Variation in the Photocurrent Response Due to Different Emissive

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C 2018, 122, 3818−3823

Variation in the Photocurrent Response Due to Different Emissive States in Methylammonium Lead Bromide Perovskites Qi Shi,†,§ Supriya Ghosh,‡,§ Abdus Salam Sarkar,‡ Pushpendra Kumar,† Zhengjun Wang,† Suman Kalyan Pal,‡,* Tõnu Pullerits,†,* and Khadga J. Karki†,* †

The Division of Chemical Physics and NanoLund, Lund University, Box 124, 22100 Lund, Sweden School of Basic Sciences and Advanced Material Research Center, Indian Institute of Technology Mandi, Kamand, Mandi, 175005 Himachal Pradesh, India

J. Phys. Chem. C 2018.122:3818-3823. Downloaded from pubs.acs.org by AUSTRALIAN NATL UNIV on 08/13/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: Thin films and crystals of methylammonium lead bromide (MAPbBr3) perovskites have strong photoluminescence (PL). Previous studies have shown that the emission arises from different states. However, the role of these states in the performance of a solar cell has not been reported. We have used photocurrent and photoluminescence microscopies (PCM and PLM) to investigate the correlation between the photocurrent (PC) and the PL behavior in the different regions of MAPbBr3 thin film solar cells. Our results show that the PC and the PL responses from the different regions in the thin film show poor correlation compared to the correlation between those of a high efficiency GaAs solar cell. Furthermore, we establish a relationship between the different emissive states and the PC and the PL responses. Out of the two emissive states at 2.34 and 2.28 eV that have been reported, only the state at 2.34 eV has a dominant contribution to the PC. Our results suggest that the emission at 2.28 eV is related to traps, which can lower the performance of the solar cells. Finally, the correlation analysis of the PC and the PL responses we have presented can be used in any solar cell made from direct band gap semiconductor to identify the loss channels in the device.

1. INTRODUCTION Metal halide perovskites have emerged as excellent materials for photovoltaic technology because of ease of fabrication as well as favorable properties such as high absorption coefficient1−5 and carrier mobilities.6−10 The most efficient perovskite solar cells have a power conversion efficiency (PCE) of 22.1%.11,12 Although solution-processed perovskites are inherently inhomogeneous, they are still highly emissive.13−16 The high PCE as well as the PL yield suggests that the perovskite solar cells may be operating close to the Shockley−Queisser limit.17 However, the high degree of correlation between the PC and the PL yields of efficient solar cells made out of direct band gap materials, such as GaAs, is achieved only in the samples that have extremely low defect density. As the solution-processed perovskite solar cells have a high density of defects,18,19 one does not expect high PC or PL from them. Thus, the role of heterogeneity and defects in the photovoltaic performance of the perovskite solar is being actively investigated.20−25 It has been shown that the structural inhomogeneity affects the PL yield and the spectra in MAPbBr3. At least two different kinds of emissions at energies of about 2.28 and 2.34 eV have been identified at different regions of MAPbBr3 crystals and thin films.21,22,26,27 Although the impact of heterogeneity on the carrier lifetimes and the photocurrent response have been studied previously,20,24,28−38 the role of the different emissive states on the yield of the PC has not been systematically investigated. © 2018 American Chemical Society

Here, we have used PCM and PLM to simultaneously measure the PC and the PL, respectively, from different regions in a MAPbBr3 thin film solar cell. The PCM and the PLM allow us to correlate the PC and the PL from the different emissive states. Our results show that the major contribution to the PC is from the emissive state at 2.34 eV, whereas the other emissive state at 2.28 eV does not show a clear contribution. We have also used the PCM and the PLM to investigate the correlation between the PC and the PL from GaAs, which shows similar behavior to the PC response from the state at 2.34 eV in MAPbBr3 thin film. Our results also indicate that the state at 2.28 eV is populated by energy transfer from the state at 2.34 eV. Therefore, although its presence helps to increase the PL, it decreases the yield of the PC.

2. EXPERIMENTAL METHODS 2.1. Device Fabrication. Fluorine-doped tin oxide (FTO)coated glass substrate with a sheet resistivity of 7 Ωm was purchased from Sigma-Aldrich, USA. The substrates were etched with zinc powder (Merck, USA) and 2 M HCl for the intended electrode pattern (2 mm × 25 mm, W × L). Patterned substrates were sequentially cleaned in detergent (5% labolene solution), deionized water, acetone, isopropyl alcohol (IPA), and ethanol for 20 min each in an ultrasonic bath. The Received: January 17, 2018 Revised: January 29, 2018 Published: February 2, 2018 3818

DOI: 10.1021/acs.jpcc.8b00542 J. Phys. Chem. C 2018, 122, 3818−3823

Article

The Journal of Physical Chemistry C

Figure 1. (A) Diagram of perovskite cell architecture employed consisting of indium tin oxide (ITO)/fluorine-doped tin oxide (FTO)/titanium dioxide (TiO2)/perovskite/poly(3-hexylthiophene-2,5-diyl) (P3HT)/gold (Au). (B) Energy-level alignment of the fabricated devices. (C) Photocurrent density−voltage (J−V) curves of MAPbBr3 perovskite solar cells with the P3HT hole conductor. (D) SEM image of MAPbBr3 perovskite solar cells.

absorption-induced PL was detected by an avalanche photodiode. Two-photon excitation was used mainly to reduce the emission due to scattered light as well as to reduce the excitation-induced effects that are commonly observed in perovskites.21 The quadratic dependence of the PC and the PL responses on the excitation intensity (shown in the Supporting Information) confirms that the signals arise from two-photon excitation. The PC response was amplified by a preamplifier (SR570, Stanford Research Systems). The intensity of the PL and the PC modulated at 3 kHz, which contains only the lightinduced PL and PC rather than the dark current induced signals, was analyzed by using generalized lock-in amplifier.42,43 The PC and the PL responses at the different positions in the sample were obtained by raster-scanning the laser focus using a piezo-driven scanner (Nano-LP Series, Mad City Labs). The PL spectra were recorded by a spectrometer (Flame, Ocean Optics).

cleaned substrates were treated with UV−ozone for 15 min to remove the organic residuals. To prepare the TiO2 layer, PMT20, received from Dyesol (Australia), was spin-coated at 3000 rpm for 60 s. Then the TiO2-coated substrates were sintered at 500 °C for 60 min with a heating ramp rate of 10 °C/min. The MAPbBr3 layer on the top of FTO/TiO2 was then coated by spin-casting at 3000 rpm for 200 s and annealed on a hotplate at 100 °C for 2 min (inside the glove box). To deposit the holetransporting layer, poly(3-hexylthiophene 2-5,diyl) (P3HT) (Sigma-Aldrich) was spin-coated on the FTO/TiO2/MAPbBr3 substrate at 3000 rpm for 30 s by using a concentration of 15 mg P3HT in 1 mL toluene. Finally, the gold back contact was deposited by thermal evaporation under the pressure of 2 × 10−6. The active area was 0.02 cm2. GaAs p−i−n diode was obtained from Kyosemi Co. (part nr. KPDG008). The diode has an active area of 80 μm × 80 μm. The external quantum efficiency of the device is about 78% at 790 nm. The J−V curve of the device illuminated by the laser is given in the Supporting Information. 2.2. Electrical Characterization. All devices were characterized in ambient condition. The photovoltaic current−voltage measurements were carried out on an OAI solar simulator (USA), composed of a Keithley 2400 source measure unit. Solar irradiation was simulated using a class AAA solar simulator (OAI TriSOL) fitted with an AM1.5 (air mass) filter and calibrated by a standard Si solar cell. 2.3. Optical Setup and Data Acquisition. The optical setup has been described elsewhere.39−41 In brief, a pulsed laser beam from a mode-locked Ti:sapphire oscillator (Synergy, Femtolasers) was used as the light source (see the Supporting Information for the schematics of the setup). The beam was split into two using a 50:50 beam splitter. The phase of each of the beams was modulated by using acousto-optic modulators such that the difference in the modulation frequency was set to 3 kHz. The two beams were collimated collinearly by using a second beam splitter. One of the outputs from the second beam splitter was sent to an inverted microscope (Nikon Ti−S) for the excitation of the solar cell. The intensity of the two-photon

3. RESULTS AND DISCUSSION The architecture of the thin film solar cell device, energy-level alignment, and the photocurrent density−voltage (J−V) curves of MAPbBr3 perovskite solar cells are illustrated in Figure 1a−c, respectively. As shown in Figure 1a, the device consists of a transparent conductive electrode and a thin TiO2 layer followed by the perovskite (MAPbBr3) and a hole-transporting layer (P3HT). The back contact is a thin layer of a thermally evaporated metal (gold) electrode. As shown in the band alignment of the fabricated device (Figure 1b), MAPbBr3 can inject the electrons to the TiO2 layer and the holes to the holetransporting material (P3HT). The injected electrons and holes are then transported and collected by the transparent conductive electrode and the metal electrode. Figure 1c depicts the measured current density versus voltage (J−V) characteristics of the fabricated device under illumination with different intensities of light. The device exhibits a short-circuit current density of 6.18 mA/cm2, an open circuit voltage of 0.81 V, and a fill factor of 41.07%, yielding a PCE of 2.07% (under 100 mW/cm2). Figure 1d shows the scanning electron microscope 3819


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the other hand, do not show a clear correlation. The Pearson’s correlation coefficient of the PC and the PL maps in Figure 2c,d is only 0.32. Moreover, the entire MAPbBr3 perovskite solar cell does not show a homogeneous response. Although the inhomogeneity in the film thickness may account for some of the variation observed in the PC and the PL, it does not explain the lack of correlation. Contact heterogeneity proposed by Eperon et al. adds randomness to the PC and the PL responses,24 which can explain the lack of correlation. However, the intrinsic difference in the PC and the PL yields from the different states in MAPbBr3 can also result in the poor correlation. To explore the intrinsic difference of the PC and the PL responses of the two emissive states in the perovskite solar cell, we have measured the PL spectra at each point in the image of Figure 2d at zero bias voltage. One of the representative spectra is shown in Figure 3a. The spectrum is fitted with two Voigt profiles centered at 2.34 eV (marked as blue) and 2.28 eV (marked as red). The area under the two profiles gives the corresponding PL intensity (in Figure 3a). The decomposition of the PL spectra into two components allows us to investigate the differences in the PC response from the two emissive states. We select positions with high PC in the PCM or high PL in the PLM according to Figure 2. The coordinates of those positions are shown in Table S1 of the Supporting Information. In general, one expects four pathways by which the excited carriers can relaxtwo pathways each corresponding to the PC and the PL from the two states at 2.34 and 2.28 eV. In all of the selected positions in the sample, we can observe the PL at 2.34 eV, but the emission at 2.28 eV does not occur everywhere. In Figure 3b, we show the PL contribution at 2.34 eV versus the PC at the positions where the emission at 2.28 eV is not present. The figure also shows that at a number of positions in the image, we obtain significant PL even when the corresponding PC is negligible. We attribute this to the poor contacts of the electrodes. In these positions, the carriers can relax only by a radiative and nonradiative recombination. Moreover, the lack of PL component at 2.28 eV means that the corresponding states are not present there, suggesting that the PC at these positions mainly originates from the states that are emitting at 2.34 eV. Figure 3c shows the PL yield versus the PC from the positions where the contribution from the state at 2.28 eV is significant. The PL yields are randomly distributed, which again suggests heterogeneity in the contacts. Although the presence of both the PL components may be taken as the evidence of the PC response from both of the emissive states, it could be misleading. The distribution seen in Figure 3c can also appear if

(SEM) image of MAPbBr3 perovskite solar cells. We can observe a heterogeneous distribution of grains with a median grain size of ∼80 μm, which is larger compared to the spatial resolution of this technique’s 1 μm. The measurement of the PC and the PL are carried out over an area of 20 μm × 20 μm in the thin film. To investigate the relation between the PC and the PL in an ideal solar cell, we first analyze the PCM and the PLM of the GaAs solar cell at zero bias as shown in Figure 2a,b,

Figure 2. (a) PC map and (b) PL map measured on the film of GaAs solar cell and (c) PC map with region A marked by a red box and (d) PL map with region B marked by a blue box measured on the film of MAPbBr3 perovskite.

respectively. The figures show an edge of the solar cell with high PC and PL from the active region (yellow) and no PC or PL from the substrate (blue). As can be seen in the figures, the regions that give high PC also give high PL. We calculate the Pearson’s correlation coefficient (r)44 of the two images using eq 1 n

r=

∑i = 1 (xi − x ̅ )(yi − y ̅ ) n

n

∑i = 1 (xi − x ̅ )2 ∑i = 1 (yi − y ̅ )2

(1)

where xi and yi denote the PC and the PL yields at different positions in the images, respectively. The correlation coefficient calculated from the two images is 0.9993. Such a high degree of correlation points to the fact that the states that contribute to the PC also contribute to the PL, which is expected in the case of the GaAs solar cell. The PCM and the PLM of MAPbBr3, on

Figure 3. PL spectrum peak fitting results as a function of photocurrent response for positions of interest (shown in Table S1) in the MAPbBr3 perovskite solar cell. (a) The PL spectrum is fitted with two Voigt profiles centered at 2.34 eV (blue line) and 2.28 eV (red line); (b) correlation between PL and PC in regions where the 2.28 eV emission does not occur; (c) correlation between PL and PC in regions shows that there is an energy transfer from 2.34 to 2.28 eV. The blue and the red dots are the areas under the corresponding Voigt profiles in (a). 3820


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Figure 4. (a) PC−PL relation in region A with the emissive state at 2.28 eV by applying different forward biases to the MAPbBr3 perovskite solar cell; (b) PC−PL relation in region B without the emissive state at 2.28 eV; and (c) PC−PL relationship obtained by applying different forward biases to the GaAs solar cell.

not contribute to the PC clearly indicates that a high PL from the perovskite solar cell does not necessarily indicate high PCE. It is important to note that the emission at 2.28 eV is absent in some regions of the solar cells (Figure 3b), which suggests that the sample preparation may be optimized to reduce the emissive traps to increase the PCE.

only the state at 2.34 eV contributes to the PC provided that it also can populate the state at 2.28 eV via energy transfer. Previous studies have shown evidence of such energy transfer processes.21,26 To test whether the state at 2.28 eV contributes to the PC, we have carried out further correlation analysis between the PC and the PL by varying the external bias. We have selected two positions in the sample to investigate the changes in the yields of the PL and the PC at different bias voltages. In one of the positions, we call it A (in Figure 4a), we only have PL at 2.34 eV, whereas in the other position, we call it B, we have significant PL from the state at 2.28 eV (Figure 4b). A forward bias is varied from 0 to 0.25 V. As we use signal at 3 kHz rather than the normal dc, it is important to make sure that the bias does not influence the impedance of the device. Therefore, we have used a small window of bias voltage (0 to 0.25 V). We have also measured the yield of the PC and the PL from the GaAs solar cell under similar bias conditions (Figure 4c). Typically, when forward bias is applied to a direct band gap solar cell, fewer photogenerated carriers are extracted as current and more carriers recombine radiatively,17 which leads to a negative correlation between the PC and the PL. In our experiment, we have used an intensity-modulated light field to excite the sample. The PC and the PL signals are detected at the modulation frequency (see Experimental Methods) to select only the contributions from the light induced current. In comparison, the dc signals contain a rather large contribution from the dark current that cannot be easily separated from the signal due to photogenerated carriers. The correlation coefficient between the PC and the PL signals obtained by varying the bias in the GaAs solar cell (Figure 4c) and region A of the perovskite sample (Figure 4a) is −0.97. On the other hand, the PC−PL correlation coefficient in region B is only −0.4. Although the coefficient is not zero, the weak correlation between the PC and the PL signals indicates that the state at 2.28 eV does not significantly contribute to the PC. Among the two emissions, the emission at 2.34 eV is close to the band gap of MAPbBr3 (about 2.36 eV),22 and hence it can be assigned to the radiative band-to-band carrier recombination. The emission at 2.28 eV is significantly smaller than the band gap energy. Its origin has been debated to be either the PL from a different phase of the material or defect-related polaronic emission. Although the variation of the yield of PC with the phase has not been reported, it is unlikely to differ significantly such that the correlation between the signals is suppressed. On the other hand, the carriers trapped in the defects do not contribute to the PC. Thus, our results suggest that the emission at 2.28 eV is due to the emission from the trapped carriers. The presence of the defects that emit but do

4. CONCLUSIONS In summary, we have presented a methodological analysis based on the PCM and the PLM of the MAPbBr3 perovskite solar cell and the GaAs solar cell to investigate the variation in the PC response from the emissive states at 2.28 and 2.34 eV. Our results show significant inhomogeneity in the PC and the PL from the MAPbBr3 perovskite solar cell compared with the GaAs solar cell. We observe a strong correlation between the PC and thePL signals from GaAs, which contrasts with the weak correlation in MAPbBr3. Our results further indicate that only the state at 2.34 eV has a significant contribution to the PC, whereas the presence of the emissive state at 2.28 eV may lower the performance of the solar cell.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b00542. Optical setup; details of the positions of interest; excitation-dependent PL and PC of the MAPbBr 3 perovskite solar cell; current density versus voltage (J− V) characteristics of the GaAs solar cell (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +91 1905 2670062 (S.K.P.). *E-mail: [email protected]. Phone: +46 46 222 81 31 (T.P.). *E-mail: [email protected]. Phone: +46 46 222 83 40 (K.J.K.). ORCID

Suman Kalyan Pal: 0000-0003-2498-6217 Tõnu Pullerits: 0000-0003-1428-5564 Khadga J. Karki: 0000-0002-0002-4163 Author Contributions §

Q.S. and S.G. contributed equally.

Notes

The authors declare no competing financial interest. 3821


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ACKNOWLEDGMENTS Financial support from the Swedish Research Council, the Crafoord Foundation, NanoLund, Knut, Alice Wallenberg Foundation and the DST (grant no. DST/INT/SWD/VR/P06/2014) under Indo-Swedish (DST-VR) joint scheme is gratefully acknowledged. S.G., A.S.S., and S.K.P. are highly thankful to IIT Mandi and AMRC (Advanced Material Research Centre) for providing research facilities. Q.S. and Z.W. are grateful to China Scholarship Council (CSC) for the financial support.

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