Document Type

Dissertation - Open Access

Award Date

2018

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical Engineering and Computer Science

First Advisor

Qiquan Qiao

Abstract

Perovskite solar cells (PSCs) offer tremendous potential for simple and low-cost solution-based fabrication with high power conversion efficiency, making it a promising renewable energy source alternative to the most common non-renewable energy sources as fossil fuels. The most widely used perovskite for solar cell applications is methylammonium lead triiodide (CH3NH3PbI3). The two structures for PSC are the regular nip device typically fabricated using Spiro- OMeTAD as a hole transport material (HTM), and the inverted pin device fabricated using PEDOT:PSS as a HTM. They have achieved over 20% power conversion efficiency. However, devices are not reproducible or stable and HTMs are expensive, thus hindering its commercialization. It is important to find out ways to achieve high quality, defect-free solution, processed perovskite film, simple and scalable synthesis of cheap hole transport material, and efficient charge extraction from perovskite absorbers, to enable high efficiency and stability of perovskite photovoltaics. The goal of this dissertation was to obtain cheap PSCs with high performance via engineering hole transport and perovskite absorber layers by doping of perovskite precursor solution using additives, substituting conventional high cost spiro-OMeTAD by the facile synthesized thiophene-based materials, synergistic combination of PEDOT:PSS with graphene oxide (GO) and polyaniline (PANI), targeting stable perovskite solar cells with high efficiency, and substituting conventional high cost PEDOT:PSS by facile synthesis of polyaniline electrochemically. These goals were achieved by investigating doping of perovskite precursor solution for nip PSCs by adding optimized amounts of TBAI3, LiI, LiTFSI, and BMImI as additives and preparing nanocomposites of PEDOT:PSS, GO, and PANI. Thin films of the doped perovskite, HTM nanocomposites, and thiophene based materials were prepared. PANI thin films were synthesized electrochemically. Structural, optical and morphological characterizations were performed for the prepared thin films, followed by fabrication of n-i-p and p-i-n PSCs using these thin films. Doping of PbI2 with a heteroatom containing molecule (1) increased its solubility in DMF and enhanced its reaction with methylamine iodide, leading to a perovskite film with higher crystallinity, (2) decreased perovskite film roughness resulting in better charge transport in the film and across the interface at the perovskite/charge transport layer, and (3) enhanced injection of electrons into the conduction band of TiO2 via adsorption of the heteroatom on the TiO2 surface, resulting in an enhancement in Jsc and a positive shift in the Fermi level (Ef) of TiO2 thus increasing VOC. These enhancements in PV parameters as a result of doping greatly enhanced PSC efficiency. Furthermore, the heteroatom of dopant improved PSC stability as it binds to the carbon radical once any C−H bond is broken by heat or light through donating an electron, thus blocking its propagation into more free radicals and improving its stability. Dithieno[3,2-b:2',3'-d]pyrrole (DTP) derivatives are one of the most important organic photovoltaic materials due to better π-conjugation across the fused thiophene rings. H16 and H18 have been obtained through a facile synthetic route by cross linking triarylamine-based donor groups with the 4-(4- methoxyphenyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (MPDTP) and N-(4-(4Hdithieno[ 3,2-b:2',3'-d]pyrrol-4-yl)phenyl)-4-methoxy-N-(4- methoxyphenyl)aniline (TPDTP) units, respectively. The H16 HTM outperforms the H18 in terms of conductivity, mobility, and hole transport at interface and this could be attributed to the high quality of film exerted by the MPDTP core in H16. The optimized device based on H16 exhibits a high power conversion efficiency (PCE) of 18.16%, which is comparable to those obtained with the state-of-the-art HTM spiro-OMeTAD (18.27%). Furthermore, long-term aging test shows that the H16 based device has good stability after two months of aging in controlled (20%) humidity in the dark. Importantly, the synthetic cost of H16 is roughly 1/5 of that of spiro-OMeTAD. The present finding highlights the potential of DTP based HTMs for efficient PSCs. A synergistic engineering between GO, PANI, and PEDOT:PSS introduced additional energy levels between perovskite and PEDOT:PSS and increased the conductivity of PEDOT:PSS. Doping PEDOT:PSS with PANI improved PCE from 11.02% to 16.29%, while doping GO with PANI improved PCE from 6.24% to 14.35%, and a composite of these three materials achieved PCE of 18.12%. The hydrophobic PANI has very poor adhesion to substrate, which is the reason for its low conductivity. The gaps between the GO microflakes trap charge carriers resulting in very poor conductivity. PANI/PEDOT:PSS/GO (1:1:1) film achieved the highest Jsc as PANI nanoparticles fill those gaps among GO microparticles, and GO improves adhesion of PANI, while PEDOT:PSS increases the film compactness. PANI and GO increased the work function of PEDOT:PSS, thus increasing the open circuit voltage of the device fabricated from the nanocomposite as HTM. VOC increased to 1.05 V for the PANI/PEDOT:PSS/GO nanocomposite-based PSC from the pristine PEDOT:PSS (VOC = 0.95 V) and PANI/PEDOT:PSS (VOC = 0.99 V) - based PSCs. The reported use of PANI in perovskite solar cells involves chemical synthesis methods that are prone to contamination with impurities as it requires several materials for polymerization and adhesion improvement with substrate, contributing to the low device efficiencies. This project will mitigate this issue by the use of electrochemical method that is low cost, less time consuming and capable of producing transparent thin films of PANI with high purity and excellent adhesion to substrates without any additives. Thus, this method enables simple and scalable synthesis of PANI as HTM alternative to PEDOT:PSS, enabling an important step towards commercialization of the pin PSC with the roll-to-roll manufacturing. Results showed that efficiency of PANI-PSC is 16.94% compared to 15.11% for the PEDOT:PSS-PSC. The PANI-PSC achieved better Jsc with lower hysteresis but with lower VOC and FF compared to PEDOT:PSS-PSC. This can be attributed to the work function of PANI < PEDOT:PSS which decreased VOC but enhanced hole extraction at the HTM/perovskite interface, thus increasing Jsc. Doping electrolyte solution with LiTFSI increased the work function of PANI, thus increasing VOC from 0.87 to 0.93V.

Library of Congress Subject Headings

Solar cells -- Materials.
Perovskite solar cells -- Materials.
Perovskite.

Format

application/pdf

Number of Pages

173

Publisher

South Dakota State University

Rights

In Copyright - Non-Commercial Use Permitted
http://rightsstatements.org/vocab/InC-NC/1.0/

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