Metamorphic and Lattice-Matched GaInP Rear-Heterojunction Solar Cells for Improved Performance at Elevated Temperatures

We present the characteristics of <italic>n</italic>-on-<italic>p</italic> front-junction (FJ) and rear-heterojunction (RHJ) GaInP solar cells at operating temperatures (<italic>T</italic>) up to 300 °C. Photovoltaic cells with efficient operation at high <italic>T</italic> may be important for satellite missions near the sun or as laser power converters for sensors operating in harsh environments. In this article, we show that the time-resolved photoluminescence lifetime (<inline-formula><tex-math notation="LaTeX">${{\tau }_{\text{TRPL}}}$</tex-math></inline-formula>) values in both lattice-matched (LM) <italic>n</italic>-Ga<sub>0.51</sub>In<sub>0.49</sub>P and metamorphic (MM) <italic>n</italic>-Ga<sub>0.37</sub>In<sub>0.63</sub>P double heterostructures are high and increase significantly with <italic>T</italic>. In contrast, the <inline-formula><tex-math notation="LaTeX">${{\tau }_{\text{TRPL}}}$</tex-math></inline-formula> values in their <italic>p</italic>-type counterparts are lower and decrease with <italic>T</italic>. We go on to demonstrate both LM and MM solar cells in FJ and RHJ configurations. The internal quantum efficiency (IQE) of MM RHJ cells increases significantly up to <italic>T</italic> = 300 °C due to the increase in both <inline-formula><tex-math notation="LaTeX">${{\tau }_{\text{TRPL}}}$</tex-math></inline-formula> and the linear increase in diffusivity with <italic>T</italic>. In contrast, the IQE for MM FJ cells is nearly unchanged as <italic>T</italic> increases, while the IQE of LM cells drops sharply across all wavelengths. RHJ cells maintain higher open-circuit voltage and fill factor than their FJ counterparts, leading to a significant efficiency advantage at <italic>T</italic> = 100–300 °C. Taken together, our work shows that MM cells perform well at elevated <italic>T</italic> and that RHJ cells are promising for high-<italic>T</italic> operation.

An unavoidable factor for photovoltaic devices operating at high T is the exponential increase in intrinsic carrier concentration (n i ) and, hence, dark current as T increases, leading to strong decrement in both open-circuit voltage (V OC ) and fill factor (FF).Despite a slight increase in short-circuit current density (J SC ) due to the decrease in bandgap energy (E g ), the overall efficiency undergoes significant degradation at high T [1], [10], [11].
Another effect at elevated T is that a greater proportion of charge carriers can attain enough energy to surmount heterojunction barriers (e.g., the window and back surface field or BSF layers), leading to increased recombination at the free surface and further increased dark current [12].The rate of thermionic emission decreases exponentially with effective barrier height [13], and previous work revealed that the performance of LM Ga 0.51 In 0.49 P solar cells at high T improved with increasing E g of the LM (Al x Ga 1-x ) 0.51 In 0.49 P passivation layer due to the higher barrier [12].However, further reduction of thermionic emission for LM GaInP cells is challenging due to material limitations that restrict the E g of the LM III-V cladding layer to ≤ 2.3 eV (LM Al 0.52 In 0.48 P).
The need to suppress thermionic emission motivates the investigation of metamorphic (MM) Ga 0.37 In 0.63 P for high-T applications.At room temperature (RT), E g of MM Ga 0.37 In 0.63 P and LM Ga 0.51 In 0.49 P are 1.7 eV and 1.9 eV, respectively, while E g of MM Al 0.38 In 0.62 P and LM Al 0.52 In 0.48 P window layers are ∼2.3 eV in both cases.The larger ΔE g in the MM cells translates to higher barrier heights than in LM cells and an exponential suppression in thermionic emission.
Another potential approach to achieve improved performance at elevated T is employing a rear-heterojunction (RHJ) configuration.Under standard conditions, RHJ cells can have a lower dark current and higher V OC than their FJ counterparts [14], [15], [16] due to the reduction of trap-assisted recombination in the space charge region.Moreover, the valence bands in phosphides have a significantly higher density of states than their corresponding conduction bands [17], [18], resulting in reduced thermal spreading of minority holes compared with electrons; i.e., at a given T, minority holes tend to occupy states close to the valence band edge.RHJ cells only collect minority holes [14] and may, therefore, exhibit lower thermionic emission loss compared with FJ cells.
In this article, we compare the performance of FJ and RHJ cells at elevated T for both LM and MM GaInP.We demonstrate that the quantum efficiency (QE) of LM FJ cells decreases significantly with T, while MM cells maintain similar QE at high T due to longer lifetimes and higher barrier heights.In contrast, QE of LM and MM RHJ cells increase with T, in agreement with our observation of increasing minority hole lifetime with T. Next, we show that the dark current advantage of RHJ cells is maintained as T increases, leading to higher V OC , FF, and efficiency than FJ cells at T = 100-300 °C.These results show that RHJ cells are promising for high-T applications and that MM materials with large heterojunction band offsets are helpful in suppressing thermionic emission.

II. EXPERIMENTAL METHODS
All solar cells and double heterostructures (DHs) in this work were grown using a Veeco Mod Gen-II solid-source molecular beam epitaxy system and annealed using an AllWin rapid thermal annealing (RTA) system.RTA processes were carried out under N 2 ambient with a ramp rate of 20 °C/s.The optimal RTA conditions (850-1000 °C, 1-30 s) for DHs and solar cells were chosen based on maximizing the steady-state photoluminescence intensity [19].In our previous studies, these cells showed comparable performance to metalorganic vapour-phase epitaxy (MOVPE)-grown cells [15], [19].
The DHs, consisting of a 500 nm GaInP active layer with RT E g of either 1.7 eV (MM) or 1.9 eV (LM), were grown at a substrate temperature of 460 °C and growth rate of 0.5 μm/hr for time-resolved photoluminescence (TRPL) studies.GaInP active layers were lightly doped with electron (hole) concentrations of n o (p o ) = 1.0 × 10 17 cm −3 ; 100 nm n-InAlP (p-AlGaInP) barriers were doped with n o (p o ) = 1.0×10 18 cm −3 .The details about the DHs are described in previous papers [19], [20].
The growth structures of MM 1.7 eV and LM 1.9 eV GaInP FJ and RHJ solar cells are illustrated in Fig. 1 ) was introduced to avoid majority hole blocking.In a previous report, we showed a low threading dislocation density of 6-7 × 10 5 cm −2 in MM Ga 0.37 In 0.63 P solar cells [19].Solar cells were fabricated similar to our previous works [21], [22] with Ti/Au and Cr/Au for front and back metal contacts.No antireflection coatings were applied.
All T-dependent measurements were performed using a variable T Linkam stage (HFS600E-PB4) with a temperature controller (T95-PE).TRPL measurements were conducted under low-level injection conditions using time-correlated single-photon counting and a 532 nm pulsed laser with a 1 mm diameter spot size, 2.5 mW average power, 6 ps pulse width, and 3.6 MHz repetition rate.Lighted current-voltage (LIV) measurements were conducted under approximate AM1.5G illumination with an ABET Technologies 10500 solar simulator to determine V OC , bandgap-voltage offset (W OC = E g /q−V OC ), J SC , FF, and efficiency (η).External quantum efficiency (EQE) and specular reflectance (R) were measured using a PV measurements QEX7 system; internal quantum efficiency (IQE) was estimated as IQE = EQE/(1−R).Band diagrams were simulated using BandProf software.

A. Temperature-Dependent TRPL Lifetime Analysis
To investigate the dominant recombination mechanisms at high T, an analysis of T-dependent TRPL lifetime (τ TRPL ) was conducted on p-and n-GaInP DHs for both MM and LM cases.Taking into account different recombination lifetimes, including the radiative recombination lifetime (τ r ), Shockley-Read-Hall (SRH) recombination lifetime (τ SRH ) [23], interface recombination lifetime (τ int ), and recombination lifetime associated with thermionic emission (τ th ) [13], the effective carrier lifetime (τ eff ) can be represented as [12] follows: This equation is applicable under low-injection conditions, where Auger recombination is negligible.The temperature dependencies of these lifetimes can be expressed as follows [24]: Note that τ r increases weakly with T, τ SRH decreases weakly with T, and τ th decreases exponentially with T. By studying τ TRPL as a function of T, the relative contributions of these recombination factors can be estimated at each specific T.
Fig. 2(a) shows that, as T increases to 400 °C, the τ TRPL values for LM and MM p-GaInP decrease by ∼4×, from 2.1 to 0.5 ns, and by ∼2×, from 2.7 to 1.3 ns, respectively.The decrease in τ TRPL suggests the dominance of SRH recombination, interface recombination, and/or thermionic emission [1] in both LM and MM p-GaInP.Specifically, a steep decline in τ TRPL , signifying the onset of dominance of τ th [12], [24], is observed above 100 °C for LM p-GaInP and above 200 °C for MM p-GaInP.The somewhat more gradual reduction in τ TRPL for MM p-GaInP suggests a better suppression of thermionic emissions compared with LM p-GaInP [12], [25].
In contrast, both MM and LM n-GaInP exhibit an increase in τ TRPL with increasing T, as shown in Fig. 2(b).The observed trend indicates the dominance of τ r and the suppression of thermionic emission of minority holes in n-GaInP.For LM n-GaInP, τ TRPL increases from 12 ns at RT before peaking at 37 ns at 350 °C, suggesting the dominance of thermionic emission effects over 350 °C.For MM n-GaInP, τ TRPL starts from 23 ns at RT and rises to 58 ns at 550 °C; MM n-GaInP was the only sample that gave an adequate signal-to-noise ratio to extract the accurate lifetimes at such high T.The high τ TRPL values in MM n-GaInP at such high T may stem from reduced thermionic emission owing to the higher valence band offset (ΔE v ) at the MM n-GaInP/MM n-AlInP interface (∼0.58 eV at RT) compared with the LM n-GaInP/LM n-AlInP interface (∼0.46 eV at RT), as illustrated by the calculated band diagrams in Fig. 3  to E g reduction as T increases.LM FJ cells undergo a slight drop in IQE as T increases to 100 °C, followed by a sharp decline at 100-300 °C [see Fig. 4(a)], consistent with the τ TRPL trend observed in LM p-GaInP DHs.This steep decrease above 100 °C differs from what was reported by Perl [26] in metal-organic chemical vapour deposition (MOCVD)-grown LM GaInP FJ cells, where only a slight reduction in IQE at 300-550 nm occurred due to a decrease in E g of the AlInP window.The discrepancy is likely due to the much shorter τ SRH in our p-GaInP DHs compared with MOCVD-grown DHs (τ TRPL ∼29 and ∼16 ns at RT and 300 °C, respectively) [12].
IQE of the MM FJ cells only experiences slight losses throughout visible wavelengths up to 300 °C that can be partially attributed to increased absorption in the window layer [see Fig. 4(b)].The enhanced retention of IQE in MM FJ cells stems from the higher τ TRPL and longer diffusion length of minority electrons in the MM p-GaInP base at high T compared with the LM p-GaInP base.Our MM FJ cells likely also benefit from a stronger suppression of thermionic emission due to the higher effective barrier height relative to LM FJ cells.

TABLE I TEMPERATURE COEFFICIENTS OF LM AND MM GAINP CELLS WITH FJ AND RHJ CONFIGURATIONS
LM FJ cells show a negative value of dJ SC dT due to a substantial decrease in peak IQE to ∼80% at 300 °C (see Table I).On the other hand, the MM FJ cells demonstrate a slight increase in J SC as T increases while maintaining a peak IQE of ∼95%, showing the benefit of MM GaInP solar cells at high-T conditions.The slightly better dV OC dT observed in MM FJ cells compared with LM FJ cells is also consistent with better τ TRPL retention in MM cells at elevated T (see Table I).C. Advantages of RHJ Over FJ Cells at High T Fig. 5(a) and (b) demonstrates that IQE increases across all wavelengths up to 300 °C for both LM and MM RHJ cells due to the increase in τ TRPL with T and the direct dependence of carrier diffusivity on T [27].Between 27 and 300 °C, LM RHJ cells show a slight increase in peak IQE from ∼92% to ∼95%.In contrast, MM RHJ cells exhibit a strong boost in peak IQE from ∼84% to ∼95%, which can be attributed to the stronger increase in τ TRPL in MM n-GaInP with T.
The LIV curves in Fig. 6   Notably, among all four configurations, MM RHJ cells show the lowest dV OC dT , the highest dJ SC dT , and the lowest W OC value of 0.970 V at 300 °C (see Table I), benefiting from the advantages of both the higher barrier height and the RHJ configuration.Fig. 7(a)-(d) summarizes all figures of merits, including integrated EQE, W OC , FF, and efficiency for all four different solar cells as a function of T. Integrating EQE with the AM1.5G spectrum across all wavelengths [see Fig. 7(a)] reveals that the MM RHJ cell exhibits the strongest increase in J SC with T .Fig. 7(b) shows a linear increase in W OC with T for all cells due to the exponential increase in dark current.The advantage of RHJ cells (circles) over FJ cells (squares) in terms of W OC becomes more prominent at higher T for both LM (open symbols) and MM (filled symbols) configurations.FF shows a qualitatively similar trend [see Fig. 7(c)], although the LM cells retain higher values than the MM cells due to their wider E g .Furthermore, at RT, the RHJ cells have similar or lower FF values than FJ cells, whereas at 300 °C, higher FF is observed in RHJ cells compared with their FJ counterparts.The efficiency versus T plot in Fig. 7(d) captures all effects observed in Fig. 7(a)-(c) and demonstrates that the efficiency of the RHJ cells surpasses the FJ counterparts at T > 100 °C.

IV. CONCLUSION
In this article, we showed that the dark current advantage commonly observed in RHJ cells when compared with FJ cells persists at elevated T, enabling RHJ cells to outperform their FJ counterparts.While RHJ cells showed similar or poorer performance compared with FJ cells at RT, significant boosts in carrier collection from the long lifetime and thermally boosted diffusion length, as well as improved V OC retention, were observed with increasing T. Taking advantage of the higher barrier height in MM GaInP, we found that the MM RHJ cells exhibited the lowest W OC values among all the cells studied in this work at T > 150 °C, showing the beneficial effects of suppressing thermionic emission.Further investigations are necessary to develop low-resistance contacts with long-term stability.

Fig. 2 .
Fig. 2. TRPL lifetimes of (a) LM and MM p-GaInP DHs and (b) LM and MM n-GaInP DHs as a function of temperature.

Fig. 3 .
Fig. 3. Valence band diagrams of (a) MM n-GaInP DHs with MM n-Al 0.38 In 0.62 P barriers and (b) LM n-GaInP DHs with LM n-Al 0.52 In 0.48 P barriers at 27 °C (dashed lines) and 300 °C (solid lines), respectively.The diagrams depict only the valence bands and ΔE V,300 °C indicates the valence band offset at 300 °C.
(a) and (b); at elevated T, the higher barrier heights in the MM are still maintained compared with LM, with ΔE v values of 0.59 eV for MM and 0.52 eV for LM at 300 °C [solid lines in Fig. 3(a) and (b)].The benefit of MM GaInP is further supported by the longer τ TRPL values observed in both MM p-and n-GaInP compared with their LM counterparts, indicating longer τ SRH , τ int , and/or τ th across the entire temperature range.B. Advantages of MM Over LM GaInP FJ Cells at High T In Fig. 4(a) and (b), IQE curves of LM and MM GaInP FJ cells show the expected shift to longer wavelengths due Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.
Fig.5(a) and (b) demonstrates that IQE increases across all wavelengths up to 300 °C for both LM and MM RHJ cells due to the increase in τ TRPL with T and the direct dependence of carrier diffusivity on T[27].Between 27 and 300 °C, LM RHJ cells show a slight increase in peak IQE from ∼92% to ∼95%.In contrast, MM RHJ cells exhibit a strong boost in peak IQE from ∼84% to ∼95%, which can be attributed to the stronger increase in τ TRPL in MM n-GaInP with T.The LIV curves in Fig.6(a) and (b) clearly show the advantages of RHJ cells over their FJ counterparts in terms of J SC , V OC , and FF at elevated T. While LM FJ and RHJ cells show similar V OC and J SC values at RT, LM RHJ cells display

Fig. 7 .
Fig. 7. (a) Integrated EQE, (b) W OC , (c) FF, and (d) efficiency of MM and LM GaInP cells as a function of T. Filled and open symbols indicate MM and LM GaInP cells, respectively.Squares and circles indicate FJ and RHJ configurations, respectively.
Table I further confirms the superior performance of RHJ cells at elevated temperatures, as evidenced by lower dV OC dT and higher dJ SC dT values compared with their FJ counterparts.Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.