Three different theoretical approaches are presented to identify pathways to organic solar cells with power conversion efficiencies in excess of 20%. A radiation limit for organic solar cells is ...introduced that elucidates the role of charge‐transfer (CT) state absorption. Provided this CT action is sufficiently weak, organic solar cells can be as efficient as their inorganic counterparts. Next, a model based on Marcus theory of electronic transfer that also considers exciton generation in both the electron donor and electron acceptor is used to show how reduction of the reorganization energies can lead to substantial efficiency gains. Finally, the dielectric constant is introduced as a central parameter for efficient solar cells. By using a drift–diffusion model, it is found that efficiencies of more than 20% are within reach.
Pathways to a new efficiency regime for organic solar cells are presented. A modest increase in the dielectric constant of the materials slows down all recombination mechanisms such that, taken together with energy level optimization, efficiencies of over 20% can be realized. Ultimately the difference in performance between organic and inorganic solar cells may vanish.
It is demonstrated that the n‐type thermoelectric performance of donor–acceptor (D–A) copolymers can be enhanced by a factor of >1000 by tailoring the density of states (DOS). The DOS distribution is ...tailored by embedding sp2‐nitrogen atoms into the donor moiety of the D–A backbone. Consequently, an electrical conductivity of 1.8 S cm−1 and a power factor of 4.5 µW m−1 K−2 are achieved. Interestingly, an unusual sign switching (from negative to positive) of the Seebeck coefficient of the unmodified D–A copolymer at moderately high dopant loading is observed. A direct measurement of the DOS shows that the DOS distributions become less broad upon modifying the backbone in both pristine and doped states. Additionally, doping‐induced charge transfer complexes (CTC) states, which are energetically located below the neutral band, are observed in DOS of the doped unmodified D–A copolymer. It is proposed that charge transport through these CTC states is responsible for the positive Seebeck coefficients in this n‐doped system. This is supported by numerical simulation and temperature dependence of Seebeck coefficient. The work provides a unique insight into the fundamental understanding of molecular doping and sheds light on designing efficient n‐type OTE materials from a perspective of tailoring the DOS.
Significantly boosted n‐type organic thermoelectric performance of donor–acceptor (D–A) copolymers by a factor of three orders of magnitude is realized by tailoring the density of states through molecular design. This strategy improves the electrical conductivity effectively and reduces the loss of the Seebeck coefficient, leading to a very good power factor of 4.5 µW m−1 K−2 for doped D–A copolymers.
The efficiency of polymer solar cells critically depends on the intimacy of mixing of the donor and acceptor semiconductors used in these devices to create charges and on the presence of unhindered ...percolation pathways in the individual components to transport holes and electrons. The visualization of these bulk heterojunction morphologies in three dimensions has been challenging and has hampered progress in this area. Here, we spatially resolve the morphology of 2%-efficient hybrid solar cells consisting of poly(3-hexylthiophene) as the donor and ZnO as the acceptor in the nanometre range by electron tomography. The morphology is statistically analysed for spherical contact distance and percolation pathways. Together with solving the three-dimensional exciton-diffusion equation, a consistent and quantitative correlation between solar-cell performance, photophysical data and the three-dimensional morphology has been obtained for devices with different layer thicknesses that enables differentiating between generation and transport as limiting factors to performance.
Abstract
The ‘phonon-glass electron-crystal’ concept has triggered most of the progress that has been achieved in inorganic thermoelectrics in the past two decades. Organic thermoelectric materials, ...unlike their inorganic counterparts, exhibit molecular diversity, flexible mechanical properties and easy fabrication, and are mostly ‘phonon glasses’. However, the thermoelectric performances of these organic materials are largely limited by low molecular order and they are therefore far from being ‘electron crystals’. Here, we report a molecularly n-doped fullerene derivative with meticulous design of the side chain that approaches an organic ‘PGEC’ thermoelectric material. This thermoelectric material exhibits an excellent electrical conductivity of >10 S cm
−1
and an ultralow thermal conductivity of <0.1 Wm
−1
K
−1
, leading to the best figure of merit
ZT
= 0.34 (at 120 °C) among all reported single-host n-type organic thermoelectric materials. The key factor to achieving the record performance is to use ‘arm-shaped’ double-triethylene-glycol-type side chains, which not only offer excellent doping efficiency (~60%) but also induce a disorder-to-order transition upon thermal annealing. This study illustrates the vast potential of organic semiconductors as thermoelectric materials.
The mechanism behind the temperature dependence of the device performance in hybrid perovskite solar cells (HPSCs) is investigated systematically. The power conversion efficiency (PCE) of the ...reference cell using 60PCBM as electron extraction layer (EEL) drops significantly from 11.9% at 295 K to 7% at 180 K. The deteriorated charge carrier extraction is found as the dominant factor causing this degradation. Temperature dependent spectroscopy and charge transport studies demonstrate that the poor electron transport in the 60PCBM EEL at low temperature leads to inefficient charge carrier extraction. It is further demonstrated that the n‐type doping of 60PCBM EEL or the use of an EEL (fulleropyrrolidine with a triethylene glycol monoethyl ether side chain) with higher electron transport capability is an effective strategy to achieve HPSCs working efficiently over a broad temperature range. The devices fabricated with these highly performing EELs have PCEs at 180 K of 16.7% and 18.2%, respectively. These results support the idea that the temperature dependence of the electron transport in the EELs limits the device performance in HPSCs, especially at lower temperatures and they also give directions toward further improvement of the PCE of HPSCs at realistic operating temperatures.
The temperature dependence of the figures of merit of hybrid perovskite solar cells (HPSCs) is dominated by the electron extraction layer (EEL). At low temperature, the device using 60PCBM as EEL shows significant lowering of the performance due to the poor electron transport capability of 60PCBM. By n‐type doping 60PCBM, highly efficient HPSCs over a broad temperature range are achieved.
The last decade saw myriad new donor polymers, among which benzodithiophene-co-thienothiophene polymers are attractive due to their relatively high power conversion efficiency in bulk heterojunction ...solar cells. We examine the effect of UV-light on the stability of these polymers. The relationship between the polymer chemical structure and the UV-stability of the cells is explored on the one hand, and on the other hand, the effect of additives on their UV-stability: 1,8-diiodooctane against 1-chloronaphthalene in the cells and 1,8-octanedithiol in solution. For example, PBDTTT-E with 18% efficiency loss is more stable than PBDTTT-ET with 36% loss throughout the exposure. While 1,8-diiodooctane acts as photo-acid and leads to accelerated degradation of the solar cells, 1-chloronaphthalene does not. Acidity is known to be detrimental to the efficiency and stability of organic solar cells. The degradation is initiated upon UV-irradiation by the cleavage of the side chains, resulting in more electron traps and by the formation of iodine, dissolved HI and carbon-centered radicals from 1,8-diiodooctane as revealed by
H NMR spectrum. The 1,8-octanedithiol spectra do not show such species. Finally, the mechanisms behind the effect of 1,8-diiodooctane are explained, paving the way for the design of new, efficient as well as stable materials and additives.
We present a new experimental technique that affords direct quantification of the fraction of charge carriers lost in poly(3‐hexylthiophene):fullerene solar cells by bimolecular recombination. ...Depending on annealing conditions up to 17% of carriers recombine bimolecularly under solar illumination. We explain our findings with a closed analytical expression for the photocurrent generated by an organic solar cell.
Among the parameters that characterize a solar cell and define its power-conversion efficiency, the fill factor is the least well understood, making targeted improvements difficult. Here we quantify ...the competition between charge extraction and recombination by using a single parameter θ, and we demonstrate that this parameter is directly related to the fill factor of many different bulk-heterojunction solar cells. Our finding is supported by experimental measurements on 15 different donor:acceptor combinations, as well as by drift-diffusion simulations of organic solar cells in which charge-carrier mobilities, recombination rate, light intensity, energy levels and active-layer thickness are all varied over wide ranges to reproduce typical experimental conditions. The results unify the fill factors of several very different donor:acceptor combinations and give insight into why fill factors change so much with thickness, light intensity and materials properties. To achieve fill factors larger than 0.8 requires further improvements in charge transport while reducing recombination.
The short exciton diffusion length associated with most classical organic semiconductors used in organic photovoltaics (5-20 nm) imposes severe limits on the maximum size of the donor and acceptor ...domains within the photoactive layer of the cell. Identifying materials that are able to transport excitons over longer distances can help advancing our understanding and lead to solar cells with higher efficiency. Here, we measure the exciton diffusion length in a wide range of nonfullerene acceptor molecules using two different experimental techniques based on photocurrent and ultrafast spectroscopy measurements. The acceptors exhibit balanced ambipolar charge transport and surprisingly long exciton diffusion lengths in the range of 20 to 47 nm. With the aid of quantum-chemical calculations, we are able to rationalize the exciton dynamics and draw basic chemical design rules, particularly on the importance of the end-group substituent on the crystal packing of nonfullerene acceptors.
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