Interplay between bimolecular recombination and carrier transport distances in bulk heterojunction organic solar cells (original) (raw)
Related papers
2012
The effect of phase separation of the donor-acceptor (DA) blend on the dominant recombination mechanism in polymer-fullerene [(poly(3-hexylthiophene) (P3HT) and phenyl-C 61 -butyric acid methyl ester (PCBM)] based bulk heterojunction (BHJ) cells has been investigated. Coarse (70-150 nm) and fine (20-25 nm) phase separated blends and corresponding devices were prepared using chlorobenzene (CB) and ortho-dichlorobenzene (1,2-DCB) as spin casting solvents respectively. Nanoscale mobility measurements indicated highly unbalanced charge transport in coarse morphology based (CB cast) devices. Linear dependence of short circuit current (J sc ) vs. light intensity (I) suggested first order monomolecular (MR) recombination in the fine phase separated devices (1,2-DCB cast) whereas sub-linearity suggested dominant role of bimolecular (BR) recombination in coarse phase separated devices (CB cast). Improved device efficiency of 1,2-DCB based devices (η ≈ 2.54 %) compared to CB (η ≈ 0.9 %) may be attributed to reduced BR recombination as a result of finer phase separation.
The Journal of Physical Chemistry Letter, 2013
Recombination in the well-performing bulk heterojunction solar cell blend between the conjugated polymer TQ-1 and the substituted fullerene PCBM has been investigated with pump−probe transient absorption and charge extraction of photogenerated carriers (photo-CELIV). Both methods are shown to generate identical and overlapping data under appropriate experimental conditions. The dominant type of recombination is bimolecular with a rate constant of 7 × 10 −12 cm −3 s −1 . This recombination rate is shown to be fully consistent with solar cell performance. Deviations from an ideal bimolecular recombination process, in this material system only observable at high pump fluences, are explained with a time-dependent charge-carrier mobility, and the implications of such a behavior for device development are discussed. SECTION: Energy Conversion and Storage; Energy and Charge Transport R ecombination is a collective label to a wide range of mechanisms that returns an excited system to its ground state. All opto-electronic devices are influenced by recombination. In some cases it is a desirable process, such as for lightemitting diodes, while in others, such as solar cells, it is a major loss mechanism. A given system usually expresses a collection of different recombination mechanisms, of which one or more can be significant. Examples of mechanisms frequently encountered in bulk heterojunction solar cells are: recombination of excitons before reaching an interface, recombination of coloumbically bound charge pairs at interfaces, recombination between trapped charges and free charges, and recombination between free charges. Among these examples, the first two are first-order processes, the third can be of either first or second order depending on conditions, and the last one is a secondorder process. In studying recombination, it is perhaps of greater importance to identify the nature of the dominant recombination process(es) than to quantify, for example, carrier lifetimes, because different processes demand different means of manipulation. Material development and device engineering can benefit greatly from knowledge about the dominant recombination pathways. Obtaining such information, however, is not always trivial. Because time scales, measurement conditions, and sample geometries tend to differ significantly, both for different recombination measurement techniques as well as between recombination measurements and operational devices, the measurement results, and consequently the conclusions therefrom, vary significantly between different studies even on the same material systems. 1−6 Here data from two of the most common and straightforward techniques for recombination studies, pump−probe transient absorption (TA) 2 and charge extraction of photogenerated carriers by linearly increasing voltage (photo-CELIV), 5 are correlated with each other. The results obtained with the two methods are shown to be mutually fully consistent, but viewed separately, the data also illustrate the risk of incomplete or even disagreeing interpretations regarding the underlying recombination mechanism due to differences in the experimentally accessible time range. Both the rate and dominating type of the recombination process in the test system based on poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ-1) 7 and [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM) are consistent with solar cell device data. Chemical structures can be found in the Supporting Information.
Journal of Physical Chemistry C, 2013
Low-bandgap diketopyrrolopyrrole-and carbazolebased polymer bulk-heterojunction solar cells exhibit much faster charge carrier recombination kinetics than that encountered for less-recombining poly(3-hexylthiophene). Solar cells comprising these polymers exhibit energy losses caused by carrier recombination of approximately 100 mV, expressed as reduction in open-circuit voltage, and consequently photovoltaic conversion efficiency lowers in more than 20%. The analysis presented here unravels the origin of that energy loss by connecting the limiting mechanism governing recombination dynamics to the electronic coupling occurring at the donor polymer and acceptor fullerene interfaces. Previous approaches correlate carrier transport properties and recombination kinetics by means of Langevin-like mechanisms. However, neither carrier mobility nor polymer ionization energy helps understanding the variation of the recombination coefficient among the studied polymers. In the framework of the charge transfer Marcus theory, it is proposed that recombination time scale is linked with charge transfer molecular mechanisms at the polymer/fullerene interfaces. As expected for efficient organic solar cells, small electronic coupling existing between donor polymers and acceptor fullerene (V if < 1 meV) and large reorganization energy (λ ≈ 0.7 eV) are encountered. Differences in the electronic coupling among polymer/fullerene blends suffice to explain the slowest recombination exhibited by poly(3-hexylthiophene)-based solar cells. Our approach reveals how to directly connect photovoltaic parameters as open-circuit voltage to molecular properties of blended materials.
Journal of Polymer Science Part B: Polymer Physics, 2012
We explore charge recombination dynamics at electron donor-acceptor heterojunctions, formed between a semiconductor polymer (PCDTBT) and a fullerene derivative (PC 70 BM), by means of combined time-resolved photoluminescence and transient absorption spectroscopies. Following prompt exciton dissociation across the heterojunction, a subset of bound electron-hole pairs recombines with a temperature-independent rate distribution spanning submicrosecond timescales to produce luminescent charge-transfer excitons (CTX). At 14 K, this slow mechanism is the dominant geminate charge recombination pathway, whereas we also observe CTX emission on subnanosecond timescales at 293 K. We thus find that at these temperatures, a fraction of the initial charge-pair population is trapped deeply such that they only recombine slowly over a broad distribution of timescales by quantum tunneling. We identify geminate polaron pairs (GPP) as a reservoir of long-lived localized states that repopulate the CTX up to microsecond timescales. The observation of such distributed geminate-charge recombination highlights the importance of the molecular nature of specific donor-acceptor electronic interactions in defining the relaxation pathways of trapped GPP. polymers solar cell is hampered if electron-hole pairs produced by photoinduced charge transfer trap into these states. 16, Moreover, many studies suggest a strong relationship between the energy of the CTX in the organic photovoltaic device and its open-circuit voltage (V OC ). To address CTX formation dynamics in an efficient organic photovoltaic device, we study a high-performance polymer:fullerene photovoltaic system that demonstrated high Additional Supporting Information may be found in the online version of the article.
The Journal of Physical Chemistry Letters, 2011
O rganic photovoltaics (OPVs) present an opportunity for the low-cost manufacture of solar cells, with devices made from a polymer blended with a functionalized fullerene in a bulk heterojunction (BHJ) structure achieving power conversion efficiencies (PCEs) exceeding 7% in the literature. There is therefore considerable focus on developing methods by which the loss processes limiting the efficient generation and collection of charge carriers in this class of cells may be understood and minimized. In particular, "corrected photocurrent" analyses, based on consideration of the device photocurrent as a function of voltage after subtraction of the corresponding dark current, are widely used tools to analyze the function of such devices. 2À6 Such corrected photocurrents are typically observed to scale linearly with light intensity; this observation has been widely interpreted as indicating that the dominant loss processes limiting device performance scale linearly with charge carrier density in the device. In this Letter, we address the corrected photocurrent analysis and the validity of its underlying assumptions. Additionally, we present a comparison of corrected photocurrent and transient optoelectronic analyses of the same device to demonstrate that the observation of such linear corrected photocurrents cannot be unambiguously employed to determine the order of the underlying loss pathways.