Z.
Ding‡
a,
J.
Kettle‡
*a,
M.
Horie
b,
S. W.
Chang
b,
G. C.
Smith
c,
A. I.
Shames
d and
E. A.
Katz
ef
aSchool of Electronics, Bangor University, Dean St, Bangor, Gwynedd, LL57 1UT, Wales, UK. E-mail: j.kettle@bangor.ac.uk; Fax: +44 (0)1248 382471
bDepartment of Chemical Engineering, National Tsing Hua University, 101, Sec. 2, Kuang-Fu Road, Hsinchu City 30013, Taiwan
cDepartment of Natural Sciences, University of Chester, Thornton Science Park, Chester CH2 4NU, UK
dDepartment of Physics, Ben-Gurion University of the Negev, P.O.B. 653, 8410501, Be'er-Sheva, Israel
eDepartment of Solar Energy and Environmental Physics, J. Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boker Campus, 8499000, Israel
fIlse Katz Institute of Nano-Science and Technology, Ben-Gurion University of the Negev, Be'er Sheva 8410501, Israel
First published on 14th April 2016
The principle remaining challenge in the research area of organic photovoltaic (OPV) materials is to develop solar cells that combine high efficiency, stability and reproducibility. Here, we demonstrate an experimental strategy which has successfully addressed this challenge. We produced a number of samples of the highly efficient PTB7 polymer with various molecular weights (Mn ∼ 40–220k). OPV cells fabricated with this polymer demonstrated significant improvement of the cell efficiency (by ∼90% relative) and lifetime (by ∼300% relative) with the Mn increase. We attribute these effects to the lower density of recombination centers (persistent radical defects revealed by EPR spectroscopy) and better photoactive layer morphology in the samples with higher Mn. Relevance of the observed correlation between the OPV efficiency and stability is discussed.
Another vital drawback of polymer OPVs is poor reproducibility of the physical and electronic properties of polymer-based semiconductors. Different samples of the same polymer might differ by the molecular weight, amount of defects and residual impurities, and consequentially, different polymer batches might perform quite differently in OPV devices.12 To resolve this limitation an extensive experimental program with a wide international cooperation is currently planned.13
Recently, Troshin et al. suggested that Electron Paramagnetic Resonance (EPR) spectroscopy is a very powerful tool for assessment of the quality of different conjugated polymers used as the electron donor in OPV12 and monitoring their photochemical and thermal degradation.11 It has been shown that different batches of the same conjugated polymer might contain substantially different amounts of radical species behaving as traps for mobile charge carriers. Indeed, a correlation between the concentrations of radicals in various batches of conjugated polymers and PCE of the OPV cells on their basis has been revealed.12 Furthermore, the relative stability of materials was suggested to be quantified from the rates of radical accumulation estimated from their EPR spectra.11
In this paper, we present photovoltaic performance and lifetime of OPV devices fabricated with PTB7 synthesized by various methods or using different commercial sources. We report that the increase in polymer molecular weight results in decrease of the density of radical defects in the material and corresponding improvement of efficiency and stability of PTB7-based solar cells. We suggest that this relationship may also apply to other conjugated polymers. Furthermore, the study of various grades of the same polymer allows us to postulate a general rule: namely, a more efficient cell should be more stable. Indeed, photoinduced degradation of PV devices under operation is caused by sunlight. The larger the proportion of the incident sunlight power that is converted to electrical power, the smaller the part remaining for activation of degradation. This positive conclusion may be very important for future development of next generation PV.
Similar procedure was applied using tetrakis-(triphenylphosphine)palladium(0) (2.0 mg, 0.003 mmol, 5 mol%) to yield deep blue powder PTB7-3 (63 mg, 98% yield). GPC: Mn = 108900, Mw/Mn = 2.56. PTB7-1 obtained from Pd2(dba)2 catalyst showed lower molecular weight of Mn = 41k than that of PTB7-3 obtained from Pd(PPh3)4 catalyst (Mn = 109k). The commercial polymers PTB7-2 and PTB7-4 showed relatively high molecular weight of Mn = 83k and very high molecular weight of Mn = 216k, respectively.
Polymer purification was carried out using the same approach adopted in an earlier paper.14 Impurity levels were measured using elemental analysis and content of the main impurities (P, B and Pd) were measured to be <0.3% (the measureable limit for our equipment).
Dark EPR spectra were measured on both initial polymer powders and films deposited on Si substrate. Prior to measurements all samples were kept under nitrogen atmosphere and in dark conditions. EPR spectra of PTB7 powders were recorded at the following instrumental conditions: non-saturating microwave power PMW = 0.2 mW, 100 kHz magnetic field modulation amplitude Amod = 0.03 mT, receiver gain = 2 × 105, number of coherent acquisitions naqc = 25. In Fig. 1(a) peak-to-peak intensities of these spectra are normalized per 1 mg and, thus, reflect real spin densities in the samples under study. Due to both smaller effective weights of the polymer films as well as strong non-resonance absorption of microwave by the silicon substrate, the PTB7 film samples provide significantly less intensive EPR signals. For better accuracy of spin density estimation EPR spectra of films as well as the corresponding detonation nanodiamond reference sample, attached to the same substrate, were recorded in partially saturating (PMW = 5 mW) and over-modulated (Amod = 0.3 mT) conditions with receiver gain = 2 × 106 and naqc = 100.
Devices were checked for initial performance prior to lifetime testing. The measurement system used to characterize the devices consisted of a Newport solar simulator with 100 mW cm−2 AM1.5G output calibrated using a silicon reference cell from RERA in the Netherlands. The open circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF) and PCE values are averaged from six cells (for each polymer). The devices were then placed under a halogen light source of 1 sun (calibrated by the silicon reference cell) for light soaking. Devices were kept at open circuit in between measurements and I–V measurements were made every 20 minutes, with more details of the procedure reported elsewhere.18 This was conducted in accordance with ISOS-L-2 standards.19 Devices were measured until the PCE reached 10% of its initial value.
Polymer | M n (PDI) | λ max [nm] | E g [eV] | HOMOd [eV] | LUMOe [eV] | |
---|---|---|---|---|---|---|
Solution | Film | |||||
a Measured by UV-vis light spectroscopy. b Measured by cyclic voltammetry (CV). c Optical band gap. d Highest occupied molecular orbital, HOMO = −(4.8 + Epa-onset − EFc). Half wave potential of ferrocene, EFc (=0.845 V vs. AgCl/Ag), was measured in MeCN solution. e Lowest occupied molecular orbital, LUMO = −(4.8 + Epc-onset − EFc). | ||||||
PTB7-1 | 41![]() |
666 | 668 | 1.67 | −5.08 | −2.95 |
PTB7-2 | 83![]() |
671 | 670 | 1.66 | −5.12 | −3.01 |
PTB7-3 | 109![]() |
647 | 640 | 1.57 | −5.18 | −3.11 |
PTB7-4 | 215![]() |
671 | 675 | 1.63 | −5.19 | −3.10 |
The absorption spectra of the PTB7 samples dissolved in THF and in thin film form are presented in Fig. S2† (see also Table 1). The absorption characteristics have some minor differences and this can be explained from the GPC data (see Fig. S1†). In particular, PTB7-3, has a small peak at the retention time around 18 min is seen, which corresponds to Mn ≈ 800k. This indicates that a high molecular weight fraction is present and this is likely to contribute to form the shoulder absorption around 750 nm. In any case, the variation in optical performance of the polymers is not significant enough to cause large variations in solar cell performance. The optical band gap (Eg) was estimated from the onset of the absorption spectra in thin film. PTB7-3 shows the lowest λmax and exhibits a broad absorption peak; therefore, it gives lower Eg of 1.57 eV than others (Eg = 1.66–1.67 eV). This is desirable from a photovoltaic point of view as it improves spectral match between the AM1.5G sunlight and the polymer absorption spectra. HOMO and LUMO levels were estimated from absorption and electrochemical measurements. For donor materials in OPV, deeper HOMO levels are preferred20 and based on this assertion, PTB7-4 is likely to be the best candidate, however, the data indicate that only a moderate difference in solar cell performance should be observed between PTB7-1–4 samples.
Electron spin–lattice and spin–spin relaxation times were estimated from the saturation curves obtained for powder samples, and found to be the same (within ±30% estimation error) for all polymers under study: Tsl = 1.4 × 10−6 s and Tss = 1.4 × 10−8 s, respectively. This relatively short Tsl value found is similar to previous reports for undoped polymers that have been exposed to air at ambient temperature.21 EPR spectra of film samples being recorded under alternative instrumental conditions (see Methods) provide the same EPR patterns as their initial powders (spectra not shown).
Fig. 1(b) and Table 2 summarize densities of paramagnetic defects (positive polarons) in the samples, Ns, and evidence that in both powders and films the Ns density correlates inversely with the molecular weight: as lower is Mn as higher is Ns. Spin densities in polymers and films are found to be practically the same (within the experimental error) which indicates the film preparation and deposition do not create additional defects in polymer structures. This means that PTB7 samples do not incorporate spin-active units in their monomer structure and the observed radical defects present mostly on the polymer chain ends. Moreover, all results obtained on films correlated well with the OPV cell performance (see Table 2 and discussion below).
Polymer samples | M n (Mw/Mn) | Defect spin density Nsa (spin per g) | V oc (V) | J sc (mA cm−2) | Fill factor | PCE (%) | t 1/2 (hours) | |
---|---|---|---|---|---|---|---|---|
Powder | Film | |||||||
a Errors in spin density determination do not exceed ±15%. b Open circuit voltage. c Short-circuit current density. d Half lifetime of the OPV under 1 sun irradiation. | ||||||||
PTB7-1 | 41![]() |
2.9 × 1017 | 2.5 × 1017 | 0.73 ± 0.01 | 10.1 ± 0.2 | 0.55 ± 0.01 | 4.02 ± 0.10 | 29.8 |
PTB7-2 | 83![]() |
1.2 × 1017 | 1.3 × 1017 | 0.73 ± 0.01 | 12.1 ± 0.2 | 0.56 ± 0.01 | 4.92 ± 0.05 | 31.5 |
PTB7-3 | 108![]() |
1.3 × 1017 | 8.0 × 1016 | 0.71 ± 0.01 | 13.9 ± 1.0 | 0.62 ± 0.03 | 6.04 ± 0.3 | 56.5 |
PTB7-4 | 215![]() |
3.6 × 1016 | 6.5 × 1016 | 0.70 ± 0.01 | 15.7 ± 0.5 | 0.61 ± 0.02 | 6.71 ± 0.29 | 98.8 |
![]() | ||
Fig. 2 J–V characteristics of OPV devices fabricated with polymers PTB7-1–4 under AM1.5G illumination. |
Second, this trend can also be equated to differences in the morphology of the photoactive layer. Lower Mn samples have smaller donor/acceptor interface area and thus provide poorer exciton dissociation efficiency. This statement is supported by AFM images of the photoactive layers (Fig. 3a, c, e and g). The highest molecular weight polymer i.e.PTB7-4 (blended with C71-PCBM) yielded the smoothest surface with Root Mean Square (RMS) roughness = 2.0 nm (Fig. 3(g)) whilst the low molecular weight one i.e.PTB7-1 showed the highest: 7.2 nm (Fig. 3(a)). The PTB7-2 and PTB7-3 polymer with moderate Mn had an intermediate roughness of 5.9 nm and 4.9 nm, respectively. The discrete surface roughness is likely due to chain relaxation. Indeed, high molecular weight polymer, with high Tg, is difficult to move. Meanwhile, low molecular weight polymers, with lower Tg, are more mobile. Therefore they could relax from an as-spun ultra-smooth surface to a relatively rough surface during annealing and/or storage. Such relaxation could lead to undesirable phase separation21 and the increased film roughness could cause the decrease in the Jsc.22
Difference in morphological degradation in the active layer of the cells (Fig. 3) may be among the reasons for the variation in Jsc degradation rates. Considering the topography before and after light soaking, it is clear that the most significant changes were observed for the lower molecular weight polymers. The RMS rose from 7.2 nm to 8.98 nm for PTB7-1 (Fig. 3a and b) and from 5.9 nm to 7.8 nm for PTB7-2 (Fig. 3c and d). At the same time, the PTB7-3 and PTB7-4 showed moderate increases of less than 0.5 nm (Fig. 3e–h). As discussed above, a drastic change of the photoactive layer morphology could lead to undesirable donor–acceptor interfaces and could be a reason for the rapid drop in Jsc in PTB7-1 and PTB7-2 during light soaking. It is clear from the AFM data that a morphological change occurred during ageing experiments, leading to potential phase segregation. At the same time, the density of recombination centers could increase. For the moment, it is difficult to distinguish which mechanism is more dominant and leading to the fall off in Jsc in Fig. 4.
On the other hand, the observed correlation between the cell PCE and lifetime suggests a general trend: the more efficient cells should be more stable. Indeed, photoinduced degradation of PV devices under operation is caused by sunlight. The more incident sunlight power converted to electrical power, the less remains for activation of degradation. During operation of polymer PVs, some fraction of the excitons generated under sunlight undergo non-radiative quenching leading to the photochemical degradation.11 Efficient PV conversion due to the donor–acceptor charge transfer suppresses this process. Indeed, adding acceptor (PCBM) to the polymer blends was found to slow considerably the polymer photochemical degradation, presumably via sub nanosecond quenching of the reactive excited state by forming a lower energy charge transfer complex.23,24 On the other hand, in small-molecule fullerene OPV cells, photochemical degradation of fullerenes was reported to occur by the same mechanism in both bi-layer and BHJ devices.25 However, the process in the BHJ is slower due to more rapid exciton quenching by charge-transfer.
At the present stage of research, it is probably too early to formulate this conclusion as a general rule. However, it should be taken into account for future development of various novel PV technologies (including OPV, and perovskite-based cells) and checked in further research.
The reported findings constitute a basis for further research that should include on one hand more detailed EPR study of encapsulated and non-encapsulated blend films in as produced state as well as after various doses of solar illumination, and on other hand analyses extended to other polymers.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ta00721j |
‡ These authors contributed equally to the writing of this paper. |
This journal is © The Royal Society of Chemistry 2016 |