Method and system for assessing trap states in optoelectronic devices

Abstract

The present invention relates to method for assessing trap states in an optoelectronic device comprising Pump-Push-Photocurrent (PPPc) spectroscopy, as well as a system for implementing the method. In one aspect, the present invention provides a method for assessing trap states in an optoelectronic device comprising the following steps: i) providing an optoelectronic device sample; ii) performing one or more Pump-Push-Photocurrent (PPPc) spectroscopy measurements on the sample wherein visible light is used as the pump source and infrared (IR) light is used as the push source, said PPPc measurements comprising: a) combining beams of visible light and IR light to be collinear; b) focussing the combined beams on an area of the sample; and c) detecting visible light-induced photocurrents and IR-induced photocurrents in the sample; wherein the PPPc measurements include continuous waveform (CW)-PPPc measurements and / or time-resolved PPPc measurements in which a time delay is provided between synchronised pump and push pulses of visible and IR light; wherein when detecting visible light-induced photocurrents during (CW)-PPPc measurements the beam of visible light in step a) is modulated and the beam of IR light is blocked; and when detecting IR-induced photocurrents during (CW)-PPPc measurements, the beam of IR light in step a) is modulated and the beam of visible light is unmodulated; and wherein when detecting visible light-induced photocurrents during time-resolved-PPPc measurements the beam of visible light in step a) is modulated and the beam of IR light is blocked; and when detecting IR-induced photocurrents during time-resolved-PPPc measurements both the beams of IR light and visible light in step a) are modulated.

Description

[0001] METHOD AND SYSTEM FOR ASSESSING TRAP STATES IN OPTOELECTRONIC

[0002] DEVICES

[0003] The present invention relates to method for assessing trap states in an optoelectronic device comprising Pump-Push-Photocurrent (PPPc) spectroscopy, as well as a system for implementing the method.

[0004] BACKGROUND OF INVENTION

[0005] Remarkable optoelectronic properties and low manufacturing costs make metal halide perovskites promising materials for next generation photovoltaic techniques. Substantial progress in improving perovskite solar cell (PeSC) performance has been achieved in the last decade, bringing the certified power conversion efficiency (PCE) to 26.1 % and making PeSCs the fastest-developing solar cell technology in the history of photovoltaics. Currently, the performance of PeSCs is approaching the Shockley-Queisser limit of a single cell and their further development requires the fundamental reduction of non- radiative recombination in PeSC devices. Electronic defects, known as carrier ‘traps’, are largely responsible for non-radiative recombination under one-sun excitation.

[0006] Deeper understanding of the nature of trap states and the dynamic behaviour of the carriers being trapped in optoelectronic devices, such as PeSCs, is critical to minimise the voltage loss to further improve the device performance. Point defects (e.g., interstitials, substitutions, and vacancies), line defects, grain boundaries in lattices, as well as impurities are considered the main sources of trap states. Structural defects can induce the formation of shallow electronic trap states (within or close to the edge of the valence band (VB) or conduction band (CB)) or deep electronic trap states.

[0007] Reducing the overall trap density in optoelectronic devices is essential for optimising device performance. Multiple strategies have been suggested to reduce the density and the influence of traps in perovskites, including film formation optimisation, crystal strain relaxation, compositional engineering, and crystallisation control. Passivating traps at the interfaces between the optoelectronic layer and charge extraction layers is a popular and effective option, which works by reducing the surface trap density. However, the energy, cross-section, and location of trap states also influence the device. Moreover, the dynamics of carrier interactions with the trap states (e.g., the time scale for carriers to fill, release and recombine from trap states) are also critical for understanding the impact of traps on performance and for further device optimisation.

[0008] Conventional optical spectroscopic techniques struggle to selectively observe the dynamics of carrier trapping in optoelectronic devices. For example, in transient absorption spectroscopy (TAS), the spectrum of trapped carriers significantly overlaps that of free carriers which generally also have a much larger population than the population in traps so that any signal from trapped carriers is swamped by the free carrier signal.

[0009] In addition, it is difficult to directly monitor carriers in trap states with photoluminescence based methods due to the non-emissive nature of recombination via these states. Instead, non-radiative trap-assisted recombination can often be deduced indirectly from a photoluminescence signal by establishing simple radiative models. CN107091822A, for example, describes a method for detecting semiconductor defects through dual-light- source stimulation photoluminescence. However, the resulting picture can be unclear with various possible scenarios for trapped carrier dynamics. Finally, TAS and photoluminescence methods are difficult to apply to working photovoltaic devices under typical solar illumination conditions.

[0010] CN113970559A describes a semiconductor deep energy level defect detection device and detection method which relies on the generation and detection of a photocapacitance signal. The setup in CN113970559A can, however, only determine the presence of deep traps, but cannot provide more detailed information, such as the population, depth, and lifetime. It is also incapable of providing time-resolved information of trapped carriers, i.e., their kinetic properties.

[0011] There remains a need for further spectroscopic techniques which address the limitations of known techniques and allow assessment of the dynamics of trapped carriers sensitively and selectively.

[0012] SUMMARY OF INVENTION

[0013] The present invention is based on the use of photocurrent detection to overcome the limitations of prior art methods. In particular, the present invention utilises “pump-push- photocurrent spectroscopy” (PPPc) as part of a method and system for assessing traps. When an optoelectronic device is exposed to an optical “pump” beam it elicits band-to- band excitation and band-edge carriers are generated as a consequence, some of which subsequently or simultaneously become trapped. Then the sub-bandgap energy photons of an IR “push” beam are absorbed by a fraction of the trapped carriers to excite them back to their (conduction) band states, as part of inter-band excitation. The IR de-trapped carriers (which might otherwise have recombined) then contribute to an additional photocurrent generated by the device. The amplitude of IR-induced current can be used to evaluate the concentration of trapped carriers in the device.

[0014] The method and system can be operated using the beams of continuous-wave (CW) lasers with slow (ps-ms) modulation to simply assess and compare the extent of charge trapping in different devices. Alternatively or additionally, short fs-ns pulses can be used as pump and push to evaluate the dynamics of trap filling and the recombination kinetics of trapped carriers as part of time-resolved PPPc measurements. The method and system of the present invention offer direct, real-time, high-resolution trap information critical for optimising device performance.

[0015] Thus, in a first aspect, the present invention provides a method for assessing trap states in an optoelectronic device comprising the following steps: i) providing an optoelectronic device sample; ii) performing one or more Pump-Push-Photocurrent (PPPc) spectroscopy measurements on the sample wherein visible light is used as the pump source and infrared (IR) light is used as the push source, said PPPc measurements comprising: a) combining beams of visible light and IR light to be collinear; b) focussing the combined beams on an area of the sample; and c) detecting visible light-induced photocurrents and IR-induced photocurrents in the sample; wherein the PPPc measurements include continuous waveform (CW)-PPPc measurements and / or time-resolved PPPc measurements in which a time delay is provided between synchronised pump and push pulses of visible and IR light; wherein when detecting visible light-induced photocurrents during (CW)-PPPc measurements the beam of visible light in step a) is modulated and the beam of IR light is blocked; and when detecting IR-induced photocurrents during (CW)-PPPc measurements, the beam of IR light in step a) is modulated and the beam of visible light is unmodulated; and wherein when detecting visible light-induced photocurrents during time-resolved- PPPc measurements the beam of visible light in step a) is modulated and the beam of IR light is blocked; and when detecting IR-induced photocurrents during time-resolved-PPPc measurements both the beams of IR light and visible light in step a) are modulated.

[0016] In another aspect, the present invention provides a system for assessing trap states in an optoelectronic device using Pump-Push-Photocurrent (PPPC) spectroscopy; said system comprising: i) an optoelectronic device; ii) a source of visible light for use as a pump source and a source of infrared (IR) light for use as a push source; iii) one or more modulators for modulating the visible light and IR light; iv) a beam combiner or beam splitter configured to combine separate beams; and v) a photocurrent detector which is a lock-in amplifier.

[0017] BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is a schematic diagram illustrates the fundamental operating principle of the PPPc technique;

[0019] Figure 2 is a schematic diagram illustrating CW- and time-resolved PPPc setups that may be utilised in accordance with the present invention;

[0020] Figure 3 shows AJIR responses of FAo.ggCso.oiPb device under different illumination conditions in CW-PPPc measurement;

[0021] Figure 4 shows an Arrhenius plot of the temperature-dependent trapped carrier concentration for FAo.ggCso.oiPb device photocurrent (JpUmP) measurements recorded as a function of temperature;

[0022] Figure 5 shows modelled nTC dynamics at the perovskite surface in the samples investigated in the Examples; Figure 6 shows the ratio of IR-induced photocurrent (A / ) to vis-induced photocurrent (Jpump) over time for PPPc measurements conducted on an LED device investigated in the Examples;

[0023] Figure 7 shows spatial amplitude mapping (photovoltaic performance) for a perovskite solar cell sample;

[0024] Figure 8 shows spatial amplitOude (signatures of defects) for a perovskite solar cell sample; and

[0025] Figure 9 shows an example of a flexible scanning architecture for implementing a mapping technique, incorporating a sample mounted on an x-y stage.

[0026] DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention offers an alternative solution to known methods by identifying electronic defect trap density and lifetime using photonic-induced photocurrent. It utilises a visible photon source to induce free carriers, some of which are captured by trap states, causing efficiency loss. These captured carriers are then stimulated by infrared photons to produce a capturable photocurrent. The analysis of the IR-photon-induced photocurrent helps to quantify trap properties. PPPc spectroscopy as it applies to perovskite solar cells is discussed in detail by the inventors in Nature Communications; 2023, 14, 8000, pp 1 to 10, the content of which, together with the supplementary information available in support of the article, is incorporated herein by reference.

[0028] PPPc spectroscopy utilised in the present invention can be readily implemented by the skilled person by providing a source of visible light (vis) for use as a pump source and a source of infrared (IR) light for use as a push source (e.g. by means of two diode lasers). The visible light pump source is a “band-to-band excitation” optical source, since it elicits excitation between valence and conduction bands. The IR light push is a “trap-to-band excitation” optical source, since it specifically elicits excitation of charge carriers from the trap states within the material bandgap (i.e. , electronic states between valence band and conduction band) to the conduction band. In the present invention, the PPPc spectroscopy measurements may be configured for use with continuous-wave (CW) push and pump beams of IR and visible light, respectively, which allows an assessment of trapped carriers on a ps to ms time scale. Additionally, in the present invention, PPPc spectroscopy measurements may be configured for time- resolved measurements using pulses of IR and visible light as push and pump beams, where timing between the pump and push is controlled, for instance, by an electrical delay generator. These measurements allow an assessment of trapped carriers on a ns to ps time scale.

[0029] The CW-PPPC measurements provide quasi steady-state information, such as density and depth of traps in a sample which is investigated, as well as mean lifetime of trapped carriers. The time-resolved PPPC measurements provide transient information, such as the kinetics of trapped carriers. Through analysing IR-induced photocurrent, it is possible to obtain information about the trapped carrier population from ns through to ms time scales, via the combination of CW- and time-resolved PPPc spectroscopies.

[0030] Figure 1 illustrates the fundamental operating principle of the PPPc technique, showing the hole extraction (for current generation) with and without IR push (all arrows indicating physical processes in terms of holes). Under exposure to visible light, a photocurrent density Jwithout-IR (also referred herein to as Jpump ) is extracted from the device. This photocurrent is composed from thermalised (cold) free carriers so that Jwithout-IR = Jcold-carrier. When the IR push beam illuminates the device and a new photocurrent density Jwith-IR is observed. Some trapped carriers are optically detrapped by the IR and contribute an additional component J bound-carrier to the photocurrent density. Bandedge free carriers will also be excited to higher energy states and some of these will also contribute an amount Jhot-carrier to Jwith-IR. However, this reduces the population of thermalised carriers contributing to Jwith-IR by a similar amount so that Jcold-carrier = Jwithout-IR - Jhot-carrier. Since most hot carriers quickly cool back to band-edge to become cold carriers typically within 1 ps - which is much faster than the time scale for charge collection at the contacts (approximately 100 ns) - Jhot-carrier is expected to be very small (and hot carrier trapping is generally not observed). Therefore, the additional photocurrent density induced by the IR beam is given by: JIR = Jwith-IR - Jwithout-IR

[0031] = [Jhot-carrier + (Jwithout-IR - Jhot-carrier) + J bound-carrier] - Jwithout-IR = Jbound-carrier

[0032] The high selectivity of PPPc, which is only sensitive to bound species in the device and which makes unbound species undetectable, has been found by the inventors to be a particularly valuable tool in the assessment of trap states in optoelectronic devices, such as solar cells.

[0033] Thus, in a first aspect, the present invention provides a method for assessing trap states in an optoelectronic device comprising the following steps: i) providing an optoelectronic device sample; ii) performing one or more Pump-Push-Photocurrent (PPPc) spectroscopy measurements on the sample wherein visible light is used as the pump source and infrared (IR) light is used as the push source, said PPPC measurements comprising: a) combining beams of visible light and IR light to be collinear; b) focussing the combined beams on an area of the sample; and c) detecting visible light-induced photocurrents and IR-induced photocurrents in the sample; wherein the PPPc measurements include continuous waveform (CW)-PPPc measurements and / or time-resolved PPPc measurements in which a time delay is provided between synchronised pump and push pulses of visible and IR light; wherein when detecting visible light-induced photocurrents during (CW)-PPPc measurements the beam of visible light in step a) is modulated and the beam of IR light is blocked; and when detecting IR-induced photocurrents during (CW)-PPPc measurements the beam of IR light in step a) is modulated and the beam of visible light is unmodulated; and wherein when detecting visible light-induced photocurrents during time-resolved- PPPc measurements the beam of visible light in step a) is modulated and the beam of IR light is blocked; and when detecting IR-induced photocurrents during time-resolved-PPPc measurements both the beams of IR light and visible light in step a) are modulated.

[0034] As part of performing PPPc spectroscopy measurements on a device sample, beams of pump visible light and push IR light are a) combined and b) focussed on an area of the sample. Reference to visible light herein refers to optical radiation of wavelength from 200 nm to 820 nm, preferably from 380 nm and 780 nm. “Visible light” in accordance with the present invention may thus include wavelengths which may be considered to be in the near-infrared range, although the majority of wavelengths for the pump optical source are in the visible region. Reference to IR-light herein refers to optical radiation of wavelength of from 830 nm to 1 mm. Preferably, pump visible light and push IR light wavelength is selected so as to have a difference in wavelength of at least 100 nm, more preferably at least 150 nm. Visible and IR-light sources may be provided by laser diode and / or LED, and preferably provided in the form of laser diode radiation sources.

[0035] Combining beams in the context of the present invention refers to overlapping in space by combining visible and IR-light beams so as to be collinearthrough the use of beam-overlap optics, for instance, in the form of a beam combiner or beam splitter configured to combine separate beams. As the skilled person will appreciate, multiple visible-light and / or multiple IR-light beams may be combined, if desired. Typically, both visible and IR-light beams pass one or more lenses and one or more spatial filter apertures (e.g. pin holes) to improve the quality of the beams, by removing noise and higher-order modes, and thereby optimising the beams for combination. The skilled person is then able to focus the combined beams by any conventional means on an area of the device sample for investigation. For example, where visible light and IR-light sources are provided by a laser diode or LED, focussing means may be provided in the form of a single- (laser diode) or multi-lens (laser diode or LED) optic, which can be manipulated by varying its focal length and the diameter of the free aperture. The area of the device over which the combined visible pump and IR push beams is focused may be any suitable size to probe the area of interest. The area of the overlapping spot is suitably from 1 E'10cm2to 100 cm2, preferably from 1 E'6cm2to 1 cm2.

[0036] As part of the method, when detecting visible light-induced photocurrents, the beam of visible light in step a) is modulated and the beam of IR light is blocked; and when detecting IR-induced photocurrents, the beam of IR light in step a) is modulated and the beam of visible light is unmodulated. As discussed below, PPPc measurements are used to determine the ratio of IR-induced photocurrent (AJIR) to vis-induced photocurrent (Jpump), since this reflects the ratio between trapped and free carriers in the device sample. In order to separately determine the IR-induced photocurrent (AJIR) from vis-induced photocurrent (JpUmp) , IR light is blocked when modulated visible light is selected to induce detectable photocurrent. For CW-PPPc, visible light is unmodulated when modulated IR light is selected to induce a detectable photocurrent (AJIR). The absence of modulation in the visible light means that a photocurrent detector is able to isolate the signal induced from modulated IR light to determine IR-induced photocurrent (AJIR). For time-resolved PPPc, visible light is also modulated when modulated IR light is selected to induce a detectable photocurrent (AJIR). Visible and IR light will be delayed by a delay generator and modulated at different frequencies. The different-frequency modulation is able to isolate the signal induced from modulated IR light to determine IR-induced photocurrent (AJIR). The modulation of light can be conducted by using an optical chopper, a fiber switch or modulator, or by applying square-wave voltage to the light sources.

[0037] The method of the invention incorporates a detection step involving low-noise photocurrent collection, preferably comprising amplification electronics to enable detection of small currents. In order to reduce noise, or the effects thereof, as far as possible, use of a lock-in amplifier as the photocurrent detector is preferred. Additional steps may also be taken to improve measurement performance, including the use of shielded cables, the removal of static electricity, and the use of low-noise electric sample holders (e.g. a Faraday cage).

[0038] The invention will now be described in reference to Figure 2, which is a schematic diagram illustrating CW- and time-resolved PPPc setups that may be utilised in accordance with the present invention. Figure 2 shows a visible pump (101) and IR push (102) diode laser beams which are combined together using a beam combiner (not shown), such as a dichroic mirror, so as to be overlapped in space and made collinear, before being focused on an area of the sample (103) in the form of an overlapping spot on the surface.

[0039] A modulator (104), such as an optical chopper (e.g. rotating disc chopper or optical shutter), is provided which modulates the IR push beam (102) in the Figure 2 representation and a reference signal is sent to a photocurrent detector (105), such as a lock-in amplifier, to detect the IR-induced photocurrent(A / R). The modulator (104) may be relocated to the pump beam for the detection of visible pump-induced photocurrent (JPUMP), as part of the PPPc measurements. A lock-in amplifier has been found by the inventors to improve signal to noise when measuring photocurrent and also assists in eliminating photocurrent generated by the visible-light pump. Thus, use of a lock-in amplifier is preferred as the photocurrent detector (105) in the present invention. Once vis-induced photocurrent (JPUMP) and IR-induced photocurrent (A / ) values are measured, the ratio of AJIR / JPUMP may be determined, from which it is possible to quantitatively assess the trap filling behaviour of the optoelectronic device, including trap density and other physical properties, as described herein. To elucidate the energetics of trap states and to evaluate trap depth temperature-dependent CW-PPPc measurements can be used. By obtaining AJIR / JPUMP data over a variety of device temperatures and then fitting an Arrhenius model to the data (implemented by known software packages such as Origin and Matlab), it is possible to assess the dominant activation energy of traps and their characteristic depth.

[0040] The method of the invention is thus capable of directly identifying trap parameters of optoelectronic devices, substantially eliminating the assumptions required by existing technologies such as photoluminescence and transient absorption spectroscopy. This allows manufacturers to conduct high-resolution, comprehensive analyses for optimising optoelectronic designs and materials for their manufacture. It also allows manufacturers to accurately assess the benefits of post-treatments on device performance.

[0041] It has been postulated by the inventors that the IR push beam can de-trap carriers via two possible mechanisms: (i) via direct optical transition from trap to band state or (ii) via thermal activation, after the energy from absorbed IR photons increases the temperature of the optoelectronic device under investigation. However, in the course of developing the invention the inventors have found (see Example 2 below) that upon heating of the optoelectronic device detectable photocurrent from illumination decreases, indicating that the increase of the IR-induced photocurrent does not originate from an IR-heating effect. This validates the conclusion that it is via a direct optical transition mechanism that the IR push beam de-traps carriers.

[0042] By varying the intensity of pump and push beams in CW-PPPc measurements, it is possible to assess trapped carrier concentration and densities of traps. IR-induced photocurrent not only relates to the density of traps, but also involves the factor of charge extraction and therefore the ratio of IR-induced photocurrent (AJIR) I vis-induced photocurrent Jpump) reflects the ratio between the trapped and free carriers. To quantitatively analyse the trap filling behaviour of an optoelectronic device, the relative concentration of trapped carriers (n-rc) can be calculated from a relationship involving the gradient of (AJIR) / (Jpump) against IR intensity. Reference is made to Supplementary Note 3 of the supplementary information published in support of Nature Communications; 2023, 14, 8000, pp 1 to 10, from the inventors, which evaluates the derivation of the above relationship and shows how relative concentration of trapped carriers (nTc) can be assessed from experimentally measured (A / R) and (JpUmp) values. To elucidate the energetics of trap states and to evaluate trap depth temperature-dependent CW-PPPc measurements can be used. By obtaining AJIR / JPUMP data over a variety of device temperatures and fitting an Arrhenius model to the data, it is possible to assess the dominant activation energy of traps and their characteristic depth.

[0043] Thus, in some embodiments, the CW-PPPc measurements comprise intensity-dependent measurements in which push and pump beam intensity is varied, frequency-dependent measurements in which the frequency of push and pump light sources is independently varied, and / or thermal-dependent measurements in which the temperature of the sample is varied. Suitable pump intensities for use in the present invention are from 1 to 1 ,000,000 mW cm-2, preferably from [100 to 10,000 mW cm-2]. Suitable push intensities are from 1 to 10,000,000 mW cm-2, preferably from [1 ,000 to 100,000 mW cm-2]. A suitable frequency range over which the CW-PPPc measurements may be conducted is from 1 Hz to 10 MHz. A suitable temperature range over which the CW-PPPc measurements may be conducted is from 3 K to 1000 K, preferably from 78 to 400K, more preferably from 78 K to 300 K.

[0044] In order to assess the dynamics of trapped carrier population, time-resolved PPPc measurements may be used employing synchronised pulsed visible- and I R-light sources. The time delay between the pump and push pulses may be achieved by any suitable means, for example by means of an electrical delay generator. Total IR-induced current value (R value from the lock-in amplifier) of the detected signal provides a good description of population dynamics. Suitably, the time-resolved PPPc measurements are conducted with pulsed laser light, wherein individual pulses of pump visible light and / or push IR light are for a duration of 1 fs to 1 ms, preferably from 100 fs to 100 ns, more preferably from 1 ps to 10 ns.

[0045] When an optoelectronic device is in a first order recombination regime, n-rc as a function of time can be calculated from the time-resolved (AJIR) I (Jpump) data. For example, it is possible to monitor the average carrier concentration in an optoelectronic device on a ns- ps time scale. A first order recombination region is observed when Jpump varies linearly with light intensity. Consequently, the skilled person is able to pre-determine a region over which that linear relationship prevails across a range of different light intensities to which a particular sample is exposed (so as to ensure the measured device is mainly under trap- assisted recombination). Thus, in some embodiments, the CW and / or time-resolved-PPPc spectroscopy measurements of the method are performed over a range of visible pump and IR push light intensities over which Vis-induced photocurrent in the sample has previously been shown to vary linearly with light intensity.

[0046] There are advantages to increasing light intensity in order to strengthen the photocurrent signal, which can facilitate detection and therefore assessment of a sample. Increasing the light intensity does, however, eventually push the Jpump values into a non-linear relationship with light intensity. In that case, it is necessary to account for the non-linear relationship when calculating n-rc as a function of time from time-resolved (AJIR) I (Jpump) PPPc measurement data. Thus, in some embodiments, the time-resolved PPPc spectroscopy measurements are performed over a range of visible pump and IR push light intensities over which Vis-induced photocurrent in the sample has previously been shown to vary non-linearly with light intensity, and the method further comprising applying a conversion factor to the obtained measurements to account for measurement in the nonlinear light intensity-photocurrent region. For example, applying drift-diffusion mode to detangle the impact of band-edge carrier and clearly resolve the dynamics of trapped carriers in the non-linear light intensity-photocurrent region.

[0047] The method of the present invention may readily be applied to in-line testing of multiple samples (for instance, on a moving conveyor) so that any inconsistencies between manufactured devices (e.g. of a particular batch) can be identified and isolated quickly. Using in-line testing to assess trap states can also be used to understand the nature of traps across a batch and help identify the extent of any post-treatments that may usefully be applied to the devices to improve performance, or otherwise compensate for the degree and nature of trap states exhibited by a particular batch.

[0048] As part of the method for assessing traps in a sample in accordance with the invention, it is also possible to map the location and nature of trap states over the landscape of the device surface to understand the extent of trap state variability and assess whether concentrations of traps are present at certain locations. Such mapping of the surface has been found by the inventors to be readily implemented by conducting the PPPc spectroscopy measurements of step ii) in one or more additional area(s) of the same device sample. Once additional PPPc spectroscopy measurements have been taken at different areas of the sample, it is possible to map a degree of variation in trap properties across the dimensions of the sample.

[0049] The mapping technique is based on continuous-wave Pump-Push Photocurrent (CW- PPPc) measurements where local (AJIR) I (Jpump) point data is converted into spatial surface maps, visualising defect distributions and non-uniform electronic responses across the sample. Such a mapping technique thus enables spatially resolved defect characterization by scanning the optical excitation across the sample surface and recording the corresponding IR and Vis induced photocurrent response at each position. Maps may be constructed (using standard scientific data-processing tools such as MATLAB or Python) encompassing spatial, temporal, spectral, and intensity dimensions of trap states and charge-carrier dynamics. Spatial amplitude mapping is, for example, illustrated in Figures 7 (photovoltaic performance) and 8 (signatures of defects) for perovskite solar cell samples. An increase in trap signal (IR induced photocurrent) generally correlates with a drop in performance (vis induced current) allowing evaluation of the loss mechanism in different areas.

[0050] Preferably, mapping is accomplished using a fiber-based beam delivery and a flexible scanning architecture (either moving fiber or moving sample), enabling high-stability measurements and scalability (e.g. for industrial or in-line inspection applications). Such a scanning configuration can be adapted based on the experimental setup or sample geometry. For example, a fiber beam combiner (carrying collinear vis / IR beams) may be moved across the surface of a sample. Alternatively, a fiber beam combiner may be kept stationary and the sample may be mounted on an optionally motorized x-y stage (as illustrated in Figure 9) and moved (in the x-y plane) relative to the fiber beam combiner. As will be appreciated, mounting on an x-y stage allows for precision positioning of a sample through precise movement in perpendicular directions over the x and y axes, meaning that different areas of the sample may be subjected to CW-PPPc measurements following movement of the mounted sample.

[0051] Both approaches achieve high spatial precision and stability, and the choice between them depends on factors such as sample size, weight, optical alignment, and desired throughput for production or laboratory measurements. The desired spatial resolution in each case may be readily achieved by modifying illumination area, whilst averaging time, modulation frequency, total measurement time can also be adjusted as desired. If applicable, higher-power infrared lasers may be used to boost the signal to allow measurements over a wider sample area.

[0052] The PPPc spectroscopy measurements may be conducted on each of the different areas of the sample at the same time using different light sources (e.g. using multiple different laser diodes) in a system configured for extracting multiple PPPc spectroscopy measurements at the same time. For instance, multiple light sources may be utilised or means for splitting the optical source into multiple beams. In the case of split beams, optical choppers with different frequencies for each split beam can be used to conduct multiple PPPc spectroscopy measurements at the same time. A single lock-in amplifier can be used for the different measurements, where the lock in amplifier is configured with multiple channels (one measurement in parallel is possible per electrode, if the lock-in amplifier has a single readout electrode only one two-channel lock in may be used - one channel for pump and one for push). Alternatively, the PPPc spectroscopy measurements on each of the different areas of the sample are conducted sequentially using the same light source. This option can be readily implemented by simply focussing the light beams on to a different area of the sample, or physically adjusting the positioning of the sample relative to the focussed light beams. As the method of the invention can be utilised with variable overlap areas upon which light beams are focused, the method may be readily adapted for different device samples sizes.

[0053] As discussed herein, the method of the invention provides a method for the high resolution detection of defects giving rise to the presence of trap states. The method is non-invasive, compatible with fully automated detection (e.g. through the use of spectroscopic software), with entirely non-destructive testing. Consequently, it is possible to conduct testing in accordance with the method of the invention, determine the nature of trap states, and consider options for post treatment of a device to mitigate the effects of traps on device performance.

[0054] Thus, in some embodiments, the method of the invention further comprises performing a post-treatment on the sample intended to reduce the concentration or mitigate effects of traps; and optionally repeating Pump-Push-Photocurrent (PPPc) spectroscopy measurements on the post-treated sample to determine the effect of the post-treatment on the nature and / or concentration of the trap states in the post-treated sample.

[0055] Any known post treatment that may be applied to optimise performance of an optoelectronic device may be utilised, the effects of which being monitorable by means of the method of assessing trap states described herein. Suitable post treatments include surface passivation, recrystallisation, and interfacial re-engineering. Examples of surface passivation include treatment with n-octylammonium iodide (OAI), phenethylammonium iodide (PEAI), phenylethylamine bromide (PEABr), or oxygen gas. Recrystallisations may be achieved through thermal annealing, solvent annealing, and / or gas treatment, familiar to the skilled person.

[0056] In another aspect, the present invention also provides a system for assessing trap states in an optoelectronic device using Pump-Push-Photocurrent (PPPc) spectroscopy; said system comprising: i) an optoelectronic device; ii) a source of visible light for use as a pump source and a source of infrared (IR) light for use as a push source; iii) one or more modulators for modulating the visible light and IR light; iv) a beam combiner or beam splitter configured to combine separate beams; and v) a photocurrent detector which is a lock-in amplifier.

[0057] The optoelectronic device which may form part of the system of the invention, or be the subject of investigation in the method of the invention, is not particularly limited and may be selected from any electronic device capable of detecting, generating, and interacting with or controlling light and typically comprising a semiconductor material. Examples include light sources such as laser diodes and light-emitting diodes (LEDs); light-to- electrical converters such as solar cells; light propagation controllers (i.e. devices that can electronically control the propagation of light); and photodetectors. Preferred optoelectronic devices useful with the present invention are selected from light-emitting diode (LEDs), solar cells, and photodetectors. More preferably, the optoelectronic device is a solar cell. Any suitable solar cell may be used in connection with the present invention, including organic, inorganic or dye-sensitized solar cells. Solar cells are well known to the skilled person and may be selected from any suitable semi-conductor material including monocrystalline, polycrystalline, or amorphous semiconductor materials. Examples of suitable solar cell materials include quantum dot materials, perovskites, silicon, cadmium telluride, copper indium gallium diselenide, and gallium arsenide, and combinations thereof. Preferably, the optoelectronic device useful in the present invention comprises a perovskite, which cation may be organic or inorganic. Examples of perovskites with organic cation include methylammonium (MA) halide perovskites (such as MAPbh, MACsPbh, MAPbBra, and MASnh) and formamidine perovskites. Examples of perovskites with inorganic cation include cesium lead halides (such as CsPbBra).

[0058] The sources of visible and IR-light for use in the system and method of the invention are most suitably provided by laser diodes, although alternative sources may be used, such as LED. Any suitable form of modulation may be applied to the visible or IR-light sources, although it is preferably in the form of external modulation, for example through mechanical methods. Suitable modulators include shutters, choppers, and spinning disks. Preferably, an optical chopper modulator is used, more preferably in combination with a chopper controller to control its frequency. Internal modulation may also be used. The system and method may also include devices to vary the fluence of pump or push pulses in the time-resolved PPPc measurements (e.g. motorised filter wheels).

[0059] Any suitable beam-combiner may be used in the system of the invention, including conventional beam splitters that are configured to combine separate beams. The skilled person is readily able to select a device for this purpose as desired. For example, a dichroic mirror may be used as the beam combiner. Alternatively, and preferably, a fiber optic (single-mode or multi-mode) is used as the beam combiner, using a fused biconical tapering process. Most preferably, a multi-mode beam combiner is used, for example a 2- to-1 fiber (such as the Thorlabs Multi-Mode Fiber Optic Coupler - MP2LS1®) which is effective for combining two beams for the desired spectroscopy measurements.

[0060] However, the beam combiner may also be a multi-mode fiber accommodating n-to-1 fiber configurations, where n is >2 (e.g. 3, 4 or 5). This flexibility allows for adjustment of the number of beams combined, which is particularly useful when working with complex setups or materials requiring different beam arrangements (e.g., multiple light sources for advanced mapping or multi-wavelength measurements). In addition, the choice of fiber and its configuration (e.g., single-mode or multi-mode) will be selected based on the material properties of the device being tested and the optical requirements for the specific application (e.g., higher power, more beams, different wavelengths).

[0061] As discussed herein, multiple CW-PPPc measurements may be performed on a sample enabling the construction of maps encompassing spatial, temporal, spectral, and intensity dimensions of trap states and charge-carrier dynamics across the sample. Thus, in some embodiments, the system may employ a fiber beam combiner and may further comprise an optionally motorized x-y stage (as illustrated in Figure 9) to which a sample may be mounted.

[0062] As discussed herein, increasing pump visible light and / or push IR light intensity can be advantageous for increasing photocurrent signal, facilitating detection in materials with low concentrations of trap states or deeply located trap states, that might be associated with weaker signals. Management of light intensity can be particularly useful to remove bimolecular effects in time-resolved measurements. Increasing light intensity can, however, mean that photocurrents detected from using such higher light intensities is across a region where light intensity varies non-linearly with photocurrent produced and detected by the system. Nevertheless, using greater light intensities can be useful when assessing device samples of superior quality, where concentration of traps is expected to be lower in comparison to devices of inferior quality.

[0063] Thus, in some embodiments, the system of the invention incorporates sources of visible and IR light that are configured to operate over a range of visible pump and IR push light intensities over which visible light-induced photocurrent produced in the sample has previously been shown to vary non-linearly with light intensity. A drift-diffusion model will be in involved to detangle the impact of band-edge carrier and clearly resolve the dynamics of trapped carriers.

[0064] As discussed above, the method of the invention may be operated on multiple areas of a device sample in order to map the location and concentration of traps across the sample. Thus, in some embodiments, the system of the invention is configured for making multiple Pump-Push-Photocurrent (PPPc) spectroscopy measurements at different locations of the optoelectronic device sequentially or contemporaneously. For example, the system may be configured with multiple different light sources, beam combiners, lock-in amplifiers (including those with multiple channels) in order to take multiple measurements contemporaneously. Alternatively, the system may be configured with a device to control the positioning of the sample device so that sequential measurements may be quickly taken at different areas of the sample. Recording and mapping of trap states can also be automated by using computational tools and spectroscopic software which may also include predictive modelling functionality.

[0065] The invention will now be discussed in reference to the following non-limiting examples.

[0066] Examples

[0067] Solar Cell Device

[0068] To control the density of traps and evaluate the effects of defect-reduction strategies in perovskite solar cells (PeSCs), the inventors fabricated FAo.ggCso.oiPbh PeSCs with and without surface passivation. The architecture of the pristine (non-passivated) device was indium tin oxide (ITO) / Sn02 / FAo.ggCso.oiPbl3 / 2,2’,7,7’-Tetrakis[N,N-di(4- methoxyphenyl)amino]-9,9’-spirobifluorene (Spiro-OMeTAD)ZAu, while that of the surface-passivated device was ITO / Sn02 / FAo.ggCso.oiPbl3 / n-octylammonium iodide (OAI) / Spiro-OMeTAD / Au. OAI, which is widely used with perovskites, served to passivate interfacial traps between the FAo.ggCso.oiPbh and the spiro-OMeTAD. These device compositions and architectures were selected for their excellent ambient (~months), light, and thermal stability without the need for encapsulation as well as for being a typical model system studied across the perovskite photovoltaic community.

[0069] To investigate how the trapped carrier behaviour changes in higher-performance devices, the inventors optimised sample devices for testing by depositing an ultra-thin polymethyl methacrylate (PMMA) layer on top of the SnO2 layer [i.e., with device architectures of ITO / SnO2 / PMMA / FA0.99Cs0.01 Pbl3 / (OAI) / Spiro-OMeTAD / Au], PMMA can passivate the bottom surface of perovskite via Lewis-base-Lewis-acid interaction between the C = O group and the uncoordinated Pb, resulting in a reduction of overall trap states. As a consequence of these optimisations, much improved power conversion efficiencies from 16.1 % to 19.8% for the pristine device and from 19.5% to 23.2% for the surface-passivated device were achieved. Precursor

[0070] A perovskite FAo.ggCso.oi bh active layer precursor was prepared by dissolving 1.81 M lead iodide (Anhydro Beads, 99.999% Sigma-Aldrich), 1.65 M formamidium iodide (Greatcell solar), 0.58 M methylamonium choloride (Xi’an Polymer Light Technology Corp.), 0.016 M Cesium iodide (99.999% Sigma-Aldrich) in 1 ml anhydrous N, N- dimethylformamide (99.8%, Sigma-Aldrich) and dimethyl sulfoxide (99.9%, Sigma- Aldrich) mixed solution with the volume ratio of 8:1. An ion-modulated radical doping of spiro-OMeTAD was prepared by mixing 90 mg ml-1spiro-OMeTAD in the chlorobenzene solution with 6 mol% of spiro-OMeTAD2 +(TFS|-)2and 18 mol% TBMP+TFS|- A PMMA solution was prepared by dissolving PMMA (Mw = ~4000) in chlorobenzene with a concentration of 2 mg ml-1.

[0071] Device fabrication

[0072] All indium tin oxide (ITO) substrates were cleaned sequentially in deionized water and ethanol for 15 min respectively, and dried by a compressed nitrogen gun. After 15 min UV-Ozone surface treatment, the SnO2 electron transport layer was deposited by spin coating a 1 :6 diluted SnO2 nanoparticle water solution (Alfa Aeser) at 4000 rpm for 30 s, followed by annealing at 150 °C for 20 min in air. For the device with PMMA layer, the PMMA solution was spin-coated (4000 rpm for 30 s) on top of the SnO2 layer and followed by annealing at 100 °C for 8 mins in a N2- filled glovebox. The 800-nm perovskite layer was then deposited via spin coating the perovskite precursor at 5000 rpm for 30 s. Within 10 s of the 5000-rpm spinning, 100 pL of chlorobenzene as the antisolvent was dropped onto the film. After spin-coating, the perovskite film was then annealed at 150 °C for 15 min in ambient air. Then 5 mg ml-1 OAI (Greatcell solar) solution in tert-butanol (anhydrous, 99.5%, SigmaAldrich) was spin-coated onto the perovskite surface at 5000 rpm and annealed at 100 °C for 3 min for the surface passivation. Later, the hole transport layers (ion-modulated radical doped spiro-OMeTAD) were deposited by spin coating at 5000 rpm for 30 s without further annealing. A metal electrode (80-nm Au) was finally deposited through thermal evaporation method under a vacuum degree higher than 3 x 10-6Torr to accomplish the solar cell fabrication. A 0.06 cm2shadow mask was used to define the effective working area of the solar cells. All the devices were used unencapsulated. Perovskite film and device characterisation

[0073] Ultraviolet-visible absorption spectra were measured with a PerkinElmer model Lambda 900. For PL quantum yield measurement, the perovskite films were directly deposited on glass substrates. The excitation source was a 635-nm CW laser and the excitation power was 100 mW. The signal was detected by the spectrometer (and / or spectrometer, Oxford Instruments) through an integrated sphere. XPS measurement (monochromatised Al Ka hv = 1486.6 eV) was carried out using a Scienta-200 hemispherical analyser with dedicated home designed and built spectrometer. Data were calibrated by referencing to Fermi level and Au 4f7 / 2 peak position of the Ar+ ion sputter-clean gold foil. The experimental condition was set so that the full width at half maximum of the clean Au 4f7 / 2 line (at the binding energy of 84.00 eV) was 0.65 eV. X-ray diffraction (XRD) patterns were obtained from an X-ray diffractometer (Panalytical X’Pert Pro) with an X-ray tube (Cu Ka, A = 1.5406 A). Photocurrent-voltage curves were measured (2400 Series Source Meter, Keithley Instruments) in a N2-filled glovebox and under simulated AM 1.5 sunlight at 100 mW cm-2irradiated by an Enlitech AAA sun simulator, with the intensity calibrated by a Si reference cell. The active area of the solar cell was 0.06 cm2. The forward photocurrentvoltage scans were measured from -0.2 V to 1 .2 V and the reverse scans were from 1 .2 V to -0.2 V, both at a scan rate of 40 mV s-1 . We conducted reverse scan before the forward scan. For the electrochemical impedance spectroscopy measurement, a STAT-I- 400 potentiostat of Metrohm LTD was used. The signal was measured starting from 1 MHz down to 1 Hz over 50 frequency points, using a potential amplitude of 20 mV, under white light (100 mW cm-2) at open-circuit voltage condition.

[0074] Example 1

[0075] CW-PPPc measurements

[0076] Two continuous waveform (CW) and constant-powered laser diodes were used as the pump and push sources in the CW-PPPc setup. The pump and push beam were combined together via a dichroic mirror. Then the collinear pump and push beam was focused on the device with an overlapping spot size of 0.0003 cm2. For AJIR measurement, an optical chopper was placed in the push path (before combination with the pump path) and its frequency was fixed at 717 Hz by a chopper controller (MC2000B, Thorlabs). For Jpump measurement, a similar optical chopper was placed at the pump path (before combination with the push path). The modulated current from device at short circuit was then recorded by a lock-in amplifier (SR830, Stanford Research Systems). The intensity-dependent measurement was conducted with an 808-nm pump beam (CPS808, Thorlabs) and a 980- nm push beam (CPS980, Thorlabs) at room temperature. The intensity of pump and push beam was controlled by a motorised filter wheel (FW212C, Thorlabs). The experiment was conducted under ambient atmosphere.

[0077] Figure 3 shows CW-PPPc for FAo.ggCso.oiPb device under different illumination. When the sample is not illuminated or illuminated by just pump or push beam, the R signal of lock-in amplifier (i.e., total IR-induced current) is negligible, because the steady-state current from pump (JpUmp) is not picked up by the lock-in amplifier and the IR push photons alone do not directly generate detectable photocarriers. When the FAo.ggCso.oiPbh device is illuminated by both pump and push beams simultaneously, the AJ / R signal soars up, which suggests that A / originates from the intragap states populated by pump generated mobile carriers. By comparing X (in-phase) and Y (out-of-phase) signal from the lock-in amplifier in Fig. 3, it may be concluded that there is no significant change of phase at various chopper modulation frequencies. The amplitude of A / R scales linearly with the intensity of IR light, indicating that AJ / R can be used as a measure of trapped carrier concentration. This linear relationship also suggests that only a small fraction of trapped carriers are depopulated by IR photons.

[0078] Example 2

[0079] Temperature dependent CW-PPPc measurements

[0080] To elucidate the energetics of trap states and to evaluate whether the passivation process has an impact on the characteristic trap depth, temperature-dependent CW-PPPc measurements were taken. The thermal-dependent measurement was conducted with a 450- nm pump beam (220 mW cm-2, CPS450, Thorlabs) and a 1550-nm push beam (5300 mW cm-2, LDM1550, Thorlabs). This could minimise the impact of absorption coefficient change of pump beam and ensure the push beam could not directly excite carriers from the valence band, under low temperatures. The device was placed at a cryostat (HFS600E-PB4, Linkam) [equipped with a liquid nitrogen cooling module (LNP96, Linkam)] to control its temperature from 240 K to room temperature. Nitrogen was fully filled in the chamber to prevent any potential degradation of perovskites. This system can also control the temperature above room temperature, which was used to heat up the device from room temperature to 80 °C (and back to room temperature) for the temperature-dependent photocurrent measurement, the results of which are shown in Fig.4. Fig. 4 shows that upon heating, the photocurrent decreases, suggesting that the increase of the A / R signal does not originate from an IR heating effect.

[0081] Example 3

[0082] Time resolved-PPPc measurement

[0083] Two pulse lasers were used as the pump and push sources in the ns-PPPc setup. The pump pulse (800 nm, 40 fs, 4000 Hz) was provided by a Trsapphire regenerative amplifier (Astrella, Coherent). The push pulse (1064 nm, 10 ns, 4000 Hz) was provided by a Picosecond Nd:YVO4 laser system (piccolo AOT1 , Innolas Laser). The pump and push pulses were combined together via a dichroic mirror. Then the collinear pump and push pulses were focused on the device with an overlapping spot size of 0.003 cm2. Motorised filter wheels (FW212C, Thorlabs) were used to vary the fluence of pump or push pulses. The time delay between the pump and push pulses was achieved by an electrical delay generator (DG645, Stanford Research Systems). For A / measurement, an optical chopper was placed at the push path (before combining with the pump path) and its frequency was fixed at 717 Hz by a chopper controller (MC2000B, Thorlabs). For Jpump measurement, such an optical chopper was placed at the pump path (before combining with the push path). The modulated device current was then recorded by a lock-in amplifier (MFLI 500 kHz, Zurich Instruments). The experiment was conducted under ambient atmosphere. Figure 5 shows the simulated concentration of n-rc at the hole extraction interface as a function of time, which qualitatively and quantitatively reproduces the experimental time-resolved-PPPc measurement data.

[0084] LED Device

[0085] Precursor preparation Perovskite precursor solution was prepared by mixing Pbl2 (0.10 M), CsBr (0.2 M), FAI (0.2 M) and 5-AVAI (0.03 M) in N,N-Dimethylmethanamide (DMF) and stirred at 45 °C for 1 hour.

[0086] Device fabrication

[0087] Pre-patterned indium tin oxide (ITO) substrates were cleaned with detergent, deionized water, and dried with nitrogen flow, followed by a 10-minute UV-ozone treatment. Zinc oxide (ZnO) layer was prepared by spin-coating on substrates at 4000 r.p.m. before transferring to a nitrogen filled glovebox. PEIE (1.5 mg / ml in IPA) was spin-coated on top of the metal oxide films at 5000 rpm. for 20 s and then annealed at 100 °C for 10 mins in the glove-box. The perovskite precursor solutions were coated at 4000 rpm for 30 s, and then annealed on a hot plate for 10 minutes at 120 °C. After cooling down, a TFB layer (12 mg / mL in chlorobenzene) was further coated as the hole transport layer at a spincoating speed of 4000 rpm. The devices were finished by evaporating MoOx(7 nm) and Au (10 nm) as electrodes in a thermal evaporator under a chamber pressure of 2x1 O'4Pa. The pixel sizes of all individual devices were 4.0 mm2.

[0088] Example 4

[0089] Time resolved-PPPc measurement

[0090] These measurements were performed with the LED device in the same manner as described in connection with the solar cell device in Example 3. Figure 6 shows the ratio of IR-induced photocurrent (A / ) to vis-induced photocurrent (JpUmp) over time.

Claims

24Claims1. A method for assessing trap states in an optoelectronic device comprising the following steps: i) providing an optoelectronic device sample; ii) performing one or more Pump-Push-Photocurrent (PPPc) spectroscopy measurements on the sample wherein visible light is used as the pump source and infrared (IR) light is used as the push source, said PPPc measurements comprising: a) combining beams of visible light and IR light to be collinear; b) focussing the combined beams on an area of the sample; and c) detecting visible light-induced photocurrents and IR-induced photocurrents in the sample; wherein the PPPc measurements include continuous waveform (CW)-PPPc measurements and / or time-resolved PPPc measurements in which a time delay is provided between synchronised pump and push pulses of visible and IR light; wherein when detecting visible light-induced photocurrents during (CW)- PPPc measurements the beam of visible light in step a) is modulated and the beam of IR light is blocked; and when detecting IR-induced photocurrents during (CW)- PPPc measurements, the beam of IR light in step a) is modulated and the beam of visible light is unmodulated; and wherein when detecting visible light-induced photocurrents during time- resolved-PPPc measurements the beam of visible light in step a) is modulated and the beam of IR light is blocked; and when detecting IR-induced photocurrents during time-resolved-PPPc measurements both the beams of IR light and visible light in step a) are modulated.

2. A method according to claim 1 , wherein the CW-PPPc measurements comprise intensity-dependent measurements in which push and pump beam intensity is varied, frequency-dependent measurements in which the frequency of push and pump light sources is independently varied, and / or thermal-dependent measurements in which the temperature of the sample is varied.

3. A method according to claim 1 or 2, wherein the visible light and / or IR light is modulated for PPPc measurements at a modulation frequency of from 30 Hz to 5000 Hz.

4. A method according to any one of claims 1 to 3, wherein the time-resolved PPPc measurements are conducted with pulsed laser light, wherein individual pulses of pump visible light and / or push IR light are for a duration of 1 fs to 1 ms, preferably from 100 fs to 100 ns, more preferably from 1 ps to 10 ns.

5. A method according to claim 4, wherein the repetition rate of pulsed visible pump and pulsed IR push light is from 500 Hz to 10 kHz, preferably from 1 kHz to 5 kHz.

6. A method according to any one of claims 1 to 5, wherein the CW and / or time- resolved-PPPc spectroscopy measurements are performed over a range of visible pump and IR push light intensities over which IR-induced photocurrent in the sample has previously been shown to vary linearly with light intensity.

7. A method according to any one of claims 1 to 5, wherein the time-resolved PPPc spectroscopy measurements are performed over a range of visible pump and IR push light intensities over which IR-induced photocurrent in the sample has previously been shown to vary non-linearly with light intensity, and applying a conversion factor to the obtained measurements to account for measurement in the non-linear light intensity-photocurrent region.

8. A method according to any one of the preceding claims, wherein the PPPc spectroscopy measurements of step ii) are conducted on one or more additional area(s) of the same sample.

9. A method according to claim 8, wherein the PPPc spectroscopy measurements on each of the different areas of the sample are conducted at the same time using different light sources.

10. A method according to claim 8, wherein the PPPc spectroscopy measurements on each of the different areas of the sample are conducted sequentially using the same light source.11 . A method according to any one of claims 8 to 10, wherein the measurements made at different areas of the sample are used to map a degree of variation in trap properties across the dimensions of the sample.

12. A method according to claim 11 , wherein a fiber beam combiner carrying collinear vis- and IR-beams is moved across the surface of a sample before each additional measurement or the fiber beam combiner is kept stationary and the sample is mounted on an x-y stage which is moved before each additional measurement.

13. A method according to any one of the preceding claims, wherein the method further comprises performing a post-treatment on the sample intended to reduce the concentration or mitigate effects of traps; and optionally repeating Pump-Push- Photocurrent (PPPc) spectroscopy measurements on the post-treated sample to determine the effect of the post-treatment on the nature and / or concentration of the trap states in the post-treated sample.

14. A method according to claim 13, wherein the post-treatment comprises a surface passivation treatment, preferably treatment with n-octylammonium iodide (OAI), phenethylammonium iodide (PEAI), phenylethylamine bromide (PEABr), or oxygen gas.

15. A method according to claim 13 or 14, wherein the post-treatment comprises recrystallisation of the sample through thermal annealing, solvent annealing, and / or gas treatment.

16. A method according to any one of claims 13 to 15, wherein the post-treatment comprises interfacial re-engineering.

17. A method according to any one of the preceding claims, wherein the optoelectronic device is selected from a solar cell, LED, and a photodetector.

18. A method according to any one of the preceding claims, wherein the optoelectronic device is a solar cell.2719. A method according to claim 18, wherein the solar cell is an organic solar cell, an inorganic solar cell or a dye-sensitized solar cell.

20. A method according to claim 19, wherein the solar cell comprises a perovskite, silicon, cadmium telluride, copper indium gallium diselenide, and gallium arsenide, and combinations thereof, preferably wherein the solar cell comprises a perovskite.

21. A system for assessing trap states in an optoelectronic device using Pump-Push- Photocurrent (PPPc) spectroscopy; said system comprising: i) an optoelectronic device; ii) a source of visible light for use as a pump source and a source of infrared (IR) light for use as a push source; iii) one or more modulators for modulating the visible light and IR light; iv) a beam combiner or beam splitter configured to combine separate beams; and v) a photocurrent detector which is a lock in amplifier.

22. A system according to claim 21 , wherein the optoelectronic device is selected from a solar cell, LED, and a photodetector.

23. A system according to claim 21 or claim 22, wherein the optoelectronic device is a solar cell, preferably an organic solar cell, an inorganic solar cell or a dye- sensitized solar cell.

24. A system according to claim 23, wherein the solar cell comprises a perovskite, silicon, cadmium telluride, copper indium gallium diselenide, and gallium arsenide, and combinations thereof, preferably wherein the solar cell material comprises a perovskite.

25. A system according to any one of claims 21 to 24, wherein the sources of visible and IR light are configured to operate over a range of visible pump and IR push light intensities over which visible light-induced photocurrent produced in the sample has previously been shown to vary non-linearly with light intensity.2826. A system according to any one of claims 21 to 25, wherein the system is configured for making multiple Pump-Push-Photocurrent (PPPc) spectroscopy measurements at different locations of the optoelectronic device sequentially or contemporaneously.

27. A system according to any one of claims 21 to 26, wherein the beam combiner is a dichroic mirror or a fiber, preferably a multi-mode fiber.

28. A system according to any one of claims 21 to 27, wherein the system comprises a beam combiner which is a fiber and wherein the system further comprises an x- y stage on which a sample may be mounted.

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