ENHANCED PERFORMANCE OF CuIn 1-x G x Se 2 SOLAR CELL THROUGH OPTIMIZATION OF ABSORBER AND BUFFER LAYER PROPERTIES USING SCAPS-1D †

This study is a follow up to our previously published article on “Numerical Simulation of Copper Indium Gallium Diselenide Solar Cells Using One Dimensional SCAPS Software”. Five more parameters were optimized which are: absorber band gap, absorber electron affinity, buffer layer band gap, buffer layer electron affinity and working temperature using the same simulation tool initially used. When the absorber bandgap was varied between 0.8 eV and 1.6 eV, the efficiency of the solar cell increases until it reached its peak at 27.81%. This occurred at absorber bandgap of 1.4 eV. Other photovoltaic parameters at this optimum value are: V oc of 1.00 V, J sc of 31.99 mA/cm 2 and FF of 87.47 %. On varying the absorber electron affinity from 4.20 eV through 4.55 eV, we obtained an optimum value of 4.45 eV at V oc of 0.82 V, J sc of 37.96 mA/cm 2 , FF of 84.99 % and an efficiency of 26.36%. The optimization of buffer bandgap resulted in an optimal value of 3.0 eV, when the buffer bandgap was varied between 1.6 eV and 3.2 eV. The photovoltaic parameters at this optimal value are: V oc of 0.80 V, J sc of 37.96 mA/cm 2 , FF of 85.22 % and an efficiency of 25.86%. The effect of buffer electron affinity was studied by varying its value between 4.00 eV and 4.40 eV and its best value was found to be 4.05 eV at photovoltaic parameters with a V oc of 0.82 V, J sc of 37.96 mA/cm 2 , FF of 84.98 % and an efficiency of 26.36 %. These optimized values in all parameters were used to simulate a solar cell which resulted to device with performances: V oc of 1.11 V, J sc of 31.50 mA/cm 2 , FF of 88.91 % and an efficiency of 31.11 %. On varying the working temperature on the optimized solar cell, the optimized device with its best performance at 270 K with Photovoltaic (PV) values of V oc of 1.15 V, J sc of 31.55 mA/cm 2 , FF of 88.64 % and an efficiency of 32.18%. The results obtained were encouraging and can serve as a guide to those involved in practical development of solar cells.


INTRODUCTION
The process of getting energy from sources like fossil fuels which has been a major source of power for most countries of the world, has been found to pose a wide range of dangers (causing health concerns, ozone layer depletion, enhanced greenhouse effect, etc.) to human existence. Considering the very important role of electrical energy in the technological advancement of the society, its demand is far greater than ever in developed and developing countries. Nations without natural deposits of oil and gas are likely to find it very difficult in the nearest future to get supplies of these products since they are expensive, fast diminishing and politically regulated [1]. A good alternative to oil and gas sources for the production of electrical energy would have been found in nuclear energy sources such as uranium and plutonium but accidents in nuclear power plants and disposal of nuclear waste are a great challenge.
The search for a renewable and clean source of energy for industrial, technological and domestic purposes has led to the ongoing efforts at exploiting energy from the sun which is known alternatively as solar energy. Over the years, researchers have been finding the most efficient ways to harness solar energy and this has led to the development of the solar cell. In order to be able to understand the operation of solar cells, numerical modeling has been used in a number of instances and it has proven to be a very important tool in this regard [2,3].
As a follow up to our previously published article [3], we made use of SCAPS to optimize additional five parameters which include: absorber band gap, absorber electron affinity, buffer layer band gap, buffer layer electron affinity and working temperature.

DEVICE MODELLING AND SIMULATION
In line with the initial device set up demonstrated in our previous work [3], we utilized a solar capacitance simulation software based on the poisson and continuity equation to achieve the desired objective. The details of the set-up and simulation can be obtained in Lawani et al. [3]. In this study, similarly, a defect from double acceptor with a gaussian energy distribution, value of concentration of 1.0E + 14 cm -3 defect, and level of energy of 0.1, 0.4, eV slightly above valence band, were introduced into the CIGS (absorber) layer. The defect can be seen to arise as a result of Cu III defect † Cite as: G.J. Ibeh which is most times noticed in CIGS absorbers [4]. The absorber band gap was varied from 0.8 -1.6 eV, absorber electron affinity was varied from 4.20 -4.55 eV, buffer layer band gap was varied from 1.6 -3.2 eV, buffer layer electron affinity was varied from 4.00 -4.40 eV. The simulated device structure is as shown in Fig 1. All other parameters were kept constant while varying the aforementioned parameters.

Effect of varying CIGS (absorber) bandgap
The effect of absorber band gap was investigated by varying the CIGS layer band gap from 0.8 eV through 1.6 eV. Figure 2 shows the variation of CIGS solar cells' photovoltaic parameters with increasing absorber layer bandgap. It was observed that J SC decreased with increasing bandgap energy. This decrease occurs because part of the solar spectrum was not harvested by the solar cells [5]. An absorber with high bandgap energy absorbs photons with low wavelengths to release electrons from the valence band to conduction band in accordance with Einstein's equation [6,7] given in equation 1 below: where h is Plank's constant, c is the velocity of light and λ g are wavelength which matches the band gap of the absorber. V OC is seen to increase with increasing bandgap of the absorber.
According to Scheer and Schock [8], decreasing the bandgap of the absorber increases photocurrent but causes a decrease in V OC . This is clear when the relationship between V OC and bandgap in equation 2 is considered.
where A is the solar cells' quality factor, q is the elementary charge, E a is the activation energy of J O (and is approximately equal to the absorber bandgap [8]), J O is saturation current density and J 00 is the reference current density of the solar cell. The observed increase in efficiency is due to the increase in V OC and fill factor [9], but as the J SC drops with increasing bandgap of absorber, the efficiency begins to fall since light absorption reduces. This is detrimental to the solar cells as their performance goes down. Table 1 gives the dependence of the solar cells' performance on CIGS layer bandgap. It shows that the highest efficiency achieved after variation of CIGS absorber band gap is 27.81 % and this occurred at a bandgap of 1.4 eV. Other photovoltaic parameters at this optimized value of band gap are: V OC of 1.0 V, J SC of 31.99 mA/cm 2 and FF of 87.47 %. The J-V curves in Figure 3(a) show that as bandgap of the absorber increases, the Jsc of the solar cells decreases but the open circuit voltage increases. Figure 3(b) depicts the QE as a function of wavelength for corresponding CIGS layer bandgap. For device with wavelength within the range 300 nm -1600 nm the absorption efficiency remains the same.
The arrangement of the curves (curves corresponding to lower bandgaps are outermost, while those curves corresponding to higher bandgaps are innermost) show the inverse relationship existing between the various bandgaps and their corresponding wavelengths, as given in Equation 1.  Table 2 shows the dependence of solar cells' performance on electron affinity. In the case where the electron affinity of the n-type layer is smaller than that of the p-type layer, a positive conduction band offset (also known as a spike) given in equation 3 is formed otherwise a cliff is formed [10] ∆ , ,

Effect of varying electron affinity of CIGS (absorber) layer
where E C,n is conduction band energy for n-type layer and E C,P is conduction band energy for p-type layer. From Table 2, V OC increases until it saturates at a bandgap of 4.35 eV ( Figure 4). The increase in V OC is very likely caused by a spike which reduces the occurrence of recombination (This is good for the solar cells as recombination degrades their performance.). This also explains the rise in efficiency of the CIGS solar cells as electron affinity of absorber increases. J SC remains constant but fill factor rises to its optimum value and begins to fall due to the formation of high spikes [11]. The optimum value of electron affinity for the absorber was found to be 4.45 eV and the metric parameters are: V OC of 0.82 V, J SC of 37.96 mA/cm 2 , FF of 84.99 % and a PCE of 26.36 %.   The J-V curves in Fig. 5(a) show that for all values of electron affinity of absorber, J SC remains the same while V OC increased until it attained its optimum value and remained constant (this is shown in areas where the curves overlap). Figure 5(b) is the representation of QE as a function of wavelength for the selected values of CIGS electron affinity.
As demonstrated, within the wavelength range of 300 nm -1200 nm, an overlap in the QE curves was observed which is attributed to the unchanged absorption efficiency within the selected values of the CIGS electron affinity. This spectral response proves that absorber (CIGS) electron affinity has only a slight effect on the metric parameter of CIGS solar cells investigated. Figure 6 shows the variation of photovoltaic parameters with buffer layer bandgap. The buffer bandgap was varied from 1.6 eV through 3.2 eV. It was discovered that while V OC was unaffected by this variation, J SC and FF rose to their peaks and remained constant thereafter. The efficiency also rose to its optimum value and then dropped. Although J SC remained constant for higher values of buffer bandgap, it increased initially with an increase in buffer layer bandgap because more photons were allowed to reach the absorber layer and create more electron-hole pairs [12]. This translates to an increase in efficiency and fill factor but efficiency eventually drops beyond buffer bandgap of 3.0 eV because as bandgap goes higher, photons cannot achieve the needed amount of energy to create enough electron-hole pairs [13] which is needed to increase efficiency. The performance of the solar cells then drops. The best value of buffer bandgap after optimization is 3.0 eV and this occurs at V OC of 0.80 V, J SC of 37.96 mA/cm 2 . FF of 85.22 % and efficiency of 25.86 % (see Table 3).   Figure 7(b) is a representation of the QE versus wavelength for the selected values of buffer (In 2 S 3 ) layer bandgap. Selecting the range of wavelength to be 300 nm -1200 nm, results to spectral overlap. We therefore speculate that In 2 S 3 layer bandgap has negligible or no effect on the performance of CIGS solar cells studied.  Figure 8 shows the variation of photovoltaic parameters with an increase in the electron affinity of the buffer layer. J SC is observed to be constant until it experiences a slight increase beyond an electron affinity of 4.4 eV. This is due to the formation of cliffs which pose no barrier to photogenerated electrons. This would have been good for the solar cells' performance but as a cliff helps to boost Jsc it causes recombination between electrons injected from buffer layer and the interface defects [11]. We therefore see that V OC , and efficiency, begin to decrease while fill factor remains unchanged. After optimization, the best value of buffer electron affinity is seen to be 4.05 eV at V OC of 0.82 V, J SC of 37.96mA/cm 2 , FF of 84.98 % and efficiency of 26.36 % as shown in Table 4.

Effect of varying electron affinity of In2S3(buffer) layer
The J-V curves (Figure 9(a)) show that between buffer electron affinities of 4.00 eV and 4.15 eV, the open circuit voltage is the same but it varies thereafter, with the curve for the highest electron affinity appearing innermost (this indicates that the open circuit voltage is least for the highest electron affinity). The short circuit current density shows little or no change. Figure 9(b) depict the QE versus wavelength of electron affinity values selected for buffer layer (In 2 S 3 ). Choosing within a range of 4.00 eV and 4.15 eV, there is a clear overlap in the QE curves.

Performance of optimized parameters
In accordance to the optimized parameters obtained in this research and those sourced from our previous work [3] (see Table 5), an efficiency of 31.13 %, current density of 31.55 mA/cm 3 , voltage of 1.11 V and fill factor of 88.91 % were obtained (see Figure 10 and Table 6). These photovoltaic parameters obtained from optimization agree very much with those of the theoretical limits quoted by Ruhle [14]. Compared with the experimental data obtained in literature [15], which shows a PCE value of 22.6 %, the optimized device in this work demonstrates an enhanced value of ~37.74 % in PCE over the reported [15].   [18] and so the temperature under which they operate could affect their performance. When sunlight falls on solar cells, their temperatures rise. In this research the working temperature of the optimized cell was varied from 260 K through 340 K. Table 7 shows the dependence of solar cells performance on working temperature while Figure 11 gives the variation of photovoltaic parameters with increasing temperature. J SC remains constant (Figure 11(b)) but V OC keeps decreasing because of an increase in reverse saturation current with temperature. The fill factor reaches its peak at a temperature of 280 K and then begins to drop. This fall is due to light induced degradation [19]. The efficiency attains its optimum value at 270 K and also drops because when operating temperatures get higher than this optimum value, the electrons in the solar cell gain energy but instead of boosting electricity generation, they become unstable and recombine before they are collected [20]. Figure 12 shows the J-V curves of the CIGS solar cells with increasing temperature.

CONCLUSION
This research work investigated the influence of absorber bandgap, absorber layer electron affinity, buffer bandgap and buffer electron affinity by means of SCAPS simulation. The efficiency of the initial device which was set up in line with our previous work yielded an efficiency of 25.85% and on optimization of the aforementioned parameters, optimal values of 1.4 eV, 4.45 eV, 3.0 eV and 4.05 eV respectively, were obtained. These values were used together with optimum values of multivalent defect density (1E + 10 cm -3 ) in absorber, absorber layer thickness (1.2µm) and buffer layer thickness (0.01 µm) obtained in our previously published work, to simulate a solar cell which turned out to be the best device having photovoltaic parameters with a V OC of 1.11 V, J SC of 31.55 mA/cm 2 , fill factor of 88.91 %. and efficiency of 31.13 %. Except for J SC , these values are higher than those obtained in our paper, where the best device had photovoltaic parameters with V OC of 0.83 V, J SC of 37.75 mA/cm 2 , fill factor of 86.26 % and an efficiency of 27.00 %. This clearly shows that the cumulative effect of optimizing more parameters of the absorber and buffer layer produces better performing solar cells (as three parameters were used to produce the optimized solar cell in our former paper whereas in this work, seven were used). Lastly, the effect of working temperature on the optimized solar cell was investigated. The study showed that the best performance of the optimized device was achieved at 270 K and the photovoltaic parameters corresponding to this optimum working temperature are; V OC of 1.15 V, J SC of 31.55 mA/cm 2 , FF of 88.64 % and an efficiency of 32.18 %. At 270 K, this best device would work optimally in cold countries of the world like Russia and Canada were temperatures can get as low as 268 K.