Charge Injection, Carriers Recombination and HOMO Energy Level Relationship in Perovskite Solar Cells

Jesús Jiménez-López,Emilio Palomares,Lydia Cabau,Werther Cambarau

doi:10.1038/s41598-017-06245-5

Abstract

We present a comparative study between a series of well-known semiconductor polymers, used in efficient organic solar cells as hole transport materials (HTM), and the state-of-the art material used as hole transport material in perovskite solar cells: the spiro-OMeTAD. The observed differences in solar cell efficiencies are studied in depth using advanced photoinduced spectroscopic techniques under working illumination conditions. We have observed that there is no correlation between the highest occupied molecular orbital (HOMO) energy levels of the organic semiconductors and the measured open-circuit voltage (VOC). For instance, spiro-OMeTAD and P3HT have a comparable HOMO level of ~5.2 eV vs vacuum even though a difference in VOC of around 200 mV is recorded. This difference is in good agreement with the shift observed for the charge vs voltage measurements. Moreover, hole transfer from the perovskite to the HTM, estimated qualitatively from fluorescence quenching and emission lifetime, seems less efficient for the polymeric HTMs. Finally, the recombination currents from all devices were estimated by using the measured charge (calculated using photoinduced differential charging) and the carriers’ lifetime and their value resulted in accordance with the registered short-circuit currents (JSC) at 1 sun.

Highlights

Since the breakthrough of PSC, a myriad of new organic semiconductor materials have been synthesized and tested as HTM11–18
In this paper we aim to study efficient PSC using three of the most representative semiconductor polymers like poly(3-hexylthiophene-2,5-diyl) (P3HT), poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) and poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’] dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7), depicted in Fig. 1, and understand the reasons that hamper these materials to reach higher efficiencies when used as hole selective contacts
As we demonstrated before[26], unlike OSC and dye-sensitized solar cells (DSSC), the measured charge in PSC depends on the acquisition method

Summary

Introduction

Since the breakthrough of PSC, a myriad of new organic semiconductor materials have been synthesized and tested as HTM11–18. Semiconductor polymers, that have provided a step forward towards OSC highest efficiencies, have not achieved performances close to those recorded for spiro-OMeTAD19. The solid state organic semiconductor used as selective contact for holes has remained the well-known spiro-OMeTAD, with the exception of poly(triarylamine) (PTAA)[20, 21], to achieve top efficiencies. Efficiency losses in solar cells account for the presence of radiative recombination within the semiconductor material and interfacial carrier recombination (non-radiative) processes[22,23,24,25]. As we demonstrated before[26], unlike OSC and dye-sensitized solar cells (DSSC), the measured charge in PSC depends on the acquisition method. The carrier recombination lifetimes have been measured by photoinduced transient photovoltage (PI-TPV) under solar cell open-circuit conditions

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Methods

Results