Solar energy is the most underrated and unused renewable energy resource. Sun is a constant source of solar radiation or electromagnetic radiations. The intensity or power of these radiations vary from place to place and depends majorly on season and time. If we see for major energy producing natural resources and the extent of their power, we have wind energy which has total recoverable power of 70-130 TWy/y and OTEC (Ocean Thermal Energy Conversion) which have recoverable energy power of 3-11 TWy/y. The total amount of recoverable energy from sun or solar radiation can amount to 23,000 TWy/y, which is far more than any other renewable source of energy, but if we talk about present condition of extraction of this energy, it only results to 18.3 TWy/y and is expected to grow to 27 TWy/y in 20506. Thus, we can confer that solar energy is the future of energy production.
Figure 2. estimated finite and renewable planetary energy reserves (Terawatt-years)6.
When it comes to harnessing the power from solar radiation (electromagnetic radiations) we need a device that can convert the energy that is radiated from sun in the form of light and heat into usable energy. It was a French physicist Edmond Becquerel in 1839, who discovered this phenomenon of generation of electricity as a function of sunlight. This whole photovoltaic effect was explained by him in Les Comptes Rendus de l’Académie des Sciences 7. According to Becquerel “shining light on an electrode submerged in a conductive solution would create an electric current”. As we know that light transmits energy across space and this energy isn’t received all at once by a receiver. The energy comes in the form of small independent and discrete energy packets called photons. A photon has energy equals to E=hc/? where, E is energy of incident photon, h is Planck’s constant, c is the speed of light and ?(lambda) is wavelength of the respective photon.
2.2: Charge dissociation in Solar Cells
In reality when an incident photon with a certain wavelength is incident on a Silicon or Germanium surface which don’t have free electrons for conduction, then the electrons present at the outermost shell gets energy from photon and go to conduction band from valence band. When electrons leave the valence band and go to conduction band a hole is left back or created at valence band and this result in creation of an exciton. The dissociation of exciton is a major step in solar cell. Exciton dissociation can be promoted by charge transfer between donor and acceptor molecules. The exciton dissociation is efficient at the interface between materials with different electron affinity EA (i.e. LUMO) and ionisation potential IP (i.e. HOMO). The difference in electron affinities creates a driving force at the interface between the two materials that is strong enough to separate charge. EA and IP of the electron acceptor should be higher than those of the donor.
?(?LUMO?_D-?LUMO?_A )>Exciton energy
Figure 3. Charge separation from an exciton at donor acceptor interface8.
Thus, free charge carriers are obtained and thus sending them towards their respective electrodes by built in potential, this creates an electro motive force which results in the production of current. After converting excess energy given by photons (from light) the electrons come back to the valence band.
Figure 4. Photovoltaic effect15.
2.3: Equivalent circuit and relation with Shockley’s equation
So, if we talk about the ideal condition and real-world condition for a solar cell, it can be represented in small signal model as: –
Figure 5. Equivalent circuit of a solar cell under light in an, a) ideal condition b) real condition 8.
If we talk about the current flowing through the cell then we get the equation which is also known as an ideal diode equation or Shockley’s diode equation by viewing the solar cell as nothing more than a P-N junction diode than we have,
Is= saturation current under reverse bias.
q= elementary charge
k= Boltzmann constant
T= temperature (K)
V= bias (V)
But, under illumination we have,
IPH = Photo generated current
The ideal circuit doesn’t take in account the resistances offered by the contact and the bulk, but when it is operated in real world condition the current will be affected by the series and shunt resistance. In simple terms, the series resistance is introduced as to consider the voltage drop and internal losses because of the flow of current ; shunt resistance takes into account the leakage current to the ground when the diode (solar cell) is reverse biased. These resistances offer some losses in the cell. From the drawing of the current voltage characteristics of a photovoltaic cell under illumination, the different typical values can be measured (fig 3). The slopes at the short circuit point and at the open circuit voltage are the inverse values of the shunt resistance and the series resistance of the equivalent circuit scheme of a solar cell respectively 8.
An important theoretical power parameter is Pmax, that is Theoretical power that can be obtained from the solar cell or module. This parameter is largely dependent on the substrate and the absorption of photons by the solar cells.
Pmax = Isc * Voc
Figure 6. I-V characteristics of solar cell under illumination a) Ideal cell b) with small shunt resistance c) with small series resistance 8.
Here, Jcc corresponds to the short circuit current density, Voc to the open circuit voltage. Jmax and Vmax correspond to the maximum power Pm delivered by the solar cell.
The Ideal cell I-V characteristics are also known as fill factor. The FF is defined as the ratio of the maximum power from the solar cell to the product of Voc and Jcc. It can be defined as
FF=MPP/(V_oc.J_CC )=(V_max.I_max)/(V_oc.J_cc )
Apart from fill factor there is another parameter that is vastly used to define the effectiveness of the solar cell and that parameter is power conversion efficiency.
?=(V_max.J_max)/P_in =FF (V_oc.J_CC)/P_in
The effect of FF and PCE (?) play a major role when one has to connect a large number of solar modules to supply the high energy demand. As we can say from basic deduction skills that if increase the number of solar panels, the power that we will be getting will be increased proportionally. Whereas, the effect of parallel and series connection of solar panel is different, if we put 2 solar panels in series the voltage will increase and if we connect them in parallel the resulting current will increase.
2.4: History of Solar Cells
New York inventor Charles Fritts created the first solar cell in 1883, by coating selenium with a thin layer of gold 5 and the power conversion efficiency was found to be somewhere between 1-2%. Soon after the first solar cell was made, lot of research started pouring in to increase the efficiency. The first solar cell commercially made and patented was by Russell Shoemaker Ohl in Bell laboratories, who used silicon in place of selenium because of high efficiency of silicon. The power conversion efficiency reached a new height of 6%. If we track down the development and generations of photovoltaics we will come across three generation of development in solar technologies.
2.4.1: 1st generation Solar Cells
1st generation solar cells were produced on wafers and the only difference between two types was the crystallization level. There are basically two types of Bulk crystalline solar cell, a) Monocrystalline solar cell ; b) polycrystalline solar cells. Mono crystalline solar cells can be viewed as the first ‘real ‘solar cells. It was developed in 1954 by Chapin et al 8. If the whole wafer is based on one crystal it is known as mono crystal whereas, if wafer consist of crystal grains, it is called multi crystal solar cell 13. The efficiency of monocrystalline solar cell is higher than the multi crystalline solar cells, but the production of multi crystalline solar cells is easier and cheaper than monocrystalline.
Figure 7. Monocrystalline Cells vs. Polycrystalline Cells 16.
The first generation of solar cells were expensive with high material usage (silicon and germanium), but they came with inherent stability which makes them still one of the top choices for practical usage. Because of the large consumption or requirement of material it was very hard to scale them up, another drawback came in because of toxic materials used in manufacturing of such solar cells which had potential of degrading the environment. The highest efficiency of 26.63 % 10 was achieved by Kaneka corporation in 2017 for heterojunction back-contact crystalline silicon solar cell 14. The second generation came into play because of vigorous research in photovoltaics sector, which was in need of a device that surpasses 1st generation in terms of better performance.
2.4.2: 2nd generation Solar Cell
The 2nd generation of solar technologies (Thin film solar cells) which started with amorphous silicon with an intention of reducing the large material consumption and thermal budget of bulk crystalline solar cells. These characteristics to surpass the 1st generation were achieved by Carlson and Wronski in 1976 9. The performance of the 2nd generation solar cells was lower than the 1st generation but the generation of excitons was enormous. The Staebler-Wronski effect is what happens when an amorphous silicon solar cells degrades in performance when subjected to illumination. So, the stability was distorted and degraded drastically. The inherent stability that 1st generation solar cells has, was absent when it came to 2nd generation solar cells. The second-generation solar cells basically have a) amorphous, b) nanocrystalline, c) CIGS (copper indium gallium diselenide) and d) CdTe (Cadmium telluride). If we talk about amorphous silicon nanocrystalline silicon solar cells which were the torch bearer of the second-generation solar cells, has also been employed in LCD (liquid crystal display), TFT (Thin film transistor) etc. The main problem of intrinsic stability presented an inherent problem with non-crystalline silicon solar cell. The amorphous silicon solar cells have light degradation as well as Staebler-Wronski effect, which degraded the performance of it drastically.
Figure 8. Amorphous Silicon solar cells 17.
CIGS (Copper Indium Gallium Diselenide) solar cells are also prepared by a low temperature gas phase deposition technique, just like the amorphous silicon solar cells. Several reports also stated that CIGS solar cells present self-healing properties towards the defect in the bulk 8. CdTe (Cadmium telluride) employs thin films and was the first of the thin film technology that became a huge success. The degradation in it is mainly because of diffusion of electron material into CdTe and electrode degradation in humidity.
Figure 9. CIGS and CdTe solar cells 18.
The main advantage of the second-generation solar cell over the first generation was better manufacturing, material usage and employment in wide area of usage whereas, the drawback was decrease in efficiency and stability of the solar cell. To improve the instability, power conversion efficiency and reducing the environmental hazard caused because of the first two generation, third generation of solar cell was found. The highest efficiency ever noted by an amorphous silicon solar cells is 10.2%, for CIGS it is 21.7 % and for CdTe it is 21.0% 10. The third generation mostly employed earth abundant materials and there are numerous combinations that can be made by varying the material, contacts and structure of the device. The third generation showed quite a promising start as it was initially made by polymers which were quite abundant, easy manufacturing and apparently less toxic. The only factors still limiting the practical usage of organic photovoltaics is the Intrinsic stability and power conversion efficiency.
2.4.3: 3rd generation Solar Cells
The 3rd generation include a) Dye- sensitized solar cells (DSSC) and b) Organic photo voltaic (OPV). The dye-sensitized solar cell was first produced in 1991 as reported by O’Regan and Gratzel whose efficiency was on par with the polycrystalline silicon solar cell and way past to that of amorphous silicon solar cells. It is simple to make and have good intrinsic stability. The causes of degradation in DSSC are dye desorption, electrolyte leakage and electrode corrosion. The main disadvantage of DSSC is that for optimum efficiency and operation a liquid electrolyte is needed, many have tried replacing it with a solid electrolyte, but it never made out to commercialisation which is still seen as an abysmal failure.
Figure 10. Dye-Sensitized solar cell 19.
Organic photovoltaics (OPV) or polymer photovoltaics employee organic materials as active material in a solar cell, which in fact is made up of donor and acceptor types materials. The degradative properties of OPV are very extensive and by far of the highest magnitude. The only reason OPV have not been dismissed is because of the fact that it efficiently promises low cost, use of earth abundant material, fast manufacturing and wide range of usage. All the solar technologies by far would be seen as stepping stones if the two major disadvantages of OPV can be removed which are stability and power conversion efficiency. If we look at the progression OPV has made in terms of efficiency we can say that, OPV has improved very drastically.
2.4.4: Comparison between various generations of Solar cells
If we compare and contrast the advantage and disadvantages of different types of solar cells based on their generation we have as follow: –
1st generation solar cells, which are crystalline silicon based solar cells have following advantages and disadvantage,
1) High conversion efficiency 1) Material usage is high
2) Inherent Intrinsic stability 2) Toxic elements present
3) Low radiation damage 3) Not scalable
4) – 4) Needed a large amount of space
5) – 5) Limited area of usage
2nd generation solar cells, which are also known as thin film solar cells have following advantages and disadvantages,
1) Easy to manufacture 1) Decreased efficiency
2) Low material usage 2)Decreased intrinsic stability
3) Wide area of usage 3) Staebler-Wronski effect seen in a-Si
4) – 4) Toxic element present
3rd generation of solar cell include, Dye-sensitized solar cells, Organic or polymer solar cells have following advantages and disadvantages,
1) Uses earth abundant material 1) Extensive degradation properties
2) Scalable 2) Low power conversion efficiency
3) No toxic element 3) Intrinsically instable
4) Wide area of application 4) –
2.5: Organic Photovoltaics (OPV)
The OPV solar cells employ polymer as an active material, although polymer has been in existence and usage since hundreds of years, it was the domain of material sciences or engineers who employee it in buildings, packaging and textiles. The first observation of photoconductivity in organic compound (anthracene) was reported in 1906 by Pochettino. Electrically polymers are insulators that is why they were of no use to electrical engineers. It was only in 1977 when Hideki Shirakawa accidently developed the first conducting polymer, Polyacetylene. After the first conducting polymer was found, the research based on it started growing. Hideki himself worked with Alan MacDiarmid and Alan Heeger to increase the conductive properties of the polymer, after a lot of research they were able to reach conductivities as high as many metals. Polyacetylene itself is very unstable and hard to work with, but after it many conductive polymers have been found such as poly (p-phenylene vinylene) (PPV) which is widely used these days in OLED (organic light emitting diode) displays and polythiophene which is widely used in photovoltaics solar cells. At the starting stages there was just one type of polymeric blend used as an active material but because of smaller band gap and high recombination rate there was sought to be a need of a polymeric blend or multi-layer polymeric structure. These days polymers such as P3HT (poly(3-hexylthiophene)) and PCBM (6,6-phenyl C61-butyric acid methyl ester) are widely used as conductive polymeric donor and acceptor medium. A typical structure of an OPV solar cell looks something like
Figure 11. Structure of a typical OPV solar cells 20.
Figure 8 shows the structure of an OPV solar cell. The active layers usually consist of an electron donor and acceptor layer. These two layers can be structured in different ways and in different ratios to attain different morphologies which results in varying characteristics of the solar cell. A transparent conducting electrode is placed on the side where the sunlight will fall. Indium tin oxide (ITO) is very commonly used for this purpose. A glass surface is also used to uphold the ITO layer and provide mechanical support. Anti-reflection coatings can help minimize losses due to non-absorption. On the other, dark side the electrode usually consists of a thin film of metal (typically Al) that has been evaporated in to the device. If we look at the structure of an OPVsc in figure 8 we can see when an incident photon reacts with the active material giving its own energy to the electrons in the valence band and thus, creating an exciton. This exciton has a very small wavelength of somewhere around 3-10nm which results in faster recombination. To alter this fast recombining properties because of smaller band gap in organic compounds there was a need of phase segregation which can help in dissociation of the electron and holes from excitons. The main problem that was seen in earlier stages of OPVsc was the problem of dissociation of charge carriers and transportation of charge carriers. Because of smaller wavelength the recombination takes place very faster and thus converting the energy of incident photons to heat. After the dissociation of charge carriers in active material the role of hole transport layers comes into play. They let their own respective charge carriers to move through them. This results in accumulation of electrons on cathode and holes on anode.
There has been a vast amount of research that has been directed towards maximising the efficiency of the OPVsc. Many permutation and combinations have been tried out with various polymeric blends to get the efficiency to reach on par with the crystallin silicon solar cells. The distinction however between various kind of polymer or organic solar cell can be made on the basis of the junction between their active material. The following are the generation in OPVsc: –
2.5.1: 1st generation OPV
1st generation: – Single layer OPVsc, it is the first model of OPVsc that came into existence. The structure of the single layer OPVsc is simple and easy to manufacture. The manufacturing of these kind of cells just involves sandwiching an organic electronic material (Conducting polymer) in between of two different electrodes. The two electrodes should be of dissimilar work function. In a typical OPVsc we use ITO (Indium Tin Oxide) as an electrode (Cathode) with high work function whereas, aluminium and gold are considered for electrode (Anode) with low work function. The two dissimilar electrodes are meant to be that way in order to set up and electric field in the active layer.
Figure 12. A structural representation of a single layer OPVsc.
When a photon is incident on to the active material it absorbs the energy and that energy is transmitted to electrons in the crystal lattice who the leave the HOMO (Highest Occupied Molecular Orbit) and goes to LUMO (Lowest Occupied Molecular Orbit) thus, creating an exciton. The dissimilar work function helps in dissociation of free charge carriers from the excitons, so formed. The electrons go to the positive electrode and holes go to negative electrode. The drawback of single layer OPVsc is that the electric field produced because as a result of two dissimilar electrodes with varied work function is never rarely enough to dissociate the charge carriers from excitons and this results in decreased power conversion efficiency is less than 1 %. The quantum efficiency was less than 1%. Due to shorter wavelength the recombination takes places because of irregular dissociation.
2.5.2: 2nd generation OPV
2nd generation: – Bi-layer or multi-layer OPVsc, it was designed in such a way to maximise the dissociation of excitons into respective free charge carriers. The major problem that was there in single layer OPVsc of lower dissociation energy was overcome in this type of solar cells by making the active layer by combing two different conductive polymers. The two conductive polymer has to be off acceptor (high electron affinity and ionisation energy) and donor (high electron affinity and ionisation energy) types respectively. With the different electron affinity and ionization energies between the acceptor and donor material, the electrostatic forces are generated at the interface. The light has to be incident on the small region of interface for maximising the generation of exciton and dissociation of free charge carriers. The combined electric field is strong enough for efficient dissociation of excitons.
Figure 13. Structural representation of a Bi-layer OPVsc.
The major drawback with bi-layer OPVsc was smaller junction interface which results in low generation of excitons. As the typical diffusion length of a generated exciton in organic material is of 3-10 nm, if the photon is incident anywhere, no in the vicinity of interface, then the exciton has to travel all the way from the donor or acceptor material thickness to the interface for dissociation. Often the thickness of the layer is much higher than the diffusion length of the charge carriers, as the polymer layer need approx. 100nm for efficient absorption of visible light spectra and thus, majority of the charge carriers will get recombined instead of getting dissociated. The quantum efficiency was nearabout 9%.
2.5.3: 3rd generation OPV
3rd generation: – Discrete heterojunction OPVsc employed a multi-layer approach for active material. Three or more layers of donor acceptor material were stacked on one another to make an active layer in which the junction interface area would be more, which will result in increased generation of excitons and even from the dissociation point of view as there will be multiple junctions the dissociation of exciton can take place at any of the nearest donor/ acceptor junction. The initial results were quite satisfying for 3 generation of OPVsc as the quantum efficiency was more than 75%. The absorption of photons or solar radiations was also increased.
2.5.4: 4th generation of OPV
4th generation: – Bulk heterojunction OPVsc are one of the latest and most effective generation of OPVsc. In these types of solar cells, the donor and acceptor material are sandwiched between two electrodes in the form of a blend. The donor and acceptor materials are first blended on nanoscale. The blends interface is quite large, and the domain size is very small probably of several microns in length which results in high amount of, dissociation of, excitons as even the shorter diffusion length excitons can make to the interface. With increase in complexity of blend came in new problem of island formation in which if we decrease the domain size of the blend the donor or acceptor materials some time get isolated and covered from all the side by the material of different polarity. Thus, the blend has to be made in such a way that after dissociation of excitons into free charge carriers the effective transportation of the charge carriers can take place. The major advantage of bulk heterojunction over the layered structural approach is that they can be made into large thickness for effective absorption of photons and the problem that arose in bi-layer OPVsc won’t rise in this because of phase segregation of acceptor and donor materials.
Figure 14. Structural representation of a bulk heterojunction OPVsc.
Since mostly the material used as electron acceptor is PC71BM is a fullerene derivative thus the light absorption capacity of the acceptor material is lower than the donor material. Fullerene compounds or derivative have low electronic tunability resulting in several restrictions being placed on the use of higher and different derivatives of Fullerene. There are many proposals give to replace this fullerene derivative with any other organic conducting polymer. The power conversion efficiency of these kind of structure has reached aroun11.6%. Apart from these above-mentioned structures there are few other structures which have been tried but have not been quite successful such as, Graded heterojunction in which the donor and acceptor materials are mixed in such a way that the overall gradient is gradual etc. Majority of OPVsc employs P3HT: PCBM where P3HT is the donor and PCBM is the fullerene derivative substance used as an acceptor. The main problem with this combination of the material as an active layer in an OPVsc is that, where P3HT is a good wide gap donor whereas the corresponding fullerene derivative PCBM has high synthetic costs, limited optical absorption, poor bandgap tunability and morphological instability of fullerene-based acceptors such as phenyl-C61-butyric acid methyl ester (PC60BM) and its C71 analogue PC70BM. SO, if the two materials i.e. donor and acceptor (other than fullerene derivative) have well-matched optoelectronic and morphological properties then we can expect the losses to be minimised and a better functional solar cell. Research has been done to find a substance that can effectively and efficiently replace the fullerene derivative without affecting the overall system performance in negative way and at the same time reducing the overall production cost. A new acceptor material has been found which is IDTBR, which can reduce both spectral overlap and morphological issues with FBR via replacement of the fluorene core with an indacenodithiophene unit. The structure of O-IDTBR and EH-IDTBR is shown in the figure 15 and 16 repsectively.
Figure 15. O-IDTBR structure 23.
Figure 16. EH-IDTBR structure 23.
The reported Power conversion efficiency of P3HT:O-IDTBR and P3HT:EH-IDTBR based solar cell has been reported to be 6.30% and 6.00% respectively 11.
2.5.5: Issues with Stability and efficiency of OPV
The only reason why OPVsc are still not manufactured in large amount and available for large scale production and usage is because of the two main setbacks or drawbacks it has. Intrinsic stability and power conversion efficiency still remains two horrific barriers for the growth of OPVsc. The lifetime of an OPVsc is to a great extent dependent on the organic material that has been used for its manufacturing. If we talk about the stability of the solar cell, the more stable a solar cell is the higher its lifetime will be as its performance won’t degrade much. Thus, we can say that if various components of a solar are working perfectly fine and are stable then there are high chances of solar cell working nicely and can will be seen as a stable device, but if the device components are not stable and not working nicely on their own then there is a very low probability that the whole device when assembled will even work or be stable. So, stability is to a great extent dependent on the degradation. If the degradation is very high thus we can say that the device is not stable. A perfectly ideal and stable solar cell will have no effect of change in performance regardless of time, which inherently shows that the effect of degradation is not much or negligible. However, in practicality the scenario is different. The extent of degradation is dependent on various factors such as variation in climate (Temperature, moisture, sun irradiance etc). Thus, stability of an OPVsc is also dependent on its surrounding or environment. If we talk about stability inside a controlled environment, then we have two types of stability: –
I) Intrinsic stability: – It can be connected to a poor performing material being used or dust particles that crept in while manufacturing resulting in change in composition of the conducting polymer. Intrinsic stability can be seen as the stability of various elements of the solar cell and also on how well they work when brought together. When various elements of a solar cell are brought together there is some form of degradation that is going to be there we can minimise it but to nullify it is very hard.
II) Extrinsic stability: – Extrinsic stability can be connected with the environmental factors such as moisture, temperature, wind and solar irradiance etc. These factors determine some severe degradation such as corrosion and cracks, which can straight away cause the failure of the device. The oxygen from the air can oxidise the metallic contacts or electrodes, interlayers are highly hydrophobic and acidic which will cause damage to the cell when exposed to moisture. To reduce or minimise these effects one of the probable solutions to prevent from all these degradations is encapsulation of the device.
2.5.6: Methods for evaluation of stability and efficiency
To test the stability of the devices in a controlled environment we have various methods that can be used to test physical and chemical stability of the device such
a) UV-Vis spectroscopy (Physical stability)
b) AFM (Physical stability)
c)Interference microscopy (Physical stability)
d)Scanning electron microscopy (Physical stability)
e) Infrared spectroscopy (chemical stability)
f) X-ray photoelectron spectroscopy (chemical stability)
Thus, if we can improve the lab base intrinsic stability of the OPVsc, we can expect the device stability to be very good. The active material i.e. conducting polymer in OPVsc gets photo oxidized under light, this phenomenon is known as photobleaching. The extrinsic stability can be increased greatly by employing good encapsulation devices and elements.
The power conversion efficiency of the OPVsc is still not on par with crystalline silicon (C-Si) solar cells or we can say it is incomparable to that of C-SI solar cells which are at the efficiency that is twice that of OPVsc. Due to smaller bandgap the dissociation is less and small interface results in low generation of excitons in spite of having higher absorption than the C-Si solar cells the power conversion efficiency is quite low. The only reason why OPVsc have not been discarded completely is because of the fact that they promise cheap production, can be employed in various applications, small or no effect on the environment and finally they promise a good alternative to C-Si solar cells as the C-Si solar cells have almost reached the saturation point.
2.6: Perovskite Solar Cell
Now that we know the extent of usefulness and limitations of OPVsc, we go and look for other materials which can possibly replace conducting polymer efficiently. Perovskites ‘Sc are the latest class of solar cells. The mineral was discovered in the Ural Mountains of Russia by Gustav Rose in 1839 and is named after Russian mineralogist Lev Perovskite (1792–1856) 12. They are basically made up of mixed organic, inorganic halide material. It has a specific crystal structure with the ABX3, here A and B are two cations and X being an anion that bonds to both. Where A atoms are larger than B atoms. Calcium Titanium Oxide (CaTiO3) is the empirical formula for perovskite type structure. The most commonly used perovskites compound is methylammonium lead trihalide (CH3NH3PbX3), here X being halogen atoms such as Chlorine, Iodine or Bromine.
Figure 17. Crystal structure of CH3NH3PbX3 perovskites 21.
For perovskite type solar cells, the exciton binding energy is only 16MeV at low temperature. At room temperature, the thermal kinetic energy is estimated to be 26MeV. Thus, the environment the perovskites Sc is in, itself can provide enough dissociation energy for excitons to become free carriers. This results in higher power conversion efficiency. The first solar cell that was made using a perovskite material was produced in 2009 with a PCE of 3.8%. According to sources the recent efficiency of perovskite type solar cells has reached a staggering 23.6% in 2017 10. Because of the recent and fast paced developments that took place in last 8-10 years, the perovskite type solar cells are seen as the future of photovoltaics which can easily replace the cutting edge that OPVsc has over C-Si solar cells.
Figure 18. structural representation of Perovskite type solar cells 22.
Figure 19. Energy level diagram of TiO2 (electron transport layer), CH3NH3PbI3 and spiro-MeOTAD (hole transport layer).
2.7: Scanning Tunnelling Microscopy (STM)
The devices that employ the functionality of scanning probe microscopy, all started from Scanning Tunnelling Microscopy in 1981. It was in IBM Zurich lab where Gerd Binnig and Heinrich Rohrer developed the first STM, for which they won Nobel prize in Physics in 1986. STM employs a very sharp tip over the surface of the substrate. A potential bias is given either to the tip or the substrate and thus even an individual atom can be scanned and imaged.
Figure 20) 3D rendered Scanning Tunnelling Microscope image of atoms25.
STM is a device that is based on conjunction of multiple principles. The most important one amongst which is the effect of quantum mechanical tunnelling, which is the same principle which lets our naked eye see an object. Another important principle behind this device is the piezoelectric effect, which is employed in the tip control. Due to the piezoelectric effect we are able to scan the surface with an angstrom level of accuracy in the images. The last principle is that of a feedback loop that is employed for the monitoring the tunnelling current and coordinates the current and the positioning of the tip.
Figure 21) Illustration of STM operation25.
In the above shown figure, the bias voltage is supplied to the tip and the electron tunnelling is from tip to the surface of the substrate. The tip is positioned with piezoelectric positioning and feedback loop is used to maintain a current setpoint and thus, creating an electronic topography image with a 3-dimensional structure.
2.7.1: Tunnelling Effect
As a lot of emphasis has been laid on the tunnelling effect it will be wiser to explain it into the depth. As stated before tunnelling is a quantum mechanical effect. According to the principle of tunnelling, a tunnelling current is generated when electrons which are moving or behaving like a wave pass through a barrier which they were not supposed r expected to. These electrons that pass through those barriers generate a tunnelling current. In general, we suppose that if an electron doesn’t have enough energy to move across a barrier it will not be able to jump through or go across it but, in quantum mechanical world, electrons are deemed wavelike structures with wavelike properties. These waves if don’t have enough energy of there own and collide with the barrier won’t die but in fact they will taper off quickly. Thus, depending upon the thickness of the barrier whether it is thin or thick electron tunnelling is affected. Suppose the barrier opposing the flow of electrons is thin and then it is expected that these electrons having wavelike properties will have a relatively higher probability of moving across it (We can also use the analogy of a Bipolar Junction Transistor where the electrons flow from emitter and some of them recombine in base but most of them go across it since base is made very thin and lightly doped). This phenomenon of movement of electron across a barrier is known as tunnelling.
Figure 22) Illustration of an Electron wavefunction25.
From the above figure we can see clearly that in the first scenario when the barrier potential that is to be overcome by the electron is higher the wave tapers off quickly. Whereas if the barrier potential (of a few microns) is made small, the probability of some electrons moving across it is relatively higher.
Figure 23) Illustration of working of STM based on tunnelling effect25.
From the above figure we can get a general idea that depending upon where the bias is supplied to and where it is measured from we can image the distance between the tip and the substrate with a great accuracy.
2.7.2: Atomic Force Microscopy (AFM)
Atomic force microscopy (AFM) is a type of Scanning Probe Microscopy (SPM) in which the properties of a substance such as height, friction, morphology etc. are measured. To measure the properties of a substance the AFM moves the probe (having a few microns thick probe tip) over the surface of the substance measuring local properties simultaneously and providing a high-resolution image of all the properties to be analysed.
AFM measures all these properties by various method or modes of operation but, they all are based on a general concept for measuring these local properties of the substance. AFM measures the force between the sample and the surface. The most important part of the device is the probe which basically is required for imaging the surface parameters. The probe has a pyramid structure, which is 3-6 um tall and 15-30 nm in end radius. Due to convolution the lateral resolution of AFM tends to be somewhere around ~30 nm but the vertical resolution can be up to ~ 0.1nm.
Figure 24) (a) New AFM Probe with sharp tip and (b) old/used AFM probe with deformed tip24.
The above images clearly show the difference between a new and an old AFM probe tip. An old tip because of regular interaction with surface and bias voltage degrades slowly depending upon the mode of its working, substrate it is working, bias potential etc. Even a Nano range discrepancy can create a big offset in the result where very high precision and accuracy is in needed in terms of the morphology of the substrate in terms of image. Depending upon the structure of the tip the image quality gets increased or decreased.
Apart from the probe and probe holder another major part of an AFM machine is the Dimension Scan Head. Scan head is the place where the probe holder fits in and where the morphology is traced onto a photo detector.
Figure 25) Dimension Scan Head of an AFM machine.
Figure 26) Schematic representation of working inside a Dimension Scan Head of an AFM 24.
From the figure 25 and 26 we can clearly see that the laser that is being emitted get reflected off the probe cantilever which actually behaves like an optical lever from above and gets projected onto a four-quadrant position photo detector. Thus, any lateral or vertical movement in the cantilever will result in the deflection of the laser and thus result in the variation of the laser position on the position-sensitive photo detector. Position-sensitive photo detector is generally a four-segment photo detector. Any angular movement in the cantilever will indicate the position of the laser spot on the detector. The lower section of the scan head holds the piezo assembly which holds two tubes. The first tube bends in X-Y plane whereas the second tube retracts or extends in Z plane
Figure 27) Piezo assembly with X-Y-Z tube .
The overall working mechanism of the AFM revolves around the feedback loop. It starts by processing the signal from the four-quadrant photo detector into a process value that represents the bending of the probe cantilever. Thus, a deflection value is sent from the detector electronics which is then compared with the desired amount of deflection or the force which is fed into the system by the software interface. The resulting difference that shows up after the comparison of the two values is known as error signal. This error signal is then needed to be changed or influenced so that it can be used for calculating the deviation of the signal from the set point. So, the error signal is then fed to the gain amplifier and then fed to a high voltage circuitry. This high voltage circuitry then send a high voltage required by the Piezo assembly to force the piezo to either retract or extend based on the error signal to maintain the setpoint value.
Figure 28) Feedback mechanism of the AFM.
Figure 29) Figure showing the variation in probe cantilever and the resulting variation on the four-quadrant position photo-detector
Figure 29 shows four different stages of Piezo movement as it encounters a heighted surface. The top left image shows the probe cantilever at normal position and thus the alignment chart showing the laser corresponding to it is fixed at the centre. In the top right image, we can see that a heighted surface has approached the cantilever while scanning and thus the laser moves away from the centre corresponding to the movement of the cantilever. In the bottom left image, the probe is at the peak of the heighted surface and thus the corresponding deflection of the laser is higher resulting in the movement of the laser point farthest from the centre of the four-quadrant position photo-detector. In the fourth image the feedback loop has adjusted the setpoint and we can see that the cantilever is not bent while still being at the top of the heighted surface which shows that the Z tube has adjusted itself to maintain the setpoint value. This way a 3D heighted structure is obtained using AFM.
2.7.3: Conductive AFM (C-AFM)
Conductive or Current sensing AFM is a mode of AFM that is widely employed to get the topography (Height and features) of the sample along with electric current flow at the point of contact of the probe tip and substance. The two main feature of C-AFM, finding the topography and electric current flow is a result of two different processes. To get the topography of the sample the laser which is deflected from the back of the optical part of the cantilever and is recorded on the position sensitive detector. To get the electric current flow a current to voltage pre-amplifier is used. As we know that C-AFM employs two different methods of detection whereas STM relies only on one. As in STM the topology is a function of the electric current flowing between the tip and sample and sometimes it is hard to describe whether it is a change because of surface morphology or because of the difference between surface conductivity. This major drawback of STM is overcome in C-AFM 27.
Figure 30) Current to voltage Pre-amplifier used in Conductive AFM27.
2.7.4: Photoconductive AFM (PC-AFM)
Photoconductivity is phenomenon in which a materials electrical conductivity is increased by the absorption of various spectrum of light. PC-AFM measure the photoconductivity along with the local electrical properties of a substrate or sample. It works in contact mode, as there has to be a contact between the sample and the probe tip unlike STM. This device can be used for the better understanding of the optoelectronic and morphological properties of a sample at nanoscale.
Figure 31) Schematic diagram of a PC-AFM26.
There is only one major difference between C-AFM and PC-AFM that is the source of illumination. In PC-AFM the source of illumination is focused through an inverted microscope. The PC-AFM is especially useful in the scenarios where the current generation is very low somewhere around a couple hundred Femto amps. Devices such as OPV have very minimal current generation at any particular point in the sample and PC-AFM can help us in mapping the photo current, difference in film morphology, determination of donor acceptor domains etc.
2.7.5: Peak Force Tunnelling AFM (PF-TUNA)
Apart from PC-AFM there are various other modes in AFM such as PF-TUNA that can be used for finding out the conductivity or current through a sample. PF-TUNA unlike C-AFM or PC-AFM is not dependent on contact mode operation i.e. it does not have to gauge the sample for finding out the morphology and electrical conductivity. The major drawback of contact mode operation is that the sample is continuously being gauged by the probe tip and which can sometimes damage the sample to a great extent since the average scale of morphology that is being traced is in the vicinity of several hundred micron or even less28. Thus, unlike earlier STM devices PF-TUNA provides high resolution and high sensitivity images without damaging the sample and prevents the probe tip from any damage too. In latest PF-TUNA device the current that can be sensed ranges from fA (Femto Amp) to ?A (Micro Amp). Thus, PF-TUNA is largely used for simultaneous measurement of electrical and mechanical property of a sample at nanometre scale.
Figure 32) Peak Force TUNA images of Polymer P3HT with embedded carbon nanotubes. (A) topography, (B) current, and (C) adhesion maps. Image scale 500 nm 29.
2.7.4: AFM Modes of Operation
All though there are three general modes of AFM operation but, the two most common modes of AFM operation are tapping mode and contact mode. The tapping mode is also known as magnetic alternating current (Mac ) Tapping mode. In contact mode we can set up a constant height and constant force from the setting and then get a scanned image of the sample, we can always fixate one and make the other one varying. In constant height mode of contact mode, the tip maintains a pre-set distance from the sample surface while scanning but as the tip faces any heighted surface or feature then the force between the tip and the sample will change and this change in probe cantilever is used to create an image of the surface topography of the sample. In fixed force mode of contact mode operation, the tip remains at every time in contact with the surface of the sample due to a known applied vertical force. If this force is more the tip can also damage the sample and itself as well. Deflection of the cantilever signals the system that the topography has changed and thus to maintain a pre-set value of force between the tip and the sample the Z piezo tube should retract or extend. To make Z piezo move a voltage is needed and this voltage is then converted into height data for imaging the scanned surface35.
The fixed force mode is not advised to use for the height imaging as there is a loss of data, but it presents more refined surface information as the feedback loop is very fast in correcting even minor changes in cantilever. Contact mode is generally used when the sample is a bit brittle or hard so that the damage to the surface and tip is very less.
Unlike contact mode, in Tapping mode there is little to no damage to the surface or the probe tip while scanning. This mode of operation is generally used for observing molecules, cell surface sample etc. In tapping mode unlike contact mode, the probe tip doesn’t stay in contact with the sample surface instead it keeps on tapping or fluctuating up and down while moving across the sample. This make the tapping mode non-vulnerable to problems associated with friction, adhesion and electrostatic forces etc. This tapping mode give high resolution but making the probe tip come in contact with the surface for a smaller time and then saves the tip and the sample from any damage due to each other by not dragging the tip around all over the surface. The probe cantilever vibrates up and down at a pre-set resonance frequency. The feedback loop adjusts the oscillation amplitude to restore the setpoint value thus resulting in variation of the oscillation amplitude when the probe comes near a heighted or featured surface35.
So, we can conclude following key points contrasting the working of contact mode and tapping mode operations of the AFM: –
In contact mode the probe scans the surface of the sample in raster scan fashion whereas in tapping mode the cantilever oscillates up and down depending upon the resonance frequency.
There is a shear force required to keep the probe in contact with the surface of the sample but, in tapping mode operation of AFM the shear force is almost negligible because the scanning action is lateral instead of raster.
In contact mode operation the damage suffered by the probe tip as well as by the sample if more as compared to tapping mode.