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Photocurrent Spectroscopy of Perovskite Layers and Solar Cells: A Sensitive Probe of Material Degradation Jakub Holovský, Stefaan De Wolf, Jérémie Werner, Zdenek Remes, Martin Müller, Neda Neykova, Martin Ledinsky, Ladislava #erná, Pavel Hrzina, Philipp Löper, Björn Niesen, and Christophe Ballif J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02854 • Publication Date (Web): 25 Jan 2017 Downloaded from http://pubs.acs.org on January 25, 2017

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The Journal of Physical Chemistry Letters

Photocurrent Spectroscopy of Perovskite Layers and Solar Cells: A Sensitive Probe of Material Degradation Jakub Holovský1,2,*, Stefaan De Wolf 3, Jérémie Werner4, Zdeněk Remeš1, Martin Müller1, Neda Neykova1, Martin Ledinský1, Ladislava Černá2, Pavel Hrzina2, Philipp Löper4, Bjoern Niesen4, and Christophe Ballif 4 1

Institute of Physics, Czech Academy of Sciences, v. v. i., Cukrovarnická 10, 162 00 Prague,

Czech Republic 2

Czech Technical University in Prague, Faculty of Electrical Engineering, Technická 2, 166 27

Prague, Czech Republic 3

King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC),

Thuwal, 23955-6900, Saudi Arabia 4

Photovoltaics and Thin-Film Electronics Laboratory, Institute of Microengineering (IMT),

École Polytechnique Fédérale de Lausanne (EPFL), Rue de la Maladière 71b, Neuchâtel, 2000, Switzerland

*corresponding author, e-mail: [email protected], +00420 220 318 516

1 ACS Paragon Plus Environment


Abstract Optical absorptance spectroscopy of polycrystalline CH3NH3PbI3 films usually indicates the presence of a PbI2 phase, either as a preparation residue or due to film degradation, but gives no insight on how this may affect electrical properties. Here, we apply photocurrent spectroscopy to both perovskite solar cells and coplanar-contacted layers at various stages of degradation. In both cases, we find that the presence of a PbI2 phase restricts charge-carrier transport, suggesting that PbI2 encapsulates CH3NH3PbI3 grains. We also find that PbI2 injects holes into the CH3NH3PbI3 grains, increasing the apparent photosensitivity of PbI2. This phenomenon, known as modulation doping, is absent in the photocurrent spectra of solar cells, where holes and electrons have to be collected in pairs. This interpretation provides insights into the photogeneration and carrier transport in dual-phase perovskites.

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Organic-inorganic halide perovskites have quickly attracted enormous attention thanks to their well-suited properties as photovoltaic absorbers and their facile fabrication, triggering unprecedented progress in reported energy-conversion efficiencies.1,2 Their wide bandgap and high operating voltage make them especially attractive for tandem applications, on crystalline silicon or copper indium gallium selenide solar cells, aiming to better exploit the solar spectrum.3–5 The excellent optoelectronic properties of perovskites may originate from the spatial separation of photogenerated electrons and holes, slowing down bi-molecular recombination,6,7 their low trap-state density,8,9 evidenced by a very steep absorption edge,10,11 their photon recycling12 and the direct/indirect13 character of their bandgap. However, in humid-ambient conditions, CH3NH3PbI3 perovskite layers are unstable, tending to decompose into PbI2,6,14,15 such that the actual film often consists of two phases of pure CH3NH3PbI3 and PbI2.16–20 These phases explain several effects observed in absorptance spectroscopy10,21,22 and Raman spectroscopy,15 where the measured overall spectra are accurately enough described as a simple summation of its components. Recently, we showed that humidity- and illumination-induced bleaching of CH3NH3PbI3 can be tracked also by absorption spectroscopy: As the absorption spectrum bleaches between 1.5 eV and 2.4 eV, the initial absorption edge at 1.5 eV vanishes such that eventually the absorption spectrum features an edge close to 2.4 eV, corresponding to the bandgap of PbI2.10 To measure absorptance spectra with high sensitivity, researchers can use photothermal deflection spectroscopy (PDS),23 which measures the absorptance transformed into heat down to 0.01%, or Fourier-transform photocurrent spectroscopy (FTPS),24 which is even more sensitive but affected by electric transport phenomena. FTPS is a spectrally resolved measurement, that – when used on solar cells – is almost equivalent to external quantum efficiency (EQE). FTPS uses a constant level of modulated photocurrent at a relatively high modulation frequency of light (around 2 kHz) and is optimized 3 ACS Paragon Plus Environment


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towards high sensitivity. Note that both PDS and FTPS are equally affected by the loss due to efficient photoluminescence. In this paper, ‘absorptance’ will always imply the optical absorptance with photoluminescence subtracted. FTPS depends almost solely on the excess carrier density and is therefore proportional to the product of carrier mobility (µ) and lifetime (τ) as approximately expressed by equation25 (1): FTPS = cAPDS τ ( µ e + µ h )  eff

(1)

In equation (1), APDS stands for the optical absorptance in the semiconductor material measured by PDS, µe and µh are the electron and hole mobilities, respectively, and c is a proportionality constant, including effects such as the intensity of illumination, electric field and sample geometry. The product µ ×τ scales with the free-carrier diffusion length L through L = kTµτ / e , where k is Boltzmann’s constant, T is the absolute temperature and e is the elemental charge. The FTPS signal is thus affected by both absorptance of the material as well as its free-carrier mobility and lifetime. When applied on solar cells, the presence of carrier-selective contacts (such as electron and hole transport materials) implies that without external voltage bias, both electrons and holes are collected at the respective contacts, i.e. a photovoltaic photocurrent (FTPScell) is measured, essentially giving similar information as a measurement of the external quantum efficiency. The current flows transversely through the sample and the carriers travel over distances of a few hundreds of nanometers (i.e. the film thickness). Alternatively, when layers on glass are measured, an external voltage bias is applied to the layer featuring now evaporated coplanar contacts which are spaced by a much larger distance than the carrier diffusion length. In this case, a photoresistor photocurrent (FTPSlayer) is measured flowing laterally over relatively large distances, contributed to by both types of carriers depending on their mobility. In this way, photocurrent spectroscopy usually offers a robust method to 4 ACS Paragon Plus Environment

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characterize photovoltaic absorbers, independent of the precise device architecture. Here, we exploit these different measurement methods to give insight in the electrical transport in polycrystalline CH3NH3PbI3 perovskites, which may also aid in the understanding of the phenomena related to degradation of this material on the microscopic level.

Samples of approximately 400-nm-thick CH3NH3PbI3 perovskite layers on glass were prepared either by the two-step sequential deposition process26 or by a one-step method.27 In parallel, semitransparent perovskite solar cells were also prepared with these absorber layers. More details about the fabrication methods are given in the supporting information (SI). Humidity- and light-induced degradation under illumination in ambient air (relative humidity 30% – 50%) was studied by PDS and FTPS. Samples were otherwise stored in desiccated dark boxes to reduce undesired degradation. J-V curves of the solar cells were recorded to track the degradation of device performance. These devices were developed as top cells for tandem applications5. Hence, thanks to the absence of an opaque back reflector in these devices, transmittance measurements could be done on all samples, including the solar cells. Firstly, we analyze perovskite layers on glass prepared by the two-step process. Gradual degradation was obtained by exposing the layers to 10-minute illumination steps under ambient conditions, followed by photoresistor photocurrent (FTPSlayer) and transmittance measurements. To measure the FTPSlayer spectra, we applied a positive voltage bias of 9 V between two coplanar 6-mm-long Al electrodes with 1.5 mm spacing, evaporated directly on top of the perovskite layer. To highlight the shape evolution, the FTPSlayer spectra shown in Fig. 1a are normalized at 2.5 eV (spectra before normalization are given in Fig. S3 in SI). In addition, after every three illumination steps (i.e. 30 minutes), a PDS measurement was taken (see Fig. 1b). Between PDS measurements, the sample was stored in N2 (between one and several days). For partially 5 ACS Paragon Plus Environment


degraded samples, the FTPSlayer as well as the APDS spectra showed step-like curves featuring two absorption edges. It is now well established that below the step at a photon energy of 2.4 eV, the absorption of pure CH3NH3PbI3 is observed, while above this step the absorption of pure CH3NH3PbI3 competes with that of the PbI2 phase.10 The material decomposition can thus be tracked over time by taking the ratio of absorptance (or transmittance) below and above 2.4 eV. We plot the relative evolution of this ratio as a function of illumination time in (Fig. 1c). As observed in transmittance (Fig. S2 in SI) several changes occur that correlate well with the

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Testing this hypothesis to our experimental results, two observations can be made: Firstly, the drop in the FTPSlayer signal is faster than the change in absorptance. For this, let us focus on the CH3NH3PbI3 phase and compare the lines “APDS> τ11 indicates

that the positive term with A2 outweighs the negative term with A2 in equation (4) and the equation now successfully describes the trend of relatively higher photoresistor photocurrent above 2.4 eV. h FTPSlayer (> 2.4eV ) = c[( APDS − A2 )τ 11 (µ1e + µ1h ) + A2τ 21µ1h X 21 ]

(4)

We suggest that this quick charge-carrier transfer from the PbI2 phase into the CH3NH3PbI3 phase might explain the alleged effect of slow (0.4 ps rate) hot-hole cooling observed by transient absorption spectroscopy.28 We presume that this charge-carrier transfer is a self-limiting process due to the induced electric field (band bending at the interface) that may help electrons to become thermally excited from PbI2 phase into CH3NH3PbI3. Additionally to the two-step material, the layers of material prepared by the one-step method were also analyzed, showing qualitatively the same behavior, see Fig. S4 in SI. To better understand the photocurrent phenomena, we also performed photovoltaic photocurrent FTPScell measurements on a solar cell. The solar cell was in a short circuit regime (no external bias voltage), requiring that both electron and hole generated by each absorbed photon should be extracted from the absorber to register a signal. In this case, the photocurrent is given by the


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performance-limiting carrier. We used a solar cell in which a small bleached spot was intentionally selected to study the interface between the CH3NH3PbI3 and PbI2 phases. For this, we focused the FTPS beam into this spot and measured the FTPS signal and transmittance (see Fig. 3). For comparison, the spectra next to the spot and at its edge were also taken. From the transmittance in Fig. 3, we can clearly see the reduction of the CH3NH3PbI3 phase and creation of the PbI2 phase. From the FTPScell, we see an overall signal loss, similar to that for the FTPSlayer. However, when a more degraded region is probed, the spectrum of the FTPScell reveals a loss above 2.4 eV, a loss that was expected by equation (3) but which contrasts with the model of FTPSlayer in equation (4). This difference is now given by the fact that in solar cell one type of carrier is limiting the transport process. In equation (4), there are fewer contributions from terms with than terms with . Also, because holes are known to be more efficiently extracted30,43 from cells, the limiting contribution must come from electrons. In this case, the equation for photovoltaic photocurrent will be

FTPScell (> 2.4eV ) = c( APDS − A2 )τ 11µ1e

(5)

Compared to the PDS data, equation (5) also gives a more pronounced drop in signal above 2.4eV than below. This is because above 2.4eV, the absorptance in the CH3NH3PbI3 phase, A1, competes with absorptance A2. The effect of inverting the step around 2.4eV was recently also observed when going from a photovoltaic to a photoresistor regime of a CH3NH3PbI3 photodetector device.44



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d) 10

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Figure 3: FTPS spectra and 1–T (T is transmittance) measured with 0.2 mm spatial resolution on a solar cell. Inset a) shows a photograph of a degraded spot close to the border of a solar cell. The curves and related insets b)-d) show measurements of (from top to bottom): pristine region, the transition region, degraded region.

After gaining a basic understanding of the photocurrent generated from the perovskite material itself, we now turn our attention to the correlation between solar cell performance and photovoltaic photocurrent spectroscopy. During cell ageing, FTPScell, transmittance and J-V curves were measured on the full area of the device (outside the spot in Fig. 3). In Fig. 4a, we can see the evolution of FTPScell spectra measured during degradation. However, within the technique’s sensitivity, we did not observe a clear indication of a step around 2.4 eV, consistently with the transmittance measurement shown in Fig. 4b no measureable compositional changes happened to the overall area (except in the spot seen in Fig. 3.). Similarly as in the case of layer on glass (Fig. 1c) the trends of compositional changes are much slower than the loss of the photocurrent. Possibly, additional mechanisms may happen in the encapsulated cell as there might be organic products of degradation, dwelling together with PbI2, as another conduction barrier.45 Looking at the spectral shape evolution, we see that the loss in photocurrent is less pronounced for the high-energy photons absorbed close to the front TiO2 electron-transporting


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layer than for the low-energy photons absorbed almost homogeneously in the absorber. This confirms that the limiting process here is the electron transport to the front electrode, while transport of holes – parallel to the grain boundaries – is supported by the mechanism of modulation doping. Consequently, the dominant problem of the accompanying deterioration of the J-V curve (Fig. 4c) is serial resistance due to restricted transport and not, for example, the saturation current due to recombination through defect states. Indeed, no indication of electronically active defect states was found by looking at the deep subbandgap photocurrent (Fig. S3, Fig. S5 in SI) even though PDS spectroscopy consistently gives significant absorption in the subbandgap region (Fig. S1 in SI). The nature of this absorption is likely from the PbI2 phase but more rigorous study will have to be carried out.

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Figure 4. FTPS spectra measured on a cell in a different state of humidity degradation a), accompanied by significant degradation of the J-V curve c). Transmittance T, plotted as 1–T, showing no loss of CH3NH3PbI3 phase below 2.4 eV b).

In summary, the CH3NH3PbI3 perovskite material often contains a PbI2 phase. This has greater implications for the material prepared by the two-step method than the material prepared in one step, likely due to the increased surface roughness observed for the two-step material used for



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this study. Unlike optical absorptance spectroscopy, photocurrent spectroscopy also reflects the carrier transport that is restricted mainly by barriers in the conduction band due to inclusions of PbI2. In a photovoltaic photocurrent measured on a solar cell without bias, when both electrons and holes have to be collected, the PbI2 phase manifests itself as the step in the photoconductivity spectrum at 2.4eV, above which the PbI2 phase absorbs light without contributing to photocurrent. On the contrary, in a photoresistor photocurrent, measured between strips of Al contacts on the perovskite layer on glass, this step at 2.4eV has the opposite direction, appearing as if photocurrent from the PbI2 phase is more efficiently collected than it is in the CH3NH3PbI3 phase. This is explained by unidirectional transfer of holes generated in the PbI2 phase into the CH3NH3PbI3 phase thus extending their lifetime by escaping from their recombination counterparts. This mechanism is known as modulation doping and supports type-II band alignment between PbI2 and the CH3NH3PbI3 phase. Finally, the photocurrent method was applied to investigate performance degradation of a solar cell, suggesting that degradation is related to restriction of carrier transport rather than by electronically active defect states. Acknowledgment The authors acknowledge financial support from Czech Science Foundation project no 1726041Y, the project KONNECT-007 of the Czech Academy of Sciences, the Czech Ministry of Education, Youth and Sports projects LM2015087 and CZ.02.1.01/0.0/0.0/15_003/0000464 Centre of Advanced Photovoltaics, Swiss National Science Foundation through Nanotera and PNR 70 program. We thank Prof. Roman Grill and Dr. Thomas Dittrich for fruitful discussions. Supporting information: Deposition recipes for both layers and cells. Details of PDS and FTPS techniques and sample preparation. Transmittance and FTPS before normalization for layer on


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glass during degradation steps. Subbandgap spectra of the cell (FTPS) and the layer on glass (PDS).

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Photocurrent Spectroscopy of Perovskite Layers ... - ACS Publications

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