Cs0.15FA0.85PbI3 perovskite solar cells for concentrator photovoltaic applications

Joel Troughton , Nicola Gasparini and Derya Baran *
King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC), Physical Sciences and Engineering Division (PSE), Thuwal, 23955-6900, Saudi Arabia. E-mail: derya.baran@kaust.edu.sa

Received 13th June 2018 , Accepted 20th August 2018

First published on 24th August 2018


Recently developed, highly stable perovskite materials show promise for use in concentrator photovoltaics where the illumination intensity far exceeds standard test conditions. Here, we demonstrate solar cell devices employing different perovskite absorber layers featuring balanced charge generation and extraction characteristics at high light intensities greater than 10 suns. Using a mixed cesium-formamidinium perovskite, we are able to achieve over 18% PCE at 1 sun and 16% PCE at 13 suns with negligable performance loss after several hours of high intensity light soaking.


Perovskite solar cells (PSCs) based on organic and inorganic components have exhibited an extraordinary rise in power conversion efficiency (PCE) in the past few years. With current PCEs in excess of 22% (ref. 1) and device stability regularly exceeding 1000 hours with little degradation,2–4 perovskite solar cells seem a promising next-generation solar technology. Despite this promise, many challenges remain before commercialisation can become a reality: issues such as current–voltage scan hysteresis call into question the validity of reported efficiencies5 and stability must be further improved.6–8 One significant route to improving both performance and stability, in particular, is cation substitution within the perovskite crystal structure. The perovskite methylammonium lead triiodide (CH3NH3PbI3 or MAPbI3) has historically been a popular choice of absorber material owing to its ease of processing and phase stability at room temperature. However, there remain issues with this material such as a phase transition at potential operational temperatures,9 as well as a sensitivity to moisture.10 An alternative to MAPbI3 perovskite is formamidinium lead triiodide11–13 (HC(NH2)2PbI3 or FAPbI3) which possesses a wider band gap and superior phase stability at elevated temperatures compared to MAPbI3.11,14,15 A crucial disadvantage of FAPbI3 is its phase instability at room temperature, allowing it to crystallise into either a preferable photoactive α-phase, or an undesirable photoinactive γ-phase.9,11 This instability has been resolved, in part, by incorporating cesium into the perovskite structure. With an ionic radius smaller than that of the MA and FA cations, the inclusion of Cs in the perovskite lattice stabilises the desirable α-phase by reducing the perovskite's Goldschmidt tolerance factor, permitting a stable α-phase to exist at temperatures in excess of 230 °C.13,16 FA-containing perovskites are now commonly reported with Cs components ranging between 5–20% molarity with respect to the other cation used.13,16–19

Beyond the optimisation of light-absorbing and charge-extracting layers within the PSC, there exists other ways of increasing device performance beyond the Shockley–Queisser limit of 30% PCE for a 1.6 eV single junction:20 One option to bypass this limit is to increase irradiation intensity beyond the ASTM AM1.5G standard 100 mW cm−2 using an array of mirrors or lenses to focus sunlight onto a photovoltaic device. Such devices are known as concentrator photovoltaics (CPV). For materials with high charge carrier concentrations and low defect densities, PCEs can be retained at elevated light levels thanks to an increase in open circuit-voltage (VOC) and a preservation of fill factor. PSCs have demonstrated exceptional performance under irradiance levels at and below the ASTM standard 100 mW cm−2 owing to the low trap density and high carrier diffusion lengths in many widely-used perovskites, which further increase under reduced illumination.1,13,21–24 However, investigation of the perovskite absorber material's performance under higher light intensities has received little attention until recently.25–27 As light intensity is increased, the rate of charge carrier generation within a solar cell too increases: for these carriers to be efficiently extracted under high light intensities, the absorber material and charge extraction layers must be capable of conveying photocurrents many times greater than those experienced under typical ‘1 sun’ operating conditions. A recent article by Lin and coworkers27 predicts carrier concentration within perovskite absorber films to be high enough so as not to be a performance barrier in CPV applications beyond 100 suns of illumination. The relative balance of charge generation, recombination and extraction rates were found to be key parameters in determining performance at high solar concentration. Although it is noted that material stability is likely to be a key challenge for such an application of perovskite materials.

In this initial study, we investigate the suitability of MAPbI3 and Cs0.15FA0.85PbI3 perovskite light absorbers for use in concentrator photovoltaics. In this case, simple ‘iodine-only’ perovskite compositions are used instead of commonly reported iodine–bromine mixes to exclude the influence of halide segregation under illumination.28 While such segregation is inhibited in Cs-stabilised FA blends, the same cannot be said for MA-only compositions which would otherwise be used as control devices.29 We measure current–voltage curves, charge extraction behaviour, transient photovoltage (TPV) and photocurrent (TPC) measurements, as well as device stability over light intensities ranging between 2 mW cm−2 (0.02 suns) and 1300 mW cm−2 (13 suns). We find that the preservation of fill factor at high light intensities permits the performance of Cs0.15FA0.85PbI3 devices to be maintained over a wide range of illumination levels: 18% at 1 sun and 16% at 13 suns. This is compared to 17% at 1 sun and below 10% at 13 suns for the more commonly used MAPbI3 perovskite. In addition, while stability trends between the two perovskites appear similar under 1 sun illumination, at higher light intensities MAPbI3 based devices begin to rapidly degrade as a result of perovskite film photo-bleaching. By comparison, the Cs0.15FA0.85PbI3 suffers no appreciable degradation in efficiency, making the material a potential choice for use in CPV applications.

Fig. 1a plots the current–voltage characteristics of two planar n–i–p structured perovskite solar cells with different absorber layers measured under AM1.5G conditions. The general structure of the device is comprised as follows: glass/ITO/SnO2/PC60BM/perovskite/spiro-OMeTAD/Au where the perovskite employed is either MAPbI3 or Cs0.15FA0.85PbI3. The latter perovskite was selected because of its reportedly high thermal stability16 – a quality that is likely to be required in CPV applications. A full account of the experimental procedures for device fabrication may be found in the ESI. We note differences in VOC between the two perovskite materials owing to a reduced band gap in the case of the Cs0.15FA0.85PbI3 perovskite. This is further illustrated in Fig. 1b, showing differences in the position of the absorption edge: we calculate the band gap of the MAPbI3 and Cs0.15FA0.85PbI3 perovskite to be 1.61 eV and 1.57 eV respectively. This narrowing of the band gap is also responsible for the Cs0.15FA0.85PbI3 perovskite's higher short-circuit current density (JSC) compared to MAPbI3 devices. The degree of hysteresis present in these devices is low, with stabilised (Fig. 1a sub figure) and JV derived PCE values in close agreement. A plot of representative forward and reverse JV scans is shown in Fig. S1, indicating less than 3% variation in PCE between scan directions, along with a table of device parameters (Table S1) and statistical spread (Fig. S2). Overall PCEs from the JV sweeps are 17.8% and 18.0% respectively for MAPbI3 and Cs0.15FA0.85PbI3 perovskites, indicating very similar performance at 1 sun illumination. In order to study the impact of different light levels on the photovoltaic characteristics of different perovskite absorber materials, we consider the evolution of JSC, fill factor and VOC as a function of light intensity. Fig. 2 illustrates key photovoltaic parameters for both MAPbI3 and Cs0.15FA0.85PbI3 perovskites at laser-induced light levels ranging between 2 mW cm−2 (0.02 suns) and 1300 mW cm−2 (13 suns). Fig. 2a shows a trend towards higher PCEs over a wider range of illumination intensities in the case of the Cs0.15FA0.85PbI3 perovskite when compared to MAPbI3. Crucially, PCEs around 18% are maintained in the Cs0.15FA0.85PbI3 device between 0.5 and 3 suns illumination, whereas the MA-containing device shows a far more severe efficiency dependency on light intensity. We observe, in Fig. 2b, a linear relationship between JSC and illumination intensity (slope = 1.00), implying weak or entirely absent bimolecular recombination in both perovskite devices at short-circuit conditions. The trend of VOC as a function of light intensity (Fig. 2c) provides a direct insight into the role of trap-assisted recombination within the different perovskite films:30,31 entirely trap-assisted recombination may be identified by a slope of 2kT/q, while a slope in the order of 1kT/q is indicative of purely bimolecular recombination under open-circuit conditions.32,33 We calculate the slope for the Cs0.15FA0.85PbI3 device to be 1.2kT/q, compared to 1.6kT/q in the case of the MAPbI3 device, indicating the presence of fewer trap states within the FA-containing perovskite and a general reduction in trap-assisted recombination compared to the more conventional MA-containing device. While differences in VOC with light intensity between the two perovskite systems certainly contribute to variation in overall device performance, the most significant factor affecting PCE is fill factor. As depicted in Fig. 2d, the MAPbI3 device shows a fill factor decrease from 0.74 at 10 mW cm−2 (0.1 suns) to 0.41 at 1000 mW cm−2 (10 suns), whereas the Cs0.15FA0.85PbI3 device delivers fill factor values between 0.75 and 0.60 over the same range. These behaviours are linked to the different capability of various perovskite materials to support balanced charge generation and extraction over a range of light intensities. It is also worth noting that the decrease in fill factor with light intensity may be a consequence of charge extraction limitations within the charge selective layers: SnO2, PCBM and spiro-OMeTAD. The investigation of these layers in the context of CPV, while critical to the success of perovskite CPV, are beyond the scope of this preliminary study.


image file: c8ta05639k-f1.tif
Fig. 1 (a) Reverse current–voltage sweeps of perovskite solar cells with different absorber layers. Subplot portrays the stabilized efficiency of the devices at VMPP (asterisks) as a function of time. (b) External quantum efficiency spectra and integrated current density for perovskite solar cells.

image file: c8ta05639k-f2.tif
Fig. 2 (a) PCE, (b) JSC, (c) VOC, (d) fill factor as a function of light intensity for different perovskite absorber layers.

To elucidate the fill factor reduction in MAPbI3 devices at high light intensity, we measure the photocurrent density (Jph) as a function of effective voltage (Veff), as shown in Fig. 3a and b. Jph is defined as Jph = JlJd where Jl and Jd are the current densities in light and dark conditions respectively. Veff is given by Veff = V0V where V0 is the compensation voltage defined as Jph(V0) = 0, and V is the applied voltage. Fig. 3a shows device photocurrent under 1 sun conditions: in this case, the response of both MAPbI3 and Cs0.15FA0.85PbI3 devices is remarkably similar, displaying a saturation of photocurrent at 0.50 V and 0.51 V respectively (Vsat), indicating similar charge extraction behaviour in the two perovskite systems. At 13 suns (Fig. 3b), the Cs0.15FA0.85PbI3 perovskite device features a similar Jphvs. Veff trend to the one observed at 1 sun, with a Vsat around 0.54 V. However, at high light levels, the MAPbI3 device exhibits a stronger photocurrent dependence on the electric field, yielding Vsat around 1.22 V. This indicates a charge extraction limitation in MAPbI3 film at high light intensity, calling into question the material's suitability for CPV applications. A quantification of the charge generation rate at maximum power point (Gmpp) may be found in Fig. S3. At 1 sun, we calculate a Gmpp for MAPbI3 and Cs0.15FA0.85PbI3 of 80% and 85% respectively. However, at 13 suns, Gmpp of MAPbI3 devices decreases to 60% whereas the optimized cells maintain a Gmpp value of 84%. Using TPV and TPC techniques, we plot charge carrier lifetime, τ as well as VOC against charge density, n for both perovskites in Fig. 3c. In agreement with our previous observations, we find similar carrier lifetimes under 1 sun conditions for both studied perovskites. These lifetimes diverge at higher illumination intensities, with Cs0.15FA0.85PbI3 maintaining notably longer lifetimes at 13 suns compared to MAPbI3. This finding is corroborated by our previous observations both in the divergence in fill factor in Fig. 2d, as well as in the lower kT/q observed in Fig. 2c for Cs0.15FA0.85PbI3 compared to MAPbI3. We identify recombination orders of 2.41 and 2.02 for MAPbI3 and Cs0.15FA0.85PbI3 respectively from the slope of the charge carrier lifetime trend. A recombination order (R) of 2, as in our Cs0.15FA0.85PbI3 device, is indicative of a device exhibiting almost entirely bimolecular recombination at open-circuit voltage conditions.34,35 The R = 2.41 in the MAPbI3 cell therefore displays a higher defect density leading to increased trapping at higher light intensities.


image file: c8ta05639k-f3.tif
Fig. 3 Photo-generated current density, Jphvs. effective voltage (V0V), Veff at 100 mW cm−2 (a) and at 1300 mW cm−2 (b) obtained from JV curves in reverse bias. (c) Shows charge carrier lifetime, τ and VOC as a function of charge density, n for MAPbI3 and Cs0.15FA0.85PbI3 perovskite solar cells, obtained using transient photovoltage (TPV) and transient photocurrent (TPC) measurements at open-circuit.

One key challenge to overcome in order to prove the viability of perovskite solar cell technology in CPV applications is that of stability. It has been reported that MAPbI3 is inherently unstable under certain conditions such as high humidity, temperature and ultraviolet radiation.7,10 In addition, there are reported structural changes for this perovskite between 54–57 °C (ref. 36) which may accelerate degradation. However there are also reports of MAPbI3 perovskite remaining stable at operational temperatures in excess of 80 °C,37–39 indicating structural stability may not be the prime factor in determining device stability. CsxFA1−xPb(IyBr1−y)3 perovskites, on the other hand, have been reported in extremely stable devices thanks in part to the exclusion of the MA cation and the phase stabilisation by Cs.16,18,40–42 We present, in Fig. 4, JV stability data at both 1 sun and 13 suns of light intensity for both perovskites. Fig. 4a shows 950 hours of stability data at 1 sun, demonstrating a high degree of stability for both compositions: following an initial drop in fill factor during the first 100 hours, performance remains relatively constant for the remainder of the experiment. This stability is attributed, in part, to the favourable environmental conditions imposed on devices: only 1 sun of illumination, combined with a nitrogen atmosphere and relatively low cell temperature (40 °C). After 950 hours of light soaking under open-circuit conditions, the Cs0.15FA0.85PbI3 device retains 83% of its original PCE, while the MAPbI3 device retains 69%. When moving to higher light intensities, the trends of the two perovskite absorbers become more divergent. Fig. 4b shows devices measured periodically under 13 suns of laser irradiation. This measurement may be interpreted either as accelerated lifetime testing for lower light conditions, or real-time testing at high light conditions. Similar to the testing at 1 sun, VOC remains consistent for both perovskites over a period of several hours: this VOC stability excludes the possibility of any significant device heating under the high illumination, as such heating would narrow the perovskite bandgap and manifest as a reduction in VOC.43 Nevertheless, we observe a deterioration in the MAPbI3 cell's JSC and fill factor over a period of hours subjected to this light intensity. This decrease is attributed to photo-bleaching of the perovskite film, as investigated by Nie and coworkers.44 In this instance, formation of light-induced, localised polarons within the band gap exist alongside photo-generated free carriers; these defects dominate the photocurrent of the device, causing a reduction in JSC and fill factor under light soaking. This photobleaching is accompanied by a slight reduction in light absorption, in the case of MAPbI3 perovskite, with no such reduction seen in the mixed cation perovskite (Fig. S4). Interestingly, we observe no significant crystallographic changes during the course of this light soaking, with no evolution of any PbI2 peaks in the XRD patterns for either MAPbI3 or Cs0.15FA0.85PbI3 perovskites (Fig. S5). The Cs0.15FA0.85PbI3 device shows superior photo-fastness and suffers no degradation of JSC after nearly 6 hours of exposure to 1300 mW cm−2 light. This finding further supports the notion that the FA-containing perovskite chemistry is more resistant to defect formation, despite the fact that the device is generating and extracting charges in excess of 300 mA cm−2 at these illumination levels.


image file: c8ta05639k-f4.tif
Fig. 4 Stability of JV parameters for Cs0.15FA0.85PbI3 and MAPbI3 solar cells under 1 sun equivalent conditions using a LED light source (a) and 13 sun equivalent conditions using a laser light source (b).

Conclusions

In summary, we present an initial study into the suitability of MAPbI3 and Cs0.15FA0.85PbI3 perovskites for use in concentrated photovoltaic systems. Using electrical characterisation techniques, we determine Cs0.15FA0.85PbI3 to be a more suitable material for exposure to high irradiation intensities: this is the result of fewer traps within the material compared to MAPbI3, leading to nearly all recombination being bimolecular in nature, and allowing high fill factors at light levels greater than 1 sun, with 16% PCE still attainable at 13 suns with 60% fill factor. In addition, the stability of the FA-containing material shows no appreciable drop in performance despite 6 hours of illumination at 13 equivalent suns. This study represents a first step towards the realisation of perovskites in CPV. However, for this to come to pass, the development of thermally stable charge selective layers must be a priority to ensure all layers within the device are sufficiently resistant to the high temperatures incurred by concentrated sunlight.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

The authors acknowledge KAUST Solar Centre Competitive Fund (CCF) for financial support.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ta05639k

This journal is © The Royal Society of Chemistry 2018