What is the difference between a solar cell and a battery?

Solar cells convert sunlight to electricity (e.g., monocrystalline Si reaches ~22% efficiency), requiring light; batteries store chemical energy (Li-ion: ~200Wh/kg), discharging via reactions without light—generating vs. releasing power.

Definition and Function

In 2023, the global newly installed solar capacity reached 295GW (data from the International Renewable Energy Agency IRENA), equivalent to installing solar panels the size of a standard football field every hour; during the same period, the global deployment of energy storage batteries also exceeded 48GWh (BloombergNEF), enough to provide 2 hours of emergency power for 8 million households.

Solar Cells

In 2023, the global newly installed solar capacity was 295GW, equivalent to laying down solar panels the size of 1.3 standard football fields worldwide every hour (calculated based on 2㎡ per module). How much electricity can these panels generate per day? Taking the Beijing area as an example, a 300W monocrystalline silicon module, under 4 hours of effective sunlight around the spring/autumn equinox, can generate 1.2 kWh of electricity – enough to run 6 air conditioners for 1 hour, or charge an electric vehicle for 30 km of range.

Materials Determine Capability: The Efficiency Calculation from Silicon Wafers to Perovskite

The "power generation capability" of a solar cell depends first on the material. Currently, over 90% of the market is crystalline silicon cells, with the remaining 10% including new players like thin-film cells (e.g., CIGS) and perovskite cells.

· What is the limit of crystalline silicon? Mainstream monocrystalline silicon cells use PERC technology (Passivated Emitter and Rear Cell), with a mass production efficiency of 22-24% (China Photovoltaic Industry Association CPIA 2023). What does this mean? A 1 square meter module (weighing about 20kg) under 1000W/㎡ illumination receives 1000Wh of energy per hour and can convert it into 220-240Wh of electricity – equivalent to "compressing" 1 kWh of electricity into 20kg of silicon wafers.

· Breakthroughs in the lab: In 2023, the TOPCon cell (Tunnel Oxide Passivated Contact) efficiency certified by Germany's Fraunhofer ISE reached 26.1%, and the HJT heterojunction cell reached 26.81%. Every 0.1% increase in these numbers can generate about 3 billion kWh more electricity annually worldwide (based on 295GW global installed capacity).

· New player Perovskite: The efficiency of laboratory tandem cells has reached 33.2% (Oxford PV 2023), but stability is poor – current outdoor tests show 15% degradation in 500 hours (compared to 20% degradation over 25 years for crystalline silicon).

How does light intensity affect power generation? Measured data tells the story.

Solar cells do not generate electricity just by "having light"; light intensity (irradiance) directly determines the output power.

· Standard Test Conditions (STC): When testing efficiency in the lab, conditions are 1000W/㎡ irradiance, 25°C temperature, and AM1.5 spectrum (simulating noon sunlight). Under these conditions, a 300W module can output at full power.

· Sluggish moments in real scenarios: On cloudy days, irradiance can drop to 200W/㎡, and the module power directly drops to 60-70W (300W × 20%); at 9 a.m., irradiance is only 500W/㎡, so power is 150-170W – at this time, charging a phone for half an hour can only charge it once (3000mAh ≈ 11Wh, a 150W module generates 22.5Wh in half an hour, enough for 2 charges).

· Side effects of temperature: The stronger the sun, the more the cell "overheats". Module power decreases by 0.3-0.4% for every 1°C increase in temperature (PID test data). At noon in summer, the module temperature can reach 70°C (25°C higher than ambient temperature), and a 300W module actually outputs only 300×(1-0.3%×25)=292.5W – equivalent to generating 2.25 kWh less after half an hour of white exposure.

How much power is left after 25 years? The math behind the degradation rate.

When buying solar cells, the biggest fear is that they "won't work well after a few years," but the industry has long calculated the degradation rate clearly.

· First-year "break-in period": After new modules are installed, the power may drop by 1-2% in the first 3 months due to material stress release (IEC 61215 standard). A 300W module will have 294-297W remaining at the end of the first year.

· Annual "slow aging": For the next 24 years, the annual degradation does not exceed 0.45%. After 25 years, is the total degradation controlled at 2%+24×0.45%=13%? No! The standard is "not less than 80% of initial power after 25 years" – so the actual degradation is 20%. A 300W module will have 240W remaining after 25 years. Under 4 hours of sunlight per day, it generates 0.18 kWh less per day.

· Which degrades slower? Lithium iron phosphate storage batteries have long cycle life, but the degradation of solar modules mainly depends on the encapsulation material. Double-glass modules (glass+glass) are more resistant to aging than single-glass modules (glass+backsheet), with 2-3% lower degradation after 25 years (TÜV Rheinland test).

Hidden skills beyond power generation: Low-light response and temperature coefficient

Good cells are not only powerful under strong light but also effective under low light and high temperatures.

· Low-light response: Some modules use thinner silicon nitride anti-reflection films, starting to generate power at 7 a.m. (300W/㎡ irradiance), one hour earlier than ordinary modules. This earns about 120 kWh more per year (based on 1 more hour per day × 300W = 0.3 kWh, 365 days × 0.3 = 109.5 kWh).

· Temperature coefficient: This parameter is written in the module specification sheet, with units of %/°C. For example, -0.35%/°C is better than -0.45%/°C – for the same 25°C temperature increase, the former's power decreases by only 8.75% (300W × 8.75% = 26.25W), while the latter decreases by 11.25% (300W × 11.25% = 33.75W).

Efficiency in cost: Is it worth 0.5 RMB/W more?

· Price comparison: In 2023, mainstream PERC modules cost 1.5 RMB/W, TOPCon modules cost 1.7 RMB/W (0.2 RMB/W more). For a 10kW power station, PERC costs 15,000 RMB, TOPCon costs 17,000 RMB, a difference of 2,000 RMB.

· Payback period: Assuming an electricity price of 0.5 RMB/kWh, a 10kW PERC power station generates 12,000 kWh annually on average (10kW × 4h × 365 days = 14,600 kWh, taking 12,000 after degradation), with an annual return of 6,000 RMB, payback in 2.5 years. TOPCon generates 12,500 kWh (3% higher efficiency, 12,000 × 1.03 ≈ 12,500), annual return 6,250 RMB, payback in 2.72 years – a difference of 0.22 years (about 2.6 months). Calculated over 25 years, TOPCon earns (12,500 - 12,000) × 25 × 0.5 = 6,250 RMB more, covering the initial extra cost of 2,000 RMB, with a net profit of 4,250 RMB.

Batteries

In 2023, global energy storage battery deployment surged to 48GWh, equivalent to storing 2 hours of electricity for 8 million households (based on 10 kWh daily usage per household). A 10kWh home energy storage battery may weigh only 100kg, but it can store 20 times the energy of a full tank of gasoline for a regular car (50L ≈ 1800 Wh/kg).

Battery "Capacity": Energy Density Determines How Much Electricity Can Be Stored

· Comparison of mainstream players: NMC lithium battery (NCM811) energy density is 250-300 Wh/kg, Lithium Iron Phosphate battery (LFP) is 150-200 Wh/kg. An intuitive example: 1kg of NMC can store 250Wh, equivalent to the total battery capacity of 4 mobile phones (each about 60Wh); while 1kg of LFP can only store 150Wh, but it is 30% cheaper (2023 cell cost: NMC 0.8 RMB/Wh, LFP 0.5 RMB/Wh).

· How does it compare to gasoline? Gasoline energy density is 12,000 Wh/kg, 40 times that of NMC. But batteries don't need refueling and there's no worry about leaks – an electric vehicle in Beijing with a 60kWh battery (about 400kg) can run 400 km, similar to a gasoline car with 50L of fuel running 500 km.

· Ambitions of new players: Sodium battery energy density has just reached 120-150 Wh/kg (CATL 2023 release), with 20% lower cost, but poor low-temperature performance – at -20°C, capacity remains only 70% (LFP remains 80% at -20°C).

Charging/Discharging "Loss": How much more does lower efficiency cost?

Batteries are not "electric energy transfer stations"; if you put in 100Wh, you definitely get less than 100Wh out. The difference can calculate the extra electricity cost over several years.

· Charge-discharge efficiency formula: Efficiency = Discharge energy / Charge energy × 100%. Lithium batteries are generally 90-95%, lead-acid batteries are 70-80%. For example, using 100Wh of photovoltaic electricity to charge a lithium battery, you can finally discharge 90Wh; charging a lead-acid battery leaves only 70Wh – charging/discharging once a day, the annual difference is (90-70) × 365 = 7300 Wh ≈ 7.3 kWh. At 0.5 RMB/kWh, that's 3.65 RMB more per year, 36.5 RMB more over 10 years (seems small, but significant when scaled to a home energy storage system).

· Cost of fast charging: Using 2C fast charging (full charge in 2 hours), lithium battery efficiency drops to 85-88% (92-94% for 1C charge/discharge). A 10kWh battery charged at 2C consumes 150-200Wh more electricity per charge than 1C, equivalent to an extra 0.07-0.1 RMB (at 0.5 RMB/kWh).

· The pitfall of temperature: Charging below 0°C can cause lithium plating in lithium batteries (internal short circuit risk), so manufacturers limit charging current – at -10°C, charging efficiency drops from 92% to 80%. Charging a 10kWh battery takes 2 hours more and consumes 2kWh more electricity (10kWh ÷ 80% - 10kWh ÷ 92% ≈ 1.3kWh).

Will it "shrink" with use? How is cycle life calculated?

· Definition of cycle count: Refers to a complete cycle from full charge (100%) to discharge to 0%, then back to full charge. NMC lithium battery (1C charge/discharge) retains 80% capacity after 1000-2000 cycles; LFP battery can withstand 3000-5000 cycles.

· How long can it actually last? For home energy storage batteries with one charge/discharge per day (shallow cycle), LFP's 5000 cycles can last 13.7 years (5000 ÷ 365 ≈ 13.7); NMC's 2000 cycles last only 5.5 years. For an electric vehicle consuming 50% charge daily (deep cycle), LFP's 5000 cycles can last 6.8 years (5000 ÷ (365 × 0.5) ≈ 27.4 years? Note: Cycle count is for full cycles. For shallow cycles, 50% discharge counts as 0.5 cycles. For example, discharging 50% daily counts as 0.5 cycles/day, so 5000 cycles would last 5000 ÷ 0.5 = 10000 days ≈ 27.4 years, but in practice, affected by temperature and charging habits, it might be discounted by 30%, about 19 years).

· "Hidden cost" of degradation: A 10kWh LFP battery, after 5000 cycles, capacity remains 80% (8kWh). Originally it could store 10 kWh, now only 8 kWh – if the household uses 5 kWh per day on average, it could originally last 2 days, now only 1.6 days, equivalent to losing 0.4 days of emergency power.

Safe or not? The temperature red line of thermal runaway

The scariest thing about batteries is "catching fire," but thermal runaway temperature and protection mechanisms can lock the risk away.

· Trigger temperature: LFP battery thermal runaway initiation temperature ≥ 500°C (Chinese Standard GB 38031-2021), NMC lithium battery is about 200°C. An experiment: piercing a battery with a nail, LFP mostly just smokes, while NMC catches fire directly.

· Protection mechanism: Formal batteries have a BMS (Battery Management System) that monitors voltage, temperature, and current. For example, if a single cell temperature exceeds 60°C, the BMS will cut power; if the voltage is below 2.5V (over-discharge), it will lock to prevent permanent damage.

· End-of-life standard: It's not scrapped when it becomes unusable, but when capacity < 80% or internal resistance > 20% of initial value (national standard). A battery used until 79% capacity can still store electricity, but generates more heat during charge/discharge, increasing safety risks, so manufacturers recommend replacement.

"Calculation Guide" for choosing batteries: Is 30% more expensive worth it?

LFP is more expensive than NMC, but may be more cost-effective in the long run.

· Cost comparison: In 2023, LFP cells cost 0.5 RMB/Wh, NMC cells cost 0.8 RMB/Wh. For a 10kWh home energy storage system, LFP costs 5000 RMB, NMC costs 8000 RMB, a difference of 3000 RMB.

· Return calculation: Assuming one charge/discharge per day, LFP lasts 13.7 years, average annual cost 5000 ÷ 13.7 ≈ 365 RMB; NMC lasts 5.5 years, average annual cost 8000 ÷ 5.5 ≈ 1455 RMB. Over 13.7 years, the total cost for LFP is 5000 RMB, for NMC replacing 3 times costs 24000 RMB – a difference of 19000 RMB, enough to buy a new electric vehicle.

Working Principle

In 2023, the global newly installed photovoltaic capacity was about 295GW, and the cumulative installed capacity exceeded 1.5TW (equivalent to the capacity of 1500 Three Gorges Dams). As the "heart" of the photovoltaic system, the efficiency of solar cells directly determines the cost of electricity generation. According to BloombergNEF data, for every 1% increase in module efficiency, the Levelized Cost of Electricity (LCOE) can decrease by 3-5%.

A crystalline silicon cell the size of a fingernail must go through 12 processes to convert solar photons below 1100nm into usable current; while the efficiency of perovskite-silicon tandem cells in the lab has broken through to 33.2% (École Polytechnique Fédérale de Lausanne 2023 data), higher than the theoretical limit of single-crystal silicon cells (about 29.4%).

Core Foundation

In 2022, Germany's Fraunhofer Institute measured a 156.75mm × 156.75mm monocrystalline silicon solar cell – under AM1.5G standard illumination (1000W/m² irradiance, equivalent to noon sunlight), it converts 7.8×10¹⁹ photons per second into 0.5V voltage, 1.8A current, and finally outputs 1.17W of power.

Light Absorption: Not all light can "knock" out electrons

For a solar cell to generate electricity, the first step is to let photons "hit" the material and transfer energy to electrons. Silicon is best at this – its bandgap (Eg) is exactly 1.1eV, corresponding to the ability to absorb photons with wavelength ≤ 1100nm (accounting for over 90% of the total energy in the solar spectrum). But for a photon to "knock" out an electron, its energy must be ≥ the bandgap, otherwise it passes right through. For example, near-infrared light with a wavelength of 1240nm (energy 1eV) hitting a silicon wafer is like a small stone hitting thick glass; it simply cannot knock out an electron.

Lab measurements with a spectrophotometer show: Monocrystalline silicon wafers have an absorption rate of up to 95% for light between 400-1,100nm, but for light above 1100nm, the absorption rate drops sharply to below 10%. What's more troublesome is that even if the photon energy is sufficient, about 3% will be directly reflected off the silicon surface – this is why an anti-reflection layer is coated on the front of the cell: a 75nm thick silicon nitride film (refractive index 2.0) can suppress reflectivity to below 1% (uncoated silicon wafer reflectivity is about 30%). But even so, for every 100 photons hitting the silicon wafer, at most 95 are absorbed, the remaining 5 are either reflected or have too long a wavelength and pass through.

Electricity Generation: The "Life and Death Sprint" of Electrons and Holes

After a photon is absorbed, a valence electron in the silicon atom gains energy, breaks free from the covalent bond, becomes a free electron, and simultaneously leaves behind a positively charged hole – this is an "electron-hole pair" (minority carrier). A monocrystalline silicon cell can generate about 1.2×10¹⁶ such "electron-hole pairs" per square centimeter per second, which sounds like a lot, but 90% don't survive beyond microsecond-level time.

The first death trap they face is recombination: Electrons and holes are like the two poles of a magnet; when they meet, they recombine, and the energy is lost as heat. Laboratory measurements with a transient fluorescence spectrometer show that the minority carrier lifetime of a PERC cell is about 120μs (microseconds), meaning an electron can "live" for an average of 120μs before recombining. If the lifetime is below 50μs, the cell efficiency directly drops by more than 2%. The second trap is resistive loss: When electrons move inside the silicon wafer, they collide with atoms, losing 0.1V voltage for every 1cm moved. Therefore, the silicon wafer cannot be made too thin (mainstream thickness is 180μm), otherwise the electrons "die" of exhaustion before reaching the electrode.

Conduction: The Built-in Electric Field is the "Electron Traffic Cop"

To make electrons obediently run to the positive electrode and holes to the negative electrode, an "electron traffic cop" – the built-in electric field – must be established inside the cell. This is achieved through the PN junction structure: dope one side of the silicon wafer with phosphorus (pentavalent element, extra electrons, becomes N-type region), and the other side with boron (trivalent element, lacks electrons, becomes P-type region). The carrier concentration difference between the two regions creates an electric field at the interface from N to P, with a strength of about 10⁴ V/cm (equivalent to a voltage difference of 100,000 volts over 1 meter).

Without it, electrons and holes would be like bees crashing around, with only 10% reaching the electrodes; with it, 90% of electrons are "swept" toward the N-region and flow out through the main busbars (width 120μm, 12BB design). But busbars that are too wide will block light – the total width of 12 main busbars is about 1.44mm, occupying 3% of the silicon wafer area, directly causing a 3% current loss. Therefore, laser grooving technology is now used to reduce the busbar width to 80μm, reducing the shading area to 2%, which can increase efficiency by 0.5%.

Efficiency Bottlenecks

In 2023, the U.S. National Renewable Energy Laboratory (NREL) updated the photovoltaic efficiency roadmap – the theoretical maximum efficiency of single-junction crystalline silicon solar cells remains at 29.4%, the "ceiling" calculated by Shockley and Queisser using a formula in 1961. But the reality is that the average efficiency of mass-produced silicon cells last year was only 23.1% (CPIA data). Even the flagship products of leading companies (like LONGi, Jinko) only touch the edge of 26%. This 3% gap hides the triple shackles of materials, processes, and cost.

Theoretical Ceiling: How was the 29.4% for single-junction silicon cells derived?

The Shockley-Queisser (SQ) limit formula is clear: Efficiency = (Open-circuit voltage × Short-circuit current × Fill Factor) / (Incident photon energy). The theoretical value of 29.4% for a single-junction silicon cell is based on three perfect assumptions:

· The absorption layer is infinitely thin (only absorbs effective photons, no extra thickness);

· No resistive loss (electron movement consumes no energy, no current leakage);

· No recombination loss (all electron-hole pairs are collected, none disappear).

For example, the silicon wafer must have a certain thickness (mainstream 180μm) to absorb enough photons, which leads to resistive loss; anti-reflection layers and electrodes block light, causing fewer photons to be absorbed than theoretically possible; the minority carrier lifetime is limited, and electrons recombine before reaching the electrode. NREL measured that an ideal single-junction silicon cell can achieve 29.4% efficiency, but once the thickness exceeds 100μm, the efficiency immediately drops below 27% – this is the harsh reality of theory.

Optical Loss: Photons escape, no electricity is generated

Photons that cannot enter, are blocked, or pass through are the primary killers of efficiency.

· Reflection Loss: An untreated silicon wafer surface has a reflectivity as high as 30% (meaning 3 out of 10 photons hitting it bounce off directly). The mainstream solution now is to coat a 75nm thick silicon nitride anti-reflection layer (refractive index 2.0), which can suppress reflectivity to below 1%. But if this film's thickness deviates by 10nm (becoming 85nm), reflectivity can rebound to 2%, directly reducing efficiency by 0.3%.

· Shading Loss: The metal busbars (main and fine) on the front of the cell block light. Taking the 12BB (12 main busbars) design as an example, the main busbar width is 120μm, and the total shading area accounts for 3%; if laser-grooved "busbarless" technology is used, the shading area can be reduced to 1%, but the process complexity doubles and cost increases by 5%.

· Transmission Loss: Photons with wavelengths exceeding 1100nm (like infrared light) simply cannot be "captured" by silicon. This light accounts for 10% of the total incident energy, and we can only watch it pass through the silicon wafer and become waste heat.

Resistive Loss: Current "leaks" on the way

Electrons moving from inside the silicon wafer to the electrode must pass two hurdles: the bulk resistance of the silicon wafer itself, and the contact resistance between the electrode and the silicon.

· Bulk Resistance: Silicon's resistivity is about 1 Ω·cm. A 156.75mm × 156.75mm silicon wafer has a bulk resistance of about 0.5 Ω·cm². If the resistance exceeds 0.5 Ω·cm², the fill factor (FF) will drop from 78% to 72% (corresponding to a 1.5% efficiency drop). TOPCon cells optimize silicon wafer doping (phosphorus doping concentration increased from 1×10¹⁶ cm⁻³ to 5×10¹⁶ cm⁻³), suppressing bulk resistance below 0.3 Ω·cm², resulting in 0.8% higher efficiency than PERC.

· Contact Resistance: The contact points between the metal electrode and the silicon wafer are like pipe joints – if the joint rusts (high interface state density), water doesn't flow smoothly. The contact resistance of PERC cells is about 10 mΩ·cm², while HJT cells use an amorphous silicon passivation layer to reduce contact resistance to 1 mΩ·cm², directly improving current transmission efficiency by 2%.

Recombination Loss: Electrons and holes "make up" again

Minority carrier lifetime (the survival time of electrons before recombination) is a critical indicator. The minority carrier lifetime of crystalline silicon must be ≥ 100μs, otherwise electrons recombine with holes just after moving a short distance, turning energy into heat.

· PERC Cell: The back side is coated with a 10nm aluminum oxide passivation layer, reducing the rear surface recombination velocity from 1000 cm/s to 100 cm/s, increasing the minority carrier lifetime to 120μs, with an efficiency of 23.8% (LONGi 2023 data).

· HJT Cell: Both sides are coated with a 5nm amorphous silicon passivation layer, with an interface recombination velocity < 10 cm/s, directly boosting the minority carrier lifetime to 1ms (Meyer Burger data), with efficiency reaching 26.81%.

· TOPCon Cell: Uses an ultra-thin oxide layer (1-2nm) + polysilicon doping, with a rear surface recombination velocity of 50 cm/s, minority carrier lifetime of 80μs, and efficiency of 25.5% (Jinko's 2023 mass production value).

Mass Production Gap: The 1% Chasm from Lab to Production Line

In the lab, perovskite-silicon tandem cell efficiency can reach 33.2% (EPFL 2023), but mass production lines don't even dare to claim 28%.

· Process Complexity: HJT cells require 12 processes (PERC only 8), among which amorphous silicon coating must be done in a vacuum environment ( 10⁻⁶ Pa), with equipment investment 30% higher than PERC (cost per GW of equipment increases from 150 million to 200 million RMB).

· Yield Loss: In the lab, 9 out of 10 cells made can meet the standard; on a production line making 100,000 cells, maybe only 95% are qualified. Every 1% drop in yield increases the cost per watt by 0.02 RMB (calculated at 0.5 RMB/W).

· Material Limitations: The lab uses imported high-purity silicon material (purity above 9N), while mass production can only use 6N silicon material (10 times more impurities), directly causing a 20% drop in minority carrier lifetime and a 0.5% efficiency drop.

Structural Evolution

In 2015, the global average photovoltaic module efficiency was only 19.1% (ITRPV data), and by 2023 it had surged to 23.5% – the 4.4% increase over these 8 years is entirely due to rounds of "makeovers" in cell structure. From PERC pushing efficiency to 23%, to TOPCon and HJT "patching and mending," to perovskite tandem directly touching 33%, each structural change is like "changing the engine" of the silicon wafer, recapturing photons and electrons that were originally wasted back into the power generation process.

PERC: The Primary Contributor Pushing Silicon Cell Efficiency Past 23%

PERC (Passivated Emitter and Rear Cell) is the "old guard" that dominated the market after 2015. Its core is adding a layer of "passivation armor" to the back of traditional cells.

· Previous Pain Point: Conventional Al-BSF (Aluminum Back Surface Field) cells had a metal aluminum back; electrons hitting this area easily "get trapped" (surface recombination velocity 1000 cm/s). Photogenerated minority carriers recombine before reaching the electrode, and efficiency was stuck below 19%.

· PERC's Improvement: The back is replaced with an "aluminum oxide + silicon nitride" stack – the 10nm thick aluminum oxide (Al₂O₃) acts like a sponge, absorbing water, "grabbing" the dangling bonds (interface defects) on the silicon surface, suppressing the rear surface recombination velocity to 100 cm/s; then a 75nm silicon nitride (SiNₓ) layer is coated on top to prevent reflection and protect the aluminum oxide.

· Data Speaks: This modification alone increased the minority carrier lifetime from 50μs to 120μs (LONGi 2018 data), increased the short-circuit current (Isc) by 3%, raised the fill factor (FF) from 75% to 78%, and mass production efficiency jumped directly from 19.5% to 23.8% (Jinko's 2023 mass production value). It's no wonder 70% of global capacity is PERC – it achieves a 4% efficiency gain with minimal process changes (2 more steps than traditional cells).

TOPCon: An upgraded version "patching and mending" based on PERC

TOPCon (Tunnel Oxide Passivated Contact) can be considered an "enhanced version" of PERC, focusing on "no shading on the back".

· PERC's Old Problem: The metal busbars on the back shade 3% of the light, causing significant current loss; the aluminum oxide layer is too thin (10nm), and the passivation effect has reached its limit.

· TOPCon's Clever Idea: No metal busbars are placed on the back; instead, a "ultra-thin oxide layer + polysilicon" structure is used – the 1-2nm tunnel oxide layer (SiO₂) allows electrons to "tunnel" through but prevents holes (positive charges) from coming over, equivalent to opening a "one-way street" for electrons; a 100-200nm thick polysilicon layer (phosphorus-doped) is coated on top, which can both passivate the interface and collect electrons.

· How Efficiency Increases: Without metal busbar shading, the current directly increases by 2%; the polysilicon layer suppresses the rear surface recombination velocity to 50 cm/s, and the minority carrier lifetime increases to 200μs (Jinko 2023 data). Current TOPCon mass production efficiency is 25.5%, 1.7% higher than PERC, with only a 0.05 RMB/W cost increase – this is why it's quickly capturing market share (reached 25% of capacity in 2023).

HJT: A different approach, using amorphous silicon to "erase" recombination loss

HJT (Heterojunction) is a "rule-breaker". It doesn't compete with PERC and TOPCon on the back of the silicon wafer but directly puts an "heterogeneous coat" on the silicon wafer.

· Structural Difference: A 5nm layer of amorphous silicon (a-Si: H) is coated on both sides of the monocrystalline silicon – one side is n-type amorphous silicon (phosphorus-doped, extra electrons), the other is p-type amorphous silicon (boron-doped, extra holes). This creates two PN junctions inside the silicon wafer, separating the movement paths of electrons and holes, drastically reducing recombination.

· Data Advantage: The amorphous silicon layer acts like an "isolation wall," reducing the recombination velocity at the silicon surface from 1000 cm/s to < 10 cm/s (Meyer Burger test data), and the minority carrier lifetime directly soars to 1ms (8 times that of PERC). Combined with low-temperature processing (≤ 200°C), no high-temperature sintering is needed, allowing thinner silicon wafers (can use 120μm), further reducing resistive loss by 0.5%.

· Efficiency Cost: But the amorphous silicon layer is too thin (5nm), requiring extremely high process precision – a 1nm uniformity error in coating causes a 0.3% efficiency drop. Current HJT mass production efficiency is 26.81% (LONGi's 2023 record), 1.3% higher than TOPCon, but equipment investment is 30% higher (cost per GW increases from 200 million to 260 million RMB), and silver paste usage is also 20% higher (low-temperature solder strips are expensive).

Tandem: Stacking two layers of cells to "use up" the spectrum

No matter how single-junction cells are improved, they cannot escape the Shockley-Queisser limit (29.4%). Tandem cells directly "stack buffs" – the top cell uses a wide-bandgap material (e.g., perovskite) to absorb short-wavelength light, the bottom cell uses silicon to absorb long-wavelength light, "utilizing" the solar spectrum in segments.

· Bandgap Complementarity: Perovskite bandgap is 1.7eV (absorbs blue-green light, 400-700nm), silicon bandgap is 1.1eV (absorbs red + infrared light, 700-1,100nm). The combination can capture over 90% of solar energy (single silicon cells only capture 70%).

· Efficiency Explosion: In 2023, École Polytechnique Fédérale de Lausanne (EPFL)'s tandem cell had a top cell efficiency of 19%, bottom cell 22%, and overall efficiency of 33.2% –7 percentage points higher than the 26% of single silicon cells. Even more exaggerated, the theoretical limit can reach 45% (perovskite-silicon double junction).

· Mass Production Stumbling Blocks: But tandem cells are still in the lab – when manufacturing perovskite layers over large areas (1m²), uniformity is poor (efficiency fluctuation ±3%); defects easily generate at the perovskite-silicon interface (recombination velocity surges); encapsulation cost is double that of single silicon (afraid of moisture penetration). However, GCL Photoelectricity has already made a 1m² tandem module with 28% efficiency, expected to enter mass production in 2025.

Energy Source

2023 data from the International Renewable Energy Agency (IRENA) shows that the global annual newly installed solar capacity has exceeded 295GW, equivalent to installing 3 standard photovoltaic power stations per hour, but these stations generate electricity entirely by "consuming sunlight"; during the same period, global lithium battery energy storage installed capacity was 45GW/90GWh (Gigawatt-hours), storing electricity from the grid or photovoltaics.

Solar Cells

In 2023, global solar power generation accounted for 4.5% of total electricity generation (IEA data), but behind this 4.5% lies strict energy conversion – a standard 1.7m×1.1m 540W photovoltaic module needs to be exposed to 6 hours of effective sunlight per day (irradiance ≥1000W/m²) to generate 3.24 kWh of electricity, enough to run an air conditioner for 4 hours. But if it encounters overcast days in Chengdu (irradiance only 200W/m²), the same panel's daily generation plummets by 87%, leaving only 0.43 kWh.

Photons hitting silicon wafers: How competitive is the "electron race" in semiconductors?

The starting point of solar cell power generation is the energy carried by photons being absorbed by the semiconductor. Taking the mainstream PERC monocrystalline silicon cell as an example, its core is a P-type silicon wafer about 180μm thick, with phosphorus diffused on the surface to form an N-type layer; the interface is the P-N junction. When photons (energy ≥1.1eV, corresponding to wavelength ≤1100nm) hit the silicon wafer, valence electrons gain energy and jump to the conduction band, generating electron-hole pairs – this step is called "photogenerated carriers". But not all photons can knock out electrons: 1200nm wavelength infrared light (common at sunset) has insufficient energy and passes directly through the silicon wafer becoming waste heat; 300nm wavelength ultraviolet light (strong light during summer exposure) has excess energy, with the excess part also turning into heat. Only photons in the middle wavelength range can generate electricity efficiently.

At this point, the built-in electric field of the P-N junction (about 10^4 V/cm) acts like a referee, driving electrons towards the N-region and holes towards the P-region, forcing them to run to the electrodes. Ultimately, a 1m² cell can collect about 150A of current (corresponding to a 400W module, voltage about 2.67V). But in the lab, LONGi's HPBC cell has perfected this step: through passivation technology to reduce recombination, increasing electron survival rate, conversion efficiency reaches 26.81% (current mass production ceiling), 3.81 percentage points higher than conventional PERC cells (23%) – don't underestimate this 3.81%; 1m² can generate 38W more, a 100MW power plant generates 1 million kWh more per year.

Light intensity directly rewrites the power generation curve: data doesn't lie

The power generation of a solar cell is strictly proportional to irradiance (light power received per unit area). An example: a 10kW residential PV system in Jiangsu, at noon in summer with 1050W/m² irradiance, outputs 9.8kW power; in the evening when irradiance drops to 200W/m², power is only 1.9kW; on a cloudy day with 150W/m² irradiance, power directly drops to 1.4kW. This isn't "approximate"; it follows the formula Power = Irradiance × Module Efficiency × Area – a 10kW module area is about 18m². On a cloudy day, 150W/m² × 23% efficiency × 18m² ≈ 6.2kW? Not exactly, actually lower because the module has a temperature coefficient (discussed later).

When irradiance is below 800W/m², non-radiative recombination in the cell intensifies, and efficiency drops an additional 2-5%. For example, at 500W/m² irradiance, a module with original 23% efficiency might actually have only 21% efficiency. Power generation per m² drops from 115W (1000W/m² × 23%) to 105W (500W/m² × 21%), a 9% decrease. This is why weather stations measure "global horizontal irradiance" – Spain's annual average irradiance is 2200 kWh/m², Germany's is only 1000 kWh/m². Installing the same 1kW module, Spain generates 1900 kWh annually, Germany only 900 kWh, a difference of more than double.

Temperature and shading: invisible "electricity thieves"

Besides, for every 1°C temperature increase, module efficiency decreases by 0.3-0.5% (monocrystalline silicon temperature coefficient is about -0.4%/°C). At noon in summer, the module surface temperature can soar to 70°C (ambient temperature 30°C + 40°C temperature rise). The original 23% efficiency drops to 23%×(1-0.4%×40)=23%×0.84=19.32%. Power generation per m² drops from 230W (1000W/m² × 23%) to 193W (1000W/m² × 19.32%). This is also why photovoltaic installations in desert areas (e.g., Saudi Arabia, summer surface temperature over 60°C) should choose modules with low temperature coefficients – some manufacturers can suppress the temperature coefficient to -0.3%/°C. At the same 70°C, efficiency only drops to 23%×0.88=20.24%, generating 5% more power.

A leaf covering 1% of the module area can cause the entire string's power to drop by 30% – this is the "hot spot effect" at play: the shaded part doesn't generate electricity but heats up due to internal resistance (temperature over 100°C), current is forced to detour, causing the unshaded parts to also lose power due to increased series resistance. Tests show that when the shading area reaches 5%, the string power loss can reach 50%.

Lab efficiency vs. field generation: separated by the "reality gap"

The 26.81% efficiency advertised by manufacturers is lab data (AM1.5 standard spectrum, 25°C, 1000W/m² irradiance). Outdoors, actual efficiency can be discounted by 20%. For example, LONGi's HPBC module has 26.81% lab efficiency, but outdoor empirical annual average efficiency is about 21% – the difference lies in spectral deviation (real sunlight has more UV and IR than AM1.5), temperature fluctuations, dust coverage (annual dust accumulation causes 2-3% efficiency loss). More realistically, after 3 years of use, module efficiency naturally degrades by 0.5% (2% first-year degradation, then 0.45% annually, from IEC standard). After 10 years, actual efficiency is only 21%×(1-0.5%)×(1-0.45%×9)≈21%×0.9775×0.960≈20%, with visibly declining generation capacity.

Batteries

In 2023, China's power battery installation volume was 658GWh (China Automotive Battery Innovation Alliance data). A 400kg NMC battery pack can store 60kWh of electricity – enough for a Tesla Model 3 to drive from Beijing to Shanghai (about 1200 km). But this electricity isn't "stored" directly; it's lithium ions repeatedly "moving bricks" between the positive and negative electrodes: when charging, lithium ions deintercalate from the positive electrode (NMC material) and insert into the layered structure of the negative electrode (graphite); when discharging, they return the same way, with electrons following through the external circuit to power phones, motors, etc.

For every 10 Wh/kg increase in energy density, electric vehicle range increases by 50 km (CATL experimental data). The porosity of the positive and negative electrode materials and the viscosity of the electrolyte must be adjusted precisely to the nanoscale.

What does chemical energy storage look like? See how positive and negative electrode materials "team up"

Batteries store the energy difference of redox reactions. Taking the most common Lithium Iron Phosphate battery (LFP) as an example: the positive electrode is lithium iron phosphate (LiFePO₄), the negative electrode is graphite (C), and the electrolyte is lithium hexafluorophosphate (LiPF₆) dissolved in carbonate solvent. When charging, an external voltage forces Li⁺ ions to come out of LiFePO₄, pass through the separator (PP/PE porous membrane), and insert into the graphite layers, forming LiC₆; when discharging, LiC₆ decomposes, Li⁺ ions slip back into LiFePO₄, while electrons travel from the negative electrode through the wire to the positive electrode, powering the light bulb, motor, etc.

The positive electrode of NMC lithium batteries (NCM811, nickel-cobalt-manganese ratio 8:1:1) contains more nickel (Ni), which can bind more Li⁺ ions, resulting in energy density surging to 240-300 Wh/kg (LFP only 160-200 Wh/kg). But nickel is too active and prone to "losing control" at high temperatures – in a 2022 vehicle recall incident, the problematic battery's Ni³⁺ oxidized the electrolyte at 180°C, triggering thermal runaway, with the temperature soaring to 800°C in 10 minutes.

Charging isn't "pouring electricity in"; it's a marathon of lithium ions "moving house" between electrodes

Charging power is not equal to "electricity storage speed"; lithium ion migration rate is the bottleneck. For example, charging an electric vehicle with a 400 km range to 80% requires moving 8kWh of electricity – based on an average voltage of 3.6V, about 2222 mol of Li⁺ ions need to migrate (1 mol = 6.02×10²³ ions). These ions must move from the positive to the negative electrode, passing three hurdles:

1. Deintercalation Resistance: The tighter the structure of the positive electrode material, the harder it is for Li⁺ to come out. CATL's Kirin battery uses "ultra-electron net" technology to thin the positive electrode particles to 5μm (conventional 10μm), reducing deintercalation resistance by 30%, increasing charging speed from 1C (full charge in 1 hour) to 4C (80% charge in 15 minutes).

2. Electrolyte Conduction: Li⁺ ions swim slowly in the electrolyte, like a traffic jam. Adding vinylene carbonate (VC) as an additive can form a smoother SEI film (Solid Electrolyte Interphase) on the negative electrode, increasing ion migration rate by 20%, and low-temperature (-20°C) charging capacity retention increases from 60% to 85% (BYD experimental data).

3. Negative Electrode Acceptance: The "parking spaces" between graphite layers are limited; charging too fast causes "congestion". Using silicon-carbon negative electrodes (10% silicon content) instead of pure graphite expands the interlayer space by 3 times, increasing fast-charging capability from 3C to 5C (Xiaomi 13 Ultra battery parameters), 10-minute charge to 100%.

Discharge rate depends on reaction rate; fast charging relies on it

A gaming laptop instantly launching a large 3D game requires the battery to release high current instantly; a drone accelerating rapidly requires the battery to withstand high-rate discharge. Behind this is the electron/ion dual conduction capability of the electrode material.

For example: an ordinary phone battery (graphite negative electrode + NMC positive electrode) has a discharge rate of 0.5C (discharges in 2 hours), but gaming phones use graphene-coated negative electrodes – graphene's electron mobility is 100 times that of copper, able to lay an "electron highway" on the electrode surface, increasing the discharge rate to 3C (discharges in 20 minutes), supporting 120W fast charging for instant game launching. However, high-rate discharge has a cost: high current causes Joule heating (Q=I²Rt), raising battery temperature. For every 10°C increase, capacity degradation accelerates by 20% (MIT research). So, playing high-load games on a gaming phone for 1 hour can raise the battery temperature to 45°C, consuming 5% more lifespan than daily use.

Cycle life has hidden nuances; shallow charge/discharge can double usage time

Battery life isn't "use until death"; it's the number of charge/discharge cycles before capacity drops to 80%. In the lab, LFP batteries under standard cycles (1C charge/discharge, 25°C) can withstand 6000 cycles.

· Temperature Killer: Charging at -10°C can easily cause Li⁺ to precipitate as metallic lithium on the negative electrode surface ("lithium plating"), piercing the separator and causing short circuit, directly halving cycle count to 3000 cycles; at 45°C high temperature, electrolyte decomposition accelerates, SEI film thickens, cycle count also drops to 4000 cycles (China Automotive Engineering Research Institute test).

· Depth of Discharge Killer: Each 100% charge/discharge (0-100% SOC) results in 80% capacity after 3000 cycles; but if only 50% charge/discharge (20-80% SOC), cycle count can double to 6000 cycles.

· Voltage Killer: Long-term storage at high voltage ( 4.2V) can "overstretch" and collapse the positive electrode material structure; storage at low voltage ( 2.5V) decomposes the negative electrode's SEI film, causing permanent lithium ion loss. Therefore, manufacturers recommend maintaining 50%-70% charge (corresponding voltage 3.7-3.9V) for long-term storage, which can extend life by 2 years.

From lab to pocket: The "real-world discount" of cycle life

The 6000 cycles advertised by manufacturers are under "ideal conditions" (25°C, 1C, 0-100% SOC). In users' hands, actual cycle count may be discounted by 30-50%. For example, a certain brand's 18650 NMC battery has 500 lab cycles (80% capacity), but if a user charges to 100% daily, uses it to 20%, in a 30°C environment, capacity may drop to 75% after 1 year – equivalent to only 180 actual cycles, a discount rate of less than 40%. This is why phone manufacturers promote "smart charging": after learning your schedule, it automatically stops charging at 80% overnight, topping up to 100% half an hour before you wake up, forcibly increasing cycle count from 500 to 800, extending life by 2 years.