Structure–property relationships of oligothiophene–isoindigo polymers for efficient bulk-heterojunction solar cells

Abstract

A series of alternating oligothiophene (nT)-isoindigo (I) copolymers (PnTI) were synthesized to investigate the influence of the oligothiophene block length on the photovoltaic (PV) properties of PnTI:PCBM bulkheterojunction blends. Our study indicates that the number of thiophene rings (n) in the repeating unit alters both polymer crystallinity and polymer-fullerene interfacial energetics, which results in a decreasing open-circuit voltage (V oc ) of the solar cells with increasing n. The short-circuit current density (J sc ) of P1TI:PCBM devices is limited by the absence of a significant driving force for electron transfer. Instead, blends based on P5TI and P6TI feature large polymer domains, which limit charge generation and thus J sc . The best PV performance with a power conversion efficiency of up to 6.9% was achieved with devices based on P3TI, where a combination of a favorable morphology and an optimal interfacial energy level offset ensures efficient exciton separation and charge generation. The structureproperty relationship demonstrated in this work would be a valuable guideline for the design of high performance polymers with small energy losses during the charge generation process, allowing for the fabrication of efficient solar cells that combine a minimal loss in V oc with a high J sc . Broader contextPolymer solar cells (PSCs) have emerged as promising candidates for low-cost, environmentally friendly energy conversion. However, a low power conversion efficiency (PCE) is still the main bottleneck for their commercial applications. In order to further improve their PCE, a detailed understanding of the underlying structure-property relationships that govern these materials is crucial. In this work, a series of alternating oligothiophene (nT)-isoindigo (I) copolymers (PnTI) were synthesized to investigate the inuence of the oligothiophene block length on the photovoltaic (PV) properties of PnTI:PCBM bulk-heterojunction blends. The driving force for the PSCs can be ne-tuned in a small range of 0-0.1 eV by changing the conjugated length of the oligothiophene block. This is a valuable guideline for polymer design to achieve high-efficiency PSCs with optimum photovoltage and photocurrent.

High Performance All‐Polymer Solar Cell via Polymer Side‐Chain Engineering

“…Similar observations have been reported before for other isoindigo polymer:PCBM BHJs. [ 36,37 ] The mobilities measured from space-charge limited current (SCLC) are listed in Table S3. The hole mobilities in the blend fi lms are all around 2 × 10 −4 cm 2 V −1 s −1 , while the acceptor polymer P(TP) has a lower electron mobility of 2 × 10 −5 cm 2 V −1 s −1 .…”

Balancing High Open Circuit Voltage over 1.0 V and High Short Circuit Current in Benzodithiophene‐Based Polymer Solar Cells with Low Energy Loss: A Synergistic Effect of Fluorination and Alkylthiolation

“…[6,40] Such CT states had been also evidenced in other polymer: PCBM and polymer: polymer mixtures previously. [41,42] Beside steady-state PL study, TRPL kinetics of neat polymer PBDTT-SF-TT and PBDTT-SF-BDD and their polymer: PC 71 BM blends had been also investigated for furthermore analyzing the charge photogeneration process in the active layer materials, as illustrated in Figure S20 in the Supporting Information. Well pronounced shorter PL lifetime from 290 ps for PBDTT-SF-TT and 230 ps for PBDTT-SF-BDD to 275 and 190 ps for their respective polymer: PC 71 BM blends again strongly proved the efficient charger transfer and separation from donor to acceptor phases.…”

Electronic Structure of π-Conjugated Materials and Their Effect on Organic Photovoltaics

“…[68,69,70] The value of the driving force could be minimized to 0-0.1 eV in optimized recipe. [68,71] Too large driving force would result low open circuit voltage, while insufficient driving force will obstruct exciton dissociation, which lead to low short circuit current.…”

“…The PL emitting from Frenkel excitons could be quenched efficiently in the blend indicating that the driving force is enough for exciton dissociation, otherwise there could be not enough driving force for exciton dissociation at the D/A interface or the excitons could not diffuse to the D/A interface before recombination. [68,71] Whether inefficient exciton diffusion or insufficient driving force is limiting the eventually exciton dissociation can be distinguished by PL measurement under bias in combination of morphology measurements like…”

“…The Frenkel exciton diffusion is not affected by an electric field, while the exciton dissociation can be enhanced via facilitated CT exciton dissociation and inhibited exciton regeneration from CT excitons under bias. [71,72] …”