# Rational Design of Double Hole Transfer Layers for Efficient CdTe Nanocrystal Solar Cells

## TL;DR

This paper introduces a new design for solar cells using double hole transport layers to improve efficiency and stability.

## Contribution

The novel use of CuI and PTAA as double hole transport layers in CdTe nanocrystal solar cells is proposed.

## Key findings

- CuI and PTAA double hole transport layers improve carrier mobility and reduce interface recombination.
- Devices with CuI-PTAA HTLs achieve a power conversion efficiency of 7.36%.
- The design shows high stability with less than 5% efficiency loss over 30 days.

## Abstract

Energy losses induced by inefficient charge transfer and large energy-level offsets at the interface limit the efficiency of CdTe nanocrystal (NC) solar cells. In this work, organic poly(triaryl amine) (PTAA) and inorganic CuI which form double hole transport layers (HTLs) are first proposed to improve the charge transfer capability of hole transport layers (HTLs) and reduce the band offset at the interface of CdTe NCs. The introduced CuI improves carrier mobility, while PTAA reduces interface recombination and reinforces the inner built-in field, resulting in low energy loss from the CdTe NC active layer to the contact electrode. Photovoltaic devices using these modified back contacts show increases in both open-circuit voltage and short-circuit current, compared to a controlled device without HTL. The CdTe NCs utilizing CuI-PTAA double HTLs demonstrate a high power conversion efficiency (PCE) of 7.36%. High stability is also demonstrated, with PCE loss being less than 5% after tracking for 30 days. This work provides an effective way to minimize energy loss at the interface of the back contact in inverted CdTe NCs solar cells, by incorporating proper hole transfer layer design.