1 January 2011 Low-bandgap small molecules for near-infrared photovoltaic applications
Author Affiliations +
Abstract
We investigate the effects of interfacial layers on the photovoltaic properties of bilayer organic heterojunction photovoltaic devices. The devices were fabricated using aluminum phthalocyanine chloride (AlPcCl) as electron donor and fullerene (C60) as electron acceptor. Two types of interfacial layers inserted between the transparent indium-tin-oxide anode and the AlPcCl layer were investigated: PEDOT:PSS and MoO3. We find that these interfacial layers have a strong influence on the device open-circuit voltage (VOC). The effects of temperature and illumination intensity on VOC were explored.
Ballarotto, Herman, and Romero: Low-bandgap small molecules for near-infrared photovoltaic applications

1.

Introduction

Aluminum phthalocyanine chloride (AlPcCl) is a low-bandgap (Eg = 1.56 eV) small-molecule organic semiconductor that is currently being explored to extend the spectral range of organic photovoltaic devices to the near-infrared. Recent works1, 2, 3 on bilayer organic photovoltaic devices using AlPcCl as the donor molecule combined with C60 as the acceptor molecule have demonstrated 2% power conversion efficiency in these devices. In these works, the type of interfacial layer formed at the indium-tin-oxide (ITO) anode was shown to have a strong influence on the device open-circuit voltage (VOC). UV ozone treated ITO substrates manifested substantially different VOC of 0.68 V (Ref. 1) and 0.84 V (Ref. 2). In devices where molydenum oxide (MoO3) was used as an interfacial layer, VOC = 0.83 V has been reported.3

In general, the power conversion efficiency (ηP) of a photovoltaic device is given by ηP = VOCJSCFF/Pin where JSC and FF are the device short-circuit current density and fill factor, respectively, and Pin is the incident light intensity. Aside from its direct effect on ηP, VOC is also known to have an influence on FF. Shockley and Queisser4 were the first to note that FF for an ideal solar cell depends only on the temperature normalized open-circuit voltage, vOC = VOC/(kBT/q), where T is the cell temperature, kB is Boltzmann's constant, and q is the electronic charge. Empirical expressions proposed by Green5 demonstrated that FF increases as vOC is raised. Therefore, in order to optimize the efficiency of AlPcCl-based bilayer organic photovoltaic devices, it is important to understand the observed effects1, 2 , 3 of the anode interface on VOC. In the present work, we address the question of whether the device open-circuit voltage is determined by the intrinsic properties of the AlPcCl/C60 donor/acceptor heterojunction or the properties of the electrode interfaces. From detailed temperature and power dependence of the device current density versus voltage characteristics, we show that both factors contribute to the device open-circuit voltage.

2.

Device Fabrication and Characterization

The materials used to fabricate our bilayer devices were acquired from various commercial sources. The device geometry is presented in Fig. 1. The substrate is glass covered with transparent conducting ITO (∼150 nm thick with sheet resistance Rs = 4–8 Ω from Delta Technologies). We investigated poly(3,4-ethylenedioxythophene) doped with poly(styrenesulfonate) (PEDOT:PSS) in aqueous solution (Baytron P, H.C. Stark, USA) and MoO3 (Aldrich, USA) as interfacial hole-transport layers in our devices. Sublimed grade bathocuproine (BCP, Aldrich) was used to form an ohmic contact between C60 and the aluminum electrode. AlPcCl (Aldrich) and C60 (99.5% purity, nano-C) were used to form the electroactive bilayers of our devices. AlPcCl was purified by thermal gradient sublimation before being used for device fabrication.

Fig. 1

Structure of the bilayer devices based on small molecules AlPcCl (donor) and C60 (acceptor).

011102_1_1.jpg

The ITO electrodes were first patterned on the glass substrates by photolithographic techniques. The ITO substrates were then cleaned by ultrasonic treatment sequentially in detergent, deionized water, and acetone followed by rinsing in isopropyl alcohol. To ensure a valid comparison of the photovoltaic characteristics of devices with different interfacial layers, substrates covered by PEDOT:PSS and MoO3 were initially prepared separately. A 50-nm-thick PEDOT:PSS film was spin-coated from the aqueous Baytron P solution onto the ITO/glass at 5000 rpm under ambient atmosphere. The air-dried film was transferred inside a glove box and heated at 150°C for 10 min to remove any residual moisture from the film. A 60-Å MoO3 thin film was thermally evaporated on another patterned ITO substrate. Both substrates were then loaded into the thermal evaporation system for subsequent depositions of the remaining identical layers in each device comprising the device structure. Films of 220 Å of AlPcCl and 425 Å of C60 were thermally evaporated onto the substrates with a base pressure of 2 × 10−6 Torr. The devices were completed by depositing 100 Å of BCP and a 1000 Å Al through a shadow mask defining the 0.05 × 0.05-in. square active area for the device. The entire device fabrication was carried out inside a nitrogen-filled glove box to avoid potential sources for photo-oxidative degradation.

On completion, the devices were mounted in a cryostat inside the glove box. The mounted device was subsequently taken outside the glove box and pumped to 5 × 10−6 Torr prior to measurement of the current density versus voltage (JV) characteristics. The measurements were performed over a temperature range of 30–300 K and illumination intensities between 12 and 194 mW/cm2. The illumination source was a simulated AM 1.5 global solar simulator (Oriel 300 W) with a maximum integrated intensity of ∼200 mW/cm2. The incident illumination intensity was adjusted using different size apertures and by varying the power of the solar simulator. The intensity was measured using an NREL-calibrated silicon photovoltaic cell as detector.

3.

Results and Discussions

3.1.

Room-Temperature Current Density versus Voltage Characteristics in the Dark and under Illumination

Figure 2 presents a comparison of the illuminated current density (JL) versus voltage (V) characteristics at room temperature of the AlPcCl/C60-based heterojunction photovoltaic devices fabricated with the two types of interfacial layers. We observe a significant increase in the open-circuit voltage from VOC = 0.61 V using the PEDOT:PSS interfacial layer to VOC = 0.81 V for the device with the MoO3 layer. The corresponding device fill factor also increased from 0.49 to 0.55. However, the short-circuit current is reduced for the device fabricated with MoO3 compared to PEDOT:PSS from 4.21 to 3.47 mA/cm2. Nevertheless, the observed increase in VOC and FF results in a significant improvement in the power-conversion efficiency from 1.33 to 1.64%.

Fig. 2

Room-temperature JL–V characteristics of AlPcCl/C60 heterojunctions with the two types of interfacial layers, MoO3 and PEDOT:PSS, under AM1.5 global solar simulator with 100 mW/cm2.

011102_1_2.jpg

To understand the differences in the J–V characteristics presented in Fig. 2, we use the typical equivalent circuit model developed to describe photovoltaic devices.6 In this model, the photocell is described by a diode in parallel with a current source, Jph, and a parallel resistance Rp, all connected in series with the resistance Rs. The diode represents the rectifying donor-acceptor heterojunction and is characterized by the reverse-bias saturation current density JS, and the diode ideality factor n. Jph(V) is the voltage-dependent photogenerated current density present in the device under illumination, and it becomes zero for devices measured in the dark. The parallel resistance Rp is related to leakage currents due to pinholes or other imperfections during device fabrication or to intrinsic recombination processes present in the active layers. The series resistance Rs accounts for the organic layer resistance together with the contact resistance formed between each of the interfaces present in the device. With this model, the current density (J) as a function of the applied voltage is given by the generalized Shockley diode Eq. 1,

[TeX:] \documentclass[12pt]{minimal}\begin{document}\begin{equation} J = \frac{{R_{\rm p}}}{{R_{\rm s} + R_{\rm p}}}{\rm}\left\{{J_{\rm s} \left[ {{\rm exp}\frac{{q{\rm (}V - JR_{\rm s} {\rm)}}}{{nk_{\rm B} T}} - 1} \right] + \frac{V}{{R_{\rm p}}}} \right\} - J_{{\rm ph}} (V). \end{equation}\end{document} J=RpRs+RpJs exp q(VJRs)nkBT1+VRpJ ph (V).
Note that Eq. 1 describes the illuminated current density (JL) when Jph ≠ 0 and the dark current density (Jd) for Jph = 0. In our analysis, we estimate the photocurrent as Jph = JLJd.

The experimental Jd versus V curve is fitted to Eq. 1 to obtain the diode parameters Rs, Rp, n, and Js. The fit to the room temperature JdV for both devices is presented in Fig. 3. Good agreement of the fit with the data is obtained using the diode parameters listed in the caption to Fig. 3. When the PEDOT:PSS interfacial layer is replaced with the MoO3 interfacial layer, the reverse-bias saturation current Js decreases by two orders of magnitude from 9.48 × 10−7 mA/cm2 to 7.84 × 10−9 mA/cm2. As we will show later, such a decrease in Js can explain the elevated VOC measured from the device with the MoO3 interfacial layer.

Fig. 3

(a) Dark JdV characteristics of AlPcCl/C60 heterojunction devices fabricated with two types of interfacial layers: MoO3 and PEDOT:PSS. The lines represent fits to the data using Eq. 1 with fitting parameters: Js = 7.84×10−9 mA/cm2, n = 1.69, Rs = 1.84 Ω cm2, Rp = 1.39 MΩ cm2 for the MoO3 device, and Js = 9.48×10−7 mA/cm2, n = 1.82, Rs = 1.36 Ω cm2, Rp = 0.48 MΩ cm2 for the PEDOT:PSS device. b) JphV characteristics of AlPcCl/C60 heterojunction devices fabricated with two types of interfacial layers: MoO3 and PEDOT:PSS. Inset: Jph as a function of effective applied voltage, VCV, where VC is the compensation voltage defined in the text.

011102_1_3.jpg

An expression for the VOC can be extracted from Eq. 1 by setting J = 0. For RsRp this yields,

[TeX:] \documentclass[12pt]{minimal}\begin{document}\begin{equation} V_{{\rm oc}} = \frac{{nk_{\rm B} T}}{q}{\rm ln}\left({\frac{{J_{{\rm ph}} (V_{{\rm oc}})}}{{J_{\rm s}}} + 1 - \frac{{V_{{\rm oc}}}}{{J_{\rm s} R_{\rm p}}}} \right). \end{equation}\end{document} V oc =nkBTq ln J ph (V oc )Js+1V oc JsRp.
The device fill factor can also be calculated from the diode parameters of Eq. 1. In general, FF is obtained numerically. However, the following empirical estimates were previously shown to be good approximations of the numerical values when VOC > 10nkBT5, 6,
[TeX:] \documentclass[12pt]{minimal}\begin{document}\begin{equation} \begin{array}{l} {\rm FF}_{\rm O} = \displaystyle\frac{{v_{{\rm OC}} - \ln ({v_{{\rm OC}} + 0.72})}}{{v_{{\rm OC}} + 1}}, \quad ({r_{\rm S} = {1 \mathord{/ {\vphantom {1 {r_{\rm P} = 0}}} \kern-\nulldelimiterspace} {r_{\rm P} = 0}}}) \\[11pt] {\rm FF}_{\rm S} = {\rm FF}_{\rm O} \left({1 - 1.1r_{\rm S}} \right) + 0.19r_{\rm S}^2, \quad \left({0 \le r_{\rm S} \le 0.4,{{{\rm}1} \mathord{/ {\vphantom {{{\rm}1} {r_{\rm P}}}} \kern-\nulldelimiterspace} {r_{\rm P}}} = 0} \right) \\[9pt] \;\,{\rm FF} = {\rm FF}_{\rm S} \left\{{1 - \displaystyle\frac{{\left({v_{{\rm OC}} + 0.7} \right)}}{{v_{{\rm OC}}}}\frac{{{\rm FF}_{\rm S}}}{{r_{\rm P}}}} \right\},\quad \left({0 \le r_{\rm S} + {1 \mathord{/ {\vphantom {1 {r_{\rm P}}}} \kern-\nulldelimiterspace} {r_{\rm P}}} \le 0.4} \right) \\ \end{array} \end{equation}\end{document} FF O=v OC ln(v OC +0.72)v OC +1,(rS=1/1rP=0rP=0) FF S= FF O11.1rS+0.19rS2,0rS0.4,1/1rPrP=0 FF = FF S1v OC +0.7v OC FF SrP,0rS+1/1rPrP0.4
where the normalized quantities are defined as vOC = qVOC/nkBT, rS = Rs/RCH, and rp = Rp/RCH. The characteristic resistance is given by RCH = VOC/JSC.

We compare the experimentally measured VOC with that given by Eq. 2 using the fitting parameters obtained from the dark current characteristics described above. From Fig. 3(b), we estimate Jph(VOC) = 1.16 mA/cm2 for the MoO3 device and Jph(VOC) = 0.31 mA/cm2 for the PEDOT:PSS device. Using Eq. 2, and noting that the first term dominates over the other two terms for the quantities inside the bracket, the expected open-circuit voltage for the device with a PEDOT:PSS interface is VOC = 0.60 V, whereas the one with a MoO3 interface has VOC = 0.82 V. These estimates are in very good agreement with the experimental values of 0.61 and 0.81 V, respectively, suggesting that the equivalent circuit model is applicable in describing the devices’ photovoltaic characteristics at room temperature.

We now account for the observed fill factor in both devices. Using the estimate of VOC derived from the dark diode characteristics, the ideal fill factor (FFO) calculated from Eq. 3 is FFO = 0.80 for the MoO3 device and FFO = 0.74 for the PEDOT:PSS device. Correction associated with the different series resistance yields FFS = 0.79 for the MoO3 device and FFS = 0.73 for the PEDOT:PSS device, demonstrating negligible effect of Rs on FF. The large value of Rp extracted from the dark JdV characteristic from both devices also results in negligible correction to the fill factor. However, the linear increase of Jph at large negative applied voltage [seen in Fig. 3(b)] suggests another Rp contribution to FF under illumination condition. Estimating this light-induced parallel resistance from the slope of the linear part of the JLV curve, we obtain Rp ≈ 850 Ω cm2 for the MoO3 device and Rp ≈ 538 Ω cm2 for the PEDOT:PSS device. Using these values, the fill factor calculated from Eq. 3 is FF = 0.61 for the MoO3 device and FF = 0.55 for the PEDOT:PSS device, values that are comparable to the corresponding measured device fill factor. This analysis shows that FF is controlled by the ideality factor and saturation current of the diode as well as a light-induced Rp contribution. The latter could come from photoconductivity of the donor and/or acceptor layers.7

The measured Jph versus V characteristics for the two different interfacial devices under investigation are presented in Fig. 3(b). In the inset of Fig. 3(b), Jph is plotted against the effective voltage across the device, VCV, where VC is the compensation voltage defined as the voltage where Jph = 0. VC is 0.62 V for device fabricated with a PEDOT:PSS interfacial layer, while VC = 0.86 V for a MoO3 interfacial layer device. We note that a scaling factor of 0.75 was used to multiply Jph for the MoO3 devices. Surprisingly, the shape of the photocurrent for both devices is nearly identical demonstrating that the photocurrent generated within the device is determined by the effective voltage, VCV, consistent with the absence of space-charge formation in the donor and acceptor layers. Thus, the differences that we observe in the JphV curves can be attributed to the differences in the built-in field imposed by the presence of the anode interfacial layers. To gain further insight into the origin of the VOC, we explore the photovoltaic characteristics of both devices at various temperatures and intensities of the incident light.

3.2.

Temperature and Power Dependence of the Open-Circuit Voltage

Figure 4 compares the temperature dependence of the open-circuit voltage for both devices under different illumination intensities. For a given incident illumination intensity, we observe a linear increase in VOC as the temperature decreases from 300 to 140 K. A maximum value for VOC is achieved at ∼140 K in both devices. Figure 4 shows this value is 1.05 V for the device fabricated with MoO3 layer [Fig. 4(a)], whereas it is 0.65 V for the device with the PEDOT:PSS interfacial layer [Fig. 4(b)]. These observations indicate that the maximum VOC value is strongly dependent on the nature of the anodic interfacial layer, while its position on the temperature scale seems to be determined by the properties of the donor/acceptor heterojunction. Upon further cooling, we observe a decrease in VOC for both devices. The insets show the comparison of the temperature dependence of VOC and the compensation voltage, VC, in both devices. VOC coincides with VC for T < 140 K with both decreasing by nearly the same value of ∼0.1 V at the lowest temperature. For T > 140 K, VOC < VC with the MoO3 device manifesting a larger offset than that of the PEDOT:PSS device.

Fig. 4

Open-circuit voltage as a function of temperature for AlPcCl/C60 devices with (a) MoO3 and (b) PEDOT:PSS interfacial layers under AM 1.5 Global solar simulator with various incident power intensity. The inset compares the temperature dependence of the open-circuit voltage (VOC) and the compensation voltage (VC) for P = 97 mW/cm2.

011102_1_4.jpg

Figure 4 also reveals a clear dependence of VOC on the incident light intensity that is observed for T > 140 K in both devices. At <140 K, this power dependence disappears. Figure 5(a) presents a semilogarithmic plot of VOC versus P, where P is the incident light intensity at three representative temperatures. The experimental data can be fitted with a linear function, and the slope can be compared to kBT/q. The extracted slope decreases rapidly with temperature from 1.20kBT/q (0.80kBT/q) at T = 290 K to 0.92 kBT/q (0.69kBT/q) when T = 200 K and 0.52kBT/q (0.47kBT/q) at T = 150 K for the MoO3 (PEDOT:PSS) device. Equation 2 relates the observed power dependence of VOC to that of Jph(VOC). Assuming that the measured photocurrent at V = VOC obeys the power-law dependence, Jph(VOC) = JT(P/Po)α, where the parameter JT depends only on temperature, then VOC is given by

[TeX:] \documentclass[12pt]{minimal}\begin{document}\begin{equation} V_{{\rm OC}} \simeq \frac{{nk_{\rm B} T}}{q}\ln \left({\frac{{J_{\rm T} \left({{P \mathord{\left/ {\vphantom {P {P_{\rm O}}}} \right. \kern-\nulldelimiterspace} {P_{\rm O}}}} \right)^\alpha}}{{J_{\rm S}}}} \right) = \frac{{nk_{\rm B} T}}{q}\left({\alpha \ln \frac{P}{{P_{\rm O}}} + \ln \frac{{J_{\rm T}}}{{J_{\rm S}}}} \right). \end{equation}\end{document} V OC nkBTqlnJTPPPOPOαJS=nkBTqαlnPPO+lnJTJS.
Figure 5(b) shows a double-logarithmic plot of Jph(VOC) versus P. We observe an experimental power low dependence of Jph(VOC) with the exponent strongly decreasing with temperature. Extracting α from Fig. 5(b) and combined with the slope of VOC versus ln(P) determined from Fig. 5(a), the diode ideality factor can be obtained using Eq. 4. The results of this analysis at various temperatures and their comparison to n derived from the Jd versus V characteristics are displayed in Fig. 6. Good agreement between the two values of n is illustrated in Fig. 6, corroborating our assumption of the power-law dependence of Jph(VOC) with P as well as demonstrating that the equivalent circuit model is a good representation of the data.

Fig. 5

Incident power (P) dependence of the open-circuit voltage (VOC) (a) and the photocurrent at V = VOC [Jph(VOC)] at three representative temperatures: T = 290 K (black), 200 K (red), and 150 K (blue). The solid circles are for the MoO3 device, and the open-circles are for the PEDOT:PSS device.

011102_1_5.jpg

Fig. 6

Comparison of the diode ideality factor obtained from the data in Fig. 5 using Eq. 4 and from the fit of the Jd versus V using Eq. 1.

011102_1_6.jpg

3.3.

Discussion

We have shown above that the observed variation in VOC associated with different interfacial layers at the anode of AlClPc/C60 bilayer organic photovoltaic devices is related to the diode saturation current density Js. Earlier works on inorganic p-n homojunctions and heterojunctions have shown that Js results from the dominant transport process associated with the p-n junction.8 For devices with finite thickness, recombination at the electrode interfaces can also influence Js. In p-n homojunctions, finite thickness correction leads to Js = JSOF, where JSO is the saturation current density for an infinitely thick diode and

[TeX:] \documentclass[12pt]{minimal}\begin{document}\begin{equation} F = \frac{{S\cosh ({{W\! \mathord{/ {\vphantom {W L}} \kern-\nulldelimiterspace} L}}) + ({{D\! \mathord{/ {\vphantom {D L}} \kern-\nulldelimiterspace} L}})\sinh ({{W\! \mathord{/ {\vphantom {W L}} \kern-\nulldelimiterspace} L}})}}{{({{D\! \mathord{/ {\vphantom {D L}} \kern-\nulldelimiterspace} L}})\cosh ({{W\! \mathord{/ {\vphantom {W L}} \kern-\nulldelimiterspace} L}}) + S\sinh ({{W\! \mathord{/ {\vphantom {W L}} \kern-\nulldelimiterspace} L}})}}, \end{equation}\end{document} F=Scosh(W/WLL)+(D/DLL)sinh(W/WLL)(D/DLL)cosh(W/WLL)+Ssinh(W/WLL),
where W is the depletion width associated with the band-bending at the p-n junction, S is the surface recombination velocity at the electrode interface, and D and L are the diffusion coefficient and diffusion length of the minority carrier at the interface.9 Applying Eq. 5 for the organic heterojunction with WL (Ref. 10), then Js ≈ (JSOL/D)S. Recall that Js is nearly two orders of magnitude lower in the device with the MoO3 interface than that with the PEDOT:PSS interface. In the approximate expression for Js, the quantities JSO, L, and D are the same in both devices. Thus, S at the MoO3/AlClPc interface is significantly lower than the one associated with the PEDOT:PSS/AlClPc interface. Such difference between the surface recombination velocities of the respective interfaces accounts for the variation in the open-circuit voltage observed in AlClPc/C60 bilayer photovoltaic devices.

An early work by Rand7 demonstrated that, in bilayer organic photovoltaic devices, VOC approaches its maximum value at low temperatures. Our current work demonstrates that this maximum value coincides with the compensation voltage VC. Malliaras et al.11 showed that VC at low temperatures is a measure of the diode built-in potential Vbi. We find that Vbi is higher in the device where MoO3 is used as an interfacial layer. Neglecting excitonic corrections, the maximum VOC (or Vbi) in organic photovoltaic devices is given by the difference between the energetic positions of the donor highest-occupied molecular orbital (EHOMO) and the acceptor lowest-unoccupied molecular orbital (ELUMO) (Refs. 7, 12). Measurements using ultraviolet photoelectron spectroscopy report values of 5.3 eV (Ref. 2) and 5.4 eV Ref. 1) for EHOMO in AlPcCl and 4.1 eV Ref. 13) and 4.5 eV (Refs. 1, 2) for ELUMO in C60. These values suggest that the maximum attainable VOC for AlPcCl/C60 heterojunction devices is ∼ 0.8 to 1.3 V. The data in Fig. 4 illustrate that this expected maximum VOC is achieved in the MoO3 device in which it is observed that VOC = 1.05 V at T = 140 K, indicating negligible voltage losses at the MoO3/AlPcCl interface. The observed higher maximum VOC (and Vbi) in the MoO3 device compared to that in the PEDOT:PSS device is consistent with the low surface recombination velocity in the former as described earlier.

An unusual feature of the data presented in Fig. 4 is the apparent peak seen in the VOC versus T curves at T = 140 K in both devices. Both devices manifest a drop from this peak VOC value by about δVOC = 0.1 V as the temperature is lowered to T = 30 K, suggesting that the effect is associated with the p-n heterojunction at the AlPcCl/C60 interface. This observation indicates a voltage loss contribution of the AlPcCl/C60 heterojunction to VOC. We argue that the presence of photoexcited charge carriers at high temperatures screens the electrostatic band bending at this p-n heterojunction reducing its deleterious effect on VOC.

4.

Conclusions

The variation in the open-circuit voltage associated with different anode interface layers in chloroaluminum phthalocyanine/fullerene bilayer organic photovoltaic devices is shown to arise from interface charge recombination rather than from the intrinsic properties of the AlPcCl/C60 donor/acceptor heterojunction. Low surface recombination velocity is achieved at the interface formed by aluminum phthalocyanine chloride with molybdenum oxide accounting for the high VOC in devices where MoO3 is used as an interfacial layer.

References

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© 2011 Society of Photo-Optical Instrumentation Engineers (SPIE)
Mihaela Ballarotto, Mihaela Ballarotto, Warren N. Herman, Warren N. Herman, Danilo B. Romero, Danilo B. Romero, } "Low-bandgap small molecules for near-infrared photovoltaic applications," Journal of Photonics for Energy 1(1), 011102 (1 January 2011). https://doi.org/10.1117/1.3528044 . Submission:
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