Photovoltaic (Solar Cell) Measurements

This chapter explains the ways to measure "how well a solar cell converts light into electricity." You apply a controlled voltage to the cell and read the resulting current that flows; this voltage-current relationship (the J-V curve) reveals the cell's entire character, like the pressure-flow performance report card of a water pump. From it, important properties such as efficiency, stability, and aging are derived.

Physical background: A solar cell is a p-n junction semiconductor that generates electron-hole pairs when light is absorbed; when these pairs are separated by the junction's internal electric field, a photocurrent arises. As external voltage is applied, the junction becomes increasingly forward-biased and a "diode current" eats back into the photocurrent; the characteristic elbowed (knee) shape of the J-V curve arises precisely from the competition between these two currents. Every metric we measure (Voc, Jsc, FF, PCE) summarizes a different aspect of this competition.

• Why it is done: to answer the questions "How well does this cell work, and does it degrade over time?" with numbers.
• What it teaches / measures: fundamental figures of merit such as efficiency (PCE = what percentage of incoming solar power is converted to electricity), open-circuit voltage (Voc = the highest voltage produced when no load is drawn), short-circuit current (Jsc = the light-generated current per unit area), and fill factor (FF = how close the curve is to a rectangle).
• Typical values and interpretation: for single-junction lab cells, Voc ~0.6 V (silicon)–~1.1 V (perovskite), Jsc ~20-40 mA/cm², FF ~0.7-0.8, and PCE ~15-26%; values below these ranges signal losses (high Rs, low Rsh, recombination).
• Common mistakes / cautions: entering the active area or irradiance incorrectly shifts all Jsc/PCE values proportionally; also, looking at a single curve and overlooking hysteresis (the scan-direction effect) leads to a misleading efficiency report.
• Where it is used: in new-material research, perovskite/silicon cell development, quality control, and long-term stability testing.

The raw J-V curve on its own is hard to interpret; this module "summarizes" that curve into a few meaningful numbers. Like a report card grade, it converts the cell's performance into standard quantities (Voc, Jsc, FF, PCE) that everyone can compare. Because the entire software produces these numbers from a single core, you see the same result wherever you look.

Physical background: The J-V curve passes through three distinctive points: where the current goes to zero (Voc, the voltage-axis intercept), where the voltage is zero (Isc/Jsc, the current-axis intercept), and the knee point where the power P=V·I peaks (MPP). An ideal cell has a sharp corner (FF→1); the series resistance Rs flattens the slope of the curve near Voc, while the shunt (parallel) resistance Rsh tilts the flat region near V≈0. In other words, the geometry of the curve directly encodes the cell's internal losses.

• Why it is done: it provides a common language for fairly comparing and reporting different cells and measurements.
• What it teaches / measures: Voc = the highest voltage produced when no load is drawn (the lower the recombination, the higher it is); Jsc = the photocurrent collected per unit area (light absorption + carrier collection efficiency); FF = Pmax/(Voc·Isc), the "fullness" of the curve, i.e. the Rs/Rsh quality; PCE = the ratio of output electrical power to incoming light power (%); Rs = contact/conduction series loss (from the slope near Voc); Rsh = leakage/shunt path (from the slope near V≈0).
• Typical values and interpretation: FF>0.75 is excellent, 0.6-0.7 is moderate, <0.5 means high Rs or low Rsh; the area-normalized series resistance (R·A) ~1-3 Ω·cm² is good, >10 Ω·cm² is poor; shunt R·A >1000 Ω·cm² is good, <100 Ω·cm² indicates serious leakage.
• Common mistakes / cautions: a metric that cannot be computed is left as None (never 0) — mistaking a value for 0 and interpreting it is an error; PCE is not produced when irradiance is 0; the Rs/Rsh fit is unreliable on insufficient data (it falls back to the local-slope method).
• Where it is used: at the end of every J-V-based measurement, in the result cards and reports.

If you scan a cell first in the forward and then in the reverse direction, the two curves do not always overlap; the difference between them is "hysteresis." The hysteresis index reduces this difference to a single number, much like measuring whether a door follows the same path when it opens and when it closes. Close to zero means the cell is stable; large means there are internal dynamics (such as ion migration).

Physical background: The divergence of the forward and reverse scans indicates a slow process inside the cell that responds to voltage with a "lag": migration of mobile ions in perovskites, the filling and emptying of traps at interfaces, or capacitive charge accumulation. Because these slow charges cannot reach equilibrium instantly during the scan, a different current is read at the same voltage in the two directions; the larger the index, the more dominant this slow dynamic.

• Why it is done: to understand whether the measured efficiency is "real" or a misleading artifact that depends on the scan direction.
• What it teaches / measures: HI = (PCE_rev − PCE_fwd)/PCE_rev, i.e. the relative efficiency difference between the two directions; also the area-based hysteresis A_hyst = the ratio of the area between the two curves to the charge under the reverse-scan curve (a quantity independent of direction choice).
• Typical values and interpretation: HI ≈ 0 is a hysteresis-free, stable device; in good perovskites |HI| < 0.05; HI ~0.1-0.2 indicates pronounced hysteresis; HI > 0.2 indicates strong ion migration / trap filling. In silicon cells HI is typically negligible.
• Common mistakes / cautions: scanning a single direction (only Forward or only Reverse) and never seeing hysteresis; or reporting only the (usually higher) reverse-scan PCE as the "real efficiency." Comparing HI without fixing the scan rate is also misleading — HI is rate-dependent.
• Where it is used: especially in perovskite cells, for reliable efficiency reporting and material stability assessment.

Viewing the same measurement with different plot types lets the eye catch different problems; for example, a logarithmic plot brings out small leakage currents while a linear plot emphasizes the overall shape. Like examining a patient with both an X-ray and a blood test, you look at the same data through multiple windows. The metric cards, meanwhile, give a live summary of the most important numbers.

Physical background: The linear J-V shows the overall shape of the curve and the sharpness of the knee (FF); the absolute |J|-V and especially the log|J|-V axis make visible the region where the current changes sign and the leakage/diode currents that are orders of magnitude smaller — the diode region becomes a straight line on the log axis. The dual J-V + P-V pane marks the MPP where power peaks, visually showing the cell's true operating point.

• Why it is done: to quickly inspect the health of the curve by eye during measurement and to spot anomalies (kinks, leakage).
• What it teaches / measures: the linear J-V shape and FF; the leakage/diode region on log|J|-V; in the dual view the point of maximum power (MPP) and live PCE/Voc/Jsc/FF cards (with Δ relative to the previous measurement).
• Typical values and interpretation: on a healthy curve the forward-bias region of log|J|-V is a straight line; a "kink" on the curve (a shoulder/dip before the knee) indicates a barrier or poor contact, and a sloped (non-horizontal) flat region near V≈0 indicates low Rsh (leakage).
• Common mistakes / cautions: forgetting the log axis hides small leakages; overlaying Forward and Reverse in a dual scan and overlooking hysteresis, or misreading the axis range because of autoscaling, are frequent errors.
• Where it is used: in every PV measurement, both for live monitoring and for later exporting plots as PNG.

Mock mode is the software simulating an artificial but realistic solar cell without a real device or cell. Like a flight simulator: you can try out all the measurement steps, learn, and test the software without risking anything. Because the simulated cell behaves according to single-diode physics, the resulting numbers are sensible and instructive.

Physical background: The synthetic curve is generated with the single-diode equivalent-circuit equation: J = Jph − J0·[exp(qV/nkT) − 1] − V/Rsh, corrected with the series resistance Rs. That is, a photocurrent source (Iph), a diode (I0, n), a parallel leakage (Rsh), and a series loss (Rs) represent the same physical elements as in a real cell. Because irradiance changes Iph (Iph ∝ E) and temperature changes the thermal voltage kT/q, the light and temperature sweeps respond realistically.

• Why it is done: to enable training, development, and software verification when no lab hardware is available.
• What it teaches / measures: how each PV mode works and what typical results look like; the profile parameters correspond directly to physics: Iph photocurrent (Jsc≈30 mA/cm² at 1 sun), I0 diode saturation current, n=1.5 ideality, R_s=1 Ω series loss, R_sh=10 kΩ leakage.
• Typical values and interpretation: the results expected with this profile are consistent with a real, mid-quality cell (Jsc≈30 mA/cm², n≈1.5 → moderate recombination, high Rsh/low Rs → good FF); when illuminated=False is selected, Iph is zeroed and only the diode + Rs/Rsh remain (dark J-V).
• Common mistakes / cautions: mistaking mock results for real device data; also forgetting that the mock profile is only effective on the mock SMU — on real hardware the profile commands are silently ignored (no-op) and the data comes from the real device.
• Where it is used: training/demos, trying out new features, and rehearsing before connecting to a device.

The core metrics summarize the cell; the advanced report extracts a much deeper "full blood panel" from the same curve. Think of it like a detailed inspection that reports not just a car's speed but also its engine, brakes, and fuel system. It adds advanced parameters such as fit quality, MPP sharpness, rectification ratio, and, in a dark measurement, the diode ideality.

Physical background: The same (V, J) data reveals different physics when read through different windows: the slope near Voc gives the series resistance (contact/conduction loss), the slope near V≈0 gives the shunt resistance (leakage), the curvature around the knee gives the MPP sharpness (FF quality), and an "S"-bend/kink in the curve indicates a barrier or poor interface that impedes charge extraction. In the dark, the slope of the ln(J)-V line gives the transport mechanism (ideality n) and its intercept gives the saturation current J0.

• Why it is done: to understand not just how well the cell behaves, but why it behaves that way.
• What it teaches / measures: fit quality (R²) = how much the extracted Rs/Rsh can be trusted; MPP sharpness / tolerance window = how flat the power is around the peak (it determines FF); kink/S-curve index = charge-extraction barrier; rectification ratio RR = the diode's forward/reverse current ratio; in the dark, n (conduction mechanism) and J0 (recombination level).
• Typical values and interpretation: good fits have R²>0.99; ideality n≈1 radiative/band-to-band, n≈2 depletion-region (SRH) recombination, n>2 trap/multi-mechanism; a good diode has a rectification ratio of 10³-10⁶; the kink/S-index should be near zero, and if large there is a barrier.
• Common mistakes / cautions: fully trusting Rs/Rsh or n/J0 derived from a low-R² fit; forgetting that in a pulsed measurement the dynamic block is skipped (because dV/dt is undefined); when the advanced analysis fails, analysis = None but the core metrics are still valid.
• Where it is used: in in-depth research, failure analysis, and device-physics studies.

This is the cornerstone of solar cell measurement: it holds the cell under artificial sunlight (a solar simulator), slowly sweeps the voltage, and reads the current. The resulting J-V curve is like the performance report card the cell delivers at full throttle, and the efficiency (PCE) is born here.

Physical background: Under light the cell produces a constant photocurrent; as the voltage increases, the junction becomes forward-biased and the exponentially growing diode current eats back into this photocurrent. That is why the curve is almost flat at low voltage (current ≈ −Jsc) and rapidly enters the knee near Voc: this "knee" is the MPP, where the power is maximum. The sharpness of the knee determines FF, its location determines Voc, and the height of the flat region determines Jsc.

• Why it is done: to answer the question "How efficient is this cell under standard light?"
• What it teaches / measures: Jsc = the light-generated current per unit area; Voc = the highest voltage set by recombination; FF = the fullness of the knee (Rs/Rsh quality); PCE = the ratio of the product of these three to the incoming light power (PCE ∝ Voc·Jsc·FF).
• Typical values and interpretation: for a single-junction cell, Voc ~0.6 V (silicon)–~1.1 V (perovskite), Jsc ~20-40 mA/cm², FF ~0.7-0.8, PCE ~15-26%; FF<0.5 or lower-than-expected Jsc/Voc signals losses (Rs↑, Rsh↓, recombination).
• Common mistakes / cautions: entering the active area/irradiance incorrectly proportionally distorts Jsc and PCE; hitting compliance and clipping the curve gives a wrong Jsc; neglecting the solar simulator's spectrum/intensity calibration (1 sun = 1000 W/m²) shifts the entire efficiency.
• Where it is used: in almost every PV study, it is the first and most frequently performed measurement.

You turn off the light completely and run the same sweep; with no photocurrent, only the cell's "diode" character remains. Like running an engine with no load and listening to its inner sound, it reveals the cell's internal quality (leakages, ideality).

Physical background: When the photocurrent is zero, the J-V curve returns to a pure diode characteristic: on a semi-logarithmic scale (ln(J)-V) the forward-bias region becomes a straight line. The slope of this line gives the ideality factor n (which mechanism the carriers recombine by), and its vertical intercept gives the saturation current density J0 (the total recombination level). The small current in the reverse-bias region, meanwhile, measures the shunt leakage.

• Why it is done: to see separately the internal losses and diode quality that the light hides.
• What it teaches / measures: ideality n = the conduction/recombination mechanism (1 ideal, 2 depletion-region SRH); J0 = saturation current density, small means low recombination; Rsh = shunt/leakage from the slope near V≈0; rectification ratio = the forward/reverse current ratio (diode quality).
• Typical values and interpretation: n ≈ 1-2 is healthy (n>2 trap/multi-mechanism); the smaller J0 the better (silicon ~10⁻¹²-10⁻⁹ A/cm²); a rectification ratio of 10³-10⁶ is a good diode; a low Rsh (large reverse leakage) indicates a shunt problem.
• Common mistakes / cautions: expecting PV performance metrics (Voc/FF/PCE) in the dark — these are undefined; doing the ln(J)-V fit in the wrong (high-bias, series-resistance-dominated) region inflates n; if full darkness is not achieved, the residual photocurrent biases J0.
• Where it is used: in diagnosing recombination and leakage sources and in comparing material quality.

It scans the cell first in one direction and immediately after in the reverse direction, then compares the two curves. The more the two curves diverge, the more "memory" the cell has; this measurement is designed to deliberately measure that divergence. It is like checking whether a spring follows the same path while being stretched and released.

Physical background: The divergence of the curves in the forward and reverse scans stems from slow processes that respond to voltage with a lag: migration of mobile ions in perovskites, the filling and emptying of interface traps, and capacitive charge. Depending on the scan direction, these slow charges are in a different initial state, so a different current is read at the same voltage. The reverse scan generally gives a higher (optimistic) FF/PCE; the difference between them is the magnitude of the hysteresis.

• Why it is done: to measure the scan-direction-dependent deviation in efficiency and test the reliability of the reported efficiency.
• What it teaches / measures: per-direction Voc/PCE; HI = (PCE_rev−PCE_fwd)/PCE_rev, the relative difference; A_hyst area-based hysteresis; also the ΔVoc/ΔJsc/ΔFF/ΔPCE direction differences (which show which metric is affected by direction).
• Typical values and interpretation: |HI| < 0.05 is negligible (stable); 0.1-0.2 is pronounced; >0.2 is strong ion migration/traps; in silicon HI ≈ 0 is expected, and a notable HI may indicate a measurement/contact problem.
• Common mistakes / cautions: reporting only the reverse PCE as the "real efficiency"; comparing two devices' HI without fixing the scan rate (settling/delay) — HI is rate-dependent; not recording the preconditioning (pre-light-soaking) state.
• Where it is used: especially in perovskite cells, in ion-migration / trap-dynamics studies.

Instead of continuously biasing and heating the cell at each voltage point, it applies a short "pulse," measures immediately, and withdraws. Like tapping a hot pan briefly instead of pressing your finger on it continuously, it reduces self-heating and slow transients, thereby capturing the "instantaneous" true behavior.

Physical background: In a continuous sweep, each point both heats the cell and gives the slow charges (ions/traps/capacitance) time to respond to the voltage, distorting the curve with thermal and transient effects. In the pulsed method the cell is mostly held at a neutral baseline voltage, and the measurement is taken only within a short pulse window — if this window is kept shorter than the response time of the slow processes, the measured J-V is largely free of thermal equilibration and transients.

• Why it is done: to obtain a cleaner, transient-free J-V when heating or slow response distorts the curve.
• What it teaches / measures: the standard J-V metrics (Voc, Jsc, FF, PCE), but in a state less affected by transient/thermal effects; pulse_width_s = the duration of the measurement window (the shorter, the more "instantaneous"), baseline_v = the rest voltage between pulses.
• Typical values and interpretation: the pulse width should be shorter than the time constant of the slow processes (typically sub-ms); a large FF/PCE difference between pulsed and continuous J-V indicates that the measured cell is sensitive to thermal/transient effects.
• Common mistakes / cautions: choosing too short a pulse and not leaving the SMU time to settle gives noisy/wrong points; the Spec v2.0 dynamic block is skipped in pulsed mode (dV/dt undefined) — do not expect capacitance in this block.
• Where it is used: for accurate characterization of transient-sensitive or heating cells.

It varies the light intensity in steps and measures only the open-circuit voltage (Voc) at each level. Because no current flows, the result is unaffected by series resistance; that is why it is like closing the traffic (the current) and measuring the road's true gradient, showing the cell's "raw" quality.

Physical background: At open circuit the net current is zero, so no current passes through the series resistance and Rs has no effect on Voc. Voc is the voltage at which photogeneration and recombination balance, and it increases logarithmically with light intensity: Voc = (nkT/q)·ln(suns) + constant. Therefore, when you plot Voc against ln(suns), the slope directly gives the ideality factor n (the recombination mechanism) and the intercept gives the 1-sun Voc. Comparing the real J-V's FF with this "Rs-free" pseudo-FF separates out the series-resistance loss.

• Why it is done: to reveal the true recombination behavior and ideality that the series resistance hides.
• What it teaches / measures: ideality_n = the ratio of the Voc–ln(suns) slope to kT/q (recombination mechanism); voc_1sun_v = Voc at 1 sun (intercept); voc_slope_per_ln = the slope; and the Rs-independent pseudo-J-V curve.
• Typical values and interpretation: n ≈ 1 low recombination, n ≈ 2 depletion-region (SRH) recombination, n>2 trap/interface dominated; if the pseudo-FF is noticeably higher than the real FF, the difference is the series-resistance loss.
• Common mistakes / cautions: uncalibrated light intensities (suns) distort the slope (n); the fit is unreliable over too narrow a suns range; remember that this n is the depletion-region recombination ideality and is not exactly the same as the J-V "knee" ideality.
• Where it is used: in diagnosing series-resistance losses and in independently verifying cell quality.

It measures a full J-V at different light intensities and extracts how the metrics depend on light. In an ideal cell the current increases in direct proportion to light; like speed increasing proportionally as you press the gas pedal. Deviations from this proportionality reveal losses.

Physical background: The photocurrent is proportional to the number of absorbed photons; therefore, in an ideal cell Jsc increases exactly linearly with the light intensity. If there is trap filling, saturation, or poor carrier collection, this linearity breaks down (Jsc becomes different from expected at low light). Since Voc increases logarithmically with light (~n·60 mV per 10× intensity), PCE generally varies slowly with light intensity.

• Why it is done: to measure whether the cell behaves consistently at low and high light.
• What it teaches / measures: jsc_linearity_r2 = the Jsc–suns linearity quality (R²); jsc_per_sun_ma_cm2 = the Jsc slope per sun (photogeneration sensitivity); pce_1sun_pct = the 1-sun efficiency; pce_final_pct = the efficiency of the highest level (the light dependence of PCE).
• Typical values and interpretation: jsc_linearity_r2 ≈ 1 (≥0.999) is ideal photocurrent linearity; a low R² indicates traps/recombination or series-resistance-limited collection; PCE dropping at high light says Rs is dominant (loss ∝ I²·Rs), while dropping at low light says Rsh/leakage is dominant.
• Common mistakes / cautions: trusting the suns axis without calibrating the light intensities; interpreting the linearity R² with too few levels (n_levels); hitting compliance at high intensity and clipping Jsc.
• Where it is used: in indoor/low-light performance and in trap/recombination diagnostics.

It holds the cell at the point where it produces the most power (the maximum power point) and tracks that power over time. The software continuously searches for the best operating point with small trials; like continuously fine-tuning a radio until you find the clearest station. If the power drops over time, the cell is unstable.

Physical background: The variation of power with voltage (P=V·I) peaks at the MPP; on the two sides of this point dP/dV changes sign. The "perturb-and-observe" (P&O) algorithm takes a small voltage step, watches whether the power increases or decreases, and keeps/reverses the step direction accordingly — thus continuously tracking the drifting MPP. Especially in perovskites the power may change with light soaking at the start; the long-term Pmax(t) curve gives the true steady-state efficiency.

• Why it is done: to measure stability under real operating conditions (continuous power draw).
• What it teaches / measures: pmax_final/best/mean_w = the final/best/mean power over the tracking; vmpp_final_v, impp_final_a = the stable operating point; pce_final_pct = the steady-state efficiency; and the fluctuation/trend (rise = light-soaking improvement, fall = degradation).
• Typical values and interpretation: in a stable cell Pmax(t) stays flat/horizontal (steady-state PCE ≈ PCE read from the J-V knee); a continuous decline indicates degradation, a rise then plateau indicates light-soaking improvement; large fluctuation indicates a poorly tuned perturb step or noise.
• Common mistakes / cautions: choosing the start voltage far from Voc so the algorithm reaches the peak late; too large a perturb step causes oscillation around the MPP, too small a step causes slow tracking; keeping the tracking duration shorter than the light-soaking settling gives a misleading (low) steady-state PCE.
• Where it is used: in stability tests and real-use efficiency assessment (especially perovskites).

It holds the cell under constant light for a long time, measures J-V at fixed intervals, and tracks how the efficiency changes over time. Like wearing a new shoe for days and observing how it settles in (or wears out), it reveals the effect of light exposure.

Physical background: Under continuous illumination, slow processes work in the cell: filling/emptying of traps, ion redistribution, and interface and metastable defect changes. These can initially increase the PCE (light-induced healing/activation) or gradually decrease it (degradation). The shape of the PCE(time) curve is the net result of these competing processes; T80/T90 are lifetime indicators that measure how long it takes for the efficiency to drop to 80%/90% of its initial value.

• Why it is done: to measure whether the cell heals or degrades under light, and how long it lasts.
• What it teaches / measures: pce_initial/final_pct = the initial/final efficiency; retention_pct = 100·final/initial (the fraction of efficiency retained); t80_s/t90_s = the time at which the efficiency first drops to 80%/90% of its initial value (lifetime metrics, linear interpolation).
• Typical values and interpretation: retention ≈ 100% (even >100% with light-induced healing) is stable/good; if retention is low or T80 is short, there is rapid degradation; never reaching T80/T90 (None) means the efficiency was maintained throughout the test.
• Common mistakes / cautions: keeping the test too short and missing slow degradation; attributing retention to the material without controlling the temperature/atmosphere (humidity, O₂); choosing too sparse a J-V interval (jv_interval) and failing to sample the early transient behavior.
• Where it is used: in stability and lifetime tests and in comparing material durability.

It works on the same logic as light soaking but tracks how the cell ages slowly over a longer time and at wider intervals. Like tracking the decline of a battery's charge-holding capacity over months, it records long-term degradation.

Physical background: Under prolonged stress, irreversible aging mechanisms accumulate: material decomposition, interface/contact deterioration, permanent trap formation, and moisture/oxygen-driven chemical change. Because these processes are slower than the reversible light-soaking transients, the measurement is done over a longer time and with sparser J-V; the PCE(time) curve generally shows a monotonically decreasing degradation trend.

• Why it is done: to quantitatively determine how long-lived the cell is in long-term use.
• What it teaches / measures: the same core as Light Soaking: pce_initial/final, retention_pct, t80_s/t90_s (lifetime), n_sweeps (number of sweeps); the only difference is the longer default duration and interval.
• Typical values and interpretation: a long T80 (a late-arriving 80% point) or retention staying high means good stability; a short T80 or rapidly falling retention means poor lifetime; remember that absolute times can be compared only under the same stress conditions (same light/temperature).
• Common mistakes / cautions: converting accelerated lab time directly to field lifetime (an acceleration factor is needed); sharing T80 without reporting the stress conditions (light, temperature, environment); taking the initial PCE from a point that has not yet settled (transient) biases retention.
• Where it is used: in reliability / durability tests and production quality assurance.

It first measures the cell's health (pre J-V), then holds it for a while at a given voltage (stress), and measures again afterward (post J-V). The difference between the two measurements shows what the biasing did to the cell; it is like inspecting a bridge for damage after holding a heavy load on it.

Physical background: Holding under a fixed bias forces the field and the carrier/ion distribution inside the cell: ions can migrate and accumulate at interfaces, traps can fill, and contacts can undergo electrochemical change. The I(t) recorded during the hold shows the "trace" of this slow rearrangement (increase/decrease); the difference between the pre and post J-V, meanwhile, reveals whether this stress left a reversible or a permanent change in performance.

• Why it is done: to measure whether electrical biasing degrades or improves the cell.
• What it teaches / measures: delta_voc_v = the change in Voc; delta_pce_pct = the absolute change in PCE (percentage points); delta_pce_rel_pct = the relative (%) change in PCE; the direction of the I(t) slope during the hold (slow process) and, via bias_voltage/bias_duration_s, their dependence on voltage/duration.
• Typical values and interpretation: Δ's ≈ 0 is a stable device (unaffected by biasing); a negative ΔPCE indicates bias-induced degradation, a positive ΔPCE indicates healing/activation; the Δ growing with longer duration points to slow ionic/trap processes.
• Common mistakes / cautions: measuring the pre/post J-V under different conditions (heated/cooled, different light) and attributing the Δ to stress; mistaking a reversible change (which reverts upon waiting) for permanent degradation; overlooking that the illumination state is assumed constant (the module does not drive light).
• Where it is used: in stability-mechanism research and operating-condition durability tests.

It brings the cell to different temperatures, measures a J-V at each, and extracts how performance changes with temperature. Solar panels heat up in the field; like testing whether a phone slows down when hot, it lets you predict behavior under real conditions.

Physical background: As temperature increases, the semiconductor's bandgap narrows slightly and the diode saturation current J0 grows exponentially; this notably lowers Voc (the dominant effect). Jsc increases very slightly because of the bandgap narrowing. The net result is that PCE decreases with temperature. The slopes of Voc and PCE versus temperature (dVoc/dT, dPCE/dT) summarize this behavior in a single number — these are generally negative.

• Why it is done: to know in advance how much efficiency the cell will lose on a hot panel in summer.
• What it teaches / measures: dvoc_dt_mv_per_c = the temperature coefficient of Voc (mV/°C, depends on J0 and recombination); dpce_dt_per_c = the temperature coefficient of efficiency (%/°C); pce_final_pct = the efficiency at the highest temperature.
• Typical values and interpretation: both coefficients are negative; in a silicon cell dVoc/dT ≈ −2 mV/°C and the efficiency drops measurably with temperature (relative ~0.3-0.5%/°C). A less negative coefficient means a cell that is more robust when hot (better for the field).
• Common mistakes / cautions: mistaking the controller's set temperature for the cell temperature (sufficient waiting is required for thermal equilibrium); deriving the slope (coefficient) from too few temperature steps; forgetting that this mode requires a temperature role on its own and that the sign of the slope (negative) is expected.
• Where it is used: in field-performance prediction and module design/rating.

It holds a single voltage constant and records how the current settles (or drifts) over time. Like opening a tap to a fixed position and watching whether the water flow stabilizes, it shows the cell's stability at that point.

Physical background: When the voltage is held constant, the slow change in the current reveals the transient processes inside the cell: discharge of capacitive charge, trap filling/emptying, ion redistribution, and thermal settling. In an ideal stable cell I(t) quickly settles to a horizontal value; a continuously drifting or slowly settling current indicates an internal dynamic (e.g. ionic) that responds to voltage with a lag.

• Why it is done: to observe whether the current is stable at a particular operating point.
• What it teaches / measures: i_mean_a/i_final_a = the mean/final current; v_mean_v/v_final_v = confirmation of the applied voltage; p_mean_w = mean(V·I) power; n_samples = the number of samples; and the settling behavior of the I(t) curve.
• Typical values and interpretation: i_final ≈ i_mean and a flat I(t) indicate stable operation; a current that peaks at the start and decays indicates capacitive/transient discharge, while a continuously drifting current indicates an unsettled (unstable) point.
• Common mistakes / cautions: keeping the total duration shorter than the settling time constant and mistaking a not-yet-settled current for "stable"; choosing too large a sampling interval and missing the early transient; choosing the applied voltage outside the protection/limit range.
• Where it is used: in stability checking, settling-time measurement, and simple durability trials.

This time the current is held constant and the change in voltage over time is tracked; it is the inverse of CV. Like pedaling at a constant speed and measuring how much power you expend, it forces the cell at a given current and observes its response.

Physical background: When a constant current is driven, the SMU continuously adjusts the voltage needed to pass that current; the slow change in V(t) shows how the cell's "difficulty" in carrying that current changes over time. An increasing series resistance, contact deterioration, or ion/trap redistribution can shift the voltage up/down. Depending on the current direction, the point corresponds to a different region of the J-V curve (generation or forward bias).

• Why it is done: to see whether the voltage stays stable when a constant current is drawn.
• What it teaches / measures: the same summary as CV — v_mean_v/v_final_v = the mean/final voltage (the main interest); i_mean_a/i_final_a = confirmation of the driven current; p_mean_w = mean power; n_samples; and the V(t) drift behavior.
• Typical values and interpretation: a flat V(t) indicates stable operation; a continuously rising voltage indicates increasing series resistance/contact deterioration, while fluctuation indicates an unstable internal dynamic.
• Common mistakes / cautions: the driven current exceeding the safety current limit causes the measurement to be rejected before it starts (Kaynak akimi guvenlik limitinin uzerinde); also, at too high a current the voltage may hit the protection limit (voltage_limit) — the current should be chosen according to the cell's capacity.
• Where it is used: in stability monitoring and current-driven operation trials.

It automatically selects, one by one, many cells placed in a switching fixture and measures them in sequence. Like automatically grading the exams of an entire class and producing a transcript, it produces batch results and statistics without manual effort.

Physical background: A switch (relay) matrix connects the same SMU in turn to the contacts of different cells; an independent PV J-V is measured for each cell and the same metrics are extracted from the single-source core. The spread of PCE from cell to cell (mean ± std) shows the fixture/process uniformity; spatial patterns in the PCE heatmap (e.g. low edges) reveal systematic variation caused by coating/process.

• Why it is done: to measure and compare many cells quickly, consistently, and without human error.
• What it teaches / measures: per-cell PCE (color/heatmap), overlaid J-V curves, and batch statistics: mean ± std (n) for PCE/Voc/FF/Jsc (summarize_batch, single source); the size of the std measures the repeatability.
• Typical values and interpretation: a small std (narrow distribution) means high production uniformity; a wide distribution or a spatial gradient in the heatmap indicates a process/coating problem; gray (unmeasured) cells indicate a faulty selection/contact.
• Common mistakes / cautions: an unvalidated relay map does not send commands on real hardware (only mock/grid) — failing to notice this; not entering the area/irradiance correctly for all cells biases the statistics; one cell's error does not stop the others (ok=False record), so overlooking this and mistaking incomplete data for complete.
• Where it is used: in production-line screening, quality control, and screening large sample sets.