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Summary As the power conversion efficiency of organic photovoltaic has been dramatically improved to over 18%, achieving long-term stability is now crucial for applications of this promising photovoltaic technology. Among the high-efficiency systems, most are using BTP-4F and its analogs as acceptors. Herein, we determine the thermal transition temperatures (Tg) of seven BTP analogs to develop a structure-Tg framework. Our results point out an unresolved molecular design conundrum on how to simultaneously achieve high performance and intrinsic stability with BTP-based acceptors. We also show that PC71BM has miscibility above the percolation threshold in PM6 and can maintain local charge percolation and improved stability in ternary devices. However, PC71BM is not miscible with BTP-C3-4F and unfavorable vertical gradients that develop during aging still degrade performance. This points to a second thermodynamic conundrum. A compound with differential miscibility in the donor polymer can only impact percolation, and a compound with differential miscibility with the BTP only impacts diffusion.

Nonfullerene all-small molecule organic solar cells (OSCs) have attracted significant attention due to their competitive performance compared to the well-studied polymer-based OSCs. Small molecules are considered to be materials of choice for industrial scale OSC production due to their relatively high purity, reproducibility, and well-defined molecular weights. Despite the recent progress in all-small molecule solar cells, the mesoscale morphology of these devices remains elusive. A fundamental understanding of the structure–property relationships of these systems is thus critical for further device performance enhancement. Here, we report a dramatic change in device performance of a nonfullerene all-small molecule bulk heterojunction system upon changing the side chains of the electron donor small molecules. Morphology analysis via resonant soft X-ray scattering shows that the average composition variation and the size of the smaller domains of the all-small molecule blends are the most influential factors that derive the photogeneration and collection efficiency of charges in efficient all-small molecule OSCs. The observed side-chain and processing initiated morphological changes resulted in distinct device efficiencies in these systems. Despite the fact that side chains are mainly meant to facilitate material dissolution, the current study clearly shows that they can also impose a significant effect on material organization and photovoltaic properties. The findings will give a clear understanding of the role of side chains and processing in material organization and device functionality, and the knowledge gained in this study is quite crucial in designing next-generation materials and devices.

Solution processable conjugated organic materials have gained tremendous interest motivated by their potential of low cost, lightweight and especially easy manufacturing of large-area and flexible electronics. Toxic halogen-containing solvents have been widely used in the processing of organic electronics, particularly organic photovoltaics (OPVs). To transition this technology to more commercially attractive manufacturing approaches, removing these halogenated solvents remains one of the key challenges. Our morphological (hard/soft X-ray scattering) and calorimetric characterizations reveal that using o-methylanisole, a certified food additive, as processing solvent can achieve similar crystalline properties and domain spacing/purity with that achieved by widely used binary halogenated solvents (chlorobenzene and 1,8-diiodooctane), thus yielding comparable photovoltaic performance in spin-casted films. To move a step forward, we further present the potential of o-methylanisole as processing solvent in th...

This work was supported by the Office of Naval Research Grant N00014-17-1-2242, National Science Foundation Award CBET-1639429, and NextGen Nano.

Fluorinated molecule 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ) and its derivatives have been used in polymer:fullerene solar cells primarily as a dopant to optimize the electrical properties and device performance. However, the underlying mechanism and generality of how F4‐TCNQ affects device operation and possibly the morphology is poorly understood, particularly for emerging nonfullerene organic solar cells. In this work, the influence of F4‐TCNQ on the blend film morphology and photovoltaic performance of nonfullerene solar cells processed by a single halogen‐free solvent is systematically investigated using a set of morphological and electrical characterizations. In solar cells with a high‐performance polymer:small molecule blend FTAZ:IT‐M, F4‐TCNQ has a negligibly small effect on the molecular packing and surface characteristics, while it clearly affects the electronic properties and mean‐square composition variation of the bulk. In comparison to the control devices with an average power conversion efficiency (PCE) of 11.8%, inclusion of a trace amount of F4‐TCNQ in the active layer has improved device fill factor and current density, which has resulted into a PCE of 12.4%. Further increase in F4‐TCNQ content degrades device performance. This investigation aims at delineating the precise role of F4‐TCNQ in nonfullerene bulk heterojunction films, and thereby establishing a facile approach to fabricate highly optimized nonfullerene solar cells.

Nanoimprinting the photoactive layer of bulk heterojunction (BHJ) organic solar cells is a promising technique for enhancing device performance via improved light absorption. Here, we demonstrate that imprinting poly(3-hexylthiophene) (P3HT) and fullerene BHJ blends leads to adverse morphological changes within the photoactive nanopattern which have been previously overlooked. In particular, nanoimprinting induces a factor of 2 difference in polymer:fullerene composition between the nanopattern posts and interconnecting flash layer that inadvertently moves the composition outside the range for optimal performance. This occurs because of the strong tendency of regioregular P3HT to crystallize since imprinting blends based on amorphous regiorandom P3HT have uniform nanopattern composition. Based on these results, we outline promising design strategies, such as nanoimprinting amorphous polymers, to serve as guidelines for fabricating high-performance nanopatterned BHJ solar cells capable of maximized light absorption.

We have investigated the photovoltaic properties of inverted solar cells comprising a bulk heterojunction film of poly(3-hexylthiophene) and phenyl-C(61)-butyric acid methyl ester, sandwiched between an indium-tin-oxide/Al-doped zinc oxide (ZnO-Al) front, and tungsten oxide/aluminum back electrodes. The inverted solar cells convert photons to electrons at an external quantum efficiency (EQE) exceeding 70%. This is a 10-15% increase over EQEs of conventional solar cells. The increase in EQE is not fully explained by the difference in the optical transparency of electrodes, interference effects due to an optical spacer effect of the metal-oxide electrode buffer layers, or variation in charge generation profile. We propose that a large additional splitting of excited states at the ZnO-Al/polymer interface leads to the considerably large photocurrent yield in inverted cells. Our finding provides new insights into the benefits of n-type metal-oxide interlayers in bulk heterojunction solar cells, namely the splitting of excited states and conduction of free electrons simultaneously.

Solid-state Dye-Sensitized Solar Cells (ss-DSSCs) are promising candidates for future low cost photovoltaic energy generation and are based on polymer/metal oxide donor/acceptor heterojunctions. However, a crucial drawback of hybrid solar cells is the use of environmental unfriendly solvents, such as toluene, chloroform, chlorobenzene, etc. in the phase of preparation. In this work towards eco-friendly processing, we use water as a solvent in the preparation of the photo-active layer for hybrid solar cells. We demonstrate eco-friendly hybrid polymer/titania solar cells consisting of water soluble polythiophene as light-absorber, donor and Hole Transporting Layer (HTL), above a TiO 2 layer that acts as an acceptor and electron conductor. The water soluble conjugated polymer materials are studied in terms of their opto-electrical and morphological properties, leading to a better understanding of the resulting photovoltaic performance. An alternative new processing method in device preparation is introduced; yielding prototype solar cells with an efficiency of 0.7%. This promising solar cell device performance can be considered as a proof-of-principle for future eco-friendly solar cells.

The morphological, bipolar charge-carrier transport, and photovoltaic characteristics of poly(3-alkylthiophene) (P3AT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends are studied as a function of alkyl side-chain length m, where m equals the number of alkyl carbon atoms. The P3ATs studied are poly(3-butylthiophene) (P3BT, m = 4), poly(3-pentylthiophene) (P3PT, m = 5), and poly(3-hexylthiophene) (P3HT, m = 6). Solar cells with these blends deliver similar order of photo-current yield (exceeding 10 mA cm−2) irrespective of side-chain length. Power conversion efficiencies of 3.2, 4.3, and 4.6% are within reach using solar cells with active layers of P3BT:PCBM (1:0.8), P3PT:PCBM (1:1), and P3HT:PCBM (1:1), respectively. A difference in fill factor values is found to be the main source of efficiency difference. Morphological studies reveal an increase in the degree of phase separation with increasing alkyl chain length. Moreover, while P3PT:PCBM and P3HT:PCBM films have similar hole mobility, measured by hole-only diodes, the hole mobility in P3BT:PCBM lowers by nearly a factor of four. Bipolar measurements made by field-effect transistor showed a decrease in the hole mobility and an increase in the electron mobility with increasing alkyl chain length. Balanced charge transport is only achieved in the P3HT:PCBM blend. This, together with better processing properties, explains the superior properties of P3HT as a solar cell material. P3PT is proved to be a potentially competitive material. The optoelectronic and charge transport properties observed in the different P3AT:PCBM bulk heterojunction (BHJ) blends provide useful information for understanding the physics of BHJ films and the working principles of the corresponding solar cells.

The increasing amount of research on solution-processable, organic donor-acceptor bulk heterojunction photovoltaic systems, based on blends of conjugated polymers and fullerenes has resulted in devices with an overall power-conversion efficiency of 6%. For the best devices, absorbed photon-to-electron quantum efficiencies approaching 100% have been shown. Besides the produced current, the overall efficiency depends critically on the generated photovoltage. Therefore, understanding and optimization of the open-circuit voltage (Voc) of organic solar cells is of high importance. Here, we demonstrate that charge-transfer absorption and emission are shown to be related to each other and Voc in accordance with the assumptions of the detailed balance and quasi-equilibrium theory. We underline the importance of the weak ground-state interaction between the polymer and the fullerene and we confirm that Voc is determined by the formation of these states. Our work further suggests alternative pathways to improve Voc of donor-acceptor devices.

In this article we report the weak but omnipresent electroluminescence (EL) from several types of organic polymer:fullerene bulk heterojunction solar cells biased in the forward direction. The light emitted from blends of some commonly used polymers and the fullerene molecule is significantly different from that of any of the pure materials comprising the blend. The lower energy of the blend EL is found to correlate with both the voltage onset of emission and the open-circuit voltage of the photovoltaic cell under solar illumination. We accordingly interpret the emission to originate from interfacial charge transfer state recombination and emphasize EL as a very valuable tool to characterize the charge transfer state present in donor/acceptor organic photovoltaic (OPV) cells.

In high performance polymer: fullerene bulk heterojunction solar cells the nanoscale morphology of interpenetrating acceptor:donor materials is optimized through appropriate preparation conditions such as annealing and choice of solvent, but this initial state-of-the-art morphology will not remain stable during long-term operation. We report the effects of prolonged storage at elevated temperatures on both the morphology and the photovoltaic performance for the model systems MDMO-PPV:PCBM and P3HT:PCBM as compared to 'High T-g PPV':PCBM based solar cells, where the 'High Tg PPV' is characterized by its high glass transition temperature (138 degrees Q. In situ monitoring of the photocurrent-voltage characteristics at elevated temperatures, in combination with a systematic transmission electron microscopy (TEM) study and complementary optical spectroscopy, reveals distinct degradation kinetics and morphological changes that indicate the occurrence of different underlying physico-chemical mechanisms. (c) 2008 Elsevier B.V. All rights reserved.

Significant efforts have lead to demonstrations of nonfullerene solar cells (NFSCs) with record power conversion efficiency up to ≈13% for polymer:small molecule blends and ≈9% for all-polymer blends. However, the control of morphology in NFSCs based on polymer blends is very challenging and a key obstacle to pushing this technology to eventual commercialization. The relations between phases at various length scales and photovoltaic parameters of all-polymer bulk-heterojunctions remain poorly understood and seldom explored. Here, precise control over a multilength scale morphology and photovoltaic performance are demonstrated by simply altering the concentration of a green solvent additive used in blade-coated films. Resonant soft X-ray scattering is used to elucidate the multiphasic morphology of these printed all-polymeric films and complements with the use of grazing incidence wide-angle X-ray scattering and in situ spectroscopic ellipsometry characterizations to correlate the morphology parameters at different length scales to the device performance metrics. Benefiting from the highest relative volume fraction of small domains, additive-free solar cells show the best device performance, strengthening the advantage of single benign solvent approach. This study also highlights the importance of high volume fraction of smallest domains in printed NFSCs and organic solar cells in general.

We investigated the electrical properties of solution processed Al-doped ZnO (AZO) nanoparticles, stabilized by mixing with a TiOx complex. Thin solid films cast from the solution of AZO-TiOx (AZOTi) (Ti/Zn ∼0.4 in the bulk and ∼0.8 on its surface) is processable in inert environment, without a need for either ambient air exposure for hydrolysis or high temperature thermal annealing commonly applied to buffer layers of most metal-oxides. It was found that the electronic structure of AZOTi matches the electronic structure of several electron acceptor and donor materials used in organic electronic devices, such as solar cells. Inverted solar cells employing a bulk heterojunction film of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester, cast on an indium-tin-oxide/AZOTi electrode, and capped with a tungsten oxide/aluminum back electrode, give rise to a nearly 70% fill factor and an optimized open-circuit voltage as a result of efficient hole blocking behavior of AZOTi. The resulting electron collecting/blocking capability of this material solves crucial interfacial recombination issues commonly observed at the organic/metal-oxide interface in most inverted organic bulk heterojunction solar cells.

We demonstrated a 35% enhancement in the efficiency of inverted solar cells as a result of increased open-circuit voltage and fill factor by adsorbing an ultrathin layer of a ruthenium dye N719 on an aluminum-doped zinc oxide (ZnO-Al) electron collecting interfacial layer. The interface modification with N719 changes the charge injection levels as indicated by ultraviolet photoemission spectroscopy. The efficiency of inverted solar cells comprising a bulk heterojunction photo-active film of poly(3-hexylthiophene) and phenyl-C 61-butyric acid methyl ester has increased from ∼2.80% to 3.80% upon employing the dye modification of the electrode interface.

Poly-3-AlkylThiophenes (P3ATs) with an n-alkyl chain length varying from C3 till C9 were synthesized by using the Rieke method. Subsequently, these materials were used to make P3AT/PCBM blends which were investigated in bulk heterojunction (BHJ) solar cells. The phase diagram of a P3H(exyl)T:PCBM blend was measured by means of standard and modulated temperature differential scanning calorimetry (DSC and MTDSC). A single glass transition is observed for all compositions. The glass transition temperature (Tg) increases with increasing PCBM concentration: from 12 °C for pure P3HT to 131 °C for pure PCBM. The observed range of Tg's defines the operating window for thermal annealing and explains the long-term instability of both morphology and photovoltaic performance of P3HT:PCBM solar cells. All regioregular P3ATs allow for efficient fiber formation in several solvents. The fibers formed are typically 15 to 25 nm wide and 0.5 to >4 μm long and mainly crystalline. By means of temperature control the fiber content in the casting solution for P3AT:PCBM BHJ solar cells is controlled while keeping the overall molecular weight of the polymer in the blend constant. In this way, fiber isolation and the use of solvent mixtures are avoided and with P3HT nanofibers, a power conversion efficiency of 3.2 % was achieved. P3AT:PCBM BHJ solar cells were also prepared from P3B(utyl)T, P3P(entyl)T and P3HT using the good solvent o-dichlorobenzene and a combination of slow drying and thermal annealing. In this way, power conversion efficiencies of 3.2, 4.3, and 4.6 % were obtained, respectively. P3PT is proved to be a potentially competitive material compared to P3HT.