Hole Transfer from Low Band Gap Quantum Dots to Conjugated Polymers in Organic/Inorganic Hybrid Photovoltaics

Abstract

We use photoinduced absorption (PIA) spectroscopy to investigate pathways for photocurrent generation in hybrid organic/inorganic quantum dot bulk heterojunction solar cells. We study blends of the conjugated polymer poly(2,3-bis(2-(hexyldecyl)quinoxaline-5,8-diyl-alt-N-(2-hexyldecyl)dithieno[3,2-b:2',3'-d]pyrrole) (PDTPQx-HD) with PbS quantum dots and find that positively charged polarons are formed on the conjugated polymer following selective photoexcitation of the PbS quantum dots. This result provides a direct spectroscopic fingerprint demonstrating that photoinduced hole transfer occurs from the photoexcited quantum dots to the host polymer. We compute the relative yields of long-lived holes following photoexcitation of both the polymer and quantum dot phases and estimate that more long-lived polarons are produced per photon absorbed by the polymer phase than by the quantum dot phase.

Size‐Dependent Charge Transfer in Blends of PbS Quantum Dots with a Low‐Gap Silicon‐Bridged Copolymer

“…By benefi tting from the employment of postfabrication ligand-exchange treatments, a further effi ciency leap to respectable levels of 3-4% was recently demonstrated. [17][18][19][20] The common denominator in all recent effi cient efforts employing PbS or other types of colloidal QDs in such hybrid devices was the employment of a low-gap copolymer as the electron-donor material. Better energy-level alignment, improved morphology, and higher hole mobilities leading to a better electron-hole transport balance may have been some of the reasons for the better performance, however more fundamental studies are needed to shed light on such attributes and provide the necessary feedback for device optimization.…”

“…Higher electron-transfer rates from the polymer-dot fi lms may be obtained via mild QD ligand-exchange processes such as those reported in recent demonstrations of effi cient polymer-PbS QD solar cells. [15][16][17][18][19][20] QD emission quenching was also investigated to probe charge transfer originating in the QDs. In the steady-state experiments the PL was quasi-resonantly excited at 850 nm so excitations were photogenerated solely in the QD component of the blends.…”

“…The composition was chosen based on optimization studies of previously reported hybrid photodiodes. [ 24 ] Hybrid solar cells require in general higher QD loadings up to 90% wt [15][16][17][18][19][20] however the simultaneous tuning of the QD size and blend in a wide range is impractical so the infl uence of the latter will be studied elsewhere. Binary and ternary fi lms as well as reference pristine nanocrystal and polymer fi lms were deposited on quartz substrates using a doctor-blade technique under ambient conditions.…”

“…Control over blend nanomorphology [11][12][13][14][15] and the shape and aspect ratio of the NCs [ 11-13 , 16 ] has proven to be critical in improving performance. A further increase in effi ciency was enabled by two important developments: i) the use of mild postfabrication chemical treatments to replace bulky insulating QD ligands with shorter capping molecules to allow better charge extraction [15][16][17][18][19][20] and ii) the employment of new low-bandgap copolymers. [15][16][17][18][19][20][21][22] The photophysics of bulk heterojunctions of a high-performance, low-gap silicon-bridged dithiophene polymer with oleic acid capped PbS quantum dots (QDs) are studied to assess the material potential for light harvesting in the visible-and IR-light ranges.…”

Recent Approaches to Controlling the Nanoscale Morphology of Polymer-Based Bulk-Heterojunction Solar Cells

“…Phenyl-C61-butyric acid methyl ester (PCBM) is the most common acceptor [42][43][44], not just because of the ease with which it forms a segregated phase with organic donors, but also because of its good electron affinity [22]. Other materials such as small organic molecules [45], inorganic semiconductor quantum dots [46][47][48][49], and solution-processable functionalized graphene [50][51][52][53][54] have successfully been used as BHJ active layers. …”