum yield (ratio of upconverted photons to absorbed photons) of 30% was observed in solution phase. [18] This suggests that the intrinsic efficiency of the TTA-UC reaction (i.e., the This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.…”

Multiple exciton generation in quantum dot-based solar cells

“…As this concept, however, is not within the main focus of this article, we refer the interested reader to other reviews addressing the topic in the literature [10].…”

Photon upconversion sensitized by a Ru(II)-pyrenyl chromophore

“…Consequently, tremendous advancements have been achieved in photochemical upconversion by using metal-containing triplet sensitizers with a variety of organic-based triplet acceptors/annihilators in both fluid solution [20,22,[29][30][31][32] and various host matrices [33][34][35][36][37][38][39][40][41][42][43]. The ultimate goal of this research is to maximize the efficiency of real-world solar powered devices by imparting sub-bandgap sensitization via integration of UC technology [8][9][10]44,45]. This change in cell design should reduce the drastic losses owing to spectral mismatch between various sensitizers and semiconductor bandgaps.…”