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. 2019 Feb 13;4(3):702–708. doi: 10.1021/acsenergylett.9b00182

Tuning the Excited-State Dynamics of CuI Films with Electrochemical Bias

Gergely F Samu †,‡,∥,*, Rebecca A Scheidt †,§, Ádám Balog ‡, Csaba Janáky ‡,∥,*, Prashant V Kamat †,§,*
PMCID: PMC6413481  PMID: 30882041

Abstract

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Owing to its high hole conductivity and ease of preparation, CuI was among the first inorganic hole-transporting materials that were introduced early on in metal halide perovskite solar cells, but its full potential as a semiconductor material is still to be realized. We have now performed ultrafast spectroelectrochemical experiments on ITO/CuI electrodes to show the effect of applied bias on the excited-state dynamics in CuI. Under operating conditions, the recombination of excitons is dependent on the applied bias, and it can be accelerated by decreasing the potential from +0.6 to −0.1 V vs Ag/AgCl. Prebiasing experiments show the persistent and reversible “memory” effect of electrochemical bias on charge carrier lifetimes. The excitation of CuI in a CuI/CsPbBr3 film provides synergy between both CuI and CsPbBr3 in dictating the charge separation and recombination.

The recent surge of interest in organic–inorganic hybrid perovskites has drawn considerable attention to related materials that are essential to assemble high-performance perovskite solar cells (PSCs).1,2 An increasing emphasis is given to the charge transport properties of the electron- and hole-transporting materials (ETMs and HTMs, respectively)3 and the quality of the formed interfaces.4−6 The top-performing PSCs mainly utilize organic small molecules or polymeric HTMs,7−10 but their inorganic counterparts offer increased stability and superior device properties (e.g., suppressed hysteresis).11 Copper-based inorganic compounds (e.g., CuI,12−17 CuSCN,18−21 CuOx,22,23 and CuCrO224,25) are especially promising candidates even though their use is inhibited by their lack of suitable solution deposition methods that produce uniform, compact, and crack-free layers. Recently, PSCs employing CuSCN as the HTM have achieved stabilized efficiencies exceeding 20% over 1000 h of continuous illumination, which is superior to spiro-OMeTAD-based devices.21 Interestingly, the spotlight has avoided CuI as a HTM so far despite the fact that it possesses hole mobility two magnitudes greater than that of CuSCN.11 Furthermore, device modeling studies suggest that comparable efficiencies to CuSCN-based devices can be achieved, also overcoming the 20% threshold.26

CuI is a transparent wide-bandgap (3.0 eV) p-type semiconductor, with a large exciton binding energy of 62 meV.27 Low-temperature preparation techniques can be used to prepare high-quality CuI films such as spin-coating,13,15,28 inkjet printing,29 spray coating,17 or iodination of Cu/CuN3 precursors.30−32 The band structure, which has considerable contributions from iodide orbitals, allows for an acceptor level near the valence band edge, leading to greater hole mobility in halide-based cuprous materials than their oxide counterparts.30 This results in remarkable hole conductivity in CuI single crystals,33 up to 43.9 cm2 V–1 s–1 with a hole concentration of 4.3 × 1016 cm–3. For annealed polycrystalline thin films, a hole conductivity greater than 20 cm2 V–1 s–1 was achieved.31,32,34 Previous studies have examined the underlying mechanism of increased hole mobility, indicating that iodide and copper vacancies at grain boundaries30 as well as hole accumulation layers due to interfacial defect band formations are the cause.35 Recently, we revealed the effect of trap states on the optoelectronic properties of CuI electrodes using spectroelectrochemistry.36 It was shown that the population of shallow trap states affects the optical properties of CuI layers, while the filling of deep trap states alters the electrical properties of the CuI layers.36

To the best of our knowledge, studies on the excited-state dynamics in CuI films are still lacking. To fill this gap and to better understand the effect of external bias on the recombination kinetics of charge carriers (or excitons) in CuI, we carried out ultrafast spectroelectrochemical measurements.37,38 This technique was used to study oxidation mechanisms on oxide semiconductors in different (photo)electrochemical reactions.39−42 We aimed to probe the effect of in situ trap state filling on the fate of charge carriers. Additionally, ex situ relaxation experiments were performed on precharged CuI electrodes. Preliminary studies on CuI/CsPbBr3 interfaces are also shown as an outlook to the application of CuI in PSCs.

Stability Window of ITO/CuI Electrodes. CuI films cast on an ITO electrode (ITO/CuI) and annealed at 150 °C for 10 min exhibit two absorption bands at 338 and 407 nm, which are in good agreement with literature values.27 These sharp absorption bands are attributed to excitonic transitions (Figure 1A).27 The broad absorption band starting from 600 nm can be attributed to iodine vacancies in the crystal structure of the deposited CuI. These form trap states over the valence band of CuI. In turn, these interstitial states can induce intraband transitions with light of lower energy than the bandgap of CuI. Direct optical excitation of a semiconductor produces electrons and holes intrinsically, whereas electrochemistry allows external injection of electrons or holes selectively into the semiconductor film. By combining electrochemistry and spectroscopy, we can obtain unique information on the excited-state properties at various levels of electron or hole injection through applied electrochemical bias.

Figure 1.

Figure 1

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(A) UV–vis absorbance spectrum of the ITO/CuI electrode, annealed at 150 °C for 10 min. (B) Spectroelectrochemical data, recorded for ITO/CuI electrodes in 0.1 M Bu4NPF6/DCM electrolyte (5 mV s–1 sweep rate). The cathodic and anodic half-cycles were recorded on separate electrodes. The absorbance of the excitonic peak at 407 nm is plotted together with the recorded current during the half-cycles.

In the present investigation, spectroelectrochemical measurements were performed first to establish the stability window of ITO/CuI electrodes in dichloromethane (DCM) containing 0.1 M Bu4NPF6 electrolyte (Figure 1B). The absorbance change at the excitonic peak at 407 nm was plotted along with the current density during the electrochemical sweep. The excitonic peak decreases when the potential increases from 0.0 to +0.4 V vs Ag/AgCl. This decrease can be reverted upon reversing the potential scan, which is related to the reversible population/depopulation of shallow trap states, as discussed in our previous study.36 The rapid decrease in the absorbance at extreme positive and negative potentials (greater than +0.80 and −0.70 V), however, is caused by degradation of the CuI film as it undergoes irreversible Faradaic processes. Thus, these two potentials define the stability window (from −0.1 to +0.6 V vs Ag/AgCl) to conduct spectroelectrochemical measurements with CuI films.

Ultrafast Spectroelectrochemistry of ITO/CuI Electrodes. The large exciton binding energy of CuI (62 meV) makes it difficult to induce charge carrier separation at room temperature following bandgap excitation. Thus, the recovery of the bleach in the transient absorption (TA) spectra seen at 406 nm is attributed to the recovery of the exciton band. Previous reports have shown that the exciton band of semiconducting materials can be reversibly altered with applied electrochemical bias due to the enhanced separation of excitons and/or band filling effects.43−45 Using this knowledge, we probed the excited-state dynamics of CuI under applied bias in order to elucidate excitonic behavior. In our systematic approach, we carried out control experiments first, to show that the presence of the electrolyte has no effect on the time-resolved spectral characteristics of ITO/CuI film electrodes (Figure S1A,B). At the same time, in the electrolyte, a slightly slower recovery of the bleach signal was observed under open-circuit conditions (Figure S1C). This can be attributed to the double-layer charging near the electrode surface. The TA spectra of ITO/CuI electrodes consist of two induced absorption (IA) bands (IA1 at 399 nm and IA2 at 419 nm) and two bleaching signals (PB1 at 408 nm and PB2 at 412 nm) shown in Figure 2A. The IA1 band located at higher energies likely arises from the absorption of the lowest excitonic state, and the IA2 is from the absorption of hot excitons.46

Figure 2.

Figure 2

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(A) Time-resolved TA spectra of ITO/CuI electrodes in 0.1 M Bu4NPF6/DCM electrolyte after 387 nm laser pulse excitation (4 μJ cm–2) at an applied potential of +0.2 V vs Ag/AgCl. (B) Bleach recovery profiles monitored at 412 nm at applied potentials from −0.1 to +0.6 V vs Ag/AgCl. (C) Potential dependence of the determined lifetimes. The recovery profiles in (B) were fitted with a monoexponential decay. The error bars represent measurements on three separate electrodes.

While the majority of the excitation involves exciton formation followed by recombination (reactions 1 and 2), a small fraction can undergo charge separation, especially in the presence of electrolyte (reaction 3). The shorter lifetime in the presence of electrolyte supports such an argument. One plausible explanation for the presence of two bleaching signals involves excitons consisting of light (PB2) and heavy (PB1) holes, caused by strains in the material.47 The expected energy difference is 20 meV according to literature data.47 From the TA spectra, 30 meV of difference was calculated between the two bleach signals, validating the proposed scenario.

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To study the effect of applied bias on the kinetics of exciton recombination, the ITO/CuI electrodes were held at different potentials within the stability window (from +0.6 to −0.1 V vs Ag/AgCl). Both bleach recoveries showed similar potential dependence; thus, for the sake of simplicity, only the recovery profiles for the PB2 are shown in Figure 2B. The kinetic traces show a monoexponential decay (first-order kinetics), which is the result of direct exciton recombination (reaction 2) and charge separation (reaction 3). The time constant of this monoexponential decay can be given by the reverse sum of the rate of these two processes. Note, however, that the large exciton binding energy (62 meV) of CuI does not favor charge separation at room temperature; thus, only a small fraction of excitons can undergo separation.27

As we change the applied potential from +0.6 to −0.1 V vs Ag/AgCl, a faster bleach recovery is observed (Figure 2C). This translates to a decrease in the lifetime from 5.2 to 1.1 ps. With increased negative bias, electron injection to CuI takes place, which fills the trap states in the material. As these vacant energy states are filled, charge carrier trapping is suppressed; thus, a decrease of the lifetime of free charge carriers is expected. In addition, as a secondary effect, the external driving force might facilitate the dissociation of excitons. This introduces new pathways for the ground state to be repopulated such as direct recombination of free charge carriers and recombination at the ITO/CuI interface. The slight tailing (distortion of the monoexponential decay) on the kinetic traces in Figure 2B at more negative bias signals this scenario. We performed the fit of all of the kinetic traces to monoexponential decay as the longer component has only a minor contribution to the overall decay.

Reversibility of Exciton Dissociation in ITO/CuI Electrodes. Interestingly, the observed decrease in excited-state lifetime of CuI was reversible. When the applied bias was reversed, we saw an increasing trend in the lifetime. The TA spectra recorded under a forward (−0.1 to +0.6 V vs Ag/AgCl) scan followed by a reverse scan (+0.6 to −0.1 V vs Ag/AgCl) are shown in the Supporting Information (Figure S2). The kinetic traces corresponding to the bleaching recovery (PB2 signal) at 0.0 and +0.6 V vs Ag/AgCl recorded during forward and reverse scan are presented in Figure 3A. The recovery kinetics observed for all recovery profiles were fitted with a monoexponential decay. The lifetimes obtained during forward and reverse bias show the reversible trend of exciton bleach recovery lifetimes (Figure 3B). A small hysteresis in the measured lifetimes observed during these two scans is likely to arise from the residual accumulation of charges and/or slowness of the system to undergo equilibration and respond quickly to the applied bias.

Figure 3.

Figure 3

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Reversibility studies of the effect of applied bias on the kinetics of the bleach recovery. (A) Bleaching recovery profiles monitored at 412 nm at the two end potentials of the stability window. (B) Potential dependence of the determined bleach recovery lifetimes [monoexponential fit of kinetic traces in (A)].

Prebiasing of ITO/CuI Electrodes and Its Effect on Excited-State Recovery. In the case of PSCs, prebiasing or light soaking prior to current–voltage measurements has a pronounced effect on the determined efficiency.48 This is mainly caused by charge accumulation at the interfaces in the solar cells. Accumulation of ions or charged species near the electrode surface are also responsible for the observed hysteresis of J–V curves.49 Such charge accumulation can also have a profound influence on the excited-state recovery of CuI films. To probe such charging effects, we prebiased the ITO/CuI electrode for 10 min, and then, we disconnected the cell from the potentiostat. The potential difference between the semiconductor Fermi level and the reference electrode is the measured open-circuit potential (EOCP). By modulating the electrical bias, we shift the Fermi level of CuI by populating/depopulating trap states in the material (Scheme 1.). The measured EOCP of the cell (immediately after disconnection) provided an estimate of the charging effect.

Scheme 1. Effect of Charging with Electrochemical Bias Monitored through the Change in the EOCP.

Scheme 1

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With time, the original dark potential is achieved.

Note that the EOCP is a measure of electrons/holes accumulated at the electrode. With increasing time, the electrons (or holes) are slowly discharged and the EOCP increases (or drops). Thus, by varying the delay time after releasing potential control, different equilibration of charges can be attained in the ITO/CuI electrode. TA spectra were recorded with different equilibration delay times while monitoring the EOCP of the electrodes. This allowed us to independently confirm the effect of accumulated electrons/holes on the excited-state recovery of the CuI film, simulating the conditions in a photoirradiated cell under open-circuit conditions.

We selected two different bias potentials (−0.1 and +0.6 V), corresponding to the end points selected in the experiments of Figure 2. TA spectra were recorded for 500 s after the bias was released. The first and last recovery traces for the 412 nm bleach during the relaxation experiment (after prebiasing at −0.1 V) are shown in Figure 4A. Interestingly, after the bias was released, the charging effect was retained, as evident from the EOCP and the bleach recovery traces. The lifetimes (from recovery traces fitted to first-order kinetics) are plotted together with the measured EOCP (Figure 4B). It is apparent that the recovery lifetime changes in the same fashion as the EOCP.

Figure 4.

Figure 4

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Bleaching recovery profiles monitored at 412 nm recorded for ITO/CuI electrodes in 0.1 M Bu4NPF6/DCM electrolyte that were charged at (A) −0.1 and (C) 0.6 V for 10 min prior to measurements. Relaxation of EOCP of the electrodes plotted together with the change in the bleaching recovery time after prebiasing at (B) −0.1 and (D) 0.6 V.

The prebiasing has a persistent effect for more than 100 s on both the charge carrier lifetime and EOCP of the ITO/CuI electrodes. When the layers were prebiased at 0.6 V for 10 min, an opposite trend was observed (Figure 4C,D); however, the magnitude of the change was much less pronounced (as expected from the determined lifetimes from Figure 2C). This signals that the effect of bias is two-fold: (i) affects the dissociation of excitons and (ii) populates/depopulates the trap states in CuI. As negative bias is applied, trap states in the material are filled. Consequently, these states become dormant in charge carrier trapping; thus, decreasing charge carrier lifetime is observed. When the electrical bias is released, the original charge carrier lifetime is recovered through the slow depopulation of these trap states. The depletion of trap states decreases the Fermi level, hence the decrease in measured EOCP (Figure 4A,B and Scheme 1 top part). When applying a positive bias (decreasing the Fermi level), the trap states get completely depleted. This slightly increases the lifetime (Figure 4D). When the bias is released, a similar recovery of the EOCP is observed (but with opposite trend; see Figure 4C,D and Scheme 1 bottom part).

CuI as a Hole-Transporter in Perovskite Solar Cells. CuI has been used as a HTM in several photovoltaic studies.12−17 In such cases, the photogenerated holes are quickly transferred to the HTM, while electrons are captured by the ETM (Figure 5A). To design more efficient solar cells, it is important to probe devices under operating conditions. During operation, the presence of external or internal electric fields may influence the excited-state dynamics of the system and thus affect the power conversion efficiency. As seen in the prebiasing experiments with CuI, the HTM is also prone to this effect and thus responsive to the electric field. To move a step further toward real solar cells, we deposited bulk CsPbBr3 films from QD suspension50 on the prepared ITO/CuI electrodes. Here we show the hole transfer from CsPbBr3 films to both ITO and ITO/CuI supports. The TA spectra of ITO/CsPbBr3 and ITO/CuI/CsPbBr3 were recorded in vacuum using 387 nm laser pulse excitation. A representative set of TA spectra is shown in Figure 5B. The recovery of the bleaching signals is compared in Figure 5C. (It should be noted that the excitonic lifetime (∼4 ps) of CuI was too short to interfere with the excited-state behavior of CsPbBr3.) Rapid recovery was observed for the bleach signal of CsPbBr3 when it was in contact with CuI (Figure 5C), thereby suggesting an additional pathway involving the fast disappearance of holes.51 The signal of the CsPbBr3 was fitted with a biexponential decay, and a greatly reduced charge carrier lifetime (weighted average) of 38 ± 11 ps was determined (in contrast to the 194 ± 28 ps lifetime for ITO/CsPbBr3 electrodes37). On the basis of the comparison of lifetimes, we expect the hole transfer rate to be ∼2.5 × 1010 s–1. The relatively faster rate constant shows that CuI can capture and transport photogenerated holes effectively.

Figure 5.

Figure 5

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(A) Schematic illustration of hole transfer from excited CsPbBr3 to CuI following the bandgap excitation. (B) Time-resolved TA spectra of ITO/CuI/CsPbBr3 electrodes in vacuum after 387 nm laser pulse excitation (4 μJ cm–2). (C) Bleaching recovery profiles monitored at 524 nm for the bleaching recovery of CsPbBr3. The inset shows the magnification of the 0–200 ps region.

The similarity of the rate constant of hole transfer to CuI with that of electron transfer rate constant from excited CsPbBr3 into TiO2 (2–4 × 1010 s–1)51 shows that we can drive away holes through CuI competitively in a PSC. Inability to maintain the electron and hole transfer rates equally in a solar cell can lead to the accumulation of one of the charge carriers and can result in lower efficiency and instability of the PSC. Our studies presented here highlight the usefulness of CuI as a HTM in a PSC. However, the accumulation of holes at the CuI/CsPbBr3 interface can be particularly disadvantageous in the case of a real solar cell as it can result in the depletion of electron trap states in CuI, which in turn affects charge carrier collection of the devices. Further follow-up studies will deal with the effect of applied bias on the excited-state dynamics of this system.

Concluding Remarks. Ultrafast spectroelectrochemical experiments revealed a monoexponential decay of exciton recombination in CuI that is strongly dependent on the applied electrochemical bias. The change in charge carrier lifetimes was completely reversible, showing the dependence of excited-state dynamics on the externally controlled charge carrier density. Prebiasing experiments revealed the long-lasting and reversible effect of charge accumulation on charge carrier dynamics. The filling of the trap states and, to a minor extent, the acceleration of exciton dissociation contributed to the decreased exciton lifetime under negative bias. While we were able to demonstrate the hole-accepting property of CuI from excited CsPbBr3, one cannot ignore its semiconducting property when employed in a solar cell. The excited-state behavior of CuI employed as a HTM is likely to influence the performance of solar cells as well.

Acknowledgments

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 716539). The Ministry of Human Capacities, Hungary Grant 20391-3/2018/FEKUSTRAT is also acknowledged. ELI-ALPS is supported by the European Union and cofinanced by the European Regional Development Fund (GOP-1.1.1-12/B-2012-000, GINOP-2.3.6-15-2015-00001). P.V.K. and R.A.S. acknowledge the support of the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Award DE-FC02-04ER15533. This is NDRL No. 5231 from Notre Dame Radiation Laboratory.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.9b00182.

  • Experimental methods and additional transient absorption spectra (PDF)

The authors declare no competing financial interest.

Supplementary Material

nz9b00182_si_001.pdf (219.5KB, pdf)

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