Development of Tandem Solid-State Dye Sensitised Solar Cells via Interfacial Engineering

Abstract

Dye-sensitised solar cells (DSCs) are an attractive photovoltaic (PV) technology due to their low cost, light weight and simple fabrication process using earth abundant materials. Specifically, organic dyes offer tuneable optical properties and high absorption coefficients. However, a limiting factor on the power conversion efficiency (PCE) is that organic dyes suffer from relatively narrow spectral absorption in comparison to other PV technologies, thus limiting the light-harvesting ability of the cells. Implementing a tandem structure in which two photoactive electrodes are incorporated for enhanced light collection represents a promising approach. However, fundamental stability issues surrounding the liquid electrolyte pose concern for the commercialisation of such devices. Therefore, the work in this thesis aims to improve the stability of tandem DSCs by establishing a proof of concept for p-n tandem solid- state DSCs (ssDSCs) where the liquid electrolyte is replaced with solid charge transporting materials. The PCE of ssDSCs is generally much lower that of liquid DSCs. Therefore an amide- based hole transporting material (HTM) with improved conductivity was tested in n-type ssDSCs with the aim to enhance device performance. Further efforts to broaden the spectral absorption of ssDSCs through co-sensitisation of the TiO2 photoanode was also conducted. This thesis presents the design, fabrication and optimisation of both n-type and p-type ssDSCs. For n-type ssDSCs, the co-sensitisation of LEG4 with L0 resulted in a notable 7% increase in PCE due to the extension of the absorption region to lower wavelengths. Employment of the amide-based HTM (termed TPABT) highlighted that further aging compared with Spiro- OMeTAD based ssDSCs was essential for dye regeneration to allow for oxidation of the TPABT. This indicated the different oxidation pathway compared with Spiro-OMeTAD, which was attributed to the closer molecular packing of TPABT. Additionally, it was found that TPABT devices exhibited significant hysteresis. However, after performing a light soaking treatment the efficiency of TPABT based devices improved from 0.33% to 1.48% and hysteresis was no longer observed. It was hypothesised that the light soaking treatment led to Li+ ion migration from coordinates inside the pores toward the TiO2/LEG4 interface. Here, it is thought the Li+ ions screen conduction band electrons after injection, resulting in enhanced photocurrents and solar cell performance. Additionally, a study on mitigating interfacial charge recombination in p-type ssDSCs was undertaken where three NiO surface treatments were tested to reduce recombination at the ETM/electrode interface. Atomic layer deposition of alumina and an aluminium alkoxide solution treatment were employed onto the NiO surface, and a chenodeoxycholic acid treatment employed both as a surface treatment onto sensitised NiO and as a co-adsorbent to P1 were conducted. Current density-voltage (J-V) measurements signified resistive behaviour in the devices, and transient absorption spectroscopy (TAS) measurements revealed charge carrier lifetimes in the sub-picosecond timescale. This was attributed to hole shallow-trapping, where the existence of excess Ni3+ states on the NiO surface trap a hole to “Ni4+” – a mixed valence state of Ni higher than 3. Therefore, it is believed that the p-type ssDSCs presented in this thesis exhibit fast recombination and inefficient hole injection into the NiO, leading to poor device performances. To assemble the tandem ssDSC a charge recombination layer (CRL) was employed between the n-type and p-type subcells to enable charge transfer from one subcell to another, and the subcells were pressed together with clamps. Efforts were dedicated to fabricating a CRL that formed an ohmic contact to minimise energy barriers for the charge carriers to flow across the CRL. A CRL composed of Spiro-OMeTAD/Ag/PEI/PCBM:PEI demonstrated the establishment of an ohmic connection. However, when employed into tandem ssDSCs the poor performance of the p-type ssDSC coupled with the low current measured through the CRL significantly inhibited the performance of the tandem device. A further study testing the mechanical adhesion of a Spiro-OMeTAD/PEDOT:PSS/PEI/PCBM:PEI CRL using a transparent conductive adhesive (TCA) showed both electrical and mechanical connection of the CRL, where UV epoxy was used to secure the glass substrates in place. When employed in tandem ssDSCs, this will eliminate the necessity to apply clamping or pressure on the subcells for operation. Overall, this thesis highlights the significance of additional research required on the p-type electrode to overcome the associated charge recombination issues encountered in p-type ssDSCs. Additionally, the importance of obtaining high electrical conductivity in the CRL was highlighted. Despite this, a tandem ssDSC exhibiting a VOC of 0.88 V and JSC of 0.005 mA cm-2 was achieved, underscoring the potential of the tandem ssDSC configuration and the ability to utilise the collective open-circuit voltages of the component devices.