Research on photovoltaic characteristics of polymer based solar cells
Date Issued
2011
Date
2011
Author(s)
Tsai, Shin-Hung
Abstract
Organic photovoltaics have received an intensive attention due to their promising features, such as low cost, light weight, simple solution-based process, and flexibility.
This thesis focuses on organic-inorganic hybrid solar cells (HSCs) and inverted organic solar cells (OSCs). Employing inorganic materials prevent photovoltaic devices from weak air stability due to the acidic, hygroscopic nature of poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) and oxidation of low work function front electrode.
First of all, a novel strategy employing core-shell nanowire arrays (NWAs) consisting of Si/regioregular poly(3-hexylthiophene) (P3HT) was demonstrated to facilitate efficient light harvesting and exciton dissociation/charge collection for HSCs. Consequently, core-shell HSCs exhibit the 61% improvement of short circuit current density (JSC) for a conversion efficiency (η) enhancement of 31.1 % as compared to the P3HT-infiltrated Si NWA HSCs with layers forming a flat air/polymer cell interface.Surprisingly, the device performance exhibits a dramatic enhancement under air mass 1.5 global (AM 1.5G) illumination. The improvement of crystal quality of P3HT shells due to the formation of ordering structure at Si interfaces after AM 1.5G illumination was confirmed by transmission electron microscopy and Raman spectroscopy. The core-shell geometry with the interfacial improvement by AM 1.5G illumination promotes more efficient exciton dissociation and charge separation, leading to η improvement (~140.6 %) due to the considerable increase in Voc from 257 to 346 mV, Jsc from 11.7 to 18.9 mA/cm2, and FF from 32.2 to 35.2 %, which is not observed in conventional P3HT-infiltrated Si NWA HSCs.
Second, we demonstrate that the power conversion efficiency of OSCs based on ZnO/P3HT:PCBM was improved by incorporating Al into ZnO acting as an electron transport layer (ETL). The Al-doped ZnO (AZO) thin film was deposited via an aqueous solution method. Compared with devices based on pure ZnO, AZO-based solar cells lead to the enhancement in power conversion efficiency by about 114% under simulated AM 1.5G full-sun illumination. It was found that the increase in the Fermi level as introducing Al into ZnO film would assist the interfacial electron transfer from active region to ETL, contributing to the improvement in fill factor. Besides, the enhancement in JSC was attributed to the higher transmission, interfacial area and electrical conductivity of AZO thin films than those of un-doped ZnO. The P3HT crystalline tends to be higher ordered on AZO ETL, which was suggested by normalized absorbance spectra, resulting in a reduction in energy loss due to insufficient donor/acceptor phase segregation. This study provides new insights into the design optimization of the ETL for inverted OSCs.
Finally, an efficient way to achieve high electron transfer efficiency from active layer to cathode has been demonstrated here, replacing ITO with sputtered AZO due to lower work function of AZO. Addressing the problem of diminishing reservation of indium was also found to be an advantage. After introducing a simple HCl etching treatment to form a rough AZO electrode, the distribution of light wave within active layer can be effectively widened, leading to a significant enhancement in EQE and JSC. With the roughened AZO film, even a thin active layer based cells can reach efficiency comparable to that with thick organic layer. This novel method provides an important scheme applicable to OSCs and benefits the practicability of OSCs by increasing the efficiency-cost ratio.
Subjects
organic-inorganic hybrid solar cells
anti-reflection
inverted organic solar cells
Al-doped ZnO
electron transporting layer
AZO electrode
SDGs
Type
thesis
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