Towards High Quality Semiconducting Nanocrystals and Surface Ligand Exchange for Solar Cell Application

(Taye Zewdu)

I) Introduction

As ´there is plenty of room at the bottom´ prediction, exploiting the very tiny building blocks of unique mechanical, optical and electrical properties for various applications have been a global trend and an active research arena of huge investment for more than a decade. [1] Nanotechnology is a trillion dollar worth market being explored for applications in materials, pharmaceuticals, electronics, chemical manufacturing, aerospace science and others. Of all other hurdles, energy has come the number one global challenge of 14 TW a year as we approached the peak oil (Hubbert´s peak) and the end of the oil age on top of which sustainability and environmental issues urge alternative and renewable power sources to take stage. [2]

Promising enough, the clean solar- energy of the future, is showing enormous breakthroughs a century after the demonstration of the first 1% solar cell. [3] Depending on the materials and configurations, they can be generally classified as inorganic, organic or hybrid under which a list of state-of-the art technologies are systematically grouped as first, second, third and next generation solar cells.

Efficiency, cost and stability determine the niche (competitiveness) of the devices converting light into electricity in the global market. Theoretical predictions showed that semiconducting nanocrystals could be promising in terms of efficiency that significantly exceeds the schockley-casler limit () and the ease of solution processing. Consequently, quantum dots, nanorods, nanowires, tetrapods and nanocrystals of different heterostructures has been incorporated in Schottky, dye-sensitized and bulk heterojunction solar cell configurations (Fig. 1). However, these types of solar cells are not yet efficient enough to show up commercially, best efficiency reported being 6%, 5.4% and 3.2% for architectures shown in the schemes below, respectively. [4] A major limitation is the electron mobility impeded by organic passivating ligands on the nanocrystals, which insulate the charge transport in the interpenetrating network. This report covers the syntheses of high quality CdSe quantum dots and tetrapods and their respective treatments to enhance electron mobility. Later is discussed some aspect of the optical and electrical device characterization with respect to J-V properties, IPCE, CE and TPV and TAS results.

II)      Syntheses and Characterizations

Optimized routes for CdSe quantum dot and tetrapod preparation were studied at different growth times and temperatures. Optical, TEM and XRD characterizations of typical samples were conducted. New synthesis methods for hollow nanoparticles and ODE-Se mediated long tetrapods were tried on the basis of collaboration.

CdSe quantum dots

The well-known rapid and hot injection synthesis was followed in all cases with a modified recipe. Briefly,  1 mM of 99.99% CdO (128.4 mg) was mixed with 6 mM of oleic acid (2 mL) and 20 mL 1-octadecene in a four-necked flask. The mixture was degased for few minutes and heated up with a reflux until the solution turns clear (usually around 240 ^oC). The Se precursor was prepared by weighing out 118 mg of Se powder in a glove bag and dissolving in 1 mL TOP (90%) under sonication for 5 minutes in a small vial. As soon as the Cd-oleate turns clear, the temperature was adjusted for injection and the argón flow and heat mantle set so as to control the growth temperature which in most cases was few degrees lower than the injection temperature.  In a typical synthesis where 0.34 mL of TOPSe was injected at 250 ^oC, the particles were grown around 245 ^oC for two minutes after which the heat mantle was quickly removed and argón flow increased to cool the sulution fast. The cleaning was done with acetone three times at 4000 rpm for 1hr, 30 and 10 minutes. The resulting precipitate was dried and redispersed in chloroform for characterization.

Fig. 2 Synthesis set up

Quantum dots of CdSe were synthesized at 190 ^oC and 250 ^oC injection temperatures at different growth times of 20 sec, 1, 2 and 5 minutes. Fig. 2 and 3 show the normalized absorbance and photoluminescence spectra of the as-synthesized CdSe nanocrystals at respective injection temperatures. At lower injection temperatures, the growth was relatively slower providing the smallest quantum dots as predicted in Table 1 by the following empirical relation.

where D is the diameter of the nanocrystal and λ is the first excitonic peak (nm). The quantum dots  show a stock´s shift about 24 nm.

Typical to these syntheses is the existence of numbers of other excitonic peaks which are favorable and an indication of the kind of crystallinity exibited. For example, the second excitonic peak for a zinc blend structure forms a shoulder which is characteristic to quantum dots. This could be evidenced from the XRD data shown in Fig. x later. These several band features which are found to be growth time and temperature dependent.

The TEM images taken with a high contrast electron microscope show a cluster of smaller nanocrystals of 2-3 nm in size.

The photoluminescence efficiency of the quantum dots were also measured by calculating the quantum yield as in equation 2 below. The fluorescence quantum yield (Φ) is the ratio of photons absorbed to photons emitted through fluorescence. In other words the quantum yield gives the probability of the excited state being deactivated by fluorescence rather than by another, non-radiative mechanism such as internal conversion and vibrational relaxation. This has been done following a guide to recording fluorescence quantum yields using the comparative method of Grabolle et al., which involves the use of well characterised standard samples with known ΦS values based on suitable procedures and achievable uncertainities. Rhodamine 6G was used as standard in these studies. The corresponding fluorescence quantum yield values are provided in Table 1. Compared to literature values (0.3-0.85), fluorescence is the determining deactivation mechanism in the nanocrystal.

Fig. 2 Optical characterization of CdSe QDs synthesized at a) 190  ^oC/170 ^oC, b) 250 ^oC/230 oC and C) 300 oC/280 oC injection and crystal growth temperatures

Table 1 Typical characteristics of CdSe coloidal quantum dots in 2 minute growth time

Temperature (oC)	1st exciton peak (nm)	PL max peak (nm)	FWHM (nm)	Size (nm)	Φ (%)
190/170	534			2.8
250/230	544	569	30.6	3.1	*37
300/280	558	585	38.21	3.4

The nanocrystals show a broader PL peak at lower growth times indicating surface defect patterns in the dynamic nucleation and growth reactions. At the shortest growth time, blue nanocrystals of higher energy bandgap as much as xxeV were synthesized at 190/170 oC.   

Tetrapods

Similar to the quantum dot synthesis, 1mM of 99.99% CdO (128.4 mg) was mixed with 6 mM of oleic acid (2 mL) and 20 mL 1-octadecene in a four-necked flask. The mixture was degased for few minutes and heated up with a reflux until the solution turns clear (usually around 250 oC). The Se precursor was prepared by weighing out 39 mg of Se powder in a glove bag and dissolving in 1.5 mL TOP (90%) under sonication for 5 minutes in a small vial. The TOPSe was mixed with 36 mg of CTAB in 2 mL of toluene. As soon as the Cd-oleate turns clear, the temperature was adjusted for injection.  In a typical synthesis where 3.5 mL of TOPSe was injected at 190 oC, the particles were grown at 170 oC for two minutes after which the heat mantle was quickly removed and argón flow increased to cool the sulution fast. Short arm tetrapods were synthesized using 103 mg of CdO and 32 mg of Se poder. The resulting solution was divided into four 40mL vials in which around 2 mL of chloroform and methanol mixture was added prior to centrifuging with approximately 20 mL of acetone for 20 minutes. Additional cleaning was done only with acetone two times at 4000rpm 10 minutes each. The resulting precipitate was dried and redispersed in chloroform or hexane for characterization.

As in the case of the CdSe quantum dots, a matrix of temperature and time controlled syntheses has been carried out to find the best experimental conditions that significantly affect the nanocrystal growth. Based on an optimized recipe, reproducibly shape-controlled tetrapods were prepared.

The absorption spectra of typical tetrapods is characterized by a sharp first excitonic peak followed by a shoulder a valley at lower wave length. Peak counts as much as eight were common in these syntheses while observing the second excitonic peak coresponding to a wurtzite structure of the tetrapods. As shown in Fig. 3, the peak broadens at higher growth temperature and time conditions which could be an indication of Ostwald ripening- the evolution of an inhomogenous structure over time.    

The flourescence of these semiconducting materials was uniquely sharper than its quantum dot counter parts. A characteristic PL is sharp, with minimum FWHM(about 20 nm), symetrical and with high emission. At lower growth times, the surface trap states are predominant because of a dynamic nucleation and growth reaction leaving incomplete covalent bonds. This can be seen from the lagging tail on the PL peak at higher wave length.

However, the choice of the experimental conditions depends on what arm length and thickness of the tetrapods is needed. In particular, long arm tetrapods are synthesized at 190 oC while the short arm tetrapods are prepared at 300 oC with a modified recipe.

X-ray powder diffraction is also crucial in studying the crystalinity of the tetrapods. This has been reported by several authors. Fig. 5 shows XRD of the as-synthesized quantum dots and tetrapods.

Fig. 4 Optical properties of tetrapods sybthesized a) 190 oC/170 oC c b) TEM, 190 oC/170 oC/2 min simple c)250 oC/230 oC d) 300 oC/280 oC

Table 2 Optical characteristics of CdSe TPs

Temperature (oC)	1st exciton peak (nm)	PL max peak (nm)	FWHM (nm)	Arm length (nm)	Φ (%)
190/170	533	549	19.9
250/230	566	582	20.4		*1.3
300/280	614	627	21.9

Fig. 5 XRD of CdSe nanocrystals of QDs (green) tetrapods (red) sythesized at 250 ^oC for 2 min 

TOP-free tetrapods

Normally, tetrapods are synthesized by a rapid hot injection of TOP-Se in the presence of CTAB. Although TOP free strategies are available for colloidal quantum dots, it was barely applied for elongated nanocrystals. The preparation of these ´super-long tetrapods´is as follows: 1 mmol of CdO is added to 2 mL oleic acid , and heated with stirring under inert conditions until a clear cadmium oleate complex is formed. The solution is cooled to room temperature but 1 mL of toluene should be added when the temperature lowers below 100 ^oC. This helps the complex remain in liquid state. Similarly, equivalent amount of Se (to Cd), is added to 20 mL of ODE and heated to 280 ^oC (or another temperature depending on which the arm length and thickness is controlled). Adding 0.1 mmol of CTAB to the prepared Cd-oleate, and injecting into the ODE-Se adduct the synthesis is monitored at different growth times. The color development corresponding to the particle size is found gradual which benefits easy control of arm length and thickness.

Hollow nanoparticles

Hollow nanoparticles/nanocrystals are spherical nano-objects formed as a direct consequence of a classic mechanism in metallergy- the Kirkendall effect. The phenomenon, induced from the difference in diffusivities of atoms, is a generic route to diverse range of hollow nanostructures reported previously; such as ZnO, Co₃S₄, CoO, CoSe₂, CoTe, Co₂P and Ni₂P nanoparticles, for example. Hollow nanocrystals and the method of making is recently patented by Alivisatos and co-workers. []The underlying mechanism of hollowing has been explained by the generation of small Kirkendall voids near interfaces as a result of different interdiffusion rates and surface diffusion of core material along the pore surface or a direct elapse of the material in the core.

Very recently, group of Wong at Rice University succeeded in developing a Kirkendell process to synthesize CdSe hollow nanoparticles of less than  20 nm in size. The HNPs thereof were ligand exchanged and applied in hybrid solar cell devices to study the charge transfer reactions.

In a typical synthesis, 2.47 g of cadmium nitrate tetrahydrate (Cd(NO3)2•4H2O, 8 mmol, purity = 99%, Sigma-Aldrich), 0.32 g of selenium powder (Se, 4 mmol, purity = 99.999%, Sigma-Aldrich), 20 mL of 1-octadecene ("ODE", purity = 90%, Sigma-Aldrich) and 0.15 g of hexadecyltrimethylammonium bromide ("CTAB", 0.4 mmol, purity >99%, Sigma-Aldrich) were added to a four-neck flask, which was then heated slowly to 190 °C at a rate of 11 °C/min. The Cd:Se:CTAB molar ratio was 2:1:0.1. The reaction flask was kept at this temperature for 5 min and cooled down to room temperature, resulting is a reddish-brown cloudy liquid. The resultant solution was centrifuged to remove the solids, resulting in a clear reddish-brown liquid. Ethanol was then added to the reddish brown liquid to crash out the HNPs. The HNP powder was washed several times with ethanol before being dispersed in chloroform. The synthesis method was also performed with no CTAB, and twice and triple the original amount of CTAB (i.e., 0 mmol, 0.8 mmol, and 1.2 mmol). 

III) Ligand Exchange Strategies

Pyridine Exchange

Presynthesized and chloroform dispersed QDs (10 mg/mL) were added to a screw top vial with a septum cap. The content was dried with a flow of Ar until the solvent is completely evaporated or QDs are left a bit wet in some cases. The QDs were redispersed with 10 mL of pyridine with a glide syringe through the septum in the fume hood (under similar stirring and inert conditions). The ligand exchange for quantum dots is exceptionally slow and requires controlled heating and cooling steps. The pyridine exchange was comparatively more successful when heating the content to not more than 100 oC for 12 hours in an oil bath while redispersing to the same concentration so that all ligands are not evaporated through the vent needle. The same concentration of NCs was left for additional 12 hours at room temperature and with stirring. Thereafter, few drops of check-up sample were taken to assure the exchange has taken place and added to 5 mL hexane. A cloudy suspension means most of the pyridine exchange was performed in which case the parent solution was filled with at least 40 mL of hexane (until it turns cloudy) and centrifuged. The tendency of QDs capped in pyridine ligands is to aggregate. The hexane washed precipitate is later redissolved in chloroform to the desired concentration. This procedure was repeated whenever the surface passivation strategy was unsuccessful (QDs remain dissolved in the washing solvent).

Pyridine exchange for TPs and HNPs was relatively facile since room temperature stirring under inert conditions of 12 hours was effective with a final hexane wash and dispersion in pyridine:chloroform (1:9).

Butylamine Exchange

The short amine ligand exchange of quantum dots was done following a prior pyridine exchange; in which case 10 mL of butylamine is added to 100 mg of nanocrystals. The content was left stirring under argon for 24 hrs at 70 ^oC. Nanocrystals obtained were precipitated with acetone. The final volume redissolved in 1:9 butylamine: chloroform solution. Similarly, the TPs and HNPs were treated for coating exchanges overnight.

a)    Oleic acid     

                

b)   Pyridine      

            

c)    Butylamine

IV)        Photovoltaic performances

HNP	Jsc	Voc	FF	Eff	Rs(Ωcm2)	Rsh (MΩcm2)
Pyridine	0.74	500	40	0.15	193	1.61
Butylamine	1.83	649	48	0.6	71	1.15

IV)         Reproducing the syntheses

Few syntheses of QDs and TPs were conducted to see the reproducibility of the routes. For the experiments carried out it turns out to be far away from the expectations in terms of homogeneity. The full width at half máximum was almost twice the expected values. This is bad since there will be energy transfer in between polydispersed nanocrystals.

Experimental results

The fact that these experiments are susceptible to any changes in the syntheses conditions imposed a bottle neck in preparing nanocrystals of typical high quality characteristics. In the course of identifying the problem, the optical properties of the newly produced quantum dots and tetrapods were compared to common characteristic features.

Observation: Changing the volume of the flask, although a slight difference in injection temperatures, blue-shifted the absorption peak somehow. However, the synthesis was expected to produce QDs (abs. 540 nm) and TPs (abs. 570 nm) with FWHM values of less than 30 and 23 nm, respectively. It should be noted though a proper heating mantle was not used which resulted in a huge drop in temperature (more than 100 oC) upon room temperature injection of the TOP-Se. Unlike, previous syntheses, the Cd-oleate was a yellowish solution at injection temperature which showed some sort of impurities in the constituting reactants. As a result, all new reagents were purchased from Sigma Aldrich to the same impurity levels as in previous syntheses at Rice.

Experimental results

In this experiment, all new reagents were utilized. Temperature control was done in-situ using adjustable thermometers. The synthesis was better in that the Cd-oleate turned almost clear after degasing the ODE for 30 min, adding OA and degasing for 10 min and finally heating the reaction volume to 250 ^oC. The experimental conditions were well controlled and the right absorption bands were observed while homogeneity of the sample remains unresolved. This was evident from the fewer number of band features and the broadness of the FWHM. However, the PL peaks were so symmetric and no evidence of surface traps at higher growth times.

VI) Conclusion

So far, different shapes of particles were reported in literature, including, dots, tetrapods, tripods, tear drops and even hollow nanoparticles. Further optimization in getting highly crystalline nanocrystals is underway. Some of the important nanostructures were such as dots and tetrapods are being optimized to obtain the best results. However, in comparison to the the other shapes, the tetrapods outperformed in solar cell applications owing to their physical orientation that increases the electron mobility in the devices. No solar cells of HNPs in DSCs are reported in literature so far. We have achieved butylamine exchanged HNPs of higher efficiency compared to pyridine exchanged counterparts.

Acknowledgement

Institute of Chemical Research of Catalonia (ICIQ)

Universitat Rovira i Virgili

Rice University

Prof. Emilio Palomares

Prof. Michael S Wong

Suravani Gullipalli

Smallest geez alphabets inscribed on silicon wafers

These are possibly the smallest 'geez fidel' inscriptions your eyes have ever seen. The image has been taken using a scanning electron microscope from the cleanest facility in Spain located at Universitat Autonoma de Barcelona (UAB) in 2010.

You can think of the area to be equivalent to the tip of your pen and this is fabricated using nano and microfabrication techniques on a silicon wafer.

Thanks to Mesfin Agidie who created the fonts in his Abinet typing software. I did the tedious task of designing each of the above alphabets point by point on a specialized patterning software which dictates the etching machine.

Special thanks to Prof. Francesc Perez Murano and Prof. Xavier Borrisie

A New Vision for A New Ethiopia

By

Kebour Ghenna

“Society in every state is a blessing, but government even in its best state is but a necessary evil; in its worst state an intolerable one”. Thomas Paine

Last week someone asked me what would I say are the three immediate problems facing Ethiopia today, and how would I want them to be addressed. The following is my candid response. But first some thought about Ethiopia today. Most of you will have seen the sickening video footage of people running away from bullets and jumping into deadly ditches at the Ireecha festivities in Bishoftu, Oromya. Blame for all of this rests squarely on the Hailemariam administration, which is no longer prepared to tolerate even the truncated display of peaceful protest. It has deliberately (or not) created the conditions for a civil war to erupt. Everywhere in Oromiya today, there is mass social and political discontent directed against the ruling party. The fight against the regime now will be difficult to contain, and the resulting crisis, for better or for worse, will affect many aspects of our lives including the unity of the country. 

It’s time for Oromos, Afars, Amharas, Tigrays, Somalis, Welaytas and the rest of us, to reach consensus on the type of society we want to live in. Indeed, if we fancy modernizing our country by reforming its institutions and replace the detrimental with the constructive we need to start this debate immediately. Well-informed people know that nations do break up like empires. Sometimes the process is inevitable, sometimes precipitated by folly. EPRDF should not commit an act of folly by attempting to direct the state on its own term i.e. by force. That would not be in anybody’s interest. Instead it should conduct considerable discussion and consultation with opposition leaders and genuine community leaders to hold the country together by strengthening the democratic influence and rights of the people. There is no other alternative.

Now, here are my eight [not three] comments [with no particular order of priority] to avoid the unwanted dismemberment of Ethiopia and move the country forward. None of these arguments are new. They have been discussed in great detail in the past twenty years.

1. Understanding of the problem: The current problem of Ethiopia, argue EPRDF politicians, is lack of good governance; and the solution, they say, is a mix of new appointments to top positions, some recruited from within the party, others coming from outside. Even on this deceptively simple issue, agreement has apparently not come easy. 

But wait...What about the other serious problem.

From where we stand, the most serious problem is not personnel, rather the unwillingness of EPRDF leaders to accept that citizens are truly tired of a ‘democracy’ dominated by TPLF as the leader of the EPRDF. Now that memories of the horrors of the Derg period are fading, people demand more political rights and participation. Change in personnel isn’t going to cut it. At best, it may give us people who think things are under control…ministers who believe the present system works…and that it will deliver transformation....but real change? Nope.

The problem is that people have lost confidence in the system: a system that divide rather than unite, a system that dominate rather than liberate. Too bad the folks at EPRDF can't see this. The question to answer is what kind of nation are we building? Are we in favor of one nation, albeit with distinct identities (like Switzerland) or rather separate multiple independent nations dominated by TPLF? This issue may seem for many settled and done and printed in black and white in the constitution, but the question of how to balance the promotion of national cultures with the creation of multinational statehood was never fully resolved in Ethiopia, or anywhere in the world for that matter. 

The increasing rebellion, injuries and casualties that we all witnessed and continue to witness are the clearest early indicators of this failure. The time to reflect and engage citizens based on consultation and co-operation about holding the state of Ethiopia together is now. Fortunately despite the crisis there is still TODAY enough support within the broader public to wish to retain and nurture the unity of the state of Ethiopia. I underline TODAY, because history tells us that on such a fundamental issue opinion can change decisively over night. 

2. Commit to revising the constitution: The Constitution of “We, the nations, nationalities and the Peoples of Ethiopia...” (which is historically and ideologically rooted in TPLF cultures) is full of ambiguities, so outmoded, so dangerously ethno-nationalist that it has already created and solidified to a varying degree, distinct states, some more homogeneous (Tigray, Somali Region, Afar) than others, together with local elites that increasingly give allegiance to their ethnic state rather than the state of ‘Ethiopia’.

Perhaps it’s time for a new political order to redefine a new Ethiopia. An Ethiopia that is negotiated among all parties of the federal state (bottom up), not on terms laid out by TPLF, but rather on terms agreed by all administratively recognized ethnic groups with equal representation regardless of size or population. It’s important for TPLF (assuming it’s committed to a multi state united Ethiopia) to immediately pledge to the revision of the constitution and facilitate the process to calling a constitutional convention. On this matter, as they say: Time is of the essence.

3. Order the military and security forces back to their barracks: Today state repression should be unthinkable. Attempting to solve what is a political problem with force and violence will not bring a long term solution. By the way, even if the EPRDF come to re-establish a new administration say, in Oromiya and Amhara, will it really be obeyed: each village will be a nest of resistance, each former associate an enemy.

4. Rule of Law: A good nation must abide by the Rule of Law. We need a Justice Ministry that understands the importance of rule of law and will be committed to it. There is too much injustice and disappointment in this country. Memoirs will show this Government to be the most corrupt in the history of Ethiopia. That’s why we must demand an honest and courageous Attorney General – one who would rather resign than do something anti-constitutional or sell out our nation. The first thing that needs to be done is release all political prisoners, establish an independent panel to investigate "thoroughly" the allegations of corruption in government and ‘private sector’. All those matters that demand audit and investigation should be audited and investigated. Corruption is the “first cousin” to prejudice. It limits high performance in management and fiscal excellence.

5. Discourage ethnic based separatist parties: How can one reconcile the fact that TPLF, which is a political movement as well as an ideology of values, concepts and ideas that advocates independence for Tigray region, can at the same time root for national unity. The same can be said of the ‘puppet’ parties organized along the same logic. Clarity is in order on this issue. The fact is Ethiopia has been good for Tigray – for its economy, its openness, its culture, its visibility and its national clout; and if TPLF has not gone the ‘independence’ way it is not out of any misty-eyed Ethiopian patriotism, but because it knows that its bread is buttered in Addis Ababa. 

6. Announce new elections and reform: The recent incidents in Oromiya and Amhara regions have left Ethiopia a more divided society than ever before. The government should take responsibility for its failure to preempt the slow burning bad governance issues causing growing discontent and anger, and leading the country to a possible violent breakup. All this because of lack of space for dissent. Indeed the total absence of any opposition in parliament made proper democratic functioning impossible in the country. New legislation could not be properly tested. Committees could not fully examine their issues. Political energy got channeled away from where it could do most good. Most critically, this situation undermined public confidence in government institutions and thereby discouraged public willingness to engage fully and constructively in democratic processes. So political dialogue is definitely very important. Democracies need dialogue to resolve issues, and Ethiopia is no exception. The EPRDF should announce an independent commission to study and recommend reforms that will make the government more representative, with some form of proportional voting system that is fair and competitive. 

7. Free the Media: Ethiopia's constitution affords its citizens freedom of speech and press, but the opacity of media regulations allows authorities to crack down on news stories by claiming that they expose state secrets and endanger the country. The definition of state secrets in Ethiopia remains vague, fostering censorship (including self censorship) of any information that authorities deem harmful to their political or economic interests. The media has to be free so that government officials can be easily exposed if they abuse or misuse the power they have. Corrective action should be immediate. 

8. Issue a new ID card without ethnic identification: Here is another example reinforcing division amongst people supposedly of one nation. An immediate Must Change policy is in order.

How can this 'movie' possibly have a happy ending? Let me just say this, each of us has a choice. We can actively participate in the great initiative to keep our country united in the interest of all the people in Ethiopia; or do nothing, and see us loose our country. As Gandhi noted: ‘The future depends on what we do in the present.’ Now, more than ever, we must uphold that responsibility. What will you do?