Pushing the Envelope of the Intrinsic Limitation of ... - ACS Publications

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Pushing the Envelope of the Intrinsic Limitation of Organic Solar Cells Nadia Camaioni*,† and Riccardo Po‡ †

Istituto per la Sintesi Organica e la Fotoreattività, Consiglio Nazionale delle Ricerche, via Gobetti 101, I-40129 Bologna, Italy Research Center for non Conventional Energies, Istituto ENI Donegani, ENI S.p.A., via Fauser 4, I-28100 Novara, Italy

‡

ABSTRACT: The photogeneration of Frenkel-type excitons, instead of pairs of free charges, is one of the main drawbacks of organic photovoltaics, when compared with the inorganic counterpart. The strong Coulomb interaction of charge carriers of opposite sign in organic materials is responsible for the complexity of the process of generation of unbound charges, affecting the photogenerated current and still not clearly understood, as well as for the free energy loss of electrons resulting in a diminished open circuit voltage. Despite this practical limitation, record power conversion efficiencies approaching 10% are currently reported for lab-scale single-junction structures made of lowbandgap electron-donating conjugated small molecules or polymers blended with electron-accepting fullerene derivatives. To go beyond, a deep understanding of charge generation dynamics, highly system dependent, is necessary for the definition of the rules for the design of high-performance organic materials for the photovoltaic application and possibly the reduction of exciton binding energy, through the increase of the dielectric constant, which definitively would overcome the practical constraints to high efficiency organic solar cells.

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rganic solar cells,1,2 made of electron-donating conjugated materials (small-molecules or polymers) mixed with electron-accepting fullerene derivatives in the so-called bulk-heterojunction (BHJ) approach,3 could be the answer to the increasing demand for low-cost, ultrathin, large-area, and flexible photovoltaic modules. They are made of unconventional, abundant, and cheap soluble materials that can be printed at low temperature over any substrate. The barrier of 10% efficiency for organic solar cells has been recently broken for tandem structures,4,5 while single-junction cells are rapidly approaching it.5 The impressive progression of the power conversion efficiency (PCE) during the past years is the result of the combined effort along three main axes: (i) the accurate control of the nanomorphology of the blended active layer;6 (ii) the synthesis of new low-bandgap electron-donating materials;7 (iii) the optimization of the device structure.8 The fine control of the nanoscale morphology of the interpenetrating donor:acceptor network is crucial to ensure the best trade-off between high generation of free charge carriers and their efficient transport to the respective electrodes. Indeed, a phase separation between the two blend components of the order of the exciton diffusion length (typically of 10−20 nm) is required for the efficient diffusion of photogenerated excitons to the donor/acceptor heterojunction, where their dissociation occurs, while a higher segregation is usually beneficial for achieving bicontinuous percolative donor and acceptor networks for effective charge transport. To this end, different approaches9 have demonstrated their effectiveness in improving the morphology of solution-deposited active layers, leading to consistent enhancements of the overall PCE of organic solar © XXXX American Chemical Society

cells. The availability of high-performance low-bandgap electron-donors, also acting as the main light absorbers because of the weak optical absorption of fullerene derivatives in the visible range, has represented another breakthrough toward high efficiency organic solar cells in these past years. Excellent absorbers with (i) a bandgap in the range 1.3−1.9 eV, (ii) high hole mobility, and (iii) deep-lying HOMO (highest occupied molecular orbital) level have been effectively designed in recent years.10 However, the advantages arising from the optimized active layer morphology and from the tuning of the absorber electronic properties could vanish in the case of inefficient extraction of charge carriers from the device. Therefore, the improvement of the device structure has also been a major concern of the past years, aimed at enhancing the selectivity and the ohmic character of both electrodes through the development of proper interfacial layers.11 The research activity in the new millennium has been dominated by the three mentioned main directions. However, steady advances in any research field is always accompanied by a deeper understanding of basic mechanisms. For BHJ solar cells, this means a better understanding of charge generation dynamics, which is necessary for providing the guidelines for the design of the next generation of high-performance materials for organic photovoltaics. Indeed, a renewed effort has been devoted just within the past 2 years to the comprehension of Received: February 18, 2013 Accepted: May 9, 2013

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The Journal of Physical Chemistry Letters

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tion,22,23 and associated with the emission originated from the recombination of charge carriers occurring through those interfacial states. The emitted radiation is found to be significantly different (of lower energy) from that of the pristine materials composing the blends, and a correlation has been established between the intensity of the emitted EL signal and the external quantum efficiency of solar cells.18 The detection of the EL emission has even been proposed as a fast and reliable tool for quality control during module production.24 The internal electric field in working solar cells could be expected to improve the dissociation of interfacial CT states into free charge carriers. However significant effects on the depopulation of CT states has been only observed under the application of a strong reverse bias.25,26 Under normal operation conditions, the internal field seems to have a negligible effect on the splitting of geminate charge pairs,27,28 consistent with the significant photocurrent loss estimated at the maximum power point of low-efficiency polymer:fullerene solar cells.26,29,30 The role of CT states in yielding free pairs of charge carriers is currently the subject of controversy. The main question is whether bound electron−hole pairs are precursors to charge separated states. The commonly held viewpoint is that the route from photogenerated excitons to free charge carriers is highly system dependent (dependent on the energetics of materials and on blend processing conditions). As expected, geminate recombination of CT states is not observed in optimized solar cells with internal quantum efficiency approaching 100%,31,32 in which almost every photon absorbed leads to a pair of collected charge carriers (indicating the absence of any recombination process, geminate or nongeminate). The local nanomorphology at the donor:acceptor interface, which could also affect the local electric field,33 is believed to play a crucial role in the formation of CT states, as confirmed by the direct comparison of as-cast and thermal annealed blends,22 as well as of active layers processed with or without additives.34 In cells with optimized morphologies, either thermal annealed or by using appropriate additives, a significant suppression of the CT exciton emission is commonly observed, when compared with the less efficient nonoptimized devices. It has been suggested that enhanced crystallinity, leading to enhanced charge transport, can favorably compete with geminate recombination,35 as well as the reduction of charge trapping states.36 Low crystallinity and lack of welldefined phase-separated donor:acceptor microstructure have also been hypothesized to be responsible for the stronger fielddependence of interfacial exciton dissociation in blends made of low-molecular weight donors,37 compared to that usually observed in common polymer solar cells. It is worth noting that just a little modification of the chemical structure can induce relevant improvements of the material properties, as demonstrated by replacing the bridging carbon atom of the cyclopentadithiophene unit of a low-bandgap conjugated polymer with a silicon atom.38 The Si-bridged polymer shows enhanced crystallinity and charge carrier mobility, compared with the carbon-bridged analogue, and a reduced formation of CT states when blended with fullerene. In general, for most efficient polymer:fullerene blends, charge delocalization is believed to play a crucial role for the formation of free charges:15,32,39,40 delocalization of charge wave functions (material dependent) promotes an increased hole−electron separation resulting in a lower binding energy, thus providing a

the mechanisms leading to the light generation of free charge carriers in this kind of solar cells, with an abundance of scientific publications on this topic. This Perspective attempts to give just an overview of the complexity of the generation mechanism of unbound charge carriers in organic solar cells, on the basis of the latest findings reported in the literature, as well as some recent indications proposed as possible strategies for limiting the losses associated with the excitonic character of this photovoltaic technology. One of the main features of organic conjugated materials is the low dielectric constant (εr ∼ 3), leading to strong Coulomb interactions between charge carriers of opposite sign, strongly affecting the process of charge photogeneration, and mainly determining the limits of organic solar cells when compared with the inorganic counterpart.12 In an organic solar cell, the photogenerated electron−hole pairs (Frenkel excitons) undergo dissociation at the interface between the electron-donating and the electron-accepting material, where the driving force to overcome the exciton binding energy (typically of the order of 0.5 eV) is provided by the energy offset of a type-II heterojunction. Upon exciton dissociation by photoinduced charge transfer,13 the electron, on the acceptor side, and the hole, on the donor material, still experience a Coulomb interaction. If the carriers of this geminate pair (here referred as a bound charge pair, or a charge transfer (CT) state, or a CT exciton)14 are not allowed to escape from each other, recombination will eventually occur (geminate recombination, Figure 1),15−18 leading to a loss for solar cells. The role of CT

Figure 1. CT excitons at the donor:acceptor interface. ΔELUMO is the energy offset between the lowest unoccupied molecular orbitals, and ΔEHOMO is that between the highest occupied molecular orbitals.

states as intermediate states between photogenerated excitons and completely unbound electron−hole pairs (charge separated states) is debated. During the past years, several groups have reported spectroscopic evidence for CT formation in polymer:fullerene blends by using a variety of steady-state and time-resolved investigation techniques (see ref 19 for a review and references therein), also including light-induced electron spin resonance.20,21 An effective way to characterize the energy of CT states is provided by the spectral analysis of the weak electroluminescence (EL) observed for several types of polymer:fullerene BHJ solar cells under forward polariza1822



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resulting in an increased open-circuit voltage (Voc).14,39 The strong correlation between Voc and the energy of the CT state (ECT) has been reported for a series of polymer:fullerene solar cells.18,22,47 Indeed, the energy required for the transformation of absorbed photons into free charges limits the maximum obtainable free energy of an electron at open-circuit conditions qVoc,12,48,49 where q is the elementary charge. A scheme showing the energetic losses in a donor:acceptor solar cell is shown in Figure 3, adapted from ref 49, where Eg is the smaller

means of efficient generation of free charge pairs. For these systems, most of photogenerated excitons undergo an ultrafast full separation, and geminate recombination does not represent a significant loss channel. So a dual-path scheme for free charge formation is proposed in these cases, with photogenerated excitons generating directly charge-separated states in parallel with CT states, which can further dissociate into unbound charge pairs.32,41 The dipolar properties of common low-bandgap polymers have been also suggested to play a role in the process of charge separation when blended with the accepting fullerene.42,43 The intrachain charge-transfer occurring between the electron-rich and the electron-deficient moiety of the polymer repeating unit results in a strong intramolecular dipole, partially separating the initial bound charges, and thus lowering their Coulomb interaction and leading to a reduced driving force for their full separation. Faster electron transfer dynamics and slower recombination rates of the charge-separated states have been observed for blends made of low-bandgap polymers with larger dipole moments, indicating that electrons and holes are less tightly bound in those systems. Rather than the dipole moment at ground state, the variation of the dipole moment from ground to the excited state Δμge, has recently attracted growing attention in the dissociation dynamics of charge carriers.44−46 It has been suggested that a large polarizability of the excited state lowers the exciton binding energy, thus enhancing the electron transfer rate across the polymer:fullerene heterojunction and ultimately leading to enhanced solar cell performance. Interestingly, a linear correlation between the efficiency of optimized polymer:fullerene solar cells and Δμge has been reported for a series of eight low-bandgap polymers with different chemical structures and energy levels.45 An even better linear correlation is established, for the same solar cells, between the short-circuit current (Jsc), directly related to the generation of free charges, and Δμge, as shown in Figure 2. Although a clear explanation is still lacking for the observed linear trends, they provide the first validation of the suggested concepts. A reduced binding energy of CT excitons, other than resulting in solar cells with low photocurrent loss, should also allow for an efficient generation of unbound charge pairs with a minimal energy offset at the donor:acceptor heterojunction,

Figure 3. Scheme of the energetic losses in an organic solar cell (adapted from ref 49).

optical bandgap of the two blend components (usually, the bandgap of the donor), and the energy ΔECS, related to ECT, is that required to drive the generation of free charge pairs, which must be provided by the energy offset at the heterojunction. The figure also accounts for other losses, further reducing the open-circuit voltage, mainly due to nongeminate recombination of charge carriers. The modulation of ECT, through an extended delocalization of the CT states, can reduce the loss for Voc. The challenge is to minimize the energy offsets at the donor/ acceptor heterojunction while maintaining efficient generation of free charge carriers. For polymer:fullerene solar cells, in which the conjugated polymer is the primary light absorber, a low offset between the lowest unoccupied molecular orbital (LUMO) levels would be desirable for reduced voltage loss. However, the highest internal quantum efficiencies are commonly reported for systems showing large LUMO offsets (0.5 −1.0 eV), while reduced offset values (0.2−0.3 eV) are less effective in the efficient generation of free charges.41,50,51 Despite the enhanced Voc at low LUMO offsets, the suppression of the dissociation efficiency of geminate charge pairs limits the obtainable PCEs, as demonstrated by experimental results. The trend of PCE of some record polymer:fullerene solar cells with the LUMO energy offset is shown in Figure 4 (adapted from ref 50), compared with the efficiency estimated for the ideal complete dissociation of geminate charge pairs (zero binding energy).50 Another effective strategy to lower the energy offset at the donor:acceptor interface, required for efficient generation of free carriers, is to reduce the reorganization energy associated with the photoinduced charge transfer across the junction.52−54 No photocurrent loss and high Voc, leading to significant enhancement for cell efficiency, have been predicted for energy offsets as low as 0.15 eV but with a low reorganization energy (0.3 eV).54 Nevertheless, the most effective and definitive approach to the issue of the energy loss associated with the photovoltaic conversion in organic solar cells would be to tackle the origin of the problem, i.e., the strong interaction between charges of opposite sign because of the low dielectric constant. It has been shown that increasing εr to 4 in a poly(p-phenylene vinylene) derivative by the introduction of ethylene oxide sidechains, a significant reduction of photocurrent loss by geminate

Figure 2. Short-circuit current density of optimized polymer:fullerene solar cells as a function of the change of the dipole moment from ground to excited state. Data taken from ref 45 and references therein. Δμge was calculated for the repeating units (shown in the figure) of the eight low band gap polymers. 1823



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negligible. However, if the effect of the dielectric constant on exciton binding energy (Eb) is also taken into account (Figure 5, red symbols), leading to a reduction of the required energy offset at the donor:acceptor heterojunction with increasing εr, much higher efficiency can be predicted (over 20% for εr of 10). Thus, if active materials with higher dielectric constant will be developed, solar cells with an efficiency comparable to that of the inorganic counterpart can be predicted, but maintaining the typical and unique advantages of organic photovoltaics: low-cost processing from solution, lightweight, flexibility, and so on.

Figure 4. Record PCEs of polymer:fullerene solar cells from the literature (red markers, with the red line as a guide for the eye) and estimated efficiencies for complete dissociation of geminate charge pairs (blue curve) versus LUMO energy offsets. (Adapted with permissiomn from ref 50. Copyright 2011 The Royal Society of Chemistry.)

If active materials with higher dielectric constant will be developed, solar cells with an efficiency comparable to that of the inorganic counterpart can be predicted.

recombination can be achieved in the related blends with a fullerene derivative.55 The low dielectric constant of organic materials, other than resulting in energy losses for charge generation, also has negative effects on the processes related to the transport of separated carriers in the bicontinuous donor:acceptor network, such as on nongeminate recombination and space-charge formation, limiting the thickness of the active layer of solar cells for more efficient collection of charge carriers. Reduced losses for charge transport would allow for thicker blends, compared to the usually 100 nm reported for optimized lab-scale solar cells, more suitable for efficient solar light harvesting as well as for the coating/printing techniques required for large-scale industrial production.56 The impact on solar cell efficiency of reduced nongeminate recombination and space-charge effects through an increased dielectric constant have been modeled54 by using experimental data and parameters of a PTB7:PC70BM system (PTB7 is poly{[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]}, while PC70BM is [6,6]phenyl-C71-butyric acid methyl ester) exhibiting a PCE of 7.4% for an optimized thickness of 100 nm.57 As shown in Figure 5 for an active layer 250 nm thick, the effect of εr on cell efficiency, solely due to reduced nongeminate recombination and space-charge effects (black symbols), would not be

The question is how to address the molecular design in order to achieve εr greater than 3. The introduction of strong polar groups or chains into the molecular structure can be an effective approach to improve the dielectric constant. Besides poly(pphenylene vinylene) derivatives bearing polar oligoethylene oxide side-chains55,58 and exhibiting a dielectric constant between 4 and 5.5 (examples are given in Figure 6), no other electron-donors with such characteristics have been specifically developed for photovoltaics. Oligoethyleneoxide-substituted polythiophenes59 could exhibit a similar improvement of the dielectric constant, compared to alkyl-substituted analogues, though the values of εr have not been provided. Similarly, the introduction of nitrile groups, either on the conjugated backbone or in the lateral chains, might represent another viable approach to achieve high dielectric constant derivatives.60 Nevertheless, the latter possibility seems more desirable, because it would not significantly affect the energy levels of molecules. Another possibility could be to mimic the strategies used for the development of organic materials for energy storage devices,61,62 for which very high dielectric constants are required. A common practice to enhance the dielectric constant of organic matrices for energy storage is the addition of high dielectric inorganic fillers, such as ceramics. However, the poor compatibility between the filler and the organic material makes this approach quite challenging for solar cell application. A more ambitious and fascinating approach toward high εr organic materials for photovoltaics could be the exploitation of tridimensional and/or bioinspired supramolecular selforganized architectures. It has been reported that hyperbranched macromolecular systems, developed for high-speed capacitors, exhibit an enhanced dielectric response in comparison with their linear analogues.63−65 An example of an hyperbranched phthalocyanine polymer is shown in Figure 6. In this case, the very high dielectric constant (around 15) has been attributed to the long-range delocalization deriving from the branched architecture, although the combined effect of nitrile groups in the molecular structure cannot be excluded. Also bioinspired highly ordered structures, associated with selforganization of the α-helix, could lead to an increased dielectric constant.66 Indeed, improved charge transport and overall photovoltaic performance have been reported for an

Figure 5. Variation of the PCE (η) with the dielectric constant by taking into account solely the effects on nongeminate charge recombination and space-charge effects (black symbols) and also considering the effect of exciton binding energy Eb (red symbols). The trend of exciton binding energy with εr is represented by the blue symbols. The values of PCE were calculated on the basis of a driftdiffusion model by using the parameters of PTB7:PC70BM solar cells, exhibiting a PCE of 7.4% (adapted from ref 54 with permission of Wiley−VCH Verlag GmbH & Co. KGaA). 1824



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Figure 6. Examples of organic materials exhibiting a dielectric constant higher than 3. The values of εr were taken from refs 55, 58, and 65.

oligothiophene-functionalized homopolypeptide,67 compared with control compounds. Although the observed improvements have not been correlated with the dielectric characteristics of the bioinspired material, they could play a relevant role. Future Issues and Challenges. Thanks to the first generation of electron-donating materials, “easily available” in academic laboratories such as poly(p-phenylene vinylene) and poly-

(alkylthiophene) derivatives, the crucial role of the morphology of the donor:acceptor blend for the performance of organic solar cells has been understood, and effective protocols have been developed to favorably change it, resulting in lab-scale devices with an efficiency of 4−5%. Following the precise indications given by Scharber and co-workers in 200668 for the energy levels of electron-donors potentially able to increase the 1825



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Notes

efficiency up to 10%, a variety of low bandgap electrondonating small molecules and polymers has been specifically developed for photovoltaics and that predicted efficiency limit, considered so ambitious just a few years ago, is going to be reached. Currently, we are likely entering the third generation of high-performance materials for organic photovoltaics, to push efficiency ever upward through a fine-tuning of the properties of the active materials and possibly to go beyond the typical limitations of organic materials in the conversion of solar energy into electricity.

The authors declare no competing financial interest. Biographies Nadia Camaioni is a senior scientist at the National Research Council (Consiglio Nazionale delle Ricerche, CNR, Italy) and teaches “Organic Materials for Electronics” at the University of Bologna. Her scientific interests deal with the electronic properties of organic conjugated materials and their applications in organic electronics and solar energy conversion. The research activity of the group she leads is mainly focused on developing innovative technologies for low-cost conversion of solar energy into electric energy, in particular, organic solar cells and luminescent solar concentrators (http://www.isof.cnr.it/?q=content/ camaioni-nadia).

Another possibility could be to mimic the strategies used for the development of organic materials for energy storage devices,61,62 for which very high dielectric constants are required.

Riccardo Po is the manager of the Solar Energy Department at the Research Center for non Conventional Energies of ENI SpA, an international integrated energy company. He graduated from the University of Pisa in 1988 and has worked in the field of polymer synthesis, modification and processing. Currently, the R&D activities of his group are focussed on new-generation photovoltaics, including polymer solar cells, DSSCs, and luminescent solar concentrators. He is coauthor of about 80 patent applications and 65 papers (http://www. eni.com/en_IT/innovation-technology/research-centres/doneganiinstitute/donegani-institute.shtml).

The deep understanding of the dynamics of charge carrier generation in organic solar cells is essential for providing the necessary guidelines for the design of the next generation of high performance materials, capable of efficient generation of charge carriers. The available knowledge suggests materials with high crystallinity, high carrier mobility, low reorganization energy, and possibly the local control of the transport and morphological properties just at the donor:acceptor interface, where the critical dissociation of photogenerated excitons occurs, for reduced binding energy of CT excitons. Likely, also a large variation of the dipole moment from the ground state to the excited state could be a strategy to suppress geminate recombination. However, the most effective way to definitively solve the problem of losses associated with the generation of free carriers would be the development of new materials with a higher dielectric constant, also leading to beneficial effects for the transport of free charge carriers. To this end, the contamination of experiences from research fields currently noninteracting (photovoltaics and energy storage applications) could be the starting point to address the molecular design of the new generation of materials with improved dielectric properties. A viable approach toward high εr organic materials could be the introduction of polar groups in the molecular structure, preferably as side-chains. Another very interesting approach could be the exploitation of hyperbranched molecular structures, able of high dielectric constants and likely with positive effects also for charge transport properties. It is worth noting that the employment of photoactive materials with much improved dielectric properties could likely involve a remodulation of the current BHJ structure of the active layer of organic solar cells, to take into account for a changed operation regime. By overcoming the inherent limitation of common organic materials (the strong Coulomb interaction between charge carriers of opposite sign), solar cells able to rival their inorganic counterparts in terms of efficiency could be possible, but with the possibility of very low production costs over large-area and flexible substrates. This remains the challenge.

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ACKNOWLEDGMENTS This work was supported by ENI S.p.A. (Contract No. 3500019603). REFERENCES

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*E-mail: [email protected]. 1826



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