# Direct observation of slow transient response of photovoltage for carrier dynamics in Cu(In,Ga)Se2 via surface photovoltage measurement

Kana Ueda, Thomas Fix, Mutsumi Sugiyama

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Kana Ueda, Thomas Fix, Mutsumi Sugiyama. Direct observation of slow transient response of photovoltage for carrier dynamics in Cu(In,Ga)Se2 via surface photovoltage measurement. Applied Physics Letters, 2025, 127 (17), pp.172107. 10.1063/5.0282041 . hal-05337476

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# Direct observation of slow transient response of photovoltage for carrier dynamics in Cu(In,Ga)Se₂ via surface photovoltage measurement

Kana Ueda

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Thomas Fix

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Mutsumi Sugiyama

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Appl. Phys. Lett. 127, 172107 (2025)

https://doi.org/10.1063/5.0282041

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# Direct observation of slow transient response of photovoltage for carrier dynamics in Cu(In,Ga)Se₂ via surface photovoltage measurement

Cite as: Appl. Phys. Lett. **127**, 172107 (2025); doi: 10.1063/5.0282041

Submitted: 23 May 2025 · Accepted: 14 October 2025 ·

Published Online: 29 October 2025

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Kana Ueda,<sup>1,2,a)</sup> Thomas Fix,<sup>3</sup> and Mutsumi Sugiyama<sup>1</sup>

## AFFILIATIONS

## ABSTRACT

A direct investigation of the carrier generation and slow transient mechanism related to the photovoltage characteristics of Cu(In,Ga)Se₂ (CIGS) was conducted using the transient surface photovoltage (t-SPV) measurement. The focus was placed on the slow transient response of the photovoltage, which reflects carrier transport processes, such as trapping/detrapping, occurring during device operation. The applicability of t-SPV to directly probe the carrier dynamics related to photovoltage characteristics was investigated for CIGS thin films, CdS/CIGS *pn* junctions, and CIGS solar cells. The results provided valuable insights into the slow transient response of the photovoltage for evaluating the device performance. In addition, the excitation intensity-dependent SPV measurements supported the t-SPV findings. Overall, t-SPV is a promising and straightforward method that may serve as a practical tool for direct performance evaluation of various optoelectronic devices.

© 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0282041

Recently, optoelectronic devices based on photoelectronic effects that utilize photovoltage characteristics, such as solar cells,<fnref n="1" /> photo detectors,<fnref n="2" /> photoelectrodes,<fnref n="3" /> and optoelectronic synaptic devices,⁴,⁵ have attracted increasing attention from the perspective of reduced energy loss during power transmission and distribution and have led to the realization of self-powered devices for the Internet of Things society.⁶ To develop optoelectronic devices, exploration of semiconductor materials and simple, yet accurate, techniques to evaluate photovoltage behavior is essential.

Chalcopyrite Cu(In,Ga)Se₂ (CIGS), known as an attractive solar cell material,⁷ has a bandgap from 1.0 eV (CuInSe₂) to 1.7 eV (CuGaSe₂) for the semiconductor alloys of CuGaSe₂ and CuInSe₂,⁸ and a high optical absorption coefficient for the direct transition.⁹ Moreover, this material has been used in various optoelectronic devices, such as photoelectrodes¹⁰⁻¹² and optoelectronic synaptic devices.¹³ However, the photovoltages of CIGS-based optoelectronic devices are lower than their theoretical photovoltage.¹⁴ One of the reasons for this

is the formation of deep-level defects, such as Ga<sub>Cu</sub> antisites, because of the CIGS alloys with Ga.<sup>15,16</sup> However, the carrier dynamics contributing to the lower photovoltages of CIGS thin films remain unclear. Therefore, a direct investigation of the carrier dynamics in CIGS thin films, independent of the junctions or device structures, is required to understand the photovoltage generation mechanisms.

Surface photovoltage (SPV) measurement is a contactless technique used to observe surface potential differences in semiconductors owing to the generation and redistribution of light-induced carriers.<sup>17,18</sup> In general, band bending in the spatial charge region occurs at the interfaces (such as the *pn* junction) and at the surfaces of semiconductors because of the localized carriers in the surface states.<sup>18</sup> Previous studies have reported a correlation between the photovoltage characteristics of thin films measured by SPV and those of optoelectronic devices.<sup>19,20</sup> Therefore, it is possible to evaluate the device performance based on the photovoltage characteristics of thin films using SPV measurements. In particular, the slow transient

<footnote n="1"><sup>1</sup>Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan</footnote>

<footnote n="2"><sup>2</sup>Research Fellow of Japan Society for the Promotion of Science, 531, Kojimachi, Chiyodaku, Tokyo 1020083, Japan</footnote>

<footnote n="3"><sup>3</sup>ICube laboratory, CNRS and Université de Strasbourg, 23 rue du Loess, 67037 Strasbourg, France</footnote>

<footnote><sup>a)</sup>Author to whom correspondence should be addressed: optoelec@rs.tus.ac.jp</footnote>
response of the photovoltage measured using the SPV measurements was investigated.

In this study, particular attention was paid to transient surface photovoltage (t-SPV) measurements.²¹,²² Light-induced carriers relax to thermal equilibrium via various transition and/or recombination processes, such as band-to-band recombination and trapping/detrapping, caused by trap states. These carrier dynamics in the transition and/or recombination processes are represented based on the transient responses of the photovoltage determined using SPV measurements.²¹,²² Therefore, the carrier dynamics related to photovoltage characteristics can be investigated based on direct observations of the transient response of the photovoltage detected using SPV measurements. Furthermore, t-SPV measurement enables the observation of fast (~ microseconds)²³ and slow transient photovoltage responses (approximately several seconds, or longer).²⁴ According to previous reports, these slow transient responses have reflected the carrier transport processes that occur during the operation of optoelectronic devices compared with the fast transient responses.²⁵,²⁶ Although previous studies have reported fast transient photovoltage responses (approximately nanoseconds) in CIGS thin films and their devices,²⁷,²⁸ only a few investigations in CIGS thin films have focused on slower responses (approximately several seconds or more). Therefore, the long-timescale carrier dynamics that contribute to the photovoltage characteristics of CIGS thin films using t-SPV measurements can be investigated by comparing them with the photovoltage characteristics of the CdS/CIGS *pn* junction and CIGS solar cell.

Polycrystalline CIGS thin films with a Ga/(In+Ga) atomic ratio of approximately 0.3 and a thickness of approximately 4 µm, which is sufficient thickness for CIGS solar cells,²⁹ were grown using a conventional “three-stage process”²⁹ with molecular beam epitaxy equipment (EW-500, EIKO Engineering Co., Ltd.) on Mo/soda lime glass substrate. The carrier concentration and trap density of CIGS were on the order of 10¹⁶ and 10¹⁵ cm⁻³, respectively.³⁰,³¹ Following CIGS deposition, chemical bath deposition (CBD) of CdS (thickness of 50–60 nm) was performed using an aqueous solution of CdSO₄ (0.015 M)–NH₃ (2.2 M)–CH₄N₂S (1.5 M).³² During the CBD process, the temperature of the solution increased from approximately 25 °C (room temperature) to 65 °C. Subsequently, in the CIGS solar cell region, undoped ZnO (thickness of 100 nm) and ZnO:Al (thickness of 300 nm) (2 wt. % Al₂O₃ doped ZnO) thin films were deposited via radio frequency magnetron sputtering without intentional heating. The carrier concentrations of CdS, ZnO, and ZnO:Al were approximately of the order of 10¹⁸ and 10¹⁸ cm⁻³, and 10¹⁹ cm⁻³, respectively.³⁰ In the CIGS solar cells, CdS and ZnO were used as the *n*-type layer and ZnO:Al was used as the transparent conductive layer.²⁹,³² The bandgaps of CIGS, CdS, and ZnO were 1.2, 2.4, and 3.3 eV, respectively.³⁰ The working functions (WFs) of the CIGS, CdS, and ZnO:Al obtained using the Kelvin probe in air were –4.7, –4.8, and –4.7 eV, respectively. Based on these fundamental values, the band structure of the CIGS solar cell is shown in Fig. 1(a). In addition, the formation of Ohmic contact at the CIGS/Mo interface is well established in the case of three-stage process growth.³³ The CIGS thin film, CdS/CIGS thin film (CdS/CIGS *pn* junction), and CIGS solar cell were prepared for SPV, t-SPV, and excitation intensity-dependent SPV measurements, as shown in Fig. 1(b).

Standard SPV, t-SPV, and excitation intensity-dependent SPV measurements were performed using a halogen source with calibrated

illumination and a Kelvin probe system (KP Technology APS03)²¹,³⁴ in air. An Au tip of diameter 2 mm was used as the current collector for direct verification of the WF of the samples. The tip was calibrated by measuring the contact potential difference (CPD) using a silver reference. The Kelvin probe system, which is a contactless method for sample measurement, was used quasi-continuously (at a measurement frequency of 10 Hz) to calculate CPD. The ΔCPD (CPD<sub>light</sub> – CPD<sub>dark</sub>)

<figure><img src="image_15.png" /><figcaption>FIG. 1. (a) Fundamental band structure of the CIGS solar cell, (b) schematic of several sample structures and (c) corresponding contact potential difference (CPD) of the Cu(In,Ga)Se₂ (CIGS) thin film, CdS/CIGS *pn* junction, and CIGS solar cell, recorded using the Kelvin probe as a function of time under dark and illuminated conditions with the enlarged view of the CPD of the CIGS solar cell.</figcaption></figure>
values were determined using a quartz tungsten halogen light source with a maximum irradiance of approximately 19 mW/cm². The ΔCPD values in the dark and illuminated cycles were essentially equal to the surface photovoltages. The same KP setup was used to illuminate the sample with white light and monitor the CPD.³⁵,³⁶ A stabilization period of over 30 s was required for the CPD before measurements. The transient response of the photovoltage focused on in this study was slow (approximately several seconds, or longer).³⁷ The time and CPD resolutions of the Kelvin probe system were 0.1 s and 3 meV, respectively. Moreover, given the thickness and bandgap of the ZnO, and CdS layers, the primary light absorption in CIGS solar cells occurs within the CIGS layer.³⁸

First, the photovoltage characteristics of the CIGS thin film, CdS/CIGS *pn* junction, and CIGS solar cell were evaluated using SPV measurements. The CPD vs time curves of the CIGS thin film, CdS/CIGS *pn* junction, and CIGS solar cell under dark and illumination conditions are shown in Fig. 1(c). To clarify the transient behavior of the CIGS solar cell immediately after light is turned off, an enlarged view of the CPD in the time range from 148 to 152 s is shown in the inset of Fig. 1(c). The changes in CPD of all the samples were observed during dark and illumination cycles. This result showed that the photovoltage characteristics of the CIGS thin films and CdS/CIGS *pn* junction can be evaluated even in the absence of a complete optoelectronic device structure. Furthermore, the ΔCPD values of the CIGS thin film, CdS/CIGS *pn* junction, and CIGS solar cell were approximately 40, 69, and 655 mV, respectively. The SPV technique measures the potential difference between the top and bottom (grounded) contacts of the junction. As a result, SPV can probe carrier dynamics not only at the surface but also within the bulk and at interfaces, capturing processes such as carrier separation in the *pn* junction and recombination of trap states.³⁹,⁴⁰ In CIGS solar cells, the properties of the CdS/CIGS interface have a considerably greater impact on their photovoltaic characteristics, compared with that of the ZnO/CdS and ZnO:Al/ZnO interfaces.⁴¹ Therefore, this study primarily focuses on the CdS/CIGS interface. Larger ΔCPD values for the CdS/CIGS *pn* junction and CIGS solar cell were observed compared with that of the CIGS thin film. Moreover, the ΔCPD value of the CIGS solar cell was also larger than that of the CdS/CIGS *pn* junction. Previous studies have reported that the ZnO layer in CIGS solar cells contributes to large band bending because of the high electric field among the ZnO, CdS, and CIGS interfaces that accelerate carrier separation among the ZnO, CdS, and CIGS interfaces.⁴² This result indicates the effect of the CIGS device structure on its photovoltage characteristics. Furthermore, transient responses of the CPD were observed in all the samples. These transient photovoltage responses likely reflect carrier dynamics that are directly related to the photovoltaic performance.²⁴ Therefore, to further investigate the photovoltage characteristics of CIGS samples, this study focuses on the transient response of the photovoltage using t-SPV measurements to understand the significantly different photovoltages observed among the CIGS samples, which arise from distinct transition processes.

The carrier dynamics related to the photovoltage characteristics of each CIGS sample are discussed by observing the transient response of the photovoltage based on the t-SPV measurements. The model of the photodepopulation of light-induced carriers is shown in Fig. 2(a).²³ The rates of the transition and excitation processes, including trapping/detrapping, are denoted by *G* or *R*, respectively. Therefore, the

<figure><img src="image_16.png" /><figcaption>FIG. 2. Schematic of (a) excitation and carrier transition processes related to light-induced carrier photopopulation and (b) corresponding transient photovoltage responses reflecting carrier dynamics.</figcaption></figure>

nonequilibrium concentration of electrons $n_p$ for p-type semiconductors varies according to the following equation:

$$ \frac{dn_p}{dt} = G - R. \qquad (1) $$

The transient photovoltage response reflecting the carrier dynamics is shown in Fig. 2(b). When the light is on ($t_0 > t > t_1$), the photovoltage eventually reaches a steady state for $G=R$. After the light is off ($t > t_1$), the transition processes primarily occur because of $G=0$, resulting in a slow transient response of the photovoltage. Therefore, the carrier dynamics in the transition processes caused by trap states can be investigated based on the observation of the slow transient response of the photovoltage using t-SPV measurements.²³,⁴³⁻⁴⁵ In addition, a previous study has reported that carrier trapping/detrapping processes caused by deep-level centers or localized states, such as surface states, can be represented as slow transient responses.⁴⁶ Hence, the performances of optoelectronic devices can be directly evaluated by observing the slow transient responses of the photovoltages of the materials using t-SPV measurements.

Subsequently, the carrier dynamics related to the photovoltage characteristics of the CIGS thin film, CdS/CIGS *pn* junction, and CIGS solar cell were investigated using t-SPV measurements. The normalized CPD of the CIGS thin film, CdS/CIGS *pn* junction, and CIGS solar cell under dark and illuminated conditions are shown in Fig. 3. It is noted that the estimated transient response times of the CPD ($t_{CPD}$) of CIGS thin film, CdS/CIGS *pn* junction, and CIGS solar cell were 37.6, 31.4, and 2.3 s, respectively, as shown in Fig. 1(c). In previous reports, $t_{CPD}$ has been calculated theoretically, showing a tendency for slower CPD transient responses compared to those of GaN epilayers.⁴³,⁴⁴ However, it is difficult to theoretically calculate the transient response times of the CPD ($t_{CPD}$) of the CIGS
<figure><img src="image_17.png" /><figcaption>FIG. 3. Comparison of normalized CPD of the CIGS thin film, CdS/CIGS pn junction, and CIGS solar cell recorded using a Kelvin probe as a function of time under dark and illuminated conditions.</figcaption></figure>

samples because they are polycrystalline films, containing grain boundaries, intrinsic defects,¹⁵,¹⁶,³¹ and multiple interfaces, which contribute to discrepancies between the experimentally observed and theoretically calculated $t_{CPD}$ values. Therefore, the $t_{CPD}$ value for each CIGS sample was determined qualitatively based on experimental data. For CIGS thin films grown using the three-stage process, there are intrinsic recombination factors in the surface of the CIGS owing to the Cu-poor CIGS phases.⁴⁷ In addition, external recombination factors, such as surface states due to oxygen or humidity, are also associated with the surfaces of air-exposed CIGS thin films.⁴⁸,⁴⁹ These recombination factors are compensated for after the deposition of the CdS layer in CIGS solar cells.⁵⁰⁻⁵² Moreover, in the CdS/CIGS pn junction and CIGS solar cell, where the surface is covered with CdS or ZnO:Al, the impact of atmospheric oxygen and moisture on the surface state density is considered to be smaller than that on the bare CIGS surface.⁵³,⁵⁴ Therefore, the slower transient response of the CPD of the CIGS thin film indicates that these transition processes are intrinsic to the CIGS thin film. In addition, the transient response of the CPD in the CIGS thin film was observed under both initial and steady-state illumination conditions. This result also indicates the intrinsic transition processes of CIGS thin films. In contrast to the CIGS thin film and CdS/CIGS pn junction, a faster transient response of the CPD was observed in the CIGS solar cell. One of the reasons is that the carrier separation at the CdS/CIGS interface is accelerated owing to the large band bending by the ZnO/CdS/CIGS structure.⁴² The result indicates that acceleration of the carrier separation owing to the ZnO/CdS/CIGS structure contributes to the suppression of the carrier recombination in the CdS/CIGS interface. In addition, the conduction band of CdS is 0.2-0.3 eV higher than that of CIGS,⁵⁵ as shown in Fig. 1(a), forming a so-called “spike”-type conduction band alignment at the CdS/CIGS interface. This conduction band offset does not significantly enhance carrier recombination at the interface.⁵⁵ Instead, this spike may cause electrons to accumulate at the interface, thereby reducing recombination through surface states. Moreover, the conductivity of the ZnO:Al window layer at the surface of the CIGS solar cell enables electrons to be rapidly extracted in the CIGS solar cell [Fig. 1(a)]. These results suggest that

several recombination factors can exist at the surface of the CIGS thin film that can be compensated by CdS and suppressed by the ZnO/CdS/CIGS structure. These discussions emphasize that the performance of optoelectronic devices can be evaluated using t-SPV measurements based on the photovoltage characteristics of not only CIGS but also other thin films, even in the absence of a complete device structure. To further investigate device performance using t-SPV measurements, it is necessary to evaluate the effects of not only the device structure but also the flatness and local variation on the transient response of CPD using SPV measurements with higher resolution and measurement frequency.

Finally, the carrier dynamics related to the photovoltage characteristics of the CIGS thin film, CdS/CIGS pn junction, and CIGS solar cell were investigated using excitation intensity-dependent SPV measurements. The photovoltages of the CIGS thin film, CdS/CIGS pn junction, and CIGS solar cell under various excitation intensities with the enlarged view of the photovoltage of CIGS samples in the photovoltage range from 0 to 100 mV are shown in Fig. 4. The photovoltage of the CIGS samples logarithmically increased with excitation intensity, as indicated by the following equation:⁴³

$$V(t) = \eta k T \ln \left( \frac{c P_o}{R_o} + 1 \right). \qquad (2)$$

$P_o$ is the excitation intensity, $c$ is a geometrical factor that accounts for the portion of photons that are absorbed in the depletion region, and $R_o$ is the rate at which carriers move from the bulk to surface per unit time per unit area. For the CIGS thin film, saturation of the photovoltage was observed at the higher excitation intensity compared with that of the CdS/CIGS pn junction and CIGS solar cell. The higher the density of trap states, such as surface states, the more light-induced carriers are required to fill these states. Therefore, the result of the excitation intensity-dependent SPV measurements indicates that

<figure><img src="image_18.png" /><figcaption>FIG. 4. Representative photovoltage response of the CIGS thin film, CdS/CIGS pn junction, and CIGS solar cell measured using a Kelvin probe under various excitation intensities with the enlarged view of the photovoltage of the CIGS samples.</figcaption></figure>
there is a higher density of trap states in the CIGS thin film compared to the CdS/CIGS *pn* junction and CIGS solar cell. Furthermore, the results also indicate that these localized states in the CIGS thin film may be compensated for by the CdS/CIGS *pn* junctions and CIGS solar cell structures. These discussions are inferred by the results of Fig. 3. In addition, the measurements are performed by varying the excitation intensity to evaluate semiconductor devices.⁵⁶,⁵⁷ Therefore, the results of the excitation intensity-dependent SPV reliably reinforced the discussion in the context of t-SPV. Overall, these results suggest that a deeper understanding of the slow transient response of the photovoltage using t-SPV measurement enables the evaluation of the photovoltaic characteristics of various optoelectronic devices based on CIGS and other materials, even in the absence of a complete device structure. Investigations based on t-SPV measurements represent a promising next step toward simple and direct evaluation methods for a wide range of optoelectronic devices.

In this study, the carrier dynamics related to the photovoltage characteristics of CIGS thin films, CdS/CIGS *pn* junctions, and CIGS solar cells were investigated using SPV, t-SPV, and excitation intensity-dependent SPV measurements to support the advancement of optoelectronic device development. SPV findings revealed that a photovoltage could be generated in CIGS thin films without a complete device structure. In addition, the t-SPV results suggested that surface recombination in CIGS thin films is compensated by CdS and further suppressed by the ZnO/CdS/CIGS structure. Therefore, the applicability of t-SPV as a direct tool to examine the carrier dynamics related to photovoltage characteristics was demonstrated. Finally, the excitation intensity-dependent SPV results provided a deeper understanding of the slower transient photovoltage responses. Overall, t-SPV presents a valuable approach for simple and direct evaluation of the fundamental properties of a broad range of optoelectronic devices.

The authors thank Professor Tokio Nakada for his contribution to the growth of CIGS. This work was supported in part by JSAP Research Fellowships for Young Scientists, JSPS KAKENHI Grant-in-Aid No. JP25KJ2113 and JST SPRING under Grant No. JPMJSP2151. The authors also thank the Research Center for Space System Innovation (SSI) and the Renewable Energy Science and Technology Research Division of Tokyo University of Science for providing the facilities necessary to conduct this work.

## AUTHOR DECLARATIONS

### Conflict of Interest

The authors have no conflicts to disclose.

### Author Contributions

**Kana Ueda:** Formal analysis (equal); Investigation (equal); Writing – original draft (equal). **Thomas Fix:** Data curation (equal); Writing – review & editing (equal). **Mutsumi Sugiyama:** Funding acquisition (equal); Investigation (equal); Supervision (equal); Writing – review & editing (equal).

## DATA AVAILABILITY

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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