Opto-electrical Characterization of ZnO thin films prepared using Plasma Assisted Reactive Evaporation process

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Architecture of solar cells based on emerging materials

Inorganic solar cells

These cells are manufactured with heterojuncion architecture since the n-type material is different from the p-type material. This fact generates discontinuity in the conduction and valence bands, as a consequence of the difference in the gap and in the electronic affinities of the two materials and additionally presence of traps in the interface caused by the differences in the network constant of the two materials .
To reduce the photocurrent losses in heterojunction-type cells, these are manufactured following a concept called optical absorbent-window layer. Figure 1.3 shows the cross-section of a cell with optical absorbent-window layer structure [26].
The optical window is constituted by the buffer layer and ZnO thin film and its main function is to facilitate that the greater amount of solar radiation reaches the absorbent layer and also to form the electric field in the SCZ that gives rise to the potential difference between the device contacts. In turn, the buffer layer fulfills the function of mechanical coupling between the absorbent layer and the ZnO layer; As the buffer layer has a high absorption coefficient in general, it must be ultrathin (of the order of 60 nm thickness) to achieve a high percentage of radiation reaching the absorbent layer.

Solar cells based on hybrid organic-inorganic compounds with perovskite structure

This type of cell has a new architecture based on the concept of heterojunction; This includes a thin layer of a n-type semiconductor material that acts as an electron transport layer (ETL), a p-type layer that acts as a hole transport material (HTM) and an active layer of a intrinsic metal-organic material characterized by having a perovskite crystalline structure (see figure 1.4).
The ZnO thin films are generally used as ETL and as Hole Transport Material, P3HT or spiro-MeOTAD; Perovskite is used as the active layer. The highest efficiencies in this type of cells have been achieved with TiO2, spiro-MeOTAD as ETL and HTM respectively; however this architecture presents low stability caused by the Perovskite degradation in a humid environment and by diffusion processes generated at the interface with TiO2. For this reason the use of ZnO as ETL is presented as one of the best options for this architecture in its step to industrial level.

Organic solar cells

The organic photovoltaic devices have a different operation than the heterojuncture type inorganic devices.
The photocurrent generation in an organic solar cell follows the following processes [55]:
1. Absorption of radiation by the active layer (constituted by a mixture of donor and acceptor material with domains of the order of 20 nm) and formation of an excited state (exciton).
2. Exciton diffusion, up to the donor-acceptor interface.
3. Dissociation of the exciton by the electric field existing in the SCZ formed near the donor / acceptor junction and generation of free carriers.
4. Carrier drag (holes in the donor material and electrons in the acceptor), induced by the electric field in the area of space charge.
5. Selective transfer of carriers from the active layer to the electrodes.
The photogeneration of electrical current in an organic cell can be improved by the selective transfer of carriers from the active layer to the electrodes; this is achieved by incorporating into the structure of the device selective electron and hole transport layers, between the interfaces active-anode and active-cathode, respectively. Additionally, the ETL allow protection of the active layer, as well as quasi-ohmic contact with the electrodes [56]. In the case of solar cells based on polymers, the best results have been obtained using PEDOT:PSS (poly-ethylenedioxythiophene: polystyrene sulfonic acid) as HTM. While ZnO has been used as ETL. In Figure 1.5 you can see the architecture of this type of cells.

Kinetics and mechanisms

The ZnO thin films are deposited using a route based on the plasma assisted reactive evaporation (PARE), which consists of evaporating Zn in the presence of oxygen, so that there takes place a chemical reaction that gives place to the formation of the ZnO. As the Zn in the presence of O2 oxidize very slowly at room temperature, it is necessary to ionize both Zn and O2 to accelerate the chemical reaction between these two species. The ionization is achieved through glow discharge (GD) that includes different ionized species that increase the rate of chemical reaction and therefore the rate of growth of ZnO film. Under the conditions of current and pressure used in the PARE process, the generated plasma is a non-thermal that implies that the most probable mechanism of ionization of the gas inside the plasma is direct ionization of neutral particles (atoms, molecules or radical) by electron impact. Plasma species that do not deposit and unreacted are pumped radially outwards. The bias voltage applied between the electrodes to generate the glow discharge induces the following processes:
i) Dissociation of O2 followed by supply of sufficient energy to ionize the atoms of oxygen present.
ii) When Zn vapor enters into the region of glow discharge can be generated zinc positively ionized species.
iii) Once the precursor species (O and Zn) are ionized, they can be neutralized by binary collision through the process of recombination and removal of ions
iv) Finally the ZnO(g) generated in the plasma is diffused toward the substrate to form the thin film; during this process the ZnO(g) can interact with the plasma and can also be ionized.

Reactor engineering

The PARE reactor was designed and developed as result of previous research work and nowadays it is available in the SM&SE laboratory of the Universidad Nacional de Colombia. For the design of the reactor that allows generating a stable GD confined in the space between the electrodes, it took into account the Paschen law that relates the voltage for the initiation of the discharge and the product of the pressure by the separation distance between the electrodes [57]. Fig. 2.1 shows a scheme of the setup to grow ZnO thin films by plasma assisted reactive evaporation. This includes the following units:
(a) Vacuum system, consisting of a mechanical pump and a trap of liquid nitrogen that allows getting a base pressure of 10−4 mbar, prior to the introduction of oxygen.
(b) Reactor where takes place the chemical reaction of precursors (O2 and Zn) giving rise to the formation of the ZnO. This includes parallel flat electrodes supported by a structure of teflon and a DC power supply (2000 V, 200 mA) regulated in both voltage and current, used to activate the GD (see insert of Fig. 2.1).
(c) Source of evaporation of zinc (effusion Knudsen cell).
(d) Electronic mass flow controller.
(e) Control unit, whose main function is to control the amount of zinc that arrives to the GD zone, which is provided by evaporation from the Knudsen cell. The functions of control, measurement, acquisition, processing and data visualization are made through a virtual instrument developed with LabVIEW. The hardware used includes: the Compact Field Point-1804 module (cFP-1804), who performs the communication with the PC through the RS232 port, an analog input module cFP-TC-120 to acquire the voltage signal from the thermocouple type K used as temperature sensor and an analog input module cFP-AIO-611 to acquire the voltage signal from de Pirani Gauge used as sensor of the change of partial pressure inside the chamber.
To get conditions to deposit in a reproducibly way ZnO films with thickness, transmittance and resistivity suitable to be used as optical window in solar cells, it is necessary to control very accurately the current of ions of the glow discharge and the flow of both oxygen and zinc that arrives to the area of the plasma. The current of ions generated during the GD and the oxygen introduced into the deposition chamber are controlled with good accuracy using a power supply regulated in current and an electronic flow mass controller, respectively; however, an accurate and reproducible control of the amount of Zn arriving to the GD zone is very difficult to manipulated mainly due to the high vapor pressure of Zn and to the fact that the temperature at which starts the evaporation of the zinc changes significantly with the room humidity and the moisture absorbed in the walls of the chamber and the electrodes.
Since the amount of Zn arriving to the GD zone is the parameter that most critically affect both the reproducibility and the opto-electrical properties of the ZnO films, it was necessary to develop a tool that allows to do a control it. This was achieved through a virtual instrument (VI) with facilities to control the flow of evaporated Zn. The VI developed include a PID-PWM algorithm to perform the control of the process in two steps, in the first one the temperature ramp of the Knudsen cell is controlled up to reaching a temperature of 400 ◦C using a K-type thermocouple (TC) as sensor. When this temperature is reached, it begins the second step that incorporates a control of the partial pressure (DP) inside the chamber that decreases when Zn starts evaporating and goes in the Glow discharge zone using, in this case a pirani gauge is used as a sensor (PT).

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Operation conditions

Figure 2.2 shows a block diagram of the control system designed for each of the stages following a schema in closed loop; from the feedback signal generated by the sensors TC and PT is determined the error signal with respect to the desired value (set point); this error signal enters to the PID control algorithm which generates an analog control signal that is modulated by amplitude of pulses through a PWM algorithm. The incorporation of the PWM algorithm was made to be able to control the power of the analog power source used to supply current to the Knudsen cell. The power of this source can only be controlled by varying the frequency of a system on/off.

Table of contents :

Chapter 1
Solar Energy Technology
1.1 State of the art of photovoltaic solar technology
1.2 Architecture of solar cells based on emerging materials
1.2.1 Inorganic solar cells
1.2.2 Solar cells based on hybrid organic-inorganic compounds with perovskite structure
1.2.3 Organic solar cells
Chapter 2
Plasma Assisted Reactive Evaporation Process
2.1 Kinetics and mechanisms
2.2 Reactor engineering
2.1.3 Operation conditions
Chapter 3
Modeling and simulation of Plasma Assisted Reactive Evaporation process
3.1 Process description and modeling
3.1.1 Flow Field
3.1.2 Reactions, mass transport and thin film deposition
3.1.3 Reactions, mass transport and thin film deposition
3.2 Simulation of a Plasma Assisted Reactive Evaporation process
3.1.1 The Finite Difference Method (FDM)
3.2.2 Construction of a discretization scheme using ADI method
3.3.3 Simulation results
Chapter 4
Opto-electrical Characterization of ZnO thin films prepared using Plasma Assisted Reactive Evaporation process
4.1 Thin film characterization fundamentals
4.1.1 Electric Characterization
4.1.2 Optical characterization
4.2 Influence of deposition conditions on transmittance and resistivity
4.2.1 Influence of the ion current of the glow discharge
4.2.2 Influence of the oxygen flow
4.2.3 Influence of the Zn flow
Chapter 5
Dynamic optimization and control of Plasma Assisted Reactive Evaporation process
5.2 Dynamic optimization fundamentals
5.1 Problem definition
5.1.1 Analytical methods
5.1.2 Numerical methods
5.2 Dynamic optimization of Plasma Assisted Reactive Evaporation process
5.2.1 Case study: plasma assisted reactive evaporation process
5.2.2 Operating conditions: plasma assisted reactive evaporation process
5.2.3 Batch time minimization with Zn flow as control variable
5.2.4 Batch time minimization with Zinc flow and 0xygen flow as control variables
Chapter 6
Regulatory Control of Plasma Assisted Reactive Evaporation process
6.1 Feedback effects
6.2 Modeling and Identification
i) First Order Systems
ii) Second Order Systems
6.3 PID controller
6.4 Control simulation of Plasma Assisted Reactive Evaporation process
Chapter 7
Conclusions and perspectives
8. References

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