[19p-PB10-1~22] 13.9 Compound solar cells

[19p-PB10-21] Carbon Nanotubes for Quantum Dot Photovoltaics with Enhanced Light Management and Charge Transport

〇Yujiro Tazawa^1,2, Severin Habisreutinger^3,4, Nanlin Zhang², Daniel Gregory², Gabriel Nagamine⁵, Sameer Kesava³, Giulio Mazzotta³, Hazel Assender², Moritz Riede³, Lazaro Padilha⁵, Robin Nicholas³, Andrew Watt² (1.CRIEPI, 2.Oxford Mater., 3.Oxford Phys., 4.NREL, 5.Univ. Campinas)

Keywords:solar cell, quantum dot, carbon nanotube

Colloidal quantum dot (CQD) based photovoltaics are an emerging low-cost solar cell technology with power conversion efficiencies exceeing 15%, i.e. high enough to be interesting for commercialisation. Well-controlled and understood charge carrier transport through the device stack is required to make the next step in efficiency improvements. In this paper, polymer wrapped single-walled carbon nanotubes (SWNTs) films embedded in an insulating polymethylmethacrylate (PMMA) matrix and capped by a thermally evaporated Au electrode are investigated as a composite hole transport layer and optical spacer. Employing transient absorption spectroscopy we show that the SWNTs enhance the charge transfer rate from CQD to CQD, ZnO, or SWNT. In order to pinpoint the underlying mechanism for the improvement, we investigate the energetics of the junction by measuring the relative alignment of the band edges, using Kelvin probe and cyclic voltammetry. Measuring the external quantum efficiency and absorption we find that the improvement is not mainly from electronic improvements but from enhanced absorption of the CQD absorber. We demonstrate experimentally and theoretically, by employing a transfer-matrix model that the transparent PMMA matrix acts as an optical spacer which leads to an enhanced absorption in the absorber layer. With those electronic and optical enhancements, an efficiency of the PbS CQD solar cells improved from 4.0% to 6.0%.