Hybrid tandem quantum dot/organic photovoltaic cells with complementary near infrared absorption

Abstract

Monolithically integrated hybrid tandem solar cells that effectively combine solution-processed colloidal quantum dot (CQD) and organic bulk heterojunction subcells to achieve tandem performance that surpasses the individual subcell efficiencies have not been demonstrated to date. In this work, we demonstrate hybrid tandem cells with a low bandgap PbS CQD subcell harvesting the visible and near-infrared photons and a polymer:fullerene-poly (diketopyrrolopyrrole-terthiophene) (PDPP3T):[6,6]-phenyl-C-butyric acid methyl ester (PCBM)-top cell absorbing effectively the red and near-infrared photons of the solar spectrum in a complementary fashion. The two subcells are connected in series via an interconnecting layer (ICL) composed of a metal oxide layer, a conjugated polyelectrolyte, and an ultrathin layer of Au. The ultrathin layer of Au forms nano-islands in the ICL, reducing the series resistance, increasing the shunt resistance, and enhancing the device fill-factor. The hybrid tandems reach a power conversion efficiency (PCE) of 7.9%, significantly higher than the PCE of the corresponding individual single cells, representing one of the highest efficiencies reported to date for hybrid tandem solar cells based on CQD and polymer subcells.

Hybrid Tandem Quantum Dot/Organic Solar Cells with Enhanced Photocurrent and Efficiency via Ink and Interlayer Engineering

“…In recent years, CQD/organic hybrid tandem solar cells have been monolithically integrated and reported by several groups. [21][22][23][24][25][26] However, one noteworthy limitation in all the reports has been the unusual placement of the low bandgap CQD subcell as the front cell, namely on the transparent electrode-coated glass substrate, limiting the ability of the tandem to generate higher photocurrent. The primary reason constraining researchers to this architecture is the incompatibility of the CQD ink solvents with underlayers, including the bulk heterojunction (BHJ) organic photoactive layer and the interconnection layers (ICL).…”

“…The primary reason constraining researchers to this architecture is the incompatibility of the CQD ink solvents with underlayers, including the bulk heterojunction (BHJ) organic photoactive layer and the interconnection layers (ICL). [22][23][24][25] The secondary, yet still important reason, is the need for repeated solid state ligand exchange in many legacy recipes for CQD active layer or for the CQD hole transporting layer (HTL). 4,5,8,11 Ligand exchange, which significantly densifies the CQD film, has been shown to cause additional chemical damage and stress-induced cracking of underlayers.…”

“…In particular, the bottom organic active layer is easily dissolved or damaged by some of the solvents typically used for CQD coating and/or ligand exchange, imposing important restrictions on the CQD ink and its coating process. [21][22][23][24][25][26] We have set out to investigate alternative CQD ink solvents and robust ICLs which can survive the CQD coating process and provide protection for all underlayers. Octane and butlyamine are the two most commonly used solvents for conventional PbS CQD solar cells.…”

“…The tandem stack was completed by vacuum evaporation of MoOx as HTL and Au/Ag as the anode. 25,33 In Figure 3(a) 12 was vacuum-deposited on PTB7:PC61BM to form the HTL. An ultrathin Au layer (0.5 nm) was deposited by thermal evaporation with careful thickness monitoring.…”

“…Figure S7(b) compares the conductance of ICL including a thin Au layer as compared to an ICL without Au and shows that the inclusion of Au reduces the series resistance of the ICL. 18,19,23,25 The device structures used for conductance measurements were the isolated interlayer stacks, including glass/ITO/MoOx/Au (0.5 nm)/AZO/Al and glass/ITO/MoOx/AZO/Al.…”